V^ X A-S / London : Pbinces Street, Soho ; October, i838. WORKS ON MEDICINE, SUEGERY, MIDWIFERY, AND THE COLLATERAL SCIENCES, PUBLISHED BY MR. CHURCHILL. NEW WORKS PUBLISHED, OR PREPARING FOR PUBLICATION, DURING THE PRESENT YEAR. 1. PRACTICAL and OPERATIVE SURGERY; with One Hundred and Thirty Engravings on Wood. By Robert Liston, Surgeon to University College Hospital. 8vo. cloth, price £1 2s. Second Edition. Just ready. 2 ON THE NATURE and TREATMENT of STOMACH and URINARY DISEASES; being an Enquiry into the Connexion of Diabetes, Calculus, and other Affections of the Kidney and Bladder with Indigestion. By Wm. Prout, M.D., F.R.S. The Third Edition, revised and much enlarged. Preparing for immediate publication. 3. THE MORBID ANATOMY of the UTERUS and its APPEN- DAGES. Illustrated with highly-finished Colored Plates, in Folio, from Drawings by Mr. Perry, with descriptive letter-press. By Robert Lee, M.D., F.R.S., Lecturer on Midwifery at St, George's Hospital. Fasciculus 1. Just ready. MR. CHURCHILL S LIST OF 4. SURGICAL OBSERV ANIONS on TUMOURS ; with CASES and OPERATIONS. By John C. Warren, M.D., Professor of Anatomy and Surgery in Harwood Universit}', and Surgeon of the Massachusetts General Hospital; in royal 8vo., with 16 colored plates. *,* From the high encomiums passed upon the above work in all the English reviews, and the flattering opinion expressed of its merits by many of the first Surgeons, J. Churchill is happy to announce its early publication in this country, having made the necessary arrange- ments with the respected author during his recent visit to England. 5. PRINCIPLES of GENERAL and COMPARATIVE PHY- SIOLOGY; intended as an Introduction to the Study of Human Physiolog)^ and as a Guide to the Philosophical pursmt of Natural Historj'. By William B. Carpenter, M.R.C.S., late President of the Royal Medical and Royal Physical Societies, and Fellow of the Royal Botanical Society of Edinburgh ; Lecturer on Forensic Medicine in the Bristol Medical School. In one volume, 8vo. With Copper Plates and Wood Engravings. Nearly ready. 6. A SYNOPSIS of the VARIOUS KINDS of DIFFICULT PARTURITION, with Practical Remarks on the Management of Labours. By Samuel Merriman, M.D., F.L.S. A New Edition, with additions, 8vo. Plates, price \2s. Nearly readt/. THE SURGEON'S VADE MECUM; containing the Symptoms, Causes, Pathology, Diagnosis, Prognosis, and Treatment of Surgical Diseases and Injuries. Illustrated with Wood Engravings. By Robert Druitt, M.R.C.S. Nearlif ready. A TREATISE on RUPTURES. By W. Lawrence, F.R.S., Surgeon Extraordinary to the Queen, and Surgeon to St. Bartholomew's Hospital. The Fifth Edition, with considerable additions. 8vo. cloth, \Qs. CLINICAL LECTURES on COMPOUND FRACTURES of the EXTREMITIES, on Excision of the Head of the Femur, &c. &c., delivered at the Westminster Hospital in the Winter of 1837-S. By G. J. Guthrie, F.K.S., Surgeon to the Hospital. 8vo. cloth, 3*. 10. PRACTICAL OBSERVATIONS on the PRESERVATION of HEALTH, and the PREVENTION of DISEASES; comprising the Author's experience on the Disorders of Childiiood and Old Kg&. By Sir Anthony Carlisle, F.R.S., late President of the Royal College "of Surgeons, and Senior Surgeon to the Westminster Hospital. 8vo. cloth, price 8*. WORKS PUBLISHED THIS YEAR. 11. A PRACTICAL TREATISE^ on FRACTURES ; illustrated with Sixty Woodcuts. By Edward F. Lonsdale, Demonstrator of Anatomy at the Middlesex Hospital School of Medicine. 8vo. price Ids. 12. A TREATISE on the Nature and Treatment of HOOPING-COUGH, and its Complications ; illustrated by Cases, with an Appendix, containing Hints on the Management of Children, with a view to render them less susceptible of this and other Diseases of Childhood, in an aggravated Form. By George Hamilton Roe, M.D., Fellow of the Royal" College of Physicians, and Physician to the Westminster Hospital. 8vo, cloth, price 8*. 13.- NOTES on the MEDICAL HISTORY and STATISTICS of the BRITISH LEGION of SPAIN; comprising the results of Gun-shot wounds, in relation to important questions in Surgerj'. By Rutherford Alcock, K.T.S., Deputy Inspector General of Hospitals, die. 8vo. price 6s. 14. COUNTER-IRRITATION; its Principles and Practice, illustrated by One Hundred Cases of the most painful and important Diseases effectually cured by External Applications. By A. B, Granville, M.D. F.R.S. 8vo. cloth, price 10*'. 6ci, 15. THE VILLAGE PASTOR's SURGICAL and MEDICAL GUIDE ; in Letters from an Old Physician to a Young Clergyman, his son, on his entering upon tile Duties of a Parish Priest. By Fenwick Skrimshire, M.D., Physician to the Peterborough Infirmary. 8vo. cloth, price 8s. 16. A MANUAL of the DISEASES of the EYE; or Treatise on Oph- thalmology. By S. Littell, M.D., of Philadelphia; revised and enlarged by Hugh Houston, M.R.C.S. ]2mo. cloth, price 5*. ir. INTERMARRIAGE; or the Slode in which, and the Causes why, Beauty, Health, and Intellect, result from certain Unions, and Deformity, Disease, and Insanity from others ; demonstrated by Delineations of the Slructm-e and Forms, and Descriptions of the Functions and Capacities, whicli each Parent, in every Pair, bestovis on Cliildren, in conformity with certain Natural Laws, and by an account of Corresponding Effects in the Breeding of Animals. Illustrated by Drawings of Parents and Progeny. By Alexander Walker. 8vo. with Plates, lis. cloth. MB. CHURCHILL S LIST OF Mr, ATKINSON. MEDICAL BIBLIOGRAPHY. By James Atkinson, Senior Sur- geon to the York County Hospital, and late Vice-President of the Yorkshire Philosophical Society. Vol. I. royal 8vo. 16s. " We have never encountered so singular and remarkable a book. It unites the German research of a Plouquet, with the ravings of Rabelais, — the humour of Sterne with the satire of Democrates, — the learning of Burton with the wit of Pindar." — Dr. Johnson's Review. Mr. BATE MAN. MAGNACOPIA ; a Library of Useful and Profitable Information for the Chemist and Druggist, Apofhecary, Surgeon-Dentist, Oilman, &c., containing several Hundred New Forms, with Comments, and a variety of other information. By William Bateman, Practical Chemist. Second Edition. 24mo. 6s. " The advantage of being as wise as one's neighbour in matters of business tends materially to the augmentation of our finances. Most of the forms given in this book are so partially known, (and many of them not at all,) that to those engaged in selling, by wholesale or retail, the saving, in many instances, will be very great indeed. In fine, the practitioner, the trader, and the consumer, meet their right-hand friend at every page." — Extract from the Preface. Mr. BE ALE. A TREATISE on the DISTORTIONS and DEFORMITIES of the HUMAN BODY ; exhibiting a concise view of the Nature and Treatment of the Principal Malformations and Distortions of the Chest, Spine, and Limbs. By Lionel J. Beale, Esq., Surgeon. Second Edition. 8vo. with plates, 12*. " We take leave of our author with every sentiment of respect, and have only to reiterate our favorable opinion of his work. It is at once scientific and practical, and presents a condensed and accurate sketch of the many points on spinal and other deformities, to which every man must frequently have occasion to refer in practice." — Medical and Surgical Journal. Mr. BENOIT. THE BOTANIST'S POCKET COMPANION ; containing a general outline of Botanical Science, with a view of the Linnsean and Natural Systems, and a Description of the Medical Plants in the Chelsea Botanic Garden. By T. T. W. Benoit. 32mo. cloth, 1*. 6cl. Mr. BLAINE. OUTLINES of the VETERINARY ART, or the PRINCIPLES of MEDICINE, as applied to a Knowledge of the Structure, Functions, and Economy of the Horse, comprehending a concise View of those of Neat Cattle and Sheep; the whole illustrated by Anatomical Plates. Fourth Edition, entirely recomposed. 8vo. £1 4*. Mr. SAMUEL COOPER. THE FIRST LINES of the PRACTICE of SURGERY ; designed as an Introduction for Students, and a concise Book of Reference for Practitioners. By Samuel Cooper, Professor of Surgery in the University of London. Sixth Edition, carefully corrected, and considerably improved. 8vo. 18*. Bi/ the same Author. A DICTIONARY of PRACTICAL SURGERY; comprehending all the most interesting improvements, from the earliest times down to the present period, &c. &c. Seventh Edition. In the Press. MEDICAL WORKS. Mr. CKOSSE. A TREATISE on the FORMATION, CONSTITUENTS, and EXTRACTION of the URINARY CALCULUS. By John Green Crosse, Esq., F.R.S., Surgeon to the Norfolk and Norwich Hospital. Being the Essay for which the Jacksoniaii Prize for 1833 was awarded by the Royal College of Surgeons in London. 4to., with numerous Plates, price £2 2s. plain, £2 12s. Qd. colored. " It 13 a work which all hospital surgeons will possess — indeed, which all surgeons who wish to be well acquainted with their profession should." — Dr. Johnson's Review. " Experience and study have done their utmost for this work. We hope its circulation will be equal to its merits." — Medical Quarterly Review. " Mr. DENHAM. VERBA CONSILII; or. Hints to Parents who intend to bring up their Sons to the Medical Profession. By W. H. Denham, F.R.C.S. 12mo. cloth, 3s. Qd. M. DUPUTTREIT. A TRANSLATION of PARISET'S ELOGE upon BARON DUPUYTREN, with Notes. By J. T. Ikin, Surgeon. ^yo.2s.Qd. Mr. EVANS. A CLINICAL TREATISE on the ENDEMIC FEVERS of the WEST INDIES, intended as a Guide for the Young Practitioner in those Countries. By W. J. Evans, M.R.C.S. 8vo. cloth, 9*. " We strongly recommend this work to every Medical Man who leaves the shores of Eng- land for the West India Islands. It is full of instruction for that class of the Profession, and indeed contains a great mass of materials that are interesting to the Pathologist and Practi- tioner of this country." Medico-Chirw. Review, 52. April, 1837. Dr. GRANVILLE. GRAPHIC ILLUSTRATIONS of ABORTION and the DISEASES of MENSTRUATION. Consisting of Fourteen Plates, from Drawings en- graved and colored by Mr. J. Perry. Representing forty-five specimens of aborted ova and adventitious productions of the Uterus, with preliminary observations, explanations of the figures, and remarks, anatomical and physiological. By A. B. Granville, M.D., F.R.S. Price £2. 2s. " We feel called upon to notice this work thus early on account of the extraordinary and unparalleled beauty of the plates. As colored productions, and in fidelity of execution, they certainly stand unrivalled; and the volume will prove not only an elegant and brilliant, but a most useful, ornament of every medical library in which it may be placed." — hancet. " This is really a splendid volume, and one which in an especial manner deserves the patron- age of the profession. The plates are beautifully executed; some of them superior, as speci- mens of art, to anything which has hitherto appeared in this country. This work is sold at what cannot be a remunerating price, especially as the number of impressions is very limited. * # » * • As we have been under the necessity of differing much and frequently from Dr. Granville, it affords us pleasure on this occasion to speak in terras of unmingled commenda- tion."— Medical Gazette. Mr. GRAY. A SUPPLEMENT to the PHARMACOPOEIA ; being a Treatise on Pharmacology In general ; including not only the Drugs and Compounds which are used by Practitioners in Medicine, but also most of those which are used in the Chemical Arts, or which undergo Chemical Preparations. Sixth Edition. Svo. lis. MR. CHURCHILL S LIST OF Dr. aULLT. AN EXPOSITION of the SYMPTOMS, ESSENTIAL NATURE, and TREATMENT of NEUROPATHY, or^ Nervousness. By James M. Gully, M.D. 8vo. boards. 6s. Mr. GUTHRIE. ON the ANATOMY and DISEASES of the URINARY and SEXUAL ORGANS ; being the First Part of the Lectures delivered in the Theatre of the Royal College of Surgeons, and in the VVestmiaster Hospital. By G. J. Guthrie, F.R.S. 8vo. Colored Plates. iOs. 6cL By the same Author. ON the CERTAINTY and SAFETY with which the OPERATION for the EXTRACTION of a CATARACT from the HUMAN EYE may be performed, and on the Means by which it is to be accomplished. 8vo. 2*. 6d. Dr. HENNBE". PRINCIPLES of MILITARY SURGERY; comprising Ohservations on the Arrangement, Police, and Practice of Hospitals : and on the History, Treatment, and Anomalies of Variola and Syphilis. Illustrated with Cases and Dissections. By John Hennen, M.D., F.R.S.E., Inspector of Military Hospi- tals. Third Edition. With Life of the Author, by his Son, Dr. John Hennen. 8vo. boards. 16*. " The value of Dr. Hennen's work is too well appreciated to need any praise of ours, We are only required, then, to bring the third edition before the notice of our readers ; and having done this, we shall merely add, that the volume merits a place in every library, and that no military surgeon ought to be without it." — Medical Gazette. Sir EVEEAED HOME. LECTURES on COMPARATIVE ANATOMY; in which are ex- plained the PREPARATIONS in the HUNTERIAN COLLECTION. Six vols. 4to., with several hundred Plates. The Executors of Sir Everard Home having directed the disposal of the above splendid work, J. Churchill became the purchaser, and now oft'ers it at less than half of the published price, the small paper for 8 guineas, published at 18 guineas ; the large paper for 12 guineas, published at 26 guineas. • ,* According to Mr, Clift's Evidence before the Committee of the House of Commons, this Work contains the substance and only remains of the unpublished Writiiigs of the cele- brated John Hunter, Dr. HOOPER. LEXICON MEDICUM, or MEDICA L DICTIONARY ; containing an Explanation of the Terms in Anatomy, Physiology, Practice of Physic, Ma- teria-Medica, Chemistry, Pharmacy, Surgery, Midwifery, and the various Branches of Natural Philosophy connected with Medicine, selected, arranged, and compiled from the best Authors. By Robert Hoopek, M.D. Seventh Edition, edited by Dr. Grant. In the Press, By the same Author. THE PHYSICIAN'S VADE MECUM ; or, Manual of the Principles and Practice of Physic ; containing the Symptoms, Causes, Diagnosis, Prognosis, and Treatment of Diseases, &c. &c. New Edition, considerably enlarged, edited by Dr. Ryan. 7s. 6d, boards. MEDICAL WORKS. Dr. JEWEL. PRACTICAL OBSERVATIONS on LEUCORRHCEA, FLUOR ALBUS, or " WEAKNESS/' with cases illustrative of a new mode of treat- ment. By George Jewel, M.D. Physician- Accoucheur to the .Royal Lying-in Hospital ; Lecturer on Midwifery, &c. 8vo. boards. 5s. " We now beg to offer Dr. Jewel our unfeigned thanks for his valuable little work. It will do more to alleviate human suffering and to secure happiness, than many brilliant discoveries : no mean praise." — Medical Gazette. By the Same. * LONDON PRACTICE of MIDWIFERY: including the most im- portant Diseases of Women and Children. Chiefly designed for the Use of Students and early Practitioners. With Alterations and Additions. 12mo. 6th edit. 6s. Qd. Mr. LAWKElSrCE. A TREATISE on the DISEASES of the EYE. By W. Lawrence, F.R.S., Surgeon to St. Bartholomew's Hospital. One thick 8vo. vol., price 18s. " We earnestly recommend this able and interesting work to the perusal of every surgeon, and every student of medicine."— Erfi>j6M»-g-ft Medical and Surgical Joiinwl, " In this work we find combined the results of the author's own practice and observation, with the science and experience of the most eminent surgeons on the Continent." — Medical Gazette. Mr. LEE. OBSERVATIONS on the PRINCIPAL MEDICAL INSTITU- TION Sand PRACTICE of FRANCE, ITALY, and GERMANY; with Notices of the Universities, Cases of Hospital Practice, &c. By Edwin Lee, Esq., for- merly House- Surgeon to St. George's Hospital. 8vo. 8s. boards. By the same Juthor. A TREATISE on some NERVOUS DISORDERS, being chiefly intended to illustrate those varieties which simulate Structural Disease. Second Edition, rewritten and considerably enlarged ; with an Appendix of Ca^-es. 8vo. 7s. Dr. LEY. AN ESSAY on LARYNGISMUS STRIDULUS, or Croup-like In- spiration of Infants. With Illustrations of the General Principles of the Pathology of the Nerves, and of the Functions and Diseases of the Par Vagum and its prin- cipal Branches. By Hugei Ley, M.D., Lecturer on Midwifery at St. Bartholo- mew's Hospital. 8vo. Plates. 15s. " One of the most important essays that has appeared in this country during the present century." — Medico-Chirurgical Review. •' Every page of the work affords proof of the uncommon industry with which Dr. Ley has investigated the subject in all its bearings; and, in our opinion, the original views he entertains of the Pathology of ' Laryngismus Stridulus,' are perfectly correct." — British and Foreign Medical Renew. M. MAG-ENDIB. MAGENDIE'S FORMULARY, for the Preparation antl Adminis- tration of certain New Remedies ; translated from the last French Edition, with Annotations and Additional Articles. By Jame.s Gully, M.D. Second Edition. 5s. 6f/. boards. " A work of remarkable succinctness and merit." — Britishand Foreign Medical Review. MB. CHURCHILL S LIST OF Dr. MACREiaHT. A MANUAL of BRITISH BOTANY; in which the Orders and Genera are arranged and described according to the Natural System of De Can- DOLLE ; witb a Series of Analytical Tables for the assistance of the Student in the Examination of the Plants indigenous to, or commonly cultivated in. Great Britain. By D. C. Macreight, M.D., Lecturer on Materia Medica and Therapeutics at the Middlesex Hospital. Small 8vo. cloth. 7*. 6d. " There is a prodigious mass of elementary matter and useful information in tliis Pocket Volume." — Medico-Chiriir. R.evieiv,July, 1838. " This very elegant little volume is a most useful accession to Botanical Literature." — Literary Gazette, July, 1838. Mr. MAPLE SON. A TREATISE on the ART of CUPPING, in which the History of that Operation is traced, tbe Complaints in which it is useful indicated, and the most approved method of performing it, described. By Thomas Mapleson, Cupper to his Majesty. A new Edition, improved. 12mo. boards, 4«. Mr. MATO. OUTLINES of HUMAN PHYSIOLOGY. Fourth Edition, with numerous Engravings on Wood. By Herbert Mayo, F.R.S., Surgeon to tbe Middlesex Hospital. 8vo. clotb. 18s. MEDICAL BOTANY. Now COMPLETE in three handsome royal 8vo. vols., illustrated by two hundred Engravings, beautifully drawn and colored from nature, price Six Guineas, done up in cloth and lettered, MEDICAL BOTANY; or, ILLUSTRATIONS and DESCRIP- TIONS of tbe MEDICINAL PLANTS of tbe London, Edinburgh, and DubUn Pharmacopoeias ; comprising a popular and scientific account of poisonous vegeta- bles, indigenous to Great Britain. By John Stephenson, M.D., F.L.S., and James Morss Churchill, F.L.S. New Edition, edited by Gilbert Burnett, F.L.S., &c. &c., Professor of Botany in King's College, London. " So high is our opinion of this work, that we recommend every student at college, and every surgeon who goes abroad, to have a copy, as one of the essential constituents of his library." — Dr. Johnson's Medico-Chirurgical Review, No. 41. " The price is amazingly moderate, and the work deserving of every encouragement." — Medical Gazette, •' The authors of Medical Botany have amply redeemed the pledge which their first num- ber imposed on them. The work forms a complete and valuable system of Toxicology and Materia Medica. It will prove a valuable addition to the libraries of medical practitioners and general readers." — Lancet. " The figures are equal, if not superior, to those of any other botanical periodical." — Loudon's Gardener's Mag. Mr.' OLIVER. THE STUDENT'S COMPANION to APOTHECARIES' HALL, or the London Pharmacopoeia of 1836, in Question and Answer. By Edward Oliver, M.R.C.S. 24mo. cloth. 4*. M. EATEE. A TREATISE on DISEASES of the SKIN. By P. Rayer, D.M.P. Translated from the French, by William B. Dickenson, Esq., Member of the Royal College of Surgeons. 8vo. price 12*. ' ' We can recommend the present translation of Bayer's Treatise as an excellent companion at the bedside of the patient." — Lancet. " The translation of Rayer has conferred a great obligation on the science of medicine in England." — Medical and Surgical Journal. MEDICAL WORKS. Dr. EEID. A MANUAL of PRACTICAL MIDWIFERY, containing a De- scription of Natural and Diflicult Labours, with their Management. Intended chiefly as a book of reference for Students and Junior Practitioners. By James Reid, M.D., Surgeon and Medical Superintendent to the Parochial Infirmary of St. Giles and St. George, Bloomsbury, and formerly House Surgeon to the General Lj'ing-in Hospital. 5s. 6d. with Engravings. " The relative diameters of the pelvis and the foetal head, and the different presentations of the child, are all usefully represented by wood engravings among the letter-press, and the book is thus particularly well calculated to effect the objects of such a work." — Lancet Dr. RTAN. THE MEDICO-CHIRURGICAL PHARMACOPEIA; or, a Con- spectus of the best Prescriptions ; containing an account of all New Medicines, Doses, ifec. ; Magendie's and Lugol's Formularies ; the Improvements in the London Pharmacopoeia. New Nomenclature ; the Treatment of Poisoning, Dislocations, Fractures, and natural and difficult Parturition. By Michael Ryan, M.D., Member of the Royal College of Physicians. Second Edition, 3s. 6d. cloth. " A vast mass of information in this little work, all useful at the bedside of sickness, or in the short hour of leisure from professional toils and anxieties." — Dr. Johnson's Review, July, 1838. Mr. SAVORY. A COMPANION to the MEDICINE CHEST ; or. Plain Directions for the Employment of the various Medicines used in Domestic Medicine. To which are added, a brief Description of the Symptoms and Treatment of Diseases ; Directions for Restoring Suspended Animation, and for Counteracting the Effects of Poisons; a Selection of Prescriptions of established Efficacy, &c. Intended as a source of easy reference for Clergymen, Master Mariners, and Passengers ; and for Families at a distance from Professional i^ssistance. By John Savory, Member of the Society of Apothecaries. 4^. neatly bound. " This is a very excellent and most useful little work from a highly respectable quarter. It will be found extremely useful in families." — hiterarj/ Gazette. Mr. SHAW. THE MEDICAL REMEMBRANCER; or, Practical Pocket Guide, concisely pointing out the Treatment to be adopted in the first Moments of Danger from Poisoning, Drowning, Apopljexy, Burns, and other Accidents. To which are added, various useful Tables and Memoranda. By Edward Shaw, M.R.C.S., one of the Medical Assistants to the Royal Humane Societv. Cloth, gilt edges, 2*. Qd. Mr. SNELL. A PRACTICAL GUIDE to OPERATIONS on the TEETH; to which is prefixed, an Historical SI A SUPPLEMENT to MYOLOGY, illustrated by colored Plates, on a peculiar construction, containing the Arteries, Veins, Nerves, and Lym- phatics, the Abdominal and Thoracic Viscera, the Brain, the Ear, the Eye, &c. and Evans, Clare-Street, Bristol. TO SIR JOHN F. W. HERSCHEL, BART., K.H. F.R.S. L. AND E. ETC., THIS VOLUME IS MOST RESPECTFULLY DEDICATED, AS A TRIBUTE DUE ALIKE TO HIS HIGH SCIENTIFIC ATTAINMENTS, AND MORAL WORTH, AND AS AN EXPRESSION OF GRATITUDE FOR THE BENEFIT DERIVED FROM HIS DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY, BY THE AUTHOR. PEEFACE. However trite may be the reason so commonly assigned by writers on any subject for presenting themselves to the public, the Author is not disposed to omit its mention as regards himself. During the course of his Physiological studies, he has felt, in common with many others, the want of a Treatise which should give a comprehensive view of the Science, embracing whatever general principles may be regarded as firmly estab- lished, and illustrating them as fully as could be done within moderate limits, yet without distracting the attention by profuse- ness of detail. He has long, therefore, kept in view the production of such a work as the present, should it not be anticipated by some other on the same plan; and in now deciding upon its publication, he has been influenced by the opinions of individuals of high eminence as Teachers of Phy- siology, as well as by the encouragement he has received from some who take an elevated station in Physical Science, and who have experienced the same deficiency. It is now generally acknowledged that Physiology can only be properly studied by a constant reference to the comparative structure and functions of many different classes of Animals ; and in most of the recent works on this Science, an outline of the development and actions of each system in the inferior tribes is prefixed to the details relating to its condition in man. This outline is filled up in the present volume, not only by amplifying the portion of it which relates to the Animal Kingdom, but also by the introduction of a similar view of the comparative structure and functions of Vegetables, which is here shown to be governed by the same laws. It is this which constitutes the peculiar feature of the work; as the author believes it to be the first attempt, in this country at least, to form anything like a VI PREFACE. systematic Comparative Physiology of Vegetables. The trans- lation of the elaborate Comparative Physiology of Tiedemann would, indeed, have occupied this ground; but it is still incom- plete, and is likely to remain so ; and the mass of details which it embraces, unconnected by comprehensive principles, renders it most tedious and embarrassing to the student. From that most valuable storehouse of facts, the present volume differs essen- tially, therefore, in plan ; this being devoted to the explanation and illustration of general Zaiys. Although his work is especially intended as an Introduction to the study of Human Physiology for the use of the Medical Student, the author has kept in view the wants of the General Reader, to whom he hopes to make intelligible some of the highest doctrines in this most interesting science. For this purpose he has given explanations of most of the scientific terms employed, in the situations where they could be most appro- priately introduced ; and reference to them is facilitated by the copiousness of the Index, which thus serves the purpose of a Glossary. He has also expressed himself in general terms in some instances where more detail might otherwise have been admitted; but he trusts that he has, by this means, avoided all chance of offending the true delicacy even of the female reader. The desire which he has felt to moderate the extent of the volume, and to make it generally acceptable both in size and price, has compelled the author unwillingly to omit the greater number of references which he had designed to introduce, and which his own experience leads him to consider as of importance in a work like the present. He has, however, retained those which concern insulated Memoirs on particular subjects, or facts of novel and peculiar interest; and the following list comprises the Systematic Treatises on the authority of which he has usually relied. Alison's Outlines of Physiology Edinb. 1836 Bostock's Elementary System of Physiology London 1836 Burdach. — ^Traite de Physiologie, traduit par Jourdain ...... Paris 1837-8 Burmeister. — Manual of Entomology, translated by Shuckhard London 1836 Burnett's Outlines of Botany London 1835 Cams. — Traite Elementaire d'Anatomie Comparee, traduit par Jourdain Paris 1835 Decandolle. — Organographie Vegetale Paris 1827 Physiologie Vegetale Paris 1832 Edwards on the Influence of Physical Agents on Life, trans- lated by Drs. Hodgkin and Fisher London 1832 EUiotson's Physiology London 1835-8 PREFACE. Vll Fletcher's Rudiments of Physiology Edinb. 1837 Grant's Outlines of Comparative Anatomy London 1835-8 Lectures published in the Lancet — Vols, xxv, xxvi. Graves's Introductory Lectures on the Institutes of Medicine, published in the London Medical and Surgical Journal, Vol. VII. Henslow's Descriptive and Physiological Botany London 1836 Kirby on the History, Habits, and Instincts of Animals (Bridgwater Treatise) London 1835 Lindley's Introduction to Botany London 1835 Introduction to the Natural System of Botany .... London 1837 Mayo's Outlines of Physiology London 1837 Miiller's Elements of Physiology, translated by Dr. Baly .... London 1838 Prout on Chemistry, Meteorology, and the Function of Diges- tion (Bridgwater Treatise) London 1834 Roget's Animal and Vegetable Physiology (Bridgwater Treat.) London 1834 Smith's Philosophy of Health London 1835-7 Solly on the Brain and Nervous System London 1836 Tiedemann's Comparative Physiology, translated by Drs. Gully and Lane London 1834 Cyclopaedia of Anatomy and Physiology Penny Cyclopaedia Reports of the British Scientific A ssociation The following, amongst others; have been occasionally con- sulted : — Adelon. — Physiologic de I'Homme Paris 1829 Blainville. — Cours de Physiologic Paris 1833 Buckland's Geology and Mineralogy (Bridgwater Treatise) .. London 1836 Cuvier. — Lemons d'Anatomie Comparee Paris 1835-8 Dutrochet. — Memaires Anatomique et Physiologiques Paris 1837 Lord's Popular Physiology London 1834 Lyell's Principles of Geology London 1835 Magendie. — Legons sur les Phenomenes Physiques de la Vie Paris 1835-8 Meckel. — Traite General d'Anatomie Comparee, traduit par Jourdain Paris 1828-38 Milne-Edwards. — Elemens de Zoologie Paris 18.34 Prichard's Researches into the Physical History of Mankind . . London 1836 Raspail. — Nouveau Systeme de Chimie Organique Paris 1833 Serres. — Anatomie Comparee du Cerveaa Paris 1827 Sprengel's introduction to the study of the Cryptogamia .... London 1807 Whewell's History of the Inductive Sciences London 1837 In the following pages is embraced the substance of an Essay on the "Laws regulating Vital and Physical Phenomena," to which was adjudged the annual Students' Prize awarded by the Medical Faculty of the University of Edinburgh for 1837; and also of an Essay on some departments of Vegetable Physio- logy which received the First Prize given by the Professor of Botany in the year 1836. The author has freely availed himself, also, of the liberal permission of the Editors of the British and VIH PREFACE. Foreign Medical Review to make what use lie deemed proper of his contributions to that journal; especially in regard to two Papers, — one on the Study of Physiology as an Inductive Science, and the other on the Functions of the Nervous System, — which have been recently honoured with a place in its pages. It would be ungrateful on the part of the author, were he not also to acknowledge his peculiar obligations to some, by whose instructions and personal guidance he has been greatly aided in his Physiological studies, and to whom whatever merit this pro- duction may possess is in a great measure due. To Dr. Hiley, his former instructor and present colleague in the Bristol Medical School, — to Professors Grant and Lindley, of London University College, — and to Professor Alison, of the University of Edinburgh, he gladly takes this opportunity of tendering his respectful thanks; and he cannot refrain from here offering the tribute of regard to his highly-valued friend Dr. John Reid, Lecturer on Physiology in the Argyle-Square Medical School, Edinburgh, to whose clearness of thought, experimental skill, and comprehensive knowledge of his Science, he feels himself in many respects deeply indebted. Whatever claims to originality the present Treatise may possess, the author is not disposed to put them forwards here. He has not hesitated to employ the language of other writers in the description of facts, wherever it seemed appropriate ; deem- ing it useless to alter, for the mere sake of rendering it his own, that which may be regarded as common property. But he believes that he has never employed the ipsissima verba of others in the expression of opinions, without acknowledgment. The facts for which he holds himself responsible may, in general, be readily distinguished. The degree of novelty which any of the inferences from them may possess, he is content to leave it to his readers to estimate; and he will only now express the hope that, as they have not been formed hastily or inconsiderately, they may not be too readily pronounced crude or unphilo- sophical. ANALYSIS OF THE CONTENTS. N.B. The Numbers refer to the Paragraphs. INTRODUCTION.— ON ORGANISED STRUCTURES. I. — Preliminary Remarks^ 1 — 7. On the Nature and Objects of the Science of Physiology. — Difficulties in the way of its advancement. — True mode of pursuing it. II. — Of Organised Structures in General, 8 — 20. The object of this Section, taken in connection with Chapter I. (Book I ), is to show the relations between Life, or Vital Action, and Organised Structure. The argument may be thus briefly stated. — The properties of any aggregation of Matter depend upon the mode in which its ultimate molecules are combined and arranged, and have not an existence separate from the matter itself (160). Tlie simplicity of our notion of the properties of inorganic matter depends upon the facility with which we may become acquainted with them, through the command which we possess over the agencies by whose operation they are manifested ; and their evident uniformity of action enables us at once to refer them to definite and comprehensive laws (4,141). These Laws simply express the conditions of action of the material bodies which they concern (147, 148). All Physical Phenomena result from the excitement of the physical properties of matter by external agents ; and, when these are not in opera- tion, no change takes place (1, 9, 10, 144). In like manner. Vital Actions result from the excitement of the vital properties of certain forms of matter by external agents or stimuli ; and are not manifested, or called into play, without the infiuence of these (10, 144, 155). These Vital properties exist only in organised tissues, and stand in the same relation to them as do the Physical properties to matter in general (140). They may be regarded as essentially dependent on the peculiar state in which the component particles exist. This state can only be induced by an action of Organ- isation efiected upon inorganic matter by a pre-existing structure ; and the change thus operated developes properties that previously lay dormant, the material particles not being, until then, in the condition required to exhibit them (10, 151, 152). In proportion as the properties thus called into exercise differ from those common to matter in general, does the organised tissue which possesses them, exhibit pecu- liarities of structure and composition unlike those presented by other forms of matter (11, 19, 159). These peculiarities are such that spontaneous decomposition of the elements has a constant tendency to take place, especially in the most highly organised structures ; but this is kept in check, in the living body, by the continual renovation which is characteristic of Vital Action (18, 214). So long as this takes place, the vital properties are retained; but if any considerable alteration of structure or composition occur, they are perverted or altogether destroyed (152 — 4). All Vital Action, therefore, is dependent on two conditions ; — an organised structure possessed of properties not manifested by inorganic matter ; and a stimulus by which these properties are excited to action. But many of the changes concerned in the main- tenance of Life are of a strictly physical character ; and it is by these that the con- nection is effected between the Organism and the external world (159 — 165). ANALYSIS OP THE CONTENTS. III. — Elementary Structure of Vegetables. Membrane and Fibre 21 Cbllttlak Tissue 23 Dotted ducts 24 Ligneous Tissue, or Woody Fibre 25 Vascular Tissue 26 Spiral Vessels 26 Ducts, spiral and annular . . 27 reticulated 29 IV. — Elementary Structure of Animals, 31. Fibre and Membrane 32 Cellular Structure Adipose Tissue 33 Cellular (areolar) Tissue 34 Serous Membranes 36 Fibrous Membranes 37 Mucous Membranes 38 Skin 39 Cartilage 40 Bone 41 Muscular Tissue 42 Nertous Tissue 44 V. — Transformation of Tissues, 45 — 47. YI. — General View of the Vegetable Kingdom, 48, 70. PHANEROGAMIA 49 Structure of Seed 50 Exogens (Oak, Eose, Bean) 51 Structure of Stem 51 Endogens (Palms, Lilies, Grasses) 52 Structure of Leaves 63 Flower 54 Gymnospermse (Pine, Zamia) 57 Rhizanthese (Rafflesia) 58 CRYPTOGAMIA 49 FiLiCES (Ferns) 59 Musci (Mosses) 60 Hepaticse (Liver-worts) 61 Characese (Stone-worts) 62 Fungi (Mushroom, Mildew) 63 Spontaneous Origin ? 65 LiCHENES (Lichens) 68 Alg^ (Sea-weeds, Confervse) .... 69 VII. — General View of the Animal Kingdom, 71 — 74, 122, 123. VERTEBRATA 73 Mammalia (Quadrupeds) 75 AvES (Birds) 76 Rbptilia (Reptiles) 77 Metamorphosis 80 Pisces (Fishes) 81 ARTICFLATA 82 Crustacea (Crab, Lobster) 84 Arachnida (Spider, Mite) 86 Insecta (Insects) 86 Larva and Pupa 87 Myriapoda (Centipede) 90 Annelida (Nereis, Leech, Earth- worm) 91 CiRRHOPODA (Barnacles) 92 RoTiFERA (Wheel- Animalcules) . . 93 Entozoa (Ascaris, Lernsea) 94 MOLLUSCA 95 Cephalopoda (Cuttle-fish,Nautilus) 96 Pteropoda (Clio, Hyalsea) 98 Gasteropoda (Snail, Slug, Cowrie) 99 Conchifera (Oyster, Pecten) .... 102 Tunicata (Ascidia, Pyrosoma) 104 RADIATA 105 Echinodermata (Sea-Urchin, Star- fish) 106 AcALEPH^ (Medusa, Beroe) 108 ACRITA 109 Cilia 110 Steeelmintha (Hydatid, Taenia) 111 Polygastrica (Infusorial Animal- cules) 113 PoLYPiFERA (Zoophytes) 115 PoRiFBRA (Sponges) 121 Comparison of Animal and Vegetable Kingdoms, 124 — 130. VIII. — Symmetry of Organised Structures, 131 — 139. ANALYSIS OF THE CONTENTS. BOOK I.— GENERAL PHYSIOLOGY. CHAPTER I. — ON THE NATURE AND CAUSES OE VITAL ACTIONS, 140 — 1G6. See Introduction, Section II. CHAPTER II. — OF VITAL STIMULI. General Considerations 167 Of Heat as a Vital Stimulus 171 Of Light as a Vital Stimulus 177 Of Electricity as a Vital Stimulus 185 Physical Conditions of the surrounding medium 188 CHAPTER III. — GENERAL LAWS OF ORGANIC DEVELOPMENT, 190 — 210. CHAPTER IV. — GENERAL VIEW OF THE FUNCTIONS, 211 — 230. BOOK IL— SPECIAL AND COMPARATIVE PHYSIOLOGY. CHAPTER V. — ON THE INGESTION AND ABSORPTION OF ALIMENT. General Considerations 231 Endosmose 244 Absorption in Vegetables 246 Absorption in Animals 257 Process of Digestion 259 CHAPTER VI. — ON THE CIRCULATION OP NUTRITIVE FLUID. General Considerations 281 Circulation in Vegetables 283 Circulation in Animals 292 CHAPTER VII. — ON INTERSTITIAL ABSORPTION. Lymphatic system of Animals 331 CHAPTER VIII. — ON THE NUTRITION AND FORMATION OF TISSUES. General Considerations 341 Nutrition in Vegetables • 343 Nutrition in Animals 356 CHAPTER IX. — ON RESPIRATION. General Considerations 869 Respiration in Vegetables 373 Respiration in Animals 386 CHAPTER X. — ON THE EXHALATION OP AQUEOUS VAPOUR. General Considerations ^27 Exhalation in Vegetables 428 Exhalation in Animals ^34 Xh ANALYSIS OP THE CONTENTS. CHAPTER XT. — ON THE SECRETIONS IN GENERAL. General Considerations 441 Secretion in Vegetables 444 Secretion in Animals 454 - CHAPTER XII. — EVOLUTION OF LIGHT, HEAT, AND ELECTRICITY. Evolution of Light in Vegetables 473 in Animals 474 Evolution of Heat. General Considerations 478 . • in Vegetables 479 in Animals 481 Evolution of Electricity. General Considerations 496 in Vegetables 499 in Animals 501 CHAPTER XIII. — ON THE REPRODUCTION OE ORGANISED BEINGS. General Considerations 51 2 Reproduction in Vegetables 518 Reproduction in Animals 528 CHAPTER XIV.— SUBORDINATE LAWS REGULATING REPRODUCTION. Distinction of Species 542 Laws of Hybridity 545 Hereditary Transmission of acquired peculiarities 547 CHAPTER XV. — SENSIBLE MOTIONS OP LIVING BEINGS. Of Contractility in general 552 Manifestations of Contractility in Vegetables 563 Muscular Contractility in Animals 669 CHAPTER XVI. — FUNCTIONS OP THE NERVOUS SYSTEM. General Characters in different Classes of Animals 561 Functions of the Symmetrical (cerebro-spinal) system 584 Functions of the ^sj/mwieifricaZ (sympathetic or ganglionic) system 595 CHAPTER XVII. — OP THE MARKS OF DESIGN IN ORGANISED STRUCTURES. Final Causes 596 Inferences of Design to be rather drawn from laws than from individual^acfs . . 598 INTRODUCTION. ON ORGANISED STRUCTURES. I. — Preliminary Remarks. 1. The most careless observer cannot fail to recognise in tlie world around him, many evident distinctions between living beings and inani- mate objects. Perbaps the most apparent and positive of these distinc- tions is rather based upon a comparison of their mode of existence, than upon an examination of their intimate structure. The ceaseless tendency to change manifested in the life of the former, stands in yet more obvious contrast with the unaltering stability of the latter, than does that peculiar arrangement of elementary particles which is called Organisation, with the regular aggregation of the ultimate atoms which the Inorganic world presents. The snow-capped mountain rears its summit to the clouds, unaffected by the lapse of the ages which have rolled by since its first elevation — a monument of Nature's power ; and the giant edifices erected by the hand of man on the plains of Egypt, bear to remote posterity the attestation of the former gi-andeur of a nation now sunk into poverty and insignificance. And what, compared with the permanence of these, is the duration of any structure subject to the conditions of vitality ? To be bom — to gi'ow — to amve at maturity — to decline — to die — to decay, is the sum of the history of every being that lives ; from man in the pomp of royalty or the pride of philosophy, to the gay and thoughtless insect that glitters for a few hours in the sunbeam and is seen no more ; from the stately oak, the monarch of the forest through successive centuries, to the humble fungus which shoots forth and withers in a day. How simply, yet how expressively, are these changes described in the words of the sacred writer, " Our life is as a vapour, which appeareth for a little time and then vanisheth away." 2. And yet, amidst the constant change and succession of individuals, we observe the form and character first impressed upon each race by the B 2 ON QBGANISED STRUCTURES. Creator of all, uninterruptedly transmitted from parent to offspring through periods of indefinite duration. " One generation passeth away" — but " another cometh" — ^like it in structure, functions, habits, food, instincts, passions, and the limit of its existence. The mistletoe flourishes on the oak of our own forests, just as when made an object of superstitious veneration in the hallowed groyes of our Druidical ancestors. The bee builds her comb with the same unvarying regularity, and stores it with the same materials now, as when her beautiful works attracted the notice of the poets and philosophers of classic ages. And man, however modi- fied by education, however various his degree of civilisation, however elevated his condition of mental and moral refinement, is yet born the same helpless dependent being, with the same dormant faculties of body and mind, as the first offspring of our original parents. 3. In the ever- varying conditions of the animated world, then, a very superficial glance Avill display to us a certain degree of regularity and arrangement ; and the more attentively we investigate the relations which its changes present, the more stable and definite is the assurance we obtain, that they are all harmonised and controlled by fixed laws, which are but simplified expressions of those conditions of action which the Creator has imposed upon organised no less than upon inorganic matter. To arrive at the knowledge of these laws, and, having attained them, to trace their application to all the countless variety of phenomena presented by the myi-iads of living beings whose beautiful forms people this globe, is the object of the science of Physiology, — using that term in its most extended sense, to which the designation Biology is perhaps rather applicable. That the most advantageous plan of studying it is that inductive method which has been successful in other sciences, will not perhaps now be disputed : yet the prevalence of a contrary system has long retarded its progress; and it is only within a recent period that the ends to be attained have been generally understood, and the most satis- factory means of pursuing them fully determined.* In this, as in the Physical Sciences, the first object of the philosophic enquirer is to collect a body of facts, by the comparison of which the general principles common to all may be deduced. Now the facts which the observation of living beings brings under our notice, are obviously of two kinds ; — one class having reference to their structure, the other to their actions ox functions. The investigation and comparison of the former class of particulars, is the object of the science of Anatomy ; whilst by the collection and generali- sation of the latter, the science of Physiology is built up. 4. The obstacles which interpose themselves to the prosecution of these sciences, result more from that difficulty in the ascertainment of facts and the observation of phenomena, which is occasioned by the peculiar conditions of living beings, than from any incapability on the part of * See Britlsli and Foreign Medical Review, vol. v., p. 317, &c. PRELIMINARY REMARKS. 3 these facts and phenomena to be comprehended within laws as stable and as definite as those of the physical sciences. Thus, although the structure of the human hodj has been carefully and minutely examined by so many thousands of anatomists, how many points are still uncertain, and how much still remains to be discovered ! Yet this structure is but one of those groups of instances — the Baconian term for phenomena — that must be collected from the many hundred-thousand species of plants and animals which the naturalist knows to exist on the surface of the globe, before we can have sufficient data for the establishment of those general and comprehensive laws, regulating the development of living organisms, which we may hope some day to ai-rive at. The difficulties that present themselves in the observation of the facts which it is the object of the science of Anatomy to ascertain and generalise, are as nothing to those which beset the path of the Physiological enquirer, who has to study the changes which all classes of living beings perform and undergo during the whole period of their existence. The sum of all the pheno- mena which constitute the Life of a single organised structure, and which result from the actions of that structiu-e, is, in like manner, to be regarded as a collection of facts^ of which each must be stated in a separate and concise form, before it can be made the subject of any general expression, founded upon the comparison of similar facts derived from the study of other living beings. Now the great difficulty in physiological investiga- tion results from the complexity of the combinations in which vital phe- nomena present themselves, and from their dependence upon one another to a degi'ee that almost entirely precludes their separate examination. Were Ave able to ascertain the changes which take place in the interior of the living body Avith the same ease that the astronomer watches the motions of a planet, or the chemist observes the formation of a precipitate, the very multiplicity of these changes, and the variety of the conditions under which they occur, would be of essential service in the determination of their laws, instead of being, as at present, sources of doubt and embar- rassment. The chemist, when desirous of establishing to which of the ingredients in a given mixture a particular effect is due, places each sepa- rately in the conditions required to produce the result : but the physiolo- gist finds that the attempt to insulate any one organ, and to reduce the changes performed by it to definite experimental investigation, necessarily destroys, or considerably alters, those very conditions under Avhich alone its functions can be normally performed. Take away an important and essential part of a living being, and it ceases to exist as such ; it no longer exhibits even a trace of those properties which it is our object to examine ; and its elements remain subject only to the common laws of matter. We cannot, like the fabled Prometheus of old, breathe into the lifeless clay the animating fire ; we cannot, by a judicious and skilful arrangement of those elements, combine them into ncAv and artificial forms B 2 4 ON ORGANISED STRUCTURES. SO as to produce new and unexpected phenomena ; and almost all our knowledge of tlie laws of Life must therefore be derived from observation only. Experiment can conduct us very little further in this enquiry than the determination of the dependence of the functions upon one another, and upon the external agents, heat, light, &c., by the action of which upon the organism the phenomena of Life are produced. But a judicious and careful sj^stem of observation will almost supply the place of experi- ment ; for the ever-varjdng forms of organised beings by which we are surrounded, and the constantly-changing conditions in which they exist, present us with such numerous and different combinations of causes and effects, that it must be the fault of our mode of study if we do not arrive at some tolerably definite conclusions as to their mutual relations. In the language of Cuvier, the different forms of animals may be regarded as " so many kinds of experiments ready prepared by Nature, who adds to or deducts from each of them different parts, just as we might wish to do in our laboratories, showing us herself at the same time their various results." 5. From such considerations as these, it will be evident that the laws of Life can only be searched for, with a probability of success, by investi- gating their operations wherever presented to us ; and that the study of Physiology can only be scientifically prosecuted, if the attainment of these laws be regarded as its ultimate object, by embracing within its range the examination of the phenomena exhibited by all classes of living beings. It is a great mistake to suppose that there is anything funda- mentally different in the character of the vital operations as performed in the animal and in the vegetable structures, or in the simpler and moi-e complicated organisms of either kingdom. An enlarged view of their functions, not based upon the observations of their conditions in one or two instances only, but derived from an extended examination of their perform- ance wherever manifested, Avill recognise an essential conformity through- out, wherever those which are really analagous are compared. There is an obvious advantage, therefore, in commencing the study of Physiology by an enquiry into the simplest manifestations of each of those functions which in the higher organisms are so complicated in their nature. From no such enquiry should the consideration of the Vegetable Kingdom be excluded ; for those vital functions which are performed by plants in common with animals, are presented by the former in a state of greater simplification than is ever exhibited by the latter ; since all the changes necessary to the support of the individual and the continuance of the species, are performed mthout the influence or interference of those powers which are possessed in a greater or less degree by the whole animal kingdom. Hence the physiologist may advantageously resort to the study of vegetable life, for the explanation of many of the proximate causes of those phenomena which are complicated in the higher forms of 1>RELIMINARY REMARKS. 5 organised beings by so great a variety of secondary influences. Of the advantage of this mode of investigation, the details hereafter to be given (chap. XV.) on the Motions of Plants compared with those of Animals, afford a characteristic illustration. In the pursuit of his science on this plan, therefore, the physiological student will learn what are the essential conditions of life ; he will see the changes indispensable to its support manifested in their simplest circumstances ; he will be able to ascertain what structures are necessary to their performance, and Avhat additions and modifications these may undergo to suit the various purposes of their existence, (see for illustration § 213 and 237) ; and thus he will be saved the necessity of unlearning many erroneous notions which he would un- avoidably imbibe from the premature study of the complex phenomena ex- hibited by the living human organism. Moreover, in those departments of Physiology which are capable of being elucidated by experiment, recourse may generally be had ^vith advantage to the lower classes of animals, and to plants ; since that bond of union Avhich links together, in the higher animals, all the changes concerned in the maintenance of the vitality of the system, is in them much less close and decided ; so that particular organs may be insulated, and the study of the conditions of their actions prosecuted to a much greater extent. It is in the investigation of the effects of those external agents upon which all the phenomena of Life are dependent (§ 144), that this mode of investigation is capable of being most advantageously employed. Thus, Dr. Edwards was enabled to arrive at many important conclusions on these points, by subjecting frogs and other reptiles to treatment which would have been fatal to animals of higher character (chap, ix.) ; and Mirbel, by watching the gro^^i:h of an humble liverwort (§ 61), to establish some beautiful principles as to the influence of light on the development of the Vegetable System. 6. The mass of facts Avhich is gi-adually being accumulated relative to the structure of living beings, and to the vital phenomena they exhibit, must be classified and arranged, before they can become subservient to the purposes of science ; and this object is accomplished in diffierent ways, according to the nature of the laws of which the philosophical enquirer is in search. Thus the Anatomist and Physiologist, whose object it is to discover the peculiarities of organised structures, their adaptations to particular uses, and the conditions of the functions to which they are subservient, analyse, as it were, the group of facts Avhich each li\dng being exhibits (§ 4) ; and, pursuing their enquiries through an extensive range of objects, classify these individual results according to their similarity with each other, and their ob\dous tendency to the same end : or, to speak in less abstract language, they compare the individual organs and functions through all the forms of animated beings in which they are manifested. The object of the Naturalist, on the other hand, is to ascertain the laws which regulate the combination of the separate organs O ON ORGANISED STRUCTURES. into living fabrics, and govern their adaptation to diflFerent modes of existence : he, therefore, viewing each organism in its totality^ arranges similarly formed beings into the same group, placing as the character common to the whole the points in which they agree, and leaving the subordinate diflferences to be added to this common character, in order to express the qualities of an individual. This classification (resembling the combination of anatomical details into descriptions of organs, and of physiological changes into functions) is but a step towards the establish- ment of general laws by which the structure of the organised kingdoms of nature is regulated. These laws, — expressing the manner in which the organs are combined and adapted to each other, the relative development or a simplicity of each, the modifications which the typical forms (§ 129) of each group may undergo according to the circumstances in which the being is to be placed, and various other conditions of its formation, — it is the object of the Naturalist to ascertain ; and any mode or system of classification which he may adopt, is valuable in proportion as it keeps the establishment of these laws in view, and facilitates the accumulation of the knowledge upon which they must be founded. The connexion between these two branches of investigation is so intimate that neither can be pursued with any probability of success, without a considerable know- ledge of the data and principles upon which the other is founded ; and he will evidently be the most likely to arrive at the discovery of important general truths in either, who includes the whole of the phenomena of life in one extensive survey. The Physiologist refers to the Naturalist for instances in which a function is performed on the same general plan, but under a great variety of circumstances, as manifested by the adaptation of the structure of the organ to the medium of existence, (e.g. the form- ation of the respiratory membrane into lungs or gills) ; whilst the Naturalist refers to the Physiologist to assist him, by the examination of the function and development of an organ, in determining its real character, to which the consideration of its form and structure alone might not lead him. The Natural System of Botany affords a beautiful example of this conjoint kind of investigation ; and there can be little doubt, from the advances recently made, that some of the most important laws, regulating the structure of living beings, and the combination of their organs, will be speedily disclosed to view. 7. Although the object of the present treatise is the exposition of Physiological principles alone, it seems desirable to preface these by such an outline of the general structure and arrangement of the organs on which the phenomena of Life are dependent, as may render subsequent details respecting their functions more intelligible. We shall first consider, therefore, what there is peculiar in the mechanical arrangement of the particles of which organised structures are composed, and in the forms which such fabrics present. The principal varieties of the primary or GENERAL CHARACTERS OF ORGANISED STRUCTURES. 7 elementary tissues of which the more complex organs of plants and animals are constructed, will then be described and compared Avith one another. And lastly, the general characters of the principal groups in each of the animated kingdoms of Nature will be pointed out, the mode in which their individual organs are arranged and combined will be explained, and their relative positions displayed. Although such know- ledge is readily accessible to the student of Natural History, the embarras des ric/tesses may not be a little perplexing to such as seek only that extent of it which will enable them to enter upon the study of Physiological Science, without being immediately checked by the want of this kind of information. It will probably be more conducive to the purpose of the proposed outline, to commence it with a description of the classes which are best kno^vn ; and to pass from these to others more simple, but whose structure is less generally understood, of which a more particular description will therefore be frequently required. In the portion of this volume, however, devoted to the consideration of the structure and functions of particular organs, an opposite method mil be adopted ; since there is an evident advantage in tracing these in their simplest manifestations, and thus determining what are their really essential conditions, before examining their more complex phenomena in beings of elevated rank in the scale of animated creation. II. — 0/ Organised Structures in General. 8. In the production of the changes which constitute the Life of every animated being, we find an agent employed which is peculiar to the bodies that exhibit such changes, and Avhich is entirely different from anything we observe in the surrounding universe. This agent is the mechanism which is termed organised structure; the designation given to it implying that it consists of separate parts or organs^ each of which is adapted to perform some distinct part in the Yital economy. The whole organised structure of any living being is termed its organism ; and the word organisation is used to imply that peculiar process by which the organism is constructed out of the materials supplied by the inorganic or mineral world, and sometimes also to indicate the state or condition of the matter upon which this process — one of the most remark- able of all the vital actions — ^has been effected. When we come to enquire into the nature of the functions which these organised structures perform, it will be seen that they all tend towards a common object — the maintenance of the integrity of the fabric. And it may be regarded therefore as the peculiarity of an organism, that its distinct parts or organs are destined thus to subserve, each in its osva. particular way, some general purpose. This, indeed, is one of the peculiarities which distinguish organised structures from inorganic matter ; for in a mineral, every particle possesses a separate individuality, and Avhatever changes 8 GENERAL CHARACTERS OF ORGANISED STRUCTURES, this undergoes in obedience to physical agencies, these changes occur in conformity to laws which apply to it as well separately as in conjunction with others; whilst in a living being, the actions of all parts of the machine are so connected together, that whatever influences one single particle of the organism on which these actions depend, mil more or less affect the entire system. Thus, we may suppose a mass of gold alloyed with a small quantity of silver, and immersed in nitric acid ; this chemi- cal agent will affect every particle of the silver as completely as if the mass consisted of nothing else, and vnll leave the gold in its previous condition, having of itself no power of dissolving it. On the other hand, a similar chemical agent applied to an organised structure, will not only destroy the integrity of the part itself, but Avill prodiice a disturb- ance of the general functions proportional to the importance of the organ which has been injured; whilst the influence of any of the ordinary external agents by which life is maintained (see chap, ii.) is exercised not only on the parts or organs with Avhich they are in immediate relation, but through them on the whole structure. 9. But it may be said that this is no more than takes place in any engine of human construction, or in the complicated machine of the universe, — that in these, as well as in li^ang bodies, there is an adapta- tion of parts to each other, and of their actions to some general purpose, — and that all forms of matter are possessed of properties by the mutual influence of which, changes may be produced, as regular and as ceaseless as those which living beings exhibit. Thus, the uniform motions of the heavenly bodies, the alternation of the seasons, the continual alterations which the crust of the earth is undergoing, the earthquakes and volcanic eruptions which so remarkably excite our attention to those alterations, and which may be regarded as more prominent indications of the same latent causes, the successive evaporation and condensation of the watery covering of the globe, the perpetual variations in the force and direction of the wind, the occasional recurrence of those violent meteorological changes which spread terror and desolation wherever they occur, but which serve such important purposes in the economy of Nature — all these phenomena, and many more which might be instanced, result, no less than the constant changes exhibited by the animated creation, from that adaptation of pai'ts to a Avhole which is so characteristic of design in the universe at large. Hence some philosophers have gone so far as to embody these phenomena into a general expression — the Life of the World ; and as far as the abstract meaning of the terms is concerned, it Avould be difficult to show that a single piece of mechanism, or the entire universe, is not organised as completely as any animated structure. But a little consideration vdll show that in the construction of a machine, man avails himself only of the ordinary or physical properties of matter ; and that in the most complicated arrangement of its parts, each single GENERAL CHARACTERS OP ORGANISED STRXJCTITRES. 9 element possesses only the same capabilities as it would have if separated from the rest. Thus the moving power of a clock is given by the gra- vitation of a weight, — that of a watch by the elasticity of a spring, — that of a steam engine by the expansion and condensation of watery vapour ; and all the rest of the mechanism is contrived only to give effect to these agencies by employing them in the manner most advantageous for the designed end. In the phenomena of the universe, again, we see nothing but the agency of the ordinary physical properties of matter. Thus the motions of the solar system all result from that universal property of matter — ^gravitation — which, originally balanced against other forces, will continue to produce the same effects as long as may be consistent "vvith the designs of the Creator. By these motions are produced all the variations of climate and season in our own globe ; and fi-om the same property which causes them, results the constant movement of the waters of our ocean. By the heat of the central luminary, again, are probably occasioned, either directly or indirectly, most of the atmospheric changes which are of such consequence to the Avell-being of the plants and animals which people this earth ; and the same agent has a most im- portant and immediate influence on the phenomena of their growth and decay. Further, it appears probable, that the supposition of an internal heat in this globe itself, coupled with the known effects of solar heat, oceanic movements, atmospheric changes, and other external agents upon its surface, (among which the influence of living beings is by no means the least, as the formation of coral reefs and islands sufiiciently exem- plifies,) will ultimately explain the constant changes which its crust is undergoing. 10. But it is only where different material bodies are brought into a relation with one another, by which their properties are called into action, that these changes are exhibited ; and so long as any mass of inorganic matter is placed out of the pale of their influence, it may remain perfectly unchanged for an indefinite period. In the mineral or inorganic world, therefore, change is the exception^ ^^ai^ permanence is the rule ; whilst in the animated kingdoms, change is constant and universal, and is indeed essential to our idea of life (§ 1).* When we compare, therefore, the constant changes which we encounter in living organised beings with the inert state of inorganic matter, we are compelled to conclude, that to whatever extent the forces which control the latter contribute to the actions going on in the former, there must be additional forces resulting * It is true that there are certain cases in which org'anised structures have remained per- fectly unchang'cd for centuries, without losing their peculiar properties ( § 155-7), and wc have no reason to believe that there is any limit to the period during which they might thus exist. But it will be shown that this can only occur when they ore not merely removed from the influence of those stimuli which would rouse their dormant vitality into active life, but are also placed out of the sphere of those decomposing- agents, whose i)ower would occasion the separation of their elements and the consequent loss of their vital properties. 10 GENERAL CHARACTERS OF ORGANISED STRUCTURES. from tlie operation of properties to which we know nothing analogous elsewhere. The degree in which these superadded forces harmonise or interfere with those common to other forms of matter, constitutes a fair and highly interesting branch of enquiry which will hereafter be pursued. (chap. I.) But it is at present sufl&cient to state, that since these properties are never exhibited by any forms of matter except those usually denomi- nated organised, our notion of an organised structure is founded not only upon the adaptation of its parts to one another, but upon the indisputable possession by each part, of independent properties, by which it is enabled to execute actions for which physical laws will by no means account. And the process of organisation implies, therefore, not only the conyer- sion of the homogeneous materials into regular and complex structures, but the simultaneous endowment of them mth vital properties. 11. Although in every animal and vegetable fabric there are many different kinds of organised tissue, differing from one another both in structure and properties, and although these again present differences according to the class of beings in which the examination is made, yet there are certain general peculiarities by which all are seen to be characterised, when contrasted with mineral or inorganic bodies ; and these peculiarities are as manifest in the humblest and simplest member of the animated creation, as in the most elevated and complex. It has been a favourite attempt amongst many naturalists, to trace a regular gradation or scale through the whole material universe ; and not only to prove that the line of separation is indistinct between the animal and vegetable king- doms, but to show that there is not such a complete division between the organised and inorganic world as physiologists think themselves justified in erecting. It is doubtless true, that the discoveries of modern science are constantly bringing to light connecting links which were previously deficient in many parts of the chain ; and that, in particular, an increased acquaintance with the various races of animals and vegetables which formerly inhabited this earth, through many successive epochs, seems likely to fill up whatever chasms are left open between the groups at present existing. But it is no indication of a philosophical spirit to attempt to discover relations where none can by possibility exist. The simplest of the aerial flags, such as the red snow, or ih.e gory dew (§ 69), :as well as the most minute and, apparently, least complex animalcule, exhibits, when carefully examined, all the characteristics of organised structure, as well as all that can be regarded as peculiar in vital actions. They grow from a germ, increase, reproduce their kind, die, and decay, as regularly as any of the higher members of their respective kingdoms ; and they present the same peculiar and definite arrangement of particles, the same combination of fluid and solid materials, the same mutual adaptation of organs, as the latter possess. 12. However close, therefore, may be the links by which the animal GENERAL CHARACTERS OP ORGANISED STRUCTURES. 11 and vegetable kingdoms are connected together, tlie relation is only a mutual one ; and between organised fabrics, and tlie products of crys- tallisation, (or of any otter mode of aggregation by wbich inorganic matter is held together, in masses great or small,) there is a total Avant of resemblance. In this instance no analogy can be traced, except what is vague and chimerical. The absurd speculations of Robinet — who described all matter as possessed of living properties, and who regarded every object in existence, whether mineral, vegetable, or animal, as resulting from the repeated efforts of nature, becoming only pro- gressively successful, to form man — can now only excite our pity and contempt. Yet this doctrine has been advocated by many continental authors, who have even represented the fantastic forms assumed by many minerals, and bearing some resemblance to different parts of the human body, as so many proofs of this long and bungling apprenticeship of nature to the art of man — making ; and there are philosophers, even at the present time, who hold doctrines, which, if cleared from their veil of mysticism, and expressed in ordinary language, would probably be found to be not dissimilar.* To show in what the state of organisation essen- tially consists, it will be necessary to contrast, in more detail, the pecu- liarities common to all beings which exhibit it, with the condition of inorganic or mineral bodies. 13. Wherever a definite form is exhibited by mineral substances, that form is bounded by straight lines and angles, and is the effect of the process termed crystallisation. This process results from the tendency which evidently exists in particles of matter, especially when passing gradually from the gaseous or fluid to the solid state, to arrange them- selves in a regular and conformable manner with regard to one another ; and there is, perhaps, no inorganic element which is not capable of assuming such form, if placed in circumstances adapted to the mani- festation of this tendency among its particles. The mineralogist is conversant with an immense variety in the forms of crystals; these, however, may all be reduced to a few primary types, from which the mathematician can deduce the rest. But, on the other hand, if the particles be not placed in circumstances favourable to this kind of union, and the simple molecular attraction, or attraction of cohesion, is exercised in bringing them together without any general control over their direc- tion, an indefinite and shapeless figure is assumed. Neither of these conditions finds a parallel in the organised creation. From the lowest to the highest of living beings, the shape is determinate for each individual, — not only as a whole, but even as to each of its component parts ; and instead of being circumscribed within angles and right lines, organised * See a Memoir on the Kingdoms of Nature, their Life and Affinity, by Dr. C. G. Carus, translated in Taylor's Scientific Memoirs ; and the ingenious but vague " Philosophic de I'Histoire Naturelle" of M. Virey. 12 GENERAL CHARACTERS OF ORGANISED STRUCTURES. fabrics usually present a rounded outline and convex surface. It is true, that in the vegetable kingdom, and to a certain extent among animals also, we find a considerable difi"erence in the external forms of objects of the same species ; but this difference is restrained within certain limits, and may usually be referred to the influence of external causes. Although, as has been stated, the characteristics of organisation are never so far absent from the living structure as to indicate a transition to the mineral world, it is interesting to remark, that, as we descend in the scale of animated creation, we find these peculiarities less striking. And with regard to/orm, it may be observed, that this seems least definite among the Sponges and Polypifera (coral-formers) among animals, and among the lowest groups of cellular plants among vegetables ; and that there is reason to believe that among these the same germ may assume a variety of distinct forms according to the circumstances under which it is deve- loped, just as the same mineral substance may present itself under a diversity of crystalline shapes. 14. With regard to size, again, nearly the same remarks apply; since the magnitude of inorganic masses is entirely dependent on the number of particles which can be brought to constitute them, and is therefore completely indeterminate ; whilst the size of living beings is restrained, like their form, within definite limits, which vary to a certain extent in each individual. And, as in the former case, the size to which the inferior members of the animated kingdoms may increase, seems but little restricted in comparison mth that of the higher classes ; so that it is of no uncommon occurrence for some species of sea- weed to attain a length of many hundred feet ; and in those enormous masses of coral which compose so many islands and reefs in the Polynesian Archipelago, or of which the debris seem to have constituted many of the calcareous rocks of ancient formation, it would be almost impossible to ascertain the limits to which a single individual might extend itself. 15. Having considered the external form and size, we have now to compare the internal arrangement or aggregation of the particles respec- tively composing organised structures and inorganic matter. And here we at once meet with a striking and remarkable difi"erence. Every particle of a mineral body may exhibit the same properties as those possessed by the whole ; and if there is a variation, it resiilts only from an impurity or admixture of some other body. The chemist, in experi- menting with any substance, cares not, therefore, except as a matter of convenience merely, whether a grain or a ton be the subject of his researches. The minutest atom of carbonate of lime has all the pro- perties of a, crystal of this substance, were it as large as a mountain. Hence we are to regard a mineral body as made up of an indefinite number of constituent particles, similar to it and to each other in pro- perties, and having no further relation among themselves than that GENERAL CHARACTERS OP ORGANISED STRUCTURES, 13 which they derive from their juxtaposition. Each particle may be considered, therefore, as having a separate individuality. The living body, on the other hand, whether of a plant or animal is made up of a number of organs, each of which has a peculiar texture and con- sistence ; and it derives its character from the whole of these collectively. By their action with each other, and with external agents, life is pro- duced ; and hence there is a relation between their elementary consti- tuents much closer than that of proximity only, namely, that of mutual dependence ; so that as no one part can continue to exist without the rest, it cannot be regarded as possessing that separate individuality which belongs to the whole system alone. Thus, the perfect plant Avhich has roots, stem, and leaves, is an example of an organised structure in which the relation of every part to the integrity of the whole is suf- ficiently obvious, since every one is aware that, if completely deprived of any of these parts, the plant will perish unless endowed with the power of replacing them ; and no one portion separated from the rest can long continue its functions. But yet, in the plant, many of these organs are but repetitions of each other, so that some may be removed ^vithout permanent injury to it, provided enough are left to maintain its present existence. In the more highly organised animal structures, however, where the greater diversity of organs forbids such repetitions, the mutual dependence of their actions upon one another is much greater, and the loss of a single part is much more likely to endanger the existence of the whole. But when we look at the lower classes of plants and animals in this point of view, it is often very difficult to fix the limits of their individuality. Thus there are some even among the Mollusca (§ 104) which unite together into aggregate masses during one period of their existence, and separate at another. And among the Eutozoa and Radiata, there are many which are so entirely composed of repetitions of the same parts, that they may be multiplied by subdivision. There are among the Sea-weeds also, and especially among the fresh-water Con- fervas, many species in which several similar parts are vmited together for a time, and afterwards spontaneously separate, so as then unquestionably to become distinct individuals. Even among the higher plants, as among the Polypifera, which so rauch resemble them in their mode of gro>\i:h and increase, it may reasonably be enquired if every bud is not to be regarded as a separate individual, since each is capable (like the polype) of main- taining its own existence when removed from its parent structui-e. It may be found not altogether an incorrect or unnatural representation of the gradation which exists in this character, to say — The indivi- duality of a mineral substance resides in each molecule ; that of a plant or inferior animal, in each member ; and that of one of the higher animals, in the sum of all the organs. 16. The next point of difference between organised structures and 14< GENERAL CHARACTERS OP ORGANISED STRUCTURES. mineral bodies is their consistence. Inorganic substances can scarcely be regarded as possessing a structure, since they are exclusively made up of one form of matter, which — whether solid, liquid, or gaseous — ^is uniform or homogeneous throughout, being composed of similar particles held together by attractions which affect all alike. It may be objected to this statement, that there are solid mineral substances to the crystal- lisation of which water is essential, and others which inclose it within cavities : but in the first of these cases, the water becomes solidified, being chemically united with the substance ; and in the second, its pre- sence is merely accidental. Far different is the organised structure of living beings ; for in this may be detected an arrangement of the ultimate particles very different from that which crystallisation produces ;'"' and it is always composed of a mixture of solid and fluid elements which are so intimately combined as to produce a degree of flexibility and tenacity strongly opposed to the rigidity and brittleness of mineral substances. And it vsdll be noticed that, wherever it becomes necessary that for the support of the fabric an extraordinary degree of firmness should be given to any portion of the structure, this quality is imparted by the deposition of earthly or saline particles, which frequently retain their crystalline form, and are e^ddently subject to no laws but those of physics and chemistry (§ 41). Thus we have carbonate of lime diffused through almost all the tissues of plants, and a copious deposition of silex beneath the surface of the grass tribe, where lightness is to be especially conjoined with strength. It has been lately shoAvn that so universally do the tissues of plants receive support from these inorganic elements deposited in their inters- tices, that, if the organised portion of the structure be carefully destroyed by the agency of heat, an earthy skeleton will remain in which the forms of all the parts vdll be distinctly marked out. In animal structures, earthy depositions are usually more concentrated into particular spots, especially where the locomotive powers are considerable ; since it is obviously essential to the exercise of those powers, that whilst the frame- work which gives attachment to the organs of propulsion should be solid and unyielding, these organs themselves, as well as other parts of the fabric, should be capable of great freedom of action. In the higher animals, therefore, we find carbonate and phosphate of lime deposited in special situations, so as to give a firm basis for the attachment of softer structures; the former ingredient predominating where the skeleton is massive and external, as in the Mollusca in general ; the latter where it is enclosed within the softer parts, and where concentration of bulk * It has been a favourite doctrine on the part of many Physiologists that the ultimate parti- cles of organised tissues have always a globular form ; there is little doubt, however, that this statement is partly based on an optical illusion ; and it seems most satisfactorily refuted by Ehrenberg', who has shown that there are animalcules of complex stinicture more minute than the so-called ultimate globules. GENERAL CHARACTERS OF ORGANISED STRUCTURES. lo ■without diminution of strength, is therefore an important object. But there are some among the loAvest, in which the adaptation for locomotive powers is no object ; and here we find the structure even more universally penetrated with calcareous matter, than that of vegetables. Thus the masses of coral, which are produced by the action of a soft and almost jelly-like animal structure, were long supposed to be the habitation of the numerous little beings which form them, rather than a portion of their 0A\Ti structure (§ 119). Moreover, it is in the parts in which depositions of this kind take place, that vital changes are least actively performed ; and we find the bones of animals, and the woody fibre of plants, to be the portions of their respective structures which resist decay the longest, and thus rank nearest to mineral substances. While, on the other hand, it is by the softest tissues that the most active functions are performed ; and these frequently lose by subsequent consolidation the properties which rendered them capable of such important duties. Thus, the spongioles of plants, by Avhich the nutritious fluid is introduced into their vessels (§ 248), are nothing but the newly-formed succulent ex- tremities of their rootlets ; and, when condensed by the addition of new materials, they become embodied into the substance of the root, and trans- fer their function to fresh prolongations of the fibres. In like manner, the cartilages of animals become consolidated by the advance of life, and their elastic pliancy gives place to rigid density. And that texture of which the offices are most important, and the furthest removed from any thing analogous in the external Avorld, the nervous matter — is the softest and the most decomposable of all the tissues of the body, and is constantly being renewed (if we so may judge of the object of the vast quantity of blood ■with which it is supplied) in the living body, in pro- portion to the demands- upon its exercise. While solidity or hardness, therefore, may be looked upon as the term of perfection in the mineral kingdom, softness often appears to be the peculiar characteristic of the most important vital or oi'ganised structures ; and this I'esults from the large quantity of fluids which enter into their texture. 17.. A peculiarity in respect to their chemical constitution is usually regarded as belonging to organised structures. This point being at present made the subject of zealous enquiry on the part of many distin- guished philosophers, and great difiference of opinion existing among them, it seems advisable to state in this place only what is positively knoA-^ai. Of the elementary constituents of living bodies, it may be observed, in the first place, that no substance is foimd in them which does not also occur in the world around. This fact is a remarkable one; but a little consideration will show that it is a necessary result of the mode in which their structures are organised, or, as it were, built up of the materials supplied from external sources. For the parent communicates to its ofi\spring, not so much the structure itself, as the power of forming 16 GENERAL CHARACTERS OF ORGANISED STRUCTURES. that structure from the surrounding elements. Of the 54 simple or elementary substances which occur in mineral bodies, only about 18 or 19 are found in plants and animals, and many of these in extremely minute proportion, although perhaps not on that account in a state of less activity (§ 500). Now with regard to these it may be observed that, while the bulk of the inorganic world is made up of the metals and their compounds, (which form the alkalies, earths, and some of the acids,) the essential ingredients of living bodies appear to be four of the non-metallic elements, viz, oxygen, hydrogen, nitrogen, and carbon ; of which the first three in their uncombined state have a gaseous form. Of these, carbon may be regarded as the most characteristic ingredient in the composition of vegetables, and nitrogen in that of animals : it was formerly supposed that the latter very rarely exists in the vegetable kingdom, but further research has shown that it is much more exten- sively diffused than was believed, though usually present only in small proportion. Scarcely any of the 54 elementary substances are found m an uncombined state in nature ; most of them exist in union with others ; and in some the tendency to remain thus combined is so strong that they can with gi-eat difficulty be obtained in a free state. Indeed of one — Fluorine — it may be said that it has never yet been obtained in a separate form, since its tendency to combine with all other bodies is so powerful, that there is no one of them, not even platinum, with which it does not unite if brought in contact with it. Its properties can therefore only be judged of from its observed effects, and fi-om the analogies which it presents to other elements. Now this tendency to combine with other bodies, or in other words this affinity — a term which we must be careful not to employ as signifying a distinct or separate force, being only the expression of a property of certain forms of matter, — is possessed by all simple substances in a greater or less degree. It is by its action that compounds are formed, and that these compounds have a tendency to unite with one another. It is by the operation of affinity, also, that compounds already in existence are decomposed; a new and more powerful set of forces being brought into action by the change of circum- stances, which occasions the separation of the elements that have the weaker attraction for one another, and their reunion into other compounds Avhere they are more firmly held together. 18. In forming our opinion as to the nature of the affinities by which the elements of living tissues are held together, it is important to recollect that those which are regarded as strictly che^nical, (being, in fact, the result of the electrical properties of bodies), are very much affected by temperature and other external influences, so as even to be reversed by them. Thus potassium at low degrees of heat has a much stronger affinity for oxygen than iron, and to obtain oxygen will decompose almost any substance into which it enters ; but at a white heat, the GENERAL CHARACTERS OP ORGANISED STRUCTURES. 17 affinity of iron is so mncli greater that it will decompose potassa (the oxide of potassium) and, by subtracting the oxygen, mil leave the metal in an uncombined state. The affinity of mercury for oxj^gen is affected in the contrary manner by heat, — this metal being oxidised by contact with the air when near its boiling point, but losing its affinity for oxygen at a higher temperature. It is scarcely a sufficient argument, therefore, for the existence of a set of vital affinities, distinct from those which hold inorganic substances in combination, to say that all organised tissues exhibit a tendency to spontaneous decomposition by the separation of their elements, or by their dissipation under simpler forms, immediately upon the loss of their vitality. That this is a usual occurrence, every one knows ; and it is so obvious as to have given rise to the well-knowTi definition of life^ that it is the power by which decomposition is resisted. But the inference from it — that the affinities which hold together the elements during life are of a different nature from those which operate in producing their subsequent separation — appears scarcely entitled to the character of a positive law. For it may be readily shown \>^ a reference to well-kno"v\Ti physiological facts, that no solid or fluid com- pounds which have a disposition to spontaneous decay after death, can continue to exist without change during life ; and that the activity of the processes of interstitial absorption (chap, vii.) and reposition (chap, viii.) seems to bear a pretty constant ratio in every case mth the natixral tendency to separation. So that the maintenance of the original com- bination may be ornng, not so much to anything peculiar in its mtal affinities^ as to the constant provision for the removal of particles in a state of incipient decay, and their replacement by others freshly united by the peculiar operations of the living system, the nature of which will be hereafter considered. Thus, we find that all the most permanent parts of the animal frame, such as the massive skeletons of the Polj^pi- ■ fera, the calcareous enclosure of the Mollusca, the bony scales of Fishes, &c., all of which are believed by geologists to have remained nearly unchanged for thousands of centuries, are completely extravascular in the living animal, that is to say, not permeated by nutritious or absorbent vessels, and undergoing no interstitial change when once formed. Next to these in order of durability, are the osseous structures of animals, and the Avoody fibre of vegetables, whose connection with the nutritive system appears rather adapted to meet the exigencies of growth, injurj^ or dis- ease, than to maintain a constant change required by the tendency to decomposition. When we examine the softer tissues, on the other hand, we find that the rapidity of interstitial change fully compensates for the increased tendency to decay ; but that if this change be, from any cause, prevented, decomposition and loss of vital properties ensue, — as in the case of spontaneous gangrene from obstructed circulation. It is interesting to remnrk also, that the liberation of carbonic acid, Avhich begins so soon c 18 GENERAL CHARACTERS OF ORGANISED STRUCTURES. after death, and is one of the first signs of putrefaction, is the most con- stant and necessary excretion of the body during life, being thrown off not only by the special respiratory apparatus, but also by the general surface. It might further he argued against the doctrine of a distinct set of vital affinities, that the circumstances under which organic compounds exist in the living body differ in so many particulars from those of dead matter, that no conclusion could be fairly drawn from the fact of their spontane- ous decomposition after death ; since inorganic chemistry affords so many examples of the occurrence of similar changes, under the influence of very, slight variations in temperature, electrical condition, light, &c. 19. It has usually been maintained that in the composition of inor- ganic substances, the elements unite together in a binary arrangement, and in a relation which admits of being very simply expressed ; and that all the more complex arrangements admit of being resolved into this simple form. Thus, sulphur and oxygen unite in the proportion of 1 to 3, to form sulphuric acid ; and sodium and oxygen in the proportion of 1 to 1, to form the alkali soda ; equivalents of each of these, if brought together, unite to form the salt termed sulphate of soda ; and this salt may unite Avith some other of analogous composition to form a double salt. On the other hand, it is usually believed that the J^?ro^^maifl3 principles^ as they are termed, of organic compounds, (that is to say, the simplest forms to which these compounds can be reduced, without altogether disuniting them into their ultimate elements), consist of three or four ingredients united together in a relation of much complexity. Thus, an equivalent of the vegetable alkali quinine is made up of 21 eq. of carbon, 12 eq. of hydrogen, 1 eq. of nitrogen, and 2 eq. of oxygen. But on this it may be observed,, that there are undoubtedly some proximate principles which consist of two elements alone ; as for instance, the compounds of hydro- gen and carbon Avhich exist as such in living bodies. Again, our igno- rance of the appropriate means of analysis is very likely to lead us astray ; since there is no improbability, but, on the other hand, an almost positive certainty, that most of the more complex organic substances might be resolved into simpler compounds, if the chemist knew how to treat them. Let us compare for example the two instances just quoted. If the chemist were at once to analyse the sulphate of soda into its ultimate elements, he would find it made up of 1 eq. of sulphur, 4 eq. of oxygen, and 1 eq. of sodium. Were this all that he knew of its com- position, he would be at a loss to say how the oxygen is distributed between the other elements. But knowing, as he does, that the salt contains two binary compounds which he can separately examine, he may say with confidence that the oxygen is distributed between the sulphur and the sodium in the proportion of 3 to 1 .''^ Now it is obvious * I have thought it better to keep to the usual opinion on the composition of salts^ though by no means insensible to the beauty of the new doctrines which have been lately offered. GENERAL CHARACTERS OF ORGANISED STRUCTURES. 19 that as long as compounds like quinine remain in their original state, we must be in total ignorance of the method in which their elements are united : but the progress of analytical research undoubtedly tends to indicate that such complex arrangements may be resolved into those of a simple binary character ; and with regard to the vegetable alkalies in particular, it is now generally admitted that they owe their power of neutralising acids to the ammonia which enters into tlieir composition. Again, camphor, which was long regarded in the light of a proximate principle, and which consists of 8 hydrogen, 10 carbon, and 1 oxygen, is now found to be an oxide of cmnphene^ a compound radical of binary composition, which mil unite Avith another equivalent of oxygen to form camphoric acid, and with chlorine, &c. into other compounds. Many similar instances might be adduced ; and it seems an unquestionable fact that every fresh discovery is tending to break down the barrier between the two classes of organic and inorganic bodies, as far as regards their method of chemical combination. 20. Investigations into the elementary arrangement of the parts Avhich primarily compose organised structures, are often attended mth much difficulty and liability to eiTor. The minuteness of the objects which are to be examined, and the changes which may be produced in them by the preparation they are necessarily made to undergo, before being submitted to microscopic inspection, not to mention the deceptions arising from imperfection in the instrument itself, or the mode of em- ploying it, have led to much discrepancy in the statements of different observers. Too often the descriptions given have not been of what has been actually seen, but of what has been imagined ; and have, mthout any intention of falsifying them, been shaped according to the precon- ceived notions of the enquirer. Hence, an examination of the characters of the primary tissues, whether of plants or animals, requires not only considerable manual skill and dexterity in the use of the microscope, but an acquaintance mth all the fallacies arising fi-om the difference between the image presented to the eye, and the object as it exists in its natural situation ; besides what is even more important, a perfect readiness to give up preconceived notions, Avhen they are inconsistent with observ- ation, and a determination to consider nothing as proved until every mode of investigation has been employed mth the same result. In the brief outline which will now be given of the characters of the principal elementary structures occurring in the fabric of plants and animals. (See Graham's Elements of Chemistry, p. 158, &c.) To those who are acquainted with these, it will be evident that their tendency is to indicate a still gTeater resemblance than that here pointed out between organic and inorganic compounds ; the sulphate of soda being- on this view formed by the union with sodium of the compound radical sulphatoxygen, which is regarded as analogous with cyanogen, and as combining, like chlorine, iodine, &c., with the metals and other bases. c 2 20 ON ORGANISED STRUCTURES, care will be taken to distinguish wliat is actually seen, from tlie theo- retical ideas of the conformation of the parts to which such observations lead. And it will be an especial object of enquiry, how far such an analogy may be traced between these characters, as to lead to the belief that any of the tissues, whether peculiar to the two kingdoms respec- tively, or occurring under different forms in both of them, are constructed upon the same plan. III. — Elementary Structure of Vegetables. 21, All the elementary or primary tissues of plants may be con- sidered as formed of membrane andjibre, either separately or conjoined; it may, however, be doubted whether even these are to be regarded as distinct elements, or whether they may not be formed by the adhesion of single globules, sometimes in expanded surfaces, sometimes in lines only. Although they frequently occur in combination, membrane is often found without any trace of fibres, and sometimes fibres may be seen without; any membranous envelope. Instances of their union may be seen in spiral cells (Fig. 5), or in spiral vessels (Fig. 12) ; and of their separate existence, in the simple membranous cell (Fig. 1), or in the curious spiral fibre surrounding the seed of the Gollomia.'^ Yegetable Membrane is of variable thickness and transparency, and though very permeable to fluids, is entirely destitute of visible pores. Many botanists have described the existence of apertures in the membrane of which some forms of cellular tissue are composed ; but sometimes these appearances seem to be pro- duced by grains of semitransparent matter adhering to the membrane, and may be removed by immersing it in nitric acid, which renders them opaque, after which, immersion in a solution of potash will restore them to their previous degTee of transparency ; and in other instances they are probably due to a diminution in the thickness of the membrane in those spots, from causes which will be presently explained. Elementary Fibre may be compared to hair of extreme tenuity, its diameter often not exceeding the X2^o"o ^^ ^^ inch. It is generally transparent and colour- less; it is usually disposed in a spiral direction (Fig. 14), and its ad- jacent threads seem to have a peculiar tendency to unite and grow together (Fig. 16). Some observers maintain that it is hollow, others that it is solid ; a question involving the conditions of a body of such extreme minuteness, however, is not easily determined. 22, The forms under which these elements and their combinations * This beautiful microscopic object is to all appearance like other seeds ; but on the out- side of its coats there is a cong'eries of elastic spiral fibres, which, in the ordinary state, are ag'g-lutinated by mucilage, and pressed together so as not to be perceptible. Immediately as the seed is wetted, however, the mucilage is dissolved, and their elasticity causes them to spring out vnth great rapidity. Some other seeds, as those of the Salvia Verbenaca CVN'ild Clary), have a similar property. PRIMARY TISSUES OP PLANTS. 21 most frequently present themselves, may be thus classified. 1. Cellular Tissue. 2. Woody or Fibrous Tissue. 3. Vascular Tissue. It will be shoAvn, however, that they may all be regarded as modifications of the same elementary forms ; since they are all developed in the young plant from a common origin, and in the adult structure many intermediate links are found which connect them by almost imperceptible transitions. Still it is important for practical purposes to distinguish these different forms of tissue ; since, when once fully formed, they do not appear sus- ceptible of mutual transformation, and their functions in the economy of the plant are entirely different. 23. That which may be regarded as the most characteristic example of cellular tissue, exists in most pulpy fruits, as well as in the pith and other soft parts of the structure. It is simply a vesicle or minute sac of a globular or spheroidal figure, containing fluid to which its colour, if it presents any, is due, the membrane of which it is formed being trans- parent and colourless ; thus, in the pith this tissue is white, in the leaves green, and in the petals of flowers it may be variously coloured. From its being composed of membrane alone, it is called membranous cellular tissue (Fig. 1). The rounded form is only exhibited when the vesicles are but loosely aggregated together, and it is then that the distinctness of their sides is most evident. When the tissue is more solid, the sides of the vesicles are pressed against each other, so as to become flattened, and to be in close apposition ; and sometimes they adhere in such a manner, that the partition between two adjacent cells seems to be but a single instead of a double membrane. If the pressure to which the vesicle is subject be equal in all directions, the form it will assume is that Avhich is mathematically termed a rhomboidal dodecahedron, that is to say, a twelve-sided solid with all its faces equal, and showing an hexagonal section when cut across. Each cell will thus be in contact Wiih. twelve others, which completely surround it mthout leaving interstices. It is not very often, however, that this form is displayed with such extreme regularity ; since there is usually in the gromng plant a disposition to elongation in the direction of increase, and to compression in the trans- verse one, so that the cells are found to have rather a prolonged form. Such are especially found in the lower tribes of plants, which have no other kind of tissue, and are destitute of vessels, the function of which is partly performed by them. Not unfrequently, cellular tissue is found to possess a cubical or prismatic shape, especially in pith (Fig. 2) ; and occasionally the vesicles are arranged in regular horizontal rows like the bricks in a house ; this last, which is called muriform cellular tissue (Fig. 3), enters into the structure of the medullary rays (§ 51) ; and the horizontal elongation of the cells which is peculiar to it, appears to con- tribute to an important function of the vegetable economy. Fluids wliich penetrate this tissue, always pass most readily in the direction of the 22 ON ORGANISED STRUCTURES. greatest length of tlie cells ; and whilst, in the growing plant, the elonga- tion of the cells, in those parts of the stem through which the upward current of sap passes, is always vertical, the downward current, which has to be conveyed from the bark to the interior of the stem, traverses these horizontal cells, which are sometimes so much lengthened as to resemble tubes (Fig. 4). But the vesicles of cellular tissue do not always consist of simple membrane; and two other kinds of cells may be noticed, not so much because interesting or important in themselves, as because they assist in explaining the character of other kinds of tissue. One of these is the spiral cell (Fig. 5), which consists of a membranous vesicle having a fibre coiled spirally around its interior; this form of tissue is occasionally met with in the coverings of winged seeds, and constitutes the entire plant of the Moss Sphagnum. Sometimes the fibre adheres so closely to the membrane that it cannot be separated, and the cell seems as if it were formed of a spirally-coiled fibre alone ; bu.t this may probably be due to the intimate union of the two elements. Another kind of vesicle occasionally met with, is that termed the dotted cell^ of which specimens are sho^vn in Fig. 6, chiefly derived from Orchi- deous plants. This is a very interesting kind of structure with reference to the explanation of others. The cell marked a is one Avhich would formerly have been supposed to possess pores or apertures in its mem- branous sides ; but the true nature of these seems to have been satisfac- torily determined by the comparison of other forms intermediate between it and the spiral cell just noticed. Thus, a cell presenting similar dots is seen at h, where they are shown to be spaces intervening between the coils of the spiral fibre, which is adherent to the membrane so closely as to form a sort of inner coat deficient at these spots, where the membrane alone forms the wall of the cell. This is evidently, therefore, a transition form between the spiral cell (Fig. 5), and the dotted cell (Fig. 6, a) which would at first sight have appeared quite different in character. 24. The size of the cellules of this tissue is very variable ; they are usually from -^^-^ to -^^-^ of an inch in diameter, but may be found of all sizes from Jq to -^-qq-q of an inch. Although other kinds of structure are mixed up with it in flowering plants, it may be regarded as constituting, either in itself or in its most simple modifications, the great bulk of the Organs in which active vital processes are being performed ; and in the greater part of the Cryptogamia, no other is found. It is capable of growth in all directions, and it consequently fills up the interstices left by the more solid parts of the fi-amework with a softer structure, which may be regarded in some measure as analogous to the flesh of animals. This is usually termed parenchyma^ and a good illustration of it may be found in leaves, where the beautiful skeleton formed by the reticulation of the veins, (Avhich may be separated by maceration from the general substance of the organ), gives support to the intervening tissues. Although fluid PRIMARY TISSUES OF PLANTS. 23 generally finds its way with tolerable facility tlirougli cellular structure, esjjecially in the direction of the greatest length of its cells, a more direct means of connection between distant parts is required when the circula- tion is active (§281). This is afforded by what has been termed Vasiform tissue, which consists merely of cells laid end to end, the partitions be- tween them being more or less obliterated, so that a continuous tube is formed. The origin of this kind of tissue has been much dispiited ; and many writers regard it as a modification of vascular rather than of cellular structure, since traces of a spiral fibre may often be seen upon its sides. It is, however, so common to see the remains of the partitions which originally closed the ends of the individual cells, disposed at intervals along the ducts (Fig. 7), that it is impossible to avoid admitting that they are generally, at least, produced in this manner ; and we occasionally find not only dotted ducts (Fig. 8) and even spiral ducts, but vessels formed by the aggregation of simple cells (Fig. 9). The vasiform is the largest of all kinds of tissue, and may frequently be detected with the naked eye, when its open mouths are exposed by a transverse section, as in the vine or cane ; it is frequently pervious for a considerable length, especially in cases where the rapidity of vegetation and the length of the stem render a rapid transmission of the fluid necessary. There is a very evident analogy between the mode of development of these canals, and that of vessels in the animal structure, Avhich appear to be first formed by a similar junction of minute cavities in particular lines. 25. Ligneous tissue or woody fibre consists of very slender transparent membranous tubes, usually tapering at their extremities, collected into bundles, and generally having no direct communication with each other except by invisible pores (Fig. 10). Mr. Slack* and Dutrochet,t how- ever, state that they have seen evident communications between the extremities of the tubes ; these are scarcely, perhaps, to be considered as of regular occurrence, but rather as the occasional results of the rupture or obliteration of the membrane by pressure. Although we find so many intermediate forms between woody fibre and cellular tissue, that there is no difficulty in tracing the gradual elaboration of the former fr-om the latter, yet the characteristic forms of the two structures differ considerably. Woody fibres, like the vesicles of cellular tissue, are closed sacs : but whilst the latter have more or less of a rounded shape, the former are elongated and attenuated so as to present altogether a different appear- ance ; at the same time they acquire a greatly increased density and firmness, although the membrane which forms their walls is really much thinner. It vidll be readily perceived that, independently of the difference in the tenacity of this membrane, a structure composed of woody fibre will bear a much greater tension than one formed of cellular tissue, from the advantage gained by the situation of the tubes with regard to each * Trans, of Soc. of Arts, vol. xlix. t Mem. Anat. et Physiolog-iqucs, torn. 1, p. 121. 24 ON ORGAiS'lSED STUUCTUKES. Other ; thus, threads of hemp and flax, each of which is a small bundle of Avoody fibres, are far stronger than those of cotton of similar diameter, which are composed of cells laid end to end. A peculiar form of woody fibre is found in the stems of the Coniferce (fir tribe) ; no dotted ducts exist in them, and the diameter of the woody fibres is much greater than usual ; along each of them is perceivable with the microscope a row of large dots, which appear to be formed by the adhesion of some little bodies to the interior of the tube. This curious structure, shown at Fig. 11, has enabled botanists to determine that many remains of fossil woods, espe- cially those of the coal formation, belonged to this order ; and as the mode in which these dots are arranged exhibits variations, each of which is peculiar to some division of the order, the fossil specimens may be closely compared, by their stems alone, with those of the present epoch. Woody fibre is apparently destined for conveying fluid in the direction of its length, and for giving firmness and elasticity to the parts of the fabric which require support. Wherever vascular structure exists, it is protected by bundles of this tissue ; and hence many parts in which they are united, such as the veins and footstalks of leaves, are spoken of as being com- posed oi Jlhro-vascular tissue. In ail plants Avith permanently elevated stems, this tissue is very abundant ; but it is not discoverable in any below the Ferns, and it exists in but small amount in herbaceous plants. It may therefore be regarded as constituting the essential organ of sup- port in all the orders of the vegetable kingdom ; and when no longer required for the conveyance of fluid, additional firmness and toughness are given to it by the deposition of various secretions within its tubes, constituting the diiference between the duramen or heart-Avood, and the alburnum or sap-wood (§ 51). After the deposition just mentioned has taken place, woody fibre seems to be removed from the active functions of vegetation, and to undergo but little change for an almost indefinite period. It is therefore somewhat intermediate in character between the bony structures of the higher animals, Avhich maintain a constant relation to the processes of nutrition by means of their circulating and absorbent systems, and the unorganised parts of their fabric, such as the hair and nails, or the shells of MoUusca and Crustacea, which, when once formed are independent of any further vital changes. Perhaps cartilage is of all the animal tissues that which bears the greatest analogy to woody fibre; owing, like it, the density which it possesses, to the deposition of a secre- tion (albumen) in the minute caAdties of a modified form of cellular tissue. 26. The third kind of elementary structure in plants is that which is denominated vascular tissue. Its essential character is the pos- session of a spiral fibre coiling AAdthin its membranous tubes from end to end ; but this fibre is not alAA'^ays to be traced with the same distinctness, and sometimes the appearance presented is rather that of ti'ansverse bars, or irregular markings. The most perfect kind of PRIMAKY TISSUES OF PLANTS. 25 vascular structure is shown in tlie spiral vessel^ wliicli consists of a tube with a conical termination at each extremity, traversed by a filament regularly coiled from one point to the other (Fig. 12.) This filament is usually single, but sometimes double, or even triple ; and in the very large spiral vessels of the Nepenthes (Chinese pitcher-plant) it is quadrujjle (Fig. 13.) The tubes in their perfect state contain air only ; they are found in the delicate membrane surrounding the pith of Exogens, and in the midst of the woody bundles occurring in the stem of Endogens ; from thence they proceed to the leaf-stalks, through which they are distributed to the leaves. By careful dissection under the microscope, they may be separated entire ; but their structure may be more easily displayed by cutting round, but not through the leaf- stalk of the strawberry, geranium, &c., and then drawing the parts asunder. The membrane composing the tubes of the vessels will thus be broken across ; but the fibres within, being elastic, will be drawn out and unrolled, as seen in Fig. 14. A very curious analogy to this structure is exhibited in the tracheas, or air-tubes of insects, which ramify by minute subdivisions through the whole of their bodies. These tubes are formed, like the spiral vessels of plants, of an external mem- brane distended by spiral fibre, which is coiled with the most beautiful regularity (Fig. 19) ; the principal difference in these two structures being that the air-tubes of plants, are closed vessels, and that their gaseous contents find their way through the delicate membrane which composes them, by the capability of permeation, which mil be subse- quently described ; while the tracheal system of insects exhibits the most beautiful and minute ramifications, formed by the subdivision of its principal trunks, which communicate directly with the atmosphere. 27. There are some peculiar modifications of the regular type of the vascular structure of plants, which deserve notice, not only on account of their intrinsic importance, but also as exhibiting analogies still more remarkable in the structure of the respiratory organs of the animal and vegetable kingdoms. The tubular vessels occurring in many parts of the stem, roots, and leaf-stalks of flowering plants and ferns, and ex- hibiting traces, more or less distinct, of a spiral structure, are called Ducts. Of these, some approach so nearly to the character of a spiral vessel, that they could scarcely be distinguished from it ; the difference between them being confined to the absence of elasticity in the spiral fibre, which prevents it from being unrolled, as in the former case, without snapping. Another form is that in which the spiral fibre is not continuous, but is broken into rings ; whence the vessel is called an annular duct. The rings in some vessels are very close, in others at considerable intervals ; but if the vessel be traced to any extent, some indications of a spiral fibre may generally, if not always, be found (Fig. 15). It appears probable, since vessels arc found in all states 26 ON ORGANISED STRUCTURES. of transition from perfectly spiral to annular, that the original tendency Avas to develope a spiral fibre ; but that the vessel during its formation was elongated more rapidly than the fibre, from its want of elasticity, could keep pace withj and that the latter was consequently broken into rings. A structure, exactly corresponding, is met with in the trachea (windpipe) of air-breathing rertebrata. This is composed of cartila- ginous rings, usually separate from one another, but united by a membrane ; thus resembling an annular duct. In some birds, how- ever, traces of a spiral arrangement of the cartilage are met with ; and this appears to be the regular structure of the trachea in the Dugong (one of the whale tribe), where we see a continuous strip of cartilage disposed in a spiral form, and occupying the place of several rings, but occasionally terminating in the usual manner (Fig. 20). 28. Another modification of vascular structure is shown in Fig. 16. It is produced by the partial adhesion of the coils of a close spire to one another, and to their enveloping membrane, so that the fibre itself is no longer distinguished, but irregular dots or spaces are left in its interstices. This is also peculiarly interesting from the analogue it meets mth in the animal kingdom. The trachese of insects occasion- ally exhibit dilatations in their course into air-sacs (Fig. 21); the walls of Avhich, in some instances, appear simply membranous ; but in other cases exhibit a distinct continuation of the spiral structure of the tubes. Most frequently, however, the membrane has the aspect represented in Fig. 22, which seems due to the same partial adhesion of the fibres as has been traced in the vegetable structure. 29. Two other forms of vascular tissue, the reticulated and dotted ducts, are represented in Figs. 17 and 18. They appear to take their origin in a spiral structure, modified by the irregular fracture of the fibre, and the subsequent adhesion of its fragments. Thus, in the reticulated duct (Fig. 17), the spire may be occasionally traced, although the general disposition of the fibre is in an irregular network, with large interspaces. These spaces are often observed to be more -contracted and definite ; and thus a transition is indicated to the character of the dotted duct (Fig. 18), of which the dots appear to be the intervals not covered by the expansion and adhesion of the frag- ments of the fibre, just as in the separate cell (§23). It seems, there- fore, that the dotted duct may be formed, either by the junction of distinct cells (of which some evidence remains in the imperfect par- titions), or upon the type of a spiral vessel, when the tube will be more continuous. The mode in which the dotted structure is acquired, is evidently the same in both cases.* The animal kingdom presents an instance of similar degeneration from the original type, in the * This view of the structure of dotted ducts, which is the one taken by Mr. Slack, manifestly tends to reconcile the conflicting accounts which have been given of their origin ; PRIMARY TISSUES OF ANIMALS. 2? bronchial ramifications of the trachea, in which the iiTegular patches of cartilage (Fig. 23) exhibit an appearance exactly conformable with that which has just been described. 30. The description which has been now given of the vegetable tissues, will suffice to show the mode in Avhich they are mutually con- nected, as well as the forms which are characteristic of each kind. Many varieties have been passed by, as not of particular interest in regard to the present object, although in a full description of the Anatomy of Plants they would receive more especial notice. It is scarcely possibly to observe the different forms which result from the varied combinations of the simple elements of tnembrane and Jibre, each of them probably having its peculiar function in the vegetable economy, without being struck mth the simplicity of the plan by which creative design has effected so many marvels, as well as with the extreme beauty and regularity of the structures which are thus produced. The comparison of such specimens of Natui-e's workman- ship as the meanest plant affords, with the most elaborate results of human skill and ingenuity, serve only to put to shame the boasted superiority of man ; for whilst every additional amj)lification of the latter, by the increased powers of our microscopes, serves but to ex- aggerate their defects, and display new imperfections, the application of such to organized tissues, has only the effect of disclosing new beauties, and bringing to light the concealed intricacies of their struc- ture. If such be the result of the study of the minute anatomy of vegetables, that of animals should still more impress our minds with astonishment and delight, from the increased variety of the forms which the same simple elements are capable of presenting, and the extraordinary complication of these (frequently so great as to baffle the most skilful enquirer), which becomes necessary for the production of the phenomena of animal life, themselves so varied and so conplex. IV. — Elementary Structure of Animals. 31. The great bulk of the fabric of animals is made up of tissues composed of the same elements as those which constitute the whole of the vegetable organism, namely, membrane and fibre; and, in fact, it may be regarded as composed of fibres alone, since the most delicate membranes (as will presently be shown) are formed out of this arrangement of organised particles. Two other elementary forms of structure are found in animals, which have no analogy AA-ith an)i:hing many Physiologists referring- them to the vascular system, whilst others, with much reason, maintain that they have originated from vesicles of cellular tissue. The spiral vessel is manifestly only an elong-ated spiral cell, from the type of which the dotted cells and dotted ducts may arise on one side, or the spiral vessel and reticulated ducts on the other ; and every grade of transition may be detected in the same plant between tiiese different forms. 28 ON ORGANISED STRUCTUEES. that exists in plants ; these are the 'inuscular fibre, and the nerwus matter. It is very interesting to remark, howerer, that these are, for the most part, restricted to the parts of the fabric which are sub- servient to the functions purely animal^ namely, sensation and voluntary motion ; and that wherever they are introduced into the apparatus of organic life (chap, iv.) it is for the purpose of adapting it to the con- ditions of animal existence. Thus, we shall find (§ 237) that one of the characteristics of animals is the possession of a digestive cavity, in which the food is stored up for the continued supply of the absorbent system, and in which it undergoes a certain degree of preparation ; this addition to the absorbent apparatus of plants being required by the locomotive propensities of animals, and also by the nature of their food. Now, for the introduction of aliment into this cavity, for the expulsion of the excrementitious matter from it, and (where the cavity is prolonged into a tube, as in the higher animals,) for the motion of its contents from one extremity to the other, an apparatus of nerves and muscles becomes necessary; but still this cannot be regarded as an essential part of the absorbent system, which is composed, as in plants, of the simple elements, membrane and fibre. A similar ex- planation might be given of the introduction of muscular fibre into the circulating apparatus of animals ; since the regularity and constancy of the movement of the blood which is required in them, rendered necessary the addition of a central impelling organ (chap, vi.) which could only be constructed of a tissue possessed of the peculiar contractile powers of muscular fibre. It is not a little curious, moreover, that there should be a perceptible and essential difi"erence in the muscular tissue employed in the vital organs, and in the locomotive apparatus (§ 43). 32. The fibres^ by the union and interlacement of which, the greater part of the animal tissues appear to be composed, are of extreme minute- ness, and require a very high magnifying power to recognise them. Those Avhich are perceptible to the naked eye, as the white glistening threads traversing fibrous membranes, are composed of many elementary fibres united together. Probably those of Avhich the cellular tissue is formed are among the most minute ; and these are said to vary from FtVo" *^ T^V^ ^^ ^ ^™® ^^ diameter. They are transparent and have edges quite smooth ; and they appear principally composed of gelatine. These fibres may unite in bundles to form larger fibres ; or they may be arranged side by side into the delicate membranous plates or lamellce, of which all membranous expansions appear formed. Many microscopic observers have maintained that these ultimate fibres are themselves made up of globules arranged end to end ; but the appearance which has led to this belief may be regarded, almost Avith certainty, as an optical illusion, proceeding in part from imperfection in the instrument employed, and in part from the preparation which the object has undergone. The ultimate PRIMARY TISSUKS OF ANIMALS. 29 fibres are distinctly visible in tlie membranous lamella? which by their interlacement form cellular tissue ; and also in the delicate membrane which forms the vesicles of fat. Some physiologists maintain that these fibres are tubular, and serve for the conveyance of the serous part of the blood ; but this can scarcely be granted, when we consider their condensed arrangement in various membranes and other structures, which exhibit a regular gradation from the superficial fascia, into which the cellular tissue passes almost imperceptibly, to the dense tendons in which so little circulation of fluid takes place. 33. The most regular and definite form of cellular tissue is that in which fatty matter is deposited, and which has therefore been denominated adipose tissue. This seems to be exactly parallel with the simple mem- branous tissue of plants (§ 23) ; consisting, like it, of isolated vesicles formed of a delicate transparent membrane. And, as in certain parts of the vegetable organism, we find the cavities filled Avith oil or gummy matter stored up in them for the future nutrition of growing parts, so does it appear that the substance which composes the fat of animals (a mixture of oil and stearine), is separated from the circulating fluid and deposited in these vesicles for a partly similar purpose. That the vesicles do not communicate with one another, is proved not only by microscopic examination, but by the fact that their contents, which are fluid in the living body, have no tendency to gravitate. They are clustered together in masses, each of which is enclosed within a distinct membrane, on which blood-vessels ramify ; these masses are again clustered together into larger ones under another envelope ; so that any mass of fat may be separated into a number of distinct nodules, which may be several times subdivided into others before the ultimate vesicles are arrived at. The diameter of these is stated by Raspail at from about y qVo ^^ "200 ^^ ^^^ inch ; they are much smaller in the young animal than in the adult. Perhaps of all the animal tissues, this is the one which most resembles those of vegetables ; and it is curious that it does so not only in structure but in chemical composition. The gelatine of which the membrane is principally composed, may be regarded as the least animalised of the proximate principles which enter largely into the organism, containing a smaller proportion of nitrogen than albumen, and much less than fibrin ; whilst both of the ingredients which constitute the fatty matter itself, are composed of oxygen, hydrogen, and carbon alone, being very analogous in the proportion of those elements to the fixed oils pro- duced by vegetables. 34. The structure which is ordinarily termed cellular tissue in the animal body, differs so much from that just described, as well as from that of plants, that it would perhaps be better if some difi^erent appellation were given to it. It consists of a net-Avork of fibres and membranous lamella?, Avoven into a minutely reticulated texture ; and a number of 30 ON ORGANISED STRUCTURES. little cells or cavities are thus formed, Avhich contain a watery fluid slightly impregnated with albumen, and very similar to the serum of the blood. These cavities are not, however, surrounded with a distinct envelope, cutting them off from one another ; biit, being merely separated by the net-work of fibres and strips of membrane which forms the solid part of the texture, they communicate freely -with each other, and fluid or air injected into one part is speedily transmitted into the neighbouring portions of the structure. There is great difficulty, therefore, in submit- ting this tissue to a satisfactory examination, since its condition after death and when removed from the body is necessarily very different from the state in which it exists in the living structure, the fluid which should distend its cavities having escaped. Some physiologists have regarded it as possessed of a very low degree of organisation, and as result- ing merely from the disengagement of gaseous particles in the midst of coagulating lymph ; whilst others have gone to the opposite extreme, and considered it as entirely composed of blood-vessels, nerves, and absorbents. Of its importance in the animal economy there can be no doubt ; but it will be found that it rather contributes to the performance of the special functions of the difi'erent organs which are formed, as it were, upon the basis of it, than executes any definite vital actions itself. As among plants, we find that the simplest form of tissue is that which constitutes in animals the entire bulk of the least organised species, and enters most largely into the fabric of the highest ; and it is very interest- ing to remark that the more nearly we approach the confines where these two kingdoms border upon one another, the more closely does their elementary structure approximate. For among the Porifera (§ 121), the body, or at least the soft portion which clothes the skeleton, consists of a number of translucent globules which are not perceptibly joined together, and thus resembles that loosely aggregated gelatinous tissue which con- stitutes some of the lowest plants. Even in the more complex Polypi- fera (§ 119) and Acalephae (§ 108), all that is evident of their organisa- tion is a soft and gelatinous matter, analogous to the pulpy substance of fruits, consisting of vesicles loaded with fluid. A similar gradation in the characters of cellular tissue may be observed in watching the develop- ment of any of the higher animals ; thus, the germinal membrane, from the folds and layers of which most of the organs of the embryo are developed (chap, xiii.), is first perceived within the egg to consist of an aggregation of globular corpuscules ; and at a later period the whole mass of the embryo presents a structure very analogous to that of the inferior animals just described. The tissues peculiar to difi'erent organs are developed within this ; and it may be observed, either in ascending the animal scale, or in watching the gradual evolution of any one organ- ism, that in proportion as these are formed, the cellular tissue becomes less predominant in the fabric, and performs a less important part in its PRIMARY TISSUES OP ANIMALS. 31 functions. This is exactly parallel to what occurs in the vegetable king- dom ; for whilst the lowest classes are entirely composed of cellular tissue, which serves every purpose in the economy, in ascending to the higher ones we encounter new tissues, adapted for special functions, which supersede it as regards these objects. The same is the case in the devel- opment of the embryo, which at first consists of a soft gelatinous mass, and afterwards presents in succession the forms of cellular structure, woody fibre, and vascular tissue. 35. Cellular tissue is diffused through the whole fabric of the adult animal, and enters into the composition of every organ ; so that it has been said, that if all the particles of other kinds of structure, and all the deposits in its interstices, could be removed, there would still be left a kind of framework in which the form and arrangement of every portion of the body would be perceptible. Hence results its uninterrupted con- tinuity through the whole body ; since it not only fills up the spaces left between the different organs, connecting them together more or less closely, according to degree of mobility which is to be permitted them, but binds together their minutest portions. Thus, the ultimate nervous filaments, and the minutest muscular fibres, are united into bundles by tissue of this character ; these bundles, again, are incorporated into larger ones by another investment of the same kind ; and even the trunks of the nerves and the bodies of the muscles possess similar sheaths. In the healthy condition of this tissue, its interstices are filled with fluid, secreted from the blood vessels ; and it appears to be upon the due relation between the distention of the cavities, and the elasticity of the fibres (both of which states are liable to be affected by diseased condi- tions of the nutrient actions), that its peculiar tone in the living body depends. This property is manifested in the resiliency of the skin, after it has been pressed and the pressure has been removed ; here the fluid contained in the cavities beneath, is at first expelled into the neigh- boui'ing interstices, and the elasticity of the fibres by which these are bounded forces it back again as soon as the compression is taken ofi: Again, the retraction of the sides of a wound made in the skin and sub- jacent tissue during life, is due to the same property, and may be ex- plained on the same principle. But when the tissue of any part becomes over-distended with fluid, as in local dropsies, either from an increase in the amount of secretion, or fi-om inactivity of the absorbent process which ought to keep it in check, the fibre loses its elasticity ; and the surface, in consequence, j)its on pressure, not immediately recovering itself when the obstacle preventing the return of the fluid to its usual situation is removed. It is not improbable, however, that in many cases the loss of elasticity is the result of diseased nutrition; and that the accumu- lation of fluid, instead of being its cause, is its consequence. A curious fact may be noticed regarding the chemical composition of cellular tissue 32 ON ORGANISED STRUCTURES. at cliflPerent periods of life, Avliicli is analogous to what lias already been mentioned of its variations in structure. In the earlj period of develop- ment, it consists almost entirely of gelatine, and hence it is that the flesh of young animals affords much more jelly by boiling, than that of adults. But in the advance of life, a deposition of albumen, a more highly animalised principle, replaces a part of it ; so that even in the chemical composition of this primary tissue, it is only progressively that the characters of the perfect animal are evolved. 36. The primitive fibres of which cellular tissue is composed, some- times arrange themselves into distinct membranes, which present a uniform surface, and have none of these interstitial cavities Avhich have just been described. Of these, the one that presents the nearest approach to the characters of cellular texture, is the serous membrane, which, indeed, differs but little from the thin and expanded plates of that tissue which are frequently met with as envelopes to different organs. It appears to be formed of fibres aggregated into bundles, which are closely interwoven together, so as to leave no appreciable in- terstices ; and it is characterised by the peculiar smoothness and glisten- ing appearance of its surface. This tissue is confined to particular parts of the body ; and it is remarkable that wherever it occurs, it forms a closed bag or sac* The simplest form of such sacs is presented by the hurs(B (purses) as they are termed, which lie beneath or around tendons, and sometimes are interposed between them and the skin ; these are frequently round, in other instances of an irregular shape, and contain a serous fluid which is secreted from their internal parietes ; and their object appears to be to protect the tendons and to facilitate and direct their motion. The arrangement of the synovial mem,hranes, as they are termed, is more complex, and will serve to illustrate that of other serous membranes, which is less readily explained. These membranes form an important part of all the joints or articulations, investing the cai-tilages which cover the adjoining ends of the bones, and affording them a, surface of exquisite smoothness and evenness, by which they may play readily upon one another. The mode in which this is accomplished, and yet the closure of the sac maintained, may be under- stood by the accompanying ideal section of a joint, Avhich shows the extremity of a bone, and of that which is articulated with it, «, a, covered with a layer of cartilage h, b ; and over this, the synovial mem- brane (marked by the dotted line) which envelopes the ends of the bones, and is then reflected back from one to the other, c, c. It is * To this general law, there are one or two comparatively unimportant exceptions. PRIMARY TISSUES OP ANIMALS. 33 SO closely united to the cartilage, that some anatomists deny its presence on its surface ; and this has indeed been rather inferred from analogy, than actually proved. In some of the joints, as that of the knee, there is a complicated apparatus of ligaments Avhich appears to be within the synovial capsule ; but they are in reality on its exterior, each being surrounded by a sheath of membrane which is prolonged from the walls of the sac. Into the cavity of the sac is secreted a fluid termed si/novia, which by its lubricity prevents friction ; it has an oily appearance, but consists, like the fluid of the bursae, of water holding alkaline matter and albumen in solution, and thus dififers but little from concentrated serum (§ 365). What are more commonly knoAvn as serous membranes line the three gi-eat cavities of the body, those of the head, chest, and abdomen ; enveloping the viscera which they contain, in such a manner as to afford them an external coat (as the synovial membrane does to the articular cartilage) ; and being also reflected over the interior of the cavity with which they are in contact (as the synovial membrane over the opposite articular surface), so as to form a shut sac, intervening between the walls of each cavity and its contents, and thus to facilitate their movements, by forming a smooth surface by which each may glide evor the other. This is evidently of peculiar importance where such constantly-moving organs as the heart and lungs are concerned. In the healthy state it seems probable that the opposite surfaces of the serous sac are in absolute contact, (that which covers the lung, for instance, touching that which lines the chest) ; and that the secretion from each, which seems almost identical "with the serum of the blood, is sufiicient only to keep it moist and smooth ; but in diseased conditions this may be excessive, and may accumulate to an injurious extent. Although serous membranes do not appear in general to possess a high degree of vitality, they are capable of taking on a state of violent inflammation, in which the fibrinous part of the blood is exuded upon their surface ; this may be organised into new membranes, and produce injurious adhesion between the opposite sides of the sac. The membrane which lines the heart and blood vessels presents many of the characters of this tissue. 37. The next elementary form of animal structure to be considered, is that which is denominated fibrous meinbrane ; this composes a great variety of organs, but must not be confounded ynih the muscular and nervous structures which are also fibrous, being only another modifica- tion of the same elements that compose cellular tissue. There are two kinds of fibrous tissue, the white and yellow. Of the white are con- structed tendons and ligaments, as well as the fibrous membranes which cover the bones and other organs. These are characterised by the pre- sence of white or greyish fibres, sufficiently large to be seen by the naked eye, possessed of considerable density, and united by membrane of a looser character. They appear to consist of bundles of primitive fibres, 34 ON ORGANISED STRUCTURES. containing a large j)roportion of albumen, and very much condensed ; sometimes they run simply parallel to each other, especially when they form membranous expansions like thefascice of muscles ; whilst, in other instances, they interlace most minutely with one another, as in the dura mater and most tendons, so as not to be unravelled except by very pro- longed maceration. The gradual transformation of common cellular tissue into fibrous membrane, may be frequently observed; and there would seem to be no essential difference between their constituent parts. And between the various forms of fibrous tissue there is a closer resem- blance than would be at first suspected ; for a tendon differs from a liga- ment in little else than the greater condensation of its fibres, (of the arrangement of which a microscopic view is given in Fig. 24), and the smaller proportion of soft tissue intervening between them. Fibrous membranes appear to possess but a low degree of vitality, the circulation through them being inactive ; and the ofiices which they perform in the economy are almost purely mechanical. They possess but little elasticity beyond that conferred by the small quantity of interfibrous cellular tissue ; but their great characteristic is their toughness, by which they are enabled to resist forces which would othermse tear or rupture them. Hence they are peculiarly adapted to enclose delicate organs like the brain ; to connect separate parts so as to preserve their mobility, as in the joints ; or to sustain a powerful strain, as where the muscles termi- nate in tendons. The yellow fibrous tissue possesses much more elasti- city than the white, and is employed in such situations as peculiarly require the exercise of this property. Thus, it composes the ligaments by which the vertebrae (or bones of the spine) are held together ; and, in cattle, the strong elastic cord which stays the head, is formed of the same. It also unites the valves of Mollusca (§ 102) in such a manner as to keep them a little apart, unless its elasticity is counteracted by the exer- cise of muscular power ; and, by a similar contrivance, the claws of the Feline tribe are kept retracted within their sheath, except when volun- tarily protruded by muscular action. The fibres which characterise the structure of these ligaments, seem to be of a different natiire from those just described, and to be in fact sui generis, presenting an obvious rela- tion in their peculiar contractility to muscular fibre, between which and cellular tissue they may be regarded as intermediate. Their disposition is seen at Fig. 25. 38. A very important modification of cellular tissue, or, at least, of its elementary constituents, is the texture denominated, from its peculiar secretion, mucous membrane. This, like serous membrane, appears formed by the close interlacement of fibres ; but the surface, instead of being smooth and glistening, is soft and unpolished, like the pile of velvet, or the rind of a ripe peach. Nor is it by any means uniform ; for there are a number of little depressions or pits into which it PRIMARY TISSUES OF ANIMALS. 35 descends ; and from the lining wliicLi it gives to these, the secretion of mucus appears to be principally formed. Unlike serous membranes, those we are now considering line the 02)en cavities of the body ; thus, one commences at the mouth (being there continuous with the skin), communicates with that which lines the nostrils and covers the spongy bones (upon which the olfactory nerve is minutely distributed), and then divides into two branches; one of these passes dovni the air-passages, and is continuous over the whole interior of the lungs ; the other lines the alimentary tube through its whole extent, communicating again with the skin at its farther extremity, and sending prolongations along the ducts of the glands which pour their secretions into it, — ^these prolonga- tions ramifying and subdividing in such a manner as to be, in fact, the essential constituents of the glands themselves (chap. xi). Another mucous membrane covers the eye, and lines the eyelids, sending a pro- longation which forms the lachrymal gland, and another which lines the lachrymal duct and thus communicates mth the membrane of the nose. Another lines the urinary passages, and forms the tubuli of the kidney (§ 465) ; and another has a similar connection "with the tubes and cavities of the generative system. All these, it is obvious, are continuous with the skin at some point or other ; and anatomical examination of the cutaneous tissue shows that it is not itself organically different from the mucous membrane, consisting, like it, of simple fibres interwoven together in all directions, and having almost identically the same chemical composi- tion,— ^gelatine predominating in both. Although in the higher animals the very different functions which the external and internal portions of this membrane (for so they may be regarded) have to perform, are so different as to lead to such modifications in their structure as render them incapable of altogether fulfilling each other's offices, they would seem, in some of the lowest, to be mutually convertible ; the lining of the stomach in the common gi-een Polype (§ 115) being capable of becoming the skin by inversion, and what was previously the skin serving equally as well as the other to line the digestive cavity. In the higher classes, the skin is the principal organ of common sensibility, the nerves of touch being minutely distributed upon it ; and it also furnishes the means for dissipating a large proportion of the superfluous fluid of the system by exhalation. On the other hand, the mucous membrane lining the ali- mentary canal is specially modified for absorption ; that of the lungs, for the interchange of gaseous ingredients between the blood and the air; that of the various glands, for the separation or elaboration of their pro- ducts from the blood, — and so on. 39. Mucous membranes are very highly organised, being copiously supplied with blood vessels, nerves, and absorbents; the nerves pre- dominating in the skin, and on the membranes lining the nose and mouth ; the blood vessels on the pulmonary and other secreting mem- D 2 3G ON ORGANISED STRUCTURES. branes ; and the absorbents on that of the intestinal canal. The secretion of mucus which constantly covers them, appears to protect them from the contact of acrid or irritating substances ; and it is often observed that if this secretion be from any cause insufficient in quantity, the membrane becomes inflamed. It usually seems to proceed from the general surface, as vi^ell as from the little pits or follicles into which that surface is pro- longed ; but when another secretion takes place from the surface, as that of the epidermis from the skin, or of the epithelium from the membrane of the oesophagus and stomach, this is probably restricted to the mucous crjrpts just mentioned. The skin of fishes is so largely furnished with these crypts, as almost to present the characters of a mucous membrane ; since the protection afforded by their secretion seems necessary for the defence of the body from the contact of the irritating saline fluid ia which it is constantly immersed. The epidermis or scarf-skin, is formed by a secretion from the surface of the true skin, presenting itself in man in the form of little scales or plates, of which the outer ones are con- stantly being thrown off, while new ones are being formed in the interior ; these have at first somewhat the appearance of cells or vesicles, but subsequently become dry and flat. The epithelium^ which covers the mucous membrane of the mouth, and extends down the oesophagus into the stomach, as well as that which covers the conjunctiva of the eye, has a very similar structure, and appears to be only a form of mucus which is capable of becoming solid, similar scales being observed occasionally diffused through the mucus of the lower part of the intestinal canal, where no epithelium is formed. Yarious other substances are formed from the skin in the same manner with the epidermis, and are therefore termed epidermic appendages. Of this kind are the nails, claws, hoofs, scales, &c. which consist of a hardened exudation from the surface, and which all have a laminated structure. Not very dissimilar in character are the hair, spines, quills, feathers, &c. found on other parts, which are secreted by follicles or bags contained in the substance of the skin, and opening on its surface ; these follicles differ but little, except in degree of development, from those which elsewhere secrete mucus. All these parts are extra-vascular, that is to say, not permeated by nutrient blood vessels or absorbents ; and they consequently undergo no change when once formed.'"" It will be hereafter seen that these extra- vascular secretions have a much more important office in the lower animals, con- * Though this is the general fact, yet there are some exceptions which it is not easy to explain. Thus, many instances are on record of the colour of the hair being suddenly changed, usually through some mental emotion. This may, perhaps, be produced by a new secretion from the hair-follicle, of some agent which is conveyed along the tube of the hair, as along any other inorganised canal, and acts chemically upon its substance. In the same manner, also, may be produced that periodical change of colour in the plumage of many birds, which ornithologists have noticed, and which is independent of change in the feathers themselves. PRIMARY TISSUES OP ANIMALS. 37 stituting their entire skeletons ; and tliat the horny casing of Insects, the massive shells of the MoUusca, the lighter calcareous tegument of the Crustacea, and the stony axes of the Polypifera, are all to he regarded in the same light as the epidermic appendages of the Vertehrata (§ 82). 40. It has heen seen that the mucous membranes are the parts of the fabric most concerned in the performance of the organic functions ; their especial office being to maintain the communication between the nutrient system and the external world, by obtaining from the latter the materials requisite for the supply of the body, and by returning to it the superfluous or injurious portions. The constant activity of the processes in which they are concerned, keeps them in intimate dependence upon the due supply of blood Avhich is the stimulus to their actions ; and accordingly we find that the circulation through them is more energetic, and more liable to be affected by temporary changes in their condition, than that of probably any other organ except the brain. Thus, the colour presented by the mucous membrane which lines the stomach, depending entirely on the degree of fullness of its minute vessels, is completely changed by the stimulus of food, which excites the secretion of gastric juice, and, consequently, the flow of blood into its capillaries ; so that the pale hue which the surface presents, when the process of digestion is not going on, rapidly changes to a rosy tint on its commencement. The hue of the skin, in like manner, is aflPected by alteration of temperature, mental emotions, &c. ; and it is an obvious consequence of this susceptibility of change, that these parts are peculiarly liable to the attacks of disease, since they are so much exposed to the influence of external agents. — At the opposite extreme, in point of intri- cacy of organisation, and activity of function, is the cartilaginous struc- ture, which assists in the support and protection of the vital organs. This, in many animals, forms the entire skeleton ; and in the early state of the human body it serves the same purpose. Even in adults, it exists in many parts where a certain degree of flexibility and elasticity are to be combined mth toughness and density; and we consequently find it covering the articular surfaces of bones, as well as giving the form and substance to the eyelids, ears, and other similar parts. In its simplest state, cartilage can hardly be said to exhibit any trace of structure, being apparently homogeneous throughout. By prolonged maceration or boiling, however, something like filaments of cellular membrane may be detected in it ; and as it may be easily shown to contain both gelatine and solid albumen, it may perhaps be regarded as formed by the deposition of the latter material in the cavities of cellular tissue. Many of the cartilages, however, exhibit an evident fibrous structure ; and here the albuminous part may in them be regarded as organised in the same manner as that which composes the white glistening lines on fibrous membranes (§ 37). Sometimes the fibrous structure prevails to so great an extent, as to give 38 ON ORGANISED STRUCTURES. an almost indeterminate character to tlie tissue ; this is the case, for in- stance, in the intervertebral substance which lies between the bones of the spinal column. The exterior of each mass appears composed of fibrous tissue, the interstices between which are filled up with cartilage ; but the fibres gradually diminish in the interior lamina, and the cartilaginous matter becomes less dense ; so that in higher animals the tissue is compa- ratively soft, and in fishes the centre is occupied Avith a sac containing fluid. The cartilages which precede, in the foetus, the portions of the skeleton which are to be subsec[uently osseous, do not exactly correspond in the ultimate arrangement of their particles, with those which are per- manently to retain their soft condition. However, in general character they are obviously the same ; and their sections, before the process of ossification has commenced, exhibit an almost homogeneous substance, with little trace of defined structure. At this period, it is doubted by many whether they possess blood-vessels of sufiicient size to convey the red particles, as they are altogether destitute of colour ; and the same doubt is entertained as to the permanent cartilages, which seem to be the parts of the fabric in which vitality is feeblest. Their elasticity prevents them from the liability to be as severely affected by mechanical violence as bones are ; and therefore they do not require the same power of repa- ration. All their actions are of a physical character simply ; and need only a structure endowed Avith the physical property of density combined with some degree of flexibility and elasticity. It is during the process of ossification that the greatest activity is exhibited in this tissue, red vessels rapidly extending through it, and effecting an important change in its character. This change appears to consist essentially, in the removal of a portion of the albumen, which previously filled up the interstices of the cellular tissue, and its replacement by particles of calcareous matter deposited by the blood, so as to convert the cartilaginous into the osseous structure now to be described. 41. The essential characteristic of the osseous or hony tissue, is its possession of a large quantity of calcareous matter, deposited in the interstices of an organised structure, in which exists an apparatus of vessels, &c. capable of performing vital changes with considerable activity. In this consists its difference from the extravascular structures of which the skeletons of the invertebrated animals are composed ; since the latter are not susceptible of being modified by any agents except those which act upon their surface (§ 100), whilst the former are as capable as any part of the organism to which they belong, of undergoing the processes of interstitial absorption and deposition (§ 232). The particles of calcareous matter cannot themselves be regarded as organised, since they retain, if not their crystalline form, at least their crystalline arrangement ; but they are strictly analogous vidth those which are deposited, in smaller proportion indeed, in the tissues of vegetables. These particles consist of PRIMARY TISSUES OF ANIMALS. 39 the carbonate and pliosphate of lime, both of which are present in variable proportion, (the latter usually predominating considerably), in all bones, and even in minute quantity in cartilage ; they certainly exist there as salts, and not in chemical combination with organic principles. The tex- ture of bone is usually fibrous ; but the fibres are frequently united into lamellaa or plates, which sometimes form regular layers, and sometimes bound cancelli or cells. In the long bones, we find in higher animals a central canal, which in Birds is hollow, and in Mammalia is filled with a fatty substance termed marrow. This canal is surrounded by very compact bony tissue, in which the laminated structure is most distinct ; whilst towards the extremities, the compact laminse become thinner, and the interior becomes filled with cancellated structure, analogous to that which intervenes between the hard external plates of the flat bones. In Reptiles and Fishes, however, there is not this distinction of parts, the bones being solid throughout, but nowhere presenting the same firmness as that of the exterior of the bones of Birds and Mammalia, their texture being of a spongy character. Whatever may be the an-angement of the constituent parts of bones, however, there can be little doubt that their texture is ultimately reducible to a form of cellular tissue, in which carti- laginous and calcareous depositions have taken place. The latter may be removed by the action of acids ; and a flexible elastic substance is then left, possessing the characters of soft cartilage, but exhibiting marks of higher organisation. A very complicated apparatus of minute canals is observed in it,* which probably mark the situation of the calcareous * For the latest details on the minute structure of bone and cartilag'e, see Miiller's Physio- logy, vol. I., p. 377, &c. These tubes (termed Haversian, from the name of their discoverer,) contain medullary matter; and, indeed, the central canal, as well as the separate cells, may be regarded as enlargements of them. Around each canal is observed a deposition of bony matter in concentric circles ; and on each circle are radial lines, which seem to divide it into narrow tubes pointing from the centre to the circumference. The teeth have been imagined by many to be extravascular structures (§ 39), or mere secretions, formed in successive laminae hke hair or quills ; or to be, at least, only connected with the circulating and absorb- ent system through the membrane which lines then* cavity. The recent enquiries of Miiller, Purkinje, Retzius, and Mr, Owen, however, (the last of whom communicated the results of his observations to the British Association, at its Newcastle meeting*,) have shown that their substance is composed of tubular fibres, which are generally arranged in a radiating manner from the centre to the circumference, so as to be perpendicular to their surface, and formed of animal membrane containing a deposition of calcareous matter. These tubes frequently ramify and inosculate with great minuteness, and communicate with cells, in which also calcareous matter seems deposited on a basis of membrane. In the teeth of higher animals, thei'e are no large branches connected with the central pulp-cavity ; so that the structure re- sembles that which a bone would present, if destitute of Haversian canals, but retaining the medullary cavity. But among fishes and other inferior species, there are seen coarse tubular ramifications of the pulp-cavity, containing a sanguineous medulla, which are continuous with those of the bone to which the tooth is attached ; and liere, therefore, the resemblance to bone is much closer. This is one of the many instances of a special structure, for a special function, not being superadded to, but elaborated from one more general, in proportion as we ascend the scale (§ 200). 40 ON ORGANISED STRUCTURES. deposit ; but what connection these have with the blood-vessels which permeate the structure does not seem distinctly ascertained. If, on the other hand, the animal portion be removed, either by heat or chemical agents, a brittle mass remains, which preserves its original form, and ex- hibits the particles of calcareous matter in a state of loose aggregation. By long-continued boiling, the organised part may be dissolved in water, and then possesses the characters of gelatine, of which principle it seems to be mostly composed ; it probably also contains some albumen, though this ingredient exists in much less proportion than in cartilage, and seems to have given place to the calcareous matter. The proportion of the animal to the mineral matter, however, varies in different species ; always bearing a relation to the age of the individual. Young bones possess considerable elasticity, but are deficient in density, assimilating more in their proper- ties to cartilage ; Avhilst those of old persons are so much consolidated by the continual deposition of mineral matter, that they become extremely hard and brittle. 42. All the tissues now described are formed, more or less evidently, upon the basis of cellular structure. It would be easy to multiply the number of elementary parts, by describing as distinct fabrics what are only modifications of others, or combinations with one another. But those which have been specified will probably be found to comprehend all the essentially different varieties which are met with in the animal kingdom, with the exception of the two which are quite peculiar to it, — the muscular and nervous tissues. And these are rather peculiar in vital properties and chemical constitution, than in their form of organ- isation. Observation of muscular structure shows that it possesses a fibrous texture; and when any muscle is particularly examined, it is found to be separable into a number of distinct fasciculi, or bundles of fibres, which are connected by cellular tissue. These again are divisible into smaller fasciculi, which are similarly united; and each of these, if carefully analysed, is found to consist of a number of dis- tinct fibres, which possess a very peculiar and characteristic structure, and are usually spoken of as the ultimate fibres of muscular tissue. (This statement applies, however, only to the muscles of voluntary motion ; for it will presently be seen that the structure of the muscles connected with the organic functions is very different.) These fibres are usually about -^^-^ of an inch in diameter, varying in dififerent organs and in different animals from about -^^-^ to -^\-q. Each appears to possess a tubular structure, and to be formed of a number of smaller filaments arranged longitudinally, bound together by transverse or circular bands. According to the late observations of Mr. Skey,* each of these filaments, of which about 90 or 100 unite to form one tubular fibre, has itself a tubular structure ; but this is a point not * Philosophical Transactions, 1837. PRIMARY TISSUES OF ANIMALS. 41 easily determined, from the extreme minuteness of the filament, Avhich is only about yfJto P'^^*' ^^ ^^ inch in diameter. That they are hound together by transverse rings is inferred by Mr. Skey, not only from the striated appearance of the fibre itself (Fig. 26), but from the fact that each separate filament exhibits indentations at distances corres- ponding mth those of the striae on the perfect fibre. The tubes of the fibres are found to contain a glutinous fluid, but its nature has not been distinctly ascertained ; by Dr. Quain it is stated that distinct globules are discernible in it. This may be reasonably doubted until these globules shall hare been shown out of the tube ; since the com- bination of the transverse striae with the markings of the longitudinal filaments is very liable, where the tube itself is vicAved, to occasion an appearance resembling globular structxu'e, the true nature of Avhich cannot be detected except by a very superior microscope. This has been the source of error among those who imagined that the muscular fibre itself is made up of globules ; and some have even maintained that these globules are the red particles of the blood, without suffi- ciently attending to the disproportion in their size. 43. The muscles of organic life present a very difierent appearance. Their fasciculi are not so distinct ; and when the attempt is made to separate them into fibres united by cellular tissue, it is found that they do not possess this structure, but that the minute filaments are interlaced into an irregular net-work (Fig. 27), in which nothing analogous to the Jibre of the muscles of voluntary motion can be detected. This, at least, is the character of the muscular structure of the alimentary canal, from the point where the oesophagus enters the thorax ; above this, the oesophageal fibres, as well as the constrictors of the pharynx, present the appearance first described. In the heart, a kind of mixed structure is seen ; a tendency to the formation of tubular fibres, in the midst of the irregularly reticulated mass of filaments, being discernible. The nature of the actions of these parts seems closely con- nected mth the arrangement of their elements. In the muscles of voluntary motion, the object is to approximate by their contraction two distinct points, upon which, therefore, all their force is concentrated ; and the filaments, in which the contractile power resides, are an'anged in the most advantageous manner to effect this pui-pose, being parallel to each other, and closely united so as to harmonise in their actions. In the muscles of organic life, on the other hand, a rapid and energetic contraction in a single direction is not what is required ; but a slower movement propagating itself gradually over an extended surface, and operating in several different directions. In the heart also, the inter- mediate condition of the structure appears related to that of the function ; for energetic and decisive contractions are here required, but these must be diffused over the surface of the caA'ity in which the resistance is 42 ON ORGANISED STRUCTURES. situated, and must operate without fixed points of attachment for the fibres, so that a reticulated arrangement is obviously most advantageous. It is stated by Mr, Skey, that the middle coat of the arteries is composed of a tissue having precisely the same appearance under the microscope as the muscular tunic of the intestines ; and that this is obviously different from the elastic fibrous tissue with which it has been associated. It does not seem, however, to be chemically identical with muscular tissue, since it is entirely deficient in fibrin, which is the characteristic ingredient of the latter. Fibrin, of which the ultimate fibres of muscles appear chiefly composed, is made up, like most of the other combinations of which the animal body is constructed, of the four elements, oxygen, hydrogen, carbon, and nitrogen ; but it contains a larger proportion of the last than any other proximate principle, and is thence considered as the most highly animalised (§ 17). It is interesting to compare its peculiar composi- tion with the special character of its function ; this being one of many facts which tend to prove that what are termed vital as well as physical properties, may be dependent upon the combination and arrangement of the elementary particles of the tissues which manifest them (chap, i). 44. The last of the animal tissues, the nervous structure, is one which has afforded a fruitful source of investigation to the microscopic enquirer, whilst the peculiarity of its functions renders it an object of especial interest to the physiologist. If any nervous trunk be care- fully examined, it will be found to consist of a number of smaller filaments, connected together by cellular tissue, and enclosed in a common membranous envelope, the neurilema. These filaments, when analysed in the same manner with the minute fasciculi of muscular substance, are found to consist of tubular fibres, which are usually, if not always, perfectly cylindrical (Fig. 29) Their cavity is filled with a sort of medulla or pith, which, when squeezed from them, has a granular consistence ; but when lying in situ, this substance is stated by Remak to be itself a continuous fibre, divisible into minute filaments. Their diameter varies, among the Invertebrata, from -^ to TiyVo^ *^^ ^ ^^^^ ' ^^* ^^ Vertebral animals the extremes are not so distant, the tubes being commonly fi'om ^^ to -^\-q of a line in diameter, A smilar fibrous structure is evident in the brain; and here the tubes seem to contain a viscous fluid, not altogether unlike the medulla of the fibres of nerves, but of less consistence. A different structure has been described by Ehrenberg, under the name of varicose tubes, which he states to exist in the brain, spinal cord, and nerves of special sensation. These tubes were so named from their not being cylindrical, but presenting dilatations at intervals, so as to resemble a string of beads (Fig. 428) ; and the appearance of these dilatations has given rise to the opinion that the brain is composed of globules. It is now, however, satisfactorily shoAvn that they are the result of PRIMARY TISSUES OP ANIMALS. 43 the pressure and other manipulations to which the objects are subjected in preparation for the microscope ; and that, if the nervous fibres of the brain and other parts are examined in a recent state, they are cylindrical, like those of the nervous trunks in general. Still there is some difference in their structure, since they exhibit this tendency to become varicose, which is elsewhere wanting. Besides these tubular fibres, which constitute the white portions of the nervous matter, there are other filaments of a grey colour, and of much smaller diameter, without distinct cavities, which exist especially in the sympathetic nerves, but which may also be detected in others. All the fibres ap- pear to maintain their separate continuity, from their origin to their termination, without any junction or anastomosis amongst each other ; it is not uncommon, however, for two or more trunks to interchange separate filaments. This kind of connection seems to exist between the two great divisions of the nervous system (chap, xv), each con- taining some fibres Avhich are derived from the other. In the grey substance of the brain, and in the ganglia of the sjonpathetic, the fibrous arrangement seems to be lost ; these portions consisting of cellular tissue, and a network of blood-vessels, in the interstices of which lie a number of globules, and into which the neighbouring fibres are pro- longed in loops. These globules are of large size in the ganglia, and seem connected together by little filaments, like the small grey fibres of the sympathetic nerve. In the brain, they appear broken down into more minute granules. Nervous tissue is very copiously supplied with blood-vessels, which not only form a large part of the grey sub- Stance, but ramify minutely in the trunks of the nerves ; and upon the constant stimulus of the circulating fluid its functions seem to depend. Nervous matter, or neurine, contains a remarkable propor- tion of water — no less than 80 per cent. ; a peculiar fatty matter into the composition of which nitrogen enters ; together Avith some sulphur and phosphorus, the proportions of which last ingredients appear pecu- liarly liable to be afi^ected by disease. V. — Transformation of Tissttes. 4)5. There exists, to a certain extent, a capability on the part of the different tissues now described, to assume each others' characteristic forms and properties. This transformation of tissues, however, is governed, like their first creation, by certain fixed laws. In particular portions of the vegetable structure, we may detect the occurrence of such changes among the regular phenomena of growth. Thus, we find vesicles of cellular tissue, which were at first isolated, subsequently becoming continuoiis ducts or canals, by the obliteration of their partitions (§ 24) ; and that this change takes place during the development of every more perfect plant, seems evident from the fact that in the embrvo state no such ducts 44 ON ORGANISED STRUCTURES. are ever found, the whole fabric being formed of cellular tissue. Again, it seems ascertained that cells and vessels formed upon the spiral tj^pe, may present great varieties of appearance at different stages of develop- ment ; the fibre which at first possessed a regular spiral form, being sub- sequently broken into rings or more irregular portions, so as to produce an annular or reticulated duct ; and these portions at a later period grow- ing at their edges, and uniting together to form that kind of internal sheath with numerous interstices, which constitutes a dotted duct (§ 27 — 29). We find in the lower parts of the vegetable scale that the function of these ducts, namely the conveyance of fluid, is performed by cellular tissue, which alone constitutes the simplest forms of plants ; and it is from cellular tissue, in the higher and more elaborated vegetable fabrics, that we find these special organs gradually developed. But we never find woody fibre replacing the ducts either in situation or in function, however extensively we prosecute our examination ; nor do we ever observe that woody fibre transforms itself into any kind of duct or vessel. These, indeed, appear to be modifications of cellular tissue entirely distinct from each other, although having a common origin ; so that when once their character is determined, it remains fixed. Although the varieties of elementary tissue are much fewer in vegetables than in animals, we are able to trace the operation of the same general law in their development, — that the transformations which they undergo in the evolution of the embryo of the higher plants, are analogous to those which are presented to us in ascending the scale of existence, from its simpler to its more complex structures. 46. In the development of a highly organised animal fabric, possessed of a multitude of dissimilar parts, out of the simple and almost homoge- nous body which constitutes its germ, it would be very interesting to trace the gradual evolution of the different tissues, as well as the organs they compose. But this subject must be here very slightly dwelt upon. It is commonly stated that all the elementary structures take their origin from cellular tissue ; this is, however, scarcely correct, since the appearance of this last, as of all the rest, is preceded by the existence of a semi-transpa- rent gelatinous matter, of which the entire embryo seems at an early period to be formed. This substance, which bears a considerable resem- blance to the granular pulp of which the lowest animals are composed, is absorbed and disappears, in proportion as more definite structure is evolved; and the formation of several tissues may be observed to take place simul- taneously in the midst of it. There can be no doubt, however, that cel- lular tissue enters into the composition of every organ in the body ; and that, in all which essentially consist of it, very important modifications may take place, either during the natural stages of gi'owth, or from the effects of disease. Thus, we find cartilage transformed into bone ; mem- branes becoming cartilaginous ; ligamentous bands becoming fibrous ; — PRIMARY TISSUES OF ANIMALS. 4.5 and so on. But these transformations are governed, both in health and disease, by certain fixed principles, of which the most general (being applicable to the vegetable as well as to the animal kingdom) was stated in the last section. But this must be understood in the latter case, as in the former, with some limitations. Thus, cellular tissue may be transformed into any tissue Avhich takes its origin from it ; but this, when once fully evolved, cannot be converted into another. When this trans- formation takes place, it is often to fulfil some special object required by the circumstances. Thus, a new serous or synovial membrane is pro- duced to obviate friction, where a new joint results from an unreduced fracture or dislocation ; a cutaneous membrane is developed, where pro- tection from the external air is necessary ; and cartilage, where elasticity and strength are required. 47. When a regeneration occurs of parts which have been destroyed by disease or injury, cellular tissue is at first formed, which is afterwards converted into the structure that is to be repaired, or into some other which replaces it in the animal scale. Thus, a divided muscle is united by a yellow fibrous tissue, like that which in some animals seems to exist as a substitute for muscle ; and cartilage, which is formed between the two ends of a broken bone, before the deposition of ossific matter, fre- quently supplies its place in animal structures. But although cellular tissue itself may be transformed into any of its modifications, these do not appear capable of being changed by disease into one another, except- ing so far as, in the progress of embryonic life, or in the animal series, similar transformations occur. Thus, cartilage may become bone, but never mucous membrane ; mucous membrane may be converted into skin, and vice versa, but neither into serous membrane. Again, it is found that all tissues which are atro^yhied, or insufficiently supplied with nutriment, have a tendency to return to the condition of cellular structure. This degeneration occurs in a great variety of instances. Sometimes it forms part of the regular succession of changes which mark the advance of life ; as when the thymus gland, ductus arteriosus, &c. of the foetus shrivel up, having no longer any function to perform. Sometimes it results from disease or want of use in the organs themselves ; as, for instance, where muscles which have been long inactive lose their contractile fibres. And sometimes it is observed in the animal species, where an organ Avhich is important in one species, ceases to be so in another that is allied to it. A curious illustration of the latter kind is affi)rded by the Ugamentiim nuchce, which gives such important assistance in the support of the head, where the neck is long and the head heavy, as in the horse or ox. lu these animals it is distinctly composed of yellow fibrous tissue ; but this structure is not so evident in the sheep, the dog, and the pig, where strength and elasticity are less required for this purpose. Few fibres are found in the ligamentum nuchae of the cat, and in man it is entirely composed of 46) GENERAL VIEW OF THE ORGANISED CREATION, cellular substance ; but in persons who are accustomed to carry heavy burdens on their heads, a fibrous structure may be detected. Lastly, transformations which result from disease not unfrequently tend to estab- lish an analogy with the usual condition, in some other animal, of the part affected. Thus, it is not uncommon to find in man bony plates existing in the fibrous membrane (dura inater) Avhich surrounds the brain, and especially in those projections of it (the falx and tentorvMn) which divide and support the different parts of that organ ; and these projections exist in a state of more or less complete ossification in many quadrupeds, especially among the Carnivora, Again, the ligamentous substance which connects the muscular fibres at the base of the heart, is not unfrequently ossified by a process of disease in man ; whilst in the ox and other ruminating quadrupeds, bone naturally exists there. It would be easy to adduce many illustrations of this kind ; but these will suffice to show that law and arrangement preside over the adaptation of these minute parts, as over that of structures apparently more important, VI, — Vegetable Kingdom. 48. It is computed that from 70,000 to 80,000 distinct species of plants, or races descended fi-om different original stocks (chap, xiv,), exist in various collections ; and probably at least as many more remain to be discovered. It is obvious that an acquaintance with their cha- racters, structure, and mutual relations, will be much facilitated by a judicious arrangement of them ; and, indeed, it can only be gained within the compass of a single life by such means. In making this arrangement, those species are first assembled into a group, termed a genus, which resemble each other in all the more important particulars, and differ only in minor details. Several genera may, in like manner, be united into a larger division, which shall embrace those that agree in the higher or more general characters, but differ in their special conformation. By continuing to pursue the same plan with regard to these divisions, we arrive at orders and classes j and we are at last brought, by uniting these, to certain primary divisions into which the whole kingdom may be at once distributed, each of which contains a large number of dissimilar groups united together by some common points of general resemblance. Whatever be the peculiar mode of classification, this plan is its found- ation ; and what are called Artificial and Natural Systems differ in this, — that the artificial method groups together plants according to their cor- respondence in some one particular character, without regard to the rest, and thus fi-equently brings together plants which differ extremely in character and properties ; — whilst the natural aims to associate in the same division those which have the greatest general resemblance to each other, and the properties as well as the structure of which are found to VEGETABLE KINGDOM. 47 present a manifest correspondence. The Artificial System of Linnaeus is undoubtedly tlie best of its kind, and tbe most easy of application. Its classes and orders are principally founded upon the number of particular parts in the flower ; and as every tyro can count these, the place of an unknown plant in the classification may at once be discovered. But when so ascertained, no absolute information has been gained respecting the structure, properties, or affinities of the individual ; and a reference to books is necessary to obtain it. A person acquainted with the characters of the Natural orders may, on the other hand, at once determine to what previously knoAvn genus a new or unknown plant is most allied, what is its place in the series in reference to others, and (which is of the most immediate practical importance) what are its poisonous or esculent properties. It is only by aiming to perfect a Natiu-al System, which shall give a faithful account of the relative conformation of the immense multitude of species dispersed over the globe, that we can have any expectation of arriving at a knowledge of the laws which regulate the structure and distribution of the vegetable kingdom ; and Linnasus was so sensible of this, that he framed his artificial system solely for the purpose of facilitating the accumulation of materials necessary to construct a natural method. 49. The primary division of the vegetable kingdom made by Linnaeus, into Phanerogamia or Flowering Plants, and Ckyptogamia or Flower- less Plants, Avas, however, a natural one ; because the members of these groups do not agree in the single condition of the j)resence or absence of flowers alone, but in various other peculiarities of structure. It must be explained, however, that it would probably be more con-ect to speak of the Cryptogamia as plants in which only one kind of apparatus is neces- sary to the formation of the embryo, and of Phanerogamia as requiring two forms of reproductive organs for the production of the seed (chap. XVII.) : but these organs, though usually contained together in each flower, are sometimes separated, as in Monoecious and Dioecious plants ; or they may exist in an obscure form, without any of those appendages which constitute what is usually denominated a flower. The Phane- rogamia have this most important difference in structure from the Cryp- togamia, that whilst the former contain woody and vascular texture in abundance, the latter are almost entirely composed of simple vesicles of cellular tissue. Hence the former are frequently denominated vascular plants, and the latter cellular ; but this distinction must not be regarded as holding good in every instance, since the higher Cryptogamia, such as Ferns and Mosses, possess not only a woody stem, but evident indications of vascular structure, although no true spiral vessels are found among them ; and there are many Phanerogamia in which no spiral vessels can be detected. This instance is only one among many which could be produced, to show the impossibility of laying down, AA-ith regard to any 48 ON ORGANISED STRUCTURES. group, characters so definite as to include all its members, without at the same time opening the door for the admission of others which may pre- sent approximations to them. In fact, there is scarcely any one pecu- liarity of structure upon which divisions have been established, that may not be found to exist in an obscure or rudimentary form in neighbouring species. Another marked peculiarity which distinguishes Phanerogamia from Crjrptogamia, is the structure of the seed of the former, as compared with the spore of the latter. A mature seed, prepared by one of the former class, contains not only the embryo of the future plant, already assuming a definite form, and exhibiting the rudiments of its future stem and root ; but also one or more temporary leaves termed cotyledons^ which assist in its development until the true leaves are evolved ; as well as a store of nutriment already assimilated by the parent, which, like the albumen of the egg, supports the growing structure until it is capable of maintaining its own. existence. This last part ordinarily con- stitutes the bulk of the seed. The spore of Cryptogamia, on the other hand, possesses no such distinction of parts ; and the commencement of the growth of the embryo which it contains is very different from the germination of a seed,* 50. The division of the Phanerogamia into subordinate groups is very readily effected, since there are many striking points of difference which separate them into two classes. Of these, the most constant and remarkable are the structure of the stem, and that of the seed ; whilst the conformation of the leaves and flowers also present some peculiarities common to the two classes respectively. The names given to these divisions, with reference to the structure of their stems, are Exogens and Endogens. The former, which includes all the trees and most of the herbaceous plants of temperate climates, is so named from the additions to the diameter of the stem being made externally to the part already formed. In the Endogens, the division which comprehends the Palms, Canes, &c. of tropical climates, and the Grasses, with most bulbous-rooted plants of this country, the ad- dition to the stem is made within the previous portion of it. The respective structures of this part will presently be more particularly described. The divisions which, previously to the discovery of this distinction, had been erected upon the character of the seed, correspond almost exactly with those just stated. Seeds usually contain either one or two cotyledons ; in the former case they are termed Monocoty- ledonous, and in the latter Dicotyledonous. The structure of the former is illustrated in Fig. 32, Avhich represents the seed of a lily or onion ; this contains the embryo, a, enveloped in its cotyledon, and surrounded by the albumen, b, which is laid up for its support. * What is the real analog-ue of the spore in flowering plants, will be hereafter consi" dered (chap, xiii). VEGETAULE KINGDOM. 49 When germination (the incipient development of the seed) takes place, the plmnula or young stem («, Fig 33) pushes itself through the co- tyledon, which continues to sheath its lower part withoiit entirely quitting the coats of the seed. All plants which have an Endogenous stem have Monocotyledonous seeds, and vice versa; so that the terms are synonymously used to characterise this great division of the vegetable kingdom. Amongst Dicotyledonous plants there is more variety in structure ; for the albumen does not always surround the embryo, but is sometimes taken into its substance, rendering the cotyledons thick and fleshy. This is the case in such seeds as those of the bean or pea (Fig. 34), where the two cotyledons a, «, are seen connected by the germ of the stem and roots, which consists of the plumula, 5, and the radicle, c. In the Lime Tree, Castor Oil plant, and many others, however, the albumen is a separate store, as in Monocotyledons, and the seed-leaves are thin and membranous. During the germination of these seeds, the upward elongation of the plumula carries the cotyledons to the surface, Avhere they acquire a green colour by their action with the air, at the same time performing all the functions of leaves, until the permanent foliaceous organs are evolved (Fig. 35). The albumen, whether contained within them, or remaining within the seed, is gradually absorbed by the young plant, which, when this is entirely exhausted, is capable of maintaining its own existence. All Dicotyledonous plants are Exogenous in the structure of their stem, but the reverse does not quite hold good ; for some Exogens, as the Pine tribe, have many cotyledons ; and others, as the Horse-chesnut, appear to have only one. In the former case, however, it is probable that the increased number may be due to the division of the original pair ; and in the latter, it is certain that there is no absence of either part, but that the cotj^ledons are united together, so as to resemble a single organ, being still really double. 51. AVe shall now consider more in detail the structure of the stem, and its differences in Exogens and Endogens. In both cases it consists in part of cellular tissue, which forms, as it were, the mould of it ; and in herbaceous plants, the soft succulent axis is composed of little else. In harder stems, however, Avoody fibre forms a larger or smaller part ; and in these we find vessels and ducts of different kinds developed to the greatest extent (§ 285). It is in the particular an-angement of these tissues, and in their mode of increase, that the difference between the Exogenous and Endogenous stems consists. The structure of the former is illustrated in Fig. 30, of which the upper part shows a horizontal or transverse section, and the lower portion a vertical section, (the same parts being represented in both under different aspects), of such a stem as the Ash, Beech, Ehu, &c. In the centre at a is seen the pith, as viewed 50 ON ORGANISED STRUCTURES. through a microscope ; this is composed of cellular tissue only, generally of a regular figure, either hexagonal or square. Surrounding the pith is a delicate memhrane consisting almost entirely of spiral vessels, seen in section at 5, h ; this is called the medullary sheath. Exterior to this is the wood^ which is composed of concentric rings, equal in niunher to the age of the tree. Each ring is made up of vessels and woody fibre : the vessels c, c, c, (whose transverse section is shown by the large apertures in the upper figure), being usuallv>|;^^i^^^^^=^:e:^h ring ; while the woody tissue 7 previously formed parts, wliich seems common to all the Crj^togamia possessed of anything like a distinct axis, the term Acrogens has been applied to the members of this division, for the purpose of bringing it into contrast with Exogens and Endogens. Ferns present no form of fructification which has an evident analogy mth flowers ; but their cor- responding organs are very interesting. The spores, (commonly supposed to be the equivalents of the seeds of Phanerogamia*), are contained in little cases of very curious structure termed T/iecce, which are developed on some part of the under surface of the leaf, being always connected with its veins. Each t/ieca, in its most perfect form, is mounted upon a little stalk, which is continued round its circumference in the form of a ring ; and this, by its elasticity, separates the divisions for the escape of the spores when ripe (Fig. 46). In some species, however, the theca is destitute both of a footstalk and of a ring, and is simply implanted on the leaf. The thecse are usually arranged in clusters, termed sori; and these are sometimes circular (Fig. 47, «), sometimes linear, as at b, and sometimes confined to the edge of the leaf. In some forms of this group, such as the Osmunda regalis, (or flowering fern, as it has been incor- rectly termed,) which is the handsomest of the British species, the sterile or leafy fronds are distinct from the fertile or spore-bearing ones, the latter losing their leafy aspect by the contraction of their margins around the thecse. This distinction also exists in the Oj)hioglossum, (adder's tongue) ; here the thecee are altogether wanting, the spores being inclosed in segments of the leaf, which are folded in to embrace them (Fig. 48). Of one of the orders which have usually been ranked among Ferns, or as allied to them, the Marsileacea^, we shall speak more particularly at a future time (§ 524). The Lycopodiacece, or Club-Moss tribe, appear intermediate between Ferns and Coniferse on one hand, especially through their fossil allies ; and between Ferns and Mosses on the other. They are related to Coniferse by the structure of their stems, especially those of their larger kinds ; and to Ferns in the abundance of the annular ducts contained in them, as well as in the characters of their reproductive system, about which there is, however, some uncer- tainty. Their general aspect most resembles that of the Mosses, espe- cially when the stems are creeping, and the leaves imbricated, or folded over each other. Their system of fructification consists of Theca^ con- taining two kinds of reproductive bodies, the relative oJBices of which are not knoAvn. The powdery matter which constitutes one of these, goes under the name of vegetable sulphur ; and from its peculiar com- bustibility, taking fire with a flash when diffused through the air, it is employed at the theatres, &c. for the purpose of producing artificial lightning. 60. The next group of Cryptogamia, that of jiosses, is as interesting * Into their real character we shall enquire in the proper place $ 519. 60 ON ORGANISED STRUCTURES, wliicJb tliey groAV, if tliis be at all impregnated with calcareous matter ; and by the deposition of it beneath their tegument, they have gained their popular name of Stone-icorts. The peculiar circulation of nutritious fluid within these tubes, to which so much attention has recently been j^aid, will be described in its appropriate place (§ 353). It is in their organs of fructification, however, that the Characese seem to rank above those tribes, with which the very simple structure of their other parts would associate them. As the true character of these organs has not yet, however, been ascertained, it is not desirable to enter here into a description of them. 63. The characters of the three lowest groups of Cryptogamia, the FUNGI, LICHENS, and ALG/E, approximate so closely to each other, that it is not easy to define them, by reference to their structure alone. They are all entirely composed of cellular tissue, and in the evolution of their reproductive system hardly seem to differ essentially. In fact, the lowest tribes of each pass into one another by almost insensible gradations. The peculiar character of the fungi, or Mushroom tribes, consists in their habitation, which is always upon dead or decaying organised matter ; and in the predominance of their reproductive system, no thallus or foliaceous expansion ever existing independently of the part which bears the spores. Lichens grow upon living vegetables, earth, or stones, in situations where they are fully exposed to light, and are not too abundantly sup- plied with moisture ; the tendency in them is to the formation of a thallus, of which the upper surface usually presents itself as a hard dry crust, whilst in certain parts of it, asci, or tubes containing spores, are united into sJdelds, which are distinct from the remainder of the expan- sion. If Lichens are removed from the influence of light, and are over- supplied with moisture, they then show a tendency to the extension of the vegetative or foliaceous portion of the thallus, and to the non-produc- tion of the fruit. This is what occurs in alg^ or sea-weeds, all of which are inhabitants of water, and which are scarcely distinguishable by any other positive character from Fungi and Lichens, than by the predominance of their nutritive system over the rejjroductive organs. All, however, meet in such simple forms of vegetation as the Protococcus nivalis or red snow (Fig. 57), the Palmella cruenta or gory dew, the Nostoc or fallen star; these consist of simple aggi-egations of vesicles without any definite arrangement, sometimes united, but capable of existing separately ; and by their own rupture give independent exist- ence to the rudiments of new individuals contained vrithin them. By some they have been placed among the Algas, by some termed Fungi, and by others Lichens ; the real truth appears to be, that in beings of such simplicity there are no definite characters by which their afiinity to one group or another is particularly indicated ; and that they are to be regarded rather as the sketches or rudimentary forms of more perfect VEGETABLE KINGDOM. 61 structures. It does not seem an improbable conclusion from certain ob- served facts, that the same germ, among these lower Cryptogamia, may assume several forms usually regarded as distinct, according to the circumstances under which it is developed (§ 65). 64. The tissue of the ftjngi is generally soft and succulent, and its duration transient. These plants are almost always found gi'owing upon dead or decaying animal or vegetable substances; and where they appear unequivocally upon living bodies, there is much reason to believe that they are generally the indications of a state of previous disease, which, by the unhealthy nutrition of the tissues, has prepared a similar nidus for their development. In their simplest form they are little jointed filaments, composed of cellules laid end to end, or collected in a mass under the cuticle of leaves or other parts ; such are all the varieties of Mould, Mildew, &c. In some of these the joints separate, and each appears capable of reproduction ; in others the cellules which contain the rudiments of the new plants are collected at one extremitj'^, whilst the others serve as a stalk (Fig. 56) ; and in the higher forms of this group, these fertile cells are collected within a special membranous envelope. Other Fungi, again, have a more determinate figure, usually rounded ; and in their substance the sporules either lie loosely mixed Avith filaments, as in the Lycoperdons or puff-balls, or contained in membranous tubes, like the asci of Lichens. In their most complete state, exemplified in the Agaric or Mushroom tribe, there is a distinct stem or axis evolved, which separates the reproductive apparatus contained in the f ileus or cap, from the nutritive or absorbent system of the root ; in these, the spores are contained in tubes, imbedded in the hytnenium or fructifying membrane, that constitutes what are termed the lamince or gills, on the under surface of the pileus (Fig. 57). Thus, a progressive complica- tion of form may be observed, without any alteration of the original characters of the simpler members of the gi-oup. The Fungi spring up with extraordinary rapidity, often acquiring the volume of many cubic inches in a single night ; and they are commonly observed to appear suddenly after thunder-storms or some other meteoric changes. From these circumstances, and from the remarkable certainty of their appear- ance upon decapng organised matter, wherever it exist, many have been disposed to question the development of Fungi from distinct germs, and to imagine that they are generated by the processes which are ante- cedent to their manifestation. It is stated in support of this doctrine, that it is possible to increase particular species with certainty, by exposing a certain mixture of organic and inorganic matter to atmos- pheric changes, as in the process adopted by gardeners for raising the edible Mushroom ; and that particular species of parasitic fungi are confined to particular leaves. It certainly is not easy to answer the questions which thence arise, why no kind of fungus but the Ar/oricus 62 ON ORGANISED STRUCTURES, campestris should arise upon the Mushroom-spawn^ as it is termed, — •why- no Puccinia but the Puccinia rosw should be found upon rose-bushes, — why this species should never be seen elsewhere, — and so on. As this is one of the most entertaining enquiries in Vegetable Physiology, and has an important bearing upon general science, we shall examine it a little more in detail. Q5. In the first place, then, it may easily be proved, that, in all the true Fungi, the reproductive system is developed to such an extraordinary extent, that the number of germs liberated from a single plant almost defies calculation. Of this any one may convince himself by examining a puff-ball in a state of maturity. On this subject Fries states, " The sporules are so infinite (in a single individual of Reticularia maxima I have counted above 10,000,000), so subtle (they are scarcely visible to the naked eye, and often resemble thin smoke), so light (raised, perhaps, by evaporation into the atmosphere), and are dispersed in so many ways, (by the attraction of the sun, by insects, wind, elasticity, &c.), that it is difficult to conceive a place from which they can be excluded." Accord- ing to this view, then, the germs of all kinds of fungi are constantly floating in the atmosphere, and one species or another developes itself, according as the nature of the decomposing matter is respectively adapted to each. It is impossible to deny that this may be the case, however improbable it may seem ; there are, however, some other circumstances to be taken into account, which may lead us to adopt the opinion in a somewhat modified form. A series of facts equally important with those just alluded to, have lately been brought to light by the researches of some German Cryptogamists, who maintain, apparently on good grounds, that the same germ may assume widely different forms according to the circumstances which influence its development ; thus, Fries asserts that out of the different states of one species ( Thelephora sulphurea), more than eiglit distinct genera have been constructed by various authors. It would seem, then, that the absolute number of species among the Fimgi is not nearly so great as has been usually supposed ; and that the kind produced by a decomposing infusion, or a bed of decaying solid matter, vnll depend as much upon the influence of the material employed, as upon the germ itself which is the subject of it. QQ. Another very important enquiry has lately been suggested ; namely, whether all the fungoid growths on the surface of living plants are really such, or whether they may be regarded as degenerations of the tissue upon which they are found. linger, a German botanist, has argued with considerable ingenuity,* that the appearances termed blight, mildew, smut, &c. or more technically Uredo, (Ecidium, Puccinia, c^-c. are to be considered as the Exanthemata (eruptive fevers), of vegetables, being essentially diseases of the stomata. He points out that they are * Annales des Sci. Nat. 1834. VEGETABLE KINGDOM. 63 most liable to occur on those portions of plants where vegetation is most active, as on the green parts in general, and on the leaves in particular ; and he remarks that on the surface of the healthy bark, we find either more perfect Cryptogamia or Phanerogamic parasites. The cellular parasites evidently flourish best Avhen the bark is approaching decay; and it may be often remarked on an old tree, that whilst the stem and principal branches are covered by mosses and lichens, these diminish and disappear as we advance towards the younger and fresher portions. The presence of these morbid appearances, seems connected with that of the stomata ; and it has been supposed to be from some obstruction to their functions, that the exanthemata arise. They usually appear at the season of the most active vegetation, namely, the spring and early sum- mer ; whilst the period of the most rapid development of the true Fungi appears to be the autumn and the commencement of winter. Admitting, what perhaps it would be difficult to controvert, that these morbid growths really possess the characters of this class (a statement which is based, not merely on external appearance, but on their structure and chemical composition), still it remains a question, which we are yet scarcely in a condition to answer without reserve, either in one way or the other, whether plants of a high degree of organisation are capable of producing, by diseased action, from various parts of their tissues, beings which present the characters of inferior orders (§ 517). However absurd some might think it, to answer such a question in the affirmative, it is to be recollected that all our knowledge of the laws of reproduction is founded upon a limited experience in the higher orders of the organised creation ; and that in the extension of these laws to the inferior tribes, very important modifications are shown to be necessary. We shall hereafter see that the function of reproduction may be considered as only a peculiar modification of that of nutrition ; and if its regular performance leads to the evolution of germs, which, when developed, resemble the parent, it is not irrational to suppose that it may be so far perverted, as to give origin to beings of simpler organisation. To this question Ave shall return when speaking of the corresponding parasites among the animal kingdom. That these entophi/tic Fungi may be com- municated from one plant to another, has been fully ascertained by the experiments of Decandolle and others. It is usually imagined that the germs liberated by one plant are taken up by the roots of others, and being carried along the cuiTent of sap, are deposited and developed in the parts where vegetation is most active ; perhaps, however, they may find a shorter entrance into the cavities of the fabric, by means of the stomata, these being the precise situations where they are subsequently manifested. Finally, it appears probable that many reputed fungi, such as various RhizomorpheEe, are accidental and irregular expansions of the tissues of G4 ON ORGANISED STRUCTURES. floAvering plants Avliich become deformed through growing in the dark, as in cellars, caverns, &c. 67- Amongst the most important to man of all the species of this extensive group, (including- probably between 4,000 and 5,000 which have been described, and many more of the tropical kinds, which, from their perishable nature, have not been subjected to accurate examination), are those which constitute dry-rot^ such as Polyporus destructor^ Merulius vastator^ Sj-c. The minute fibrils of these fungi insinuate themselves between the fibres of the wood on which they grow, separate and soften them, and thus bring on premature decay ; for the further they insinuate themselves, the more easily can air and moisture gain access ; and as the successive crops of spores develope themselves in these chinks, larger and larger clefts are formed. The distensile power of some Fungi is so great, as to raise heavy stones beneatk which they grow, and to rend the trunks of large trees. In one instance which occurred in the town of Basing- stoke, a paving-stone twenty-one inches square, and weighing eighty- three pounds, was raised an inch and a half out of its bed, by a toadstool six or seven inches in diameter ; and nearly the whole of the pavement of the town was disturbed in a similar manner. 68. The hard persistent crusts of lichens, which seem scarcely to undergo any alteration in the lapse of many years, contrast forcibly with the fugitive character of the last class. There can be little question that the greater part of this tribe derive their nourishment from the atmos- phere and its contained moisture alone ; flourishing, as they do, upon sterile rocks, without a particle of previously organised matter in their neighbourhood. There are some species which usually grow uj)on trees, without seeming to derive any more nutriment from them than the moisture of their surface ; since they will flourish equally well on a damp wall. There are other sj)ecies allied to the Fungi, however, which vegetate on matter already undergoing decomposition, or preparing to decay. An attempt has been made to prove that some particular kinds of Lichens are confined to certain trees ; and much has been AATitten on their use in distinguishing the different kinds of bark, especially those of the Cinchonacese. It may be doubted, however, whether this difference is not principally due to locality, and to the adaptation of the quantity of the superficial moisture and exposure to light, furnished by different trees, to the wants of the respective species of Lichens ; since there is no reason to believe that they imbibe any of the proper juices of the plants to which they adhere. It is well established that by far the greater number vegetate indifferently on all kinds of trees, as well as upon rocks ^ but there is no doubt that some trees bear them in much greater abundance than others. Thus, the Beech, Elm, Sycamore, and Lime, are comparatively seldom found infested with the common heard- VEGETABLE KINGDOM. 65 moss^ which clothes so profusely the Fir, Ash, Oak, or Birch ; so that the poet's epithet of " rude and moss-grown beech" is by no means appro- priate. The early development of the Lichens, is favoured by darkness ; but, for their ultimate perfection, a considerable quantity of light is required. The development of the shields, which is occasioned by ex- posure to this agent, is frequently accompanied by so great a change in the general appearance of the plant, that the same species growing in dark and moist places, in which the fructification was not evolved, has been considered to belong to a distinct genus from the perfect specimen. There seems, indeed, from late observations, to be nearly the same uncer- tainty of form among the Lichens, as prevails in the Fungi ; the same germs presenting many difibrent appearances, according to the mode and degree of their development. The sporules which are developed from the shields, appear capable of multiplying the species ; whilst the powdery matter, which is frequently produced in little cup-Hke bodies raised above the surface of the thallus, as well as the separated particles of the plant itself, appear capable of independent existence, in various less definite forms (Fig. 58). 69. AVe now arrive at that which is usually regarded as the lowest tribe of the vegetable creation, and some members of which present the greatest approximation to the Animal kingdom. The ALGiE or Sea- weeds are distinguishable fi-om Lichens and Fungi, more by their aquatic habi- tation and its consequent influence on their growth, than by any definite character. Like the Fungi, they present many grades of organisation. Thus, the Protococcus, Palmella, and other species, which constitute the greenish or reddish mucous slime that is often seen on the damp parts of hard surfaces, closely resemble the lower tribes of Fungi ; being nothing but an aggregation of solitary cells, (each of which may be regarded as a distinct individual), in the midst of a semi-fluid matter, which partly or wholly envelopes them (Fig. 59). Proceeding a little higher, we find these united into filaments, but still preserving the power of separation, as in the Diatoma tenue (Fig. 60) ; and higher still are the true Con- fervcB, in which the vesicles are permanently united, and enveloped in a common membrane (Fig. 61). It is in this section, that we find some of the most remarkable instances of spontaneous motion, occurring in the fully developed plant. The Oscillatorise exhibit very uniform and evident vibrations ; the Fragillarias, to which the Diatoma belongs, have no apparent motion as long as the riband-like threads remain entire, but separate with a sort of starting movement ; and many other instances might be mentioned. The more complete Algae, or Sea-weeds properly so called, assume very definite forms, the cellular tissue which composes them being arranged with great regularity ; and they sometimes attain an enormous extent of development, forming vast submarine forests of the most luxuriant vegetation. Thus, the Chorda filum, a species common 66 ON ORGANISED STRUCTURES. in the North Sea, is frequently found of the length of thirty or forty feet ; and, in the neighboui-hood of the Orkneys, it forms meadows through which a boat forces its way with difficulty. This is nothing, however, to the size of the prodigious Macrocystis pyrifera^ which is reported to be from 500 to 1,500 feet in length ; the long and narrow leaves having an air- vesicle at the base of each, the stem not being thicker than the finger, and the upper branches as slender as common packthread. This deve- lopment of the nutritive surface takes place at the expense of the fructi- fying apparatus, which is here quite subordinate ; its structure will be detailed hereafter. Algse pass into Lichens by the Lichenoid species of the former, which vegetate on rocks occasionally submerged by the tide. These two orders, so closely resembling one another in every character but their locality, may in a philosophical arrangement be classed together under the term of Protophyta or simplest plants ; whilst the Fungi, which are separated by their habitation, reproductive system, and other peculiarities, constitute a distinct group. It is to be noticed with regard to the last-named order, that though they approach more nearly to the animal kingdom in chemical composition than any other tribe of plants, they present few instances of that power of spontaneous motion, which is so remarkable a characteristic of the Algse. Much difficulty has naturally arisen from this tendency, in drawing the line between the two kingdoms ; since it is in many instances impossible to determine the precise character of the motions perceived, and structure often affords no definite and satisfactory information. There are, therefore, many tribes whose place in the scale has not yet been determined. It is curious that among the Diatomese, we should find the same kind of affinity to the Mineral kingdom, as is indicated in the massive calcareous skeletons of the Polypifera; their joints containing large angular crystals which occupy a large part of their cavity. 70. The affinities of the principal divisions of the Vegetable Kingdom may be generally expressed in the following manner : — Q EXOGENS EnDOGENS. 1 i i t Ferns, 8^c. — Protophyta. — Fungi. ^ V ACROGENS. Starting from the simplest Algas and Lichens, we may pass, on one side, through the Hepaticae and Mosses, to the Ferns, the highest among the Acrogens or Cryptogamia. From Mosses and Ferns the transition is easy to Exogens, through Lycopodiacese and Gymnospermse. Exogens and Endogens have many connecting links ; and from the latter group, ANIMAL KINGDOM. 67 the return to tlie Fungi is direct by tlie RMzantheae ; whilst the simplest forms of the Fungi bring us back again to the Protophyta. VII. — Animal Kingdom. 71. A similar cursory view of the Animal Kingdom will now be taken, with the object not only of furnishing a key to subsequent des- criptions, but also of pointing out the very curious links of affinity, by which the respective groups are connected, and which demonstrate in so evident a manner the Unity of the Design with which the whole system was constructed. The foUomng passage, from the writings of a distin- guished Zoologist, seems peculiarly applicable as an introduction to this subject. " No one who believes in the existence of an Omnipotent Creator, can suppose for a moment, that the innumerable beings which He has created were formed without a plan. If an architect sat do^vn and made innumerable models of cornices, entablatures, columns, friezes, and all those ornaments used in a stately building, yet vdthout any design of subsequently combining them, we should naturally say, however much we might admire the parts, that his work was imperfect. Let us apply this reasoning to the Creation : however perfect an animal may be in its structure, it Avould still only resemble one of the ornaments we have just alluded to. It is beautiful in itself; but it is only when we attain some glimpse of the station it occupies with its fellows, and of the manner in which it is combined into one great whole, that we see this beauty in its true light. No rational being can therefore suppose that the great Architect of the world has created its inhabitants without a plan."* 72. Now, to discover this plan, — ^by ascertaining the laws by which such infinite variety of form is combined wdth such general uniformity of structure, — ^is the object of the researches of the Naturalist (§6). It is obvious that it would be useless to look for their attainment in any process, which does not include a very comprehensive survey of the whole animal kingdom, and which does not found its arrangements upon a general view of the structure and functions of each group, rather than upon any individual peculiarities. From the more intimate relation however, which subsists between the different functions of animals, than amongst those of plants, it will often happen that a classification which is really artificial (§ 48), because based on the indications afforded by a single character, may be also natural. Thus, the division of the Mammalia by Linnagus, into orders founded upon the arrangement of the teeth, was really a most natural one ; because the adaptation of the teeth to the carnivorous, herbivorous, insectivorous, or omnivorous habits of the animal, and to the several varieties of these, is necessarily accompanied by an adaptation of their general structure to their * Swainson on the Geog'raphy and Classification of Animals, p. 319. p 2 68 ON ORGANISED STRUCTUEES. respective methods of obtaining their food, and of converting it to the purposes of nutrition. Again, it may happen that some particular external character is so constantly associated with certain peculiarities of internal conformation, that from the appearance of the one we may predicate the existence of the other, although no essential or necessary connection between them can be discerned. Thus, a Naturalist on hearing that a particular specimen is supported on two legs only, and is covered with feathers, at once knows that it is a Vertebrated animal, possessed of warm blood, a complete double circulation, highly developed lungs, complicated digestive apparatus, oviparous in its reproduction, destitute of teeth but furnished with a horny bill — in short presenting all the characters peculiar to the class of Birds. But this knowledge is simply the result of his experience, that no animal possessing different internal structure is ever covered with feathers ; and he cannot assign any direct reason for this invariable connection. When, however, the habits of the animal are taken into account, the structure of the feathers may, to an acute Ornithologist, be a pretty certain indication of the place of an unknown bird in the scale ; for he can judge from their peculiarities whether it belong to a family remarkable for its strong and •rapid, or its slow and heavy flight ; or whether, as in the case of the Ostrich tribe, the wings are altogether undeveloped. In the former case the possession of feathers is a peculiarly artificial character ; whilst in the latter, their conformation has an evident bearing on the general peculiarities of the species, and must therefore be admitted as of import- ance in a natural classification ; since it is obviously important for the practical emplojrment of any system whatever, that its divisions should be indicated by easily-recognised external marks, although they can only he founded upon a full comparison of internal structure. It is the object of the naturalist, therefore, to discover what peculiarities of external appearance are constantly associated with differences in internal con- formation ; in order that he may not be obliged to examine the latter, in every case in which a classification, already formed, is brought into use. It must be kept in mind, however, that no truly natural sj^stem can be established, which does not embrace all the peculiarities of internal conformation which anatomical research can discover ; since the most important affinities or differences may there be detected, which are not indicated in the slightest degree by external characters. 73. The Animal kingdom was formerly divided into two primary groups ; the Vertebrata, possessing a jointed spinal column, within which a principal portion of the nervous system is inclosed ; and the Invertebrata, which are destitute of any such structure. The first division included only Mammalia, Birds, Reptiles, and Fishes ; the second comprehended all the Insect and Vermiform tribes, the Mollusca or Shellfish, as well as the lowest and simplest of the animal creation. ANIMAL KINGDOM. G9 But it is now generally acknowledged that this method is by no means a natural one, since the Invertebrata contain at least three and perhaps four groups, differing as much from each other as that of Vertebrata does from either of them, and therefore entitled to hold the same rank with the latter. The primary groups or sub-kingdoms, are, therefore, to be regarded as consisting of — I. Vertebrata, which are characterised, as before mentioned, by the possession of an internal bony column, com- posed of jointed pieces or vertehrce, within which, or their modifications, the central organs of the nervous system are inclosed ; to this column all the other bones in the body are more or less directly attached ; and these are covered by soft flesh, which partly consists of the muscles by which they are moved. Of all animals, their structure is most com- plicated ; they all possess the power of active locomotion, and enjoy the senses of taste, smell, sight, and hearing, as well as that of touch. In some the blood is warm, in others cold ; but in all it is of a red colour. These characteristic peculiarities undergo various modifications among the lower forms of this group, by which the affinities to the other types are indicated. — II. Annulosa or Articulata, animals in which the hard parts or skeleton are external, and formed into jointed rings. This is the character of a large number of classes included in this division, which present, with much difference in complexity, a very general con- formity of structure. Thus, from the soft and simple Vermiform tribes, such as the Leech or Earthworm, we pass by almost insensible gradations to the Centipede, and from this, to the highly organised Insects and Crustacea. Although some of the animals contained in this division border upon the lowest of the whole kingdom, yet others are inferior only to the Vertebrata in the complexity of their organisation. A dis- tinct mouth and eyes are almost universally present. The muscles that execute the movements of the body, are attached to the interior of the hard envelope, which, where distinct members are developed, incloses them as well as the trunk. In some of the Annulosa the blood is red, in others it is nearly colourless ; and among the Insect tribes there is a power of generating heat, almost as great as among any of the Vertebrata. The locomotive powers are usually very considerable ; and the general structure of the body is peculiarly adapted to the predominance of this faculty. There is one group, however, which approaches the Mollusca in which this tendency is reduced to a subordinate condition, in con- formity with other peculiarities of its organisation (§ 92). — III. Mol- lusca, or Shellfish, with allied species unpossessed of a testaceous cover- ing. This group also includes many animals of high organisation, such as the Cuttle-fish, which approach the Vertebrata A'ery closely in struc- ture and general characters ; as well as many whose conformation is very simple. Instead of long jointed bodies equally developed on the two sides, they almost always present an irregular rounded form, Avith no 70 ON ORGANISED STRUCTURES, distinct members j or, when such are developed, they are but fleshy tentacula (as the arms of the Cuttle-fish), or tubercles (as ihefoot of the snail), quite different from the complex jointed limbs of insects or Crus- tacea. In none are the locomotive powers developed to a high degree ; many remain affixed during nearly their whole lives to other substances ; and in most, the nutritive system appears to predominate above the animal functions. Some have a distinct head, with eyes, ears, and mouth; whilst others are destitute of any organs of special sensation, and the entrance to their alimentary canal is not indicated by its situation on any prominent part of the body. — lY. Radiata, or radiated animals, a group which formerly included a vast quantity of most heterogeneous materials ; uniting the comparatively symmetrical and complex Star-fish and Echini, Avith those simple beings which form the transition to the vegetable kingdom. The group is now restricted, however, to those animals, which, as the term imports, have their organs arranged in a radiated or star-like form around the orifice to the stomach. Such are Star-fish, the Sea-urchin tribe, and some of the Medusae or jelly-fish. The locomotive powers are usually inconsiderable, and the organs of sensation indistinct, though rudiments of eyes are siispected to exist ; but the nutritive processes appear to be performed with great activity, and some of the softest and most delicate of these animals are known to seize upon and digest the hard bodies of others much higher in organ- isation. The tribes which have been separated from this group are comprehended in the last division — V. Acrita, which must be cha- racterised rather by the absence of the peculiarities which separate the other sub-kingdoms, than by any positive distinctions which its members Present. There is but little indication amongst the animals which com- pose it, of any connected nervous system, and none of any special organs of sensation. The tissues are almost homogeneous, and seem equally irritable and contractile in nearly every part ; they are whitish and semi- transparent, and appear to be nourished by direct absorption from the surrounding medium, or from the cavity into which food is received, without the intervention of any circulating apparatus consisting of regu-lar vessels ; although in some cases there seems to be a motion of fluid through canals excavated in the soft substance of the body. From this simple organisation it results that all the functions are very much blended together ; and that, as there is no special organ for each, every part serves a number of distinct purposes to nearly an equal degree. The power of locomotion is in general very slight ; and where it is possessed to any great extent, as by the geminules of the Sponges and Polypes (§ 116, 121), it seems difficult to say how far it is to be regarded as voluntary, and how far it is the mere result of peculiarities of organ- isation, as in plants. A large proportion of this division is entirely fixed during all but the earliest stage of life ; and many exhibit so little indi- ANIMAL KINGDOM. 71 cation of sensibility or voluntary power, as to render it doubtful wbether they do not belong to tlie vegetable ratber than to the animal kingdom. Just as among the border tribes of the Protophyta, there is frequently great difl&culty in determining the precise characters of individuals of this division ; for it may be regarded as including a large nvmiber of animals in which the characters of higher groups are adumbrated, or, as it Avere, sketched out ; but which have not attained a sufficient degree of general development to deserve a place amongst them. 74. Now it is found that every one of these groups may be charac- terized by the form and development of its nervous system; and as this has an obvious relation with all the functions, both animal and nutritive, it is probably the best single character which could be adopted, (see Chap. xvi). Thus, Yertebrated animals have a nervous cord, inclosed in the spinal column which supports the back; and this is dilated by the addition of new parts into the brain contained within the cavity of the head, where also it is connected with the organs of special sensation. Hence, they may be termed spini-cerehrata. Annulose or Articulated animals, again, present, as the typical form of their nervous system, a double cord studded at intervals with ganglia or knots; this runs along the lower or abdominal surface of the body, protected, however, by its general envelope, and is connected with similar ganglia within the head, which are large in proportion to the development of the organs of special sensation, but which never correspond entirely "with the brain of verte- brata. In those species in which the locomotive apparatus is most connected with one part of the body, as in insects, the ganglia no longer present their regular disposition through the whole trunk, but are con- centrated in its neighbourhood, so that their peculiar arrangement is less evident; but in these, at an early period of life, the typical conformation is witnessed, to express which, the term diplo-neura has been applied to this sub-kingdom. In the MoUusca, the nervous system is principally concentrated around the entrance to the alimentary canal, forming a circle of ganglia, through which the oesophagus passes, and which is connected with other ganglia, disposed without symmetry among the viscera, or in the neighbourhood of the organs of locomotion, if such should be specially evolved. In some of the highest of this division, the nervous system approaches very closely in its aiTangement to the form it presents in the lowest vertebrata, and receives a corresponding protection by a rudimen- tary internal skeleton; but, in general, it is more connected Avith the immediate supply of the nutritive functions, and wants that symmetrical arrangement and close connection with the locomotive organs, which may be regarded as characters of elevation in the nervous system of the Articulata. From the general plan of the distribution of their ganglia, Mollusca have been termed cyclo-gangliata. The Radiata present such a form of nervous system as might be expected, when the peculiarity of the 72 ON ORGANISED STRUCTURES. arrangement of their organs is considered. It is composed of a filamen- tous ring, which surrounds the mouth, and sends oif branches to the different divisions of the body; a shght gangHonic enlargement being usually perceptible where these fibres are given off. Hence these animals have been termed cydo-neura. Among the Acrita, no definite or con- nected nervous system is discoverable, except in those species which border upon the neighbouring divisions. Most naturalists imagine that globules of nervous matter are incorporated with the individual tissues. The probability of this supposition will be hereafter considered (Chap. xvi). At present, the classes or subdivisions of these primary groups will be described with somewhat more of detail. 75. The MAMMALIA, which unquestionably assume the highest rank in the whole animal creation, as well as amongst the vertebrated classes, are particularly distinguished from all others, not only by producing their young alive (which is done by some species much lower in the scale), but by their supporting them by suckling for some time after birth, — whence their name. This, indeed, is the only single obvious, and, at the same time, universal character which is peculiar to them: for though they may be described as warm-blooded animals, breathing air, and having a complete double circulation, this would include Birds also; or if they were characterised as four-legged animals which live on the ground, they would be associated with Reptiles; and if the hair or fur which generally clothes the body be assumed as a distinctive peculiarity, it would scarcely hold good, since it is absent from the surface of such as are covered with scales, like the Armadillo, and something much resembling it is exhibited by the degenerated feathers of some birds which approach nearest in character to the Mammalia. The general structure of this class is suffi- ciently well known to render it unnecessary to dwell upon it in this place ; and it does not come within our purpose to enter into its sub-divisions or orders. To examine the mode in which the typical structure of the group is adapted to all the different conditions in which its members are respectively to exist, tracing the conversion of a terrestrial mammiferous animal into one destined to range among the finny tribes of the ocean, or to skim through the air on wings like a bird, would at any time be a most interesting pursuit. Nor would it less tend to raise our ideas of the Unity of Nature's Design, to see these modifications evidenced in many of the smaller sub-divisions, though not carried out to the same com- pleteness as in the principal orders. Thus, we have not only, in the Cetacea or whale tribe, a whole order of mammalia adapted to the conditions in which fishes alone ordinarily exist, and, in the Cheiroptera or bat tribe, a similar adaptation to the life of birds; but, among the Carnivora we find the seal and its allies, among the Rodentia the beaver, among the Pachydermata the hippopotamus (and probably a still more remarkable animal, the dinotherium, an extinct species forming a link ANIMAL KINGDOM. 73 between this order and the Cetacea), and among the Edentata the extra- ordinary ornithorhyncus or duck-billed platypus, — all more or less aquatic in their habits ; and, in like manner, we observe the flying lemur among the Quadrumana, the flying squirrel among the Rodentia, and the flying opossum among the Marsupialia, — all of which have, to a certain extent, the power of supporting and moving themselves in the air, by means of expanded membranes connected with their limbs or tails. "Well marked as the characters of the mammalia appear to be, they exhibit a very dis- tinct transition to the class of birds, by the order Monotremata, one species of which, the ornithorhyncus, has already been mentioned; as well as, in a less degree, by the Marsupialia. In the animals of the latter tribe, such as the kangaroo and opossum, the young leave the uterus at a very early period of development, and are conveyed into the marsupimn or pouch of the mother, where they remain attached to the nipple for a considerable time, scarcely exhibiting signs of sense or motion, and inca- pable of maintaining a separate existence. In the Monotremata, the embryos seem never to acquire the same direct connection with the parent which they possess within the uterus of the higher mammalia, and in this respect they appear to resemble the eggs of birds, though the period and mode of their birth has not yet been ascertained; the mouth is entirely destitute of teeth; and in the ornithorhyncus it is provided with a horny bill, like that of birds; this animal also possesses spurs on its hinder feet, like those of a cock. 76. The class of birds is distinguished by the possession of a com- plete double circulation and warm blood, at the same time that their generation is oviparous ; by their covering of feathers, which, however, sometimes degenerate almost into bristles or quills, like those of the porcupine, or into scales like those of fishes ; by the position of their bodies upon two feet only, and the modification of the anterior members for wings, (this, however, being by no means constant) ; by their want of teeth, whilst the bones of the jaw are covered Avith a horny bill ; and by various other characters of less importance. The senses of sight, smell, and hearing seem to be more acute than those of taste and touch ; it is for their locomotive powers, however, that this class is most remarkable. It is pretty certain that some species can fly at the rate of 100 miles an hour, and maintain this velocity for some time ; but although the power of flight is that which most evidently distinguishes birds from other Vertebrata, it is by no means possessed to the same degree by all. In the tribe of C'ursores or runners, for instance, the wings are not sufficiently developed to raise the body from the ground ; yet it is believed that they assist, by beating the air, the action of the powerful legs, by which an Ostrich is able to keep pace Avitli a fleet horse. In the Penguin, again, the wing resembles in form the fin of a fish, and the feathers assume the appearance of narroAV scales l}nng one 74 ON ORGANISED STRUCTURES. over the other; as instruments of flight they are of course entirely useless ; but when the bird is once in the water (which it rarely leares), the fin-like wings become a pair of powerful oars, capable of propelling the body at a prodigious rate. All the accounts given by navigators favour the belief that the Penguins, however helpless on land, are yet the svsdftest family of swimmers in the feathered creation, rivalling the swallows in the rapidity with which they pursue their prey. The modifications which the typical structure of the Bird undergoes to meet the various conditions of its existence, can scarcely be regarded, however, as equally considerable with those presented by the Mammalia ; they are principally due to the relative development of the anterior and posterior extremities; and the law of balancing of organs (§ 208) is no where better illustrated than in the comparison of the legs and wings of the Ostrich with those of the Swallow. The Ostrich and its allies present many points of transition to the Mammalia, not only in their external covering, but in their internal conformation ; some of these will be hereafter noticed. With Reptiles, a class differing from that of Birds in almost all its prominent characters, no animals now living would seem to indicate a connection ; but here we have a remarkable instance of the necessity of including extinct forms in our classification, in the fact, that in the fossil genus Pterodactylus, there is such a singular union of the characters of the two classes, that much controversy has taken place as to the one in which it should be located. Birds have been called the Insects of Vertebrated classes ; and when we come to describe the position of Insects among the Articulata, it will be seen that the expression is not inappropriate. 77. The class of reptiles, which is next to be considered, presents more diversity of form among its separate orders, than any other among Yertebrata. Nothing would seem more unlike than Tortoises, Lizards, Serpents, and Frogs ; yet the differences between them are not in reality so great as to prevent their association into one class, distinguished by the characters which are common to all. Reptiles are cold-blooded animals, having a heart with only three cavities, and an incomplete circulation, (only a portion of the blood transmitted to the body having previously passed through the respiratory organs) ; they usually breathe, in their adult state, by lungs ; though some of them respire by gills in their early condition, and a few retain them during life. This deficiency in the oxygenation of the blood, combined >vith the slowness and feebleness of the circulation, is connected ^^dth general inactivity of the nutritive functions, as well as with obtuseness of sensations and slug- gishness of locomotion. It is a curious result of the feeble exercise of these functions, that they may be suspended for a considerable time without apparent injury to the animal ; and that parts separated from the body retain, for a long period, the low degree of vitality which they ANIMAL KINGDOM. 75 usually exhibit in connection with it. Although, at present, Reptiles appear to perform a comparatively insignificant part in the economy of Nature, especially in temperate climates, where their numbers are com- paratively few and their powers feeble, — we learn from the records of Geology, that there was a period in the earth's history, long antecedent to the creation of Birds and Mammalia, when gigantic animals of this class not only constituted the chief tenants of the earth, but extended their dominion over the waters of the sea. With regard to the external appearance of the Reptiles in general, it may be remarked, that their low degree of animal heat requires no fur or feathers to retain it ; and that those which are most injuriously afiected by a high external temperature, namely the Frog tribe, have a soft naked skin, by transpi- ration from which the body may be kept cool, and the noxious influence resisted. Where a scaly covering exists, as in the Tortoises, Lizards, and most Serpents, its individual parts are to be regarded as appendages of the same kind with the horns, hair, or feathers of the higher classes, being formed by a corresponding set of organs. 78. The order Chelonia, or Turtle tribe, is characterised by the absence of teeth, the horny covering of the jaws which resembles that of birds, the possession of four feet, and the inclosui-e of the body in a shell-like covering. The want of teeth, claws, or other weapons of offence, is thus compensated by their means of passive resistance. The shell, as it is commonly called, is composed of two distinct portions ; the upper one, which is termed the carapace^ is usually more or less arched, and is composed of a bony expansion of the ribs, which are consolidated into a firm structure, and covered with the horny plates that constitute the true shell ; whilst the lower plate, termed the plastron^ is nothing but a peculiar development of the sternum or breast-bone, which, instead of being prolonged forwards into a heel^ to give attachment to large muscles as in birds, is extended laterally for the protection of the subjacent parts. This order passes, by a very remarkable species, the Emys serpentina, (alligator-tortoise, or snapping-turtle), into that of the Sauria or Lizards, which is characterised by its elongated body covered with scales, the possession of teeth, and the presence of legs, of which four is the tj'jiical number. Whilst all the Chelonia are herbivorous, many of this order derive their support from the animal kingdom alone, and their carnivorous tendency is indicated by the character of their teeth ; some of the largest among the extinct species, however, appear to have been vegetable feeders, possessing teeth more adapted for grinding and bruising than for cutting and tearing. Most of them are modified for progression on land ; though the crocodiles chiefly inhabit the Avater, for propulsion through which, their fin-like tail is adapted : whilst the Draco volans, a harmless and beautiful little inhabitant of tropical woods, and the only living representative of the fabulous Dragon, passes 76 ON ORGANISED STRUCTURES. part of its time in fluttering from branch to branch hy means of its wing-like appendages (§ 194) ; and from the skeleton of the Pterodacty- lus, its powers of flight must be regarded as having been considerable (§ 193). This extinct animal would appear, from the conformation of its teeth, to have been insectivorous ; and thus it seems to have repre- sented, both in character and functions, the class of Birds, which was not then called into existence. Two other remarkable extinct genera belong to this order, which exhibit peculiarities of organisation indicative of affinities to distant groups. The Ichthyosaurus immediately connects fishes Avith lizards, as its name imports; presenting the head and teeth of the latter, combined with the vertebral column of the former; as well as a union of other organs, apparently heterogeneous, but without doubt perfectly adapted to the conditions of its dwelling. The Plesiosaurus appears to have been a still more singular animal, uniting to the head of a lizard and teeth of a crocodile, a long neck like the body of a serpent ; a trunk and tail having the proportions of an ordinary quadruped ; the paddles of a whale ; and the ribs of a chameleon, the peculiarity in form of which seems connected Avith very great distensibility of the lungs. 79. The transition of form from the Saurian tribes to the next order, that of Serpents, is made out by very evident links. Although Lizards have usually four legs, some species have only two ; and the two which are deficient, are the anterior in one species, and the posterior in another. In the Anguis fragilis or slow- worm, no extremities appear outwardly, but they may be demonstrated by careful preparation of the skeleton ; and amongst the undoubted members of the order Ophidia or serpents, rudiments of extremities may be detected in several instances. Although apparently so different from the Saurian reptiles. Serpents are to be dis- tinguished by little but the absence of extremities ; as in the possession of teeth, and the scaly covering of their bodies, they completely corres- pond with them. The elongated form of their bodies reminds us of the vermiform tribes among the Annulosa, which they may be considered as representing among the Yertebrata ; and they correspond mth them in a very curious particular, Avhich exists in no other tribe of Yertebrata, namely, the periodical exuviation of the skin. It would scarcely be sup- posed that a link could be found which should connect the Ophidia with the frog tribe ; yet this exists in the Ccecilia (naked serpent), an animal destitute of a scaly covering, having a soft skin like that of the Batrachia, and resembling the latter in many points of internal organisation. The Batrachia themselves are among the most remarkable tribes of animals in the whole kingdom, however despised the members of it may be. Besides the naked skin, which is the principal character that distinguishes them in the adult form from other reptiles, and the imperfect separation of the heart into three cavities, they possess another peculiarity, which is regarded by many naturalists as sufficient to constitute them a distinct ANIMAL KINGDOM. 77 class rather than order, namely, the change of form or metamorphosis which they undergo, in their growth from the young to the adult state. All the animals belonging to this order, such as the frog, toad, water- newt, and some others less known but still more remarkable, resemble fishes in their tadpole or imperfect state, but afterwards assume more or less of the reptile form ; they thus act as a connecting link of a very peculiar kind between the tAvo classes. The tadpole, when it has just emerged from the egg, is essentially a fish ; it is deficient in members, moving solely by its tail; it breathes by gills, and all its organs are adapted to aquatic respiration ; its brain and nervous system, its cir- culating and digestive apparatus, are all those of a fish. As the animal grows, the body increases in size, while the tail remains stationary ; the legs are put forth, the hind pair appearing first ; the gills are superseded in function by the lungs ; the tail becomes rudimentary ; the gills dis- appear ; and the animal quits the water in the form of a frog, breathing air, and depending for locomotion on its extremities alone. Some of this order, however, which undergo a complete metamorphosis, remain aquatic, such as the common Salamander or water-neTsi; ; these, however, do not breathe by gills in the adult state, but take air into the lungs at the surface of the water, Avhich they are therefore occasionally obliged to visit. There is a period in the development of the tadpole, at which there is a kind of balancing between the organs which are disappearing, and those which are being evolved ; when the lungs and gills exist simultaneously, and the legs as well as the tail are employed for pro- gression. This state is transitory in the common tadpole, and only exists for a short time ; but in some animals of this order it remains permanent^ their development, as it were, being checked ; so that they never assume the complete reptile form, but retain the gills along with imperfect lungs, and the tail united with short extremites. Of these curious animals more Avill be said hereafter (§ 409). A cimous link between the Batrachia and the Chelonia has recently been discovered in South America, being nothing less than a frog furnished with a carapace and plastron ; by this, the circularity of the group of Reptiles is com- pletely established. The Batrachia have no weapons, either of offence or defence ; taken as an order, they are certainly as harmless to man as any tribe of animals ; and though the forms of many of the species offend against our notions of beauty, and their love-songs give them the cha- racter of " horrible musicians," there is certainly nothing to justify the aversion and prejudice with which they are ordinarily regarded. 80. It may not be inappropriate to stop here for an instant, to enquire hoAV far the existence of a metamorphosis can be regarded as a character sufficient to establish this or any other group into a distinct class, if the general structure of the adult do not wan-ant the distinction. It will hereafter be stated (§ 202) as a general law of the organised 78 ON ORGANISEB STRUCTURES. creation, that in the evolution of each of the individual organs of the higher tribes, a series of conditions is passed through, which bear an evident analogy with those that are permanent in the lower parts of the scale. In the human embryo, for instance, the first appearance of the nervous system resembles that which exists in the lower vermiform tribes; and at a subsequent period, the brain presents the successive characters peculiar to the fish, reptile, bird, and mammiferous animal, in their adult states. The heart and circulating system, the respiratory system, and many others, Avill be sho^vvn to undergo a corresponding series of changes. The later portions of these occur in the embryo of the Marsupialia after it has quitted the uterus, but whilst still remaining attached to the exterior of the parent ; and in the egg of birds, the whole development of the embryo takes place independently of the parent, subject only to the warmth with which it may be artificially supplied. Now the difference between the metamorphosis of the Batra- chia, and the changes which occur in the embryos of Birds and Mam- malia, consists in this : — that in the latter case, the 4ife of the foetus being maintained by nutriment either continually supplied by the parent, or stored up in the ovum, there is no necessity for that harmony between the corresponding states of its different organs, which is essential to a being that is to maintain an independent existence ; and the development of each, therefore, goes on without reference to the corresponding state of the others. In the tadpole of the frog, or the larva of the insect, on the other hand, there is that harmony ; the embryo does not receive sufficient nutriment from its parent stored up within the egg, to enable it to arrive at its full development before quitting its envelope ; it comes forth, therefore, in a state which, as regards its ultimate condition, is imperfect; but in this state it is enabled to maintain its existence, by procuring and assimilating its own food, since its organs are functionally adapted to each other, though universally presenting, for a time, the characters of the class below. The changes which the tadpole undergoes in its conversion to a frog, or the larva in its metamorphosis to the per- fect insect, are not different in kind from those which all animals of complex organisation present at some period of their existence, although peculiar in their combination and synchronism ; they cannot, therefore, be regarded, in a classification based upon philosophical principles, as sufficient of themselves to characterise a class. 81. The last class of the vertebrata is that of pishes, which are cold- blooded animals, inhabiting the water, and breathing by gills during the whole of life; possessing but two cavities in the heart, having the body covered with cartilaginous or bony scales, and the extremities metamor- phosed into expanded fins. Their whole structure is adapted for pro- gression in water, and the movements of propulsion are principally executed by the lateral action of the spine, whilst the fins are used for ANIMAL KINGDOM. 79 the purpose of balancing the body, and of modifying its direction. The peculiar construction of the spinal column endows it with great flexibility, at the expense, however, of strength; but the latter is not required in beings whose bodies are universally buoyed up by the surrounding ele- ment. The tail is flattened vertically, and increased power is given to the stroke of the body by the prolongation of the spinous processes of the vertebral column into the dorsal fin; fishes are thus remarkably distin- guished from the cetacea (their representatives among air-breathing animals), the flattening of whose tail is horizontal, for the purpose of bringing the body to the surface for respiration by means of its vertical stroke. The pectoral fins of fish answer to the arms of man, and the ventral fins, which are connected with the- pelvic bones, to his legs; besides their uses in steering, they assist in raising the fish vertically through the water; and in species Avhose habits frequently require this kind of motion, the ventral fins are brought much forwards, and are even situated anteriorly to the pectoral. As we have seen birds modified to inhabit the water, so do we find fish adapted in some degree to rise in the air; the flying-fish being enabled, by means of the stroke of its expanded fins on the surface of the water, to skim over it for a considerable dis- tance, though not to execute a sustained flight in a medium of such rarity contrasted vdth its usual element. Fishes are subdivided into the osseous and cartilaginous; the former being possessed of a bony skeleton, whilst the firamework of the latter is comparatively soft. Although many carti- laginous fishes present a high degree of organisation, and even produce their young alive, (hatching the eggs within the oviduct), others exhibit the simplest forms of vertebrated structure, and seem to pass towards both the Mollusca and the Annulosa. Thus, we find species, especially among the extinct gi-oups, in which the whole body is enveloped in a covering of dense bony scales, and the internal cartilaginous skeleton is scarcely more developed than that Avhicli exists as a protection to the nervous system of the higher Cephalopodes ; and to this class another transition is exhibited in the tentacular appendages prolonged from the mouth of some of the cyclostome fishes, which evidently represent the arms that, among the cuttle-fish, are developed to so great an extent, and constitute the principal organs of locomotion. There are other species, again, which in the entire absence of members, the non development of any hard protection to the nervous system, the uniform size of the latter from one extremity of the column to the other, the general softness of their tissues, and the flexibility of the body, bear a very close resemblance to the Vermiform tribes; such are the lamprey and myxine (hag), in the former of which the spinal column is a simple cartilaginous tube, and in the latter the nervous cord has only a membranous envelope, and no eyes or distinct jaws are developed. There are also some species of fishes which exhibit a distinct transition in the structure of their teeth, vei*te- 80 ON ORGANISED STRUCTURES. bral column, and respiratory organs, to the lizard tribes; of these the only li\dng representative is the Lepidosteus or bony pike of the North American lakes; but, from the researches of Agassiz, it is probable that they were formerly rery numerous. 82. The difference which has been already pointed out between the internal skeleton of the Yertebrata, and the hard external tegument of the Articulata, is a very remarkable one, and deserves further notice. In the latter group we often observe the trunk presenting exactly the same divisions into head, thorax, and abdomen, as in the former; and where distinct articulated members exist, the joints of their horny or calcareous sheath correspond in number and situation with those of the higher vertebrated tribes, the grand difference being that in the former case they are all external, and in the latter internal. We shall enquire, then, in what this difference consists in a philosophical point of view. The skeleton of an animal is that harder portion of its tissues, which is destined to afford protection to the more delicate and important organs, and support to the soft parts of the body in general ; under this defini- tion, therefore, we may include, not only the hony apparatus of the Vertebrata, but also the dense scaly covering by which some even of these are protected externally, and which sometimes (especially among extinct races of fishes) appears of more importance to the support and protection of the animal, than its soft internal skeleton. The same definition will include the calcareous tegument of the crab, the horny casing of the insect, the more massive shell of the oyster, and even the stony stem of the coral. In forming our estimate of the relation of these structures to the systems with which they are respectively connected, we must take into account the whole conformation of the animal, and not hastily decide that parts are to be regarded as dissimilar, which, though apparently diverse in form and character, are constructed by modifications of the same parts, and fulfil the same ofiice in the economy of the animal. In the Yertebrata, the integrity of the brain and spinal marrow, the centres of nervous energy, is so essential to the life of the system, that the preservation of these organs is the chief object of the skeleton. These important parts are therefore inclosed in a bony case, formed of several portions united by ligaments, so as to combine flexibility with strength. As appendages to this neuro-skeleton, as it may be termed, we find a set of bones giving firmness to the extremities, which are organised for locomotion in various ways, and in the higher Yertebrata are adapted for other purposes also; but these cannot be regarded as essential parts of the skeleton, since we find them absent in the whole of the order Ophidia, rudimentary in many of the Lizards which approach nearest to them, and in Fishes giving up their peculiar function to the tail. In some of the latter class, in which the internal skeleton affords but slight protection to the nervous system, from its softness and want of resisting power, the support required by the ANIMAL KINGDOM. 81 more delicate organs of the body is given by a peculiar modification of the skin, which is beset with plates of bone and enamel, forming a dermo- sJceleton. Among the Invertebrata, the neuro-skeleton is entirely absent, except in a few of the highest species, where it exists in a rudimentary state (§ 96), the dermo-skeleton being highly developed and supplying its place. The nervous system in these animals is less concentrated than in the Vertebrata; there are many centres of power instead of one. It does not, therefore, require such a specific protection, since injury to one part does not necessarily involve the destruction of the animal ; and the skeleton is consequently adapted, by its external position, to the protection of the whole of the soft tissues of the body, and not to that of the nervous sys- tem alone. The high development of the locomotive powers in the Articulata requires that this dermo-skeleton should be adapted by its numerous joints to their free exercise, equally with the neuro-skeleton of the Vertebrata ; and it is this adaptation, (by the division of the cover- ing of the body into distinct rings or segments, and its prolongation into articulated members,) that constitutes the dificrence between the light but firm tegument of the Annulose tribes, and the more dense and massive protection of the Mollusca, which is in reality formed, like the other, by a secretion from the membrane that answers to the skin. 83. This leads us to advert to the very important difference in cha- racter between the skeleton of the Vertebrata and that of the Invertebrated classes. The bone which constitutes the former is a true living structure, traversed by blood-vessels, absorbents, and nerves; and is subject to that continual renovation by which all the living tissues of the animal body are characterised. The dermo-skeleton of the latter, on the other hand, when once formed, undergoes no further change: it is adapted to the increasing size of the body, either by being periodically cast and renewed by those animals whose entire surface it covers, as by Crustacea; or by addition made to its edges, where these are free, and do not entirely enclose the body, as in the Mollusca; or by similar additions made to the jointed edges of its individual parts, as in that curious class, the Cirrho- poda, which represent the Mollusca among the Articulated tribes. In all these cases, the skeleton is to be regarded as, when once formed, indepen- dent of the nutritive system, or extra-vascular, like the nails, horns, hair, scales, or feathers, of Vertebrata, which are all distinct rudimentary forms of the dermo-skeleton; and the latter is earned to its highest development where no more completely organised internal fabric is yet evolved.* * Besides the peculiarities common to the Articulated classes, which have been formerly mentioned, there is one which deserves especial notice, when they are compared with ^'e^te- brata. The double nervous cord of the Articulata has been stated (§ lA) to run along the abdominal surface of the animal, and would thus appear to hold an opposite position in the body from that which the corresponding- part of the nervous system possesses in the Vertebrata. But, when closely examined, it is found that this inversion is common to other parts of the fabric, the intestine being situated in the back, the respiratory organs hanging from the abdo- G 82 ON ORGANISED STRUCTURES. 84. It is not very easy to say wMcli class of the Articulata is to be regarded as the highest; some possessing marks of superiority in one sys- tem, and some in another. Insects, for example, breath air, and have a very complex organisation; but their nervous system never presents itself in the same concentrated form as that of the Crustacea, which may be briefly described as annulose animals, resembling insects in their general form, but possessing four or more pairs of legs* usually encased in a jointed calcareous shell, and breathing by gills instead of air-passages; they have compound eyes like those of insects, and are furnished mth two pairs of antennae. The crab, lobster, and cray-fish are well-known illustrations of this class, which was formerly included by Linnasus, with that of Arach- nida, under the general division of Insects; but there are many other forms less familiar, which serve to connect it with the neighbouring groups. The dense envelope of these animals would interpose too great a resistance to the increase of the body, were not the means provided for its periodical renewal. The exuviation, or throwing-oflf the old shell, is pre- ceded by evident illness on the part of the animal, which retires to its hiding place at the time. The soft skin is soon covered with a sort of mucous exudation, which contains a large quantity of calcareous matter, and speedily hardens; and it would seem that the earthy deposit in the stomach, commonly termed crabs-eyes, disappears at this period, being, in fact, a kind of reservoir of lime, stored up against the time of want. In the process of exuviation it is not uncommon for the animal to lose part of a claw, which is speedily replaced by a new one from the broken joint. The second articulation from the body is the part at which the fracture most frequently occurs, and is probably the only one from which the new gi'owth can issue; since if the claw be broken off below that point, the animal itself effects the removal of the upper portion, either simply casting it off by violent muscular contraction, t or striking it against some hard body. The Crustacea in general, at the time of their first emersion from the egg, differ considerably from their adult form, especially in the number of their legs; for there is a remarkable power of modification in the anterior pairs, which are sometimes true legs, sometimes changed into claws, and sometimes converted into jaws; and they are found in some species in every stage of transition between these extreme forms. The changes which they undergo during their development can hardly be regarded as amounting to a metamorphosis like that of insects, although men, &c.; so that a lobster placed upon its back will exhibit a conformity in the situation of all the essential organs with the Vertebrata. This view is confirmed by the fact that the yolk-bag", to which the animal is attached in the egg", opens into the intestine by an umbiheus situated on the dorsal surface, and not on the abdomen, as in Vertebrata (§ 537) ; as well as by the relative position of the motor and sensory columns of the nervous system (§ 570). t Some specimens of the Gecarcinus or land crab in the Zoological Gardens were observed to throw off their smaller legs with great ease, in order to escape from any one who injudi- ciously took them up by those members. ANIMAL KINGDOM. 83 some naturalists have described them as such. The members of the dif- ferent orders of Crustacea have, however, a much greater resemblance to one another in their early states than subsequently; for it is only in the progress of their growth that the characters which distinguish the genera and species evolve themselves; and hence there has been much confusion amongst naturalists regarding them. In the lower orders especially, there is frequently a kind of premature liberation of the embryo from the egg, so that the animal has subsequently to undergo a part of the same series of changes, which the higher species pass through before quitting the ovum. This subject is one of the most curious in the physiology of these animals, but it has not yet been fully investigated. The highest order, that of Decapodes, which contains the Lobster, Crab, &c. indicates a transition towards both the Arachnida, and the Myriapoda; for whilst the short-tailed, soft-bodied crabs look like enormous aquatic spiders, some of the long-tailed lobster tribe conduct us, through an order in which the segments of the body are still more distinct and equal, (as in the common wood-louse), to the centipedes. Others, again, indicate an evident affinity with the Cirrhopoda (§ 92), especially to the transitory form of the latter; and there are species but little elevated above the higher Entozoa (§ 94). 85. The next class, termed arachnida from the general resemblance of its members to Spiders, also differs from the true Insects by possessing more than six legs ; eight being here the usual number. The head, like that of many of the Crustacea which approach them in form, is united with the thorax ; but is destitute of antennje. The eyes of this class are peculiar, as not possessing the compound structure which characterises those of Insects and Crustacea, but as resembling the simpler though more perfect organs of vision in the Yertebrata. The respiration is also peculiar, being performed by organs Avhich are neither lungs nor gills, but present analogies to both (§ 402) ; they are, however, adapted for breathing air, most of this class being inhabitants of the land. Many interesting animals are included in it ; such as the Spiders, the structure and employment of whose sjiinning organs are so curious ; the Scorpions, remarkable for their enormous claws, which resemble so much those of the Crustacea, and for their long jointed tail terminated by a sting ; and the Acari or mites, which are either parasitic upon other animals, or exist upon decaying organised matter.'"" The legs of the Arachnida are * The Spiders are probably the most remarkable among the whole animal kingdom for the variety of their means of obtaining- their prey. Some of them bore habitations for them- selves in wood, earth, or any other penetrable masses ; lining them with a silken tapestry of most beautiful texture, which is adapted to resist humidity ; and guarding their entrances by trap-doors, furnished with a hinge, so accurately fitted as not to be perceptible externally, and closing of themselves when the animal has passed through. Within these tubes, the little inhabitants lie in wait for their prey, dart out upon it when near, and drag it back to their dens to devour it at leisure. Others, again, form large nets of the most beautiful G 2 84 ON ORGANISED STRUCTURES. renewed after injury, in the same manner as those of the Crustacear The links which connect them with that class have already been noticed; those which form the transition to the Insect races are less evident ; although the affinity which exists between the parasitic Acari and some not very dissimilar forms amongst Insects, cannot be very distant. 86. Although inferior in certain individual points of structure to the classes already described, that of insects avouM certainly appear to present, in the most elevated degree, the combination of those pecu- liarities which are characteristic of the Articulata in general, and should therefore be regarded as its typical group. Its predominance in number of species above all others is not a little remarkable ; according to Mr. Swainson the probable estimate would reach 550,000, whilst the total remaining species of the animal kingdom at present existing on the globe do not amount to 30,000, or scarcely one-twentieth of the whole. The name of this class is derived from the well marked division into head, thorax, and abdomen which its members usually present. It is cha- racterised by the possession of six legs, compound eyes, antennae, and wings, in the perfect state ; and also by the existence of a metamorphosis through two distinct forms, previous to the attainment of that which ultimately characterises it. The caterpillar or larva which afterwards changes to a beetle, a butterfly, or a wasp, bears no resemblance what- ever to its perfect or imago form, and is in fact allied in almost every particular of its conformation to a class far beneath, namely, the Anne- lida; so that a naturalist, who should not be aware of their ulterior metamorphosis, would unquestionably associate the larvae of many Insects with that class. After what has been already said (§ 80) of the nature of this change in a philosophical point of view, the applica- tion of the same principles to the present case can be a matter of no difficulty. The alteration in the whole character of the animal is no less evident in the metamorphosis of Insects than in that of the Batra- chia. In the larva condition, its whole energies seem concentrated upon the nutritive functions ; and the increase in the weight of the body is very rapid; whilst in the perfect insect, the reproductive system is called into exercise, the body no longer increases in size, and the evolu- tion of an active locomotive apparatus induces an entire change in its habits. Although nearly the whole of this class are produced in the state of eggs like other Annulosa, a few are brought forth alive, the egg having been hatched within the body of the parent. One peculiarity texture and regular conformation, near which they lie hid, until warned of the neighbourhood of their game by a line passing from the centre of the web to their place of concealment, from which they dart forth, and do not hesitate to attack the largest insect entang-led in their toils. The hunting spiders, which are unprovided with such means, are possessed of peculiar agility, and spring upon their prey like tigers, seldom missing their aim. ANIMAL KINGDOM. 85 attending the eggs of Insects is, that they frequently increase in size after being laid, probably by imbibing moisture, like plants, from the surrounding medium. 87. The Larva when it first emerges, bear but a rery small propor- tion to its subsequent bulk. That of the silkworm weighs, when hatched, j ^ ^^ of a grain ; previously to its first metamorphosis it increases to ninety-five grains, or 9,500 times its original weight. During its growth, it throws off its skin several times, like the Crustacea ; and the rudiments of the organs to be subsequently evolved are gradually formed within. The body of the larva is divided into thirteen segments or rings, of Avhicli the anterior one constitutes the head ; the rest are nearly similar to one another, and, in general, each is furnished with a pair of short legs. There is obviously but little necessity for the development of the locomotive powers in this condition, since the food is always vege- table, and the egg is deposited by the parent in such a situation that a supply shall be mthin reach of its progeny as soon as required. There is a remarkable correspondence between the different kinds of caterpillars and the different orders of Annelida. Some are, like the leech, destitute of eyes and members, and move by suckers at the ends of the body ; others have the legs moderately developed and possess eyes ; others are aquatic and breathe by gills ; and we may regard the caddis-worm^ which con- structs a casing for itself by glueing together bits of stick or straw, and grains of sand or gravel, as the analogue of the Tubicolse (§ 91), which form a similar artificial protection to their soft elongated bodies. In many insects, however, the larvae differ but little fi-om the perfect state, except in the absence of wings, their development having proceeded much further before they quitted the ovum. The larva first changes itself into the state of Pupa or chrj^salis ; previously to which it often spins a silky bag. or cocoon, in which it encloses itself. The pupse of the different orders of insects differ much in form and in degree of torpor. Some have the whole body inclosed in a horny case without vestige of members, and are totally inactive, except when disturbed ; whilst others retain their legs, and possess their locomotive powers almost undiminished, scarcely seeming, in fact, to pass into the pupa state at all. The metamorphosis is said in the latter case to be incomplete, not because the ultimate form of the insect is less perfect, but because the change is less distinct ; those which exhibit all the phases described, in their most evident form, being said to undergo a com^plete metamorphosis. 88. In the Imago or perfect state, the Insect still retains the thirteen segments which are characteristic of the group ; they are, however, no longer similar, but combined into separate divisions. The head, which is regarded as the first segment, is quite distinct from the body ; the thorax contains the three succeeding rings closely united together, and these always retain their legs, with the addition of the Avings, Avhich are 86 ON ORGANISED STRUCTURES. developed from the third and fourth segments ; the abdomen consists of the nine remaining rings, more or less consolidated, and entirely destitute of legs. The especial function of the perfect insect is the continuance of the species ; and the wings enable it to seek its mate, and to obtain a situation fit for the deposition of its eggs. Many insects, as the silkworm-moth, do not eat after emerging from the pupa state, and die as soon as they have fulfilled this object ; and in few is there any marked increase in bulk during this stage of their existence. It has been well observed that there is a beautiful correspondence between the metamorphosis of insects, and the development of flowers. Every species of plant exhibits itself, in the course of the year, in different states. First are seen the succulent stems adorned with the young foliage ; next emerge the flower buds ; then the calyx opens, and permits the tender and lovely blossoms to expand. The insects destined to feed upon each plant must be simultaneous in their development. If the butterfly came forth before there were any flowers, she would in vain search for the nectar that forms her food ; and if the caterpillar was hatched after the leaves had begun to wither, it could not exercise its functions in devouring them. The eggs of many insects are laid in the autumn, and remain unchanged during the winter; their development being excited, like the evolution of plants, by the genial warmth of the spring. Others, again, pass the chrysalis part of their existence at the same season, and come forth as perfect insects early in the ensuing year. The senses of the Imago seem usually acute, especially those of vision and smell, although the special organ for the latter has not been detected. There is a difference of opinion with regard to their hearing, some ento- mologists believing that they are entirely deaf, whilst others attribute this function to their antennm or feelers. That the latter are delicate organs of touch, can scarcely be doubted by those who have watched the employ- ment of them ; and that, by their use, individuals have the power of communicating mth each other, is regarded as probable by those who have observed the habits of insects living in societies, as the Bee and Ant. 89. The characters which have been mentioned as peculiar to this class undergo considerable modifications in some of its border groups. Thus, the Podura or spring-tail belongs to an order which does not undergo any metamorphosis, and is deficient in wings ; some of its species approach the Myriapoda in structure and habits. The Pediciihis (louse) and similar parasitic insects may, in like manner, be considered as forming an osculant or connecting group ; being allied in many respects to the parasitic Acari among the Arachnida: so that the place of one curious species, the Nycterihia (bat-louse) is doubtful, Latreille having placed it among Insects, and Dr. Leach considering it as belong- ing to the Acari. The true Insects are usually divided primarily into ANIMAL KINGDOM. 87 two extensive groups, the Haustellata and the Mandihulata ; the former obtaining their food hy suction through a haustellmm or proboscis, and the latter biting it by their mandibles or jaws. The latter division, however, presents many approaches to the former ; some of its members, as the bee, not using the jaws for mastication, but collecting the food by the tongue, which is so elongated as to serve almost as a proboscis." 90. We must now quit this most important and extensive group, and pass on to the next class, the myriapoda, towards which some links of transition have been pointed out, both among Crustacea and Insects. In fact, some species among the former, in Avhich the segments are nearly equal, and the number of legs great and uncertain, pass almost insen- sibly into this class. There is here no division of the trunk into thorax and abdomen ; and the head is not always very distinctly separated from it. The segments of the body are numerous, and nearly equal, each possessing a pair of legs. The head is furnished wth antennae and simple eyes ; and, in the poisonous species, the second pair of legs is formed in the shape of a claw, through an aperture in the point of which, the poison is made to issue from its reservoir. In this class there is sometimes a kind of metamorphosis, the animal acquiring several additional segments and legs after quitting the egg. One of the orders is harmless ; the other contains the Centipede, which is the best kno^v^Ti illustration of the class. The largest specimens brought to this country are a foot long ; though some are described as attaining the length of a yard. 91. The ANNELIDA present us with a still further simplification in * From the rank and importance of this class in the animal king'dom, and from the differ- ences of structure and constitution presented by its various forms, it seems desirable to indicate its principal subdivisions, to v^hich reference will be occasionally made hereafter. That of Mr. Ku-by, being- founded on adult structure, and not on the nature and degree of the metamorphosis, appears to possess the best title to be here adopted. Excluding- some orders of minor importance, the principal ones may be thus arranged, each being regarded as holding in its sub-class, a corresponding position with the parallel one in the other : — HAUSTELLATA. MANDIBULATA. 1. Diptei-a (Gnat, Gad-fly, &c.) 5. Hymenoptera (Bee, Wasp, Ant) 2. Lepidoptera (Butterflies, Moths) 6. Neuroptera (Dragon-fly, Ephemera) 3 Homoptera (Cicada, Lantern -fly) 7, Orthoptera (Grasshopper, Locust) 4. Hemiptera (Boat-fly, Bug) 8, Coleoptera (Beetles). These orders are all named according to the character of their wings. In the 1st, only one pair of these organs is developed ; but the rudiments of the other are discernible. In the 2nd, the wings, which are membranous, are overspread -with a downy covering, which con- sists of delicate scales of the most elaborate beauty. In the 3rd, the wings are all nearly alike, but their texture is somewhat coriaceous or horny. In the 4th, the anterior or upper pair resembles horn or leather at the base, but is membranous near the tip. In the 5th and 6th, both pairs are completely membranous ; the veins which support them being, in the former, disposed like the ramifications of blood-vessels, and, in the latter, crossing each other nearly at right angles. In the 7th, the upper pair is somewhat coriaceous, the lower mem- branous. And in the 8th, the upper wings being entirely coriaceous or liorny, and not being used in flight, are termed elytra or wing-cases ; beneath these, the second pair, which are membranous, and frequently of considerable size, are folded transversely as well as longitudi- nally, so that, when the elytra are closed, they are completely excluded from view. 88 ON ORGANISED STRUCTURES. the form of the body, the segments being usually less distinct, and the head not separated from the trunk; we observe also the gradual dis- appearance of members,^ — ^from the highest species which approach the Myriapoda in their possession of jointed appendages, do^vn to those simple worms in which the body is entirely smooth. Thus, the Nereis or sea-centipede, and its allies, have one or two pairs of long, jointed, bristle-like appendages to each of their segments, which appear to serve as legs when the animal creeps over the surface of the bed of the ocean ; whilst the respiratory tufts (§ 392), by their movements, serve as its organs of propulsion in water. Some of these present ajB&nities not only to the Myriapoda, but to the Isopod Crustacea. Another order, the Tuhicolce, which includes the Sabellas, Serpulse, &c., has a long naked body indistinctly divided into segments, and its respiratory and locomo- tive appendages collected together at the head ; the remainder being enclosed in a tube which the animal either excavates from hard sand, or constructs by the agglutination of bits of sand, shells, &c., or forms, like the Mollusca, by a calcareous exudation from its body. In fact, until the peculiar characteristics of the Articulata were understood, the Serpula was placed among Mollusca. In still lower grades, such as the earth-Avorm, Ave find locomotion performed almost entirely by the body, but assisted by bristles on the surface, of which four belong to each segment. In the lowest order, of AA^hich the leech is an example, Ave find no trace whatever of appendages ; the division into segments is scarcely perceptible, and locomotion is entirely performed by the body, which is furnished AAdth suckers at each end. At the same time that the body and appendages are thus gradually simplified, there is a progressive diminution in the peculiarities of the head ; that of the Nereidans being distinct, with eyes, jointed organs like antenna, and a proboscis armed Avith jaws: while in the leech it is nothing more than the entrance to the intestinal canal, not being in the least separated from the body, and being unprovided with regular jaws, eyes, or other organs of special sensation ; the sensibility being here diffused over the whole surface. 92. The class cirrhopoda (Barnacles) was long regarded as apper- taining to the Mollusca, on account of the shell-like covering of the body, its fixed condition, the absence of a distinct head, and other peculiarities; it Avas, however, removed to the Articulata on account of the diplo- neurose character of the nervous system ; and all the knoAvledge Avhich has been acquired since that time of its structure and development, confirms the propriety of this alteration. The body presents some indi- cation of division into segments, and possesses six pairs of jointed arms ; but the head is indistinctly defined, and has neither eyes nor tentacula. Their conformation is symmetrical, or nearly so ; and thus differs from that of all but the highest Mollusca (§ 138). The most curious point in their organisation is the structure of the shell ; this is composed of several distinct pieces, each having the poAver of increase at its edges, so as to ANIMAL KINGDOM. 89 enlarge the capacity of the whole ; and it is thus adapted to the increasing size of the animal, without that periodic exuviation which is common to the hard envelopes of the Articulata in general, but which would not have suited the Molluscous character of these animals. Of the chemical constitution of these it may he remarked, that whilst the shells of the Crustacea are composed of a mixture of phosphate and carbonate of lime, and the hard envelopes of Insects of a peculiar animal principle termed chitine (combined with phosphate of lime where any calcareous matter exists), the shells of the Cirrhopoda consist, like those of Molluscous animals, almost entirely of carbonate of lime, and possess a more distinct crystalline structure than those of crabs. Each division of the shell has a groove along its edges, into which the mantle (as it is called in the MoUusca, answering to the skin of other animals,) is prolonged, for the purpose of depositing additional matter when required. One division of this class, the Balani (acorn-shells), have the bases of their pyramidal shells fixed upon rocks or other large masses of matter ; whilst the Lepades (Barnacles) attach themselves to floating bodies by a mem- branous tube, sometimes of considerable length. They appear to obtain their food by the whirlpool which they create by the motion of their jointed arms and of the cilia (§ 110) mth which they are fringed. The changes which the animals of this class undergo during the progress of their development, have recently been made a peculiar object of enquiry; and the very unexpected result has been, that the young animals on their liberation from the egg are found to possess a form much more analogous to that of the lower Crustacea, than that which they are ultimately to assume. Four stages have been described by Burmeister as being pre- sented by the animal subsequently to its emersion from the Qg^^. In the first, it is possessed of no hard covering, has two long antennae, and three pairs of arms tipped with bristles, by which it freely moves through the water ; and it is believed to be furnished with eyes. At a subsequent time, the animal fixes itself by its antennee, and the shell, of leathery consistence, begins to be formed in one piece at the back of the body ; and at this period the eyes are very distinct and brilliant. In the third stage, the divisions of the shell begin to appear, and it more completely encloses the animal, at the same time becoming more solid by the deposi- tion of calcareous matter. Soon after the animal completely fixes itself, the old integuments, together Avith the antennae and eyes, are thrown ofi^. The fourth stage is that in which the development is completed. Although the ciliated arms of adult Cirrhopoda evidently possess gi-eat sensibility to touch, no organs of special sensation can be detected in them. Some observers have remarked, however, that they shrink from a strong light brought to shine upon them suddenlj"^ ; and Dr. Coldstream has noticed the closure of the opercula of the Balani, on the movement of the hand or other part of the body in their vicinity. 90 ON ORGANISED STRUCTURES. 93. Besides these classes, whicli are all that are included by many naturalists in this sub-kingdom, there are two others which present its characters in a less degree, and the situation of which may be regarded as uncertain. One of these classes is that of rotipera, the complex struc- ture of the animals composing which, was long overlooked, OAving to the minuteness of their size. The class is a very circumscribed one, including only the wheel-animalcules^ and their allies, which have been separated from the common Infusoria, on account of their highly developed nervous and muscular systems, as well as of the complexity of their masticating and digestive apparatus. These beautiful little animals derive their name from the circular arrangement of their vibratile cilia (§ 110), which appear when in motion like revolving wheels, and by the action of which they not only move from place to place, but, when they have fixed them- selves by the sucker at one extremity, create a vortex in the surrounding water, which brings the food to their mouths, and probably aerates their circulating fluid. Some species, when their wheels are closed up, present an appearance not very dissimilar to that of the leech, the body being then elongated, having some trace of segments, and furnished with a sucker at each end ; whilst others approach the simple Crustacea, both in form and structm-e. The common Vorticella rotatoria (Fig. 77) has a wheel on each side of its prolonged head, and two spots on the latter, believed to be eyes. Within the body is a very curious masticating apparatus, which is seen to move with great energy and regularity Avhen the wheels are in action. The nervous cord is very distinct, being usually double above, where it surrounds the oesophagus, and single below; and it possesses ganglia, particularly at its upper part, where the principal movements of the body are executed. In proportion to the complexity of their organ- isation, this class is endowed with remarkable tenacity of life, many of its members being capable of revival after entire desiccation.* * This fact has been doubted by some high authorities, especially by Ehrenberg-, who states that he has never succeeded in producing this revival. The foliowing statement of my own experience on the subject may not therefore be undesirable. In the summer of 1835, 1 placed a dozen specimens of the Vorticella rotatoria in a drop of water, on a slip of glass, and allowed the water to dry up, which it did speedily, the weather being hot. On the next day I exam- ined the glass under the microscope, and observed the remains of the animals coiled up into circles, a form which they not unfrequently assume when ahve, but so perfectly dry that they would have splintered in pieces if touched with the point of a needle. I then covered them with another drop of water; and in a few minutes ten of them revived, and speedily began to execute all their regular movements with energy and activity. After remaining alive for a few hours, I again allowed the water which covered them to dry up, and renewed it on the follow- ing day with the same result. This process I repeated six times ; on each occasion one or two of the animals did not recover, but two survived to the last; and with these I should have experimented again, had I not accidentally lost them. It is possible that the species on which Ehrenberg, and other foreign naturalists have experimented, may not be the same as that which I and other English observers have used. This tenacity of life appears peculiarly adapted to the habits of the animal, which prefers shallow waters that are liable to be occasionally dried up. ANIMAL KINGDOM. 91 94. The second of tlie classes alluded to is that of the entozoa or parasitic worms; this, however, includes animals as various in structure as the different vertebrated animals inhabiting the same country; and although it has been made a subject of distinct study by many naturalists, there can be little doubt that it should be divided into at least two piin- cipal groups, the situation of which in the animal scale is widely different. One of these contains those species which are possessed of a distinct intestinal tube, vsdth an orifice at each end, and of a nervous system more or less developed. These may perhaps be considered as deserving a place among the Articulata, especially as many of them show traces not only of a division of the body itself into segments, but also of the evolution of articulated members and of organs of special sensation. This is the case among the very curious Lernece recently described particularly by Nordnann, as attaching themselves to the eyes of fishes; they approach the lower Crustacea in complexity of structure, possessing not only dis- tinct jaws, bnt rudimentary antennae, and having the body divided into segments, of which three form a thorax separate from the head and abdomen, and of which each is furnished Avith a pair of rudimentary legs. Others, again, belonging to this division, bear more resemblance to the lower Annelida; such are the Filaria or Guinea- worm, which buiTows beneath the skin in tropical regions, and the Ascaris lumbricoides or Round-worm of the intestines. These last are included in the division Nematoidea of Rudolphi, the Vers cavitaires of Cuvier, the Ccelelmintha of Mr. OAven. If they be admitted as a distinct class into the sub-king- dom Articulata, they certainly connect it very closely Avith that of Acrita ; the latter including, amongst its classes, the other division of Entozoa, in which no distinct traces of a nervous system or of members are to be detected, and in Avhich the intestinal canal, Avhere it exists, is merely hollowed out of the general soft tissue of the body. 95. It is the necessary result of any natural system of classification, that in pursuing one type of organisation through all the forms in AA'hich it manifests itself, we are led from the highest and most complicated, to creatures of such simplicity as to be, in reality, of a loAver rank than others belonging to a group Av^hich, considered as a Avhole, is beloAv that in Avhich they are included. Even amongst the Yertebrated classes, there are, as we have seen (§ 81), some species Avhich must be regarded as inferior in general character to the more elevated among the Articulata, and Avhich actually present the greatest affinity Avith members of the loAver classes of the latter. The typical character of the Vertebrata is unquestionably much higher than that of the Articulata, and yet it may be presented in such a degraded form as scarcely to be recognisable. In the same manner, although the active locomotive poAvers and acute sensations of the Articulata in general, Avould seem to entitle them to a place in the animal series above that assigned to the Mollusca, a large proportion 92 ON ORGANISED STRUCTURES. of the beings included in tlie latter group must be regarded as much more elevated than the simpler vermiform tribes we have been last considering, amongst which the typical characters of the sub-kingdom are presented in their least evident condition. The range of animal forms comprehended in the sub-kingdom Mollusca, is so great, that it would be difficult to include them by any character common to all. The highest class approaches Fishes in many points of its organisation; and in the lowest, we not only lose many of the peculiarities of the division, but we find a number of distinct individuals associating to form a compound animal, as is the case with many among the Acrita (§104). In all the Mollusca, the body itself is of soft consistence, as its name imports, and is enclosed in a soft elastic skin, lined wdth muscular fibres, which is termed the mantle. This skin is frequently not applied closely to the body, but forms a membranous bag, having apertures for the admission of the sur- rounding water to the mouth and respiratory organs, which are situated within it, as well as for its subsequent ejection, and also for the protusion of the head and foot where these organs exist; sometimes these apertures are for particular purposes extended into proboscis-like tubes (Fig. 83). It is from the surface of the mantle that the calcareous matter is exuded that forms the shell (where the animal is furnished with this protection), which possesses the same relation to it that the casing of the Crustacea holds in reference to their true skin; it occupies the place of the rete-mucosiim or coloured layer in the skin of higher animals, and is covered in its natural state by an epidermis analogous to the scarf-skin of man, which is, however, generally removed in preserved specimens, since it impairs the beauty of their exterior. The Mollusca possess, in general, a complicated digestive and circulating apparatus; but they are very imperfectly provided vnth the organs of sensation and voluntary motion. The greater number, indeed, are formed for an existence as completely stationary as that of the Zoophytes, which grow like a tree fi-om a fixed base; and are dependent for their nourishment on the supplies of food casually brought vdthin their reach by the waves and currents of the ocean. As among the Cirrhopoda, however, even the species which are afterwards to become attached, swim about freely in their imma- ture state. The shell is most solid and massive in those species which lead an inactive life; and is usually very light and thin, or altogether deficient, in those whose powers of locomotion are greater. As it does not inclose the whole body, there is no occasion for the exuviation which takes place in the covering of the Crustacea, or for the division into seg- ments and addition to the edges of each, which are necessary to meet the wants of the Cirrhopoda; and accordingly we find that the size of the shell is progressively increased by deposits of new matter from the mantle, lining its interior and extending beyond the margin (where the mantle is usually thickened into a glandular structure), — this extension ANIMAL KINGDOM. 93 taking place whenever the wants of the animal require svich an addition to its covering. 96. Among the cephalopoda or Cuttle-fish tribe (so named on account of the position of the feet around the head), we find not only links of connection with Fishes, but also some curious analogies with more remote groups. Thus, the animals of this class possess a beak composed of two firm horny mandibles, like those of the parrot in form ; and are furnished with a muscular stomach like the gizzard of birds. The beak encloses a large fleshy tongue ; and in the head are also situated well developed eyes, a distinct organ of hearing, and what is probably a rudimentary form of the organ of smell. There are two distinct groups in this class, which are particularised by the number of their gills ; but their general structure is so different as to require sepa- rate notice. In the highest of these divisions, which contains the common Sepia (Cuttle-fish), Loligo (Calamary), &c., there is (with the exception of the Argonaut or Paper Nautilus) no external shell enclosing the body; although a rudiment of it, which is frequently quite horny from deficiency in calcareous matter, exists within the folds of the mantle on the back. Where this is the case, the nervous system, which pos- sesses in these animals a very elevated character, would be almost entirely destitute of protection, were it not partly enveloped by cartilaginous plates, which may be regarded as the first indication of the neuro-skele- ton, manifested where the dermo-skeleton is least developed. All the Cephalopoda which are destitute of an external shell are provided, in the ink-bag, with a remarkable means of escape from their enemies ; the dark pigment contained in it being ejected upon the slightest alarm, and by diffusing itself rapidly through the water, serving to conceal them effectually. The locomotive apparatus of this division consists not only of the arms (eight or ten in number) disposed round the head, but, among the long slender-bodied cuttle-fish in which these arms are least developed, of fins attached to the sides of the body (Fig. 78, «, a), and furnished with cartilaginous supports, which seem to be the rudiments of the more perfect members of fishes ; by these they are able not only to propel themselves through the water, but even, it is believed, to spring out of it like the flying-fish. In the common Octopus or Poulp the feet are connected for some distance round the mouth by membranes and muscles which form a kind of circular fin : whilst in the Argonaut, the first pair of arms is provided with two expanded membranes, which the animal has been supposed to erect into the air as sails ; and tliis use of them has been a subject of poetic imagery in all ages. According to Mr. Owen, however, as the same appendages are possessed by two other species, of Avhich neither inhabits a shell, and in which the expanded membranes could not be used " to waft the animal along the surface of the ocean, as has been said and sung of the Argonaut from Aristotle to 94 ON ORGANISED . STRUCTURES. Cuyier, from Callimachus to Bjrron," the physiologist is compelled to ahandon the idea as altogether a poetic fiction. 97. The second division of the Cephalopoda contains those which inhabit a shell, and which, from their comparative inactivity, and their general inferiority of development, as well as from particular points in their organisation, may be regarded as connecting the group already described with the inferior Mollusca. Instead of the few long powerful arms which the Cuttle-fish exhibits, the true Nautilus and its allies have the mouth surrounded with very numerous short and comparatively feeble tentacula, which resemble those of many Gasteropoda. It is thus seen that ih&feet or arms by which this class is characterised, have really no analogy to corresponding parts in Yertebrata, but are simply an excesssively developed form of a structure which is common to other tribes of Mollusca, and of which traces may be found in fishes (§ 81). The organs of sensation in this division appear less acute than those of the naked Cephalopoda (those unprovided with an external shell) ; and that of hearing seems altogether absent. Like other testaceous Mollusca, the animals of this division possess no organs of rapid locomotion. The structure of their shell is, however, peculiarly interesting. In all species at present knovna, it is spiral, and divided by transverse sefta or partitions into chambers, in the largest and external one of which, the body is enveloped (Fig. 79). "When its bulk has increased so as to be too great for the chamber, the animal forms a new one by prolonging the mouth of the shell ; and at the same time, it throws a septum across the portion it has quitted. It still retains a communication, however, with the empty chambers by means of a membranous tube, termed the siphuncle, a, a, which passes through all the septa, and is capable of considerable distension."' A spiral chambered shell, although forming the prominent character of this gToup, is not, however, altogether restricted to it. For even the flat bone of the Cuttle-fish exhibits traces of a corresponding structure ; and in the Spirula, a shell very similar to that of the Nautilus is enveloped within the body, the animal itself resembling a Sepia. And, among the extinct species, although affinity of structure -undoubtedly places the Ammonites and Nautilites in the same position with the * By this structure, the animal appears to be enabled to rise or fall in the water at pleasure. It would seem that the specific gravity of the body and shell are so nicely adapted to that of their element, that a very little diflPerence will cause them to swim or sink. When the animal is at the surface and wishes to sink, it forces into the siphuncle a quantity of water previously contained in the pericardium or bag inclosing the heart ; the distension of the siphuncle com- presses the air in the chambers, and the bulk of the exterior of the body being thereby diminished, its specific gravity is increased and it consequently sinks. When it wishes to rise, it has only to withdraw the pressure from the pericardium ; the elasticity of the air in the chambers forces the water back from the siphuncle into the external cavity, and thus by increasing its total bulk renders its specific gravity again less than that of the water. This account, which has been recently given by Dr. Buckland m his Bridgwater treatise, is the only satisfactory explanation yet offered of the use of this apparatus. ANIMAL KINGDOM. 95 existing Nautilus, the Belemnite, wliicli possessed a conical chambered shell, must certainly be associated with the Sepia, since the remains of the ink-bag (Avhicli is possessed by none but naked Cephalopoda) are found in connection with it. A very interesting point in the structure of the naked Cephalopoda, is the organisation of the suckers, vfith which their arms are copiously provided ; these are adapted to lay firm hold of any object to which they are applied, by the creation of a vacuum beneath. The food of most of this class appears to consist of Crustacea, animals which might have been supposed peculiarly difficult for them to master ; but they probably overcome their prey by mnding their arms around their claws and legs, w^iose motion they prevent by their suckers, and then tear off the shell with their firm horny mandibles. 98. The PTEROPODA form a small class of Mollusca, of which little need be said; they derive their name fi-om the fin-like expansions of the mantle on each side of their bodies, on the surface of which the gills are situated ; but these fins are never supported by rays. The head is provided with tentacula, but seldom with eyes. The body is frequently unprotected ; and where a shell exists it is very delicate and almost transparent. Their habits are active, and they are often found smmming in mjaiads near the calm surface of the ocean. " Their delicate struc- ';ure" says Dr. Grant " is ill adapted to encounter an agitated sea, or the dangers of a rocky or shallow shore ; and it is only in the vast and deep ocean that their elegant forms and colours and their graceful motions delight the mariner's eye, when the glassy sm-face of the still sea reflects the rays of the setting sun." One of the most common species of this class is the little clio horealis (Fig. 80), which exists in such multitudes in the arctic latitudes as to constitute the chief food of the whale. The shells of two species afford indications of a transition towards the Cepha- lapoda; one resembling in its straight conical form the belemnite and many other extinct genera of that class, and the other having a partially formed chamber at the lower closed extremity ; and similar evidence is afforded by their internal structure. 99. Of the large number of species included in the class gasteropoda (so named firom the situation of the muscular disk upon which the animal creeps, in the neighbourhood of the digestive organs), some are entirely naked or destitute of shells; others possess a small shell, covering one part of the body or imbedded in the back, as in the slug; whilst others are almost entirely enveloped in shells, varying in form from the simple cone of the Limpet, to the convoluted spiral of the Snail, and the still more complex fabric of the Murex, It is perhaps amongst the tribes of this class that we find the characters of the Mollusca in general most prominently displayed; the high development of the nutritive apparatus, combined with sluggish powers of locomotion; and the consequent defi- ciency of that resemblance between the two halves of the body which is essential in an animal adapted to rapid movement, and which, in the 96 ON ORGANISED STRUCTURES. higher Mollusca, has triumphed oyer that unequal disposition of their organs which is common to all the rest of the group (§138). In all the more perfect forms of this class (which are usually carnivorous), the head and eyes are distinctly retained; but in the naked species (which are mostly vegetable feeders), these organs are not evolved. Many curious transitions might be pointed out between dijfferent groups, indicated by the form of the shell. Thus the passage from the simple cone of the Patella (limpet) to the spiral of the snail, is evident in such as the Pileopsis where the point of the cone is prolonged and somewhat convoluted (Fig. 81); and the gradations are so close as to make it difficult to draw a distinct line of separation. From the spiral we may return again to the long straight form of the shell, by the Scalaria preciosa in which the turns of the spire touch each other only by the ribs; and by the Vermetus (Fig. 82), and Magilus, in which the commencement only of the shell possesses a spiral form, the remainder being prolonged into a straight tube, so as to have led to the opinion of their affinity with the Serpulce, which among the Annelides form a shell by no means dissimilar. The Magilus is an animal which fixes itself on coral beds, and as their thickness increases, it is obliged to prolong its shell to their surface; sometimes to such an extent, that the animal leaves altogether the spiral portion first formed, which it fills up, more or less completely, by a deposition of solid calca- reous matter, and entirely resides in the tube. The Vermetus, which is similarly circumstanced, throws a septum across the part which it has quitted, closely resembling that of the chambered shells among the Cephalopoda, and, in fact, dijBfering only by the want of a siphuncle. Instances of a similar tendency occur among other Mollusca (§ 102). The shell, it must be recollected, is simply an exudation from the skin; and the characters of the animal alone can be regarded in a classification strictly natural. In the naked species of Gasteropoda, especially those which inhabit the land, the skin is thick and dense, so as to afford a certain degree of protection; in others, which have no shell externally, a small one is imbedded wdthin their substance; and amongst those which have an external shell, every variety is presented in the degree in which it is capable of affording protection to the entire animal. "Where the head and respiratory organs, which are usually situated near the entrance to the shell, are capable of being entirely drawn within it, there is not unfrequently a tubular prolongation of the mantle, adapted to a channel in the columella or central pillar, round which the spire turns, for the purpose of conveying water to these organs without the necessity for their quitting the shell.* * The term mantle being frequently employed, as it might appear, synonymously with skin, it may be well to explain again that it is a portion of the skin concerned in the secretion of the shell, differing from the rest in its thick and glandular character ; and sometimes it is prolonged considerably farther than any of the organs which it encloses, either, as in the present instance, to form a tube, or to increase the surface of the shell. ANIMAL KINGDOM. 97 100. The shell in all cases is enlarged by additions to its interior surface, a new layer being thrown out by the mantle, Avhich projects beyond the former ones, and thus increases both the length of the spire or cone, and the diameter of its outlet. In terrestrial shells, when full growth has been attained, a rim or margin is formed around the apertui-e, which serves to strengthen the whole fabric ; Avhile in the marine species, which attain to much larger dimensions, the growth is effected at distinct periods, each of which is indicated by a well defined margin. This margin is sometimes fringed with spines, as in the Murex, formed by prolongations of the mantle ; and the dissimilar num.ber of these spines has led to the establishment of many distinct species, which, when the habits of the animal were better kno'V'VTi, have proved to be but different forms of the same. For it now appears that the animal has not only the power of forming new spines, but of removing old ones, especially such as would interfere ^vith the continued groAvth of the shell. How the absorption of shelly matter from their base, which causes them to drop off, is effected, is still unexplained ; various analogous phenomena may be witnessed among other species, portions of the shell first formed being wholly or partly removed. Sometimes the walls of the older portion are thinned for the purpose of lightening the shell, and the same object seems to be attained by other inhabitants of spiral shells in a different manner; these Aidthdraw their bodies from the highest part of the cone, throw a partition across the cavity, and then allow the point (which, not being internally supported, is brittle, and appears to have been purposely thinned,) to be broken off, leaving the shell decollated as it is termed.* It must be borne in mind, however, that the changes thus effected in the shell are not the consequence of any interstitial absorption, such as takes place in the osseous structures of Vertebrata; but result from the same kind of power of superficial absorption, as appears to be exercised by many Gasteropoda upon cal- careous rocks, which they perforate for their hal^itations, as well as upon the substance of their o^vn shells. It is believed by many that this power consists in the secretion of an acid which decomposes the substance ; and by others that it is the result of an electrical action which separates the components in another method. 101. Several of the aquatic species of this class form not merel}^ a spiral shell, but an accurately fitted cover to its mouth, so attached to the body that when the latter is entirely withdraA\Ti, the operculum^ as it is called, completely encloses it. Sometimes this is horny, but not unfrequently calcareous ; and occasionally it bears so large a pro- portion to the shell as almost to appear like a second valve, such as is characteristic of the Conchifera. Some of the land species also possess * Mr. Stutchbury informs me that he has seen the Bulimus forcibly strike tlie apex of its shell ag-ainst a stone, for the purpose of decollating- itself. H 98 ON ORGANISED STRUCTURES. an operculum ; but in general they are destitute of it, and form during hybernation a temporary closure to the mouth of the shell by a viscid secretion, which becomes hard and includes a bubble of air ; behind this a second and even a third similar partition are occasionally found, as in the common snail. In a marine species allied to the snails (Janthina), the matter secreted by the mantle, which in other cases forms either a permanent operculum or a temporary partition, appears destined for a very different purpose. When the sea is calm, according to the state- ment of Bosc, these beautiful violet-snails may be seen collected in large bands, swimming over the surface by means of a floating apparatus con- sisting of air vessels of unequal size, produced by a membranous secre- tion from the foot. When the sea is rough, the animal absorbs the air from its vesicles, changes the direction of its foot, contracts its body, and lets itself sink. It does the same when in danger from any enemy ; and like the naked Cuttle-fish (which the peculiar thinness of the shell causes it to resemble in the want of other protection), colours the water by the emission of a blue fluid, which serves to conceal it. 102. The next class to be considered is that of conchifera, which includes all the Mollusca whose shell is composed of two principal pieces, or those usually termed hivalve. It is not upon the structure of the shells, however, that the division is formed ; but upon that of the animals contained in them, which differs essentially from that of the individuals composing the class last described. They have been termed Acephalous Mollusca, from the circumstance of the head being undistinguished from the rest of the body, in any way but by the presence of the mouth ; for no special organs of sensation are possessed by them, except perhaps those of taste. It would seem, however, that even in these, there is some sensibility to light ; and in a few species, which are endowed with more than the usual locomotive powers of the class, some traces of eyes may be discovered. The Pectens, for example, are free smmmers, and fi-om their rapid and desultory motions have been termed the butterflies of the ocean; the manner in which these motions are performed, espe- cially on the approach of danger, indicates the possession of a sense analogous, at least, to that of ordinary vision. In general, however, the Conchifera are peculiarly inactive ; a large proportion of them remain fixed to the spots they have originally selected ; either immediately, by the attachment of the shell itself, — or by the intervention of the hyssus, a cord formed by a series of brown silken threads, loosely intertwined, connecting the foot of the animal, by which it is spun, with rocks or other secure places. The two valves of which the shell is composed, are connected by a hinge formed of teeth that lock into one another; and this joint is sometimes very perfect, and so peculiar in its character, that even Avhen the shells are dry, it allows free motion of the valves without permitting them to be separated. In general, however, the ANIMAL KINGDOM, 99 retention of the valves in apposition to each other, is due to a liga- ment connected to the hinge in such a manner, that its elasticity keeps the valves somewhat apart, unless counteracted by the action of the muscle in the interior of the shell, which draws them together. This is a very beautiful provision for the performance of the animal functions without difficulty or effi)rt; for when undisturbed, the ligament keeps the valves open ; but when danger is apprehended, or circumstances require it, the adductor muscle contracts, overcomes the resistance of the hinge, and shuts the valves close, until they may be opened with safety. One of the earliest signs of the loss of vitality in Conchifera, is the unusually Avide gaping of the shell, which arises from the con- tinuance of the elasticity of the ligament, (which does not disappear as long as its structure is undecomposed), unbalanced by the vital con- tractility of the muscle. The valves are formed and increased by suc- cessive layers secreted from the mantle, just as among the Gasteropoda ; but here we find them attaining much greater size and solidity. It has been observed, however, that the quantity of calcareous matter thus deposited as a protection to the animal, varies with the character of the element it inhabits ; thus, a species which in calm water forms but a light delicate shell, will sometimes produce one of a solid and massive character, if its habitation be among the agitated waves of the ocean. A curious provision exists among Conchifera, for adapting the capacity of the shell to the size of the body, which reminds us of the facts already mentioned regarding other Mollusca (§ 97 and 99). An oyster kept without food, will frequently expend its last energies in secreting a new pearly layer, at a distance from the old internal surface of the concave valve, corresponding to the diminution of bulk it has experienced during its fast. The Spondylus repeatedly does the same thing, so that its concave valve, when cut across, exhibits a large number of regular chambers, which bear an evident analogy vnth those of the Nautilus ; the object here, however, as in the Magilus, is to prevent the animal from being imbedded by the growth of the coral to which its shell is attached. 103. Although usually so sluggish, many of the Conchifera possess considerable muscular power, which is manifested in the force mth which they draw together the valves, and sometimes in the powerful action of the foot. Thus, the common cockle can take considerable leaps, by suddenly extending this organ, which was previously bent at an acute angle ; and Mr. Stutchbury has mentioned to me, that the first specimen of Trigonia which he discovered on the coast of New South "Wales, having been placed in the stern of the boat fi-om which he was dredging, leapt over the gunwale, a height of about six inches, into the sea. This feat argues a power of vision on the part of the animal. Bosc states that the animals of the genus Venus may be seen in calm weather sailing H 2 100 ON ORGANISED STRUCTURES. on the surface of the waters, using one of their valves as a boat, and the other as a sail. No special organs of locomotion, however, seem to he evolved in this or other cases where the animal is unattached ; the action of the foot appearing to produce the more rapid and violent movements ; while the constant ingestion on one side, and ejection on the other, of the currents of water which are to pass over the respiratory organs, and to supply the digestive system, would seem to produce the slower and more equable motions. This passage of water occasionally takes place by means of two restricted openings in the sac of the mantle, which are even prolonged into tubes or siphons; in many species, however, the divisions of the mantle lining the two valves are not connected at their edges to any great extent, so that the water has free entrance to the cavity within the shell. To this class belong the greater part of the boring-shells, which have so remarkable a power of excavating rocks, timber, &c. ; the means by which they produce the effect are still obscure, some considering it to be by mechanical, and some by chemical action. The tendency among some of the Gasteropoda to the formation of an enlarged operculum like a second valve, has already been noticed ; the transition would likewise be established by the genus OrUcula, which has one valve formed like the shell of the Patella ; so that a Norwegian species in which the lower valve is particularly thin, and unattached by a hinge to the upper, has been described as belonging to the last order. The passage to the lower group of Mollusca, the Tunicata, is so direct,, that Lamarck and other naturalists have united the two classes under the general title of Acephala. 104. The TUNICATA, or naked Acephala, seem to establish the tran- sition belween the Mollusca and Acrita, by connecting the class last described with the Polypiferous tribes, — not only through their individual structure, but in the instances they present of the association of a number of single and independent beings to form a compound animal. The Tunicata are of soft consistence, unpossessed of a shell, but having their organs enclosed in a stiff leathery envelope or mantle, which has two openings, one for the ingestion of water to the mouth and gills, the other for its ejection. In the general structure of their organs, the higher species, which are usually free, approach very closely to the Conchifera ; whilst the simple ones, which are attached to rocks, &c., border as closely upon the Poljrpes. The external tunic possesses considerable con- tractility, which appears to be under the control of the animal ; since, when alarmed, it ejects the water contained in its cavity with consider- able force. The Ascidia (Fig. 83) is a species of this class which occurs in the northern seas and attaches itself singly to rocks ; but the more remarkable are the Pyrosoma and Salpe of warmer latitudes, which usually exist as aggregate animals. The single animal has a form some- what elongated, the oral aperture being at one end, and the anal at the ANIMAL KINGDOM. 101 other, instead of being in proximity as in tlie Ascidia. A large number associate themselves into the form of a hollow cylinder (Fig. 84), each individual having the oral opening connected with the central passage, and the other situated externally. In the Atlantic species, this tube is usually about five inches long ; and in the Mediterranean it sometimes attains the length of fourteen. These animals are highly phosphoric, and when floating on the surface of the ocean, exhibit not only a dazzling light, but the most brilliant succession of colours. They do not appear to possess any independent power of locomotion, except that conferred by the direction of the current of water, which is ahvays entering one extremity of the tube, and, after passing through the bodies of the little animals which compose it, is ejected externally and somewhat in the opposite direction. Although closely attached to one another, these associated animals are capable of being separated by a smart shock applied to the sides of the vessel in which they are swimming ; and it appears that at a certain period of their existence, this separation takes place spontaneously, their association being only maintained during their young state, when, perhaps, it is required for their mutual support and protection from injury. A similar phenomenon has been already noticed in the vegetable kingdom (§ 69). 105. We now arrive at the sub-kingdom Radiata, Avhich, although decidedly inferior to the Mollusca in general organisation, cannot be regarded as succeeding them in the descending scale, since it rather possesses affinities with the Articulata, The peculiar character of the animals composing this division of the Animal creation, consists, as has been mentioned, in the radiated disposition of their parts round a common centre ; and these parts are usually but repetitions of one another, so that one or more may be removed without injury to the functions of the remainder. Such beings form the natural links of transi- tion from those more highly elaborated structures, in which every organ is of a difierent character, and dependent for the due performance of its functions upon the integrity of the rest, — to those more simple animals, in which the diflferent parts are so completely repetitions of one another, as not only to be capable of removal without injury to the welfare of the system at large, but even to possess the poAver of maintaining an indepen- dent existence.* * In the first part of liis General Outline of the Animal Kingdom, just published, Professor Jones adopts the desig'nation Nematoneura proposed by Mr. Owen for this division, as cha- racterising the filamentous condition of its nervous system, opposed on one side to the Acrita, in which none can be detected, and on the other to the hig-her groups in wliich g-anglia are discernible. In this group of Nematoneura, Prof. Jones associates with the Ecliinoder- mata, the Cavitary Entozoa and Epizoa (§94) and the Rotifera (§93) already described among' the Articulata, as well as the Cilated Polypes, which will be presently mentioned (§117); and he places the Acalephae among' the Acrita, with the Polygastrica (§ 113). Sterelmintha (§ 111), inferior Polypes (§ 115), and Sponges (§ 121). Tliis classification is 102 ON ORGANISED STRUCTURES. 106. The first and most highly organised class of Radiated animals is that of ECHiNODERMATA (prickle-skinned), v/hich derives its name from the spiny integument of some of its species, and comprehends those well-known animals, the Echinus (sea-urchin), Asterias (star-fish), as well as many others. The Echinus, one of its most perfect forms, is nearly glohular in shape : and the shell, which is composed principally of carhonate of lime cemented hy animal matter, is made up of a number of polygonal plates, which are susceptible of receiving addition at their edges, and thus of keeping pace with the growth of the animal. It is beautiful to observe how completely this structure is adapted to its wants; for as it cannot get quit of its envelope periodically like the crab, or add to its edges like the MoUusca, it is manifest that the animal would speedily outgrow its habitation, were not some means provided for the continued extension of the latter. In this provision there is a manifest resemblance to the means by which the Cirrhopoda add to the capacity of their shell ; and, indeed, there is so much correspondence in various particulars between these two groups, that they cannot be considered as very widely separated, although manifestly belonging to dificrent divisions of the animal kingdom. The plates composing the shell of the Echinus are of two kinds (Fig. 85). The larger or tubercular plates, are thickly studded on the outside with little hemispherical protuberances, on which the holloAV extremities of the spines work, in the manner of a ball and socket joint. The small or amhulacral plates have no tubercles, but an immense number of minute holes, through which the animal has the power of putting forth a series of tubes terminated by suckers, which are of great use to it in walking or seizing its prey. These tubes are formed of very delicate membrane lined by muscular fibres longitudinally disposed. When the animal wishes to move forwards, it prolongs them by injecting them with Avater from its interior cavity ; it then attaches the suckers at their extremity to some fixed object; and, by the contraction of the muscle, shortens the tube, and draws the body in the desired direction. founded entirely upon the presence or supposed absence of a nervous system ; and the author cannot help thinking- that this, being- but a single character, must lead to an artificial system of classification if followed alone. In the hig-her animals, the conformation of the nervous system is so intimately related to that of the whole fabric, that the one necessarily imphes the other. But in these lowest classes, the functions of org-anic Ufe predominate so much over the animal faculties, that the structure upon which the latter are dependent is often obscure. The division Nematoneura, as above specified, contains animals differing- from one another in almost every character but the filamentous appearance of the nervous cords ; and many of its members would be with difficulty excluded by any definition from the Articulata. It appears to him that the Radiated type of structure is as well marked, and as deserving- of a separate rank, as the Molluscous or Articulated ; and that if each division be made to include the lowest forms in which its general type of structure is discernible, but few will remain to be associated in the lowest and therefore most heterog-eneous of the whole. Thus, the Articulata might perhaps not unfairly embrace nearly all of the Entozoa ; the Mollusca, in like manner, might include the Ascidiform Polypes (§ 117) ; and among' the Radiata mig-ht be placed not only the Acalephae, but the Asteroid Polypes (§ 119). ANIMAL KINGDOM. 103 The spines are also capable of being slowly moved at the will of the animal, by means of the system of contractile fibres involved in the skin which covers their bases. In the higher Echinodermata, the intestinal canal has tAvo distinct orifices, Avhich in the Echinus are at opposite sides of the shell ; and in the Spatangus and other inferior species, they are nearer each other ; whilst the Asterias possesses but one opening, Avhich serves both for the ingestion of food into the stomach, and for the expul- sion of the faecal portions. In the Echinus, the oral orifice is guarded by a very beautiful and complex apparatus of teeth, which is moved by powerful muscles. The food of these animals, consisting of small shell- fish and Crustacea, is brought to the mouth by means of the tubular feet ; these, having once gained an attachment to the prey by means of the sucker at their extremities, do not quit their hold until it is conveyed to the mouth. From the sluggish habits of the Echinus and the alertness of the motions of the animals which form its nutriment, it would appear difficult for it to seize upon them ; but when once they allow themselves to be touched by one of the suckers of their enemy, they are soon seized by a great number of others, and speedily reduced to a pulp in the powerful grinding machine to the action of which they are subjected. In the Spatangus, on the other hand, the dental apparatus is absent ; and the stomach is filled mth sand, from which the animal appears to remove the nutritive particles mixed with it (§ 240). 107. The different species of this class present a very gradual and cui'ious transition in form, of which the leading types, represented in Fig. 86, may here be noticed. From the almost globular Echinus^ in which the two orifices of the alimentary canal are opposite one another, we may pass to the flattened Spatangus, where traces of a pentagonal figure appear, and in Avhich the intestinal tube terminates nearer the mouth. Thence we are led by the Clypeaster and Scutella, which are flattened and pentagonal, to the common Asterias, in which there is a central body with five or more arms, and a single orifice only to the stomach. Amidst all this change of form, it is curious to observe how the relative situation of the ambulacral plates remains the same. In the Echinus, all the vital organs are concentrated, as it were, into one mass ; in the Asterias they are distributed through the arms; but in some of the lower forms of the class, we find them again withdi'awn to the centre, that the arms may undergo an extraordinary multiplication and sub- division. Thus, in the Euryale and C'omatida, the arms are much increased in number, and give off branches which ramify and subdivide into minute filaments. Closely connected with this part of the class is the remarkable family of Crinoidea, whose fossil remains are so abundant in the older formations, and which have been supposed to be at present entirely extinct. Some small specimens of Pcntacrlnus, are, hoAvever, occasionally found in the Bay of Cork ; and a larger and very beautiful >104 ON ORGANISED STKUCTURES. species lias recently been brouglit from the West Indies.* These animals strongly resemble a Comatula, placed on a long jointed footstalk, which is attached to the bottom of the sea; and it has recently been stated that the Pentacrinus europceus (Fig. 87) is the young of a species of Comatula, which is attached during its early life, and afterwards swims about freely. We shall find that from some of the most ramifying forms of Pentacriijus, the transition is not difficult to the Coralline Polypes; and the Echino- derma are connected to the Articulata, not only by the Balani (acorn- shells) among the Cirrhopoda, but by two peculiar forms presented in their own class, the Holothuria and Siponculus; in these the tegument is not calcareous but leathery, and the body is elongated instead of being extended radially, so that the Siponculus might really be taken for a vermiform animal (Fig. 88). 108. The ACALEPHJE (sea-nettles) are among the softest-bodied of animals, seeming to melt away entirely when taken out of the water. They are composed of a soft gelatinous structure without any hard support, except in a few instances. The common Medusa or jelly-fish (Fig. 89) is a familiar example of the class. It possesses a radiated form, having a large mushroom-shaped disc, which contains the digestive organs, with various filamentary appendages or tentacula, depending from it. These seem to be constructed somewhat on the plan of the feet of the Echinoderma, being tubular, furnished with suckers, and connected with the internal cavity, from which they are injected with fluid when their prolongation is required. The general mass of the disc is cellular, uniform, and very soft; the quantity of solid matter in it is very small, a Medusa, which when taken out of the water weighs fifty ounces, being reduced when dry to five or six grains. Some traces of muscular structure may, however, be observed in the tegumentary membrane, especially round its margin ; and by the contraction of these, the movements of the mantle are pro- duced which propel the animal through the water; other species, however, have different means of locomotion. The Beroe (Fig. 90) swims by means of the cilia (§ 110) with which it arms are fringed. The little Velella possesses a cartilaginous skeleton, formed of a vertical and a hori- zontal plate; the body of the animal is placed beneath the former, Avhile the latter acts as a sail, being exposed to the action of the gentle breeze, when the animal floats on the surface of the sea in calm weather. Some species of this group are very abundant in the arctic regions, and form an important article of food to the whale. The Physalia (Portuguese man- of-war) is very common in tropical seas; it is furnished with an air blad- der of an oval shape placed at the upper part of the body; and also with a membrane of a beautiful purple colour, which acts as a sail, like the crest of the Velella. These animals are met with in great numbers in the * Of this, two very splendid specimens, one of them the most perfect known to exist, are contained in the Museum of the Bristol Institution. ANliAIAL KINGDOM. 105 Atlantic ocean, and more especially in its warmest regions and at a con- siderable distance from land. The following animated account of the MedusEe has been given by M. Peron: "Among the animals of this family, we find the most important functions of life performed in bodies, which oflfer to the eye little more than a mass of jelly. They grow frequently to a large size, so as to measure several feet in diameter; and yet we cannot always determine what are their organs of nutrition. They move mth rapidity, and continue their motions for a long time; and yet we cannot always satisfactorily demonstrate their muscular system. Their secretions are frequently very abundant; and yet the secreting organs remain to be discovered. They seem too weak to seize any vigorous animal, and yet fishes are sometimes their prey. Their delicate stomachs appear to be Avholly incapable of acting upon such food, and yet it is digested within a very short time. Most of them shine by night with great brilliancy; and yet we know little or nothing of the agent which produces so remarkable an effect, or of the organs by which it is elabo- rated. And lastly, many of them sting the hand that touches them ; but how, or by Avhat means they do so, still remains a mystery," It will be seen, therefore, that the peculiar nature of their tissues, the singular arrangements of their organs, and the anomalies in their functions present as many objects of interesting enquiry to the physiologist, as the Avon- derful variety and striking elegance of their forms, and their splendid colouring exhibit to the admiration of the naturalist. Some among the Acalephee exhibit a decided tendency towards the character of the Mollusca, whilst the greater number are evidently radiate in their struc- ture; the Actinia (Sea-anemone), Avhich has by some naturalists been placed in this group, must be regarded as rather belonging to the Poly- pifera (§ 120), but it forms a close link of connection between them. 109. These two classes are the only ones in which the radiated type of conformation distinctly exists ; we have now to examine those lowest and most dissimilar forms of animal structure, in which no definite cha- racter can be traced as universally pervading the group in Avhich they are associated, but which appear like sketches or adumbrations of fabrics higher and more remote from each other. They agree, however, in the general simplicity of their structure, and in the absence of any decided characters which would justify their assignment to other divisions of the animal kingdom. The tissues of the Acbita all present a still more homogeneous appearance, than in the simplest of the tribes we have yet described; for not only does there seem to be an absence of nervous filaments, but of muscular or fibrous structure ; the alimentary canal even, where it exists, is not possessed of distinct walls bounding its cavity, but seems channelled out of the soft parenchyma; and where anything like a circulation of nutritive fluid goes on, it takes place in similar reticulated canals unprovided with proper tunics. In these and 106 ON ORGANISED STRUCTURES. many other respects, tlie animals composing this division resemble the early condition of the embryo of one of the higher classes ; and just as the rapidity of the changes this undergoes in the progress of its develop- ment, is proportional to the simplicity of its structure, and to the shortness of the period which has elapsed since its evolution com- menced, do we find among the Acrita a peculiar tendency to advance into close approximation with the genera respectively belonging to the higher classes with which they are connected. From this circumstance results the great difficulty which has been felt in assigning definite characters to the division at large ; for whatever type of conformation be made the basis of these characters, it is fotind to undergo the most important modifications, where it presents itself in affinity vrith those of the other divisions towards which transitions are made. The tendency to repetition of similar parts among the Radiata has been already noticed; and a similar one exists, to a certain extent, in the lower Articulata, where the difiierent segments of the body are almost alike. Among the Acrita, this tendency, so characteristic of the vegetable kingdom, is carried to a still greater extent, and is often exhibited in a very curious manner. Thus, the Sponges, of which the yoimg gemmules swim freely about, are fixed at a later period of life ; and in forming their calcareous, siliceous, or earthy skeleton (§ 121), seem to lose the few characteristics of animal life which they before possessed, and in its construction are limited to the repetition of a single spiculum. The In- fusorial animalcules, again, are termed Polygastrica, from the repetitions of the digestive cavity which occupy the principal part of their bulk. Among the Sterelmintha or lower Entozoa, we find a similar repetition of the reproductive system, each joint of the body of the Tcenia (tape- worm) being the seat of a separate ovary, though all are nourished by continuations of one simple system of nutritious tubes. And in the Polypes, the respective mouths and stomachs appear to be to a certain extent independent; being connected together by the gelatinous flesh which clothes the exterior of the axis or lines its tubes (§ 115), but being capable of separation without injury to the general structure, and without the destruction of their own existence. 110. As it is in the animals belonging to this division, that the organs termed cilia appear to perform the most important part, in relation both to the nutritive and animal functions, this would seem the proper place to introduce a more particular description of them. Cilia, then, are little hair-like filaments, covering the surface and fringing the edges of various parts, both external and internal, which are in con- tact with fluid ; in which fluid they produce, by their vibrations, currents which may serve various important purposes in the economy of the animal. In the active and free-moving Infusorial Animalcules, the cilia on the exterior of the body are the principal, if not the only organs ANIMAL KINGDOM. 107 of locomotion ; in the Polypes, fixed to a particular situation and unable to go in search of their food, the currents which they create in the sur- rounding element bring it mthin reach of their tentacula or arms ; and in all animals modified for respiration in water, from those simple struc- tures in which no particular part of the surface seems appropriated to this function, to Fishes and the larvse of the Batrachia, their movements appear to have an important relation with it, in constantly renewing the stratum of water in apposition Avith the aerating siirface. Cilia are even found on the mucous membrane lining the trachea and ramifying air-passages of the higher Vertebrata ; and their use appears there to be, to convey the secretions and foreign matters, if such should be present, along the surface. They have also been observed in the upper part of the alimentary canal of Reptiles, throughout its whole extent in Mol- lusca, and in the stomach and its appendages in the Asterias ; as Avell as in many other situations. The presence of cilia, Avhen they are moving with rapidity, can frequently be only inferred from the eddies which they produce in the neighboviring fluid. Sometimes the return-stroke, which is made more slowly, can be seen when the direct stroke is too rapid to be followed ; this is particularly the case in the wheels of the Rotifera (§ 93), which appear to revolve continuously in one direction, from the observer being only able to trace one set of the vibratory movements of the rings of cilia which compose them. In general, however, the cilia may be best seen when their motion slackens; and their shape, size, arrangement, and manner of moving, may then be distinguished with tolerable accuracy. Their figure is that of slender filaments, sometimes a little flattened, and tapering gradually from the base to the point. Their size is extremely variable, the largest being about -^^-q of an inch long, and the smallest being stated at xs^oo^- They are generally arranged in regular order, sometimes in straight rows, sometimes spirally or in circles ; and they are usually set pretty close together. When in motion, each cilium appears to bend from its root to its point, returning again to its original state, like the stalks of corn when depressed by the wind ; and when a number are affected in succession with this motion, the appearance of progressive waves following one another is produced, as when a corn-field is agitated by frequent gusts. The motion of the cilia seems to be quite independent of the Avill of the animal, being seen after death, and proceeding with perfect regularity in parts separated from the body ; its duration varies according to the species in Avhich it is observed, and is influenced by many external circumstances ; it has been seen fifteen days after death, in the Tortoise, when putrefaction was far advanced ; and in the River-mussel it seems to endure Avith similar pertinacity. It is the opinion of Dr. Grant that the cilia are tubular organs like the feet of the Echinoderma, and that their movements are OAAang to injection of Avater from elastic tubes running along the base ; but this seems scarcelj^ 108 ON ORGANISED STRUCTURES. consistent with the fact that their vibrations continue when entirely detached from the circulating system. Dr. Sharpey, who has particu- larly investigated this curious subject,* is disposed to believe with other observers, that the motion is produced by the action of muscular fibres, connected with the base of the cilia, and probably traversing their sub- stance also ; their return being perhaps due to their own elasticity Avhen the muscle is relaxed. It is very properly urged that the minuteness of the parts is no argument against this supposition, which seems to derive some weight from the correspondence between the duration of the ciliary motion after death, and the persistence of muscular irritability in like circumstances. 111. It is a matter of little consequence with which class we begin the description of the individual groups included among the Acrita, since they cannot be regarded as possessing much affinity with each other, or any regular gradation of structure. We may first speak of the sterel- MiNTHA or Vers ParenchymaUuoe of Cuvier, a class formerly included among the Articulata, but separated from the Entozoa already described on account of the absence of nervous filaments, the homogeneous cha- racter of the textures, and the nature of the boundaries to the digestive cavity. This in the Entozoa (§ 94) is furnished with a distinct tunic, whilst in these simpler tribes it is only channeled out of the substance, and occasionally appears altogether absent. Mr. Owen separates some of the least organised of this tribe, and unites them with the Vibrios and other vermiform animalcules, (which are only found in decomposing fluids, and have been usually placed among the Polygastrica), to form another division which he calls Protelmintha ; but they may probably be regarded as belonging to the present type reduced to its lowest condition. Of these, the eels of Paste or Yinegar, and others which are parasitic in living vegetables, are characteristic examples ; they appear to possess a straight intestinal canal, but no distinct stomachs, and are destitute of external cilia. To this group belongs the very curious parasite which has been lately discovered to be of no unfrequent occurrence in the muscles of the human body, the Trichina spiralis (Fig. 91); which is a little worm-like animal about the -^^ of an inch in length, lying coiled up in a cyst formed by the inflammation of the cellular tissue that exists between the fibres of the muscle. It is curious that this parasite has only been found in the muscles of animal life; and that even where these are thickly beset with them, the muscles of organic life, namely, the heart and the alimentary tube, are entirely free. It was at first supposed that their existence was connected with a generally-enfeebled state of the system ; but they have been since found in healthy men who have died suddenly, as well as in the bodies of those who have suffered from ex- hausting diseases. * See Cyclopaedia of Anatomy, Art. Cilia. ANIMAL KINGDOM. 100 112. A parasite of eyen greater simplicity inhabiting the animal body, is the common Hydatid or Acephalocyst, which consists of a globu- lar membranous bag, containing a limpid colourless fluid. This so much resembles certain cysts which are occasionally formed in an abnormal state of the nutritive process in the animal system, • that much doubt has been felt whether it should be regarded as possessing an independent existence. The best observers agree in stating that the Hydatid is impassive under the application of stimuli of any kind, and that it manifests no contractile power, either partial or general, save such as evidently results from elasticity; in short, that it neither feels, nor moves, nor exhibits any distinctly animal faculty. Its power of reproduction, however, by the formation of gemmee or buds between its layers, shows it to be entitled to the rank of an independent being; the young Hydatids being thrown off either internally or externally, according to the species. If the views formerly stated (§ QQ')^ however, on the subject of the parasitic fungi, should be ultimately received as an established doctrine, it will not be diilSicult to apply them to such structures as the present, which approach so near to the morbid growths spontaneously arising in the bodies of higher animals. Other species, such as the Cysticercus^ have a more complex and definite form; and possess a head armed with spines and suckers for the imbibition of nutriment. This parasite has been observed mthin the eye of man. Some of these, by their elongated bodies, return again to the vermiform character; and amongst the most remarkable of type, the Taenia solium (tape-worm) may be mentioned. This animal has sometimes attained the length often feet; its breadth varies from a quarter of a line at its anterior part to three or four lines at its posterior part, where it again gradually diminishes. The head is small, and possesses four mouths, surrounded by a double circle of small hooks. The segments or divisions of the body are very numerous, sometimes amounting to seve- ral hundreds; but they are all connected by the nutritive canal proceeding from the mouth, although the reproductive apparatus is repeated in each part. It has been the opinion of some naturalists that each segment of the Ttenia might be regarded as a separate animal. This, however, can- not be received; as it is found that the existence of the head is essential to the life of the body ; and that, if broken off" with some joints attached, it continues to grow and to form new ones, Avhilst those which have been separated from it die, and are expelled from the body. 113. The next class which may be noticed is that of polygastrica or Infusorial Animalcules; this was formerly supposed to contain the sim- plest members of the animal kingdom; but it is now kno^^'n, from the researches of Ehrenberg and others, to possess, in general at least, an organisation of much complexity. Wherever any decaying organised matter exists in a fluid state, and is exposed to air and warmth, it "vWU speedily be found peopled with minute inhabitants of the most varied 110 ON ORGANISED STRUCTURES. forms, and diversified movements, possessed of considerable activity, and evidently endowed Avith an energetic system of nutrition. They are, therefore, by no means so nearly allied to vegetables as those inactive and simple creatures, the sponges and their neighbouring species. The cause of the spontaneous appearance of these animalcules where no germs were previously suspected to exist, and where it could not be supposed that they had been conveyed, has been a matter of much speculation. Many have had recourse to the supposition that they formed, in a latent state, a part of the living tissues of the animal and vegetable structures, from the decomposition of which they were evolved; and others have even supposed them to have arisen from accidental combinations of inorganic elements. As yet, however, somewhat of the same obscurity hangs over their origin, as envelopes the propagation of the Fungi; since there is some reason to believe that amongst the Polygastrica, also, the same germ may be developed into different forms, according to the character of the infusion from which it derives its support. But these little animals are not con- fined to infusions of organised matter; they are found in the stagnant waters around our cities; in the waters of rivers, harbours, lakes, and, even, it is believed, in every fluid drop of the ocean. From their minute size and extensive distribution, therefore, there is reason to suppose that they are the most numerous living beings that exist on the face of the globe. Their tissue is usually soft and gelatinous; but not unfrequently they possess a transparent envelope which appears to be of a homy consist- ence, but which, in many species, is now found to consist almost entirely of silex. From the late researches of Ehrenberg, it appears that whole rocks of the mineral termed Tripoli or rotten-stone (an impalpable powder used in the arts for polishing), are composed of the siliceous shields of a species of Navicida, which seem to differ little from those now existing. Even where the shields cannot be separated in a distinct form, traces of them and of other similar remains are found, as in the consolidated nodules of various flints, opals, &c. It is scarcely possible to imagine the countless multitudes of these beings, which must have existed in former ages, for their very exuviw to have thus accumulated.* 114. The character of the Polygastrica is derived, as their name imports, from the number of their stomachs, which are little dilatations of the alimentary canal, excavated, as it were, from their soft cellular parenchyma. This canal sometimes possesses two distinct orifices (as in the Enchelispupa, Fig. 92), of which the mouth only is usually fringed with cilia ; but most frequently the lower extremity of it returns to the point from which it set out, and the same external orifice communicates both * It is peculiarly interesting' to trace such occurrences in prog-ress at the present time. The author has seen water, brought from a lake in the island of St. Vincent, crowded with the shields of races of Naviculae at present inhabiting it; and the mud which is being deposited in abundance at the bottom of the lake, is almost entirely composed of them. ANIMAL KINGDOM. Ill with the entrance and the termination of the canal (Fig. 77, a). The little digestive sacs are very numerous in some species ; more than a hundred have been seen in the Paramcecium (Fig. 94), filled at the same time ; and there may have been many more unseen from their emptiness. The method of viewing them is to introduce into the water some colouring matter, such as carmine or indigo, in a state of minute division ; its particles are then received into the intestinal canal, and are very evidently seen through the transparent tissues which surround it. Some among these animalcules, however, seem to possess a more complex structure. Ehrenberg has announced the existence of many distinct organs in them, but it may be questioned whether they are yet altogether demonstrated. In this interesting class we find many different modes of reproduction, which will be more particularly described hereafter (chap, xiii.) ; and the diversities of form and movement which its various species exhibit are of the most extraordinary character. The latter have been thus described: — "Several swim with the velocity of an arrow, so that the eye can scarcely follow them ; others appear to drag their body along with difficulty, and to move like the leech ; and others seem to exist in perpetual rest ; one will revolve on its centre, or the anterior part of its head ; others move by undulations, leaps, oscillations, or successive gyrations ; in short there is no kind of animal motion or other kind of progression that is not practised by animalcules." Although we may not immediately perceive an object for the existence of such countless multitudes of living beings, there can be little doubt that they serve a most important purpose in the economy of Nature, by supplying food to the larger tenants of the waters ; Polypes, MoUusca, Crustacea, and even Fish seem greatly indebted to them for their nutriment ; and even the larger animalcules prey upon the smaller ones, — the Vorticella, for instance, creating by its wheels a current which draws them into its mouth, just like (it has been amusingly remarked by Spallanzani, partis componere magna^) a certain species of whale, which after having driven herrings into a bay or strait, by a blow of its tail produces a whirlpool of vast extent and great rapidity, which precipitates them doA\Ti its open mouth. 115. There is probably no group in the Animal Kingdom more heterogeneous in character than that which has been formed into a class under the title of polypifera or Zoophjrtes. It is only recentlj^, how- ever, that an increased acquaintance with the structure of its members has revealed the incongruity of their association ; and at present there- fore we must be content to retain their general designation, until a sub- division shall have been agreed upon. Peculiar interest attaches to this class in the eyes of the Physiologist as well as of the Zoologist ; for, as has been remarked,* " they present to him the simplest independent structures * Johnston's History of Britisli Zoophytes. 112 ON ORGANISED STRUCTURES. compatible with the existence of animal life, enabling him to examine some of its phenomena in isolation, and free from the obscurity which greater complexity of anatomy entails. The means of their propagation and increase are the first of a series of facts on which a theory of genera- tion must rise ; the existence of vibratile cilia on the surfaces of the membranes, which has been shown to be so general and influential among animals, was first discovered in their study ; and in them are first detected the traces of a circulation carried on independently of a heart and vessels." In describing the principal types of structure, which may be most readily distinguished, it will be convenient to begin with one of the inferior, and, at the same time, best known species. The common Hydra viridis (green poljrpe, Fig. 95) is one of the simplest forms of structure, evidently animal, with which we are acquainted. It consists of nothing but a granular and apparently homogeneous membrane, com- posing a bag, which may be regarded as a stomach ; its single aperture or mouth being fringed with tentacula, or tendril-like filaments, which are very irritable and contract upon anything which touches them, endeavouring to draw it towards the entrance of the digestive sac. These tentacula are not fringed with cilia ; and therein consists an important difference between this polype and higher species of whose form it may be regarded as a sketch. Although so simple in its struc- ture, its digestive powers are very energetic, and it appears to exercise considerable force in conveying to its mouth the living animals which it frequently seizes. The contractility of the whole body is very remark- able, and causes the animal to assume entirely different forms at different periods. No trace of fibre is discoverable in its tissues, which seem entirely composed of globules united together by a jelly-like matter. The tentacula of one species CII. fusca) have been seen to extend from less than a line to a length of eight inches ; and it is not uncommon to see the body ten or twelve times longer at one period than at another, varying in form between that of a long narrow cylinder and that of a tubercle or button. AVhilst the want of cilia on its tentacula prevents the creation of currents for the purpose of bringing a constant supply of food to the mouth, and thus affords less choice to the animal, the body is so constructed as to be capable of accommodating itself to a prey of very variable size ; and, in like manner, the absence of this special means of aerating the fluids, is compensated by the exposure of every part of the tissue, both by its internal and external surface, to the surrounding element. A striking proof of the simplicity of the structure of this Polype is the fact, that it may be turned inside out like a glove ; that which was before its external tegument becoming the lining of its stomach, and vice versa. Another very curious result occurs from the same cause, — the extraordinary power which one portion possesses of reproducing the rest. Into whatever number of parts a Hydra may be ANIMAL KINGDOM. 113 divided, each Avill retain its vitality, and give origin to a new and entire fabric, so that thirty or forty individuals may be formed by the section of one. The regular mode of reproduction in this animal, however, bears no analogy to this. Little bud-like processes are developed from its external surface, which are soon observed to resemble the parent in character, possessing a digestive sac, mouth, and tentacula ; for a long time, however, their cavity is connected with that of the parent, but at last the communication is cut off, and the young polype quits its attach- ment, and goes in quest of its own maintenance. 116. The first sub-division of the Polypifera, termed (by Dr. John- ston) Hydroida or Hydraform, includes, with the simple genus just de- scribed, all those compound structures in which a number of polypes similar to it are associated together. Of this group the common Sertu- laria (Fig. 96) is a characteristic illustration. The polypidom, or solid framework formed by a secretion fi-om the soft tegument of the animal, consists of a tubular horny stem, enlarged at the extremities of its branches into sheaths ; within these the individuals can retract them- selves, although when in search of their food they extend beyond it. Each single polype resembles a hydra in every important respect but this ; — the stomach, instead of being closed at the bottom, communicates with, the interior of the stem and branches ; and the membrane forming the sac may be regarded as a jjrolongation of that which lines these tubes.* The pulp contained in the hollow stem, rather than the polype itself, appears to be the essential part of the animal ; for the latter is not only formed subsequently to it in the first instance, but frequently dies, and is reproduced by it. Although reproduction sometimes takes place by buds in these associated poljrpi, as in the Hydra, a more special ap- paratus is evolved for this purpose. At certain periods there are formed from particular spots on the stem of the Sertularia and its allies, expan- * When the stem and branches are examined with a high magnifying power, a current of granular particles is seen running along the axis ; which, after continuing one or two minutes in the same direction, changes and sets in the opposite one, in which it continues about as long, and then resumes the first ; thus alternately flowing down the stem to the extremities of the branches and back again. The change of direction is sometimes immediate ; but at other times the particles are quiet for a while, or exhibit a confused whirling motion for a few seconds, before the change takes place. The current extends into the stomachs of the polypi, in which, as well as in the mouth, a continual agitation of particles is perceptible. When these particles are allowed to escape from a cut branch, they exhibit an apparently spontaneous motion. JMo contraction of the tube or of the stomach seems concerned in the production of the currents ; and their rapidity and constancy appear intimately connected with the activity of the nutritive processes taking place in the parts towards which they are directed. In the Tuhularia, another polype with naked tentacula, currents of a similar kind have been observed ; but in this genus the stem is divided by nodes or partitions, into distinct cavities like the elongated cells of the Chara (§ 62). As on the walls of those cells, a number of slightly spiral dots are seen, in the line of which the current appears to move, passing down one side, crossing at the septum, and ascending the other witli an even and uniform motion, just like the globuliferous fluid of the Chara (§ 353). I 114 ON ORGANISED STRUCTURES. sions of its structure, somewhat resembling those which encase the polypes, but usually larger (Fig. 96, h). They are provided with a lid; and in their cavity are seen a number of gelatinous globules, which are at first connected by cords mth the soft tissue at the base of the cell, but afterwards separate from it ; and having acquired cilia on their surface, and being liberated by the falling-off of the lid, they swim forth, and after a little time attach themselves to some body which will serve for the support of a new structure. The vesicle, when thus emptied of its contents, soon drops off, like the seed-vessel of a plant after its functions are performed. Each reproductive gemmule consists of two substances, a thin cuticle or envelope, and a contained pulp. The former seems the rudiment of the future horny sheath; and, in the early stages of development, it is distended and moulded by the growth of the pulp Avithin. The latter at first increases longitudinally, and then forms a polype, which bursts its envelope, and commences the active exercise of its functions. The external membrane becomes hardened into the cell, within which the polype can retract itself, and then undergoes no farther change. This division has also been termed (by Dr. FaiTc) Nudibrachiata, from the deficiency of cilia on the arms or tentacula. 117. Another division of Polypifera may be termed Ciliobrachiata (Dr. Farre) from the presence of cilia upon its tentacula ; or Ascidioida (Dr. Johnston), from its afiinity mth the Tunicata, to which it is closely allied. On a superficial inspection, no very striking difference would be observed between the characters of this group and those of the one just described; but the minute examinations of Milne-Edwards, and Dr. Farre,* have disclosed in the former a degree of complexity of structure, which it would seem scarcely possible to have imagined. In the Bowerbankia densa (Fig. 97), for example, we find a horny transparent sheath enclosing the poljrpe, the upper part of which is so flexible as to be capable of being drawn inwards by the action of muscles, thus closing the mouth of the cell. The animal contained in it possesses ten tenta- cula, fringed with cilia, which surround the mouth of the large open tube, a, that forms the entrance to the digestive sac ; this leads by a narrow orifice to a globular cavity, b, which seems analogous to a gizzard, having thick sides lined internally with tooth-like processes ; below this is the true stomach, c, a large bag in whose parietes are situated a number of follicles for the secretion of bile, which tinges this part of a rich brown colour. From the upper part of the stomach, not far from the first opening, the intestine passes off by a distinct orifice, EVELOPMENT. 165 it seems that the wing of this extraordinary animal was extended, not over the whole member, as in the Bird, — nor over the hand, as in the Bat, — hut over one of the fingers only, which was immensely elongated in proportion to the rest. In the Flying-fish, again, the pectoral fins may be regarded as, in some sort, its wings; though it does not appear that the animal has the power of raising itself by means of their action on the air, the impulse being given at the moment of quitting the water. These fins are distinctly analogous to the anterior members of higher Vertebrata; but the bones of the arm and fore-arm are scarcely developed, while the hand is expanded, and joined immediately, as it were, to the trunk. 1 94. A very different structure prevails among those imperfect wings, which serve rather to support the animals which possess them, in their movements through the air, than to propel them in that medium. Thus, in the Flying Squirrels, Flying Lemurs, and Phalangers or Flying Opos- sums, there is an extension of the skin between the fore and hind legs, which, by acting as a parachute, enables the animals to descend with safety from considerable heights. In the Draco Volans, on the other hand, the wings are affixed to the sides of the back, being supported by prolongations of the ribs, and are quite independent of the extremities. Here we have still the same function and general form ; but it would evidently be absurd to say that the organs are of the same real character. Among the Invertebrated classes, there is still greater variety in the con- struction of the organs which make use of the resistance or impelling power of the air as a means of locomotion. Details on this subject have already been given in various sections of the Introduction ; but in addi- tion to what was there stated regarding the wings of Insects, it may be mentioned that there seems now sufficient reason to regard them as ap- pendages to the respiratory system.* * That they bear no real analogy to the wings of Vertebrata, would appear almost self- evident, when their structure is compared ; and yet there are Entomolog^ists who have main- tained that the wing- of an Insect is a modification of its leg'. A very little attention to the relative positions of these parts and the history of their development, will disprove this doc- trine ; whilst the true nature of the wing will be stated in its proper place (§ 398). Any one who compares the skeleton of the wing of a Bat or Bird with that of the fore-leg of a terrestrial quadruped, will see an obvious analogy in the essential parts of which each is composed, every bone which exists in one being discoverable ( though not always in a separate form) in the other ; whilst few unprejudiced persons could trace in the minutely-ramified nerves which support an insect's wing, any resemblance to one of its simply-articulated members. The segments which form the body of a caterpillar never possess more than one pair of legs on each ; but towards the latter period of their Larva condition, the rudiments of the wings may be detected beneath the skin, and these become more evident in the Pupa. When the perfect insect emerges, it is found that only three pairs of legs are retained by it, these being attached to the three segments of the thorax, whilst the nine segments of the abdomen have lost all trace of members. It is to the second and third segments of the thorax that the vnngs also are attached. Now if these wings had taken the place of the legs which disappear during the metamorphosis, there might have been some ground for regarding them as analogous organs ; but if their position be fau'ly considered, a resemblance which is at best so obscure must be 166 GENERAL PHYSIOLOGY. 195. These instances will sliow the caution which must be exercised in deciding upon analogies between organs, from correspondence in external form and function merely. Many similar ones might readily be adduced from the animal kingdom ; but the vegetable world affords them in even greater abundance. To take a very simple case ; — ^the tendril is an organ developed to serve a particular purpose, that of supporting the plant by tmning round some neighbouring prop ; but this varies much in its real character, being in the Vine a transformation of the peduncle or flower-stalk, in the Pea a prolongation of the petiole or leaf-stalk, in Gloriosa the point of the leaf itself, whilst in the singular genus Stro- phanthus it is actually the point of the petal which becomes a tendril and twines round other parts. But it is now time to speak of the other class of instances, in which a real similarity of character is concealed under a marked difference of form, and even of function. Of this, the Respiratory apparatus affords an excellent illustration. Few uninstructed observers would perceive any resemblance between the gills of a fish and the lungs of a quadruped, or between the elegant tufts on the body of a sand-worm, and the air tubes I'amifying through the structure of an insect ; and those who are in the habit of forming exclusive notions upon a hasty survey, might be led to deny that any real analogy could exist. When the character of the function is investigated, however, with the structure it requires for its performance, it becomes evident, that in order to bring the circulating fluid into the due relation with the atmosphere, all that is needed is a membrane which shall be in contact with the air on one side and with the fluid on the other. And this key, applied to the examination of all the forms of respiratory apparatus which exist in the animal kingdom, shows that they all possess the same essential cha- racter, and that their modifications in particular instances (which will hereafter be specially described, chap, ix.) are only to adapt them to the conditions of the structure at large. It has been seen that in one case, that which is obviously a part of the respiratory structure is made sub- iservient to the function of locomotion ; and in the swimming-bladder of fishes, which is now certainly ascertained to be a rudimentary lung (§ 406), we have a still more remarkable proof of the necessity of dis- regarding function in investigations of this kind. In Vegetable Physio- logy, again, innumerable instances of the same kind might be adduced, from amongst the ever- varying forms which the same elements assume in the leaves and flowers ; but these must sufiice for the present purpose, 196. The most general, then, of all the laws which have been yet discovered to regulate the structure of organised beings, is founded upon abandoned. On attending to their evolution also, it is found that in their early condition they evidently form part of the respiratory system, and are developed at the same rate with it, being only fully expanded at last, after their tubes have been forcibly distended with air (§ 396) ; and that in some aquatic insects, they actually serve as gills during the larva state. ON THE LAWS OP ORGANIC DEVELOPMENT. 167 the careful study of these analogies; and is commonly denominated the law of Unity of Composition. It is important to state this law in an unexceptionable form, since much objection has justly been made to that in which it has been propoimded by some physiologists. It may, how- ever, be remarked in limiyie that, for the broad acceptation in which it will be here explained, a strong argument of an a priori character may be adduced. If it be admitted as a principle every where prevailing throughout creation, that every end is attained by the best adapted means (a principle which not only revelation but reason in every way supports), it Avill necessarily result that, Avhere the function or purpose is identical, the structure of the organ by Avhich it is to be performed should be always essentially the same, but that the disposition of its parts should vary with the circumstances in which that function is to be performed. It is to the apparent alterations which result from these diversities in arrangement, that the observations of those writers apply, who have traced in them the indications of Omnipotent control over the elements employed. Thus, in the words of Richard Baxter, "Art and means are designedly multiplied that we might not take it (the order of creation) for the effect of chance : and in some cases the method itself is different, that we might see it is not the effect of surd necessity." And in the same excellent spirit it has been remarked by a modern writer, that "a certain definite mode of being is generally adapted to a certain definite end. But no absolute necessity binds the means to the end. The mode generally adopted may be, and doubtless is, the best; but the varieties of modes adapted to similar con- ditions demonstrate that the end has not influenced and controlled the contriving and adapting power which might have chosen another mode, and which does occasionally adapt Avidely different modes to the same purposes."* A deeper enquiry into the subject will show us, that there is everywhere o, fundamental unity prevailing through all the varieties of any particular structure; and it need scarcely be be argued that the original employment of a means which' should be capable of modification so as to suit every end, implies at least as high a degree of Creative Wisdom and Power, as the creation of new means in partictdar cases to which the plan first adopted might prove inapplicable. 197. It is necessary to bear in mind that the law of Unity of Compo- sition applies in its most general sense to the organic systems only, and does not embrace the locomotive and sensorial organs Avith these. It is obvious that this must be the case, since Plants are entirely deficient in the structures composing the latter ; and it is to be recollected also that these organs are not destined to produce any immediate change in the composition or state of the individual (except so far as regards his psychi- cal condition), and only influence his relation Avith the external world. But it will be found that the same principle taken in a more restricted * Duncan on Analog'ies, p. 25. 168 GENERAL PHYSIOLOGY. sense applies to these organs also ; since, although the locomotive apparatus varies so much in the different classes of animals, its essential characters are the same throughout each of the principal groups into which the king- dom may he divided. Throughout the whole animated Creation^ then., the essential character of the organs which all possess in common., remains the same; whilst the mode in which that character is nfianifested 'caries with the general plan upon which the being is constructed. Thus, in the lowest plants, as in the embryo-state of animals, the whole surface is modified for absorption of nutrient fluid; and the only change in the character of this absorbent surface in the higher vegetables consists in its restriction to a certain part of the stru^cture, the root, which is developed so as to bring it into most advantageous employment. In animals, a change of a differ- ent character has become necessary to adapt the function to the conditions of their being; and we find the absorbent points distributed not upon the external surface, but upon an inversion of it, adapted to retain and pre- pare the food (§ 237). Still the same fundamental imity exists; and the spongiole of the vascular plant, and the origin of the absorbent vessel in the animal, have precisely the same essential character with the membrane which constitutes the general surface of the Sea- weed or Red Snow. The advance from the lowest to the highest form in each kingdom is extremely gradual, as will be hereafter shown (chap, v.); and it will also appear that there are links of connection between the two principal modifications of the structure, a plant exhibiting something like the digestive cavity and absorbent system of the animal (§ 239), and certain animal forms absorb- ing from their general sm-face like the lowest plants. 198. This law may be applied, not only in the general method just pointed out, but in more restricted cases. Thus, where a particular func- tion is performed on two or more different plans, it Avill be found that each is steadily followed out through a number of varying circumstances, with such modifications only as may be necessary to adapt it to them. Throughout the Vegetable Kingdom for example, we observe the absorb- ent surface external, and among Animals internal. In aquatic animals again, we find the respiratory surface prolonged externally into gills; whilst in the air-breathing classes it is extended internally, so as to form tubes or cells exposing a large amount of surface. In the flowers of Phanerogamia, a certain number of different organs may fairly be regarded as universally present, either in a developed or rudimentary condition; and in the osseous skeleton of Vertebrated animals there is, in like man- ner, a correspondence of essential and even of subordinate parts. We do not find, in making such comparisons, that when any new modification of the function is to be performed, a structure entirely new is provided for it; for the end is always attained by a corresponding modification in the structure already present. Thus, where a plant requires the means of retaining fluid for absorption, a pitcher is provided by the metamorphosis ON THE LAWS OF ORGANIC DEVELOPMENT. 169 of the leaf; and where a bird has to be endowed with powers of flight, a wing is constructed out of its anterior member. 199. Every organ, therefore, which has a fundamental tjq^e common to all animated beings, has also a more special type possessed by each of the great classes into which they may be divided. And those organs which are restricted to certain divisions of either kingdom, have types peculiar to the various classes in which they appear. Thus, the organs of support and protection which constitute the skeleton, are external throughout the sub-kingdoms Mollusca and Articulata, exhibiting an approach, however, in their highest forms, to the internal position which they occupy in Vertebrata. In the latter division, the spinal column and its appendages, which essentially constitute the osseous skeleton, undergo many remarkable modifications, none of which, hoAvever, obscure the original type. Thus, the skull is but an expansion of the three highest vertebrae, in order to afford space for the development of the contained brain, and of the organs of sense ; and however strange such a statement may appear to those who are only acquainted with the skull of man, the fact is evident where the brain is little developed, as among fishes and the lower reptiles. On the other hand, the tail is a continuation of the vertebral column, generally deprived of some of its parts, and often having several of its joints consolidated, as in the human sacrum and coccyx. Where no members appear, as in serpents, some rudiments may often be traced, although totally inapplicable to the purpose of locomo- tion, as in the slow-worm, and the boa. It is a most beautiful exempli- fication of this law of uniformity of composition, combined with the one which -will be next stated, that we never find any organ totally absent, which is possessed in a prominent degree by the members of adjacent groups. Thus, the rudiments of teeth, which are never developed, and at a later period cannot be detected, are found in the embryo of the whale, and are also observed during the development of the jaws of many birds. In the abdominal muscles of the Mammalia are found white cartilaginous lines, indicating the situation of the abdominal sternum and ribs of Lizards, of which these lines are the representation ; still it is not impossible that they may serve the purpose Avhich has been attributed to them, — that of preventing the contraction of the whole length of those muscles into a knot, which might press injuriously on the viscera. It is by no means uncommon for an organ to serve, in the lower stages of its formation, a purpose quite different from that of its perfect form ; thus, the swimming bladder of a fish is a rudimentary lung, but instead of being subservient to respiration, it ministers to an entirely different function. 200. Connected mth this law is that of Progressive Development. In the early stages of formation in every animal or vegetable, Ave may observe as great a dissimilarity to its ultimate condition as exists between 170 GENERAL PHYSIOLOGY. the lower and higher members of each kingdom. And if we watch the progress of evolution, we may trace a correspondence between that of the germ in its advance towards maturity, and that exhibited by the per- manent conditions of the races occupying different parts of the ascending scale of creation. This correspondence results from the operation of the same law in both cases. If we compare the forms which the same organ presents in different parts of the series, we shall always observe that it exists in its most general or diffused form in the lowest classes, and in its most special and restricted in the highest, and that the transition from one form to the other is a gradual one. Thus, to refer again to the organs of absorption; — ^these we find diffused over the whole exterior in the simplest plants and animals, so that the sui-face becomes, as it were, all root ; whilst they are restricted to a very small proportion of it in vascular plants, and in the higher animals. The function, therefore, which was at first most general, and so combined with others performed by the same surface as scarcely to be distinguishable fi-om them, is after- wards found to be confined to a single organ, or specialised by separation from the rest ; these having, by a similar change, been rendered dependent on distinct organs. It follows, therefore, that there is a greater variety of dissimilar parts in the higher organisms than in the lower ; and hence the former may be said to be heterogeneous^ whilst the latter are more homo- geneous, approaching in some degree the characters of inorganic masses. This law is, therefore, thus concisely expressed by Von Bar, who first announced it in its present form. " A heterogenous or special structure arises out of one more homogeneous or general j and this hy a gradual change" The details which will be given in the second division of this work, — relative to the evolution of structure and the complication of function, witnessed in studying the development of each system, both in the ascending scale of creation, and in the growth of the embryo, — ^will so fully illustrate this law that more need not here be said of its application. 201. This law holds good with respect to function as well as to structure ; indeed it must inevitably do so, since all alteration in structure must be accompanied with more or less change in its properties. But observation of the functions of the more complex forms of animated beings leads to the knowledge of another law, which in some degree restricts the operation of the one just mentioned. It may be stated as follows. — In cases where the different functions are highly specialised, the general structure retains, more or less, the primitive community of function which originally characterised it* As this law also will be copiously illustrated in subsequent chapters, it is unnecessary here to do more than point out its mode of application. The absorbent system has been shown to be one of those most highly specialised (or, in other words, having a * See Edinburg-h Philosophical Journal, July, 1837. ON THE LAWS OP ORGANIC DEVELOPMENT. 171 separate organ most exclusively devoted to it) in the more complex organisms ; yet it is never entirely resti-icted to its special organ. For, as in the simplest or most homogeneous beings the entire surface participated equally in it, so in the most heterogeneous every part of the surface retains some connection with it; since, even in the highest plants and animals, the common external integument admits of the passage of fluid into the interior of the system, especially when the supply afforded by the usual channels is deficient. In the same manner we find that, whilst in the lowest animals the functions of excretion are equally performed by the whole surface, there is in the highest a complicated apparatus of Glandu- lar organs, to each of which some special division of that function is assigned ; but as all these glands have the same elementary structure, and differ only in the peculiar adaptation of each to separate a particular constituent of the blood, it is a necessary result of the law just stated, that either the general surface of the^skin or some of the special secreting organs should be able to take on, in some degree, the function of any gland whose duty is suspended ; and observation and experiment fully bear out this result, as Avill hereafter appear (chap. xi). 202. Allusion was just now made to the correspondence which is discernible between the transitory forms exhibited by the embryos of the higher beings, and the permanent conditions of the lower. When this was first observed, it was stated as a general law, that all the higher animals in the progress of their development pass through a series of forms analogous to those encountered in ascending the animal scale. But this is not correct; for the entire animal never does exhibit such resemblances, except in a few particular cases to Avhich allusion has already been made (§ 80); and the resemblance or analogy which exists between individual organs has no reference to their forms, but to their condition or grade of development. Thus, we find the heart of the Mam- malia, which finally possesses four distinct cavities, at first in the condition of a prolonged tube, being a dilatation of the principal arterial trunk, and resembling the dorsal vessel of the Articulated classes; subse- quently it becomes shortened in relation to the rest of the structure, and presents a greater diameter, whilst a division of its cavity into two parts, a ventricle and an auricle, is evident, as in Fishes; a third cavity, like that possessed by Reptiles, is next formed, by the subdivision of the auricle previously existing; and lastly a fourth chamber is produced by the growth of a partition across the ventricle; and in perfect harmony with these changes are the metamorphoses presented by the system of vessels immediately proceeding from the heart. In like manner, the evolution of the brain in man, is found to present conditions which may be successively compared with those of the Fish, Reptile, Bird, lower Mammalia, and higher Mammalia; but in no instance is there an exact identity betAveen any of these. 172 GENERAL PHYSIOLOGY. 203. Since the doctrine, so far as it is correct, refers to individual organs alone, and not to those collections of them which go to form living structures, it is no objection to it to say, as may be fairly done, that neither the embryo of man, nor that of any other among the higher animals, resembles a lower animal to such a degree as to be mistaken for one; for, however similar may be the apparent origin of each being, the changes which it undergoes from its very commencement have a definite end, — the production of its perfect and specific form. Such an admission, therefore, can have no tendency to confound the established distinctions in Natural History. But this correspondence may, as already stated, be regarded in the light of a result or corollary from the more comprehensive law at first laid down; since, if the evolution of particular organs discloses the same plan, when traced upwards from their simplest and most general forms, — whether in the lowest being, or in the embryo of the highest, — ^their progressive stages must present resemblances in condi- tion. As already mentioned (§ 80), there are certain cases in which the limitation is removed, and the whole being is made to correspond in what must be regarded as its embryo condition with the form and structure characteristic of an inferior class. This is for the purpose of enabling it to maintain its own existence at an earlier period than would otherwise be practicable; and the means by which this is effected without the addition of any new structure, or the infraction of any law of develop- ment, are not a little curious. Thus, to adapt the embryo frog to the life of a fish, requires a provision for aquatic respiration; and this is made simply by developing to a greater extent in the tadpole, those rudiments of gills which all the higher animals possess in common with it. In the Larva of the insect, again, which, at its emersion from the egg, bears so small a proportion to its ultimate magnitude (§ 231), the germinal membrane, that in other ova is spread over and progressively absorbs the yolk or store of nutriment supplied by the parent (§ 534), speedily becomes a large intestine, into which the food is taken in prodigious quantities by the mouth : and when it has served this purpose, a part of it is metamorphosed into generative organs, which in the perfect Insect are destined to continue the species, and which are elaborated in other animals from the same source — the germinal membrane — before their first entrance into the Avorld. In the form in which the law of pro- gressive development has been here stated, it will be found applicable to the Vegetable kingdom, as well as to the animal; the progress of indivi- dual organs from a more general to a more special type, being discernible as well in the development of the embryo as in ascending the scale. But it would be quite impossible to maintain the position that any of the stages of growth presented in the evolution of a flowering plant, are altogether comparable with the permanent forms exhibited by Lichens, Fungi, Mosses, &c. ON THE LAWS OF ORGANIC DEVELOPMENT. 173 204» Another law, of less comprehensive application, has been estab- lished by the study of the evolution of the higher organisms, and is called that of excentric development. It is observed that the parts of the struc- ture most distant from the median plane, are in general more advanced than those nearer the centre; thus, the ribs are ossified earlier than the sternum or vertebral column, — ^tlie parietal bones sooner than the central portions of the sphenoid or occipital. But it appears to have a more extended application than this; for there is much evidence to prove that the formation of all the organs in Vertebral animals takes place on a double system, not only those which are permanently double being thus evolved, but those which subsequently appear as single and even asjmi- metrical organs consisting, at an early period, of two separate and equal halves. Thus, the spinal column and all the bones placed on the central line, are originally divided longitudinally, the points of ossification not being on that line, but on each side of it; in some of the lower animals, the lateral halves remain separate, as in the case of the lower jaw of most sei'pents; and in man it is not uncommon to find a permanent division in particular bones, especially the frontal, which can scarcely be regarded as amounting to a malformation. But the application of this law is still more extraordinary, when it is considered in relation to organs which in their perfect form are not only single, but are placed off the median plane of the body, so as not to consist of two equal halves. The liver, for instance, is almost entirely confined in adult man to the right side of the body; but in the foetus, its two lobes are at one period equally balanced betAveen the two (§ 139). The heart is, at its first formation, placed on the median line, as in the Articulata, and consists of two equal halves; while the large vessels connected with it, the aorta and vena cava, are actually double. Many similar instances might be adduced; but those afforded by monstrous conditions, or inal/ormations are most illustrative. 205. Of the malformations Avhich occur in the higher animals, a great variety may be referred to arrest of development; and this may operate in several ways. It leads to the permanent assumption of a condition, in particular organs, which should have been transitory only; and thus a resemblance will arise between the condition of that organ in the mal- formed being, and that which is characteristic of some inferior gi-ade. This is peculiarly striking in the malformations of the circulating system, of which many instances will hereafter be adduced. But it may also present itself in a deficiency of structure on the median line, such as occa- sions hare-lip, cleft palate, bifid uvula, absence of the commissures of the brain, approximation of the two eyes in one socket, the disease termed spina hi/Ida (which results from deficiency of the posterior part of the rings of the vertebrse), and many others. These and other deformities are no longer regarded by the philosophic anatomist with the hon*or and dis- gust which they once inspired, and which thoy still excite in the vulgar 174 GENERAL PHYSIOLOGY. mind; since they afford the most appropriate and convincing evidence of the Uniformity of design which runs through creation; and, if properly employed, become the most stable foundation for the prosecution of the enquiry into the laws through which that Design has operated. 206. In the Yegetable kingdom, the study of monstrosities has been peculiarly effectual in the elucidation of the laws regulating the metamor- phoses of organs, or the dissimilar forms which the same elements may assume. Thus, it is found that parts of the flower which have, in their ordinary state, least of the foliaceous appearance, such as the stamens or carpels, revert to the form of leaf (which may be regarded as the type of them all), under some alteration in the conditions of their development, which is not yet fully understood. Again, the forms of flowers, which in some species are characteristically deficient in symmetry, exhibit a ten- dency to assume that regularity which may be regarded as typical of the structure. Thus, the common Snap-dragon has an irregular form of corolla, which is denominated labiate, from the two large lips bounding its mouth, and is furnished with a single long spur; it is by no means uncommon, however, to find specimens in which the corolla has become perfectly regular, each petal being similar in form, and each furnished with a spur. At the same time, the stamens, which in this species are four and didynamous (two long and two short), become five, and all of the same length; and it is thus shown that the suppression of one stamen and the shortening of two others, which is characteristic of this group, does not result from any essential alteration in the plan of structure, but merely from a deficiency in the evolution of certain parts of which the rudiments exist. In the Nasturtium, again, Avhich in its usual state possesses one spurred petal only, the tendency to regularity is exhibited, sometimes by the disappearance of the spur, sometimes by the development of it on other petals. Innumerable examples of the same kind might be adduced; but these are sufficient to prove the importance of attention to them. 207. Another subordinate law, which however has an extensive appli- cation, is that of the balancing of organs, alluded to by Paley and other authors as the "principle of compensation." This, like other generalisa- tions, has been carried too far by many writers who have dwelt upon it. Thus, it has been stated in the following most objectionable form; — -that the extraordinary development of one organ occasions a corresponding deficiency in another, and vice versa. It is perfectly true that in a great majority of cases the extraordinary/ development of one organ is accom- jyanied by a corresponding deficiency of development in another ; but the development and the deficiency are both parts of one general plan, and neither can be regarded as the cause or the effect of the other. Thus, in the human cranium, the elements which form the covering or protection of the brain are very largely developed, whilst those which constitute the face are comparatively small. In the long-snouted herbivorous quadru- ON THE LAWS OP ORGANIC DEVELOPMENT. 175 peds or reptiles, on the other hand, the great development of the bones of the face is coincident with a very small capacity of the cerebral cavity. In the bat, we find the anterior extremity widely extended, so as to afford to the animal the means of rising in the air; whilst the posterior is very much lightened, so as not to impede its flight. In the kangaroo, on the other hand, the posterior members are very large and powerful, enabling the animal to take long leaps; whilst the fore paws are proportionably small. The mole, again, requires for its underground bun-ows the power of excavating with its fore-feet, whilst the hind legs are used for propul- sion only; and the relative development of these members follows the same proportion as in the bat, although the plan in the two cases is widely different. Moreover it is obvious that, from the peculiar habits of this animal, eyes would be of little or no use to it; and accordingly we find them merely rudimentary, and no cavity in the skull for their reception ; whilst to compensate for the want of them, the ethmoid bone, which con- tains the organ of smell, is amazingly developed. In other classes of animals similar illustrations abound; and the relation between the internal and external skeletons, already alluded to (§ 82), is a striking proof of the extensive application of this principle. 208. Another law, propounded by Cuvier, and supported by other authors, is that of the harmony of forms, or the coexistence of elements. It implies that there is a specific plan, not only for the formation, but for the combination of organs; that there is a constant harmony between organs apparently the most remote; and that the altered form of one is invariably attended Avith a corresponding alteration in the others. That this statement is true as far as it goes, no one can deny; and the researches which have been based upon it have been most successful in repeopling the globe, as it were, with the forms of animals which have long been extinct, but which can be certainly predicated even from minute fragments of them. A general comparison of the skeleton of the carnivorous with that of a herbivorous quadruped, mil show the manner in which this enquiry is pursued. The tiger, for example, is furnished with a cranial cavity of considerable dimensions, in order that the size of the brain may correspond with the degree of intellect which the ha1)its of the animal require. The face is short, so that the power of the muscles which move the head may be advantageously applied. The fi-ont teeth are large and pointed; and by the scissors-like action of the jaw, they are kept constantly sharp. The lower jaw is short, and the cavity in which its condyle works is deep and narrow, alloAving no motion but that of opening and shutting; the fossa in which the temporal muscle is imbed- ded, is very large; and the muscle itself is attached to the jaAV in such a manner as to apply the power most advantageously to the resistance. The molar teeth are sharp and adapted for cutting and tearing only. The spi- nous processes of the vertebra^ of the back and neck are very strong and 176 GENERAL PHYSIOLOGY. prominent, giving attachment to powerful muscles for raising the head, to enahle the animal to carry off his prey. The bones of the extremities are disposed in such a manner as to alloAV the union of strength with freedom of motion; the head of the humerus is round, and the fore-arm has the power of pronation and supination, indicated by the character of the arti- cular surfaces. The toes are separate, and armed with claws, which are retracted when not in use by a special apparatus that leaves its mark upon the bones. — On the other hand, in the conformation of the herbivorous quadruped, we are at first struck mth the diminished capacity of the cra- nium, and the size of the bones of the face. The jaws are long, and have a great degree of lateral motion, the glenoid cavity being broad and shal- low; and whilst the pteregoid fossa, in which the muscles which rotate it are lodged, is of large size, the temporal fossa is comparatively small, no powerful biting motions being required by the nature of the food or the mode of obtaining it. The front teeth are fewer and smaller; but the surfaces of the grinding teeth are extended, and kept constantly rough by the alternation of bone and enamel. The extremities are more soHdly formed, and have but little freedom of motion, the shoulder being scarcely more than a hinge-joint; the toes are consolidated and inserted into a hoof, which is double or single, according as the animal ruminates or not. The whole body is heavier in proportion, the nutritive system being more complicated; and the muscles which enable the tiger to lift considerable weights in his mouth, are here necessary to support the weight of the head itself. 209. A little consideration will show that the existence of this adap- tation of parts is nothing more than a result of other laws of development. It is evident that if it were deficient, the race raust speedily become extinct, the conditions of its existence being no longer fulfilled; these conditions being, for the whole organism, what the vital stimuli already described are for its individual properties. An animal with the carnivo- rous propensity of the tiger, for instance, and the teeth or hoofs of a horse, could not remain alive, from the want- of power to obtain and prepare its aliment; nor would a horse be the better for the long canine teeth of the tiger, which would prevent the grinding motion of his jaws required for the trituration of the food. The statement above given cannot, therefore, be regarded as a law, since it is nothing more than the expression, in an altered form, of the fact that, as the life of an organised being consists in the performance of a series of actions Avhich are dependent upon one another, and all directed to the same end, whatever seriously interferes with any of those actions must be incompatible with the maintenance of existence. The splendid discoveries of Cuvier and other anatomists, who have succeeded in determining from minute fragments of bones the cha- racters of so many extraordinary species of remote epochs, have resulted only from the union of a sagacious application of this fact, with the ON THE LAWS OF ORGANIC DEVELOPMENT. 177 laborious comparison of these remains and the similar parts of animals at present existing. Until the laws of formation are discovered, which have operated in producing one result as well as the other, no briefer process than this can be adopted. 210. That these laws may be most advantageously pursued while disregarding for a time the particular connection of organs with the functions they appear designed to serve, will be hereafter shown (chap. XVII.); at present it may be remarked that those who have dwelt most upon this adaptation of the structure of living beings to the external conditions in which they exist, appear to have forgotten that these very conditions might be regarded, with just as much propriety, as specially adapted to the support of living beings. We have as much ground to believe that this earth, with all its varieties of season, temperature, light, moisture, &c., was adjusted for the maintenance of plants and animals upon its surface, as that these plants and animals were created in accordance mth its pre-existing circumstances. The Natural Philoso- pher does not regard it as a sufficient explanation of the astronomical or meteorological changes which he witnesses, that they are for the benefit of the living inhabitants of the globe; and yet, as it has been already shoAvn, they furnish conditions of vital action as important as those affi)rded by organised structure. The Philosophical Anatomist, therefore, does not regard the object or function of a particular structure as a sufficient account of its existence; but, in attaining the laws of its formation independently of any assumption of an end, he really exhibits the primary Design in a much higher character than in deducing it from any limited results of its operation. CHAPTER IV. GENERAL VIEW OF THE FUNCTIONS OF ANIMATED BEINGS, AND THEIR MUTUAL RELATIONS. 211. It has been stated (§ 4 — 6) to be the object of the Physiologist, to ascertain the laws regulating the changes which constitute the Life of organised beings ; and that in order to arrive at any certain general con- clusions respecting them, he must collect and compare all the facts of similar character, with which the study of the animated creation furnishes him. The changes which occur during the Life of any one being, are of themselves inadequate to furnish the required information ; since this presents us only with a group of dissimilar phenomena, incapable of comparison with each other, or permitting it but to a low degree. Were 178 GENERAL PHYSIOLOGY. we to derive all our notions of Physiology from the history of one of the simple cellular plants, we should obtain but very vague ideas as to the character of its different nutritive processes ; since we cannot separate these from one another, and investigate them apart. And, on the other hand, Ave should be apt to form very erroneous conceptions of the essential conditions of these processes, were we to study them only in their most complex form and specialised condition, and reason thence as to their dependence upon particular kinds of structure. It is only, then, from a comprehensive survey of the whole organised creation, — embracing the unobtrusive manifestations of life which Nature displays at one ex- tremity of the scale (as if to show the simplicity of her operations), as Avell as those evident actions which every moment displays to us in her most elaborate works, — that any laws possessing a claim to general application can be deduced. 212. A careful examination, however, of the vital operations of the human system, or of any other of similar complexity, will reveal much more regarding the essential conditions of life, than a superficial glance could ascertain from them. Thus, if the series of phenomena be enquired into, which constitute the Function of Respiration, it will be found that, whilst some of these are indispensable to the continuance of life, and can only be performed under the conditions supplied by the organised system, other actions are merely superadded for the purpose of facilitating them; and these, if from any cause not performed by the mechanism contrived for their production, may be artificially imitated, with a degree of success exactly proportional to the perfection of the imitation. The essential part of the function of Respiration is the aeration of the blood; that is to say, an interchange of ingredients between the fluid and the air, resulting from its exposure, either directly to the atmosphere, or the gases diffused through water. All the changes which are associated as partaking in the function, share in it only by contributing to this, the real constituent of it. The alterations in the capacity of the chest, which are effected by the actions of the diaphragm and of the external muscles, have only for their object the renewal of the quantity of air in contact with the membrane through which the blood is exposed to it. These actions are really a part of the functions of the Muscular and Nervous systems; and are only associated under that of Respiration, on account of their obvious tendency towards its essential purpose. They have no share in the production of the aeration of the blood, except by supplying its conditions; and if these conditions can be supplied independently of them, the essential part of the function will be performed as when they were concerned in it.* * Thus, in Asphyxia (§ 152 note) the deficient supply of arterialised blood to the brain soon paralyses its functions; and the nervous stimulus required for the respiratory movements being withheld, those movements cease. But, if the chest be artificially inflated, and emptied again by pressure, and these alternate movements be sufficiently prolonged to re-excite the circula- GENERAL VIEW OP THE FUNCTIONS. 179 213. By an analysis of this kind applied to the other functions, similar conclusions might be arrived at respecting their essential charac- ter; for it will appear in every one of them, that some of the changes which are thus grouped together are essential, and others superadded. But these conclusions do not possess the same certainty as if they were founded upon a broader basis; nor are they so easily attained. For, to revert to the instance just quoted, observation alone of the vital pheno- mena of the lower animals, will reveal what could only be determined in man by experiment. Until an experiment (the insufflation of the chest) had been found successful in continuing the aeration of the blood, it could not be certainly known that the respiratory movements had not some further share in the function than that of mechanically renewing the air in apposition Avith the circulating fluid. But when the conditions of the function are examined in the lower animals, it is found that these are varied (the essential part being every where the same) to suit the respective circumstances of their existence. Thus, Reptiles, having no diaphragm, are obliged to fill the lungs with air by a process which resembles swallowing. In Fishes and other aquatic animals, to have introduced the required amount of the dense element they inhabit into the interior of the system, would have occasioned an immense expenditure of muscular power; and the required purpose is answered by sending the blood to meet the Avater, Avhich is in apposition Avith the external surface. And in those simple gelatinous creatures, in Avhich the fluids appear equally diffused through the AA^hole system, their required aeration is effected by the simple contact of the water with the general surface: the stratum in immediate apposition Avith it being rencAved, either by their OAATi change of place; or, if they are fixed to a particular spot, through the means they possess of creating ciu-rents, by Avhich their supply of food is brought to them. And, going still further, Ave find in Plants, the essen- tial part of the function of Respiration performed Avithout any movement AvhatcA^er; the Avide extension of the surface in contact Avith the atmo- sphere affording all the requisite facility for the aeration of the circulating fluid. 214. A brief but comprehensive vieAV of the vital phenomena com- mon to all living beings, as well as of those peculiar to the Animal kingdom, exhibiting their mutual dependence, and distinguishing their essential conditions from those which are superadded or accidental, Avill assist in the due appreciation of the details hereafter to be given as to tion of the blood through the lung-s, by aerating- that which had been stagnated there, tlie whole train of vital actions may be again set in motion. Or, if the cessation of the respiratory movements results from a cause primarily affecting the nervous system — as when narcotism is induced by poisoning witli opium — and the blood be, in consequence, stagnated in the lungs by the want of aeration, this change, so essential to the continuance of vitahty, may be prolonged by artificial respiration, until the narcotism subsides, unless the dose have been too powerful. N 2 180 GENERAL PHYSIOLOGY. their individual character. In all living beings, the appropriation of alimentary matter from w^ithout, — its conversion into a nutritions fluid, of which the elements supply materials for the growth and renovation of the fabric, and thus maintain its vital properties, — and the excretion of the particles unfit for these purposes, — constitute the sum of the vital acts by which the existence of the individual is immediately supported. If due allowance be made for the differences occasioned by the possession of the faculties of sensation and voluntary motion, it will be found, on close examination, that there is a fundamental correspondence in the mode in which these processes are performed in all the members both of the Animal and Vegetable kingdoms; their apparent differences resulting from the necessary adaptation of their organs to the respective conditions of existence. These functions are, therefore peculiarly vital; and are spoken of as constituting the organic or vegetative life of the being. 215. But the maintenance oi individual life is not all that is required from the powers of an animated body. It has been stated (§ 146) to be a general law of organisation, that all organised structures must be produced by others previously existing; no living being ever taking its origin from spontaneous combinations of inorganic matter.* Since, then, the limited duration of each individual existence would soon occasion the extinction of the race, if no provision were made for perpetuating it, the necessity for a succession of new creations can only be obviated by the endowment of each organism with the means of preparing a germ, which, when sufficiently mature, may support an independent existence, execute all the vital changes which are characteristic of the form which it has assumed, and in its turn originate new beings by a similar procees. This function, which is common to all living beings, is termed that of Reproduction. 216. Now, it may be observed, before proceeding further, that there is a certain degree of antagonism between the nutritive and reproductive functions, the one being executed at the expense of the other. The reproductive apparatus derives the materials of its operations through the nutritive system, and is entirely dependent upon it for the continuance of its function. If, therefore, it be in a state of excessive activity, it will necessarily draw off from the individual fabric some portion of the * In cases which have been thoug-ht to support the contrary doctrhie, such as the appear- ance of parasitic Fungi (§ 64, 66) on decaying' organised substances, the production of animalcules in decomposing infusions (§ 113), or the existence of Entozoa in the animal tissues (§ 111), a different explanation may fairly be offered; as it is impossible to prove in any of these cases that germs were not in reality present. The admission that the beings which appear wdth such remarkable uniformity in these situations, have really an origin from pre-existing organisms, becomes much easier if it be admitted on the other side (as it seems reasonable to do), that the form of the being which is there produced need not always corres- pond with that which afforded the germ. This subject vvdll hereafter be more fully enquired into (chap. XIII.). GENERAL VIEW OF THE FUNCTIONS. 181 aliment destined for its maintenance. It may be universally observed that where. the nutritive functions are particularly active in supporting the individual, the reproductive system is in a corresponding degree undeveloped, — and vice versa. Thus, it has been stated that in the Algae, the dimensions attained by single plants exceed those exhibited by any other organised being ; and in this class the fructifying system is often obscure, and sometimes even undiscoverable. In the Fungi, on the other hand, the whole plant seems made up of reproductive organs; and as soon as these have brought their germs to maturity, it ceases to exist. In the flowering plant, moreover, it is well known that an over-supply of nutriment will cause an evolution of leaves at the expense of the flowers, so that what actually would have been flower-buds, are converted into leaf-buds; — or the parts of the flower essentially concerned in reproduc- tion, namely, the stamens and pistil, are converted into foliaceous expansions, as in the production of double flowers from single ones by cultivation; — or the fertile florets of the disk in composite species, such as the Dahlia, are converted into the barren but expanded florets of the ray. And the gardener who wishes to render a tree more productive of fruit, is obliged to restrain its luxuriance by pruning, or to limit its supply of food by trenching round the roots. 217. The same antagonism may be witnessed in the Animal Kingdom; but as a third element (the sensory and locomotive apparatus) here comes into operation, it is not always so apparent. It appears to be a universal principle, however, that during the period of rapid growth, when all the energies of the system are concentrated upon the perfection of its indivi- dual structure, the reproductive system remains dormant, and is not aroused until the comparative inactivity of the nutritive functions allows it to be exercised without injury to them. Thus, in the larva condition of the insect, the assimilation of food and the increase of its bulk seem the sole objects of its existence (§ 87); its locomotive powers are only adapted to obtain nourishment that is within easy reach, to which it is directed by the position of its egg, and by an unerring instinct that seems to have no other end. The same is the case, more or less, with all young animals; although there are few in which voracity is so predominant a characteristic. In the Imago, or perfect insect, on the other hand, the fulfilment of the purposes of its reproductive system appears to be the chief and often the only end of its being. The increased locomotive powers which are conferred upon it, are evidently designed to enable it to seek its mate; its instinct appears to direct it to this object, as before to the acquisition of food; it now shuns the aliment it previously devoured with avidity, and frequently dies as soon as the foundation is laid for a new generation, without having taken any nutriment from the period of its first metamorphosis. In the adult condition of the higher animals, again, it is always found that, as in plants, an excessive activity of the J 82 GENERAL PHYSIOLOGY. nutritive functions indisposes the system to the performance of the repro- ductive; a moderately-fed population multiplpng (ceteris paribus) more rapidly than one habituated to a plethoric condition. 218. There is no reason to believe that vegetables possess anything like the consciousness which we know to exist in ourselves, and which, from the analogy of its manifestations, we believe to exist in other ani- mals. In man, this consciousness is but the foundation of a series of mental operations, in which many different classes of actions may be recognised. These may altogether be spoken of as resulting from his psychical endowments. Whether those endowments are to be regarded as properties,' resulting from certain dispositions of matter, or whether matter affords only the mechanism by which they are brought into relation with the external Avorld, is a question more of speculative interest than of practical importance. That in descending the animal scale, this mecha- nism becomes less and less complex, and the psychical powers themselves less numerous and varied, until they are at last reduced to little else than mere consciousness, is acknowledged by all. And it is thus seen that the Animal Kingdom has no more distinctive separation from the Vegetable, than its principal groups exhibit amongst each other. The psychical endowments of animals, in whatever degree possessed, require for their exercise the means of information as to the condition of the external world, which is afforded by the faculty of sensation; as well as the capa- bility of altering their o^^ti relation to it, which is effected by the apparatus of locomotion. These functions then are peculiarly animal in their cha- racter, and they are spoken of as constituting animal life, in contradis- tinction to those of organic life. 219, When the animal functions are reduced to their lowest degree, it is often very difficult to obtain satisfactory evidence of their being exer- cised at all; since their manifestations scarcely differ from those organic changes which may occur without their participation. Thus, when the tentacula of the Hydra or other polypes are touched, they suddenly con- tract, and endeavour to enclose the substance which affects them; but fi'om such actions alone, we should have no right to infer that either sensation or volition are possessed by this being, since the Sensitive plant and the Dionoea perform movements precisely analagous (chap. xv.). We observe, too, that the gemmules of the Sponges and Polypes swim about for some time previously to fixing themselves, apparently with a choice of direction, and a perception of obstacles; yet the sporules of many Alg^ are almost equally active, and it would be difficult to point out any essential difference between the two instances. In many cases, too, the influence of external causes, such as movement of the fluid itself or the attraction of other bodies, renders it impossible to determine what are voluntary motions, and what are therefore to be regarded as possessing an exclusively animal character. GENERAL VIEW OF THE FUNCTIONS. 183 220. All physiologists agi'ee that structure alone presents no certain diagnostic mark, hy which the simplest and most approximated groups of the two kingdoms may be distinguished. In the higher classes of animals we may detect certain organs which we know hy experience to minister to their peculiar functions; hut these, becoming gradually simplified as we descend the scale, at last disappear altogether (at least to our means of observation), whilst some degree of the functions appears still to remain. Again, there are certain modifications Avhich the apparatus of organic life is seen to undergo in the higher animals, for the purpose of adapting it to the conditions of their existence; and upon these some naturalists have relied as diagnostic peculiarities (§ 239). But these also are perceived to disappear, as the psychical endowments of the beings, and the organs by which they are brought into relation with the external world, occupy a less prominent situation. This has been already seen with regard to the symmetrical disposition of the parts of the fabric; and it will be hereafter shown in more detail, when the individual organs are described. 221. On the other hand, in proportion to the complexity and extent of the psychical endowments of each species of animals, may their influ- ence over the conformation of the organic structure be perceived; so that it becomes more and more removed from that which is presented by vege- tables, the chief end of whose existence appears to be the elaboration of such a structure from the elements furnished by the inorganic world. In man, the being that possesses the largest share of these capabilities, the whole apparatus of organic life would seem destined but to serve for the maintenance of the animal functions. The processes of nutrition are here chiefly directed towards perfecting the nervous and muscular systems, and bringing their functions into most advantageous operation; whilst in many of the lower animals this would seem quite a subordinate object, the extension of the organs of vegetative life being, as in plants, the direction taken by their development. Hence it may be stated generally, that the more exalted is the animality of any particular being (or, in other words, the more complete the manifestation of characters peculiar animal), the more closely are the organic functions brought into relation Avith it.* 222. Yet, however intimate may be the bond of union between the organic and animal functions, the former are never immediately dependent upon the latter; although, as it has been already shown (§ 31 and 212), * A simple illustration will render this evident. In certain of the lower tribes of animals, whose locomotive powers are feeble and general habits inactive, the circulation of nutritive fluid is carried on nearly in the same manner as in plants ; there is no central organ for pro- pelling it through the vessels, and ensuring its regular and equable distribution ; and its motion appears dependent upon the forces created in the individual parts themselves. In the higher classes, on the other hand, the comparative activity of all tlie functions, and the peculiar dependence of those of animal life upon a constant supply of this vital stimulus, require a much more elaborate apparatus, and especially a central power, by which the movements of tlic fluid in the individual parts may be harmonised, directed, and controlled. 184 GENERAL PHYSIOLOGY. they sometimes depend upon them for the conditions of their maintenance. There is no good reason to belieye that " nervous agency " is essential to the processes of nutrition and secretion in animals, any more than to the corresponding processes in plants. This is a question which may be more certainly determined by observation than by any experiment which can be made. That they are very readily influenced by changes in the condition of the nervous system, is universally admitted; and it is the intimacy of this connection which has given rise to the idea of a relation of dejjendence, and which prevents that idea from being disproved. In order to cut off all nervous communication from any portion of the organism — a gland for example, — so violent an operation is required (involving no less than the complete division of the blood-vessels, on which a plexus of ganglionic nerves is minutely distributed), that it is impossible to say that the dis- turbance of the function may not be owing to the shock produced on the general system. Observation, however, shows us that these processes are performed in the most complex and elaborate manner by vegetables, in which all the attempts that have been made to prove the existence of a nervous system have signally failed, (these attempts seeming to have been only excited by an indisposition to admit the possibility of any vital actions being independent of "nervous influence"); — ^that the lowest animals appear equally destitute of a nervous apparatus destined to influ- ence them; — and that in the higher classes we find such an apparatus developed, just in proportion as the necessity arises, from the complication and specialisation of the organic functions, for their being harmonised and kept in sympathy with each other and with the conditions of the animal system, by some mode of communication more certain and direct than that afforded by the circulating apparatus, which is their only bond of union in plants.* 223. The Absorption of alimentary materials is the first in the train of Vital operations, and is common to Plants and Animals, although per- formed under somewhat different conditions in the two Kingdoms. The plant derives its support immediately from the surrounding elements; it is fixed in the spot where its germ was cast; and it neither possesses a will to move in search of food, or any locomotive organs for so doing. By the peculiar structure of its roots, however, it is endowed with some power of obtaining aliment not immediately within its reach (§ 249). The animal possesses a recipient cavity, in which its food, consisting of matter pre- viously organised, undergoes a certain preparation or digestion, before it is taken up by the absorbents, which are distributed on its sides. The * This, it may be safely affirmed, is all that has yet been proved of the functions of the sympathetic or ganglionic system of nerves; and any hypothesis which presumes further, must be regai'ded as unphilosophical, because unnecessary to explain facts. The ontts probandi certainly rests with those who maintain what is contrary to the important analogy just adduced, and not with those who frame their opinions in harmony with it. GENERAL VIEW OF THE FUNCTIONS. 185 introduction of food into this cavity, or its ingestion^ seems more and more dependent upon the animal functions, in proportion as we ascend the scale. The ciliary movements of the lower classes of animals, which produce currents of such rapidity in the water that surrounds them, and thus bring a supply of food to the entrance of the digestive cavity, are probably to be regarded as of the same involuntary character as those which exist in the higher (§110). In animals of more complex structure, the process of obtaining food requires a much greater variety of move- ments, which are evidently dependent on the muscular and nervous systems; but these may still be regarded as not involving changes of a strictly mental character. Rising still higher, however, we find the psychical endowments of the animal evidently concerned in procuring its support; and in man, where they exist in their greatest amount, the com- parative imperfection of the bodily structiu'e increases the reliance upon them (chap. v.). 224. The alimentary materials taken up by the absorbent system, are carried by the Circulation into all parts of the fabric. This movement, so evident in the highest classes of plants and animals, becomes less necessary in the lower, where the absorbent surface is in more immediate relation with the parts to be supplied with nourishment (§ 281). Besides afford- ing this continued supply, the circulating system carries the vital fluid to the organs destined to separate from it the impurities it may have con- tracted in its course, to maintain the necessary balance between its different ingredients, and to effect other changes whose nature is little understood. This function, in animals as in plants, is entirely independent of the will, and, in its usual condition, is unaccompanied with consciousness. The muscular apparatus is concerned in it only to harmonise it with the con- ditions of animal existence (§ 31); and nervous agency merely brings it into sympathy wdth other operations of the corporeal and mental systems. (chap. VI.). 225. Besides conveying to the various tissues the materials required for their renovation, and serving as the stimulus necessary to the perform- ance of their functions, the current of circulating fluid takes up, in animals especially, the particles which have discharged their duty in the structure, and which are either to be rendered again subservient to the process of nutrition, by admixture vdth alimentary matter newly absorbed, or to be separated from the general mass, and carried out of the system. This function is termed Interstitial Absorption; and it is performed, in the higher animals, by a special vascular apparatus (chap. vii.). 226. The alimentary materials first taken up by the absorbents must undergo various changes before they become part of the organised fabric. It is, in fact, in these changes that the Life of the being essentially con- sists; those already mentioned being merely subservient to them. There is much difliculty in tracing them Avith precision, either in the animal or 186 GENERAL PHYSIOLOGY. vegetable sjrstems. The first which is perceptible, is the formation of organisahle products by a new combination of the elements supplied by the food. This appears to commence, in vegetables, as soon as these elements are absorbed; and the same may probably be said mth regard to animals, though the preparatory process of digestion seems to partake in it. The organisation of the combinations thus formed, appears to commence whilst they are still diflFased through the circulating fluid; and vital properties are probably simultaneously communicated to them. The elaborated sap of plants, and the chyle and blood of animals, contain these organisahle products in abundance, but not in a state of mere admixture; and the existence of regular globules in them resulting from incipient organisation, appears to be a characteristic of nutritious fluids. From these materials the individual tissues of the fabric are created and renewed, by the process of Nutrition; each deriving from the blood the portion Avhich its composition requires. This process is influenced, through the nervous system, by conditions of the mind or of the general fabric; but it does not seem to depend upon that system for its mainten- ance (chap. VIII.). 227. In order to preserve the circulating fluid in the state required for the due performance of its important functions, means are provided for separating and carrying out of the system whatever may be superfluous or injurious in its constituent parts; as well as for elaborating from it certain fluids having a destined use in the economy. These changes may be comprehended in the general term Secretion^ the former constituting the function of Excretion. This is one no less important to the health of the system than the absorption of aliment; and in proportion to the com- plexity of the structure, we find a multiplication of the excreting organs, as well as a variety in their products. The loss of fluid by Exhalation^ and of superfluous carbon by Respiration* are constant, however, in all living beings. The evolution of heat and light appear to result, where they occur, fi-om a peculiar modification of this function; and that of electricity depends upon similar changes. These changes seem to have no more immediate dependence upon the nervous system than that of nutrition; and they will take place, to a certain extent, after the final extinction of the animal powers. They are, however, most intimately dependent upon the maintenance of the circulation, and soon come to an end if that ceases; but it is probable that, in particular cases, they are kept up by the capillary circulation, after the action of the heart is extinct (chap. ix. — xii.). 228. The essential difiference between the function of Reproduction and that of Nutrition consists in this, — that in the latter case the alimentary materials are appropriated in renovating the structures of the * Reasons will hereafter be given ($ 369) for regarding this function as a branch or subdivi- sion of that of Excretion. GENERAL VIEW OF THE FUNCTIONS. 187 indimdual, — whilst in the former they are applied to the production of a new structui-e, Avhich is for some time a part of the parent heing, hut which subsequently becomes a 7iew individual. In the lowest classes of Plants and Animals we often find these two functions completely blended together (§ 513)> In many of the inferior tribes of each kingdom, a single organ only is necessary for the production and maturation of the germ; and the process then goes on as regularly and uninterruptedly as any of the nutritive functions. Even Avhere two distinct kinds of organs are necessary, it may proceed "ndthout any interference of will or excite- ment of consciousness on the part of the individual; for these may be united in the same being, as in hermaphrodite flowers; or, if separated, as in monoecious species, their functions may be made to concur by external influences.* There are some animals in which the two classes of organs are united in the same individual; and the actions necessary to bring them in relation are probably of a purely instinctive character, like those which are designed to bring food to the digestive organs. But in the higher classes, where the organs exist in separate individuals, the will, excited by a powerfully-stimulating propensity, is evidently the instrument by which they are brought into relation with one another; and in man, where this propensity is connected Avith a nobler and purer passion, not only the will, but the highest powers of the intellect are put in action to gratify it. But even here, the essential part of the function is as completely independent of mental influence as in the plant or simplest animal (chap, xiii., xiv.). 229. The function of Muscular Contraction^ to Avhich nearly all the sensible motions of the higher animals are due, is one which has an important connection Avith almost every one of the vital operations; although, as already explained, this connection is mostly of an indirect character. The property of contractility on the application of a stimulus, is not, however, confined to animals; since it is possessed by many of the vegetable tissues, and has an important relation with their nutritive processes. In the lowest animals, also, it seems generally diffused through the system; and appears to be, in like manner, generally excited by external stimuli. But in the higher classes, it is concentrated in a special texture, and is called into operation by a new stimulus, the nervous power, which originates in the individual itself. By this means it is brought under subordination to the will, and is made the instrument of changing the relations between the bodily system and the external world (chap. XV.). 230. The functions of the nervous system are twofold. First, to bring the conscious mind, (using that term in its most extended sense, to denote the psychical endowments of animals in general,) into relation * Thus, the pollen of one flower is often conveyed to the stigma of another at some distance, by the agency of the wind, insects, &c. 188 GENERAL PHYSIOLOGY. witli the external world; by informing it, through the medium of the organs of sensation, of the changes which material objects undergo; and by enabling it to act, by other appropriate organs, upon various beings, animate and inanimate. And also, to connect and harmonise dijfferent actions in the same individual, without necessarily exciting any mental operation. But, in the words of a profound -writer on this subject, "mental acts, and bodily changes connected with them, are not merely superadded to the organic life of animals, but are intimately connected or interwoven with it; forming in the adult state of all but the very lowest animals, part of the conditions necessary to the maintenance of the quantity, and of the vital qualities, of the nourishing fluid on Avhich all the organic life is dependent."* "The nervous system," it is elsewhere remarked by the same author, "lives and grows within an animal, as a parasitic plant does in a vegetable; with its life and growth, certain sensations and mental acts, varying in the different classes of animals, are connected by nature in a manner altogether inscrutable to man; but the objects of the existence of animals require that these mental acts should exert a powerful controlling influence over all the textures and organs composing an animal." It will hereafter be shown that this influence is exerted just in proportion to the development of the nervous system; and that, whilst in the lowest animals the functions of organic life are per- formed nearly under the same conditions as in plants, they are, in the highest classes, dependent for their maintenance upon its operations (chap. XVI.). The remainder of this volume wiU be devoted to the examination of these functions in detail, constituting the department of Special Physiology. * Alison's Physiology. Supplement, p. 3. BOOK IL SPECIAL AIN^D COMPAEATIVE PHYSIOLOGY. CHAPTER V. INGESTION AND ABSORPTION OF ALIMENT. General Considerations. 231. It has been stated that the peculiar characteristic of living beings in general, is the power which each possesses of maintaining, for a certain period, its regular external form and internal structure, in defiance of the common physical properties of its component parts; whilst, at the same time, all these parts are undergoing, in greater or less degree, alterations both in composition and conformation. If Ave consider the development of any one germ, from the time of its first quitting the parental system to its final decay, we observe that it is not so much the structure itself which is furnished by the parent (as from a superficial view of the reproductive function in the higher animals we might be led to suppose), as the capability of forming that structure, by the conversion of the materials supplied by the external Avorld into organised tissues endowed Avith pecu- liar and diversified properties, — which process is termed assimilation. These materials, then, constitute the aliment necessary for the first devel- opment of the living system; and in proportion to the activity of its operations Avill be the occasion for their supply. Thus, the larvse of the flesh-fly produced from the eggs laid in can-ion are said to increase in weight 200 times in the course of twenty-four hours; and their voracity is consequently so great, that it was maintained by Linn^us that three individuals and their immediate progeny (each female giving birth to at least 20,000 young, and a few days sufficing for the production of a third generation,) would devour the carcase of a horse AAith greater celerity than a lion (see also § 87). A still more extraordinary instance of rapid growth 190 SPECIAL AND COMPARATIVE PHYSIOLOGY. is found in the vegetable Kingdom. The Bovista giganteum, a large Fungus of the puff-ball tribe, has been known to increase in one night from the size of a mere point to that of a huge gourd, estimated to contain 47,000,000,000 cellules. 232. But the supply of aliment is not required only for the original development of the organism, but for the continued maintenance of the perfect structure and properties of its various parts. The tendency to decomposition exists not only in dead animal or vegetable matter, but in the living tissues; and, as it has been already stated (§ 18), it does not seem improbable that the peculiar influence of Yitality is not so much exercised in resisting that tendency (as some have supposed), as in pro- viding for its effects by the removal or deportation of all particles in a state of incipient decay. It seems at any rate certain, that the process of interstitial absorption, by which these particles are carried off, is performed in each tissue with an activity proportioned to its tendency to spontaneous decomposition; and it is very evident that the supply of new alimentary materials must be at least equal in quantity and regularity. This may, therefore, be regarded as the principal source of the continued demand for nutriment in the adult system. 233. Since living beings have not only to maintain their existence in the midst of favourable external conditions, but are liable to the occasional infliction of disease and to damages resulting from mechanical violence, they would be irrecoverably injured by such attacks, were it not that, in the nutritive system, the means are provided for repairing their effects. The regeneration of various organs and tissues, after what appeared their total destruction, is a process no less remarkable than their first formation, and no less evidently displays the foresight of the original Designer. In many of the harder parts both of animals and vegetables, Avhich exhibit so little tendency to spontaneous decay as to be capable of preservation after death to an almost indefinite period, the absorption of old particles and the deposition of new take place so slowly in the natural condition as to be scarcely perceptible; but when disease or injury calls the actions of repa- ration into play, they are effected with a rapidity and certainty not sm- passed in any other parts of the system. The other demands upon the nutritive functions which arise out of the conditions of the organised structiire are principally connected with its physical relations. Thus, where the external tegument is dense, and does not possess within itself the power of adaptation to the altered form of the body, it must be thrown off and replaced by a new one, as we see in the Crustacea (§ 84). Again, the constant movements of the body necessarily produce a waste or wearing away of the materials both of its harder and softer structures, just as in any piece of mechanism; hence arises one cause of the increased demand for nutriment which is pro- duced by continued muscular exertion. This general statement will ABSORPTION. GENERAL CONSIDERATIONS. 191 suffice to sIlow the connection between the activity of the different vital processes, and the dependence of the being upon the supply of new materials for its structures ; since upon the perfection of these structures it relies for the performance of all its vital actions. 234. Amongst the general differences between the Animal and Vege- table Kingdoms, none are more striking than those existing between the aliments on which they are respectively supported, and the mode of their ingestion or introduction into the system. The essential nutriment of plants appears to be supplied by the inorganic world, and to consist of water, Avith certain saline impregnations, and carbon. The water is derived partly from the fluid which percolates the soil, and partly from the moisture of the atmosphere. The carbon is principally obtained from the carbonic acid of the air; but most plants require for their healthy growth that it shall be introduced by the roots also. In all soils of mode- rate richness there exists a large quantity of the remains of organised structures, the upper layer of which is constantly undergoing some degree of decomposition by contact with the atmosphere, and carbonic acid is formed in it. The water which traverses such a soil, therefore, will become charged with this gas, just as when it flows fi-om a spring in which a similar disengagement has taken place from other causes; and this state of solution appears to be that in which carbon may be most advantageously introduced into the vegetable system. It does not seem probable that the organic matter which rich soils contain is itself applied to the nutrition of the plant without this previous decomposition; for it is found that those which afford the most steady and equable supply of carbonic acid are the most favourable to vegetable growth; and that whilst it is sometimes necessary to retard the decomposition of animal manures (Avhich may disengage carbonic acid so rapidly as to gorge the plants) by the addition of charcoal, it is advantageous to excite fermentation in others by the addition of yeast. 235. The opinion that carbon and water constitute the essential food of plants is confirmed by the fact that many, even of the more highly organised species, will grow in circumstances where no other kind of nutriment is accessible to them; and no one is ignorant that the simpler forms of Lichens will appear on barren rocks in the midst of the ocean, increasing by absorption from the atmosphere alone, and preparing by their decomposition a nidus for the reception of the germs of higher orders of vegetation. The only class of plants which seems to be dependent for its support upon matter already organised, is that of Fungi, — a group of peculiar interest, whether we regard the rapidity and luxirriance of their growth, the varied forms they assume, the importance of the offices they perform, or the universalitj^ of their diffusion, either as actively vegetating plants, or as dormant germs ready to be developed upon every oppor- tunity. Like insects, they have been denominated the " scavengers 192 SPECIAL AND COMPARATIVE PHYSIOLOGY. of nature," from their utility in removing the noxious products of decofn- position; and they present us with two curious analogies with the animal kingdom, hoth resulting, no douht, from the nature of their aliment. The large quantity of carbonic acid with which their absorbent system furnishes them, prevents the necessity of their deriving any addi- tional supply of it from the atmosphere; but on the contrary, like animals, they have only to get rid of what is superfluous. The pro- portion of nitrogen contained in their tissues is much greater than in any other vegetable; so that fungin, a proximate principle which may be obtained from them, is as highly azotized as animal flesh. In the subsequent details relative to the structure and functions of the nutritive system of plants, the mode in which carbon is assimilated from the atmo- sphere will be described under the head of Respiration (although not properly a part of that function), since the conditions of vegetable growth require that the portion of the structure exposed to the air should perform this office, as well as that more strictly appertaining to it, — the aeration of the circulating fluid. Some curious superadded organs occasionally met with, which seem to shadow forth the stomach and digestive system generally regarded as peculiar to animals, will be presently described (§ 239). 236. Vegetables, then, seem to constitute the intermediate link in the scale of creation, between the inorganic world and the animal kingdom; and although in a few instances they are partially dependent upon the latter for their existence, it cannot be doubted that the general balance is greatly in favour of the supplies they afford. To furnish these supplies would indeed appear to be the great purpose of their being: for, as Dr. Roget has well observed, "the only final cau.se which we can assign for the series of phenomena constituting the nutritive functions of vegetables, is the formation of certain organic products calculated to afford suste- nance to a higher order of beings. The animal kingdom is altogether dependent for its support, and even existence, upon the vegetable world. The materials of animal nutrition must, in all cases, have previously been combined in a peculiar mode, Avhich the powers of organisation alone can effect." 237. It is a general law of vitality that the materials of nutrition can only be introduced into the living system in the fluid state; and although the ingestion of solid aliment by the higher animals might seem to contra- dict such a principle, a little examination into the character of their nutritive organs will show that they are framed in conformity with it. In addition to the absorbent system with which plants are furnished, and which is the medium of communication between the organic life and the external world, nearly all animals are provided with cavities for the recep- tion of their food, and for its reduction to a state fit to enter the vessels. The necessity for these cavities arises out of the nature of the aliment ABSORPTION. GENERAL CONSIDERATIONS. 193 required by animals, wliicli usually pre-exists in a form more or less solid; and also from the intervals whicli occur betAveen the periods at which it is obtained. Whilst the roots of vegetables are fixed in the soil, and ramify through it in pursuit of their nutriment, animals, whose locomotive poAvers are necessary for the search after the food they require, may be said to carry their soil about \Adth them; for their absorbents are distri- buted on the walls of the digestive cavity, just as those of plants are externally prolonged into the earth. This caAdty is in all instances formed by a reflexion of the external surface, of Avhich the hydra (§ 115) may be regarded as presenting us with the simplest example. It is merely a bag with one opening, which may be regarded as all stomach. A higher form is that in which the caAdty has two orifices, and thus becomes a canal, such as is found in many of the infusorial animalcules; and all the complicated intestinal apparatus of the higher animals may be considered as a more extended deA'elopment of this simple type. The food Avhich is intro- duced into it is acted upon mechanically by the motion of the walls, and chemically by the secretions poured from the surface; so that the nutritious parts of it are separated from that which may be rejected, and are reduced to a fluid form. 238. That the process of digestion is really of no higher character than this, and that it has nothing to do AAdth organising or vitalising the materials submitted to it, appears from a priori considerations, and from experiment also. The substances contained in the intestinal canal are usually as much exterior to the system, as if they were placed in contact with the skin; for Ave cannot regard them as introduced into it until they have been absorbed; and, up to this period, they hold precisely the same relation to the lacteal vessels, as the fluid which has percolated the soil bears to the roots of the plants Avhich ramify through it. The experiments Avhich have recently been performed on artificial digestion have precisely the same bearing (§ 258, 261). 239. Although the possession of a digestive cavity has been regarded by some physiologists as a prominent characteristic of animals, so that it has been gravely proposed to define an animal as " un estomac servi par des organes" it is by no means a universal endoAvment ; for although there is no doubt that many of the minuter beings, Avhich Avere formerly supposed to be destitute of such an organ, are really possessed of it in no very simple form, there is no less doubt that many others, during a part of their existence at least, are nourished by absorption from the exterior surface alone (§ 280). The difficulty of establishing a Avell-defined limit between the animal and vegetable kingdoms, upon a distinction of this kind, is increased by the fact that among plants Ave find many adapta- 194 SPECIAL AND COMPARATIVE PHYSIOLOGY. tions of structure for tlie reception and preservation of aliment ; some of wliicli it would not be easy to exclude from any definition Ave might frame of a stomach. Concavities in different parts of the surface, fitted for the collection of the moisture caught from rain or condensed from dew, may frequently be observed ; and these vary in the completeness of their structure, from the simple hollow formed in the leaf of the Tillandsia (wild pine of the tropics), or of the Dipsacus (teasel), to the extraordinary ascidia of the pitcher-plants. The exact method in which the fluid thus obtained is applied to the nutrition of the plant is not always evident. Sometimes the channelled leaves seem to convey it to the roots, by which it is absorbed in the usual manner. The function of the pitcher of the Nepenthes (Chinese pitcher-plant) has not been certainly determined; as it is difficult to ascertain how much of the fluid which it contains is col- lected from the atmosphere by the downy hairs that line its interior, and how much is secreted by the plant itself. The object of the pitchers of the Disckidia (Fig. 102) is, however, less doubtful, and their structure far more complicated. This curious plant grows by a long creeping stalk, Avhich is bare of leaves until near its summit; and as, in a dry tropical atmosphere, the buds at the top would have great difficulty in obtaining moisture through the stem, a sufficient supply is provided by the pitchers, which store up the fluid collected from the occasional rains. " The cavity of the bag," says Dr. Wallich,* " is narrow, and always contains a dense tuft of radicles, which are produced from the nearest part of the branch, or even from the stalk on which the bag is suspended, and Avhich enter through the inlet by one or two common bundles. The bags generally contain a great quantity of small and harmless black ants, most of Avhich find a Avatery grave in the turbid fluid which frequently half fills the caAdty, and AA^hich seems to be entirely derived from AAith- out." The earth has been justly spoken of as the common stomach of A^egetables, supplying them Avith nutriment ready to be taken up by their absorbent system ; in this curious plant the failure of its regular means of support has called forth the addition of an organ, Avhich, like the stomach of animals, serves as a receptacle for the' supplies it may occa- sionally obtain. According to Mr. Burnett,t in the pitcher of the Sar- racenia a process still more like that of animal digestion goes on; for it appears that the fluid it contains is very attractive to insects, Avhich, having reached its surface, are prevented from returning by the direction of the long bristles that line the cavity. The bodies of those Avhich are droAvned seem, in decaying, to afford a supply of nutriment as favourable to the groAvth of the plant, as a similar process on the leaves of the Avell- known Dionwa muscipula (Venus' fly trap), — to the health of which, a supply of animal food appears to be essential. 240. Although such instances as these may seem to contradict the * Plantae Asiaticoe rariores, vol. ii. p. 35. t Brande's Journal, vol.vi. ABSORPTION. GENERAL CONSIDERATIONS. 1 95 general statement, that plants derive the materials of their nutrition from the inorganic world, yet the}'- probably do so more in appearance than in reality. In all cases where previously organised matter influences their growth, it is only whilst in a decomposing state, during Avhich it is sepa- rated into its ultimate elements or very simple combinations of them (§ 234). In animal digestion, on the contrary, the proximate principles contained in the food appear to be immediately subservient to the form- ation of others of a higher order; and whatever tendency to disunion its elements might have previously manifested, this is immediately checked by the antiseptic qualities of the gastric fluid. We find in the animal kingdom, also, many apparent exceptions to the general statement which has been made respecting the source of their nutrition ; for it often ap- pears as if they derived their support in part, at least, from the inorganic world. Thus, the Spatangus (§ 106) fills its stomach Avith sand, but really drives its nutriment from the minute animals contained in it. The earth-worm and some kinds of beetles are known to swallow earth, but only to obtain from it the remains of organised matter Avhich are mixed with it. In fact, the inorganic matter thus taken into the stomachs of these animals no more contributes to their nutrition, than the gravel swallowed by graminivorous birds, or the chalk eaten by a hen preparing to lay.* 241. The particular articles, which constitute the food of the different races of animals are as various as the races themselves. Some tribes in almost every division of this kingdom are maintained solely by vegetable food; and wherever plants exist, we find animals adapted to make use of the nutritious products Avhich they furnish, and to restrain their luxuriance within due limits. Thus, the Dugong browses upon the submarine herb- age of the tropics; whilst the Hippopotamus roots up with his tusk the plants growing in the beds of the African rivers; the Giraffe is enabled by his enormous height to feed upon the tender shoots Avhicli are above the reach of ordinary quadrupeds; the Rein-deer subsists during a large part of the year upon a lichen buried beneath the snoAv; and the Chamois finds a sufl&cient supply in the scanty vegetation of Alpine heights. Many species of animals, especially among the Insect tribes, are restricted to par- ticular plants; and, if these fail, the race may for a time disaj)pear. But there is probably not a species of plants which does not furnish nutriment for one or more tribes of insects, either in their larva state or perfect con- dition, by which it is prevented from multiplying to the exclusion of * Among' the human race some savage nations are in the habit of introducing: larg'e quanti- ties of earthy matter with their food ; and this sometimes throug'h ig-norant prejudice, but more frequently to give bulkiness to the aliment, so that the stomach may be distended, — as among the Kamschatdales who mix saw-dust or earth with their train oil. This example has been followed in civilized countries in times of scarcity ; thus in the year 1832 a famine in Degernii, on the borders of Lapland, occasioned the flour and bark of trees, of which the bread was made, to be mixed up with siliceous earth, (> 2 196 SPECIAL AND COMPARATIA^B PHYSIOLOGY. others. Thus, on the oak not less than 200 kinds of caterpillars have been estimated to feed; and the nettle, which scarcely any beast will touch, supports fifty different species of insects, but for which check it would soon annihilate all the plants in its neighbourhood. The habits and economy of the different races existing on the same plant are as various as their structure. Some feed only upon the outside of the leaves; some upon the internal tissue; others upon the flower or on the fruit; a few will eat nothing but the bark; while many derive their nourishment only from the woody substance of the trunk. It is very curious to observe that many plants injurious to man afford wholesome nutriment to other animals; thus, Henbane, Nightshade, Water Hemlock, and other species of a highly poisonous character, are eaten greedily by different races of quadrupeds. Some cattle, again, will reject particular plants upon which others feed with impunity. 242. Every class of the animal kingdom has its carnivorous tribes also, adapted to restrain the too rapid increase of the vegetable-feeders, by which a scarcity of their food would soon be created, — or to remove from the earth the decomposing bodies which might otherwise be a source of disease or annoyance. The necessity of this limitation becomes evident if we consider the rapid multiplication which the prolific tendency of the herbivorous races would speedily create, until checked by the famine that would necessarily result from their inordinate increase. Thus, the my- riads of insects which find their subsistence in our forest trees, if allowed to increase without restraint, would soon destroy the life that supports them, and must then all perish together; but another tribe (that of the insectivorous birds, as the Woodpecker,) is adapted to derive its subsist- ence from them, and thus to keep within salutary bounds the number of these voracious little beings. A very curious instance of the nature of the checks and counter-checks, by Avhich the "balance of power" is main- tained amongst the different races, is mentioned by Wilcke, a Swedish naturalist. A particular species of Moth, the Phalcena strohilella has the fir cone assigned to it for the deposition of its eggs; the young cater- pillars, coming out of the shell, consume the cone and superfluous seed; but, lest the destruction should be too great, the Ichneumon strohilella lays its eggs in the caterpillar, inserting its long tail in the openings of the cone until it touches the included insect, for its body is too large to enter. Thus it fixes its minute egg upon the caterpillar, which when hatched destroys it.* 243. It has been said that all alimentary matter, in order to be in- troduced into the living system, must be presented to it in a fluid form; * The Chapter on the " Economy of Nutritive Matter" in Dr. Roget's Bridgwater Treatise, and those of Mr. Lyell's Principles of Geology, on the " Equilibrium of Species," may be referred to for a more extended view of this very interesting- subject than the limits of the pre- sent work will permit. ABSORPTION. GENERAL CONSIDERATIONS. 197 and that the reduction of it to that form is one object of the digestive processes of animals. The changes involved in its passage through the external integument, or that modification of it specially adapted for the purpose, constitute the function of Absorption. Before considering the particular conditions under which it is performed in the different classes of organised beings, it will be right to enquire what is its essential cha- racter, and how far physical laws may be applied to its elucidation. It was formerly the general opinion that Absorption is always effected by vessels, the open mouths of which, being in contact with the fluid, might imbibe it by capillary attraction, suction, or some other means. But anatomical enquiry has sho^vn that in no one instance are absorbent vessels thus brought into immediate relation with the fluid to be received by them; but that the transmission always takes place through some tissue of a membranous character. Thus, we shall find that the skin of the higher animals, and the cuticle covering the general surface in plants, participate more or less in the function of Absorption, even where a special system is adapted to its performance ; that in the inferior tribes, the external integument (with the reflexion of it which lines the digestive cavity in animals) is its sole medium ; and that neither in the roots of plants, nor in the walls of the intestinal canal in animals, do the vessels terminate in open mouths, all the fluid which enters them having to traverse the tissue by which they are closed. The notion that mere ca- pillary attraction has anything to do with the absorption of fluid into living systems is therefore completely untenable; but there is a remark- able phenomenon to which the term of Endosmose has been given by its discoverer Dutrochet, which, occurring under conditions supplied by in- organic materials alone, bears so strong a resemblance to this vital func- tion that it is scarcely possible to disbelieve its partial concern in it. The following is a general statement of the phenomena in question. 244. If into a tube, closed at one end with a piece of bladder or other membrane, be put a solution of gum or sugar, and the closed end be immersed in water, a passage of fluid wdll take place from the exterior to the interior of the tube through the membranous septum; so that the quantity of the contained solution will be greatly increased, its strength being proportionably diminished. At the same time, there mil be a counter-current in the opposite direction; a portion of the gummy or saccharine solution passing through the membrane to mingle -with the exterior fluid, but in much less quantity. The first current is termed endosmose, and the counter-current exosmose. The increase on either side will of course be due to the relative velocity of the curi-ents; and the changes will continue until the densities of the two fluids are so nearly alike as to be incapable of maintaining it. The greater the original dif- ference (provided that the denser fluid be not actually viscid, but be capable of mixing with the other), the more rapidly and powerfully Mill 198 SPECIAL AND COMPARATIVE PHYSIOLOGY. the process be performed. The best means of experimenting upon these phenomena is aflForded by a tube narrow above, but widely dilated below, so as to afford a large surface to the membrane, compared with that of the superincumbent column, which mil then increase in height with great rapidity. By bending this tube into the form of a syphon, and introducing into its curve a quantity of mercury, the force as well as the rapidity of the endosmose between different fluids may be estimated Avith precision. Although it is not universally true that the activity of the process depends upon the difference in density of the two fluids (for in one or two cases the stronger current passes from the denser to the lighter), it seems to be so with regard to particular solutions, as those of gummy or saccharine matter. No endosmose takes place between fluids which will not mingle, such as oil and Avater ; and very little between such as act chemically on each other. Although an organic membrane forms the best septum, yet it has been found that thin laminae of baked pipe clay Avill suffice for the evident production of the phenomenon ; and that porous limestones possess the same property in an inferior degree. It is evident, then, that hoAvever obscure may be the nature of the pro- cess, and however difficult it may be to explain it on physical laAvs, these alone are concerned in it.* 245. It may reasonably be enquired hoAv far the passage of fluid through membranes or tissues in the living body may be explained on this principle. It has been maintained that this is a purely Adtal change, because it does not occur except during the continuance of life. But it may be alleged, on the other side, that if Ave regard the other vital actions as furnishing the conditions of endosmose, the absorption of fluid may itself be considered as only an instance of the phenomenon. That this is the case in the Vegetable kingdom, subsequent details will show, and there will be no difficulty, therefore, in understanding why the process should cease Avith life : the function of Absorption in Animals cannot, however, be so conveniently studied, and its true character has not yet been satisfactorily elucidated. Still there seems much reason to believe that it is here, also, due to physical laws acting under conditions supplied by the living system • for transudation readily takes place through dead as Avell as living animal membranes, even where these, instead of forming a distinct septum, as in the production of Endosmose, are in contact A^dth the tissues on the other side. Thus, Lebkiichner found that oil of tur- pentine and camphor placed on the skin of a rabbit, 1 2 hours after death, communicated, in the space of 10 hours, their peculiar odours to a paper placed on the internal surface of that membrane. A solution of prussiate * For further information on this curious subject, see the Article Endosmosis in the Cyclo- paedia of Anatomy and Physiology, and Dutrochet's Memoires, Anatomiques et Physiolo- g'iques, torn. i. ABSORPTION IN VEGETABLES. 1.99 of potass penetrated in 5 lioui-s; sulphuric acid in 6 houi's; and acetic acid in 24 hours. Ink, and a solution of muriate of soda, had not passed in 24 hours ; and a solution of ammoniuret of copper required two days for its transit.* These experiments go far to confirm the view, which will be hereafter stated (§ 250), that what has been termed the selecting lyotcer of absorbent surfaces, by which they take up some fluids (saline solutions for instance) and reject others, is not due so much to their pecu- liar vital properties, as to the physical relations between their tissues and the substances brought into contact with them. Again, Magendie im- mersed the amputated paw of a rabbit in ink, and the cellular membrane became coloured. He formed a bag from a piece of human skin, the epidermis being internal, and then filled it with water; — transudation took place rapidly: but when the experiment was reversed, and the epidermis placed externally, it became raised into a blister; thus showdng that, from some physical causes, the passage of fluids takes place through it much more readily in the internal than in the external direction, t It is very easy to explain on this theory why absorption should take place so much more rapidly and energetically during life than after death; since the quantity of fluid which first penetrates the membrane is con- veyed no further into the system, unless there is a demand for it; and it therefore saturates the tissues with which it is in contact, and prevents the admission of more. But when the fluid so absorbed is constantly being drawn ofi" for the purposes of the economy, a continual demand for a renewed supply is created, and thus the action becomes one of the most regular of those subservient to Life. Absorption in Vegetables. 246. In the lowest orders of Plants we find this function performed under its most simple conditions. The division of Aphyllous (leafless) Cryptogamia, including the Algae, Lichens, and Fungi, presents a remarkable similarity of internal structure, concealed under great diver- sity of external form. The substance of all is composed of vesicles more or less firmly united to each other, and but slightly altered from their original spheroidal form; and the envelope which surrounds them can seldom be regarded as a distinct structure, as it generally difi"ers but little from the remainder of the cellular tissue. The simplest forms of alg^e, such as the Protococcus nivalis (Fig. 59), consist of individual cellules, each of which seems to be in itself capable of nutrition and reproduction ; but in the higher genera, the plant is composed of a mass of such cellules united together, sometimes in single rows, as in the (Jonfervce (Fig. 61), sometimes in a more definite and expanded form, as in the Sea-weeds in * Madden on Cutaneous Absorption, p. 33. t The first Volume of the "Lemons sur les Pheuomenes Pliysiques de la \'ie," contain? a great mass of evidence of the same character. 200 SPECIAI/ AND COMPARATIVE PHYSIOLOGY. general. In all of these, hoAvever, the whole surface appears to be endowed with the power of absorption to nearly an equal degi'ee; and although the semblance of a stem and roots occasionally presents itself, yet these seem to have no other function than to give the means of attachment to the leaf-like expansion, which performs not only the nutritive but the reproductive function (§ 520, 521) on all parts of its surface. In the lichens, there is altogether a great similarity of form and structure to the Algse; but the difference in their locality appears to produce a separate appropriation of portions of the surface to the nutritive and reproductive functions. The upper surface of these plants, being exposed to the sun and air, becomes hard and dry, a condition which seems to favour the evolution of the fruit; whilst it is by the lower surface, which is usually soft and pale, that the nutriment is probably introduced into the system. The latter is not unfrequently fui-nished with hair-like appendages, which may be regarded as prolongations of its surface; and these not only serve to fix the plant, but appear to be much concerned in the absorption of its aliment, being so much developed in some Lichens which are located upon the ground as almost to resemble roots. In the fungi we find a still smaller portion of the general surface adapted to absorb fluid, and more especial prolongations of it for the purpose. The lower forms of this group (§ 64), however, seem to imbibe their aliment by their Avhole surface; but in the more complex structures, the reproductive system is separated from the nutritive by the intervention of a stalk (as in the Mushroom), whose base is prolonged into hairs or radical fibres, by which the decajdng matter, that constitutes the food of this remarkable group of plants, is introduced into the system. In some species, too, the whole surface is covered vtith hair, which may assist their very rapid development by absorption of fluid from the atmosphere. 247. In the biosses and their allies we find a somewhat higher form of the same structure. From the base of the stem there usually proceed slender radical filaments, which sometimes ramify through the soil to a considerable extent; and other similar filaments are frequently deve- loped from the sides of the stalk, and from the lower surface of the leaves. In Mosses that exist on rocks, however, these filaments are but little developed, and appear to serve rather for support than for absorp- tion of nourishment, which must in such circumstances be derived from the atmosphere through the leaves. It is well known that these are very permeable to fluid, and that Mosses will thus recover the appearance of life after being long dried; from the same cause, these beautiful little plants are enabled to vegetate rapidly during a moist season, whilst their tenacity of life enables them to withstand a subsequent drought. In ascending through these tribes of the Cryptogamia, then, we may trace a gradual development of separate absorbent organs, and may observe the specialisation of the function, by its restriction to one particular part of the ABSORPTION IN VEGETABLES. 201 surface, instead of being diffused over the whole. Still, however, we find that when these special organs are not developed, or are insufficiently supplied mth nutriment, the general surface can take on its original function and thus supply the deficiency. 248. It is probable that in all the Cryptogamia, except the Ferns (which, possessing a vascular structure, seem to resemble flowering plants in the essential conditions of their nutrition,) the Avhole surface of the radical fibre is endowed >vith the power of absorption; in the Phanero- GAMiA, however, it seems to be through the newly-formed, succulent extremities alone that fluid is admitted; and the function is, of course, more actively performed by them in proportion to the diminution in the amount of surface they expose. The root presents a great variety of forms in different plants ; there are, however, some parts which are essen- tial, and others merely accessory. The simplest form, as well as the most essential part, consists of single fibres; these occasionally exist alone (as at the base of the hyacinth bulb), but more often proceed from ramifying branches of woody texture (as in most trees and shrubs), or from tubers (as that of the turnip). Each fibre appears to differ from those just mentioned as existing in cellular plants, by the possession of a bundle of vessels which occupies its centre: and the extremities of these tubes are covered with loosely formed cellular tissue, through which it appears that fluid passes into them. This structure is well seen in the radical fibre of Lemna or duckweed (Fig. 76). The spongiole, as this point has been termed, is sometimes spoken of as a distinct organ; but it is nothing more than the growing point of the root, which, with a few exceptions, lengthens only by additions to its extremity. The soft lax texture of the newly-formed part, causes it to possess in an eminent degree the power of absorption; but as the fibre continues to grow, and additional tissue is formed at its extremity, that which was formerly the spongiole becomes consolidated into the general structure of the root, and loses almost entirely its peculiar properties. That it is to the spongioles that the principal absorbing power of the root is due, was fully proved by the experiment of Senebier. He fixed two roots in such a manner that the extremity of one was in contact with water, whilst of the other every part except the extremity was immersed. He found that the first root absorbed nearly as much as usual, whilst the second scarcely took up a sensible quantity. It is not improbable that the relative absorbent power of the spongioles and of the general surface of the root may vary in different plants, according to the character of the texture of each, and the situation in which it grows; but it appears to be a general fact that in vascular plants the spongioles are the organs specially destined for intro- ducing the fluid nutriment into the system. 249. There are evident limits to the supjily of alimentary materials to the roots of plants, as long as they remain in the same spot; and some change must take place to ensure its continuance. As the plant cannot 202 SPECIAL AND COMPARATIVE PHYSIOLOGY, remove itself to a new situation, its wants are provided for by the simple elongation of its radical fibres; and their extension takes place, not by increase throughout their whole length, but by addition of fresh tissue to their points. This addition, being made in the direction of least resist- ance, enables the fibrils to insinuate themselves into the firmest soil, and even to overcome the obstacle presented by solid masonry; for however narrow the crevice may be into which the filament enters, the subsequent expansion of the tissue by the infiltration of fluid is so great, as to enlarge the opening considerably, and even to rupture masses of stone. This tendency to increase in the direction of least resistance, will also evidently cause the root to grow towards a moist situation; and by keeping this in view, many of the facts regarding the so-called instinct of plants, which at first sight appear so remarkable, may be satisfactorily eplained. There are some cases, however, for which our present amount of knowledge does not enable us to account. 250. The absorbent power of the spongioles appears limited by the size of their pores, which, although hitherto undetected, must have a sen- sible diameter. If the roots be immersed in coloured solutions, they take up the most finely divided particles, leaving behind the larger molecules, which are only absorbed when the spongioles have been damaged. The pores are liable to be blocked up by fluids which are of too viscid or glu- tinous a consistence to pass readily through them; and if the roots are immersed in a thin solution of gum or sugar or neutral salts, the watery particles are absorbed in the greatest degi-ee, so that the portion which is left contains a larger proportion of the ingredient in solution. The power of selection, however, would seem to extend beyond this; since of two sub- stances equally dissolved, some plants will take one, and some the other ; and some neutral salts are rejected altogether. It does not appear that the selecting power is employed to prevent matter from being introduced into the tissue of the plant, which is capable of exerting a deleterious influence upon it; for many substances are taken up by the roots, which speedily put a stop to vital action, if opportunity is not afforded for their excretion. From the little that is at present known on the subject, it seems a reasonable inference that the rejection of any particular ingredient of the fluid in contact with the roots, results either from the want of adaptation in the form or size of its molecules to the pores of the spon- gioles; or from an organic change effected by it on their delicate tissue, such as is proved by the experiments of M. Pay en* to occur when tannin enters into the solution, even very minute proportion. 251. The quantity of fluid absorbed, and the force with which it is propelled upwards in the stem, vary not only in different species and indi- viduals, but in the same plant at different perids of the year and even of the day. The former seems intimately connected with the activity with which the other processes of vegetation are being carried on, and especially * Ann. des Sci. Nat. N. S. Botan., vol. iii. p. 5, &c. ABSORPTION IN VEGETABLES. 203 to depend upon the quantity of vapour transpired from the leaves (chap, x.); all the causes which increase transpiration may therefore he considered as stimulants to absorption also. The force of the roots in the propulsion of the sap is sufficiently proved by the celebrated experiments of Hales on the vine. By gages affixed to the stem during the "bleeding season," when the sap rises rapidly, he found that a column of mercury 26 inches high, equal to a column of water nearly 31 feet, might be supported by the propellent force of the absorbent organs; but if the upper part of the plant was cut off, this power soon diminished, and after a time ceased altogether. 252. There would seem much reason to believe that the mere act of Absorption, in this and other cases, is due to the physical property already referred to as possessed by many organised tissues, — viz. the capability of producing endosmose (§ 244). The succulent extremities of the spon- gioles serve as the medium required for this process; but it may be reasonably enquired whence the other condition is furnished, namely that difference in density of the fluids on the opposite sides of the septum, which is necessary for the commencement and continuance of the action. This is, in the first instance, supplied by the store of nutritious matter obtained by the embryo from its parent, and contained Avithin its tissues ; and, at a later period, when the plant is supporting an independent exist- ence, by the admixture of a portion of the dense elaborated or descending sap, with the crude and watery ascending fluid. If this be the true explanation of the phenomenon, a counter current ought to exist, and an exosmose of the fluids within the system should take place into the sur- roimding medium. That this is actually the case is proved by the fact that an excretion of the peculiar products of the species may be always detected around the roots of the plant (§ 454) ; a fact of very important practical applications. The cessation of this action of admixture (a change evidently depending upon other vital actions) at the death of a plant, fully accounts for the non-continuance of endosmose, which is also checked if the superincumbent column of fluid be not drawn off by the leaves. It has been very justly remarked by Professor Henslow that "if we suppose the plant capable of removing the imbibed fluid as fast as it is absorbed by the spongioles, then we may imagine the possibility of a supply being kept up by the mere hygroscopic property of the tissue; much in the same way as the capillary action of the wick in a candle maintains a constant supply of wax to the flame by which it is consumed."* 253. It is an axiom in Vegetable Physiology which has been laid down by De CandoUe "that when a particular function cannot, according to a given system of structure, be sufficiently carried into effect by the organ which is ordinarily appropriated to it, it is performed whoUj or in part by another." This is but a result of the general principle which has been * Cabinet Cyclop.xdia. Botany. Pag'e 177. 204 SPECIAL AND COMPARATIVE PHYSIOLOGY, already laid down (§ 201); and the reason that it is more evident in the Vegetable than in the Animal kingdom is simply, that in the former the specialisation of function is nowhere carried so far as in the latter; so that any part of the general surface of a plant can perform in a considerable degree the functions of all the rest. We might then a priori expect that whilst the roots are, in the usual condition of the perfect plant, the organs by which its fluid nutriment is absorbed, and the leaves its organs of transpiration and respiration, some traces of the primitive community of function enjoyed by the general surface of the simpler tribes, would be found in the capacity of each of these organs to perform in a certain degree, if required, the function of the other. Thus, it is evident that w^hen the roots are either absent or imperfect, or are implanted in an arid or barren soil, serving merely to fix the stem (as happens with many Orchidece and the generality of aerial parasites), the plant must derive its chief supply of nutriment through the absorption performed by the leaves, or in leaf-less plants (as the Cacti) through the general surface. And it must be obvious to all who have observed the manner in which plants faded by the intense action of light and heat are refreshed by the natural or artificial application of moisture, that absorption takes place, in these instances also, by the general sm-face, as well as by the roots. 254. Various experiments have been devised with the view of deter- mining the relative extent to which the plant is supplied by these two channels; but the proportion appears to depend upon the circumstances of its growth. Thus, Bonnet took some specimens of Mercurialis, and immersing the roots of part of them in water, he placed others so that only their leaves touched the fluid. A small shoot of each plant was kept from contact "odth water, and after the experiment had proceeded for five or six weeks, those which had derived all their nutriment through the leaves were nearly as vigorous as those which had imbibed it by the roots. It is by the under surface of the leaf, where the cuticle and cellular tissue beneath are least compactly arranged, that absorption is performed with the greatest rapidity; and the downy hairs "with which some plants are plentifully furnished seem to contribute to this function, acting like so many rootlets. These prolongations of the surface are usually wanting in such plants as grow in damp shady situations, where moisture already exists in abundance; but in hot, dry, exposed localities, where it is necessary that the plant should avail itself of every means of collecting its food, we find the leaves thickly set mth them; and this difference may be observed in the same species of plant according to the soil and climate in which the individuals exist, and even in the same individual if transplanted. A very curious adaptation of the leaf of the Oleander to the same purpose mil be hereafter described (§ 428). 255. In tracing the gradual evolution of the special absorbent system of the more perfect plants, we may observe many interesting relations DIGESTION AND ABSORPTION IN ANIMALS. 205 between the progressive stages of its development, and the permanent forms of the system in the lower orders. Thus, the embryo at its first appearance within the ovule (§ 52.5, 6) is nothing but a single cell, like that of the protococcus, in the midst of the store of semifluid nutriment prepared by its parent, which it gradually absorbs by its whole surface, just as do the simplest cellular plants. At the time of the ripening of the seed, we find the rudiment of the future root, Avhich is developed during germination; but in the early stages of this process, the radicle simply prolongs itself into the gi'ound, and appears to be equally capable of im- bibing moisture thaough its whole length, like that of the Fungi or Mosses. It is not until the true leaves are evolved, that the root begins to extend itself by ramification, then first protruding perfect fibrils, com- posed of woody fibre and vessels, and terminated by spongioles. 256. Thus, then, in the development of the absorbent system of vege- tables, the first Avhich Ave have been called upon to study in detail, we have perceived the application of the laws Avhich have been already enun- ciated (chap. III.); for it has been found that whether we trace its various forms through the ascending scale of the different ti'ibes of plants, or Avatch the progress of its evolution in the more perfect orders, it is constantly to be observed that the special structure and function arise by a gradual change out of one more general; and that even AA'here the special foiin is most highly developed, the general structure retains, more or less, the primitive community of function Avhich originally characterised it. Digestion and Absorption in Animals. 257. It has been already stated that the conditions under Avhich the function of Absorption is performed in animals, are so far different from those Avhich affect it in plants, that a preparatory process of Digestion becomes necessary for the reduction of the food to the fluid form required for its entrance into the system. This process is effected in cavities of the body, which are bounded by a continuation of its external surface, modi- fied, by its secreting power, to supply the means necessary for the solution of the aliment, and, by its absorbent faculty, for the selection of the part of it capable of contributing to the nutrition of the fabric. It has been already shown that so long as this aliment is unabsorbed, it cannot be regarded as introduced into the system; since it merely holds the same relation AAdth the absorbent vessels, as the nutritious fluid surrounding the roots of plants. Both are liable to be influenced by the secretions poured out from the surfaces Avith Avhich they are in contact; and though Ave have no positive CAddence that vegetables ever prepare their food by such means, its occasional employment may be infeiTcd from the fact, that the Fungi have been observed to hasten the decomposition of substances on which they have made their appearance. The chief peculiarities, then, in the preparation of the food of Animals, consist in the mechanical 206 SPECIAL AND COMPARATIVE PHYSIOLOGY. influence to which it may be subjected, by the peristaltic and masticatory actions of the alimentary canal and its appendages; and, in the tempera- ture to which, in the higher animals at least, it is subjected. With the process of absorption, strictly so called, the organisation of the consti- tuents of the alimentary fluid, and their endowment with vital properties, may be regarded as commencing in animals as well as in plants. 258. That the process of digestion has really this character, — that it is no more dependent upon the vital powers of the stomach, than as far as these are concerned in the secretion of its solvent fluid, and the main- tenance of its temperature and movements, — appears from the most recent experiments, as well as from a priori reasoning. For, as will presently be stated, the gastric juice seems to be as energetic out of the stomach as in it, provided that the other conditions, namely, the warmth and the motion, are also supplied (§ 261); and, where sudden death takes place in an healthy animal, the stomach itself is not unfrequently dissolved, if it be not distended with food on which the solvent will more readily act. This seems to be an unanswerable argument in favour of the smvjAj 2)Jiysical nature of the process of digestion; since it is absurd to suppose that any lingering vitality in the organ itself can have an in- fluence in disorganising its OAvn tissues. 259. The first act in the digestive process is the mechanical reduction of the food which has been ingested, to a state which will render it more easily afi^ected by subsequent chemical processes. This is accomplished by the acts of mastication and insalivation, for which provision is made in most of the higher classes of animals. Mastication is not always, however, performed in the mouth; for though that is the situation of the teeth in Mammalia and Reptiles, the pharynx (or funnel-like entrance to the gullet) is their seat in Fishes, and the stomach in Crustacea. A gizzard, or hard muscular stomach with cartilaginous walls, answers the purpose of mastication in graminivorous Birds, Cephalopoda, and pro- bably also in the higher Polypes (§ 117); and in the first of these classes, insalivation is performed, not in the mouth, but in the crop, a dilatation of the oesophagus in which the food is retained for this purpose. In general it will be found that the more analogous is the character of the food to that of the animal juices, the less preparation of this kind does it undergo. Thus, the carnivorous Mammalia have teeth and jaws more adapted for cutting and tearing than for mastication; whilst the her- bivorous species, which are deficient in teeth of that character, have the remainder so constructed as to present a large uneven surface for the trituration of their aliment, and jaws capable of that peculiar rotatory movement which can give most eflcct to their employment. In Birds, again, the predaceous species are destitute of any mechanical means of reducing their food to the semifluid state which vegetable substances must acquire before they can be acted on by the gastric juice; and the DIGESTION AND ABSORPTION IN ANIMALS. 207 only preparation wliicli it undergoes, is the separation of the hair, feathers, claws, and other indigestible parts, which are disgorged from the crop without being alloAved to pass through the alimentary canal, 260. There are many animals for Avhose food such preparation does not seem necessary; its soft consistence and high organisation (which increases its tendency to decomposition) rendering it easily soluble. Such are the whale, which is destitute of teeth, and whose gigantic swallow is furnished Avith an enormous filter for straining off those minute inhabi- tants of the ocean of Avhich such mjT:iads are necessary for its subsistence ; and there are many Mollusca, and even animalcules, which, in their mode of obtaining their food, as well as in the voracity of their appetites, seem like whales in miniature. Whether or not the saliva, which in most animals that masticate their food, is mixed with it dui-ing the process, has any other than a mechanical agency, is not fully ascer- tained; many have imagined that it possesses a solvent power on the organised substances through which it is diffused, superior to that which water alone would exercise; but the only fact knoA^iii on this point is that it has the property, like gastric juice, of changing starch into sugar. 261. Various experiments have been made at different times on the solvent power of the gastric juice, and on the influence of the motions of the stomach on its effects; but none are so satisfactory as those of Dr. Beaumont, who availed himself of the opportunity afforded him by the remarkable state of Alexis St. Martin (a man who, though in perfect health, had a fistulous opening in his left side, which permitted insj^ec- tion of the interior of the stomach, and the removal of its solid or fluid contents), to settle many points Avhich previous contradictory statements had left doubtful, as well as to add much to what Avas already received. The food which has been propelled dovvii the oesophagus enters the cardiac orifice of the stomach in successive waves; and there it is sub- jected to a series of operations, of Avhich chemical solution is imdoubtedly one of the most important. The gastric juice by which it is effected, is poured out of minute follicles or secreting cavities in the coats of the stomach; but some animals, Avhose food is peculiarly difficult of digestion, appear provided with a more special glandular apparatus for its elabora- tion, such as exists in the Beaver (§ 459). This fluid is secreted only when the coats of the stomach are irritated by the contact of matter in- gested by it; and it can therefore only be obtained in a pui-e state, by introducing some insoluble body which shall cause its formation, and shall also absorb it as fast as it is poured out. For this purpose a piece of sponge has been frequently employed, which has been swallowed, and when satui-ated, has been drawn up by a thread fastened to it ; but Dr. Beaumont was enabled to accomplish the same object in a more satis- factory manner by introducing an India-rubber tube through the opening, 208 SPECIAL AND COMPARATIVE PHYSIOLOGY. which served Loth to irritate the membrane by its contact, and to conduct away the fluid as fast as secreted, without admixture. The gastric juice thus obtained was found to have a reducing power but little inferior to that of the stomach itself, when its solvent action was assisted by heat and agitation ; and a homogeneous fluid, closely resembling the chyme of the alimentary canal, was produced by these means. Similar efi"ects have been obtained by an artificial gastric juice, which has been formed (by Miiller and Schwann) of a mixture of dilute acetic or muriatic acid with mucus of the stomach; the simplest way of manufacturing it being, to macerate a portion of mucous membrane in the acid. But this, although it appears effectual with many substances, is resisted by others. Neither acids nor mucus, however, will act alone; but the correspondence of their united effect, so far as it goes, with that of the gastric juice, can leave no doubt that the operation of the latter is of a chemical nature. 262. The chyme thus formed is not absorbed without further prepara- tion; and it is in the separation of the portion of it which will become subservient to nutrition, from that which is only fit for rejection, that the operation of the bile seems most important. Dr. Beaumont mentions that a mixture of biliary and pancreatic secretions (both of which he was able to procure by means of his elastic tube) with chyme, separated the latter into a turbid milky fluid, which he regarded as chyle, and a flaky precipitate, which appeared of an excrementitious character. Of the nature of the changes which the food undergoes in its progress along the intestine, we know, however, but very little. The nutritious portion is gradually taken up by the absorbent vessels, which are distributed copi- ously on the mucous lining of the tube. In the Invertebrata, it would appear that that the general vascular system performs this office, the absorbed fluid at once entering the current of the circulation; but, in higher animals, a more special provision for this purpose is observed, in the system of lacteal absorbents, which are delicate vessels distributed on the mucous surface of the intestine, and destined for this function alone. The fluid which they absorb, termed chyle (which will be described here- after § 357) is conveyed in a general receptacle, where it is mixed with the lymphatic fluid absorbed from the system at large (§ 331); and both then enter the general circulating mass. These absorbent vessels may be regarded as strictly analogous to the roots of plants. They do not open by patulous orifices on the surface of the intestine; but ramify among the villi of the mucous membrane, which are little filamentous processes of delicate structure, that give to its surface the fleecy appearance it ex- hibits when highly magnified. In Fig. 103 is seen one of these villi mth its absorbent vessel, which may be contrasted with the absorbent termina- tion of the radical fibres of plants formerly described (§ 248 and Fig. 76). It is only in the higher animals, however, that these villi exist ; in the lower tribes, the surface of the membrane is increased by its being plaited DIGESTION AND ABSORPTION IN ANIMALS. 209 into simple folds; or it may be altogether smooth, — but its extent is then proportionably greater. 263. It is curious to observe, in the progress of the food along the alimen- tary canal of higher animals, the gradual removal of it from connection with the functions of animal life. To procure it in the first instance, is one important oifice of these functions; and the highest exercise of the loco- motive, sensorial, and intellectual powers is often required for this purpose. Its introduction into the mouth is an act of pure volition in man, whilst the masticatory movements to which it is there subjected may be regarded as having been originally voluntary, but as afterwards so completely habit- ual as to be scarcely dependent on the will, although not removed from its control. The act of deglutition or swallowing is of a very curious nature, being the result of a nervous influence in which the vdll is not concerned : when the solid or fluid contents of the mouth are brought in contact with the surface of the pharynx, the impression made upon the sensory nerve is transmitted to the upper part of the spinal cord; and an instinctive motor impulse is propagated along the motor fibrils, by which the muscular movements requisite for the action of swallowing are excited. How far this process necessarily involves consciousness and sensation on the part of the animal will be hereafter enquired (chap. xvi.). A similar action causes the propulsion of food down the oesophagus (gullet) ; and the move- ments of the stomach are in part, if not wholly, excited in the same man- ner. Beyond the stomach, however, the connection of the motions of the alimentary canal and the nervous system ceases, the peristaltic movements of the intestines appearing to depend upon the stimulus directly applied to their muscular coat by the contact of food; although they may be in some degree controlled by a system of muscles disposed around the outlet of the canal, which are, like those at its entrance, partly involuntary, and partly under the direction and restraint of the will. 264. In the lower animals, however, the process is much more simple. The very action of introducing the food into the stomach appears to be, in many instances, the result of direct stimulation, without the intervention of a nervous system. Thus, the movement of the cilia, which fulfils this purpose in so many animalcules, is known to be completely involuntary and unproductive of sensation in higher tribes; and analogy would seem to show that the contraction of the tentacula of the Hydra and other polypes, is of no higher character (§ 115). Where the nervous system is first distinctly concerned in such actions, it is probably only in combining and harmonising them; and as long as they are constant and uniform, always occurring under the same circumstances, and excited by the same stimuli, it seems more philosophical to regard them as purely instinctive, like the action of deglutition in man, and as not of themselves implying anything like will on the part of the animal, which can only exist where p 210 SPECIAL AND COMPARATIVE PHYSIOLOGY. there is intellect/* A brief sketch may now he given of the principal forms of digestive apparatus in the different classes of animals; hut the extent of this subject renders it necessary to enter but little into details. 265. The class porifera presents us with what may be regarded as the same simple form of an absorbent system as that which prevails among the Algee. Every part of the surface of the soft gelatinous flesh of the Sponge, appears equally endowed with the power of appropriating to itself the nutritious materials contained in the water which is in apposition with its external surface and circulates through its ramifying canals. These canals constitute the simplest means by which the absorbent surface may be increased without prolonging them externally; and the movement of fluid through them may be regarded as uniting the capillary circulation of the higher animals with the propulsion of food over the absorbent surface of the intestinal tube, — a special circulating apparatus not being here interposed between the part of the system where the fluid is absorbed, and that in which it is applied to the purposes of nutrition (§ 281). The small quantity of alimentary materials contained in the waters of the ocean, renders necessary in this class a rapid and continuous ingestion of successive portions; and as the animal does not possess the power of appropriating solid masses to the supply of its wants, there is no necessity that the food should be delayed in digestive cavities, for the purpose of undergoing any change preparatory to its absorption. Minute flocculent films may be observed in the fluid which issues from the vents or foecal orifices; and these may be regarded as composed of excrementitious par- ticles thrown off from the interior surface. 266. The method in which the Hydra and other polypifera obtain their food, presents a remarkable contrast to that just described. The sides of the digestive cavity are probably endowed, in most of these ani- mals, with an equal power of absorption throughout; but the food is generally introduced in solid masses, frequently in a living state, and must long be submitted to the influence of the digestive process before it can be * It may be said that although the tentacula of the Hydra, when the animal is hungry, con- tract upon the slightest touch, they allow themselves to be stimulated without responding-, after repletion with food ; and that sensation and will are thus implied. But, on the other hand, it may be urged that a parallel phenomenon occurs in man, which is certainly independent both of will and sensation. For it appears from the experiments and observations of Dr. Beaumont, that the secretion of gastric juice does not continue in proportion to the quantity of food taken into the stomach, although at first excited by its contact, but to the wants of the system ; so that when suflScient nutriment has been provided for absorption, no further active process of digestion goes on, although the will, inattentive to the dictates of Nature, continues to transmit to the stomach more food than is dissolved at the time. Vegetables, again, cease to absorb when the structure is replete with food, and there is no continued demand for it. The more we pursue our researches into the actions of plants and of the lower tribes of animals, the more are we struck with the beauty of the adaptations by which the influence of a capricious will, which would often be to the injury of the system, is rendered unnecessary. DIGESTION AND ABSORPTION IN ANIMALS. 211 assimilated. In tliese two classes, tlien, we have two opposite characters of the digestive apparatus distinctly exhibited : — in one the food is already introduced into the cavities in a fluid form (as in plants), and so largely diluted that no further preparation for its absorption is necessary, all that is required being a continued supply of it; — whilst in the other, the food is obtained at distinct intervals, and in a form which requires energetic digestive actions to render it fit for absorption. In the Hydra, the trans- parency of the tissues, and the absence of any firm envelope, allow the process to be distinctly watched. The prey is frequently, and indeed generally, introduced aKve; and its movements may be observed after it has been swallowed. In a little time, however, its outline appears less distinct, and a turbid film partly conceals it ; the soft parts are soon dis- solved and reduced to a fluid state; and any firm portions which the body may contain, are rejected through the aperture by which it entered. When the process of digestion is complete, the granules, of which the tex- ture of the animal seems principally composed (§ 115), are observed to be tinged with the colour of the dissolved substance, although the fluid which surrounds them remains transparent. A movement of these granules seems concerned in the distribution of the absorbed matter through the fabric ; sometimes they are seen to be forced into the tentacula, whence they are driven, by a sort of reflux, back to the body. 267. In the associated sj)ecies of Polypifera, there is considerable diversity in the degree in which the functions of the individual Polypes are connected together. In many of the simpler Alcyonians (§ 119), which are, like Sponges, provided with polypes at their orifices, a general circu- lation of the products of digestion takes place through the whole fabric. The same is the case, although to a less extent, in the Sertularia and other Hydraform Polypes, as already mentioned (§ 116); while in the complex Ciliohrachiafa, each Polype seems to live for itself alone (§117). The digestive process exhibits itself in the latter -with a considerable advance towards its more perfect types; for we find not only a second orifice to the alimentary tube, but a gizzard for mechanically reducing the food, a secreting apparatus for the production of bile, and a distinct separation of the stomach from the lower part of the intestinal canal. This conformation evidently conducts us to the highly-developed digestive system of the Mollusca; whilst in the isolated Actinice (§ 120), we are led towards the Radiata. The stomachs of these animals are very capacious and disten- sible; so that, like the ht/dra, they can enclose prey many times larger than themselves, which their copious secretions enable them speedily to dissolve, the excrementious matter being thrown out by the oral orifice. The ramifying tentacula of these animals, which surround the central disk containing the stomach, remind us of the minutely-divided arms of the Comatula and other stellated species of Echinodermata, into which the digestive cavity does not extend (§ 107, 270). p 2 212 SPECIAL AND COMPARATIVE PHYSIOLOGY. 268. The number of tlie digestive sacs in the class polygastbica has abeady been noticed (§ 114) as its distinguishing characteristic, and some of their forms detailed. In the Monas, a minute animalcule, formerly supposed to be destitute of any cavity whatever, several stomachs open into a common mouth, surrounded by cilia (Fig. 93); but, in most other species, the digestive sacs are connected with a tube which has an equal relation to all (Fig. 77, a). No special absorbing organs are yet developed in these soft-bodied creatures; and the extension of the alimentary canal through the whole system, by which the nutrient materials are directly conveyed to every part, seems to prevent the necessity for any such provision.* 269. Among the acalepha, the digestive system, although formed upon a simple type, exhibits a very complex arrangement. In the Medusa (Fig. 89), the mouth, situated on the lower surface of the disk, in the centre of the four tentacula, leads to a capacious stomach, partly divided into four portions by the ovarial sacs, which have separate external ori- fices. From this central cavity, prolonged canals ramify minutely through the tissues, and are especially distributed on the margin of the mantle, where the aeration of the fluid seems principally effected. In other spe- cies, such as the BMzostoma, the stomach has no large orifice, but imbibes its fluid by vessels contained in the tentacula, and opening by minute pores on the surface; before these openings were discovered, the cavities were supposed to be filled by endosmose. The ramifying canals are here even more complex, and are distributed most minutely on the free margin of the mantle, the propulsive movements of which evidently assist in its aeration. Here we perceive an enormous extension of the digestive cavity, compensating for the absence of a special vascular system, which is not yet developed. In the Beroe (Fig. 90) and other allied species, there is * The account of the digestive apparatus of Polygastrica given in the text is based upon the statements of Ehrenberg. The author is much disposed, hovi^ever, to agree with Professor Rymer Jones (Outline of the Animal Kingdom, p. 57) in questioning the correctness of these observations. The belief in the existence of a number of distinct sacculi opening from a com- mon intestinal tube, is founded upon the appearance of animalcules which have been fed with coloured particles, and which exhibit numerous coloured globules in their substance, that have been supposed to be cavities into which the matter from without is immediately introduced by the canal proceeding from the mouth. "During the last two hours," says Prof. J., "we have been carefully examining some beautiful specimens of the ParamcEcium aurelia, an animalcule which, from its size, is peculiarly adapted to the investigation of these vesicles ; and so far from their having any appearance of connection with a central canal, they are in continual circula- tion, moving slowly upwards along one side of the body, and in the opposite direction down the other, changing moreover their relative positions with each other, and resembling in every respect the coloured granules which have been described as visible in the g'elatinous paren- chyma of the Hydra" (§226). With regard to one rare animalcule, the Enchelis caudata, which recently came under the author "s notice, he is fully convinced that it possesses a single large cavity like that of the Hydra ; this he has seen partly filled with the green CercaritB which the creature had devoured, and he was at the same time able to trace distinctly the boundaries of the empty portion of the digestive sac, which occupied nearly the whole body. DIGESTION AND ABSORPTION IN ANIMALS. 213 an alimentary canal passing through the body, with a capacious dilatation, serving as the stomach, which sometimes occupies nearly its whole bulk. When there is no food in it, both orifices remain open; and as the animal smms Avith its moiith forwards, a constant current of water is passing through : but when alimentary matter touches the Avails of the stomach, its orifices are immediately contracted, and the digestive process begins. Ramified biliary follicles appear to surround the stomachs of some of the higher species, assisting the process of digestion by the secretion they form. 270. Among the echinodermata, we find an important addition to the digestive system, in the development of a distinct circulating appa- ratus; and in proportion to the perfection of this do we observe the absorbent surface diminished, as in plants. In the common Asterias, the stomach has biit one orifice, and not only occupies the central disk, but sends coecal prolongations into the rays. Upon these we find the absorbent vessels, AA'hich may be regarded as veins, minutely ramifying. In the Comatulct and Pentacrinus, these coeca are rudimentary, the stomach being confined to the disk; and this also contains two convolu- tions of a cylindrical intestine, Avhich terminates by a separate orifice near the mouth. Rising through the Clypeaster and Spatangus (§ 107) to the Echinus, Ave observe the two orifices becoming more and more dis- tant, until, in the last, they are situated on opposite sides of the body. In the Holothuria, as in the Echinus, the intestinal tube varies little in diameter from one extremity to the other ; in some species Ave find not only biliary but salivary follicles; and the absorbent veins are distributed on the intestine through the mesentery, or membrane Avhich binds it to the Avails of the general cavity of the body. The firm tegument of these animals must almost, if not entirely, check that absorption of fluid through the exterior siu-face, which, in the classes preAdously mentioned, appears to perfoi-m a most important part in nutrition. 271. There is considerable uniformity in the structure of the digestive apparatus throughout the sub-kingdom Articulata. It usually par- takes of the character of the body itself, being elongated and narroAA', with little dilatation in any part; this is in conformity Avith the general habits of the group, Avhich are carnivorous; and it will be found, here as elsewhere, that the more highly organised is the food, the more simple is the apparatus required to reduce it. In all but the very simplest Entozoa (Avhich in the present arrangement have been consigned to the group of Acrita), there are two orifices to the alimentary canal; and these are situated near the opposite extremities of the body. In some of these, hoAvever, the head, Avhich is generally furnished Avith curved spines or hooks, does not appear so much concerned in the nutrition as in the attachment of the animal; and nourishment seems more derived by general superficial absorption, than by the mouth. As long, in fact, as 214 SPECIAL AND COMPARATIVE PHYSIOLOGY. the integument remains soft, and the alimentary surface unprovided with definite absorbent A^essels, the former seems almost as important to nutri- tion as the latter. In these parenchymatous worms, therefore, we return to the simplest condition of the nutritive apparatus, in which the aliment is brought into immediate relation with the tissues to be supplied. It is very interesting to remark that, in some of the lowest of the Vermiform tribes, the entrance to the digestive canal is not by one orifice but by several, which seem to act as so many polypes among Zoophytes. In the Tcenia (tape-worm), there are four of these, leading to two canals which remain separate during their whole length, but are connected by transverse canals in each segment. 272. In the higher species of Entozoa, as most of the other Articu- lata, we find a vascular system superadded to the digestive, and thus superseding the necessity of the ramification of the latter through the body. The intestine not being in them merely channelled out of the tissues, but having the character of a distinct tube, is attached to the walls of the cavity in which it lies, by a mesentery^ as in the higher Echinoderma; and in this, the absorbent vessels, which form as yet only a portion of the general vascular system, are distributed to its surface. In Fig. Ill are shown, on one side the digestive system, and on the other the vascular apparatus, of the curious Diplo-zoon paradoxum, a parasite infesting the gills of fishes. In some of this class we observe the first rudiment of a liver, in the cmca (tubes closed at one extremity) which are prolonged from the intestinal canal. These are observed gra- dually to become more numerous, as we ascend the scale, and to open into some definite point in the alimentary tube, which is always in the neighbourhood of the dilatation that may be regarded as a stomach, where such exists. At the same time we find masticating organs super- added, which are furnished with rudimentary salivary glands, having a similar coecal form, like those of the Echinus. Some of these parts are represented in Fig. 104, which shows the jaws, «, «, stomach, 5, and biliary coeca, c, c, of the Diglena lacustris, one of the rotifera (§ 93). In the ANNELIDA, and myriapoda, the alimentary canal usually retains its straight form, but exhibits, in the higher orders at least, a more definite separation into parts. The mouth gradually becomes more com- plex in structure, being endowed A^dth distinct organs for mastication or for suction; the oesophagus is usually naiTow, and then dilates into a larger cavity — the stomach, — which is frequently provided with a firm muscular coat, like the gizzard of birds. Below this, the intestine is usually narrower, but sometimes dilates again near its termination, as in higher animals. Where the canal is more uniform in size throughout, as in the leech and earthworm, the biliary coeca are short and numerous, and disposed along nearly its whole length, instead of being restricted to the neighbourhood of the stomach. They may always be distinguished, DIGESTION AND ABSORPTION IN ANIMALS. 21.5 hoAvevef, as secreting organs, from sucli prolongations of the digestive cavity itself as we observe in the Asterias ; since the contents of the tube are never seen to pass into them, and they exhibit the yellow colour peculiar to their secretion. It might be expected from the general Mol- luscous form and condition of the cirrhopoda that the characters of their digestive system should assimilate with those exhibited in that division. This is indeed the case; for we here find a development of the salivary glands and liver quite disproportionate to the general perfection of their structure as Annulose animals, — these organs, as we shall presently see, being evolved in the Mollusca in the inverse proportion to their posses- sion of animal powers. 273. In insects we find the digestive apparatus presenting nearly the same characters as in higher animals ; and the variations in its conforma- tion which adapt it to the respective kinds of food upon which the differ- ent species exist, are extremely well marked. The two modes in which food is obtained in this class, have already been noticed (§89); but it may here be added that in each case the mouth is constructed of the same parts, which form either mandibles armed with teeth for cutting and tear- ing, or a long delicate proboscis for suction, according to the requirements of the animal. These parts often change their form during the metamor- phosis, when the food of the imago differs from that of the larva. "Where the food is subject to much trituration in the mandibles, the salivary glands are large; but they still exist only in the condition of prolonged coeca (chap. xi.). The oesophagus (a. Fig. 105) is usually narrow above, and dilates below into a cro/>, ^, Avhich, like that of birds, seems destined to commence the digestive process by macerating the food in the fluid secreted by its follicles. Below this is a muscular stomach or gizzard^ c, for mechanically reducing the food; but the development of this depends on the nature of the aliment, and it is altogether absent in those which live by suction. The true stomach, d, however, is never wanting, and is always distinctly separate, in the adult state, from the rest of the canal. It is surrounded by biliary coeca, which usually open near its termination. The form and size of the lower intestine vary much in different species ; being straight and nan-ow in carnivorous insects, and convoluted with occasional dilatations in the vegetable-feeders. In most adult insects, we observe very long convoluted and often branched coeca, e, e, which open into the intestinal canal, at a variable distance between the stomach and its termination. These have been usually regarded as analogous to the liver; and yet their entrance below the part where digestion was proceed- ing, seemed incompatible with what is known of the uses of the bile in other instances. It has been shown, however, by analysis of their con- tents, that they are to be regarded as urinary organs ; and that the fluid they pour into the canal is strictly an excretion, as the position of their 216 SPECIAL AND COMPARATIVE PHYSIOLOGY, orifice would indicate.* Tlie digestive apparatus of the arachnid A and CRUSTACEA, wliicli are all carnivorous, resembles that of the predaceous insects, in the shortness and simplicity of the alimentary canal; and the dilatations on it are nowhere considerable. The liver now begins, how- ever, to assume a more concentrated form, the follicles and coeca being aggregated into lobules of solid appearance (§ 459); and in the higher Crustacea the entrance to the stomach as well as the mouth is guarded Avith teeth, which are moved by powerful muscles, and have a firm calca- reous structure. 274. The development of the digestive system of the Molluscous classes presents a remarkable contrast Avith that Avhich we have been just considering. Whilst the generally acute sensations and active locomotive powers of the Articulated tribes enable them to go in search of their prey, and to select that which they are capable of digesting with the greatest facility, — the Mollusca are usually either fixed to one spot, or confined, by the want of means of active locomotion, within a very narrow range, and their perceptions seem proportionably obtuse. Being, therefore, depend- ent upon casual supplies for their support, their digestive organs are adapted to much greater variety of food, and to act upon organised matter of a kind much inferior to the tissues of the animals themselves. Even in the lowest of this group, we observe a form of the alimentary canal nearly as complex as that of Insects. Thus, in the Cynthia^ one of the TUNICATA (Fig. 83), we see the oesophagus (the entrance to Avhich, c, lies at the bottom of the sac of the mantle, into whose cavity water is con- stantly being received by the apertures, a,) leading to a wide stomach, iratory organs being furnished, like that which distributes it to the system at large, Avith an impelling cavity, by which a. constant and regular current is maintained. This structure, however, is peculiar to that order of Cephalopoda which, in the symmetrical distribution of its organs, its deficiency of external shell, and its possession of a rudimentary internal skeleton, as Avell as in other particulars, exhibits so many points of resem- blance to Fishes (§ 96). In the Nautilus tribe, on the other hand, the general structure is more analogous to that of the other Molluscs: and accordingly we find, fi-om the account of Mr. Owen, that the vascular system presents nearly the same arrangement as in the Gasteropoda. The veins that return the blood from the system enter a common sinus, which has not, however, a muscular character, and does not possess contractile powers; and from this the branchial vessels proceed, which, after expos- ing the blood to the respiratory surface, conduct it back to the heart or systemic ventricle.* 309. Although in fishes we find the same simple conformation of the heart as that which exists in the Molluscous classes, the alteration in its position, relatively to the other parts of the vascular system, occasions its influence on the function of the circulation to be greatly modified (Fig. 120). The blood which is expelled fi-om the single ventricle is carried at once to the gills, the principal trunk subdividing into four or five branches on each side, which run along the branchial arches (§ 405), sending ramifications to every filament. After being thus aerated, it is collected by confluent vessels into the great systemic artery, which then distributes it to the different organs of the body; and thence it is returned to the auricle by the veins, which before entering it exhibit large dilata- tions or sinuses. (Similar cavities exist among the Cephalopoda.) Although this circle appears sufiB.ciently simple in its character, it yet pos- sesses some peculiarities which are worth notice, especially as they seem to foreshadow more important modifications in higher classes. Two or three small arteries are usually seen passing ofi"from the branchial arches, so as to convey the pure aerated blood directly to the head, instead of transmitting it to the general systemic trunk. It will be hereafter shown that a * It is not a little curious that the principal vein, just before entering' the sinus, should com- municate with the abdominal cavity by small apertures existing- between the muscular fibres which traverse it, just as in the Aplysia. From various parts of the venous system, both in the Nautilus and in the Cuttle-fish, a curious series of follicles or little sacs is seen to pro- ceed, forming' spongy masses, sometimes of considerable size. The use of these is not certainly known, some regarding them as secreting organs, and others as temporary reservoirs of venous blood, like those which are found in the Cetacea and other diving animals. CIRCULATION IN ANIMALS. 243 similar provision exists in the Crocodile, and has a very important pur- pose in its economy; and that the same condition is manifested up to the termination of the embryo state of the higher Vertebrata, including the human species. Although we still find the respiratory and general circu- lation united in this class, they hold the same relation to one another as in the classes in T^hich a complete double circulation exists, Avhose heart possesses four cavities instead of two. The passage of blood through the respiratory organs is sometimes called the "lesser circulation;" but there is more than one instance in the animal economy, in Avhich the circulating fluid is made to pass through a circuitous track for the purpose of being purified by the elimination of some of its contents; and these would be alike deserving of the term. Thus, in Fishes the blood which is being returned to the heart from the tail and posterior part of the body, is transmitted through large venous trunks, partly to the liver, and partly to the kidneys, (sometimes almost entirely to the latter), in which organs these vessels ramify for the purpose of causing the separation of their peculiar secretions. After this process has taken place, the blood is con- veyed to the heart by large venous trunks into which the smaller branches again unite. Thus, the portal circidation, as it is termed, holds precisely the same relation to the general circulation in Fishes, as did the respira- tory circulation in the MoUusca; being interposed, for the purification of the blood which has circulated through the system, between its capillaries and the heart. It has not any special impelling organ for the purpose of transmitting the blood through it; miless a contractile portion which has been described as existing in the caudal vein of the Eel can be regarded as subservient to this function. This portal circulation exists in the same form in Reptiles : but in Birds ' and Mammalia the kidneys, like other organs, are supplied Avith arterial blood from which their secretion is formed; still the liver is connected Avith the venous system, and the portal circulation continues to have an important office in the piu-ification of the blood, — an office which seems especially necessary in the foetus before the action of the lungs has commenced. 310. Quitting now those classes which are modified for existing in water and passing on to the Reptiles, Birds, and Mammalia, we find that very important modifications of the circulating system are necessary to adapt the animal to the conditions of atmospheric respiration. It is evident that the blood will be aerated much more rapidly Avhen exposed to the air itself, than when merely submitted to the small quantity which is dif- fused through the watery element. If, therefore, the whole amount of the circulating fluid be thus exposed, the changes which it undergoes will be performed with such increased energy that, if the other vital processes be made to conform to them, a warm-blooded animal is produced at once. But as the reptiles are intended to lead a life of comparative inertness, and to exist in circumstances which Avould be fatal to animals of higher R 2 244 SPECIAL AND COMPARATIVE PHYSIOLOGY. organisation, the respiratory process is reduced in amount by the peculiar structure of the vascular system now to be described. The single ventricle of the heart gives off arterial trunks which pass both to the lungs and to the system at large; so that a part of the blood which has been expelled by each stroke is sent to supply the requirements of the nutritive system, and a part is separated for aeration. The pure arterialised blood which returns from the lung is conveyed to one auricle, whilst the venous blood which is transmitted by the systemic A^eins enters the other; these two auri- cles are hence not repetitions of one another, but have distinct functions. Both empty themselves into the ventricle, where the blood derived from these different sources is mixed, and fi-om which one part is again sent to the body, and another transmitted to the lungs (Fig. 121). 311. This is the general type of the circulating system in the class of Reptiles, but there are some very curious modifications of it, which connect it with the vascular apparatus of Fishes on one hand, and with that of Birds and Mammalia on the other. The connection with Fishes, it is evident, will be established by the order BatracMa or Amphibia, Avhich in their early or larva condition are in every respect analogous to the mem- bers of that class. Their circulation is for a time performed exactly upon the same plan, the blood being transmitted from the simple bilocular heart to the branchial arches, and after aeration being circulated through the system. The transition from this condition of the vascular organs, to that which they present in the perfect Reptile state of the animal, when they are conformable to the general type of the class, is so curious as to be worth a some Avhatminutedescription; more especially as, in all the higher animals, a series of changes precisely analogous takes place during the early stages of their development. It will be rendered intelligible by the accompanying Figures 122-4. In Fig. 122 is seen the arrangement of the parts before the metamorphosis has commenced. Three branchial trunks (1, 2, 3,) pass off on each side of the heart, terminating in a mi- nute capillary network which is contained in the branchial arches, and by which the blood is aerated during the aquatic existence of the animal; from this network the returning vessels take their origin, which unite into trunks, one for each gill; and of these the first supplies the head, while the second and third join to form the great sj^stemic artery. A, as in fishes. But besides these vessels, there are some small undeveloped branches, which establish a communication between each branchial artery and the returning trunk that corresponds with it. There is also a fourth small trunk, 4, given off from the heart, which unites Avith another small branch from the aorta, to be distributed upon the (as yet) rudimentary lungs. After the metamorphosis has begun, hoAA'ever, by Avhich the ani- mal from a fish has to be converted into a reptile, the branches that connect the arteries of the gills Avith their returning trunks are much increased in size, so that a large part of the blood floAvs continuously through them CIRCULATION IN ANIMALS. 245 without being sent to the gills at all, and the branchial vessels are them- selves relatively diminished; at the same time, the fourth trunk, which was before the smallest, becomes the largest, so that an increased propor- tion of blood is sent to the lungs. By a continuance of these changes, the branchial vessels gradually become obliterated, and the communicating branches, Avhich were at first like secondary or irregular channels, now form part of the continuous line of the circulation, the upper one sending off the cerebral vessels, the second and third uniting to supply the trunk, and the fourth passing as before to the lungs. 312. In the Proteus the arrangement of the vascular system per- manently resembles that which has been represented as intermediate between the larva and perfect condition of the frog. This animal is provided with kmgs slightly developed, as well as with permanent gills; and the blood which is expelled from the ventricle is partly transmitted through the gills, partly finds its way directly into the aorta by means of the communicating branches, and a small quantity is transmitted to the lungs; the latter is returned perfectly arterialised to the pulmonary auri- cle, and is afterwards mixed in the ventricle with the venous blood trans- mitted to the systemic auricle. In many of the higher Reptiles we find not only two auricles, but the cavity of the ventricle more or less perfectly divided into two; sometimes the septum is complete, as in the Crocodile; and in other cases it affbrds only a partial separation, which is still per- haps sujB&cient to modify the direction of the currents of the blood. Thus, in the Lacerta ocellata (spotted lizard) where the ventricle is partly divided, the right side of it, into which the systemic auricle discharges itself, principally gives off the j>ulmonary trunks, so that a large pro- portion of the venous blood returned from the system is transmitted to the lungs for aeration; and. this being returned to the pulmonic auricle is conveyed to the left side from which the systemic arteries proceed. As long as there is any direct communication, however, between the two sides of the heart, it is obvious that a part of the blood returned fi-om the systemic veins may be sent immediately into the aortic trunks without being previously arterialised; and in proportion to the degree in Avhich the septum is complete, Avill be the approach of the animal towards the condition of the warm-blooded Vertebrata. The distribution of the vessels, however, has a considerable effect upon the character of the fluid with Avhich individual organs are supplied ; for in Reptiles which mani- fest this separation to a considerable extent, a part of the blood trans- mitted to the system has still a venous character, whilst that which is furnished to the brain and upper part of the body is purely arterial. The contrivance by which this is effected is curious and interesting. The aortic trunk does not arise singly, but by two origins, one of which is connected Avith the right and the other Avith the left side of the ventricle; the latter receives chiefly the arterial blood from the left or pulmonary 246 SPECIAL AND COMPAKATIVE PHYSIOLOGY. auricle, and tliis giyes off branches wliicli convey it without admixture to the head; while the main trunk passes on to unite with the second aortic arch that arose from the right side of the heart, and consequently is filled with blood almost entirely venous, which has been discharged from the system into the right auricle. This second arch, before its union with the first, however, gives off a large branch which is distributed to the intestines and other viscera, and which, therefore, contains venous blood with little admixture of arterial; the common aortic trunk formed by the union of the two arches conveys mixed arterial and venous blood to the remainder of the trunk and members. It is beautiful to observe how by these simple contrivances the greatest economy of material is obtained, Avhilst each organ is supplied with blood of a character best fitted to maintain its functions. 313. The Crocodile presents us with a condition of the vascular system still more allied to that of warm-blooded Yertebrata; the ventri- cular septum being complete, and the circulation, as far as the heart is concerned, being truly double. Still, however, whilst the principal aortic trunk arises from the left ventricle, which contains nothing but arterialised blood, a second arch arises from the right (or venous side) along with the pulmonary artery of which it might almost be considered a branch; and this, after giving off its intestinal branches, enters the first trunk, which has already fiu-nished the cerebral arteries with pure arterial blood, and transmits the mixed fluid to the rest of the system (Fig. 125). There is another communication between the trunks arising from the two sides of the heart, by means of an aperture which passes through their adjoining walls just after their origin; so that although the blood in the heart is entirely venous on one side and arterial on the other, it undergoes admix- ture in the vessels according to the character of the functions to which it is to minister. We shall presently see a remarkable analogy to this dis- tribution of the vascular system, exhibited in the foetal condition of Birds and Mammalia. 314. In the highest form of the circulating system, that possessed by the warm-blooded Vertebrata, there is a complete double circulation of the blood, each portion of it which has passed through the capillaries of the system being aerated in the Ivmgs, before being again distributed to the body. This is effected by a form of the vascular apparatus of which we saw a sketch in the Cephalopoda, and to which a near approach is exhibited by the higher Reptiles. The heart consists of four cavities, two auricles and two ventricles; those of the right or venous side having no direct communication Avith those of the left or arterial side; and the vessels proceeding from them being entirely distinct, and having no con- nection whatever except at their capillary terminations. The blood trans- mitted by the great veins of the system to the right auricle or receiving cavity, having passed into the ventricle or propelling cavity, is transmitted CIRCULATION IN ANIMALS. 2-i7 by it through, the pulmonary arteries to the lungs of the two sides.* After being there arterialised by exposure to the atmosphere, it is brought back to the left auricle ; and having been poured by it into the corresponding ventricle, is transmitted by the great systemic artery or aorta to the most distant parts of the body(rig. 126). The heart is therefore completely duplex in structure, and, so far as its functions are concerned, might be regarded as consisting of two distinct portions; for economy of material, however, these are united, the septum of the ventricles serving as the wall to each. In the Dugong (one of the Whale tribe) however, the heart is bifid, and presents this division into two separate organs not only functionally but structurally (Fig. 127). 315. Various peculiarities in the distribution of the vascular system w^hich are presented by different orders of Quadrupeds and Birds, would be worth notice if our limits permitted. Of these one of the most re- markable is the modification both of "the venous and arterial trunks existing in the Cetacea and other diving animals, which are occasionally prevented from respiring for some time, and in which, therefore, the arte- rialisation of the blood is checked. Various arteries of the trunk are here found to assume a ramified and convoluted form, so that a large quantity of blood may be retained in the reservoirs formed by these plexuses; whilst the venous trunks exhibit similar dilatations, capable of being dis- tended with the blood Avhich has been transmitted through the system, so as to prevent the heart being loaded with the impure fluid, whilst the lungs have not the power of arterialising it. In some diving animals this object is effected, not so much by a number of venous plexuses, as by a single great dilatation of the vena cava before it enters the heart. Fre- quently the force vnth which the blood is sent to particular organs seems to be purposely diminished by the division of the trunk that conveys it into a number of smaller vessels, which, after a tortuous course, unite again and are distributed in the usual manner. A structure of this kind is found in the arteries of the brain of the long-necked grazing animals, to which the l)lood would be transmitted with too great an impetus, owing to the additional influence of gravitation, were it not retarded by this con- trivance. A similar distribution of the arteries is found in the trunks supplying the limbs of the Sloths, and of other animals Avhich resemble them in tardiness of movement. In other cases, the arterial canals are specially protected from compression by surrounding organs, in order that * In must be borne in mind here and elsewhere that the term artery is used to denote a vascular tube carrying' blood from the heart, whilst the word vein designates one which con- veys it towards the heart : but tliat arterial blood means that which has been rendered florid by the respiratory process (§ 418) in whatever direction it may be travelling' ; and that by venous blood is meant the dark impure fluid of which the vital properties have been impaired by circulation through the capillaries of the system. The pulmonary arteries convey venous blood from the heart to the lungs ; and the pulmonary veins retnrn arterial blood from the lungs to llie heart. 248 SPECIAL AND COMPARATIVE PHYSIOLOGY. there may be no obstruction to tbe passage of blood througli them, and that they may be protected from injury; thus, in the fore leg of the Lion, where all possible force and energy is to be attained, the main artery is made to pass through a perforation in the bone, that it may be secured from the pressure of the rigid muscles, which, when in a state of contrac- tion, might othermse haye altogether checked the current through it. In most Quadrupeds, as in man, the right anterior extremity is more directly supplied with blood from the aorta than the left; so that the superior strength and activity of this limb is not altogether the result of habit and education,, as somei have supposed; in Birds, however, where any ine- quality in the powers of the two wings would have prevented the necessary regularity in the actions of flight, the aorta gives off its branches to the two sides with perfect equality. Some further peculiarities in the distri- bution of the arterial system will be hereafter noticed (§ 330). 316. Having now traced the vascular system to its highest form, it is proper to enquire how far this differs from the simple condition in which it was at first manifested. There can be no doubt that in the higher animals, possessed of a distinct muscular heart, this is the chief agent in keeping up, by its successive contractions and dilatations, the motion of the blood through the vessels. But a careful survey of all the phenomena of the circulation Avould seem to lead to the conclusion that the impulse of the heart is not the 07ili/ means by which the motion of the blood is continued, but that the changes which this fluid undergoes in the capilla- ries have some share in its production, and have at any rate a very considerable modifying effect upon the quantity transmitted through the individual organs. We have seen that in Yegetables the nutritive circu- lation is entirely capillary; that in the lower Animals it is chiefly so; that even in Insects it appears but little dependent upon the action of the central recipient and impelling cavity; but that in the higher tribes the capillary power* is more and more subordinated to the heart's action. The following are some of the facts which appear to support the conclu- sion that, even in the highest animals, this capillary power is not oblite- rated, but is merely superseded by the energy of the central organ, which it was necessary to endow with an amount of force sufiicient to govern and harmonise the numerous actions going on in different parts of the system. * By using this expression, the author does not mean to imply that any motions of the capillary vessels are of mechanical assistance to the passage of fluid through them, — a doctrine which neither common sense nor experience in any degree support ; but he merely employs it to designate the agent, whatever may be its nature, which is immediately concerned in the independent motion of the blood through the capillaries, and which is evidently the product of the organic changes it undergoes in them. According to Dr. Alison the movement is owing to a new set of vital attractions and repulsions to which these changes give rise; this must be regarded as a hypothetical explanation merely, and liable to the objection that, in the present state of our knowledge of physical causes, we are not entitled to declare that the effect is not due to these. CIRCULATION IN ANIiMALS. 249 317. In many warm-blooded Vertebrata, and still more in the cold- blooded Reptiles, (amongst which the vitality of individual parts much longer survives injury to the general system), motion of blood in the capillaries has been seen to continue some time after the heart has ceased to act, or has been removed, or after the great vessels have been tied ; and this motion may be immediately checked by certain applications to the parts themselves. After most kinds of sIoav natural death, the arterial trunks and left side of the heart are found to be comparatively empty, and the venous cavities full of blood. This effect has been ascribed to the contraction of the arterial tubes; but it is impossible that it can be alto- gether due to that cause, since their calibre is never found to have dimi- nished in any very evident degree; it must rather result from the continuance of the capillary movement after the general systemic circula- tion has ceased. The continuance of various processes of secretion and even of nutrition subsequently to general or somatic death, affords an excellent proof of this lingering vitality; and it is scarcely possible that these can be maintained "without some degree of capillary circulation. There are some kinds of sudden death, however, in which the vitality of the whole system appears to be simultaneously destroyed, and the blood remains in the vessels as it was at the moment of decease. Again, it has been stated that in an ampututed limb the circulation of blood through the capillaries has been seen to persevere (under the influence of heat) for ten or fifteen minutes. Microscopic examination of the circulation in the living animal discloses many irregularities in the capillary currents, which it is impossible to attribute to any influence derived from the vessels that supply them; thus, the velocity of two currents in neighbouring channels is often very different, their direction changes, and some of them even occasionally stop and recommence again without any perceptible mechan- ical cause. 318. Amongst the most remarkable proofs of the influence of the capillary circulation on the general distribution of the blood, is one derived from the observation of organs which undergo periodical changes in activity. Thus, Avhen the uterus commences to develope itself during pregnancy, the capillary circulation is of course performed with unusual activity, and occasions an increased demand for blood, which is supplied by an increase in the diameter of the trunks that transmit fluid to the organ; and this is entirely independent of any increased energy in the heart's action, which would have affected the "whole system alike. The same may be said of the occasional development of the mamma? for the secretion of milk; and of similar changes in other organs, of which the activity is periodical. In diseased states, also, of particular portions of the system, which do not occasion any appreciable alteration in the heart's action, the quantity of blood sent to the part is much increased, and the pulsation of the arterial trunk leading to it is evidently stronger than that 250 SPECIAL AND COMPARATIVE PHYSIOLOGY. of any other vessels in the system. These phenomena, and many others which might he mentioned, are evidently analogous to one formerly men- tioned as having heen ascertained by experiments on plants (§ 289); and, when taken in connection, they seem to indicate Avithout much doubt, that the quantity of blood sent to individual organs, and the force with, which it is transmitted, vary more with the degree of attraction exercised upon it by the vital processes taking place in them, than with the vis a tergo derived from the impulsive power of the heart. Another remarkable proof of the influence of the capillary on the general circulation is derived from the phenomena of Asphyxia or suffocation; since it now seems dis- tinctly ascertained that the check given to the circulation, and thence to all the other functions, arises from the stagnation of the blood in the capillaries of the lungs, by the cessation of that reaction between the fluid and the air, which seems requisite, not only to maintain its normal con- stitution and properties, but to promote its movement through the vessels (see notes to § 152, 212). Some other arguments for the independent nature of the capillary circulation, may be drawn from the spontaneous motions exhibited by the globules of the blood when removed from the body or liberated from vessels; but a more particular account of them mil be given at a future time (§ 363). 319. In the development of the embryo of the higher vertebrated ani- mals, moreover, there is a period at which a distinct movement of red blood is seen, before any pulsating vessel can be detected to possess an influence over it; and in the formation of new membranes, which is one of the results of inflammation, the lymph, that is poured out in a fluid state and gradually acquires a solid consistence, presents channels in which globiUes are seen to move before these become connected with the vessels of the neighbouring parts. Finally, instances not very unfre- quently occur, of embryos having attained nearly their full development, which have been unpossessed of a heart, and in which the circulation has been, as it were, entirely capillary; and although in most, if not all, of these cases, the monster has been accompanied by a perfect child, the heart of which may have been suspected to have influenced its own circu- lation, yet in one of those most recently examined, the occurrence of this has been disproved. From a careful examination of the vascular system, it appeared impossible that the heart of the tmn foetus could have caused the movement of blood in the imperfect one; and this must, therefore, have been entirely similar to the circulation of elaborated sap in plants, being maintained by the nutritive changes occurring in the capillaries, — an effect not the less certain because we are as yet unable to explain it satisfactorily.* * For the details of this interesting case, which was communicated by Dr. Houston, of Dub- lin, to the British Association, in 1836, see the British and Foreig'n JMedical Review, vol. ii., p. 596, and the Diiblin Medical Journal for 1837. CIRCULATION IN ANIMALS. 251 320. The evolution of that circulating system which has been described as peculiar to the higher classes of Vertebrated animals, is not completed until the moment of birth; and the progressive changes which the vascular apparatus undergoes in the development of the foetus of Birds and Mam- malia afford a most beautiful illustration of the principles already laid dovsTi, respecting the correspondence between the transitory stages of each system in the higher animals, and the forms permanently exhibited by the lower. It has been seen that in the organs of circulation, as well as in all others, the tendency, as we rise from their lowest to their highest condi- tion, is one of centralisation. In the simplest animals, as in plants, whatever motion of fluid takes place is effected by each individual part by and for itself; whilst in the complex and highly developed structures that occupy the other extremity of the scale, the development of a power- ful organ of impulsion, the influence of which extends over the whole system, has superseded the diffused agency by which the circulation was previously maintained. This progress from a more general to a more special type is equally manifested in the vascular system of the embryo ; and the analogy which thus arises between the forms it presents at differ- ent epochs of its development, and those presented by the lower tribes of animals, is not superficial only, but extends even to minute particulars. The eggs of Birds afford the best opportunity for studying the early changes which it undergoes, and these have been described mth gi-eat minuteness; such a sketch of them only will here be given as will serve to demonstrate the principles alluded to. On the surface of the yolk-bag of a fresh egg, a little semi-opake spot about ^ of an inch in diameter may be readily detected; this is termed the cicafricula or germ-spot, and it is here that the first changes are performed in which the development of the embryo consists (§ 535). This afterwards extends itself into the fferminal membrane, which gradually spreads over and encloses the yolk, and on the central portion of which, the embryo is developed. This membrane soon exhibits a subdivision into three laminae, of which the middle one, termed the vascular layer, gives origin to the circulating apparatus, and the development of it alone will be here described. 321. During the early period of incubation, the thickened portion of the vascular layer that surrounds the germ becomes studded mth nume- rous irregular points and marks of a dark yellow colour; and as incubation proceeds, these points become more apparent, and are gradually elongated into small lines, which are united together, first in small groups, and then into one network, so as to form what is called the Vascular area. The newly formed vessels, which are at first simply channelled out like the proper vessels of plants, gradually become more distinct, acquiring regular walls, and containing a fluid of a darker colour; the small branches of the network aiTange themselves like the fibrils of a leaf on each side of the embryo, and terminate in two vessels whicli pass into its structure. 252 SPECIAL AND COMPARATIVE PHYSIOLOGY. Towards the circumference of tlie area, tlie smaller ramifications open into a circular trunk wliich. bounds the space (Fig. 128). The first rudi- ment of the heart appears about the 27th hour, and is of a tubular charac- ter, being formed by a longitudinal fold of the vascular layer; for some time it is simple and undivided, extending, however, through nearly the whole length of the embryo; but the posterior part may be regarded as corresponding with the future auricle, since prolongations may be perceived extending from that part into the transparent area, which indicate the place where the veins subsequently enter. Although the development has proceeded thus far at about the 35th hour, no motion of fluid is seen in the heart or vessels until the 38th or 40th hour. When the heart, Avhicli is evidently at this period strictly analogous to the dorsal vessel of the Annelida, first begins to pulsate, it contains only colourless fluid mixed with a few globules. A movement of the dark blood in the circumference of the vascular area is at the same time perceived; but this is independent of the contractions of the heart, and it is not until a subsequent period that such a communication is established between the heart and the dis- tant vessels, that the dark fluid contained in them arrives at the central cavity, and is propelled by its pulsations. This fact, which has a very im- portant bearing on the theory of the circulation, and which has been denied by some observers (amongst others by Dr. Allen Thomson), appears to have been positively established by the latest researches of Von Baer,* 322. The contraction of this dorsal vessel (for so it may be termed) begins, as in the Annelida, at its posterior extremity, and gradually extends itself to the anterior; but between the 40th and 50th hours, a separation in its parts may be observed, which is effected by a constriction round the middle of the tube, and the dilatation of the posterior portion into an auricular sac, and that of the anterior into a ventricular cavity. Between the 50th and 6'Oth hours, the circulation of the blood in the vascular area becomes more vigorous, and the action of the ventricle is no longer continuous with that of the auricle, but seems to succeed it at a separate period. At the same time the tube of the heart becomes more and more bent together until it is doubled; so that this organ now becomes much shorter relatively to the dimensions of the body, and is more confined to the portion of the trunk to which it is subsequently restricted. A change somewhat similar but less in amount has been shown by Mr. Newport to take place in the dorsal vessel of many insects * He says that there is no doubt of the blood being- formed before the vessels. The formation of the blood g-oes on in every part of the body ; and when formed, it is pnt in motion by some unknown cause that impels it in the proper direction, until at leng-th it reaches the central formation of blood, around which is developed a tubular canal afterwards to be further modi- fied and changed into a heart. The first motions of the blood are toioards the heart, and con- sequently the first vessels formed are vems; a fact of itself sufficient to disprove the hypothesis that the motive power which presides over the circulation resides exclusively in the ventricles of the heart, Uber Entwickelungsgeschicte der Thiere, &c. Kbnigsberg', 1837. Part ii.p. 126. CIRCULATION IN ANIMALS. 253 at the time of their last metamorphosis.* The convex side of the curve which the tube presents (Fig. 129) is that which subsequently becomes the apex or point of the heart; and, between the 60th and 70th hours, this is seen to project forwards from the breast of the embryo, much in the situation it subsequently occupies. About the same time, the texture of the auricle differs considerably from that of the ventricle; the auricle retaining the thin and membranous walls which it at first possessed; while the ventricle has become stronger and thicker, both its internal and external surfaces being marked by the interlacement of muscular fibres, as in the higher MoUusca. About the 65th hour, the development of the heart may be regarded as corresponding with that of the fish; the auricle and ventricle being perfectly distinct, but their cavities as yet quite single. The heart of the dog at the 21st day bears a great resemblance to that of the chick at the 55th or 60th hour; it consists of a membranous tube twisted on itself and partially divided into two principal cavities, besides the bulb or dilatation which at this period is found at the commencement of the aorta, and which is peculiarly developed in Fishes. 323. The blood-vessels which are first observed in the body of the embryo, as well as in the vascular area, appear formed in isolated points, which gradually coalesce so as to form tubes; no difference is at first observed between the characters of the arteries and those of the veins, and these are only to be distinguished by the direction of the currents of blood circulating through them. Subsequently, however, (about the fourth or fifth day of incubation), the coats of the arteries begin to appear thicker than those of the veins, and the distinction between them soon becomes evident. After the principal vessels are formed, the deve- lopment of new ones no longer appears to take place in disunited points, but to be effected by the prolongation of loops from those already existing. This process has been described by several observers, as witnessed in the finny tail and external gills of the common tadpole and water newt. In these animals, the course of the blood is at first very simple. In the early stages of development there is no capillary network on the tail, but a simple arterial trunk which runs to the end of it, and there joins a return- ing vein. At a later period, it is well knoA^Ti that the tail is covered by a network of minute vessels, communicating with the primary artery and vein, in which the blood is conveyed through the whole substance of the organ. The development of these vessels has been shown to be owing to the prolongation of communicating branches formed between the primary trunks. These communicating branches pass at first directly from the artery to the vein; but they become gradually longer and as- sume a looped form, extending from the middle to the lateral expanded portions of the tail; other loops are formed in succession from these, and new ones again from them, until, in the course of ten or more days, the ' • Koget's Physiology, \^ol. ii. p. 245. 254 SPECIAL AND COMPARATIVE PHYSIOLOGY. whole of the finny part of the tail is corered by beautiful minute arteries and veins. The loop of the vessel, when short and newly formed, has at first more the appearance of artery than vein, as the blood passes through it in jerks; as the loop elongates, however, and new branches proceed from it, the blood moves in jerks only in that part of the loop which communicates with the arterial trunk, whilst, in the part connected with the returning vein, the stream of blood becomes uniform. 324. The development of the vessels in the filamentous gills of the aquatic Salamander takes place on precisely the same plan; and their distribution in the leaf-like gills of the adult Proteus (Fig. 130) is evi- dently the result of a similar process. A corresponding series of changes has been observed in other organs. Thus, the anterior extremities of the Salamander commence as little tubercles sprouting behind the head and almost destitute of circulating blood. Soon after their appearance, a single vessel is seen winding round their extremities, Avhich returns to the body without giving off any branches. The toes are soon observed to bud forth from the end of the limb, and each of them receives a small loop from the original vessel. Communicating branches are likewise thrown across the joints, and as the limb becomes larger, numerous capillary vessels are formed in the same manner as the primitive trunks. The same appearances have been observed in the first evolution of the extremities of the chick, and also in the embryo of the rabbit and other mammiferous animals; so that there appears reason to believe that, after the circulation has once been fully established, the development of new vessels takes place universally on this plan, except where their formation is the result of a diseased action (§ 364). Some microscopic observers state that new loops may occasionally be seen to form during the ordinary processes of nutrition, in parts which have already attained their full development. 325. Having traced the evolution of the heart of the chick up to the grade which it presents in fishes, we may now enquire what is the condition of the other parts of the vascular system at the same time. At the end of the second day, the aorta, which arises by a single trunk, is seen to have divided into two canals (1, 1, Fig. 131) which separate from one another to enclose the pharynx, and then unite again to form the trunk. A, which passes down the spine. During the first half of the third day (about the 60th hour), a second pair of arches, 2, 2, is formed, which encompasses the pharynx in the same manner; and towards the end of the third day, two other pairs of vascular arches, 3, 3 and 4, 4, are formed; to that the pharynx is now encompassed by four pairs of vessels which unite again to supply the general circulation. These evidently coi-respond Avith the branchial arteries of fishes, although no respiratory apparatus is connected with them; and in fact the distribution of the vascular system of the bird on the fourth and fifth days exactly resembles that presented by CIRCULATION IN ANIMALS. 255 many cartilaginous Fishes, as well as by the tadpoles of the Batrachia. The first arch is obliterated about the end of the fourth day ; but a vessel which is sent from it to the head and neighbouring parts, and which afterwards becomes the carotid artery, c, continues to be supplied through a communicating vessel, 6, from the second arch. While the first pair is being obliterated, a fifth, 5, is formed behind the four which had previously existed, proceeding in the same manner as the fourth from the ascending to the descending aorta. On the fourth day, the second arch also becomes less, and on the fifth day is wholly obliterated; whilst the third and fourth become stronger. From the third arch, now the m.ost anterior of those remaining, the arteries are given off which supply the upper extremities, b, h; and the vessels of the head are now brought into connection with it, by means of the communicating branch, 7, which previously joined the third with the second arch. When these vessels are fully developed, the branch, 8, by which these arches formerly sent their blood into the aorta, shrinks and gradually disappears, so that by about the 13th or 14th day the whole of the blood sent through the two anterior branches is carried to the head and upper extremities, instead of being transmitted to the descending aorta as before. There now only remain the fourth and fifth pair of branchial arches, the development of which into the aorta and pulmonary arteries will be described in connection with the changes which are at the same time going on in the heart. 326. During the fourth day, the cavities of the heart begin to be divided for the separation of the right and left auricles and ventricles. About the eightieth hour the commencement of the division of the auricle is indicated externally by the appearance of a dark line on the upper part of its wall; and this, after a few hours, is perceived to be due to a con- traction, which, increasing downwards across the cavity, divides it into two nearly spherical sacs. Of these the right is at first much the largest and receives the great systemic veins ; the left has then the aspect of a mere appendage to the right, but it subsequently receives the veins from the lungs when these organs are developed, and attains an increased size. The septum between the auricles is by no means completed at once; a large aperture (which subsequently becomes ihe foramen ovale) exists for some time at its lower part, so that the ventricle continues to communi- cate freely with both auricles. This passage is afterwards closed by the prolongation of a valvular fold which meets it in the opposite direction; it remains pervious, however, until the animal begins to respire by the lungs, and sometimes is not completely obliterated even then. From late observations it would appear that the division of the ventricle commences some time before that of the auricle. Although some variation exists in the statements of different authors on the mode in which this is eftected, the general fact appears to be that a septum is gradually developed within the cavity by a projection arising- from its inner wall ; and this progress- 256 SPECIAL AND COMPARATIVE PHYSIOLOGY. ively acquires firmness, and rises higher up, until it reaches the entrance to the bulh of the aorta, where some communication exists for a clay or two longer. At last, however, the division is complete, and the inter- ventricular septum becomes continuous with the inter-auricular, so that the heart may be regarded as completely a double organ. The progressive stages presented in the development of this septum are evidently analo- gous to its permanent conditions in the various species of Reptiles (311-3). The changes which occur in the heart of the Mammalia are of a precisely similar character; and, as they take place more slowly, they may be watched with greater precision. Soon after the septum of the ventricles begins to be formed in the interior, there appears a corresponding notch on the exterior, which, as it gradually deepens, renders the apex of the heart double. This notch between the right and left ventricles continues to become deeper until about the eighth week in the human embryo, when the two ventricles are quite separated from one another, except at their bases (Fig. 132); this fact is very interesting from its relation with the similar permanent form presented by the heart of the Dugong. At this period, the internal septum is still deficient, so that the ventricular cavities communicate with each other, as in the chick on the fourth day. After the eighth week, however, the septum is complete, so that the cavities are entirely insulated; whilst at the same time their external walls become more connected towards their bases, and the notch between, them is diminished; and at the end of the third month the ventricles are very little separated from one another, though the place where the notch previously existed is still strongly marked. 327. Returning again to the distribution of the arterial trunks, we are now prepared to follow their final modification, by which they are adapted to the existence which the individual is soon to commence as an air- breathing animal. The first, second, and third branchial arches have been shown to be replaced by the brachial and carotid arteries, and to have lost all communication with the aorta except at its commencement, where they arise with the other trunks from its dilated bulb. This remains a single cavity after the ventricles are distinct; but towards the end of the fifth or the beginning of the sixth day in the chick, the bulb becomes flattened, and the opposite sides adhere together so as to separate it into two tubes running side by side. Of these, one communicates with the left, and the other with the right ventricle. The former, which subsequently becomes the aorta, is continuous with the fourth branchial arch on the right side only; but from this the carotid and brachial arteries arise by two principal trunks (Fig. 131). This arch becomes gradually larger, so as to form the freest mode of communication between the heart and the descending aorta; it subsequently becomes, in fact, the arch of the aorta. The trunk, which is connected with the right ventri- cle, on the other hand, and which subsequently^ becomes the pulmonary CIRCULATION IN ANIBIALS. 257 artery, transmits its blood through the fourth arch of the left side (the two primary tubes twisting round each other) and the two fifth arches; the latter were seen in the tadpole to pass to the lungs exclusively from the first, and to increase in development only as the supplies of blood to the branchice were cut off. The fifth arch on the left side is gradually obliterated, so that the pulmonary artery, P, is ultimately formed by the fourth arch of the left side and the fifth arch of the right. The original prolongation of the former trunk, 9, to meet the descending aorta, still remains; so that a portion of the blood sent from the right ventricle is transmitted through this communicating branch directly into the descend- ing aorta, just as in the adult crocodile. After the first inspiration, hoAvever, the whole of the blood transmitted through the pulmonary artery passes into the lungs, and does not enter the aorta until it has been returned to the heart; and this communicating vessel soon shrinks and becomes impervious. The general plan of the changes Avhich occur in the vascular system of the Mammalia is the same as that which has been described in Birds; the differences being only in detail, — as for instance that the aortic arch is formed not from the right but from the left bran- chial vessel. 328. Up to the period of the hatching of the egg in Birds, and the separation of the foetus from the parent in the Mammalia, the circulation retains some peculiarities characteristic of the inferior type which is per- manent in the reptile tribes. Of the blood which is brought by the venous trunks to the right auricle, part has been purified by transmission to the respiratory surface (the membrane lining the egg in birds, and that forming the placenta in mammalia), whilst a part has been vitiated by circulation through the system. The former is brought from the abdomen by the ascending vena cava, mixed with the blood which has circulated through the lower extremities; whilst the descending cava brings back that which has passed through the capillaries of the head and upper extremities, and which, having received no admixture of arterial blood, is not fit to be again transmitted in the same condition. It will be recollected that a communication still exists between the two auricles, the foramen ovale yet remaining pervious; and by a fold of the lining membrane of the right auricle, forming the Eustachian valve, the ascending and descending currents are so directed that the former (con- sisting of the most highly arterialised blood) passes at once into the left auricle, whilst the latter flows into the right ventricle. From the left auricle, the arterial blood is propelled into the left ventricle, and thence through the arch of the aorta to the vessels of the head and upper ex- tremities, a comparatively small part finding its way into the descending aorta. The venous current is propelled through the pulmonary artery; but the lungs not yet being expanded, little of it is transmitted to these organs, and the greater part finds its way through the ductus arteriosus 258 SPECIAL AND COMPARATIVE PHYSIOLOGY. into the descending aorta, where it mixes with the remainder of the first mentioned portion. This trunk not only supplies the viscera and lower extremities, (which are thus seen to receive, as in reptiles, blood of which only a portion has been oxygenated), but sends a large proportion of its contents to the umbilical vessels, by which it is conveyed to the oxygenating organ, and returned again to the venous trunk of the abdo- men. The peculiar course taken by the blood through the heart, which was suspected from anatomical investigation, has been recently demon- strated by means of coloured injections of plaster-of-paris by Dr. J. Reid.* Another peculiarity in the foetal circulation is the mode in which the blood passes through the liver. In the adult Mammalia, as in Birds and all other Yertebrata, the blood which has circulated through the intestinal viscera is collected into a large venous trunk, the vena porta, which subdivides again into capillary vessels in the liver; the object of this arrangement is not the nutrition of that organ, which is effected by the branches of the hepatic artery, but the supply of its secreting surface for the elimination of the bile; and the hepatic vein consequently receives, and conveys to the ascending vena cava, both the blood which has been transmitted through the nutritive capillaries of the liver by the hepatic artery, and that which, after ramifying through the nutritive capillaries of the intestines, has traversed the secreting capillaries of the liver. The vena porta in the foetus, however, receives not only the venous blood of its abdominal viscera, but the arterial blood sent from the respiratory surface; and as it would not be desirable that the whole of this should pass through the liver before being transmitted to the heart, an immediate passage into the vena cava is provided for a part of it in the ductus venosus, which, not being required after birth, shrivels into a ligament. 329. The knowledge of the different stages of the development of the vascular apparatus enables us to explain many of the malformations which it occasionally presents. One of the most common of these gives rise to the malady termed cyanosis or the blue disease; this results from the foramen ovale which establishes a communication between the auri- cles remaining open after pulmonary respiration has been established, so that a considerable portion of the blood transmitted to the right cavity passes into the left, without being previously arterialised by passage through the lungs. Persons thus affected have always a livid aspect, from the quantity of venous blood circulated through the arteries; they are deficient in muscular energy, and in power of generating heat, and they are seldom long lived. A consequence partly similar would pro- bably have resulted from a curious malformation mentioned by Kilian, had the infant remained alive; in this case, the aortic arch had not been developed, so that the primary aortic trunk only gave off the vessels to * Edinb. Med. and Surg. Journ. vol. xliii. pp. 11 and 308. CIRCULATION IN ANIMALS. 259 the head and upper extremities; whilst the communicating branch between the pulmonary artery and descending aorta, which usually is of a secondary character, constituting the ductus arteriosus, was here the only means by which the blood could be transmitted to the latter, so that the circulation through the lower part of the trunk and extremities would have been entirely yenous. A malformation of this kind in a diminished degree has not been found incompatible with the continuance of life; several cases being on record in which the ductus arteriosus has remained pervious, and has brought part of the blood from the pulmonary artery to the descending aorta. Cyanosis is of course, as in the foririer instance, the result of this imperfect arterialisation; and the individual is reduced, as far as his vascular system is concerned, to the condition of the Croco- dile. An an-est of development at an earlier period may cause still greater imperfections in the formation of the heart. Thus, the septum of the ventricles is sometimes found incomplete, the communication between the cavities usually occurring in the part which is last formed, and which in most reptiles remains open. In other cases it has been altogether wanting, although the aorta and pulmonary artery were both present and arose side by side from the common cavity; and this form of the circu- lating apparatus is evidently analogous to that presented by Reptiles in general. A still greater degradation in its character has been occasionally evinced; for several cases are now on record in which the heart has pre- sented but two cavities, an auricle and a ventricle, and thus corresponds with that of the Fish; in one of these instances the child had lived for seven days, and its functions had been apparently but little disturbed. The occasional entire absence of the heart has already been noticed; and coexistent with this, there is always great deficiency in the other organs, the brain and sometimes the liver and stomach being undeveloped. The bifid character of the apex, which presents itself at an early period of the development of the heart, and is permanent in the Dugong, sometimes occurs as a malformation in the adult human subject; evidently resulting, like the others which have been mentioned, from an arrest of develop- ment. On similar principles some occasional peculiarities noticed in the distribution of the vessels may be accounted for, of Avhich a striking ex- ample Avill be presently given. The Vena Cava is occasionally observed to consist of two parallel trunks, which are sometimes partially united, and then separate again; the explanation of this fact is different, and is to be sought for in the history of the development of the venous sj^stem in general. We have seen that in many of the loAver animals, such as the Crustacea, where the arteries are perfect canals having distinct coats, the veins seem to be merely channels through the tissues having a much less definite character: in like manner, at an early period of the foetal deve- lopment of the higher animals, several small vessels are found where one vein subsequently exists; and, if the coalescence of these has been from s 2 260 SPECIAL AND COMPARATIVE PHYSIOLOGY, any cause checked, they will remain permanently separated to a greater or less extent. 330. Several interesting varieties have heen detected in the arrange- ment of the principal trunks given off from the aorta; and though we cannot account for them on the principles already mentioned, it is not a little curious that nearly all of these irregular forms possess analogues in the arrangements which are peculiar to some or other of the Mammalia. The mode in which the cerehral and brachial vessels usually arise in the human subject is shown in the adjoined figure, a, where a, b, is the arch b a b a of the aorta, 1 and 2 the trunks of the right carotid (which supplies the head) and the right subclavian (which is distributed to the upper extre- mity), arising by a common trunk — the arteria innominata; while the left carotid, 3, and the left subclavian, 4, arise separately. At b is seen a distribution which is rare in the human subject, the two carotids aris- ing by a common trunk, and the right as well as the left subclavian being given off separately; this is the regular arrangement of branches in the Elephant. It is not so unusual for all the branches to arise from single trunks, as at c; and this appears to be the regular type in some of the Cetacea. Sometimes, again, there is an ai'teria innominata on each side, subsequently dividing into the carotid and subclavian, as at d; and on this plan the branches are distributed in the Bat tribe, and also in the Porpoise, A not unfrequent variety in the human subject, is for both carotids to arise with the right subclavian from a single trunk, as at E, the left subclavian coming off by itself; this is observable as the regular form among many animals, being common among the Monkey tribe, the Carnivora, the Rodentia, &c. Another variety which is not unfrequent is shown at f, the vertebral artery on the left side, 5, which usually arises from the subclavian, springing directly from the aorta ; it is on this plan that the branches are given off in the Seal. A form which is very uncommon in man is that represented at g; here the aorta divides at once into an ascending vessel, from which the two subclavian and two carotid arteries arise, and a descending trunk ; this is the regular distribu- tion of the vessels in ruminating animals, and appears to be most general in Mammalia possessing a long neck. Lastly, at h, is seen a form which evidently results from an arrest of the usual changes in the arterial trunks ON INTERSTITIAL ABSORPTION. 26'1 described in § 325, 7 ; the aorta continuing to possess a double arch, from the ascending part of which the subclavian, external carotid, and internal carotid arteries are given off on each side, the single descending trunk being formed by the union of the two original branches. This, it will be recollected, is the normal type of formation in Reptiles.* CHAPTER VII. On Interstitial Absorption. 331. The circulating system already described not only serves to con- vey to parts of the organism remote from the absorbent surface the alimentary materials required for the nutrition of their tissues : but, in the loAver tribes of animals, it returns to the central reservoir the portion of the circulating fluid which has not been so employed ; and is also the means of conveying to it, for the purpose of subsequent excretion, those particles of the solid structure which, fi-om tendency to decomposition or some other cause, are not fit to be retained in it. Moreover, the general vascular system seems occasionally concerned in the absorption of fluid fi-om the external surface as well as from the walls of the digestive cavity (§ 279). But in the Vertebrated classes, which possess a special set of vessels for the absorption of chyle from the intestines (§ 262), we also find a system of tubes of corresponding structure ramifying through every part of the system, to which the function of absorption seems more particularly dele- gated. The lymphatics^ as they are termed, are distributed through almost every tissue in the body, and are believed to form a large propor- tion of the fibres of cellular structure; they are especially abundant beneath the skin, where they form a close network so universally diffused that if successfully injected it is scarcely possible to find a spot not tra- versed by them. The minutest of these tubes are, however, much larger than the capillary vessels connecting the arteries and veins; and it seems now generally allowed that they do not take their origin in them (as some have maintained), but that they commence, like the lacteals, mthout open extremities, their contents being derived by imbibition or endosmose fi-om the surrounding tissues. The minute lymphatic canals xmite, like the veins and lacteals, into larger trunks ; and by these the fluid which is taken * In the foreg'olng account of the development of the vascular system, the author has availed himself freely of the valuable papers of Dr. Allen Thomson, in the Edinb. Philos. Journal, vols. IX. and x.; in the sketch of the malformations of the heart, he has made use of the paper of Dr. Pag'et in the Edinb. Med. and Surg'. Journal, vol. xxxvi. ; and the last paragraph, with the accompanying figures, has been entirely derived from the magnificent work of Tiede- mann on the Arteries. 262 SPECIAL AND COMPARATIVE PHYSIOLOGY. up by tlie absorbent extremities, is conveyed to tbe principal veins. The number and mode of tbe communications between the lymphatic and venous systems differs in each, class of animals; it will be seen that they are most numerous in Fishes, but that the separation between the two kinds of vessels becomes more complete, until, in Man and the higher Mammalia, the contents of the lymphatics are united with those of the lacteals, and are poured into the venous system by one trunk only. 332. The lymphatic system is exhibited in its simplest and most dif- fused form in pishes, the lowest class in which it has been observed. Indeed it has not been detected in some of those vermiform species of which the conformation is so simple, and in Avhich the traces of verte- brated structure are so slight. The minute vessels are distributed exten- sively through both the superficial and deep-seated parts of the body; but their coats are peculiarly thin and soft, and their tubes, which are destitute of valves, extremely distensible. Numerous plexuses are formed by the convolutions and anastomoses of their trunks, in different parts of the body, especially around the veins, which may be regarded as the first indications of the so-called glands which are presented in the higher classes. Although a considerable proportion of the lymphatic trunks unite with the lacteal vessels to form principal canals (corresponding with the thoracic duct in higher animals), which empty their contents into the systemic veins near the heart, there are many other communications between the two systems, as Fohmann appears to have satisfactorily demonstrated. A pulsating cavity has been described by Dr. M. Hall, as belonging to the vein of the tail in the eel ; but it is more probably an appendage of the lymphatic system, like those to be presently described in Reptiles. 333. Reptiles present several interesting peculiarities in the con- formation of their lymphatic system. As in fishes, the vessels are generally destitute of valves, though these may occasionally be observed in the large trunks; and the extended plexuses do not yet exhibit the concentration which they present in Birds and Mammalia. In this class, pulsating dilatations of the lymphatic trunks, or lymphatic hearts^ have been dis- covered in different parts of the body. In the frog there are two pairs of these, one situated just under the skin, through which its pulsations are readily seen in the living animal, immediately behind the hip joint; and the other pair is more deeply seated at the upper part of the chest. The former receive lymph from the posterior part of the body, and pour it into the veins proceeding from the same part ; the latter collect that which is transmitted from the anterior part of the body and head, and empty their contents into the jugular vein. Their pulsations are totally independent of the heart and of the acts of respiration, since they continue after the removal of the former, and for an hour or two after the apparent death and complete dismemberment of the animal. Neither are they synchro- ON INTERSTITIAL ABSORPTION. 263 nous with eacli other on the two sides of the body, nor always performed in the same space of time; for the pulsations are not only generally irre- gular, but sometimes exhibit long and frequent intermissions; when in constant action they occur about sixty times in a minute. A similar pair of vesicles has been detected in salamanders and lizards, where they are situated near the root of the tail, and are connected, in like manner, with the veins of the loAver extremity; they have also been discovered in ser- pents, where they lie under the last rib. One of these, which has lately been described in the Python bivitatus, a large species, is represented laid open in Fig. 133. It is about nine lines in length and four lines in breadth; and has a thick muscular coat with four muscular columns running across its cavity. It communicates with three lymphatic trunks, and with two veins; and all the orifices are provided with valves formed by folds of the smooth membrane that lines the cavity. In the turtle, however, there is no communication between the pelvic veins and the lymphatic trunks, and these pulsating vesicles do not exist. In the Reptiles in general, the lymphatic system is stated by Panizza to attain a prodigious development, when its extent is contrasted with that of the blood-vessels : but this exten- sion is probably rather apparent than real; and, like a similar extension of the respiratory system in insects and birds, arises from the want of that concentration which it elsewhere exhibits. It seems probable that some vital changes are produced in the fluid both of the lacteals and Ijnnph- atics, during its passage through their tubes; and the prolongation of these appears essential to their performance (see chap. viii.). 334. In BIRDS we find the lymphatic system existing in a more perfect form, its trunks being provided with valves, and the diffused plexuses being replaced by glands or ganglia, which seem to be only another aspect of the same structure. These can scarcely be compared with the other organs designated by that name, since their function does not appear to consist in the separation of any products from the blood which passes through them. They consist of a number of convolutions of lymphatic trunks closely approximated to one another, through which veins also are dispersed. Although direct communicatons between these two systems of tubes have been alleged to exist in the lymphatic glands, the statement is probably erroneous; the passage of fluid from one to the other, which can be made to take place by forcible injections, being due either to a rupture of the coats of adjoining vessels, or to the transudation of fluid through those invisible pores which must exist wherever absorption is performed. It seems probable that in the lymphatic glands some mutual changes are effected between the blood and the lymph, through the walls of their respective vessels; but of the nature of these, we are entirely ignorant. Similar formations occur on the lacteal trunks; and they generally exhilnt the same degree of complexity vnih those of the lymphatics. The absorbents of Birds terminate principally by two thoracic ducts, one on 264 SPECIAL AND COMPARATIVE PHYSIOLOGY. each side, which enter the jugular veins by several orifices; there are, however, two other entrances, as in Reptiles, into the veins of the lower extremity. These are connected with two large dilatations of the lymph- atic trunks, which are evidently analogous to the lymphatic hearts of reptiles, but which do not seem to have any power of spontaneous movement. In the Goose, they are about the shape and size of a kidney- bean, and are situated in the angle between the tail and the thigh. They were supposed by Panizza to possess an automatic power of alternate contraction and dilatation; but this motion has been shown by Miiller to be due to the respiratory actions, being synchronous with them, and ceasing when they are interrupted. 335. In MAMMALIA, both lacteals and lymphatics terminate entirely in the thoracic ducts, of which that on the left side is usually the largest, the right trunk receiving only the lymphatics of the right side of the head and upper extremity, with those of the right lung and right side of the liver. These terminate at the angle formed by the junction of the sub- clavian and internal jugular veins, on the two sides respectively; and it is a beautiful instance of mechanical adaptation, that this should be a point of much less resistance to the entrance of a fresh current, than if the aperture had been made in the side of a singla trunk. Although these are the only two canals by which the lymphatics usually communicate with the veins in man, their number is greater in many species of Mam- malia, although they all terminate in the same part of the venous system. Thus, the left thoracic duct often resembles rather a plexus of vessels than a single tube, branches proceeding from it and then reuniting, and at last terminating in the veins by several apertures. Sometimes it consists throughout of two tubes, which anastomose with each other and with the duct on the right side, and terminate separately in the veins; and in the Pig a branch of communication is sent off to the vena azygos, which is a small trunk running in proximity with it along the spinal column. All these modes of distribution occur as irregularities of conformation in the human subject, the former not being uncommon; the last, however, is rare. In the lymphatic system of the Mammalia we witness its most concentrated and highly developed form; the vessels are copiously pro- vided with valves; and their parietes are firmer than in the lower classes. Instead of the extensive plexuses of Fishes, we find small dense lymph- atic glands disposed in different parts of the system; these are more numerous than in Birds, and are most abundant in the lymphatics con- nected with organs which may receive or imbibe substances from without, such as the digestive cavity, the lungs, and skin; whilst they are smaller and more scattered upon the absorbents which arise from deep-seated organs, the substance of the limbs, &c. It is somewhat curious that where two lymphatic vessels unite, the common trunk is frequently not larger than either of those which form it; and thus the large canals do ON INTERSTITIAL ABSORPTION. 265 not exhibit by any means tbe same pi-oportion to the primary branches of which they are composed, as the great veins or arteries to their capillaries. 336. The peculiar characters of the lymph, and the sources from which it is derived, will be considered in the next chapter. Although a part of it is certainly derived from the fluid portions of the blood which have transuded through the sides of the capillaries for the nutrition of the adjacent tissue, there appears little doubt that the lymphatics may, like the lacteals, take up fluid brought into relation to them fi-om -without. In fact, the cutaneous lymphatics are to the external surface what the lacteals are to that portion of it which is reflected iuAvards to bound the digestive cavity (§ 262); and it is a little remarkable that Ave find both sets of vessels developed at the same part of the animal scale, the func- tions of each having been previously performed by the general circulating system. That it is by means of the lymphatics, and not by the veins, that substances applied with friction to the skin are chiefly absorbed, appears evident from the circumstance that if these be of an imtating cha- racter, red streaks appear in the course of the lymphatics, and the neigh- bouring glands are swollen : their absorbent power is also evinced by the fact that the branches sui-rounding collections of peculiar animal fluids have been seen filled Avith those fluids; thus, when the bile-ducts have been obstructed, the lymphatics of the liver haA^e been seen to contain fluid of a yelloAV colour Avhich contained the components of bile. The absorbent power of the lymphatics of the skin is shown by an experiment of Schre- ger's. Having tied a bandage round the hind-leg of a puppy, the limb was kept for twenty-four hours in tepid milk; at the expiration of this period the lymphatics were found full of milk, — the veins contained none. In repeating this experiment upon a young man, no milk could be detected in the blood draAvn fi-om a vein. A striking experiment Avhich led to the same conclusion is mentioned by Miiller. He placed a fi-og Avith its posterior extremities entirely immersed in a solution of prussiate of potash, and kept it so for tAvo hours. He then washed the animal carefully, and having Aviped the legs dry, tested the lymph taken from under the skin with a persalt of iron; the lymph assumed immediately a bright blue colour, Avhile the colour of the serum of the blood Avas scarcely perceptibly affected by the test. In a second experiment, in Avhich the frog was kept only one hour in the solution, the salt could not be detected in the lymph. 837. There is doubt, hoAvever, that in many cases the A^eins participate in the function of absorption even more actively than the IjTnphatics. Thus, Avhen a solution of prussiate of potash Avas injected into the lungs by Mayer, it Avas detected AAathin five minutes in the serum of the blood, and long before it could be traced in the chyle; and it Avas found in the left side of the heart (to Avhich the pulmonary veins appeared to have conveyed it) before a trace of it could be detected in the right caA'ities ; 266 SPECIAL AND COMPARATIVE PHYSIOLOGY. while, if tlie absorption had been effected by the lymphatics, the course of the lymph being first into the venous blood of the body, the salt absorbed ought to be first detectible in the right cavities of the heart. It has been also found that when the veins have been laid bare, and the poisonous substances have been applied to their exterior, their usual effects upon the system have followed. It might not perhaps be dijB&cult to reconcile these apparently contradictory results, by attention to the predominance of each system of vessels in the part to which the absorbed substance has been applied. Thus, as already mentioned, the ramification of the lymphatics in and beneath the skin is most universal; whilst the very object of the lungs being to expose the blood to the air, its capillaries necessarily approximate more closely than the lymphatics to the absorbent surface, and will consequently be more directly affected by external agents. With regard to the last experiment it is to be recollected, that the impedi- ment arising from the tissues in which the vessel was enveloped being removed, it might be expected that it should readily imbibe fluid through its thin parietes; and it can scarcely be deemed improbable that, if a lymphatic trunk of the same size existed in the body, it would absorb fluid applied to its surface Avith even greater rapidity. 338. But absorption takes place, not merely from the surface of the body or its cavities, but from the tissues themselves. One of the most evident examples of this process is the wasting of the body from the absorption of fat, which takes place during almost all diseases, and during hybernation. The removal of fluids which have been effused through the tissues, as in dropsical swellings, or of the colouring matter of the bile which has been deposited in jaundice, are instances in which this function tends to repair the effects of disease; but there are many cases in which its too great activity in itself constitutes disease, by causing a loss of sub- stance which impairs the continuity of the tissue. There are other circumstances, again, in which interstitial absorption, taking place as one of the regular series of vital changes, without a corresponding deposition of nutritive materials, produces atrophy or wasting of an organ; but this is sometimes a natural condition, and effects those alterations in the rela- tive proportions of different parts of the body which are so remarkable at progressive stages of growth. Thus are produced the disappearance of the tail of the tadpole, when it is metamorphosed into a fi'og; the removal of the membrane which closes the pupil in the foetus; the formation of cells or even of large cavities in bones which were originally filled with medul- lary pulp; the wasting of the thymus gland from infancy to the twelfth year, and many other effects of the same kind. 339. It may still be doubted to what extent these phenomena are occasioned by the lymphatics, or how far the veins partake in their production. In bony tissue, lymphatics have never yet been demonstrated; still, however, their existence cannot be altogether denied, since cellular ON INTERSTITIAL ABSORPTION. 267 tissue is the basis of the structure, and this is always permeated by absorbents. If they are really absent, the process of absorption, which is often very actively performed in the bones, is manifestly due to the veins. Their cells are developed in the child long after the bone is formed, and increase in size by the agency of the same process; some of these cells subsequently attain large dimensions, as those which form the frontal and sphenoidal sinuses, which are not developed until the period of youth; and in birds the long bones, which are at first filled with a spongy medulla, are afterwards completely hollowed, and brought into connection with the respiratory organs (§ 412). The roots of the first teeth are absorbed at the time they are shed; but this is perhaps due, not to inter- stitial absorption taking place in their own substance, but to the same kind of process as that by which the Mollusca are enabled to modify the form of their shells (§ 100). When atrophy is a general condition of the system, a certain order is usually observed in the wasting of the tissues ; fat being absorbed first, then cellular tissue, and then muscle, bone, cartilage, and tendon. Various artificial agents may produce the same effect with disease; thus, long continued pressure, by putting a stop to nutrition, may cause every tissue to be absorbed. Iodine, on the other hand, whilst it probably impairs the nutritive functions, stimulates the absorbent processes themselves, and hence is advantageously employed for the removal of indolent tumours, especially those connected with the lymphatic glands. 340. Of the causes of the motion of the fluid contained in the lym- phatic trunks in those higher animals which are unprovided with propelling cavities, little satisfactory explanation can be given. It is probably due in part to the force created by absorption at their origins, like the ascent of the crude sap in the stems of plants; and it may be assisted by the occasional pressure resulting from muscular action on the trunks, which will force their contents in the direction permitted by the valves. But these explanations, as well as all others which have yet been offered, are insufficient to account for the fact that if a ligature be put on the thoracic duct, the lower part will be distended even to bursting. 268 SPECIAL AND COMPARATIVE PHYSIOLOGY. CHAPTER VIII. NUTRITION AND FORMATION OF TISSUES. General Considerations. 341. The nature of the absorption of alimentary fluid, and the means of its transmission, when required, to distant parts of the system, having now been considered, the question next arises how the nutritive ingre- dients thus introduced are applied to the development and maintenance of the several portions of the structure. The conversion of the inorganic elements which constitute the food of Yegetables, into tissues of complex formation, and possessed of qualities entirely diiferent from those of their components, is a process in which several stages may be traced Avith con- siderable distinctness; and although the aliment intoduced into the Animal system never exists in a state of corresponding simplicity, yet the alteration which it undergoes in composition and properties are scarcely inferior in extent or peculiarity of character. With regard to the nature of these changes, our knowledge is very limited; it is derived principally from the study of their most evident effects; but there can be little doubt that the imperfection of our present means of observation has caused, and will continue to produce, great ignorance of what may be in reality their most important particulars.* 342. In cases in which the processes of nutrition are manifestly of a complex nature, and where the conversion of the alimentary materials into organised tissues does not so immediately follow their absorption as in the simplest forms both of the animal and vegetable kingdoms, a con- siderable alteration may be traced in the character of the nutritious fluid between the time of its first reception into the system and its application to its ultimate purpose. This alteration consists in the formation of certain new combinations of its elements, into substances ready to be assimilated by the various tissues, — that is, to be converted by organis- ation into part of their own structure. It is probable that even in beings whose simplicity of conformation prevents us from discerning any change intervening between the absorption of aliment and the growth and re- novation of the tissues, the same process really takes place; as it would seem to be a general law of organisation that no solid textures can as- similate, or convert into living structures like their own, matter which * Thus, the recent application of polarised light to the examination of vegetable juices, has shown that during the progress of vegetation, important differences may be detected in the nutritious principles, which were not previously supposed to exist ; that it is not improbable that its more extended employment may have a powerful influence in disclosing to us some of the most recondite processes of Nature's laboratory. See Taylor's Scientific Memoirs, vol. i. p. 584, et seq. NUTRITION AND FORMATION OF TISSUES. 269 has not been previously formed into combinations differing essentially from those which exist in the inorganic world. Thus, we find in the blood a yariety of ingi'edients, most of them peculiar to animal bodies, which have been produced subsequently to the first reception of its materials into the digestive cavity, and which are prepared for the re- paration and maintenance of the several tissues; and, in like manner, the elaborated sap of vegetables contains other principles peculiar to the vegetable structure and adapted to its maintenance. These are called, therefore, organisahle products; and are partly identical with the sub- stances denominated by the chemist, proximate principles: amongst the latter, however, are found many which do not furnish materials for organisation, but are rather its results, — being the constituents of the various secretions, which are either carried out of the system altogether, or stored up within it for particular purposes. 343. Of the means by which the simple ingredients that enter the absorbent system are converted into organisable products, little is posi- tively known; but the result of late chemical researches certainly favours the idea that the affinities by which their elements are held together are not diiferent from those which operate in the production and changes of the combinations presented to us in the inorganic world; and that, being subject to the same laws, they may be made to exhibit analogous pheno- mena. In forming an opinion on this subject, it is necessary to keep in view that the conversion of organisable products into organised tissues is a process entirely different from the production of the former, and takes place under the laws of vitality alone. In fact, the power of communi- cating to nutritious matter their own structure and properties, which is the most obvious characteristic of living beings in general, is also peculiar to each of their component textures. Thus, from the same circulating fluid of uniform character in every part of the body, is developed in one spot muscular fibre, in another nervous tissue, in another solid osseous matter, and so on; the new matter, in every instance, being deposited in continuity with the previously-existing structure. An organised character is not, however, peculiar to living solids; for some traces of it may be detected in the circulating fluid, Avhich is also possessed of properties that must be considered as vital, since they differ from any which a mere mechanical admixture of the ingredients could present. Thus, the phe- nomena of the coagulation of the blood cannot be satisfactorily explained mthout this admission; and those exhibited by the descending or ela- borated sap of vegetables seem to place it in the same light (§346). It would seem, then, that the solid parts, which most unequivocally exhibit this peculiar character, not only obtain from the nutritive fluid the materials necessary for the reparation and extension of their structure, to which they communicate their vital properties, — but, in absorbing aliment from without, and converting it into forms most adapted to their mainte- 270 SPECIAL AND COMPABATIVE PHYSIOLOGY. nance, also endow it, whilst still fluid, with qualities which prepare it for its final assimilation. In tracing the alterations which occur in the ali- mentary fluid, from the time of its first ahsorption to its ultimate desti- nation, we have therefore not only to enquire into the changes in the chemical relations of its constituents, but into the traces of organised structure and vital properties which it manifests. It is obvious that these changes can be more distinctly studied in proportion as their different steps are separated from one another; and accordingly it will be desirable to examine them first as they occur in the higher classes of plants and animals, and then to apply the results thus obtained to those of more simple conformation. Nutrition in Plants. 344. In the usual condition of most Yascular plants, there is no doubt that the greatest proportion of the fluid imbibed into the system is derived from the soil surrounding the roots; and that it holds in solution carbon in various forms, which is ultimately to enter into new combinations with the other elements, for the production of the organisable products, gum, sugar, &c. which are the principal sources of the maintenance of the tissues. It would seem that the fluid thus absorbed is in all plants nearly the same under corresponding circumstances; though of the mineral ingredients of any soil, some will be selected most abundantly by one plant, some by another. The sap in ascending the stem soon, however, undergoes an alteration, by dissolving the secretions which had been laid up from the previous year; and this admixture, whilst it furnishes a necessary condition for the continuance of the absorption, may also not improbably be an importfint aid to the process of conversion of the crude materials which is taking place at the same time. The ascending sap, when examined sufficiently near to the roots, is almost uniformly found to be of very low density, and to possess no characteristic properties in dififerent orders of plants. In its upward ascent, however, its specific gravity increases, from the cause just mentioned; and the quantity of sugar and gum contained in it is sensibly greater. How far these are newly-formed products, or are merely dissolved by the fluid in its passage through the stem, it is not easy to say. It is probable, however, that the greatest addition made to the solid tissues of vegetables is effected by the absorption of carbon from the atmosphere by the surface of the leaves; and the real process of the conversion of the oxygen, hydrogen, and carbon thus obtained from the surrounding elements, into organisable products, cannot be said to take place until the crude sap arrives there. 345. The crude sap brought to the leaves consists of little more than water ; a very abundant absorption of which is necessary to introduce into the system a sufficient quantity of the mineral and other ingredients which are so sparingly diffused through it. The separation of a large NUTRITION IN PLANTS. 271 proportion of fluid by the process of exhalation is, therefore, one of the principal changes required for the concentration of its nutritious contents; and it will be subsequently shown (§ 431) that the quantity retained in the system frequently bears a most inconsiderable relation to that originally received into it. A plant in active vegetation and exposed to the influence of solar light, obtains a considerable supply of carbon from the atmosphere, by means subsequently described (§ 373), and thus is capable of adding to the amount of its organisable materials; for these consist of little else than carbon united with the elements of water in varying proportions. The sap elaborated by these processes has undergone a very evident change in its character; for instead of being a thin watery liquid nearly the same in all plants, it is dense and viscid, and contains not only the materials necessary for the nutrition and the formation of the difi"erent parts, but the products characteristic of each order which constitute its peculiar secretions. It is well known that the juices expressed from the leaves always contain these principles, although sometimes in a diluted form, a particular part of the structure being frequently adapted to separate and store them up. The crude sap may afibrd a refreshing beverage, whilst rising in abundance, although that which is descending after elaboration cannot be tasted with impunity. Thus, the inhabitants of the Canary islands tap the trunk of the Euphorhia canariensis, and draw off the ascending current for this purpose, although the proper juice of the plant is of a very acrid character. 346. In the ascending sap there is but little trace of organisation, nor does the fluid exhibit any properties which can be regarded as vital. Some traces of globules have beec observed high up in the stem; but these may have been derived from the previously assimilated matter. In the descending sap, or proper juice, on the other hand, globules are very abundant; and they may be seen to move not only within the canals of the living plant, but even when the fluid is drawn from it. According to the observation of Amici* the glutinous sap of the vine when removed from the stem assumes, during the species of coagulation which it under- goes, regular forms, closely analogous to those of the lower Confervse on the one hand, and to the elementary tissues of which it supplies the materials, on the other. When wounds have been made in the course of its flow, it is evidently from the exudation which takes place at their edges that the regeneration of substance takes place; and it scarcely admits of a doubt, therefore, that the ingredients it contains are not mere chemical combinations of elementary bodies into organisable products, but that they are to a certain extent possessed of vital properties, which have been probably communicated to them simultaneously with the traces of organisation they exhibit. The proper juice, then, seems to contain the materials of the solid parts which compose the structure of the * Annales des Sciences Naturelles, torn. xxi. 272 SPECIAL AND COMPARATIVE PHYSIOLOGY. plant, as well as of its various secretions and excretions: the former existing to a certain extent in the crude sap, and being but little prone to spontaneous decomposition; the latter appearing to be compounds of a higher order, being more removed from inorganic substances in form and composition, and in general more liable to the separation of their elements. The gum and sugar contained in the crude sap, with the additional carbon derived by the leaves from the atmosphere, would seem to be readily convertible into the materials essential to the growth of the plant, and these are formed under almost any circumstances which permit the maintenance of its existence; but it requires a perfect state of the vegetative powers, and the presence of the necessary external stimuli in a high degree, to produce some of those peculiar secretions of which such a remarkable variety exists in the vegetable kingdom. 347. The organisable product most universally existing in the proper juices of plants, is Gutn. It is found in the bark and wood of all trees, and is present in such abundance in several as to flow from the bark Avhen wounded, or when its surface cracks; this exudation is, therefore, to be considered in the light of an accidental hemorrhage, and not of a regular excretion. The following reasons may be specified for regarding gum as the essential ingredient in the nutritious fluid. 1. It exists in all vascular plants without any known exception. 2. It is found in all their organs, particularly in the bark where some of the special secretions appear to be formed. 3. Its properties seem favourable to the life of plants, which grow readily in a solution of it, if not too viscid. 4. Its composition, which is merely carbon in union with water, is such as might be expected from the action of the leaves upon the crude sap. 5. This composition differs little from that of the substances which form the basis of the organised textures; and these substances are convertible into gum by sim- ple chemical processes. — Various modifications of this principle exist in different vegetables; but they may all be considered as combinations of pure gum \\dth other principles. 348. Sugar is considered by Dr. Prout as being, from its crystalline form and simple constitution, the most allied of any of the organic pro- ducts to inorganic combinations. Several varieties of it may be obtained from different sources; that of the cane, which seems the purest, and which may probably be regarded as the type of the rest, is composed, according to Dr. Prout, of 9 atoms of carbon and 8 of water. The sugar of honey contains 9 atoms of carbon and 12 of water, and may thus be regarded as a compound of water and pure sugar, or a hydrate of sugar ; other forms of this principle, differing somewhat in their sensible proper ties, might probably be represented in the same manner. There are some instances in which sugar appears to be the first organic compound formed by the combination of the external elements, as when abundantly existing in the ascending sap of trees, — the maple, for example; frequently, how- NUTRITION IN PLANTS. 273 ever, its formation is the result of other processes, as when it is produced by the conversion of starch in the manner to be presently mentioned. It appears to be the form of nutriment best adapted for the development of rapidly-growing succulent parts, (thus the sugar in the stem of the cane is exhausted by flowering, and that which is so abundantly contained in the cortical system of the beet, is ultimately carried into the upper part of the plant, and similarly dimirdshed by its inflorescence) ; and whenever a store of nutriment has been previously laid up for their maintenance, it is made to assume the form of sugar before being applied to its destined purpose. 849. Neither of these principles ever exhibit traces of organisation, and they may therefore be regarded as strictly organisahle products; but there is a peculiar form of the first, which appears in some respects intermediate between its usual condition and that of the living tissues which are formed from it. This is Fecula or starch, a principle very universally difinsed through the vegetable kingdom. Starch, when removed from the plant, exists in the form of minute granules, each of which, if examined mth the microscope, is found to consist of a little vesicle, having an insoluble envelope formed of a kind of organised membrane, and containing within it a substance very analogous to gum. These grains resist the actions of many chemical agents; but when exposed to a heat of about 160°, the pellicle bursts, and its contents are liberated; and this is the explanation of the fact tha,t starch once dissolved in hot water can never be restored to its original form. Fecula may in fact be considered as little else than gum divided into minute portions, each of which is enclosed in a membranous cell; and in this state it appears to ansAver very important ends in the vegetable economy. It is remarked by De CandoUe that "while gum itself may be considered the nutrient principle of vegetation, diffused freely through the structure of the plant, and constantly in action, starch is apparently the same substance stored up in such a manner as not to be readily soluble in the circulating fluids," thus forming a reservoir of nutritious matter, which is to be consumed — like the fat of animals (which it closely resembles in structure) — in supporting the plant at par- ticular periods. Thus, we find it stored up in the seeds of most species, either forming a separate albumen as in the Grasses, or taken into the structure of the embryo and constituting the mass of the fleshy cotyledons, as in the Leguminosae, &c. (§ 49, 50); in each of these cases it serves as a magazine of food for the nutrition of the embryo, until the development of those organs which enable it to maintain an independent existence. Similar reservoirs are occasionally formed by the enlargement of the stem into tubers, for the nutrition of the buds to be developed from them, — as in the Potatoe, Arrowroot plant, &c.; or by the accumulation of the same material in fleshy roots, bulbs, &c., fi-om which stems rapidly grow up. Fecula is also found abundantly in the soft interior (improperly called pith) of the stem of the Sago Palm and other Endogens, where it seems 274 SPECIAL AND COMPARATIVE PHYSIOLOGY. destined to assist the evolution of the young leaves; and in the fleshy expansions of the flower-stalk (termed receptacles)^ on which, in many orders, the flower is situated, and in Avhich it seems to answer a corres- ponding purpose. 350. In all these cases, the immediate end of the accumulation of fecula is that it may he ready for the nutrition of the young germ hefore it is capable of obtaining food for itself; and it may be observed that the deposit continues to increase as long as the plant is in active vegetation, — arrives at its maximum,^ — and then, remaining stationary during the winter, begins to decrease in the spring. The deposition of fecula fulfils, therefore, an obvious purpose in the Vegetable economy; but we cannot doubt the wise and benevolent intention of the Creator, in thus providing a store of nutritious and palatable food for man in situations whence he can so easily obtain it; and it is interesting to remark that, as it almost always exists in an insulated form, it may be obtained in a state of purity from many vegetables which would otherwise be very poisonous. Before it can be applied to the nutrition of the plant, however, its condition must be changed. Thus, in the germination of seeds, it is converted into sugar, which is the form of aliment best adapted to the development of the embryo; the same change takes place in the tuber of the potatoe; and, from the researches of Dunal (§ 881), it seems probable that the starch deposited in the receptacle is converted during the period of flower- ing into sugar. This conversion is a process which the chemist can imitate; for if the fecula be first heated, so that its vesicles may be ruptured, and then treated with dilute sulphuric acid, it is converted into sugar. This change is effected in the vegetable economy by the operation of a secretion called diastase^ which seems to be formed for the express purpose, and may be obtained in a separate state most readily from the neighbourhood of the eyes or huds of the potatoe. It is stored up in that situation for the purpose of being conveyed, by the vessels connected with the bud, into the substance of the tuber, when the demand for nutrition is occasioned by the development of the shoot; and, in the laboratory of the experimenter, it produces exactly the same eficcts as in the vegetable economy. It is probable that the secretion of diastase takes place in every instance in which fecula previously deposited is to be reabsorbed. 351. Lignin appears to be a modification of gum which constitutes the basis of the solid tissues of plants, and never exists but in an organ- ised state; it must, therefore, be considered as taking a higher rank than the substances previously described. It may be converted into a substance resembling gum by admixture with strong sulphuric acid; and, on boiling the liquid for some time, the gum disappears, and a saccharine principle is generated. Much diversity has existed in the statements of dififerent chemists as to its elementary composition, probably arising in part from NUTRITION IN PLANTS. 275 the deposition of other secretions within its cavities. According to the statements of Dr. Prout, it consists of 9 atoms of carbon to 6 of water; and it diifers from gum, therefore, in containing a larger proportion of carbon. This fact accounts for the influence of light upon the density of wood; since, as will be subsequently mentioned (§ 373), it is under that stimulus alone that the fixation of carbon from the atmosphere can take place.* 352. The mode in which the organisable products are converted into living tissues must ever remain a matter of great obscurity. Some curious facts are kno^vn, however, of which a summary may be advantageously given. It has been already stated (§22) that all the vegetable tissues may be regarded as taking their origin from the cellular, since this ap- pears to be the only one existing in the germs and first-formed structures even of the most highly-organised plants. Although at a late period, therefore, it would seem to be a general law that each tissue is developed in connection vnth one similar to it, this does not prevent textiu'es ap- parently heterogeneous from being evolved from those which were pre- viously simple and uniform, since they are all but modifications of one another. 353. For the development of cellular tissue to any extent, a single original vesicle seems all that is required. A most remarkable instance of its rapid growth has already been mentioned (§ 231); and although amongst flowering plants there is not the same proportional multiplication of cells, yet their number is often very quickly increased. Thus, the leaf of the Urania speciosa (one of the Banana tribe) has been known to lengthen four or five inches in one day, the vesicles being developed at the rate of about 4000 or 5000 per hour. The evolution of new cells at the extremities of those previously existing may be easily watched in the Ckara tribe, where they are aiTanged in single rows so as to constitute the tubular filaments of which those plants are composed. In expanded membranes formed of aggregated rows of cells, the new vesicles seem to be interposed between the old ones. This appears from the observations of Mirbel on the extension of the beautiful little fringe which surrounds the mouth of the urn containing the gemmules or buds of the Marchantia polymorpha (§ 61).t The circulation of fluid which has been observed in * Hence it is that wood, not only of different kinds of trees, but of different individuals of the same species, differs so much in density. It is well known that for toughness and durabi- lity, the stems which have grown in exposed situations, thoug-h stunted and irregular, are much superior to those which have been luxuriantly developed in close and shady woods. Homer tells us that the heroes of old used to cut the wood for their spears from trees growing in exposed situations; and a recent traveller in Holland mentions that the beeches growing in thick groves, so close together as to exclude every ray of the sun, as well as to impede tlie action of the atmosphere, are good for nothing but fire-wood, the trunks riving and splitting in every direction, when brought out of the forest. t Nouv. Ann. du Mus6e, Tom. i. T 2 276 SPECIAL AND COMPARATIVE PHYSIOLOGY. the separate cells of tlie Characece^ as Avell as in other plants, seems undoubtedly connected, not only with the nutrition of the individual vesicle, but with the development of that which is taking its origin from it. Each joint (Fig. 55) consists of a single cell composed of a mem- branous envelope, within which is arranged a layer of green granules covering every part, except two longitudinal lines which remain nearly colourless. During the healthy state of the plant, a constant motion of the semi-fluid matter, containing numerous jelly-like globules, is seen to take place within this green layer; the current passing up one side, changing its direction at the extremity, and flowing doMTi the other, — the ascending and descending streams being bounded by the transparent lines just mentioned. These lines appear to result from the adhesion, at those points, of an internal membranous sac to the outer envelope ; and the space between the inner and outer vesicle will thus be divided into two cavities, which communicate with each other at the ends of the cell. An imaginary transverse section of one of these tubes (Fig. 55, a) will illustrate this curious structure; a, «, being the outer envelope, h, h, the layer of green granules, c, c, the internal sac adhering to the outer one at d, d, and leaving the spaces e, e, within which the fluid circulates. The globules are of various sizes, being sometimes very small and of definite figure, and some- times existing as large irregular masses which appear to be formed by a union of smaller ones. There is little doubt that the layer of granules is formed by the adhesion of some of the circulating globules to the outer membrane and to each other, since they are always found to correspond closely in size. Thus, at b are seen the globules floating in their fluid, and at c is shown their regular disposition when lining the cell. It scarcely seems unlikely that diu-ing their circulation they undergo those changes which take place on a larger scale in the ingredients of the gene- ral circulating fluid of vascular plants, becoming gradually organised by their relation with the living structure which envelopes them; and that the green layer is the intermediate condition between the first formation of organisable products and their conversion into the actual tissue of the cell. No passage of fluid from one cell to another ever seems to take place; and if a long tubular vesicle be divided by a ligature, a separate movement is seen in each of the divisions. The globules effused from a cut cell have themselves a spontaneous motion. 354. Although this circulation has been most attentively watched in the plants of the Chara tribe, in which the cells are so large as to exhibit it in a very evident manner, it is by no means confined to them, since it has been observed in the individual vesicles forming the hairs and other transparent parts of higher plants ; and it might probably be detected at some period of the growth of every vesicle whose situation permits it to be watched, since it seems closely connected with the nutrition of the indivi- dual cell, and the production of new ones from it. Thus, the transparent NUTRITION IN PLANTS. , 277 scales at the foot of the leaf-stalks of the Hydrocharis morsus-rano) or fi-og-bit, a common aquatic plant, exhibit precisely analogous phenomena in their in(ii\ddual cells, though the presence of an internal membrane is not quite so evident; and the same may be observed in a section of its stem, — the motion being for a time checked by the violence, but soon recovering itself in the cells which have not been injured, if the portion be immersed in vs^ater. In the beaded hairs of the Tradescantia virginica or Virginian spider- wort, a similar circulation may be easily "v^atnessed ; as well as in the elongated cells which sometimes singly form hairs, as in the Penstemon ; and in the transparent radical fibres of Marchantia, Mosses, &c. This movement of fluid in the individual cells must not be confounded with the general circulation of the plant, as it is perfectly dis- tinct both from the ascent of the sap in the vessels of the stem, and from the distribution of the elaborated fluids by their passage through their special canals as already described (chap. vi.). Where each cell elabo- rates its own nutriment, as in the Chara, it is more distinct, and probably takes place with greater energy, than in those Avhich are supplied with fluid that has already been partly assimilated. Little granules are seen adherent to the walls of most vesicles of cellular tissue; and when the membrane is ruptured by violence, and they are effused into water, they are seen to have a spontaneous motion like that of the globules of the Chara. To what this motion is owing, it is as yet impossible to deter- mine; but similar phenomena will be shown to occur in regard to the animal fluids (§ 363). 355. The details just given of the mode of increase observed in the development of cellular tissue, express nearly all that is certainly known of the processes of nutrition in the lower Cryptogamia. It may be added, however, that many Fungi exhibit a circular gi-owi;h in concentric rings, OAving to the exhaustion of the nxitriment from the soil on which they vegetate, and the consequent death of the central first-formed portions, whilst the edges continue to extend over ncAv spaces. This is the method in -which, fairy rings are produced. It is generally found that when the vesicles are regular in shape, their increase takes place equally in all directions; whilst, if they be of a prolonged form, the new cells are developed from their extremities. This is equally true of the cellular tissue of vascular plants; for whilst that which is mixed with the wood and forms a large part of the bark has a tendency to lengthen vertically, those of which the medullary rays are composed are necessarily extended in a horizontal direction, to maintain that communication between the centre and circumference of the stem which would otherAAase be cut off by its increase in diameter. As to the mode in which woody fibre is formed, there is a considerable difference of opinion amongst vegetable physiologists. Between the last layers of wood and bark in Exogens there is formed every year fi-om the descending sap a glutinous secretiou, 278 SPECIAL AND COMPARATIVE PHYSIOLOGY. termed the cambium, which exhibits traces of incipient cellular organisa- tion. After a time, parallel rows of woody fibre are found in this situation, intermixed with cellular tissue; and these subsequently diyide into two distinct layers, of which one forms the outer ring of alburnum, whilst the other constitutes the interior lamina of the bark enveloping it. These remain in contact until the new formation of cambium in the suc- ceeding year. It is the opinion of many physiologists that the woody fibre, as well as the cellular tissue, is formed out of the cambium; but there are many reasons which seem to render this opinion untenable. The most consistent account of its development is that given by Du Petit Thouars, who, followed by Lindley, regards the fibrous tissue as formed in the leaves and growing downwards from them into the cambium, just as the roots are prolonged into the soil. This view would liken the woody fibres to the roots of the buds; and such a comparison, though at first sight improbable, is fully borne out by facts of no unfrequent occurrence. Thus, it often happens that a stem dies, whilst some of the buds upon it continue to vegetate, and send down woody bundles in the usual situation, which, after reaching the ground, form true roots. A graft, even, has thus maintained its existence after the death of the stock, completely enveloping the latter in the wood it has formed. Again, in Endogens, where there is no bark or cambium, the woody fibres may be traced from the leaves into the centre of the stem. It is difl&cult, therefore, to resist the conclusion that this tissue is organised in the leaves, although it may derive the means of increase from the nutritious juices which it traverses in its descent. It is, then, from the growth of this structure that the roots are developed, and the longitudinal increase given to the stem; whilst the cellular portion of the stem, from which the buds take their origin, is capable of extension in any direction, and of thus accommodating itself to the distribution of the woody bundles. Nutrition in Animals. 356. In tracing the gradual conversion of the alimentary materials ingested by Animals, into the organised structures which compose their fabric, it is advantageous, as in the case of Vegetables, to study in the first instance the cases in which the dificrent changes are most widely separated from each other. The most attentive observation of the life of the Sponge would probably never reveal much more than is at present known respecting the nutrition and growth of its tissues; whilst, on the other hand, the function which there appears so simple is shown, when we turn to the higher classes of animals, to be one in which several dis- tinct stages may be traced. The general facts relating to the solution and reduction of the food by the process of digestion have been already stated (§ 257 — 262); but it is now to be enquired what is the precise phange eflFected in its constituent parts, and what new products are found NUTRITION IN ANIMALS. 279 in the chyme which is the result of these actions. Although the chyme is usually homogeneous, and exhibits little or no trace of the character of the food which has been swallowed (unless it contain absolutely insoluble matter), it is not identically the same whatever be the nature of the ali- ment, as some have maintained. The characteristic ingredient of it is always, Ylqw&n&x^^q prooaimate princifle (§ 19), or organisahle product (§ 342), termed Albumen. This appears to hold the same place in the Animal economy with gum in the Vegetable; being the material at the expense of which most, if not all, of the other products are formed. It is that which is stored up in the egg by the parent, for the nutrition of the embryo, — composing a large part of the yolk., and existing nearly pure in the white, which, therefore, affords us an opportunity of studying its properties. It appears to consist of 8 eq. of carbon, 7 eq. of hydrogen, 3 eq. of oxygen, and 1 eq. of nitrogen; and it may be probably regarded as the least azotised of the materials of the animal tissues. Its charac- teristic property is that of coagulating by heat, acids, electricity, &c. A temperature of about 150° converts the transparent, semi-fluid mass, into a firm white, semi-opake, and somewhat elastic substance, which, when cautiously dried, shrinks up and assumes the appearance of horn. In its fluid condition, albumen is soluble in water, if combined, as it always is in nature, with a small quantity of soda; but after coagulation it no longer possesses this property, so that a turbidity is produced by heat in a solution containing only a thousandth part of albumen. A substance much resembling it, both in chemical constitution and the power of co- agulation, is found in some vegetables, especially in seeds. But the albumen of chyme doest not exist in precisely the same condition; since white-of-egg, taken into the stomach, is found to undergo important changes in the process of digestion. It is first coagulated by the action of the gastric juice, of which the acid appears to be the essential ingre- dient in this process. The curdy mass assumes a gelatinous appearance; and, by the combination of the mechanical and chemical powers of the stomach, it is gradually converted into a fluid which subsequently becomes chyme. Although still existing as albumen, it is now incapable of co- agulating firmly, and thus dififers essentially from that which is met vAih. elsewhere. According to Dr. Prout, all the materials which serve as aliment to animals, are converted by the digestive process into com- pounds of an albuminous or an oleaginous nature. The former is always present in the contents of the stomach, when the food has been composed of similar elements; but if it have been of a vegetable character, the albumen more resembles that found in plants than the true animal prin- ciple. After the products of digestion have been submitted to the action of the bile and pancreatic fluid, however, true albumen is always dis- tinctly present. Many forms of vegetable aliment, Avhich are composed of substances allied to sugar in their composition (hence termed by Dr. 280 SPECIAL AND COMPARATIVE PHYSIOLOGY. Prout the saccharine group), appear to be converted during digestion into an oilj matter, Avhich is a constant ingredient of chyle, hut which seems to he partly changed, before entering the general current of the cir- culation, into other principles. Although albumen, in its uncoagulated state, is perfectly homogeneous, globules are found in the neighbourhood of the positive pole when it is being coagulated by electricity, and a similar character may be traced in Avhite-of-egg, when dried in a thin layer, as well as in other albuminous fluids. It is not surprising, there- fore, that globules should exist abundantly in the fluid part of the chyme; and their presence can scarcely be regarded as a proof of the vitalising powers of the stomach, since it may be doubted whether albu- men can exist pure (that is, uncombined with the alkali which ordinarily keeps it in solution,) under any other form. The globules of the blood are quite distinct from these, possessing a definite structure, as well as peculiar vital properties. 357. The change which the chyme undergoes in the duodenum (the first portion of the intestine leading from the stomach) appears principally due to the admixture of the secretions there poured into it, whose efi^ect has been already described (§ 262). The milky fluid which is thus separated and taken up by the absorbents, is termed chyle; and it differs in many important particulars from the first product of digestion. These differences, however, become more decided in proportion to the part of its course in which we examine it; — ^the properties of the fluid drawn from the lacteals near their origin being allied to those of the con- tents of the duodenum; — whilst the characters of the chyme drawn from the thoracic duct more resemble those of the blood. This change is evi- dently analogous to that which has been already noticed (§ 344) as occurring in the ascending sap of vegetables. The chemical constitution of the chyle, as well as its degree of turbidity, depend in part upon the character of the food; the latter being principally due to the admixture of oily fluids with the albuminous matter which may be regarded as its essential constituent. The chyle formed from animal food is usually most opake (containing a large quantity of fatty matter), and passes most rapidly into decomposition. That formed by herbivorous animals is more transparent, and frequently quite clear. The milky character is sometimes communicated to the serum of the blood, especially if the food contain much oleaginous matter, and the chyle be, from any cause, rapidly propelled into the current of the circulation. The oily particles, being suspended in the fluid in a state of minute division, cause it to present a globuliferous appearance; but, besides these particles, other globules may be observed in it, to Avhich the turbidity (according to the observations of Miiller) is partly omng. These globules are usually pretty regular in form, but of variable size; generally, however, they are about half or one-third the diameter of the red particles of the blood. It NUTRITION IN ANIMALS. 281 can scarcely be supposed that these are identical with the glohules which have been stated to exist in the chyme; since the vessels which take them up must terminate in pores of a sensible diameter, which all ob- servers now agi'ee not to be the case. It has been argued, however, that, as the blood of kittens and of puppies, if drawn at a certain time after sucking, exhibits turbidity of the serum on coagulating, the globules of milk must have entered the absorbents; but this argument becomes gi'oundless when it is remembered that oleaginous food will produce the same effect in all cases. 858. Two hypotheses have been offered to account for the presence of globules in the chyle. According to one (that of Doellinger), the villi of the mucous membrane of the alimentary canal (§ 262) are constantly undergoing solution, Avhere permeated by absorbent vessels, and are repro- duced on their intestinal surface by the aggregation of the nutritive mate- rials in contact with them. The immediate variation in the character of the chyle according to that of the food is, however, an evident objection to this doctrine; and, moreover, it is scarcely to be believed that the nutrient materials should be made to form part of an organised tissue (that of the villi), only to be again disintegrated. According to the other hj'pothesis, the globules are formed in the lacteals themselves, by a vital influence of the tissue of these vessels upon their contents. If this be the case, such influence must be exercised immediately as absorption has taken place; since globules may be found in the network of lacteals which is distiibuted on the surface of the intestines themselves. There is nothing discordant, however, in such a supposition, mth what we observe else- where ; for no other account than this can be given of the formation of red particles of the blood, Avhich will be shown to have a comparatively elaborate structure. It is not improbable that a more attentive obser- vation of the process of absorption in plants, and of the size of the par- ticles of colouring matter which will pass through the spongioles, might elucidate this cui'ious question. 359. It is in its power of spontaneous coagulation, however, and in the colour which it presents, that the peculiarities of chyle, in the later part of its course, are chiefly manifested. This coagulation will not take place in chyle taken from the smaller lacteals, and it rarely occurs even in that which has passed through the mesenteric glands (§ 335); but that which is drawn from the thoracic duct separates, in about ten minutes, into a coagulum, which consists of the globules connected by a transparent sub- stance (probably fibrin), — and a serous fluid, containing fi-om 3 to 5 per cent, of solid matter (chiefly albumen). The presence of the fibrin has been variously accounted for. It may either result from the gradual con- version of the albumen of the chyle, effected by some unknown action of the lacteal vessels themselves; or it may be produced by an actual admixture of some of the constituents of the blood, by their permeation of the coats 282 SPECIAL AND COMPARATIVE PHYSIOLOGY. of the capillaries, which are in immediate relation with the lacteal tubes in the so-called mesenteric glands. But these glands are nothing more than prolongations of the lacteal tubes, on the walls of which blood-vessels ramify ; and it seems the more probable supposition, that the presence of fibrin in the chyle results from the elaboration of the albumen previously contained in it, by processes of whose nature little is knoAvn, but whose conditions are open to investigation.* Fibrin would seem, as will be pre- sently stated, to be a more highly-organised form of albumen, and to pos- sess the capability of exhibiting actions of a character otherwise than physical. The red colour of the coagulum of chyle taken from the thoracic duct, is another indication of the approach of that fluid to the qualities of the blood. This becomes more evident after the coagulum has been for a short time exposed to the air; and it is much more perceptible in chyle taken from the summit of the thoracic duct than in that which is derived from its lower extremity. It is possible, however, that there is not an actual formation of red particles in the chyle during its ascent, but that these are derived from the spleen; since the lymphatics coming from this viscus are sometimes charged with fluid of a red- wine colour. Still it appears by no means unconformable to what we have elsewhere observed, to suppose that the presence of fibrin and of red colouring matter in the chyle are indications of the process of assimilation which is gradually being effected in it. 360. This admixture of lymph, the fluid taken up by the absorbents of the system at large, with the chyle in the thoracic duct, is a point which must not be forgotten in any account of the properties of the latter. This fluid is usually quite transparent, and contains no fatty matter. A delicate coagulum forms in it within about 1 0 minutes after its removal from the body, and this appears to consist almost entirely of the fibrin which was previously fluid, — a few small globules being inclosed in it ; but the larger proportion of these remains suspended in the serous fluid, which contains a considerable proportion of albumen. The quantity of coagulum is very small, being, when first formed, not much more than TT7 of the weight of the fluid; and being reduced by drying to ■^^-^. When dry, the coagulum presents a fibrous appearance. The globules are much smaller than the colouring particles of the blood, and are by no means abundant. In the frog, they are not above \ of the size of the blood-globules. It will be presently seen that the lymph is nearly identi- cal in composition with the fluid portion of the blood; and it is net * That the walls of vessels have some unknown influence on their fluid contents, which can only he regarded as of a vital character, appears from the following' experiment amongst others. If two ligatures be placed round a blood-vessel, at the distance of two or three inches, the blood between them, although of course removed from the current of the circulation, vnll not coagulate for some time ; but, if it be drawn from its receptacle, it undergoes the ordinary- changes. NUTRITION IN ANIMALS. 283 improbable that its principal soui'ce is from the separation of this portion (the liquor sanguinis § 364) in the capillary circulation, a larger quantity permeating the tissues than is required for the purposes of nutrition, and the superabundant part being taken up, along with other matters, by the lymphatics. It may also be presumed that the admixture of this highly elaborated fluid with the crude matter of the chyle has an important effect in assimilating the latter, and in preparing it to enter the general current of the circulation, — just like the admixture of the previously- formed secretions of plants with their ascending sap (§ 252). 361. In what the precise difference consists between Fibrin and albumen, it is not easy to say. The most obvious character of the former is its tendency to spontaneous coagulation, Avhich can scarcely be regarded as otherwise than a vital property; and it will be seen that this frequently serves a most important purpose in the nutritive processes (§ 364). Fibrin may be most readily obtained in a pure state by stirring fresh-drawn blood mth a stick, so as to prevent its coagulation in the usual manner; a fibrous mass will be gradually collected, which may be freed by wash- ing from the red particles mixed with it, and consists of fibrin nearly pure. This substance is regarded as composed of 6 eq. of carbon, 5 eq. of hydrogen, 2 eq. of oxygen, and 1 eq. of nitrogen; on comparing these proportions with those of the elements of albumen, it will be seen that the nitrogen remains the same, whilst the relative quantity of the other constituents is diminished, so that this principle may be regarded as more highly azotised than the other. Its peculiarly animal character is mani- fested also in its tendency to pass rapidly into decomposition. After coagulation, it is entirely insoluble in water at common temperatures; but a very minute proportion is dissolved by the long-continued action of boiling water. Many attempts have been made to show the identity of fibrin and albumen; but however similar these principles may be in ulti- mate composition, there can be little doubt that they hold a very different relation to each other in the animal economy. Fibrin enters largely into the constitution of the muscles; but it is not easy to obtain it from them, since it is intimately mixed with the elements of cellular and vascular tissue. It is a fact of some interest in relation to subsequent enquiries (§ 503) that the fibrin of muscles, when triturated, becomes so strongly electric, that its particles repel each other and adhere to the mortar. Phenomena Avill be presently mentioned which render it probable that certain vital endoAvments possessed by the fibrin of muscles exist also in that of the blood. 362. In proportion to the perfection of organisation which the ali- mentary materials are ultimately to attain, appears the degree of elabora- tion which they require. It has been akeady stated (§ 262) that, in the Invertebbata, the absorbed fluid at once enters the general current of the circulation; but the blood of many of these tribes may be compared 284 SPECIAL AND COMPARATIVE PHYSIOLOGY. rather with the lymph than with the blood of higher animals, having hut few globules, and coagulating but feebly when drawn from its vessels. In the Yertebrata, there can be little doubt that the long course which the chyle traverses in the lymphatic system, contributes to prepare it for entering into the composition of the highly-elaborated fluid that supplies both the alimentary materials of their tissues and the necessary stimulus to their vital actions. The Blood of all the higher classes presents a decided red colour, when drawn from the arteries; and this is changed in the course of circulation into the dark purple tint which it exhibits in the veins. The colour exists only in the globules or corpuscles which are carried in its stream, the fluid portion being perfectly transparent and colourless; it appears due to the union of a minute quantity of iron with an animal compound, but not, as was formerly supposed, to oxide of iron alone. When the minute streams are examined in the living animal, flowing through the capillary vessels in which the globules usually arrange themselves in single file, they appear quite colourless; and it is only when these particles are collected into a mass that their tint becomes manifest. The composition of these two constituents of living blood, — the globules^ and the fluid portion or liquor sanguinis^ — will now be examined separately. 363. The form and size of the coloured particles vary considerably in different classes, but they are never so spherical as to deserve the appella- tion globules commonly given to them. In man and the other mammalia they are round flat discs, resembling pieces of money in form; but their thickness is porportionably greater, being about i or ^ of their diameter. In BIRDS, REPTILES, and PISHES, they are almost always elliptical in form; the long diameter being usually about twice the short one. The average thickness bears about the same proportion to the short diameter; but a central prominence is observed on each side in many Reptiles and Fishes. This prominence is seen in the red particles of Frogs' blood (which are four times as long as those of the blood of man) to be occasioned by the presence of a central nucleus, which differs in chemical properties from the matter that surrovinds it. Although the existence of a similar nucleus in the blood of Birds and Mammalia has been doubted, it seems now to be established ; its presence in the red particle is not indicated, however, by a prominence, but by a dark spot which has been erroneously regarded as a depression.* Other globules are found in the blood, * There can be no doubt that many errors as to the character of these bodies have arisen from the mode in which they have been studied. They can only be fairly examined in the serum of the blood itself (§ 365) ; since admixture of pure water chang-es their flattened form into a spherical one, and renders circular those which were elliptical. After a time, most of the coloured portion is dissolved, and the nuclei remain ; but this eifect is produced much more rapidly by acetic acid. Various circumstances lead to the belief that each particle is contained in a membranous envelope of great deHcacy, which encloses, first the colouring matter in a semi-fluid state, and then the nucleus ; and that the effects of water and other NUTRITION IN ANIMALS. 285 howerer, wliicli seem identical witli those of chyle and lymph. They hare been supposed to constitute the foundation, as it were, of the red- particles, being usually of about the same size mth the nuclei; but they are not always of the same shape. These lymph globules may not only be found in blood drawn from the body, but they may be watched in the general current of the circulation, — especially in tadpoles, where they are sooner introduced into the veins than in higher animals. According to the observations of Ascherson* the velocity of the two sets of particles is different; the lymph globules moving slowly, especially in the neighbour- hood of the sides of the vessel, where they are delayed by friction ; whilst the red particles, being endoAved mth more perfect smoothness of surface, and a considerable degree of elasticity, are not so easily retarded by slight obstacles. There is some variation in the size of the red particles, even in the same individual; but none ever attain twice the average diameter. In the embryos of Mammalia, however, they are usually much larger than in the adult. In the Invertebrata the globules seem to bear much resemblance to those of the lymph, in their want of a definite form, and in the roughness of their surface. In many species they occur very scantily, and their entire absence from some has been asserted. Their form often changes a good deal during circulation; but in some of the higher species, which border most upon Vertebrata, it seems more defi- nite, and nuclei have been observed in them. The foUomng are the average diameters of the blood-particles in different species, stated in parts of an inch. Fowl long d{am.-j--^-^-xi ^^^o^'i stW Tortoise — Man from -joV 9 *^ W77 Cat Vtf Shark fi-om -htVt *^ "si^ Scorpion ^tiJ - Wis Astenas tsTs ~ T^Vr T2 195 Frog ^T5 Cuttle-fish... 2TV9 Snail WiT-nr The nuclei of the red particles appear to consist of a substance resembling fibrin or coagulated albumen in most of its properties. The coloured envelope also has much affinity with fibrin in chemical composition; its characteristic property, however, is the change Avhich its colour is capable of undergoing under the influence of various agents. Thus, when brought into relation with oxygen, whether pure or diluted, its florid tint is heightened, and carbonic acid is at the same time generated. IMany saline solutions have a con-esponding effect; and this is shown most evi- dently when the colouring particles are suspended in a saline mixture (such as their own serum), and exposed to oxygen at the same time. Most acids, on the contrary, render it dark; and carbonic acid seems to reag'ents are due to an action of endosmose taking place through this membrane, by which its contents are increased, so far as even to rupture it, — or diminished, so as to cause a real depression on its surface. * British and Foreign Medical Review, vol. vi. p. 219. 286 SPECIAL AND COMPARATIVE PHYSIOLOGY. possess this power in a high degree. The application of these facts will be seen when the changes effected in the blood by Respiration are described (§ 418). This cruorin, as the colouring matter has been termed, is coagulated by heat and the mineral acids; and it then resem- bles fibrin in many of its chemical relations. When blood has been removed from the body, and diluted so as to prevent its coagulation, the particles are seen to be for some time in a state of constant movement. This is twofold, — the corpuscles moving towards each other, so as to arrange themselves in regular rows, — and each having a continual whirl- ing motion of its own. The former may, perhaps, result from physical causes only; the latter, however, closely resembles that which has been abeady mentioned as occurring in the globules of the nutritive fluid of plants (§ 354), and which is witnessed in a still more remarkable form elsewhere (§ 519). It has been said that their movement cannot be the result of vitality, since it may continue, under favourable circumstances, for some time after the blood has been removed from the body. But it is to be remembered that the lower the degree of the vitality natural to each part, the longer is it usually retained; and that the ciliary move- ments (§ 110) might on the same ground be represented as purely physi- cal, which no one has yet asserted. 364. The liquor sanguinis^ or fluid portion of the circulating blood, is that in which the tendency to coagulate exists; and it is probably that which is chiefly concerned in supplying nutriment to the tissues, — the globules, so far as can be ascertained, being merely passive in the circula- tion. It consists of water, holding in solution fibrin and albumen with saline matter; and it also contains a small amount of fatty particles, though in less proportion than the chyle. Many other ingredients may be detected in it; but these seem to have the most important connection with the nutritive processes. The liquor sanguinis may be separated from the globules by a filter sufficiently fine to retain the latter; and it will then coagulate alone. This constituent of the blood is occasionally separated from it in the living system, under the form oi coagulahle or plastic lymph^ which is sometimes effused as a product of inflammation, and sometimes for the simple purpose of reparation, and appears to con- sist of the liquor sanguinis in a concentrated form, the proportion of fibrin in it being especially large. "When it is poured out on the surface of membranes, it has a tendency to become organised; new vessels, with lymphatics, and perhaps even nerves, are gradually formed in it; and these acquire connections with the neighbouring parts. Serous membranes are particularly liable to such effusions; and the false membranes thus formed very commonly produce adhesions between their adjacent siu'faces. The newly-organised tissue at first presents the characters of simple cellular structure; but it gradually, in many instances at least, assimilates itself to the tissue with which it is connected. There seems no doubt NUTRITION IN ANIMALS. 287 that the formation of vessels usually commences in the substance of the coagulated fluid thus effused, and that the canals gradually form connec- tions with those of the neighbouring parts; but what is the precise manner in which this process is effected still remains a mystery. It is almost impossible to consider it without admitting that the liquor sanguinis is as completely possessed of vitality as any solid tissue in the body; since it exhibits properties which no merely chemical admixture can possess. Fibrin is sometimes efixised in a similar manner into the substance of organs, and, by becoming organised, converts that into a solid mass which previously possessed a spongy texture; — as when the lungs are hepatised by inflammation, and their air-cells obliterated. It is by an analogous process of organisation in effixsed fibrin that the reparation of injuries takes place; and whatever may be the nature of the tissue to be subse- quently regenerated, the effusion of coagulable lymph, followed by its conversion into cellular structure, is always the primary change. SQ5. The blood drawn from the living body does not long remain in the fluid form in which it exists in the vessels ; but undergoes a process of spontaneous coagulation, separating into a solid crassamentum or clot, and a fluid serum. The former is composed of the red particles held together by the fibrin, through which they are usually diffused pretty equably; if, however, the coagulation be retarded, either by the condition of the sys- tem at the time the blood was drawn, or by artificial means, the red par- ticles will sink, leaving the surface of the coagulum nearly fi'ee from colour, so as to form what is termed the huffy coat. The serum consists of the watery portion of the blood, which holds in solution the greater part of the albumen with the saline matter and most of the other elements; it is coagulable by heat like other albuminous fluids. This is not unfre- quently separated from the blood in the living state; the natural secretions of serous membranes (§ 36) appearing to differ but little from it; and the collections which sometimes take place to a great amount in their cavities, presenting nearly all its characters. In inflammatory conditions of these membranes, more or less fibrin is generally contained in the serous effu- sion, the flakes which are suspended in it giving it a turbid appearance. The spontaneous coagulation of the fibrin can scarcely be regarded in any other light than as a result of the vital properties mth which this princi- ple is endowed. It has been shown to serve a most important purpose in the economy of the living S3fstem ; the fluid form, which the fibrin retains as long as it exists within living vessels, enabling it to be conveyed wherever it may be neeeded for the purposes of nutrition or reparation, — whilst, as soon as it is deposited by them in connection with parts already organised, it manifests a tendency to partake of their structure.* The * A curious instance of the tendency of fibrin to become organised came under the autlior's observation wliilst Clinical Clerk to Dr. Watson at the Middlesex Hospital. A patient labour- ing' under endocarditis (inflammation of the lining membrane of the heart) having been bled, 288 SPECIAL AND COMPARATIVE PHYSIOLOGY. vitality of the tissues witli which it is contact, however, seems to have an important influence on its condition. Blood will not remain fluid in a portion of a vessel isolated by ligatures (§ 859, note) longer than the healthy structure and properties of its coats are retained; and it is hy its tendency to coagulate in any part where these are impaired that hemor- rhages are often prevented or checked. Again, it has been found that violent injuries of the nervous centres (especially the breaking down of the brain and spinal cord) very rapidly produce coagulation of the blood whilst it is yet moving in the vessels, by destroying the vitality of their coats. On the other hand, coagulation does not take place after some kinds of sudden death, in which the vitality of the whole system appears to have been instantaneously destroyed (as when an animal is killed by lightning or by a violent electric discharge) ; and, in these cases, the usual stiffening of the muscles (a change that seems to result from a similar change in their constitution, which may be regarded as a last eflbrt of vitality,) does not take place. Of the various conditions which hasten or retard the coagulation of the blood, more cannot here be said; but the subject is one of much interest and importance. 366. Of the means by which the nutritious materials of the blood are converted into organised tissues, and of the mode in which this takes place, little can be said in addition to what has been already stated. There can be no doubt that the constituents of all these tissues exist in the blood, in a form very nearly allied, except so far as their organisation is concerned, to that which they possess in the solid parts; since its elements may be obtained from them by chemical analysis. Thus, the fibrin is particularly found in muscles, albumen in cellular tissue and its modifications, the oily particles in the fat and in the neryous matter, phosphate of lime in the bones, — and so on. But another proximate principle is abundantly yielded by many tissues, which cannot be detected (except, perhaps, in very small quantity) in the blood. This is gelatine^ familiarly known as glue. Its characteristic properties are its solubility in warm water, and its coagulation on cooling; and its precipitation by tannin as a new compound, on the perfect formation of which the conversion of skin into leather depends. By long-continued boiling, gelatine may be obtained, in greater or less proportion, from almost all the animal tissues; but it is doubted by many chemists whether some chemical change is not thus efi"ected in their constitution, and whether gelatine, as such, exists in the living body. It may be regarded as composed of 7 eq. of carbon, 7 eq. of hydrogen, 3 eq. of oxygen, and 1 eq. of nitrogen; and it thus dififers from albumen in the deficiency of one proportional of carbon. The conversion of the albumen the coagulum exhibited at its edges a numher of little wart-like processes, exactly resembling in character the softer granulations which are seen on the cardiac valves after death from this disease. NUTRITION IN ANIMALS. 289 of the blood, therefore, into the gelatine of the tissues (or into whatever principle occupies the place of that which is known as gelatine in a separate form) may not improbably be the source of the carbon set free during the circulation of the blood through the system (§ 418). 367. It is in the capillary vessels only that the nutritive changes take place between the blood and the tissues which they permeate. It has been maintained by some that these vessels have no distinct coats, and that they are merely channels through which the fluid moves. The latest observations, however, permit little doubt that, in the perfect tissues of the higher animals, each ramification, however minute, is a regular tube possessing distinct walls of its own; but that in the lower tribes, as in the newly-forming tissues of the higher, the simpler condition is retained in which these exist in plants (§ 288). The movement of fluid through them appears in all instances to be much influenced by the activity of the nutrient processes, and in some cases to depend entirely upon them. Each tissue seems to derive from the blood the materials it requires, and to convert it into an organised structure like its o^vn. More minute details on their respective modes of groivth and increase cannot, however, be given, until the difiiculties which impede the observation of them shall have been much diminished. The actual processes of organisation proba- bly correspond, so far as they take place, pretty closely in all animals; the diff^erences between the higher and low^er tribes consisting more in the preparatory stages, and in the varieties which arise at a later period. 368. Although, as formerly stated (§ 226), the function of Nutrition does not seem to have any immediate dependence upon the nervous system in Animals, any more than the con-esponding process in Plants, there can be no doubt that it is greatly influenced by changes in the con- dition of that system, whether these be the result of states of mind or those of other parts of the material organism. Thus, if a limb be para- lysed by the division of its nerve, it loses after a time its healthy firmness, its muscles become pale and flabby (sometimes even showing little trace of true fibres, and losing their contractility), and it is peculiarly liable to be injuriously afi"ected by changes of temperature or by external applica- tions, which produce no perceptible change in sound parts. Again, if the nerves supplying mucous membranes be divided, these parts are very liable to become inflamed; for the secretion is no longer formed Avhich protects them from the contact of irritating matter. That the general function is liable to be influenced by the state of the mind, every one must have observed; still this may be affected, not only immediately by the influence of the nerves, but by an imperfect preparation of the alimentary materials in the digestive cavities, A^hicli may result from deficiency or want of solvent poAver in their secretions, — the formation of these last being manifestly influenced, like nutrition, by the condition of the system at large. The doctrine of the dependence of animal nutrition 290 SPECIAL AND COMPARATIVE PHYSIOLOGY. on the influence of tlie nervous centres is opposed to the fact that, in the early evolution of the organism, its processes go on most energetically, long before these nervous centres are formed, and even before the presence of nervous matter can be detected; and that the formative processes by which new structures are created in the adult appear equally removed from their direction. In the present state of our knowledge we can only refer these processes to the property which living structures appear to possess, of converting into textures, similar in character to their own, certain proximate principles endowed with a capacity of being thus organised. CHAPTER IX. RESPIRATION. General Considerations. 369. Although this has usually been considered in the light of a dis- tinct function, there is no longer reason for so regarding it; since the structure of glands being now more fully understood, that of the respira- tory organs is perceived essentially to correspond with them, whether these are formed into external prolongations — such as the leaves of plants or the gills of aquatic animals — or into the internal cavities which constitute the lungs of the air-breathing Yertebrata. The modifications which exist have all reference to the peculiar conditions required for this function,— namely, the exposure of the nutritive fluid to atmospheric air (either in a pure state or as contained in water) through the medium of a thin mem- brane; and it is very obvious that provision must be made, not only for the continual transmission of this fluid through the respiratory apparatus, but for a continual renewal of the air in contact with the outer stirface of the membrane. It appears that the interchange of ingredients between the circulating fluid and the atmosphere, which constitutes Respiration, has for its chief object the liberation of a portion of the superfluous carbon of the system, in the gaseous form which it assumes when united with oxygen. There is no doubt, however, that the quantity of oxygen and nitrogen in the structure, as well as of carbon, is partly maintained at its proper standard by the same means; and that, in plants, carbon is intro- duced into the system from the atmosphere, as well as excreted by it. All these changes may be comprehended under the general term of the aeration of the circulating fluid, and would seem to take place under the same general conditions; but the liberation of carbonic acid, by the union of the superfluous carbon of the system with the oxygen of the atmosphere, RESPIRATION. GENERAL CONSIDERATIONS. 291 is that which peculiarly constitutes respiration; and upon that, as will be hereafter seen, the maintenance of the temperature of the system is pro- bably dependent. 370. The aeration of the nutritious fluid, would appear to be, like absorption, a change dependent on physical laws, and occurring in con- formity with them, when the requisite conditions are supplied by the structures of an organised being, and by the functional alterations which the Hying state involves. The physical laws alluded to, and the pheno- mena which are exhibited in conformity -with them, will be now briefly described. 371. All gases of dififerent densities, which are not disposed to unite chemically with one another, have a strong tendency to mutual admixture. Thus, if a vessel be partly filled with hydrogen, and partly with carbonic acid, the latter, which is 22 times heavier than the former, will not remain at the bottom, but the two gases will be found in a short time to have uniformly and equably mixed; and it is on this principle that the consti- tution of the atmosphere is every where the same, although the gases which compose it are of very diflFerent specific gravities. The same ten- dency exists also Avith regard to two volumes of the same gas or mixtui-e, possessing dijBferent degrees of heat, and therefore different densities, until by their mutual penetration the temperatui'e becomes the same throughout. So strong is this tendency to admixtuire on the part of different gases, that it will take place when a membrane or other porous medium is interposed between them. When, for instance, a bladder of hydrogen is placed in an atmosphere of carbonic acid, a certain quantity of hydrogen will pass out; but a much larger proportion of cai'bonic acid will enter, so as to distend the bladder even to bursting. This interchange, therefore, evidently resem- bles the endosmose and exosmose of fluids ; and although the tendency to admixture of the two gases is the fundamental cause of their movement, the nature of the septum has so much influence over the phenomenon as sometimes to reverse the results. When plaster-of-paris is employed as the medium of diffusion, the exchange will take mth simple relation to the relative densities of the gases; and a general law has been ascer- tained by Professor Graham, which applies to all instances, — that the replacing or mutual-diffusion volumes of different gases vary inversely as the square-roots of their densities. Thus, if a tube, closed at one end with a plug of plaster-of-paris, be filled Avith hydrogen, the gas will soon be entirely removed, and will be replaced by something more than one fourth of its bulk of atmospheric air; the density of hydrogen being about -[L that of the atmosphere. But when organic membranes are emploj'^ed, the result is much influenced by the relative facility with which each gas permeates the septum. Thus, carbonic acid passes through moist bladder much more readily than hydrogen ; and, in consequence, the result occurs which has been mentioned above, and Avhich seems contrary to the law u 2 292 SPECIAL AND COMPARATIVE PHYSIOLOGY. just stated. It would not seem improbable that the phenomenon of endosmose is dependent upon laws precisely similar, and that its ano- malies are of a kind which experiments on the gases would much elucidate. 372. Further, it is found that if a fluid be charged with any gas which it will absorb (as, for example, water with carbonic acid), it will speedily part with it when exposed to the attracting influence of another gas, such as atmospheric air; and the more different the densities of the two gases, the more rapidly, and with more force, -will this take place. As in the former instance, this attraction will go on with little interruption, through a porous membrane; and part of the exterior gas will be absorbed by the fluid (if of a nature to be so imbibed) in place of that which has been removed. These simple phenomena will be found a key to the explana- tion of the changes which take place in the aeration of the circulating fluid by exposure to air; for it seems a universal fact that carbonic acid existing in that fluid is exhaled and replaced by absorbed oxygen; and that an exhalation and absorption of nitrogen take place in animals, and perhaps also in plants. Respiration in Plants. 373. As already stated (§ 235), two distinct changes, both nearly con- stant throughout the Vegetable kingdom, have been associated under this function. The air being the chief source whence carbon is supplied to the living plant, the introduction of that element has been confounded with the contrary change, Avhicli is also necessary for the continued health of the structure, and which corresponds exactly with the respiration of Ani- mals. The introduction of carbon is efi"ected by the power which the surfaces of plants possess (especially those Avhich are green) of decompo- sing, under the stimulus of light, the carbonic acid contained in the atmosphere. This process is one which our knowledge of the application of physical laws can but little elucidate, and we must be content to regard it as a phenomenon of an essentially vital character. Its conditions may, however, be advantageously enquired into. If we place some fresh leaves in an inverted jar containing an atmosphere charged with 7 or 8 per cent, of carbonic acid, and expose them to strong sun-light for a few hours, it will be found that a large proportion of the carbonic acid will have dis- appeared, and will be replaced by pure oxygen. If, on the contrary, we cause a plant to groAV in a dark situation, with even a less proportion of carbonic acid in the atmosphere around it, it will soon become sickly and die; and if in common air, under similar circumstances, it will lose its colour, and thus be etiolated or blanched, from the want of the supply of carbon which it can only obtain under the influence of light. It is found that no degree of artificial light will produce this change; and that the proportion of carbonic acid in the atmosphere is that which is most RESPIRATION IN PLANTS. 293 favourable to groAvth under tlie average amount of the stimulus wliicli this climate affords. 374. As to the organs hy which this process of digestion, as it may reasonably he called, is performed by the different classes of plants, it is difficult to speak with certainty. In the Phanerogamia, the green sur- faces of the leaves, stems, &c., are those by which the fixation of carbon is chiefly, if not entirely effected. In general, it is by the upper side of the leaf that the greatest amount of this function is performed; as would be supposed from its greater exposure to the light, and as is evinced by its brighter colour. But in other cases, the two sides are equally exposed to light, and then their colour is the same. In the ferns, mosses, &c. there is the same separation of parts as in the flowering plants; and the process is here also, without doubt, performed by the green parts of the surface. Of the inferior Cryptogamia, however, we know very little. The fungi would not seem to depend upon the atmosphere for any part of their supply of carbon, which is altogether furnished by their peculiar aliment (§ 235); and these plants scarcely ever present any green surface, and flourish most in situations to Avhich light has but little access. The same may be said of the Cuscuta (broom-rape) and other parasitic plants of more complex structure, that live upon the prepared juices they derive from the plant to which they attach themselves. There can be no doubt that lichens derive the carbon which enters into their structure almost entirely from the atmosphere, and, that the algje are supported, in like manner, by the carbonic acid contained in the circumambient water; but experiments are yet wanting to ascertain the precise conditions under which its assimilation is effected. Few Lichens have any green surfaces; and although many of the Algge are very brilliantly coloured, yet we find them occasionally existing at such depths as forbid us to believe that light is the only stimulus under which they can attain this appearance. Thus, Humboldt found near the Canaries a species of Fucus which was bright grass-green, although it had grown at a depth of 190 feet, where the light could only have been y^'^ ^th part as intense as at the surface. The sim- pler forms of Algse, especially the Confervce, which inhabit fresh water, * A very ingenious theory has been raised by M. Brong-niart upon the fact that an increased quantity of carbon may, under particular circumstances, be assimilated by V^egetables. He supposes that, at the epoch of the g-rowth of those enormous primeval forests which supplied the materials of the coal formation, the atmosphere was highly charged with carbonic acid, as well as with humidity ; and that from this source the Ferns, Lycopodiacete, and Coniferae of that era were enabled to attain their gigantic development. He imagines that they not only thus converted into organised products an immense amount of carbonic acid, which had been previously liberated by some changes in the mineral world, but that, by removing it from the atmosphere, they prepared the earth for the residence of the higher classes of animals. The hypothesis is a very interesting one, and well deserves consideration ; but it may reasonably be enquired whence the increased light was derived which would be necessary (unless the laws of the vegetable economy at that period were different from those now in operation) for the increased assimilation of carbon to any such extent. 294 SPECIAL AND COMPARATIVE PHYSIOLOGY. appear to exercise an important influence in maintaining it in a state fit for the support of animal life; since it seems probable that they absorb the products of the decomposition of that foul matter by which all ponds and streams are constantly being polluted, and at the same time yield a supply of oxygen to the water.* 375. The change which, strictly speaking, constitutes the respiration of vegetables is not, like that we have been describing, an occasional one; but is constantly taking place during the whole life of the plant, and appears to be more immediately necessary to its healthy existence. This consists in the disengagement of the superfluous carbon of the system, either by combination Avith the oxygen of the air, or (which is most likely) by replacing with carbonic acid the oxygen that has been absorbed from it. The respiration of vegetables is performed in part by their dark surfaces, and partly also by the covering of the leaves. It does not cease by day, by night, in sunshine, or in shade; and it has been shown that the leaves continue to disengage carbonic acid, even under the circumstances when the Jiwation of carbon is most actively per- formed.t If the function be checked, the plant soon dies, — as when placed in an atmosphere mth a large proportion of carbonic acid, and without the stimulus of light which enables it to decompose the deleterious gas. Plants which are being etiolated by the want of light, absolutely diminish in the weight of their solid contents, owing to the continued excretion of carbon by the respiratory process, although their bulk may be ijiuch increased by the absorption of water; and if the proportion of carbonic acid in the surrounding air be increased by its want of renewal, they become sickly and die, from the impediment to their respiration. The parallel, therefore, between plants and animals appears to be complete as regards the influence of carbon upon their gi'owth; for to both it is deleterious when breathed, and to both it is invigorating to the digestive system when absorbed as food. 376. It becomes a question of much interest to ascertain the relative amount of carbon thus absorbed and excreted by Vegetables. If it be true, as was stated (§ 234), that a large part of the solid materials of * It is a notorious fact that fishes are never so healthy in reservoirs destitute of aquatic plants, as in ponds and streams in which they abound. Besides the use of these plants in setting- free oxyg-en in the water, it is not impossible that the jelly-worts (as some of them are occasionally called) enable the fluid to retain a larg-er quantity of gas in mixture with it, than pure water would do; for it appears that, like some mucilaginous beverag-es (beer or ale, for example), such water gives out by heat or in a vacuum a larger proportion of air than it naturally con- tains. Burnett's Botany, p. 77. The floating islands which are constantly being formed in the lake Solfatara in Italy, exhibit a striking example of the luxuriance of cryptogamic vege- tation in an atmosphere impregnated with carbonic acid. These islands consist chiefly of Conferva and other simple cellular plants, which are copiously supphed with nutriment by the carbonic acid that is constantly escaping from the bottom of the lake with a violence which gives to the water an appearance of ebullition. See Sir H. Davy's " Consolations in Travel," 3d ed., p. 116. t Journal of the Royal Institution, N.S., vol. i. RESPIRATION IN PLANTS. 2i)5 tlieir tissues is derived from tlie atmosphere, it would be evident that the quantity of carbonic acid in the air must be diminished by their growth. This, from the best-conducted experiments, appears to be the case; for the amount of carbon assimilated by a healthy plant during a period of ordinarily clear weather is found to exceed that exhaled by respiration, although the former is an occasional change, and the latter a constant one. From the data presented to us by Dr. Daubeny it is shown that a plant consisting of leaves and stems, if confined in the same portion of air, day and night, and duly supplied with carbonic acid gas during sun- shine, "will go on adding to the proportion of oxygen present, as long as it continues healthy; the slight diminution of oxygen and increase of carbonic acid which talce place during the night, bearing no considerable ratio to the degree in which the opposite effect occurs by day. 377. There is one tribe of plants, the fungi, in which we see the effects of a Respiration performed almost as actively as that of animals, and unobscured by any opposing changes. From the late experiments of Marcet, it appears that growing mushrooms absorb from the air a large quantity of oxygen; a portion of which appears to combine with the carbon of the plant, and thus to form the carbonic acid which replaces it; whilst the rest seems to be retained in its structure. The large quantity of carbonic acid disengaged from the soil in which alone the Fungi thrive, renders it necessary that the superabundant carbon of the plant should be constantly removed by the atmosphere, instead of any addition being received through that medium (§ 235). 378. The balance of nutrition, therefore, between the Animal and Vegetable kingdoms is thus maintained in a very perfect and interesting manner. Plants convert the carbonic acid of the atmosphere into organ- ised tissues, although the conditions of their growth require that part of the materials so introduced should be restored to the surrounding medium; these tissues serve for the nutrition of the whole animal kingdom, Avhich is immediately or remotely dependent upon them; and the large amount of carbonic acid which is constantly being excreted from their bodies in the living state, (perhaps owing to a slow decomposition of their struc- tures, § 18), with that disengaged during their final decay, restore to the atmosphere the ingredients Avhich are required for the maintenance of fresh generations of organised beings. 379. "With regard to the changes effected by vegetation upon the principal constituent of the air — ^nitrogen — no very certain or definite statement can be made. It has long been knoAATi that this element enters largely into some of the products of secretion; but it has been usually supposed not to constitute a part of the organised structures themselves. The experiments of M. Payen already alluded to (§ 250), however, shoAved that the tissue of the absorbent vessels, especially near the extre- mities of the roots, is acted on l)y tannin in the same manner as animal 296 SPECIAL AND C031PARATIVE PHYSIOLOGY. membrane, and therefore probably contains an azotised principle. And tbe recent analyses of Mr. Rigg,* seem to have proved the existence of this element as an essential constituent of all those parts of the vegetable structure, which perform the most important offices in the economy. When the required amount of this gas is not taken up by the roots in a state of solution, it must be absorbed from the atmosphere; but as all the vrater which is imbibed contains more or less of common air, it is probable that a sufficient supply is generally thus obtained. Indeed, the few experiments which have been performed with express view to this subject lead to the belief that azote is more frequently exhaled than absorbed. Wherever the plant is supplied with rich animal manures, the fluids absorbed from which contain more azote than it can assimilate, we should expect to find it disengaged in some quantity; this is the case in the Fungi, although their tissue contains more nitrogen than that of any other tribe of plants. 380. There are two periods during the life of plants in which the function of Respiration appears to go on with remarkable activity. The first of these is germination, or the development of the young plant from seed (§ 50), which requires that the starch laid up by the parent for the support of the embryo should be converted into sugar, the latter being the form in which it is applied to the purposes of nutrition. This con- version (§ 350) involves the liberation of a quantity of carbon, which is disengaged precisely in the manner in which it is set free at a later period of vegetable life, — namely, by its combination with the oxygen of the surrounding atmosphere. Germination takes place most readily in the dark, since this most essential part of the change would be antagonised by the influence of light. The young plant is, therefore, much in the condition of one which is being etiolated; and it is accordingly found that, during the early period of germination, the weight of the solid con- tents of the seed diminishes considerably, though its bulk increases by the absorption of moisture. This is its state until the cotyledons or seed- leaves have arrived at the surface, and temporarily perform the functions of leaves. It is an interesting fact that, after many trials, germination has been found to take place most readily in an atmosphere consisting of 1 part oxygen and 3 parts nitrogen, which is nearly the proportion of the air we breathe. If the quantity of oxygen is much increased, the carbon of the ovule is abstracted too rapidly, and the young plant is feeble; if the proportion is too small, carbon is not lost in sufficient quantity, and the young plant is scarcely capable of being roused into life. 381. The changes which place during Jloweriti^ are very similar to those occurring in germination. A large quantity of oxygen is converted into carbonic acid by the action of the flower; and it is believed that the fecula or starch, previously contained in the disk or receptacle (§ 349), is * Proceedings of the Eoyal Society, May, 1838. RESPIRATION IN PLANTS. 297 changed by tliis process into saccliarine matter adapted for the nutrition of the pollen and young ovules, the superfluous portion flowing ofi" in the form of honey. It is remarkable that this analogy between germination and flowering holds good, not only in their products, but in the condi- tions essential to their development. Neither will commence except in a moderately warm temperature; both require moisture, for flowers will not open unless well supplied Avith ascending sap ; and the presence of oxygen is in each case necessary. It has been well ascertained that the carbon- isation of the air bears a direct relation to the development of the gland- ular disk, and that it is principally eff^ected by the essential parts of the flower, or organs of fructification. Thus, Saussure £ound that the Arum Italicum, whilst in bud, consumed in twenty-four hours, 5 or 6 times its own volume of oxygen; during the expansion of the flower, 30 times; and during its withering, 5 times. When the floral envelopes were removed, the quantity of oxygen consumed by the remaining parts in proportion to their volume was much gi-eater. In one instance the sexual apparatus of the Arum Italicum consumed in twenty-four hours 132 times its bulk of oxygen. Saussure also observed that double flowers, in which petals replace sexual organs, vitiate the air much less than single flowers in Avhich the sexual organs are perfect. (See also § 480). 382. Besides the means of aeration which the transmission of the nutritive fluid to the external surface afibrds, the more highly organised plants seem to have the power of admitting air into cavities existing in the leaves, (especially beneath their inferior cuticle Fig. 69) through their stomata; and in this manner a much larger extent of membrane is exposed to its influence. The peculiar organisation which is probably subservient to this purpose will be hereafter described (§ 429) under the head of exhalation^ for which function it appears more particularly designed. But, superadded to this, we find in the Phanerogamia a sys- tem of tubes apparently designed to connect the interior of the structure with the external air. These are the spiral vessels (§ 26), which, in their perfect form, are never found to contain any but gaseous fluids. In exo- GENS they usually exist in only one pai-t of the stem, being confined to the medullary sheath, a delicate membrane, principally formed by them, which immediately surrounds the pith. In endogens they are more universally distributed through the stem, forming part of every bundle of fibro-vascular tissue. In each case, however, they traverse the stem for the purpose of entering the leaves; and they seem to communicate with the intercellular passages, and, through their medium, if not more directly (as some have supposed), with the external air. We have already noticed the cmious analogy between these respiratory tubes and the trachcw of insects; and although their exact office is not fully ascertained, there can be little doubt that they contribute in some way to the aeration of the 298 SPECIAL AND COMPARATIVE PHYSIOLOGY. internal fluids. It has been found that they contain a larger quantity of oxygen by 7 or 8 per cent, than that which exists in the atmosphere. 383. In a great number of the aquatic tribes, both among the simpler and the more highly organised plants, we find cavities expressly adapted for the inclusion of air, which would seem designed to give buoyancy to the structure. Thus, the roots of the Utricularia are furnished with a number of bladder-like vesicles; the whole surface of the Fucus vesiculosus (bladder- wrack) is studded with similar ones ; whilst in the leaves of the Duck-weed or Water-lily, or the stem of the Lymnocharis we find hollows surrounded with regularly-built-up tissue, evidently answering the same purpose. These present an obvious analogy to the air-bags with which various aquatic animals are furnished, from the vesicles of the Physalia (Portuguese man-of-war. Fig. 140) to the swimming bladder of fishes. As the air which they contain is seldom identical in composition with that of the atmosphere, it has been conjectured that they have some con- nection with the function of Respiration; but on this point no certain conclusions have been obtained. It is desirable, however, that these regular air-passages should be distinguished from the irregular hollows which are occasionally found — as in the stems of gi'asses, umbelliferous plants, &c. — and which simply result from the expansion of the external tissues faster than the interior can be filled up by the materials ready. 384. Although the leaves are to be regarded as the special organs of aeration in the plants furnished with them, yet there is no doubt that the remainder of the surface is more or less concerned in this function ; the green parts probably assimilating carbon wherever they exist, and the dark portions, especially the roots, disengaging carbonic acid. That the access of oxygen to the roots is necessary for the health of the plant is well known; but this may be required ftr the decomposition of the organic matter which surrounds them. It has often been found that, if an addi- tional stratum of soil be laid over the roots of a large tree, either they will send up fibres nearly to the surface; or, if they be not strong enough to do this, the tree will perish. 385. Regarding the progressive evolution of the respiratory system in plants, much might here be said which will perhaps be more advanta- geously deferred to the account of their development in general (chap. XIII.). It may be remarked, however, that the early condition of the embryo of the flowering plants resembles, in its want of special organs, the simple vegetation of the cellular Cryptogamia, although it difi^ers in the mode in which nutriment is supplied; the latter deriving it by their unassisted powers from the surrounding elements, whilst the former is provided with it by the parent. At the first period of the germination of the seed, a curious analogy may be traced between the gTowing embryo and the tribe of Fungi. Both are supplied with nutriment previously RESPIRATION IN ANIMALS. 299 organised, the one from its parent and the other by the decay of animal or vegetable matter; both are developed most rapidly when supplied with warmth and moisture, and in the absence of light; and both liberate carbon to a large amount without assimilating any from the atmosphere. By the time, however, that the cotyledons have risen to the surface and acquired a gi'een colour, the plant has advanced a stage in its growth, and the respiratory system has now arrived on the level of the Marchantia (§ 61), possessing, like it, stomata and intercellular spaces, but being destitute of spiral vessels. These do not appear until true leaves are evolved; and as soon as this last stage in the development has taken place, the cotyledons, which may be regarded as temporary respiratory organs, decay away. When we have traced the evolution of the respira- tory system of animals in a similar manner, we shall observe a most interesting coiTespondence between the consecutive phenomena, as they occur in the two kingdoms. Respiration in Animals. 386. In the Animal Kingdom, we find Respiration exerting a more immediate, though perhaps not in reality a more powerful influence over the system, than in Vegetables. The dependence of the organism on the constant stimulus of the circulating fluid is more evident in proportion as, in ascending the scale, we meet with greater variety and activity in the vital operations. The maintenance of the vivifying powers of this fluid by its exposure to the atmosphere is, therefore, demanded more urgently than the mere supply of its deficiency by the ingestion of fresh aliment; and it is accordingly found that many animals are capable of subsisting a considerable time without nourishment, whilst there are few which do not speedily perish, or whose vital actions at least are not checked, when deprived of air. The correspondence between the activity of this function in any individual system, and its general vital energy, must be evident to the discriminating observer; the comparative energy of the respiration in the active and rapacious eagle, and in the timid and indolent tortoise, afibrd a ready illustration of the connection. The development of the locomotive powers, and the degree of heat maintained in the sysfem, which may be regarded as pretty constant indications of the general activity of its organic functions, will be found peculiarly connected with that of respiration. In making comparisons of this kind, however, we must bear in mind that the absolute amount of respiration does not depend upon the comparative bulk of the organs, but on the extent of surface by which the blood is exposed to the action of the air; so that the minutely- partitioned cellular lungs of a rabbit present greater opportunity for the aeration of the blood than the capacious undivided sacs of a turtle ten times its size. 387. The organs appropriated to the perfonnaiice of the function of 300 SPECIAL AND COMPARATIVE PHYSIOLOGY. respiration in the various classes of the Animal Kingdom, appear at first sight so very different, that a superficial observer would hardly trace any analogy between them (§ 195). A little reflection, however, will show, that all their forms are reducible to the simple element of which the respiratory organs are constructed in the Vegetable kingdom; — an exten- sion of the external surface, peculiarly adapted, by its permeability to gases, for the interchange of ingredients, between the circulating fluid brought in contact with one side of it, and the atmosphere which it touches on the other. This extension usually takes place internally or externally according as the animal is to be an inhabitant of the air or the waters. In animals modified for atmospheric respiration, the air enters the system to meet the blood; a peculiar set of movements, more or less complicated, being appointed for its constant renewal by successive inha- lation and expulsion. In those adapted to an aquatic residence, a different plan is required. The small quantity of air contained in the water is all that the respiratory system employs; and it would have been a useless expenditure of muscular exertion to have provided means for the constant inspiration and expiration of a large amount of so dense a fluid. In most aquatic animals, therefore, the aerating surface is extended out- wardly, instead of being prolonged inwards; and the blood is propelled through it so as to come in relation with the surrounding medium, the portion of which in contact with it is constantly being renewed, either by the natural movements of the animal, or by others more expressly con- trived for the purpose. In tracing upwards the different forms of the respiratory apparatus through the principal classes of animals, we shall observe the same gradual specialisation which has been noticed in the other systems; for, beginning mth the lowest, it will be seen that the general surface is the organ of respiration as well as of other functions; whilst, in the highest, the aeration of the blood is almost entirely effected in one central apparatus adapted to it alone, although the general surface is not altogether destitute of participation in it. 388. In the simplest forms of animal life, which are all aquatic, the almost homogeneous tissues are immediately nourished by absorption from without, as in the porifera ; and the constant movement of fluid through their ramifying canals answers the purpose of aerating their tissues, as well as of supplying them mth nutriment. In the infusoria, which also seem unprovided with special respiratory organs, we detect an appa- ratus which ministers not only to locomotion and the ingestion of food, but to the aeration of the fluid constituents of the organism, by perpe- tually renewing the surrounding water. The roAvs or tufts of the vibratile cilia (§ 110) by which these objects are fulfilled, are variously distributed in different species; in the gemmules of the Sponges and Polypes, which are destitute of internal canals of any kind, the surface of the body is covered with them; but in the Infusoria they are usually RESPIRATION IN ANIMALS. 301 disposed around the moutli; and tliey are arranged as a fringe on the tentacula Avhich border this orifice in the polypifera. We have no reason to believe that any minute distribution of capillary vessels exists in species so simply organised; and the supposition that each of these cilia, like the filament of a fish's gill, is composed of bloodvessels prolonged into the water for the purpose of aerating their contents, is scarcely tenable. It is, however, by no means improbable that the internal pro- longation of the surface which lines the digestive cavity, may be comiected with the respiratory function as well as Avith that of absorption (a combi- nation which we find in the foliaceous expansions of plants), in the cases where no more special structure is evolved. " The bodies of these animals," as Dr. Grant has remarked, "are not yet covered Avith solid shells, or with dense impervious scales, or with other hard materials which would exclude the general respiratory influence of water, and necessitate the formation of gills and lungs; but consist of the soft cellular tissue in Avhich all higher organisations are at first developed. The few kinds which are furnished with a thm transparent or silicious pellicle, have the power of extending the ciliated part of their body from beneath it: and thus of effecting all the required respiration." 389. The same observation will probably apply to the acaleph^, the soft external tegument of whose bodies would seem to afford sufiicient means for the aeration of the nutritious fluid, where the constant change of the medium to which it is exposed is provided for by the organs of motion. It has been mentioned that, in the Medusa and other similar animals, prolongations of the digestive canals ramify on the margin of the mantle, which, being the most moveable part of the body, exposes them to a constant interchange of the external element with which they are in relation (§ 269); and the special vascular system developed in the Ces- tutn Veneris^ Beroe, &c. sends similar prolongations along the ciliated margins, which appear destined rather for the aeration of their fluid than for the purjjoses of nutrition (§ 294). In this beautiful and interesting class, we not unfrequently meet with large sacs containing air, which often, in fact, constitute the bulk of the animal, and most attract the attention of observers (Fig. 140). Whether or not these serve any other purpose than that of giving buoyancy to the structure, and of occasionally receiving the impulsion of the wind, is still uncertain; nor has the gas contained in them been analysed. The animals appear to have consider- able power over their degree of distension ; for whole fleets of the elegant "Portuguese men-of-war," which variegate with their brilliant colours the surface of the ocean on a calm day, will suddenly sink into the water and disappear when a storm is threatened. 390. In the class echinodermata a distinct respiratory apparatus is evolved, which is required, not only by the increased energy of the animals, manifested in their poAverful muscular contractions, and l)y the 302 SPECIAL AND COMPARATIVE PHYSIOLOGY. development of a special circulating system, but by tbe condensation of the external tegument, which is no longer capable of serving, as in the classes we have been considering, for the aeration of the fluid portion of the tissues it encloses. Contrary to the general principle which has been stated, that in aquatic animals the circulating system is prolonged out- wardly, bringing the blood to meet the air contained in the dense element, we find that the respiratory apparatus of this class consists of a large cavity, from which a series of tubes ramifies minu.tely (in the higher species at least) through the body, and conveys the aerating fluid into every part of the structure. This cavity embraces the intestinal canal and other viscera, the exterior Avails of Avhich are therefore in contact with the fluid it contains; it is obviously analogous to the peritoneal cavity of higher animals; and though this is generally a closed sac, jet some traces of a similar conformation may be discovered in the Crocodile. The membrane which lines it in the Echinodermata is sufficiently muscu- lar to execute the movement necessary for the transmission through its ramifying prolongations of the water which it inspires; and in the Holo- tJmria, the leathery covering of which admits of more distension than the hard envelope of the Star-fish or the unyielding shell of the Echinus, so much water is sometimes taken in that the bulk of the animal is several times increased, and, by contraction of the cavity, the fluid may be expelled with considerable force. 391. The Molluscous classes presents great variety in the form and situation of their organs of respiration, although they are, with but few exceptions, inhabitants of the water. Most of these tribes are remarkable for the slowness of their movements, and many of them are entirely fixed ; and it is beautiful to observe how all of them, even the most inert, are provided with means of renewing the fluid in immediate contact with their bodies, so as to aerate and renovate the blood. Although the form and position of the gills varies much in the different classes, their general structure is the same in all; — they consist of delicate membi'anous folds or tufts (prolongations of the external surface) minutely reticulated with blood-vessels, and covered Avith vibratile cilia, by whose action constant and regular currents are produced. These gills are usually situated within the cavity of the mantle, and are in fact expansions of the delicate mem- brane which lines it (like the valvules conniventes formed by the reduplica- tion of the mucous membrane of the intestinal tube); and the entrance and exit of the water they require, are provided for by appropriate orifices, which are themselves fringed Avith cilia (§ 95). Sometimes the propulsion of the fluid is assisted by the general movements of the body; but not unfrequently the ejection of the expired water in a regular current is the principal means of locomotion with Avhich the animal is endowed. In other instances, the gills are situated on the exterior of the mantle, and are formed in a corresponding manner by an extension of its membrane RESPIRATION IN ANIMALS. 303 into folds or tufts copiously supplied vnth blood-vessels. This is the case in many gasteropoda and pteropoda; and whilst, in the least per- fect species, it is found that the general surface of the mantle, whether internal or external, seems adapted by its softness and yascularity for sharing in the aeration of the blood, — in those of higher organisation, in which a more powerful heart is developed, the branchial tufts or laminae are restricted to particular parts, and the function appears confined to them alone. The branchi*, when external, are generally disposed in such a manner as to be most influenced by the motions of the animal; thus, in the PTEROPODA they are situated on the fin-like processes by which these beautiful little Molluscs propel themselves through the ocean. Many of the GASTEROPODA are terrestrial, and are consequently modified for aeri- form respiration. In the Snail, for instance, we find an opening on the right side of the body, which leads to a highly vascular sac destined to receive atmospheric air; this sac is placed nearly in the middle of the back, the position in which we find the air-bag in fishes; and though the sm-face it exposes is much smaller than that presented by tufted gills, it does not conduce less to the aeration of the blood, since the air is brought to it in a pure state, and not diluted by diffusion in Avater. 392. In ascending through the series of Articulated animals, from the simple parasitic worms, to the highly-organised Insects or Crustacea, we find the respiratory apparatus assuming a more complicated form; and it is in this series that we first meet with beings capable of maintaining an active existence in the air. In the simple entozoa, no special respiratory apparatus is evolved; and it is obvious that whatever aeration their fluids require must be performed by the external envelope, or by the reflexion of it that lines the digestive cavity. In the aquatic orders of the extensive class ANNELIDA, liowever, Ave find a special prolongation of the surface, adapted to that purpose. This sometimes assumes the form of delicate feathery tufts, disposed in a radiated manner round the head, and often displaying the most splendid variety of colours, — as in the iSerpula'^ (Fig. 144). In another portion of the class Annelida, the ramified tufts are disposed at intervals along the body of the animal, as in the common iSandworm (Fig. 145) or in the Nereis (Fig. 141). A third form of the respiratory apparatus exists in those species of the Annelida which are adapted to live in air as well as water, such as the Leech or the Earth- worm. Here Ave lose the external gills or branchial tufts of the purely aquatic tribes, and find in their place a series of small bags, opening from their sides by minute orifices termed stigmata, and extending into the * There are few sights more striking to the observer of nature in tropical regions, than the unexpected view of a bed of coral in shallow water, having its surface scattered with the bril- liant tufts of the Serpulffi which have formed their habitations in it ; tlie glowing and variegated tints of which, when lighted up by the mid-day sun, and contrasted with the sombre hues of the suiTounding rocks, present an appearance compared to which the most beautiful garden of carnations (which flower the animals mucli resemble in form) sinks into insignificance. 304 SPECIAL AND COMPARATIVE PHYSIOLOGY. interior of the body. No communicating tubes exist, however, between the sacs; nor do they send ramifying prolongations to distant organs; but this simple inflexion of the external surface cannot but be regarded as the rudimentary form of the complex respiratory apparatus of Insects. It is desirable to remark the connexion between the functions of respiration and locomotion in this class; the first indications of the evolution of spe- cial appendages for the latter purpose being discernible in those particularly adapted for the former. The motion of the branchial tufts of the Nereis is obviously one means of its propulsion through the water; although its progression is, no doubt, effected principally by the serpentine movements allowed by the general flexibility of the body. In som« species, one of the filaments is prolonged and straightened into what is called a cirrJms (Fig. 142, 3, «), which possesses an obvious tubular structure, and is evi- dently the rudiment of the regularly-articulated members possessed by the succeeding classes. In the tribes modified for aerial respiration, the traces of the external organs are sometimes found in the setce or bristles, of which a certain definite number are attached to every segment, (the earth- worm possessing two pairs on each side), and which obviously serve as organs of locomotion ; the species which are deficient in them, such as the leech, have the segments of the body very short and numerous, and thus possess greater flexibility of the trunk. 393. In the myriapoda, the respiratory and locomotive systems are more definitely separated from each other. The increased hardness and want of flexibility of the tegumentary covering, requires both a more spe- cial apparatus for the aeration of the blood, and a more decided develop- ment of organs of propulsion. The prolonged setce, therefore, of the higher Annelida here become regular jointed legs, endowed with consi- derable muscular powers; and as all of this class are inhabitants of the air, the respiratory surface is prolonged inwards in the form of canals ramifying through the body; but these trachece or air-tubes (§ 26), which arise from distinct stigmata, seldom have much communication with each other. In this form we may observe an intermediate condition between the insulated sacs of the air-breathing Annelida, and the complex distri- tribution and frequent anastomosis of the tracheal system of Insects. In some of the higher species, however, two longitudinal canals have been observed, connecting together all the separate systems of tracheae, such as will be presently shown in the larva of Insects. 394. In studying the respiratory system of insects, we shall have occasion to observe several peculiar modifications which it undergoes for particular purposes, whilst its essential character remains unaltered; and we shall have also an opportunity of noticing the varieties of form and function which the same apparatus may present at difi"erent periods of life, and under changes in external conditions. The muscular energy required for the locomotive powers of the perfect Insect, and the general RESPIRATION IN ANIMALS. 305 activity of the organic processes, necessarily involve a large amount of communication between the nutritious fluid and the atmosphere; hut, on the other hand, the low development of the circulating system would prevent the aeration from being accomplished with sufficient rapidity by the transmission of the blood through one particular organ. The dif- ficulty is obviated by the introduction of the vivifying agent into every part of the body, by means of a complex and minutely-distributed system of tubes, which appear to ramify through even the smallest and most delicate organs, and which bring the air into immediate relation with all their tissues. This structure answers another purpose; for, by means of the distention of the body by gaseous fluid, its specific gravity is reduced, and it is maintained in the atmosphere with less exertion. We shall find indications of a similar adaptation in Birds, the insects of the Vertebrated classes, as they have been justly denominated. The extent of respiratory surface thus created is such, that the amount of the aerating changes per- foi-med by an insect in a state of activity, is not less in proportion to its bulk than that efi^ected by the most energetic of the Vertebrata. It is impossible to view this subject philosophically without being struck by the fact, that this very high degi'ee of respiratory power is given, not by a sudden advance to a more complicated and perfect system of organs, such as exist in the Vertebrated classes of animals, but by a extension of the comparatively simple plan of which we observed the first traces in the Annelida; thus affording a beautiful example of the great law of regular progression in the development of organs, which has few apparent and perhaps no real exceptions. Nor would it be easy for any reflecting mind to contemplate the manner in which the air is thus brought into contact with the blood in the minutest textures of the body,'"' without a feeling of admiration at the contrivance shoA^Ti in the compensation of the limited circulation of the fluids by the extensive distribution of the respiratory apparatus; and at the means by which the necessary lightness, elasticity, buoyancy, and muscular energy are imparted to the bodies of these beautiful and interesting inhabitants of the air. 395. In the Larva condition of such aerial insects as undergo a com- plete metamorphosis, and are therefore most different in their early state fi-om their ultimate character, the respiratory system much corresponds ivith the type it had attained in the higher Myriapoda. We find it entirely consisting of ramifjdng trachece, connected with the external air by the stigmata that open on the sides of the body; and freely com- municating with each other, especially by the two longitudinal tubes * A French Microscopist, M. Bernard-Deschamps, imagines, not without show of proba- bility, that the tracheae are even continued into the scales which clothe the membrane of the wings. Many of these, after their coloured lamina has been removed, exhibit a series of hnes directed towards the point by which the scale is attached to tlie wing. (Ann. des Sci. Nat, N. S. Zool. iii), , X 306 SPECIAL AND COMPARATIVE PHYSIOLOGY. which traverse its length, and into which the stigmata open hy short straight passages (Fig. 147). Of the peculiar structure of these tubes a description has already been given (§ 26); and the change which they undergo in the metamorphosis of the insect will therefore be now briefly stated. Just as the Larva is passing into the Pupa state, the larger trachcEe exhibit dilatations at intervals, which are subsequently developed into expanded sacs that sometimes attain considerable size. The efforts which the animal makes at the moment of transformation to rupture its skin by the distention of its body, appear to contribute towards the expansion of these sacs, the formation of which had previously com- menced.* One remarkable portion of the tracheal system, also, the incipient evolution of which may be detected in the Larva state, now shows an increased tendency to prolongation; — that namely which forms the wings. It may be regarded as absurd to maintain that the wings of insects are a part of the respiratory apparatus; but that such is really the case, is shown by the consideration of their perfect structure, and of the history of their development (§ 194<7iote). During the first metamorphosis of the Sphinx ligustri, as observed by Mr. Newport, the Avings, Avhich at the moment of slipping off the larva skin were scarcely as large as hemp- seeds, have their trachcce distended mth air; and, at each inspiration of the insect, are gradually prolonged over the trunk by the propulsion of the circulating fluid into them. The enlargement of the tracheae may also be observed in the antennw (§ 88), which, just before the change Avere coiled up within the sides of the head, but are now extended along the sides and abdomen. 396. The full development of the respiratory apparatus only takes place, however, after the last metamorphosis; when the wings become fully distended with air, and prepared for flight by the active respiratory movements of the body; and the expansion of the pulmonary sacs proceeds to a greater extent. It may frequently be noticed that for some hours or even days after the perfect Insect has emerged from the pupa state, it makes no effort to fly, but remains in almost the same torpid condition with that it has quitted; when stimulated to move, hoAvever, it makes a few deep inspirations, its wings rapidly become fully expanded, and it soon trusts itself in the element which was intended for its habitation. The pulmonary sacs sometimes attain a very large size, and communicate AA'ith each other so freely as to appear like continuous cavities. This is well seen in Fig. 21, which exhibits the respiratory apparatus of the abdomen of the Humble-bee; and in Fig. 146, Avhich shoAvs that of the Scolia hoTtorum. They vary considerably, however, in different species and tribes; being usually most developed in those insects that sustain the longest and most poAA'erful flight, which are generally those whose larva condition has been most imperfect, and in which there has been originally * Newport on the Respiration of Insects, Phil. Trans. 1836. RESPIRATION IN ANIMALS. 307 no appearance of these enlargements. They are almost entirely absent in the insects destined to live upon the ground; or in them are little larger than the slight expansions found in the early conditions of such as undergo no complete metamorphosis. There can he little doubt that one use of these cavities is to diminish the specific gravity of the Insect, and thus to render it more buoyant in the atmosphere; but it would not seem impro- bable that they are intended to contain a store of air for its use while on the wing, as the spiracles are at that time closed, so that none can enter from mthout. 397. The various provisions which are made for the respiration of such insects as inhabit the Avater are of a nature too interesting to be passed by. In those aquatic Larvae which breathe air, we often find the last segment of the abdomen prolonged into a tube, * the mouth of which remains at the surface while the body is immersed. The larvae of the gnat may often be seen breathing in this manner, which calls to mind the elevation of the trunk of the elephant when crossing rivers that entirely conceal his head and body. Sometimes this air-tube, which is to be regarded as a prolonged spiracle, is several inches in length, and its mouth is furnished with a fringe of setm (or bristles), which entangle bubbles of air sufficient to maintain respiration when the animal descends entirely to the bottom. The large trachese proceeding from this tube convey the air through the body in the usual way. Most aquatic Larvae which are unpossessed of such an air-tube, have their spiracles situated only at the posterior extremity of the body, and may be seen apparently hanging from the surface, whilst taking in the necessary supply. All perfect insects being adapted to aerial respiration only, many curious contrivances may be witnessed among such as inhabit the water, for carrying down a sufficient supply of oxygen to aerate their blood Avhilst under the surface. Some enclose a large bubble beneath the elytra (wing-cases) Avhich, not being closely fitted to the exterior of the body, leave a cavity into which the spiracles open. Others have the whole under surface of the body covered mth down, which entangles minute bubbles of air in such large quantity as to render the insect quite buoyant, and to oblige it to descend by creeping along the stem of a plant, or l)y a strong muscular effort. A very beautiful contrivance for a similar pur- pose is that of the diving-spider, which remains for a considerable j)eriod under water by means of a reservoir that it constructs of silken thread agglutinated together, open at the bottom like a diving bell, and attached to neighbouring stones or plants, and which it gradually fills with air by carrying down successive bubbles beneath its body. In this habitation it spends the winter in a state of partial torpidity, and the quantity of air it has enveloped in this curious manner is sufficient to maintain its respiration. 398. There are some Larva?, however, more particularly adapted to X 2 308 SPECIAL AND COMPARATIVE PHYSIOLOGY. aquatic respiration by the development of the tracheal system externally into branchial plates or tufts, the object of which is not so much to cany the circulating fluid into contact with the water, as to absorb from that element the air which it contains, and which is then carried into the internal respiratory apparatus. Sometimes the membrane which covers the trachea?, and Avhich is a prolongation of the external surface, is con- tinuous, so that the gills have a foliaceous appearance like that of the wings ; but, in other cases, it is divided, so that the branchiae more resem- ble the filamentous tufts of the Nereis. Their position is constantly varying; sometimes they are attached to the thorax, sometimes to the abdomen, sometimes even situated within the intestine; but in every case they have an important relation with the movements of the animal, and are frequently the sole organs of progression with which it is furnished. Thus, the sudden darting motion of the Larvse of the Lihellula (dragon- fly) is caused by the violent ejection from the intestine of the Avater which has been taken in for the sujiply of the gills it contains, Avhence it is received into the tracheal system. A very little examination into the structure of the wings will show that it is essentially the same as that of the expanded gills of aquatic larvae; each consisting of a prolongation of the superficial covering of the body over a system of ramifying nerves or ribs, which are principally composed of tracheae in connection with those of the interior of the fabric. Hence Oken followed by Blainville, termed the wings aerial gills, — an idea which, however ridiculed by suc- ceeding writers on Entomology, will be found to be supported by the strictest analogies in structure, situation, and development. It is only by taking an extensive view of comparative structure that we can have any hope of arriving at accurate results; and great care is necessary to dismiss from the mind all prejudice in favour of a particular organisation as a standard or type of the rest. If we suppose an Entomologist to form his vieAvs of the structure of Animals in general from that of the Articulata, he would expect to find the wing of a bat or bird constructed on the model of that of an insect. Yet he would not be acting more absurdly in main- taming that this organ is developed out of the respiratory sj^stem in Verte- brated animals (especially considering its remarkable connection with this system in birds), than many entomologists Avho have been led, by their previous acquaintance with other types of structure, to consider the wing of an Insect as a modification of its leg (§ 194, note). 399. In the Crustacea, the respiratory organs are universally adapted for an aquatic mediimi, and are consequently developed in the form of gills, which are usually placed on the under surface of the body, and con- nected with some very moveable parts, as there is not yet any special means adapted for propelling currents of water over them. The dififerent orders of this class, however, exhibit so many interesting gradations of development of the respiratory system, that they can scarcely be overlooked RESPIRATION IN ANIMALf?. 009 in a sketch like the present; — especially since these gradations exactly correspond, as Milne Edwards has admirably shown, mth the transitory forms Avhich each indiyidual of the higher species presents in the progress of its development. In the lowest tribes no special aerating surface is evolved, nor do any of the other organs appear to undergo such modifications as would fit them for assisting in the discharge of the func- tion; it must be concluded, therefore, that the process of respiration is carried on by the whole exterior of the body. In other orders, again, the last joints. of the legs are flattened out into a surface which is soft and vascular, and which, by its action upon the water, appears calculated to facilitate the influence of the air upon the nutritious fluid. Proceeding higher, we find a particular portion only of the extremity devoted to respi- ration; but this is developed to an increased extent, and the water in con- tact with its surface is incessantly renovated by currents set in motion by the abdominal members. The next stage in the specialisation of this function is the restriction of the branchial apparatus to the abdominal members, which are entirely devoted to it, and cease to have other uses. In a still higher order, the gills have assumed more of the character which they present in Fishes and some Mollusca; the laminated or leaf- like form which they at first possessed, having given place to one in which the surface is greatly extended by miiuite subdivision into delicate filaments. 400. The most developed form of respiratory apparatus possessed by Crustaceans is that which exists in Crabs, Lobsters, and other Decapods. In this order not only is the function thrown upon particular organs created expressly for the purpose, but these organs are lodged and pro- tected mthin especial cavities, and the renewal of the water necessary to their operation is secured by the motion of distinct appendages. These cavities are formed by a reduplication of the external tegument, and are provided with two orifices, one for the introduction and the other for the expulsion of the fluid. In those Crustacea Avhich are adapted to live for a time on land, these orifices are very small, so that a trifling amount of evaporation can take place from them; and it appears that in all species the gills can be subservient to aerial as well as to aquatic respiration, pro- vided their surface is kept moist, — the asph3rxia of the animals in a dry atmosphere being due to the desiccation of the membrane, and its conse- quent ujifitness for the performance of its functions. There are other species which not only live habitually out of water, but are infallibly drowned if kept long immersed in that fluid. These are the land-crahs^ which are esteemed among the greatest delicacies of the West Indian Islands, and are sometimes regularly fattened for the table. The mem- brane lining their branchial cavities is sometimes disposed in folds capable of serving as reservoirs for a considerable quantity of water, and sometimes presents a spongy texture equally well adapted for storing up the fluid '4 310 SPECIAL AND COMPARATIVE PHYSIOLOGY. that is necessary to keep the organs of respiration in a state of humidity required for the performance of their functions. Land-crahs are never known to remove far from damp situations; and this humidity may be either derived from the atmosphere, or may be secreted, as in higher ani- mals, fi-om the circulating fluid. It can scarcely be doubted that the spongy lining of the branchial cavity in these Crustacea is peculiarly sub- servient to aerial respiration; and it appears owing to the check given to its activity that the land-crabs are drowned when plunged under water. A more highly-developed form of this type of respiratory system is found in the next class. 401. The stages in the development of the branchial apparatus of the Astacus Jluviatilis (river-crab) have been so beautifully traced by M. Milne-Edwards in connexion with the various forms of the same in adult species of different tribes, that it seems advantageous to notice them here for the sake of ready comparison, rather than to defer the account of them to the general description of the progressive evolution of the system in the embryo of higher animals. At the earliest period of embryotic life, no trace of branchiae can be discovered; but when they are first evolved, during the process of incubation, they consist of simple laminated expan- sions, occupying the situation of the extremities of the maxillary append- ages; these soon subdivide, and one part assumes a cylindrical form, and seems no longer to belong to the apparatus, — whilst branchial filaments begin to appear on the other, which are subsequently prolonged into com- plete gills. During this interval the thoracic extremities have made their appearance, and they also become furnished with branchial appendages. At a subsequent time, a narrow groove or furrow is seen along the under edges of the thorax, the margins of which, in no long period, are prolonged so as to meet each other and enclose the gills; openings being left for the entrance and exit of water, which are at first large, but subsequently become contracted to the proper size. It is thus evident that the lining membrane of the cavity, as well as that which covers the filaments of the branchise, is but a prolongation of the external tegument. We cannot avoid perceiving in this conformation, a transition from the branchial to the pulmonary form of the respiratory apparatus; a transition which is still more evident in the structure of the next class. 402. In the lower tribes of the arachnid A, which approximate most nearly to the Insect type, such as the Acari (cheese-mite, &c.) the respi- ratory apparatus is constructed on the plan which prevails in that class; being composed of a system of tracheae, ramifying through the body, and opening externally by stigmata. In the more perfect forms, however, such as the Spider, Scorpion, &c., the circulating system is more deve- loped, and there is no longer occasion for such universal aeration of the individual parts, that of the nutrient fluid being sufficient. Accordingly the respiratory apparatus exists in a more concentrated state, which ap- RESPIRATION IN ANIMALS. 311 proxiinates nearly to that which has been described as possessed by the higher Crustacea; but, being adapted for aerial respiration only, it must be regarded as belonging rather to the pulmonary than to the branchial system. The stigmata in these animals, instead of opening into a pro- longed set of ramifying and anastomosing tubes, enter at once into distinct sacs, disposed along the sides of the abdomen, to which the air has there- fore ready access. The interior of these cavities is not smooth, however, like that of the pulmonary sacs of Insects, but prolonged into a number of duplicatures or folds; these lie close to each other like the laminae of gills, and may be regarded either as analogous to them, or as rudiments of the partition of the cavity into minute cells, as in the lungs of higher animals. From these analogies to both classes of organs they are denominated by Audouin pulmonary hranchice. The following figures Avill serve as a plan of the transition which is thus effected between one form of respi- ratory apparatus and another. At a, is seen the character of the simple foliaceous gill, Avhich is evidently a mere external prolongation of the general surface, a, b; at h, a similar internal prolongation or reflexion, forming the simple pulmonary sac of the leech or earthworm; c represents a gill formed by the minute subdivisions of the surface into filaments, so that it is greatly extended, as we find it in fishes; and d shows a similar extension of the internal sxirface by the partition of the cavity, (by which is effected, within a small space in the lungs of Vertebrated animals, that which the economy of the Insect condition required to be performed by an apparatus of much greater extent) ; lastly, at e is shown a plan of one of the respiratory cavities in the Crustacea, or of the pulmonary branchiae of the Arachnida, exhibiting the transition abeady described, in the location of the gill-like processes upon the concave walls of a cavity formed, like that of the lungs, by an internal prolongation of the tegu- mentary surface, 403. In this slight sketch, then, of the development of the organs of respiration in Invertebrated classes, it will be observed that, whilst the entire covering of the animal is subservient to this function in the lowest tribes, portions of the surface specially modified for the aeration of the blood are found in the higher: these being disposed, according to the medium which the animal is destined to inhabit, in the form of gills or pulmonic cavities; and situated in the most convenient position for receiv- ing the fluid, and for submitting it to the influence of the surrounding 312 SPECIAL AND COiMPARATIVE niYSIOLOGY. element. Although amongst some of these animals the hranchial appa- ratus reaches nearly the highest development which it attains under any circumstances, we only observe the sketch, as it were, of the imlmonary organs of the higher Vertehrata, which never lose their diffused character in the classes we have been considering. In no case do the respiratory organs communicate with the mouth, Avhich is an organ solely appro- priated, in these lower tribes, to the reception and subdivision of the food; and it may also be remarked that the movements by which the aeration of the blood is assisted are, in most cases, those of the body at large. 404. Amongst the Vertebrata we observe a similar diversity of form in the respiratory organs to that which the inferior classes have presented to us; and the differences in the general economy of the system, with which the amount of the function is connected, are mani- fested in even a more striking degree. In the slow-moving Reptile, as in the Mollusca, where the respiration is feeble, it may be suspended for a time without inconvenience; but to the active inhabitants of the air. Birds as well as Insects, in Avhom this function is necessarily performed mth great energy, its suspension is quickly fatal. If a bird be kept in a limited quantity of air until it ceases to respire, and we then place suc- cessively in the same atmosphere, a dormouse, a frog, and a snail, each of these animals will continue to breathe for some time in an atmosphere which its predecessor had vitiated too much to continue to support its own respiration. 405. Although the respiratory apparatus in fishes retains the type which characterised it in the inferior aquatic classes, it undergoes great increase both in extent and importance. In order to keep up with the rapid advance in the development of the other systems, the respiration requires to be conducted, though by means of an aquatic element, with great velocity and effect. For this purpose it is not sufficient that fishes should have merely filamentous tufts hanging loosely at the sides of the neck; but it is requisite that they should have the means of rapidly and constantly propelling large streams of water over their surface, and of forcing the whole blood of the system through the respiratory apparatus, to be submitted to the action of the air that is contained so scantily in the water. The former of these ends is effected by the connexion of the gills Avith the cavity of the mouth, the muscles of which send a rapid cur- rent of water through the branchial passages; and the latter, by the alter- ation in the position of the heart, which is placed so as to affect the respiratory organs previously to the system at large (§ 309). The gills in most fishes are disposed in fringed laminse, the fibres of which are set close together like the barbs of a feather (Fig. 149), and attached on each side of the throat in double rows, to the convex margins of four or five long bony or cartilaginous arches which are very similar to the ribs. RESPIRATION IN ANIMALS. 313 The extent of surface exposed by these gills is very great; Dr. Munro computed that in the skate it is at least equal to that of the human body. In the osseous fishes, the gills are concealed by a valvular covering, called the operculum, which allows free exit to the water impelled through the mouth. In the cartilaginous fishes, the gills are more completely enclosed, and the water which passes over them finds its way out through five small openings on each side of the neck, which are called branchial openings (Fig. 148); these, as will be hereafter seen, may be detected at an early period of the development of all higher animals, not excepting man him- self. During the embryo condition of both of the principal divisions of Fishes, the gills may be seen hanging loosely from the back part of the neck; for, in osseous fishes, they have attained considerable development before the prolongation of the integument has been formed into the valve which covers them; and in the cartilaginous fishes, the branchial openings are at first large, and the filaments of the gills are prolonged much beyond them, — other filaments also, which subsequently disappear alto- gether, being produced from their edges. 406. In considering the respiratory organs of Fishes, the air-bladder must not be omitted, this being now generally regarded as the rudimentary form of the complex lungs of the higher Vertebrata. In many Fishes, as in the embryo of Mammalia, it is a simple shut sac, placed along the middle of the back; in others, it has a division of its cavity by one or two membranous partitions. This air-bag usually communicates with some part of the alimentary canal near the stomach, by means of a short wide canal termed the ductus pneumaticusj but sometimes, as in the sword-fish, it has no manifest opening, and we find it connected with a glandular and highly vascular organ, which has been supposed to secrete the gas that it contains. The true character of the structure is most remarkably sho-v^Ti in the Lepidosteus or bony-pike of the North American lakes (§ 81). This curious fish, which presents many points of approxi- mation to the lizard tribe, has the air-bladder divided into two sacs that possess a cellular structure, — ^the trachea that proceeds from it opening high up in the throat, and being surmounted with a glottis. As many fishes are known to swallow air and eject it as carbonic acid gas, it Avould scarcely seem imjDOSsible that where a communication exists between the alimentary canal and the air-bladder, the latter organ is concerned in the change: for the process of respiration is performed by an action resembling swallowing in frogs and other Amphibia which possess no ribs or diaphragm; and in those curious species which are modified for both aerial and aquatic respiration, the lungs are scarcely more highly-organ- ised than the air-sacs of the Lepidosteus. 407. The uses of the air-bladder in those fishes which possess no ductus pneumaticus are involved in some obscurity. That it is not imme- diately connected Avith the function of respiration appears sufficiently 314 SPECIAL AND COMPARATIVE PHYSIOLOGY. evident; and this seems one of the instances, of which many might be pointed out both in the vegetable and animal kingdoms, where the rudi- mentary form of an organ that attains its full development in other classes, is adapted to discharge some office quite diflferent from that to which it is destined in its perfect state. The gas which it contains is composed of the same elements as atmospheric air, namely oxygen, nitro- gen, and carbonic acid; but these are mixed in proportions that are very liable to variation. It has been said that oxygen is deficient in the con- tents of the air-bladder of fresh-water fishes, and is predominant in that of fishes which remain at considerable depths in the sea. This organ is altogether absent in fishes accustomed to remain at the bottom, and whose movements are slow; whilst it is of large size in those remarkable for vehement and prolonged movements, especially in Flying-fish of various species. It is generally supposed that the fish is enabled by means of the air-bladder to alter its specific gravity, by compressing the bag or permitting its distension; but experiment shows that, after the organ has been removed, a fish may still retain the power of raising or lowering itself in the water. 408. The transition which has already been described, as occurring between the class of Fishes and that of reptiles, and as being manifested, not only in the permanent and complete forms, but during the progress of the development of individual organs, is nowhere more beautifully indicated than in the respiratory apparatus. All of the order Batrachia (otherAvise called Amphibia), when young and imperfect, inhabit the water solely, and are in fact j!>ro teinpore fishes. Their organs of respira- tion are of course formed on the aquatic type, consisting of branchise; and, in their early development, they undergo the same change with those of fishes. In all instances they are at first external, hanging like tufts from the neck; and this state continues in the Proteus, Siren, and other species of the family of perennihrancMate amphibia (which retain their gills through life). In those, however, whose development proceeds further, as frogs, salamanders, &c., they are subsequently more or less enclosed by a fold of the skin, which forms a membranous valve, analo- gous to the bony operculum of fishes. In frogs, the branchial cavity thus formed is closed completely on the right side, and the water which passes into it is ejected through the opening that remains in the left. As the tadpole advances towards the final change which is to convert it from a fish into a reptile, the gills entirely disappear, and lungs are developed, by which it breathes for the remainder of its life. These lungs are not, however, minutely subdivided like those of Birds or Mammalia; a large part of their cavity is simple; and the appearance of partitions is almost restricted to the top (Fig. 150). It appears as if, in the family of peren- nihrancMate amphibia, the development had been checked just at the period of the transformation ; for we find their permanent form exactly RESPIRATION IN ANIMALS. 315 coiTespondiug with that which is transitory in those that undergo a com- plete metamorphosis, and resembhng that which has been artificially- rendered permanent in the latter by the due regulation of the vital stimuli (§ 183). It is not a little curious that the habitation of the least-developed of these animals, the Proteus^ subjects it to exactly the same conditions as those by which Dr. Edwards found that he could retard the development of the frog; and, until analogous species were found elsewhere, it was believed to be the Larva of some more perfect Reptile. 409. The members of this family of Perennihranchia (which are the only true ampJiihious animals) possess lungs more or less developed; those of the Proteus being very similar to the air-bags of fishes, whilst those of the Siren exhibit some degree of partition into cells. The tube by which they open into the mouth bears greater analogy to the ductus pneumaticus than to the trachea of higher animals, being simply mem- branous without an appearance of rings; and the glottis in which it terminates is a mere slit in the throat. Thus, the transition from the simple closed sac of fishes to the more complex subdivided lung of the frogs and land-salamanders is perceived to be very gradual; whilst, at the same time, the point of connexion between the respiratory cavity and the alimentary tube may be observed to ascend, by similar gradations, from the stomach or some neighbouring part to the oesophagus, and at last to the mouth. Although all the animals which retain their gills at the same time that they acquire lungs, are more or less adapted both to aerial and aquatic respiration, the relative degree of the two varies with the comparative development of their organs. Thus, in the Siren, the pulmonic respiration is more extensive and important than the branchial; but the reverse is the case in the Proteus. In taking leave of respiration by gills, it must not be forgotten that, even in the most developed condi- tion of their structure, these organs are covered Avith vibratile cilia of precisely the same character as those which seemed to be the only organs connected with this function in the lowest and simplest animals. In both cases, however, their purpose would seem to be the same, viz., to create currents over the surface on which they are fixed, which shall con- stantly renew the stratum of fluid in apposition with it. In the larva condition of the Amphibia, they are not confined to the gills, however, but act over the whole body; and, in the adult, the general surface appears peculiarly connected with the function of respiration, the soft moist skin being an excellent medium for the exposure of the blood to the air. Experimental proofs of the degi'ee in which the general surface of Fishes and Batracians may be regarded as sharing in the process of aeration, will be hereafter given (§ 422). 410. In Serpents^ we usually find a long cylindrical sac, only divided into cells at its upper part, and generally extending along the tail; in 316 SPECIAL AND COMPARATIVE PHYSIOLOGY. some genera, liowever, tliis sac is double; and where there is only one, it is that on the right side which is developed, the other remaining in its rudimentary state. From the great capacity of the respiratory sac, the mobility of their ribs, and the power of their intercostal muscles, Serpents are capable of rapidly inspiring and expiring a large quantity of air, by which the want of an extensive surface is compensated, and energy is imparted to their muscular exertions. It is the prolonged expulsion of the air after the lung has been fully inflated, that gives rise to the con- tinued hissing sound by which these animals sometimes alarm their prey. In the aquatic Serpents, the large volume of air contained in the body serves to render it buoyant, and at the same time supplies the wants of the animal during a prolonged immersion. Serpents may be regarded as representing, in their general conformation, the lower Articulated classes among Vertebrata, whilst Birds evidently typify the Insect tribes. The prolongation of the lung through nearly the whole extent of the body, and its low degi'ee of development, indicated by its almost entire want of cellular subdivision, forcibly remind us of the pulmonary sacs of the Leech or Earthworm. 411. In the Saurian reptiles, we still find a very imperfect subdivision of the pulmonary sacs; but they are equally developed in both sides of the body. In the lower genera of this order, there is scai'cely any appearance of cells; but when we have advanced upwards to the Crocodile, we find the lungs, though externally small, subdivided to a great degree of minute- ness by internal partitions; and we also find a rudimentary condition of the diaphragm, which is entirely wanting in all the inferior genera, the lungs frequently extending in them through the whole trunk. In the Chameleon for instance, as well as many other Lizards, the lungs extend far beneath the skin; and, by their fulness or emptiness of air, give rise to the plump or lean appearance, either of which these animals have the power of assuming by the simple processes of inspiration or expiration. It is not a little curious that in the Crocodile are found two openings, leading from the surface to the interior cavity of the abdomen, which is lined by the peritoneum. This structure is evidently similar in character to that which has been described in the Holothuria (§ 390); whether it is adapted to the same purpose is not yet fully ascertained. It has been supposed by GeofiP. St. Hilaire, that the superior energy of the Crocodile when immersed in water is due to the penetration of that fluid into the abdominal cavity, and the consequent conversion of the peritoneum into an additional respi- ratory surface. Whether this be correct or not, it is worthy of notice that the sternum is prolonged over the front of the abdomen, and the sides fortified with ribs like the thorax; a structure of which the indications are readily traced in the linca alba and linece transversales on the abdominal muscles of Mammalia. The structure of the lungs in Turtles and other Chelonia, is very similar to that exhibited by the higher lizards; the sacs RESPIRATION IN ANIMALS. 317 are very ca,pacious and liave few subdivisions; and tliey materially assist, by the quantity of air they contain, in buoying up the heavy trunk of these animals when sailing on the surface of the water. The Chelonia, like the inferior Reptiles, are obliged to distend the lungs by a process resembling swallowing; the diaphragm being nowhere developed in a sufiicient degree to be capable of producing active inspiratory movements. Thus, however paradoxical it may appear, a reptile can be prevented from respiring by holding its mouth open. 412. The respiratory apparatus of birds is intermediate in the perfection of its development between that of the Reptiles and that of the Mammalia. In this class, as in Insects, it extends through a great part of the body; large sacs connected with the lungs being contained in the abdomen, and even continued beyond the cavity of the trunk, as under the skin of the neck and extremities.^' Even the bones are made subservient to this function; for though at an early period they possess a spongy texture, like those of the Reptiles, and are filled with thin marrow, they subsequently become hollow, and their cavi- ties communicate with the lungs; in the aquatic species, however, the original condition is retained through life. In those Birds of which the bones are thus permeated by air, the trachea may be tied and the animal still continue to respire by an opening made in the humerus or even the femur. The lungs are confined, as in Tortoises, to the back part of the cavity of the trunk ; they are of a spongy texture, but much less minutely subdivided than those of Mammalia. No diaphragm exists in Birds, except in the Ostrich, Avhich forms a transition to the class Mammalia; and, from the manner in which the lungs are connected with the walls of the chest, the state of distention is the natural or passive condition, and * Various surmises have been formed on the particular uses of these air-sacs in the economy of the bird; and it does not seem improbable that, besides contributing- to the function of respi- ration by the extension of surface they afford, they have some subsidiary purposes. One of the most evident is that of rendering the body specifically lighter, as in Insects; and this will be obviously assisted by the great heat of the system, which rarifies the contained air. Again, the distension of the air cells assists in keeping the wings outstretched ; as is shown by the fact that inflation of those situated in the neighbourhood of their muscles is followed by their expan- sion ; this must be a most important economy of muscular action in birds which hover long in the air. Their evident analogy to the pulmonary sacs of insects is confirmed by their rela- tively larger dimensions in birds of long continued and rapid motion, than in the slow-moving- tribes which are almost confined to the earth or waters. It has been remarked in addition that "the same air which exerts its renovating- influence upon the blood, supports all the more deli- cate structures which it reaches and surrounds, as a cushion of the most perfect softness and elasticity ; so that by the most rapid motion, and the most violent twitches which the body receives in the changes and turnings of that motion, there can be no concussion of the parts more immediately necessary for the life of the birds." It would scarcely seem improbable that the large air-cells which are found extending beneath the integument of the wliole body, espe- cially the under surface, of the Pelican and Gannet, serve to deaden the concussion which the system must experience when the bird, after raising itself to considerable height in the air, lets itself suddenly fall upon the water in pursuit of its (inny prey. 318 SPECIAL AND COMPARATIVE PHYSIOLOGY. the act of respiration is forced. It is beautiful to observe that in Birds, as in insects, the great extension of the respiratory surface is given by a sim- ple increase in the capacity and prolongation of the sacs, and not by that concentration of it into a small bulk which is effected by the minute par- titioning of their cavity, and which indicates the highest form of the respiratory organs. Another analogy to the character of the respiratory system of insects is this: in insects, the Avhole of the aeration is effected by bringing the air in contact with the blood actually circulating through the system; whilst, in the higher air-breathing animals possessed of a more centralised apparatus (whether consisting of lungs or gills), the blood is transmitted through it by a special adaptation of the vascular system, in the intervals of its circulation through the body. In Birds is presented a curious adaptation of the latter more elevated type to the conditions of their existence; for, whilst the air introduced into the lungs acts upon the blood transmitted by the pulmonary vessels, that which fills the air-cells and cavities of the bones comes into relation (as in Insects) with the capillaries of the system at large. 413. The respiration of mammalia is not, like that of Birds, extended through the system, but is restricted to the lungs; and as a perfect diaphragm is now developed, which completely separates the thoracic from the abdo- minal cavity, these organs are confined to the former. Although their bulk is proportionally so much smaller than that of the pulmonary sacs of birds, or even reptiles, the actual amount of surface over which the blood is exposed to atmospheric influence, is beyond comparison larger, owing to their very minute subdivision into cells (Figs. 151, 2). The want of capacity, too, is compensated by the active movements of inspiration and expiration, which constantly and most effectually renew their contents; for, by the contraction of the diaphragm, and the elevation of the ribs, the cavity of the thorax is greatly enlarged, and the air rushes into the lungs to fill up, by distending them, the vacuum thus created; and the diaphragm being relaxed, and pushed upwards by the contraction of the abdo- minal muscles upon the contained viscera, and the ribs being at the same time depressed, the cavity of the chest is again diminished and the con- tents of the lungs expelled. It has been ascertained by experiments made for the purpose of discrimination between the lungs which have been dis- tended by natural inspiration, and those which have been artificially inflated (a point of much importance in criminal enquiries as to Infanti- cide), that in the former case a much more minute injection of the ultimate air-cells takes place, than in the latter; and that while portions of the lung Avhich have been artificially inflated may be compressed in such a manner as to sink in water, the air cannot be expelled in a similar manner from lungs which have once breathed naturally, without their structure being entirely broken down. This fact serves to show the superiority of a mode of respiration like that of the Mammalia over the deglutition of air prac- RESPIRATION IN ANIMALS. 319 tised by Reptiles. The lungs are greatly developed in all the more power- ful Mammalia, as in the carnivorous species; but they are comparatively smaller in their extent of surface in the feeble and inferiorly-organised herbivora. The varieties of these organs presented by the different orders of quadrupeds relate chiefly to their exterior divisions, and to their greater or less capacity; the plan of structure being nearly the same in all. It is interesting to remark, however, that in every case the lungs are largest on the right side; we have seen that in Sei-pents, where only one lung is developed, it is also the right; and even in the air-breathing Gasteropoda the pulmonic cavity is on the same side. This fact seems to have a con- nexion Avith the superior energy of the members on the right side, Avhich is by no means confined to man, or acquired by habit, as some have supposed. 414. We observe in the respiratory system of Mammalia the highest degree of connexion between the organic and animal functions which is any where exhibited. The mere act of the aeration of the blood is as completely independent of the animal powers in them, as in the simplest beings in this kingdom, or as the corresponding process in plants. But to give sufficient opportunity for the energetic performance of this func- tion which is required by the higher animals, an immense extension of surface becomes necessary; and as this extension obviously could not be produced, consistently with the other conditions of their existence, by a proportional increase of their external superficies (as in plants), it is obvious that some means must be provided for constantly renewing the air in contact with the delicate partitions of the minutely-divided cells of the internal organs. This is accomplished by the respiratory movements, Avhich are performed by the muscular and nervous systems; but these are not more immediately connected with the aeration of the blood, than is the action of the heart which propels that fluid to the lungs (§ 212, 3). 415. The preceding sketch of the progressive evokition of the respi- ratory system in the animal scale may seem to have been extended to a disproportionate degi'ee; but fuller details have, been entered into on this subject than have been elsewhere given, since it is one peculiarl}^ adapted to furnish illustrations of the general laws which have been previously enunciated. The function of aeration is one capable of being particularly well defined; and, as any structure adapted to it becomes at once a respi- ratory organ, there can be no difiiculty in tracing the analogies between the corresponding parts in different animals, except in cases where they have undergone a metamorphosis for the sake of being adapted to some other purpose, as the wings of Insects, or the swimming-bladder of Fishes. It has been seen that the fundamental character of the respiratory organs is everywhere the same, however different their external form; and that it is only the disposition of their parts that is varied in accordance \\-ith the circumstances in which their function is to be performed. The pro- 320 SPECIAL AND COMPARATIVE PHYSIOLOGY. gressive specialisation of tlie function has been traced in ascending the series, by marking the evolution of a particular apparatus for its exercise, and the restriction of it to that apparatus; in no instance has any sudden change in character been witnessed; but, in the classes adjoining those in which a new organ was to be introduced, has been found some adumbra- tion of it; yet even where the function is most highly specialised, the general surface is found to retain in some degree its participation in it, as will be presently shown (§ 422). "VVe shall now briefly trace the evolution of the respiratory apparatus in the embryo of the higher Verte- brata; reserving, as before, the account of the earliest changes in the ovum to a future period (chap, xiii.), and leaving until that period the description of the organs which are peculiar to the foetal condition, and which serve only to assist in the conversion of the nutriment supplied from the parent system, as during the germination of seeds. 416. At about the third day of the develojiment of the chick, four pairs of clefts or transverse slits are observable behind the mouth, in the situation of the branchial apertures of fishes; and at the same time, the branchial vessels are developed from the aorta, as already described (§ 325). One of the apertures is intermediate between each pair of vascular arches, just as in the gills of fishes and tadpoles. Nothing like branchial tufts, however, are developed; and the appearance described is very transitory, the vessels changing their direction and condition within two days. The development of perfect gills would have been useless, as the animal has not to maintain its ovm existence like the tadpole, but subsists, until the time of the perfect evolution of its respiratory system, upon the store of aliment furnished by the parent. It is evident, how- ever, that the history of this evolution is so far the same as in Reptiles and Fishes. The lung first appears as a simple closed sac lying at the posterior and lowest part of the thorax; it soon becomes bifid, and pre- sents a cavity, which does not, however, for some time communicate with the intestinal tube, the trachea and bronchi being last developed. The history of the evolution of these organs in the Mammalia is precisely analogous. It is usually at about the sixth of the period of uterine gesta- tion that the rudiments of the branchial apparatus are seen, as marked by the shortness and thickness of the neck, the penetration of the sides of the pharynx by the branchial clefts, and the division of the aorta into vessels corresponding in number and distribution with the branchial arteries of fishes. These general features have been observed in the embryos of most orders of Mammalia, not excepting man himself; and they are probably common to all. A few days after the appearance of the fifth arch, which is the last developed, the neck begins to elongate, the apertures are closed gradually on the outside, while the vascular arches undergo those changes by which the permanent arterial branches arising fj-om the heart are formed. The lungs in Mammalia are developed RESPIRATION IN ANIMALS. 321 mucli in the same manner as in Birds. They are not discernible hefore the period when the branchial apertures begin to close; a single mass is first perceived, which is soon divided into the rudiments of a right and left lung by a longitudinal groove ; and the trachea and bronchi are sub- sequently developed, as in birds. Scarcely a more beautiful illustration of the Unity of Design manifested in the creation of different classes of animals could be adduced than this hidden but not obscured correspond- ence; and the inferences to be drawn from it could hardly be more admirably expressed than in the subjoined passage from the eloquent pen of Professor Powell.* Nor is the analogy confined to animals alone; for it is impossible to compare the stages of the evolution of the perfect respi- ratory apparatus in the higher forms of the two kingdoms, mthout being struck A\dth their essential correspondence. In the flowering plant we have seen a temporary' respiratory organ, the cotyledon^ first developed, just as the branchi^ of a tadpole; and disappearing altogether when the evolution of the permanent aerating apparatus renders it unnecessary. And just as the system which is the permanent one of the lower tribes of animals, is transiently indicated in the early development of the higher, Avill it subsequently appear (§ 526) that the foliaceous expansions of the inferior stemless Crj'ptogamia are to be regarded as the analogues of the cotyledons of flowering plants, and thus, like the gills of aquatic animals, continue to perform their functions during life in a degree adapted to the wants of the system. 4 J 7. That which has been said of the correspondence of the essential structure of the respiratory apparatus, through all its varieties of external form, will apply with equal truth to its function also ; for, in whatever * " In the gradual stages of the process here unveiled, we perceive organs bestowed appa- rently without discrimination as to the future destiny of the creature : adapted in many to no perceptible end ; in fact positively useless and superfluous. All notion of final causes seems excluded ; and all idea of adjustment to a purpose, violated. Even the suppression of a use- less organ, and the substitution or super-induction of one which is useful, seems a circuitous and unnecessarily complex process of obtaining the end ultimately accomplished. But when welook attheregitto-ifi/ o/i/ie sj/stem on wliich all this is planned; when we consider that these useless or abortive org-ans are, in all cases, constructed on one simple model ; when we observe the precise order in which they disappear, exactly in accordance with the destined difference of function in the different species; when we trace the undeviating' scheme on which the new modifications are respectively super -induced; when we regard the determinate scale, according to which the whole process is unalterably carried on;— then we shall be urged with an increasing and accumulating force of conviction to the conclusion that all tliis arrange- ment, however ap])arently complex, is in reality an astonishing instance of conformity to laws of the most recondite simphcity : that every step in the process, however apparently super- fluous, is in strict accordance with a great principle of uniformity : that every stage in the ti-ansformation, however, in first appearance, destitute of a direction to a purpose of utility, yet if it answer no other, has its direct application in filling up a place in tiie universal harmony and incomparable Unity of design, which pervades all organised nature. The very singularity of the provision, well considered, evinces the enlarged preservation of analogy : the very objection and difficulty of the case is converted into an evidence in favour of the argument from symmetry." Connexion of Natural and Revealed Truth, p, 145. V 322 SPECIAL AND COMPARATIVE PHYSIOLOGY. tribe of animals the clianges composing it have been investigated, they are found to be of a very uniform character. The object of these changes appears to be in all instances the liberation of carbon from the blood in a gaseous state, the communication to it of oxygen, and the exchange of nitrogen on one side or the other. It will be more convenient to enquire into the particular character of these changes in the distinct form in which they are presented to us in the higher animals, before proceeding to inves- tigate their more obscure manifestations in the inferior tribes. These changes may be examined either in the circulating fluid, or in the air to which it has been exposed. 418, The most obvious difference between the fluid brought to the lungs for aeration after passing through the capillaries of the system, and that which has undergone the process, — or in short between venous and arterial blood — is its colour, which is dark purple (sometimes called black) in the former, and bright red in the latter. The altera- tion in colour may be produced by agitating venous blood with oxy- gen, or even by exposing it for a time to the atmosphere; in the latter case, hoAvever, the surface only acquires the arterial tint. The bright scarlet colour may also be given by the admixture of neutral salts ; whilst the addition of acids renders it still darker and prevents the change. "When venous blood is placed under the vacuum of an air- pump, a small quantity of carbonic acid gas is given out; but a larger amount, sometimes one-sixth of the whole volume, is evolved when the blood is agitated with atmospheric air, hydrogen or nitrogen. Gas may be extracted also from arterial blood by means of the air-pump, and this is found to consist of a larger proportion of oxygen. From the experiments of Magnus, the latest and most satisfactory on the subject, it appears that the oxygen in arterial blood amounts to about ^ or ^ of the quantity of carbonic acid which it contains, whilst in venous blood it bears the pro- portion of at most ^ and often only i. The relative quantity of nitrogen is extremely variable. It appears that these gases exist in the blood in a state of solution, as atmospheric air is found in river and sea water; but it is not improbable that a feeble chemical union may take place between the oxygen and the colouring particles, since it appears to be by its action upon them, rather than by the extraction of carbonic acid, that the change of tint is produced. 419. The changes in the air which has been respired are capable of being examined with greater accuracy. They may be considered under four heads: — 1. The disappearance of oxygen, which is absorbed. 2. The presence of carbonic acid, which has been exhaled. 3. The absorption of nitrogen. 4. The exhalation of nitrogen. The oxygen which disappears is usually more than is contained in the carbonic acid expelled, so that it must be actually absorbed into the system; and this we find to be especially the case in the lower classes of Vertebrata, and in all young RESPIRATION IN ANIMALS. 323 animals. The quantity varies in sucli proportion tliat it sometimes exceeds the third part of the carbonic acid formed, and is sometimes so small that it may be disregarded, — the difference depending, not only on the constitution of the species, but on the comparative degree of develop- ment, and on individual varieties among adults. This fact, Avhich was first established by the admirable experiments of Dr. Edwards, explains the result obtained by Messrs. Allen and Pepys, who found the quantities of ox3'gen lost, and of carbonic acid produced, to be the same; from which it was inferred that the ofl&ce of the oxygen was merely to remove the carbon from the system. These gentlemen took the greatest care to obtain accurate results; but their experiments were made on two species only, — man and the guinea pig. It is evident that the absorption of oxygen is necessary to communicate to the blood its powers as a vital stimulus; since animals do not long support life without it, although the usual quantity of carbonic acid be removed by other means. With regard to the production of carbonic acid, there is now quite suflEicient evidence to prove that it is not generated by the contact of oxygen and carbon in the lungs, as was formerly supposed; since it is not only found to exist in venous blood, but in the products of the respiration of gases entirely free fi'om admixtiu-e A^dth oxygen. Such an experiment can only be performed on animals which can sustain for a time the absence of the stimulus of oxygen. That snails confined in hydrogen mil generate carbonic acid was long ago shown by Spallanzani; but the recent experi- ments of EdAvards, Miiller, &c. upon frogs are more satisfactory, both from their superior accuracy, and from their freedom from the objection which might be raised against the others, on the ground of the Ioav place of their subjects in the animal scale. It appears that, when confined in hydrogen, fi-ogs will give out carbonic acid, for a time at least, as rapidly as in atmospheric air; and that the quantity generated in nitrogen is not much inferior. 420. These results are evidently conformable with the principles formerly stated as regulating the mutual diffusion of gases. Giving to its energetic reaction Avith carbonic acid (occasioned by it^ gi'eat difference in specific gravity) hydrogen removes it from the blood with greater force than any other gas; so that venous blood will give off carbonic acid when exposed to an atmosphere of hydrogen, even after it has been submitted to the exhausting poAver of a vacuum. It is obvious, hoAVCA'cr, that, for the continued generation of carbonic acid, oxygen must be supplied from AA'ithout, as there is no superfluity of it in the system. The folloAving, therefore, appears to be the history of the changes AA^hich the blood undergoes in its passage through the body. In the capillaries of the lungs it becomes charged with oxygen, Avhich it carries into those of the system; in the course of the actions Avhich there occur betAA'een the nutritious fluid and the textures it supports and stimulates, part of the oxygen y 2 324 SPECIAL AND COMPARATIVE PHYSIOLOGY. disappears and carbonic acid takes its place; tlie venous blood, therefore, returns to tbe lungs holding this in solution, together with the unabsorbed oxygen; and, in the capillaries of the lungs, the former gas is removed by the atmosphere, and replaced again by oxygen, — the interchange being entirely in accordance Avith the physical principles alread}^ stated.* 421. With regard to the absorption and exhalation of nitrogen. Dr. Edwards has shown that both these processes are constantly going on, but that their relative activity varies in different species and at different parts of the year. It appeared that an increase in the volume of nitrogen in the respired air took place in most young animals, and during the summer months ; but that, in the autumn and winter, there is a considerable absorption when adult animals are employed. It is a curious question which is yet undecided, whether herbivorous animals absorb more nitrogen from the atmosphere than those of carnivorous habits; for, as nitrogen scarcely exists in vegetables, but enters largely into the constitution of all animal bodies, it does not seem unlikely that this is the source from which it is derived, when not contained in the food. It is probable, however, that no animal could exist long, if fed on purely unazotised principles. 422. The function of Respiration is not confined to the lungs, even in animals which possess them in their most developed form. The blood which circulates through the capillaries of the skin is aerated by commu- nication with the atmosphere, wherever there is no impediment offered by the density of the tegumentary covering. In Amphibia, especially frogs, the cutaneous respiration is of such importance to the animal, that, if impeded by covering the skin Avith oil or other unctuous substance, death will take place almost as soon as if the lungs are removed; and the animal may be supported for a considerable time by it alone, if the tem- perature be not too high. In such circumstances it is found that carbonic acid is generated in an atmosphere of hydrogen, as by pulmonary respira- tion. In like manner, if Birds or Mammalia are enclosed in vessels out of Avhich their heads protrude, carbonic acid will be found to replace a portion of the oxygen; and the same result has been obtained by the similar enclosure of a limb of the human body. Animals whose respira- tion is aquatic do not decompose the water they breathe, but merely abstract the oxygen from the air contained in it; for if one of this class be placed in a limited quantity of water, from which it soon exhausts the air, or in water fr-om which the air has been expelled by boiling, it dies almost as soon as an animal whose respiration is aerial when placed in a vacuum. If, however, the surface of the water be in contact with the * This view of the function of Respiration was given in a paper which the author published in the West of Eng-land Journal in the year 1835, as that which best accorded with the facts then known. It has been fully confirmed by subsequent experiments, especially those of Mag-- nus, and he is most happy to find it now sanctioned by the eminent authority of Prof. Miiller. RESPIRATION IN ANIMALS. 325 atmosphere, it will absorb air fronf it; and tbe life of the animal will be longer, the more fully the quantity thus obtained compensates for that which is consumed. 423. When a Fish, in a limited quantity of aerated water, has reduced the proportion of air until its respiration has become difficult, it rises to the surface and takes in air from the atmosj)here; and, if prevented from doing so, it dies much sooner. The air thus taken in probably acts upon the lining membrane of the intestines; for, after being expelled, it is found to contain a large proportion of carbonic acid. The death of fishes when taken out of the water is partly due to the very rapid loss of the fluid of the body by transpiration (§ 435); and partly to the collapse of the gills, which prevents the air from having access to their surface, and to their desiccation, which incapacitates it from acting upon the blood beneath. Many Fishes are provided with a special apparatus for keeping the gills moist and free when exposed to the air, and such are able to live a consi- derable time out of water, especially in a humid atmosphere; thus, eels Avill leave their pools when dried up, and wind through the grass in search of water. The Doras of Guiana and the Hydrargyra of Carolina migrate in large bodies, under similar circumstances, and always direct themselves towards the nearest water, although they have no perceptible way of dis- covering it; and the climbing perch of Tranquebar not only creeps upon the shore, but ascends the Fan Palm in search of the Crustacea which constitute its food. The life of Fishes unprovided Avith any special modi- fication for the purpose, may be prolonged for some time by raising the opercula and keeping the branchial fringes separate, at the same time that evaporation is checked by a humid atmosphere around. The respiration of some of the inferior aquatic tribes, such as Crustacea, MoUusca, and Annelida, has been examined mth similar results. According to the researches of Humboldt and Gay Lussac, the air contained in water is richer in oxygen than that of the atmosphere; the proportion being 32 per cent, in the former, and but 21 in the latter. 424. The respiration of Insects has recently been made the subject of accurate research by Mr. Newport; and the results Avhich he has obtained coiTespond in a remarkable manner Avith those of Dr. EdAvards's experi- ments on Yertebrated animals under different conditions. In those tribes Avhich undergo a complete metamorphosis, the proportion of air consumed by the larva is much smaller than that Avhich the perfect insect requires, AA'hen their relative bu^lk is alloAved for, and their condition is the same as to rest or activity. If a larva of the common butterfly, for instance, has arrived at its full size at the time of making the observation, it appears to respire in a given time more than the perfect insect; but the result is lia- ble to this fallacy — that the former is at least tAVO-thirds larger than the latter, and is almost ahvays in a state of activity, Avhilst the latter is fre- quently in a state of quiescence. This fact is evidently analogous to one 326 SPECIAL AND COMPARATIVE PHYSIOLOGY. ascertained by Dr. Edwards, that, in the higher animals, a greater quan- tity of oxygen is required in the adult state in proportion to the size of the respiratory apparatus, than in the infant condition. Again, many larv£e can support a degree of privation of oxygen which would he fatal to the perfect insect; thus, there are some which inhabit the bodies of other insects, or are buried deeply in the soil, or seek their subsistence in noxious and unaerated places, all of which situations would be soon destructive to life in an advanced condition. This, too, finds its parallel in the history of the Vertebrated classes : for Dr. Edwards found that puppies soon after birth will recover after submersion in water for 54 minutes, thus bearing the privation of oxygen much better than the adult animal. The amount of respiration in the perfect Insect depends chiefly upon its state of activity or excitement. When its movements are rapid and forcible, the aeration of the tissues must be performed to a greater extent than when it is at rest; and the difiPerence is manifested, as well by the respiratory motions, as by the amount of oxygen consumed. Thus, the number of respirations in an Humble Bee (Bombus terrestris), while in a state of excitement soon after its capture, was from 110 to 120 in a minute; after the lapse of an hour they had sunk to 58, and subsequently to 46. Moreover a specimen of the same insect, confined in a limited quantity of air, produced in one hour after its capture, whilst still in a state of great activity, about ^ of a cubic inch of carbonic acid; and during the whole twenty-four hours of the succeeding day, the animal evolved a quantity absolutely less. The amount of respiration in the pupa state is much less than in any other condition of the insect, which will readily be understood when its com- plete inactivity is remembered; the state of the animal at that time may be considered (as far as its respiration is concerned at least) in the same light as the hybernation of warm-blooded Vertebrata (§ 156). 425. In Insects, as in other animals, the activity of respiration is increased with elevation of the temperature of the surrounding medium. This has been shown in a very striking degree with regard to the Amphi- bia, by the researches of Dr. Edwards. It has been already mentioned that the cutaneous respiration of frogs is sufficient for the temporary sup- port of life; and this holds good, not only when they are inhabiting the air, but even when immersed in water, provided the temperature be low. The air in the latter case must have a very feeble vivifying effect, on account of the small proportion of it diffused through the fluid; but it suffices to maintain the life of the animal as long as the temperature is beloAV 50°. If, however, a slight increase of heat takes place, pulmonary respiration is necessary, and the animal takes in air at the surface of the Avater. During the heat of summer, pulmonary respiration aided by cuta- neous respiration in water is not sufficient to counteract the effect of the high temperature ; and cutaneous respiration in air becomes so necessary, that frogs confined to the water at this time almost certainly die. The EXHALATION. GENERAL CONSIDERATIONS. 327 influence of temperature is seen also on the existence of fishes in limited quantities of water; and the degree of heat which obliges frogs to increase their respiration by quitting the water entirely, causes fishes to take in air from the surface, as may be frequently witnessed during the summer, especially in small collections of water. They sometimes quit their ele- ment almost entirely for a time, that the skin and branchiae may be exposed to the yivifying action of the air. 426. It has been mentioned that during the development of the ovum, like that of the seed, the process of respiration is actively carried on. It is performed through the membranous tegument of the egg, or the porous covering of calcareous matter which in some tribes it possesses. If an egg be varnished over, so as to render it impermeable to gases, or be placed in irrespirable media, the development of the embryo is checked, though it may be renewed if the privation of oxygen has not been of too long con- tinuance. As the watery portion of the albumen evaporates and the remainder is taken into the system of the foetus, the quantity of the air originally existing in the egg becomes much increased; previously to incu- bation it contains as much as 25 or 27 per cent, of oxygen, but subse- quently about 6 per cent, appears to have been converted into carbonic acid. In many cases this aeration is performed iinder peculiar circum- stances, and special provision is accordingly made for it (§ 538). CHAPTER X. EXHALATION OF AQUEOUS VAPOUR. General Considerations. 427. As all the alimentary materials taken into living bodies for the nutrition of their solid tissues are in a fluid form, being either dissolved in or mixed with water, it is evident that a large quantity of that liquid must be superfluous, and that means must be provided for carrying it out of the system. This is partly accomplished, in animals more especially, by its combination vnth. various other ingi-edients, — which have either been introduced in greater quantity than the processes of nutrition require, or have already served their purpose in the vital economy, — into the fluid excretions, for the elaboration and deportation of which various structural contrivances are adapted. But besides the means thus affbrded for the diminution of the superfluous fluid of the system, Ave find that the external surface has this special function imposed upon it, and that the disen- gagement of nearly pure aqueous vapour, though partly the effect of 328 SPECIAL AND COMPARATIVE PHYSIOLOGY. simple evaporation, is principally dependent upon a true process of secre- tion, by wliicli it is liberated from the circulating fluid. Tliis is most evident in plants, wliere tlie quantity of fluid absorbed bears a mucb larger proportion to the amount of the solid matter contained in it than in ani- mals; and where, from the little opportunity which there is for the intro- duction of superfluous nutriment, and the comparatively slight tendency to decomposition in the solid structures, the necessity for a constant excre- tion of other ingredients unfit to be retained is much less. Exhalation in Plants. 428. The soft and succulent tissues of Vegetables, if freely exposed to the atmosphere, would soon lose so much of their fluid as to be incapable of performing their functions; and in all plants, therefore, which are subject to its influence, we find a provision for restraining such injurious effects. In the alGjE, however, and other tribes constantly immersed in water, or in a very moist atmosphere, no such loss can take place in their natural condition, and no means are required to prevent it. The outer layer of cells composing their integument differs but little from those which it holds together, except in density; and it is accordingly found that such plants, Avhen exposed to a dry air, speedily desiccate. All plants whose natural residence is the air, however, are covered with a membrane of peculiar character, which is termed the ctUicle. This is composed of cellular tissue, the vesicles of which are arranged with great regularity, and in close contact \nt\ each other; but they differ from those of the paremchyma beneath, in being colourless or nearly so, and in containing air instead of fluid. The form of these vesicles is different in almost every tribe of plants; thus in the cuticle of the Iris (Fig. 72) they have straight walls and regular angles, whilst in that of the Apple (Fig. 73) their boundaries have a sinuous character. In most European plants, the cuticle contains but a single row of these cellules, which are moreover thin-sided; whilst in the generality of tropical species, there exist two, three, or even four layers of thick-sided cells, as in the Oleander (Fig. 70), the cuticle of which, when separated, has an almost leathery toughness. In this plant the cuticle is also covered with hairs, which may not only serve as an additional resistance to exhalation, but probably assist in absorption also (§ 254); and these hairs not only clothe the surface, but line the cavities which replace the stomata on the lower side of the leaf (Fig. 70, e, e). This difference in conformation is obviously adapted to the respective conditions of growth; since the cuticle of a plant indigenous to temperate climates would not afford a sufficient pro- tection to the interior structure, against the rays of a tropical sun; whilst the diminished heat of this country would scarcely overcome the resist- ance afforded by the dense and non-conducting tegument of a species fwmed to exist in warmer latitudes. From the researches of Ad. EXHALATION IN PLANTS. 329 Brongniart it appears that, externally to this membrane, there exists a very delicate transparent pellicle, without any decided traces of organ- isation, though occasionally somewhat granular in appearance, and marked by lines which seem to be the impressions of the junction of the cells With Avhich it was in contact. He thinks that he has traced this membrane where the real cuticle does not exist, as on the apex of the stigma, and the general surface of submerged vascular plants. It is per- forated by apertures leading to the stomata, where they exist in the cuti- cle; and would seem to bear a very close analogy to the epidermis of animals (§ 39). 429. In the cuticle of most plants which possess this structure dis- tinctly formed, there exist minute openings termed stotnata, which are bordered by cellules of a peculiar form, distinct from those of the cuticle, and more resembling in character those of the tissue beneath. These boundary cells are usually kidney-shaped (Figs. 72, 73, a, «), and the opening between them oval, as at c; but, by an alteration in their form, the opening may be contracted or completely closed. They are sometimes more numerous, however, and the opening angular; and in the curious Marchantia polymorplia^ their structure is extremely complicated. The openings in the cuticle of this plant are surrounded by five or six rings placed one below the other, so as to form a kind of funnel or chimney, each ring being composed of four or five cellules (Fig. 52). The lowest of these rings appears to regulate the aperture, by the contraction or ex- pansion of the cells which compose it. Wherever stomata exist in the cuticle, they are always found to open into cavities in the tissue beneath, which are thus brought into immediate relation with the external air (Fig. 71, c). In the Marchantia these chambers are very large and sur- rounded by regular walls; whilst in the leaves of higher plants they exist simply as intercellular spaces, left by the deficiency of the tissue. Stomata do not exist where there is no regular cuticle; and they are consequently not found upon the lower cellular plants, and but very rarely on Mosses. They are not formed upon the cuticle of any plants growing in darkness, nor upon the roots nor the ribs of leaves; but they exist in general on all foliaceous expansions, and on herbaceous stems, especially on those of which the surface performs the functions of leaves, as in the Cacti. They are most abundant on the under surface of leaves, except Avhen these float on water, and then they are found on the upper side alone; but they exist equally on both surfaces of erect leaves, as in the Lily tribe and Grasses. As a general fact they are least abundant on succulent plants whose moisture is to be retained in the system; and they arc frequently so imperfectly formed as not to have any tendency to open, especially on the leaves of those adapted to exist in hot and dry situations. In the Oleander, which has to bear the parched atmosphere of a Barbary summer, the stomata of the lower surface arc replaced by cavities lined with hairs. 330 SPECIAL AND COMPARATIVE PHYSIOLOGY. the probable function of which has just been explained. In all instances where stoniata exist, the tissue beneath is very loosely arranged, and con- tains many intercellular spaces; in the greater number of leaves there- fore, the most closely-packed cells will be found on the upper side (Fig. 69, &, b); and it is from this that the darker colour of the superior surface is principally derived. If a leaf be placed in water, and the pres- sure of the air above be taken off, a number of minute globules will be seen to escape from these cavities, and to stud its exterior with brilliant points, 430. The loss of fluid from the surface of plants may take place, as has been said, by simple evaporation, or hj exhalation. The quantity of the former will be regulated by the degree of moisture in the tissue exposed to the atmosphere, and by the compactness of its arrangement. Thus, although the simpler terrestrial cellular plants have no true cuticle distinct from the subjacent tissue, their external layer of cells is generally of so dense a consistence as to be almost impervious to water; so that their moisture is very slowly evaporated. The process is one quite independent of vitality, and is, indeed, the means by which dead plants are dried up, and by which the gradual loss of weight takes place from fruits, tubers, &c., that undergo no other alteration. It will, therefore, be influenced by those obvious external causes under the control of which the process is universally performed, — namely, variations in temperature and in the humidity of the surrounding medium. Exhalation, on the other hand, is a change which only continues during the life of the plant, and appears to be closely connected with the performance of its other vital functions. If a piece of glass be held near the upper surface of a leaf in full growth in a hot-house, it is scarcely dimmed after some time; but if in proximity with the lower surface of the same leaf, it is speedily bedewed with moisture, which accumulates in a short time so as to form drops. This rapid transpiration of fluid appears to take place through the stomata, as it is now satisfactorily proved that it bears a strict relation (other things being equal) with the number of stomata in the plant, or on the particular part of it made the subject of examination. 431. Various experiments have been made at different times, with the view of ascertaining the quantity of water thus transpired fi-om different plants, and the circumstances most favorable to the process. With regard to quantity, the results obtained by Dr. Woodward* are among the most worthy of attention, although probably the earliest on record. Four plants of spearmint were placed with their roots in water, and in a situation fully accessible to light, during 5Q days (from June 2nd to July 28th); and the following table exhibits the quantity of water which each plant absorbed, (proper allowance being made for the evaporation from the sur- face of the fluid), and its increase in weight at the end of the experiment. Pliilos. Trans. 1699. EXHALATION IN PLANTS. 331 The difference must of course be tlie quantity exhaled, and would scarcely express the whole amount of it, as part of the increase in weight Avould be due to the fixation of carbon from the atmosphere. Original Weight. Gai No. 1. 127 grs. No. 2. 110 grs. No. 3. 74 grs. No. 4. 92 grs. 128 grs 139 grs 168 gi-s 284 grs, Water expended. Difference. 14,190 grs. 14,06*2 grs. 13,140 grs. 13,001 grs. 10,731 grs. 10,563 grs. 14,950 grs. 14,666 grs. Nos. 3 and 4 were immersed in wa- ter with a little earth at the bot- tom. These experiments give satisfactory evidence of the very large proportion of the absorbed fluid which is given out again by transpiration; and, joined with others by the same individual, they show that the activity of this function is much greater in summer than in the autumn. A valuable series of experiments, communicated by Guettard to the Academic Royale in 1740, confirms this conclusion. He stated that transpiration is so much less active during the winter than at other parts of the year, even in ever- greens, that a laurel parts mth as much fluid in two days in summer, as during two months in -winter. He also maintained that transpiration goes on much more rapidly under the influence of light and a moderate degree of heat, than at a high temperature and without light. One of his most striking experiments is that upon the Cornus Mascula (cornel), the young shoots of which he found to lose twice their own weight of water daily. The experiments related by Hales in his essays on Vegetable Statics will ever remain, like those which he performed on the animal circulation, a monument of his skill and perseverance. The results which he obtained from the accurate observation of a specimen of Helianthus annuus (sun- flower) during 15 days, are those most frequently quoted by succeeding authors; but there are many others scarcely less interesting. This plant was 3 5 feet high, Aveighed 3 lbs., and the surface of its leaves was esti- mated at 5616 square inches. The mean perspiration during the whole period was found to be 20 oz. per day; but on one warm dry day it was as much as 30 oz. During a dry warm night it lost 3 oz.; when the dew was sensible though slight, it neither lost nor gained ; and by heavy rain or dew it gained 2 or 3 oz. The following table shows the results of similar experiments on other plants. Subject. Cabbage Vine Apple Lemon Plantain Wean Transpiration. ' 19 oz. 5 5 oz. 9 oz. 6 oz. 5 oz. Greatest Transpiration. Depth. 25 oz. aV Surface. 2736 sq. in. 1820 sq. in. 1589 sq. in. 2557 sq. in. 2024 sq. in. The last column shows the mean quantity of water transpired from equal areas in the difi'erent plants (its depth being stated in parts of an inch) 6| oz. rh 15 oz. 1I2 8 oz. 248- 11^ oz. xb 332 SPECIAL AND COMPARATIVE PHYSIOLOGY. for the sake of ready comparison. That of the sun-flower would be ■j^3- and is shown, therefore, to be less than half that of the Cabbage. The Lemon may be remarked to have exhaled far less than any of the others ; and the same observation seems true with regard to evergreens in general. The mean transpiration from the skin of the human body in health, vnth the exhalation from the lungs, may be stated at about 45 to 50 oz. in twenty-four hours. The external surface may average about 2160 sq. in.; but the surface of the mucous membrane of the lungs cannot be estimated. An experiment performed by Bishop Watson will assist in giving an idea of the extraordinary amount of change performed by this function in plants. He placed an inverted glass vessel, of the capacity of 20 cubic inches, on grass which had been cut during a very intense heat of the sun, and after many weeks had passed without rain; in two minutes it was filled with vapour, which trickled in drops down its sides. He collected these on a piece of muslin Avhich he carefully weighed ; and, repeating the experiment for several daj^s between twelve and three o'clock, he estimated as the result of these enquiries, that an acre of grass land transpires in 24 hours not less than 6400 quarts of water. This is probably, however, an exaggerated statement; as the amount transpired during the period of the day in which the experiment was tried, is far greater than at any other. 432. All experiments point to the conclusion that light is the chief stimulus to exhalation. Thus, it was shown by Senebier that if plants, in which the process is being vigorously performed, are carried into a darkened room, the exhalation is immediately stopped; and that the absorption by the roots is checked almost as completely as if the plant had been stripped of its leaves. Again, from the experiments of Dr. Daubeny, it appears that exhalation is stimulated by the coloured rays of the solar spectrum in proportion to their illuminating not to their heating power, these two being separated by the prism. Dr. D. further states that exhalation is not promoted by the most intense degree of artificial light, in which he contradicts the opinion expressed by Decandolle.* Still, it must be acknowledged that heat also, especially when combined with dryness of the atmosphere, has a greater efi'ect upon the loss of fluid than light only. Thus, it is well known that plants perspire in a sitting- room, the air of which is constantly dry but which is imperfectly illumi- nated, so much more than in the open air exposed to the direct rays of the sun, that it is impossible to keep many kinds alive in such a situation. It Avould not seem improbable, then, that the eff'ect of light is confined to the opening of the stomata, which it is known to perform; and that the large quantity of fluid discharged from them may be due to the effect of simple evaporation from the extensive surface of succulent and delicate tissue which is thus brought into relation Avith the air, and to the con- stant supply of fluid from within by which it is maintained in a moist * Philosophical Magazine, May, 1836. EXHALATION IN PLANTS. 333 condition. Electricity appears to possess, like liglit, a direct stimulating power over tlie exhaling organs of plants. It lias been generally admitted that the electric state of the atmosphere has a considerable influence in hastening the growth of many vegetables (§ 186). DecandoUe states that experiments with artificial electricity satisfactorily prove, that plants sub- mitted to its influence exhale more by a fourth or a third than similar ones not electrified; and in some cases, especially when sparks are drawn, the water has been seen to accumulate in drops. 433. If plants are exposed to a light of too great intensity, especially if they are not at the same time well supplied with water, their tissue becomes dried up by the increased exhalation which then takes place, and which is not sufficiently counterbalanced by absorption, so that their vege- tation is materially checked; — a fact of which we see abundant evidence in dry sandy soils and exposed situations. If, on the contrary, the leaves are shaded, and the roots take up much moisture, the growth of the plant is active and luxuriant, but its tissue is soft; — an eficct partly owing to the retention of fluid, and partly to the diminution of the quantity of carbon fixed from the atmosphere. If a plant be kept for some time in total darkness so that it becomes etiolated (§ 373), its texture is soft and suc- culent, and its tissue is distended Avith the moisture it has absorbed and with which it cannot part; and if this state be allowed to continue too long, the leaves disarticulate and drop off, and the plant dies of dropsy. Succulent plants naturally require most light to secure for them a regular discharge of moisture; hence Mr. Knight enforces the propriety of expos- ing as many leaves as possible in the Melon frame to the action of the sun's rays. There are some of this character which possess so few stomata, that they may be preserved out of the ground for many days and even weeks, Avithout perishing from Avant of moisture; and it sometimes hap- pens that Sedums and other such plants push considerable shoots when placed under pressure Avhilst being prepared for the Herbarium. The quantity of fluid lost by Transpiration, though ultimately dependent upon the degree of moisture supplied to the roots, does not appear to be in- creased by the propellent force of the sap ; and this, observes the sagacious Hales, " holds true in animals, for the perspiration in them is not always greatest in the greatest force of the blood ; but then often least of all, as in fevers." The water exhaled is very nearly pure, so that Avliat is furnished by dificrent species varies but little in taste or odoui*. Duhamel remarked, however, that fluid thus obtained sooner becomes foul than ordinary water. Senebier analysed the liquid Avhich he had collected by the exhalation of a vine at the commencement of the summer, and found that 40 oz. contained scarcely 2 grains of solid matter; and in a similar experiment on fluid collected at the end of the summer, 105 oz. gave but little more than 2 grains, or about ^ sii o o^ P^^* °^ solid matter. 334i SPECIAL AND COMPARATIVE PHYSIOLOGY. Exhalation in Animals. 434. The loss of fluid wliich is constantly taking place from the sur- face of all animals inhabiting the air, or at least from some part of it, appears due, like the exhalation of plants, partly to its physical, and partly to its vital conditions. There can he no doubt that from all soft moist surfaces evaporation will take place in a warm and dry atmosphere; and the quantity of fluid lost in this manner will be in strict relation with the temperature of the surrounding medium, and the rapidity with which it is supplied to the evaporating surface. This process will of course be impeded by a humid state of the atmosphere, and entirely checked by contact of water — whether warm or cold — with the part which previously efifected it. But there is another process by which fluid is exhaled from the surface, and which possesses the character of a true ewcretionj this is effected by the separation from the blood of a watery fluid, usually con- taining a small quantity of saline and animal matter in solution, through the medium of a set of minute glands imbedded in the substance of the cutis or true skin. Each of these little bodies consists of a convoluted tube, in the neighbourhood of which the blood-vessels ramify minutely; this tube is continued to the surface of the skin as an excretory duct (Fig. 153), traversing the remaining thickness of the cutis and epidermis in a spiral manner, and opening by a very minute pore on the exterior of the latter, passing through it so obliquely that a kind of valve is formed by the membrane over its orifice. When the transudation of the sweat or sensible perspiration is observed with a glass, as it occurs on the palms of the hands or the tips of the fingers, the first drop from each pore will be seen to be preceded by an elevation of this little valve. These ducts are visible in the form of delicate fibres passing from the cutis to the epi- dermis, when the latter is torn off; their diameter is stated by Dr. Madden* to be -^^q of an inch, the canal occupying about one-third of their breadth, t It has not yet been ascertained how low in the animal scale these organs exist; the only species in Avhich they have been hitherto detected being included in the class mammalia. 435. No investigations have yet been made upon the function of exhalation in the aquatic Invertebrata, with the view of determining to what extent it is one of the regular processes of their economy. Although simple evaporation will of course be prevented by the contact * Essay on Cutaneous Absorption, p. 19. t Another apparatus has been described by Dr. Wallace as being part of the exhalent system, — namely, a set of " epidermoid glands" situated between the inner and outer layer of epidermis, which he states to exist at the points from which the drops of sweat are seen to issue. The author is disposed to agree with Dr. Madden (Op. Cit. p. 24), however, in beheving that Dr. W. has been deceived on this point, and that the supposed glands are nothing more than the shrunk and contracted ducts of the true secreting organs of the perspiration. EXHALATION IN ANIMALS. 33;5 of their surfaces with Avater, there is no reason to suppose that a secretion of fluid may not take place from them, as from the skin of the higher animals under similar circumstances. When exposed to the air, all those Avhich are formed of soft tissue, unprotected by a hard enyelope, are rapidly desiccated, and usually perish; but, that the whole of the fluids of the body may thus be lost by evaporation, and vitality still remain, is shown by the statement formerly made respecting the rotifera (§ 93). It is evident that such animals are, when exposed to the atmosphere, in the same condition with the Algee among plants, which lose Aveight so rapidly owing to the softness of their tissues and the Avant of a cuticle. Even amongst those AA^hich are provided Avith a hard envelope, there is always a peculiar tendency to evaporation from some parts of the surface; thus, a very rapid exhalation of fluid takes place fi-om the gills of the CRUSTACEA, Avhich Avould Speedily offer a fatal impediment to the per- formance of their functions, if a special provision Avere not made for preserAdng their membrane in a humid condition (§ 400). From the experiments of Dr. EdAvards on fishes, it appears that the loss of fluid by evaporation from the general surface of the body and fi-om the gills, when the animal is exposed to the air, is so great as to be one of the chief causes of its death. Sometimes the impediment to respiration, Avhich is produced by desiccation of the gills, is the immediate cause of death; but where this is prevented, and the action of these organs continues during life, the surface parts with so much fluid by evaporation that the body becomes stiff and dry, and preAdously to death loses from -^ to -^-g part of its Aveight. It has been shoAATi that if the loAver part only of the body be immersed in Avater, no absolute diminution in weight of the Avhole takes place, and life is prolonged, although death seems at last to result fi-om the unfavourable influence of dry air upon the branchial apparatus ; but if, on the other hand, the head and gills be immersed and the trunk suspended in air, life may be almost indefinitely prolonged, although the drying of the surface of the part of the trunk exposed to the air AA'as as marked as in the case Avhere these animals Avere entirely exposed to the atmosphere, and AA'here they died after a considerable diminution in weight. 436. It is among terrestrial animals that the process of exhalation assumes a higher rank amongst the Adtal functions; and, CA-en in the lowest orders, we find it exercising a very important influence on the condition of the system. Thus, in insects, it has been ascertained by Mr. Ne^vport, that the transpiration of fluid takes place to a considerable extent; and this not only in the species which have a soft external tegu- ment, but among those which have the body encased in a dense horny envelope, such as the beetle tribe. It is of course diflicult to ascertain Avhat proportion of the loss of fluid takes place in each case from the external surface, and from the prolongation of it that lines the air 336 SPECIAL AND COMPARATIVE PHYSIOLOGY. passages, which in this class are so extensive and minutely ramified; probably it is from the respiratory membrane, as in the Crustacea, that the principal liberation of it occurs. The peculiar object of the dis- engagement of fluid in the form of vapour, is evidently the reduction of the temperature of the surface from which it is set free. Animals Avhich inhabit the water have no need of any special provision for keeping down the temperature of their bodies within a certain limit; since the rapidly- conducting power of the medium is sufl&cient to reduce any superfluous amount of caloric which may be generated. The tenants of the deep, therefore, have very little power of maintaining a temperature above it, unless they are provided, like the whale tribe, with a layer of non-conduct- ing fat, or, like diving birds, Avith a downy covering possessed of a similar property (§ 491). Moreover, the vicissitudes of temperature in large collections of water are never great, so that there is no demand from this soui'ce for a means of regulating the temperature of the individual inha- bitants. But an animal living upon the surface of the earth, exposed to constant and extensive atmospheric changes, and deprived of the power of rapidly parting with its heat, when superfluous, by mere contact Avith a conducting medium, has need of some special means not only of generating caloric, but also of getting quit of it. The former will be hereafter described in detail (chap, xii.); the latter is simply effected by the secretion of SAveat from the surface, which, being poured out of the perspiratory ducts in a fluid form, and carried off as a vapour by the atmosphere, necessarily renders latent a large quantity of caloric, and thus diminishes the sensible heat of the exhaling body. The observations of Mr. NcAvport on Insects shoAV that they have the power of thus reducing their temperature AA^hen excessively raised by a continuance of rapid movements, or AV'hen the heat of the surrounding medium is too great (§ 490). 437. It is among the BatracMa, however, that the exhalation of fluid from the surface is carried on to the most evident degree, and seems to ansAver the most important purpose in the economy; and it is here, therefore, that its conditions may be most advantageously studied. The experiments of Dr. EdAvards on this subject are extremely interesting, and a brief account of them will now be given. He found that Avhen a frog was placed in a dry calm atmosphere, the loss of weight during different succeeding hours varied considerably, but with a marked ten- dency to progressive diminution : that is to say, the more fluid the animal had lost, the less actively did exhalation go on. The actual quantity lost Avas influenced by various external agents, such as the rest or movement of the air, its temperature, and degree of humidity. Thus, frogs, hung in the draft of an open AAdndow, lost double, triple, or quad- ruple the amount exhaled by others placed at a closed Avindow in the same room. The influence of the humidity of the air AA''as tested by EXHALATION IN ANIMALS, 337 placing animals of the same kind in a glass vessel inverted over water ; and it was ascertained that exhalation, if not then entirely prevented, was reduced to its minimum. On the other hand, when the dryness of the air was maintained by quicklime during the progress of the experiment, the diminution of weight was found to be increased, the perspiration being from five to ten times gi'eater in dry air than in extreme humidity, according to the duration of the experiment. The influence of tempera- ture is shown principally in increasing the transudation or secretion from the skin; since the amount of fluid lost in a heated atmosphere differs but little whether the medium be humid or dry, and increases in much more rapid proportion than mere evaporation would do. When frogs were placed in an atmosphere saturated with humidity, by which mere evapo- ration would be almost or entirely suppressed, the loss by transudation between 32° and 50° was very slight, as also between 50° and 68°; but between 68° and 104° it was so great, that at the last-named degree its amount Avas b5 times that at 32°. The secretion is not even altogether suppressed by immersion in water. When fi-ogs are exhausted by excessive transpiration and are placed in water, they speedily repair the loss by absorption from the surrounding fluid (§ 279); and the quantity thus gained sometimes amounts to one-third of their entire weight. 438. From his experiments on the higher animals. Dr. Edwards obtained results of a similar kind; but the influence of changes in exter- nal conditions was not quite so marked. The distinction between the simple evaporation which takes place in obedience to physical laws, and the transudation which is the result of a secreting process, must be kept in view in order to account for their efi"ects under dififerent circumstances. It might, at first sight, appear to correspond vdth that between insensible or vaporous, and sensible or liquid transpiration; but this is not altogether true, since the secretion of the skin, if not very abundant, may pass off in the same form with the vapour which arises from its surface. The degree of evaporation fi-om the skin of warm-blooded Vertebrata is modified, as in the Batrachia and other cold-blooded animals, simply by the tempera- ture, degree of humidity, movement, or pressure of the surrounding medium. Wholly to suppress it, the air must not only be of extreme humidity, but also at a temperature not inferior to that of the animal; since, if the air be colder, it will be warmed by contact with the body, and thus be capable of holding an additional quantity of aqueous vapour in solution. Although cold, therefore, diminishes or even altogether suppresses transudation, evaporation will continue to a certain extent. In man, as in the Batrachia, it seems probable that heat alone stimulates the function of secretiofi from the skin ; so that at moderate temperatures and in ordinary states of the atmosphere the quantity transuded is not more than one-sixth of that Avhich is evaporated : whilst at an elevated tempo- z 338 SPECIAL AND COMPARATIVE PHYSIOLOGY, rature, especially if the air be already humid, the amount of secretion will much surpass that lost hy evaporation; but if the air be dry and sufficiently agitated, evaporation may increase nearly in the same ratio.* 439. The amount of fluid exhaled in the form of vapour from the lungs appears to be usually somewhat more than that transpired from the surface. There is no reason to believe that it is liberated in any other way than by evaporation, under the peculiarly favourable circumstances afforded by the delicacy and permeability of the respiratory membrane, its constant supply of fluid blood, and the frequent renewal of the air in contact with it. It is obvious that changes in the external conditions will have much less influence upon its amount than upon the quantity evaporated from the skin; since the temperature of the air in the pulmo- nary cells will be nearly uniform under all circumstances (in the healthy state at least), and its movements are uninfluenced by the variations of the atmosphere. If, however, the external air were saturated with mois- ture, and of the same temperature with the body (so as to be unable to acquire by its heat an increased capacity for vapour), it is obvious that the evaporation from the lungs, as well as that from the skin, will be entirely checked. 440. From the experiments of Lavoisier and Seguin it appears, that the maximum quantity of fluid exhaled from the cutaneous and pulmonary surfaces in man is 5 lb., the minimum being If ft.; and that the mean quantity exhaled per minute is 1 8 grs., of which 1 1 pass off by the skin and 7 by the lungs. There is much difficulty in attaining correct informa- tion on this subject, however, owing to our ignorance of the amount absorbed from the atmosphere; and that, under favourable circumstances, the quantity of fluid exhaled from the skin may be much greater in a short time than these results would lead us to believe, appears from the late observations of Dr. S. Smith, t These were made upon labourers at the Phoenix Gas Works, who are employed twice a day in draAving and charging the retorts and in making up the fires, which usually occupies about an hour; the labour is performed in the open air, but is attended Avith much exposure to heat. On a foggy and calm day at the end of * It has been stated as the result of the experiments of MM. Delaroche and Berg'er, that not only the heat but the humidity of the atmosphere stimulates transudation ; since they uni- formly found that air excessively hot, and charged with extreme humidity, excited a more abundant perspiration than dry air at a higher temperature ; but this result may be due to the accumulation of fluid on the surface, in the former condition, which would have been rapidly dissolved by the air in the latter. It is suflEciently evident, however, that a humid state of the atmosphere does not check the secretion of fluid in the skin ; and the same may be said of the contact of warm fluid. There is good reason to believe that the loss of weight which fre- quently takes place in the warm bath, though partly to be accounted for by the continuance of pulmonary exhalation, also results in part from cutaneous secretion, — the diminution having been, in one of Dr. S. Smith's experiments, as much as 8 oz. in half an hour, although the body was previously in a state of exhaustion from labour in a heated atmosphere. t Philosophy of Health, vol. ii., 322, &c. SECRETION. GENERAL CONSIDERATIONS. 339 November, wlien the temperature of the external air was 39°, and the men continued at their work for an hour and a quarter, the greatest loss observed was 2 ib. 15 oz.; and the average of eight men was 2 lb, 1 oz. On a bright clear day in the middle of the same month, when the tem- perature of the air was 60° with much vdnd, the greatest loss was 4 ib. 3 oz.; and the average was 3 ib. 6 oz. And on a very bright and clear day in June, when the temperature of the external air was 60° without much wind, the greatest loss (occurring in a man who had worked in a very hot place) was 5 lb. 2 oz.; and the average during the hour was 21b. 8 oz. If, as seems probable, a large proportion of the fluid thus rapidly exhaled would be speedily replaced by absorption from the atmosphere, it is obvious that no calculation of the total daily amount can be accurate which is based only on the relative quantities of the ingesta and egesta. CHAPTER XI. SECRETION. General Considerations. 441. Although the function of Secretion might not, at first sight, appear so universal in organised beings as those already described, there can be little doubt that it is no less essential to their existence, since there is reason to believe that it takes place under some form in every living structure. The term Secretion implies a separation of some portion of the constituents of the organism; and although it has been usually employed to designate the elimination from the c\xc\x\vhich is a receptacle adapted to receive the germ, and to forward its development by means of the store of nutriment it contains. In the Marsilea, the thecce containing the spores (analogous to the anthers of flowering plants) are enclosed, AA'ith the ovules, in a common envelope (Fig. 74); and the communication between them seems to be direct (Fig. 75). The spores will not of them- selves produce new plants, neither will the OAoiles; since the germs con- tained in the former require to be assisted in their development, and the latter must be fertilised by the introduction of a germ. This process appears here a very simple one, being effected by the direct communica- tion which exists between the two organs. In the flowering plants, of which this may be regarded as one of the least developed forms, a more complex apparatus is usually found; but it only serves the same purpose in a different manner. 525. According to the views here taken, therefore, the essential part of the reproductive system of the Phanerogamia consists, as in the Crj^togamia, of an organ for the production of vesicles containing germs, here termed the anther; and the part which distinguishes it is super- added, for the purpose of giving that assistance to the early development of these germs, Avhich seems in all instances to be required where a com- plex and highly-organised structure is ultimately to be produced. Late researches on the process of fertilisation have shoAvn that the function of the parts Avhich constitute the pistil, is simply to convey to the ovules contained in the ovarium at its base the germs liberated from the pollen- grain. When this is emitted by the Anther (as the spoi-e from the theca of a Cryptogamous plant), it does not immediately become subservient to the influence of external agents, and owe its subsequent evolution to the nutriment which it obtains from the surrounding elements alone; but the germs it contains are received into another part of the structure, and sup- plied not only with present aliment, already prepared for organisation, but with a store which may serve to continue their development for some time after their final separation from the parent. The changes Avhich take place in the pollen-grain when it is brought in contact with the moist surface of the stigma, are exactly equivalent to those which have been described as occurring in the spore. The outer envelope separates in one or more points; and the inner tunic is protruded in the form of 2d 402 SPECIAL AISID COMPARATIVE PHYSIOLOGY. tubes, which contain some of the granules that might have been previously seen freely moving Avithin their cell. These tubes insinuate themselves (in the manner represented in Fig. 68) along the lax tissue of the style, and may be traced to the ovarium. There they enter the openings which, up to that time, have been left in the membranes of the ovules, in whose cavities nothing but a quantity of fecula and mucilaginous fluid previously existed; but one of the granules in the pollen-tube thus introduced into each ovule gradually increases at the exjsence of these materials, and finally either occupies the whole ovuluni by the absorption of the albumen into its cotyledons, or shares it with the separate albumen (§ 50). The maturity of the seed is a period of cessation in its actions; and it then arrives at a state of development in which it may remain dormant for a considerable period, — until, in fact, the stimuli requisite for its further evolution shall be supplied to it. 526. In the early development of the embryo of the Phanerogamia within the ovule, we may trace an essential correspondence with the evo- lution of the germs of the Ferns or other Cryptogamia; and we may also trace a general analogy between its transitory conditions at different epochs and those which are permanent in the lower classes. In its earliest recognisable appearance, it is a mere vesicle filled with a whitish fluid, and may be regarded as representing the Protococcus or some other equally simple plant ; but as new cells are developed, it becomes analogous to Algae of somewhat more complex structure. It soon begins, however, to expand laterally, so as to form a Cotyledon, which will be single or double according to the class to which it belongs (a. Figs. QQ, 7). When the seed is mature and separated from the parent, it requires warmth, moisture, and the presence of oxygen to stimulate its further development, and to enable it to convert into organised structure, the store of aliment which it contains. Where the cotyledons are fleshy (the albumen having been taken into their substance), they shrivel and fall off as soon as their store is exhausted; but where they are leafy (the albumen remaining separate), they come to the surface, acquire a green tint, possess stomata on their cuticle, and perform for a time the functions of true leaves, until these are evolved. As yet, the fibro-vascular system is but imperfectly developed, the young plant consisting of but little more than cellular sub- stance; and at this period it may be regarded as exactly representing the Marchantia and other similar plants, which possess stomata on their fronds or foliaceous expansions; since these fronds, being analogous to the primary fronds of Ferns, are truly permanent Cotyledons. '^' In a short time the plumula (§ 50) ascends, bearing with it the rudimentary leaves, which, becoming developed, repeat in a much more perfect manner t The very curious analog'ies to the animal kingdom here presented, have already been pointed out in § 385 and 417 ; and the former may be referred to for a curious correspondence between the condition of a g-erminating seed and a growing- Fungus. REPRODUCTION IN ANIMALS. 403 tlie functions previously performed by the cotyledons, and commence tlic formation of Avoody fibre. The plant is now arrived at a stage of its groAvth which may be compared with that of the Ferns; it is not until a somewhat later period that we can trace the true spiral vessels, which are confined to floAvering plants; and Ave must Avait for full maturitj^ before that special form of the reproductive system is evolved, which marks the entire completion of the development. 527. In tracing the progressive evolution of the special Reproductive apparatus in Plants, we observe that although it is gradually separated from the nutritiA^e system, in proportion as Ave ascend the scale, it is never entirely disconnected with it. It Avas formerly stated that all the parts of the floAver may be regarded as metamorphosed leaves (§ 54); or, more correctly, as metamorphosed forms of the elements of AA'hich leaves are the types. Even the stamens and carpels are proved, by the frequent occur- rence of monstrosities, to have this character. The former often present the appearance of leaflets thickened at their edges by the formation of pollen; and these reproductive vesicles are themselves found, by obserA^a- tion of their early development, to differ but little in essential character or mode of production from any. other form of cellular tissue. The car- pels, moreover, are proved to be leaves, not only by such monstrosities as the one formerly mentioned, but by the fact of their bearing ovules at their edges; for these OAOiles are essentially huds^ (as may be seen in par- ticular abnormal instances), like those developed from the edges of various ■leaves, such as those of the Malaxis paludosa (Bog-orchis), and Bryutn calycinum (one of the air-plants of the tropics), Avhich are capable of developing themselves either separately or Avhile still attached to the parent structure. The special reproductive organs of the Cryptogamia might probably be reduced to similar elements, if their monstrosities Avere observed; thus,, the sori oi the Ferns have been seen to be replaced by clusters of leaflets, each of them representing a metamorphosed theca. Reproduction in Animals. 528. Although among the loAver tribes of the Animal kingdom aa'c may recognise the same simple and general condition of the reproductive system, as that Avhich has been shoAATi to exist in plants, it is in the higher classes much more completely specialised, and its relation yvith the nutritive system is much less obvious. This is an CAndent result of the peculiar complexity and heterogeneous character of the more perfect animal organisms, Avhich prevent the employment of the mode of propaga- tion, by mere extension of some part of the original structure, that is carried to so great an extent even in the highest plants. The onl>^ forms under AA^hich this mode of reproduction (Avhich is but a peculiar operation of the nutritive system) manifests itself, have been already noticed (§ 514) ; but some additional particulars may here been mentioned. In the 2i) 2 404 SPECIAL AND COMPARATIVE PHYSIOLOGY. gemmiparous propagation observed in many of the polypes, the new being- is obviously nothing but an increased development of a part of the parent structure, and exactly corresponds Avith the bud of a plant; a similar mode of increase seems to exist in some of the simpler Entozoa, where the young sprout from the interior of the cavity of the parent, and swim about, after their separation, in its contained fluid. The fissiparous generation, as it is called, is evidently but another form of the same plan; the parent structure not putting out a smaller and younger bud, but dividing itself into parts of which each has the power of reproducing the Avhole. It is among the infusoria that this mode is most characteristi- cally seen. Thus, the Paramoecium divides itself transversely, the division at first appearing like a notch, and gradually extending itself across the body, until the halves are completely separated (Fig. 94, a). Some species of Vorticellse divide themselves longitudinally in like manner (Fig. 77, &) ; and instances still more curious might be mentioned. Amongst many higher animals this mode of increase is practised, as already stated; but it is seldom that a more special reproductive apparatus is not also deve- loped. The object of this apparatus, in animals as in plants, is to form and mature a germ, Avhich, from the time of its first organisation, is destined to be the rudiment or embryo of a neAV being, and which is separated from its parent, (in the first instance at least, § 515 note\ in a form altogether dissimilar to that which it is ultimately to assume.* 529. The dififerent means provided in the Animal kingdom for the evolution and maturation of germs, and the early processes of develop- ment in these, will now be considered in their general aspects. As in the Yegetable kingdom, it will be found that there is throughout an essential correspondence in the function, however different its manifestations may appear. There is yet much uncertainty, however, regarding its condition in many of the lower classes of animals; and it will therefore be better to confine the present outline to the description of the principal types which may be recognised as distinct, than to attempt to define the groups to Avhich these respectively belong. The great distinction in the character of the reproductive bodies liberated fi-om the parent, corresponds with that already pointed out in the vegetable kingdom ; for we find in the lowest classes of animals, as the sponges and polypifera, an evolution of * These views regarding' the essential difference between that general condition of the Repro- ductive function which is only one application of the nutritive processes, and the special form of it more commonly understood as such, were suggested to the author by an extended com- parison of the modes of propagation in plants and animals ; and he imagined himself to be unsupported in them by any other authority. He is most happy to find, however, that so eminent a physiologist as Burdach has taken the same view. This author has employed dis- tinct terms to characterise the different types of the function, which may be best understood from their French synonymes ; — generation accrementitielle, or propagation by addition to the fabric of the individual, designating the first; — and generation secrementitielle, or propagation by separate g-erms, being applied to the second. — Ehrenberg also has recently expressed the opinion that the gemmiparous and fissiparous modes of reproduction are essentially the same. REPRODUCTION IN ANIMALS. 405 gemmules, evidently analogous to the granules emitted by the AlgSB (§ 51.9-20), which are formed by one set of organs only, and appear to consist of nothing but the germ of the future being, unprovided with any supply of nutriment for assisting its early growth. In the higher animals an ovuin or egg is produced, like the seed of Phanerogaraia, by the con- currence of two sets of organs, which are sometimes united in the same individual, (as in many MoUusca), but are more commonly separated. This ovum, however, is usually thrown off from the parent at an earlier period of the development of the embryo than the seed of plants; but a large store of nutriment is provided for it, upon which it subsists until competent to obtain its owa support. Sometimes the ova are retained within the body of the parent until the young are hatched, so that they come forth alive; the animal is then said to be ovo-vim2yarous. In the true viviparous form of reproduction, which is confined to the mammalia, the ovum is never furnished with more than a very small store of nutri- ment for the incipient development of the embryo ; this being adapted to gain a new and peculiar attachment to the parent, which affords it a direct and continual supply, 530. The connection between the general and special modes of Repro- duction is most plainly seen amongst some Animalcules, which, like the lowest Algae, propagate themselves in a manner which may be considered either as an extension of the parent structure by budding, or as a form- ation of the germs of new beings not yet restricted to some one part of the fabric as in higher organisms. Thus, the Volvox globator (Fig. 93, a) pro- duces its offspring from every part of its interior surface; these, after a time, quit their attachment, and swim about freely within the body of the parent, which finally ruptures to give them exit. Not unfrequently, however, it occurs that a third generation is seen within the second, before the escape and diffusion of the latter. This, then, exhibits the special function of Reproduction in its most diffused condition. It is obviously a parallel case to that of the Protococcus (§ 519) and simple ConfervEe (§ 520); the reproductive cell here constituting the entire animal, so that its rupture and the death of the parent are contemporaneous; whilst, in the more complex fabrics of higher animals, the reproductive organs are so far separated from the nutritive, that their functions do not interfere.""' In other species of animalcules, the germinal granules, as they have been termed, are liberated from some particular spot in the parent structvu'e; and their development may be watched after their separation. It not unfrequently happens that, in their early condition, their form is so dis- similar to that which they are subsequently to assume, that they have * The reproduction of this animal has been ievm^A fisslparovshy some authors, and §'<>»)- miparous by others. In referring it to this type, the author lias been influenced by its evident affinity with that of the Protococcus ; and, having' sliown the former to be merely the most diffused form of sporuliferous propagation in plants, he thinks that this will ai)pear to hold a similar relation with the higher kinds of reproduction in animals. 406 SPECIAL AND COJIPARATIVE PHYSIOLOGY. been mistaken for distinct species. According to Burdacli, tlie Hydra propagates itself in the same manner, in addition to the mode of repro- duction formerly noticed (§ 115); germinal granules being separated from its exterior surface in the autumn, which pass the -winter in a state of inactivity, and are developed in the spring. In the associated Polypes, the special reproductive apparatus for the production of these germs or gemmules, has abeady been noticed (§ 116). Although their receptacle has been usually termed the ovarium, the term is incorrectly applied to them, being appropriate only to the organ in which true ova are developed in higher animals; and from the very curious correspondence which may be seen in their structure to the theca of Mosses, it might not be undesir- able to apply that designation to them. Several other classes of inferior animals appear to propagate themselves in a similar manner; but the limits of this type are by no means ascertained. Probably, it is common to all the Acrita and Radiata; and to the inferior Articulata and Mol- lusca. Sometimes the germinal bodies are evolved from special recepta- cles; sometimes they are dispersed through the whole structure, lying in . the interstices of the different organs and tissues. 531. The peculiarity of the development of one of these germs, as distinguished from that of an ovum or egg, is exactly parallel to that which has been already noticed in the Vegetable kingdom. In the former case the bodies are homogenous, and the Avhole of their substance is converted, in the progress of their development, into the new animal; Avhilst in the latter, there is an evident distinction of parts, the germ being accompanied with a store of nutriment prepared by the parent, and the whole enveloped in one or more membranous tunics. The development of these simple germs has not yet been observed in more than a few instances. That of the Polypes has already been described (§ 116-8); and in the Sponges, Avhose reproduction has been attentively observed by Dr. Grant, the process is not dissimilar, though still more simple. The gemmules are here developed in the interior of the structure, and find their way into the large canals, through which they are emitted. At that time they appear like globules of gelatinous matter, presenting no trace of the cavities or canals which are subsequently formed (§ 280). 532. In all Animals which form a true ovum, whether that be fully developed within the body or not, the concurrence of two sets of organs, analogous to those described in plants, is a necessary condition; the ofBice of one being to prepare the ovum with its nutritious store and membranous envelopes; and of the other, to communicate to that ovum a fertilising influence. What the nature of that influence is — whether to introduce the germ which has been prepared by itself, or merely to stimulate the evolution of that already contained in the ovum, — has not yet been fully ascertained. The analogy supplied by the vegetable kingdom would cer- tainly countenance the first supposition; but this is perhaps a case in Avhich it Avould be dangerous to push analogy too far. Although the REPRODUCTION IN ANIMALS. 407 fertilising influence is usually communicated to tlie ova "wliilst yet contained in the ovarium, as in plants, there are many instances in which it is not applied until after they have been extruded frona the canal which conveys them from it; this is the case in most Fishes and Batrachia, and perhaps also in some other classes. In animals whose reproduction is performed on the first plan, the ova are not unfrequently extruded in a sterile condition, if from any cause the system is in a state of activity, and the fertilising influence be withheld. In the common fowl, for instance, barren and imperfect eggs are not unfrequently laid during the whole season, if the bird be highly fed, and no fecundation take place ; but this is well known to be very injurious to its health. Such eggs, though apparently the same in structure as those which have been fertilised, soon decompose, Avhen submitted to the heat of incubation, instead of under- going the changes accessory to the development of the embryo. This is analogous to what occurs to the seeds of plants, if the influence of the pollen be not communicated to them. In describing the ovum, and the changes it undergoes during its early development, it will be convenient to refer principally to that of Birds, pointing out what is deficient in that of inferior classes, and what is difi"erent or superadded in viviparous animals. 533. The ovum, Avhile yet contained in the ovarium, may be termed for distinction the ovidicm. It consists of the foUoAving parts. — 1. A dense transparent membrane containing the yolk, and thence termed the ^olk- hag. — 2. The yellow fluid mass known as the yolk. This, at an early period is composed of an oily mattei', mixed Avith a number of minute and transparent globules; in Reptiles and Fishes, hoAvever, these ingre- dients exist separately, and appear to have different functions. At a more advanced period, the yolk contains a mviltitude of larger globules of regular spherical form and perfect transparency; and the presence of these distinguishes the ovulum of oviparous animals, in Avhich the store of nutriment is destined to supply the embryo with the means of its deA^elop- ment for a considerable period, from that of the Mammalia, in AAdiich its function is speedily superseded by the new attachment that the ovum forms with the parent. — 3. A layer of granules adherent to the interior surface of the yolk-bag, and apparently forming part of it. In one por- tion, this layer, AA'hich is elscAvhere transparent, becomes thickened and opake, so as to form Avhat has long been known as the cicatricula or germ-spot. The centre of this spot is, hoAvever, perfectly diaphanous, and free from any granular appearance. This is occupied by — 4. The germinal vesicle (discovered by Purkinje), a very minute cellule, fr-equently not above ^^^ of a line in diameter; it contains a pellucid lymph; and on one of its sides, Avhich are formed of extremely delicate membrane, another spot has recently been detected (by Wagner) Avhich is too minute for analysis ; it is termed by him the germinal-spot (see Fig. 13G). 408 SPECIAL AND COMPARATIVK PHYSIOLOGY. 534. The last-named parts appear to be those essential to the ovulum, and the next to be merely envelopes superadded for particular purposes. In Insects, the germinal vesicle and sjwt constitute the entire ovulum; and, on the other hand, in Mammalia, a new envelope is added to those that exist in birds, — namely, the Graafian vesicle; this, however, does not leave the ovarium, but bursts to permit the escape of the ovulum into the oviduct. It is in their early condition that the ovula of Mammalia most resemble those of Birds; since, at a subsequent period, the increased development of the yolk in the latter concurs with the other alterations connected Avith the mode of its future development, to produce a dis- similarity. A most important change, however, which occurs in all instances at a certain period of the evolution of the ovum, whether fertilised or not, is the rupture of the germinal vesicle, and the consolida- tion of the granular layer within the yolk-bag into a membrane, from which the embryo, however complicated its oi'gans and systems may afterwards be, altogether originates. This is called the germinal mem- brane^ or hlastoderma ; and at the part of it in which the vesicle lay, there still remains a transparent space or area. The ovule of Birds, on entering the oviduct, becomes encased in a secretion from its lining membrane, which is termed the albumen^ and known as the white of the egg. The layers of this albumen first deposited become consolidated into a membrane, which is in close apposition, therefore, with the yolk-bag; and, when all the albumen is deposited, a similar membrane is formed around it, lining the shell that afterwards covers its surface. The com- plete ovum consists, therefore, of the shell, (which is well known to be a porous structure composed of calcareous matter cemented by animal glue), the membrane lining it and enveloping the albumen, the albumen itself, the membrane separating it from the yolk-bag, and lastly, the ovulum Avith its yolk and germinal membrane, in Avhich last part important changes are now commencing. The parts are essentially the same in other oviparous Vertebrata, although somewhat differently arranged: in the Batrachia, for instance, the germinal membrane, instead of being confined to one spot, nearly envelopes the yolk; and in Reptiles, as well as Fishes, there is usually an absence or partial deficiency of calcareous deposit on the exterior. In Mammalia, the ovulum, in descending through the oviduct into the receptacle or matrix in which it is to undergo its continued development, also receives an additional envelope, the chorion, which afterwards performs a most important part in its new connection -with the parent. 535. In the perfect eggs of the common Fowl, before incubation has commenced, the cicatricula is of a round form, a whitish colour, and generally about ^ of an inch in diameter (Fig. 174). After incubation has proceeded for 7 or 8 hours, a small dark line, termed the primitive trace, may be seen upon it; one extremity, which is rather swollen. REPRODUCTION IN ANIMALS. 409 corresponds nearly to the centre of the transparent area (Fig. 175). This primitive trace constitutes the first appearance of the embryo, the large extremity being the situation of the head. As incubation proceeds, the cicatricula expands, and the transparent area becomes more pellucid and defined. About the 12th or 14th houi-, the germinal membrane, (still of merely granular consistence), becomes divided into two layers, termed the serous and mucous^ of which the former is situated immediately under the yolk-bag, and the latter in contact with the yolk. Between these, as formerly stated (§ 320, 1) the vascular layer is found; but this does not exist separately until between the 20th and 24th hours, and seems to be formed by a division of the mucous layer. It is in the serous layer that the primitive trace exists: at first it occupies a long furrow, the mem- brane being thickened into two ridges (Fig. 137, b); about the 20th hour, this furrow is converted into a canal open at both ends, by the junction of its margins, as at c; and soon after, the larger extremity of it is closed. In this canal a semi-fluid matter is subsequently deposited, which becomes the rudiment of the spinal cord and brain. The parts of the serous layer which surround it gradually become thicker and more solid; and before the 24th hour, four or five small round opaque bodies are seen, which become the rudiments of the dorsal vertebrae. 536. Up to this period the layers of the germinal membrane have continued nearly flat and uniform; but about the 25th hour, when they cover nearly a third of the circumference of the yolk, they begin to exhi- bit various folds, which afterwards serve for the formation of the cavities of the body; The parts of the germinal membrane which lie bej^ond the extremities of the embryo are folded in so as to make a depression on the yolk; and their folded margins gradually approach one another (Fig. 138, 9) under the abdomen, which lies next the interior of the egg. The layers of the germinal membrane are bent doAvn also towards the sides of the spinal canal (Fig. 137, d); so that there is formed under each end of the embr3^o a short sac or cavitj^, which communicates v\ath the yolk by an opening common to both (Fig. 139, a). These sacs indicate the rudimentary state of the intestinal tube; the anterior corresponding to the oesophagus, the posterior to the lower part of the large intestine. At the anterior fold of the germinal membrane, a considerable space is left between the serous and mucous layers, which is occupied by a dilated portion of the vascular layer, forming the first rudiment of the heart (Fig. 139, h); this is seen about the 27th hour. As the subsequent development of the individual systems will have been particularly des- cribed under their respective heads, they need not be further traced here. But the changes which the remaining parts of the egg undergo must not be passed oA'er, as they are extremely interesting and curious. 537. The Vascular Area (§321) is the part in which the blood appears to be formed from the subjacent yolk; and it furnishes tlie nutritious 410 SPECIAL AND COMPARATIVE PHYSIOLOGY. fluid to tlie embiyo by means of two principal trunks, called the omphalo- inesenteric arteries. Tlie intestinal cavity, wliicli has been seen to be, in its first formation, but a part of that in wbicli the yolk is retained, remains continuous with it in all oviparous animals. In birds, the sac of the yolk, (formed by the expansion of the germinal membrane within the original yolk-bag, so as to become a complete envelope,) is gradually drawn into the body of the embryo, as incubation advances ; and, at the period of the exit of the chick, it is entirely contained -within the abdomen. In many fishes, however, the yolk is not taken into the embryo when it bursts its envelope, and the little fish swims about with the bag depending from the abdomen, and exposed to the contact of water, which will be presently seen to have an important influence upon it. In the mammalia, on the contrary, the function of the yolk is merely temporary, being superseded by the formation of a vascular connection with the parent; its proper sac and vessels, however, are discernible at an early period; and, in the orders which most nearly approach Birds in the structure of their reproductive apparatus, this is proportionably larger; but, instead of being withdrawn into the body, its connection with the intestinal tube becomes gradually obliterated, and it remains on the umbilical cord as a small vesicle, which may be distinguished during the early period of uterine gestation. 538. But in order that the nutritious matter stored uj) by the parent may be converted to the nutrition of the embryo, it is necessary that it should undergo some changes in which atmospheric air is concerned, as in the germination of seeds (§ 380); and during the development of the foetus, its blood requires aeration as much as that of the adult animal. "In the early stages of development there appears to be what may be called a General or Interstitial respiration, or a change essential to life, j)roduced by oxygen in all the substance of the embryo, or of its accessory parts, which, as the foetus is more perfectly formed, takes place in parti- cular organs only. As soon as a peculiar nutritive fluid, and a central propelling organ are produced, this fluid is exposed on the expanded sur- face of the yolk, to the influence of the respiratory medium, either directly or through the coverings of the ovum."* It will be convenient to trace the evolution of the respiratory system of the egg, first in Fishes, and then in the higher Yertebrata. In most of the osseous fishes, the blood of the foetus is transmitted to the vascular area, which gradually extends over the whole yolk, by a prolongation of the intestinal veins passing through the liver. It ramifies on the sac of the yolk, where it is aerated, at first through the thin membranes of the ovum, and subse- quently by direct contact with the surrounding element. It then returns ■* See the excellent paper, by Dr. Allen Thomson, on the development of the vascular system in the Foetus of Vertebrated Animals, from which this section has been principally derived. Edinb. New Philos. Journal, vols, ix, and x. REiniODUCTION IN ANIMALS. 411 to the heart by another set of vessels, which enter the vena cava. As the yolk becomes diminished in size, and the permanent respiration is established, the blood passes more directly from the liver to the heart, by the enlargement of vessels which Avere at first capillary into venous trimks, just as in the metamorphosis of the Batrachia (§311). In the cartilaginous Fishes, hoAvever, the blood sent to the respiratory surface is derived from an arterial trunk; and this is the case in all the higher Vertebrata. Most of the Fishes in this tribe, such as the Rays and Sharks, are ovo-viviparous, retaining their ova in the body for a longer or shorter time after development begins. In these, the membrane lining the oviduct of the parent is very vascular, and its blood is probably aerated by the introduction of the surrounding element into the abdomi- nal cavity; so that the aeration of the fluids of the ovum may take place through its means. This may be regarded as a sketch of the plan which is more fully developed in viviparous Reptiles, and carried to its highest extent in Mammalia. Where the ovum is extruded at an early period of development, apertures are found in the angles of its horny covering, through which a current of Avater is permitted to pass over the vascular membrane.'"' 539. In the early period of the development of the Batrachia, the same means of aerating the blood are adopted as in fishes; but near the epoch of their maturity, Ave find the traces of another organ that is formed by an extension of the intestine near its posterior termination, Avhich expands so as to occupy a considei'able space in the abdomen; on its membranous walls (Avhich subsequently constitute the urinary bladder of the animal), a plexus of arteries ramifies; and the blood which has passed over it, is conveyed through the liver into the vena cava. In the Lizards respiration is carried on by the sac of the yolk during the first half of incubation : but the vesicular membrane extended from the foetus, Avhich is called the allantois, gradually expands itself between that sac and the general envelope or chorion; and this, during the remainder of the foetal life, serves as its aerating surface, entirely superseding the sac of the yolk, Avhich seems to be subsequently concerned only in the absorp- tion of nutriment. The greater part of the allantois is left in the egg Avhen the foetus emerges from it; and a small part of its root only remains to form the ui'inary bladder of the adult animal. Many Lizards and Serpents retain their ova in the oAdduct, until the allantois is suffi- ciently expanded to caiTy on respiration; and, in those Avhich are completely ovo-viviparous, such as the Viper, the allantois becomes closely united with the vascular lining of the oviduct, so as to expose the venous blood of the foetus to the oxygenised blood of the parent. In some of the Turtles and Serpents, a large proportion of the allantois + This is well seen in the ova of the ray and dog'-fish, so common on our shores, and called by the fishermen " sea-devils" and "fairies' purses." 412 SPECIAL AND COMPARATIVE PHYSIOLOGY. remains to form tlie ui-inary bladder of the adult animal; and, as it appears tliat water is introduced into its cavity from without, it probably serves as an auxiliary to the function of respiration during the whole of life. The respiration of the embryo of birds within the egg is performed upon precisely the same plan (Fig. 176). The results of this change upon the surrounding air have been already mentioned (§ 426). 540. It now remains to state the peculiarities in the development of the ovum in mammalia. Even in this class, the ovulum originally con- tains a yolk-bag, within which the germinal membrane expands, just as in oviparous animals; and, in passing through the oviduct, it gains an additional envelope, the chorion (§ 534). But the oviduct, instead of immediately conveying the ovum out of the body, deposits it in the receptacle provided for its further development, namely the uterus. During its early period of increase mthin this matrix, the allantois is formed, and possesses the same situation and function as in oviparous animals, being interposed between the foetus and the enveloping chorion; and the latter being in contact with the vascular lining of the uterus, the venous blood of the foetus is arterialised by the influence of that of the parent communicated through it. The relative sizes of the allantois, and of the sac of the yolk, or umbilical vesicle as it is termed in Mammalia, vary much in different orders, and are usually in an inverse proportion to one another. Thus, in the JRodentia, which presents many other charac- ters of degradation, the umbilical vesicle is so large, and the allantois so little developed, that the latter is almost imbedded in the folds of the former, instead of enveloping it. In the Carnivora, on the other hand, the allantois almost entirely incloses the umbilical vesicle as well as_the foetus. But the allantois in Mammalia is never more than a temporary respiratory organ; for it speedily gives place to one peculiar to this class, Avhich is elaborated out of the chorion. In the ruminating species, in which its formation may be most easily traced, it takes place in the following manner. The vessels which ramify on the outer layer of the allantois, which is in contact with the chorion, gradually prolong them- selves into the latter membrane, and sprout, as it were, from its surface, so as to give it a flocculent appearance. At the same time, similar changes take place in the lining membrane of the uterus at certain points; and its processes, which are almost entirely composed of blood-vessels, interlace with those of the chorion, so that the blood of the foetus may be submitted to the arterialising action of that of the mother. There does not appear, however, to be any more direct communication between the two vascular systems, than through the parietes of their respective vessels; but this is precisely analogous to Avhat occurs in other organs, as in the lungs and glands. In what precise form the foetus derives its nutriment from the parent, is not yet ascertained; but it is probably the liquor san- guines only which transudes, the red particles being formed by the foetus Reproduction in animals. 413 itself. The number and extent of tlie points of connection between the embryo and the parent differ considerably in the various orders: in the inferior tribes and in the early condition of the higher, they are diffused oyer a considerable portion of the chorion; in the advanced stages of the gestation of the latter, however, they are concentrated into one part, forming what is termed the placenta. This is composed of a spongy parenchyma, containing many cells into which the maternal blood passes by vessels prolonged from the lining of the uterus; and the vessels of the foetus ramify minutely on the walls of the cells. This organ has therefore an evident analogy with the lungs of the perfect animal, — the maternal blood occuppng the place, and performing the function, of atmospheric air. It is most interesting to observe, as we ascend the animal scale, one structure thus superseded by another, adapted to the increased extent of the function progressively required; and to compare this with the coitcs- ponding changes which take place during the foetal development of the higher Mammalia, those structures having there a temporary office, which are the only ones concerned in the development of the lower classes. 541. In the greater number of Mammalia, the ovum is retained in the uterus until the foetus assumes nearly the form of the adult, and is capable of maintaining its own existence, if the digestive system is supplied mth appropriate nutriment. That furnished by the mammary glands (which are supplementary additions to the essential reproductive apparatus of this class), is the most appropriate, but it is not usually indispensable. In the Marstcpinlia, however, the embryo quits the uterus in a compara- tively imperfect state, resembling a worm in form and appearance; and, being conveyed to the marsupium, it remains attached to the nipple, almost without motion, for a considerable period. There is still a degree of uncertainty as to the mode in which the ova of the Ornithorhyncus and other Monotreinata are connected with the uterus during their stay in its cavity: but it seems probable that no placenta is formed, the general sur- face of the chorion acting as the aerating membrane (§ 75). Note to § 517. An interesting- case, in which a Mxicor (one of the inferior Fungi constituting' Mould) develoiied itself in the form of a Conferva (belonging' to the Ai.g.'e) in a fluid medium, but was subsequently recog'nised by its fructification, is related by Mr. Berkeley in the Magaz. of Zool. and Botan. vol. ii, p. 340. The rank which this gentleman holds as a Mycologist precludes all doubt as to the genuineness of the fact. 414 SPECIAL AND COMPARATIVE PHYSIOLOGY. CHAPTER XIV. SUBORDINATE LAWS REGULATING THE EXERCISE OP THE REPRODUCTIVE FUNCTION. DISTINCTION OF SPECIES. PROPAGATION OF SPONTANEOUS OR ACQUIRED PECULIARITIES. 542. "When we contemplate the immense number of diyersified forms which the study of the organised creation brings under our notice, and witness these forms perpetuated, as it would seem, by the process of reproduction, so as to constitute distinct races, the question naturally arises whether all these had a different origin; or whether the characters of any of them have been so modified in the course of time, as to lead to the belief in the diversity of origin of those which were at first really identical. When it can be shown that two races have had a separate origin, they are regarded as of different species; and, in the absence of proof, this is inferred when we see some peculiarity of organisation, cha- racteristic of each, so constantly transmitted from parent to offspring, that the one cannot be supposed to have lost, or the other to have acquired it, through any known operation of physical causes. It cannot be regarded as an unimportant question to the naturalist to ascertain what these con- stant distinctions are ; whilst it is an investigation of high interest in a physiological point of view, to trace the modifying influence of external circumstances upon the structure and functions of living beings, and to enquire how far the result of such influences may be transmitted heredi- tarily, so that the difference produced by them may be perpetuated. Where races which have originally sprung from a common stock present marked differences, they are spoken of as varieties ; and the variety may be transient^ from its peculiarity manifesting a tendency to disappear, — ox permanent^ where it continues to be transmitted without change. The uncertainty of the limits of species is daily becoming more and more evident; and every naturalist is aware that a very large number of races are usually considered as having a distinct origin, when they are nothing more than permanent varieties of a common stock. Whilst the exertions of the enterprising discoverer are adding to our already enormous list of species, from the unexplored resources of foreign lands, the skill of the horticulturist and of the breeder is exerted to produce new varieties of species already in our catalogues : and it has unfortunately too often hap- pened, that a new specific name has been invented for the latter as well as for the former; and that a mere hybrid or transient variety has thus taken the rank of a species, to the confusion of all true principles of arrange- ment. The philosophic naturalist, on the other hand, aims to reduce the number of species by investigating the degree of variation which each is liable to undergo, the forms it assumes at different periods of its existence. DISTINCTION OF SPECIES. 415 the permanent characters by which it may be distinguished during its whole life, the habits which are natural to it, the degree in which these may be changed by the influence of circumstances; — and, in fine, he endeavours to become acquainted with the tohole natural history of a reputed species, before separating it from another to which it may be closely allied. 543. Many examples may be given of the success with which this mode of investigation is now being prosecuted. The belief which is gain- ing groimd that many diversified forms of the simpler Cryptogamia may arise from similar germs developed under different circumstances, has already been noticed (§ 65-8). The same may, perhaps, be surmised wdthout improbability of the Infusorial Animalcules; and with respect to these, patient observation has already done much in reducing the numbe^r of species amongst the forms previously kno\'\Ti (whilst the improved powers of the microscope have revealed many new ones), by showing that the same individual may present very diversified appearances at dif- ferent times, owing to the variable distention of its digestive cavities, and the changes which it undergoes in the process oi Jissiparous reproduction (§ 528). Among the higher plants, the experiments of Mr. Herbert on the primrose, cowslip, oxslip, and polyanthus (which he proves to be all varieties of one species), are sufficient evidence of the important results which would probably accrue from a similar investigation in other quarters. In Zoology, again, the very interesting paper of Mr. Gray" may be referred to, as proving the great influence of external circum- stances in modifying the form of shells; it is there shown, among other instances, that what have been regarded as six distinct species of Murex (§ 100) are in reality but different states oi one ; and Mr. Stutchbury has been equally successful in reducing the number of species of Patella, Cyprsea, and Oliva, by attending to the changes of form which each individual undergoes in the progress of its development. Many instances might be related in proof of the uncertainty of reputed specific distinctions among higher classes. Insects have been seen presenting the characters of different species on the two sides of the body; and it is noAV certain that an erroneous multiplication of species among Birds, especially in the migrating tribes, has been occasioned by their change of plumage at dif- ferent seasons. And finally, to return to the Vegetable kingdom, the uncertainty of all principles of arrangement founded upon arbitrary cha- racters has been demonstrated by the fact recently published,t that the flowers and pseudo-bulbs of three distinct genera of Orchideous plants have been produced by the same individual. :{: * Philos. Transactions, 1833. t Linn. Trans, vol. xvii. X This fact has also come under the author's own notice in the Durdham Down Nm-sery, near Bristol, two of the genera being- the same as in the instance just quoted, but the third a different one, so that /our may thus be regrarded ns of the same species. 416 SPECIAL AND COMPARATIVE PHYSIOLOGY. 544'. It 1ms been formerly stated (§ 72) tliat the Naturalist endeavours to simplify the acquirement and pursuit of his science, by the adoption of easily-recognised external characters as the basis of his classification; but these can only be safely employed, when indicative of peculiarities in internal structure Avhich are found to be little subject to variation, and M'hich are not liable to be affected by the influence of physical causes. The colour of flowers, for example, is liable to so much alteration from the influence of soil and climate, that it is seldom regarded as of itself any test of the unity or diversity of species : in moths and butterflies, on the other hand, the uniform appearance of particular spots on the wings is held sufiicient to constitute a specific character, because it is never known to vary; and it would probably be found associated, if the examination were pushed far enough, Avith some unequivocal differences in the con- figuration of internal organs. Amidst all these difficulties attending the discrimination of species from structural characters alone, it is not unrea- sonable to enquire if there are any other means of effecting the object mth greater certainty. This subject has been fully considered by Dr. Prichard in his elaborate work on the Physical History of Man; all that can be here considered are the laws regulating the intermixture of species, and the propagation of hereditary or acquired peculiarities. 545. The conclusions Avhicli have now been attained on the first of these points, and Avhich (if stated in a sufficiently general form) are equally applicable to both the Animal and Vegetable kingdoms, may be regarded as one of the most valuable tests which naturalists possess. In plants, the stigma of the flower of one species may be fertilised with the pollen of an allied species; and, from the seeds produced, may be raised plants of an intermediate character. But these hybrid plants mil not long continue the race ; for, although they may ripen their seed for one or tAvo generations, they will not continue to reproduce themselves beyond the third or fourth. But, if the intervention of one of the parent species be used, its stigma being fertilised AA'ith the pollen of the hybrid, or mce versa, a mixed race may be kept up for some time longer; but it aa^II then have a manifest tendency to return to the form of the parent whose intervention has been employed. Where, on the other hand, the parents Avere themselves only varieties, the hybrid is only another variety, and its poAvers of reproduction are rather increased than diminished; so that it may continue to propagate its OAvn race, or may be used for the pro- duction of other varieties, almost ad infinitum. In this AA^ay many beautiful new varieties of garden floAvers have been obtained, especially among such species as have a natural tendency to change their aspect.* * There are many instances in which foreign plants have been introduced into this country, and have received different specific names, but have been found capable of producing- fertile hybrids ; in these cases a more accurate examination of the original locality has g^enerally shov/n that the pai'ents were nothing more than permanent varieties, or even hybrids naturally PROPAGATION OF HYBRIDS. 417 Amongst animals, tlie limits of hybridity are more narrow, since the hybrid is totally unable to continue its race with one of its own kind; and although it may be fertile with one of its parent species, the progeny will of course be nearer in character to the pure blood, and the race will ultimately merge into it.* In animals, as among plants, the mixed off- springs originating from different races within the limits of the same species, generally exceed in vigour, and in the tendency to multiply, the parent races from which they are produced, so as often to gain ground upon the older varieties, and gradually to supersede them. Thus, the mixture of the European races with the Hindoo and South American has produced tribes of such superior characters of body, and of such rapid tendency to multiplication, that there is reason to believe that they Avill ultimately become the dominant powers in the community.+ The general principle, then, is that beings of distinct species, or descendants from stocks originally different, cannot produce a mixed race which shall pos- sess the capability of continuing itself; whilst the union of varieties has a tendency to produce a race superior in energy and fertility to its parents. 546. In examining into the characters of the different species of Plants and Animals -with which different regions on the earth's surface are peopled, the naturalist soon becomes aware that there are many kinds which are restricted to particular localities, whilst others are diffused ex- tensively or even universally over the globe; — that there are some spots (especially insular ones), of which the aboriginal inhabitants are almost entirely different from those elsewhere found; — and yet that amongst these there will always be found species holding the same rank with regard to the remainder, and thus representing each other in different coimtries. Thus, the species of plants and animals originally inhabiting the eastern and western hemispheres were probably almost entirely different, until the agency of man changed their geographical distribution; and almost the same may be said of the species north and south of the Equator. On the other hand m«w, and his constant attendants the dog and the fly, exist in every quarter of the globe. Again, Ave find in New Holland no quadrupeds which do not belong to the order Marsupialia or Monotremata (§ 75), with the exception of a dog which is believed to have been intro- duced by man, and to have run wild; and none of these species are found occurring- between other varieties. This is particularly the case with many of the South American genera, such as that elegant g-arden flower, the Calceolaria ; and this is probably the explanation of the almost indefinite number of splendid varieties, well known to horticul- turists, which may be obtained from the South American Amaryllis. * One or two instances have been mentioned in which a mule has, from union with a similar animal, produced offspring' ; but this is certainly the extreme limit, since no one has ever maintained that the race can be continued further than one generation, without admixture with one of the parent species. t Several additional instances of this kind are related in Dr. Prichard's work, vol. i, p. 147 and in Mr. Combe's Constitution of Man. chapter v. 2 E 418 SPECIAL AND COMPARATIVE PHYSIOLOGY. elsewhere. The greater part of the plants also belong to new genera; and those included in the genera already kno-v^oi constitute distinct species, — with scarcely any exception but among the Crjiptogamia, the distribution of which seems more extended than that of flowering plants. The Flora of insular situations, if at a great distance from land, contains very few species which occur elsewhere. Thus, among the flowering plants of St. Helena, which is so far removed even from the western shores of Africa, there have been found, out of 61 native species, only tivo or three which exist in any other part of the globe. From these and many similar facts it appears fair to conclude, that every species of plant and animal had originally a distinct locality, from which it has been dispersed, according to the capabilities possessed by its structure of adapting itself to changes in its external conditions, its own locomotive powers, and the degree in Avhich it is subject to external agencies. "What is a rare plant," says Decandolle, "but one which is so organised that it can only live in a par- ticular locality, and which perishes in all others; such a plant is incapable of assuming difierent forms. What, on the other hand, is a common plant? It is one robust enough to exist in very dififerent localities, and under very different circumstances, and which will therefore put on many different forms."* Plants, then, are liable to run into varieties in propor- tion as they are more robust, more common, or more cultivated; and some native species are, from this cause, domesticated with greater difl&culty than many exotics. Precisely the same may be said of animals ; those which have the power of adaptation to differences of temperature, food, &c. are most universally diffused ; while those which can only exist within narrower limits of variation are restricted to the neighbourhood of their original locality. 547. It becomes a most interesting question, then, to determine what are the changes which may be produced by the influence of external cir- cumstances, and how far these are hereditarily transmissible. On this subject, a few facts may be stated which will give an insight into the nature of the enquiry; but it is one which deserves more attention than it has yet received, since it is not only essential to the correctness of all Natural-history classifications, but is connected with some of the highest questions in Physiological science. One of the most obvious distinctions, Avhere it is well marked, is that of size; and yet a little examination will show that it is one most open to fallacy. Thus, a plant only a few inches high in a poor dry soil, may become much larger in a damp rich one ; and this is a very common effect of cultivation. On the other hand, by starvation naturally or artificially induced, plants may be dAvarfed, or reduced in stature : thus, the DaJdia has been diminished from six feet to two; the Spruce Fir, from a lofty timber tree to a pigmy bush; and many of the trees of plains become more and more dwarf as they ascend * Library of Useful Knowledge. Botany, p, 138. PROPAGATION OP AC'dUIHED PECULIARITIES. 419 mountains, till at length they exist as mere underwood. That a similar influence vnll be productive (within narrower limits, however,) of corres- ponding effects in the animal kingdom, no one can be ignorant; and a very curious illustration is given by Mr. Gray of the effect of external conditions upon the size of Mollusca, in the fact that there is so much difference of size between individuals of Bulimics rosaceus on the coast and on the mountains of Chili, that the latter have been described as distinct species. He also mentions that the Littorina petrcea found on the sea side of Plymouth Breakwater acquires, from its superior exposure to light and heat, and probably also from the greater supply of nutriment which it obtains, twice the size which is common to individuals living on the north side within the harbour.* It is interesting to remark that these great variations occur in animals which, from their fixed condition, and the preponderance of their nutritive system, have most alliance with the vegetable kingdom; and it seems probable that a diminution of the vital stimuli, which in them only reduces the growth, would be fatal to other tribes whose animal powers are more active, and which have therefore greater means of suiting their external conditions to their bodily con- stitution. 548. Other modifications in the form and relative size of indi\'idual parts are very common in Vegetables, where the tissues are so simple, and the different organs so much alike in elementary constitution. Thus, cultivation often converts a single flower into a double one, by the metamorphosis of its stamens into petals, or by the development of a row of petals previously abortive, or by the change of the small tubular florets of a composite flower (like those composing the disk or eye of the Dahlia) into flat expanded florets Avhich constitute the ray. Cultivation has a similar effect in obliterating the spines, prickles, and thorns, from the surface of many plants; a change which was fancifully, but not improperly, termed by Linngeus "the taming of wild fruits." The instances of such alterations effected by external agency in the vegetable kingdom, are almost innumerable; but it is very difiicult to say how far the varieties thus created may become permanent by their hereditary transmission. The usual principle is, that propagation by seeds will only reproduce the species, the race not being continued with any certainty. In most plants which have been much altered by cultivation — such as the Apple, the Cabbage, or the Dahlia — the seeds, if dropped on a poor soil, will produce plants which approximate to the original type of the species; Avhilst from the seeds of the Cerealia (corn-grains), which are believed to have been originally grasses of some very different aspect, no other forms are ever produced which might assist in the solution of the curious problem of their origin. It is not improbable that, as among animals, varieties which arise from some peculiarity in the constitution of the being itself, * Gray, in Philos. Trans., 1833, p. 786. 2 R 2 420 SiPECIAL AND COMPARATIVE PHYSIOLOGY. are more liable to be reproduced in tbe offspring, than those which are simply the result of external agencies. It is evident, at least, that here also the capability of undergoing such modifications is that which renders the species most truly valuable to man. 549. Amongst animals, the various breeds of domestic cattle, of the horse, dog, &c. aiford abundant evidence of the modifying influence of external conditions; since there is no doubt that they have originated from single stocks, and that their peculiarities have been engrafted, as it were, upon their specific characters. Between the Shetland pony and the Arabian racer, for example, or between the Newfoundland dog and the Italian greyhound, there would seem much greater difi"erence than between the Lion and Tiger (the sculls of which are so much alike that even Cuvier was not always able to distinguish them), or between various other species of the Feline tribe, which, from the incapability of domestication, have not been exposed to such influences. That these domesticated races, however different their external characters, have a common origin, is proved by the fact that, whenever they return to a state of nature, — as is the case with the dogs introduced by the Spaniards into Cuba, and the horses and wild cattle which now overspread the plains of South America, — the differences of breed disappear, and a common form is possessed by all the individiials. It is not a little curious, too, that instincts which must have remained dormant for many generations during the domesti- cated condition of the race, should re-appear when this change takes place in its habits; thus, among the Avild horses of South America there is the same tendency to associate in herds under the protection of a leader, as among those of Asia whose ancestors have never been reduced to sub- jection. " It seems reasonable to conclude," as Mr. Lyell has justly- remarked, " that the power bestowed on the horse, the dog, the ox, the sheep, the cat, and many species of domestic fowls, of supporting almost every climate, was given expressly to enable them to follow man through- out all parts of the globe, in order that he may obtain their services and they our protection." " Unless some animals had manifested in a wild state an aptitude to second the efforts of man, their domestication would never have been attempted. If they had all resembled the wolf, the fox, and the hyaena, the patience of the experimentalist would have been exhausted by innumerable failures before he at last succeeded in obtaining some imperfect results; so, if the first advantages derived from the culti- vation of plants, had been elicited by as tedious and costly a process as that by which we now make some slight additional improvement in cer- tain races, we should have remained to this day in ignorance of the greater number of their useful qualities." 550. How all the varieties of breeds have been produced Avhich are noAV so striking, is a question much more easily asked than replied to satisfactorily. That peculiarities of structure sometimes arise independently PROPAGATION OF ACQUIRED PECULIARITIES. 421 of external agencies can scarcely be doubted; thus, it is by no means un- common to find individuals of tbe human species with six fingers and six toes; and such peculiarities are more likely to be continued hereditarily than those which have been acquired. Sometimes advantage has been taken by man of accidental varieties of this kind, for some purpose useful to him, and he has exerted his skill to perpetuate them. The foUomng example is of comparatively recent occurrence. In the year 1791 one of the ewes on the farm of Seth Wright in the state of Massachusets, pro- duced a male lamb, which, from the singular length of its body and the shortness of its legs, received the name of the otter breed. This physical conformation, incapacitating the animal from leaping fences, appeared to the farmers around so desirable that they Avished it continued. Wright detei-mined on breeding from this ram, and the first year obtained only two with the same peculiarities. The following years he obtained greater numbers; and, when they became capable of breeding with one another, a new and strongly-marked variety, before unknown to the world, was established.* This shows the influence which the circumstance of a scanty population may have formerly had in the production of varieties, both in the human and other species. At the present time, any pecu- liarity which may occasionally arise speedily merges by intermixture, and returns to the common standard; but it may be imagined that, in the older ages of the world, some race in which a peculiarity existed, may have been so far separated from the rest as to necessitate frequent union among its members, so that the character would be rendered still more marked instead of disappearing; and, being propagated for a few genera- tions, would be rendered permanent. Acquired peculiarities, on the other hand, are seldom reproduced in the offspring, unless they have a relation Avith the natural habits and physical wants of the species; but, when this relation exists, they may be transmitted as regularly as the specific character. Thus, in dogs, the relative perfection of the organs of sight and smell, perhaps also of hearing, varies much in different breeds, and their mode of hunting their prey undergoes a corresponding change; but in these cases no new instinct is developed, the difi'erence merely consisting in the relative proportion of those already existing; and the new peculiarities have an intimate relation to the habits of the animal in a wild state, t It is impossible not to recognise in many acquired habits, * Phil. Trans. 1813. t In a mongrel race of dogs employed by the inhabitants of the banks of the ]\Iag-dalena almost exclusively in hunting the white-lipped Pecari, a peculiar instinct appears to have become hereditary, like that of the pointers and other dogs of this country. The address of these dogs consists in restraining- their ardour, and attaching themselves to no animal in parti- cular, but keeping the whole herd in check. Now among these dogs some are found which, the very first time they are taken to the woods, are acquainted with this mode of attack ; whereas, a dog of another breed starts forward at once, is surrounded by the Pecari, and, whatever may be his strength, is destroyed in a moment. ]Mr. Lycll mentions that some 422 SPECIAL AND COMPAKATIVE PHYSIOLOGY. however, something more than a relation to the instincts necessary for the preservation of the species; they evidently arise, in part at least, from the connection of the race with man. This is more particularly exemplified in the instance of the breed of shepherds' dogs, which often display an extraordinary hereditary sagacity respecting their peculiar vocation; as well as in cases Avhich have been frequently mentioned, where the descend- ants of dogs to which peculiar tricks have been taught, have displayed an unusual aptitude for learning the same. It may then be considered that the capability of undergoing such modifications, is a part of the psychical as well as structural character of the dog, even in a wild state ; and that his relation to man may have as impoi'tant an influence on his heredi- tary propensities, as the supply of their physical wants has on animals of other species. The same may, perhaps, be said of the horse, in the races of Avhich we find peculiar habits transmitted from parent to offspring, which are the pure results of human instruction. It is from the want of this relation towards either the natural habits of the species or their sub- serviency to man, that habits acc^uired by other animals do not become hereditary. Thus, pigs have been taught to hunt and point game Avith great activity and steadiness, and other learned individuals of the same species have been taught to spell; but these acquirements have in no instance been transmitted to the ofi'spring, not being the result of the deve- lopment or modification of any instinctive propensity naturally existing. In like manner, however artificially the forms of domesticated animals may have been altered in all the individuals of successive generations, the usual character of the species and variety is maintained in each one of the offspring; unless, as sometimes happens, this alteration happens to coincide with natural varieties of the species. Thus, instances are on record in which dogs, that have been deprived of their tails by accident or design, have produced puppies with a similar deficiency; but as breeds of tail-less dogs have spontaneously ai-isen, there would be a stronger tendency to the perpetuation of the acquired peculiarity, than when no such peculiarity naturally occurred. It has also been asserted, however, that cats deprived of their tails will often produce one or two tail-less kittens at each birth; and that a cat which had its tail distorted by acci- dent, has been known to transmit the deformity to some of its offspring. Englishmen engaged in conducting' the operations of the Real del Monte company in Mexico, carried out with them some greyhounds of the best breed, to hunt the hares which abound in that country. The great platform which is the scene of sport, is at an elevation of about 9000 feet above the level of the sea, and the mercury in the barometer stands habitually at the height of about 19 inches. It was found that the greyhounds could not support the fatigues of a long chase in this attenuated atmosphere ; and before they could come up with their prey they lay down gasping for breath : but these same animals have produced whelps which have grown up, and are not in the least degree incommoded by the want of density in the air, but run down the hares with as much ease as the fleetest of their race in this country. Some curious instances of a similar propagation of acquired peculiarities connected with the natural habits of the raice are given us by Mr. Knight, Phil. Trans. 1837. I'HOPAGATION OF ACQUIRED PECULIARITIES. 423 These are certainly exceptions to the general rule, but must not be left out of view; there can be no doubt that much has yet to be learned of the influence of the state of the parent upon the development of the off- spring; and that, though credulity and the love of the marvellous have been the occasion of many strange fictions being transmitted to us, Ave are by no means justified in rejecting the doctrine without further enquiry."' 551. Any one who takes an extensive survey of the psychical as well as corporeal peculiarities of the human race must discover that both are susceptible of a higher degree of education than those of any other tribe of animals; and it is in consequence of this, that man has surmounted the obstacles interposed by his naked and defenceless condition, and found the means of existence in every part of the globe. And in general it may be observed, that the more difficulties are presented by circumstances to the supply of his instinctive wants, the more are his intellects called into exercise for their gratification. Thus, the conditions of civilised life are more calculated to excite the dormant energies of his mind, than the pastoral habits of the Nomade tribes, scarcely now advanced beyond patriarchal simplicity, or the easily satisfied Avants of the Indian hunter or the Polynesian fisherman. If, again, this power of self adaptation had been confined to the mind of man, whilst his body continued unable to resist changes in its external conditions, or to perform those actions which his new circumstances might require, his race must as necessarily have ceased long ago to exist, except in spots peculiarly favoured by Nature, as if, with his present organisation, he had been made dependent upon those mere instincts, which are just capable of maintaining his life when supplied by the ministration of others. The educability of man's bodily frame is in fact scarcely less remarkable than that of his psychical powers. Although each of his organs of sensation is naturally inferior in acuteness to the corresponding organ of some other animal, it may be rendered by constant practice so far superior to the usual standard as to convey a degi-ee of information greater than that Avhich brutes can attain. Thus, the experienced seaman announces with confidence the proximity of land, or the aspect and direction of a vessel, which the ordinary voyager cannot discern; and the watchful ear of the North American Indian distinguishes the tread of friends or foes when his civilised companion is unconscious of their neighbourhood. That these acquired powers are sometimes propagated as hereditary instincts, seems probable Avhen we remember that, among some savage nations of North America and New Holland, precisely the same notion of direction is manifested, as is evinced, in a degree scarcely more remarkable, by the lower animals; individuals fre- quently traversing pathless forests for the first time Avithout SAverving in the least from the direct line towards the point at Avhich they are aiming. No one, avIio has sufficient opportunity of observation, can doubt that the * Montgomery on the Signs of Pregnancy, p. 16. 424 SPECIAL AND COBIPARATIVE PHYSIOLOGY. intellectual faculties wliich have been developed by cultivation, are gene- rally transmitted to the offspring in an improved state; so that the descendant of a line of educated ancestors will probably have a much higher capacity for instruction than the child that springs from an illiterate race. CHAPTER XV. SENSIBLE MOTIONS OF LIVING BEINGS. 552. The power of executing movements, without the direct applica- tion of mechanical force, cannot be in itself regarded as a characteristic of the Animal kingdom; since many evidences of it are seen among Vege- tables. This power must, it is obvious, depend upon a property inherent in some of the tissues of the organism, of contracting luider the influence of peculiar stimuli; and there is no more difficulty in imagining a tissue to be possessed of such a propertj'^, than in acknowledging its power to separate from the circulating fluid the elements of its nutrition, and to convert them into an organised fabric. This property of contractility on the application of a stimulus, may be readily distinguished from the elasticity which is simply due to the mechanical relation of the particles composing the tissue; the latter being retained as long as there is no evident decomposition, whilst the former is ah essentially mtal endowment. An elastic ligament, when stretched, tends to contract only in virtue of the mechanical force which has been created in it; but a muscle which contracts upon the stimulus of a simple touch, or one of a still less mechanical nature, can do so only by a property of its oyvti. This pro- perty is difiiised, in various degrees, through a large proportion of the Vegetable as well as the Animal kingdom. It is probably possessed by all the tissues actively concerned in the nutrition and reproduction of the beings belonging to the former; and it is manifested under the influence of the vital stimuli (heat, light, moisture, &c.), as well as, in some pecu- liar cases, in obedience to impressions of a mechanical nature. In the lowest and simplest animals, whatever degree of contractility is possessed, appears to be almost equally diffused through the system; and we can neither discover in them any structure specially endowed with this pro- perty, nor anything resembling a nervous system fitted to call it into exercise. In proportion as we ascend the scale, however, we find a distinct muscular structure evolved, in which the general contractility of the body becomes, as it were, concentrated; and, in proportion to its development and complexity, it supersedes the corresponding but more SENSIBLE MOTIONS OF LIVING BEINGS. 425 feeble powers of the remainder of the tissues. It is noAV almost entirely subjected to the nervous system ; and all those parts of it, Avhich are not connected with the functions of organic life merely, are rendered sub- servient to the will, and thus become the instruments of its operation upon the place and condition of the body. 553. It is among the lowest classes of Plants that some of the most curious and inexplicable motions are witnessed. Those which occur in connection with the reproductive functions have already been noticed; but there are others no less interesting. Thus, in the plants of the gi'oup of Oscillatorice, belonging to the class of algyE, the filaments have a movement of alternate flexion and extension, writhing like worms in pain; sometimes they appear to twist spirally, and then to project themselves forward by straightening again. These movements are greatly influenced by temperature and other external circumstances; in heat and solar light they are more active than at a low temperature and in shade; and they are checked by any strong chemical agents, which also put a stop to the motions of the animalcules inhabiting the same Avater. Another group of Algae, the Nostochince, manifests similar properties. Its members are generally composed of several distinct portions, which unite, like some of the compound animals, dm'ing a part of their existence, and afterwards separate; these have considerable power of spontaneous movement, the causes of which it is equally difficult to detect. 554. In many of the higher Plants, evident movements may be ob- served,— sometimes taking place in obedience to the ordinary vital stimuli, and forming part of the regular series of phenomena of growth and repro- duction;— and sometimes being performed in respondence to excitement of a mechanical kind. The immediate connection of these movements with the organic functions, in the first class of instances, and the indication they would seem to give of consciousness and sensibility in the second, have led many persons to seek for an explanation of them in the fancied attribute of a nervous system. But it will be seen, if the question be fairly investigated, that, whilst no evidence of its presence is furnished by the minutest anatomical research, no argument for its operation can be deduced from the phenomena. In the simplest and most intelligible instances of sensible motions in plants, the change is the result of the contraction of the part to which the stimulus is applied. Thus, if the base of the filament of the Berberry be touched with the point of a pin, the stamen immediately bends over and touches the style. In this case, the movement is produced by the peculiar contractility of the tissue on the interior side of the filament, which, when called into operation by the application of a stimulus, necessarily occasions the flexion of the stalk. This peculiar irritability has a relation with the functions of the flower; since, when called into play (as it frequently is) by the contact of insects, the fertilisation of the stigma will be assisted. Many similar instances 426 SPECIAL AND COMPARATIVE PHYSIOLOGY. might be adduced, in which a corresponding operation is connected with the process of reproduction in Plants. 555. There are cases of more complexity, however, in which an irrita- tion of one part produces motion in a distant and apparently unconnected organ. Thus, in the Dioncea inuscipula (Venus' fly-tray), the contact of any substance with one of the three prickles which stand upon each lobe of the leaf, will occasion the closure of the lobes together, by a change taking place in their leaf-stalk. And in the Mimosa pudica (sensitive plant), any irritation applied to one of the leaflets will occasion, not only its own movement towards its fellow, but the depression of the rib from which it springs; and, if the plant be healthy, a similar depression will be produced in the principal leaf-stalk, and even in the petioles of other leaves. Now, in animals, such a propagation of a stimulus would un- doubtedly be efifected by the nervous system; and it might be plausibly argued from analogy that it could not be performed without a similar apparatus in plants. Let it be first enquired, however, how the indivi- dual functions of the more complex and specialised structures among Vegetables are harmonised and brought into relation with one another. The whole system of the plant, it must be recollected, is immediately de- pendent upon external stimidi for its maintenance. All its vital properties are closely connected with the support of its organic life, and the continu- ance of its race: all its energies are directed towards these ends. Each organ possesses, to a considerable extent, an independent vitality; and each, when separated from the rest, can perform its own function, as long as the conditions essential to it are supplied. All the functions, however, are blended and harmonised in the most perfect plant by means of the circulating system; and, from the ordinary phenomena of vegetable nutri- tion, there is no reason to believe that any other bond of union exists, since they may be all referred to the vital endowments of the several parts thus brought into connection Avith one another. 556. Now with regard to the movements under consideration, it is beautiful to observe, that Nature, in effecting a new purpose, has accom- plished it, not by adding an entirely new structure, but by modifying those already existing. The irritability of which they are the result, ap- pears to be of precisely the same character with that just now described in the Berberry. In fact it seems but an exaltation of that common to most of the vegetable structure, which exhibits itself under various forms; thus, the leaf of the wild Lettuce exudes, when the plant is in flower, the milky juice contained in its vesicles, if these be irritated by the touch; and the contraction of the poison-gland of the Nettle, when the tubular hair which surmounts it is pressed, appears to be another mani- festation of the same property. This irritability has been sho^vQ to operate upon distant parts, in the case of the Mimosa, and probably also in the Dionsea, through the circulating system. Where each leaflet SENSIBLE JIOTIONS OP LIVING BEINGS. 427 is implanted upon its rib, there is a little swelling or intumescence; tliis is more evident Avliere the lateral ribs join the central one; and it is of considerable size at the base of the petiole, vrhere it is articulated vnth the stem. The experiments vvliich have been made upon its properties, have been performed, therefore, in the latter situation ; but the description of their results will apply equally well to the rest. The intumescence consists of a succulent tissue, which, on the upper side, appears very dis- tensible, and on the loAver very irritable. In the usual position of the leaf or leaflet, the distension of the tAvo sides seems equally balanced; but any means which causes an increase of fluid on the upper side, or a con- traction of the vesicles on the lower, Avill obviously give rise to flexion of the stalk. The latter effect may be readily produced by touching that part of the intumescence itself; and then the leaf or leaflet wiU be depressed by the contraction of the part immediately irritated, just as in the case of the stamen of the Berberry. The same result follows the stimulation of this part by an electric spark, by the concentration of the sun's rays upon it with a burning glass, or by chemical agents; and if, instead of applying a temporary stimulus, whose effect is speedily recovered from, a notch be made in the lower side of the intumescence, the balance between its resistance and the expansive tendency of the upper side is then permanently destroyed, and the stalk remains depressed. Now, supposing the lower side to be in its usual condition, flexion of the stalk may result also from Avhatever distends the vesicles of the upper part of the intumescence; and this is the mode in Avhich the movement is usually effected. For a stimiilus applied to any part of the leaf will cause a con- traction of its vesicles; and the fluid expelled from them is carried by the circulating system to the distensible portion of the intumescence belonging to each leaflet, and to that of the petiole itself. The experiments of Dutrochet have completely established that it is to the vasctdar system alone that this propagation of stimulus is due; and these harmonise most completely with what was previously known of the influence of this system in the vegetable economy. 557. It appears, then, that these evident motions are readily explicable on the supposition that contractility is a property of various tissues of plants, and that this may be excited by stimuli of a physical nature. To suppose more, Avould be unphilosophical because unnecessary. There are other movements, hoAvever, arising fi-om causes Avhich originate in the system itself, of Avhich some notice should be taken. Such are, the fold- ing of the floAvers and drooping of the leaves, knoAvn as the sleep of plants. These phenomena seem due to a diminution in the actiAdty of the vital processes by which the turgescence of the soft parts of the structure is maintained; and this diminution appears partly to result from the AvithdraAval of the usual stimuli, especially light, and to be in part of a periodical character. For it is found that artificial light and AA'armth 428 SPECIAL AND COMPARATIVE PHYSIOLOGY. ■will cause many flowers and leaves to erect themselves for a time; and tliat, by proper management, the usual periods may he altogether reversed. But the phenomenon cannot he altogether explained on this principle; since there are many plants of which the flowers only expand in the night, and which must he kept in darkness to prevent them from closing. Much would seem due to the law oi periodicity^ in conformity with which living beings in general appear to be organised (§ 157); for in almost all we find some periodical cessation or diminution of all the functions, which, although modified as to its period and degree by change in exter- nal circumstances, cannot be altogether done aAvay with. One other spontaneous vegetable motion may be instanced, as of a very inexplicable character, — that of the Hedysarum gyrans, a Bengalese plant. Each petiole supports three leaflets, of which the central one is large and broad, and the two lateral ones, which are situated opposite to one another, small and naiTow. The position of the central leaflet appears peculiarly influenced by light: for in the daytime it is usually horizontal; by the action of strong solar light it is raised towards the stalk; whilst in the evening it bends downwards; and it is manifestly depressed if placed in the shade only for a few minutes. The small lateral leaves are in inces- sant motion; they describe an arch forwards towards the middle leaflet, and then another backwards towards the footstalk; and this by revolving on their articulation with the petiole. They pass over the space in 30 or 40 seconds, and then remain quiet for nearly a minute; the leaflets do not move together, but in opposite directions, one usually rising while the other is sinking; the inflexion downwards is generally performed more rapidly and uniformly than that upwards, which occasionally takes place by starts. These movements continue night and day; being slower, how- ever, in cold nights, and more rapid in warm and moist weather. They seem less affected by mechanical or chemical stimuli than do those of any other plant; and continue for a longer time in separated parts. 558. One class of spontaneous vegetable movements has been shown by Dutrochet to be due to the action of Endosmose (§ 244) in the organs which execute them. This is particularly the case in various seed-vessels, which burst when ripe in such a manner as to eject their contents with force, — as in the instance of the Momordica elaterium (common squirt- ing-cucumber.) His experiments upon the capsule of the Balsam termed Irnpatiens noli-me-tangere are particularly interesting. The valves of this capsule, when the fruit is ripe, suddenly spring from each other and curl inwards, scattering the seeds to some distance. Now an examination of the tissue of the valves shows that the outer part consists of much larger vesicles than the inner; and that the fluids contained in it are the densest. By the law of Endosmose, the fluids contained in the tissue of the interior will have a tendency to pass into the vesicles of the exterior; and it will distend them in such a manner as to produce a disposition in SENSIBLE MOTIONS OF LIVING BEINGS. 429 that side to expand, wlien permitted to do so, whilst the inner side has an equal disposition to contract. This at last occurs from the separation of their edges consequent upon their ripening; and then each valve rolls inwards. If, however, the valves be placed in a fluid more dense than that contained in the exterior vesicles, such as syrup or gum-water, these will he emptied on the same principle, and the valves will become straight, or even curl outwards. 559. For the rapid and energetic movements which the purposes of animal existence require, a special tissue, the muscular, is endowed in a very high degree with the property of contractility; and provision is made in the nervous system for calling that property into exercise, either in obedience to the will, or to external stimuli acting on remote parts of the organism. It was formerly shown that muscular tissue exists in two con- ditions; and that the form which it presents in those parts of the appa- ratus of organic life, in which it is introduced for particular purposes (§ 31 & 43), is much less characteristic than that which it possesses in the locomotive or animal organs. In the former case it is excited to action, like the contractile tissue of plants, by stimuli immediately applied to it; thus, the movements of the alimentary tube, fi'om the stomach downwards, are solely dependent upon the contact of its contents with the mucous membrane; and a stimulus applied to any of its fibres, excites a continous action along their course for some distance. There is no reason to believe that this automatic action is dependent upon nervous influence; although it cannot be doubted that it is much affected, like the nutritive processes, by the condition of the corporeal and mental system. It is obviously necessary that this communication should exist, to maintain harmony of action throughout the whole machine. The sympathetic nerve appears to be its channel; and the action of the heart is, as every one knows, peculiarly liable to be aflfected by variations in the state of mind or body." 560. By the contraction of muscular fibre, in obedience to the stimulus of innervation, are produced the movements of the locomotive apparatus * It will be perceived that the Hallerian doctrine of irritability, as a ■wis imita or independent property of muscular fibre, is here unreservedly adopted, in opposition to that which maintains that not only contraction is produced by the stimulus of nervous influence, but that the pro- perty of contractility is communicated by the operation of the nervous system. It would have been foreig'n to the purpose of this work to have entered upon a full discussion of this very interesting question ; but it is hoped that it will appear that the doctrine here maintained is consistent with itself, and with the analogies drawn from Vegetable life, as well as with what is known of the vital endowments of other tissues. Rloreover it is supported by the latest and best-conducted experiments. Thus, Dr. J. Reid has shown that the exhausted irritability of a muscle is recovered as speedily when its nerve is divided, as when it is entire, provided that its nutrition be not impaired j and Dr. Madden has ascertained, on the other hand, that narcotics acting- through the nerves, destroyed their power of stimulating the muscles, long before the irritability of the muscles themselves was impaired. See 4th and 7th Reports of British Association. 430 ' SPECIAL AND COMPARATIVE PHYSIOLOGY. by which the relation of the organism with the external world is effected; as well as those motions in the system itself which are indirectly con- cerned in the maintenance of the organic functions, such as those of Respiration. But the fibre, although subjected to a new and special stimulus, is not insensible to the more general one, for a mechanical or chemical application will occasion its contraction; but this change is confined to the fibre stimulated, and is not propagated by continuity as in the case just mentioned.* All the movements of the fabric in general appear, in the higher animals at least, to be strictly under the control of the will; and hence the muscles which execute them are usually termed voluntary. Those concerned in maintaining the organic functions, on the other hand, though capable of being more or less controlled and directed by the will, are not dependent upon it, and may take place in opposition to it; thus, the acts of Respiration cannot be restrained by any effort of the will, beyond a certain period (§591). In these cases, the nervous system appears to act simply the part of a conductor, conveying to its central organs the stimulus which its sentient extremities have received, and transmitting downwards a motor influence in respondence to it (§ 592). Now as almost every muscle in the body may be excited either by this direct stimulus, or by one acting through the will, the decision as to its voluntary or involuntary character obviously depends upon the relative frequency and force with which these two modes are brought into opera- tion. Thus, the diaphragm is constantly being called into involuntary action, and is, comparatively, but little influenced by the will; whilst the muscles of the limbs are rarely the subjects of involuntary stimulus, and are at all other times completely under the control of volition. * An interesting' remark on this question occurs in the writings of Galen, which shows the correctness of his views on the subject of muscular action. He observes that the relation of nervous action with muscle constitutes that an animal organ, which, as far as its own structure and properties are concerned, is a physical organ only, (that is, belongs to the apparatus of organic life). As to the mechanical adaptations by which the force generated by muscular contraction is brought into such varied and advantageous operation, space forbids anytliing being here added to what has been already stated in the Introduction regarding the means of locomotion possessed by different classes of animals. Many interesting details on this subject will be found in Roget's Physiology, vol. i. FUNCTIONS OF THE NERVOUS SYSTEM. 431 CHAPTER XVI. FUNCTIONS OF THE NERVOUS SYSTEM. 561. A general view of tlie structure and offices of the nervous system in Animals has already been given (§ 44, 230); and it has been stated that there is no valid reason to believe that anything analogous to this system exists in Vegetables (§ 222, 554). The foUoAving chapter Avill, therefore, be devoted to the consideration of the principal forms which it presents in the Animal kingdom, and the Functions to which it ministers. By the nervous trunks a communication is maintained between all parts of the fabric to which they are distributed, and certain central organs, in which the changes take place that immediately give rise to sensation or originate motion. These centres are the parts termed the hrain and spinal cord in Vertebrated animals; but they consist of several distinct organs, which are found in a separate form in the inferior classes, and are termed ganglia. Just as it is the function of the absorbents to convey to the centre of the circulation, from all parts of the surface or the interior of the body, the fluid Avhich they have absorbed, is it the property of cer- tain of the nervous fibrils to transmit to the central sensoriu?n the changes produced at their extremities. Until the mind becomes conscious of these impressions, no sensation is produced; and, to whatever motor changes they may give rise, as long as the mind is unconcerned in them, their character is the same. "We shall hereafter see (§ 594) that there is a very important class of muscular movements in the animal body, the excitement of which is quite independent of any mental influence. The nature of the sensations produced mil obviously depend upon the charac- ter of the impressions propagated to the sensorium; and this is, no doubt, modified by the peculiarities in the origin of the difi^erent sensory nerves. In the skin, it would appear that the bundles of fibres which supply it subdivide and ramify most minutely, so as to form a very close and beautiful network, in which no free extremities can be detected. In the nervous expansions, however, Avhich form the essential part of the organs of special sensation (§ 585), it appears that the nerve divides at once into its ultimate fibres, and that these run side by side without interlacement, each terminating in a little enlargement or jyapilla, on which the impression is probably made. Analogy Avould lead to the belief that similar terminal fibrils exist in the papillae of the skin and tongue, which seem principally composed of vascular structure enclosing nervous twigs. The network in the substance of the skin is not formed by the inosculation of the ultimate fibres themselves, which seem never to unite; but by the separation and reunion of the larger fibres, which consist of fasciculi of those more minute. 432 SPECIAL AND COMPARATIVE PHYSIOLOGY. As far as is at present known, it seems that each fibre runs a distinct course from the circumference to the central organs; and that it termi- nates in the grey matter which is found in all ganglia, and Avhich may, indeed, he regarded as essentially constituting them. This principally consists of vascular structure, with Avhicli the nervous fibres are brought into peculiar connection; but what is the precise relation betAveen them is not yet ascertained. The motor fibres, which originate in the same part, run towards the circumference, and convey to the muscles the in- fluence originating in the centre. They also seem to maintain a perfect separation through their entire course; although their trunks occasionally anastomose and exchange filaments with one another. Each trunk, on reaching the muscle to which it is distributed, sends out successive branches, which run across the course of the muscular fibres, and then, bending in\A^rds so as to form loops, return to the trunk again. Of the mode in which the sensory impressions are propagated from the circumference to the centre, and the motor stimulus from the centre to the circumference, physiologists are as yet entirely ignorant. Many have supposed that it is by a movement of the fluid which the nervous tubes contain, — an idea which derives some support from the fact that the conducting power of a nerve is destroyed by tying it, whilst it is still capable of propagating a current of electricity. Of the changes immediately concerned in the production of impressions and sensations, we are, if possible, still more ignorant, having no facts Avhatever on which even to build an hypothesis. What has been hitherto said refers to the division of the nervous system concerned in the reception of im- pressions, the production of sensations, and the stimulation of muscles to contraction; and as these are all purely animal functions, it has been called the nervous system of animal life. There is another set of nerves, however, which constitute what is termed the sympathetic or visceral system; this is distributed to the various nutritive organs, and is evidently connected with the functions of organic life, although, on the exact degree to which it participates in them, physiologists are not yet agreed. Reason will hereafter be given for the belief that it is not concerned in the sympathetic movements of the voluntary muscles, as was formerly supposed; but there can be little doubt that it is the vehicle of the sym- pathetic communication between the organs of nutrition, secretion, &c., and of the involuntary action of the mind upon them. This is sometimes called the nervous system of organic life; but we must not be misled by this expression into the belief that the organic functions are dependent upon its action (§ 222). 562. The group of acrita is regarded as comprehending those classes in which no definite nervous system can be discovered. It is * For an account of the recent microscopical researches into the structure of the nervous system, see the Brit, and For. Med. Rev., No. xii. FUNCTIONS OF THE NERVOUS SYSTEM. 433 generally believed, liowevei', that, in the animals which belong to it, the nervous matter is present in a " diffused form" — that is to say, incorporated ■\vith the tissues; but it would be difficult to assign a valid reason for such a gratuitous supposition. An arrangement of this kind cannot be required to confer on the individual parts of the organism their vital pro- perties, since these exist to as great an extent in beings which are allowed to be entirely destitute of it, namely the entire Vegetable kingdom. The simplest office of a nervous system is, as we have seen (§ 559), to establish a communication between parts specially modified to receive impressions, and others particularly adapted to respond to them. Where every portion of the body has similar endowments, there can be no object in such a communication; just as, Avhere every part of the surface is equally capable of absorption, and every part of the tissue equally per- meated by nutrient fluid, there is no necessity for a circulating system. The motions exhibited by animals of these lowest classes would seem to be scarcely less directly dependent upon external stimuli than those of plants; being, in fact, the result of the general diffusion of that exalted degree of irritability which is restricted in most plants to particular parts of the structure. Thus, the contractile tentacula of the Hydra close upon any object placed Avithin their reach; but so does the fly-trap of the Dioneea; and it is not difficult to imagine that a similar mechanism may operate in both cases. At any rate there is no necessity for attributing such phenomena to a nervous system, when we can neither discover any traces of it, nor discern anything in them which cannot be accounted for in other ways. It may reasonably be asked, then, upon what ground this polype or any similar creature is regarded as belonging to the animal kingdom; and it is not easy to give a definite reply to such a question. Although, however, the greater part of the motions, not only of the indi- vidual members, but of the whole body, seem to be performed in obedience to such stimuli as govern the actions of plants, observation of the living poljrpe will show that all its motions are not of this character, but that some are probably to be reckoned as voluntary, and as indicating that consciousness on the part of the individual, which must, in the present state of our knowledge, be regarded as a peculiar characteristic of animal existence. On the other hand, it could scarcely be proved that the movements of the gemimdes of the Polypes and Sponges (§ 121) are of any higher character than those of the reproductive particles among the AlgEe; and the employment of cilia for the purpose can hardly be regarded as establishing such a distinction, since the movements produced by their action are known to be involuntary in the higher animals (§ 110). 563. We have at present no certain means, it must be acknowledged, of appreciating the degree of sensibility possessed by the lowest members of the Animal kingdom. The motions which follow the impressions of external agents are our only means of judging of its possession by a parti- 2f 434- SPECIAL AND COMPARATIVE PHYSIOLOGY. cular being; and the analogies which have just been mentioned seem to indicate that, if these motions are accompanied by sensation, they are not dependent upon it. Much error has probably arisen from comparing the manifestations of life exhibited by creatures of this doubtful character, with those of the highest animals ; and thence inferring the presence of a nervous system with its appropriate organs, because motions are witnessed in the former which bear some analogy to those of the latter. But, when it is considered how completely vegetative is the life of such beings, and how closely all their motions are connected with the performance of their organic functions, it would seem obvious that the general comparison should be made with plants rather than with animals; and that we should seek the assistance of principles of a higher character, only when those we already possess are insufficient to explain the phenomena. A nervous system would seem to be required only in a being possessed of a number of distinct organs, whose actions are of such a character that they cannot be brought into mutual relation, without a more immediate and direct communication than that afforded by the circulating system, which as we have seen, is the only bond of union between distant parts that plants possess. In the lowest and simplest animals, whatever degree of contractility exists, appears to be almost ec[ually diffused through the system; and we neither find any special sensory organs, adapting one part more than another to the reception of impressions, nor do we observe any portion of the structure peculiarly endowed -with the power of motion; neither can we discover anjrthing like a nervous system fitted to receive such impressions, and to excite respondence to them in distant parts. To use the forcible expression of Sir Gilbert Blane — "Mr. Hunter, by a happy turn of expression calls the function of the nervous system inter- nuncial. It is evident that some such principle must exist in the compli- cated system of the superior animals, in order to establish that connexion which constitutes each individual a whole." But where all the parts act for themselves, there is, as we have seen, no necessity for such an inter- nuncial communication; and consequently, althovigh when united their functions all tend towards the maintenance of the system to Avhich they belong, they are capable of being separated from it and from each other without these functions being necessarily abolished. It is thus that we may account for the divisibility of many of the animals belonging to the group under consideration, which shows, in a remarkable degree, an af&nity for the vegetable kingdom. 564. The Acrita, however, present links of transition to higher groups (§ 109); and the gradation of structure is manifested no less in the nervous system than in other organs. Thus, in the Actinia (§ 120) the base is traversed by nervous filaments, disposed in a radiated manner among the muscular partitions, and having small ganglia at intervals (Fig. 177); and thus is obviously sketched out the form in which this system appears FUNCTIONS OF THE NERVOUS SYSTEM. 43.5 in the Radiated classes.* And, among the higher steeelmintha (§ 111), such as the Echinorh^/ncus, nervous filaments may be detected traversing the body longitudinally, and thus conducting us towards the Articulated series. In the polygastrica no connected nervous fibres have been cer- tainly traced; but red spots may be frequently observed, which, from their resemblance to the eyes of animals a little higher in the scale, are supposed to be visual organs. This is, however, but a conjecture; since, although many of the motions of these animals are obviously influenced by light, it is impossible to say that this agent does not act upon them in the same manner as upon Plants. 5Q5. Among the Radiata we find these rudiments gradually assum- ing a more distinct and complex form. It is probable that a connected nervous system exists in all the acalephje, although the softness of their tissues renders it difficult of detection. According to Ehrenberg, two nervous circles may be detected in the Medusa; — one running along the margin of the mantle, and furnished mth eight ganglia, from which fila- ments proceed to the eight red spots which he supposes to be eyes,^ — whilst the other is disposed around the entrance to the stomach, and furnished with four ganglia, from which filaments proceed to the tenta- cula. In the Beroe it is stated by Dr. Grant that a nervous ring exists round the mouth, furnished with eight ganglia, from each of which a fila- ment passes towards the other extremity of the body, while others are sent to the lips and tentacula. It must be acknowledged, however, that it is very difficult to arrive at certain conclusions as to the characters of such organs in animals whose texture is so delicate; and so many mis- takes have been committed, that it would seem better to wait the results of more extended enquiry, before the exact characters of the nervous system in this class shall be decided on. In the echinodermata, how- ever, its manifestations are much less equivocal. In the Asterias, for instance, we find a ring of nervous matter surrounding the mouth (Fig. 178), and sending three filaments to each of the arms; of these one seems to traverse its length, and the two others to be disti'ibuted on the coecal prolongations of the stomach. In the species examined and figured by Tiedemann, no ganglionic enlargements of this ring seem to exist; but they are usually evident at the points where the branches diverge. In the Echinus the arrangement of the nervous system follows the same general plan; the filaments which diverge from the oral ring being distributed (in the absence of arms) to the complicated dental apparatus, whilst others pass along the course of the vessels to the diges- tive organs. This apparatus seems, therefore, to unite in itself the cha- racters of the two nervous systems which are distinct in higher animals; * This description and fig'ure are given on the authority of Spix (Ann. du ]Mus6e, torn, xiii); many other observers, how^ever, have denied that any connected nervous system exists in this animal. 2 P 2 436 SPECIAL AND COMPARATIVE PHYSIOLOGY. one being subservient to the functions of animal life, and the otter being connected with the maintenance of the several vital actions. The transi- tion between the Radiata and Articulata, presented by the Holothuria and Siponculus, is peculiarly w^ell marked in the nervous system of these animals; for the ring which encircles the mouth is here comparatively small, but two filaments traverse the length of their prolonged bodies, running near its abdominal surface which is their situation in the Arti- culated classes (§83, note). 566. It is peculiarly interesting to compare the character of the nervous system of the Radiated classes with that of higher animals of more heterogeneous structure. We here find the body consisting of a number of parts, of which each is similar to the rest; and each is con- nected with a distinct ganglion, that seems subservient to the functions of its own division alone, and to have little communication with the rest. This is the case, indeed, not merely with the tribes we are now consider- ing, but with the lower vermiform species; the only difierence being, that the individual portions are here disposed in a radiate manner round a common centre, whilst in the latter they are longitudinally arranged. But when the different organs are so far specialised as to be confined to dis- tinct portions of the system, and each part consequently becomes possessed of a different structure, and is appropriated to a separate function, this repetition of parts in the nervous system no longer exists; its individual portions assume special and distinct offices; and they are brought into much closer relation to one another by means of the commissures or con- necting fibres, Avhich form a large part of the nervous masses in the higher animals. It is evident that, between the most simple and the most complex forms of this system, there must be a number of inter- mediate gradations, — each of them having a relation with the general form of the body, its structure and economy, and the specialisation of its distinct functions. This will be found, on careful examination, to be strictly the case; and yet, with a diversity of its parts, as great as exists in the conformation of any other organs, its essential character will appear to be the same throughout. 567. Among the Molluscous classes, no repetition of parts like that just described can be said to exist; and the nervous system partakes of the general want of symmetry in the body, which seems so characteristic of the predominance of the vegetative organs in these animals (§ 138). Its ganglionic centres are principally disposed round the mouth, since its actions appear destined to little else than the supply of the digestive organs; and their size is usually proportional to the development of the organs of special sensation which are connected with them, and to the energy of the masticatory movements required for the reduction of the food. Where, however, unusually active powers of locomotion are pos- sessed, we commonly find ganglia situated in the neighbourhood of the FUNCTIONS OP THE NERVOUS SYSTEM. 437 organs clestiiiecl to serve this piirj)ose. From the circular arrangement of the nervous centres around the mouth, the term cyclo-gangliate has been applied to this form of the system; but, as this arrangement is by no means constant, the designation hetero-gangliate^ which implies their irre- gular disposition, is perhaps to be preferred. In the Tunicata, the nervous system exists under a very simple form. We find in the Cynthia (Fig. 83), for example, a single small ganglion, /, situated between the two openings of the mantle. This sends two branches which encircle the oral aperture, «, giving ofi" filaments to its sensitive tentacula, and meet again beyond it; they then continue as a broad cord, g^ along the back of the mantle, and are connected Avith other ganglia situated among the viscera, which seem to form part of the sympathetic system of nerves. In the coNCHiPERA we may trace the same general plan, but a much higher development of the nervous centres. In Fig. 179 is sketched the nervous system of a Unio^ where are seen two quadrangular ganglia, lying aboA^e the oesophagus, and connected by a transverse fibre. From these are given off filaments to the mouth, others which run laterally to the edges of the mantle, and others that descend among the viscera, to which they give branches in their course; these last unite again to form one or two ganglia, which are always larger than the anterior masses, and which supply the whole posterior part of the animal, and the outlet of the mantle, with nervous filaments. This animal belongs to the order pos- sessed of a double adductor muscle (§ 102), and exhibits more lateral symmetry than the species possessed of only one, of which the common Muscle (Mytilus edulis) is an example. In this animal (Fig. 180) we find two ganglia lying in proximity mth the mouth, and connected by a filament which encircles the oesophagus and sometimes forms a small ganglion above it; another ganglion is occasionally formed in the same manner just beneath them. Nervous columns are sent from each lateral ganglion along the body; and these approximate in the situation of the foot, where they form another pair of ganglia, connected by a tranverse filament, and varying in size with the development of that organ, Avhich they supply Avith nerves. The columns continue their separate course backwards, and again approximate in the neighbourhood of the adductor muscle, where a third pair of ganglia is situated, which is often, however, united into one mass; from this filaments proceed, which supply the muscle, the outlet of the mantle, and all the posterior parts of the body. Besides these, small ganglia have been observed on the filaments of the visceral nerves; and this system becomes more distinct from the moto- sensitive, being connected with it only at particular points. .568. As the head is not otherwise indicated, in these two classes of MoUusca, than by the position of the mouth, and does not possess any organs of special sensation, it is not to be wondei'ed at that the ganglia connected Avith the oesophagus should not be larger than those of other 438 -SPECIAL AND COMPARATIVE PHYSIOLOGY. parts of the body, and should be even inferior in size to those more con- nected with powerful and active muscles. But in the higher classes it is very different; and in proportion as we meet with evidence of the posses- sion of the senses of sight, hearing, &c., do we observe a greater concen- tration of the ganglionic system towards their neighbourhood. Although eyes have been asserted to exist among some of the more active Conchifera, they are not confined to any single part of the body, but are disposed along the free margins of the mantle, — an interesting intermediate condition between the diffused sensibility to light, which is probably possessed by the whole of the surface in the inferior tribes, and the concentration of the sense into one portion of it observed in other cases. Among the GASTEROPODA, two cycs only exist, and these are placed on the anterior part of the body, in the neighbourhood of the mouth. In the lower species of this class, however, the general distribution of the nervous system is not very dissimilar to that which has been last described. Thus, in the Carinaria (Fig. 181) we observe lobed ganglia, connected by a transverse band, lying at the sides of the oesophagus; and these send off the optic nerves and tentacular filaments. Besides other branches trans- mitted to the nighbouring organs, two principal trunks are sent backAvards (as in the Muscle), which unite in a large ganglion situated among the viscera; from this, nerves proceed to the foot and posterior part of the trunk. A separate set of visceral filaments, connected but at one point with the S3rmmetrical system, has also been described. In the Bulla (Fig. 182) however, we find the oesophageal ganglia much larger in pro- portion to the abdominal; and the nervous matter forms a kind of collar encircling the oesophagus, so that a considerable portion is above that canal, and may be regarded as approximating in character to the brain of higher animals. Another small ganglion is situated anteriorly to this ring; and two of considerable size, connected with the cephalic ganglia by large cords, are found in the neighbourhood of the foot. Other small ganglia are disposed among the viscera, and seem to belong to the sym- pathetic system. In some other species of this class, the nervous system attains a still higher grade of development; the greater part of its gan- glionic centres being placed above the oesophagus, and the foot as well as the rest of the body deriving its nerves from this mass, instead of from a subordinate ganglion, 569. The nervous system of the Cephalopoda exhibits an obvious approach towards that of vertebrated animals, in the concentration of the cephalic ganglia into one mass, which, though stiU perforated by the oesophagus, lies almost entirely above it, and is sometimes protected by plates of cartilage which constitute the rudiment of a neuro-skeleton (§82). In the JVautilus, however, and other species composing the inferior order of this class, the general distribution of this system cor- responds pretty closely with that seen in the higher Gasteropoda. The FUNCTIONS OP THE NERVOUS SYSTEM. 439 oesopliagus is still encircled with a ganglionic ring (Fig. 183), of which the upper part gives off the optic nerves, whilst the lower supplies the mouth and tentacula, and sends trunks backwards into the shell. The trunk which supplies the internal tentacula, and what is regarded as the olfactory organ, has a small ganglion situated upon it ; and other ganglia, which probably belong to the sympathetic system, are found on the nerves distributed to the viscera. In the Cuttle-fish, and other naked species, whose habits are more active and general organization higher, we find a somewhat different arrangement (Fig. 184). The organ of vision here attains an increased development and importance ; an organ of hearing evidently exists ; and the whole surface of the body is pos- sessed of sensibility. The cerebral mass, therefore, attains a much increased size, and several smaller ganglia, connected with the organs of sense, are found in its neighbourhood. The portion of the oesophageal collar that remains below the aperture for the passage of the tube is now relatively small. From it proceed outwards two large trunks which pass to the mantle, and which enter two ganglia before their final distribution. Two central trunks pass from it towards the intestines ; and ganglia are found also upon the ramifications of these, which probably belong to the sympathetic system. The appearance of ganglia on the nerves that supply the mantle is evidently connected with the increased locomotive powers possessed by that organ in the order we are considering ; and they are particularly evident in those in which the lateral fins are much developed. It is stated by Dr. Sharpey* that the nerves of the arms of the Cuttle-fish have a structure perfectly similar to that of the ab- dominal cord of the Articulata, — consisting of two pairs of trunks, one of which has ganglionic enlargments corresponding with the suckers, whilst the other passes over these without contributing to their formation. It would seem probable, from considering the origins of the cephalic nerves in this class, that the greater part of their cerebral mass is to be regarded as analogous to the optic lobes or ganglia of Yertebrata, which will be seen to constitute the largest portion of the brain in many Fishes. The infra-oesophageal part, from which the auditory and respiratory nerves arise, and which is continuous with the two large trunks dis- tributed to the system, probably correspond with the medulla oblongata (§ 578). "We do not perceive any part analogous to the spinal cord of Vertebrata, which is an organ possessed of independent poAvers distinct from those of the brain, and to which, therefore, a mere nervous trunk, however large, cannot be rightly compared. "We have traced, in the Cephalopoda, the highest development of a nervous system formed to minister to the nutritive functions only ; we shall now follow that of the Articulata, in which the locomotive powers are so predominant; * Miiller's Physiology, p. 676. 440 SPECIAL AND COMPARATIVE PHYSIOLOGY. and we shall afterwards find that the Vertebrata combine the types characteristic of both. 570. The plan on which the nervous system is distributed in the sub-kingdom Articulata exhibits a remarkable uniformity throughout all its classes; whilst its character gradually becomes more elevated as we trace it from the lowest to the highest divisions of the group. It usually consists of a double nervous cord, studded with ganglia at intervals ; and the more alike the different segments, the more equal are these ganglia. The two filaments of the nervous cord are sometimes at a considerable distance from one another, and their ganglia distinct (Fig. 197) ; but more frequently they are in close apposition, and the ganglia appear single and common to both (Fig. 189). That which may be regarded as the typical conformation of the nervous system of this group is seen in Fig. 190, which shows the ganglionic cord of the Scolopendra (Centipede). This is shown to run from one extremity of the body to the other, and to present nearly the same proportions throughout ; each ganglion is in connection with one segment, and has little to do with any others ; the two filaments of the cord diverge towards the head, to enclose the oesophagus, above which we find a pair of ganglia that receive the nerves of the eyes and antennse. We shall find that, in the higher classes, the inequality in the formation and ofiice of the different segments, and the increased powers of special sensation, involve a con- siderable change in the nervous system, which is concentrated about the head and thorax, and thus approaches that of Vertebrata. And, in the simplest Vermiform tribes, Ave lose all trace of ganglia, the nervous cord passing without enlargement from one extremity to the other. In all of the Articulated classes, the nervous cord appears to run, not along the back, as in Vertebrata, but along the abdominal surface of the body. This anomaly is explained, however, by the fact formerly mentioned (§83 note), that all the organs in these classes appear similarly inverted, so that they may really be regarded as in a corresponding position with those of Vertebrata, when the animal lies upon what is commonly called its back, but which is really its abdomen. This view is supported by the relative position of what are believed to be the inotor and sensory portions of the nervous cord in these classes. When we examine into the structure of this column, wherever it is well developed, Ave find that it consists of tAvo distinct tracts ; only one of these enters the ganglia ; the other passes over them (Fig. 192, b, c). The first is usually regarded as the sensory, the second as the motor column. The parts of the spinal cord in Vertebrata appropriated to these ofiices are so disposed, that the sensory tract lies nearest the surface of the back, and the motor column in proximity to the viscera. This corresponds Avith AA'hat Ave find in the Articulata, when the general inversion of their bodies is alloAved for ; FUNCTIONS OF THE NERVOUS SYSTEM. 441 since the ganglionic portion of tlie cord is nearest what appears the abdominal surface of the animal, and the motor column lies upon it. Besides these tracts, however, another usually exists, which lies between the motor column and the viscera ; this, too, passes over the ganglia without entering them ; and its nerves, Avhich are principally distributed to the respiratory organs, usually come off at intermediate points. The relative position of these parts is explained by Fig. 192, b, which represents a ganglionic portion of the cord viewed on the side in contact with the viscera, and shoAvs the narrow respiratory tract lying on the motor column, and this again passing over the ganglion which belongs to the sensory portion alone ; and c gives a side vicAV of the same. Nothing precisely corresponding to this respiratory tract is found in the spinal cord of Vertebrata, since the respiratory system is not in them distributed so remarkably throughout the whole body ; a portion of a large and very important nerve, the par vagun, which arises from the upper part of the spinal cord, and is distributed to the lungs, would seem to be its real analogue. 571. A very brief sketch of the gradual development of this system in the lower classes of Articulata will be here sufficient ; since it is in Insects that its chief peculiarities are manifested. In the Strongylus, one of the entozoa, we find (Fig. 185) a single cord running from one extremity of the body to the other, but separating into two portions to encircle the orifices of the alimentary canal. This is destitute of ganglia; but it sends off slender filaments, at short intervals, which encompass the body. In the lowest Annelida, such as the earth worm (Fig. 186), the nervous system is almost exactly similar, except that two distinct ganglia are found anterior to the oesophagus, from Avhich nerves proceed to the mouth. In the rotifera, notwithstanding their minuteness, a nervous system may be distinctly traced. Fig. 187 shoAvs that of the Hydatina, Avhich consists of a circle of ganglia surrounding the entrance to the alimentary canal, and giving off filaments to the poAverful muscles of the jaws and to the ciliary apparatus of the wheels, and of a nervous cord that proceeds backwards to the posterior extremity of the body. In the species just mentioned, this cord is single and destitute of ganglia; but in others it is evidently double, and one or tAVO pairs of ganglia exist upon it. In the cirrhopoda we find another variety in the dis- tribution of the nervous system, the same essential type, hoAA'ever, — the double ganglionic cord — being retained ; and it Avas the discoA^ery of this conformation that first led to the suspicion that these animals should be classed with the Articulata, and not as formerly AAdth the Mollusca. At Fig. 188 is shoAvn the nervous system of the common Barnacle (Anatifa). A slender nervous ring surrounds the oesophagus, and sends filaments to the neighbouring parts, but scarcely forms a ganglion above it, — this creature being, in its fixed adult state, destitute of the eyes 442 SPECIAL AND COMPARATIVE PHYSIOLOGY. and antennae wWcli it possessed when in the condition of free-moving Crustacea (§ 92). On the columns which traverse the body, ganglia are developed at the base of each pair of members. In the higher forms of the ANNELIDA we are led to the condition of the nervous system which has been spoken of as t3rpical of the group of Articulata ; foi% whilst the soft-skinned species, in which there are neither organs of special sensation, nor distinct members for propulsion, have, like the Earthworm, scarcely any ganglionic enlargements on the nervous cord, the higher tribes, in which the division into segments becomes distinct, and in which the animal relies for locomotion more upon the action of its members than upon that of its trunk, have ganglia regularly disposed at intervals corresponding with the division into segments. This con- formation is shown in Fig. 189, which is the nervous system of the Aphrodita (sea-mouse) ; where we perceive two small supra-oesophageal ganglia, corresponding with the imperfectly-developed eyes and antennae of this animal ; and a series of ganglia disposed along the cord with considerable regularity, becoming smaller and closer, however, as they approach the posterior part of the body. There is evidently but little difference, except in the relative development of the cephalic ganglia, between this system and that of the myriapoda just described (Fig. 190). Whilst the symmetrical system is thus attaining an increased develop- ment, traces of the sympathetic or visceral system present themselves, in the form of nervous filaments embracing the dorsal vessel, and lying among the viscera ; and these are occasionally found to be possessed of minute ganglia. 572. The nervous system of insects, like the rest of their organs, presents very different aspects at the different stages of their metamor- phosis; and these have a peculiarly interesting relation with the general characters and habits of the animals. The Larva or caterpillar, it has been formerly stated (§ 86), may be regarded as, in almost every respect, on a level with the higher Annelida; all its segments are equal, or nearly so; all are usually provided with legs, and alike concerned in the function of locomotion; and its nervous cords, with their ganglia, are consequently disposed with great uniformity. The number of segments being ahvays 13 (including the head as one), that of the ganglia is usually the same. The cephalic ganglia, placed in front of the oesophagus, are small in pro- portion to the size they subsequently attain (Figs. 191 and 194), in con- formity with the low development of the organs of special sensation. The first ganglion of the trunk, placed immediately beneath the head, sends nerves to the first pair of legs; and all the others are similar to it. In the Sphinx Ugustri (privet-hawk-moth), whose nervous system is repre- sented in these figures, the two last ganglia are consolidated into one, as frequently happens.* Throughout the whole column of the larva, the * See Newport in Phil. Trans., 1832 and 1834. FUNCTIONS OP THE NERVOUS SYSTEM. 443 separation of its lateral halves is evident; and this is a character peculiar to the lower articulated tribes; for, in the perfect Insects, Crustacea, &c., its divisions approximate so closely as to leave no space between them. The small respiratory filaments are seen to come off a little above the ganglionic nerves, and these are distributed to the stigmata, and to the muscles concerned in respiration, whilst the others ramify on the general surface and supply the locomotive organs. Besides these systems, how- ever, another may be detected, which appears to have its analogy in Vertebrata. At Fig. 191, A, is an enlarged representation of the cephalic ganglia and oesophageal ring of the larva of the Sphinx; and two filaments are shoAvn to proceed from the lower side of these ganglia, and to meet in a small central ganglion from which a nervous trunk proceeds. This trunk passes downwards along the oesophagus and stomach, on the walls of which its branches are distributed; and it appears to correspond with the portion of the par vagum which has a similar distribution in Yerte- brata. In the latter sub-kingdom, the par vagum supplies the lungs and heart as well as the stomach; but it is not surprising that the extended character of the respiratory organs in Insects should have occasioned the amplification of the part of the nervous system appropriated to it, into what is apparently a distinct portion of the apparatus. Besides this, we observe two small ganglia connected with nerves which come off on the side of the cephalic ganglia, and these appear to belong to the true sympathetic or visceral system, which here becomes connected with the sensori-motor nerves, sends filaments to the organs of sense, and commu- nicates with the respiratory nerves, just as in Vertebrated animals. 573. When the larva is about to assume the Pupa state, a very remarkable series of changes takes place in the nervous system, the result of which is shown in Fig. 192. The ganglia are rapidly approxi- mated, in accordance with the sudden diminution in the length of the body; but the cords themselves are not yet shortened, so that they assume a sinuous form, and, in the thoracic region, the lateral halves are more widely separated than before. No great change is yet seen in the ganglia themselves; but the oesophageal ring is much contracted; and the filaments proceeding to the rudimentary wings, which now make their appearance, begin to attain a considerable size. At Fig. 192, a, is an enlarged representation of a portion of the thoracic column, showing the transverse or respiratory nerve lying on the median line (whilst the sensori-motor cords diverge), and sending off its lateral branches between the ganglia. The Sphinx ligustri remains for several months in the Pupa state; and the progressive changes in its nervous system may, therefore, be very advantageously watched. It appears that, between the time of the first and that of its second metamorphosis, very considerable changes gradually take place, which all tend toAvards its final development. At Fig. 1 93 is represented what may be regarded as its characteristic form 444 SPECIAL AND COMPARATIVE PHYSIOLOGY. in the pupa state. It is seen that the inter-ganglionic cords have now adapted themselves to the shortened dimensions of the hody, and that they lie straight as in the larva. The cephalic ganglia are shown to have greatly increased in size, and to he in such close proximity with the first ganglion of the trunk, that the oesophageal aperture is now much con- tracted. The second and third ganglia of the trunk, from which the nerves pass to the wings, are considerably enlarged; whilst the fourth and fifth have coalesced into one mass, to which the sixth also closely approxi- mates. The abdominal columns are but little altered; their ganglia, however, are now somewhat smaller in proportion to the rest. 574. The condition of the nervous system in the Imago or perfect insect is shown in Fig. 194. The cephalic ganglia have now undergone an enormous increase in development, the part connected with the eyes being particularly enlarged; and they extend over the oesophageal canal so much as to conceal it, uniting themselves closely with the first ganglion of the trunk. The second ganglion has entirely shifted its position and receded towards the middle of the thorax; the third has quite disap- peared, seeming to have coalesced in part with the second, and in part with the one below it, as well as with their connecting cords. The next ganglion seems to contain the nervous matter, — not only of the fourth and fifth, which have evidently coalesced to form it, — but of the sixth and seventh, which have become obliterated, though their nerves are still given off from the cord. The remaining ganglia have undergone but little change.* 575. We see, then, that the tendency of the metamorphosis is to concen- trate the ganglionic portion of the nervous system in the head and thorax; the former being the position of the organs of special sensation, the latter the situation of the locomotive system. A lateral concentration may be frequently observed, as well as a longitudinal one; for in some larvae the two cords are quite distinct, and are separated by a considerable interval; and these approximate in the Imago into a single column. There are many Insects in which the concentration is carried much farther than in the instance now described; the abdominal ganglia being almost entirely obliterated, and the nervous centres restricted to the head and thorax. This is partly the case in the Melolontha (cock-chaffer), whose nervous system is represented in Fig. 195. The cephalic ganglia * The origins of the nerves supplying- both pairs of wings have here united ; and the same structure is found in the Bee and other Hymenoptera remarkable for rapid flight. On the other hand, in many Insects which are not remarkable for velocity or equability of motion, the nerves supplying each wing originate separately, and have little communication, just as in the larva of the Sphinx ; and in the Coleoptera, in which the upper pair, or elytra (§ 89, note), are motionless during flight, the nerves frequently remain entirely separate. Hence it is not unfairly argued by Mr. Newport that this common origin of the nerves is subservient to the uniformity and equability of the actions of the wings required in Insects of rapid and powerful flight. FUNCTIONS OF THE NERVOUS SYSTEM. 445 are here seen to have great lateral development, and to approximate closely to the first ganglion of the trunk. The small lateral ganglia, also, Avhich belong to the sympathetic nerve, are considerably developed. Three contiguous ganglionic masses exist in the thorax, from which nerves radiate to the Avings and legs, and others pass backwards into the abdomen, where no ganglia exist. The gi'eatest concentration exists, however, in the orders Homoptera and Hemiptera. In Fig. 196 is shoAvn the thoracic portion of the nervous system of the Banatra linearis^ a species of the former tribe allied to the common Notonecta or boat-fly ; and it is here seen that, besides the first, or infra-oesophageal ganglion, there is but one nervous centre in the trunk, fi-om which filaments are sent to the whole body. 576. The Crustacea present us with nearly as great a variety in the forms of the nervous system as do the Insect tribes. In some of the least-developed species of this class, the nervous filaments are scarcely perceptible. In many more, in Avhich the equality of the segments of the body indicates an affinity with the class Myiiapoda, the nervous system almost exactly resembles that of the Centipede or higher Annelida, This is the case in the Talitrus locusta (sand-hopper) whose nervous system is represented in Fig. 197. The tAvo cords are seen to be at an unusual distance from one another, and even the ganglia are widely separated, although connected by a transverse filament. The cephalic ganglia are but little larger than the rest, Avhich are very uniform in size and position. In higher orders, hoAvever, aa^c perceive, as in Insects, a ten- dency to the concentration of the ganglia in the thorax, and to the increase in the size of those representing the brain. This is seen in the Lobster, where, although none of the ganglia are obliterated, the last seven are small in comparison with the first five. But it is in the short-bodied Crabs that this concentration becomes most apparent. "We here find, as in some Insects, but one thoracic mass, from Avhich the Avhole of the trunk is supplied Avith nerves, as in the Maia squamado, Fig. 198; and this con- formation evidently leads us toAvards the Mollusca, in AA'hich there is a similar tendency to the concentration of the nervous matter around the oesophagus. The distribution of the nervous system in the arachnida is not dissimilar to that of the Crustacea — the Spiders of the sea. In the long-bodied Scorpions there is a large mass surrounding the oesophagus, formed by the union of the cephalic Avith the first thoracic or infra- cesophageal ganglion, from Avhich the nerves of the five pairs of legs are given ofF; and, posteriorly to this, are seven small ganglia disposed at regular intervals along the trunk. In the Spiders (Fig. 199), on the other hand, we find the cephalic ganglia distinct, but small; and these communicate Avith a large star-shaped mass in the front of the thorax, which appears to be formed by the union of at least four pairs of ganglia, and Avhich sends off nerves to the legs; from this proceeds a double cord 446 SPECIAL AND COMPARATIVE PHYSIOLOGY, which swells, at its termination, into an enlargement that gives off bran- ches to the other organs. 577. The principal varieties in the distribution of the nervous appara- tus in the Invertebrated classes having now been described, we are prepared to enter upon the consideration of its conformation in Verte- BRATA. It has been already remarked (§ 139) that, in this division of the Animal kingdom, the locomotive system of the Articulated classes may be regarded as united with the nutritive apparatus of the Mollusca; and this union is nowhere more remarkable than in the nervous system. We have traced in the latter group a circle of ganglia surrounding the oesophagus, specially connected with the organs of sense, and, therefore, with the function of nutrition; we have seen these becoming, in the higher species, almost supra-oesophageal; and the nervous cords which proceed from them conduct their influence to every part of the body, no other ganglia but those of the visceral system being developed, except where extraordinary locomotive powers exist. In the Articulata, on the other hand, we have seen that the ganglia connected with the organs of special sensation and surrounding the oesophagus, are usually quite sub- ordinate to those connected with the locomotive apparatus, in the neigh- bourhood of which the greatest concentration takes place; and that where this is diffused (as in the Annelida, Myriapoda, and larvae of Insects) throughout the whole body, each segment appears much more dependent upon its own ganglion, than upon any influence it derives from the cephalic mass, whose function is probably to harmonise and direct the actions of all. Now, in the Yertebrata we find both these types of struc- ture united; for the cerebral mass obviously corresponds with that of the higher Cephalopoda (the lowest Fishes scarcely exhibiting any advance in its character); whilst the spinal cord, being possessed of independent powers, must be regarded as something very different from a mere bundle of nerves, such as passes off from the cerebral mass in the Mollusca, and will be shown to correspond with the ganglionic cord of the Articulata. In tracing the development of the nervous system from the lowest to the highest forms it assumes in this division of the Animal kingdom, it is necessary, for the right understanding of its character, to lay aside all preconceived notions derived from the study of the human brain alone; since the extraordinary difference in the proportions of its parts, from those which Ave meet Avith elsewhere, Avould otherwise be a source of great confusion, 578. That which may be regarded as the most essential part of the nervous system in Vertebrata is the nervous cord commonly known as the spinal marrow, with its continuation in the cranium as far as its junction with the hemispheres of the brain (termed the medulla oblongata^ ; this is altogether called the cerebrospinal axis. With it all the nerves are connected ; — the sensory nerves terminating in it, and the motor FUNCTIONS OP THE NERVOUS SYSTEM. 447 nerves passing out from it. A ganglionic enlargement is always found in the neighbourhood of the junction of the sensory nerves with this cord, — sometimes on the column itself (Fig. 200), and sometimes on the nerves near their roots (Fig. 212). Those in connection with the nerves of special sensation, namely the optic and olfactory ganglia, are particu- larly large in many of the lower Yertebrata ; and they constitute the principal part of what is there known as the brain. Two distinct tracts may be discovered in this column, connected with the distinct functions of motion and sensation. All the nerves which minister to these functions conjointly, have roots arising from both columns, as shown in Fig. 212; and on the sensory roots of the spinal nerves, there is a ganglionic enlargement, in which the motor portion has no concern."' The motor column of the spinal cord is usually spoken of as the anterior one, from its position in man ; and the sensory column as the posterior. The former is in proximity with the viscera, whilst the latter is nearest the dorsal surface ; these evidently corresponding with the position of the two portions of the ganglionic column of the Articulata (§ 570.) 579. The portions of the nervous system which seem to be peculiar to the Vertebrated classes are the cerebral lobes or hemispheres^ and the cerebellum. The former (&, Figs. 200-219) constitute the mass of the brain in the Mammalia ; but in Fishes they are usually inferior in size to the optic ganglia, c. As we ascend from the lowest to the highest Yertebrated animals, do we observe an increased development of these organs with respect to the cerebro-spinal axis and the nerves and ganglia appertaining to it, which seems to bear a pretty close relation to the degree of intelligence of the animal ; their surface becomes convoluted (Fig. 211) so as to augment the quantity of cortical or grey matter ; and the complexity of the arrangement of the fibres of the medullary or white portion greatly increases. The cerebral lobes are connected with both tracts of the spinal cord ; and, fi-om the points of union, fibres may be seen diverging towards all parts of their surface. The Cerebellum {d, Figs. 200-219), which is always situated beneath the hemispheres, is an organ of whose precise functions we are obliged to confess our ignorance ; in the lowest classes it forms a single mass placed on the * The nerves arising' from the spinal portion of the cerebro-spinal axis have all double roots ; but those of special sensation, which take their origin within the skull, are not incorporated with any motor trunk ; and the motor nerves of the eye, and of the greater part of the face, are not united with any sensory filaments. The loss of the sensibility, or of the capability of motion, of particular organs, may be produced, therefore, by dividing the sensory or motor roots of their nerves, if these arise from the spine, or by dividing their distinct trunks, if their function be single ; and disease or injury of these parts produces corresponding effects. Cases of palsy of the face, in which the sensibility is retained whilst the muscular power is lost, or in which muscular power is retained and sensibility lost, are by no means rare ; but instances of the same affection of parts of the trunk are not so common, since any afleetion of a nerve in its course will here implicate both the motor and sensory filaments, wliich can only be separately acted on at their origins. 448 SPECIAL AND COMPARATIVE PHYSIOLOGY. median plane ; whilst, in the higher, it is divided into tAvo hemispheres. It is connected with both columns of the spinal cord ; and experiment leads to the belief that its office is in part to combine the individual actions of diflferent members into the complex and nicely-balanced movements required for progression of various kinds. We may now briefly glance at the relative development and position of these parts in the dififerent classes of Yertebrata. 580. In FISHES, although the head is generally large in proportion to the trunk, and its cavity capacious, only a small part of it is filled with the brain, which as yet appears but a slightly-developed prolongation of the spinal cord. The interval between the walls of the skull and the surface of the brain is filled with fluid contained in a closed serous mem- brane, the arachnoid'"' The most antei'ior of the ganglia contained in the head are those connected mth the olfactory nerves (a, a, Fig. 200) ; these are sometimes separated by peduncles from the rest of the brain, especially in cartilaginous fishes, such as the Ray (a, «, Fig. 202). Behind these are the hemispheres of the brain 5, 5, which are usually small in proportion to other parts ; they have no ventricles or cavities in their interior, nor convolutions on their surface. We next come to the optic lohes or ganglia, c, c, which are not unfrequently larger than the hemispheres ; these may be regarded as analogous to the principal part of the cephalic ganglia in Invertebrated animals ; but, in the higher classes, they will be seen to diminish in proportion to the development of the hemispheres. Even in the more powerful cartilaginous Fish, such as the sharks, the hemispheres are already so far prolonged back- wards as partly to conceal them. Behind these we find the cerebellum,, d, which is but a simple transverse band in the lowest cyclostome fishes, and bears but a small proportion to the optic lobes in the Conger (Fig. 201) and others of that tribe; whilst, in the muscular Rays and Sharks, it is prolonged forwards so as partly to cover the optic lobes, and backwards on the spinal cord. Still, however, it is only the central portion which is yet developed, the hemispheres being entirely absent. The spinal cord difi"ers much in its proportions in dififerent tribes of this class. In the Eel and other Vermiform fishes, it is of nearly uniform size throughout ; and, in the lowest of these, the cerebral ganglia are scarcely more prominent upon it than those of the leech or caterpillar. In proportion as distinct locomotive members are developed, do we find enlargements of the spinal cord corresponding with the origins of their nerves, just as in the ganglionic column of Insects ; and Avhere the * This is one of the many instances in which a condition which is the result of disease in man is found to be the natural state of some of the inferior tribes. Amongst other cases of the same kind, the adhesion of the heart to the pericardium, and the dilatation of the air- cells of the lung-s might be instanced ; the former being the natural condition in Fishes, and the latter in Reptiles, FUNCTIONS OF THE NERVOUS SYSTEM. 449 anterior members are very powerful, as in the Trigla (gurnard), these enlargements have an evidently ganglionic character (Fig. 200). Ac- cording to Mr, Owen, they are connected with sensory organs of peculiar character, superadded to the pectoral fins. In such species as the Lophius (frog-fish), in which the nutritive system is enormously developed at the expense of activity of locomotion, and the animal thus constructed more upon the Molluscous type, the nervous centres are confined to the neigh- bourhood of the head ; for the true spinal cord soon separates into a bundle of nerves which act only as conductors. Throughout the whole of the spinal column in Fishes, there is a canal which marks its division into two lateral halves, as in the Articulata; and this canal is particularly wide beneath the cerebellum, — the position in which the oesophagus passes through it in Insects, &c., — but is contracted in the higher classes into ikL^ fourth ventricle. 581. In REPTILES (Figs. 203-5) we observe a considerable advance in the development of the hemispheric ganglia, and proportional diminu- tion of those connected merely with the sensory nerves. The former contain hollows or ventricles within, into which their enveloping mem- brane is continued, and which, therefore, increase the general extent of surface. The cerebellum is still a simple mass but slightly developed in respect to the hemispheres. The nervous system of Batrachia, like all their other organs, presents, in the tadpole state, the characters of that of fishes ; and these are partly retained by the perennibranchiate species during the whole of their existence. In Fig. 218, are sho-wn the brain and spinal cord of a young tadpole; where the cerebral hemispheres, h, b, are shown to be of small size, and to be separated by a considerable interval from the optic ganglia, c, c ; whilst the cerebellum, d, is but a transverse band, and the spinal cord narrow, although slightly dilated in parts. As the members are formed, however, and the whole con- dition advances, the posterior and middle portions of the spinal cord, from which their nerves are derived, enlarge considerably (Fig 219) ; at the same time this column is shortened relatively to the length of the body, being withdrawn from the tail which formerly contained it ; and the development of the cerebral hemispheres proceeds, until, in the adult frog, the parts of the brain have the proportions represented in Fig 204. (The olfactory ganglia, which are here small, are not shown in the figure, being concealed by the hemispheres, in apposition with whose under surface they lie). In birds we find the centres of the nervous system attaining a greatly-increased lateral development, and filling up the whole of the cavity which contains them. It is in the cerebral hemis- pheres that the principal increase is manifested ; and these extend not only laterally, but so far backwards as nearly to cover the optic lobes (Fig. 210), which, as well as the olfactory ganglia, are proportionably reduced in size. The cerebellum now exhibits a considerably increased 2g 450 SPECIAL AND COMPARATIVE PHYSIOLOGY. development, especially in birds of powerful flight, and those which remain long on the wing ; and we find not only a central mass, but rudiments of lateral portions or hemispheres. Still the surface of the brain is un- marked by convolutions ; and the distribution of its fibres is very simple. In this class, however, we first meet with the rudiments of the great transverse commissure (corpus callosum), a band of fibres which unites the two hemispheres of the cerebrum. 582. The same general course of development may be observed in the different orders of mammalia. The size of the cerebral hemispheres increases in every direction, so that they completely cover the olfactive and optic ganglia, which are now comparatively minute,"" and often partly conceal the cerebellum, which has also attained a great increase in deve- lopment, and is possessed of hemispheres in addition to its central mass. The surface of the brain is now marked by convolutions, which increase in number and in depth as we ascend from the lowest to the highest orders; being almost absent in the Monotremata and Rodentia, and but shallow in the Cetacea and Ruminantia, Avhilst they are strongly marked in the Carnivora and Quadrumana, and most of all in man. The internal arrangement of the fibres of the hemispheres also gradually becomes more complex; for, besides those which ascend from the sensory columns to the convolutions, and the corresponding ones which descend to the motor columns, there are others which establish the communication between the two hemispheres, and another set, again, (which is the most complex of all) that brings the different parts of the same hemisphere into connection with one another. It is in the development of the last-named set of fibres that the brain of man is so superior to that of all other animals; since there are several in which it is larger relatively to the bulk of the body. In this respect, again, we may trace the gradual ascent in the character of the organ through the different orders of Yertebrata; for the brains of Rodentia and Marsupialia are nearly as destitute of these unit- ing tracts or commissures, as are those of Birds. The spinal cord, like all other parts of the nervous system, is larger in proportion to the bulk of the animal, than in other classes ; but it is much smaller in reference to the brain. Its extension through the vertebral column, and the degree of its enlargement where the nerves for the members are given off, vary, as in other cases, with the character and development of the different loco- motive organs. Nothing has been yet said of the development of the Sympathetic or visceral system of nerves in the Yertebrated classes. This advances, \\OMve\er, pari passu yi'ith. the cerebro-spinal; and in Mammalia * The former is known in man as the bulbous expansion of the first pair of nerves, that lies upon the cribriform plate of the aethmoid bone. In reality, however, the nerves commence from this ganglion, the trunk which connects it with the brain being analogous to the peduncle seen in the cartilaginous fishes. The optic ganglia are known in man as the corpora quad- rigemina. FUNCTIONS OF THE NERVOUS SYSTExM. 451 it becomes a system of great complexity, having two large ganglia (the semilunar) in the abdomen, from which filaments are distributed to all the digestive organs, and a regular series along the spine. It com- municates with each of the spinal nerves near their roots, as well as with most of the cerebral; and is believed to interchange filaments with them. It forms a plexus which is minutely distributed upon the large vascular trunks, and which probably accompanies their ramifications into every part of the system. 583. A brief sketch will now be given of the embryonic development of the nervous system in Birds and Mammalia, for the purpose of showing that the same remarkable con-espondence exists in this, which has been demonstrated in former instances; and, were fuller details here admissible, the correspondence would be still more evident. It must be recol- lected, however, that, in all comparisons of this kind, we are not to look for similarity in external form or size, but in the grade of development, and the relative condition of difi'erent parts. The first appearance of nervous matter in the embryo of the chick, is a simple white line, running along the primitive trace (§ 535), and thus evidently analogous to the fila- mentous cord in the lower Annelida and Entozoa. At the 21st houx of incubation, this cord is seen to be double (Fig. 206), as in the higher Articiilata; and the lower part begins to be enclosed in the rudiments of vertebrae. The slight curves at its upper part indicate the situation where the cerebral vesicles are subsequently to appear. The formation of these is seen commencing in Fig. 207, which represents the nervous system at the 40th hour; and their more advanced condition at the third day is shown in Fig. 208. Here we perceive the rudiments of the cerebellum, the large optic lobes, the small hemispheres, and the olfactory ganglia, disposed in one line, as in Fishes. The advanced condition of the brain on the 14th day, when it has nearly assumed its permanent form, is shown in Fig. 209. The early formation of the nervous system in the Mammalia probably follows much the same plan; but there are obvious difficulties in the way of becoming minutely acquainted with it. At Fig. 213 is shoA'VTi the aspect of the nervous centres in the human embryo at the 7th week of development; at Fig. 214, the same at 9 weeks; and at Fig. 215, the same at 12 weeks. Although none of these bear any great external resemblance to the figures formerly given, a careful exa- mination shows that they may be regarded as analogous to the brains of difi'erent tribes of Fishes, The fourth ventricle (§ 580), is seen to be still open, as in many of that class. At Fig. 216 is shown the brain of a foetus of 14 or 15 weeks, which bears a general correspondence with that of Reptiles; the cerebral hemispheres being enlarged, and partly covering the optic lobes, while the cerebellum is still in a single mass, and the fourth ventricle scarcely closed. Finally, at Fig. 217 is given a side view of the brain of a human embryo of 27 weeks, in which the cerebral lobes 2 G 2 452 SPECIAL AND COMPARATIVE PHYSIOLOGY. are seen to have "V'VTappecl themselves round tlie optic ganglia, and the cerebellum to have gained lateral development as well as a furrowed sur- face. Slight depressions, indicative of commencing convolutions, are seen on the surface of the cerebrum; and, altogether, this condition of the brain much resembles that which is permanent in the Rodentia. The occurrence of monstrosities, in which the central organs of the nervous system have been deficieirt while the nervous trunks have been distributed as usual, shows that the formation of the latter is quite independent of the former, just as the formation of the capillaries is independent of the heart (§ 321), And Mr. Newport's observations on the progressive ap- pearance of the nerves supplying the wings of Insects, during their meta- morphoses, also lead to the conclusion that the development of the trunks proceeds from the circumference towards the centre. 584. To enter into any detail on the functions of the nervous system would be inconsistent with the plan of the present work; and all that can here be given is a very general sketch of the different classes of actions in which it is concerned. It must be recollected that most of our knowledge on this subject is derived from the observation of its functions in man; and that we are unable to reason, except by analogy, as to the phenomena presented by the lower animals. And so difficult is it to arrive at any certainty regarding the changes concerned in these phenomena, that it is even now a disputed question whether particular motions, which may be excited by stimulating parts of the surface in animals whose brain has been removed, and which at first sight appear to indicate consciousness and will, are or are not independent of sensation. We may consider the functions of the nervous system under the following heads. 1. Its recep- tion of external impressions^ and communication of them to the sensorium, where they give rise to sensations. 2. The origination in the nervous centres, and the propagation along the motor trunks, of an influence which stimulates muscles to contraction. 3. The operations of the mind, which are excited by sensations, and which, to produce any action upon the corporeal system, must terminate in giving rise to a motor impulse. 4. The establishment of a connection between the organic functions, by which they are brought into harmony with one another, and influenced by certain mental conditions. 585. The reception of external* impressions is efi'ected by the nerves termed sensory ; the ramifications of which are minutely distributed upon all parts of the surface of the body. What is the nature of the change produced in them, by which the impression is conducted along their cords to the nervous centres, Ave can only guess at; but we know that such a process takes place, since division of the cord prevents any impression * The term external is here employed in the usual metaphysical sense, implying' that which does not originate in the mind. The impression may be produced by some change in the corporeal structure itself, as well as by the phenomena of the external world. FUNCTIONS OF THE NERVOUS SYSTEM. 453 made upon its extremities from being felt; and mechanical irritation of the di Added extremity that is in connection -with the brain, gives rise to a sensation, which is refen-ed to the part of the body to which the trunk is distributed. The sensation of tact is commonly spoken of as general; since the capability of exciting it is possessed by the whole surface in man, and probably also in all but the very lowest classes. The sensations of sight, hearing, smell, and taste, are designated as special, being occa- sioned only by impressions made upon particular organs, which are adapted to receive them and them only."^ These organs we find progressively evolved as we ascend the animal scale; but though we have reason to believe that the sensations to which they respectively minister, can only be excited in a perfect form where they exist, there is good reason to believe that animals destitute of them have a diffused sensibility to the agents Avhich excite them, — being, for example, conscious of the influence of light, although not able to see objects. This, then, may be regarded as one of the many instances in which a special structure is elaborated out of one more general. Although all the sensory nerves terminate in the cerebro-spinal axis of Vertebrata, there would seem good reason to believe that, as long as the impression which they convey is confined to that organ, it cannot produce sensation, (even although it may excite motions, (§ 591), until transmitted to the cerebral hemispheres.t Of the nature of the change by which the mind is rendered conscious of the impression conveyed by the nerves, we are as ignorant as of that con- cerned in the impression itself. 586. The next division of the functions of the nervous S3^stem is that concerned in the production of motion. As in the former case a stimulus originating in the circumference of the body was propagated towards the centre, we here find an influence excited in the centre, either by mental action or some other change, transmitted to the circumference, and this operating on the muscles, by exciting them to contraction. It has been already stated (§ 559) that these organs appear possessed of the property of contractility, which may be called into action by stimuli of various kinds; and that nervous agency is one means (and, in the living body, the principal means) by which their contraction may be produced. That the influence of the will in causing muscular contraction is conveyed by the motor trunks, is at once demonstrated by the interruption occasioned by the division of the nerve; and mechanical in-itation of the cut extremity of the part which supplies the muscles, is followed by their contraction. It has been frequently supposed that the influence thus propagated is of an electrical kind, since this agent is capable of imitating it; — muscular * The sense of tasle, however, may probably be regarded as a refined kind of touch. t Many eminent physiologists still hold a contrary doctrine, imagining that, when the brain is removed, sensibility exists in the spinal cord. For a full discussion of this question, and of the docti'ine of excited actions, see the Brit, and For. Med. Rev. vol. v. p. 486, et seq. 454 SPECIAIi AND COMPARATIVE PHYSIOLOGY. contractions being produced by a galvanic current transmitted along the Jnotor nerves. It may be objected to tbis doctrine, how^ever, tbat other stimuli besides galvanism are capable of occasioning muscular contraction when applied to the nerves; that no unequivocal manifestation of electri- city has ever been produced by nerves along which the motor influence is being powerfully transmitted (as evidenced by the muscular contractions it excites); and that many of the conditions of the operation of the two agents are so dissimilar that their identity seems scarcely admissible.* The motor influence seems really to issue from the cerebro-spinal axis, which gives rise to all the motor nerves; but, when excited by the will, it probably originates in the cerebral hemispheres. It may, however, be produced quite independently of the will, and Avithout any influence from the brain, in modes that will presently be explained (§ 590). 587. The complexity of the operations of the inind^ and the impossi- bility of deriving, from the study of the lower animals, any assistance which can be relied upon in their analogies, have hitherto been a com- plete bar to the successful investigation of them as a portion of the func- tions of the nervous system. It is yet quite uncertain how far mental acts are dependent on or connected with any changes in its condition; and we only know that they can neither be excited in the first place, nor effect any change upon the material structure of the body, except through its intervention. All acts of thought are either immediately or remotely dependent upon sensations; and, if all their inlets were closed from the first, the mind would remain dormant, like the seed buried deep in the earth. The activity of the mind is just as much the consequence of ex- ternal impressions by which its faculties are called into play, as is the life of the body the result of the excitement of its several vital properties by external stimuli; and just as many animals are capable of retaining a certain degree not only of vitality but of vital action, when deprived for a time of these stimuli, (as in hybernation), so could the mind which had once been roused retain its powers by the recall of its former sensations, though debarred from the excitement of new ones. 588. The acts of mind in which the intellectual faculties are con- cerned can only produce an influence on the corporeal structure, by an exertion of the will, which, being propagated from the brain to the cerebro-spinal axis, excites in it a motor impulse that is propagated to the muscles. But various mental operations are independent of the employ- ment of the intellect, and can produce an influence on the motor nerves by some channel distinct from the will. Of this kind are the emotional actions, which, though aroused by sensations, are independent of the will, and often strongly opposed to it. It is only when the emotions are strongly excited, however, that the actions performed in obedience to them have this character; if less vehement, or partially subdued by the * See Alison's Physiology, p. 117. FUNCTIONS OP THE NERVOUS SYSTEM. 45.7 will, the excited emotion merely stimulates tlie intellectual processes to the formation of a desire (of which it then becomes an element), and from this an act of volition results. The distinctness of the channels of emotional and volitional actions is beautifully evinced by the occasional effects of disease; for cases have occurred in which muscles have been entirely paralysed to the influence of one, and have yet been susceptible of the other. In the well-regulated mind of man, in which the passions, emotions, and propensities, (which are all conditions of an analogous nature, varying in the degree in which they are connected with the ope- rations of the intellect, or with the performance of the organic functions), are kept under due control, few of the actions will be of this involuntary nature; but, in proportion as they predominate, whether in health or dis- ease, the individual loses his freedom of will, and his actions approach towards an instinctive character. 589. Putting aside, then, the actions in which intellect and volition are concerned, and of whose nature we must be, for the present, content to acknowledge our ignorance, we may next enquire into the causes and conditions of the other movements which we witness in the animal body, and which, — as we trace in all of them a direct respondence to an external stimulus, unmodified by the will of the individual, and not directed by him towards a definite end, — may be included under the general term instinctive. These will be seen to predominate greatly in the lower classes of animals ; and to be, indeed, in many instances, almost the only actions manifested by them; and, whilst in man they are rendered partly sub- ordinate to his powerful reason, they display themselves in full force during childhood, or when the mind is weakened by disease. But it will be desirable to analyse these more closely. 590. In the lowest and simplest class of excited movements, the nervous system would not appear to be concerned. They result from stimuli directly applied to the muscle; and are evidently of the same character with the motions of plants. Of this kind are the motions of the heart, and of the alimentary tube below the stomach, in the higher animals; and probably, as already shovra (§ 562), the greater part of the movements of the Hydra and other creatures of equal simplicity, in which no connected nervous system can be traced. They are all immediately connected with the functions of organic life, (to suspend which, even for a short time, would be fatal) ; and they are incapable of being controlled, directed, or antagonised by the Avill. Some of them are influenced, however, by mental emotions, &c., probably through the sympathetic nerve (§ 595). 591. In the next class of excited movements, the nervous system ap- pears to act the part of a conductor of stimuli from the spot on which the impression is made, to the muscles M'hich are to be called into action. These require for their performance the integrity of the nervous circle; 4^56 SPECIAL AND COMPARATIVE PHYSIOLOGY. that is, of the sensory nerves wliich receive the impression, of the portion of the central organ (the cerebro-spinal axis) to which it is conveyed, of the portion from which the motor trunk originates, and of that trunk itself. We are quite ignorant of the cause why certain motions should be always excited by certain impressions; but it is well to bear in mind that most fi-equently the sensory nerves, which convey the stimulus to the central axis, enter it in pretty close proximity with the motor trunks, through which the movements are excited. In the spinal nerves, indeed, the two systems united in the same cord are often alone concerned; so that a single segment of the spinal column is all that is wanted to com- plete the circle, and movements may be excited in the part of the body which it supplies, when all the rest of the column has been removed. This is the case, to even a more striking degree, in the Articulata; for each segment which contains a ganglion will, in such a creature as the Centipede, continue to execute regular movements for some time. But it may be easily proved that no direct communication between the sensory and motor filaments is concerned in producing them, by destroying the portion of the spinal cord with which these are connected; when they will no longer be excited. 592. The movements of this class may seem but remotely connected with the maintenance of the functions of organic life; but they are essen- tial to its continuance in all save the lowest animals. Those concerned in Respiration will afford an apposite illustration. They are excited by a stimulus, originating in the lungs (being occasioned by the presence of venous blood in the pulmonary vessels), and conveyed by a sensory nerve (a portion of the par vaguin) to the upper part of the spinal cord. A part of the motor nerves concerned in stimulating the respiratory muscles to action arise in the neighbourhood; others are given off lower down; but to all is the motor influence equally transmitted, and this as well without the brain as with it. If the circle be anywhere interrupted, however, the respiratory movements will cease, and the aeration of the blood will be consequently checked, although no mechanical impediment exist to the entrance of air or blood into the lungs. Many other actions of the same kind are constantly involved in ministering to the organic functions, although not immediately essential to them ; of this kind is the process of swallowing formerly described (§ 263). Others, again, are for the protection of the body from injury; as when the pupil contracts, from the influence of light on the retina; or when a limb is withdrawn from a flame suddenly applied to its surface. These movements are generally capable of being, for a time at least, restrained or antagonised by the will; but they cannot be altogether controlled by it. In the greater number of cases, sensation is produced by the impression which excites them ; and hence it has been supposed to be a necessary link in the chain of actions. If it be true, however, that no impression can produce FUxXCTIONS OP THE NERVOUS SYSTEM. 457 sensation, unless it be propagated to tlie cerebral hemispheres (or what- ever part corresponds with them in Invertebrata), it is evident that sensation cannot be necessary to their performance, since they will all take place when the brain has been removed. It is obviously difficult to prove that sensations do not exist in animals on which experiments are made, and whose movements would appear to indicate them; but the question seems decided by the occurrence of cases of disease in man, in which movements of limbs, that were quite insensible and palsied to the will, have been excited by stimuli applied to them; and in which the pupil has been excited to contraction by the stimulus of light upon the retina, of which the mind was not conscious. This class includes all the movements that have been termed sympathetic^ and a part of those com- monly called instinctive. A very interesting example of them is the act of sucking in the infant, which appears to be directly excited by the contact of the nipple with the lips, without the consciousness of the individual being necessarily involved; for instances have occurred in which it has been energetically performed by infants born without brain; and a similar result has followed the removal of the brain of puppies. A large proportion of the movements of the Articulated tribes, which are so uniform as to forbid the idea of judgment and mil being concerned in them, probably possess a similar character.* 593. The highest class of excited movements is that in which sensa- tions do partake, although still without the operation of the judgment or will. In these, the organs of special sense are chiefly concerned ; and the actions in question appear to have a tendency to the preservation of the system and the perpetuation of the race. Of this class, the involun- tary movements directed towards the acquirement of food, the construc- tion of habitations, the balancing of the body, &c., seem to be examples; but few of these are involuntary in man ; and the inference that they are instinctive in the lower animals principally rests upon the uniformity of their occurrence, especially when contrasted Avith the variability of those which depend upon reasoning processes. Still we find that, even in man, there are many motions destined to the preservation of the body from danger, which have been wisely rendered independent of his un- certain and capricious will. Thus, in one of the cases formerly alluded to (§ 588), the eyelids, which could not be moved by the wiU of the individual, closed involuntarily on the sudden approach of a body towards the eye, or on the application of a strong light. The instincts which minister to the supply of the organic functions are, in adult man, rendered subservient to volition, by which they are controlled, and to which they act as a stimulus. It is easy to perceive the final cause for this change. If the organisation of the human system had been adapted to perform all the actions necessary for the continued maintenance of * See, on this subject the Brit, and For. Med. Rev. loc. cit. 458 SPECIAL AND COJIPARATIVE PHYSIOLOGY. its existence, with the same certainty and freedom from a voluntary effort as we perceive where pure instinct is the governing principle, and if all his sensations had given rise to intuitive perceptions, instead of those perceptions being acquired by the exercise of his mind, it is evident that external circumstances could have created no stimulus to the improve- ment of his intellectual powers, and that the strength of his instinctive propensities would have diminished the freedom of his moral agency. Although, therefore, to all the actions immediately necessary for the maintenance of his own existence, and for the continuance of his race, a powerful instinct strongly impels him, these propensities could not be gratified, if the means were not provided by the exercise of the mental powers, which he enjoys in a degree far exceeding those of any other terrestrial being. 594. In tracing the progressive complication of the psychical mani- festations during the early life of the human being, a remarkable cor- respondence may be observed with the gradual increase in mental endowments which is to be remarked in ascending the Animal scale. The first actions of an infant are evidently of a purely instinctive charac- ter, and are directed solely to the supply of its physical wants; they are thus analogous to those of the lowest animals possessed of a nervous system, which are entirely governed by instinct. The new sensations which are constantly being excited by surrounding objects, call into exercise the dormant powers of mind; perceptions are formed, and notions thus acquired of the character and position of external objects ; and the simple processes of association, with its concomitant — memory, are actively engaged during the first months of an infant's life. At the same time an attachment to persons and places begins to manifest itself. All these are the characteristics of the great majority of the lower Ver- tebrata, as far, at least, as our knowledge of their springs of action enables us to form a judgment. As the infant advances in age, the powers of observation are strengthened; the perceptions become more complete ; those powers of reflection are called out which prompt him to reason upon the causes of what he observes, and to perform actions resulting from more complicated mental processes than those which guide the infant ; and, at the same time, we observe the development of the moral feelings, but these are manifested only towards beings who are the objects of sense. Among the more sagacious quadrupeds, it is easy to discover instances of reasoning as close and prolonged as that which usually takes place in early childhood ; and the attachment of the dog to man is evidenly influenced by moral feelings of Avhich the latter is the object. " Man," it was expressively said by Burns, " is the God of the dog." Up to this point, then, we observe nothing peculiar in the char- acter of man ; and it is only when his higher intellectual and moral endowments begin to manifest themselves, especially those relating to FUNCTIONS OF THE NERVOUS SYSTEM. 451) an invisible Being, that we can point to any obvious distinction between the immortal •^'vxv of man, and the transitory irvevf.ia of the brutes that perish. May we not regard these as here existing but as the germs or rudiments of those higher and more exalted faculties, which the human mind shall possess, when purified from the dross of earthly passions, and enlarged into the comprehension of the whole scheme of Creation, the soul of man shall reflect, without shade or diminution, the full effulgence of the Love and Power of its Maker ? 595. One more function of the nervous system still remains to be considered; namely, its influence on the organic processes. This has already been generally pointed out (§ 222, 368). Although there is not sufficient evidence to Avarrant the belief that either the processes of nu- trition and secretion, or the niotions of the heart and alimentary canal, are dependent upon the nervous system, there is no doubt that they are greatly influenced through its medium by conditions of the body or mind; and the sympathetic or asymmetrical system, whose branches accompany the blood vessels throughout the whole body, besides being abundantly distributed to the heart and abdominal viscera, seems to be the channel of their operation. All the sympathies between the actions of the organs concerned in the Yital functions are probably effected through its medium. Of this kind are the acceleration of the heart's action when a local in- flammation occurs in a distant part, the secretion of milk about the time of parturition, and the formation of other secretions for the protection of exposed surfaces.* Whatever be the precise nature of the actions of these nerves, it is quite certain that they are not, in their natural state, subservient to sensation ; and that the very slight motions which the muscles they supply may be sometimes excited to perform by irritating them, may be fairly attributed to the cerebro-spinal filaments they con- tain. The latter seem, however, to receive an influence from certain invo- luntary states of mind, particularly those of an emotional character, by which the organic functions are modified (§ 368). * It is remarkable that palsy of the cerebro-spinal nerves supplying- some parts should check the protective secretion, and thus occasion inflammation. This is the case not only in the eye, of which the outer membrane is, in the healthy state, acutely sensible to any un- usual stimulus, but in the bladder, of which the lining' membrane does not seem sensible unless diseased . It must be supposed to be by an influence communicated through these to the sym- pathetic, that the secretion is stimulated in the natural state ; and it may perhaps be trans- mitted through those filaments derived from the sympathetic, which every cerebro-spinal nerve contains ; whilst the acute sensibility of some parts when diseased, to which none but sympathetic nerves are distributed, may be accounted for by the presence of cerebro-spinal filaments in them (Miiller's Physiology, pp. 668-672). 460 EVIDENCES OF DESIGN. CHAPTER XVII. ON THE EVIDENCES OF DESIGN PRESENTED BY THE STRUCTURE OF ORGANISED BEINGS. 596. If little has been expressly said upon this subject in the fore- going pages, it is because it has been thought that when the perfect adaptation that exists between all the minute details of each member of the aninaated world, and the harmony of the parts they have to perform in the grand system of the Universe, were being explained and demon- strated, it might be safely left to the mind of the reader to draw those inferences, which it is perhaps impossible for any soundly-judging person to avoid making, who is unwarped by the pride of human reason, or by that tendency to practical disregard of them, which, in so many instances, is mistaken by the individual himself for a valid argument on the side of disbelief. When we consider the universality of this adaptation, so con- stant that it cannot be the effect of chance, — ^the beautiful harmony of the details, uninterrupted by the slightest discordance, — and the consum- mate perfection of the whole, so complete as to forbid the idea of a limited power, — it seems scarcely possible to an-ive at any other rational conclu- sion, than that the Universe with all that it contains is the work of one Almighty and Benevolent Creator. 597. Much has been said and written on the study oi final causes in Physiology, or the examination of the particular uses of each organ, and its adaptation to the objects of the system of Avhich it forms a part. No doubt can be entertained that, when the belief in Universal Design is once established, its pursuit into particular instances may often lead to enquiries which would otherwise have been neglected, and may put us on the right track in the conduct of those enquiries. Thus, Harvey states himself to have been excited to his researches on the movement of the blood, which terminated in the splendid discovery of its double circula- tion, by the contemplation of the valves in the veins; and Sir C. Bell was led to his discoveries on the functions of different portions of the nervous system, by a feeling of curiosity as to the object of the double roots of the spinal nerves. But we are not to rest satisfied with the obvious purpose of a particular structure as affording us the supposed reason for which it was created. As well might we think it (to take Bacon's examples) a sufficient account of the clouds that they are for watering the earth, or "that the solidness of the earth is for the station and mansion of living creatures." "The physical philosopher," says Mr. Whewell,* "has it for his business to trace clouds to the laws of evaporation and condensation; * Bi'idgwater Treatise, p. 353. EVIDENCES OF DESIGN. 461 and to determine the magnitude and mode of action of tlie forces of coliesion and crystallisation by whicli the materials of the earth are made solid and firm. This he does, making no use of the notion of final causes; and it is precisely/ because he has thus established theories inde- pendently of any assumjMon of an end, that the end, when after all it returns upon him and cannot be evaded, becomes an irresistible evidence of an intelligent Legislator." 598. The philosophic Physiologist, who is not deterred by the clamour of bigotry and prejudice, will follow precisely the same course. The adap- tation which he discovers in particular instances may well serre both to awaken his curiosity, and to lead him to suspect a pre-existing Design. But he will obtain a much more elevated vicAv of the nature of Creative Power, if he carry his enquiries farther. He must disregard for a time, as in physical philosophy, the immediate purposes of the adaptations which he Avitnesses; and must consider these adaptations as themselves but the results or ends of the general laws for which he should search. The observation of the facts upon which he establishes these laws may have been suggested, and the phenomena themselves brought to light, by the perception of this harmony and adaptation in individual cases; but instances in which it is apparently deficient may be as valuable to him, when considered in this point of view. What, for example, would have been the present state of the science of Vegetable Morphology, which explains the metamorphoses of the organs composing the flower (§ 54), if the philosophic botanist had adopted the final cause or function of the different parts as his guide in investigating the laws of their structure, instead of tracing that structure through all its regular and in-egular forms Avith a total disregard of their function? In considering the laws of the organised world, we have abundant opportunities of observing how diversified, both in their forms and uses, are the various types which the same rudiments may present; and that, even when imdeveloped, such rudiments appear as the necessary result of these laws, and assist man in the attainment and comprehension of them. It is evident, then, that we are not to judge of the value of facts in Physiology by their immediate and obvious bearing upon the phenomena of Vital Action; for those which would seem to be of the most trifling consequence, if vicAved in this light only, are often found, when properly applied, to possess an unexpected and momentous import. They are like the marks in the forest by which the American Indian at once detects the passage of friends or foes. A broken twig, a torn leaf, a flattened blade of grass, are signs which an ordinary traveller would pass Avithout observation; but, to the practised eye of the denizen of the woods, they are alike certain and expressive. In proportion to our attainment of the generali- sations to which we are thus led, Ave acquire fresh proofs of the Omnipo- tence of Creative skill. For, at every successive step, are Ave able to 462 ' EVIDE>rCES OF DESIGN. comprehend new relations betAveen facts that previously seemed confused and insulated, new objects for what at first seemed destitute of utility; and in the same proportion will the contemplative spirit be led to appre- ciate the vastness of that Designing Mind, which, in originally ordaining the laws of the animated world, could produce such harmony and adapta- tion amongst their innumerable results. To use another very forcible expression of Mr. Whewell's (which he, applies, however, only to physical science, regarding Physiology as excluded from it) "the notion of design and end is transferred by the researches of science, not from the domain of our knowledge to that of our ignorance, but merely from the region of facts to that of laws." 599. To avoid all chance of being misunderstood in these views, it may not be useless to adduce, in illustration of them, one of the most obvious and simple adaptations everywhere presented in the structure of Animals,' — that of the muscles to the skeleton. We constantly find in pursuing our anatomical enquiries, that, for the advantageous attachment of muscles to bones, some particular form of the latter is provided; and that, where much power or a particular direction is required, a consider- able prominence is given to the point of attachment. The teleologist, who rests satisfied with the evident object of this adaptation as a sufficient reason for its occurrence, would say with truth that each of the bony pro- cesses was intended for the attachment of a muscle; and he might safely rest upon this intention as a ground for inferring the form and direction of certain muscles of extinct animals, from the prominences which are found upon their fossilized bones. He might go further, and maintain that the formation of this prominence is occasioned by the existence of the muscle; and might allege, in support of his view, the well known fact, that the osseous points of attachment are strongly developed in those persons who have much exercised their muscular system. On the other hand, the philosophic anatomist, fully acknowledging the adaptation between the osseous and muscular systems, would disregard it for the time, whilst seeking for the laws regulating the development of these systems; which laws he would aim to deduce from the observation of all the forms of each, both normal and abnormal, imperfect and complete. Thus, he would find that almost every one of the important processes in the human skeleton exists as a separate bone in some of the inferior animals; and that the complicated muscular system of man gradually simplifies itself in proportion as the skeleton exhibits more repetition of similar parts, and is, in consequence, adapted to a less diversity of actions. Supposing, then, that the physiologist has succeeded in establishing such laws independently of any assumption of an end " that end, Avhen after all it returns upon him, becomes an irresistible evidence of an intelligent Legislator." For it may be safely left to the judgment of any candid and reflecting person, whether it does not imply a far higher degree of Creative EVIDENCES OF DESIGX. 463 "Wisdom and Power to suppose that, in the estahlishment of the laws of osteology and myology (themselves probably subordinate to some higher generalisation), all the results of each were foreseen and harmonised, so that every muscle, developed in accordance with the laws of its system, should find an attachment in the osseous process resulting from the action of the laws of its system, — than to imagine that the formation and adapta- tion of each separate muscle, and of each individual process, required a distinct efibrt of creative skill. 600. It has been one object of the foregoing pages to show that vital properties are as essentially connected with certain forms of matter, as are those usually denominatd fliysical with matter under its more com- mon aspects. One more question yet remains. It is possible that the physical and vital properties of matter, which are at present our ultiniate facts or axioms, may be included within a more general expression com- mon to both? On this subject we can only speculate; bvxt the pro- bability appears decidedly in the affirmative. It has already been remarked that the rapid progress of generalisation in the physical sciences renders it probable that, ere long, a single formula shall comprehend all the phe- nomena of the inorganic world (§ 141); and it is not, perhaps, too much to hope for a corresponding simplification in the laws of the organised creation, although its progress is necessarily retarded by the many obstacles which the nature of the subject presents to the philosophic enquirer. Every step which we take in the progress of generalisation, increases our ad- miration of the beauty of the adaptation, and the harmony of the action, of the laws we discover; and it is in this beauty and harmony that the contemplative mind delights to recognise the wisdom and beneficence of the Divine Author of the Universe. This, in fact, is one of the highest results to which the exercise of our intellectual faculties should lead; and we cannot but believe that the Creator, in endowing us with these facul- ties, intended that they should conduct us nearer to the conception of his Infinite mind. But, at the same time, the vastness of the prospect thus disclosed can scarcely fail to impress us with the most humbling conscious- ness of our owQ insignificance. 601. If, then, we can conceive that the same Almighty j'?^);^ which created matter out of nothing, impressed upon it one simple law which should regulate the association of its masses into systems of almost illimitable extent, controlling their movements, fixing the times of the com- mencement and cessation of each world, and balancing against each other the perturbing influences to which its own actions give rise, — should be the cause, not only of the general uniformity, but of the particular variety of their conditions, governing the changes in the form and structure of each individual globe protracted through an existence of countless centuries, and adjusting the alternation of " seasons and times, and months and years," — should people all these worlds with living beings 464 ' EVIDENCES OP DESIGN. of endless diversity of nature, providing for their support, tlieir happiness, their mutual reliance, ordaining their constant decay and succession, not merely as individuals but as races, and adapting them in every minute particular to the conditions of their dAvelling, — and should harmonise and blend together all the innumerable multitude of these actions, making their very perturbations sources of new powers; — when our knowledge is sufficiently advanced to comprehend these things, then shall we be led to a far higher and nobler conception of the Divine Mind than we have at present the means of forming. But, even then, how infinitely short of the reality will be any view that our limited comprehension can attain, seeing, as we ever must in this life, " as through a glass, darkly;" — ^how much will remain to be revealed to us in that glorious future, when the Light of Truth shall burst upon us in unclouded lustre, but when our mortal vision shall be purified and strengthened so as to sustain its dazzling brilliancy. EXPLA]^[ATIO]^ OF THE PLATES. N.B. When no paragraph is referred to, the one last named is understood. PLATE I. Vegetable Tissues. FIG. 1. Membranous cellular tissue, § 23. 2. Cubical and prismatical cellular tissue. 3. Muriform tissue of medullary rays § 23, 61, 286. 4. Tubular form of the same, from a fossil wood. 5. Fibrous cellular tissue, from Orcliideous plant, § 23. 6. Dotted cellular tissue from the same, — a, showing merely dots, — b, exhibiting traces of spiral fibre, 7. Section of a duct from fossil wood, showing remains of partitions, § 24. 8. Dotted duct formed by aggregation of dotted cells. 9. Simple duct formed, in like manner, from simple cells. 10. Woody fibres clustered in a bundle, § 25. 11. Glandular woody fibre of coniferous wood. 12. Spiral vessel with single fibre, § 26. 13. Spiral vessel of Nepenthes with quadruple coil. 14. Spiral fibres drawn out of the vessels. 15. Annular duct, exhibiting remains of spiral fibre, § 27. 16. Close spiral duct, showing interstices between adhesion of spiral fibre, § 28. 17. Reticulated duct, § 29. 18. Dotted duct formed on the type of the vascular system. Animal Structures. 19. Trachete of Insects, § 26, 395. 20. Spiral cartilage from trachea of Dugong, § 27. 21. Dilated Air-sacs of JBombiis terristris, Humble-Bee, § 28, 396. 22. Appearance of the membrane lining Air-sacs, § 28. 23. Bronchial tubes of human lung, § 29. * 24. Arrangement of fibres in tendon, with muscular fibre, § 37. 25. Arrangement of fibres in elastic tissue of ligamentum nuchce. 26. Single muscular fibre from voluntary muscle, § 42. 27. Muscular structure of organic life, not united into fibres, § 43. 28. Varicose nerve tubes of the brain, ^ 44. 29. Cylindrical tubuli of nervous fibres. Vegetable Structures. 80. Horizontal and vertical sections of an Exogenous stem of 3 years growth; a, pith; b, b, spiral vessels constituting medullary sheath; c, c, dotted ducts; d, d, woody fibre; e, e, bai'k, § 61. 31. Horizontal and vertical sections of an Endogenous stem; a, a, cellular tissue; b, b, spiral vessels; c, c, ducts; d, d, woody fibre, § 52. PLATE 11. 32. Seed of Monncotijledon (Scirpus supinus); a, embryo; b, albumen, § 50. 2 H 466 ' EXPLANATION OF THE PLATES. FIG. 33. Germination of ditto; a, plumula; ft, cotyledon; c, radicle. 34. Seed o{ Dicotyledon (Bean) ; a, a, cotyledons ; &, plumula; c, radicle. 35. Germination of ditto. 36. Vertical section of Fossil wood, crossing direction of Medullary Rays; a, dotted duct, with remains of partitions; b, b, woody iibres; c, c, cut ends of Medul- lary rays, § 51. 37. Different forms of leaves exhibiting the same character o{ venation, ^ 53. 38. Forms of anthers; a, lily; b, lemna; c, potato; cZ, berberry; e, ginger; y, sage, §55. 39. Pistil of Coriaria myrtifolia, showing distinct carpels and styles. 40. Carpel of doiible Cherry; a, natural form, and section showing position of ovules; 6, monstrous form of ditto. 41. Section of pistil of Vaccinium amcenum; a, calyx; 6, ovarium; c, style; d, stigma. 42. Section of ovarium of T/iamne« zmi/?ora; a, calyx; 6, ovarium. 43. Transverse section of ovarium of Viola tricolor (heartsease). 44. Flower oiRaffl<^sia Arnoldi, % 58. 46. Portion of stem of Tree-Fern; a, a, scars of fallen leaves, § 59. 46. Tiiecte o^ Fern; a, closed; b, open, and dispei-sing spores. 47. Sori on fronds oi Ferns; a, circular; b, elongated. 48. Fronds of Op/izogrZossMm (adder's-tongue); «, sterile or leafy ; &, fertile or sporu- liferous. 49. Theca, &c. of Moss ; a, theca ; b, operculum; c, peristome ; d, columella, § 60. 50. Gemrage of Marcliantia ; a, early state ; b, commencing to form roots, § 61, 180. 51. Advanced bud, self-inverted, with stomata above, and roots below, § 180. 62. Section of Stoma and air-chamber of Marcliantia ; a, rings of cells, § 429. 53. Pelta of Marchantia, bearing thecee associated at their bases, § 61 ; a, b, c, early development of the spores, § 523. 54. Stem and branches of JVitellaflexilis, § 62. 66. Diagram of circulation in ditto; A, imaginary transverse section of tube; b, glo- bules in circulation; c, tlie same adherent, § 35-3. 56. Simple forms oi Fungi; a, Monilia glauca; b, Aspergillus penicillatus, § 64. 57. One of the highest tribe, ^manzto ??it{scaria; a, pileus, with its laminse or gills; b, portion of hymenium, with c, the asci or sporuliferous tubes, § 64, 522. 58. Lichen cupularis (cup-moss), with section showing position of sporuliferous tubes, §68 69. Red snow ( Protococcus 7iivalis) ; a, a, vesicles containing germs; &. &, the same after their rupture; c, the liberated germs becoming developed, § 69, 519. 60. Vesicles of Diatoma tenue, united and separating, § 69, 515. 61. Filaments of Conferva rivularis; A, magnified vesicles of Conf. cerea, emitting germs, § 520. 62. Development of these germs ; a, b, c, successive stages. 63. Early development of spores of Fern ; a, b, c, successive stages, § 523. 64. Further evolution of the primary frond. 65. Subsequent evolution of the permanent frond, a, and roots, b. 66. Early development of embryo of Monocotyledon (Potamogeton) ; a, first appear- ance of cotyledon, § 526 67. Ditto of Dicotyledon (QSnothei'a) ; a, a, cotyledons. 68. Section of pistil of Antirrhinmn during fertilisation; «, cs, pollen grains; b, b, pollen tubes insinuating themselves between c, c, vesicles of style and stigma, §525. 69. Vertical section of leaf of Ajjple ; a, a, cells of upper cuticle ; b, b, closely-packed parenchyma beneath it ; c, c, looser parenchyma below ; d, d, cells of under cuticle, § 429. 70. Similar section of leaf of Oleander; a, a, upper cuticle possessing three rows of vesicles ; e, e, chambers in lower cuticle, lined with hairs, § 428,9. 71. Vertical section of stoma of l7'is ; a, a, green cells bounding orifice; b, b, cells of parenchyma; c, air-chamber. T2. View of ditto from above ; a, a, cells of the stoma ; e, opening between them. 73. Similar view of stoma of Ajople ; a, a, cells of the stoma ; b, b, cells of the cuticle ; e, opening of the stoma. 74. Involucrum of Marsilea laid open; a, a, smaller reproductive bodies, thecse, or anthers ; b, b, larger ones, or ovules, § 524. 75. An ovule, b, with its anthers, a, a, separated. 76. Radical fibre of Lemna (Duckweed); a, vessels; b, unformed cellular tissue covering their mouths and constituting the spongiole, § 248. EXPLANATION OF THE PLATES. 467 PLATE III. Animal /Structures. FIG. 77. Vorticella rotatoria; a, Vort. convallaria; b, the same dividing, § 93, 528. 78. Calamary (Loligo vulgaris) ; a, a, fins; h, ink-bag; e, funnel, § 96, 274. 79. Section oi Nautilus; a, a, siphuncle, § 97. 80. Clio borealis, § 98. 81. Pileopsis, § 99. 82. Magilus ; a, young state of the same. 83. Cynthia; a, entrance to mantle ; &, anal aperture ; c, entrance to oesophagus ; d, stomach ; e, intestine ; f, ganglion; g, dorsal cord, § 104, 274, 567. 84. Pyrosoma; § 104. 85. Shell of Echinus ; A, portion enlarged ; a, tubercular plates ; b, ambulacral plates, § 106. 86. Echinodermata ; A, Spatangus; B, Clypeaster; c, Asterias, § 170. 87. Pentacrinus Europseus. 88. Holothuria. 89. Medusa aurita, § 108. 90. Beroe pileus ; a, a, ciliated tentacula ; b, mouth ; e, orifice of intestine. 91. Trichina spiralis, § 111. 92. Enchelis pupa, showing alimentary canal, § 114. 93. Monas termo ; a, Volvox globator, § 268, 630. 94. Paramoecium aurelia; a, the same dividing, § 114, 268 n, 528. 95. Hydra viridis, and H. fusca, in different states, § 115. 96 (See PL IV.). Sertularia ; a, polype-cells ; b, ovaria ; c, polypes, § 116. 97. Bowerbankia densa; a, oesophagus ; 6, gizzard ; c, stomach; d, orifice of intes- tine, § 117. 98. Alcyonium eros; a, mouth; b, communicating tube; c, gemmuliferous tube, §119. 99. Alcyonidium elegans ; A, section of ditto, showing interior chambers. 100. Isis hippuris ; a, jointed axis ; b, flesh with polypes. 101. Section of Actinia ; a, cavity of stomach ; b, surrounding chambers, § 120. 102. Pitcher of Dischidia ; oi, exterior; &, section showing rootlets, § 239. 103. Villus of intestine with absorbent vessel, § 262. 104. Digestive organs of Diglena lacustris ; a, a, jaws ; b, stomach ; c, c, biliary coeca, § 272. 105. Alimentary canal of Cicindela campestris ; a, oesophagus ; b, crop ; e, gizzard ; . i 575. 196. i^ Hemipterons Insect). 197. r : Sand-hopper, Isopod Cnistaeeons animal), i576. 198. Maia fquamado (Crab). 199. Spider. In the loBoTong figures, the letter a. points to the olfactory gsnglia ; i, to the cerebral hemispheres or ganglia ; f , to the optic lobes or ganglia ; d, to the eerebdlmn; and e, to tbe spinal cord. 200. Brain of Trigta lyra (Gniifflrd), § 580. 201. 3furmtm eomger (Conger Eel). 902. Saia rmbus {Bsv). 203. Grey lizard, * 581. SOi. Frog. 305. Te^ido m^das, (Green TraHe). 206-9. DCT'«3<^pfment of fTervons system in CMck. § 583. 210. Brain of Casso^rarv, ^ .381 . 211. lion, i 58-2.' 212. Origins of nerres from spinal eoixi, % 578. 213. XerroTis srstem of human embryo at sexen ■weeks, i 583. 214. Brain of embryo at nine weeks. 215. twdTC weeks. 2I6L fifteen weeks. 217. twenty-seven weeks. S18-9. Brain and spinal cord of Tadpcde, § 561. CORRIGEXDA. Kge 13,Bae3S,fo? -'£stian»ai" lead ^£^lnBoa.~ &, Vme 3 tnm Guttata, for "Fig. ^aS," read 'Fig. 33." ^ Bae la, far ■'Cbap. ST." reai "Clap, xn." fl», Bk » fi^lHtlBn, ftr "FSs- ST" i»i "Kg- SO." as, Sk 13, tar'TewmeiMt (^ 82), aad JE^iZu", read " TermeOu tuA MagOMt (Fig. &): 1^ nmi^l^fe, ftr "J^MuiUBy" waA '^AefioBS." S4L, Was 19 a— tattBM, fcr "OutUviytMir read "catrfyae." mJ 2 ', 5 y y V, I y y W/J \ K y ^ / / Hcute, %. CDCnCDl* 1 I rr -±. Cmypvttr, a b. C aipentcr , del. ;\ I'-A,. C arpenter , d.el.'. 5- J. Xima^r. folp- DEX. N.B. The numbers refer to the paragraphs. Where an asterisk is affixed to any reference, it indicates the place in which the subject is particularly explained. Absorbent system, evolution of in Ani- mals, 280 ; in Plants, 255 Absorption, of aliment, 223*, 243-5, 358; interstitial, 18, 225*, 232, 331, 338 ; from surface, 253-4, 279, 336 ; by veins, 337 AcALEPHiE, 34, 108*. 122; circulation in, 294; digestion in, 269; nervous system in, 565 ; phosphorescence in, 474 ; res- piration in, 389 Acari, 85*, 89, 402 Acephalocyst, 112*, 517 Acini, 459 AcRiTA, 73*, 74, 109*, 122, 124, 128; nervous system in, 562 ; repetition of parts in, 109 ACROGENS, 59, 70 Actinia, 108, 120*, 122, 267, 564 Actinijvnn Polypes, 120 Actions, Vital, 3, 146, 152, 155 , Physical in living beings, 159 — 165 Adductor muscle, 102, 567 Adipose tissue, 33, 457 Aeration of nutrient fluid, 370, 414 Affinities, chemical, 18, 164, 165, 443 , vital, 18, 163, 164 Aggregation of organised structures, 15 Air-bladder of Fishes, 406,7 Air-sacs, of Birds, 412 ; of Insects, 28, 396 Albumen, 37,40,1, 356* , of seeds, 49, 50*, 349, 526 Alburnum, 25, 51*, 285,6 Alcyonian PoJypes, 119*, 136, 267 AiG^., 63, 69*, 124; absorption in, 246; circulation in, 283; reproduction in, 520; spontaneous movements in, 553; spores of, 219, 520*, 562 AUantois, 539, 540 Aliment, sources of demand for, 231-3 Anatomical science, nature of, 3, 598 Analysis of phenomena, 4, 212, 213 Analogies, how to be recognised, 191-195 Animals, distinguished from plants, 218, 220, 221, 239; dependent on plants, 269; evolution of electricity in, 501, 510 ; heat of, 481—495: light of, 474-6 Animal kingdom, 71-3, 122,3, 128 Annelida, 91* ; circulation in, 298,9 ; digestion in, 272; nervous system in, 571; phosphorescence in, 474; res- piration in, 392 Annular duct, 27 Annulosa, see Articulata. Antennas, 88*, 395, 570,1 Anthers, 55*, 524,5 Aorta, 300, 325, 339 Apple, cuticle of, 428,9 Aquatic insects, respiration of, 397 Arachnida, 85*; circulation in, 303; digestion in, 273 ; nervous sj'stem in, 576; respiration in, 402 Arachnoid membrane, 580 Area, transparent, 534 ; vascular, 321,537 Arrest of development, 205 Articulata, 73,4*, 82*, 122,3, 127,8; circulation in, 297 ; nervous system in, 570-6 ; respiration in, 392 ; secretion in, 462, 465, 467 ; symmetry in, 137 Artificial Classification, 48, 72 Arum, 53, 381,480 Asci (spore-tubes), 63,4, 522 Ascidia, 104*, 305 Ascidiform Polypes, 117 Asphyxia (suffocation), 152 n, 161, 318, 400, 495 Assimilation, 231, 342 Associated Mollusca, 104, 305 «. Polypes, 116-20 Astacus fluviatilis, 401 Asterias, 106*.7, 136, 270, 295, 565 Asteroida, 119*,20 Atrophy of tissues, 338,9 Auricle, 304 472 INDEX. Balancing of organs, law of, 207 Barnacle, 92*, 571 Basingstoke, pavement of, 67 BatracTiia, 79; circulation in, 311,2; digestion in, 276 ; exhalation in, 437 ; embryonic development in, 539; nervous system in, 581 ; temperature in, 484 Bee, 487-490 (see Bomlous). Benzule,445 Berberry, 654 Beroe, 108*, 269, 294, 389, 565 Bilateral symmetry, 131*, 137, 139 Bile, 262, 463* Binary composition, 19 Biology, 3 Birds, 76; circulation in, 315; digestion in, 277 ; embryonic development in, 535-9; lymphatic system in, 334 ; nerv- ous system in, 681 ; respiration in, 412 ; temperature in, 491 Bladder, gall, 463; urinary, 465 Blastoderma (see Germinal membrane). Blight, 66. Blood, characters of, 362-6 ; difference between venous and arterial, 41 8 ; in- dependent movement of, 317-21 Blood-vessels, formation of, 322,3 Bombus, 396, 424, 486, 488 Bone, structure of, 41 Bovista giganteum, 231 Brain, 74, 561, 368 (see Cerebrum). Branchial arch, 303, 405; openings, 405, 416; tufts, 392-8 Branchise, pulmonary, 402 Bronchial tubes, 29 Eryum calycinum, 527 Buccinum undatum, 138 Buffy coat, 365 Bulimus, 100 ?i, 547 Cacalia septentrionalis, 449 Cacti, 253, 429 Caddis-worm, 87 Calcareous deposits, 16, 41, 62, 69, 84, 92, 95, 106, 118-21, 500 Calyxj 54 Cambium, 355 Camphene, 19, 449 Capillarity, 497 Capillary circulation,227,288, 293 n, 297*, 316-8,367,420 Carbon, fixation of, 373,4 ; excretion of from Plants, 375,6, 384 ; from Animals, 378, 386, 418,9*, 442, 463, 494,5 Carnivora, 47, 75*, 208, 278, 330, 340, 582 Carpels, 55*, 527 Cartilage, 40 Catalytic actions, 165, 443 Caterpillar (see Larva). Cells of Plants, 23 ; spiral, 24 Cellular Plants, 49 Cellular tissue, of Animals, 32,5* ; trans- formation of, 46; of Plants, growth of, 353,4; transformation of, 45 Cephalopoda, 81, 96* ; circulation in, 307,8; digestionin,274; nervous system in, 569 ; secretion in, 458, 462-5 ; sym- metry in, 132 Centipede, 90,671 Cerealea, 548 Cerebellum, 579-83 Cerebrum, 579-83 Cestum Veneris, 294, 389 Cetacea, 75*, 315, 330, 491, 582 Chambered shells, 97, 102 Chameleon, 411 Characece, 62 ; circulation in, 284 n, 353 Characters, external, 544 Chelonia, 78*, 276, 411, 484 Chemical consitution, 17 ; affinities, 18, 162 Cholesterine, 463, 471 Chorion, 534, 540 Chromule, 452 Chrysalis (see Pupa). Chyle, 262, 275, 357*, 358 Chyme, 261 *,2, 356 Cicatricula, 320, 533*, 535 Cilia, 92,3, 108, 110*, 117 m, 121, 264, 388, 409 Ciliobrachiata, 117, 267 Circular system, 130; symmetry, 131-6 Circulating system, evolution of, in Ani- mals, 320-8 ; in Plants, 291 ; malform- ations of, 329,30 CiECTJLATiON,224*,281*,2; oflatex; 286- 290; in embryo, 319, 325-8; complete double, 314 ; capillary, 297 ; respira- tory, 292, 420; portal, 309 CiKRHOPODA, 83, 92* ; digestion in, 272 ; nervous system in, 571 Cirrhus, 392 Classification, 48, 71,2, 544 Climbing plants, 134 Coagulation, of blood, 343, 365* ; of albu- men, 356 ; of chyle, 369 ; of elaborated sap, 346 Coeca, 270, 278* Ccelelmintha, 94 Co-existence of elements, 208 Coleoptera, 89 n, 487 Cold, influence of on Animals, 174; on Plants, 172,3 Collomia, 21 Colocasia odora, 480 Colour of plants, 54, 374, 452* ; of blood, 363 Columns, motor and sensory, 570, 578 Comatula, 107*, 136, 267, 270 Combustion, spontaneous, 477 ?i Commissures, 666*, 581, 582 Compensation, principle of, 207 Complete metamorphosis, 87 Composition, unity of, 196-9 CoNCHiFERA, 102; circulation in, 305; digestion in, 274 ; nervous system in, 667 Conditions of Vital Action, 168 Condor, 188 Conductors of nervous influence, 563, 591 Co7iferv(B, 62, 69*, 246, 374, 520, 541 n Coniferce, 25, 57* 473 Connection of Animals with Plants, 124-8; of Animal groups, 122,3 ; of Plants, 70 Consciousness, peculiar to Animals, 218 Consistence of organised structures, 16 Contractility, 167, 229, 552*, 557, 586 Coral, red, 119; stony, 120 Cornus mascula, 431 Corolla, 54 Corpora quadrigemina, 582 Corpus callosum, 582 Cotyledons, 49, 50*, 380, 416, 526* Crassamentum, 365 Crepuscular animals, 181 Crinoidea, 107*, 119 Crocodile, 313, 390, 411 Crop, 259, 273, 274, 277 Crustacea, 83, 84* ; circulation in, 303 ; digestion in, 273 ; exhalation in, 435 ; metamorphosis of, 84; nervous system in, 576 ; phosphorescence of, 474 ; res- piration in, 399-401 ; secretion in, 459- 462 Cryptogamia, 49; absorption in, 246,7j nutrition in, 355 ; symmetry of, 132 Crystallisation, 13, 69, 466 Cuticle of Plants, 428, 429 Cuttle-fish, 96*, 308 n, 569 Cyanosis, 329, 494 Cydo-gangliata, 74, 567 Cyclo-neura, 74 Cylindrical nerve-tubes, 44 Cyntliia, 274, 567 Deathf molecular, 153,4 ; somatic, 152,3, 317 Decajjoda, 84, 400 Decollation of shell, 100 Deglutition, 263,4 Deportation, 232 Dermo-skeleton, 82,3 Development, arrest of, 205 ; balance of, 207 ; eccentric, 204 ; progressive, 200-3 Diaphragm, 412, 413, 560 Diastase, 350 Diatoma, 69, 515 DicotyledoTis, 52, 626 Diffusion of gases, 371*, 420 Digestion, 237, 238, 240, 257*, 261 ; of coats of stomach, 258 ; in plants, 374 Digestive cavity, 237*, 239, 257 ; forma- tion of in embryo, 280 Dioecious plants, 49, 55* Dionaea, 219, 239, 555*, 562 Diplo-neura, 74*. 570 Diplozoon, 278, 298 Dischidia, 238 Disk, 349, 881 Distribution of species, 546. Diving animals, 315, 436 ; spider, 397. Dorsal vessel, 300, 301, 321, 322 Dotted ducts, 24, 27 Dracontium, leaf of, 53 Draco volans, 78, 194 Dropsical tissue, 35, 433 Dry-rot, 67 JDucts of Plants, 24 ; annular, 27 ; closed, 28 ; dotted, 24, 27 ; spiral, 27 ; reticu- lated, 29 Ductus arteriosus, 47, 238* ; pneumati- cus, 406 ; venosus, 328 Dugong, 27, 314, 326 Dura mater, 37, 47 Duramen, 25, 51*, 285 Earthworm, 91*, 240,272, 299, 392, 482 Echinodermata, 106 ; circulation in, 295,6 ; digestion in, 270 ; nervous sys- tem in, 565 ; symmetry in, 136 Echinus, 106*,7, 136, 270, 295, 565 Educability of animals, 550 ; of man, 551 Eggs, influence of light on, 183 ; of warmth, 492; respiration of, 426 (see Ovum) Ehrenberg, referred to, 113,4, 268 n, 565 Elasticity, 40, 159, 552 Electrical organs of Fishes, 507 Electricitt, influence of, on Animals, 187 ; on Plants, 186 ; evolution of, in Animals, 501-10; in Plants, 499, 600 ; sources of, 496-8 ; development in man, 502 ; in Fishes, 504-10 ; in muscles, 503 ; influence of nerves on, 508,9 ; uses of, 510 Elytra, 89* n, 397, 574 n Enchelis, 114, 268 n Endogens, 50, 70, 127 ; circulation in, 287 ; growth of, 355 ; stem of, 52 Endosmose, 160,1, 243,4*,5, 252, 442, 454 Entophytic Fungi, 66, 517 Entozoa, 94*, 517 ; absorption in, 271,2 ; circulation in, 293,7 ; nervous system in, 571 Epidermis, 39*, 95 ; appendages to, 39 Etiolation, 373,5, 433*, 447 Evaporation, 430,34,9 Euphorbia, 345, 446, 454, 473 Eustachian valve, 328 Exanthemata of Plants, 66 Excentric development, 204 Excretion, 227*,434, 441* ; in Animals, 462-6 ; in Plants, 453 ; by roots, 454 Exhalant glands, 434 Exhalation, 227, 427* ; in Animals, 434-40 ; in Plants, 428-33 ; dependence of, on light, 432 ; on heat, 437,8 ExoGENS, 50, 70, 128 ; circulation in, 285,6 ; growth of, 355 ; stem of 52 Exosmose, 244*, 454 Experiment, uses of in Physiology, 4, 5, 213, 222 Exuviation of Crustacea, 84 Fairy rings, 355 Falx, ossification of, 47 Fasciaj, 32, 37* Fat, structure of, 33 Fecula, 349 ; conversion of, 350, 380 Ferns, 59, 70 ; absorption in, 248 ; cir- cidation in, 284 ; reproduction in, 523 Fertilisation, 524,5, 532 jFiiire, elementary, of Animals, 31,2; of 2i 474 Plants, 21 ; glandular, 25*, 57 ; mus- cular, 31, 42«, 43 ; woody, 25 Fibrin, 43, 369, 361*, 364, 365 Fibrous membrane, 37 Fibro-cartilage, 40 Fibro-vascular tissue, 25, 285 Final causes, 210, 455, 597* Fire-flies, 475,6 Fishes, 81 ; circulation in, 309 ; diges- tion in, 276 ; electricity in, 504-10 ; embryonic development in, 539 ; exha- lation in, 435 ; lymphatic system in, 332; nervous system in, 580; phos- phorescence of, 474,7 ; temperature of, 483 Fissiparous reproduction, 528,9 n, 543 Fixation of carbon, 373-5 Floral envelopes, 54, 381 Flower, structure of, 54, 55 ; symmetry of, 133 ; changes produced by, 381 , 480 Fluids, passage of through tissues, 160,1 245* Flustra, 117 n Foetus, circulation in, 328 Follicles, simple, 458 ; compound, 459 Food of Animals, 240,2 ; of Plants, 234-6 Foramen ovale, 326-9 Form of organised structures, 13 Fraxinella, 453 Freezing of Fishes , 156, 483 Frond, primary, 523,6 Fruits, ripening of, 451 Fucus, 374, 383, 521 Functions i 3* ; animal, 218 ; organic, 214 ; 222 ; reproductive, 215 ; specialisation of, 200*, 247, 256, 279, 399, 415, 456, 518 ; antagonism of, 216,7 Fungi, 63,4*-7, 70, 125, 235, 517 n ; ab- sorjotion in, 246; circulation in, 283; respiration of, 377 ; reproduction in, 522 Fungin, 235 Ganglia, 44, 74, 561* Gasteropoda, 99 ; circulation in, 306 ; digestion in, 274 ; nervous system in, 568 ; respiration in, 391 ; symmetry in, 138 Gastric juice, 261, 277 Gelatine, 35,8, 40,1, 366* Gemmfe of Cryptogamia, 60,1*, 180, 522 Gemmiparous reprodiiction, 528,9 n Gemmules, of Sponges, 121, 219, 529 ; of Polypes, 116,8, 219, 529, 562 Generation, equivocal, 517 ; spontaneous, 516 Germinal membrane, 34, 203, 320, 534*-6 vesicle, 533,4 Germination, 50*, 380, 480, 526 ; influence of electricity on, 186 Germ-spot, 320, 492, 533*,5 Gills, 195, 391,2,9, 405 ; aerial, 398 Gizzard, 117, 273,4,7 Glands of Plants, 448 ; of Animals, 456- 461 ; evolution of in embryo, 472 ; di- mensions of ultimate portions, 470 Globules in fluids of plants, 346, 353,4 ; of chyle, 357-8 ; of lymph, 360 ; of blood, 362,3 Glow-worm, 474,5 Gluten, 450 Graafian vesicle, 434 Granules, reproductive in Plants, 519-21, 529 germinal in Animals, 530,1 Grey matter of nerves, 44, 561, 579 Gum, 346,7* Gymnosperm(s, bl*, 70 Gymnotus, 504-10 Harmony of forms, law of, 208 Haustellata, 89 Heart, formation of, 321-6 ; malforma- tions of, 329 ; propelling influence of, 316 Heat, action on plants, 173 ; action on animals, 174 ; degree of, consistent with life, 171,5 ; evolution of, 478 ; in plants, 479,80 ; in animals, 481-95 ; sources of, 494,5 Hedysarum gyrans, 557 Helianthus, 431, 473 Helianthoida, 120 Hemispheres of brain, 580-3 Hemiptera, 89 n*, 576 HepaticcB, 61 (see, Marchantia) Heterogeneous structures, 200, 513 Hive-bee, temperature of, 487-490 Holothuria, 107*, 122, 270, 296, 390, 411, 565 Holly, leaf of, 53 Homogeneous structures, 200, 513 Homoptera, 89 n, 575 Humble-bee, 396, 424, 486,8 Hybernation of Animals, 101, 156*,7, 175, 489. 493 ; of Plants, 157 Hybrids, production of, 545 Hydatid, 112 (see, Acephalocyst) Hydra, 38, 115*, 219, 237, 264 n,6, 562 Hydraform Polypes, 116, 267 Hydrangea, flower of, 54 Hymenium, 64, 522 Hymenoptera, 89 n*, 485 Ichneumon strobilella, 242 Imago, 88*, 273, 574 Impatiens noli-me-tangere, 558 Impressions, 561 Incomplete metamorphosis, 87 Incubation of bees, 488 ; of birds, 492 Individuality of parts, 15 Induction, 191 Inflammation, 364,6 Infusoria (see, Polygastrica) Ingestion of aliment, 223*, 234 Injuries, reparation of, 84,5, 233, 364*, 514 Ink-bag, 96*, 274, 465 Insalivation, 259, 260 Insects, 86-9* ; circulation in, 300 ; di- gestion in, 273 ; electricity of, 504 ; exhalation in, 436 ; metamorphosis of. 475 86, 203; nervous system in, 572-5; number of, 86 ; phosphorescence of, 475,6; respiration in, 394-8, 424,5; secretion in, 460-5 ; temperature of, 485-90 Instinct of Plants, 249 ; of Animals, 122, 589-93 ; acquired, 549-51 Intellectual faculties, 588 Intercellular spaces, 283,5, 449 Invertebral substance, 40 Invertebrata, 73*, 83 ; blood in, 362, 3 ; exhalation in, 435 ; nutrition in, 362 ; respiration in, 403 ; temperature of, 482 Inversion of Articulata, 83 n Involuntary movements, 560, 586,9-93 Iris, leaf of, 428 Janthina, 101, 465 Kidney, 460,5,6 Kiernan, Mr., referred to, 461 Lacerta ocellata, 312 Lacteal absorption, 262, 275* Laminaria buccinalis, 124 Lamprey, 81, 405 Land-crab, 84 w, 449 Larva, 86,7*, 203, 273, 424, 572 ; respi- ration of, 395-7 ; temperature of, 485 ; voracity of, 87, 203, 231 Latex, 286,8, 444 Laws, of Nature, 147,8; of Vital Action, 3, 190; generalisation of, 141, 190 Leaves, arrangement of, 133 ; structure of, 53, 428 Leech, 91*, 272, 392, 482 Lepidoptera, 89 n, 485 Lepidosteus, 81, 406 Libellula, 398 Liber, 51*, 286 Lichens, 63,8*, 124, 235, 517 ; absorp- tion in, 246 ; circulation in, 283 ; re- production in, 517, 522 ; secretion in, 451 Lichens, 63,8*, 124,235,517 ; absorption in, 246 ; circulation in, 283 ; reproduc- tion in, 517, 522 ; secretion in, 451 Life, 141,2,6, 167,8 Ligamentous structure, 37 ; in Conchifera, 37,102*; in FelinEe, 37 Ligamentum nuchas, 37, 47* Light, influence of, on Animals, 181-4, 408; on Plants, 178-80,373,4,432,3, 444; evolution of in Animals, 474-7 ; in Plants, 473 Lignin, 351 Lime (see Calcareous deposits) Liquor sanguinis, 360,4 Littorina petrsea, 647 Liver, 458,9, 462,3 Lizards, 78*, 199, 276, 539 Lobules, 459-61 Lycopocliacece, 57,9* Lymph, composition of, 360 ; movement of, 340, 359 ; coagulable, 364 Lymphatics, 331 ; lymphatic glands, 334 ; hearts, 333 Malaxis paludosa, 527 Malformations, 204*-6, 329, 330 Mammalia, 75* ; circulation in, 315 ; digestion in, 278 ; embryonic develop- ment in, 540 ; lymphatic absorption in, 335 ; nervous system in, 582 ; respira- tion in, 413,4 ; secretion in, 462-8 ; temperature of, 491 Mammary secretion, 469, 541 Man, educability of, 551 ; development of mind in, 594 Mandibulata, 89 ; mandibles, 89, 96, 273 Mantle, 95*,99 n, 104, 569 Marchantia, 61*. 353, 429, 523 ; influence of light- on, 180 MarsileacecB, 59, 524* Marsupialia, 75*, 492, 541,6, 582 Mastication, 259, 260 Medulla oblongata, 569, 578* Medullary rays, 23, 51* ; sheath, 51, 282 Medusa, 108*, 269, 389, 474, 565 Melolontha, 487, 675 Membr-ane, elementary of Animals, 31 ; of Plants, 21 ; fibrous, 37 ; mucous, 38*, 39, 262 ; serous, 36, 580 ; synovial, 36 Mercurialis, 264 Mesentery, 270*,1 Metamorphosis, 80 ; of Batrachia, 79*, 311,408; of Insects, 86-8, 672-4; in Plants, 54, 527 Microscopic investigation, 20, 394 n Milkiness of sap, 446 Mimosa, movements of, 219, 566,6 Mind, operations of, 587 ; development of, 579, 694 Mineral deposits in organised structures (see Calcareous and Siliceous) Modifications induced, 537 MoLLtrscA, 73*,4, 95, 122-8 ; circulation in, 304 ; ligament of, 37 ; nervous sys- tem in, 567-9 ; phosphorescence of, 474 ; respiration in, 391 ; reproduction in, 530 ; secretion in, 462-7 ; symmetry in, 138 ; temperature of, 482 Momordica elaterium, 558 Monocotyledons, 50, 626 Monoecious Plants, 49, 55* Monotremata, 76*, 541,6, 582 Monstrosities, 205,6, 614 Mosses, 60* ; absorption in, 247 ; circu- lation in, 284 ; reproduction in, 523 Motions, spontaneous of Plants, 653-8 ; of blood, 318 ; of gemmules, 116, 121 ; of sporules, 520 Motor column, 570,8 ; nerves, 561 Mould, mildew, &c., 64,6, 541 n Mucous membrane, 38*,9, 262 ; crypts, 458, 468 ; layer, 535 Murex, 99, 100*, 543 Muscles, structure of, 42,3 ; contractility of, 559; heat of, 481; electricity of, 503 Mtriapoda, 90 ; circulation in, 299 ; 2 I 2 476 digestion in, 272 ; nervous system in, 571 ; respiration in, 293 Myxine, 81 Natural classiiication, 48, 72 Naturalist, objects of the, 6, 48 Nasturtium, monstrosity in, 206 ; light of, 473 Naviculae, 113 Nautilus, 97*, 308, 569 Nematoidea, 94 Nepenthes, 26, 239 Nereis, 91*, 392,8, 474 Nerves, structure of, 44, 561 ; motor, 561, 586; sensory, 661, 515; sympa- thetic, 222, 561, 582, 695 Nbrvoxjs system of Animal life, 561 ; of organic life, 661 ; functions of, 230. 561-3, 584-595 ; evolution of, 683 Neurine, 44 Neuro-skeleton, 82,3 Nitrogen, present in Vegetables, 17; in- fluence of, on vegetation, 379; on Animals, 421 Nostoc, 63, 553 Nuclior adnata, 116 Nurse-bees, 488 Nutrition-, 226*, 341-3, 367-8 Nycteribia, 89 Observation of vital phenomena, 4,6, 212, 213, 222 Odours of animals, 468; of plants, 449 CEsophagus, 263 Oils, secretion of, 445,9, 450, 468 Oleander, 428,9 Olfactive ganglia, 578, 580-3 Operculum of Fishes, 405 ; of Gasteropo- da, 101,3; T)f Mosses, 60 Ophidia, 79 (see Serpents) Opium, 446,454,5 Opposite leaves, 133 Optic gangha, 569, 578, 580-3 Orbicula, 103 OrchidecB, 23, 263, 449, 643 Organic compounds, 162; functions, 31, 40, 214 Oi'ganisation, 8; of fluids, 166; a vital process, 162,6, 343*, 352, 367 Organisable products, 343,9, 362 Organised structures, 8 ; actions of, 1 40- 166 ; aggregation of, 16 ; consistence of, 16 ; constitution of, 17 ; durability of, 18 ; form of, 13 ; size of, 14 ; ten- dency to decay in, 18 Organism, 8 Ornithorhyncus, 75, 641 Oscillatorice, 69, 653 Ossification, 40 Ostrich, 76, 492, 465 Otter-breed, 550 Ovarium, of Animals, 533 ; of Plants, 55; of Polypes, 116 Ovo-viviparous reproduction, 529, 538 Ovules of Plants, 524-7 Ovuliim of Animals, 533 Ovum, 529-32 Oxygen, absorbed by Animals, 419, 420 ; by Plants, 376 ; given out by Plants, 373,4 Pachydermata, 76, 278 Pseony, flovrer of, 64 Pancreas, 458, 464 PapaveracecB, 446, 464,5 ParamcEcium, 114, 268 n, 528 Parasitic Fungi, 66 ; grovsfths, 517 Parenchyma, 24, 53 Parotid gland, 459, 461 Particles, coloured of blood, 362,3 Par vagum, 570, 572 Patella, 99, 543 Pavement of Basingstoke, 67 Pennatula, 119*, 136, 476 Pentacrinus, 107*, 136, 270 Perennibranchia, 408,9 Pericardium of Cephalopoda, 97 n Periodical changes, 157, 557 Permanence, of laws of Nature, 148 ; of forms of organised beings, 2 ; of inor- ganic masses, 1, 10; occasional, of or- ganised structures, 10 n, 155-7 Perspiration, 434 Petals, 54 Petiole, 53 Phalsena strobilella, 242 Phanerogamia, 49 ; absorption in, 248 ; reproduction in,625,6; symmetry of, 133 Pharynx, 259 Phosphorescence, 473-7 Physalia, 108, 383,9 Physiological science, 3 ; difficulties in, 4 ; proper mode of pursuing, 5-7, 210, 211 Picromel, 463 Pileus of Fungi, 64, 522 Pine-tribe, 25, 50, 57* Pistil, 65, 525 Pitchers of Plants, 198, 239 Pith, 23, 57* Placenta, 540 Planorbis, 138 Plastic lymph, 364 Pollen, 55*, 518, 619,526 POLYGASTRICA, 109, 113*; digestion in, 268 ; circulation in, 293 ; reproduction in, 628-30 , respiration in, 388 ; sensi- bility of, 564 Polypidom, 116*, 118, 120 POLYPIFEKA, 34, 69, 1]6*-120; circula- tion in, 116 n, 293 ; digestion in, 266,7 ; reproduction in, 529 ; respiration in, 388 ; symmetry of, 133 PoRiFERA, 34, 121*, 136; absorption in, 265 ; movement of fluid in, 121, 265 ; phosphorescence of, 476 ; reproduction in, 529 ; respiration in, 388 Pressure, influence of, 188 Primitive trace, 536, 583 Primrose, cowslip, &c., 543 Principle, 141 ; vital, 140-3 Progressive development, law of, 200 Property, use of tlie term, 150 INDEX. 477 Properties, of inorganic matter, 10, 17, 43, 600} vital, 10, 43, 149-154, 166,7, 343, 600 Protelmintha, 111 Proteus, 312, 324, 408,9, 484 Protococcus, 63, 69*, 246, 255, 518,9 Pkotophyta, 69*, 70, 124 Proximate principles, 19*, 342,3 Pterodactylus. 76, 78*, 193 Pteropoda, 98* ; digestion in, 274 ; res- piration in, 391 Pulmonary branchiae, 402 Pulmonic cavities, 391,2, 402, 406, &c. Pupa, 87*, 485-8, 673 Pyrosoma, 104, 474 Python bivitatus, 333 Qiuxdrumana, 75 Quinary system, 130 Quinine, composition of, 19 Races, hybrid, 545 Eadiata, 73*, 105*, 122, 125, 128; nervous system in, 565 ; symmetry of, 136 Rafflesia, 58 Raphides, 445 Ray, 276, 464, 538, 580 ; electric, 504 Receptacles, 349, 381 Red snow, 11, 63, 69 (see Protococcus) Regeneration of parts, 47, 84,5, 514 Reparation of parts, 233, 364,5 Repetition of parts, 15, 109, 136, 566 Reposition, interstitial, 18 Representation, doctrine of, 130 Reproduction, 215*, 228*, 511-7; gene- ral form of, 513-5, 528 ; special form of, 513, 519, 529 ; of parts, 84,5, 614 Reproductive cells, 619 ; granules, 519- 521, 530 Reptiles, 77 ; circulation in, 310-3 ; digestion in, 276 ; lymphatic system in, 333 ; nervous system in, 581 ; respira- tion in, 408, 411 ; temperature of, 484 Resinous secretions, 449 Respiration, 212,3, 227*, 369-72, 386,7; artificial, 213, 495 ; connection of with light, 184, 375 ; by general surface, 384, 422 ; of embryo, 426, 538-40 ; influence of temperature on, 425 Respiratory nerves, 570, 572 ; system, evolution of in Animals, 416,6; in Crustacea, 401 ; in Plants, 385 Reversion, 138, 139 Revivification of organised beings, 60, 61, 93 n, 165,6, 435 Rhizantliece, 58, 70 Bodentia, 76, 278, 640, 582 Roots, 248-60; excretions from, 464,5 Rotation of crops, 465 Ruminating stomacli, 278 Salamander, 79, 324, 458, 514 Salivary glandf , 459, 467 ; secretion, 259, 260, 467 Sap, ascending, 344; ascent of, 286, 9; elaborated, coagulation of, 346 ; move- ment of, 286, 288, 290 Sarracenia, 239 Sauria, 78*, 199, 276, 411, 484 Scorpion-trilDe, 85*, 302, 402, 576 Secretion, 227*, 441-3 Seed, 49*, 349, (see Germination) ; vita- lity of, 155, 158, 173 Selecting power, 245, 250, 276, 443 Sensation, 218, 561*, 584,5, 592,3 Sensibility of Acrita, 563 Sensitive plant, 219, 555 Sensorium, 661 Sensory columns, 570,8; nerves, 561 Serous layer, 635 ; membrane, 36 Serpents,'79*, 276, 410, 484, 539 Serpula, 91*, 99, 392 Sertularia, 116, 476 Serum, 365; serosity, 466 Setse, 392*, 393, 397 Shark, 276, 464, 638, 680 Shell of Annelida, 91 ; Cirrhipoda, 83, 92*; Crustacea, 83, 84*; Echinoder- mata, 106; Mollusca, 39, 83, 95*; chambered, 97, 102 Shields of Lichens, 63, 68*, 517 Siliceous deposits, 16, 121 Silurus, 504,7 Siphuncle, 97 Siponculus, 107*, 122, 565 Siren, 408,9 Size of organised structures, 14; alter- ation in, 547 Skeleton, 82, 199 Skin, secretions of, 434, 468 Sleep, of Animals, 156 ; of Plants, 557 Snail, 99, 101, 306, 391, 482 Solfatara, 374 n Spatangus, 106*, 107, 240 Specialisation, principle of, 200*, 247, 266, 399, 415, 456, 518 Species, distinction of, 542-4 ; distribution of, 546 Sphinx ligustri, 396, 372-4, 486 Spider-tribe, 85*, 302, 402, 576 Spinal cord, 74, 561 Spini-cerebrata, 74 Spiral vessels, 26*, 382; tendency, 99, 126, 131-8 Spondylus, 102 Spongioles, 16, 161, 248*, 250, 289 Spontaneous combustion, 447 n ; division , 615 Spore, 49, 69, 65, 519; of Algae, 219, 520,1 ; development of, 523 Stamens, 55*, 206, 527 Stem, structure of, 51,52; spiral growth of, 134 Sterelmintha, 109, 111*, 136; absorp- tion in, 271 ; nervous system in, 564 Stigma of Plants, 55*, 525 Stigmata of Insects, 392, 395 Stimuli, 144*, 146, 165, 167-70 Stomach, nature of, 237,9 ; human, 278; in camel, 278 ; ruminating, 278 ; ad- umbration of in plants, 230 478 INDEX. Stomata, 382, 429*. Style, 55*, 525 Swallowing, 263, 592 Swimming-bladder, 195, 406 Sword-fish, 276, 406 Symmetry, 131-9 Sympathetic movements, 692; nerves, 222, 561*, 582, 595 Syncope (fainting), 152, n Synovial membrane, 36*, 46, 457 Systems, natural and artificial, 48, 72, 105, n; circular, 129 ; quinary, 130 Tadpole, 79, 183, 581 Taenia, 109, 112*, 271 Tannin, 250, 455 Teeth, structure of, 41, w; situation of, 259 Temperature of Animals, 482-92 ; of flowers, 480 ; of seeds, 480 ; of trunks, 479 Tendinous structure, 37 Tendril, various forms of, 195 Tentacula, 81, 97, 108, 115 Tentorium, ossification of, 47 Tetraodon, 504 Thallus, 61*, 62, 68 Thecse, 59*, 60, 521, 523,4 Thoracic duct, 334,5 Thymus gland, 47 Tissues, of Animals, 31 ; adipose, 33, 457; areolar, 34 ; cartilaginous, 40 ; cellular, 34 ; fibrous, 37 ; muscular, 42 ; mucous, 38 ; nervous, 44 ; osseous, 41 ; serous, 36 ; of Plants, 21 ; cellular, 23 ; ligne- ous, 25 ; muriform, 23 ; vasiform, 24 ; vascular, 26 Tone of living tissues, 35, 457 Torpedo, 504-10 Torpidity (see Hybernation) TracheEB, 26*, 37, 127, 382, 393*-8 Transformation of tissues, 45-7 Transposition of viscera, 139 Trichina spiralis, 111 Trichiurus, 504 Trigla, 580 TropcEolum, 206, 473 TubicolEe, 87, 91* Tubularia, 116 w TuNiCATA, 104; circulation in, 305; digestion in. 274; nervous system in, 567 Type of natural group, 129 Ulva clathrata, 520 Umbilical vesicle, 540 Umbilicus, 83, n Unger, referred to, 66 Unity of Composition, law of, 196, 197* ■ fundamental of structure, 196 Urania speciosa, 353 Uredo, 66 Uric acid, 466,471 Urine, urea, 466 Urn of Mosses, 60 Uterus, 540 ; temperature of, 481 Vapour, quantity of exhaled by Plants, 431 Man, 440 Variation, tendency to, 546-550 Varicose nerve-tubes, 44 Varieties, 542 ; propagation of, 550 Vascular tissue of Plants, 26 area, 321-3, 637 layer, 320, 635-7 Vasiform tissue of Plants, 24 Vegetable kingdom, 48, 70, 128 Vegetables distinguished from Animals, 218-21 ; electricity of, 499, 300 ; heat of, 479, 480 ; phosphorescence of, 473 Vegetating Wasp, 517 w Velella, 108 Venation of leaves, 53 Vena porta, 328*, 461, 463 Ventilation of bee-hive, 490 Ventricle of heart, 304 Ventricles of brain, 580,3 Ventriculus succenturiatus, 277, 458 Venus, 103 Vermetus, 99 Vermiform fishes, 81*, 122, 680 Fens cavitaires, 94 j)arenc}iymateux, 111 Vertebkata, 73*, 74, 82, 122, 123, 128 ; absorption and digestion in, 276,6 ; nervous system in, 577-9 ; respiration in, 404 ; symmetry in, 139 Vertebrae, 37 Vessel, spiral of Plants, 26 Villi of mucous membrane, 262, 358 Visceral nerves (see Sympathetic) Vital actions, 3, 146, 152, 155 principle, 140, 141 properties, 10, 43, 140, 149, 151-4, 600 Vitality, 149; of blood, 343, 365; of seeds, 155 Viviparous reproduction, 515, 629, 540 Voluntary motions, 660, 588 Volvox globator, 530 Vorticella rotatoria, 93*, 114 Vorticils, 133 Water, influence of salt, 189 Whale, 188 m, 199, 260 (see Cetacea) White matter of nerves, 44, 561, 579 of egg, 356, 534* Will, influence of, 560, 688 Wings of Vertebrata, 193, 194 Insects, 88, 194 n, 398 WoUaston, his theory of secretion, 531 Wood, formation of, 351 n, 356 Woody circles in Exogens, 61 fibre, 25 ; glandular, 26, 57 Yolk-bag, 533*, 537-9 Yolk of egg, 366, 633*, 537, 538 Young animals, respiration in, 424; tem- peratui-e of, 492 PRINTED BV PHILP AND EVANS, 29, CLAHE-STREET, BRISTOL,