iHb LIBRARY .OF THE DENTAL DEPARTMENT, UNIVERSITY OF CALIFORNIA. This book must be returned within four days. Fine, five cents each day for further detention. \rsrsisjsjsrsmmrsiSfsmismi=tfHiHJsmisisrsisfBfmsfsisisisfSfSj-afSJsmi (3^ ^^I^^i^, ^r^'^^ CARPENTER'S ELEMENTS OF PHYSIOLOGY, ELEMENTS PHYSIOLOGY, INCLUDING PHYSIOLOGICAL ANATOMY. WILLIAM B. CARPENTER, M.D., F.R.S., F.G.S., EXAMINER IX PHYSIOLOaT AND COMPARATIVE ANATOMT IN TOE UNIVERSITY OF LONDON; AND AUTHOR OP "THE PRINCIPLES OF HUMAN PHYSIOLOGY," AND "THE PRINCIPLES OF GENERAL AND COMPARATIVE PHYSIOLOGY," ETC. SECOND AMERICAN, FROM A NEW AND REVISED LONDON EDITION. WITH ONE HUNDRED AND NINETY ILLUSTRATIONS. PHILADELPHIA: BLANCHARD AND LEA. 1851. K C. SHERMAN, PRINTER. A^ecesijiou yo./..y.V^ n iwJ /65 /v •^%> L ^«S? ^j: //>, '^S^^'^l ■^y or. It, "y AMERICAN PUBLISHERS' ADVERTISEMENT. The present volume has been printed simultaneously with the Second London Edition. The numerous alterations which Dr. Carpenter has introduced, and the thorough manner in which he has revised every portion of it, have rendered unnecessary any notes or additions. The efforts of the publishers have, therefore, been directed towards obtaining a correct reprint, to accomplish which, its passage through the press has been supervised by Dr. F. G. -Smith, Lecturer on Physiology in the Philadelphia Association for Medical Instruction. It is, therefore, con- fidently presented to the Profession, as in every way worthy of the high reputation which it has obtained as an elementary text-book. Philadelphia, October, 1851. PREFACE The present volume owes its origin to a desire on the part of the Publisher, that an elementary treatise on Physiology should be added to the series of »admirable Students' Manuals, on the various depart- ments of Medical Science, which he had previously issued. In carrying this desire into execution, the Author has endeavoured to avoid inflicting upon the class for whose use the Treatise is especially intended, the injury of placing in their hands such a superficial and imperfect sketch of the science, as, whilst affording them but a limited amount of knowledge of its facts, should leave them very ill-informed as to its general doctrines. His object has rather been to convey to the Student as clear an idea as possible of those Principles of Physiology which are based on the broadest and most satisfactory foundation, and to point out the mode in which these principles are applied to the explanation of the . phenomena presented by the living actions of the Human body. In this manner has the Author desired to prepare him for that more detailed study of the latter, which becomes necessary when Physiology is pursued (as it ought to be) in connexion with the changes produced in the living body by Morbific and Remedial Agents, and is thus taken as a guide in the study of the causes, prevention, and treat- ment of Disease — which should be the primary object of attention with every one who undertakes the Practice of his Profession. Although this Manual combines in some degree the scope of the Author's " Principles of Physiology, General and Comparative," and of his "Principles of Human Physiology," yet it cannot be regarded as a mere abridgment of them, having been written for the most part with very little reference to them, and with every desire to make it complete in itself. As the matter of which these volumes are composed is itself condensed to the utmost practicable degree, it is manifestly impossible that the present Manual should contain more than a mere* outline of the subjects of which they treat. To them, therefore, he Vlll PREFACE. would refer such of its readers, as may desire further information upon various topics which are here only slightly touched upon ; and in them, also, will be found references to various original authorities, the intro- duction of which would be incompatible with the limited scope of a treatise like the present. The Author has only to add, that he feels most grateful for the kind appreciation which this Manual has experienced ; and that in the prepa- ration of the present Edition, he has used his best endeavours to render it still more worthy of a favourable reception. The whole treatise has been subjected to a most careful revision ; many statements which the advance of science has shown to be doubtful or erroneous, have been omitted or corrected; and a considerable amount of, new matter has been introduced. Of the First, Eleventh, and Twelfth Chapters, more especially, a considerable proportion has been entirely rewritten ; and the Author ventures to believe that the doctrines which they contain will enable such as may master them to obtain a clearer comprehension of the facts of Physiological Science, than they could previously have acquired. Regent's Park, London, September, 1851. TABLE OF CONTENTS. BOOK I. GENERAL PHYSIOLOGY. I. On the natuke and objects of the Science of Physiology 1. General Characters of Organized Structures 2. Distinctive Characters of Vital Actions 3. Of the Forces concerned in the production of Vital Phenomena 4. Of Degeneration and Death ..... 5. General Summary ...... II. Of the Exteknal Conditions of Vital Activity . 1. Of light, as a Condition of Vital Activity 2. Of Heat, as a Condition of Vital Activity 3. Of Electricity, as a Condition of Vital Activity . 4. Of Moisture, as a Condition of Vital Activity III. Of the Elementary Parts of Animal Structures . 1. Of the Primary Components of the Animal Fabric . PAQE. 17 18 25 44 53 56 68 61 71 94 97 105 107 119 127 2. Of the Simple Fibrous Tissues 3. Of the Basement or Primary Membrane 4. Of Simple Isolated Cells, employed in the Organic Functions . 130 5. Of cells connected together, as permanent constituents of the Tissues 154 6. Of Cells coalesced into Tubes, with Secondary Deposit . . 196 BOOK II. , SPECIAL PHYSIOLOGY. IV. Of Food, and the Digestive Process ..... 286 1. Sources of the Demand for Aliment .... 286 2. Of the Digestive Apparatus, and its Actions in general . . 252 3. Of the Movements of the Alimentary Canal . . . 257 4. Of the Secretions poured into the Alimentary Canal, and of the Changes which they eflFect in its contents . . . 264 5. Of Hunger, Satiety, and Thirst . . . .274 V. Of Absorption and Sanguification ..... 277 1. Of Absorption from the Digestive Cavity . . . 277 2. Of the Passage of Chyle along the Lacteals, and its admixture with the Lymph collected from the General System . . 281 8. Of the Spleen, and other Glandular appendages to the Lymphatic System ....... 286 4. Of the Composition and properties of the Chyle and Lymph . 292 5. Of Absorption from the External and Pulmonary Surfaces . 296 6. Of the Composition and Properties of the Blood . . .297 TABLE OF CONTENTS. CHAPTER. VI. Of the Circulation of the Blood .... 1. Nature and Objects of the Circulation of Nutrient Fluid 2. Diflferent forms of the Circulating Apparatus . 3. Action of the Heart .... • 4. Movement of the Blood in the Arteries . 5. Movement of the Blood in the Capillaries . 6. Movement of Blood in the Veins VII. Op Nutrition ..... 1. Selecting Power of Individual Parts 2. Varying Activity of the Nutritive Processes 3. Of Death, or Cessation of Nutrition 4. Disordered Conditions of the Nutritive Processes VIII. Of Kespiration ...... 1. Essential Nature and Conditions of the Respiratory Process 2. Different forms of the Respiratory apparatus in the lower Animals 3. Mechanism of Respiration in Mammalia, and in Man . 4. Chemical Phenomena of Respiration 5. Effects of Insufficiency, or Suspension, of the Aerating Process IX. Of Secretion . . . . 1. Of the Secreting process in general, and of the Instruments by which it is effected 2. Of the Liver and the Bile 3. Of the Kidneys, and the Urine 4. Of the Cutaneous and Intestinal Glandulae 5. General Summary of the Excreting Processes X. Of the Development of Light, Heat, and Electricity, in the Animal Body ........ XL Generation and Development 1. General View of the Nature of the Process 2. Action of the Male . 3. Action of the Female XII. Of the Nervous System . 1. General view of the operations, of which the Nervous System is the Instrument ...... 2. Comparative Structure and Actions of the Nervous System 3. Functions of the Spinal Cord and its Nerves 4. Functions of the Medulla Oblongata 5. Functions of the Sensory Ganglia 6. Functions of the Cerebellum 9. Functions of the Cerebrum 8. Functions of the Sympathetic System XIII. Of Sensation, General and Special . 1. Of Sensation in general 2. Of the Sense of Touch . 3. Of the Sense of Taste 4. Of the Sense of Smell . . : 6. Of the Sense of Hearing 6. Of the Sense of Sight . XIV. Of the Voice and Speech LIST OF WOOD ENGRAVINGS. 1. Simple isolated Cells, containing reproductive molecules 2. Fibrous structure of exudation-membrane ; after Gerber 3. Fibrous membrane lining egg-shell (original) 4. White fibrous tissue of areolar tissue and tendon ; after Gerber . 5. White fibrous tissue of ligament ; after Gerber 6. Yellow fibrous tissue of ligamentum nuchee ; after Gerber 7. Development of fibres from cells ; after Lebert 8. Ideal Section of a Joint ...... y. Capillary vessels of Skin ; after Berres 10. Capillary vessels of Intestinal villi ; after Berres . 11. Capillary vessels around orifices of Mucous follicles ; after Berres 12. Capillary vessels around follicles of Parotid Gland ; ditto 13. Distribution of Sensory nerves in Skin ; after Gerber 14. Primary membrane, with germinal spots ; after Goodsir . 15. Primary membrane, showing component cells ; after Goodsir 16. Cells from Chorda Dorsalis of Lamprey, after Quekett 17. Multiplication of Cartilage-cells by duplication; after Leidy 18. Parent-cells, with contained secondary cells, of cancerous structure; after Lebert ......... 19. Cells from fluid of Herpes ; after Addison .... 20. Oblique section of Epidermis ; after Henle .... 21. Epidermic cells from Conjunctiva ; after Gerber 22. Portion of Choroid-coat, showing pigment cells ; ditto. 23. Separate Pigment-cells ; after Mandl ..... 24. Detached epithelium-cells from mucous membrane of mouth ; after Lebert 25. Pavement-epithelium from bronchial tubes : after Lebert 26. Layer of cylindrical epithelium, with cilia ; after Henle . 27. Follicles from liver of Crab, with contained secreting cells ; after Goodsir 28. Follicles of Mammary gland, with contained secreting cells ; after Lebert 29. Secreting Cells of Human Liver ..... 30. Formation of Spermatozoa within cells ; after Wagner 31. Diagram of Intestinal Mucous membrane, in intervals of digestion; after Goodsir ........ 32. Extremity of Placental villus ; after Goodsir . . . , 33. Progressive stages of cell-growth, in Shell-membrane (original) 34. Progressive stages of coalescence of cells, in Shell-membrane (original) 35. Fusiform tissue of plastic exudations ; after Lebert 36. Areolar and Adipose tissue ; after Mandl .... 37. Capillary network around Fat-cells : after Berres 38. Cartilage of Mouse's-ear; after Quekett .... 39. Section of Cartilage ; after Schwann ..... 40. Distribution of vessels on surface of Cartilage ; after Toynbee 41. Nutrient vessels of Cornea; after Toynbee .... 42. Shell of Echinus (original) ...... 43. Sections of Shell of Pinna (original) .... 44. Tubular shell-structure from Anomia (original) 31 115 115 119 120 120 121 123 127 127 127 127 127 128 130 131 132 132 134 141 141 143 143 145 145 146 148 148 148 150 152 153 156 156 158 158 158 161 162 163 165 167 169 170 45. Cancellated structure at extremity of Femur ; after Toynbee . . 173 xu LIST OF WOOD ENGRAVINGS. 46. Lacunas of Osseous substance ; after Mandl 47. Section of Bony Scale of Lepidosteus (original) . 48. Network of Haversian canals, from vertical section of Tibia ; after Mandl 49. Transverse section of long bone; after Wilson 60. Section of Cartilage, near seat of Ossification; after Wilson 51, Section of Cartilage, at the seat of Ossification ; after Wilson 52. Vessels of Dental Papilla ; after Berres 63. Oblique Section of Dentine ; after Owen 54. Vertical Section of human molar tooth ; after Nasmyth 55. Development of Teeth ; after Goodsir 66. Structure of Hair of Musk-deer and Sable (original) 57. Structure of the Human Hair; after Wilson 58. Fasciculus of striated Muscular fibre ; after Mandl . 69. Non-striated Muscular fibres ; after Bowman 60. Striated Muscular fibre separating into fibrillte 61. Muscular fibre cleaving into disks; after Bowman 62. Transverse section of muscular fibres ; after Bowman 63. Structure of ultimate fibrillse of striated fibre (original) . 64. Nucleated fibres from non-striated muscle ; after Wilson 65. Fusiform contractile cells ; after Kolliker . • . 66. Nuclei in striated muscular fibres of foetus ; after Bowman 67. Capillaries of muscle ; after Berres 68. Distribution of nerves in Muscle ; after Burdach 69. Components of gray substance of brain ; after Purkinje . 70. Capillaries of Nervous centres ; after Berres . 71. Structure of Ganglion of Sympathetic; after Valentin 72. Distribution of Sensory nerves in lip ;^after Gerber . 73. Capillaries at margin of lips ; after Berres 74. Section of Human Stomach ..... 75. Mucous coat of small intestine, showing Villi, and orifices of follicles ; after Boehm ........ 76. Peyerian glandula ; after Boehm .... 77. Stomach of Sheep ....... 78. Section of Stomach of Sheep, showing derai-canal ; after Flourens 79. Lobule of Parotid Gland ; after Wagner .... 80. Gastric glandulse after Wagner .... 81. Orifices of Gastric tubuli ; after Boyd .... 82. Distribution of Capillaries in Intestinal Villus ; after Berres 83. Commencement of Lacteal in Intestinal Villus ; after Krause 84. Diagram of Lymphatic gland ; after Goodsir . 85. Epithelial cells of intra-glandular Lymphatic ; after Goodsir 86. Course of Thoracic duct ..... 87. Appearance of inflamed Blood; after Addison 88. Vascular area of Fowl's egg ; after Wagner . 89. Diagram of the Circulation in Fish 90. Diagram of the Circulation in Reptile . 91. Diagram of complete Double Circulation. 92. Anatomy of Human Heart and Lungs 93. Capillaries of Nervous centres, after Berres 94. Capillaries of Glandular follicles ; after Berres 95. Capillaries of Conjunctival membrane: after Berres 96. Capillaries of Choroid coat ; after Berres 97. Capillaries around orifices of mucous follicles; after Berres 98. Capillaries in Skin of finger ; after Berres 99. Capillaries in fungiform papilla of Tongue ; after Berres. 100. Doris, showing branchial tufts ; after Alder and Hancock 101. One of the arborescent processes of gills of Poris; ditto 102. Respiratory apparatus of Insects .... 103. Diagram of diflFerent forms of Respiratory Apparatus (original) 104. Capillaries of Gill of Eel (original) .... 105. Section of Lung of Turtle; after Boj anus 106. Capillaries of Human Lung (original) 107. Simple glandular follicles ; after Miiller . 108. Embryonic development of Liver ; after MUller LIST OF WOOD ENGRAVINGS. xm FIG. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144. 145. 146. 147. 148. 149. 150. 151. 152. 153. 154. 155. 156. 157. 158. 159. 160. 161. 162. 163. 164. 165. PA0E. Rudimentary Pancreas, from Cod ; after Miiller Mammary Gland of Ornithorhyncus ; after Miiller . Meibomian Glands ; after Miiller ..... Portion of Cowper's Gland ; after Miiller Lobule of Lachrymal Gland ; after Miiller Hepatic Follicles from Crab ; after Goodsir . Ultimate follicles from Mammary gland ; after Lebert . Surface of Lobule of Liver of Squilla ; after Miiller Interior of Lobule of Liver of Squilla ; after Miiller Liver of Tadpole ; after Miiller . . • • Distribution of Blood-vessels in Lobules of Liver ; after Kiernan Connexion of Lobules of Liver with Hepatic vein ; ditto Distribution of Hepatic ducts, around Lobules of Liver ; after Kiernan Secreting Cells of Liver ...... Development of Kidney, in embryo of Lizard ; after Miiller . Kidney of foetal Boa ; after Miiller . . . .• Portion of Kidney of Coluber ; after Miiller .... Fasciculus of tubuli uriuiferi of Bird ; after Miiller Section of Kidney ....... Section of portions of Kidney, slightly magnified ; after Wagner Distribution of vessels in Kidney ; after Bowman Vertical section of Skin ; after Wilson .... Hepatic Cells gorged with Fat ; after Bowman Various stages of development of Haematococcus binalis ; after Hassal Successive stages of development of simpler Algae ; after Kiitzing . Diagram representing the three principal forms of the Generative process in Plants (original) ....... Successive stages of Segmentation of the vitellus of Ascaris ; after Bagge Anatomy of the Testis . . . . . . The Uterus and its appendages ...... Successive stages of segmentation of Mammalian vitellus ; after Coste Formation of the Mulberry mass ; after Coste .... Plan of Early Uterine Ovum ; .after Wagner .... Germ and surrounding parts ; after Coste .... Vascular Area of Fowl's Egg ; after Wagner .... Diagram of Ovum, at commencement of separation of digestive cavity ; after Wagner ........ Diagram of Ovum, showing the formation of the Amnion ; after Wagner Diagram of Human Ovum, in second month, showing the Allantois ; after Wagner ........ Diagram of the Circulation in the Ovum at the commencement of the forma tion of the placenta ; after Coste ..... Extremity of Placental Villus ; after Goodsir External membrane and cells of placental villus ; after Goodsir Diagram illustrating the arrangement of the placental decidua ; after Goodsir Plan of the Foetal Circulation . . . . . Termination of portion of milk-duct in follicles ; after Sir A. Cooper Portion of the Ganglionic tract of Polydesmus ; after Newport Human embryo at sixth week ; after Wagner Dissection of the Medulla Oblongata, showing the connexions of its tracts ; after Solly (altered) .... Diagram of the relations of the Cerebrum to the Sensory Ganglia, as horizontal section (original) .... Diagram of the relations of the several parts of the Encephalon, as vertical section (original) .... Capillary network at margin of Lips ; after Berres Distribution of tactile nerves in Skin ; after Gerber . Capillaries of fungiform papilla of Tongue ; after Berres Distribution of Olfactory nerve ; after Wilson Diagram of the Auditory apparatus; after Wilson Refraction of rays of light through convex lens Formation of images in eye .... Capillary network of Retina ; after Berres Structure of the Larynx ; after Willis several EXPLANATION OF PLATE L The Figures in this Plate represent the Cells floating in the various animal fluids ; and they are all, with the exception of Figs. 4 and 5, copied from the representations given by M. Donn^ in his "Atlas de I'Anatomie Microscopique." These representations are transcripts of Daguerreotype pictures, obtained from the objects, by a solar microscope, with a magnifying power of 400 diameters. Fig. 1. Red Corpuscles of Human Blood, viewed by their flattened surfaces (§ 215). Fig. 2. Red Corpuscles of Human Blood, adherent by their flattened surfaces, so as to form rolls ; — at a, the entire surfaces are adherent ; at 6, their surfaces adhere only in part. Fig. 3. Red Corpuscles of Human Blood, exhibiting the granulated appearance which they frequently present, a short time after being withdrawn from the vessels. Fig. 4. Colourless Corpuscles of Human Blood (| 214). Fig. 6. The same, enlarged by imbibition of water. Fig. 6. Red Corpuscles of Frog's Blood (§ 215). Fig. 7. The same, treated with dilute acetic acid ; the first effect of which is to render the nucleus more distinct, as at 5 ; after which the outer vesicle becomes more transparent, and its solution commences, as at a. Fig. 8. The same, treated with water ; at a is seen a corpuscle nearly unaltered, except in having the nucleus more sharply defined ; at b, others which have become more spherical, under the more prolonged action of water ; at c, the nucleus is quitting the centre, and approaching the circumference, of the disk ; at d it is almost freeing itself from the envelope ; and at e it has completely Fig. 9. Globules of Mucus, newly secreted (§ 237). Fig. 10. The same, acted on by acetic acid. Fig. 11. Globules of Pus, from a phlegmonous abscess (^ 637). Fig. 12. The same, acted on by acetic acid. FLATE 1 (i> m 9 fc^ -v^#«' PLATE 21 14 SinrJairs iith. /^'2- EXPLANATION OF PLATE IL The Figures in this Plate represent the principal forms of the Nervous Centres in diffe- rent classes of animals. The 1st is copied from a Memoir by M. Blanchard ; the 2d, 3d, and 4th, from Mr. Newport's delineations ; the 5th to the 13th from the work of M. Guillot on the Comparative Anatomy of the Encephalon in the different classes of Verte- brata ; and the last two from the work of M. Leuret on the same subject. Fig. 1. Nervous System of Solen; a, a, cephalic ganglia, connected together by a trans- verse band passing over the (Esophagus, and connected with the other ganglia by cords of communication ; b, pedal ganglion, the branches of which are distributed to the powerful muscular foot ; c, branchial ganglion, the branches of which proceed to the gills d, d, the siphons e, e, and other parts. On some of these branches, minute ganglia are seen ; as also at/, /, on the trunks that pass forwards from the cephalic ganglia (^ 852). Fig. 2. Nervous System of the Larva of Sphinx ligustri; a, cephalic ganglia ; 1-12, ganglia of the ventral cord (§ 856). Fig. 3. Thoracic portion of the Nervous System of the Pupa of Sphinx ligustri; a, 6, c, three ganglia of the ventral cord ; d^ d, their connecting trunks ; e, e, respira- tory ganglia (^ 862). Fig. 4. Anterior portion of the Nervous System of the Imago of Sphinx ligustri; a, cepha- lic ganglia; b, b, eyes ; c, anterior median ganglion, and d, d, posterior lateral ganglia of stomato-gastric system ; g, /, large ganglionic masses in the thorax, giving origin to the nerves of the legs and wings (§ 863). Fig. 5. Brain of the Perch, seen from above (g 869). Fig. 6. The same, as seen from below. Fig. 7. Interior of the, same, as displayed by a vertical section. The following references are common to the three preceding, and to the succeeding figures. a, «, Olfactory lobes or ganglia. b, b, Cerebral ganglia or Hemispheres. c, c, Optic lobes. d, Cerebellum. e, Spinal Cord. /, Pineal gland. g, Lobi inferiores (their precise character not determined). h, Pituitary body. i, Optic Nerves. XVI EXPLANATION OF PLATE II. Fig. 8. Brain of the Common Lizard, seen from above (§ 871). Fig. 9. The same, as seen from below. Fig. 10. The same, as displayed by a vertical section. Fig. 11. Brain of the Common Goose, as seen from above (§ 872). Fig. 12. The same, as seen from below. Fig. 13. The same, as displayed by a vertical section. Fig. 14. Brain of the Sheep, viewed sideways (| 873). Fig. 15. The same, as displayed by a vertical section. In addition to the parts indicated by the preceding references, we have here to notice ; — k, the corpus callosum ; I, the septum lucidum ; and m, the Pons Varolii. BOOK I. GENEKAL PHYSIOLOGY. CHAPTER I. ON THE NATURE AND OBJECTS OF THE SCIENCE OF PHYSIOLOGY. 1. The general distribution of the objects presented to us by external nature, into three kingdoms — the Animal, the Vegetable, and the Mine- ral,— is familiar to every one ; and not less familiar is the general distinc- tion between living bodies, and dead inert matter. True it is, that we can- not always clearly assign the limits which separate these distinct classes of objects. Even the professed Naturalist is constantly subject to per- plexity as to the exact boundary between the Animal and the Vegetable kingdoms ; and the distinction between Animal and Vegetable struc- tures, on the one hand, and Mineral masses on the other, — or between living bodies, and aggregations of inert matter, — is by no means so obvious in every case, as to be at once perceptible to the unscientific observer. Thus, a mass of Coral, if its growing portion be kept out of view, or a solid Nullipore attached to the surface of a rock, might be easily confounded with the mineral bodies to which they bear so close a resemblance ; and a minute examination might be required to detect the difference. Nevertheless, a well-marked distinction does exist, between the organized structures of Plants and Animals, and the inorganic aggregations of Mineral matter ; as well as between the condition of a living being, whether Animal or Plant, and that of dead or inert Mineral bodies. It is upon these distinctions, which are usually obvious enough, that the sciences of Anatomy and Physiology are founded;: these sciences taking cognizance, — the former, of those structures which are termed organized^ — and the latter, of the actions which are peculiar to those structures, and which are distinguished by the term vital. It will be desirable to consider, in a somewhat systematic order, the prin- cipal ideas which we attach to these terms ; as we shall be thus led most directly to the distinct comprehension of the nature and objects of Phy- siological science. 2 18 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 1. General Characters of Organized Structures. 2. Organized structures are characterized, in the first place, by the peculiarities of their form. — Wherever a definite form is exhibited by Mineral substances, it is bounded by straight lines and angles, and is the effect of the process termed crystallization. This process results from the tendency which evidently exists in particles of matter, espe- cially when passing gradually from the fluid to the solid state, to arrange themselves in a regular and conformable manner in regard to one ano- ther. There is, perhaps, no inorganic element or combination, which is not capable of assuming such a form, if placed in circumstances adapted to the manifestation of this tendency among its particles ; but if these conditions should be wanting, and the simple cohesive attraction is ex- ercised in bringing them together, without any general control over their direction, an indefinite or shapeless figure is the result. — Neither of these conditions finds a parallel in the Organized creation. From the highest to the lowest, we find the shape presenting a determinate character for each species or race, with a certain limited amount of variation amongst individuals ; and this shape is such, that, instead of being circumscribed within plane surfaces, straight lines, and angles, organized bodies are bounded by convex surfaces, and present rounded outlines. We may usually gather, moreover, from their external form, that they are composed of a number of dissimilar parts, or organs; which are combined together in the one individual body, and are cha- racteristic of it. Thus in the Vertebrated or Articulated animal, we at once distinguish the head and extremities from the trunk, which consti- tutes the principal mass ; and where there exist no external organs of such distinctness, as in some Molluscs, the rounded character of the general form is sufficiently characteristic. The very simplest grades of animal and vegetable life present themselves under a shape, which ap- proaches more or less closely to the globular. It is among the lower tribes of both kingdoms, that we find the greatest tendency to irregular departures from the typical form of the species ; and thus is presented an approach, on the one hand, to that inclefiniteness which is characte- ristic of uncrystalline mineral masses ; and, on the other, to that variety of crystalline forms which the same mineral body may present, accord- ing to the circumstances which influence its crystallization. 3. With regard to size, again, nearly the same remarks apply. The magnitude of Inorganic masses is entirely indeterminate, being altoge- ther dependent upon the number of particles which can be brought together to constitute them. On the other hand, the size of Organized • structures is restrained, like their form, within tolerably definite limits, which may nevertheless vary to a certain extent among the individuals of the same species. These limits are least obvious in vegetables, and in the lower classes of animals. A forest-tree may go on extending itself to an almost indefinite extent ; certain species of sea-weed attain a length of many hundred feet, and their growth does not appear to be restrained by any limit ; and the same may be said of those enormous masses of coral, which compose so many islands and reefs in the Poly- OF ORGANIZED STRUCTURES IN GENERAL. 19 nesian Archipelago, or of which the debris seem to have constituted many of the calcareous rocks of ancient formation. But in these cases, the increase is produced by the multiplication of similar parts, which, when once completely evolved, have but little dependence upon one ano- ther, and might be almost considered as distinct individuals. Thus, each bud of a tree, if placed under favourable circumstances, can main- tain its life by itself, and can perform all the actions proper to~ the species. Each polype of the coral mass, in like manner, at first pro- duced by a process of budding from the original stock, comes in time to be completely independent of it, and of those with which it is associated. And in the sea-weed, each portion of the frond is an almost precise repe- tition of every other, and grows for and by itself ; neither receiving from nor communicating to, any other part, the materials of its organic structure. Thus among Plants and the lower Animals, we find an indefiniteness in point of size, depending upon the tendency to multiplica- tion of similar parts, which has been designated as vegetative repetition. 4. It is, however, in the internal arrangement or aggregation of the particles, respectively composing Organized structures and Inorganic masses, that we find the difference between the two most strongly marked. — Every particle of a Mineral body (in which there has not been a mixture of ingredients) exhibits the same properties as those possessed by the whole ; so that the chemist, in experimenting with any substance, cares not, 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, for instance, has all the properties 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 properties, and having no further re- lation among themselves than that which they derive from their juxta- position. Each particle, then, may be considered as possessing a sepa- rate individuality ; since we can predicate of its properties all that can be said of the largest mass. — The Organized structure, on the other hand, receives its designation from being made up of a number of dis- tinct parts or organs, each of which has a texture or consistence peculiar to itself; and it derives its character from the whole of these collectively. Every one of these, as we shall hereafter see, is the instrument of a certain action or function, which it performs under certain conditions ; and the concurrence of all these actions is required for the maintenance of the structure in its normal or regular state, and for the prevention or the reparation of those changes, which chemical and physical forces would otherwise speedily produce in it, from causes hereafter to be ex- plained. Hence ther.e is a relation of mutual dependence among the parts of an Organized structure ; which is quite distinct from that of mere proximity. Thus, the perfect plant, which has roots, stem, leaves, and flowers, is an example of an organized structure, in which the rela- tion of the different parts to the integrity of the whole is sufficiently obvious ; since, when entirely deprived of either set of these organs, the race^ must perish, unless the plant have within itself the power of re- placing them. 5. It is not only in Zoophytes and other aggregate Animals, that we 20 NATURE AND OBJECTS OF PHYSIOLOGHCAL SCIENCE. notice the tendency to "vegetative repetition;" for it may be observed in many animals which can be divided without the destruction of their lives, — especially among the Radiated, and the lower Articulated tribes. Where such a repetition exists, some of the organs may be removed with- out permanent injury to the structure ; their function being performed by those that remain. Thus it is not uncommon to meet with specimens of the common five-rayed Starfish, in which not only one or two, but even three or four, of the arms have been lost without the destruction of the animal's life; and this is the more remarkable, as the arms are not simply organs of locomotion or prehension, but contain prolongations of the stomach. In the bodies of the higher animals, however, where there are few or no such repetitions (save on the two sides of the body), and where there is consequently a greater diversity in character and function between the different organs, 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. Such structures are said to be more highly organized than those of the lower classes ; not because the whole number of parts is greater, — for it is frequently much less ; but because the number of dissimilar parts, and the consequent adap- tation to a variety of purposes, is much greater, — the principle of divi- sion of labour, in fact, being carried much further, a much larger class of objects being attained, and a much greater perfection in the accom- plishment of them being thus provided for. 6. Keeping in view, then, what has just been stated in regard to the divisibility of a Tree or a Zoophyte into a number of parts, each capable of maintaining its own existence, we may trace a certain gradation from the condition of the Mineral body to that of the highest Animal, in regard to the character in question. Thus, the individuality of a Mi- neral substance may be said to reside in each molecule ; that of a Plant or Zoophyte, in each complete member ; and that of one of the higher Animals, in the sum of all the organs. The distinction is much greater, however, between the lowest organized fabric and any mineral body, than it is between the highest and the lowest organized structures ; for, as we shall hereafter see, the highest and most complicated may be regarded as made up of an assemblage of the lowest and simplest ; whose structure and actions have been so modified as to render them mutually depen- dent; but which yet retain a separate individuality, such as enables them to continue performing their functions when separated from the mass, so long as the proper conditions are supplied. 7. Between the very simplest Organized fabric, and every form of Mineral matter, there is a marked difference in regard to intimate struc- ture and consistence. Inorganic substances can scarcely be regarded as possessing a structure ; since (if there be no admixture of components) they are uniform and homogeneous throughout, whether existing in the solid, the liquid, or the gaseous form ; being composed of similar parti- cles, held together by attractions which affect all alike. Far different is the character of Organized structures ; for in the minutest parts of these may be detected a heterogeneous composition, — a mixture of solid and fluid elements, which are so intimately combined and arranged, as to impart such peculiarities to the tissues, even in regard to their physi- OF ORGANIZED STRUCTURES IN GENERAL. 21 cal properties, as we never encounter amongst Mineral bodies. In the latter, solidity or hardness may be looked upon as the characteristic con- dition ; whilst in Organized structures, softness (resulting from the large proportion of fluid components) may be considered the distinctive quality, being most obvious in the parts that are most actively concerned in vital operations. This softness is evidently connected with the roundness of form characteristic of Organized fabrics, which is most evident when the tissues contain the greatest proportion of fluid ; whilst the plane surfaces and angular contours of Mineral bodies are evidently due to the mode in which the solid particles are aggregated together, without any intervening spaces. 8. The greatest solidity exhibited by Organized fabrics, is found where it is desired to impart to them the simple physical property of resistance ; and this is attained by the deposition of solid particles, often of a mineral character, in tissues that were originally soft and yielding. It is in this manner that the almost jelly-like substance, in which all the organs of animals originate, becomes condensed into cartilage, and that the carti- lage is afterwards converted into bone ; it is in the same manner, also, that the stones of fruit, and the heart-wood of timber-trees, are formed out of softer tissues. But, as we shall hereafter see, this kind of con- version, whilst it renders the tissue more solid and durable, cuts it off from any active participation in the vital operations ; and thence reduces it to a state much more nearly analogous to that of mineral bodies. This resemblance is rendered more close by the fact, that the earthy deposits frequently retain a distinctly crystalline condition; so that, when they are present in large proportion, they impart a more or less crystalline aspect to the mass, and especially a crystalline mode of frac- ture, which is evident enough in many shells. It must not be hence concluded, however, that such substances are of an inorganic nature ; all that is shown by their crystalline structure being, that the animal basis exists in comparatively small amount, and that the mode in which the mineral matter was deposited has not interfered with its crystalline aggregation. 9. It is not to be disputed that a certain degree of homogeneity is apparently to be found in the minutest elements, into which certain Organized tissues are to be resolved. Thus, in the membranes which form the walls of Animal and Vegetable cells, the highest powers of the microscope fail in detecting any such distinction of fluid and solid com- ponents, as that which has been described as characteristic of organized structures. Nevertheless it is indubitable that such distinct components must exist; and this especially from the properties of these membranes in regard to water. For it is one of the most remarkable facts in the whole range of science, that a membrane, in which not the • slightest appearance of a pore can be discovered under the highest powers of the microscope, should be capable of allowing water to pass through it ; and that, too, with no inconsiderable rapidity. The change which these membranes undergo in drying, is another proof that they are not so homogeneous as they appear, and that water is an element of their struc- ture, not merely chemically, but mechanically. The same may be said in regard to the fibres^ which form the apparently ultimate elements of 22 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. the simple fibrous tissues in Animals, and which are also met with in the interior of certain cells and vessels in Plants. These fibres would appear to be of perfectly simple structure ; yet we know from the loss of fluid, and the change of properties which they undergo in drying, that water must have formed part of their substance. — It may be remarked, how- ever, in regard to both these elementary forms of Organized tissue, that the simplicity of their function is in complete conformity with the appa- rent homogeneousness of their structure ; for the cell-membrane is chiefly destined to act, like the porous septum in certain forms of the voltaic battery, as a boundary-wall to the contained fluid, without altogether interfering with its passage elsewhere ; the forces which produce its imbibition or expulsion being probably situated, not in this pervious wall, but in the cavity which it bounds. And, in the same manner, the func- tion of the fibrous tissues, to w^hich allusion was just now made, is of dn entirely physical character ; being simply to resist strain or pressure, and yet to allow of a certain degree of yielding by their elasticity. 10. In all cases in which active vital operations are going on, we can make a very obvious distinction of the structures subservient to them, into liquid and solid parts ; and it is, indeed, by the continual reaction which is taking place between these, that the fabric is maintained in its normal condition. For, as we shall hereafter see, it is liable to a constant decomposition or separation into its ultimate elements : and it is conse- quently necessary that the matters which have undergone that disinte- gration should be carried ofi", and that they should be replaced by new particles. These processes of removal and replacement, with the various actions subservient to them, make up a large proportion of the life of all Organized beings. Now as all the alimentary matter must be reduced to the liquid form, in order that it may be conveyed to the situations in which it is required, and as all the decomposed or disintegrated matter must be reduced to the same form in order to be carried ofi", the inter- mingling or mutual penetration of solids and liquids in the minutest parts of the body is at once accounted for. We shall hereafter see that a cell^ or closed vesicle, formed of a membranous wall, and containing fluid, may be regarded as the simplest form of a living body, and the simplest independent part or instrument of the more complex fabrics (§ 30). 11. Organized structures are further distinguished from Inorganic masses, by the peculiarity of their cJieinieal constitution. This pecu- liarity does not consist, however, in the presence of any elementary sub- stances which are not found elsewhere ; for all the elements, of which organized bodies are composed, exist abundantly in the world around. It might have been supposed that beings endowed with such remarkable powers as those of Animals and Plants, — powers which depend, as we shall hereafter see, upon the exercise of properties to which we find nothing analogous in the Mineral world, — would have had an entirely diff'erent material constitution ; but a little reflection will show, that the identity of the ultimate elements of Organized structures with those of the Inorganic world, is a necessary consequence of the mode in which the former are built up. For that which the parent communicates, in giving origin to a new being, is not so much the structure itself, as the power of forming that structure from the surrounding elements ; and it I OP ORGANIZED STRUCTURES IN GENERAL. 23 is by gradually drawing to itself certain of these elements, that the germ becomes developed into the complete fabr,ic. Now, of the sixty- two simple or elementary substances, which are known to occur in the Mineral world, only about eighteen or nineteen are found in Plants and Animals ; and many of these in extremely minute proportion. Some of these appear to J^e merely introduced, to answer certain chemical or mechanical purposes ; and the composition of the parts which possess the highest vital endowments, is for the most part simpler and more uniform. 12. The actual tissues of Plants, when entirely freed from the sub- stances they may contain, have been found to possess a very uniform composition, and to agree in their chemical properties. The substance which forms the principal part of the thickness of the walls of the cells, vessels, &c., of which the Vegetable organism is composed, is identical with Starch in the proportion of its components ; but as these are in a different state of aggregation, it is distinguished as Cellulose, It con- sists of 12 Carbon, 10 Hydrogen, and 10 Oxygen ; or, in other words, of Carbon united to the elements of water, in the proportion of eight of the former to seven of the latter. It may be very easily converted into gum or sugar, by chemical processes, which effect the removal or the addition of the elements of water. Now there is no compound known to exist in the Inorganic world, which bears the remotest analogy to this ; and we have no reason to believe that it could be produced in any other way, than by that peculiar combination of force which exists in the growing Plant. But although Cellulose is the predominating component of the Vegetable fabric, yet it is not the most essential ; for late researches have shown, that within what has been ordinarily consi- dered as the cell-wall, is a delicate membrane, termed the " primordial utricle," which is really the original cell-wall, from the exterior of which the layer of cellulose is secreted. And it is a very interesting fact, that the composition of this membrane corresponds with that of the proper cell-walls of the Animal tissues; it being, in fact, a proteine compound (§ 13). Hence every act of Vegetable growth involves the production of this substance also, which is still more removed in its composition from ordinary Inorganic compounds. 13. The composition of the Animal tissues, when freed from the fluids they may contain, or from the solid matters which may have been deposited within them, is nearly as uniform. We may distinguish among them two chiei proximate principles, which appear under various modifications in a great variety of dissimilar parts, and which seem capable of conversion into other principles by the addition or subtrac- tion of some of their constituents. The first and most important of these, named Proteine,^ consists of 40 Carbon, 31 Hydrogen, 5 Nitro- gen, and 12 Oxygen ; and although its composition is so complex, it appears^ to act like a simple body, in uniting with Oxygen, Acids, &c., in definite proportions, as well as with Sulphur and Phosphorus ; with * Although it may be doubted whether Proteine has ever been actually obtained in a separate state, yet the term maybe conveniently applied to that composite base, which is united with different equivalents of sulphur and phosphorus in the albuminous com- pounds. 24 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. which last, indeed, it is always found combined, in the Albumen, Fibrine, &c., that are commonly regarded as the organic constituents of the Animal tissues. — The second of the chief proximate principles, termed G-elatine, is largely diffused through the Animal body ; but the tissues which are composed of it possess a simple fibrous structure, and a purely mechanical function ; and no vital action seems to ta^e place in them, subsequently to their first production. It consists of 13 Carbon, 10 Hydrogen, 2 Nitrogen, and 5 Oxygen ; and it is principally characte- rized by its solubility in hot water, and by the insolubility of its com- pound with tannic acid. 14. We shall hereafter dwell more in detail upon the Chemical Con- stitution of the Animal tissues and products (chap, hi.) These sub- stances are only noticed here, in illustration of the general statement, that the "proximate principles" of Animal and Vegetable bodies (that is, the simplest forms to which their component structures can be reduced, without altogether separating them into their ultimate ele- ments), are of extremely peculiar constitution ; being made up of three or four elements, of which the atoms do not seem to be united two by two, or by the method of binary composition, but of which a large number are brought together to form one compound atom, of ternary/ or quaternary composition. This compound atom, like Cyanogen, and many others derived from Organic products, acts like a simple or ele- mentary one in its combinations with other substances. — It is worthy of remark, however, that, in this respect as in others, the Vegetable kingdom is intermediate between- the Animal and the Mineral. For whilst Proteine and Gelatine are remarkable, not only for containing four elements, but for the very large number of atoms of these compo- nents which enter into the single compound atom of each ; the Cellu- lose of Plants is much simpler in its composition, since it includes only three elements, and the numbers which represent their proportions are smaller. And further, the proportions of the components of Cellulose are themselves such as suggest the idea of simplicity in their method of combination, — the union of water and carbon in the common binary method ; — an idea which is confirmed by the mode of its original pro- duction, which indicates a direct union of carbon with water ; as well as by the fact, that the chemical difference between Cellulose and nume- rous other substances found in Plants, may be represented by the simple addition or subtraction of a certain number of atoms of water, and that the chemist can effect an actual conversion of the former into certain of the latter, by means which are calculated to effect such an addition or subtraction. 15. We shall hereafter see that Vegetables are intermediate between the Animal kingdom and the Inorganic world in another most important particular — the nature of the chemical operations they effect ; for it is their function to combine the oxygen, hydrogen, carbon, and nitrogen, of the Inorganic world into Organic Compounds ; which not only serve as the materials of their own growth, but also as the food of Animals, whose existence is entirely dependent upon them, since they possess no such combining power. It is from the Water, Carbonic Acid, and Ammonia, supplied by the atmosphere and by the soil in which they are fixed, that OP VITAL ACTIONS IN GENERAL. 25 Plants derive these elements. On the other hand, the Animal, making use of the ternary and quaternary compounds which have been elabo- rated by Plants, is continually restoring their elements to the Inorganic world, in the very forms which they originally possessed; for, as we shall hereafter see, the excretion of Water, Carbonic acid, and Ammo- nia is constantly taking place in the Animal body during life, aa^the result of those changes in which its peculiar activity consists. And thus is sustained that balance between Animal and Vegetable nutrition, which is found to be the more wonderful and complete, the more care- fully it is scrutinized. 2. Distinctive Characters of Vital Actions. 16. We are now arrived at the second head of our inquiry, — namely, the nature of those actions^ which distinguish living beings from masses of inert matter, and which are designated as Vitcd^ to mark their dis- tinctness from Physical and Chemical phenomena. There can be no doubt whatever, that, of the many changes which take place during the life^ or state of vital activity ^ of an Organized being, and which inter- vene between its first development and its final decay, a large propor- tion are effected by the direct agency of those forces, which operate in the Inorganic world ; and there is no necessity whatever for the suppo- sition, that these forces have any other operation in the living body, than they would have out of it under similar circumstances. Thus the propulsion of the blood by the heart through the large vessels, is a phe- nomenon precisely analogous to the propulsion of any other liquid through a system of pipes by means of a forcing pump ; and if the ar- rangement of the tubes, the elasticity of their walls, the contractile power of the heart, and the physical properties of the fluid, could be precisely imitated, the artificial apparatus would give us an exact repre- sentation of the actions of the real one. The motor force of the muscles upon the bones, again, operates in a mode that might be precisely repre- sented by an arrangement of cords and levers ; the peculiarity here, as in the former case, being solely in the mode in which the force is first generated. So, again, the digestive operations which take place in the stomach are capable of being closely imitated in the laboratory of the chemist ; when the same solvent fluid is employed, and the same agencies of heat, motion, &c., are brought into play. Moreover we shall here- after see reason to believe, that the peculiar form of Capillary Attrac- tion, to which the term "endosmose" is applied, performs an important part in the changes which are continually taking place in the living body. 17. But after every possible allowance has been made for the opera- tion of Physical and Chemical forces, in the living Organism, there still remain a large number of phenomena, which cannot be in the least ex- plained by them, and which we can only investigate with success, when we regard them as resulting from the agency of forces as distinct from those of Physics and Chemistry, as they are from each other. It is to these phenomena that we give the name of Vital; the forces from whose operation we assume them to result, are termed vital forces ; and the properties, which we must attribute to the substances manifesting 26 NATURE AND OBJECTS OP PHYSIOLOGICAL SCIENCE. those forces, are termed vital properties. Thus we say that the act of contraction in a muscle is a vital phenomenon ; because its character appears totally distinct from that of a Physical or Chemical action ; and because it is dependent upon other vital changes in the muscular substance. The act is the manifestation of a certain force, the posses- sion of which is peculiar to the muscular structure, and which is named the Contractile force. But that force is only exerted under certain con- ditions, and these may only recur at long intervals, though the capacity for exerting it be always present in the organized tissue ; this capacity is termed ^property ; and thus we regard it as the essential peculiarity of living Muscular tissue, that it possesses the vital property of Con- tractility. Or, to reverse the order, the muscle is said to possess the property of Contractility ; the property, when the appropriate conditions are supplied, gives rise to the Contractile Force ; and the force produces, if its operation be unopposed, the act of Contraction. 18. It may be said that the distinction here made is a verbal one ; and that a very simple thing is thus made complex ; but it will be pre- sently seen that it is necessary, in order to enable us to take correct views of the nature of Vital phenomena, and to understand their rela- tions to those of the Inorganic world. And, in fact, the distinction between the property, the force, and the action, become apparent upon a little consideration. Of the property we are altogether unconscious, so long as it is not called into exercise ; we could not, for example, de- termine by the simple exercise of any of our senses, whether a certain piece of muscle retained, or had lost, its contractility. When the pro- perty is called into action by its appropriate stimulus, we may convince ourselves that a force is generated, even if no sensible action is pre- sented ; thus, if we were to hold the two extremities of a muscle so firmly, as to prevent them from approximating in the least degree when its contractility was excited, we should be conscious of a powerful force tending to draw our hands together ; and we might measure the amount of that force, by mechanical means adapted to determine the weight it would sustain. And lastly, if no obstacle be interposed to the act of contraction, it then becomes obvious to our senses, by the change in the shape of the muscle, and by the approximation of its two extremities, as well as of the bodies to which they are attached. 19. The advantage of this method of viewing the phenomena of Life is best shown, by turning our attention for a moment to the mode of investigation practised in Physics and Chemistry. Thus, when a stone falls towards the earth, we say that this is an act or phenomenon of Gravitation. The force with which the stone tends to fall to the ground, whether it actually falls or not, is called the force of Gravitation ; and we speak of the tendency which every substance has to act in this mode, as the property of Gravitation. Now from observation of the Moon's motion, it is shown that she too is drawn towards the Earth ; her ellip- tical path around it being the result of the combined action of the cen- trifugal or tangential, and of the centripetal or gravitative forces. And it is further established, that not only does the Moon gravitate towards the Earth, but the Earth gravitates towards the Moon ; so that, if the two bodies were entirely free from the action of all other forces, they OF VITAL ACTIONS IN GENERAL. 27 would fall towards each other (the distance traversed by each being in proportion to the size of the other), and would meet in their common centre of Gravity. Hence it is evident that the attractive force is similar in both bodies ; and our idea of the property of Gravitation must be extended, therefore, from the Earth to the Moon. Again, we find ample reason to believe that the same force acts between the Sun and the Planets, — between the Planets and the Sun, — and amongst the Planets themselves ; and further, careful experiment shows that masses of matter upon the Earth's surface are not only attracted by it, and attract it in their turn, but that they attract and are attracted by each other. Hence we arrive at the idea of the universality of this property of mutual at- traction ; and we perceive that, in spite of varieties in the actions it produces, and of difi'erences in the amounts of the forces to which it gives rise, the property is the same throughout ; so that we can predict all these actions, and anticipate the forces which will be developed, from the simple general expression of the property of Mutual Attraction, and of the conditions under which it is manifested, — constituting the Law of Gravitation or Mutual Attraction. 20. Now in this case of Mutual Attraction, we have no opportunity of witnessing the dofmant condition of the property in any mass of matter ; for, as nothing is wanting but the presence of another mass to call this property into operation, it is always generating force, and giving rise to actions. If we could conceive of the existence of but a single mass of matter in the universe, we shall at once see that though possessed of the property of mutual attraction, it would not be able to exercise it, so as to generate an attractive force, or to produce a move- ment. 21. But we will turn to another case ; in which there is a closer analogy to the condition of living beings ; and by which, therefore, the view here put forth may be more clearly illustrated. When a magnet (itself a bar of iron, having no peculiarity of appearance) draws towards it a piece of iron, we say that a Magnetic action or phenomenon takes place ; further, we speak of the power which produced the movement, as the Magnetic force ; and we attribute this force to a certain property inherent in the Magnet, by virtue of which it draws towards itself all pieces of iron that are within the sphere of its operation, and we speak of the iron bar as endowed with the property of Magnetic attraction. Now we cannot ascertain the presence or absence of this property in a certain bar of iron, by any difi*erence in its aspect, its specific gravity, its chemical properties, nor, in fact, by any other mode than the putting it in circumstances adapted to call the Magnetic property into action if it really exist : — thus we dip it into iron filings, and judge by their ad- hesion whether it is capable of attracting them ; or, as a still more deli- cate test, we ascertain whether it is capable of exerting any repulsive power on a delicately-suspended needle already magnetized. Again, a needle, or bar of iron, which exhibits this magnetic power of attracting other portions of the same metal, exhibits another power, which would seem at first sight totally distinct ; namely, that of constantly turning one of its extremities towards the north, and the other towards the south, when it is so supported as to be free to do so. And yet there is no 28 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. doubt whatever, that this directing power is only another manifestation of the same magnetic attractiveness ; depending on the relation between the magnetized bar, or needle, and the Earth, which must itself be re- garded as a great magnet. Hence the idea of a peculiar kind of mutual attractiveness, — existing in only a limited class of bodies, capable of being excited in one by a certain agency on the part of the other,* — and re- quiring for its exercise or manifestation a certain set of conditions, with- out which no phenomenon results, — is that which we regard as funda- mental in the Science of Magnetism. 22. We may now turn from these departments of Physical Science to Chemistry ; and here we shall find that the investigation is carried on upon the very same plan. In fact, the whole science of Chemistry is founded upon the idea of a certain attractiveness or affinity existing among the ultimate particles or molecules of the different elementary substances ; and therefore entirely distinct from the homogeneous attrac- tion, which holds together the particles of the same mass, or from the gravitative attraction, which operates alike upon all masses, whatever be their composition. Thus we say that Sulphuric acid and Potash have an affinity for each other; because they unite when they are brought together, and form a new compound. This is a Oftemical action or phe- nomenon. Now we know that they tend to unite with a certain force ; a force, however, which cannot be measured mechanically, and which can only be expressed by comparing it with some other force of the same kind. Thus we say that the mutual affinity of Sulphuric acid and Potash is greater than that of Nitric acid and Potash ; because, if we pour Sulphuric acid upon Nitrate of Potash, the Sulphuric acid detaches (as it were) the Potash from its connexion with the Nitric acid, forms a new compound with it, and sets the Nitric acid free. Hence we say that it is a property of Sulphuric acid to have a very strong affinity for Potash. This property exists in every particle of Sulphuric acid that exists, whether free or combined ; but it does not manifest itself, except when called into operation by the contact of Potash ; and it then de- velopes a force, which may completely change the combinations previ- ously existing, and give rise to new ones. 23. Now of this property we are not informed by any of the other properties of Sulphuric acid ; and we only recognise its existence by the action which is the result of its exercise. If a new element or com- pound be discovered, the chemist is totally unable to predict its force of affinity for this or that substance ; and he can only guess by analogy, what will be its behaviour under various circumstances. Thus if it have the external properties of a Metal, he presumes that it will correspond with the Metals in possessing a strong affinity for oxygen, sulphur, &c.; whilst if it seem analogous to Iodine, Chlorine, &c., he infers that it will be a supporter of combustion, that it will form an acid with hydrogen, and so on. But, even though such guesses may be made with a certain amount of probability, nothing but experience can show the positive degrees of affinity which the substance may have for others of different kinds ; and the experimental determination can only be made, by observ- * As when one magnet is made by another; or when iron rails, pokers, &c., become magnetic by the influence of the earth. OF VITAL ACTIONS IN GENERAL. 2^ ing the actions of the body when placed in different circumstances, from which we judge of its properties^ and of the forces to which these proper- ties give rise when they are called into operation. 24. It is hoped that the propriety of the distinct use of the terms Vital Action, Vital Force, and Vital Property, will now be evident ; and that the student will be prepared to attach distinct ideas to each^f them. It is the business of the Physiologist to study those actions or phenomena, which are peculiar to living beings, and which are hence termed vital: — he endeavours to trace them to the operation of specific forces acting through organized structures, just as the Astronomer traces all the movements, regular and perturbed, of the heavenly bodies, to the mutual attraction of their masses, acting concurrently with their force of onward rectilinear movement ; or as the Chemist attributes the different acts of combination or separation, which it is his province to study, to the mutual afiinities of the substances concerned : — and the physiologist, like the astronomer or the chemist, seeks to determine the laws according to which these forces act, or, in other words, to express the precise conditions under which they are called into play, and the actions which they then produce. It is only in this manner, that Phy- siology can be rightly studied and brought to the level of other sciences. There can be no doubt that its progress has been greatly retarded by the assumption, that its phenomena were all to be attributed to the operation of some general controlling agency, or Vital Principle ; and that the laws expressing the conditions of these phenomena, must be sought for by methods of investigation entirely distinct from those which are employed in other sciences. But a better spirit is now abroad; and the student cannot be too strongly urged to discard any ideas of this kind as absolutely untenable ; and to keep steadfastly in view, that the laws of Vital Action are to be attained in the same manner as those of Physics or Chemistry, — that is, by the careful collection and com- parison of vital phenomena, and by applying to them the same method of reasoning, as that which is used in determining the forces and pro- perties on which other phenomena depend. True it is, that we can scarcely yet hope to reach the same degree of simplification, as that of which other sciences are capable ; and this on account of the very com- plex nature of the phenomena themselves, and the difiiculty of satisfac- torily determining their conditions. The uncertainty of the results of Physiological experiments is almost proverbial : that uncertainty does not result, however, from any want of fixity in the conditions under which the vital forces operate, but merely from the influence of diffe- rences in those conditions, apparently so slight as to elude observation, and yet sufiiciently powerful to produce an entire change in the result. And, owing to that mutual dependence of the different actions of the orga- nized structure, to which reference has been already made (§ 5), we cannot seriously derange one class of these actions, without also deranging, or even suspending others : — a circumstance which obviously renders vital phenomena much more difficult of investigation than those of inorganic matter. 25. All sciences have their "ultimate facts ;*' that is, facts for which no other cause can be assigned than the Will of the Creator. Thus, in 30 NATURE AND OBJECTS OP PHYSIOLOGICAL SCIENCE. Physics, we cannot ascend above the fact of Attraction (which operates according to a simple and universal law) between all masses of matter ; and in Chemistry, we cannot rise beyond the fact of Affinity (limited by certain conditions which are not yet well understood) between the par- ticles of different kinds of matter. When we say that we have explained any phenomenon, we merely imply that we have traced its origin to properties with which we were previously acquainted, and shown that it takes place in accordance with the known laws of their operation. Of the existence of the properties, and the determination of the conditions of their action, we can give no other account, than that the Creator willed them so to be ; and, in looking at the vast variety of phenomena to which they give rise, we cannot avoid being struck with the general harmony that exists amongst them, and the mutual dependence and adaptation that may be traced between them, when they are considered as portions of the general economy of Nature. There is no difference in this respect between Physiology and other sciences ; except that the number of these (apparently) ultimate facts is at present greater in physiology than it is in other departments, because we are not at present able to include them all under any more general expression. But, as will presently appear, a considerable degree of simplification appears practicable in our view of them ; and although we may not be able to say why the structure called Muscular should possess contractility, and why the structure called Nervous should be capable of generating and conveying the force which excites that contractility to action, we may draw, from the study of the conditions under which they respectively manifest themselves, some indications of the existence of a common tie, such as that which binds together the planetary masses, at the same time that it weighs down the bodies on the surface of the earth towards its centre. 26. In the study of any branch of science, it is most desirable to com- mence with definite views of the nature of the phenomena with which it is concerned ; and such are best gained by the examination of these phenomena under their simplest aspects. This course is most especially necessary in Physiology : since the complexity of the conditions under which its phenomena usually present themselves, often tends to mask their real character, causing that to be regarded as essential which is only accidental or contingent, and vice versd. It is extremely difficult, however, and frequently impossible, for the Physiologist to isolate these several conditions, and to study them separately, in the way that the Chemical or Physical investigator would do ; and his best course is to take advantage of those "experiments ready prepared by Nature," which he finds in the variety of forms of living organized beings, with which the globe is so richly peopled. Now it is in the simplest forms of Cryptogamic Vegetation, that the phenomena of Life present themselves under their least complicated aspect ; for we shall find in the operations of each of the simple cells of which such Plants are composed (all of them resembling one another in structure and actions), an epito7ne, as it were, of those of the highest and most complex Plant ; whilst those of the higher Plants bear a close correspondence with those which are immediately concerned in the Nutrition and Reproduction of the Animal OP VITAL ACTIONS IN GENERAL. 31 body. And when we come to consider the proper Animal functions, we shall find that they are not so far removed in their essential nature from those of Plants, but that they may be ranked under the same category, and regarded as diflferent manifestations of the same original forces. A Qell^ then, in Physiological language, is a closed vesicle or minute bag, formed by a membrane in ^ig- 1- which no definite structure can be discerned, and having a cavity which may contain matters of varia- ble consistence. Such a cell constitutes the whole organism of such simple plants as the Protococcus nivalis (' red snow'), or Palmella eruenta (' gory dew') ; for although the patches of this kind of vege- tation, which attract our notice, are made up of vast aggregations of such cells, yet they have no depen- dence upon one another, and the actions of each are J^-p^« i'°l?dtt"f 'mX an exact repetition of those of the rest. In such cuies. a cell, every organized fabric, however com- complex, originates. The vast tree^ almost a forest in itself, and the feeling, thinking, intelligent man^ spring from a germ, that differs in no obvious particular from the permanent condition of one of these lowly beings. But whilst the powers of the latter are restricted, as we shall see, to the continual multiplication of new and distinct individuals like itself, those of the former enable it to produce new cells that remain in' closer connexion with each other ; and these are gradually converted, by various transformations of their own, into the diversified elements of a complex fabric. The most highly-organized being, however, will be shown to consist in great part of cells that have undergone no such trans- formation, amongst which the different functions performed by the indi- vidual in the case just cited, are distributed, so to speak ; so that each cell has its particular object in the general economy, whilst the history of its own life is essentially the same as if it were maintaining a separate existence. 27. We shall now examine, then, the history of the solitary cell of one of the simplest Cryptogamic Plants, from its first development to its final decay ; in other words, we shall note those Vital Phenomena, which are as distinct from those of any inorganic body, as is its organ- ized structure (simple as it appears) from the mere aggregation of par- ticles in a mineral mass. In the first place, the cell takes its origin from a germ, which may be a minute molecule, that cannot be seen without a microscope of high power.* This molecule appears, in its earliest condition, to be a simple homogeneous particle, of spherical form ; but it gradually increases in size ; and a distinction becomes apparent between its transparent exterior and its coloured interior. Thus we have the first indication of the cell-wall, and the cavity. As the enlargement proceeds, the distinction becomes more obvious ; the cell-wall is seen to be of extreme tenuity and perfectly transparent, and to be homogeneous in its texture ; whilst the contents of the cavity are * The modes in which new cells may be generated are various ; but the above exam- ple is purposely drawn from one of those simple Algee, whose usual mode of multipli- cation is by ♦* zoospores." (See the Author's " Principles of Physiology," U 139-144.) 82 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. distinguished by their colour, which is very commonly either green or crimson. At first they, too, appear to be homogeneous ; but a finely- granular appearance is then perceptible ; and a change gradually takes place, which seems to consist in the aggregation of the minute granules into molecules of more distinguishable size and form. These molecules, which are the germs of new cells, seem to be at first attached to the wall of the parent cell; afterwards they separate from it, and move about in its cavity ; and at a later period, the parent-cell bursts and sets them free. Now, this is the termination of the life of the parent cell ; but the commencement of the life of a new brood ; since every one of these germs may become developed into a cell, after precisely the foregoing manner, and will then in its turn multiply its kind by a simi- lar process. 28. By reasoning upon the foregoing history, we may arrive at cer- tain conclusions, which will' be found equally applicable to all living beings. In the first place, the cell originates in a germ or reproductive body, which has been prepared by another similar cell that previously existed. There is no sufficient reason to believe that there is any ex- ception to this rule. So far as we at present know, every Plant and every Animal is the ofispring of a parent, to which it bears a resemblance in all essential particulars ; and the same may be said of the individual cells, of which the Animal and Vegetable fabrics are composed. But how does this germ, this apparently homogeneous molecule, become a cell ? The answer to this is only to be found in its peculiar property, of drawing materials to itself from the elements around, and of incorpo- rating these with its own substance. The Vegetable cell may grow wherever it can obtain a supply of water, carbonic acid, and ammonia ; for these compounds supply it with oxygen, hydrogen, carbon, and nitro- gen, in the state most adapted for the exercise of the combining power, by which it converts them into those new compounds, whose properties adapt them to become part of the growing organized fabric. Here, then, we have two distinct operations ; — the union of these elementary sub- stances into that composite protoplasma, which seems to be the imme- diate pabulum of the Vegetable tissues ; — and the incorporation of that product with the substance of the germ itself. 29. The first of these changes mai/ be, and probably is, of a purely Chemical nature ; and analogous cases are not wanting, in the pheno- mena of Inorganic Chemistry, in which one body. A, exerts an influence upon two other bodies, B and c, so as to occasion their separation or their union, without itself undergoing any change. Thus platinum, in a finely-divided state, will cause the union of oxygen and hydrogen at ordinary temperatures ; and finely-powdered glass will do the same at the temperature of 572°. This kind of action is called catalysis. A closer resemblance, perhaps, is presented by the act oi fermentation ; in which a new arrangement of particles takes place in a certain compound, by the presence of another body which is itself undergoing change, but which does not communicate any of its elements to the new products. Thus, if a small portion of animal membrane, in a certain stage of de- composition, be placed in a solution of sugar, it will occasion a new arrangement of its elements, which will generate two new products, OF VITAL ACTIONS IN GENERAL. 33 alcohol and carbonic acid. If the decomposition of the membrane have proceeded further, a dijQTerent product will result ; for instead of alcohol, lactic acid will be generated. And in a further stage of decomposition, the ferment is the means of producing butyric acid (the fatty acid of butter). There appears no improbability, then, in the idea, that the influence exerted by the germinal molecule is of an analogous nature ; and that it operates upon the elements of the surrounding water ahd^ carbonic acid, according to purely chemical laws, uniting the carbon with the elements of water, and setting free the oxygen. This result of the nutritive operations of the simpl§ cellular plants may be easily veri- fied experimentally, by exposing the green scum, which floats upon ponds, ditches, &c., and which consists of the cells of a minute Crypto- gamic Plant, to the influence of light and warmth beneath a receiver ; it is found that oxygen is then liberated, by the decomposition of the carbonic acid contained in the water. We shall presently have to return to the consideration of the Chemical phenomena of living beings ; and shall pass on, therefore, to consider those to which no such explanation applies. 30. The second stage in the nutritive process consists in the appro- priation of the new products thus generated to the enlargement of the living cell-structure ; a phenomenon obviously distinct from the pre- ceding. It is well to observe, that this process, which constitutes the act of Orgamzatio7i, may be clearly distinguished in the higher Plants and Animals, as consisting of two stages ; — the first of these being that of assimilation, which consists in the further preparation or elaboration of the fluid matter, by certain alterations whose nature is not yet clear, so as to render it plastic or organizahle ; — the second being the act of formation, or the conversion of the organizable matter into the solid texture, in which process the properties that distinguish that texture come to manifest themselves. Thus, for example, we do not find that a solution of dextrin (or starch-gum) is capable of being at once applied to the development of Vegetable tissue, although it is identical in com- position with cellulose ; for it must first pass through a stage, in which it possesses a peculiar glutinous character, and exhibits a tendency to spontaneous coagulation, that seems like an attempt at the production of organic forms. And in like manner, the albumen of Animals is evi- dently not capable of being applied to the nutrition of the fabric, until it has been first converted into fibrin ; which also is distinguished by its tenacious character, by its spontaneous coagulability, and by the fibrous structure of its clot. Now, in both these cases, there is probably some slight modification in chemical composition, that is, in the proportions of the ultimate elements ; but the principal alteration is evidently that which is eff'ected by the rearrangement of the constituent particles ; so that, without any considerable change in their proportions, a compound of a very diff"erent nature in generated. Of the possibility of such changes we have abundant illustrations in ordinary Chemical pheno- mena ; for there is a large class of substances, termed isomeric, which, with an identical composition, possess chemical and physical properties of a most diverse character. 31. But we cannot attribute the production of Fibrin from Albumen, 3 34 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. the organizable from the unorganizable material, to the simple operation of the same agencies as those which determine the production of the different isomeric compounds ; for the properties of Fibrin are much more vitally distinct from those of Albumen, than they are either che- mically ov physically ; that is, we find in the one an incipient manifes- tation of Life^ of which the other shows no indications. The spontane- ous coagulation of fibrin, which takes place very soon after it has been withdrawn from the vessels of the living body, is a phenomenon to which nothing analogous can be found elsewhere ; for it has been clearly shown not to be occasioned by any mere physical or chemical change in its constitution ; and it takes place in a manner which indicates that a new arrangement of particles has been effected in it, preparatory to its being converted into a living solid. For this coagulation is not the mere ho- mogeneous setting^ which takes place in a solution of gelatine in cooling ; nor is it the aggregation of particles in a mere granular state (closely resembling that of a chemical precipitate), which takes place in the coagulation of albumen : it is the actual production of a simple fibrous tissue^ by the union of the particles of fibrin in a determinate manner, bearing a close resemblance to the similar process in the living body (§ 188). We say, then, that the coagulation of Fibrin, and the produc- tion of a fibrous tissue, are the manifestation of its vital properties, rather than the direct result of chemical or physical agencies ; because no substance is known to perform any such actions, without having been subjected to the influence of a living body ; and because the actions themselves are altogether different from any which we witness else- where. 32. The act of Formation seems to consist of a continuation of the same kind of change, — that is, a new arrangement of the particles, pro- ducing substances which differ both as to structure and properties from the materials employed, but which may be so closely allied to them in chemical composition, that the difference cannot be detected. Thus, from the " protoplasma"* of Plants are generated, in the process of cell- development, the membranes which constitute the walls of the cells: chemically speaking, there seems to be no essential distinction between these substances ; and yet between the living, growing, reproducing cell, and the gelatinous, semifluid matter in which they are imbedded, how wide the difference ! So in the Animal body, we find that the composi- tion of the proper muscular tissue scarcely differs, in regard to the pro- portion of its elements, from the fibrin, or even from the albumen, of the blood ; and yet what an entire rearrangement must take place in the particles of either, before a tissue so complex in structure, and so peculiar in properties, as muscular fibre, can be generated ! 33. Both in the Plant and the Animal, we find that tissues presenting great diversities both in structure and properties, may take their origin in the same organizable material ; but in every case (at least in the ordinary processes of growth and reparation) the new tissue of each kind is formed in continuity with that previously existing. Thus in * This term is now commonly employed to designate that combination of starchy and albuminous matters, in which all newly-forming cells appear to originate. See g 28. OF VITAL ACTIONS IN GENERAL. 35 the stem of a growing tree, from the very same glutinous sap or cam- bium, intervening between the wood and the bark, the wood generates, in contact with its last-formed layer, a new cylinder of wood ; whilst the bark produces in contact with its last-formed layer, a new cylinder of bark ; the woody cylinder being characterized by the predominance of ligneous fibre and ducts, and the cortical by the predominance_of_ cellular tissue. In like manner we find that, in animals, muscle pro- duces muscle, bone generates bone, nerve developes nerve, in continuity with itself, all at the expense of the materials supplied by the very same blood. 34. The Nutrition of tissues, by the organization of the materials contained in the nutrient fluid with which they are supplied, may be superficially compared, therefore, to the act of crystallization, when it takes place in a mixed solution of two or more salts. If in such a solu- tion we place small crystals of the salts it contains, these crystals will progressively increase by their attraction for the other particles of the same kind, which were previously dissolved ; each crystal attracting the particles of its own salt, and exerting no influence over the rest. And it is curious that if either of the crystals be broken, the new deposit will take place upon it in such a mode as gradually to reproduce its characteristic form. But it must be borne in mind, that such a resem- blance goes no further than the surface ; for the growth of a crystal cannot be really regarded as in the least analogous to that of a cell. The crystal progressively increases by the deposit of particles upon its ^'exterior ; the interior undergoes no change ; and whatever may be the size it ultimately attains, its properties remain precisely the same as those of the original nucleus. On the other hand, the cell grows from its original germ by a process of iifterstitial deposit ; the substance of which its wall is composed extends itself in every part ; and the new matter is completely incorporated with the old. 35. Moreover, as the increase proceeds, we see an evident distinction between the cell-wall and its cavity ; and we observe that the cavity is occupied by a peculiar matter, different from the substance of the cell- wall, though obviously introduced through it. Of the essential differ- ence which may exist in composition, between the cell-wall and the con- tents of the cavity, we have a remarkable example in the case of the simple Cryptogamic plant, which constitutes Yeast, and which differs in no essential part of the history of its growth from the examples already referred to. The principal component of its cell- walls is nearly identical with ordinary cellulose f whilst the contents of the cells are closely allied in composition to proteine. Again, in the fat-cells of Animals, the cell-wall is formed from a proteine compound ; whilst the oily con- tents agree, in the absence of nitrogen, and in their general chemical relations, with the materials of the tissues of Plants. It is evident, then, that one of the inherent powers of the cell, is that by which it not only combines the surrounding materials into a substance adapted for the extension of its wall, but that which enables it to exercise a similar combining power on other materials derived from the same source, and to form a compound, — of an entirely different character, it may be, — which occupies its cavity. Now this process is as essential to our idea 86 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. of a living cell, as is the growth of its wall ; and it must never be left out of view, when considering the history of its development. 36. Every kind of cell has its own specific endowments ; and gene- rates in its interior a compound peculiar to itself. The nature of this compound is much less dependent upon the nutrient materials which are supplied to the cell, than upon the original inherent powers of the cell itself, derived from its germ. Thus we find that the "red snow" and " gory dew" invariably form a peculiar red secretion ; and that they will only grow where they can obtain, from the air and moisture around, the elements of that secretion. Again, the "yeast-plant" invariably forms a secretion analogous to animal proteine ; and it will only grow in a fluid which supplies it with the materials of that substance. Hence the "red snow" would not grow in a fermentible saccharine fluid; nor would the "yeast plant" vegetate on damp cold surfaces. Yet there is little difference, if any, between their cell-walls, in regard to chemical composition. — So, also, we shall find hereafter, that one set of cells in the Animal body will draw into themselves, during the process of growth, the elements of bile ; another, the elements of milk ; another, fatty matter ; and so on : the peculiar endowments of each being derived from their several germs, which seem to have an attraction for these sub- stances respectively, and which thus draw them together ; whilst the cell-wall appears to have a uniform composition in all instances. 37. The term Secretion^ or setting apart, is commonly applied to this operation, to distinguish it from Nutrition or growth ; but it is obvious from what has now been stated, that the act of secretion is in reality the increase or growth of the cell-contents, just as the process of en- largement is the increase or growth of the cell-wall ; and that the two together make up the whole proce^ of Nutrition, which cannot be pro- perly understood, unless both are taken into account. It is to be remem- bered, however, that the contents of the cell may not be destined to undergo organization ; indeed we shall find hereafter, that the main use of certain cells is to draw off from the circulating fluid such materials as are incapable of organization ; and the operation may be so far attri- buted, therefore, to the agency of Chemical forces. But we shall find that, in other instances, the cell-contents are destined to undergo orga- nization, and this either within the parent cell, or after they leave it ; here, then, we must recognise a distinct vitalizing agency, as exerted by the cell upon its contents. 38. This organizing or vitalizing influence must be exerted upon a certain portion of the contents of every cell that is capable of repro- ducing itself; for it is in this manner that those germs are produced, in which all the wonderful properties are inherent, that are destined to manifest themselves, when they are set free from the parent-cell. This power of Reproduetio7i is one of those, which most remarkably distin- guishes the living being ; and we shall find that, in the highest Animal, as in the humblest Plant, it essentially consists in the preparation of a cell-germ, which, when set free, gradually developes itself into a struc- ture like that from which it sprang. The reproductive molecules or cell-germs are formed, like the tissue and the contents of the parent-cell, from the nutrient materials which it has the power of bringing together I OP VITAL ACTIONS IN GENERAL. 37 and combining ; and in their turn they pass through a corresponding series of changes ; and at length produce a new generation of similar molecules, by which the race is destined to be continued. Notwith- standing the mystery which has been supposed to attach itself to this process, it is obvious that there is nothing in reality more difficult to understand in the fact, that the parent-cell organizes and vitalizes the protoplasma which it has elaborated, so that it should form the germ of a new individual possessing similar properties with itself; than in its incorporating the same material with its own structure, and causing it to take a share in its own actions. 39. Finally, the parent-cell having arrived at its full development, having passed through the whole series of changes which is character- istic of the species, and having prepared the germs by which the race is to be propagated, dies and decays ; — that is, all those operations, which distinguish living organized structures from inert matter, cease to be performed ; and it is given up to the influence of chemical forces only, which speedily occasion a separation of its elements, and cause them to return to their original forms, namely, water, carbonic acid, and ammonia. It is not, however, in the dead organism alone that this decomposition occurs ; for it is certain that interstitial death and decay are incessantly taking place during the whole life of the being ; and that the maintenance of its healthy or normal condition depends upon the constant removal of the products of that decay, and upon their continual replacement. If, on the one hand, those products be retained, they act in the manner of poisons ; being quite as injurious to the welfare of the body, as the most deleterious substances introduced from without. On the other hand, if they be duly carried off, but be not replaced, the conditions essential to vital action are not fulfilled, and the death of the organism must be the result. 40. Now it is to be observed that, as Plants obtain the chief materials of their growth from water and carbonic acid, taking the carbon from the latter and setting free the oxygen, so do they require, as the condi- tion for their decay, the presence of oxygen, which may reunite with the carbon that is to be given back to the atmosphere. If secluded from this, the vegetable tissues may be preserved for a long time without decomposition. Generally speaking, indeed, they are not prone to rapid decay, except at a high temperature ; and hence it is that we have so little evidence, in Plants, of the constant interstitial change, of which mention has just been made. Its existence, however (at least in all the softer portions of the structure), is made evident by the fact, that a con- tinual extrication of carbonic acid takes place, to an amount which sometimes nearly equals that of the carbonic acid decomposed, and of the oxygen set free, in the act of Nutrition (§ 28). The latter operation is only effected under the stimulus of sunlight ; the former is constantly going on, by day and by night, in sunshine and in shade ; and if it be impeded or prevented by want of a due supply of oxygen, the plant speedily becomes unhealthy. Now this extrication of the products of interstitial decay is termed Excretion. It is usually confined in Plants to the formation of carbonic acid and water, by the union of the particles of their tissues with the oxygen of the air, — a process identical with 38 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. that which occurs after the death of the entire structure. But in Ani- mals it is much more complicated ; owing to the larger number of con- stituents in their fabric, and to the much greater variety in the propor- tions in which these are combined ; hence the products of interstitial decomposition are much more numerous and varied, and several distinct modes are devised for getting rid of them. Moreover, as the animal tissues are much further removed than the Vegetable from the composi- tion of Inorganic bodies, they are subject to much more rapid and con- stant decay ; and we shall find that this decay is so considerable in amount, as to require on the one hand a very complex excretory appa- ratus to carry off the disintegrated matter, and on the other a large supply of nutrient materials to replace it. 41. The preceding history may be thus summed up. i. The Vege- table cell-germ or reproductive molecule draws to itself, and combines together, certain inorganic elements ; and thereby produces a new and peculiar compound. This compound, however, exhibits no properties that distinguish it from others, in which ordinary Qhemical agencies have been concerned ; and we may, therefore, regard the first act of the cell- germ as of a purely chemical nature. We shall presently see that che- mical agencies are undoubtedly concerned in it, to a very considerable degree. The Animal cell-germ does not possess the same Chemical power ; it is not capable of decomposing the water, carbonic acid, and ammonia, which include the elements of its tissues ; and it is entirely dependent for its growth, upon the supply of nutriment previously pre- pared for it by the agency of the vegetable kingdom, many species of which possess the power of generating a large amount of proteine-com- pounds within their cells, though they do not organize them. ii. The cell -germ then exerts an Assimilating agency upon the pabulum thus prepared ; by which a new arrangement of its particles is produced. This new arrangement gives new and peculiar qualities to the fluid, which show that it is something more than a mere chemical compound, and that it is in the act of undergoing the process of organization, ill. The Formation of this elaborated pabulum into tissue then takes place ; its materials are withdrawn from the fluid, and incorporated with the solid texture ; and in thus becoming part of the organized fabric, they are caused to exhibit its own peculiar properties. IV. At the same time, another portion of this pabulum is gradually prepared to serve as the germ of a new cell, or set of cells, by which the same properties are to be exhibited in another generation, v. By an operation resembling that concerned in the first preparation of the pabulum, certain products, more or less differing from it in character, but not destined to undergo organization, are formed in the cavity of the cell. vi. A decomposition or disintegration of the organized structure is continually going on, by the separation of its elements into simpler forms, under the influence of purely Chemical agencies ; and the setting free of these products by an act of excretion, is thus incessantly restoring to the Inorganic world a portion of the elements that have been withdrawn from it. vii. When the term of life of the parent-cell has expired, and its reproductive molecules are prepared to continue the race, the actions of nutrition cease ; those of decomposition go on unchecked ; and the death of the OF VITAL ACTIONS IN GENERAL. 39 structure, or the loss of its distinguishing vital properties, is the result. By the decomposition, which then takes place with increased rapidity, its elements are restored to the Inorganic world ; presenting the very same properties as they did when first withdrawn from it ; and becoming capa- ble of being again employed, by any successive numbers of living beings, to go through the same series of operations. 42. Thus, then, we see that our fundamental idea of the properdTes" of the simplest Living being consists in this ; — that it has the capability of drawing into its own substance certain of the elements furnished by the inorganic world : — that it forms these into new combinations (which the chemist may find out methods of imitating) ; — that it rearranges the particles of these combinations, in that peculiar mode which we call or- ganization ; — that in producing this new arrangement, it renders them capable of exhibiting a new set of properties, which we call vital, and which are manifested by them, either as connected with the parent or- ganism, or as appertaining to the germs of new structures, according to the mode in which the materials are applied; — that, notwithstanding its peculiar condition, it remains subject to the ordinary laws of Chemistry, and that decomposition of its structure is continually taking place ; — and finally, that the duration of its vital activity is limited ; the changes which the organic structure undergoes, in exhibiting its peculiar actions, being such as to render it (after a longer or shorter continuance of them) incapable of any longer performing them. 43. There is abundant evidence, that the duration of the Life, or state of Vital Activity, of an organized structure, is inversely propor- tioned to the degree of that activity ; and consequently that Life is shortened by an increased or abnormal activity ; whilst it may often be prolonged by influences which diminish that activity. Thus, we shall hereafter find reason to believe, that the duration of life in the Muscular and Nervous tissues of Animals is entirely dependent upon the degree in which they are exercised ; every call upon their activity causing the death and disintegration of a certain part ; whilst if they be allowed to remain in repose for a time, only that amount of decomposition will take place, to which their chemical character renders them liable. Again, we may trace the connexion between the vital activity of a part and the duration of its life, by comparing the transitory existence of the leaves of a Plant, which are its active organs of nutrition, with the comparative permanence of its woody stem, the parts of which, when once completely formed, undergo very little subsequent change. The most striking manifestation of this connexion, however, is afforded by that condition, in which, without any appreciable amount of vital activity or change, an organized structure may remain unaltered for centuries ; not only presenting at the end of that time its original structure, but being pre- pared to go through its regular series of vital operations, as if these had never been interrupted. This state may be designated as Dormant Vitality. It difi'ers, on the one hand, from Life; because Life is a state of Activity. On the other hand, it differs from Death ; because Death implies not merely a suspension of activity, but a total loss of vital properties. Now in the state of Dormant vitality, the vital pro- perties are retained ; but they are prevented from manifesting them- k 40 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. selves, by the want of the necessary conditions. When these conditions are supplied, the state of vital activity is resumed, and all the functions of life go on with energy. 44. Of this Dormant Vitality it may be well to adduce some exam- ples ; which may assist in impressing on the mind of the student the general views here put forth. This condition is manifested in the most remarkable manner by the seeds and germs of plants ; many of which are adapted to remain, for an unlimited period, in a state of perfect repose, and yet to vegetate with the greatest activity, as soon as ever they meet with the necessary conditions. Thus the sporules of the Fungi, which can only develope themselves in decaying organized mat- ter, seem universally diffused through the atmosphere, and ready to vegetate with the most extraordinary rapidity, whenever a fitting oppor- tunity presents itself. This at least appears to be the only feasible mode of explaining their appearance, in the forms of Mould, Mildew, &c., on all moist decaying substances ; and that there is no improbabi- lity in the supposition itself, is shown by the excessive multiplication of these germs, a single individual producing not less than ten millions of them, so minute as when collected to be scarcely visible to the naked eye, rather resembling thin smoke, and so light as to be wafted by every movement of the atmosphere ; so that, in fact, it is difficult to imagine any place from which they can be excluded. — Moreover, it is certain that an equally tenacious vitality exists in the seeds of higher plants. Those of most species inhabiting temperate climates are adapted to remain dormant during the winter ; and may be easily pre- served, in dry air, and at a moderate temperature, for many years. Some of those which had been kept in the Herbarium of Tournefort during upwards of a century, were found to have preserved their ferti- lity. Cases are of no unfrequent occurrence, in which ground that has been turned up, spontaneously produces plants dissimilar to any in their neighbourhood. There is no doubt that in some of these cases, the seed is conveyed by the wind, and becomes developed in spots which afford congenial soil, as already remarked in the case of the Fungi. Thus it is commonly observed that clover makes its appearance on soils which have been rendered alkaline by lime, by strewed wood-ashes, or by the burning of weeds. But there are many authentic facts, which can only be explained upon the supposition, that the seeds of the newly- appearing plants have lain for a long period imbedded in the soil, at such a distance from the surface as to prevent the access of air and moisture ; and that, retaining their vitality under these circumstances, they have been excited to germination when at last exposed to the re- quisite conditions. Thus Professor Lindley states as a fact, that he has raised three raspberry-plants from seeds taken from the stomach of a man, whose skeleton was found thirty feet below the surface of the earth, at the bottom of a barrow which was opened near Dorchester ; as his body had been buried with some coins of the Emperor Hadrian, there could be no doubt that the seeds were 1600 or 1700 years old. Again, there are undoubted instances of the germination of grains of wheat, which were enclosed in the wrappers of Egyptian mummies, OF VITAL ACTIONS IN GENERAL. 41 perhaps twice that number of years ago ; the wheat being different in some of its characters from that now growing in the country. 45. These facts make it evident that there is really no limit to the duration of this condition ; and that when a seed has been thus pre- served for ten years, it may be for a hundred, a thousand, or ten thou- sand, provided that no change of circumstances either exposes it to de- cay, or calls its vital properties into activity. Hence in cases wEere seeds have been buried deep in the earth, not by human agency, but by some geological change, it is impossible to say how long anteriorly to the creation of man they may have been produced and buried ; as in the following very curious instance. — Some well-diggers in a town on the Penobscot River, in the State of Maine (New England), about forty miles from the sea, came at the depth of about twenty feet upon a stra- tum of sand ; this strongly excited curiosity and interest, from the cir- cumstance that no similar sand was to be found anywhere in the neigh- bourhood, and that none like it was nearer than the sea-beach. As it was drawn up from the well, it was placed in a pile by itself; an unwil- lingness having been felt to mix it with the stones and gravel which were also drawn up. But when the work was about to be finished, and the pile of stones and gravel to be removed, it was necessary also to remove the sand-heap. This, therefore, was scattered about the spot on which it had been formed, and Avas for some time scarcely remem- bered. In a year or two, however, it was perceived that a number of small trees had sprung from the ground over which the heap of sand had been strewn. These trees became in their turn objects of strong interest, and care was taken that no injury should come to them. At length it was ascertained that they were Beach-Plum trees ; and they actually bore the Beach-Plum, which had never before been seen, except immediately upon the sea-shore. The trees had therefore sprung from seeds, which were in the stratum of sea-sand, that had been pierced by the well-diggers. By what convulsion they had been thrown there, or how long they had quietly slept beneath the surface, cannot possibly be determined with exactness ; but the enormous length of time that must have elapsed since the stratum in which the seeds were buried formed part of the sea-shore, is evident from the accumulation of no less than twenty feet of vegetable mould upon it. 46. Numerous instances will be related in the succeeding Chapter, of the occurrence of a similar condition in fully-developed Plants, and even in Animals of high organization. In some of these it is a regular part of the history of their lives, coming on periodically like sleep ; whilst in others it is capable of being induced at any time, by a withdrawal of some of the conditions essential to vital activity. In regard to all of them, however, it may be observed, that the vitality can only be retained, when the organized structure itself is secluded from such influences as would produce its decay. Thus, the hard dry tissue of the seed is but little liable to decomposition ; and all that is usually required for the prevention of change in its structure, is seclusion from the free access of air and from moisture, and a steady low or moderate temperature. If a seed be exposed to air and moisture, but the temperature be not high enough to occasion its germination, it will gradually undergo decay, 42 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. and will consequently lose its vitality. The animal tissues are more liable, as already mentioned, to spontaneous decomposition; and the only instances in which they can retain their vitality for a lengthened period, without any nutritive actions, are those in which all decomposi- tion is prevented, either by the action of cold, or by the complete depri- vation of air or of moisture, — as when Frogs, Snakes, &c., have been preserved for years in an ice-house, or Wheel-Animalcules have been dried upon a slip of glass. 47. The class of phenomena last brought under notice, serves to ex- hibit in a very remarkable manner the dependence of all Vital Action upon certain other conditions, than those furnished by the organized structure alone. Thus a seed does not germinate of itself ; it requires the influence of certain external agencies, namely, warmth, air and moisture ; and it can no more produce a plant without the operation of these, than warmth, air, and moisture could produce it without a germ prepared by a pre-existing organism. Now when we come to study these conditions, we find that they may be arranged under two catego- ries, the material and the dynamical. Thus, a seed cannot germinate without sufficient water to bring the contents of its cells into a state in which their chemico-vital reactions can take place ; and it must be sur- rounded with an atmosphere containing oxygen, since without the pre- sence of this element the necessary chemical transformations cannot go on. Thus oxygen and water are the material conditions required by the germinating seed ; in almost every other case, alimentary matters are required in addition ; but these the seed possesses within itself. Even if supplied, however, with an unlimited amount of water and oxy- gen, the seed cannot germinate, unless it be acted on by Heat ; and this, in fact, may be considered with great probability, as supplying the force^ of which not merely the chemical transformations, but the growth and development of the tissues of the Plant, are the manifestation. This view will be more fully developed hereafter (Sect. 3). 48. This dependence of Vital actions upon certain external Agencies, as well as upon the properties of the organism which manifest them, is no greater than the dependence of any of the phenomena exhibited by an Inorganic substance, upon some other agency external to itself. In fact, no change whatever can he said to he truly spontaneous; that is, no property can manifest itself, unless it be called into action by some stimulus fitted to excite it. Thus, when spontaneous decomposition (as it is commonly termed) occurs in an organized or an inorganic substance, it is due to the forces generated by the mutual attraction between cer- tain elements of the substance, and the oxygen of the atmosphere ; and this attraction is sufficient to overcome that which tends to hold toge- ther the particles in their original state. If the air be totally excluded, decay will not take place ;* because no new force comes into operation, to cause a separation of the components from their original modes of union. The influence of the Dynamical conditions which are essential * On this principle meats, vegetables, and even liquid soups, are now largely pre- served, for the use of persons undertaking long voyages ; by enclosing them in tin cases, carefully soldered up. There is no limit to the time, during which decomposition may thus be prevented. I OF VITAL ACTIONS IN GENERAL. 43 to Vital activity, will be fully explained in the next Chapter ; and at present it will be sufficient to remark, that the degree in which they are supplied possesses a well-marked influence upon the amount of activity and energy manifested in the actions of the organized structure ; that there is a limitation in the case of each of them, as to the degree in which it can operate beneficially, the limitation being usually narrower and more precise, according to the elevation of the being in the scale ;~ that an excessive supply may be destructive to the vital properties of the organism, by over-stimulating it, and thus causing it to live too fast, or by more directly producing some physical or chemical change in its condition ; and that a deficiency will keep down or suspend all vital activity, leaving the structure to the unrestrained operation of those agencies which are always tending to its disintegration, and consequently occasioning a speedy loss of the vital properties, — save in those cases in which they may be preserved in a dormant condition, and which are exceptions to the general rule, that the death or departure of the vital properties follows closely upon the cessation of vital actions. 49. Our fundamental idea of Life, then, is that of a state of constant change or action ; this change being manifested in at least two sets of operations; — the continual withdrawal of certain elements from the inorganic world ; — and the incorporation of these with the peculiar struc- tures termed organized, or the production from them of the germs that are hereafter to accomplish this. As the conditions of this continual change, we recognise the necessity of an organized structure on the one hand, or of a germ which is capable of becoming so ; whilst we also per- ceive the necessity of a supply of certain kinds of matter from the inor- ganic world, capable of being combined into the materials of that struc- ture, which may be designated as the alimentary substances ; and, fur- ther, we see that the organism can exert no influence upon these, except with the assistance of certain dynamical agencies, such as light, heat, &c., that supply the forces ov powers without which no change can occur. And to these forces, acting under the conditions which the Organized body alone can supply, may be attributed (as will hereafter appear) the phenomena which we distinguish as Vital. 50. But just as we find among Inorganic bodies, that various kinds are to be distinguished by their difierent properties, whilst all agree in the general or essential properties of matter, so do we find that living organized substances are distinguished by a variety of properties inherent in themselves, whilst they all agree in the foregoing general or essential characters. ^ In many instances, the difference of their properties is as obviously coincident with differences in their structure and composition, as it usually is among the bodies of the Mineral world : thus we find the property of Contractility on the application of a stimulus, to be for the most part restricted to a certain form of organized tissue, the Muscular; and we find that the property by which that stimulus is capable of being generated and conveyed to a distance, seems to be restricted to another kind of tissue, the Nervous. In a great number of cases, however, very obvious diff'erences in properties manifest themselves, when no perceptible variation^ exist, either in structure or composition; thus it would be impossible to distinguish the germ-cell of a Zoophyte from that of Man, 44 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. by any difference in its aspect or composition ; yet neither can be deve- loped into any other form than that of the parent species, and they must be regarded, therefore, as essentially different in properties. In the same manner we shall find that, in the same organized fabric, there are very great varieties in the actions of its component cells, which indicate a similar variety in their properties ; and yet they are to all appearance identical. But there can be no reasonable doubt, that differences really exist in such cases ; though our means of observation are not such as to enable us to take cognizance of these, by the direct impressions they make upon our senses. Analogous instances are not wanting in the Mineral world ; for the Chemist is familiar with a class of compounds designated isomorphous, in which, with perfect similarity in external form and physical properties, there is a difference, more or less com- plete, in chemical composition, and consequently in the effects of re- agents. 51. Whatever may be the peculiar vital properties possessed by an organized tissue, we find that they are always dependent upon the main- tenance of its characteristic structure and composition, by the nutritive operations of which we have spoken ; and that their existence forms a part, as it were, of the more general phenomena of its Life. They manifest themselves with the first complete development of the tissue ; they are retained and exhibited so long as active nutritive changes are taking place in it ; their manifestation is weakened or suspended if the nutritive operations be feebly exerted; and they depart altogether, whenever, by the cessation of those actions, and the uncompensated influence of ordinary Chemical forces, the structure begins to lose that normal composition and arrangement of parts, which constitutes its state of organization. Hence we may regard these peculiar properties as con- formable, in all the essential conditions of their existence, with those more general properties, which have been previously dwelt upon as characterizing a living organized structure. 3. Of the Forces concerned in the Production of Vital Phenomena. 52. In prosecuting his inquiry into the causes of those phenomena of Living organisms, which, being of a totally different order from those of Inorganic matter, are distinguished as Vital, the Physiologist must take as his guide those methods of investigation, which have proved successful in other departments of scientific research. If he turn, then, to the sciences of Mechanics, Optics, Thermotics, Electricity, Magnetism, or Chemistry, he finds that the phenomena which they respectively comprise are referable to the operation of certain forces, and that what are termed the laws of those sciences, are nothing else than expressions of the conditions of action of those forces. Thus in Mechanics we have principally to do with the motion of masses of mat- ter, and our idea of force is chiefly derived from our own experience of the exertion of a power in producing or resisting motion ; whilst the ' laws' of Mechanics are nothing else than expressions of the conditions, under which the forces or powers that produce motion opeijate upon matter. So in Optics, we have to do with the force which we term light ; OF VITAL FORCE IN GENERAL. 45 and the laws of Optics are expressions of the conditions under which that force is propagated, and of its action on material substances. In Thermotics, again, we have to do with the force of Heat ; and its laws are expressions of the circumstances under which heat is propagated, and of the changes which it occasions in the substances it affects. So in the sciences of Electricity and Magnetism^ we have to do with the forces known under those names ; and with the laws expressive of their action" upon matter. And the scientific Chemist refers all the phenomena with which he is concerned to the operations of Chemical Affinity^ and endea- vours to deduce from observation of the phenomena the laws of the operation of this force. — So the Physiologist will be justified in assuming a Vital Force (or Forces) as the power which operates in producing Vital phenomena ; and will most legitimately pursue his science, in inquiring into the conditions under which that force operates. 53. The analogy of the Physical Sciences may be advantageously pursued further. — Although we are accustomed to speak of the power that produces Mechanical Motion, of Light, of Heat, of Electricity, of Magnetism, and of Chemical Afiinity, as distinct forces^ yet it has gra- dually become apparent that very intimate relations subsist between them, and that they are, in fact, mutually convertible ; so that one force (a) operating upon a certain form of matter, ceases to manifest itself, but developes another force (b), in its stead ; whilst, in its turn, the second force (b) may be reconverted into the first (a), or into some other (c), which, again, may reproduce either the first (a), or second (b), or some other (d or e). — It was in the case of Electricity and Magnetism, that this reciprocal relation, which is designated as ' correlation,' was first clearly apprehended. If an electric current be passed round a piece of soft iron, that iron becomes magnetic, and remains so as long as the current is circulating : on the other hand, from a magnet put in motion, an electric current may be obtained. Hence we are accustomed to con- nect these two forces under the term Electro-Magnetism ; but they can be easily shown to be quite distinct in their modes of operation on matter ; and their relation is not really more intimate than that of other forces. For Heat may be developed by Electricity ; as when a galvanic current, sent through a thin platinum wire, heats it to ignition, or even fuses it. Conversely, Electricity may be developed by Heat ; as when heat is applied to bars composed of dissimilar metals in contact with each other. Again, if Mechanical Motion be retarded, as in friction, we immediately have a development either of Heat or of Electricity ; heat alone being developed, when the two rubbing surfaces are composed of precisely the same substance ; and electricity being produced, when these substances are different. And it is for the most part through the medium of Heat or Electricity, that the force of Mechanical Motion is ' correlated' to Light, Magnetism, and Chemical Affinity. 54. The idea of correlation also involves that of a certain definite ratio, or relation of equivalence, between the two forces thus mutually inter- changeable ; so that the measure of force B, which is excited by a certain exertion of force A, shall, in its turn, give rise to the same measure of force A, as that originally in operation. Thus, when an electric current is set in motion by galvanic action, we have a conversion of chemical 46 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. force (which has manifested itself in the decomposition of the water and the oxidation of the zinc) into electrical ; but the electrical current may, in its turn, be made to produce chemical decomposition ; and the amount of this kind of change which it will effect, bears a precise correspon- dence (cceteris paribus) with the amount of zinc which has undergone oxidation in the galvanic cell. In like manner, when water at 212° is converted into steam, the heat which it receives is no longer manifested as heat, but mechanical force is developed in its stead, and this in a certain definite ratio, so that the ' mechanical equivalent' of heat is capa- ble of being exactly determined : so soon, however, as the steam, losing its elasticity by condensation, returns to the condition of water, the original equivalent of heat is again developed, its mechanical force being no longer manifested.* 55. Now in every case in which one force is thus converted into another, the change is effected through the medium of a certain form of matter, or material substratum. This may be, in some cases, of almost any description whatever ; as when Heat is produced by the friction or retarded motion of solids, liquids, or even gases ; or when Motion (as shown in expansion) is produced by the application of heat to any kind of material substance. But in other cases, the change can only be effected through some special form of matter ; or if several substances may serve as its medium, there is some one which is greatly superior to all the rest, in the readiness with which a certain force manifests itself through it. Thus iron is the only substance through which Electricity can be converted into Magnetism ; and the development of magnetic force, therefore, can only take place through this medium. So, Heat is more readily converted into Electricity through a combination of bis- muth and antimony, than through any other metals ; and the affection of Light by magnetic force (discovered a few years since by Prof. Fara- day), though producible through any transparent substance, can be made much more obvious when the magnetism is made to act upon a peculiar glass composed of vitrified borate of lead, than through the medium of any other substance yet known. This speciality in the action of diffe- rent substances, when subjected to the same forces, is a fact of funda- mental importance ; and it is on it, indeed, that our notion of their several properties depends. 56. Now as the properties of every kind of matter require certain conditions for their manifestation, our acquaintance with them entirely depends upon whether the conditions of their action have been afforded. Thus, to go back to a former illustration, supposing a new chemical =* The above statement is an expression of the simple facts of the case, which, when thus understood, render the hypothesis of " latent heat" altogether unnecessary. This hypothesis, however ingenious, will doubtless share the fate of many other such attempts to substitute a form of words for realities. It supposed the 966 degrees of heat expended in converting a certain amount of water at 212° into steam at 212°, to become altogether inactive or latent ; and gave no account whatever of the mechanical force which is produced in that act of conversion. The idea of an inactive force, in fact, is one that cannot be entertained ; for if a force ceases to be active, it is no longer < force.' And it cannot be imagined that force, any more than matter, should cease to exist; it wwsi manifest itself under some other aspect. — For a complete exposition of the mutual relations existing among the above-named agents, see Prof. Grove's treatise *' On the Correlation of the Physical Forces." OP VITAL FORCE IN GENERAL. 47 element to be discovered, we could not know its properties in regard to heat, electricity, or magnetism, the mode of its combination with other elements, the nature and properties of the compounds produced, their reactions with other compounds, &c., until we have tried a complete series of experiments upon it, — that is, until we have placed it in all the circumstances or conditions requisite to manifest the properties, wiiJi_ which we seek to become acquainted, or whose absence we seek to de- termine if they do not exist. Now we might have made all the experi- ments we could devise upon such a body ; and yet we might have failed in detecting some remarkable and distinguishing property inherent in it, simply because we had not placed it in the requisite circumstances for the manifestation of this peculiarity. Further, even in the elements or compounds with which we are best acquainted, it is very possible that properties exist, of which w^e as yet know nothing, simply because they have not yet been called into action by the requisite combination of conditions. For example, no one would have thought it possible, a few years since, that water could be frozen in a red-hot metallic vessel ; and yet this is now known to be effected with ease and certainty, in the proper combination of conditions. 57. Again, the properties of a compound substance are, in general at least, altogether different from those which present themselves in either of its components; so that we could not in the least degree judge of the former from the latter, or of the latter from the former. What more different, for example, than the physical and chemical properties of Water, from those of either the Oxygen or the Hydrogen that enter into its composition ? Or what more different than the properties of a neutral salt, from those of the acid and alkali by w^hose union it is pro- duced ? — Further, the properties of a substance may be completely changed, by an alteration in its condition as regards Heat or any other of the forces, already mentioned. For example, the particles of water have so strong an attraction for each other, at a low temperature, as to become aggregated in a crystalline form, and to produce a dense solid mass ; at somewhat a higher temperature, their mutual attraction is so slight, that a very small amount of mechanical force is sufficient to sepa- rate them, and they move upon each other with the utmost freedom ; whilst at a still higher temperature, they manifest a power of mutual repulsion, which increases with the greatest rapidity with every augmen- tation of temperature. Yet when the temperature of the substance is lowered to its former standards, we observe that it first returns to the liquid, and then to the solid form ; and that, in those states, it manifests all the properties which before characterized it. Not merely the phy- sical, but the chemical properties of bodies may be affected by a change in their mechanical condition. Thus, it is well known that oxygen and iron, at ordinary temperatures, have a mutual affinity, which is only sufficient to produce a slow combination between them ; whilst at high temperatures, that affinity is such as to cause their rapid and energetic union. Now if iron, in a state of very minute division, such as it pos- sesses when set free from the state of oxide by means of hydrogen, at the lowest possible temperature, be brought into contact with oxygen or even with atmospheric air, at ordinary temperatures, it immediately becomes 48 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. red-hot, and is converted into an oxide. The minuteness of the division, predisposing to chemical union, appears to be the occasion of our power of causing many substances to combine, when one or both are in the nascent state (that is, when just set free from some other combination), which could not be made to unite in any more direct manner ; thus, when a quantity, however minute, of any preparation of arsenic is dissolved in fluid in which hydrogen is being generated, the hydrogen will detach the metal from its previous combination, and will pass forth in union with it, as arseniuretted hydrogen, a compound which cannot be formed by the direct union of the elements. In like manner, in that mechanical mixture of three finely-divided substances, which we call gunpowder, the rapidity with which combustion is propagated through the largest col- lection of it, is entirely dependent upon the minute subdivision of its components, and the very close approximation of their particles. Hence it may be very correctly said, that the true chemical properties of the substances are not manifested, except when they are in a state of very minute division ; and that these are in fact obscured, by the aggregation of the particles into masses. Thus, then, we are at no loss to discover examples, in the Inorganic world, of an alteration in the sensible pro- perties, both Chemical and Physical, of the bodies composing it, by a change in the conditions in which they are placed. And it may be stated as a general fact, that we never witness the manifestation of new properties in a substance, unless it has undergone some change in its own condition, of which altered state these properties are the necessary attendants. 58. Now if we apply the same methods to the phenomena of Life, we shall see that they will lead to a mode of viewing them, which will con- siderably tend to the simplification of Physiological science. In the first place we have to look at these phenomena as the results of certain forces^ acting through those forms of matter which we term Organized ; and these forces we shall provisionally designate as Vital. Thus in the growth of the simple Vegetable cell, as already described (§§ 26-41), we trace the operation of a force closely allied to ordinary chemical affinity^ but so far difi*erent that it can only be exerted through a living organism ; of a force of assimilation or vital transformation ; and of a force of organization and complete vitalization. Now although we may provisionally designate these as distinct forces, on account of the diver- sity of their manifestations, it is impossible not to see that they are mutually dependent, and that they form the successive elements of a continuous series of phenomena belonging to the same category, that of cell-life; and further, we observe that they operate under the same conditions, namely, the presence of a cell-germ and of the materials of its growth, and the action of light and heat. Again, in the multiplica- tion of the original cell, by whatever method performed, we cannot but trace the continued action of forces of the same character ; since this operation takes place as a continuation of the process of growth, and under precisely the same influences. Further, we occasionally meet with examples, even among the simplest forms of Vegetation, of very active movement ; thus the filaments or elongated cells of the OscillatO' rice are continually bending themselves backwards and forwards, with a OF VITAL FORCE IN GENERAL. 49 regular rhythmical undulation; and the "zoospores" of the Confervece are propelled through the water by the rapid vibration of the cilia with which they are furnished (§ 234). Now that such a production of a purely physical change is a manifestation of vital force, is obvious from this, — that it takes place only while the vitality of the organism endures, and that it is dependent upon the very same conditions as the other vital operations require ; and it is further interesting to remark in the case of the "zoospores," that it seems to take the place of the operations of growth, for these do not commence until the movement of the spore has ceased. The spiral filaments, again, which have been discovered in most of the higher Cryptogamia, and which seem to per- form the same function with the spermatozoa of Animals (§ 240), have a similar spontaneous movement, which must be looked upon as an ex- pression of their vital force. Many cases of motion produced by a change of form of certain contractile cells, might be cited from among the higher tribes of the Vegetable kingdom ; these movements being sometimes rhythmical and spontaneous, as in the Hedysarum gyrans^ — sometimes taking place only in respondence to stimulation, as in the Dionoea museipula (Venus's fly-trap), — and sometimes occurring as part of the series of ordinary vital phenomena, although producible also by stimulation, as in the Mimosa pudiea (sensitive plant), which regularly closes its leaves at night, but will do so at any time when they are touched or otherwise irritated.* These movements only take place during the life of the Plant ; and it is particularly observable in the last-named species, that the facility with which they may be excited in any individual is closely related to the activity of its vegetating pro- cesses. Thus even in the Plant, we see that the Vital forces manifest themselves, not merely in growth, but in movement. 59. When we examine the structure of one of the higher Plants, we find that, although the principal part of its fabric is still made up of unmetamorphosed cells, yet that certain portions of it have undergone histological transformation; that is, its primordial cells have lost their original character, having been changed into other kinds of tissue. This transformation takes place to a much greater extent in the Animal body ; in which the variety of actions to be performed is much larger, and in which we accordingly find a much greater variety of tissues de- veloped as their instruments. But however widely these tissues may depart from their original character, we find that the process of trans- formation takes place under the same conditions as that of growth, and must be regarded as a continuation of it ; being, in fact, the special manifestation of vital force in one set of cells, as multiplication is in another, or as motion in another. And we shall find, that, in proportion as this transformation takes place, do the tissues lose their proper vital endowments ; for it may be stated as a general fact, that even in the most complicated and elaborate Animal organism, all the most active vital ope- rations are performed by tissues which retain their original cellular con- stitution with little or no change. — Further, it is to be observed, that as it is the peculiar character of such organisms that each of their parts should * For a fuller analysis of these phenomena, see the Author's "Principles of Physio- logy, General and Comparative," chap. xix. 4 56 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. be appropriated to some distinct office which it is specially adapted to per- form, so do we find that the cells which become the instruments of some one particular kind of operation seem to lose their other endowments ; as if the expenditure of the vital force of each cell upon any one pur- pose, unfitted it for any other agency. Of this we shall meet with numerous examples hereafter ; it will be sufficient here to refer to two of the most characteristic. It is necessary for every act of Secretion, that a set of cells should be formed within the ultimate follicles of the Gland which is the instrument of the function (§ 238); and these ulti- mate follicles are really to be regarded as parent-cells, which produce the true secreting cells in proportion as the materials of their growth are supplied by the blood. Now these parent-cells themselves possess no secreting power, their vital forces being entirely expended in the production of the true secreting cells. On the other hand, the true secreting cells possess no reproductive power, but die and are cast off when they have reached their maturity ; as if their whole vital force were expended in the secreting process, which is nothing else on their parts than an act of growth. So, again, the cells which constitute the fibrillse of Muscular fibre, and of whose change of form the contraction of the muscle is the result (§ 336), exercise no power of chemical transfor- mation, undergo no histological change, and appear to be entirely desti- tute of the power of self-multiplication ; the expenditure of their vital force in the act of muscular contraction involves their death and disinte- gration ; and their renewal appears to be accomplished by a production of new cells from the nucleus of the Myolemma (§ 338), which, itself pos- sessing no contractile power, retains its reproductive capacity. 60. Hence, then, we have reason to believe, that all the truly Vital phenomena, however diversified, are but results of the operation of one and the same Force, whose particular manifestations are determined by the nature of the material substratum through which it acts : the same fundamental agency producing simple growth in one case, transformation in another, multiplication in a third, mechanical movement in a fourth, whilst in a fifth it developes nervous power, which may itself operate in a variety of different modes. Such a view seems fully justified by the consideration, (1) that all these forces are exerted, even in the most highly-organized living being, through a common instrumentality, the simple cell ; (2) that the entire assemblage of cells making up the totality of any organism, have all a common parentage, being lineally descended from the single primordial cell in which it originated ; and (3) that they are manifested in connexion with each other, as parts of the life of each individual cell, in those simple organisms which are the lowest members of the two kingdoms respectively, and in which there is no separation or specialization of function. 61. The question next arises, — what is the source of the Vital Force, of which the phenomena of Life are the manifestations ; and under the guidance of the ideas derived from Physical Science, we shall have no difficulty in referring it to the operation of those external agencies, the influence of which has long been known to be essential to Vital action, and which have been usually designated by the term Vital Stimuli. Thus, the growing Vegetable cell cannot decompose carbonic acid, OF VITAL FORCE IN GENERAL. 51 except when acted upon bj Light ; and the amount of this change which it effects, is in strict ratio (eceteris paribus), with the illuminating power of the rays which it receives (§ 86). So, again, neither Plants nor Animals can maintain their activity, except under the continual influence of a certain measure of Heat ; and the amount of that activity will be shown to bear a constant ratio, in all those tribes which have no independent power of sustaining it, to the quantity which they receive from external sources (chap. ii. Sect. 2) ; this being true, not merely of the general rate of the Vegetative actions of growth and development, but also of those manifestations of vital power which are peculiar to Animals. Thus we may say, that Light and Heat acting upon the organic germ, become transformed into Vital force, in the same manner as Heat acting upon a certain combination of metals becomes Electricity, or as Electricity acting upon iron developes itself as Magnetism ; and we shall find that this view is in complete harmony with all the pheno- mena of Vital action. Moreover, the Vital force thus engendered fre- quently manifests itself in producing Physical or Chemical phenomena ; thus completing that mutual relationship, or correlation, which has been shown to exist among the Physical and Chemical forces themselves (§§ 53, 54). Of this we have already seen an instance in the movements pro- duced by muscular contraction and by ciliary vibration. The production of heat by certain Plants and by warm-blooded Animals, is another appo- site exemplification of the same principle. But the most remarkable illustration is undoubtedly derived from the Nerve-force ; which, whilst itself a peculiar form of the general Vital force, and capable of affecting all the other manifestations of the same force (as in the modifications which it produces in the processes of Nutrition and Secretion, as well as in exciting Muscular Contraction), is capable of developing Electricity as well as Light and Heat, and is also capable of being called forth by the action of Light, Heat, Electricity, Chemical Afiinity, or even Mechan- ical Motion, on the Nervous tissue. It is a most remarkable confirma- tion of the views here advanced, that the nerve-force, which must be accounted, in its relations to Mind, as the highest of all the forms of Vital force, should yet be the one which is most directly and intimately related to the Physical forces, — the '^ correlation" even of Electricity and Magnetism not being more complete, than the "correlation" of Electricity and Nerve-force may be shown to be (§ 396). 62. Thus, then, not only are the materials drawn from the Inorganic world by vital agencies, given back to it again by the disintegration of the living structures of which they form a part ; but all the forces which are operative in producing the phenomena of Life, being first derived from the Inorganic universe, are returned to it again under some form or other. ^ The Plant forms those organic compounds, at the expense of which Animal life (as well as its own), is sustained, by the decomposi- tion of carbonic acid, water, and ammonia ; and the light, by whose agency alone this process can be effected, may be considered as meta- morphosed into the peculiar affinity, by which the elements of these compounds are held together. The heat vfluch. Plants receive, acting through their organized structures as Vital force, serves to augment these structures to an almost unlimited extent, and thus to supply new 62 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. instruments for the agency of light and for the production of organic compounds. The whole nisus of Vegetable life may be considered as manifested in this production ; and, in effecting it, each organism is not only drawing material, but force, from the universe around it. Sup- posing that no Animals existed to consume these organic compounds, they would be all at last restored to the inorganic condition by sponta- aeous decay, which would reproduce the carbonic acid, water, and am- monia, from which they were generated. In this decay, however slow, heat and light are given out, in the same amount as when more evidently produced in the ordinary combustive process ; and this sometimes occurs even during the life of the plant, whose vital movements, also, may be considered as restoring to the Inorganic universe a certain measure of the force they have derived from it under other forms. Thus in making use of the stores of Coal which have been prepared for his wants by the luxuriant Flora of past ages, Man is not only restoring to the atmo- sphere the carbonic acid, the water, and the ammonia, of the Carboni- ferous period, but is actually reproducing and applying to his own purposes, the Light and Heat which were operating to produce the growth of vegetation at that remote period in the Earth's history. 63. But the organic compounds which the agency of Light and Heat upon the Vegetable structures has produced, are designed for a much higher purpose, than that of being merely given back to the Inorganic universe by decay or combustion ; and the forces which hold together their elements have a much more exalted destiny. In serving as the food of Animals, a part of them become the materials of their organized tissues, and the instruments through which the nervous and muscular forces are developed; whilst another part are applied to sustain the combustive process, by which the heat of the higher classes is main- tained quite independently of the external supply of that force. The greater part of the Animal kingdom, however, is dependent, like the Vegetable, upon the Inorganic Universe, for the Heat which serves as its organizing force ; and it is only under the constant influence of this agent, that the operations of growth, development, and maintenance can take place. The Animal is not dependent like the Plant upon Light ; and this is obviously because that agent is chiefly concerned in that preliminary operation, by which the organic compounds are generated as the pabulum of the growing tissues ; in fact, the embryo within the germinating seed, which, like the animal, is nourished upon matter previously prepared for it, is most rapidly developed in the absence of light, up to the time when, its store being exhausted, its further sup- plies must be obtained by its own instrumentality. — The Vital activity of Animals, then, may be considered as chiefly sustained by the Che- mical forces subsisting in their food, which are set free when the ele- ments are reconverted to their original state ; and by the Heat which they derive from external sources, or from the combustion of a part of their food. These forces may be considered as in a state of continual restoration to the Inorganic Universe, during the whole life of Animals, in the heat, light, electricity, still more in the motion, which they deve- lope ; and, after their death, in the production of heat and light during the processes of decay. During Animal life, there is a continual resto- OF DEGENERATION AND DEATH. 53 ration to the mineral world, of the carbonic acid, water, and ammonia, which have been appropriated by Plants ; and it will hereafter appear, that the amount thus given off by the animal organism bears a close correspondence, on the one hand, with its degree of vital activity, as shown in the amount of heat and motion which it generates, and, on the other, with the amount of the organic compounds which it consumes as food. So that, on the whole, there is strong reason to believe that tEe entire amount of force (as of materials), received by an animal during a given period, is given back by it during that period, provided that its condition at the end of the term is the same as it was at first ; and fur- ther, that all the force (like the material), which has been expended in the building up of the organism, is given back by its decay after death.* 4. Of Degeneration and Death. 64. We have seen that the general history of the phenomena of Life is fully conformable with the view, that the Vital properties of a tissue (that is, the properties in virtue of which the forces that act upon it are caused to manifest themselves in Vital action), are dependent upon that state of combination and arrangement, which is termed Organiza- tion. As long as each tissue retains its normal or regular constitution, renovated by the actions of absorption and deposition through which that constitution is preserved, and surrounded by those other conditions which a living system alone can afford, so long, we have reason to be- lieve, it will retain its vital properties, — and no longer. And just as we have no evidence of the existence of vital properties in any other form of matter than that which we call organized, so have we no reason to believe that organized matter can retain its regular constitution, and be subjected to the appropriate forces, without exhibiting vital actions. The advance of pathological science renders it every day more probable (indeed the probability may now be said to amount to almost positive certainty), that derangement in function^ — in other words, an imperfect or irregular action^ — always results, either from some change of struc- ture or composition in the tissue itself, or from some corresponding change in the forces by which the properties of the organ are called into action. Thus, when a Muscle has been long disused, in can scarcely be excited to contraction by the usual stimulus, or may even be altoge- ther powerless ; and minute examination of its structure shows it to have undergone a change, which is obvious to the microscope, though it may not be perceptible to the unaided eye, and which results from imperfect nutrition. Or, again, convulsive or irregular actions of the Nervous system may be produced, not by any change in its own composition, but by the presence of various stimulating substances in the blood, although their amount be so small that they can scarcely be recognised. ^ Q^, As there is a constant tendency, in the Animal tissues more espe- cially, to spontaneous decay, so must the maintenance of the vital pro- * The whole of this subject is more fully developed in the Author's "Principles of Physiology, General and Comparative," chaps, iii. and v. ; and in a Paper on "■ The Mutual Relations of the Vital and Physical Forces," contained in the Philosophical Transactions for 1850. 64 NATURE AND OBJECTS OP PHYSIOLOGICAL SCIENCE. perties depend upon their continual regeneration by the nutritive opera- tions. Hence we have no difficulty in accounting for the Death of the whole system, on the cessation or serious disturbance of any one impor- tant function ; for any such check or change must suspend or disorder the nutrient processes, in such a degree that they can no longer main- tain the normal constitution of the several tissues. But as there is a great variety in the rapidity of the decomposition of the tissues, when the act of nutrition is suspended, so do we witness a corresponding variety in the duration of their vital properties, after that permanent severance of the chain of functions, which is distinguished as somatic death, — i. e., the death of the hodt/ as a whole. It is by the Circulation of the Blood, that the connexion of the diiferent functions is essentially maintained ; that fluid being not only the material for the nutrition of the tissues, but in many cases supplying also the stimulus to their activity. Hence with the permanent cessation of the Circulation, somatic death must be regarded as taking place. 6Q. Yet after this, we observe that vitality lingers in the tissues ; and that it departs from them only as they lose their proper composition. Thus we find that, although the Nervous centres cannot originate the stimulus necessary to produce Muscular contraction, after the Circula- tion has ceased, — yet the Nervous fibres can convey such a stimulus, long after somatic death ; so that contractions may be excited in muscles by the application of galvanism, or of mechanical or chemical stimulants, to the trunks that supply them. The molecular death of the Nervous tissue, therefore, has not yet taken place. After a time, however, this power is lost ; the tissue no longer exhibits its distinguishing vital pro- perties ; and incipient decomposition and change of structure manifest themselves. Yet for some time after this, the Muscular tissue, especially in a cold-blooded animal, continues to possess its peculiar contractility ; for contractions may be excited in it, by stimuli directly applied to itself, long after the nerves have ceased to convey their influence. Sometimes, indeed, the contractility of muscle endures, until changes in its struc- ture and composition become evident to the senses ; thus the heart of a Sturgeon, removed from the body, and hung up to dry, has been known to continue alternately contracting and dilating, until the movement pro- duced a crackling noise, in consequence of the dryness of the texture. Again, there is evidence, that various processes of nutrition and secre- tion may go on, for some time after somatic death, and even after the removal of the organs from the body, provided a sufficient quantity of blood remains in them ; and the blood itself retains its vitality, so as not to coagulate, whilst contained in the vessels of tissues still living. 67. Hence it is, that parts which have been completely separated from the body may often be reunited with it, if they were previously in a healthy state, and too much time have not elapsed ; thus, there are many cases on record, in which fingers, toes, noses, or ears, that have been accidentally chopped ofi^, have been made to adhere and grow as before, by bringing the cut surfaces into contact, even some hours after their severance. It is evident, then, that the parts so severed cannot have lost their vitality ; , since no treatment could produce union between a dead mass and a living body. And we are fully justified in assuming, OF DEGENERATION AND DEATH. 55 that, in cases where attempts at such reunion have not been successful, the death of the separated part has resulted from the too prolonged interruption of its regular nutritive operations, whereby chemical and physical changes have taken place in it, and destroyed the peculiar struc- ture and composition of its several parts. The ordinary phenomena of Death, therefore, as well as those of Life, bear out the views which have been here advanced. 68. But it has been maintained by those who consider Vitality as something superadded to an Organized Structure, essentially indepen- dent of it, and capable of being subtracted from it, that Death frequently takes place under circumstances, which leave the organism as it was ; so that " the dead body may have all the organization it ever had whilst alive." For such an assumption, there is not the least foundation. In nearly all cases in which death takes place as a result of disease, the connexion between changes of structure and composition, either in the tissues or in the blood, and such a loss of the vital properties of some part or organ as is sufficient to bring the Circulation to a stand, is so palpable as to require no proof; and in by far the greater majority of cases in which it is not at once obvious, a more careful scrutiny will reveal it. It must be confessed on both sides, that our means of inves- tigation, and our knowledge of the normal structure and composition of the tissues and the blood, are not yet sufficient to enable us to detect minute shades of alteration, nor to assert what extent of change is incon- sistent with the continuance of life. But as no one has yet shown, by the careful and exact microscopical and chemical examination of the solids and fluids of a dead body, that it has all the organization it had whilst alive, the assertion above quoted is totally unwarranted by experience, and is contradicted by all our positive knowledge of the matter. (See§187.)_ 69. But it has been urged, that Death may result from the sudden operation of some agency of an immaterial character, which leaves no trace behind it, — such as a powerful electric shock, or a violent mental emotion. Here, too, the argument entirely fails. It is impossible that a powerful electric shock could be transmitted through a mass like the animal body, composed of elements in such a loose state of combination that they are always undergoing decomposition, without producing impor- tant chemical changes in it ; and its imperfect conducting power renders- it equally liable to physical disturbances. As a matter of fact it has been noticed, that the bodies of animals killed by electricity pass into decomposition with unusual rapidity ; showing that the ordinary chemical affinities of their components have received a powerful stimulus ; and it has also been ascertained, that when eggs in process of development have had their vitality destroyed by an Electric shock, the minute vessels of the vascular area (§ 551) have been ruptured. — ^Nor is it more difficult to explain the immediate cause of death, as a result of Mental emotion. In some cases, an obvious physical change has been produced, by the too violent action of the heart, the movements of which are stimulated by the emotion ; thus, even in a healthy person, rupture of the heart or aorta has been known to take place, — an occurrence to which those affected by previous disease of that organ are much more liable. Where 56 NATURE AND OBJECTS OP PHYSIOLOGICAL SCIENCE. there is any disorder in the heart's action, resulting from thickened valves, narrowed orifices, &c., the physical influence of mental emotion can be easily accounted for. But it must be admitted that cases have occurred^ in which no such explanation can be offered ; sudden death having taken place without any perceptible structural cause. We are not obliged, however, to have recourse to any hypothesis, for an explana- tion of even these cases, which is not borne out by ample analogy. For it is well known that mental emotions, acting through the nervous force, exert a powerful influence over the composition of the fluids of the body, and are capable of instantaneously altering these. Thus in many human beings, and still more in the lower animals, alarm or agitation will occasion the immediate disengagement of powerfully odorous secretions, which must have resulted from new combinations suddenly formed ; and a fit of passion may immediately occasion such a change in the milk of a nurse, as renders it a rank poison to the infant. There is no reason to doubt, therefore, that the blood itself may undergo changes of analo- gous character from the same cause ; and that it may become a violent poison to the individual himself, instead of being the source of whole- some nutriment, or the stimulus to vital activity. — But the effect of Electricity, of mental Emotion, or even of Mechanical force, may be exerted more dynamically than organically ; destroying the vital powers, by antagonizing the forces that produce them, without occasioning any perceptible material change. This, in fact, we see in the state of pros- tration or ' shock,' induced by sudden and violent impressions of almost any description. 5. General Summary. 70. To conclude, then ; — we only know of Life, as exhibited by an Organized structure, when subjected to the operation of certain forces which call it into activity ; and we only know of Vitality, or the state or endowment of the being which exhibits that action, as conjoined with that particular aggregation and composition which we term Organiza- tion. We have seen that the act of Organization, and the consequent development of peculiar properties in the tissues which are produced by it, can only be attributed to the vital force of a pre-existing organism ; and hence it is, that whilst the operation of Physical forces upon an organized body gives rise to vital phenomena, no such phenomena can be manifested as the result of their action upon any kind of inorganic matter. It is in fact, the speciality of the material instrument thus fur- nishing the medium of the change in their modus operandi, which es- tablishes, and must ever maintain, a well-marked boundary-line between the Physical and the Vital forces. According to the views here pro- pounded, the Vital force is as different from Heat or Electricity, as they are from each other ; but just as Heat, acting under certain peculiar conditions, is capable of transformation into Electricity, whilst Electri- city is capable, under certain other conditions, of being metamorphosed into Heat, so may either of these forces, acting under conditions which an Organized fabric alone can supply, be converted into Vital force, whilst, in their turn, they may be generated by Vital Force. GENERAL SUMMARY. 57 71. Starting, then, with the abstract notion of one general Force, we might say that this Power, operating through Inorganic matter, mani- fests itself in those phenomena which we call electrical, magnetical, chemical, thermical, optical, or mechanical ; the agents immediately con- cerned in these being so connected by the relation of reciprocal agency, or " correlation," that we must regard them as fundamentally the same. But the very same Force or Power, when directed through Organized structures, effects the operations of growth, development, metamorphosis, and the like ; and is further transformed, through the instrumentality of the structures thus generated, into nervous agency and muscular power. If we only knew of Heat, for example, as it acts upon the or- ganized creation, the peculiarities of its operation upon inorganic matter would seem no less strange to the physiologist, than the effects here attributed to it may appear to those who are only accustomed to con- template the physical phenomena to which it gives rise. Of the exis- tence of Force or Power, we can give no other account than by referring it, as we are led by our own consciousness to do, to the exertion of a Will; and this unity among the Forces of Nature is the strongest possible indication of the Unity of the Will of which they are the ex- pressions. And further, the constancy of the actions which result from them, when the conditions are the same, — that is, their conformity to a fixed plan, or (in the language commonly employed) their subordination to laws, — indicates the constancy and unchangeableness of the Divine Will, as well as the Infinity of that Wisdom,' by which the plan was at first arranged with such perfection, as to require no departure from it, in order to produce the most complete harmony in its results. 72. So also, if we endeavour to assign a cause for the existence of a cell-germ, we are led at first to fix upon the vital operations of the pa- rental organism by which it was produced ; and for these we can assign no other cause than the peculiar endowments of its original germ, brought into activity by the forces which have operated upon it. Thus we are obliged to go backwards in idea from one generation to another ; and when at last brought to a stand by the origin of the race, we are obliged to rest in the Divine Will as the source of those wonderful pro- perties, by which the first germ developed the first organism of that race from Materials previously unorganized, this organism producing a second germ, the second germ a second organism, and so on without limit, by the uniform repetition of the same processes. Yet we are not to suppose that the continuation of the race is really in any way less dependent upon the Will of the Creator, than the origin of it. For whilst Science leads us to discard the idea that the Deity is continually interfering, to change the working of the system He has made,— since it everywhere presents us with the idea of uniformity in the plan, and of constancy in the execution of it, — it equally discourages the notion entertained by some, that the creation of matter, endowed with certain properties, and therefore subject to certain actions, was the final act of the Deity, as far as the present system of things is concerned, instead of being the mere commencement of his operations. If it be admitted that matter owes its origin and properties to the Deity, or, in other words, that its first existence was but an expression of the Divine Will, 58 EXTERNAL CONDITIONS OF VITAL ACTIVITY. what is its continued existence, but a continued operation of the same Will ? To suppose that it could continue to exist, and to perform its various actions, by itself, is at once to assume the property of self-exist- ence as belonging to matter, and thus to do away with the necessity of a Creator altogether ; — a conclusion to which it may be safely affirmed that no ordinarily constituted Man can arrive, who reasons upon the indications of Mind in the phenomena of Nature, in the same way as he does in regard to the creations of Human Art. CHAPTER 11. OP THE EXTERNAL CONDITIONS OF VITAL ACTIVITY. 73. It has been shown in the preceding Chapter, that the most general conditions of Vital phenomena are twofold ; — one set being supplied by the organized structure, which is endowed (in virtue of its organization) with certain peculiar properties, but which is inert so long as it is alto- gether secluded from the influence of external agents ; — whilst the other is derived from external sources, and consists in a supply of those ma- terials of which the organized structure is built up, and in the operation of iho^Q forces by which the organism is made to appropriate those ma- terials, which are the sources of its peculiar powers. We might thus, in a rough and rude way it is true, compare the living body to a set of machinery adapted to convert cotton from the raw material into a woven fabric. Each portion of the machinery does its own special work, in virtue of its peculiar construction ; e. g. one part cards, another spins, and a third weaves ; but their actions are closely related and even mu- tually dependent. Further, their operations all result from one and the same force or power ; and their products may consequently be regarded as the expressions or manifestations of that Force, which acts through the diff'erent portions of the mechanism, each in its own peculiar mode. Now. such a machine can produce no result, without the concurrence of these conditions ; namely, the perfectly constructed organism (for so in the wide sense of the term it may be designated), a supply of the raw material on which it is to operate, and an adequate moving power. And it is to be observed, that the amount of its product will depend rather upon the power, than upon the material supplied ; for whilst its activity cannot be increased by any augmentation in the quantity of the material, beyond that amount which it has power to employ, it can be promoted by a more energetic application of the force, as well as retarded by its diminution ; the amount of material appropriated being increased or diminished accordingly. 74. In like manner, it is requisite to distinguish, among the external conditions whose concurrence is necessary to produce a Living Organism, between those which furnish the materials requisite for its construction and maintenance, and the forces or powers on which its operations are dependent ; in other words, between the Material and Dynamical con- EXTERNAL CONDITIONS OF VITAL ACTIVITY. 59 ditions of Vital Activity. Under the former group must be comprised, not merely the Alimentary substances which are capable of being con- verted into portions of the solid fabric, but also those which are used (among the warm-blooded animals) for the maintenance of the bodily heat by the combustive process. In addition, we have to include the Water which is requisite to maintain the due proportion of liquid in the organized fabric ; and the Oxygen, whose presence in the surrounding medium is essential in various modes to the maintenance of its vital activity. The dependence of Vital Activity upon Food and Oxygen will be fully considered hereafter (chaps, iv. and viii.); and in the present Chapter it will be only necessary to take account of the demand for Moisture (Sect. 4). 75. The Forces to whose operation we can most clearly trace the phenomena of Life, are Light and Heat^ of which the latter is the one whose agency is the most universal, and most immediately connected with the acts of growth and development. The agency of Light is indispensable for the first production of organic compounds by the in- strumentality of the Vegetable fabric ; but it would possess no efiicacy whatever, without the simultaneous operation of Heat , and when these compounds have been generated, we find that they can be applied to the purposes of Vegetable nutrition, no less than to the nutrition of Animals, without the aid of Light ; as is seen in the fact, that the ger- mination of seeds takes place in darkness, and that the formation of new wood in a stem takes place beneath a thick covering of bark. A very large proportion of the vital operations of Animals have no direct dependence upon Light ; yet it is entirely through its operation upon Plants, that they derive the materials of their nutriment ; so that La- voisier was fully justified in the assertion that " without Light, nature were without life and without soul ; and a beneficent God, in shedding light over creation, strewed the surface of the earth with organization, with sensation, and with thought." As an example of the very direct relation which subsists between the amount of Light and Heat acting on an organism, and the amount of vital change produced, it may be well to advert to the statement of Boussingault, that the same annual plant, in arriving at its full development, and going through the process of flowering and of the maturation of its seed, everywhere requires the same amount of Light and Heat, whether it be grown at the equator or in the temperate zone; the whole time occupied being inversely to the intensity of these forces, and the rate of growth having a relation of direct equivalence to it. — We have little certain knowledge of the degree of the ordinary dependence of Vital Activity upon Electricity ; although there can be no doubt that it is capable of exerting a most important influence upon the living organism. 76. In regard to all these Forces it may be observed, that the de- pendence of Vital Action upon their constant influence is greater in proportion to the high organization of their structure, and vice versd ; so that beings of simple organization are capable of enduring a depri- vation of them, which would be fatal to those higher in the scale. This will be partly understood, when it is borne in mind that the higher the development of the living being, the more complete is the distribution 60 EXTERNAL CONDITIONS OF VITAL ACTIVITY. of its diiFerent actions amongst separate organs, — the more close, there- fore, is their mutual dependence, — and the more readily, in consequence, are they all brought to a close by the interruption of any one. But there is no doubt, that the actions of even the individual parts of the higher organisms require for their excitement a greater supply of these powers, than the similar actions of the corresponding parts in the lower : whilst if these forces be exerted upon the lower with the intensity that is required for the higher, they destroy the vital properties of the tissues altogether, by the excess of their action. This distinction is most ob- vious in regard to the relative influence of Heat, upon warm-blooded and cold-blooded animals ; of which examples will be given hereafter. 77. It may also be observed of the influence of these, as of that of other forces whose agency is less general, that it is rather relative than absolute ; being frequently dependent upon the degree of change, rather than upon the measure of the actual amount. This constitutes a marked difierence between the influence of these forces on mere chemical com- pounds, and their operation on bodies endowed with vitality. In the former case their action is always uniform ; thus the same amount of heat, the same exposure to light, the same charge of electricity, would be required to produce a given Chemical efi'ect, how often soever the action might be repeated. But this is not the case with living bodies ; since an increase or diminution in the intensity of Heat, which, if made suddenly, would be scarcely compatible with the continuance of Life, may be so brought about, as to produce no marked change in its phe- nomena,— the organism possessing a certain power of adapting itself to conditions which are habitual to it, and thus allowing great changes in these conditions to be gradually efi*ected, without any serious disturb- ance.— Thus of two individuals of the same species, one may become torpid at a temperature of 60°, because it has been accustomed to a temperature of 70° ; whilst another, habituated to a temperature of 60°, would require to be cooled down to 50°, in order to induce torpidity; the influence of temperature upon the vital condition being proportioned, more to the variation from the usual standard, than to the actual degree of heat or cold in operation. Yet the first of these individuals might be gradually habituated to live in the same temperature with the second ; and to require the same amount of further depression for the induction of torpidity. (See § 132.) 78. It is a very curious fact, that, whilst the lower classes of living beings are more capable than the higher of bearing the deprivation of these Vital stimuli, they are at the same time more liable to alterations in their own structure and development, in consequence of variations in the degree of their agency, or from other causes external to them- selves. Thus the forms of the lower tribes of Plants and Animals are liable to be greatly afi'ected by the conditions under which they grow ; and these especially modify their degree of development. It seems as if the formative power were less vigorous in the lower, than in the higher classes; so that the mode in which it manifests itself in the former is more dependent upon external influences ; whilst in the latter it either predominates over them, causing the regular actions to be per- formed, or gives way altogether. — The same principle applies to the LIGHT, AS A CONDITION OP VITAL ACTIVITY. 61 % early condition of the higher organisms ; their embryos, like those beings of permanently-low type which they resemble in degree of deve- lopment, being liable to be affected by modifying causes, which the perfect beings of the same kind are able to resist. It ig in this way that we are to explain the influence which the female parent exerts upon the embryo, during the period through which it is dependent upon her for the materials for its development. 1. Of Light J as a Condition of Vital Activity. 79. The importance of this agent, not only to the Vegetable but to the Animal World, is not in general sufficiently estimated. Under its influence alone can that fiirst process be accomplished, by which Inor- ganic matter is transformed into an Organic compound, adapted by its nature and properties to form part of the organized fabric. The fol- lowing is an example of the simplest phenomenon of this kind ; and it demonstrates the influence of Light the more clearly on account of that simplicity. "If we expose some spring-water to the sunshine, though it may have been clear and transparent at first, it presently begins to assume a greenish tint ; and, after a while, flocks of green matter col- lect on the sides of the vessel in which it is contained. On these flocks, whenever the sun is shining, bubbles of gas may be seen, which, if col- lected, prove to be a mixture of oxygen and nitrogen, the proportion of the two being variable. Meanwhile the green matter rapidly grows ; its new parts, as they are developed, being all day long covered with air-bells, which disappear as soon as the sun has set. If these observa- tions be made upon a stream of water, the current of which runs slowly, it will be discovered that the green matter serves as food for thousands of aquatic Insects, which make their habitations in it. These insects are endowed with powers of rapid locomotion, and possess a highly- organized structure ; in their turn they fall a prey to the Fishes which frequent such streams."* Such is the general succession of nutritive actions in the Organized Creation. The highest Animal is either directly dependent upon the Vegetable Kingdom for the materials of its fabric, or it is furnished with these by some other Animal, this again (it may be) by another, and so on ; the last in the series being always necessitated to find its support in the Vegetable kingdom, since the Animal does not possess the power of causing the Inorganic elements to unite into even the simplest Organic compound. This power is pos- sessed in a high degree by Plants ; but it can only be exercised under the influence of Light, We shall now examine, more in detail, the conditions of this influence, both in the instance just quoted, and in others drawn from the actions of the higher Vegetable organisms. 80. The "green matter of Priestley," (as it is commonly called), which makes its appearance when water of average purity is submitted to the action of the Sun's light, and which also presents itself on the surface of walls and rocks that are constantly kept damp, is now known by Botanists to consist of cells in various stages of development, — the * Prof. Draper, on the Forces which produce the Organization of Plants ; p. 15. 62 EXTERNAL CONDITIONS OF VITAL ACTIVITY. early forms, it may be, of several different species of Confervae. That these cells all originate from germs, and not merely from a combina- tion of inorganic elements, appears not only from general considera- tions, but also from the fact that, if measures be taken to free the water entirely from any possible infusion of organic matter, and to admit into contact with it such air alone as has undergone a similar purification, no green flocks make their appearance, under the prolonged influence of the strongest sunlight. We find, then, that the presence of a germ is one of the conditions indispensable to the chemical transformation in question. It may be asked how it can be certainly ascertained that lights and not heat, is the essential condition of this process ; seeing that the two agents are combined in the solar beam. To this it may be replied, that a certain moderate amount of heat is undoubtedly ne- cessary ; but that no degree of heat without light will be effectual in producing the change, as is easily proved by exposing the water to warmth in a dark place. Moreover, when a certain measure of light is afforded, variations in the amount of heat make very little difference ; but we shall presently see that under the same degree of heat, the amount of the change is directly proportional to the intensity of the light. Although, therefore, heat furnishes an essential condition, it cannot be questioned that light is the chief agent in the process, by which the germ brings into union the elements to be employed in the development of its own fabric. 81. The next question is, — What are these elements, and whence are they obtained ? All water that is long exposed to the atmosphere ab- sorbs from it a certain amount of its constituent gases ; but these do not enter it in the proportions in which they are contained in the atmosphere itself; their relative quantities, in a given measure of water, being proportional to the facility with which they are respectively ab- sorbed by the liquid. Thus, carbonic acid is most readily absorbable ; oxygen next, and nitrogen least so. From the experiments of Prof. Draper, it would appear that, notwithstanding the very small propor- tion of carbonic acid contained in the atmosphere (usually not more than l-2000th part), it forms as much as 29 per cent, of the whole amount of air expelled from water by boiling. Of the residue, one-third consists of oxygen, and the remaining two-thirds of nitrogen ; so that the proportion of the oxygen to the nitrogen is as one to two, instead of being one to four, as in atmospheric air. The absolute quantity of this water-gas, contained in any measure of water, is subject to varia- tion with the temperature ; the quantity being diminished as the tem- perature rises. Now when water thus impregnated with carbonic acid, oxygen, and nitrogen, and containing the germs of aquatic plants, is exposed to the sun's light, a development of vegetable structure takes place, indicated by the green flocculent appearance, as already men- tioned. If the changes, which are now occurring in the water, be ex- amined, we find that the carbonic acid is diminishing in amount ; and that oxygen is being evolved. The growing mass increases in volume and weight ; and after a time exhausts the whole carbonic acid origi- nally contained in the water. If it be then prevented from receiving an additional supply, the process stops ; but as conducted naturally, INFLUENCE OP LIGHT ON PLANTS. 63 there is a free exposure to the atmosphere, through which carbonic acid is diffused ; and hence, as fast as it is removed by decomposition, it is restored by absorption. 82. Here then are the conditions and materials ; what i^ the result ? As a consequence of the conjoint action of light and of a vegetable cell- germ, with a moderate degree of heat, upon carbonic acid and water, we find a vegetable structure produced, whose fabric chiefly consists of carbon, united with the elements of water. Whether this union is really as simple and direct as is implied by this expression, or whether the same proportions of ogygen, hydrogen, and carbon are united in a dif- ferent form, is not a matter of consequence to the present inquiry ; the general fact being, that by the decomposition of the carbonic acid, oxy- gen is set free, and carbon is made to unite with the elements of water ; so as to form an organic compound, which is appropriated by the Vege- table organism as the material for its growth. — How far Light is also concerned in the production of the proteine-compounds which are gene- rated by Plants, not merely for the use of Animals, but also as part of the material of their own growth, has not been yet ascertained ; but it is probable that these are not the less dependent upon its agency for their formation, since they are formed under the same circumstances with the preceding. 83. The process whose conditions we have thus examined, is carried on in the individual cells, that compose the highest and most complex Plants, precisely as in those which constitute the entire organisms of the lowest. Thus if a few garden-seeds of any kind be sown in a flower-pot, and be caused to germinate in a dark room, it will soon be perceived that although they can grow for a time without the influence of light, that time is limited ; the weight of their solid contents diminishes, although their hulk may increase by the absorption of water ; their young leaves, if any should be put forth, are of a yellow or gray-white colour, and they soon fade away and die. But if these plants are brought out suffi- ciently soon into the bright sunlight, they speedily begin to turn green, they unfold their leaves, and evolve their different parts in a natural way ; and the proportion of their solid contents goes on increasing from day to day. If the fabric be then subjected to chemical analysis, it is found to contain oxygen, hydrogen, carbon, and nitrogen ; united in various proportions, so as to form compounds that differ in the various species, though some, — such as gum, starch, cellulose, and albuminous matter, — are the same in all. If the plants be made to grow in closed glass vessels, under such circumstances that an examination can be accurately made as to the changes they are impressing on the atmo-^ sphere, it is discovered that they are constantly decomposing its carbonic acid, — appropriating its carbon, and setting free its oxygen, — so long as they are exposed to the influence of sunshine or bright daylight. They also appropriate a part of the minute quantity of ammonia which is diffused through the atmosphere ; extracting its nitrogen to employ it in the production of their azotized compounds. It is capable of being demonstrated by experiment, that these changes are confined to the green surfaces of plants, and therefore to the leaves or leaf-like organs, to the young shoots, and to the stems of herbaceous plants, or of those in which 64 EXTERNAL CONDITIONS OF VITAL ACTIVITY. (as in the Cactus tribe) the leaves are wanting and the enlarged succu- lent stem supplies their place. When these surfaces cease to become green, the decomposing action also ceases ; carbon is no longer fixed and oxygen set free ; but, on the contrary, carbonic acid is exhaled : this is the case when the leaves change colour, previously to their fall, in the autumn. The compounds which are thus generated in the green surfaces, are conveyed to the remote parts of the fabric, by the circu- lation of the sap, and become the materials of their nutrition ; and thus the green cells of the leaves have exactly the same function, in minis- tering to the growth of the fabric of the largest tree, which the green cells of the humble Conferva perform in regard to themselves alone. 84. It has been already mentioned, that the decay which is always taking place in the softer vegetable structures, gives rise to a continual production of carbonic acid, even in the living plant ; this process, which must be regarded as a true Respiration, is effected, as in Animals, by the union of the carbon of the Plant with oxygen derived from the atmo- sphere ; and it is carried on, not by the green parts only, but also, per- haps chiefly, by the darker surfaces. Being antagonized during the day by the converse change just described, it can only be made sensible, by placing the plants for a time in an atmosphere in which no carbonic acid previously existed; and it will then be found that, even in full daylight, a certain amount of that gas is exhaled. The fact, however, becomes much more obvious at night, or in darkness ; since the decom- position of the surrounding carbonic acid by the green surfaces is then completely at a stand, and the full effect of the respiratory process is seen. Moreover, when a plant becomes unhealthy, from too long con- finement in a limited atmosphere, it begins to exhale more carbonic acid than it decomposes ; and the same is the case, as just now stated, in regard to leaves that have nearly reached the term of their lives. It does not admit of question, however, that, under ordinary circumstances, nearly the whole carbon of a slow-growing plant is derived from the car- bonic acid of the atmosphere ; either directly through the leaves, or indirectly by absorption through the roots ; and that there must be a vast surplus, therefore, of the carbonic acid decomposed, over that which is exhaled, during the whole life of the tree, — that surplus being in fact represented by the total amount of carbon contained in its tissues. 85. It is probable that the minute amount of Carbonic Acid at present contained in the atmosphere, is as much as could be beneficially supplied to Plants, under the average amount of light to which they are sub- jected, over the whole globe, and throughout the year. It has been clearly shown, that, under the influence of strong sunlight, an atmo- sphere containing as much as 7 or 8 per cent, of carbonic acid may be not merely tolerated by Plants, but may be positively beneficial to them, producing a great acceleration in their growth ; but as soon as the light is withdrawn, it acts upon them most injuriously, causing them speedily to become unhealthy, and , altogether destroying their vitality, if they are long subjected to it. Under more cloudless skies than ours, how- ever, the continual supply of a larger quantity of carbonic acid, than our atmosphere contains, is found to be quite compatible with healthy vegetation ; especially in the case of Cryptogamic plants, which (as will INFLUENCE OF LIGHT ON PLANTS. 65 be presently shown) require a less amount of this agent than those of a higher kind. Thus in the lake Solfatara in Italy, an unusual supply of carbonic acid is afforded by the constant escape of that gas from fissures in the bed of the lake, with a violence that gives to the water an appearance of ebullition ; and on its surface there are numerous floating islands, which consist almost entirely of Confervae and other simple cellular plants, growing most luxuriantly on this rich pabulum. And it has been remarked, that the vegetation around the springs in the valley of Gottingen, which abound in carbonic acid, is very rich and luxuriant ; appearing several weeks earlier in the spring, and continuing much later in the autumn, than at other spots in the same district. Many circumstances lead to the belief, that at former epochs in the Earth's history, the atmosphere was much more highly charged with carbonic acid than at present ; and that to this circumstance, in con- junction with a more intense and constant influence of light and heat, we are to attribute that extraordinary luxuriance of the vegetation of those periods, of which we have most abundant evidence, in the vast beds of disintegrated vegetable matter — Coal — that are of such value to Man, and in the remains which have been more perfectly preserved to us, and which indicate that not only the general forest mass, but many of the individual forms, attained a degree of development, which cannot now be paralleled even between the Tropics. 86. Various experiments have been recently made, with the view of determining more precisely the conditions under which Light acts, in producing the chemical changes that have been now discussed. These experiments for the most part agree in the very interesting result, that the amount of carbonic acid decomposed by plants subjected to the dif- ferently-coloured rays of the solar spectrum, but otherwise placed in similar circumstances, varies with the illuminating power of the rays, and not with their heating or their chemical power. The method adopted by Prof. Draper, which seems altogether the most satisfactory, consisted in exposing leaves of grass, in tubes filled with water which had been saturated with carbonic acid (after the expulsion of the previously dis- solved air by boiling), to the influence of the different rays of the solar spectrum, dispersed by a prism ; these were kept motionless upon the tubes for a sufl&cient length of time to produce an active decomposition of the gas in the tubes which were most favourably influenced by the solar beams ; and the relative quantities of the oxygen set free were then measured. It was then evident that the action had been almost entirely confined to two of the tubes, one of them being placed in the red and orange part of the spectrum, and the other in the yellow and green. The quantity of carbonic acid decomposed by the plant in the latter of these, was to that decomposed in the former, in the ratio of nine to five ; the quantity found in the tube that had been placed in the green and blue portion of the spectrum, would not amount, in the same proportion, to one; and in the other tubes, it was either absolutely nothing, or extremely minute. Hence it is obvious that the yellow ray, verging into orange on one side, and into green on the other, is the situation of the greatest exciting power possessed by light on this most important function of plants ; and as this coincides with the seat of the 6 66 EXTERNAL CONDITIONS OF VITAL ACTIVITY. greatest illuminating power of the spectrum, it can scarcely be doubted that light is the agent here concerned ; more especially as the place of greatest heat is in the red ray, and that of greatest chemical power is in the hlue^ both of which rays were found to be quite inert in the ex- periment just quoted. It must not be supposed from this experiment, however, that the yellow ray, and those immediately adjoining it, are the only sources of this power in the Solar spectrum ; since it proves no more than that, when the leaves were exposed to a highly carbonated atmosphere, they could only decompose it under the influence of these rays. It is certain, from other experiments, that plants will grow, in an ordinary atmosphere, under rays of different colours ; and it appears that the amount of carbon they severally fix, bears a constant proportion to the illuminating powers of the respective rays. 87. Although this fixation of carbon by the decomposition of carbonic acid, is the most universally dependent, of all the processes of the Vege- table economy, upon the influence of Light, yet it is not the only one, especially among the higher Plants, in which that agent becomes an important condition. Of the whole quantity of moisture imbibed by the roots, and contained in the ascending sap, a large proportion is exhaled again by the leaves ; a small part only being retained (together with the substances previously dissolved in the whole) to form part of the fabric. Now upon the rapidity of this Exhalation depends the rapidity of the absorption ; for the roots will not continue to take up more than a very limited amount of fluid, when it is not discharged again from the opposite extremity (so to speak) of the stem. The loss of fluid by the leaves appears to be a simple process of evaporation, depending in great part upon the temperature and dryness of the surrounding air ; this evaporation, however, does not take place solely, or even chiefly, from the external surface of the leaves, but from the walls of the passages which are channeled-out in their interior. Into this complex labyrinth, the outer air finds its way through orifices in the cuticle, which are termed stomata; and through these it comes forth again, charged with a large amount of vapour communicated to it by the extensive moist surface, with which it comes into contact in the interior of the leaf. Now the stomata are bounded by two or more cells, in such a manner that they can be opened or closed by changes in the form of these ; and this alte- ration is regulated by the amount of Light, to which the leaves are sub- jected. When the stomata are opened under the influence of light, the external air is freely admitted to the extended surface of moist tissue within the leaf, and a rapid loss of fluid is the result ; more especially if the temperature be high, and the atmosphere in a dry state. On the other hand, if the stomata be closed, the only loss of fluid that can take place from the internal tissue of the leaves, is through the cuticle ; the organization of which seems destined to enable it to resist evaporation, so that the exhalation is almost entirely checked. The influence of light upon this important function is easily shown by experiment. If a plant, which is actively transpiring and absorbing under a strong sun- shine, be carried into a dark room, both these operations are almost immediately checked, even though the surrounding temperature be higher than that to which the plant w^as previously exposed. INFLUENCE OF LIGHT ON PLANTS. 67 88. The effect of the complete and continued withdrawal of Light from a growing plant, is to produce an etiolation or blanching of its green surfaces : a loss of weight of the solid parts, owing to the conti- nued disengagement of carbon from its tissues, unbalanced by the fixa- tion of that element from the atmosphere ; a dropsical distension of the tissues, in consequence of the continued absorption of water, which Js not got rid of by exhalation ; a want of power to form its peculiar secre- tions, or even to generate new tissues, after the materials previously stored up have been exhausted ; in fine, a cessation of all the operations most necessary to the preservation of the vitality of the structure, of which cessation its death is the inevitable result. A partial withdrawal of the influence of light, however, is frequently used by the Cultivator, as a means of giving an esculent character to certain Plants, which would be otherwise altogether uneatable ; for in this manner their tis- sues are rendered more succulent and less "stringy," whilst their pecu- liar secretions are formed in diminished amount, and communicate an agreeable flavour instead of an unwholesome rankness of taste. 89. There is one period in the life of the Flowering Plant, however, in which the influence of Light is rather injurious than beneficial ; this is during the first part of the process of germination of seeds, which is decidedly retarded by its agency. This forms no exception, however, to the general rule ; since the decomposition of the carbonic acid of the atmosphere, and the fixation of carbon in the tissues, do not constitute a part of the operation. On the contrary, the embryo being nourished, like an animal, by organic compounds previously elaborated and stored up in the seed, the chemical changes which take place in them involve the opposite action, — the extrication of carbon, which is converted into carbonic acid by uniting with the oxygen of the atmosphere. It is obvious, then, why light should not only be useless, but even prejudicial, to this process ; since it tends to fix the carbon in the tissues, which ought to be thrown off. As soon, however, as the cotyledons or seed- leaves are unfolded, the influence of light upon them becomes as impor- tant, as it is on the ordinary leaves at a subsequent time ; their surfaces become green, and the fixation of carbon from the atmosphere com- mences. Up to that point, the young plant is diminishing day by day (like a plant that is undergoing etiolation), in the weight of its solid contents ; although its bulk has increased by the absorption of water. From the time, however, that its cotyledons begin to act upon the air, under the influence of light, the quantity of solid matter begins to in- crease ; and its augmentation subsequently takes place, at a rate pro- portional to the amount of green surface exposed, and the degree of light to which it is subjected* 90. The influence of Light upon the direction of the growing parts of Plants, upon the opening and closing of flowers, &c., is probably due to its share in the operations already detailed. Thus the green parts of Plants, or those which effect the decomposition of carbonic acid (such as the leaves and stems), have a tendency to grow towards the light ; whilst the roots, through whose dark surfaces carbonic acid is thrown out by respiration, have an equal tendency to avoid it. That the first direction of the stems and roots of plants is very much 68 EXTEKNAL CONDITIONS OF VITAL ACTIVITY. influenced in this manner, appears from the fact, that, by reflecting light upon germinating seeds, in such a manner as that it shall only fall upon them from below, the stems are caused to direct themselves downwards, whilst the roots grow upwards. — There can be no doubt, however, that Light has also a more direct influence on the develop- ment of particular organs in certain Vegetables. Thus when the gem- mules'^ of the Marchantia polymorpJia (one of the Hepaticce or Liver- worts), are in process of development, it has been shown by repeated experiments, that stomata are formed on the side exposed to the light, and that roots grow from the lower surface ; and that it is a matter of indiff"erence which side of the little disk is at first turned upwards, since each has the power of developing stomata, or roots, according to the influence it receives. After the tendency to the formation of these organs has once been given, however, by the sufiiciently prolonged in- fluence of light upon one side, and of darkness and moisture upon the other, any attempt to alter it is found to be vain ; for if the surfaces be then inverted, they are soon restored to their original aspect by the twisting growth of the plant. 91. The same amount of this agent is not requisite or desirable for all Plants ; and we find in the difi'erent habitats which are characteristic of diff'erent species, even amongst our native plants, that the amount congenial to each varies considerably. Generally speaking, the succu- lent thick-leaved Plants require the largest amount ; their stomata are few in number ; and the full influence of light is requisite to induce sufficient activity in the exhaling process ; accordingly we find them growing, for the most part, in exposed situations, where there is nothing to interfere with the full influence of the solar rays. On the other hand, plants with thinner and more delicate leaves, in which the ex- haling process is easily excited to an excessive amount, evidently find a congenial home in more sheltered situations ; and there are some which can only develope themselves in full luxuriance in the deep shades of a plantation or a forest. By a further adaptation of the same kind, some species of Plants are enabled to live and acquire their green colour under an amount of deprivation which would be fatal to most others ; thus in the mines of Freyburg, in which the quantity of light admitted must be almost infinitesimally small, Humboldt met with Flowering Plants of various species ; and Mustard and Cress have been raised in the dark abysses of the collieries of this country. 92. Generally speaking, however, the Cryptogamia would seem to be better adapted than Flowering Plants to carry on their vegetating processes under a low or very moderate amount of this agency. Thus Humboldt found a species of sea-weed near the Canaries, which pos- sessed a bright grass-green hue, although it had grown at a depth of 190 feet in the sea, where, according to computation, it could have received only 1-1 500th part of the solar rays that would have fallen upon it at the surface of the ocean. Many Ferns, Mosses, and Lichens * These gemmules are analogous to the buds of higher plants ; and they consist of little collections of cells, arranged in the form of flat disks ; which are at first attached by footstalks to the parent plant, but afterwards fall off, and are developed into new individuals. INFLUENCE OF LIGHT ON PLANTS. 69 seem as if they avoided the light, choosing the northern rather than the southern sides of hedges, buildings, &c., for their residence; so that the former often present a luxuriant growth of Cryptogamic vegetation, whilst the latter are comparatively bare. It must not he supposed, however, that they avoid light altogether, but only what is to them an excessive degree of it. The avoidance of light seems to be much stronger in the Fungi, which grow most luxuriantly in very dark situations ; and the reason of this is probably to be found in the fact that, like the germinating seed (§ 89), they form rather than decom- pose carbonic acid ; their food being supplied to them from the decay- ing substances on which they grow ; and the rapid changes in their tissues giving rise to a high amount of Respiration, — a change exactly the converse of that, on which, as we have seen, Light exerts such a remarkable power. 93. In regard to the agency of Light upon the functions of Animals, comparatively little is certainly known. It is evident that the influence it exerts on those chemical processes which constitute the first stage of Vegetable nutrition, can have scarcely any place in Animals ; because they do not perform any such acts of combination, but make use of the pro- ducts already prepared for them by Plants. Hence, we do not find that the surface of Animals undergoes that extension, for the purpose of being exposed to the solar rays, which is so characteristic a feature in the Vegetable fabric, and so important in its economy. Still there can be no doubt, that the degree of exposure to light has a great influence upon the colours of the Animal surface ; and here we seem to have a manifestation of Chemical agency, analogous to that which gives colour to the Vegetable surface. Thus it is a matter of familiar expe- rience, that the influence of light upon the skin of many persons, causes it to become spotted with brown freckles ; these freckles being aggre- gations of brown pigment cells, which either owed their development to the agency of light, or were enabled by that agency to perform a che- mical transformation which they could not otherwise efi"ect. In like manner, the swarthy hue, which many persons acquire in warm cli- mates, is due to a development of dark pigment-cells diffused through the epidermis (§ 229) ; and an increased development of the same kind gives rise to the blackness of the Negro-skin. There can be no doubt that the prolonged influence of light upon one generation after another, tends to give a permanent character to this variety of hue ; which will probably be more easily acquired, in proportion to the previously-exist- ing tendency to that change. Thus it is well known that a colony of Portuguese Jews, which settled at Tranquebar about three centuries ago, and which has kept itself distinct from the surrounding tribes, can- not now be distinguished as to colour from the native Hindoos. But it is probable that a similar colony of fair-skinned Saxons would not, in the same time, have acquired anything like the same depth of colour in their skins. 94. It can scarcely be questioned, that the brilliancy of colour which is characteristic of many tribes of animals in tropical climates, especially Birds and Insects, is in great part dependent, like the brightness of the foliage and fruit of the same countries, upon the brightness of the light 70 EXTERNAL CONDITIONS OF VITAL ACTIVITY. to ivhich their surfaces are exposed. When birds of warm climates, distinguished by the splendour of their plumage, are reared under an artificial temperature in our own country, it is uniformly observed that they are much longer in acquiring the hues characteristic of the adult ; and that these are never so bright as when they have been produced by the influence of the tropical sun. And it has been also remarked, that if certain Insects (the Cockroach for example), which naturally inhabit dark places, be reared in an entire seclusion from light, they grow up almost as colourless as Plants that are made to vegetate under similar circumstances. 95. There is reason to believe that Light exercises an important in- fluence on certain processes of development in Animals, as well as in Plants. Thus, the appearance of Animalcules in infusions of decaying organic matter is much retarded, if the vessel be altogether secluded from it. The rapidity with which the small Entomostracous Crustacea (Water-Fleas, &c.) of our pools, undergo their transformations, has been found to be much influenced by the amount of light to which they are exposed. And it has been ascertained that, if equal numbers of Silk- worm's eggs be preserved in a dark room, and exposed to common day- light, a much larger proportion of larvae are hatched from the latter than from the former. The most striking proof of the influence of Light on Animal development, however, is afi'orded by the experiments of Dr. Edwards. He has shown that if Tadpoles be nourished with proper food, and be exposed to the constantly renewed contact of water (so that their respiration may be freely carried on, whilst they remain in their fish-like condition), but be entirely deprived of light, their growth continues, but their metamorphosis into the condition of air-breathing animals is arrested, and they remain in the condition of gigantic tad- poles. It is interesting to remark, that the Proteus anguineus, an ani- mal which closely corresponds in its fully-developed form with the transition stage between the Tadpole and the Frog, finds a congenial abode in the dark lakes of the caverns of Styria and Carniola, and in the underground caverns that connect them ; thus showing its adapta- tion to a condition, which keeps down to the same standard the develop- ment of an animal, that is empowered under other circumstances to ad- vance beyond it. Numerous facts, collected from diff'erent sources, lead to the belief that the healthy development of the Human body, and the rapidity of its recovery from disease, are greatly influenced by the amount of light to which it has been exposed. It has been observed, on the one hand, that a remarkable freedom from deformity exists amongst nations who wear very little clothing ; whilst, on the other, it appears certain that an unusual tendency to deformity is to be found among persons brought up in cellars or mines, or in dark and narrow streets. Part of this difierence is doubtless owing to the relative purity of the atmosphere in the former case, and the want of ventilation in the latter ; but other instances might be quoted, in which a marked variation presented itself, under circumstances otherwise the same. Thus, it has been stated by Sir A. Wylie (who was long at the head of the medical stafi" in the Russian army), that the cases of disease on the dark side of an extensive barrack at St. Petersburgh, have been uniformly, for many INFLUENCE OF HEAT ON PLANTS. 71 years, in the proportion of three to one, to those on the side exposed to strong light. And in one of the London Hospitals, with a long range of frontage looking nearly due north and south, it has been observed that residence in the south wards is much more conducive to the welfare of the patients than in those on the north side of the building. 96. These facts being kept in view, it is easy to perceive that there must be differences among the various species of Animals, as among those of Plants, in regard to the degree of light which is congenial to them. Among the lowest tribes, in which no special organs of vision exist, there is evidently a susceptibility to the influence of light, which appears scarcely to deserve the name of sensibility, but which seems rather analogous to that which is manifested by Plants ; thus among those Polypes which are not fixed to particular spots, and amongst Ani- malcules, there are some species which seek the light, and others which shun it. And it appears from various observations upon the depths at which marine animals are found, especially from the extensive series of facts collected by Prof. E. Forbes,* that there are a series of zones, so to speak, to be met with in descending from the surface towards the bottom of the ocean, each of which is characterized by certain species of animals peculiar to itself, whilst other species have a range through two or more of the zones ; — the extent of the range of depth, in each species, bearing a close correspondence with the extent of its geographical distribution. Now there can be no doubt, that the restriction of par- ticular species to particular zones is due in great part to the degree of pressure of the surrounding medium ; but there can be as little doubt, that the variation in the degree of Light also exerts a most important influence, the solar rays in their passage through sea water being subject to a loss of one half for every seventeen feet. From the results of Prof. Forbes's researches, it appears that no species of Invertebrated animals habitually live at a greater depth than 300 fathoms ; and although Fishes have been captured at a depth of from 500 to 600 fathoms, it is probable that they had strayed from their usual abodes. 2. Of Heat, as a Condition of Vital Activity. 97. The most perfectly-organized body, supplied with all the other conditions requisite for its activity, must remain completely inert, if it do not receive a sufficient amount of Heat. The influence which this agent exerts upon living beings, is far more remarkable than its effects upon inorganic matter ; although the latter are usually more obvious. We are all familiar with its power of producing expansion, — with the liquefaction which is the consequence of its application to solids, — with the evaporation which it occasions in liquids, — and with the enormous repulsive force which it generates among the particles of vapours ; but it is not until we look deeper than the surface, that we perceive how immediate is the dependence of every action of Life upon this myste- rious agent. The temporary or permanent loss of vitality, in parts of the body subjected to extreme cold, is a " glaring instance" of the effect * Report on the Invertebrata of the ^gean Sea, in Transactions of British Associa- tion, 1843. 72 EXTERNAL CONDITIONS OF VITAL ACTIVITY. of its withdrawal. This change, however, is not immediate. Its first step is a mere depression of the vitality of the part, involving a partial stagnation of the capillary circulation, diminution of sensibility, and want of muscular power. But the continued action of cold on the sur- face, not compensated by a sufficient generation of heat within, causes the circulation of the part to be completely suspended, its small vessels contract so that they become almost emptied of blood, its sensibility and power of movement are destroyed, — in a word, its vital activity is com- pletely suspended. In such a state, a timely but cautious application of warmth may produce the gradual renewal of the circulation, and the restoration of the other properties of the part, which are dependent upon that function ; but any abrupt change would complete the mischief which the cold has begun ; and would altogether destroy, by the violence of the reaction, the vitality which was only suspended, causing the actual death of the part. Hence, when the extremities are frost-bitten, nothing can be more injurious than to bring them near a fire ; whilst no treat- ment has been found so safe and efiectual as the rubbing them with snow. 98. The influence of Heat upon Vital activity, is attested on a larger scale, by the striking contrast between the dreary barrenness of Polar regions, and the luxuriant richness of Tropical countries, where almost every spot to which moisture is supplied teems with Animal and Vege- table life. And the alternation of Winter and Summer in temperate climates, may be almost said to bring under our own view the opposite conditions of those two extreme cases. The efi"ect of the withdrawal of Heat is most obvious in the Vegetable kingdom ; since all its operations are dependent upon a certain supply of that agent ; and in no case are Plants possessed of the power of generating that supply within them- selves,— excepting in certain organs which do not impart it to the rest of the structure. When the temperature of the air falls to the freezing- point, therefore, we find all the operations of the Vegetable economy undergoing a complete suspension ; yet a very trifling rise will produce a renewal of them. It is not only in Evergreens, that the vital pro- cesses continue to be performed to a certain extent during the winter ; for there is abundant evidence that, even in the trunk and branches of trees unclothed with leaves, a circulation of sap takes place, whenever there is even a slight return of warmth. In this manner, the leaf-buds are gradually prepared during the milder days of winter, so as to be ready to start forth into full development, with the returning steady warmth of spring. 99. The influence of Heat upon Vegetation is easily made apparent by experiment ; in fact experimental illustrations of it, on a large scale, are daily in progress. For the Gardener, by artificial warmth, is not only enabled to rear with success the plants of tropical climates, whose constitution would not bear the chilling influence of our winter ; — but he can also, in some degree, invert the order of the seasons, and produce both blossom and fruit from the plants of our own country, when all around seems dead. This process of forcing, however, is unfavourable to the health and prolonged existence of the plants subjected to it; since the period of repose, which is natural to them, is interrupted ; and they are caused, as it were, to live too fast. The same result occurs, i INFLUENCE OP HEAT ON PLANTS. 78 when a plant or tree of temperate climates is transported to the tropics. Within a very short period after one crop of leaves has fallen off, a new one makes its appearance. This goes through all its changes of develop- ment and decay more rapidly than it would do in its native clime ; and in its turn falls oflf, and is speedily succeeded by another. Hence the fruit-trees of this country, transported to the East or West Indies, bear abundant crops of leaves, — three, perhaps, in one year, or five in two" years, — but little or no fruit ; and the period of their existence is much shortened. 100. As Plants are almost wholly dependent upon the temperature of the surrounding medium for the supply of Heat necessary for their gfowth, many regions must have been devoid of Vegetable life altogether ; if there were not a remarkable adaptation, in the wants of difierent species, to the various degrees of temperature of the habitations prepared for them. Thus we see the Cacti and Euphorbiae attaching themselves to the surface of the most arid rocks of tropical regions, luxuriating, as it would seem, in the full glare of the vertical sun, and laying up a store of moisture from the periodical rains, of which even a long-continued drought is not sufficient to deprive them. The Orchideous tribe, on the other hand, whose greatest development occurs in the same zone, find their congenial habitation in the depths of the tangled forests, where, with scarcely an inferior amount of heat, they have the advantage of a moister atmosphere, caused by the exhalations of the trees on which they cling. The majestic Tree-Fern, again, reaches its full development in insular situations ; where, with a moist atmosphere, it can secure a greater equability of temperature than is to be met with in the interior of the vast tropical continents. None of these races can develope themselves elsewhere to their full extent at least, unless their natural conditions of growth are imitated as far as possible ; and in proportion as this imitation can be made complete, in that proportion may the plant of the tropics be successfully reared in temperate regions. 101. There are some examples of the adaptation of particular forms of Vegetable life to extremes of temperature, which are interesting as showing the extent to which this adaptation may be carried. In hot springs near a river of Louisiana, of the temperature of from 122° to 145°, there have been seen to grow, not merely Confervas and herbaceous plants, but shrubs and trees ; and a hot-spring in the Manilla Islands, which raises the thermometer to 187°, has plants flourishing in it, and on its borders. A species of Qhara has been found growing and repro- ducing itself in one of the hot-springs of Iceland, which boiled an egg in four minutes ; various Confervse, &c., have been observed in the boiling- springs of Arabia and the Cape of Good Hope ; and at the island of New Amsterdam, there is a mud-spring, which, though hotter than boiling- water, gives birth to a species of Liverwort. On the other hand, there are some forms of Vegetation, which seem to luxuriate in degrees of cold, that are fatal to most others. Thus the Lichen, which serves as the winter food of the Rein-deer, spreads itself over the ground whilst thickly covered with snow ; and the beautiful little Frotoeoecus nivalis, or Red Snow, reddens extensive tracts in the Arctic regions, where the perpetual 74 EXTERNAL CONDITIONS OF VITAL ACTIVITY. frost of the surface scarcely yields to the influence of the solar rays at Midsummer. 102. It is, for the most part, among the Cryptogamic tribes, — the Ferns, Mosses, Liverworts, Fungi, and Lichens, — that the greatest power of growing under a low temperature exists ; and we accordingly find that the proportion of these to the Phanerogamia, or Flowering Plants, increases as we proceed from the Equator towards the Poles. It has been estimated by Humboldt, that, in Tropical regions, the num- ber of species of Cryptogamia is only about one-tenth that of the Flow- ering Plants ; in the part of the Temperate zone which lies between lat. 45° and 52°, the proportion rises to one-half ; and the relative amount gradually increases as we proceed towards the Poles, until, be- tween lat. 67° and 70°, the number of species of Cryptogamia equals that of the Phanerogamia. Among the Flowering Plants, moreover, the greatest endurance of cold is to be found in those, which approach most nearly to the Cryptogamia in the low degree of their development ; thus the Glumaceous group of Endogens, including the Grasses, Rushes, and Sedges, which forms about one-eleventh of the whole amount of Phanerogamic vegetation in the Tropics, constitutes one-fourth of it in the Temperate regions, and one-third in the Polar ; and the ratio of the Gymnospermic group of Exogens, which chiefly consists of the Pine and Fir tribe, increases in like manner. Still the influence of a high temperature is evident even upon the Cryptogamia and their allies ; for it is only under the influence of the light and warmth of tropical climes, that the Ferns, — the highest among the former, — can develope a woody stem, and assume the character of trees ; and it is only there that the tall Sugar-Canes, and the gigantic Bamboos, which are but Grasses on a large scale, can flourish. 103. It appears, then, that to every species of Vegetable there is a temperature which is most congenial, from its producing the most favourable influence on its general vital actions. There is a considera- ble diff*erence between the power of growing and of flourishing, at a given temperature. We may lower the heat of a plant to such a degree, as to allow it to continue to live ; yet its condition will be unhealthy. It absorbs food from the earth and air, but cannot assimilate and con- • vert it. Its tissue grows, but becomes distended with water, instead of being rendered firm by solid deposits. The usual secretions are not formed ; flavour, sweetness, and nutritive matter, are each diminished ; and the power of flowering and producing fruit is lost. We see a diffe- rence in the amount of heat required for the vegetating processes, even in the various species indigenous to our own climate ; thus the common Chickweed and Groundsel evidently grow readily at a temperature but little above the freezing point, whilst the Nettles, Mallows, and other weeds around them remain torpid. But the difi*erence is much more strongly marked in the vegetation of diff'erent climates; showing an evident adaptation of the tribes indigenous to each, to that range of temperature which they will there experience. Instead of being scantily supplied with such of the tropical plants as could support a stunted and precarious life in ungenial climates, the temperate regions are stocked with a multitude of vegetables which appear to be constructed expressly INFLUENCE OF HEAT ON PLANTS. 75 for them ; inasmucli as thece species can no more flourish at the Equa- tor, than the equatorial species can in these Temperate regions. And such new supplies, adapted to new conditions, recur perpetually as we advance towards the apparently frozen and untenantable regions in the neighbourhood of the Pole. Every zone has its peculiar vegetables ; and while we miss some, we find others making their appearance, as if to replace those which are absent. 104. Thus in the countries lying near the Equator, the vegetation consists in great part of dense forests of leafy evergreen trees. Palms, Bamboos, and Tree-Ferns, bound together by clustering Orchidse and strong creepers of various kinds. There are no verdant meadows, such as form the chief beauty of our temperate regions ; and the lower orders of Vegetation are extremely rare. It is only in this torrid zone that Dates, Coffee, Cocoa, Bread-fruit, Bananas, Cinnamon, Cloves, Nut- megs, Pepper, Myrrh, Indigo, Ebony, Logwood, Teak, Sandal-wood, and many others of the vegetable products most highly valued for their flavour, their odour, their colour, or their density, come to full perfec- tion. As we recede from the Equator, we find the leafy evergreens giving place to trees with deciduous leaves ; rich meadows appear, abounding with tender herbs ; the Orchidese no longer find in the atmo- sphere, and on the surface of the trees over which they cluster, a suffi- ciency of moisture for their support, and the parasitic species are re- placed by others which grow from fleshy roots implanted in the soil ; but aged trunks are now clothed with Mosses : decayed vegetables are covered with parasitical Fungi ; and the waters abound with Confervae. In the warmer parts of the temperate regions, the Apricot, Citron, Orange, Lemon, Peach, Fig, Vine, Olive, and Pomegranate, the Myrtle, Cedar, Cypress, and Dwarf Palm, find their congenial abode. These give place, as we pass northwards, to the Apple, the Plum, and the Cherry, the Chestnut, the Oak, the Elm, and the Beech. Going fur- ther still, we find that the fruit trees are unable to flourish, but the tim- ber-trees maintain their ground. Where these last fail, we meet with extensive forests of the various species of Firs ; the Dwarf Birches and Willows replace the larger species of the same kind ; and even near or within the Arctic circle we find large flowers of great beauty, — the Mezereon, the yellow and white Water-Lily, and the Globe-flower. Where none of these can flourish, where trees wholly disappear, and scarcely any flowering-plants are to be met with, an humbler Cryptoga- mic vegetation still raises its head, in proof that no part of the Globe is altogeth.er unfit for the residence of living beings, and that the empire of Flora has no limit. 105. But distance from the Equator is by no means the only element in the determination of the mean temperature of a particular spot, and of the vegetation which is congenial to it. Its height above the level of the sea is equally important ; for this produces a variation in the amount of heat derived from the Sun, at least as great as that occa- sioned by difference of latitude. Thus it is not alone on the summits of Hecla, Mount Blanc, and other mountains of Arctic or temperate regions, that we find a coating of perpetual snow ; we find a similar covering on the lofty summits of the Himalayan chain, which extends 76 EXTERNAL CONDITIONS OP VITAL ACTIVITY. to within a few degrees of the Tropic of Cancer ; and eVen on the higher peaks of that part of the ridge of the Andes, which lies immediately beneath the Equator. The height of the snow-line beneath the Equator is between 15,000 and 16,000 feet above the le(Vel of the sea ; on the south side of the Himalayan ridge it is about 15,500 feet,- but on the north side it rises to 18,500 feet ; and in the Swiss Alps it is about 8000 feet. Its position is very much affected, however, by local cir- cumstances, such as the neighbourhood of a large expanse of land or of sea; hence the small quantity of land in the Southern Hemisphere, renders its climate generally so much colder than that of the Northern, that in Sandwich Land (which is lat. 59° S., or in the same parallel as the north of Scotland) the whole country, from the summits of the mountains down to the very brink of the sea-cliffs, is covered many fathoms thick with everlasting snow ; and in the Island of Georgia (which is in lat. 54° S., or in the same parallel as Yorkshire), the limit of perpetual snow descends to the level of the ocean, the partial melting in summer only disclosing a few rocks, scantily covered with moss and tufts of grass. Yet the highest mountains of Scotland, which ascend to an elevation of nearly 5000 feet, and are four degrees more distant from the equator, do not attain the limit of perpetual snow ; this is reached, however, by mountains in Norway, at no greater elevation. 106. If, then, Temperature exert such an influence on Vegetation as has been stated, we ought to find on the sides of lofty mountains in tropical regions, the same progressive alterations in the characters of the Plants that cover them, as we encounter in journeying from the equatorial towards the polar regions. This is actually the case. The proportion of Cryptogamia to Flowering Plants, for example, is no more than one-fifteenth on the plains of the Equatorial region ; whilst it is as much as one-fifth on the mountains. In ascending the Peak of Tene- riffe, Humboldt remarked as many as five distinct zones, which were respectively marked by the products which characterize different climates. Thus at the base, the vegetation is altogether tropical; the Date-Palm, Plantain, Sugar-Cane, Banyan, the succulent Euphor- bia, the Dracaena, and other trees and plants of the torrid zone there flourish. A little higher grow the Olive, the Vine, and other fruit- trees of Southern Europe ; there Wheat flourishes ; and there the ground is covered with grassy herbage. Above this is the woody region, in which are found the Oak, Laurel, Arbutus, and other beau- ful hardy evergreens. Next above is the region of Pines ; characterized by a vast forest of trees resembling the Scottish Fir, intermixed with Juniper. This gives place to a tract remarkable for the abundance of Broom ; and at last the scenery is terminated by Scrofularia, Viola, a few Grasses, and Cryptogamic plants, which extend to the borders of the perpetual snow that caps the summit of the mountain. 107. The effects of Temperature on Vegetation are not only seen in its influence upon the Geographical distribution of Plants, that is, in the limitation of particular species to particular climates ; for they are shown, perhaps even more remarkably, in the variation in the size of individuals of the same species ; when that species possesses the power of adapting itself to widely different conditions, which is the case with INFLUENCE OF HEAT ON PLANTS. 77 some. Thus the Oerasus Virginiana grows in the Southern States of North America as a noble tree, attaining one hundred feet in height ; in the sandy plains of the Saskatchawan, it does not exceed twenty feet; and at its northern limit, the Great Slave Lake, in lat. 62°, it is re- duced to a shrub of five feet. Another curious effect of heat is shown in its influence on the sexes of certain Monoecious flowers ; thus Mr. Knight mentions that Cucumber and Melon plants will produce none but male or staminiferous flowers, if their vegetation be accelerated by heat ; and all female or pistilliferous, if its progress be retarded by cold. 108. The injurious influence of excessive Heat can be, to a certain extent, resisted by Plants, through the cooling process kept up by the continual evaporation of moisture from their surface. But the power of maintaining this cooling process entirely depends upon the supply of fluid, with which the plant is furnished. If the supply be adequate to the demand, the efiect of heat will be to stimulate all the vital opera- tions of the plant, and to cause them to be performed with increased energy ; though, as we have already seen, this energy may be such as to occasion a premature exhaustion in its powers, by the excessive luxu- riance which it occasions. But if the supply of water be deficient, the plant is burnt up by the continuance of heat in a dry atmosphere ; and it either withers and dies, or its tissues become dense and contracted, without losing their vitality. Thus it has been remarked, that shrubs LHowing among the sandy deserts of the East, have as stunted an ap- {)earance as those attempting to vegetate in the Arctic regions ; their leaves being converted into prickles, and their leaf-buds prolonged into :horns instead of branches. — The influence of excessive heat in destroy- ing life, can sometimes be traced through the direct physical changes which it occasions in the vegetable tissues. Thus it has been ascer- tained that grains of corn will vegetate, after exposure to water or vapour possessing a considerable degree of heat ; provided that heat do not amount to 144° in the case of water, and 167° in that of vapour. At these temperatures, the structure of the seed undergoes a disorga- nizing change, by the rupture of the vesicles of starch which form a large part of it ; and the loss of its power of germinating is therefore readily accounted for. The highest temperature which the soil usually possesses in tropical climates, is about 126°, though Humboldt has once observed the thermometer rise to 140°. Seeds imbedded in such a soil, tlierefore, may not lose their vitality, although they will not germinate ill such temperatures. The temperature most favourable to germination probably varies in diff"erent species, and is one of the conditions that produces their adaptation to difi'erent climates. Thus it appears that Covn will not germinate in water at a higher temperature than 95°, wliilst Maize will germinate in water at 113° ; and, as is well known, Maize will flourish in countries in which Corn cannot be grown. 109. We must not confound the power which Plants possess of vege- Hating, or exhibiting vital activity, under widely-difierent degrees of temperature, with the power of retaining their vitality in a dormant .condition, which many of them possess in a very remarkable degree. I When the external temperature is much below the freezing-point, it is impossible that any vegetating processes can go on; since the Plant 78 EXTERNAL CONDITIONS OF VITAL ACTIVITY. does not possess the power of generating heat within itself. Now such a complete cessation of activity is quite compatible, in many instances, with the preservation of the organized structure in a condition perfectly unchanged, and, in consequence, with the continuance of its peculiar properties ; so that these properties may be again called into operation, when the temperature shall have risen. But in other cases, the plant may be hilled by the intensity of the cold ; that is, the return of warmth will not excite it to activity. We have occasion to notice, in every severe winter, the difference in this respect amongst the plants which are cultivated in our own climate ; some of them being killed by a hard frost, the effects of which are resisted by others, even though their situa- tion be more exposed. In general it will be found, that the cold acts most powerfully (as might be expected) upon plants which are not indi- genous to our country, but which have been introduced and naturalized from some warmer regions. But it is worthy of note, amongst other pe- culiarities in the relation of Heat and Vegetation, that many plants are readily killed by a low temperature, which yet flourish well under a very moderate amount of warmth; so that they will grow in situations where the mean temperature of the year is low and the summers cool, provided the winters are not severe ; whilst they cannot be preserved without special protection, in situations where the winters are colder, even though the summers should be much hotter, and the mean temperature of the whole year should be considerably higher. Thus there are shrubs grow- ing in the Botanic Garden of Edinburgh, which cannot be safely left in the open air in the neighbourhood of London, and which would be most certainly killed by the winter-cold of Central France. 110. It does not admit of doubt, that the destructive influence of a very low temperature upon the Vitality of Plants, is immediately exerted through its chemical and physical effects upon the tissues and their con- tents. Thus it will produce congelation of their fluids ; and the expan- sion which takes place in freezing will injure the walls of the containing cells, — distending, lacerating, or even bursting them. The same cause will probably occasion the expulsion of air from some parts which ought to contain it ; and the introduction of it into other parts which ought to be filled with fluid. And a separation will take place, in the act of freezing, between the constituent parts of the vegetable juices ; which will render them unfit for discharging their functions, when returning warmth would otherwise call them into activity. Hence we are enabled in some degree to account for the differences in the power of resisting cold, which the various species of Plants, and even the various parts of the same individual, are found to possess. For, other things being equal, the power of each plant, and of each part of a plant, to resist a low temperature, will be in the inverse ratio of the quantity of water contained in the tissue ; thus a succulent herbaceous plant suffers more than one with a hard woody stem and dense secretions ; and young shoots are destroyed by a degree of cold, which does not affect old shoots and branches of the same shrub or tree. Again, the viscidity of the fluids of some plants is an obstacle to their congelation, and therefore enables them to resist cold ; thus it is, that the resinous Pines are, of all trees, those which can endure the lowest temperature. The dimen- 4 INFLUENCE OF HEAT ON ANIMALS. 79 sions of the cells, too, of which the tissue is composed, appear to have an influence ; the liability to freeze being diminished by a very minute subdivision of the fluids. And when the roots are implanted deep in the soil, where the temperature does not fall by many degrees so low as that of the surface, the fluidity of the sap may be maintained, in spite of an extremely cold state of the atmosphere. 111. It is in Cryptogamic plants, that the greatest power of sustain- ing Cold exists; as might be inferred from what has been already stated in regard to their geographical distribution. The little Fungus {Torula Cerevisice) which is one of the principal constituents of Yeast, does not lose its vitality by exposure to a temperature of 76° below zero ; though it requires a somewhat elevated temperature for its active growth. It Avould appear that Seeds are enabled to sustain a degree of cold, without the loss of their vitality, which would be fatal to growing plants of the same species ; thus grains of corn, of various kinds, will germinate after being exposed for a quarter of an hour to a temperature equal to that of frozen mercury. It is not difiicult to account for this, when the close- ness of their texture, and the small quantity of fluid which it includes, are kept in view. The act of Germination, however, will only take place under a rather elevated temperature ; and we find in the Chemical changes which it involves, a provision for maintaining this, when the process has once commenced. 112. The influence of Heat upon the vital activity of Animals, is quite as strongly marked as we have seen it to be in the case of Plants ; but the mode in which it is exerted is in many instances very difi'erent. In those animals which are endowed with great energy of muscular move- ment, and in which, for the maintenance of that energy, the nutritive functions are kept in constant activity, we find that a provision exists for the development of heat from within, so as to keep the temperature of the body at a certain uniform standard, whatever may be the climate in which they live. Their energy and activity are, in fact, so dependent upon the steady maintenance of a high temperature in their bodies, that, if this be not kept up nearly to its regular standard, a diminution or even a complete cessation of vital action takes place, and even a total loss of vitality may result. In these warm-blooded animals, as they are termed, we do not so evidently trace the eff'ects of Heat, because they are constantly being exerted, and because external changes have but little influence upon them, unless these changes are of an extreme kind. But if those internal operations, on which the maintenance of the tem- perature is dependent, are from any cause retarded or suspended, the efi'ect is immediately visible, in the depressed activity of the whole system. In the class of Birds, whose muscular energy, and whose ge- neral functional activity, are greater and more constant than those of any^other animals, the temperature is pretty steadily maintained at from 108° to 112°; and we shall presently see, that a depression of the heat of the body to about 80° is fatal. Among Mammalia, the temperature is usually maintained at from 98° to 102° ; and it seems that in them too a. depression of about thirty degrees is ordinarily fatal. 113. In the difi'erent tribes of Birds and Mammals, we find a very diversified power of generating heat ; and on this depends their adapta- 80 EXTERNAL CONDITIONS OF VITAL ACTIVITY. tion to various climates. Where the usual temperature of the atmo- sphere is but little below the normal standard of the body, a small amount of the internal calorifying power is required ; and accordingly we find that animals which naturally inhabit the torrid zone, cannot be kept alive elsewhere, except, like the Plants of the same regions, by ex- ternal heat. On the other hand, the animals of the colder temperate and frigid climes are endowed with a much greater internal calorifying power ; and their covering is adapted to keep in the heat which they generate. Such animals (the Polar Bear for example) cannot be kept in health, in the summer of our own country, unless means are taken for their refrigeration. The constitution of Man seems to acquire, by habitation to a particular set of conditions through successive genera- tions, an adaptation to differences of climate, of which that of few other animals is susceptible ; and thus we find different races of human beings inhabiting countries, which are subject to the extremes of heat and cold. The Hindoo or the Negro, suddenly transported to Labrador or Siberia during the depth of winter, would probably sink in the course of a few days, from want of power to generate within his body a sufficient amount of heat, to resist the depressing influence of the external cold ; whilst on the other hand, the Esquimaux, suddenly conveyed to the hottest parts of India or Africa, would speedily become the subject of disease, which would probably terminate his life in a short time. It is in the inhabitant of temperate climates, who is naturally exposed during the seasonal changes of his year, to a wide range of external temperature, that we find the greatest power of sustaining the extremes of either cold or heat ; and yet, even in such, the continued exposure to either extreme, during a long series of years, will so much influence the heat-producing power, that the constitution does not adapt itself readily to a change of conditions. 114. We see, then, that the variations observable between diff"erent races in this respect, are only exaggerations (so to speak) of the alter- nations which an individual may undergo in the course of a few years ; and it is easy to understand how such an adaptation may take place to an increased extent in successive generations ; — this being the regular law, not merely in regard to Man, but in regard to other animals placed under new conditions, to which they have a certain, but limited, power of adapting themselves. Thus we find that a European, who has lived for several years in the East or West Indies, suffers considerably from the cold, when he first returns to winter in his native country ; his con- stitution having, for a time, lost some of its power of generating heat. After a few years' residence, however, this power is commonly recovered to its original extent, unless the age of the individual be too far ad- vanced ; but his ^hildi^en, if they have been not only born, but brought up, in the hotter climate, experience much greater difficulty in adapting themselves to the colder one. 115. The conditions on which the power of maintaining the heat of the body, in despite of external cold, is dependent, will become the sub- ject of inquiry hereafter (chap. x). It is sufiicient here to state, that this power is the- result of numerous Chemical changes going on within the body ; and especially of a process analogous to combustion, in which I INFLUENCE OF HEAT ON ANIMALS. 81 carbon and hydrogen, taken in as food, are made to unite with oxygen derived from the atmosphere. It is dependent, therefore, as to its amount, upon the due supply of the combustible material on the one hand, and of atmospheric air on the other. If the former be not fur- nished either by the food, or by the fatty matter of the body (which acts as a kind of reserved store laid up against the time of need), the_ heat cannot be maintained ; and it is in part for want of power to digest and assimilate a sufficient amount of this kind of aliment, that animals of warm climates cannot maintain their temperature in colder regions. On the other hand, if the supply of oxygen be deficient, as it is when the respiration is impeded by diseased conditions of various kinds, there is a similar depression of temperature. 116. Now if, from either of these causes, the temperature of the body of a Bird or Mammal (except in the case of the hyhernating species of the latter, to be presently noticed) be lowered to about 30° below its usual standard, not only is there a cessation of vital activity, but a total loss of vital properties ; in other words, the death of the animal is a necessary result. This occurrence is preceded by a gradually-increasing torpidity ; which shows the depressing influence of the cooling process upon the functions in general. The temperature of the superficial parts of the body is, of course, first affected ; the circulation is at first retarded, causing lividity of the skin ; but, as the temperature becomes lower, the blood is almost entirely expelled from the surface by the contraction of the vessels, and paleness succeeds. At the same time, there is a gradu- ally-increasing torpor of the nervous and muscular systems, which first manifests itself in an indisposition to exertion of any kind, and then in an almost irresistible tendency to sleep. At the same time, the respi- ratory movements become slower, from the want of the stimulus that should be given by the warm current of blood to the Medulla Oblongata, which is the centre of those movements ; and the loss of heat goes on, therefore, with increased rapidity, until the temperature of the whole body is so depressed, that its vitality is altogether destroyed. IIT. But when there is a deficiency of the proper animal heat, the vital activity of the system may be maintained by caloric applied by external sources. This fact is of high scientific value, as giving the most complete demonstration of the immediate dependence of the vital func- tions of warm-blooded animals upon a sustained temperature ; and its practical importance can scarcely be overrated. It rests chiefly upon the recent experiments of Chossat, who had in view to determine the circumstances attending death by Inanition, or starvation. He found that, when Pigeons were entirely deprived of food and water, their average temperature underwent a tolerably regular diminution from day to day ; so that, after several days (the exact number varying with their previous condition), it was about 4i° lower than at first. Up to this time, it seems that the store of fat laid up in the body supplies the requisite material for the combustive process ; so that no very injurious depression of temperature occurs. But, as soon as this is exhausted, the temperature falls rapidly, from hour to hour ; and as soon as the I total depression has reached 29^° or 30°, death supervenes. Yet it was found by M. Chossat, that when animals thus reduced by starvation, 82 EXTERNAL CONDITIONS OF VITAL ACTIVITY. whose death seemed impending (death actually taking place in many instances, whilst the preliminary processes of weighing, the application of the thermometer, &c., were being performed), were subjected to artifi- cial heat, they were almost uniformly restored, from a state of insensi- bility and want of muscular power, to a condition of comparative activity. Their temperature rose, their muscular power returned, they took food when it was presented to them, and their secretions were renewed ; and, if this artificial assistance was sufficiently prolonged, and they were supplied with food, they recovered. If the heat was withdrawn, however, before the time when the digested food was ready, in sufficient amount, to supply the combustive process, they still sank for want of it. 118. Various important practical hints may be derived from the con- sideration of these facts. There can be no doubt that, in many diseases of exhaustion, the want of power to sustain the requisite temperature, is the immediate cause of death ; the whole combustible material of the body having been exhausted, and the digestive apparatus not being able to supply what is required. Now where this is the case, there is no doubt that life may be prolonged, and that recovery may be favoured, by the judicious sustentation of the temperature of the body. This may be effected either by internal or by external means. Of the internal, the most efficient is undoubtedly the administration of Alcoholic fluids; which, for reasons hereafter to be given {§ 495), will be absorbed into the circulating system, when no other alimentary substance can be taken in ; and which, moreover, exert a favourable influence by their specific stimulating eff'ect upon the nervous system. It is a matter of familiar experience, that, in such conditions of the body, the quantity of alcohol which may be administered with positive and evident benefit, is such as would in ordinary circumstances be productive of the most injurious results ; and this is fully accounted for by the reflection, that it is burnt off as fast as it is taken in. But a most important adjunct in all such cases, — and in many instances a substitute for alcohol when the latter would be inadmissible, — will be found in the application of external heat; and especially in the subjection of the whole surface to its influence, by means of the hot-air bath. This is a valuable portion of the treatment, in the recovering of persons who have been reduced to insensibility by suffocation of any kind ; and especially in cases of drowning, since the heat of the body is rapidly withdrawn by the conducting power of the water. Indeed it may be stated as a general rule, that, where the tem- perature of the body is lowered from any cause, external heat may be advantageously applied ; and much evidence has lately been produced to show, that the reparative processes by which extensive wounds are healed, go on more favourably under the contact of warm dry air, than with any other application. 119. On the other hand, where the object is to keep down a tendency to a too violent action, the local application of moderate cold is found to be of the greatest value ; all surgeons of eminence being now agreed upon the efficacy of water-dressing in restraining the inflammatory pro- cess, especially in cases of wounds of the joints, in which this action is most to be apprehended. The general application of cold to the surface, by means of continued exposure to cool air, or by a short immersion in INFLUENCE OF HEAT ON ANIMALS. 83 cold water, is frequently in the highest degree beneficial, by imparting tone to the system, i. e., by producing a firmer condition in the solids which were previously relaxed^ and more especially by calling into action the tonicity of the walls of the blood-vessels, which imparts to them an increased resistance, and thus favours the regular and vigorous circu- lation of blood, upon principles which will be hereafter stated (§ 609). But so far from producing any permanent depression in the temperature of the body, this measure has a tendency to elevate it, by the increased vigour it produces in the circulation ; hence the glow which is experi- enced after the use of the cold bath. If this efi'ect be not produced, and a chilling of the body, instead of an invigorating warmth, be the result of the use of cold, it is evident that this cannot be beneficial. The inju- rious results of the too-prolonged application of even a moderate degree of cold, are seen in the depression of temperature, without a correspond- ing reaction, which is the consequence of an immersion in water of 50^^ or bt)° prolonged for several hours ; and still more in that chilling of the whole surface, frequently productive of the most serious consequences, which arises from the evaporation of fluid from garments that have been moistened, either by perspiration from within, or by the fall of rain or dew upon their exterior. There is no doubt that the obstruction to the continuance of the perspiration, presented by a covering already satu- rated with moisture, is one cause of the injurious results that so com- monly follow such an occurrence ; but there is as little doubt that the chilling influence of the external evaporation has a large share in pro- ducing them. For experience shows that, if the evaporation be pre- vented by an impenetrable covering, the contact of a garment thoroughly saturated with moisture is not productive of the same injurious conse- quences. 120. The practical importance of the due comprehension of the prin- ciples, upon which Heat and Cold should be employed, in the treatment of disease and the preservation of health, has required this digression. We now proceed to consider the influence of temperature upon a certain group of warm-blooded animals ; which ofiers a remarkable peculiarity in this respect, — their power of generating heat being for a time greatly diminished or almost completely suspended ; the temperature of their bodies following that of the air around, so that it may be brought down nearly to the freezing-point ; their general vital actions being carried on with such feebleness as to be scarcely perceptible ; and yet the vital properties of the tissues being retained, so that, when the temperature of the body is again raised, the usual activity returns. This state, which is called hybernation, appears to be as natural to certain animals, as sleep is to all, and it corresponds with sleep in its tendency to periodical return. 121. No account can be given of the causes to which it is due ; but the condition of the animals presenting it offers several points of much interest. There are some, as the Lagomys, in which it appears to differ but little from deep ordinary sleep ; they retire into situations which favour the retention of their warmth ; and they occasionally wake up, and apply themselves to some of the store of food, which they have pro- vided in the autumn. In other cases, a great accumulation of fat takes 84 EXTERNAL CONDITIONS OF VITAL ACTIVITY. place within the body in autumn, favoured by the oily nature of the seeds, nuts, &c., on which the animals then feed ; and this serves the purpose of maintaining the temperature for a sufficient length of time, not indeed to the usual standard, but to one not far below it. The state of torpor in these animals is more profound than that of deep sleep, but it is not such as to prevent them from being easily aroused ; and their respiratory movements, though diminished in frequency, are still performed without interruption. But in the Marmot^ and in animals which, like it, hybernate completely, the temperature of the body (owing to the want of internal power to generate heat) and the general vital activity, are proportionably depressed ; the respiratory movements fall from 500 to 14 per hour, and are performed without any considerable enlargement of chest ; the pulse sinks from 150 to 15 beats per minute ; the state of torpidity is so profound, that the animal is with difficulty aroused from it ; and the heat of the body is almost entirely dependent upon the temperature of the surrounding air, not being usually more than a degree or two above it. When the thermometer in the air is somewhat below the freezing-point, that placed within the body falls to about 35°; and at this point it may remain for some time, without any apparent injury to the animal, which revives when subjected to a higher temperature. When, however, the body is exposed to a more intense degree of cold, the animal functions undergo a temporary renewal ; for the cold seems to act like any other stimulus in arousing them. The respiratory movements and the circulation increase in activity, so as to generate an increased amount of heat ; but this amount is insufficient to keep up the temperature of the body, which is at last depressed to a degree inconsistent with the maintenance of life ; and not only the sus- pension of activity, but the total loss of vital properties, is the result. 122. Now the condition of a hybernating Mammal closely resembles that of a cold-blooded animal, in regard to the dependence of its bodily temperature upon external conditions. There is this important diffe- rence, however ; — that the reduction of the temperature of the former to 60° or 50° is incontpatible with a state of activity, which is only exhi- bited when the temperature rises to nearly the usual Mammalian standard ; — whilst a permanently low or moderate temperature is natural to the bodies of most cold-blooded animals, whose functions could not be well carried on under a higher temperature. Thus all the muscles of a Frog are thrown into a state of permanent and rigid contraction, by the immersion of its body in water no warmer than the blood which naturally bathes those of the Bird ; and we find, accordingly, that cold-blooded animals which cannot sustain a high temperature, are provided with a frigorifying rather than with a calorifying apparatus. Although we are accustomed to rank all animals, save Birds and Mammals, under the general term cold-hlooded, yet there exist among them considerable diver- sities as to the power of generating heat within themselves, and of thus rendering themselves independent of external variations. Thus among Reptiles, it appears that there are some which can sustain a temperature several degrees above that of the atmosphere, especially when the latter is sinking ; and among Fishes, it is certain that there are species, — the Thunny and Bonito for example, — which are almost entitled to the INFLUENCE OF HEAT ON ANIMALS. 85 name of warm-blooded animals, their temperature being kept up to nearly 100°, when that of the sea is about 80°. It is uncertain, however, to what extent it would be depressed, by a lowering of that of the sur- rounding medium. The greatest power of developing heat in cold- blooded animals appears to exist, when their bodies are reduced nearly to the freezing-point ; and when that of the surrounding air or Avater is much below it. Thus Frogs have been found alive in the midst of ice whose temperature was as low as 9°, the heat of their own bodies being 33° ; and it has been observed that even Animalcules contained in water that is being frozen, are not at once destroyed, but that each lives for a time in a small uncongealed space, where the fluid seems to be kept from solidifying, by the caloric liberated from the Animalcule. 123. The peculiar condition of the class of Insects, in regard to its heat-producing power, exhibits in a very striking manner the connexion between an elevated temperature and vital activity. In the Larva state of Insects, the temperature of the animal follows closely that of the surrounding air, as in the cold-blooded classes generally ; but it is usually from | to 4"^ above it. In the Pupa condition, which is one of absolute rest in most insects that undergo a complete metamorphosis, the temperature scarcely rises above that of the surrounding medium ; except at nearly the close of the period, when it is about to burst its envelopes and come forth as the perfect Insect. The temperature which different Insects possess in their Imago state, varies in part ac- cording to the species, and in part with the condition of the individual in regard to rest or activity ; but the same principle is evidently ope- rating in both cases, since the variation existing amongst different species, in regard to their heat-producing power, is closely connected with the amount of activity natural to them. The highest amount is to be found in the industrious Hive-Bee and its allies, and in the elegant and sportive Butterflies, which are almost constantly on the wing in search of food; next to these come the Beetles. of active flight ; and lastly tHose which seldom or never raise themselves upon the wing, but pursue their labours on the ground. The temperature of individual Bees has been found to be about 4° above that of the atmosphere, when they are in a state of repose ; but it rises to 10° or 15°, when they are excited to activity. When they are aggregated together in clusters, however, the temperature which they possess is often as milch as 40° above that of the atmosphere. When reduced to torpidity by cold, they still generate heat enough to keep them from being frozen, unless the cold be very severe ; and they may be aroused by moderate excitement to a state of activity, in which the temperature rises to a very conside- rable elevation. Now although the increased production of heat is in these cases, as in hybernating Mammals similarly aroused, the conse- quence of the increased activity, there can be no question that it is a condition necessary to the continuance of that activity ; since we find that, if the temperature of the body be again reduced by external cold, the activity cannot be long maintained. 124. Whilst the foregoing facts exhibit the connexion between an elevated temperature, and the most active condition of the muscular and nervous systems, in cold-blooded animals, there is abundant evi- 86 EXTERNAL CONDITIONS OF VITAL ACTIVITY. dence of the same kind in regard to the influence of Heat upon the pro- cesses of nutrition and development. Thus the time of emersion of Insect-larvae from their eggs, — or in other words, the rate at ■which the previous formative processes go on, is entirely dependent upon the tem- perature. In the case of the Bird, we find that, if the temperature be not sufiicient to develope the egg, chemical changes soon take place, which involve the loss of its vitality ; or if the temperature be reduced, so low as to prevent the occurrence of those changes, the loss of heat is in itself destructive of life. But this is not the case in regard to the eggs of cold-blooded animals in general ; for, like the beings they are destined to produce, they may be reduced to a state of complete inac- tion by a depression of the external temperature ; whilst a slight eleva- tion of this renews their vital operations, at a rate corresponding to the warmth supplied. Hence the production of larvae from the eggs of Insects may be accelerated or retarded at pleasure ; and this is, in fact, practised in the rearing of Silk-worms, in order to adapt the time of their emersion from the egg to the supply of food which is ready for them. The same may be said in regard to the eggs of other cold- blooded animals ; those, for example, of the minute Entomostracous Crustacea (Water-Fleas, &c.), which people our ditches and ponds. In many of these, the race is continued solely by the eggs, which remain dormant through the winter ; all the parents being destroyed by the cold. The common Daphnia pulex produces two kinds of eggs ; from one, the young are very speedily hatched ; but the others, which are produced in the autumn, and enveloped in a peculiar covering, do not give birth to the contained young until the succeeding spring. They may be at any time hatched, however, by artificial warmth. 125. We sometimes find special provisions for imparting to the eggs a temperature beyond that which is natural to the bodies of the parents ; thus it has been shown that in Serpents, the temperature of the poste- rior part of the body rises considerably, when the eggs are lying in the oviduct, preparatory to being discharged, — evidencing a special heat- producing power in the surrounding parts at this period, which is obvi- ously for the purpose of aiding the maturity of the eggs. The Viper, whose eggs are frequently hatched in the maternal oviduct, so that the young are brought forth alive,^ is occasionally seen basking in the sun, in such a position as to receive its strongest heat on the parts that cover the oviduct. Certain Birds have recourse to substitutes for the usual method of incubation. The Tallegalla of New Holland is directed by its remarkable instinct, not to sit upon its eggs, but to bring them to maturity by depositing them in a sort of hot-bed, which it constructs of decaying vegetable matter. The Ostrich is believed to sit upon its eggs, when the temperature falls below a certain standard, but to leave them to the influence of the solar heat when this is sufficient to bring them to maturity; and this statement derives confirmation from a similar fact observed in a Fly-catcher, which built in a hot-house during several successive years, — the bird quitting its eggs when the temperature was high, and resuming its place when it fell. In all these cases, as in many more which might be enumerated, we observe the influence of an elevated temperature upon the processes of development ; and the pro- INFLUENCE OF HEAT ON ANIMALS. 87 visions made by Nature, in the physical or mental constitution of ani- mals, for affording that influence. The development of heat around the oviduct of the Serpent is a process over which the individual has no control, being entirely dependent upon certain organic changes ; whilst the imparting of warmth to its eggs by the Bird, either from its own body or through artificial means, is committed to the guidance of its Instinct, — which same instinct leads it to suspend the process whernt~ is not necessary. 126. Phenomena of an equally interesting and instructive character may be observed in the history of the Pupa-state of Insects ; which, in those that undergo a complete metamorphosis, may be almost charac- terized as a re-entrance into the egg. In fact we shall obtain the most correct idea of the nature of that metamorphosis, by considering the Larva as an embryo, which comes forth from the egg in a very early and undeveloped condition, for the sake of obtaining materials for its continued development, which the egg does not supply in sufficient amount. When these have been digested and stored-up in the body, the animal becomes completely inactive, so far as regards its external manifestations of life ; and it forms some kind of envelope for its protection, which may not be unaptly compared to the shell or horny covering of the egg. Within this are gradually developed the wings, legs, and other parts which are peculiar to the perfect Insect ; whilst even those organs, which it pos- sesses in common with the Larva, are for the most part completely altered in character. When this process of development is completed, the Insect emerges from its Pupa-case, just as the Bird comes forth from the egg; then only does its Insect life begin, its previous condition having been that of a Worm ; and the alteration of its character is just as evident in its instinctive propensities, as it is in its locomotive and sensorial powers. 127. Now this process of development is remarkably influenced by external temperature ; being accelerated by genial warmth, and retarded by cold. There are many Larvae, which naturally pass into the Pupa state during the autumn, remain in it during the entire winter, and emerge as perfect Insects with the return of spring. It was found by Reaumur, that Pupae, which would not naturally have been disclosed until May, might be caused to undergo their metamorphosis during the depth of winter, by the influence of artificial heat ; whilst, on the other hand, their change might be delayed a whole year beyond its usual time, by the prolonged influence of a cold atmosphere. In order to hasten the development of the pupae of the Social Bees, a very curious provision is made. There is a certain set, to which the name of Nurse- bees has been given, whose duty it is to cluster over the cells in which the Nymphs or Pupae are lying, and to communicate the heat to them, which is developed by the energetic movements of their own bodies, and especially by respiratory actions of extreme rapidity. The nurse-bees begin to crowd upon the cells of the nymphs, about ten or twelve hours before these last come forth as perfect Bees. The incubation (for so it may be called) is very assiduously persevered in during this period by the Nurse-bees ; when one quits its cell, another takes its place ; and the rapidity of the respiratory movements increases, until they rise to k 88 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 130 or 140 per minute, so as to generate the greatest amount of heat just before the young bees are liberated from the combs. In one in- stance, the thermometer introduced among seven nursing-bees stood at 92 J°; the temperature of the external air being 70°. We observe in this curious propensity a manifest provision for accelerating the deve- lopment of the perfect Insect, which requires (as already pointed out) a higher temperature than the larva, in virtue of its greater activity. The Nurse-bees do not station themselves over the cells which are occupied by the larvae ; nor do they incubate the nymph-cells with any degree of constancy and regularity, until the process of development is approach- ing its highest point. 128. The influence of variations in the Heat of the body upon its vital activity, is further manifested by the very remarkable experiments of Dr. Edwards ; who has shown that Cold-blooded animals live much faster (so to speak) at high temperatures, than at low; so that they die much sooner, when deprived of other vital stimuli. Thus when Frogs were confined in a limited quantity of water, and were not permitted to come to the surface to breathe, it was found that the duration of their lives was inversely proportional to the degree of heat of the fluid. Thus when it was cooled down to the freezing-point, the frogs immersed in it lived during from 367 to 498 minutes. At the temperature of 50°, the duration of their lives was from 350 to 375 minutes ; at 72°, it was from 90 to 35 minutes ; at 90°, from 12 to 32 minutes ; and at 108° death was almost instantaneous. The prolongation of life at the lower tempe- ratures was not due to torpidity, for the animals perform the functions of voluntary motion, and enjoy the use of their senses ; but it is occasioned by their diminished activity, which occasions a less demand for air. On the other hand, the elevation of temperature increases the demand for air, and causes speedier death when it is withheld ; by increasing the general agility. The natural habits of these animals are in correspon- dence with these facts. During the winter, the influence of a sufi&cient amount of aerated water upon their exterior serves to maintain the re- quired amount of respiration through the skin, so that they are not obliged to come to the surface to take in air by the mouth. As the season advances, however, their activity increases, a larger amount of respiration is required, and the animals are obliged to come frequently to the surface to breathe. During summer, the yet higher temperature calls forth an increased energy and activity in all the vital functions ; the respiration must be proportionably increased ; the action of the air upon the cutaneous surface, as well as upon the lungs, is required ; and if the animals are prevented from quitting the water to obtain this, they die, as soon as the warmth of the season becomes considerable. The result of experiments on Fishes, in regard to the deprivation or limited supply of the air contained in the water in which they are immersed, is exactly similar ; the duration of life being inversely as the temperature. And precisely the same has been ascertained w^ith respect to hyberna- ting Mammals ; which, as already remarked, are for a time reduced, ii all such conditions, to the level of cold-blooded animals. 129. The energy of the reparative actions of Animals is much influ- enced by temperature, as might be inferred from what has been just INFLUENCE OF HEAT ON ANIMALS. 89 said of their nutritive and developmental operations. Thus the rate at which regeneration of lost parts, like that of the ordinary process of budding, takes place in the common Hydra (Fresh-water Polype), is in close accordance with the temperature in which it lives ; and in like manner, the healing of wounds in Frogs takes place more rapidly in summer than in winter. In many of the higher animals, indeed, -it appears that the complete regeneration of parts requires a higher tem- perature than is necessary to sustain the ordinary vital activity. Thus it has been found that the common Triton (water-newt) can reproduce a limb that has been cut off, if it be kept at a temperature of from 58° to 75°; but cannot do so if a less amount of heat be afforded to it. And in like manner, the snail can regenerate its head, if it be kept in a warm atmosphere, but not at a low temperature. Now it has been justly re- marked by Mr. Paget, that the process of development seems to require a higher amount of vital force than simple growth ; and we see that the relation already pointed out between Heat and Vital force, here holds good in such a marked degree, as to afford a strong confirmation of the idea of their mutual relationship. 130. It is quite conformable to the same principle, that we should find Cold-blooded animals able to sustain the deprivation of food during a much longer period, at cold temperatures, than at warm. The case is precisely the reverse, however, in regard to most Warm-blooded ani- mals ; since in them a due supply of food is a condition absolutely ne- cessary (as we have already seen) for the maintenance of that amount of bodily heat, whose loss is fatal to them ; and exposure to a low tem- perature will of course more speedily bring about that crisis. Hence it is that Cold and Starvation combined are so destructive to life. But in this respect also, the hybernating Mammals correspond with the cold- blooded classes ; their power of abstinence being inversely as the tem- perature of their bodies. 131. We have seen that the animals termed cold-blooded are greatly influenced as to the temperature of their bodies, by the temperature of the surrounding medium ; although many of them are endowed with the power of keeping themselves a certain number of degrees above it. Now the consequence of this is, that all of them which are subject to any considerable and prolonged amount of cold, pass into a state of more or less complete inactivity during its continuance ; which state bears a close correspondence with the hybernation of certain Mammalia. Among the Reptiles of cold and temperate countries, this torpid state uniformly occupies a considerable part of the year ; as it does also with Insects, terrestrial Molluscs, and other Invertebrated animals, which are subject to the influence of the cold. On the other hand. Fishes, Crustacea, and other marine animals, do not usually appear to pass into a state of tor- pidity ; the temperature of the medium they inhabit never undergoing nearly so great a degree of depression, as that of the atmosphere. The amount of change necessary to produce this effect, or on the other hand to call the animals from a state of torpidity to one of active energy, differs for different species ; and there is probably a considerable diffe- rence even among individuals of the same species, according to the tem- perature under which they habitually live. Thus one animal may remain I yU EXTERNAL CONDITIONS OF VITAL ACTIVITY. torpid under a degree of warmth which will be sufficient to arouse another of the same kind, accustomed to a somewhat colder climate ; because the stimulus is relatively greater to the latter. 132. It was observed by Mr. Darwin, that at Bahia Blanca in South America, the first appearance of activity in animal and vegetable life, a few days before the vernal equinox, presented itself under a mean tem- perature of 58°, the range of the thermometer in the middle of the day being between 60° and 70°. The plains were ornamented by the flowers of a pink wood-sorrel, wild peas, evening primroses, and geraniums ; the birds began to lay their eggs ; numerous beetles were crawling about ; and lizards, the constant inhabitants of a sandy soil, were darting about in every direction. Yet a few days peviously, it seemed as if nature had scarcely granted a living creature to this dry and arid country ; and it was only by digging in the ground that their existence had been discovered, — several insects, large spiders, and lizards, having been found in a half-torpid state. Now at Monte Video, four degrees nearer the Equator, the mean temperature had been above 58° for some time pre- viously, and the thermometer rose occasionally during the middle of the day to 69° or 70°; yet with this elevated temperature, almost equivalent to the full summer heat of our own country, almost every beetle, several genera of spiders, snails, and land-shells, toads and lizards, were still lying torpid beneath stones. We have seen that at Bahia Blanca, whose climate is but a little colder, this same temperature, with a rather less extreme heat, was sufficient to awake all orders of animated beings ; — showing how nicely the required degree of stimulus is adapted to the general climate of the place, and how little it depends on absolute tem- perature. 133. We may learn much from the Geographical distribution of the diiferent species of cold-blooded animals, in regard to the influence of temperature on Animal life. No general inferences of this kind can be found upon the distribution of warm-blooded animals ; since their own heat-evolving powers make them in great degree independent of external warmth. And it is probably from the distribution of the marine tribes, whose extension is less influenced by local peculiarities, that the most satisfactory deductions are to be drawn. In regard to the class of Crustacea, which is the one that has been most fully inves- tigated in this respect, the following principles have been pointed out by M. Milne Edwards ; and they are probably more or less applicable to most others. I. The varieties of form and organization manifest themselves more, in proportion as we pass from the Polar Seas towards the Equator. II. The diff'erences of form and organization are not only more nume- rous and more characteristic in the warm than in the cold regions ofi the globe ; they are also more important. III. Not only are those Crustacea, which are most elevated in thei scale, deficient in the Polar regions ; but their relative number increases rapidly as we pass from the Pole towards the Equator. IV. When we compare together the Crustacea of diff*erent parts of the world, we observe that the average size of these animals is con- INFLUENCE OF HEAT ON ANIMALS. 91 siderably greater in tropical regions, than in the temperate or frigid climes. V. It is where the species are most numerous and varied, and where they attain the greatest size, — in other words, where the temperature is most elevated, — that the peculiarities of structure which characterize the several groups, are most strongly manifested. — — VI. Lastly, there is a remarkable coincidence between the tempera- ture of diflferent regions, and the prevalence of certain forms of Crus- tacea. 134. Now although, as appears from the foregoing general state- ments, the number of species of Crustacea inhabiting the colder seas bears a very small proportion to that which is found within the tropics, and although the species formed to inhabit cold climates are so far in- ferior both as to size, and as to perfection of development, yet it doefe not follow that the same proportion exists in regard to the relative amount of Crustacean life in the two regions ; for this depends upon the multiplication of individuals. In fact it may be questioned whether there is any inferiority in this respect ; so abundant are some of the smaller species in the Arctic and Antarctic, as well as in the Temperate seas. Thus we see that a low range of temperature is as well adapted to sustain their life, as a higher range is to call forth those larger and more fully-developed forms, which abound in the tropical ocean. There is an obvious reason why the seas of the frigid zones should be much more abundantly peopled than the layid ; the mean temperature of the former being much higher. And it would almost seem as if Nature had intended to compensate for the dreariness and desolation of the one, by the profuseness of life which she "has fitted the other to support. 135. The influence of Temperature in producing a variation in the size of individual Animals of any one species, is not so strongly marked as it is in the case of Plants ; for this reason, perhaps, that an amount of continued depression or elevation, which might be sustained by a Plant, but which would exert a modifying influence upon its growth, would be fatal to an Animal formed to exist in the same climate. Instances are not wanting, however, in which such a modifying influence is evident ; and these, as might be anticipated, are to be met with chiefly among the cold-blooded tribes. Thus the Bulimus rosaceus, a terrestrial mol- lusc, is found on the mountains of Chili of so much less a size than that which it attains on the coast, as to have been described as a distinct species. And the Littorina petrcea found on the south side of Plymouth Breakwater, acquires, from its superior exposure to light and heat (though perhaps also from the greater supply of nutriment which it obtains), twice the size common to individuals living on the north side within the harbour. — The following circumstance shows the favourable influence of an elevated temperature, in producing an unusual prolific- ness in Fish; which must be connected with general vital activity. Three pairs of Gold-fish were placed, some years since, in one of the engine-dams or ponds common in the manufacturing districts, into which the water from the engine is conveyed for the purpose of being cooled ; the average temperature of such dams is about 80°. At the end of three years, the progeny of these Fish, which Were accidentally 92 EXTERNAL CONDITIONS OF VITAL ACTIVITY. poisoned by verdigris mixed with the refuse tallow from the engine, were taken out by wheelbarrowfuls. It is not improbable that the unusual supply of aliment, furnished by the refuse grease that floats upon these ponds (which would impede the cooling of the water, if it were not consumed by the Fish), contributed with the high temperature to this unusual fecundity. 136. Although h very low temperature is positively inconsistent with the continuance of vital activity^ in Animals as in Plants, yet we find that even very severe cold is not necessarily destructive of the vital propei'ties of organized tissues ; so that, on a restoration of the proper amount of heat, their functions may continue as before. Of this we have already noticed an example, in the case of frost-bitten limbs ; but the fact is much more remarkable, when considered in reference to the whole body of an animal, and the complete suspension of all its func- tions. Yet it is unquestionably true, not only of the lowest and sim- plest members of the Animal kingdom, but also of Fishes and Reptiles. In one of Captain Ross's Arctic Voyages, several Caterpillars of the Laria Rossii having been exposed to a temperature of 40° heloiv zero, froze so completely, that, when thrown into a tumbler, they chinked like lumps of ice. When thawed, they resumed their movements, took food, and underwent their transformation into the Chrysalis state. One of them, which had been frozen and thawed four times, subsequently became a Moth. The eggs of the Slug have been exposed to a similar degree of cold, Avithout the loss of their fertility. It is not uncommon to meet in the ice of rivers, lakes, and seas, with Fishes which have been completely frozen, so as to become quite brittle ; and which yet revive when thawed. The same thing has been observed in regard to Frogs, Newts, &c. ; and the experiment of freezing and subsequently thawing them, has been frequently put in practice. Spallanzani kept Frogs and Snakes in an ice-house for three years ; at the end of which period they revived on being subjected to warmth. 137. It does not appear, however, that the same capability exists, in the case of any warm-blooded animals ; since if a total suspension* of vital activity take place in the body of a Bird or Mammal for any length of time, in consequence of the prolonged application of severe cold, re- covery is found to be impossible. The power which exists in these ani- mals, however, of generating a large amount of heat within their bodies, acts as a compensation for the want of the faculty possessed by the cold-blooded tribes ; since they can resist, for a great length of time (if I in their healthy or normal condition), the depressing influence of a tem- perature, sufficiently low to produce a complete suspension in the acti- vity of the latter. 138. It only remains to say a few words regarding the degree of heat which certain Animals can sustain without prejudice, and which even i appears to be genial to them. Among the higher classes, this range i seems to be capable of great extension. Thus many instances are on record, of a heat of from 250° to 280° being endured, in dry air, for ai considerable length of time, without much inconvenience; and persons * In the case of hybernating Mammals, the suspension is not total ; and if it be ren- j dered such, the same result follows as in other instances. INFLUENCE OF HEAT ON ANIMALS. 93 who have become habituated to this kind of exposure, can (with proper precautions) sustain a temperature of from 350° to 500°. In all such cases, however, the real heat of the body undergoes very little eleva- tion ; for, by means of the copious evaporation from its surface, the ex- ternal heat is prevented from acting upon it. But if this evaporation be prevented, either by an insufficiency in the supply of fluid from- within, or by the saturation of the surrounding air with moisture, the temperature of the body begins to rise ; and it is then found, that it cannot undergo an elevation of more than a few degrees, without fatal consequences. Thus in several experiments which have been tried on different species of warm-blooded animals, for the purpose of ascertain- ing the highest temperature to which the body could be raised without the destruction of life, it was found that as soon as the heat of the body had been increased, by continued immersion in a limited quantity of hot air (which would soon become charged with moisture), to from 9° — 13° above the natural standard, the animals died. In general Mammals die, w^hen the temperature of their bodies is raised to about 111°; the heat which is natural to the bodies of Birds. The latter are killed by an equal amount of elevation of bodily heat above their natural standard. 139. Hence we see that the actual range of temperature, within which vital activity can be maintained in warm-blooded aniniuls, is ex- tremely limited ; a temporary elevation of the bodily heat to 13° above the natural standard, or a depression to 30° below it, being positively inconsistent, not merely with the continuance of vital operations, but also with the preservation of vital properties : and a continued departure from that standard, to the extent of only a very few degrees above or below it, being very injurious. The provisions with which these animals are endowed, for generating heat in their interior, so as to supply the external deficiency, and for generating cold (so to speak), when the ex- ternal temperature is too high, are therefore in no respect superfluous : but are positively necessary for the maintenance of the life of such ani- mals, in any climate, save one whose mea7i should be conformable to their standard, and w^hose extremes should never vary more than a very few degrees above or below it. Such a climate does not exist on the surface of the earth. 140. The range of external temperature, within which cold-blooded animals can sustain their activity, is much more limited, as well in regard to its highest as to its lowest point ; notwithstanding that the range of bodily heat, which is consistent with the maintenance of their life, is so much greater. In those which, like the Frog, have a soft moist skin, which permits a copious evaporation from the surface, a considerable amount of heat may be resisted, provided the air be dry, and the supply of fluid from within be maintained.* But immersion in water of the temperature of 108°, is almost immediately fatal. In many other cold- blooded animals, elevation of the temperature induces a state of tor- * The Frog has a remarkable provision for this purpose ; in a bladder, which is structurally analogous to our Urinary bladder, but which has for its chief function to contain a store of fluids for the exhaling process. It has been noticed that, when this store is exhausted by continued exposure of the animal to a warm dry atmosphere, the bladder becomes full again, when the animal is placed in a moist situation, even though it take in no liquid by its mouth. i 94 EXTERNAL CONDITIONS OF VITAL ACTIVITY. pidity, analogous to that which is produced by its depression. Thus the Helix pomatia (Edible Snail) has been found to become torpid and motionless in water at 112°; but to recover its energy when placed in a colder situation. It would seem to be partly from this cause, but partly also from the deprivation of moisture, that the liottest part of the tropical year brings about a cessation of activity in many tribes of cold- blooded animals, as complete as that which takes place during the winter of temperate climates. 141. The highest limit of temperature compatible with the life of Fishes has not been certainly ascertained : and it appears probable that there are considerable variations in this respect amongst different species. Thus it is certain that there are some which are killed by immersion in water at 104°; whilst it is also certain that others cannot only exist, but can find a congenial habitation, in water of 113°, or even of 120°; and examples of the existence of Fishes in thermal springs of a much higher temperature than this, have been put on record. Various fresh water Mollusca have been found in thermal springs, the heat of which is from 100° to 145°. Rotifera and other animalcules have been met with in water at 112°. Larvae of Tipulse have been found in hot springs of 205° ; and small black beetles, which died when placed in cold water, in the hot sulphur baths of Albano. Entozoa inhabiting the bodies of Mammalia and of Birds must of course be adapted to a constant tem- perature of from 98° to 110° ; and they become torpid when exposed to a cool atmosphere. These lowly organized animals seem more capable of resisting the effects of extreme- heat, than any others ; — at least if we are to credit the statement, that the Entozoa inhabiting the intestines of the Carp have been found alive, when the Fish was brought to table after being boiled. In all such cases, it is to be remembered, that the heat of the animal body must correspond with that of the fluid in which it is immersed ; and we have here, therefore, evident proof of the com- patibility of vital activity, in certain cases, with a very elevated tempe- rature. Additional and more exact observations, however, are much wanting on this subject. 3. Of Electricity, as a Condition of Vital Activity. 142. Much less is certainly known with respect to the ordinary influ- ence of this agent, than in regard to either of the two preceding ; and yet there can be little doubt, from the effects we observe when it is pow- erfully applied, as well as from our knowledge of its connexion with all Chemical phenomena, that it is in constant though imperceptible opera- tion. Electricity differs from both Light and Heat in this respect ; — that no manifestation of it takes place so long as it is uniformly diffused, or is in a state of equilibrium ; but in proportion as this equilibrium is disturbed, by a change in the electric condition of one body, which is prevented, by its partial or complete insulation, from communicating itself to others, in that proportion is a force produced, which exerts itself in various ways according to its degree. The mechanical effects of a powerful charge, when passed through a substance that is a bad conductor of Electricity, are well known ; on the other hand, the chemical OF ELECTRICITY AS A CONDITION OF VITAL ACTIVITY. 95 effects of even the feeblest current are equally obvious. The agency of Electricity in producing Chemical change is the more powerful, in pro- portion as there is already a predisposition to that change ; thus, the largest collection of oxygen and hydrogen gases, or of hydrogen and chlorine, mingled together, may be caused to unite by the minutest elec- tric spark, which brings into the condition required for their active exercise, the mutual aflSnities that were previously dormant. Hence it cannot but be inferred, that its agency in the Chemical phenomena of living bodies must be of an important character : but this may probably be exerted rather in the way of aiding decomposition, than of producing new combinations, to which (as we have seen) Light appears to be the most effectual stimulus. Thus it has been shown that pieces of meat, that have been electrified for some hours, pass much more rapidly into decomposition, than similar pieces placed under the same circumstances, but not electrified. And in like manner, the bodies of animals that have been killed by electric shocks, have been observed to putrefy much more readily than those of similar animals killed by an injury to the brain. It is well known, moreover, that in thundery w^eather, in which the electric state of the atmosphere is much disturbed, various fluids con- taining organic compounds, such as milk, broth, &c., are peculiarly dis- posed to turn sour ; and that saccharine fluids, such as the wort of brewers, are extremely apt to pass into the acetous fermentation. 143. The actual amount of influence, however, which Electricity exerts over a growing Plant pr Animal, can scarcely be estimated. It would, perhaps, be the most correct to say, that the state of Electric equilibrium is that which is generally most favourable ; and we find that there is a provision in the structure of most living beings, for maintaining such an equilibrium, — not only between the different parts of their own bodies, but also between their own fabrics and the surrounding medium. Thus a charge given to any part of a Plant or Animal, is immediately diffused through its whole mass ; and though Organized bodies are not sufficiently good conductors to transmit very powerful shocks without being them- selves affected, yet a discharge of any moderate quantity may be effected through them, without any permanent injury, — and this more especially if it be made to take place slowly. Noav the points on the surfaces of Plants appear particularly adapted to effect this transmission ; thus it has been found that a Ley den jar might be discharged by holding a blade of grass near it, in one third of the time required to produce the same effect by means of a metallic point ; and an Electroscope furnished with Vegetable points has been found to give more delicate indications of the electric state of the atmosphere, than any other. Plants designed for a rapid growth have generally a strong pubescence or downy covering ; and it does not seem improbable that one purpose of this may be, to maintain that equilibrium between themselves and the atmosphere, which would otherwise be disturbed by the various operations of vegetation, and especially by the process of evaporation, which takes place with such activity from the surface of the leaves. 144. There appears to be sufficient evidence that, during a highly electrical state of the atmosphere, the growth of the young shoots of certain plants is increased in rapidity ; but it would be wrong thence to L 96 EXTERNAL CONDITIONS OF VITAL ACTIVITY. infer that this excitement is useful to the process of Vegetation in gene- ral, or that the same kind of electric excitement universally operates to the benefit or injury of the Plant. From some experiments recently made it would appear, that potatoes, mustard, and cress, cinerarias, fuchsias, and other plants, have their development, and, in some in- stances, their productiveness, increased by being made to grow between a copper and a zinc plate, connected by a conducting wire ; while, on the other hand, geraniums and balsams are destroyed by the same in- fluence. The transmission of a series of moderate sparks through plants, in like manner, has been found to accelerate the growth of some, and to be evidently injurious to others. It is not unreasonable to suppose, that, as a great variety of chemical processes are constantly taking place in the growing plant, an electric disturbance, which acts as a stimulus to some, may positively retard others ; and that its good or evil results may thus depend upon the balance between these individual effects. This would seem the more likely from the circumstance, that, in the process of Germination, the chemical changes concerned in which are of a simpler character. Electricity seems to have a more decided and uni- form influence. The conversion of the starch of the seed into sugar, which is an essential part of this change, involves the liberation of a large quantity of carbonic, and of some acetic acid. Now as all acids are negative, and as like electricities repel each other, it maybe inferred that the seed is at that time in an electro-negative condition ; and it is accordingly found that the process of germination may be quickened, by connexion of the seed with the negative pole of a feeble galvanic appa- ratus, whilst it is retarded by a similar connexion with the positive pole. A similar acceleration may be produced by the contact of feeble alkaline solutions, which favour the liberation of the acids ; whilst, on the same principle, a very small admixture of acid in the fluid with which the seed is moistened, is found to produce a decided retardation. 145. It is well known that Trees and Plants may be easily killed by powerful electric shocks ; and that, when the charge is strong enough (as is the case of a stroke of lightning), violent mechanical efi"ects, — as the rending of trunks, or even the splitting and scattering of minute fragments, — are produced by it. But it has also been ascertained, that charges which produce no perceptible influence of this kind, may destroy the life of Plants ; though the efiect is not always immediate. In par- ticular it has been noticed, that slips and grafts are prevented from taking root and budding. There can be little doubt that, in these in- stances, a change is efi'ected in the chemical state of the solids or fluids ; although no structural alteration is perceptible. 146. In regard to the influence of Electricity upon the Organic func- tions of Animals, still less is certainly known ; but there is evidence that it may act as a powerful stimulant in certain disordered states of them. Thus in Amenorrhoea, a series of slight but rapidly-repeated electric shocks will often bring on the catamenial flow ; and it is certain that chronic tumours have been, dispersed, and dropsies relieved by the ex- citement of the absorbent process, through similar agency. In fact, there is strong reason to believe, that Electricity may be advantageously OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 97 employed remedially in many states of disordered nutrition ; in virtue of its power of modifying the operations of the Vital forces. 147. The closest relations of Electricity, however, are with the proper Animal functions; for these, as will be shown hereafter, are more directly and obviously subject to its influence, than are the Organic. Thus Electricity, when transmitted along a Nerve, whether sensory olil motor, a nerve -of "special" or one of "common" sensation, is capable of calling forth all the actions of which that nerve is the instrument ; and, when brought to bear on a Muscle, it immediately excites a con- tractile movement. It is probably through the influence of this agent upon the Nervous system, that electric states of the atmosphere induce in certain individuals a degree of languor and depression, which cannot be accounted for in any other way. An instance is on record, in which the atmosphere was in such an extraordinary state of electric disturbance, that all pointed bodies within its influence exhibited a distinct luminosity ; and it was, noticed, that all the persons who were exposed to the agency of this highly electrified air, experienced spasms in the limbs and an ex- treme state of lassitude. 148. Animals, like Plants, are liable to be killed by shocks of Elec- tricity ; even when these are not sufiiciently powerful to occasion any obvious physical change ii:! their structure. But, as formerly mentioned (§ 69), there can be no doubt that minute changes may be produced in their delicate parts, which are quite sufficient to account for the destruc- tion of their vitality, even though these can only be discerned with the microscope. The production of changes in the Chemical arrangement of their elements, is, however, a much more palpable cause of death ; since it may be fully anticipated beforehand, and can easily be rendered evident. To take one instance only ; — it is well known, that albumen is made to coagulate, i. e., is changed from its soluble to its insoluble form, under the influence of an electric current ; and it cannot be doubted that the production of this change in the fluids of the living body (almost every one of which contains albumen), even to a very limited extent, is quite a sufficient cause of death, even in animals that are otherwise most tenacious of life. " I once discharged a battery of considerable size," says Dr. Hodgkin, " through a common Earth-worm, which would in all probability have shown signs of life long after minute division. Its death was as sudden as the shock ; and the semi-transparent sub- stance of the animal was changed like Albumen which has been exposed, to heat." 1 4. Of Moisture J as a Condition of Vital Activity. 149. Independently of the utility of Water as an article oi food^ and of the part it performs in the Chemical operations of the living body, by supplying two of their most important materials (oxygen and hydro- gen), there can be no doubt that a certain supply of moisture is requi- site, as one of the conditions without w^iich no vital action can go on. It has been already remarked, indeed, that one of the distinguishing peculiarities of Organized structures, is the presence in all of them of solid and liquid component parts ; and this in the minutest portions of 7 98 EXTERNAL CONDITIONS OF VITAL ACTIVITY. the organism, as well as in the aggregate mass. And in all the vital^ as well as in the chemical actions, to which these structures are subser- vient, the presence of liquid is essential. All nutrient materials must be reduced to the liquid form, before they can be assimilated by the solids ; and, again, the solid matters which are destined to be carried ofif by excretion, must be again reduced to the liquid state, before they can be thus withdrawn from the body. The tissues in which the most active changes of a purely vital character are performed, — namely, the Nervous and Muscular, — naturally contain a very large proportion of water ; the former as much as 80 and the latter 77 per cent. On the other hand, in tissues whose function is of a purely mechanical nature, such as Bone, the amount of liquid is as small as is consistent with the maintenance of a certain amount of nutrient action in its interior. By the long-continued application of dry heat to a dead body, its weight was found to be reduced from 120 pounds to no more than 12 ; so that, taking the average of the whole, the amount of water, not chemically combined, but simply interstitial, might be reckoned at as much as 90 per cent. It is certain, however, that much decomposition and loss of solid matter must have taken place in this procedure ; and we shall pro- bably estimate the proportion, more accurately, if we regard the weight of the fluids of the huma,n body as exceeding that of the solids by six or seven times. 150. There is a great variation in this respect, however, among dif- ferent tribes of living beings. There are probably no highly organized Animals, whose texture contains less liquid than that of Yertebrata (unless, it may be, certain Beetles); but there can be no question that, among some of the Zoophytes, the proportion of solids to liquids is just the other way. In those massive coral-forming animals, which seem to have been expressly created for the purpose of uprearing islands and even continents from the depth of the ocean, we find the soft tissues confined to the surface, and all within of a rocky hardness. It is not, however, correct to say (as is commonly done), that the coral-polypes " build up" these stony structures as habitations for themselves ; for the stony matter is deposited, by an act of nutrition, in the living tissue of these animals, just as much as it is in the bones of Man. But the parts once consolidated henceforth remains dead, so far as the animal is con- cerned ; they are not connected with the living tissues by any vessels, nerves, &c., their density prevents them from undergoing any but a very slow disintegrating change, so that they require and receive no nutrient materials ; and they might be altogether removed, by accident or decay, without any direct injury to the still-active, because yet unconsolidated, portions of the polype structure. 151. There is a close correspondence, in this respect, between the condition of the stony or horny stem of a Coral, and the heart-wood of the trunk of a Tree ; for the latter, becoming consolidated by internal deposit, for the purpose of afi'ording mechanical support, is thenceforth I totally unconnected with the vegetative operations of the tree, and might be removed (as it frequently is by natural decay) without afi'ecting them. In all the parts, in which the nutrient processes are actively going on, do we observe that the tissue contains a large proportion of water ; afid OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 99 that, if the succulent portions be dried up, their vital properties are de- stroyed. Thus it is in the soft tissue at the extremities of the radicles or root-fibres, that the function of absorption takes place with the greatest activity ; so that these parts have received the name of spon- gioles: it is in the cells which form the soft parenchyma of the leaves, that the elaboration of the sap takes place, the fixation of carbon frora- the atmosphere, and the preparation of the peculiar secretions of the plant : and it is in the space between the bark and the wood, which is occupied (at the season of most active growth) by a saccharine glutinous fluid, that the formation of the new layers of wood and bark takes place. Now, as soon as these parts become consolidated, they cease to perform any active vital operations. The &pongioles, by the lengthening of the root-fibres, become converted into a portion of those fibres, and remain subservient merely to the transmission of the fluids absorbed; the leaves gradually become choked by the saline and earthy particles contained in the ascending sap, which they have had no power of excreting, and they wither, die, and fall off; and the new layers of wood and bark, when once formed, undergo but little further change, and are subser- vient to little else than the transmission of the ascending and descending sap to the parts where they are to be respectively appropriated. 152. There are some remarkable instances in both the Animal and Vegetable kingdoms, of an immense preponderance in the amount of the fluids over that of the solids of the structure. This is characteristic of the whole class of Acalephce or Jelly-Fish^ giving to their tissues that softness from which their common name is derived ; these animals, in consequence, are unable to live out of water ; for when they are removed from it, a drain of their fluids commences, which soon reduces their weight to a degree that destroys their lives, — a Medusa weighing fifty pounds being thus dried down to a weight of as many grains. The most remarkable instances of a parallel kind among Plants, are to be found in the tribe of Fungi ; certain members of which are distinguished by an almost equally small proportion of solid materials in their textures, presenting a most delicate gossamer-like appearance to the eye, and possessing such little durability, that they come to maturity and undergo decay in the course of a few hours. These are not inhabitants of the water, but will vegetate only in a very damp atmosphere. 153. As we find various Plants and Animals very differently con- structed, in regard to the amount of fluid contained in their tissues, so do we also find them dependent in very different degrees upon a con- stant supply of external moisture. There is no relation, however, be- tween the succulence of a plant, and the degree of its dependence upon water ; in. fact, we commonly find the most succulent plants growing in the driest situations ; whilst the plants, which are adapted to localities where they can obtain a constant supply of fluid, are not usually re- markable for the amount of water in their own structure. This, how- ever, is easily explained. We find the most succulent plants, — such as the Sedums or Stone-crops of our own country, and the Cacti and Uuphorbice of the tropics, — in dry exposed situations, where they seem as if they would be utterly destitute of nutriment. The fact is, how- ever, that they lose their fluid by exhalation very slowly, in consequence 1"#0 EXTERNAL CONDITIONS OF VITAL ACTIVITY. of their small number of stomata ; whilst, on the other hand, they absorb with great readiness during rainy weather, and are enabled, by the fleshiness of their substance, to store up a large quantity of moisture until it is required. In some parts of Mexico, the heat is so intense, and the soil and atmosphere so dry, during a large part of the year, that no vegetation is found at certain seasons, save a species of Cactus ; this affords a wholesome and refreshing article of food, on which travel- lers have been able to subsist for many days together, and without which these tracts would form impassable barriers. On the othei* hand, the plants of damp situations usually exhale moisture almost as fast as they imbibe it ; and consequently, if their usual supply be cut off or diminished, they soon wither and die. Plants that usually live entirely submerged, are destitute of the cuticle or thin skin, which covers the surface in other cases ; in consequence of this, they very rapidly lose their fluid, when they are removed from the water ; and they are hence dependent upon constant immersion in it for the continuance of their lives, although their tissues may not be remarkable for the amount of fluid which they contain. 154. There are some Plants which are capable of adapting themselves to a great variety of situations, difi'ering widely as to the amount of moisture which their inhabitants can derive from the soil and atmo- sphere ; and we may generally notice a marked difi'erence in the mode of growth, when we compare individuals that have grown under oppo- site circumstances. Thus a plant from a dry exposed situation, shall be stunted and hairy, whilst another, of the same species, but developed in a damp sheltered situation, shall be rank and glabrous (smooth). But in general there is a certain quantity of moisture congenial to each species; and the excess or deficiency of this condition has, in conse- quence, as great an influence in determining the geographical distribu- tion of Plants, as the amount of light and heat. Thus, as already remarked, the Orchidege and Tree Ferns of the tropics grow best in an atmosphere loaded with dampness ; whilst the Cactus tribe, for the most part, flourishes best in dry situations. The former become stunted and inactive, if limited in their supply of aerial moisture ; whilst the latter, if too copiously nourished, become dropsical and liable to rot. Among the plants of our own country, we find a similar limitation ; a moist boggy situation being indispensable to the growth of some, whilst a dry exposed elevation is equally essential to the healthy development of others. There is a beautiful species of exotic Fern, the Trichomanes speciosum; the rearing of which has been frequently attempted in this country and elsewhere, without success ; but which only requires an atmosphere saturated with dampness, for its healthy development, being easily reared in one of Mr. Ward's closed glass-cases. In this, as in similar examples, it is only necessary to imitate as closely as possible the conditions under which the species naturally grows ; and sometimes this can only be accomplished, by surrounding the plant with small trees and shrubs, so as to give it a moister atmosphere than it could otherwise attain. Professor Royle mentions the growth, under stich circumstances, of a fine specimen of the Xanthochymus dulcisy one of the Gruttiferce or Gamboge-trees, in the garden of the King of Delhi ; OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 101 this tree is naturally found only in the southern parts of India ; and the success of its cultivation in this northerly situation is entirely due to its being sheltered by the numerous buildings within the lofty palace wall, surrounded by almost a forest of trees, and receiving the benefit of perpetual irrigation from a branch of the canal wjiich flows through the garden. 155. In regard to the influence of external moisture upon Animal life, there is much less to be said ; since the mode in which fluid is re- ceived into the system is so entirely diff'erent. It may be remarked, however, that Animals habitually living beneath the water, like sub- merged Plants, are usually incapable of sustaining life for any length of time when removed from it, in consequence of the rapid loss of fluid which they undergo from their surface. It is, however, by the desic- cation of the re^'piratory surface, preventing the due aeration of the blood, that the final result is for the most part occasioned ; since we find that when there is a special provision to prevent this, as in the case of certain Fishes and Crustacea, the animals can quit the water for a great length of time. There can be no doubt that the amount of Atmospheric moisture is one of those conditions, which are collectively termed climate, and which influence the geographical distribution of Animals, no less than that of Plants. But it is difficult to say how far the variations in moisture act alone. There can be no doubt, however, of their operation ; for every one is conscious of the efi'ect, upon his health and spirits, of such variations as take place in the climate he may inhabit. The two principal modes in which these will operate, will be by accelerating or checking the exhalation of fluid from the skin and from the pulmonary surface; for when the air is already loaded with dampness, the exhaled moisture cannot be carried off with the same readiness as when it is in a condition of greater dryness ; and it will consequently either remain within the system, or it will accumulate and form sensible perspiration. 156. Now each of these states may be salutary, being the one best adapted to particular constitutions, or to different states of the same individual. A cold drying wind shall be felt as invigorating to the relaxed frame as it is chilling to one that has no warmth or moisture to spare; on the other hand, a warm damp atmosphere, which is refresh- ing to the latter, shall be most depressing to the former. All who have tried the effect of closely-fitting garments, impervious to njoisture, are well aware how oppressive they soon become ; this feeling being de- pendent upon the obstruction they occasion to the act of perspiration, by causing the included air to be speedily saturated with moisture. When the fluids of the system have been diminished in amount, either by the suspension of a due supply of water, or by an increase in the excretions, there is a peculiar refreshment in a soft damp atmosphere, or in a warm bath, which allows the loss to be replaced by absorption through the general cutaneous surface. The reality of such absorption has been placed beyond all doubt, by observations upon men, who had been exposed to a hot dry air for some time, and afterwards placed in a warm bath ; for it was found that the system would by this unusual 102 EXTERNAL CONDITIONS OF VITAL ACTIVITY. means supply the deficiency, which had been created by the previous increase in the transpiration. 157. The effect of a moist or dry atmosphere, then, upon the Animal body, cannot be by any means unimportant; although, as we shall hereafter see, there exists in it a series of the most remarkable provi- sions for regulating the amount of its fluids. The influence of atmo- spheric moisture, however, is most obvious in disordered states of the system. Thus in persons who are subject to the form of Dyspepsia called atonic, which is usually connected with a generally-relaxed con- dition of the system, a very perceptible influence is experienced from changes- in the quantity of atmospheric moisture ; the digestive power, as well as the general functions of the body, being invigorated by dry- ness, and depressed by damp. Again there is no doubt that, where a predisposition exists to the Tuberculous Cachexia, it is greatly favoured by habitual exposure to a damp atmosphere, especially when accom- panied by cold : indeed it would appear, from the influence of cold damp situations upon animals brought from warmer climates, that these two causes may induce the disease, in individuals previously healthy. On the other hand, there are some forms of pulmonary complaints, in which an irritable state of the mucous membrane of the bronchial tubes has a large share ; when this irritation presents itself in the dry form, a warm moist atmosphere is found most soothing to it ; whilst a drier and more bracing air is much more beneficial, when the irritation is accompanied by a too copious secretion. 158. Although, as already stated, no .vital actions can go on without a reaction between the solids and fluids of the body, yet there may be an entire loss of the latter, in certain cases, without necessarily destroy- ing life ; the structure being reduced to a state of dormant vitality, in which it may remain unchanged for an unlimited period ; and yet being capable of renewing all its actions, when moisture is again supplied. Of this we find numerous examples among both the Vegetable and the Ani- ilial kingdoms. Thus the Mosses and Liverworts, which inhabit situa- tions where they are liable to occasional drought, do not suffer from being, to all appearance, completely dried up; but revive and vegetate actively, as soon as they have been thoroughly moistened. Instances are recorded, in which Mosses that have been for many years dried up in an Herbarium, have been restored by moisture to active life. There is a Lycopodium (Club-Moss) inhabiting Peru, which, when dried up for want of moisture, folds its leaves and contracts into a ball ; and in this state, apparently quite devoid of animation, it is blown hither and thither ' along the surface by the wind. As soon, however, as it reaches a moist situation, it sends down its roots into the soil, and unfolds to the atmo- sphere its leaves, which, from a dingy brown, speedily change to the i bright green of active vegetation. The Anastatica (Rose of Jericho) is the subject of similar transformations ; contracting into a ball, when dried up by the burning sun and parching air ; being detached by the wind from the spot where its slender roots had fixed it, and rolled over the plains to indefinite distances ; and then, when exposed to moisture, unfolding its leaves, and opening its rose-like flower, as if roused from sleep. A blue Water-Lily abounds in several of the canals at Alexan- OF MOISTURE AS A CONDITION OF VITAL ACTIVITY. 103 dria, which at certain seasons become so dry, that their beds are burnt as hard as bricks by the action of the sun, so as to be fit for use as car- riage roads ; yet the plants do not thereby lose their vitality; for when the water is again admitted, they resume their growth with redoubled vigour. 159. Among the lower Animals, we find several of considerable com- plexity of structure, which are able to sustain the most complete desic- cation. This is most remarkably the case in the common Wheel-Ani- malcule; which may be reduced to a state of most complete dryness, and kept in this condition for any length of time, and which will yet revive immediately on being moistened. The same individuals may be treated in this manner, over and over again. Experiments have been carried still further with the allied tribe of Tardigrades ; individuals of which have been kept in a vacuum for thirty days, with sulphuric acid and Chloride of Calcium (thus sufi'ering the most complete desiccation the Chemist can effect), and yet have not lost their vitality. It is sin- gular that in this desiccated condition, they may be heated to a tem- perature of 250°, without the destruction of their vitality ; although, when in full activity, they will not sustain a temperature of more than from 112° to 115°. Some of the minute Entomostracous Crustacea, which are nearly allied to the Rotifera, appear to partake with them in this curious faculty. Many instances are on record in which Snails and other terrestrial Mollusca have revived, after what appeared to be complete desiccation ; and the eggs of the Slug, when dried up by the sun or by artificial heat, and reduced to minute points only visible with the Microscope, are found not to have lost their fertility, when they are moistened by a shower of rain, or by immersion in water, which restores them to their former plumpness. Even after being treated eight times in this manner, the eggs were hatched when placed in favourable cir- cumstances ; and even eggs in which the embryo was distinctly formed, survived such treatment without damage. — That such capability should exist in the animals and eggs just mentioned, shows a remarkable adap- tation to the circumstances in which they are destined to exist ; since were it not for their power of surviving desiccation, the races of Wheel- Animalcules and Entomostraca must speedily become extinct, through the periodical drying up of the small collections of water which they inhabit ; and a season of prolonged drought must be equally fatal to the terrestrial Mollusca. 160. It would seem that many cold-blooded animals are reduced, by a moderate deficiency of fluid, to a state of torpidity closely resembling that induced by cold ; and hence it is, that during the hottest and driest part of the tropical year, there is almost as complete an inactivity as in the winter of temperate regions. The common Snail, if put into a box without food, constructs a thin operculum or partition across the orifice of the shell, and attaches itself to the side of the box : in this state it may remain dormant for years, without being affected by any ordinary changes of temperature : but it will speedily revive if plunged in water. Even in their natural haunts, the terrestrial Mollusca of our own cli- mates are often found in this state during the summer, when there is a continued drought; but with the first shower they revive and move L 104 EXTERNAL CONDITIONS OF VITAL ACTIVITY. about. In like manner it is observed that the rainy season, between the tropics, brings forth the hosts of insects, which the drought had caused to remain inactive in their hiding-places. Animals thus rendered torpid seem to have a tendency to bury themselves in the ground, like those which are driven to winter quarters by cold. Mr. Darwin men- tions that he observed with some surprise at Rio de Janeiro, that, a few days after some little depressions had been changed into pools of water by the rain, they were peopled by numerous full-grown shells and beetles. 161. This torpidity consequent upon drought is not confined to In- vertebrated animals. There are several Fish, inhabiting fresh water, which bury themselves in the mud when their streams or pools are dried up, and which remain there in a torpid condition until they are again moistened. This is the case with the curious Lepidosiren, which forms so remarkable a connecting link between Fishes and the Batrachian Rep- tiles : it is an inhabitant of the upper parts of the river Gambia, which are liable to be dried up during much more than half the year; and the whole of this period is spent by it in a hollow which it excavates for itself deep in the mud, where it lies coiled up in a completely torpid condition, — whence it is called by the natives the sleeping-fish. When the return of the rainy season causes the streams to be again filled, so that the water finds its way down to the hiding-place of the Lepidosiren, it comes forth again for its brief period of activity ; and with the- ap- proach of drought, it again works its way down into the mud, which speedily hardens around it into a solid mass. In the same manner, the ProteuSj an inhabitant of certain lakes in the Tyrol, which are liable to be periodically dried up, retires at these periods to the underground pas- sages that connect them, where it is believed to remain in a torpid con- dition ; and it thence emerges into the lakes, as soon as they again be- come filled with water. The Lizards and Serpents, too, of tropical climates appear to be subject to the same kind of torpidity, in conse- quence of drought, as that which aff"ects those of temperate regions during the cold of winter. Thus Humboldt has related the strange acci- dent of a hovel having been built over a spot where a young Crocodile lay buried alive, though torpid, in the hardened mud; and he mentions that the Indians often find enormous Boas in the same lethargic state ; and that these revive when irritated or wetted with water. All these examples show the necessity of a fixed amount of fluid, in the animal structure, for the maintenance of vital activity : whilst they also demon- strate, that the preservation of the vital properties of that structure is , not always incompatible with the partial, or even the complete abstrac- tion of that fluid ; the solid portions being then much less liable to de- composition than by heat, or by other agencies, than they are in their ordinary condition. ELEMENTARY PARTS OF ANIMAL STRUCTURES. 105 CHAPTER III. OF THE ELEMENTARY PARTS OF ANIMAL STRUCTURES. 162. In the investigation of the operations of a complex piece of Me- chanism, and in the study of the forces which combine to produce the general result, experience shows the advantage of first examining the component parts of the Machine, — its springs, wheels, levers, cords, pulleys, &c., — determining the properties of their materials, and ascer- taining their individual actions. When these have been completely mastered, the attention may be directed to their combined actions : and the bearing of these combinations upon each other, so as to produce the general result, would be the last object of study. 163. This seems the plan which the Student of Physiology may most advantageously pursue, in the diflScult task of making himself acquainted with the operations of the living Organism, and with the mode in which they concur in the maintenance of Life. He should first examine the properties of the component materials of the structure, in their simplest form : these he will find in its nutrient fluids. He may next proceed to the simplest forms of organized tissue, which result from the mere solidi- fication of those materials, and whose properties are chiefly of a mechani- cal nature. From these he will pass to the consideration of the struc- ture and actions of those tissues that consist chiefly of cells; and will investigate the share they take in the strictly vital operations of the economy. Next his attention will be engaged by the tissues produced by the transformation of cells ; of which some are destined chiefly for affording mechanical support to the fabric, and others for peculiar vital operations. And he will be tljen prepared to understand the part which these elementary tissues severally perform in the more complex organs. A due knowledge of these elementary parts, and of their physical, chemi- cal, and vital properties, is essential to every one who aims at a scientific knowledge of Physiology. True it is, that we may study the results of their operations, without acquaintance with them ; but we should know nothing more of the working of the machine, than we should know of a cotton-mill, into which we saw cotton-wool entering, and from which we saw woven fabrics issuing forth ; or of a paper-making-machine, which we saw fed at one end with rags, and discharging hot-pressed paper, cut into sheets, at the other. The study of these results affords, of course, a very important part of the knowledge we have to acquire respecting the operations of the machine ; but we could learn from them very little of the nature of the separate processes effected by it; still less should we be prepared, by any disorder or irregularity in the general results, to seek for, and rectify, the cause of that disturbance in the working of the machine, by which the abnormal result was occasioned. 164. Now just as in a cotton-mill, there are machines of several different kinds, adapted to effect different steps of the process by which the raw material is converted into the woven fabric, so do we find that in the complex Animal fabric there is a great variety of organs for per- L 106 ELEMENTARY PARTS OF ANIMAL STRUCTURES. forming the several changes, by which the fabric itself is built up and maintained in a condition fit for the performance of its peculiar opera- tions. These operations are the phenomena of sensation, of spontaneous motion, and of mental action. They are the great objects of Animal existence ; just as the combination of elements into organic substances, that are to furnish the materials of the Animal fabric, seems to be the great purpose of Vegetative Life. The vital phenomena which are peculiar to Animals, are manifestations of the properties of certain forms of organized matter, — the Nervous and Muscular tissues, — which are restricted to themselves ; just as those Avhich are common to Animals and Plants, are effected by organized structures which are found alike in both kingdoms. Here, then, we have one essential distinction between these kingdoms ; — namely, the presence in Animals of a peculiar appa- ratus, and the consequent possession by them of peculiar endowments, which are totally wanting in Plants. There are, it is true, many species, indeed whole tribes, in which it is impossible to say with certainty, how far sensibility and spontaneity of action may be justly inferred from the movements they exhibit ; so that, their structure being so simple as to afford no distinctive characters, our assignment of them to the Animal or to the Vegetable kingdom must be determined entirely by the mode in which they obtain the materials of their nutrition (§§ 62, 63). 165. All the operations, then, which are common to Animals and Plants, are concerned in the building up of the organized fabric, in the maintenance of its integrity, and in the preparation of the germs of new structures, to compensate for the loss of the parent by death. These operations, as formerly explained (§41), involve a series of very distinct processes ; which, though all performed by the simple cell of the humblest plant, are distributed in more complex structures through a number of parts or organs, whose several actions are almost as separate as those of the dissimilar machines of the cotton-mill, — although, like them, sus- tained by the same powers, and so far mutually dependent, that neither of them can be suspended without in a short time putting a stop to thei rest. Now just as in each of the machines of the cotton-mill we may have similar elements, — such as wheels, levers, pulleys, bands, &c., put together in different methods, and consequently adapted for different pur- poses, as carding, spinning, weaving, &c., so shall we find in the animal body, that these different organs are composed of very similar elements, and that the individual actions of these elementary parts are the same; but that the difference of result is the consequence of the variety in their arrangement. Thus we shall find that the growth of cells, their absorp- tion of certain matters from the surrounding fluids, and their subsequent liberation of these by the bursting or liquefying of the cell-wall when their term of life is come to an end, are means employed in one part of the body to introduce nutrient materials into the current of the circu- lation, whilst in another the same processes are used as means to with- draw, from that very same current, certain substances of which it is necessary to get rid. Now certain combinations of elementary struc- ture, adapted to the performance of a set of actions tending to one pur- pose, and thus resembling one of the machines of a cotton -mill, is termed an organ ; and the sum-total of its actions is termed its function. Thus ALBUMINOUS COMPOUNDS. 107 we have in the function of Respiration, which essentially consists of an interchange of oxygen and carbonic acid between the air and the blood, a multitude of distinct changes, some of them of a character apparently not in the least related to it, but all necessary, in the higher and more complex fabric, to bring the blood and the air into the necessary relation. The sum-total of these changes constitutes the function of Respiration-^ and the structures by which they are effected are organs of Respiration. 166. The entire Organized structure, then, may be regarded as made up of distinct organs, having their several and (to a certain extent) independent purposes ; and these organs may be resolved, in like man- ner, into simple elementary parts, whose structure and composition are the same, in whatever part of the fabric they occur. And in like man- ner, the phenomena of Life, considered as a whole, may be arranged under several groups or functions, according to the immediate purpose to which they are directed ; and yet in every one of these groups, we shall find repeated the same elementary changes which are concerned in the rest. Thus in the act of Respiration, the same kind of muscular movements, the same sort of nervous agency, are concerned, as con- tribute to the ingestion of the food ; together with a circulation of blood, similar to that which supplies the materials for the nutrition of the tissues. Hence we see the propriety of applying ourselves first to the consideration of the elementary parts of the living structure, and of the properties by which they effect the changes, that are characteristic of its several organs. 1. Of the Primary Components of the Animal Fabric. 167. As we can best study the primary components of the Animal fabric, by investigating their properties before the process of Organiza- tion begins, or whilst it is taking place, we must have recourse for this purpose to the nutrient fluid, — the Rlood,— in which these are contained in the state most completely prepared for the reception of the vitalizing influence. The same substances may be found, in an earlier stage of preparation, in the Chyle and Lymph ; and also in the Eggs of ovipa- rous animals. The circumstances attending the development of the latter afford, indeed, the most satisfactory proof of the convertibility of the simple chemical product. Albumen, with certain inorganic substances, into every form of organized structure. For the white of the egg con- sists of nothing else than albumen, combined with phosphate of lime ; whilst the yolk is chiefly composed of the same substance, mingled with oily matter, and a minute quantity of sulphur, iron, and some other inorganic bodies. Yet this albumen and fatty matter are converted, after the lapse of a few days, under the agency of an elevated tempe- rature upon the germ, into a complex fabric, composed of bones, mus- cles, nerves, tendons, ligaments, cartilages, fibrous membranes, fat, cellular tissue, &c., and endowed with the properties characteristic of all these substances, which, when brought into consentaneous activity, manifest themselves in the life of the chick. — In tracing these wonder- ful transformations, therefore, we should rightly commence with Albu- men. k 108 CHEMICAL COMPOSITION OF ANIMAL TISSUES. 168. Albumen exists, not merely in the "white and yolk of the egg, but also in the various liquids which supply the materials for the nutri- tion of the Animal tissues. Thus it is found in the Chyme, or product of the digestive operation, whenever Animal flesh, or any Vegetable substance corresponding with it in composition, has been taken in as food. And it is absorbed from the alimentary canal into the Chyle and Blood, of whose solid constituents it forms a very large proportion. In its soluble state. Albumen is always combined with a small quantity of free soda, with which it seems to be united as an acid with its base ; and to this state of combination, its solubility is regarded by most Chemists as being due. When the fluid in which it is dissolved is eva- porated at a low temperature (not exceeding 126°), the Albumen, or rather Albuminate of Soda, may be dried, without losing its solubility ; when dried, it may be exposed to a temperature of 212°, without under- going change ; and it forms, when again dissolved in water, the same glairy, colourless, and nearly tasteless fluid as before. . When a higher temperature is employed, however, the Albumen passes into the insolu- ble form ; and presents itself either as a cloudy or flocculent precipitate, or as a firm consistent coagulum, according to the strength of the origi- nal solution. The same condition regulates the amount of heat requisite for the purpose ; thus if the quantity of albumen be so great that the liquid has a slimy aspect, a temperature of 145° or 150° is sufficient for the purpose, and the whole becomes solid, white, and opaque; but in a very dilute condition, boiling is required, and the albumen then sepa- rates in the form of white finely-divided flocks. In either case the soda and other soluble salts are separated from the albumen, and remain dissolved in the water. When the coagulation of Albumen takes place rapidly, the coherent mass seems quite homogeneous, and shows no trace of anything like definite arrangement ; but when the process is more gradual, minute granules present themselves, which do not, however, exhibit a tendency towards any higher form of structure. The insolu- ble coagulum, or pure Albumen, dries up to a yellow, transparent, horny substance ; which, when macerated in water, resumes its former whiteness and opacity. — Pure Albumen may also be obtained from the solid mass which remains when an albuminous fluid is dried at a low temperature, by reducing it to a fine powder, and then washing it with cold water on a filter ; common salt, with sulphate, phosphate, and car- bonate of soda, are dissolved out ; and a soft swollen mass remains upon the filter, which has all the characters of Albumen obtained by precipi- tation, except that it is readily soluble in a solution of nitrate of potash, ■^hich will not dissolve the latter substance. 169. Albumen may be thrown down from its solution, in a coagulated state, not merely by heat, but by Alcohol and Creasote, and by most Acids when added in excess, so as to do more than neutralize the alkali. Nitric acid is particularly efficacious in occasioning its coagulation ; on the other hand. Hydrochloric and Acetic acids, and common or tribasic Phosphoric acid, do not precipitate it, these acids having the property of dissolving pure Albumen. When albumen is dissolved in hydrochlo- ric acid, a pinkish hue is at first seen ; the liquid then becomes of an intense purple colour, and, on applying heat a little longer, an intense ALBUMINOUS COMPOUNDS. 109 blue ; after standing for some time, it again assumes its former pink or claret colour. — In the precipitation of Albumen bj an acid, definite compounds are formed between the two ; in which the Albumen acts the part of a base. On the other hand, it serves as an acid in its combina- tions with the caustic alkalies, and is held in solution by them. Most of the metallic salts, as those of Copper, Lead, Mercury, &c., form- insoluble compounds with albumen, and thus give precipitates with its solution ; hence the value of white of egg as an antidote, in cases of poisoning with Corrosive Sublimate. The simplest method of detecting the presence of soluble Albumen in very small quantity, is to boil the liquid, and add nitric acid ; if turbidity is then produced, the existence of albumen may be inferred. A more delicate test of the presence of Albumen, however, is the precipitate which is given by the addition of the ferro-prussiate of potash to the liquid, when this has been first acidulated with acetic acid. 170. Albumen is readily decomposed by the action of the fixed alka- lies ; a disengagement of ammonia being occasioned by the addition of caustic potass to even a very weak solution of albumen. This may be made evident by adding a solution of sulphate of copper, a deep purple colour being produced by the action of the liberated ammonia upon the metal ; and thus the addition of liquor potassse and a solution of sul- phate of copper, forms a very delicate test for the presence of albumen. If Albumen, or any albuminous compound, be heated with caustic potash, it is completely decomposed; not, however, being resolved at once into its ultimate constituents, or altogether into simple combina- tions of them, but in great part into other organic compounds. One of these, termed Leucin, is a crystalline substance, which forms colourless scales, destitute of taste and odour ; it is soluble in water and alcohol, and sublimes unchanged. It consists of 12 Carbon, 12 Hydrogen, 1 Nitrogen, and 4 Oxygen. There is not at present any evidence that it is produced in the living body ; and the chief interest which attaches to it arises from the fact, that it may be procured from Gelatine as well as from Proteine ; which indicates a certain relationship between these two substances. Another compound abtained by the same reaction, is called Tyrosin; it crystallizes in brilliant needles; and its composition is 16 C, 9 H, 1 N, 5 0. — Besides these substances. Ammonia, with Formic and Carbonic Acids, are produced; the acids unite with the potash emj^loyed to effect the decomposition ; and the ammonia is set free. — The action of caustic potash upon Albumen has also the effect of liberating sulphur, which unites with the potash, and remains in the solution, where its presence may be recognised by the black precipitate formed on the addition of a solution of acetate of lead. The existence of unoxidized Sulphur in albumen is shown by the familiar fact of the blackening of a silver spoon by a boiled egg, which is due to the forma- tion of an alkaline sulphuret during coagulation. Albumen may also be shown to contain Phosphorus in an unoxidized state. In its soluble state. Albumen is very commonly united with Phosphate of Lime, about two per cent, of which may be taken up by it ; and it is chiefly in this mode, that this very important substance is introduced into the Animal body. k I 110 CHEMICAL COMPOSITION OF ANIMAL TISSUES. 171. The ultimate composition of the Albumen of the blood may be stated as follows : — Carbon, - - - . - 548 Hydrogen, - - - - - 71 ^ Oxygen, ----- 212 Nitrogen, ----- 159 Sulphur, ----- 7 Phosphorus, ----- 3 1000 . It has been maintained by Mulder, that the sulphur and phosphorus may be completely separated from the substance formed by the union of the first four elements ; and to this substance, he gave the name of P.roteine, believing it to be the base of the whole series of albuminous compounds, which are supposed to consist of proteine, united with sul- phur and phosphorus in varying proportions. It does not appear, how- ever, that proteine has ever been really obtained in an isolated form ; and its existence must at present be considered as hypothetical. It may be convenient, however, to use the phrase ^' proteine-compounds," to designate the whole series of Animal and Vegetable substances, which are capable of being converted into Albumen in the digestive process : and in this sense alone will it be here employed. — The atomic constitution of Proteine is considered by Liebig to be represented by the formula — 49 Carbon, 36 Hydrogen, 14 Oxygen, 6 Nitrogen. whilst by Mulder the following formula is adopted — 40 Carbon, 31 Hydrogen, 12 Oxygen, 5 Nitrogen. jM Both these formulae are sufficiently conformable to the relative propor- tions of the components ; but it has not been yet determined which best represents the combining equivalent of the substance. 172. Nearly allied to Albumen is the, substance termed Oaseine, i which replaces it in milk ; and this is specially worthy of notice here, because it is the sole form in which the young Mammal receives Pro- teine into its body, during the period of lactation. Like Albumen, this substance may exist in two forms, the soluble, and the insoluble or coa- gulated ; and it further agrees with it, in requiring, as a condition of its solubility, the presence of a free alkali, of which, however, a very small quantity suffices for the purpose. It differs from Albumen, how- ever, in this : that it does not coagulate by heat, and that it is precipi- tated from its solution by Acetic acid. Caseine is further remarkable for the facility with which its coagulation is effected by the contact of certain animal membranes, as in the ordinary process of cheese-making. This change is considered by some Chemists to be due, however, not to any direct action of the membrane upon the caseine, but to its influence in converting some of the milk-sugar into lactic acid, which, separating the alkali of the caseine, will occasion the precipitation of the latter. The only difference which can be detected between Albumen and Case- ine, in regard to the proportions of their elements, consists in the ab- j sence of Phosphorus, and the smaller proportion of Sulphur, in the! latter ; but this can scarcely be the cause of the foregoing differences in J ALBUMINOUS COMPOUNDS. Ill their properties. Caseine appears even to surpass Aljbumen in its power of combining with the phosphates of lime and magnesia, and of render- ing them soluble. 173. Albumen and Caseine, then, may be regarded as constituting the raw materials, at the expense of which the organized tissues of the Animal fabric are built up ; and we have sufficient evidence, in the de-^- velopment of the Chick from the egg, and of the young Mammal from milk, that they may be transformed into any of the proteine-compounds which are to-be found in the Animal body. There is good ground to believe, however, that, for the formation of all the Animal tissues, the presence of fatty matter, as well as of an albuminous compound is es- sential ; and it is a circumstance worthy of note, that in both the fore- going cases, fatty matter is mingled with the albumen, in the aliment destined for the development of the young animal. This subject, how- ever, will be more fully discussed hereafter. 174. The Animal derives the materials of its nutrition, however, not only from the Albuminous compounds furnished by flesh, eggs, and milk, but also from those which are supplied by the Vegetable kingdom. Every growing Plant, as already mentioned (§12), forms albuminous compounds by the combination of inorganic elements, as the pabulum of its own tissues ; but many Plants generate them in much larger proportion, and store them up in their cells; and it is from such, that Animals derive their largest supply of nutritive material. Thus the gluten of wheat, or the tenacious mass which is left after the removal of the starch by wash- ing, is principally composed of a substance which is closely allied in composition and properties to the Albumen of animals ; being moderately soluble by water, coagulated by heat, and precipitated by acids (except the phosphoric and acetic) and by metallic salts ; and this is designated Vegetable Albumen. From the seeds of Leguminous plants a substance termed Legumin may be separated, which corresponds with Caseine in not being precipitated by heat, and also in the absence of phosphorus ; hence it is sometimes designated as Vegetable Caseine, Various other modifications of the albuminous principle are found in Plants ; but the foregoing are the most important in their relations to the nutrition of Animals. Like flesh, cheese, &c., they are reduced by the digestive process to the state of soluble Albumen; and in this form they are taken up, and carried into the circulation. 175. Next in importance to the Albuminous compounds as a consti- tuent of the Animal fabric, is Gelatine; which is obtained by the action of boiling water on White Fibrous tissue^ and on the various membranes, &c., of which this is the chief component. The composition of Gelatine is much simpler than that of the Proteine-compounds, so far, at least, as regards the number of atoms of its several elements ; for it consists of 13 Carbon, 10 Hydrogen, 5 Oxygen, 2 Nitrogen. This composition is the same, whether the Gelatine be obtained from isinglass, from fibrous membranes, or from bones. The distinctive characters of Gelatine are its solubility in warm water, its coagulation on cooling into a uniform jelly, and its formation of a peculiar insoluble compound with Tannic acid. Gelatine is very sparingly soluble in cold water ; though pro- : longed contact with it will cause the Gelatine to swell up and soften. Ik 112 CHEMICAL COMPOSITION OP ANIMAL TISSUES. Its power of forming a jelly on cooling is such, that a solution of one part in 100 of water will become a consistent solid. And its reaction with Tannic acid is so distinct, that the presence of one part of Gelatine in 5000 of water is at once detected by infusion of Galls. There can be no doubt that Gelatine does not exist exactly as such in the Fibrous tissues ; since none can be dissolved out of them by the continued action of cold water, and it usually requires the prolonged action of hot water, to occasion their complete conversion. There afre some substances, how- ever, in which this is not requisite ; and from which the -gelatine may be extracted within a shorter time. This is the case, for example, w^ith the air-bladder of the Cod and other fish; which when cut into shreds and dried, is known as Isinglass. It is the case also with the substance of bones, from which the calcareous matter has been removed. In both instances it would seem that the state of organization is very imperfect ; the fibrous structure being by no means well-marked. When the fibrous arrangement is more complete, the solubility of the tissue is much di- minished. Hence it would seem that the particles have a difi*erent ar- rangement in the tissues, from that which they have in the product obtained by boiling. Their ultimate composition, however, is the same; for when any serous membrane, or other tissue principally composed of the white fibrous element, is analysed by combustion, the elements are found to have the same proportion to each other as in Gelatine, allow- ance being made for the small admixture of other substances. The action of Tannic acid, too, is the same on the organized tissue, as it is on the gelatine extracted from it ; and hence results its utility in pro- ducing an insoluble compound, not liable to undergo decomposition, in the substance of the Skin, converting it into leather. . 176. It is not yet known how Gelatine is produced in the Animal body. There can be no doubt that it may be elaborated from Albumen ; since we find a very large amount of Gelatine in the tissues of young animals, which are entirely formed from albuminous matter ; and also in the tissues of herbivorous animals, which cannot receive it in their food, as Plants yield no substance resembling gelatine. Carnivorous animals, however, will receive it ready formed, as part of their aliment. There is no reason to believe that it is capable of being converted into Albu- men ; and consequently it can never be applied to the nutrition of the albuminous tissues. If Gelatine be boiled for some time in caustic potash, it is decomposed, with an escape of ammonia ; and two new compounds, leucine, and glycoeoll or gelatine-sugar, are generated. The production of leucine from Gelatine, by the action of the same reagent as that which caused its generation from Albumen, is a fact of much importance ; as showing that, notwithstanding their difference of composition and characters, a certain similarity in the arrangement of their ultimfvte elements still subsists between these two bodies. Glycoeoll is an organic base of great interest from its relations to other substances ; as will be shown hereafter. It has a strong sweet taste, and is very soluble in water, from which it may be crystallized like ordinary sugar. Its compo- sition is comparatively simple ; being 4 Carbon, 4 Hydrogen, 1 Nitrogen, 3 Oxygen. 177. A peculiar modification of Gelatine, which presents itself in ALBUMINOUS COMPOUNDS. 113 Cartilage, is distinguished as Cfhondrine. This requires longer boiling than gelatine for its solution in water ; but the solution fixes into a jelly in cooling, and dries by evaporation into a glue that cannot be distinguished from that of gelatine. Like gelatine, it is thrown down from its solution by alcohol, creasote, tannic acid, and bichloride of mercury ; but it is also precipitated by acetic acid, alum, acetate of lead, and protosulphate of iron, which do not disturb a solution of gelatine. — It is curious that, in proportion as Cartilages become fibrous, their Chondrine gives place to Gelatine ; and during the progress of ossification, the Chondrine seem& to be entirely replaced by Gelatine, of which the fibrous basis of the* bony tissue is composed. 178. The foregoing may be considered as the chief among the " raw materials" of the Animal fabric; and it is now to be shown, that while the Gelatinous components merely become subservient to the formation of tissues, whose structure is the simplest possible, and whose function is purely mechanical^ the destination of the Albuminous compounds is much higher ; it being at the expense of the latter that those tissues are generated, which are the instruments of the purely vital operations of the Animal economy. In their progress towards the state of complete organization, however, we find that they pass through an intermediate condition, which is one that requires special consideration. The fluids that are formed at the expense of the Albuminous matters which have been digested, and absorbed, contain a substance, which is so closely related to Albumen in its ultimate Chemical composition, as not to be distin- guishable from it with any degree of certainty,* but which, though still fluid whilst circulating in the living vessels, exhibits a decided tendency to assume the organized form, and manifests properties which are so diff"erent from those of inorganic matter, that they must be regarded as vital. This substance is Fihrine. It is found in the Chyle, or crude blood, soon after this is taken up from the food ; it presents itself in gradually increasing proportion, as the Chyle slowly passes along the Lacteal vessels, and through the Mesenteric glands, towards the termi- nation of the Absorbent system in the Venous ; and it is also found in the fluid contents of that other division of the Absorbent system, the Lymphatics, which is distributed through the body at large, and which seems to have for its chief office to take up, and to reintroduce into the circulating current, such particles contained in the fluids of the tissues, as do not require to be at once cast out of the body, but may be again employed in the process of Nutrition. But it is found, above all, in the Blood ; the fluid whose ceaseless and rapid course through the body sup- plies to every element of the structure the materials of its growth and * According to some analyses, Fibrine differs slightly from Albumen in ultimate com- position, the proportions x)f its several constituents in 1000 parts being as follows ; — Carbon 54G, Hydrogen 70, Oxygen 220, Nitrogen 157, Sulphur 4, and Phosphorus 3. The difference in their vital relations, hovrever, is far greater than any such difference in their chemical composition can account for, and can only be justly attributed to the forces brought to act upon the fibrine during its circulation in the vessels of the living body. It has been recently maintained, that Fibrine is not to be regarded (as here represented) as Albumen in the transition-stage of incipient organization, but that it is a product of the disintegration of the tissues, only received back into the blood in order that it may be carried out of the system through the excretory channels. For a discus- sion of this hypothesis, see the Brit, and For. Med. Chir. Review, vol. vii. pp. 153, 473. 8 114 CHEMICAL COMPOSITION OF ANIMAL TISSUES. development : and the varying proportions in which it presents itself there, are evidently closely connected with the formative powers of that fluid. It is also a principal element of certain colourless exudations^ which are put forth from wounded or inflamed surfaces, or which are deposited in the interstices of inflamed tissues ; these exudations, when possessed of a high formative property (that is, a readiness to produce an organized tissue), are said to be composed of coagulahle or organi- zahle lymph, which is nothing more than the fibrinous element of the blood, in an unusually concentrated state. We shall first notice the Chemical properties of Fibrine ; and shall then inquire into those, which present the first dawnings or indications of Vitality. 179. Like the other Proteine-compounds, Fibrine may exist in solution, or in an insoluble form ; but there is this important dif- ference,— that its soluble form is not a permanent one, and can- not be maintained in any fibrinous fluid that has been drawn from the living vessels, without the influence of reagents, which totally destroy its peculiar properties. All investigations of a Chemical nature, there- fore, must be made upon insoluble Fibrine ; and this may be obtained in its purest state, by whipping fresh blood with a bundle of twigs, by which operation it will be caused, in coagulating, to adhere to the twigs in the form of long, white, elastic filaments, with scarcely an admixture of foreign matter. Wh-en dried in vacuo, or at a gentle heat, it becomes translucent and horny ; and in this condition, it closely resembles coagu- lated albumen. It further resembles that substance, in being soluble in very dilute caustic alkali^ and in phosphoric acid ; and the solutions exhibit many of the properties of the similar solutions of albumen. "When the Fibrine of venous blood is triturated in a mortar with a solu- tion of nitrate of, potash, and the mixture is left for twenty-four hours or more, at a temperature of from 100° to 120°, it becomes gelatinous, slimy, and eventually entirely liquid. In this condition, it exhibits all the properties of a solution of Albumen which has been neutralized by acetic acid. It coagulates by heat ; it is precipitated by alcohol, corro- sive sublimate, &c.; and, when largely diluted, it deposits a flocculent substance, not to be distinguished from insoluble albumen. The close Chemical relation of Fibrine and Albumen is further proved by the ready conversion of the former into the latter in the act of digestion ; Animal flesh, which consists of Fibrine, being reduced to the form of Albumen with the same facility, as the Vegetable compounds which resemble the latter much more closely in the first instance. The Fibrine of arterial blood, however, cannot be reduced to the fluid form by solution with nitre ; and this appears to be due to its oxidized condition ; for in a solu- tion of Venous fibrine in nitre, contained in a deep cylindrical jar, and having its surface freely exposed to the air, a fine flocculent precipitate is gradually seen to form ; and this, when collected, is found to have the properties of arterial fibrine. The Fibrine of Animal flesh agrees with that of venous, rather than with that of arterial blood. Fibrine, like Albumen, unites with acids as a base, forming definite compounds ; and with bases as an acid. It also possesses the property of uniting with the earthy phosphates ; of which from 0*7 to 2*5 per cent, are found in the ash that is left after its combustion. FIBRINE. 115 1 : 80. We see, tlien, that when considered in its simply-Chemical rela- tions, Fibrine does not differ in any essential particular from i\.lbumen ; and that the chief point of obvious variation, is the spontaneous coagu- lation of the former, when it is removed from the living body. There is, however, in the structure of the coagulum itself, a most important difference; for instead of consisting of a homogeneous structureless mass, or of a simple aggregation of minute granules, it is found by the Microscope to possess a definite fibrous arrangement, the fibres crossing one another in every direction. In the ordinary coagulum or clot of Blood, these fibres do not present any great degree of firmness : they may be hardened, however, by boiling ; and their arrangement then becomes more definite. They may be seen much more clearly, however, in the "buffy coat" of Inflammatory blood ; in which there is not only an increased proportion of Fibrine, but the Fibrine itself seems to have undergone a higher elaboration, that is, to have proceeded still further in the change towards regular organization. In this state, the process of coagulation is unusually slow ; the clot formed by the fibrous tissue is much more solid ; and it continues for some hours, or even days, to increase in solidity, by the mutual attraction of the particles composing the fibres, which causes them, to contract and to expel the fluid contained in their interstices. 181. The most perfect fibrous structure originating in the simple coagulation of fibrine, is to be found, however, in those exudations, which take place either from inflammation, or from a peculiar forma- tive action, destined to repair an old tissue or to produce a new one. Fig. 3. Fig. 2. ) 1 Fibrous structure of inflammatory exudation from peritoneum. Fibrous membrane, lining the egg-shell, and forming the animal basis of the shell itself. ' Thus in Fig. 2 is shown the fibrous structure of a false membrane, i formed by the consolidation of a fibrinous exudation from the surface of I an inflamed peritoneum. And in Fig. 3 is displayed a similar fibrous i structure (in which, however, the fibres have more of a reticulated ar- trangement), which incloses the fluid contents of the egg, and enters 'into the composition of the shell itself. As the ovum (which, at the ;time of its quitting the ovarium, consists of the yolk-bag only) passes along the oviduct of the parent, it receives its coating of albuminous 'matter, of which layer after layer is thrown out by the vessels of the oviduct. When a sufficient supply has thus been furnished, it appears 116 CHEMICAL COMPOSITION OF ANIMAL TISSUES. that fibrinous instead of albuminous matter is poured forth ; and this, in coagulating, forms a very thin layer of fibrous tissue, which enve- lopes the albumen., Layer after layer is gradually added ; and at last, by the superposition of these layers, that firm tenacious membrane is formed, which is afterwards found lining the egg-shell. The process is then continued, with this variation, that carbonate of lime is also se- creted from the blood in a chalky state, and its particles lie in the in- terstices of the fibrous network, and give it that solidity which is characteristic of the shell. If they be removed by the agency of a w^eak acid, or if the bird be not sufficiently supplied with lime at the time of laying, the outer membrane has the same consistence as the inner: and either may be separated, after prolonged maceration, by dexterous manipulation, into a series of layers of a fibrous matting^ like that represented in Fig. 3. 182. It is scarcely possible to deny to such a tissue the designation of an organized structure, even though it contains no vessels, and may not participate in any further Vital phenomena. We shall hereafter find, that a tissue presenting very similar characters forms a large part of the Animal fabric ; and that the vessels with which it is copiously supplied, have for their object nothing else than the removal of its dis- integrated or decaying portions, and the deposition of new matter in a similar form (§194). In the production of new parts, we find this simple fibrous tissue performing the important function of serving as a matrix or bed for the support of the vessels ; and as, by the more gra- dual transformation of the nutritive materials they bring, new and more permanent tissues are formed, the original one gradually undergoes dis- integration, and all traces of it are in time lost. This would appear to be the history of the Chorion of the Mammalian ovum ; which is at first nothing else than a fibrous unvascular bag, formed round the ovum in its passage through the Fallopian tube, precisely after the manner of the shell-membrane of the Bird's egg; but which is afterwards pene- trated by vessels proceeding from the embryo, and in time acquires a new structure (chap, xi.) 183. The completeness of the production of such a fibrous tissue depends in part, as we have seen, upon the degree of elaboration which the Fibrine has undergone ; but in great part also upon the na- ture of the surface, on which the coagulation takes place. Thus we never find so perfect a membrane formed by the consolidation of the Fibrine out of the living body, — on a slip of glass for example, — as when it takes place on the surface of a living membrane, or in the in- terstices of a living tissue. This may perhaps be accounted for by the fact, that the coagulation takes place much more slowly in the latter case than in the former, and that the particles tnay thus have more time to arrange themselves in the definite fibrillation, which seems to be their characteristic mode of aggregation: just as crystallization takes place best when the action is slow ; and as a substance, whose particles would remain in an amorphous or disunited form if too rapidly precipitated from a solution, may present a most regular arrangement when they are separated from it more slowly. Of this view it would seem to be a confirmation, that the most perfect fibrillation out of the ^R( COAGULATION OF FIBRINE. 117 dy is usually seen' in those cases, in which coagulation takes place least rapidly. 184. The conditions under which the spontaneous coagulation of Fibrine takes place, are best known from the observation of that pro- cess as it occurs in the Blood ; and although this fluid, as we shall here-;- after see, is of a very complex nature, yet as the Fibrine alone is con- cerned in its coagulation, and as that act appears to take place in the same manner as if no other substance was present, there appears to be no objection to the employment of the phenomena of Blood-coagulation as the basis of our account of the properties of Fibrine. — There can be no doubt, from Microscopical observation of the circulating Blood, that Fibrine is in a state of perfect solution in the fluid ; and in this condi- tion it remains, so long as it is in motion in the living body. That its fluidity, however, does not depend only upon its movement, is evident from two facts ; — first, that no kind of motion seems efi'ectual in pre- venting the coagulation of the blood, after it has been drawn from the vessels ; — and second, that a state of rest within the living body does not immediately produce coagulation ; a portion of blood, included be- tween two ligatures in a living vessel, remaining fluid for a long time ; and blood that has been reduced to a state of complete stagnation by inflammatory action, being often found in a fluid state after some days. On the other hand, it seems certain that the state of vitality of the parts with which the blood is in contact, has a great influence in pre- serving its fluidity ; thus it has been found that, if the brain and spinal cord of an animal be broken down, and by this measure the vitality of the body at large be lowered, clots of blood are formed in their trunks within a few minutes. Nevertheless, a mass of blood eff'used into a cavity of the living body, undergoes coagulation almost as . soon as it would in a dead vessel ; but this may be accounted for by the very small surface which is in contact with the blood, as compared with the mass of the latter. It must be remembered that the circulating blood is con- tinually being subdivided into countless streams ; and that each of these passes through the living tissue, in such a manner that all its particles are in close relation with the living surface. Moreover it is probable that the form of matter which we term Fibrine, never remains long in that condition, in the ordinary state of the system ; being continually withdrawn by the nutritive processes, and as continually re-formed from the Albumen, by an elaborating action hereafter to be considered. Hence we may regard the state of motion through living vessels, as essential to the permanent continuance of fibrine in the fluid form. 185. The length of time, however, during which Fibrine may remain uncoagulated, after it has been withdrawn from the living body, varies according to various conditions ; some of which are not well understood. In the first place, as already remarked, the more elaborated and more concentrated the condition of the Fibrine, the more slowly does it usually coagulate. Thus when a large quantity of blood is drawn, at one bleeding, into several vessels, that which flows first takes the longest time to coagulate, and forms the firmest clot : whilst that which is last drawn coagulates most rapidly, and with the least tenacity. The coagu- lation is accelerated by moderate heat, and retarded by cold ; but it is k 118 CHEMICAL COMPOSITION OF ANIMAL TISSUES. not prevented even by extreme cold ; for if blood be frozen immediately that it is drawn, it will coagulate on being thawed, — thus preserving its vitality, in spite of the freezing process, like the organized structures of many of the lower animals. Again, the coagulation is accelerated by exposure to air ; but it is not prevented, though it is retarded, by com- plete exclusion from it. Various Chemical agents retard the coagula- tion, without preventing it ; this is the case especially with solutions of the neutral salts. The coagulation is not so firm, however, or the fibrillation so perfect, after the use of these ; and there can be no doubt that they modify the properties of the fibrine, by acting chemically upon it. 186. After remaining in this condition for a certain length of time, the Fibrine undergoes a further change, which is evidently the result of decomposition ; the coagulum becomes soft, and exhibits appearances of putrefaction. This takes place the more rapidly, as the first coagula- tion was less complete. Thus in the imperfectly-elaborated Fibrine of the Chyle, the coagulum is sometimes so incomplete, that it does not separate itself from the serum, and liquefies again in half an hour. In certain states of disease, the solidifying properties of the Fibrine are very much impaired ; so that it soon liquefies and decomposes. In these cases, there is scarcely any trace of the characteristic fibrous arrange- ment of the particles. On the other hand, the fibrinous coagulum of inflamed blood, as it is more solid, is also more persistent than that of ordinary blood ; and the greatest persistency of all is seen in the fibrous network formed by exudation, as in the cases just now mentioned. 187. The coagulating power of Fibrine, — in other words, its peculiar vital property, — may be destroyed by various causes operating within the living. body; so that the blood remains fluid ^after death. These may be classed under three heads. In the first place, the vitality of the fibrine may be destroyed by substances introduced into the blood from without ; which have the power of acting in the manner of ferments, and which occasion an obvious chemical change in its condition. Such is the case in those severe forms of Fever which are termed malignant; and especially those which result from the contact of putrescent matter, as Glanders, Pustule maligne, &c. Secondly, it may be impaired or altogether destroyed by morbid actions originating in the system itself, and depending upon irregular nutrition or imperfect excretion ; thus the blood has been found fluid after death in severe cases of Scurvy and Purpura, also in cases of Asphyxia (consequent upon the retention of carbonic acid in the blood), and in the bodies of overdriven animals. The same result may follow, Thirdly, from violent shocks or impressions, which suddenly destroy the vitality of the whole system at once ; these may be such as are obviously capable of producing a chemical or me- chanical change, as in the case of death by Lightning or by a violent Electrical discharge ; or they may act through the nervous system, in a manner not yet clearly understood, as when death results from con- cussion of the brain, from a blow upon the epigastrium, from violent mental emotion, or from a coup de soleil. — It is not to be supposed that the non-coagulability of the Blood is a phenomenon by any means inva- riable under the foregoing circumstances; but it has been occasionally SIMPLE FIBROUS TISSUES. 119 2. Of the Simple Fibrous Tissues. observed in all of them. We must not mistake for the absence of coagu- lating potver, the remarkable retardation of the act of coagulation which ; sometimes occurs. Thus, the blood is occasionally found in a fluid con- dition in the bodies of persons that have been dead for some days ; and yet when withdrawn from the vessels, it coagulates. I ■^f 188. A large part of the Animal fabric, especially among the higher r classes in which the parts have the greatest amount of motion upon one another, is composed of tissues, which seem as if they consisted of nothing else than fibres, of the simple character already described, woven together in various ways, according to the purposes they are I destined to serve. These fibres are altogether different from those • hereafter to be described as constituting the Muscular and Nervous tis- sues, and must not be confounded with them. The former are solid, and possess none but physical properties ; the latter are tubular, and are distinguished by their peculiar vital endowments, which seem chiefly, if not entirely, to reside in the contents of the tubular fibre. The Sim- ple Fibrous tissues, of which we have now to treat, appear to have it for their sole office in the animal body to bind together the other elementary parts into one whole, without uniting them so closely as to render them immovable ; and we find the same elements arranged in very different modes, according to the purposes they are destined to fulfil. Thus in the Tendons, by which the muscles are connected with the bones, and impart motion to them, the only property required is that of resisting strain or tension in one direction ; and in these we find the fibres dis- posed in a parallel arrangement, passing continuously in straight line between the points of attachment. In the Ligaments which connect the bones together, and which also have for their purpose to afford resistance to strain, but which are liable to tension in a greater variety of directions, we find bundles of fibres crossing each other according to these directions ; and in some instances we find the ligaments endowed also with a certain degree of elasticity. The structure of the strong Fig. 4. Simple fibrous tissue; a, fibres of areolar tissue; 6, tendinous fibres. Fibrous Membranes, which form the envelopes to different, organs and bind together the contained parts, is very similar ; each of these mem- branes being composed of several layers of a dense network, formed by I 120 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. the interweaving of bundles of fibres in different directions. In the FihrO'CartilageSj we find a mixture of the characteristic structure of Ligament with that of Cartilage ; bundles of fibres, similar to those which constitute the former, being disposed among the cells which are the chief organized constituents of the latter. In certain Eibro-Carti- lages, however, these fibres are endowed with a high degree of elasticity. 189. These two qualities, — that of resistance to tension without any yielding — and that of resistance combined with elasticity, — are charac- teristic of two distinct forms of Fibrous tissue, the ^Vhite and the Yellow. The White Fibrous tissue presents itself under various forms, being some- times composed of fibres so minute as to be scarcely distinguishable ; and sometimes presenting itself under the aspect of bands, usually of a flattened form, and attaining the breadth of l-500th of an inch. These bands are marked by numerous longitudinal streaks, but they cannot be torn up into minute fibres of determinate size ; hence they must be re- garded as made up of an aggregation of the same elements as those which liiay become developed into separate fibres. The fibres and bands are occasionally somewhat wavy in their direction. The tissue, which is perfectly inelastic, is easily distinguished from the other by the effect of Acetic acid, which swells it up, and renders it transparent, at the same time . bringing into view certain oval corpuscles, which are supposed to be the nuclei of the cells that were concerned in the formation of the tissue. 190. The Yellow Fibrous tissue exists in the form of long, single, elastic, branched filaments, with a dark decided border, which are dis- posed to curl when not put on the stretch. They are for the most part between l-5000th and l-10,000th of an inch in diameter ; but they are often met with both larger and smaller. They frequently anastomose, so as to form a network, as shown in Fig. 6. This tissue does not un- Fig. 6. Fig. 6. Fasciculus of fibres of white fibrous tissue; from lateral ligament of knee joint. Yellow fibrous tissue from ligamentum nuchns; a, the fibres drawn apart, to show their reticulate arrangement; 6, the fibres in situ. dergo any change, when treated with acetic acid. It exists alone (that is, without any mixture of the white) in parts which require a peculiar elasticity, such as the middle coat of the Arteries, the Chordae Vocales, the Ligamentum Nuchas (of Quadrupeds) and the Ligamenta subflava ; it enters largely into the composition of certain parts, which are com- SIMPLE FIBHOUS TISSUES. 121 monly regarded as Cartilaginous, such as the external ear; and it is also a principal component of other tissues to be presently described. 191. These tissues are very different in Chemical composition. Those which are composed of the White fibrous element, — namely, Tendons, Ligaments, &c. — are almost entirely resolved by long boiling into G-ela- tine; and this substance is also largely obtained from the Skin, and from Mucous and Serous membranes, of which, as we shall presently see, that element is a principal component ; whilst it is also yielded in great quantity by Bones, whose animal basis is almost entirely gelatinous. 192. The composition of the Yellow fibrous tissue appears to be alto- gether dissimilar. It scarcely undergoes any change by prolonged boil- ing ; it is unaffected also by the weaker acids ; and it preserves its elasticity, if kept moist, for an almost unlimited period. According to : Scherer, it consists of 48 Carbon, 38 Hydrogen, 6 Nitrogen, and 16 Oxygen ; and he considers it to be composed of an atom of Proteine with two atoms of water. (See § 171.) 193. The simple Fibrous tissues appear to be 1 very little susceptible of change in the living body ; and we find them very sparingly supplied with blood-vessels. In the solid Tendons, the bundles of straight parallel fibres are a little separated from each other by the intervention of the Areolar tissue to be presently noticed ;. and this permits the sparing access of vessels to their interior. In ■ the Fibrous Membranes and Ligaments, this is ! found in somewhat larger amount ; and the vascu- larity of these tissues is rather greater. Two dif- : ferent views of their mode of development have I been taken by those who have studied it. By some it has been maintained that the White fibres are first developed as cells, which progressively be- come elongated and solidified ; their nuclei at the j same time disappearing, until brought into view by acetic acid (Fig. 7) ; and the Yellow fibres have j been supposed to have a similar origin. By others I it has been considered that the White fibres are produced by the direct i fibrillation of a blastema or plastic exudation (§ 181), and that the Yellow I proceed from the nuclear particles which this contains; no development j of cells being requisite for the production of either. The recent inqui- ries of Mr. Paget and others tends to show that both these methods are I adopted in the production of the fibrous tissue which is developed for ' the repair of injuries in the adult body ; the former being seen in the . reparation of external wounds to which air has access ; the latter in the 'organization of "coagulable lymph" effused into internal cavities, or into the interstices of tissue, altogether secluded from it. 194." The great use of the foregoing tissues appears to be, to afford a ! firm resistance to tension ; by which they may either communicate motion, j a$ in the case of Tendons ; or restrain it, as in thfe case of Ligaments ; i or altogether prevent it, as in the case of Aponeuroses and Fibrous i Membranes. With this firm resistance, a considerable amount of elas- Development of fibres from cells: — a, circular or oval nu- cleated cells; 6, the same be- coming pointed; c, the same become fusiform, the nuclei being still apparent; d, the same elongating into fibres, the nuclei having disappeared. 122 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. ticity may be combined. But we have now to notice a tissue, in which a v«ry different arrangement of the same elements presents itself; and the object of this is, to bind together the elements of the different fabrics of the body, and at the same time to endow them with a greater or less degree of freedom of movement upon one another. The tissue, which is called the Areolar, consists of a network of minute fibres and bands, which are interwoven in every direction, so as to leave innumerable areolce or little spaces, which communicate freely with one another. Of these fibres, some are of the yellow or elastic kind ; but the majority are composed of the white fibrous tissue, and, as in that form of ele- mentary structure, they frequently present the form of broad flattened bands, or membranous shreds, in which no distinct fibrous arrangement is visible. The interstices are filled during life with a fluid, which resembles very dilute serum of the blood ; consisting chiefly of water, but containing a sensible quantity of common salt and albumen. This tissue (which has been frequently but erroneously termed Cellular) is very extensible in all directions, and very elastic, from the structural arrangement of its elements. It cannot be said to possess an}^ dis- tinctly vital endowments ; for although it has a certain amount of sensi- bility, this merely depends upon the presence of nerves which it is con- veying to other parts ; and the small amount of contractility which it shows, depends rather upon the muscular tissue of the vessels that tra- verse it. 195. As already mentioned, we find this tissue in almost every part of the body ; thus it binds together the ultimate fibres of the Muscles into minute fasciculi, unites these fasciculi into larger ones, these again into larger ones which are obvious to the eye, and these into the entire muscle. Again it forms the membranous septa between distinct mus- cles, or between muscles and fibrous aponeuroses. In like manner it unites the elements of nerves, glands, &c. ; binds together the fat-cells into minute bags, these into larger ones, and so on ; and in this manner •penetrates and forms a considerable part of all the softer tissues of the body. But it is a great mistake to assert, as it was formerly common to do, that it penetrates the harder organs, such as bones, teeth, carti- lage, &c. Its purpose obviously is, to allow a certain degree of move- ment of the parts which it unites ; and hence we find it entering much more largely into the composition of the Mammary gland (w^hich, from its attachment to the great pectoral muscle, must have its parts capable of being shifted upon one another), than into that of the Liver, Kid- neys, &c. It also serves as the bed, in which blood-vessels, nerves, and lymphatics may be carried into the substance of the different organs ; and it often undergoes a degree of condensation, in order to form a sheath for the larger trunks, which gives it . almost the characters of a Fibrous Membrane. 196. The quantity of fluid in the interstices of Areolar tissue is sub- ject to considerable variations; but these depend rather upon the state of fulness or emptiness of the vessels which traverse it, and upon the condition of the walls of those vessels, than upon any change in the tissue itself. It has been shown that, when an albuminous fluid is in contact with an animal membrane, the watery part of the fluid will pass AREOLAR TISSUE. 123 through by transudation ; but that the albuminous matter will be for the most part kept back, so that only a very small proportion of it is to be found in the transuded liquid. This appears to be a sufficient ex- planation of the presence of a weak serous fluid in the cavities of areolar tissue ; and there is not any necessity, therefore, to imagine the exist- ence of a secreting power, either in the areolar tissue itself, or in the walls of the capillaries which traverse it. When there is a want of firmness or tone in the walls of the vessels, producing (as we shall here- after see, § 609) an increased pressure of the contained fluid on their walls, and diminished resistance, the watery part of the blood will have an unusual tendency to transudation : and we accordingly find that it then distends the areolae, and produces dropsy. The physical arrange- ment of the parts of the tissue is so much altered, that its elasticity is impaired ; and it consequently p)its on pressure, — that is, when pressure has made an indentation in the surface, this is not immediately filled up when the pressure is withdrawn, but a pit remains for some seconds or ven minutes. The free communication which exists among the inter- tices, is shown by the influence of gravity upon the seat of the dropsi- al efi'usion ; this always having the greatest tendency to manifest itself the most depending parts, — a result, however, which is also due to he increased delay that takes place in the circulation in such parts, hen the vessels are deficient in tone. This freedom of communication fcis still more shown, however, by the fact, that either air or water may e made to pass by a moderate continued pressure, into almost every art of the body containing Areolar tissue ; although introduced at only single point. In this manner it is the habit of butchers to inflate veal : and impostors have thus blown up the scalps and faces of their children, in order to excite commiseration. The whole body has been thus distended with air by emphysema in the lung; the air having escaped from the air-cells into the surrounding areolar tissue, and thence, by continuity of this tissue with that of the body in general at the root or apex of the lungs, into the entire fabric. 197. The structure of the Sey^oiis and Synovial Membranes is essentially the same with that of Areolar tissue. It is the peculiar character of these mem- branes to form closed bags or sacs, having a very Smooth and glistening inner surface, and containing a fluid more or less allied in composition to the serum of the blood. The disposition of the Syno- vial membranes may be understood by studying one of the simpler forms of the joints, such as is repre- sented in the accompanying diagram; but although originally continuous over the surfaces of the Articu- lar Cartilages, the Synovial membrane does not con- tinue to be distinctly traceable after the joint has come into play, and its vessels retreat from the portion over brams covering the arti \ • -t •, o ^ culated surfaces, and wiucn the two surfaces are exposed to friction, but passing from one to the form a circle round its margin, from which the Carti- ''*^^''' lage is nourished (§ 278).~The arrangement of the Serous membranes Fig. 8. Ideal seijtion of a Joint ; — a, a, the extremities of the two articulated bones; b, b, the layers of carti- lage which cover them; c, c, c, c, synovial mem- 124 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. is usually much more complicated. These line the three great cavities of the body, — the head, chest, and abdomen, — together with their sub- divisions ; enveloping the viscera which these contain, so as to afford them an external coating over every part save that by which they are suspended ; and being then reflected over the interior of the cavity, so as to form a shut sac intervening between its outer w^alls and its contents. The chief purpose of this appears to be, to facilitate the movements of the contained organs, by forming smooth surfaces which shall freely glide over each other ; this is evidently of great importance, where such constantly moving organs as the heart and lungs are concerned. 198. The free or unattached surface of these membranes is covered with a layer of cells; but these constitute a distinct tissue, the Epithe- lium^ of which an account will be given hereafter. The epithelium lies upon a continuous sheet of membrane, of extreme delicacy, in which no definite structure can be discovered ; the nature of this, which is called the basement or primary membrane^ will be presently considered (§ 206). Beneath this is a layer of condensed Areolar tissue, which constitutes the chief thickness of the serous membrane, and confers upon it its strength and elasticity ; this gradually passes into that laxer variety, by which the membrane is attached to the parts it lines, and which is commonly known as the sub-serous tissue. The yellow fibrous element enters largely into the composition of the membrane itself; and its fila- ments interlace in a beautiful network, which confers upon it equal elas- ticity in every direction. The membrane is traversed by blood-vessels, nerves, and lymphatics, in varying proportions ; some of the synovial membranes, especially that of the knee-joint, are furnished with little fringe-like projections, which are extremely vascular, and which seem especially concerned in the secretion of the synovial fluid. The fluid of the serous cavities is so nearly the same as the serum of the blood, that the simple act of transudation is sufficient to account for its presence in their sacs ; on the other hand, that of the Synovial capsules, and of the Bursas Mucosae which resemble them, may be considered as serum with from 6 to 8 per cent, of additional albumen. 199. The elements of Areolar tissue enter largely also into two other textures, which perform a most important share in both the Organic and the Animal functions ; — ^namely, the Mucous Membranes and the Skin. These textures are continuous with each other ; and may, in fact, be con- sidered as one and the same, modified in its different parts according to the function it is destined to perform. Thus it is everywhere extremely vascular ; but the supply of blood in the Skin is chiefly destined for the nervous system, and is necessary to the act of sensation ; whilst that of the internal skin or Mucous Membrane is rather subservient to the pro- cesses of absorption and secretion. This tissue is continued inwards from the external surface of the body, by the several orifices and outlets of its cavities ; and it is further continued most extensively from its primary internal prolongations, into the inmost recesses of the glandular structures. 200. Thus the G-astro-intestinal mucous membrane commences at the mouth, and lines the whole alimentary canal from the mouth to the anus, where it again becomes continuous with the skin ; and it sends off as ^^&rar SKIN, AND MUCOUS MEMBRANES. * 125 ranches, the membranous linings of the ducts of the salivary glands, pancreas, and liver ; these membranes proceed into all the subdivisions of the ducts, and line the ultimate follicles or caeca in which they ter- minate. Again, the Bronchio-pulmonary mucous membrane commences at the nose, and passes along the air-passages, down the trachea, through- the bronchi and their subdivisions, to line the ultimate air-cells of the lungs ; communicating in its course with the gastro-intestinal. Another mucous membrane of small extent commences at the puncta lachrymalia, lines the lachrymal sac and the nasal duct, and becomes continuous with the preceding. Another, which may be considered a kind of offset from either of the first two, passes up from the pharynx along the Eustachian tube, and lines the cavity of the tympanum. 201. Near the opposite termination of the alimentary canal, more- over, we have the Gienito-urinary mucous membranes ; these commence in the male by a single external orifice, that of the urethra ; — passing backwards along the urethra, the genital division is given off, to line the seminal ducts, the vesiculae seminales, the vasa deferentia, and the secre- ting tubuli of the testis ; another division proceeds along the ducts of the prostate gland, to line its ultimate follicles, and another along the I ducts of Cowper's glands ; whilst the urinary division lines the bladder, passes up along the ureters to the kidney, and then becomes continuous with the membrane of the tubuli uriniferi. In the female, the urinary division commences at once from the vulva ; whilst the genital passes along the vagina into the uterus, and thence along the Fallopian tubes to their fimbriated extremities, where it becomes continuous with the serous lining of the abdominal cavity, the peritoneum. 202. Besides the glandular prolongations here enumerated, there are many others, both from the internal and external surface. Thus we have the Mammary mucous membrane, commencing from the orifices of the lactiferous ducts, passing inwards to line their subdivisions, and forming the walls of the ultimate follicles. In the same manner the Lachrymal mucous membrane is prolonged from the conjunctival mucous membrane, which covers the eye and lines the eyelids, and which is con- tinuous with the skin at their edges. There are several minute glands, again, in the substance of the skin, and in the walls of the alimentary canal, which need not be here enumerated; but which contribute immensely to the extension of the surface of the mucous membrane, a prolongation of this being the essential constituent in every one. In their simplest form, these glandulae are nothing more than little pits or depressions of the surface ; these are found both in the Skin and Mucous membrane, and are particularly destined for the prouduction of their protective secretions, hereafter to be described. 203. We have seen, then, that the essential character of the Mucous membranes, as regards their arrangement, is altogether different from that of the serous and synovial membranes. For whilst the latter form shut sacs, the contents of which are destined to undergo little change, the former constitute the walls of tubes or cavities, in which constant change is taking place, and which have free outward communications. Thus in the gastro-intestinal mucous membrane, we have an inlet for the reception of the food, and a cavity for its solution, the walls of which 126 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. are endowed in a remarkable degree with absorbing power, whilst they are also furnished with numerous glandulse, which pour the solvent fluid into the cavity. On the other hand, it has an outlet, through which the indigestible residuum is cast forth, together with the excretions from the various glands that pour their products into the alimentary tube. In the bronchio-pulmonary apparatus, the same outlet serves for the introduc- tion and for the expulsion of the air ; and here, too, is continual change. In other cases, there is but a single outlet ; and the change is of a simpler character, consisting merely in the expulsion of the matters eliminated from the blood by the agency of the glands. Now it is, as we shall see hereafter, im the digestion and absorption of food, on the one hand, and in the rejection of effete matters on the other, that the commencement and termination of the nutrient processes consist ; and these operations are performed by the system of Mucous membranes, including in that general term the Skin, which is an important organ of excretion, besides* serving as the medium through which sensory impressions of a general character are received by the Nervous system. 204. The Mucous Membrane may be said, like the Serous, to consist of three chief parts ; — the epithelium or epidermis covering its free sur- face ; — the subjacent basement-membrane ; — and the areolar tissue, with its vessels, nerves, &c., which forms the thickness of the membrane, and connects it with the subjacent parts. The Epidermis and Epithelium alike consist of cells ; but the function of the former (which consists of several layers, of which the outer are dry and horny) is simply protec- tion to the delicate organs beneath ; whilst that of the latter is essen- tially connected with the process of Secretion, as will be shown hereafter. The basement-membrane resembles that of the serous membranes ; but its separate existence is unusually evident in some parts where it exists alone, as in the tubuli uriniferi of the kidney ; whilst it can with diffi- culty be demonstrated in others, as the skin. The Areolar tissue of Mucous membranes usually makes up the greatest part of their thick- ness ; and it is so distinct from that of the layers beneath, constituting the sub-mucous tissue, as to be readily separable from them. It differs not in any important particular, however, from the same tissue else- w^here ; and the white and fibrous elements may be detected in it in varying proportions, in different parts, — the latter being especially abundant in the skin and lungs, which owe to it their peculiar elas- ticity. Hence the Mucous membranes yield Gelatine in abundance, on being boiled. The skin also appears to contain some of the non-striated Muscular fibre {§ 337), in varying proportions in its different parts. 205. The relative amount of Blood-vessels, Nerves, and Lymphatics, as already mentioned, is subject to great variation, according to the part of the system examined. The first, however, are most constantly abundant, being required in the Skin for sensation (Fig. 9), and in the Mucous membranes for absorption and secretion (Figs. 10, 11, 12). In fact we might say of many of the mucous membranes, especially those of the glands, that their whole purpose is to give support to the secreting cells, and to convey blood-vessels into their immediate neighbourhood, whence these cells may obtain materials for their development. The Skin is the only part of the whole system which is largely supplied with BASEMENT OR PRIMARY MEMBRANE. 127 Nerves (Fig. 13), except the Conjunctival membrane and the Mucous membrane of the mouth and nose ; hence the sensibility of the internal Fig. 9. Fig. 10. Distribution of Capillaries at the surface of the skin of the finger. Distribution of Capillaries in the Villi of the Intestine. mucous membrane is usually low, although its importance in the organic functions is so great. The Skin is copiously supplied with Lymphatics ; Fig. 11; Fig. 12. Distribution of Capillaries around follicles of Mucous Membrane. Distribution of Capillaries around the follicles of Parotid Gland. and the first part of the alimentary canal with Lacteals ; some of the glandular organs are also largely supplied with Lymphatics. — The Fig. 13. Distribution of the tactile nerves at the extremity of the human thumb, as seen in a thin perpendicular section of the skin. Areolar tissue, whether existing separately, or forming a part of the Serous and Mucous Membranes, is capable of being vgry quickly and completely regenerated ; indeed, we often find that losses of substance in other tissues are replaced by means of it. ■^K 3. Of the Basement or Primary 'Membrane. f ^206. In many parts of the Animal body, we meet with membranous : expansions of extreme delicacy and transparency, in which no definite 128 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. Structure can be discovered ; and these seem, like the simple fibres already described, to have been formed, rather directly from the nutri- tive fluid, than indirectly by any previous process of transformation. Hence we may regard such membranes and fibres, as constituting the most simple or elementary forms of Animal tissue. The characters of membranes of this kind were first pointed out by Mr. Bowman and Prof. Goodsir ; by the former of whom it was termed baseinent-mem- brane, as being the foundation or resting-place for the epithelium-cells which cover its free surface (§ 231) ; whilst by the latter it was termed the primary membrane, as furnishing the germs of those cells. These terms appear equally appropriate, and may be used indifferently. — In its very simplest form, the basement-membrane is a pellicle of such extreme delicacy, that its thickness scarcely admits of being measured ; it is to all appearance perfectly homogeneous, and presents not the slightest trace of structure under the highest powers of the microscope, appearing like a thin film of coagulated gelatine. Examples of this kind may be easily procured, by acting upon the inner layer of any bivalve shell with dilute acid ; this dissolves away the calcareous matter, and leaves the basement-membrane. In other cases, however, the membrane is not so homogeneous ; a number of minute granules being scattered, with more or less of uniformity, through the transparent substance. And we not unfrequently find, in place of these uniformly distributed granules, a series of distinct spots, arranged at equal or variable distances, and in different directions, as shown in Figure 14. Moreover, the membrane thus constituted is disposed to break up into portions of equal size, each of which contains one of these spots ; whilst in the more homo- membraSeV tleh^^Z. goncous forms prcviously described, we find no such uZti^^tl lirj^ tendency, no appearance of any definite arrangement spots, or nutritive centres j^eiuff perceptible whou thcv are torn. — Hence it would diffused over it. -c ^ n i • ^ n ^ t seem as it the first and simplest lorm were produced by the simple consolidation of a thin layer of homogeneous fluid ; the second, by a layer of such fluid, including nuclear granules ; and the third, by the coalescence of flattened cells, whose further development had been checked, but whose nuclei continue to perform their peculiar ' functions (§ 212). We find the primary membrane, under one or other of these forms, on all the free surfaces of the body, beneath the epithe- lial or epidermic cells. Thus, as already mentioned, it constitutes the outer layer of the true Skin ; it lines all the cavities formed by Mucous membranes, and is prolonged into all the ducts and ultimate follicles and tubuli of the Glands which are connected with them (§ 199) : indeed it may be said in many instances to be the sole constituent of the walls of these follicles and tubuli, the subjacent tissue not being continued to their finest ramifications. Again, it forms the innermost layer of the Serous and Synovial membranes ; and it also lines the blood-vessels and ^ lymphatics, forming the sole constituent of the walls of their minutest ■ divisions. P 207. In every one of these cases, we find the free aspect of the Pri- BASEMENT OR PRIMARY MEMBRANE. 129 mary Membrane in contact with cells, which form a more or less conti- nuous layer upon its surface. These cells can only receive their nutri- ment by the imbibition of fluid, through the primary membrane, from the blood brought to its attached surface, by the capilLary vessels of the tissue with which it is in relation. Thus in the Skin and Mucous_ membranes, a very copious supply of blood is brought to the attached surface of the primary membrane, by the minutely-distributed capilla- ries which form a large part of the subjacent tissue ; and it is from these that the epidermis and epithelium draw their nourishment, through the primary membrane. In like manner, the ultimate follicles and tubuli of the Glands are surrounded by a copious network of capillaries (Fig. 12) ; and it is from these, through the primary membrane, that the cells of these follicles draw their nourishment. Hence this membrane, in every instance, forms a complete septum, on the one hand between the stream of blood in the vessels and the surrounding tissues, since it forms the lining even of the minutest capillaries ; and on the other between the fluids in the interstices of the substance of the true skin, the mucous membranes, &c., and the cells covering their free surfaces. It is evi- dent, therefore, that whilst bounding these tissues and restraining the too-free passage of fluids from their surfaces, it allows the transudation of a sufficient amount for the nutrition of the cells which lie upon it ; and, as we shall presently see, these cells frequently pass through all their stages of growth so rapidly, that a very free supply of nutriment must be required by them. Hence, notwithstanding its apparent homo- geneousness, the primary membrane must have a structure which readily admits the passage of fluid. In this respect it corresponds with the membrane, which forms the wall of the cells of both Animal and Vege- table tissues ; for this also appears completely homogeneous and struc- tureless, when seen under its simplest aspect, and yet allows the free passage of fluids from one cell to another. 208. But it is probable that this membrane performs a much more important office than that of simply limiting the fluids, whilst allowing the requisite transudation. We can scarcely account for the new pro- duction of cells, which (as will presently appear) is continually taking place on its surface, without referring to it as the originator of these cells, — that is, as the source of their germs. The new generations of cells cannot here be developed by the reproductive powers of the old ones (§ 212) ; since the latter are often completely cast ofi" entire, before they liberate the reproductive granules ; or they undergo changes which evidently unfit them for such a purpose.. Thus in the Epidermis we shall find that they become flattened into dry scales, forming an almost horny layer on the surface of the body ; whilst the new cells are origina- ting beneath, from the surface of the basement-membrane (§ 224 and Fig. 20). Hence we cannot find any other origin for 4:hese cells, than in the basement-membrane itself; and there seems every probability that the granules, which have been mentioned as being frequently diffused through it, are in reality the germs of cells to be developed frdm its surface ; whilst the distinct spots are collections of similar granules, each of which may give origin to a large number of such cells, which spring from them as from a centre. We shall presently see that these " germinal centres" 9 130 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. closely resemble the nuclei of cells in general, from which it is unques- Fig. 15. tionable that the new crops of cells may arise (§ 212). The only difference is, that in the latter case, the groups of new cells are for a time contained within the parent-cell (Fig. 18) ; whilst in the former they are developed on the free surface of the basement-mem- brane. In Eig. 15 is shown a portion of the same Component cells of membrane as that represented in Fig. 14 ; but havinsj primary membrane, t^ .^. ,., O with adherent epithe- bccn rendered transparent by acetic acid, its real nature as a layer of flattened nucleated cells is more obvious ; the nucleus or germinal spot of the central cell has given origin to a cluster of oval epithelial cells, of which five still adhere to it. 209. Hence we are probably to regard this primary or basement-mem- brane as a transitional^ rather than as a peTtnanejit structure ; and to look upon it as furnishing the germs of all the cells, which are developed upon its surface ; as well as serving for the medium, through which they are supplied with nutriment. It must be continually undergoing disin- tegration, therefore, on its free surface : and must be as continually renewed, at the side in relation with the blood-vessels. 4. Of Simple Isolated Cells, employed in the Organic Functions. 210. 'The active functions of the Animal body are performed, to a much greater extent than was until lately believed, by the agency of simple isolated cells ; of which every one grows and lives quite inde- pendently of the rest, just as if it were one of the simplest Cellular Plants (§ 30) ; but of which all are dependent upon the general nutritive fluid for the materials of their development, imbibing it from the cur- rents that circulate in their neighbourhood. It may be said, indeed, that all the Vegetative functions of the body, — all the processes of Nutrition and Reproduction, — all those operations, in short, which are common to Plants and Animals, — are performed in the Animal and Vegetable structures by the very same means, the agency of cells ; and this is true, not only of the healthy actions, but of various morbid operations, in which the unusual development of cells, possessing peculiar endowments, performs a most conspicuous part. Hence it will be neces- sary to enter somewhat at large into the history of cell-development in the Animal body : and the various modifications under which this process may take place. In fact, a knowledge of the Physiology of Cells may be regarded as the foundation of all 'accurate acquaintance with that department of the Science, which relates to the Nutritive and Repro- ductive processes ; and it has a considerable bearing, as we shall see hereafter, upon the history of the purely Animal functions. 211. The history of the Animal cell, in its simplest form, is essen- tially that of the Vegetaljle cell of the lowest kind : excepting in so far as it is dependent for its nutriment upon organic compounds previously elaborated, instead of generating these for itself. It lives for itself and hy itself ; and is dependent upon nothing but a due supply of nutri- ment, and of the appropriate stimuli, for the continuance of its growth and for the due performance of all its functions, until its term of life be expired. In whatever method it originates (and we shall presently see SIMPLE ISOLATED CELLS. 131 that the life of an independent cell may commence in variotis modes), it attracts to itself, assimilates, and organizes, the particles of the nutrient fluid in its neighbourhood ; converts some of them into the substance of its cell-wall, whilst it draws others into its cavity ; in this manner the cell gradually increases in size ; and whilst it is itself approaching the term of its life, it may make preparation for its renewal, by the deve- lopment of reproductive particles in its interior, which may become the germs of new cells, when set free from the cavity of the parent. In the interior of most Animal cells, usually attached to some part of their wall, is seen a collection of granular matter, which is called the nucleus (Fig. 16 a). This appears to be the centre of the vital forces of the cell ; being the part through which it specially exerts its agency on the sub- stances brought under its influence ; and being also the chief instrument in the reproductive operation. There is reason to believe, indeed, that t clear bodies may exert their vital power, and may efi'ect the transfor- Fig. 16. "O^^ /u^l^ 1 1 1 J Vf \^'^^:^:/\ Cells from chorda dorsalis of Lamprey, a, a, nuclei. mation or new arrangement of organic compounds, without the forma- tion of a cell-membrane ; the purpose of the latter, indeed, being appa- rently to bound or limit the substances drawn together by the nucleus, and to cut them ofi" from others in their neighbourhood. When the for- mation of a cell is complete, and it is not destined to reproduce its kind, the nucleus frequently disappears ; this is the case, for example, with the Red corpuscles of the Blood of Mammalia (§ 215), and also with Fat- cells (§ 257). — So far as is yet known, however, the composition of the cell-wall is everywhere the same ; being that of Proteine. It is in the nature of the contents of the cell, that (as among the cells of Plants) the greatest diversity exists ; and we shall find that the purposes of the dif- ferent groups of cells, in the Animal economy, depend upon the nature of the products they secrete, and upon the mode in^ which these products are given back, after they have been subjected to the action of the cells. 212. New cells may originate in one of the two principal modes :• either directly from a pre-existing cell ; or by an entirely new production in the midst of an organizable blastema. The development of new cells from a pre-existing cell, again, may take place in one of two modes ; either by the subdivision of the parent-cell, or by the production of a number of new cells in its interior ; the nucleus^ in each case, appearing to perform an important part in the process. Of the multiplication of cells by subdivision, we have a characteristic example in the growth of Cartilage, which repeats in adult age the process by which the develop- ment of the " germinal mass" takes place at the earliest period of embry- 132 STRUCTURte AND ENDOWMENTS OF ANIMAL TISSUES. onic life (chap, xi.) The process of subdivision seems to commence in the nucleus, which tends to separate itself into two equal parts ; and each of these draws around it a portion of the contents of the cell, so that the cell-wall, which is at first merely inflected inwards by a sort of hour-glass contraction, at last forms a complete partition between the two halves of the original cavity (Fig. 17, a-d). The process of subdi- vision may be again repeated, either in the same or in a contrary direc- tion, 80 as to produce four cells, either linearly arranged (f, g, h), or clustered together (e) ; and this duplication may take place until a large mass has been produced by the subdivision of a single original cell. — In Fig. 17. Multiplication of Cartilage-cells by duplication:— a, original cell ; b, the same beginning to divide; c, the same sho-wing complete division of the nucleus ; D, the same with the halves of the nucleus separated, and the cavity of the cell subdivided ; e, continuation of the same process, with cleavage in contrary direction, to form a cluster of four cells ; F, a, h, production of a longitudinal series of cells, by continuation of cleavage in the same direction. other cases, however, the nucleus appears to break up at once into several fragments, each of which may draw around it a portion of the contents of the parent-cell, which becomes invested by a cell-wall of its own ; and thus the cavity of the parent-cell may at once become filled with a whole brood of young cells, without any successive subdivision. Of this pro- cess we frequently have examples in the case of morbid growths, in which the multiplication of cells often takes place with great rapidity (Fig. 18). Fig. 18. Parent-cells, a, a, of cancerous structure, containini; secondary cells, b, b, each having one, two, or three, nuclei, c, c. Generally. speaking, the former method seems to prevail in structures which have a comparatively permanent destination ; whilst the latter is adopted in cases in which the life of the cells thus generated is but transi- t SIMPLE ISOLATED CELL^. 133 tory, or in which they are not destined to reproduce themselves. Thus the follicles of Glands (§ 238) are but parent-cells, in whose wall an opening has been formed for the liberation of the cells of the new gene- ration (which are the real instruments of the secreting process) as fast as they are formed; and from the nuclei or "germinal spots" of these — parent-cells, which occupy the blind extremity of the follicles, successive crops of young cells are generated, at the expense of the fresh materials which the nuclei are continually drawing from the blood. So the nuclear particles scattered through the "basement-membranes" (§ 208), probably give origin to the epithelial cells developed upon their free surfaces ; even though these nuclei have never been themselves included within cells. 213. In the production of cells de novo in the midst of an organiza- ble blaste7na, or plastic exudation, we cannot trace with the same dis- tinctness the instrumentality of pre-existing cells. This blastema, when first effused, presents the appearance of a homogeneous, semi-fluid, sub- stance ; as it solidifies, however, it becomes dimly shaded by minute dots ; and as it is acquiring further consistence, some of these dots seem to aggregate, so as to form little round or oval clusters bearing a strong resemblance to cell-nuclei. These bodies appear* to be the centres of the further changes which take place in the blastema ; for if it be about to undergo a development into a fibrous tissue, they seem to be the cen- tres from which the fibrillation takes place ; whilst if a cellular struc- ture is to be generated, it is from them that the cells take their origin. The first stage of the latter process appears to consist in the accumula- tion round each nucleus of the substance which the cell is to include ; and around this the cell-membrane is subsequently developed. Such is the mode, then, in which the development of new structures, for the filling-up of losses of substance, is provided for ; and it appears from the observations of Mr. Paget, that whilst the immediate fibrillation of the blastema takes place in the' case of effusions which are secluded from the air and which undergo organization under the most favourable circumstances, a production of cells takes place when the blastema is poured out upon the surface of an open wound, where the contact of air, and other sources of irritation, interfere with the organizing pro- cess, and occasion a tendency to degradation in the newly-generating tissue. — Such a production of cells de novo in the midst of an organiza- ble blastema, does not constitute, however, any real exception to the general rule, of the dependence of the life of ever^ cell upon that of a pre-existing cell. For it is pretty certain that the blastema is itself the product of the formative agency of certain cells expressly provided for its elaboration ; and it does not seem improbable that these cells, in bursting and setting free the plastic fluid which they have prepared, should diffuse through it their own nuclear or germinal particles in a state of solution or extremely minute division ; and that these, attract- ing each other in the act of solidification, should act as new centres of cell-growth, just as if they were still contained within the parent-cell. 214. The very simplest and most independent condition of the Ani- mal Cell is probably to be found in the Blood, the Chyle, and the Lymph ; in all of which liquids we meet with floating cells, which are 134 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. completely isolated from one another, and which are consequently just as independent as the vesicles of the Red Snow or other simple cellular Plants. Indeed in the nature of their habitat^ we may compare them with the Yeast-Plant ; for as this will only vegetate in a saccharine fluid containing vegetable albumen, so do we find that these floating cells will only grow and multiply in the albuminous fluids of animals. In their general appearance they very closely correspond with the figure already given as the type of the simple cell. Their diameter is pretty uniform in the different fluids of the body, and even in diff'erent animals ; being for the most part about l-3000th of an inch. They are some- times nearly spherical, and sometimes flattened; when they present the latter shape, they may be made to swell out into the spherical form (see Frontispiece^ Figs. 4 and 5) by the action of water, which they im- bibe according to the laws of Endosmose, — the thinner fluid, watei:, passing towards the more viscid contents of the cell, and mingling with them. By the continuance of this kind of action, the cell will be caused to burst. These cells, which are known as the corpuscles of the Chyle and Lymph, and as the White Corpuscles of the Blood, are observed to contain a number of minute molecules in their interior [Front. Fig. 4) ; Fig. 19. Colourless cells, •with active molecules, and fibres of fibrine, from Herpes labialis. and at a certain stage of their development, — probably that which im- mediately precedes the maturation and rupture of the parent-cell, — these molecules may be seen, with a good Microscope, in active move- ment within the cavity. The action of a very dilute solution of potash causes the immediate rupture of these cells, and the discharge of the contained molecules, which are probably the germs of new cells of a similar character. And when they rupture spontaneously, which they are much disposed to do under the influence of contact with air, the fluid which they set free shows an obvious tendency to assume a fibrous arrangement. — The cells which are found in many fibrinous exudations resemble the colourless corpuscles of the blood in all essential particu- lars (Fig. 19). Hence it may be concluded that they belong to the same class ; being probably developed from granular germs set free from the blood, along with the matter of the fibrinous exudation itself. 215. Besides the cells already mentioned, the blood of Yertebrated animals also contains others, which are distinguished by their red colour and flattened form. These are equally isolated, and lead an indepen- RED CORPUSCLES OF BLOOD. 135 dent life ; undergoing all their changes whilst floating in the rapidly- circulating current. These Red Corpuscles can scarcely be said to exist in the blood of Invertebrated animals, and their proportion in the blood of Vertebrata varies considerably in the several groups of that sub-kingdom ; they are altogether wanting in the blood of the Amphi-^ oxus or Lancelet, which, although essentially a Fish, has many pecu- liarities that connect it with the lower divisions of the Animal series. They present, in every instance, the form of a flattened disk, which is circular in Man and in- most Mammalia (Front. Fig. 1), but which is oval in Birds, Reptiles, and Fishes, and in a few Mammals [Front. Fig. 6). This disk is in both instances a flattened cell, whose walls are pel- lucid and colourless, but whose contents are coloured. Like the cor- puscles already described, they may be caused to swell up and burst, by the imbibition of water ; and the perfect transparency and the homo- geneous character of their walls then become evident. {Front. Fig. 8, e.) — These Red Corpuscles are not only distinguished from the others by the colour of their contents ; they are also characterized by the ab- sence of the separate molecules, which formed so distinctive a feature in the preceding; and in Oviparous Vertebrata by the presence of a distinct central spot or nucleus, which appears to be composed of an aggregation of minute granules, analogous to those elsewhere diff'used through the interior of the cell. The nucleus (where it exists) may be easily obtained separate from the cell-wall and its contents, by treating the red corpuscles, with water. The first efi'ect of this is to render the nucleus rather more distinct,. as is seen by contrasting the corpuscle which has been thus slightly acted on [Frofit. Fig. 8, a), with the un- altered coYipnscle (Front. Fig. 6) of the same animal. After a short time, the corpuscle swells out and becomes more circular (Front. Fig. 8, h); and in a short time longer, the nucleus is seen, not in the centre of the disk, but near its margin (Front. Fig. 8, c, d). Finally, the wall of the cell ruptures ; the nucleus and its other contents are set free ; and whilst the colouring matter is difi'used through the surrounding fluid, the cell-walls and the nuclei are separately distinguishable. (Front. Fig. 8, e.) — It is remarkable, however, that the Red corpuscles of the blood of Mammals should possess no obvious nucleus ; the dark spot which is seen in their centre (Front. Fig. 1), being merely an efiect of refraction, in consequence of the double-concave form of the disk. When the corpuscles are treated with water, so that their form becomes first flat, and then double-convex, the dark spot disappears ; whilst, on the other hand, it is made more evident when the concavity is increased by the partial emptying of the cell, which may be accomplished by treating the blood-corpuscles with fluids of greater density than their own contents. 216. The size of the Red Corpuscles is not altogether uniform in the same blood ; thus it varies in that of Man from about the l-4000th to the l-2800th of an inch. But we generally find that there is an ave^ rage size, which is pretty constantly maintained among the difierent individuals of the same species ; that of Man may be stated at about l-3400th of an inch. The round corpuscles of the Mammalia do not in general depart very widely from this standard ; except in the case of 136 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. the Musk-Deer, in which they are less than l-12000th of an inch in diameter. It is in the Camel tribe alone that we find oval corpuscles among Mammals ; these have about the same average length as the round corpuscles of Man, but little more than half the breadth. — In Birds, the corpuscles are occasionally almost circular ; but in general their diameters are to each other as 1 J or 2 to 1. The size of the cor- puscles is usually greater according to the size of the Bird ; thus among the Ostrich tribe, the long diameter is about l-1650th of an inch, and the short diameter l-3000th ; whilst among the small Sparrows, Finches, &c., the long diameter is about l-2400th, and the short frequently does not exceed half that amount. — It is in Reptiles that we find the largest red corpuscles ; and it is in their blood, therefore, that we can best study the characters of these bodies. The blood-disks of the Frog, from the facility with which they may be obtained, are particularly suitable for the purpose ; their long diameter is about the 1-1 000th of an inch, whilst their short or transverse diameter is about l-1800th. The curious Proteus, Siren, and other allied species, which retain their gills through their whole lives, are distinguished by the enormous size of their blood-disks. The long diameter of the corpuscles of the Proteus is about l-337th of an inch ; they are consequently almost distinguisha- ble with the unaided eye. The long diameter of the corpuscles of the Siren is about l-435th of an inch, and their short diameter about l-800th ; the long diameter of the nuclei of these corpuscles is about 1-lOOOth, and the short diameter about l-2000th of an inch, — so that the nuclei are about three times as long, and nearljfc twice as broad, as the entire human corpuscles. 217. The relation between the White or Colourless and the Red Corpuscles of the Blood can only be determined by attentively watching their development, and tracing them through all the stages of their growth. Although our knowledge on this subject is far from complete, yet there seems much reason to believe, from the observations of Mr. Wharton Jones on the difierent forms of blood-cells presented in the several classes of animals, and from those of Mr. Paget and other phy- siologists on the several gradations of structure exhibited in the blood- cells of Mammalia, that the red corpuscles have their origin in the colourless^ and that the diff'erent forms of blood-cell presented in diffe- rent groups of animals are, in fact, progressive stages in the same de- velopmental process, which may be checked at any one of them. — Thus among the lower Invertebrata, the cells which are observed to float in their circulating fluid, seem to be little else than aggregations of gra- nules, presenting a tuberculated surface ; no cavity can be distinguished ; and it is with difficulty that the presence of a distinct cell-wall can be demonstrated. This form, which is designated by Mr. Wharton Jones as the "coarse granule-cell," presents itself also among the chyle and lymph-corpuscles of Vertebrated animals, and is occasionally met with in their blood. — In other Invertebrata, the blood-cell undergoes a fur- ther development ; for the cell-wall becomes more distinct, and the gra- nules are so much more minute as to give to the entire cell a somewhat nebulous aspect, its surface being now smooth instead of tuberculated. This form of corpuscle, also, termed by Mr. Wharton Jones the " fine RED CORPUSCLES OF BLOOD. 137 granule-cell," is found in the chjle and lymph, and occasionally in the blood, of Vertebrated animals. — The next stage in the history of deve- lopment, is the aggregation of the granules into a distinct nucleus^ and the clearing up of the general cavity of the cell ; and thus is formed the " colourless nucleated cell," which is the highest grade that the- blood-cell attains in the Invertebrated series ; the number of such cells being greater in each class, the closer is its approximation to the Ver- tebrated sub-kingdom. This phase presents itself also in the blood of Yertebrata, as a transition stage between the chyle- and lymph-corpus- cle, and the proper blood-disk or red-corpuscle. — Thus, then, we see that the cells -which are found in the circulating fluid of Invertebrated animals, correspond rather with those of the Chyle and Lymph of Yer- tebrata, than with those which are characteristic of the Blood of the latter. 218. The next stage of development seems to consist in the acquire- ment of the peculiar red colour ; and in the change of form, from the spherical to the flattened or discoidal. Thus is produced the "" coloured nucleated cell," which is the characteristic grade of the blood-disk of the Oviparous Yertebrata in general. This grade may be occasionally seen in the blood of the adult Mammal, as the transition-stage between the colourless nucleated cell, and the non-nucleated cells which are proper to it ; but it is more easily made out in the blood of the em- bryo.— The non-nucleated red corpuscles which are characteristic of Mammalia, are regarded by Mr. Wharton Jones as the escaped nuclei of the preceding, which have undergone development into cells; but it seems much more probable, that they are the same cells in a yet more advanced stage of development, the nuclei having been absorbed, as often happens in the case of other cells. 219. There can be no doubt that, like all other cells, each Blood- corpuscle has its proper term of life, and that it degenerates and dies when this is expired ; if it were not, therefore, for the continual pro- duction of new cells, in the manner just described, the Blood would soon lose its due proportion of these components, since there is no reason to believe that the fully-formed red corpuscle can regenerate its kind, although multiplication by subdivision may take place in an earlier stage of its development. When much blood has been drawn from the body, the proportion of red corpuscles in the remaining fluid is at first considerably lowered ; the fluid portion of the blood being replaced almost immediately, whilst the floating dells require time for their regeneration. Their amount progressively increases, however, until it has reached its proper standard, provided that a due supply of the materials be afibrded. We shall presently see that one of these materials is Iron ; and it is well known that iron administered internally is an important aid in recovery from severe hemorrhages, as well as a valuable remedy for certain constitutional states, in which there is a diminished power of producing red corpuscles. Thus in Chlorosis, under the administration of iron, the amount of red corpuscles in the blood has been doubled within a short period. In the healthy state of the system, the constant production, and the constant death and disin- tegration, balance one another. In some instances (as in Chlorosis), 138 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. the production is not sufficient to make up for the loss by death ; and the total amount in the blood undergoes an extraordinary diminution, sometimes even to less than a quarter of their proper proportion. In other cases, under the influence of excessive nutriment (as in the state termed Plethora), the proportion of Red Corpuscles is increased beyond the normal amount ; and in this condition, the loss of a small quantity of blood may be a preservative from the evils to which it is incident, from Hemorrhage of various kinds. 220. The Red Corpuscles make their first appearance in the blood of the Embryo, however, long before the formation of chyle and lympl^ commences ; and they appear to be formed by the metamorphosis of some of the cells which constitute the inner layer of the germinal mem- brane (chap. XI.) These cells are at first nearly spherical, ^nd are full of particles of a yellowish substance like fatty matter ; in the midst of which, though somewhat obscured by them, a central nucleus may be seen. The development of these embryo-cells into the oval red corpus- cles of the Oviparous Vertebrata, is stated by Mr. Paget to be effected by the gradual clearing-up, as if by division and liquefaction, of the contained particles, the acquirement of the blood-colour and of the ellip- tical form, the flattening of the cell, and the more prominent appearance of the nucleus. This first set of blood-disks is nucleated in Mammalia, as well as in Oviparous Vertebrata ; and they occasionally present indi- cations of being in course of multiplication by subdivision. They gradually disappear from the blood, however, when the chyle and lymph-corpuscles first present themselves in the circulating current; and thenceforth the Red corpuscles seem to be formed at the expense of the latter alone. It is curious that this change should usually coin- cide, in the Tadpole, with the time at which the external branchiae dis- appear ; and, in warm-blooded animals, with the period at which the branchial fissures are closed in the neck, and the course of the circula- tion is altered (chap, vi.) 221. The chemical composition of the Red Corpuscles presents cer- tain peculiarities which require notice ; that of the White, or Colour- less, however, has not been specially examined. When the Red Corpuscles are separated from the other constituents of the blood, and are treated with water, their contents are speedily diffused through the fluid (§ 215) ; and from this may be abstracted two distinct substances, which are designated GlobuUne and Ilcematine.— The former does not seem*to differ from Albumen in any greater degree, than may be attri- buted to the presence of the walls and nuclei of the corpuscles, from which it cannot be separated ; and it is probably common to the White, as well as the Red. — It is in the Red alone, however, that the Hcematine exists. The composition of this substance is notably different from that of the proteine-compounds ; the proportion of carbon to the other ingre- dients being very much greater ; and a definite quantity of iron being an essential part of it. Its formula is 44 Carbon, 22 Hydrogen, 3 Ni- trogen, 6 Oxygen, and 1 Iron. When completely separated from Albu- minous matter, it is a dark brown substance, incapable of coagulation, nearly insoluble in water, alcohol, ether, acids, or alkalies, alone ; but readily soluble in alcohol mixed either with sulphuric acid or ammonia. uu i BED CORPUSCLES OF BLOOD. 139 The solution, even wlien diluted, has a dark colour ; and possesses all the properties of the colouring matter of venous blood. The iron may be separated from the hsematine by strong reagents which combine with the former, and the latter still possesses its characteristic colour. This hue cannot be dependent, therefore, on the presence of iron in the state of peroxide ; as some have supposed. On the other hand, the iron is most certainly united firmly with the ingredients of the hgematine, as contained in the red corpuscles ; for this may be digested in dilute sul- phuric or muriatic acid for many days, without the least diminution in the quantity of iron, the usual amount of which may be afterwards ob- tained by combustion from the haematine that has been subjected to this treatment. This experiment seems further to prove, that the iron cannot be united with the hgematine in the state of either protoxide or peroxide, as maintained by Liebig ; since weak acids would then dissolve it out. Regarding the nature of this compound, and the changes which it undergoes in respiration, there is still much to be learned ; and until these points have been more fully elucidated, the precise uses of the Red Corpuscles in the animal economy cannot be understood. There is evidence, however, that the production of Hsematine is (like the pro- duction of the red colouring matter of the Protococcus nivalis, § 26), a esult of chemical action taking place in the cells themselves ; for no bstance resembling Haematine can be found in the liquid in which ese cells float, and scarcely a trace of iron can be detected in it; hilst, on the other hand, the fluid portion of the chyle holds a large "quantity of iron in solution, which seems to be drawn into the red cor- puscles, and united with the other constituents of haematine, as soon as ever it is delivered into the circulating current. 222. It has been usually supposed, until recently, that the diff'erence in colour between Arterial and Venous blood is due to different states of combination of the Haematine they respectively contain, with Oxygen and Carbonic acid. For in its passage through the capillaries of the systenj, the arterial blood loses its bright florid hue, and assumes the dark purple tint which distinguishes ordinary venous blood ; and the converse change takes place in the capillaries of the lungs, the original florid hue being recovered. Now it is certain that the blood, in its change from the arterial to the venous condition, loses oxygen, and becomes charged with an increased amount of carbonic acid, although its precise mode of combination is not known ; on the other hand, in its return from the venous to the arterial state, the blood gives off this additional charge of carbonic acid, and imbibes oxygen. The change of colour, under similar conditions, takes place out of the body, as well as in it. Thus if venous blood be exposed for a short time to the air, its surface becomes florid ; and the non-extension of this change to the interior of the mass is evidently due to the impossibility of bringing air into relation with every particle of the blood, in the manner which the lungs are so admirably contrived to effect. If venous blood be exposed to pure oxygen, the change of colour will take place still more speedily ; and it is not prevented by the interposition of a thick animal membrane, such as a bladder, between the blood and the gas. On the other hand, i^rterial blood be exposed to carbonic acid, it loses its brilliant hue, 140 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. and is rendered as dark as venous blood ; or even darker, if exposed very completely to its influence. The simple removal of this carbonic acid is not sufficient to restore the original colour ; for this removal may be effected by hydrogen, which has the power of dissolving out (so to speak) the carbonic acid diflfused through the blood ; but the arterial hue is not restored unless oxygen be present, or saline matter be added to the blood. — Recent observations seem to render it probable that these variations are due, not so much to changes of composition in the Hasmatine, as to changes of form in the Corpuscles which contain it. For when the hsematine has been separated and difi'used through water, it is neither darkened by carbonic acid, nor brightened by oxygen, unless some corpuscles be floating in the solution. And it appears that the efi*ect of oxygen, like that of saline solutions, is to contract the corpuscles and to thicken their walls, thus, by altering their mode of reflecting light, making them appear bright red ; whilst carbonic acid, like water, may be seen to occasion a dilatation of the corpuscle, and a thinning of its walls (which are at last dissolved by it), in a degree that is probably sufficient to account for the darkening of the hue of the mass. 223. These changes in the condition of the Red corpuscles (whatever their precise nature may be), taken in connexion with the fact, that these bodies are almost completely restricted to the blood of Vertebrata (whose respiration is much more energetic than that of any Invertebrated animals save Insects, which have a special provision of a diff'erent cha- racter), and that their proportion to the whole mass of the blood corre- sponds with the activity of the respiratory function, — leave little doubt that they are actively (but not exclusively) concerned as carriers of Oxygen from the lungs to the tissues, and of Carbonic acid from the tissues to the lungs ; and that they have little other direct concern in the functions of Nutrition, than the fulfilment of this duty. Their com- plete absence in the lower Invertebrated animals, in the earliest condi- tion of the higher, and in newly-forming parts until these are penetrated by blood-vessels, seems to indicate that they have no immediate con- nexion with even the most energetic operations of growth and develop- ment ; whilst, on the other hand, there is abundant evidence, that the normal activity of the animal functions is mainly dependent upon their presence in the blood in due proportion. 224. Next in independence to the cells or corpuscles floating in the animal fluids, are those which cover the free membranous surfaces of the body, and form the Epidermis and Epithelium. Between these two structures there is no more real difi'erence than there is between the Skin and the Mucous membranes. The one is continuous with the other ; they are both formed of the same elements ; they are cast off and renewed in the same manner ; the history of the life of the indivi- dual cells of each is nearly identical ; but there is an important difi'e- rence in the purposes which they respectively serve in the general eco- nomy. The Epidermis or Cuticle covers the exterior surfaces of the body, as a thin semitransparent pellicle, which is apparently homoge- neous in its texture, is not traversed by vessels or nerves, and was formerly supposed to be an inorganic exudation from the surface of the I SIMPLE ISOLATED CELLS — EPIDERMIS. 141 trne s"kin, designed for its protection. It is now known, however, to consist of a series of layers of cells, which are continually wearing off at the external surface, and are being renewed at the surface of the true skin ; so that the newest and deepest layers gradually become the oldest and most superficial, and are at last thrown off by slow desqua- mation. Occasionally this desquamation of the cuticle is much more rapid ; as after Scarlatina and other inflammatory affections of the Skin. 225. In their progress from the internal to the external surface of the Epidermis, the cells undergo a series of well-marked changes. When we examine the innermost layer, we find it soft and granular ; consisting of nuclei^ in various stages of development into cells, held together by a tenacious semi-fluid substance. This was formerly consi- dered as a distinct tissue, and was supposed to be the peculiar seat of the colour of the skin ; it received the designation of rete mucosum. Passing outwards, we find the cells more completely formed ; at first nearly sphe- rical in shape ; but becoming polygonal where they are flattened against one another. As we proceed further towards the surface, we perceive that the cells are gradually more and more flattened, until they become mere horny scales, their cavity being obliterated ; their origin is indi- cated, however, by the nucleus in the centre of each. This flattening appears to result from the gradual desiccation or drying up of the con- tents of the cells, which result from their exposure to the air. Thus each cell of the Epidermis is developed from the nucleus on the surface of the basement-membrane — which nucleus is probably furnished by the membrane itself (§ 208), — and is gradually brought to the surface by the development of new cells beneath, and the removal of the superficial layers ; whilst at the same time it is progressively changed in form, until it is converted into a flattened scale. The accompanying repre- Fig. 20. Fig. 21. Oblique section of Epidermis, showing the progressive development of its component cells;— a, nuclei, resting upon the surface of the cutis vera /; these nuclei are seen to be gradually developed into cells, at 6, c, and d; and the cells are flattened into lamellae, forming the exterior portion of the epidermis ate. Horny Epidermis, from conjunctiva covering the cornea; a, single scales; b. single lamina of epithelium ; below is seen a double layer of the sentation of an oblique section of the Epidermis, exhibits the principal gradations of its component structures. graaations 142 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 226. The Epidermis covers the whole exterior surface of the body ; not excepting the Conjunctiva of the eye, on which, however, it has more the character of an Epithelium, and the Cornea, on which it par- ticipates in the horny character of the Epidermic covering (Fig. 21). The continuity is well seen in the cast skin or slough of the Snake ; in which the covering of the front of the eye is found to be as perfectly exuviated as that of any part of the body. The number of layers varies greatly in different parts ; being usually found to be the greatest where there is most pressure or friction. Thus on the soles of the feet, par- ticularly at the heel and the ball of the great toe, the Epidermis is ex- tremely thick ; and the palms of the hands of the labouring man are distinguished by the increased density of their horny covering. It would seem as if the irritation of the skin stimulated it to an increased production of this substance. The. Epidermic membrane is pierced by the excretory ducts of the sweat-glands, and of the sebaceous follicles, which lie in the true skin and immediately beneath it ; or we should rather say that it is continuous with the delicate epithelial lining of these. The Nails may be considered as nothing more than an altered form of Epidermis. When examined near their origin, they are found to consist of cells which gradually dry into scales ; and these remain coherent together. A new production is continually taking place in the groove of the skin, in which the root of the nail is imbedded ; and pro- bably also from the whole subjacent surface. 227. The Epidermis, when analysed, is found to differ from the pro- teine-compounds in its composition ; but not in any very striking de- gree. The proportion of its elements is considered to be 48 Carbon, 39 Hydrogen, 7 Nitrogen, 17 Oxygen; and this corresponds exactly with the composition of the substance of which Nails, Horn, Hair, and Wool are constituted. It seems probable, however, that the cell-walls are formed, as elsewhere, of Fibrine ; and that the horny matter is a secretion in their interior, which is drawn from the elements of blood during their growth and development. 228. The Epidermis appears solely destined for the protection of the true Skin; both from the mechanical injury and the pain which the slightest abrasion would produce ; and from the irritating effects of ex- posure to the external air and of changes of temperature. We perceive the value of this protectiouy when the Epidermis has been accidentally removed. It is very speedily replaced, however ; the increased deter- mination of blood to the Skin, which is the consequence of the irrita- tion, being favourable to the rapid production of Epidermic cells on its surface. 229. Mingled with the Epidermic cells we find others which secrete colouring matter instead of horn ; these are termed Pigment-cells. They are not readily distinguishable in the epidermis of the White races, except in certain parts, such as the areola around the nipple, and in freckles, naevi, &c. But they are very obvious, on account of their dark hue, in the newer layers of the Epidermis of the Negro and other coloured races ; and, like the true Epidermic cells, they dry up and become flattened scales in their passage towards the surface, thus con- stantly remaining dispersed through the Epidermis, and giving it a dark SIMPLE ISOLATED CELLS. — EPIDERMIS. 143 tint when it is separated and held up to the light. In all races of men, however, we find the most remarkable development of Pigment-cells on the inner surface of the Choroid coat of the eye, where they form seve- ral layers, known as the Pigmentum nigrum. Here they have a very regular arrangement ; which is best seen where they cover the bleocL vessels of the Choroid coat in a single layer, as shown in Fig. 22. Fig. 22. W^Mm ■WtP^'fM^^^ Fig. 23. • ^ Mj Corpuscles of Pigment, magnified 300 diameters ; — a, cell ; ft, nucleus. ment-cells, where they cover, a, a, a, the veins, in a single layer : 6, ^, ramifications of the veins near the ciliary ligament, covered with less regular pigment-cells ; c, c, spaces between the vessels, more thickly covered with pigment-cells. When examined separately, they are found to have a polygonal form (Fig. 23, a), and to have a distinct nucleus (6) in their interior. The black colour is given by the accumulation, within the cell, of a number of flat rounded or oval granules, measuring about 1-20, 000th of an inch in diameter, and a quarter as much in thickness ; these, when separately viewed, are observed to be transparent, not black and opaque ; and they exhibit an active movement when set free from the cell, and even whilst enclosed within it. The pigment-cells are not always of a simple rounded or polygonal form ; they sometimes present remarkable stellate prolongations, under which form they are well seen in the skin of the Frog. — The Chemical nature of the black pigment has not yet been made evident ; it has been shown, however, to have a close relation with that of the Cuttle-fish ink or Sepia, which derives its colour from the pigment-cells lining the ink-bag ; and to include a larger proportion of Carbon than most other organic substances, — everj" 100 parts contain- ing 58J of this element. 230. That the development of the Pigment- cells, or at least the for- mation of their peculiar secretion, is in some degree due to the influence of Light, seems evident from the facts already mentioned (§93). To these it may be added, that the new-born infants of the Negro and other dark races do not exhibit nearly the same depth of colour in their skins, as that which they present after the lapse of a few days ; which seems to indicate that exposure to light is necessary for the full development of the characteristic hue. An occasional development of dark pigment- cells takes place during pregnancy in some females of the fair races ; thus it is very common to meet with an extremely dark and b^road areola k 144 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. round the nipple of pregnant women ; and sometimes large patches of the cutaneous surface, on the lower part of the body especially, become almost as dark as the skin of the Negro. On the other hand, individuals are occasionally seen with an entire deficiency of pigment-cells, or at least of their proper secretion, not merely in the skin, but in the eye ; such are termed Albinoes ; and they are met with as well among the fair, as among the dark races. The absence of colour usually shows itself also in the hair ; which is almost white. 231. The Epithelium may be designated as a delicate cuticle, covering the free internal surfaces of the body ; and apparently designed, in some instances, simply for their protection ; whilst in other cases, as we shall presently find, it serves purposes of far greater importance. It has long been known that the Epidermis might be traced continuously from the lips to the mucous membrane of the mouth, and thence down the oesopha- gus into the stomach ; and that in the strong muscular stomach or giz- zard of the granivorous birds, it becomes quite a firm horny lining. But it has been only ascertain^ed by the use of the Microscope, that a continuous layer of cells may be traced, not merely along the whole sur- face of the mucous membrane lining the alimentary canal, but likewise along the free surfaces of all other Mucous membranes, with their pro- longations into follicles and glands; as well as on Serous and Synovial membranes, and the lining membrane of the heart, blood-vessels, and absorbents. The Epithelial cells, being always in contact with fluids, do not dry up into scales like those of the Epidermis ; and they differ from them also in regard to the nature of the matter which they secrete in their interior. In this respect, however, the Epithelial cells of dif- ferent parts are unlike one another, fully as much as any of them are unlike the cells of the Epidermis ; for we shall find that all the secretions of the body are the product of the elaboration of Epithelium cells ; and consequently there are as many varieties of endowment, in these important bodies, as there are varieties in the result of their action. 232. The Epithelium covering the Serous and Synovial membranes, and the lining of the blood-vessels, is composed of flattened polygonal cells (resembling those shown in Fig. 23), lying in apposition with each other, so as to form a kind of pavement ; hence this form is termed pave- ment- or tessellated-^^iihoimm. There is no reason to believe that it possesses any active endowments in these situations ; since it does not appear to be concerned in the elaboration of any peculiar secretion. It has been already pointed out (§ 196), that the fluid of serous membranes is separated from the blood by a simple act of mechanical transudation (which often takes place to a great extent after death) ; the walls of the blood-vessels do not appear to be concerned in forming any peculiar secretion ; and the only product of this kind, which indicates any special endowment in the epithelium-cells, is the synovia, which is probably elaborated by the cells covering the vascular fringes of the synovial membrane, formerly mentioned (§ 198). The cells draw it from the blood, during the progress of their growth, form it as a secretion within them- selves, and then cast it into the general cavity of the joint (when their term of individual life is ended), either by the rupture or the liquefac- tion of their walls. In other cases, it would seem as if the epithelial cells were not frequently cast ofl" and renewed, but possessed a considera- SIMPLE ISOLATED CELLS. — EPITHELIUM. 145 le permanency. It is to be remembered that, in the healthy state of the serous and synovial membranes, and in that of the lining membrane of the blood-vessels and absorbents, they are entirely secluded from sources of irritation ; and that they lead a sort o^ passive life, very dif- ferent from the active life of the mucous membranes. In fact, it woul4 appear to be the sole object of the serous membranes, to enclose and suspend the viscera, in such a manner as to allow of the access of blood- vessels, nerves, gland-ducts, &c.; and at the same time to permit them the required freedom of motion, and to provide against the irritation of opposing parts, by furnishing an extremely smooth and moistened sur- face, wherever friction takes place. Hence we find membranes, with all the characters of serous surfaces; in the false joints- formed by ununited fractures, and in other similar situations. 233. The Epithelium of the Mucous membranes and their prolonga- tions, is found under two principal forms, the tessellated, and the ci/lin- drical. An example of the Tessellated form is shown in Fig. 24, which shows the separate epithelium cells of the mucous membrane of the mouth, as they are frequently met with in saliva. The cells are not always so polygonal in form, however ; sometimes retaining their rounded or oval form, and being separated by considerable interstices, so that they can scarcely be said to form a continuous layer. A specimen of this kind is seen in Fig. 25, which represents a group of epithelium cells from one of the smaller bronchial tubes. This form of tessellated epithelium is more commonly met with, where the secreting operations are more active, the life of the cells consequently shorter, and the renewal of them more frequent ; so that they have not time, so to speak, to be developed into a more continuous layer. The Cylinder-Epithelium is very differently constituted. Its component cells and cylinders, which are arranged side by side ; one extremity of each cylinder resting upon the basement-mem- Fig. 25. Separated Epithelium-cells, a, with nuclei, b, and nucleoli, c, from mucous membrane of mouth. Pavement-Epithelium of the Mucous Membrane of the smaller bronchial tubes; a, nuclei with double nucleoli. brane, whilst the other forms part of the free surfjice. The perfect cylindrical form is only shown, when the surface on which the cylinders rest is flat or nearly so. When it is convex, the lower ends or basfes of the cells are of much smaller diameter than the upper or free extremi- ties ; and thus each has the form of a truncated cone, rather than of a cylinder. (Fig. 26). This is well seen in the cells, which cover the villi of the intestinal canal. (Fig. 29). On the oihex hand, where the cylin- der-epithelium lies upon a concave surface, the free extremities of the oells may be smaller than those which are attached. Sometimes each 10 146 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. cylinder is formed from more than one cell, as is shown by the nuclei it contains ; although its cavity seems to be continuous from end to end. And occasionally the cj^linders arise by stalk-like prolongations, from Fig. 26. Vibratile or ciliated Epithelium;— a, nucleated cells, resting on their smaller extremities; b, cilia. a tessellated epithelium beneath. The two forms of Epithelium pass into one another at various points ; and various transitional forms are then seen, — the tessellated scales appearing to rise more and more from the surface, until they project as long-stalked cells, truncated cones, or cylin- ders. 234. Both these principal forms of Epithelial cells are frequently observed to be fringed at their free margins with delicate filaments, which are termed cilia ; and these, although of extreme minuteness, are organs of great importance in the animal economy, through the extra- ordinary motor powers with which they are endowed. The form of the ciliary filaments is usually a little flattened, and tapering gradually from the base to the point. Their size is extremely variable ; the largest that have been observed being about l-500th of an inch in length, and the smallest about l-13000th. When in motion, each filament appears to bend from its root to its point, returning again to its original state, like the stalks of wheat when depressed by the wind ; and when a number are afiected in succession with this motion, the appearance of progressive waves following one another is produced, as when a wheatfield is agitated by frequent gusts. When the ciliary motion is taking place in full activity, however, nothing whatever can be distinguished, but the whirl of particles in the surrounding fluid; and it is only w^hen the rate of movement slackens, that the shape and size of the cilia, and the manner in which their stroke is made, can be clearly seen. The motion of the cilia is not only quite independent (in all the higher animals at least) of the will of the animal, but is also independent even of the life of the rest of the body ; being seen after the death of the animal, and pro- ceeding with perfect regularity in parts separated from the body. Thus isolated epithelium cells have been seen to swim about actively in water, by the agency of their cilia, for some hours after they have been detached, from the mucous surface of the nose ; and the ciliary movement has been seen fifteen days after death in the body of a Tortoise, in which putrefaction was far advanced. In the gills of the River Mussel, which are among the best objects for the study of it, the movement endures with similar pertinacity. 235. The purpose of this ciliary movement is obviously to propel fluids over the surface on which it takes place ; and it is consequently limited in the higher animals to the internal surfaces of the body, and always takes place in the direction of the outlets, towards which it aids lit m I SIMPLE ISOLATED CELLS. — EPITHELIUM. 147 in propelling the various products of secretion. The case is different, however, among animals of the lower classes, especially those inhabiting the water. Thus the external surface of the gills of Fishes, Tadpoles, &c., is furnished with cilia ; the continual movement of which renews the water in contact with them, and thus promotes the aeration of the blood. In the lower Mollusca, and in many Zoophytes, which pass their lives rooted to one spot, the motion of the cilia serves not merely to produce currents for respiration, but likewise to draw into the mouth the minute particles that serve as food. And in the free-moving Animal- cules, of various kinds, the cilia are the sole instruments which they possess, not merely for producing those currents in the water which may bring them the requisite supply of air and food, but also for propelling their own bodies through the water. This is the case, too, with many larger animals of the class Acalephae (Jelly-fish), which move through the water, sometimes w^ith great activity, by the combined action of the vast numbers of cilia that clothe the margins of their external surfaces* In these latter cases it would seem as if the ciliary movement were more under the control of the will of the animal, than it is where it is con- cerned only in the organic functions. In what way the will can influence it, however, it does not seem easy to say ; since the ciliated epithelium- cells appear to be perfectly disconnected from the surface on which they lie, and cannot, therefore, receive any direct influence from their nerves, f the cause of the movement of the cilia themselves, no account can be given; they are usually far too small to contain even the minutest fibrillse of muscle ; and we must regard them as being, like those fibrillse, organs sui generis, having their own peculiar endowment, — which is, in the higher animals at least, that of continuing in ceaseless vibration, during the whole term of the life of the cells to which they are attached. The length of time during which the ciliary movement continues after the general death of the body, is much less in the warm-blooded than in the cold-blooded animals ; and in this respect it corresponds with the degree of persistence of muscular irritability, and of other vital endowments. 236. The Tessellated-Epithelium, as already mentioned, covers the Serous and Synovial membranes, the lining membranes of the blood- vessels and absorbents, and the Mucous membranes with their glandular prolongations, except where the cylinder-epithelium exists. It presents itself, with some modifications presently to be noticed, in the ultimate follicles of all glands, and also in the smaller bronchial tubes. In this latter situation it is furnished with cilia ; and these are also found on the cells of the tessellated epithelium, which covers the delicate pia mater lining the cerebral cavities. The Cylinder-Epithelium commences at the cardiac orifice of the stomach, and lines the whole intestinal tube ; and, generally speaking, it lines the larger gland-ducts, giving place to the tessellated form in their smaller ramifications. A similar epithelium, furnished with cilia, is found lining the air-passages and their various offsets, — the nasal cavities, frontal sinuses, maxillary antra, lachrymal ducts and sac, the posterior surface of the pendulous velum of the palate and fauces, the eustachian tubes, the larynx, trachea, and bronchi, — becoming continuous, however, in the finer divisions of the latter, with 148 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. the ciliated pavement-epithelium. The upper part of the vagina, the uterus, and the fallopian tubes, are also furnished with a ciliated Cylin- der-Epithelium. The function of the cilia in all these cases appears to be the same ; that of propelling the viscid secretions, which would otherwise accumulate on these_^ membranes, towards the exterior orifices, whence they may be carried off. 237. The simplest office which the Epithelium-cells of Mucous mem- branes perform, appears to be that of elaborating a peculiar secretion termed Mucus ; which is destined to protect them from the contact of air, or from that of the various irritating substances to which they are exposed, in consequence of their peculiar position and functions. This Mucus is a transparent semifluid substance, distinguished by its peculiar tenacity or viscidity. It is quite insoluble in water ; but is readily dis- solved by dilute alkaline solutions, from which it is precipitated again by the addition of an acid. A substance resembling Mucus may be pro- duced from any fibrinous exudation, or even from pus, by treating it with a small quantity of liquor potassse. The secretion of Mucus, like the formation of Epidermis, appears to take place with an activity pro- portioned to the degree of irritation of the subjacent membrane. On many parts of the mucous surface, a sufficient supply is afforded by the epithelium-cells which cover it ; but in other situations, especially along the alimentary canal, the demand is much greater, and it is probably supplied not merely by the cells of the surface, but by those lining the crypts or follicles which are formed by involutions of it. 238. The Epithelium-cells, which are thus being continually renewed on the Mucous surfaces, commonly seem to haVe their origin in the granular germs diffused through the basement-membrane ; but it is dif- ferent in regard to the cells of the follicles, which seem rather to occupy their cavity than merely to line their walls, and which appear to be in course of continual production from a germinal spot, or collection of re- productive granules, at the blind extremity of the follicle. This is the case in the ultimate follicles of the more complex glands ; which may be regarded as so many repetitions of the simple crypts or follicles in the substance of the mucous membranes ; — the only difference being, that Fig. 27. Fig. 28. Fig. 29. Two follicles from the liver of Carcinus manas (Common Crab), with their con- tained secreting cells. Ultimate follicles of Mammary gland, with their secreting cells, a, a;—b, b, the nuclei. the former pour their secretion into a branch of a duct, which unites the other ramifications to form a trunk ; and this trunk conveys them to their destination in some cavity lined by a mucous membrane ;— whilst the simple follicles or crypts at once pour forth their secretion upon the Secreting cells Human Liver; a, m cleus; 6, nucleolus; oil-particles. SIMPLE ISOLATED CELLS. — SECRETING CELLS. 149 ^nmce of the membrane. The accompanying figure (27) represents two follicles of the liver of the Common Crab, which are seen to be filled with secreting cells ; it seems evident, from the comparative sizes of these cells in different parts, that they originate at the blind extremity of the follicle, where there is a germinal spot ; and that, as they recede from that spot, they gradually increase in size, and become filled with their characteristic secretion, being at the same time pushed onwards towards the outlet by the continual new growth of cells at the germinal spot. In Fig. 28 are shown the corresponding ultimate follicles of the Mammary gland ; filled, like the preceding, with secreting cells. 239. The whole of the acts, then, by which the separation of the dif- ferent Secretions from the Circulating fluid is accomplished, really con- sist in the growth and nutrition of a certain set of cells, usually covering the free surfaces of the body, both internal and external, or lining cavi- ties which have a ready communication with these by means of ducts or canals.* These cells differ widely from one another, in regard to the kind of matter which they appropriate and assemble in their cavities ; although the nature of their walls is probably the same throughout. Thus we find biliary matter and oil, easily recognisable by their colour and refracting powder, in the cells of the liver; milk in the cells of the Mammary gland ; sebaceous or fatty matter in the cells of the sebaceous follicles of the skin ; and so on. All these substances are derived from the blood ; being either contained in it previously, or being elaborated from its constituents by a simple process of transformation, — as, for example, that which converts the albumen of' the blood into the caseine of milk. Hence they may be considered as the peculiar aliments of the several groups of cells ; whose acts of nutrition are the means of drawing them off, or secreting them, from the general circulating fluid. When they have attained their full growth, and accomplished their term of life, their walls either burst or dissolve away, and thus the contents of the cells are delivered into the cavity, or upon the surface, at which they are required. Now as all the canals of the glands open either directly outwards upon the surface, or into cavities which communicate with the exterior, it is evident that the various products of the action of these epithelial cells must be destined to be cast forth from the body. This we shall find to be the case ; some of them, as the bile and urine, being excretions, of which it is necessary to get rid by the most direct channel ; whilst others, like the tears, the saliva, the gastric fluid, the milk, &c., are separated from the blood, not so much for its purification, but because they are required to answer certain purposes in the economy. 240. Now whilst thus actively concerned in the Nutritive functions of the economy, and exercising in the highest degree their powers of selection and transformation, these Secreting cells appear to have nothing to do with the operation of Reproduction. We have seen that they do not even regenerate themselves ; all their energies being, as it were, con- centrated upon their own growth ; and the successive production of new broods of them being provided for by other means. Throughout the organized creation, it appears to be necessary that the true act of Gene- WKk* Th ■ * The Synovial secretion is perhaps the only one which is poured into a closed sac. 150 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. ration should be performed by the reunion of the products of two dis- tinct orders of cells ; the coalescence of which produces a germ, that is the starting-point of a new organism ; — thus differing from the act of Reproduction by gemmation or budding, which essentially consists in an extension or out-growth from the original organism, by the subdivision of cells in the manner already described (§ 212). Among the lowest Cellular Plants, in which every cell is apparently similar to the rest, this operation is effected by the "conjugation" of any pair (as it would seem) of the cells which have been produced by multiplication from the original germ. But in all the higher organisms, both Vegetable and Animal, we find that certain cells are set apart for this purpose ; and that there is an obvious distinction between the "germ-cells," from within which the germ ultimately makes its appearance, and the " sperm- cells," which communicate to them a fertilizing influence. Still in the lower tribes, both of Plants and Animals, we find that "sperm-cells" and " germ-cells" are developed in the midst of the ordinary tissues of the body; and it is only as we ascend the scale, and find the principle of division of labour carried out in other ways, that we meet, as in Man, with particular organs set apart for their evolution, and find these organs appropriate respectively to distinct individuals. 241. The spermatic cells of Man are developed within the tubuli of the Testicle ; where they appear to hold exactly the same relation to the membranous walls of those tubuli, as do the secreting cells to the tubes and follicles of the proper Glands, being, in fact, the representa- tives of their epithelial cells (Fig. 30, a). Each of these developes in its interior a variable number of secondary cells, or " vesicles of evolution ;" and within every one of these is produced a single thread-like body, dilated at one extremity, and possessed of a remarkable self-moving power, which is termed a Spermatozoa. Sometimes the vesicles of evolu- tion remain inclosed within the parent-cell, until their spermatozoa have been completely developed, and have been set free by their rupture (5) ; Fig. 30. Formation of Spermatozoa within seminal ceUs; a, the original nucleated cell; b, the same enlarged, with the formation of the Spermatozoa in progress; c, the Spermatozoa nearly complete, but still enclosed within the cell. and thus, when they have all performed their office, the parent-cell con- tains nothing but a bundle of spermatozoa (c), whose dispersion takes place as soon as its cell-wall gives way. From the very peculiar motion they possess, the Spermatozoa were long regarded qs distinct and SIMPLE ISOLATED CELLS. — REPRODUCTIVE CELLS. 151 [ependent animalcules ; it is now generally admitted, however, that they have no more claim to a distinct animal character, than have the ciliated epithelia of mucous membrane, which will likewise continue in movement when separated from the body. Similar bodies are formed by all the higher Cryptogamic Plants ; and it appears from late re- searches that their office, as in Animals, is to fertilize the contents of the "germ-cells," with which their self-moving power brings them into contact (chap, xi.) It is a curious fact that the seminal cells, in which the Spermatozoa are formed, are ejected from the gland in certain Crus- tacea, not only before they have burst and set free their Spermatozoa, but even long before the development of the Spermatozoa in their inte- rior is completed; — thus affording a complete demonstration of their independent vitality. 242. The " germ-cells," in like manner, are very commonly developed among the lower Animals as the epithelia of the tubes or follicles which constitute the ovary ; but in Man and the higher Animals, the ovary is a solid organ, and the germ-cells are developed in its substance, lying in the midst of the dense fibrous tissue which forms its parenchyma. These germ-cells, which are known as "ovisacs," like the sperm-cells, develope secondary cells or ova in their interior ; each ovisac, however, producing but a single ovum. The ovum, again, contains a tertiary cell, the germiT^al vesicle^ whose contents appear to mingle with those of the sperm-cell in the act of fecundation, so that the fertilized germ is the result ; the remaining contents of the ovum being the nutritive materials, at the expense of which this germ undergoes its first development (chap. XI.) 243. We now proceed to a class of cells, which are equally indepen- dent of each other, which begin and end their lives as cells, without undergoing any transformation, but which form part of the substance of the fabric, instead of lying upon its free surfaces and being continually cast off from them. Still their individual history is much the same as that of the cells already noticed ; and they differ chiefly in regard to the destination of their products. The first group of this class deserving a separate notice, is that which effects the introduction of aliment into the body ; of those kinds of aliment, at least, which are not received in solu- tion by any more direct means. Along the greater part of the intestinal tube, from the point at which the hepatic and pancreatic ducts enter it, to the rectum, we find the mucous membrane furnished with a vast num- ber of minute tufts or folds, by which its free surface is vastly extended ; these are termed villi. They may be compared to the ultimate root- fibres of trees, both in structure and function.; for each of them gives origin to a minute lacteal or chyle-absorbing vessel, which occupies its centre ; whilst it also contains a copious network of blood-vessels (Fig. 10, p. 127), which appears likewise to participate in the act of absorp- tion, by taking up substances that are in complete solution. Now at the end of every villus, there may be seen, whilst the process of digestion and absorption is going on, a cluster of minute opalescent globules, in the midst of which the origin of the lacteal is lost. These globules, whose size varies from 1-lOOOth to l-2000th of an inch, are composed of a milky fluid, which is evidently the same with that which is found in 152 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. the lacteals ; and it is maintained by Prof. Goodsir, who first brought them into notice, that these globules are really cells^ and that it is by Fig. 31^ Diagram of mucous membrane of Jejunum, when absorption is not going on; a, epithelium of a villus; b, secreting epithelium of a follicle; c, c, c, primary membrane, with its germinal spots or nuclei, d, d; c, germs of absorbent vesicles ; /, vessels and lacteals of villus. their, growth and nutrition that the milky fluid, or chyle, is selected from the contents of the digestive cavity. Their function, therefore, would be precisely the converse of that of the secreting cells already described ; whilst the history of their individual lives is the same. These absorbent cells draw their materials from the fluid in the digestive cavity, instead of from the blood ; and when they burst or liquefy, they set free their contents where they may be taken up by a lacteal and conveyed into the circulating current, instead of pouring them into a cavity through which they will be shortly expelled. In the intervals of the digestive process, however, the extremities of the villi are comparatively flaccid ; and instead of cells, they show merely a collection of granular particles (Fig. 31, e), which are considered by Prof. Goodsir to be cell-germs. There is considerable doubt, however, whether these supposed cells are anything else than oil-globules ; and whether the real agents in the selection of chyle are not the epithelium-cells covering the villus (a), within which chylous-looking globules have been occasionally seen, when digestion was going on. 244. Although the Mucous membrane of the intestinal tube is the only channel through which insoluble nutriment can be absorbed in the completely formed Mammal, and the only situation, therefore, in which we meet with these absorbent cells, there are other situations in which similar cells perform analogous duties in the embryo. Thus the Chick derives its nutriment, whilst in the egg, from the substance of the yolk, by absorption through the blood-vessels spread out in the vascular layer of the germinal membrane surrounding the yolk ; which vessels answer to the blood-vessels and lacteals of the permanent digestive cavity, and are raised into folds or villi as the contents of the yolk-bag are diminished. Now the ends of the vessels are separated from the fluid contents of the yolk-bag, by a layer of cells ; which seems to have for its object to select and prepare the materials supplied by the yolk, for being received into the absorbent vessels. 245. In like manner, the embryo of the Mammal is nourished, up to ABSORBENT CELLS. 153 le time of its birth, through the medium of its umbilical vessels ; the ramifications of which form tufts, that dip down, as it were, into the maternal blood, and receive from it the materials destined to the nutri- tion of the foetus, besides eifecting the aeration of the blood of the latter, by exposing it to the more oxygenated blood of the mother. Now around the capillary loop of the foetal tuft, there is a layer of cells, closely re- sembling the absorbent cells of the villi ; and these are enclosed in a cap of basement-membrane, which completes the foetal portion of the tuft, and renders it comparable in all essential respects to the intestinal villus. It is again surrounded, however, by another layer of membrane and of cells, belonging to the maternal system ; — the derivation and arrange- ment of which will be explained hereafter. The maternal cells (b, Fig. |2), may be regarded as the first selectors of nutriment from the circu- Fig. 32. Extremity of a placental villus : — a, external membrane of the villus, continuous with the lining mem- brane of the vascular system of the mother ; b, external cells of the villus, belonging to the placental decidua; c, c, germinal centres of the external cells.; d, the space between the maternal and foetal portions of the villus; c, the internal membrane of the villus, continuous with the external membrane of the chorion; /, the internal cells of the villus, belonging to the chorion; g, the loop of umbilical vessels. lating fluid of the parent : the materials, partially prepared by them, are poured into the cavity (d) surrounding the extremity of the tuft ; and from this they are taken up by the foetal cells (/), ^which further elabo- rate them, and impart them to the capillary loop [g) of the umbilical vessels. 246. Thus we see that the several functions of Selection, Absorption, Assimilation, Respiration, Secretion, and Reproduction, are performed by the agency of cells in the Animal as in the Vegetable kingdom, — in the complex Human organism, as in the humblest Cryptogamic Plant : the only difference being, that in the latter there is a greater division of labour, different groups of cells being appropriated to different functions, in the general economy, whilst the history of their own processes of nutrition and decay is everywhere essentially the ^ame. Thus we have seen that the Absorbent cells, at the extremities of the intestinal or pla- cental villi, select and draw into themselves, as the materials of their own growth, certain substances in their neighbourhood ; which are still as much external to the tissues of the body, as are the fluids surrounding the roots of plants. Having come to their full term of life, they give up their contents to the absorbent vessels, which carry them into the general current of the circulation, where they are mingled with the fluid previously assimilated, — the blood. Whilst passing through the vessels, they are subjected to the action of the various cells (all of which we have seen to be successive phases of the same type) which float in the circu- lating current; and by these they seem to be gradually assimilated, Or 154 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. converted into a substance of a more directly organizable character. The special function of the red corpuscles peculiar to Vertebrated animals, though not yet accurately known, seems intimately connected with the process of Respiration. Next we have various groups of cells, external to the vessels, on the free surfaces of the body ; whose office it is to draw from the blood certain materials, which are destined for Secretion or separation from it ; either for the sake of preserving that fluid in its re- quisite purity, or for answering some other purpose in the system. These cells grow at the expense of the substances which they draw into themselves from the blood ; and on their dissolution, they cast forth their contents on the free surfaces communicating with the exterior of the body, to which they are in time conveyed. And, lastly, we have a special set of O-enerative cells, destined in the one sex to prepare the germs of new beings ; and in the other to elaborate a product essential to their fertilization. 247. The cells which are thus the active instruments of the Organic functions, are usually produced and succeed one another with a rapidity proportional to the energy of those functions, though the causes which influence their gronvth and decay are not always evident. Thus it is certain that, cceteris paribus, the rate of production of the Secreting cells depends upon the abundance of the materials supplied by the circu- lating current, which they are destined to eliminate from it. But this is by no means the sole condition of their development ; for, as we shall see hereafter, these materials may accumulate unduly in the blood, through the insufficient activity of the cells which are destined to sepa- rate them ; whilst, on the other hand, the presence of certain substances in the blood appears to accelerate their production. Of these stimuli, Mercury is one of the most powerful ; and we have continual opportu- nities of Avitnessing its efi*ects, in giving an increased activity to the secreting actions. There is probably not a gland in the body, which is not in some degree influenced by its presence in the blood ; but the liver, the kidneys, the salivary glands, and the glandulae of the intestinal canal, appear to be those most afi'ected by its stimulating powers. The action of the glands, in other words the development of the secreting cells, appears to be influenced by mental emotions ; being sometimes accele- rated, and sometimes retarded, through their agency. This is especially the case in regard to the secretion of Milk, Tears, Saliva, and Gastric juice. It seems probable that the influence thus manifested is partly exerted through the capillary circulation, which is known to be powerfully afi'ected by mental emotions, as in the acts of blushing and erection ; and that the increased production of the secretion is im^mediately due to the increased flow of blood to the gland. But there are other phenomena which show that the development and actions of the secreting cells are more directly influenced by the nervous system ; these will be hereafter considered (chap. IX.) 5. Of Cells connected together as permanent constituents of the Tissues. 248. We now pass on to consider those Cells, which enter as compo- nent elements into the solid and permanent fabric of the body, and which I CELLS CONNECTED TOGETHER IN SOLID TISSUES. 155 do not take so active a part in its vital operations. These we shall find to be usually more or less closely connected together, either by a general enveloping membrane^ or by an intercellular substance^ which is inter- posed between their walls, and holds them together by its adhesive properties. 249. The presence of a general enveloping membrane (where it is not a secondary formation) appears to depend upon the persistence of the original cell-walls ; which, instead of liquefying or thinning away, when distended by the multiplication of cells in their interior, are thickened or strengthened by additional nutrition. Such is perhaps the case with the sacculi in which the cells of Adipose tissue ( § 257) are often found clustered together; but this condition is usually much more obvious in many tumours, whose development depends upon an abnormal process of growth. 250. Where such enveloping membranes are wanting, we frequently find the component cells of the permanent tissues of Animals (like those of the higher plants) held together by an intercellular substance ; which generally presents no distinct traces of organization ; and which usually consists of Gelatine, or of a substance allied to it in composition. The proportion of this substance to the cells may vary in different cases ; and very different characters may thus be presented by a tissue made up of the same elements. Thus the subjoined figure (33) represents a portion of one of the animal layers included between the calcareous laminae of a bivalve shell ; in which we see on the one side a number of nuclei or incipient cells, scattered through a bed of homogeneous intercellular sub- stance, and bearing but a very small proportion to it ; whilst the opposite end exhibits a set of polygonal cells, in close contact with each other, the intercellular substance being only represented by the thick dark lines, which mark the boundaries of the cells, and which are rather thicker at the angles of the latter. Between these two extremes, we observe every stage of transition. 251. The presence of a very large amount of intercellular substance, through which minute cells are scattered at considerable intervals (Fig. 33, a), is characteristic of various forms of Cartilage ; and more par- ticularly of that soft semi-cartilaginous structure, of whfch the Jelly-fish are for the most part composed. In other forms of cartilage, we find the cells more developed, and in closer proximity to each other, the proportion of the intercellular substance being at the same time dimi- nished (as seen at h and c, Fig. 33) ; but it is not often, save in the embryonic structures, that we find the cells in such, close proximity, and the intercellular substance so nearly wanting, as at d. Such examples do occasionally present themselves, however, even in the soft tissues. Thus the chorda dorsalis, which replaces the vertebral column in the lowest Fishes, and of which the analogue is found in the embryos of the higher Yertebrata, is made up of a structure of this kind (Fig. 16), The true Skin in the Short Sun-fish, is replaced by a similar layer of cellular tissue, which extends over the whole body, varying in thickness from one-fourth of an inch to six inches. And in the Lancelot (a little fish which is destitute of so many of the characters of a Vertebrated animal, that its right to a place in that division has been doubted), a considerable portion of the fabric is made up of a similar parenchyma. 156 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 252. Now we shall find that one method, by which the requisite firm- ness and solidity are given to the animal fabric, consists in the depo- Fig. 33. Portion of shell-membrane, showing the origin of cells in the midst of horny intercellular substance; a, nuclei; 6, incipient cells ; c, the same further advainced, but separated by intercellular substance ; d, the, cells become polygonal by mutual pressure. sition of earthy substances in the interior of such cells, by a peculiar secreting action of their own. Thus in Shell, we find them completely filled up with carbonate of lime ; and in the enamel of Teeth with phos- phate of lime. When this is the case, there is a tendency to an apparent coalescence of the cells, by the obliteration of their partitions ; or rather, perhaps, by the removal of the whole intercellular substance from between them, the actual cell-walls being so very thin, that they are not distin- guishable. The incipient stages of this coalescence, as seen in another portion of th^ same membrane as that represented in the last figure, are shown in Fig. 34. At a, the nucleated cells are very distinct ; and are separated by a large quantity of intercellular substance. At h, they approach each other more closely, the amount of intercellular substance being less ; the widest intervals are seen at the angles of the cells. At c, the approximation is much closer ; and the cell-walls are scarcely dis- tinguishable at the points where they come into immediate contact. Proceeding further, we observe that the partitions are much less com- plete ; so that the originally distinct cellular character of the membrane Fig. 84. Portion of shell-membrane, showing the gradual coalescence of distinct cells ; at a, the cells separated by intercellular substance; at b, the partitions are thinner; and at c, they almost disappear. is chiefly indicated by the bright nuclei, which are regularly dispersed through it, and by the triangular dark spots, Avhich show the remain's of '^l( FUSIFORM CELLS. 157 e intercellular substance, at the angles where three cells join each other. The coalescence may be traced further than it is shown to do in the figure ; so that if it were not for the evidence afibrded by the transition-stages here represented, it would be difficult to prove that the membranous layer had its origin in cells. __ 253. These facts, respecting the gradual coalescence of cells, explain not merely certain appearances presented in Tooth, Shell, &c. (here- after to be described) ; but also those which are exhibited by the Base- ment-membrane, as already detailed (§ 206). 254. There is no evidence, in the preceding case, that the cavities of the cells coalesce ; and there is no reason why they should do so. But we often find such a union, where the production of a continuous tube is required. The long straight open ducts, through which the sap of Plants rises in the stem, are unquestionably formed by a coalescence of the cavities of cells of a cylindrical form, placed regularly end to end ; and it seems probable that the network of anastomosing vessels, through which the elaborated sap finds its way to the various parts of the vege- table fabric, is formed, in like manner, by the coalescence of cells, arranged obliquely and transversely in regard to one another. In like manner, the capillary Blood-vessels of Animals are usually believed to originate in rows of cells, the cavities of which have run together by the obliteration of the transverse partitions ; as the persistent nuclei of such cells may be occasionally brought into view in the walls of the capilla- ries. And the same appears to be the origin of the tubular fibres of Muscular and Nervous tissue, which contain the elements characteristic of those tissues ; these elements, — the fibrillse of muscle and the granular pith of the nerve-tube, — being evidently the secondary products of parent- cells, which seem to remain as their investing tubuli, in the walls of which the original nuclei are often to be seen (§ 338 and 388). 255. Besides these changes, the original cells may often undergo marked alterations of form ; and this quite independently of any pressure to which they may be subject. Thus the pigment-cells, as already men- tioned (§ 229), frequently exhibit a curious stellate form ; arising from the development of radiating prolongations, which are put forth from the original spheroid. A form which is frequently assumed by the cells that are developed in fibrinous or plastic exudations, and which is also met with in the cells of tumours, both malignant (or Cancerous) and non-malignant, is that which has received the designation oi fusiform or spindle-like, from its prolonged shape and pointed extremites. The A^arious stages of transition, which may be observed between the simple rounded cell and the fusiform cell, have been shown in Fig. 7 ; and it is there seen that, when the transformation has gone to its utmost extent, the nucleus of the cell is no longer visible, so that it bears a close re- semblance to a simple fibre. Such cells are found amongst the simple fibrous tissues , and, in the opinion of many, they give origin to them. The appearance of tissue composed of fusiform cells, is shown in Fig. 35 ; this is seldom met with as a permanent part of the normal fabric ; but it is a frequent product of morbid action. 256. We now proceed with the description of the various tissues in the Human body, which are composed of cells united or transformed 158 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. in the foregoing manner ; and we shall commence with Adipose or Fatty tissue, which may be considered as a sort of link, connecting the permanent tissues with those which are more actively concerned in the processes of Nutrition, Secretion, &c. Fig. 35. Fig. 36. Fusiform tissue of plastic exudations ; a, fusiform bo- dies without nuclei ; b, nu- cleated fusiform ceils; c, granular intercellular sub- stance. Areolar and Adipose tissue; a, a, fat-cells; 6, b, fibres of areolar tissues. 257. The Adipose tissue is composed of isolated cells, which have the power of appropriating fatty matter from the blood, precisely in the same manner as the secreting cells appropriate the elements of bile, milk, &c. These cells are sometimes dispersed in the interspaces of the Areolar tissue ; whilst in other cases they are aggregated in distinct masses, — constituting the proper Adipose tissue. In the former case they are held in their places by fibres, that traverse the areolae in different direc- tions ; whilst in the latter, each small cluster of fat-cells is included in a common envelope, on the exterior of which the blood-vessels ramify ; and these sacculi are held together by areolar tissue. We are thus probably to regard each fatty mass in the light of a gland, or assemblage of secreting cells, penetrated by blood-vessels, and bound together by fibrous tissue ; but having its follicles closed instead of open (which appears to be the early condition of the follicles of all glands, § 238) : and consequently retaining its secretion within itself, instead of pouring it forth into a channel for excretion! Capillary network around Fat-cells. 258. The individual fat-cells always present a nearly spherical or sphe- roidal form ; sometimes, however, when they are closely pressed together. ADIPOSE TISSUE. 159 ie^t)ecome somewhat polyhedral, from the flattening of their walls against each other. Their intervals are traversed by a minute net-work of blood-vessels (Fig. 37), from which they derive their secretion ; and it is probably by the constant moistening of their walls with a watery fluid, that their contents are retained without the least transudation^ although they are quite fluid at the temperature of the living body. If the Avatery fluid of the cell-walls of a mass of Fat be allowed to dry up, and it be kept at a temperature of 100°, the escape of the contained oily matter is soon perceptible. — By this provision, the fatty matter is alto- gether prevented from escaping from the cells of the living tissues, by gravitation or pressure ; and as it is not itself liable to undergo change when secluded from the air, it may remain stored up, apparently unal- tered, for almost an unlimited period. 259. The consistency, as well as the Chemical constitution, of the fatty matter contained in the Adipose cells, varies in different animals, according to the relative proportions of three component substances which may be distinguished in it, — Stearine, Margarine, and Oleine. The two former are solid when isolated, and the latter is fluid ; but at the ordinary temperature of the warm-blooded animal, they are dissolved in it. Of these, Stearine is the most solid ; and it is the most largely present, therefore, in the hardest fatty matter, such as mutton-suet. It is crystalline like spermaceti ; it is not at all greasy between the fingers, and it melts at 143°. It is insoluble in water, and in cold alcohol and ether : but it dissolves in boiling alcohol or ether, crystallizing as it cools. The substance termed Margarine exists along with stearine in most fats, but it is the principal solid constituent of Human fat, and also of Olive oil. It corresponds with Stearine in many of its proper- ties, and is nearly allied to it in Chemical composition ; but it is much more soluble in alcohol and ether, and it melts at 118°. On the other hand, Oleine, when pure, remains fluid at the zero of Fahrenheit's ther- mometer ; and it is soluble in cold ether, from which it can only be separated by the evaporation of the latter. It exists in small quantity in the various solid fats ; but it constitutes the great mass of the liquid fixed oils. The tendency of these to solidification by cold, depends upon the proportion of stearine or margarine they may contain. 260. All these substances are neutral compounds formed by the union of Stearic, Margaric, and Oleic acids, respectively, with a base termed Glycerine ; this base may be obtained from any fatty matter, by treat- ing it with an alkali, which unites with the acid 'and forms a soap, setting free the Glycerine. They contain no Nitrogen ; and their pro- portion of Oxygen is extremely small in comparison with their amount of Carbon and Hydrogen : thus Stearine has 142 Carbon and 141 Hy- drogen to 17 Oxygen : and in the other substances the proportions are similar. The fatty bodies appear to be mutually convertible ; thus mar- garic acid may be procured from stearic acid, by subjecting it to dry distillation ; and there is ample evidence that animals supplied with one of them may produce the others from it. 261. Since these Fatty matters are abundantly supplied by the Vege- table kingdom, and are found to exist largely in substances which were not previously supposed to contain them, it is not requisite to suppose, , noipre 160 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. that Animals usually elaborlite them by any transforming process -from the elements of their ordinary food.. The mode in which they are taken into the blood, and the uses to which they are subservient, will be here- after investigated : but it may be here remarked, that the portion sepa- rated from the circulating fluid to form the Adipose tissue, is only that which can be spared from the other purposes, to which the fatty matters have to be applied. Hence the production of this tissue depends in part upon the amount of Fatty matter taken in as food ; but this is not en- tirely the case, as some have maintained ; for there is sufficient evidence that animals may produce fatty matter by a process of chemical trans- formation, from the starch or sugar of their food, when there is an uh- usual deficiency of it in their aliment. 262. The development of Adipose tissue in the body appears to an- swer several distinct purposes. It fills up interstices, and forms a kind of, pad or cushion for the support of moveable parts ; and so necessary does it seem for this purpose, that, even in cases of great emaciation, some fat is alway-s found to remain, especially at the base of the heart around the origin of the great vessels^ and in the orbit of the eye. It also assists in the retention of the animal temperature by its non-con- ducting power ; and we accordingly find a thick layer of it, in those warm-blooded mammals that inhabit the seas, — either immediately be- neath their skin, or incorporated with its substance. Its most important use, however, is to serve as a reservoir of combustible matter, at the expense of which the respiration may be maintained when Other mate- rials are deficient ; thus we find that the respiration of hybernating animals is kept up, during the period when they cease taking food (§ 121), by the consumption of the store of fat which was laid up in their bodies, previously to their passing into that state; and it is also to be noticed that herbivorous animals, whose food is scanty during the winter, usually exhibit a strong tendency to such an accumulation, during the latter part of the summer, when their food is most rich and abundant, in order to supply the increased demand created by the low external temperature of the winter season. Other circumstances being the same, it appears that the length of time during which a warm-blooded animal can live without food, depends upon the quantity of fat in its body; for the rapid lowering of its temperature, which is the immediate cause of its death (§ 117), takes place as soon as the whole of this store has been exhausted. Of the means by which the fatty secretion is taken back again into the current of the circulation, when it is required for use in the system, we know nothing whatever. 263. In order that it may be applied to the maintenance of the ani- mal heat, the fatty matters must be received back into the blood ; and although we have no certain knowledge of the mode in which this is accomplished, yet it may be surmised to be as follows. The Blood nor- mally contains a certain amount of fatty matter, held in solution by combination with its alkali ; and should this be exhausted by the com- bustive process, the circulating current will draw into itself a fresh sup- ply from the interior of the fat-cells ; — it having been shown by Mat- teucci that oleaginous particles will pass through animal membranes by CARTILAGE. 161 idosmose, to diffuse themselves through an aqueous liquid, provided the latter be alkaline. 264. In the simpler forms of Qartilage^ we have an example of a tissue of remarkable permanence, composed entirely of cells scattered through an intercellular substance. This substance has received the distin- guishing appellation of Qhondrine^ which marks it as the solidifying" ingredient of Cartilage (§177). All the Cartilages of the foetus, — those which are to be converted into bone, as well as those which are to remain unossified, — are composed of it ; and yet, as soon as the process of Ossification commences, the chondrine is replaced by Gelatine, which is the sole organic constituent of the animal basis of bones. The per- manent cartilages, however, still contain only Chondrine ; but if acci- dental bony deposits should take place in them (as frequently happens in old persons, especially in the cartilages of the ribs), the Chondrine gives place to Gelatine. This change of composition is coincident, as we shall hereafter see, with a complete change in texture ; the basis of bony tissue not being Cartilage (as commonly imagined), but consisting of a substance much more nearly allied to the white fibrous tissue. — It is only in the pure cellular cartilages, in which the intercellular sub- stance presents no trace of organization, that Chondrine occurs. Those of the ^5ro-cartilages (§ 269), in which the intercellular substance has the characters of the White fibrous tissue, yield gelatine on boiling, in the manner of the ligaments and tendons ; whilst those which contain much of the Yellow or elastic tissue, undergo very little change by boil- ing, and only yield, after several days, a small quantity of an extract which does not form a jelly, but which has the other chemical properties of Chondrine. 265. Besides the organic compounds already described, most Carti- •lages contain a certain amount of mineral matter, which forms an ash when they are calcined. This ash contains a large proportion of car- bonate and sulphate of soda, together with carbonate of lime, and a small quantity of phosphate of lime ; as age advances, the proportion of the soluble compounds diminishes, and the phosphate of lime predo- minates. This is especially the case in the costal cartilages, which almost invariably become converted into a semi-ossified, substance, in old persons ; and it is remarkable that, even before they have themselves become thus condensed, they are united by ossific matter, when they have undergone fracture. Fig. 38. ^ Cartilage of Mouse's ear. When a pure Cellular Cartilage is examined microscopically, 11 162 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. its cells are seen to lie, sometimes singly, and sometimes in clusters of | two, three, or four, in cavities excavated in the intercellular substance. These occur at very variable distances ; for in some instances they are packed together as closely as the cells of a vegetable parenchyma (Fig. 38); whilst in others the principal mass is composed of intercellular substance, through which the cells are interspersed at wide intervals. From the various appearances which may be observed in the same car- tilage, at different stages of its growth, it would appear that the compo- nent" cells multiply by the doubling process already described (§ 212) ; that they then separate from one another, each of them drawing towards itself (as it were) an envelope of intercellular substance ; and that, by the repetition of the same process, the number of cells in the cartilage may be indefinitely multiplied. 267. Various stages of this history are shown in the accompanying figure (Fig. 39), which is taken from a section of the cartilaginous bran- chial ray of the larva or tadpole of the Rana esculenta, or Edible Frog. In the centre of the figure are shown three separate cells, which have evidently been at one time in closer proximity with each other. In one of these cells, the nucleus is seen to be developing two new cells in its interior ; and a continuation of this process would give rise to the appear- ance shown at 5, where two cells are shown in close contact, being evi- dently the offspring of the same parent. Now if each of these cells in like manner developes two others within itself, a cluster of four will be developed, as shown at a ; and after a time, intercellular substance being Section of the Branchial cartilage of Tadpole ; a, group of four cells, separating from each other ; b, pair of cells in apposition; c, c, nuclei of cartilage cells; d, cavity containing three cells. accumulated around each, their walls will separate, and they will acquire the character of distinct cells. It would seem as if, in other cases, one of the first pair of cells developes another pair in its interior, whilst the other (from some unknown cause), does not at once proceed to do so ; and thus only three cartilage-cells instead of four are clustered together in the cavity, as seen at d. 268. The primitive cellular organization now described is retained in some Cartilages through the whole duration of their existence. This is the case, for example, in most of the articular cartilages of joints ; in the cartilaginous portion of the septum narium, in the cartilages of the alae and point of the nose, in the semilunar cartilages of the eyelids ; in the cartilages of the larynx (with the exception of the epiglottis), the CARTILAGE. 163 cartilages of the trachea and bronchial tubes, the cartilages of the ribs, and the ensiform cartilage of the sternum. When partial ossific depo- sits take place, it is usually in the substance of cellular^ rather than in that oi fibrous cartilage. 269. When the intercellular substance, instead of being homogeneous^ has a fibrous character, the tissue called Fihro-Cartilage is produced ; and this may be either elastic or non-elastic, according as the yellow or the white form of fibrous structure prevails. In some instances, the fibrous structure is so predominant over the cellular, that the tissue ^^as rather the character of a ligament than of a cartilage. The white Jfebrous structure is seen in all those cartilages, which unite the bones by synchondrosis, and which are destined not merely to sustain pressure, but also to resist tension. This is the case especially in the substances which intervene between the vertebrae, and which connect the bones of ■bhe pelvis; these in adult Man are destitute of cartilage-corpuscles, ex- ^Bept in and near their centres ; but in the lower Vertebrata, and in the ^nrly condition of the higher, the fibrous structure is confined to the ^Rxterior, and the whole interior is occupied by the ordinary cartilage- Hfcorpuscles. The yellow-fibrous or reticulated structure is best seen in T the epiglottis, and in the concha of the ear ; in the former of these, ' scarcely any trace of cartilage-corpuscles remains ; and in the latter, the cellular structure is only to be met with towards the tip. 270. We have seen that the elements of the cellular tissues hitherto described, do not come into direct contact with the blood-vessels. The Epidermic and Epithelial cells are separated from them by the conti- nuous layer of basement-membraile, which forms the surface of the true skin, of the mucous membranes, of the glandular follicles produced from them, &c. In like manner, the cells of Adipose tissue are formed within membranous bags ; around which the blood-vessels form a minute network. The cells of Cartilage are not nourished in any more direct manner; and are sometimes at a considerable distance from Fig. 40. _^ Vessels situated between the attached synovial membrane, and the articular cartilage, at the point where the ligamentum teres is inserted in the head of the os femoris of the human subject, between the third and fourth months of foetal life ;— a, the surface of the articular cartilage ; h, the vessels between the articular cartilage and the synovial membrane ; c, the surface to which the ligamentum teres waa ~**-iChed; d, the vein; 6, the artery. e nearest vessels. It is certain that the substance of the permanent cellular Cartilages is not permeated, in a state of health, even by the minutest nutrient vessels ; none such being brought into view under the 164 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. highest magnifying power. They are, however, surrounded by vessels (Fig. 40), which form large ampullce or varicose dilatations at their edges, or spread over their surfaces ; and it is by the fluid which is drawn from them by the Cartilage-cells, that the latter are nourished. The nutrition of a mass of Cartilage thus seems to bear a strong resem- blance to that of the thick fleshy Sea-weeds, which are in like manner composed entirely of cells, with intercellular substance disposed between them in greater or smaller amount. The cells in nearest proximity to the nutrient fluid, draw from it the requisite materials, and transmit these to the cells in the interior of the mass, receiving a fresh supply in their turn from the source' in their own neighbourhood. When the Ar- ticular or other cellular Cartilages are inflamed, however, we find ves- sels passing into their substance ; but these vessels are formed in an entirely new tissuCj which is the product of the inflammatory process, and cannot be said to belong to the Cartilage itself. 271. The temporary Cartilages, which have a like cellular structure, but which are destined to undergo metamorphosis into Bone, are equally destitute of vessels when their mass is small ; but if their thickness ex- ceed an eighth of an inch, they are permeated by canals for the trans- mission of vessels. Still these vessels do not ramify with any minute- ness in the 'tissue; and they leave large islets, in which the nutritive process must take place on the plan just described. 272. The Fibro-Cartilages, formed as it were by the intermingling of two distinct elementary structures, have a degree of vascularity propor- tioned to the amount of the fibrous tissue which they contain ; but these vessels do not penetrate the cellular' portions. Adhere such are distinct from the mixed structure. 273. The Cartilaginous tissue appears to be more removed than almost any other in the body from the general tide of nutritive action. Its properties are simply of a physical character ; and they are not im- paired for a long time after the death of the tissue, its tendency to de- composition being very slight, so long as it is exposed to ordinary tem- peratures. It is protected by its toughness and elasticity from those mechanical injuries to which softer or more brittle tissues are liable ; and consequently it has little need of any active power of reparation. When loss of substance occurs as a result of disease or accident, this seems never to be repaired by real cartilaginous substance ; but the space is filled up by a fibrous tissue developed from the reparative blastema (§ 213). It is in this tissue that the new vessels are found, which have been erro- neously supposed to penetrate the cartilage when it becomes inflamed ; the fact being, that the vessels are restricted to the "false membrane" formed in the inflammatory process, which takes the place of the carti- laginous tissue that has disappeared in consequence of imperfect nutri- tion or degeneration. 274. The Cornea of the Eye bears a superficial resemblance to Cartilage; but it corresponds rather with Fibrous Membranes in its elementary structure. Besides its anterior or conjunctival layer, which consists of epithelium, and its posterior layer of cells constituting the epithelium of the aqueous humour, the Cornea has been shown by Mr. Bowman to consist of three layers, of which the anterior and the poste- rior (which are very thin) have some of the characters of the yellow CELLS CONNECTED TOGETHER. — CORNEA. 165 elastic tissue, whilst the middle one, which forms its principal thick- ness, is composed of white fibres interlaced together in such a manner Fig. 41. »H&f^^^^^^ L ""Vi^ -. '■ ^ ^^Hn' J^< ^ ^ --'v" \ J \ z^X^- 1 S^ ^tHiSL ^ki^t (& Mi r'ff A TyjWWJ. tJn'' / ^HHi u^ irit Trit.' "-5 '£^^'^1 rA. H/^ifeCOv viWX ^^fe ^^Mm ^*°?P-i\ Nutrient Vessels of the Cornea;— A, Superficial vessels belonging to the Conjunctival membrane, and mtinued over the margin of the Cornea; b, Vessels of the Sclerotic, returning at the margin of the Cornea. as to form numerous lamellse, their interspaces or areolae having the form of tubes regularly arranged and constricted at intervals, so as not to be unlike rows of cartilage-cells, for which in fact they have been mistaken. — Two sets of vessels, a superficial and a deep-seated, surround the margin of the cornea. The former (Fig. 41, A,) belong rather to the Conjunctival membrane, which forms the outer layer of the cornea ; and they are prolonged to the distance of l-8th or half a line from its margin, then returning as veins. The latter (b) do not pass into the true Cornea, but terminate in dilatations from which veins arise, just where it becomes continuous with the sclerotic. In diseased conditions of the Cornea, however, both sets of vessels extend themselves through it. Notwithstanding the absence of vessels in the healthy condition of the corneal tissue, incised wounds of its substance commonly heal very readily, as is well seen after the operation for Cataract ; but there is a danger in carrying the incision around a large proportion of its margin, lest the tissue should be too much cut ofi" from the supply of nutriment afi'orded by the ampullae of the vessels that surround it. 2T5. The Crystalline Lens of the Eye approaches Cartilage, in its structure and mode of nutrition, more nearly than any other tissue. It may be separated into numerous laminae; which are composed of fibres that lock into one another, by their delicately-toothed margins. Each of these fibres appears to be made up of a series of cells, linearly arranged, which coalesce at an early period. The lens is not per- meated by blood-vessels ; at least after it has been completely formed ; these being confined to the capsule. During the early part of foetal life, and in ii;iflammatory conditions of the Capsular membrane, both its anterior and its posterior portions are distinctly vascular ; but at a later period, only the posterior half of the Capsule has vessels distri- buted upon its surface. It has been shown by optical experiments 166 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. devised for the purpose, that a moderate vascularity of the posterior capsule does not interfere with distinct vision ; whilst if the anterior capsule were traversed by vessels, the picture on the retina would be no longer clear. — The substance of which the lens is composed, ap- pears to be soluble Albumen, or perhaps more closely resembles the Globulin of the blood. 276. The Vitreous body, which fills the greater part of the globe of the eye, also seems to possess a cellular structure ; the cells containing a fluid, which is little else than water holding in solution a small quantity of albumen and saline matter ; and the membrane which forms their walls being so pellucid as to be scarcely distinguishable. Indeed the cellular character of this substance is chiefly inferred from the fact, that when its capsule or enveloping membrane is punctured, even in several places, the contained fluid does not speedily drain away, — as it would do if it were merely contained in the interstices of an areolar tissue. The blood-vessels which traverse the Vitreous body do not send branches into its substance ; and it must derive its nutriment from those which are distributed minutely upon its general envelope, and probably also from the large plexiform vessels of the ciliary processes of the Choroid coat. 277. Before proceeding to describe the structure of Bone, to which it seems natural to pass on from Cartilage, it will be useful to advert to the modes in which the tissues of Invertebrated animals are consolidated by deposits of solid matter, in order that they may afibrd the requisite support and protection, without that interstitial growth which is pecu- liar to the skeletons of the Vertebrated classes. — Commencing with the Polypifera, or Coral-forming animals, we observe that their strong axes or sheaths are destined only to give support to their softer structures, and that the parts once consolidated undergo no subsequent change. It was formerly imagined, that the stony Corals were "built up" by the animals which form them, somewhat in the same manner as a Bee con- structs its cell. But it is now fully demonstrated, that the calcareous matter (which here consists solely of the Carbonate of Lime) is combined with the living tissue ; and that the most solid mass of Coral thus has an organized basis. The proportion of earthy to animal matter, how- ever, is so great in these structures, that very little, if any, nutrient changes can take place in their tissues, when once it has become con- solidated. Such changes are not, however, required. The substance thus developed by the attractive pow-er of the soft gelatinous tissues, which draw into themselves the small quantity of calcareous matter dis- solved in the surrounding water, is so little disposed to undergo change, that it will maintain its solidity for centuries ; and even when acted on by water or by heat, it does not undergo disintegration, for its calcareous particles arrange themselves in a new method, and become converted into a solid crystalline rock. Such rocks, the product of the metamor- phosis of ancient coral-formations, make up a large proportion of the external crust of the earth. The solid stem or sheath, once con- solidated, appears to undergo no further change in the living Coral- structure ; for its increase takes place, not by interstitial but by super- ficial deposit, — that is, not by the diffusion of new matter through its I SHELLS OF MOLLUSCA. 167 whole substance, separating from each other the parts formerly de- posited, but by the mere addition of particles to its surface and extre- mities. In this manner the growth of a solid Coral-structure may go on to an enormous extent ; the surface at which the consolidating action is going on, being the only part alive, that is exhibiting any^ vital change ; and all the rest of the mass being henceforth perfectly inert. 278. In the class of Echinodermata, which includes the Star-fish, Sea-Urchin, &c., we find the calcareous structure presenting a very elaborate organization ; as an example of this, we shall select the shell of the Echinus, commonly known as the Sea-Egg. This shell is made up of a number of plates, more or less regularly hexagonal, and fitted together so as completely to enclose the animal, except at two points, one of which is left open for the mouth, the other for the anus. On the surface of these plates are little tubercles, for the articulation of the spines, which serve as instruments of defence and of locomotion. The substance of the shell and of the spines is exactly alike ; being a sort, of areolar tissue, consolidated by the deposition of calcareous matter, and having an innumerable number of interspaces or minute cancelli, freely communicating with each other. The arrangement of this calcareous network in the spines is most varied and elaborate ; and causes thin sections of them to be among the most beautiful of all microscopic ob- jects. The external and internal surface of each plate, in the shell of the living Echinus, is covered with a membrane, from which its nutrition Fig. 42. ooc ^ O O o OOOOo'cTo O OjQy O-P ooDOoo o 9_2^ o o o o o — ooooo/ OOOy Portion of the shell of the Echinus, showing at a the constituent plates, and at 6 the calcified areolar tissue, of which they are composed. is derived ; this membrane dips down into the spaces between the ad- jacent plates ; but it does not penetrate the substance of the plates themselves, nor does it transmit vessels to their interior. A simi- lar membrane covers and encircles the spines; and it also connects these with the shell, being continuous with the membrane that envelopes the latter. Thus each plate and spine is itself completely extra-vascu- lar ; but it is enclosed in a soft membrane, which furnishes (whether by vessels or otherwise, has not yet been ascertained), the elements of its nutrition. 279. But we do not here find any evidence of interstitial growth ; nor is there any reason why such should be required. For the tissue 168 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. of which it is composed, although of such extreme delicacy, is of greal^ permanence, and does not exhibit the slightest tendency to decay, how- ever long it is preserved ; so that, when once consolidated, it appears to undergo no further change in the living animal. The growth of the animal, however, requires a corresponding enlargement of its enveloping shell ; and this is provided for by the simple process of superficial de- posit, through the subdivision of the whole shell into component plates. For by the addition of new matter at the edge of each plate, by the consolidation of a portion of the soft membrane that intervenes between the adjacent plates, the whole shell is enlarged, without losing its globular form. At the same.time it is strengthened in a corresponding degree, by the consolidation of the soft tissue at the surface of each plate. And, in like manner, the spines are enlarged and lengthened by the progressive formation of new layers, each on the exterior of the preceding ; so that a transverse section exhibits a number of concentric rings, like those of an Exogenous tree. — Thus even in the growth of this complex and elaborate structure, we recognise the principle of superficial deposit, which we shall find to be universal amongst the hard parts of the Invertebrata : notwithstanding that, at first sight, it would have appeared impossible to provide on this plan for the gradual en- largement of a globular shell, completely enclosing the animal, and therefore required to keep pace with the latter in its rate of increase. 280. Among the Mollusca, we find the body sometimes altogether destitute of solid organs of support, protection, or locomotion, — as is the case, for example, in the Slug ; and the movements are feeble and the habits inert, the muscles having no fixed points for their attachment, and acting without any of the advantages of leverage. In other cases, we find the body more or less completely protected by a Shell ; which is sufiiciently large in some instances to cover the body completely, whilst in others it afi'ords only a partial investment. The plan on which this shell is formed, however, is very different from that which has just b6en described; being much less complex. The Univalve shells, or those formed in one piece, are always of a conical form ; the cone being sometimes simple, as in the Limpet ; in other cases being spirally coiled, , as in the Snail. Now the base of this cone is open ; and through this, the animal can project its movable parts. When its increasing size requires additional accommodation, it is obvious that an addition to the large end of the cone will increase its diameter and its length at the same time ; so as to afford the required space, without any alteration in the form or dimensions of the older and smaller portions of the cone. This last, indeed, is frequently quitted by the animal, and remains empty ; being sometimes separated from the later portions, by one or more partitions thrown across by the animal, — as is seen especially in the Nautilus and other chambered shells. Besides the new matter added to the mouth of the shell, a thin layer is usually formed over its whole interior surface ; so that the lining of the new part is continuous with that of the old. — In the Bivalve shells, we trace this mode of in- crease without any difficulty ; especially in such shells as that of the Oyster, in which the successive laminae remain distinct. Each lamina is interior to the preceding, being formed on the living surface of the SHELLS OF MOLLUSCA. 169 ttnimal ; b]it it also projects beyond it, so as to enlarge the capacity of the shell ; and as the separation of the valves affords free exit to those parts of the animal, which are capable of being projected beyond the shell, there is obviously no need of any other provision- to maintain the shell in its natural form. — Thus in the shells of the Mollusca, increase takes place at the surfaces and edges only. 281 . The proportion of organic and calcareous matter in Shell differs considerably in the various tribes. The former is sometimes present in such small amount, that it can scarcely be detected ; and the condition of the calcareous matter then obviously approaches that of a crystalline deposit. But in other instances, the animal basis is very obvious; remaining as a thick consistent membrane, after all the calcareous mat- ter has been dissolved away by an acid. This membrane is formed of an aggregation of cells arranged with great regularity (Fig. 43, a) ; the cavities of which are filled with carbonate of lime in a crystalline state. Fig. 43. I Prismatic cellular structure of shell of Pinna:— a, surface of lamina; b, vertical section. The form of the cells approaches the hexagonal ; their diameter varies in different shells from 1-lOOth to l-2800th of an inch ; their thickness also is extremely variable, even in different parts of the same shell. Thus we sometimes meet with a lamina of such tenuity, as not to mea- sure 1-lOOth of an inch in thickness, whilst in other instances, a single layer may have a thickness of half an inch, or even (in certain large fossil species) of an inch or more. In this case, the cells, instead of being thin flat scales like the tessellated-epitheliuiii (§ 233), are long prisms, somewhat like the cells of the cylinder-epithelium (Fig. 26), with their walls flattened against each other. The appearance which is then presented by a vertical section of them, is represented in Fig. 43, h: The long prismatic cells are there seen to be marked by delicate trans- verse striae, and these, taken in connexion with other indications, appear to show, that every such prism is in reality formed by the coalescence of a pile of flat cells, resembling those which are seen in the very thin laminoe just described ; so that the thickness of the layer depends upon the number of the cellular laminae which have coalesced to form its component prisms. This character is of interest, as representing on a Daagmfied scale a corresponding appearance in the Enamel of human 170 . STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. . Teeth, which we shall presently find to be formed upon the very same plan (§ 318). 282. We are to regard this kind of shell-substance, therefore, as formed by the secreting action of the epithelial cells covering the mantle of the animal, — which membrane, though it answers in position to the skin, has the soft, spongy, glandular character of a mucous membrane. These draw calcareous matter into their cavities, as a part of their own process of growth : this matter being supplied from the fluids of the vascular surface beneath. Now when these calcigerous cells are sepa- rated by intercellular substance, they remain distinct through the whole of their lives, and they form by their cohesion a tenacious membrane, that retains its consistency after the removal of the calcareous matter. But this is only the case in certain groups of shells, chiefly belonging to the bivalve division. When the intercellular substance is wanting, and the cells come into close contact, their partitions become indistinct on account of their extreme tenuity ; and not unfrequently a fusion of the whole substance appears to take place, by the dissolution of the original cell-walls, so that it becomes more or less homogeneous, — traces of the original cellular structure being here and there distinguishable (§ 252). 283. Sometimes where this fusion has taken place, so as to obliterate the original cell-structure, we find the almost homogeneous substance traversed by a series of tubuli, not arranged, however, in any very defi- nite direction, but forming an irregular network (Fig. 44). These tubes vary in size from l-2000th to l-20,000th of an inch; but their general diame- ter, in the shells in which they most abound, is l-4500th of an inch. In the larger tubuli, something of a bead-like structure may occasionally be seen : as if their interior were occupied by rounded granules arranged in a linear direction. Although it might be supposed that this structure is destined to convey nutrient fluid into the substance of the shell, yet there is no evidence that such is the fact ; and, on the contrary, there is ample evidence that, even in sh ells most copiously traversed by these tubuli, no processes of interstitial growth or renewal take place. The perma- nent character of the substance of all Shells, when once it is fully formed, is as remarkable as that of Coral : and as the adaptation of their size. ^^H ^^R ^r^^^^^S%^5^^^fc ^^^H ^s^^^^^^s ^^^^^^HP ^^H R^R^^^E ^8 m^^^^^B^ K Tubular shell-structure, from Anomia. to that of the animals to which they belong, is entirely effected by addi- tions to their surfaces and edges, no interstitial deposit can have a share in producing it. SHELLS OF CRUSTACEA. 171 !84. Among the Articulated classes, we still find that the skeleton is altogether external, and belongs therefore to the cutaneous system ; but it is formed upon a very different plan from the shells of the Mollusca, being closely fitted to the body, and enveloping every part of it ; conse- quently it must increase in capacity, with the advancing growth of the contained structures. Moreover it is destined not merely to afford sup- port and protection to these, but to serve for the attachment of the mus- cles by which the body and limbs are moved : and the hard envelopes of the latter serve, like the bones of the Yertebrata, as levers by which the motor powers of the muscles are more advantageously employed. Again, the hard envelopes of the body and limbs are not formed of dis- tinct plates, like those of the Echinus-shell ; but are only divided by sutures at the joints, for the purpose of permitting the requisite freedom of motion. It might have been thought that here, if anywhere, a process of interstitial growth would have existed, to adapt the capacity of the envelopes to the dimensions of the contained parts, as the latter increase with the growth of the animal ; but, true to the general principle, that epidermic structures are not only extra-vascular, but that they undergo no change when they are once fully formed, we find that the hard enve- lopes of Articulated animals are thrown off, or exuviated, when the con- tained parts require an increase of room ; and that a new covering is formed from their surface, adapted to their enlarged dimensions. 285. This is well known to occur at certain intervals in Crabs, Lob- sters, and other Crustacea ; which thus exuviate not merely the outer shell, with the continuation of the epidermis over the eyes, but also its ' internal reflection, which forms the lining of the oesophagus and stomach, and the tendinous plates by which the muscles are attached to the lining of the shell. A similar moulting may be observed to occur in some of the minute Entomostracous Crustacea of our pools, every two or three days, even after the animals seem to be full grown. During the early growth of Insects, Spiders, Centipedes, &c., a similar moult is frequently repeated at short intervals ; but after these animals have attained their full growth, which is the case with Insects at their last change, no fur- ther moulting takes place, the necessity for it having ceased. This moulting is precisely analogous to the exfoliation and new formation of the Epidermis, in Man and most other Vertebrata ; differing from it only in this, that the latter is constantly taking place to a small extent, whilst the former is completely effected at certain intervals, and then ceases. We have examples of a periodical complete m/oult in Yertebrata, however, among Serpents and Frogs. 286. The structure of the hard envelopes of Articulated animals cor- responds with that of the Epidermis and its appendages in Man. The firm casings of Beetles, for example, are formed of layers of epidermic cells, united together, and having their cavities filled by a horny secre- tion. The densest structure is found in the calcareous shells of the Crustacea; which consists of a substance precisely analogous to the Dentine of Teeth (§ 311); covered on the exterior with a layer of pigment-cells. The calcareous matter consists chiefly of carbonate of lime ; but traces of the phosphate are also found. The animal basis has a firm consistent structure, resembling that of teeth. A thin ver- 172 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. tical section shows the tubuli running nearly parallel, but with occasional undulations, from the internal surface towards the external; but no traces of the original calcigerous cells can be detected in the fully-formed shell, the process of fusion having gone so far as to obliterate them. The manner in which these tubuli are formed, will be presently con- sidered, under the head of Dental substance. 28T. Now the condition of the osseous skeleton of Vertebrated animals is altogether different. Its purpose is still only mechanical ; its sole use being, to afford support and protection to the softer textures, and to form inflexible levers by the action of the muscles, upon which motion may be given to the different parts of the fabric. But it forms a part of the internal substance of their bodies ; and as these grow in every part, and not merely by addition to this or that portion, so must the Bones also, in order to keep pace with the rest of the structure. Hence we find them so formed, that the processes of interstitial deposition may be continually going on in their fabric, as in that of the softer tissues ; and the changes in their substance do not cease, even when they have acquired their full size. The subsequent continuance of these changes appears destined, not so much to repair any waste occasioned by decom- position,— for this must be very trifling in a tissue of such solidity, — as to keep the fabric in a condition, in which it may repair the injuries in its substance occasioned by accident or disease. The degree of this reparative power is proportional, as we shall presently see, to the activity of the normal changes, which are continually taking place in the bone ; and is thus much greater in youth than in middle life, and greater in the vigour of manhood than in old age. 288. The structure of Bones. is well adapted to demonstrate the dis- tinction between the tissues themselves, and those subsidiary parts, by which they are connected with the rest of the fabric. We have seen that Cartilage is essentially non-vascular ; that is, even when it exists in a considerable mass, it is not traversed by vessels, but is nourished by absorption from the fluids contained in the vessels distributed on its exterior. Now every mass of Bone is penetrated by vessels ; nevertheless these do not penetrate its ultimate substance, and may be easily sepa- rated from it, leaving the bone itself as it was. In fact, as Prof. Goodsir , observes, " a well-macerated bone is one of the most easily made, an( at the same time one of the most curious anatomical preparations. I( is a perfect example of a texture completely isolated ; the vessels, nerveJ membranes, and fat, are all separated, and nothing is left but the non** vascular osseous substance." Precisely the same may be said of th( substance of a Tooth, from which the vascular lining of the pulp-cavitj has been removed ; for it then possesses neither vessels, nerves, noi lymphatics ; and yet, as we shall presently see, it has a highly-organize( structure, peculiar to itself. 289. The general characters of Osseous texture vary according to the shape of the Bone, and the part of it examined. Thus in the long bonesi we find the shaft pierced by a central canal, which runs continuously from one extremity to the other ; and the hollow cylinder which sur-i rounds this is very compact in its structure. On the other hand, the dilated ends of the bone are not penetrated by the large central canal i STRUCTURE OF BONE. 173 nor are they composed of solid osseous substance. They are made up of cancellated structure, as it is termed ; that is, of osseous lamellae and fibres interwoven together (like those of areolar tissue, on a larger scale) so as to form a multitude Fig. 45. ; of minute chambers or cancelU, freely communicating with each other, and with the cavity of the shaft ; whilst the whole is capped with a thin layer of solid bone. Again, in the thin flat bones, as the scapula, ; we find the two surfaces composed of solid osseous : texture, with more or less of cancellated structure interposed between the layers. And in the thicker flat bones, as the parietal, frontal, &c., this cancel- I lated structure becomes very distinct, and forms the ! diploe ; this, however, is sometimes deficient, leaving ! a cavity analogous to the canal of the long bones : \ whilst the plates which form the surfaces of the bone j (the external and internal tables of the skull), re- I semble in their thickness and solidity, as well as in I the intimate structure presently to be described, the I shaft or hollow cylinder of those bones. Finally, we ; frequently meet (especially in the Ethmoid and Sphenoid bones) with j thin lamellae of osseous substance, resembling those which elsewhere ! form the boundaries of the cancelli ; these consist of but one layer of , bony matter, and show none of the varieties previously adverted to ; they are not penetrated by vessels, but are nourished only by their surfaces ; and they consequently exhibit to us the elements of the osseous struc- ture in their simplest form. It will be desirable, therefore, to commence ; with the description of these. j 290. When a thin natural lamella of this kind is examined, it is found to be chiefly made up of a substance which is nearly homogeneous, ' sometimes exhibiting indistinct traces of a fibrous arrangement ; this, however, may be generally resolved by prolonged boiling, into an assem- • blage of minute granules, varying in size from l-6000th to l-14,000th Fig. 46. Extremity of Os femo- ris, showing, cancellateil structure :— a, thin layer of bone, in contact with the articular cartilage ; b, cancelli. Lacunje of Osseous substance, magnified 500 diameters :— a, central cavity; 6, its ramifications. of an inch, which are more or less angular in shape, and seem to cohere by the medium of some second substance, which is dissolved by the boiling. They are composed of Calcareous salts, apparently in chemical union with the Gelatine that forms the basis of the osseous substance. In the midst of this granular substance, a number of dark spots are to be observed, the form of which is very peculiar. In their general out- line, they are usually somewhat oval ; but they send forth numerous radiating prolongations of extreme minuteness, which may be frequently 174 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. traced to a considerable distance. These spots, known as the osseous corpuscles (sometimes termed the Purkinjean corpuscles^ after the name of their discoverer), are highly characteristic of the true bony structure, being never deficient in the minutest parts of the bones of the higher animals, although those of Fishes are frequently destitute of them. These corpuscles were formerly supposed, from their dark appearance, to be opaque, and to consist of aggregations of calcareous matter which would not transmit the light : but it is now quite certain, that they are lacunce or open spaces ; and that the radiating prolongations from them, which are far smaller than the minutest capillary vessel, are canaliculi, or delicate tubes. Of these canaliculi, some may be seen to interlace freely with each other, whilst others proceed towards the surface of the bony lamella ; and thus a system of passages, not by any means wide enough to admit the blood-corpuscles, but capable of transmitting the fluid elements of the blood, or matters selected from them, is established through the whole substance of the lamella. 291. The lacunae of Human bone have an average length of l-1800th of an inch ; and they are usually about half as wide, and one-third as thick. The diameter of the canaliculi is from l-12,000th to l-20,000th of an inch. Their size and form differ greatly, however, in the different classes of Vertebrata ; so that it is usually possible to refer a mere frag- ment of bone to its proper group, by an examination of its minute structure. The succeeding figure represents the arrangement of these lacunae and canaliculi in the bony scale of a fish (the Lepidosteus) ; which is almost the only existing representative of a large class of bony- scaled Fishes, that formerly tenanted the seas. Its lacunae will be seen to differ greatly in form from those of human bone ; and the canaliculi which proceed from them are much fewer in number. — The purpose of this penetration of the osseous texture by such a complicated apparatus of tubuli, can scarcely be anything else than the maintenance Fig. 47. Section of the bony scale of Lepidosteus ;— a, showing the regular distribution of the lacunaB and of the connecting canaliculi; b, small portion more highly magnified. of its vitality by the continual percolation of nutrient fluid, drawn into the system of lacunae and canaliculi from the neighbouring blood-vessels. Thus the nutrition of the ultimate osseous texture, though carried on upon the same general plan with that of Cartilage, differs in this : — that there is a provision in Bone for the ready transmission of nutrient matter through its texture, by means of minute channels, which does not exist in Cartilage ; a difference obviously required by the greater solidity of the substance of the former, which does not allow of the diffused imbibition, that is permitted by the softer and moister nature of the latter. We shall presently find that these channels are only I STRUCTURE OF BONE. 175 formed at a late stage of the development of bone, where the remaining tissue has acquired its completest consolidation. 292. Now, as already remarked, the simple structure just described is found, not merely in the delicate plates which form the thinnest part of certain bones in Man, but also in those lamellae, which form the walls of the cancelli of the larger and thicker bones. Every one of these lamellae repeats, in fact, the same history. The cancelli are lined by a membrane derived from that of the cavity of the shaft, over which blood-vessels are minutely distributed ; between these blood-vessels and the osseous texture is a layer of cells ; and from the materials selected and communicated by these, each lamella is nourished, through its system of radiating canaliculi and nutritive centres. The cancelli, at the time of their formation in the foetal bone, are entirely filled with such cells ; which appear (as will be presently explained) to be the descendants of the cells of the original cartilage ; but in the adult bone, a large proportion of them are filled with fatty matter, which they secrete into their cavities. — The vessels of the cancellated struc- ture at the extremities of the long bones are derived from those of the medullary cavity, which is penetrated by large trunks from the exterior ; and in the flat bones, they form a system of their own, connected with the vessels of the exterior by several smaller trunks. 293. The solid osseous texture which forms the cylindrical shafts of the long bones, and the thick external plates of the denser flat bones, is not cut ofi" from nutritive action in the degree in which it might seem to be ; for it is penetrated by a series of large canals, termed the Haversian (after Clopton Havers, their discoverer), which form a net- work in its interior, and which serve for the transmission of blood- vessels through its substance (Fig. 48). These canals, in the long bones, run for the most part in a direction parallel to the central cavity ; and they communicate with this, with the external surface, with the cancelli, and with each other, by frequent transverse branches; so that the whole system forms an irregular network, pervading every part of the solid texture, and adapted for the establishment of vascular communications throughout. The diameter of the Haversian canals varies from l-2500th to l-200th of an inch, or more ; their average diameter may be stated at about l-500th of an inch. They are lined by a membrane which is continuous with that of the external surface, and which carries this inwards, so to speak, to form the lining membrane of the cen- tral cavity, and of the cancelli ; — and the cavity of the tube encloses a single twig of an artery or vein. Thus we may consider the whole Osseous texture as enclosed in a membranous bag; on which blood-vessels are minutely dis- ^"Su^ol thTSlS o^l^<^'^^ih^^ tributed, and which is so carried into the bone ^"^^1^^, "s^'diiatauroi by involutions and prolongations, that no part another venous canai. of the latter is ever far removed from a vascular surface. Havorsir.n Canal, seen on a lon- gitudinal section of the compact 176 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 294. In the adult bone, the cells which fill the remaining cavity of these canals secrete fatty matter. This is particularly evident in the case of the central cavity, which may be considered as an immensely dilated Haversian canal, where they constitute the medulla or marrow. It does not appear that these take any active part in the nutrition of the bone ; indeed, in the bones of Birds the shaft is entirely hollow, and air is admitted into it from the lungs, so that its lining membrane is rendered subservient to the aeration of the blood. 295. The arrangement of the elementary parts of the osseous sub- stance around the Haversian canals is very interesting and beautiful. When a transverse section of a long bone is made, the open orifices of the longitudinal canals present themselves at intervals, sometimes con- nected by a transverse canal where the section happens to traverse this. Around these orifices, we see the osseous matter arranged in the form of cylinders, which appear to be marked by concentric circles (Fig. 49, 1, 2). Now when one of these circles is minutely examined, it. is found to be made up of a series of lacunae, analogous to those already described ; these, however, are seldom or never so continuous as to form a complete circle. The long sides of the lacunae are directed, the one Fig. 49. Minute structure of bone, drawn with the microscope from nature. Magnified 300 diameters. 1. One of the Haversian canals surrounded by its concentric lamellae. The lacuna are seen between the lamellae; but the radiating tubuli are omitted. 2. An Haversian canal with its concentric laminae, lacuna?, and radiating tubuli. 3. The area of one of the canals. 4, 4. Direction of the lamellae of the great medullary canal. Between the lamellae at the upper part of the figure, several very long lacunae with their tubuli are seen. In the lower part of the figure the outlines of three other canals are given, in order to show their form and mode of arrangement in the entire bone. towards the Haversian canal (3) in the centre, the other towards the circular row next beyond it. And when the course of the canaliculi is traced, it is found that these converge on the inner side towards the central canal, inosculating with those of the series next within, whilst those of the outer side pass outwards in a radiating or diverging direc- tion, to inosculate with those of the series next external. Thus a com- plete communication is formed, by means of this system of radiating canaliculi and intervening lacunae, between the central canal and the I COMPOSITION OF BONE. 177 outermost cylindrical lamella of bony matter ; and each of these lamellae derives its nourishment from the vessels of the central canai, through the lamellae which intervene between itself and the vascular membrane lining that tube. 296. Thus every one of the Haversian canals is the centre of a cylindrical ossicle, which is complete in itself, as far as its elementary structure is concerned, and which has no dependence on, or connexion with, other similar ossicles. These are arranged, however, side by side, like sticks in a faggot ; they are bound together by a thin cylinder of bone, on the exterior of all, which derives its nourishment from the periosteum, or enveloping membrane : in like manner, the hollow bundle is lined by a similar cylinder, which surrounds the great medullary cavity, and is nourished by its vascular membranes; and the spaces that here and there intervene between the ossicles are filled up with laminae, which are parallel to those of the external and internal cylinders, and which seem to derive their nutriment from them (Fig. 49, 4). In this manner, the whole structure acquires great density and solidity. — The structure of the outer and inner tables of the skull, and of other thick solid layers of bone, is precisely similar ; except that the Haver- sian canals have no such definite directions, and form an irregular net- work. 297. Thus we see that each of the lamellse of bone surrounding an Haversian canal, or bounding the cancelli, may be regarded as a repe- tition of the simple bony plate, which draws its nourishment direct from the vascular membrane covering its surface, by means of its system of lacunae and canaliculi. The membrane lining the Haversian canals, cancelli, and central medullary cavity, is an internal prolongation of that which clothes the exterior ;— just as the mucous membranes, with their extensions into glandular structures, are internal prolongations of the true skin. Every Haversian canal and every cancellus are repe- titions of each other in all essential particulars, their form alone being different. The central medullary canal is but an enlarged Haversian canal or cancellus. And the whole cylindrical shaft is a collection of ossicles, each of which is a miniature representation of itself, being a hollow cylinder, with a central vascular cavity. 298. The principal features of the Chemical constitution of Bone are easily made evident. After all the accessory parts have been removed, and nothing remains but the real osseous texture, tjiis may be separated, by simple processes, into its two grand constituents, — the animal basis, and the calcareous matter. The latter may be entirely removed by maceration of the bone in dilute Muriatic or Nitric acid ; and a substance of cartilaginous appearance is then left, which, when submitted to the action of boiling water for a short time, is almost entirely dissolved away, and the solution forms a dense jelly on cooling. The same sub- stance. Gelatine, may be obtained by long boiling under pressure, from previously unaltered bone ; and the calcareous matter is then left in a friable condition. By submitting a bone to a heat sufficient to decom- pose the animal matter, without dissipating any of the earthy particles, we may obtain the whole calcareous matter in situ; but the slightest iolence is sufficient to disintegrate it. The bones of persons long 12 1T8 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. buried are often found in this condition ; their form and position being retained until they are exposed to the air, or are a little shaken, when they crumble to dust. The proportion of the earthy matter of Bones to the animal basis may be differently stated, according as we include, in our estimate of the latter, the contents of the medullary cavity, the Haversian canals, and the cancelli, or confine ourselves to that portion only of the animal matter which is united with the calcareous element in the proper osseous tissue. According to the recent experiments of Dr. Stark,* the relative amount of the two elements, in the latter estimate, is subject to very little variation, either in the different classes of animals, or in the same species at different ages, the animal matter composing about one-third, or 33J per cent., and the mineral matter two-thirds, or 66f per cent. The degree of hardness of bone does not altogether depend, therefore, on the amount of earthy matter they may contain ; for the flexible, semi-transparent, easily-divided bones of Fish contain as large an amount of animal matter, as the ivory-like leg-bones of the Deer or Sheep. The usual analyses of Bone, however, have been made upon the former kind of estimate : and they show that the proportion of the earthy matter to the whole of the animal substance contained in bone varies much in different animals, in the same animal in different ages, and even in different parts of the same skeleton. The reason of this will be apparent, when the history of the growth of Bone has been explained ; since there is a gradual filling-up of all the cavities at first occupied by fat-cells, vessels, &c., which does not cease with adult age, but which continues during the whole of life. In this manner the bones of old persons acquire a high degree of solidity, but they become brittle in proportion to their hardness. From the same cause, the more solid bones contain a larger proportion of bone-earth than those of a spongy or cancellated texture ; the temporal bone, for exam- ple, containing 63i per cent., whilst the scapula possesses only 54 per cent. In the former of these bones, the proportion is nearly the same as that which exists in pure osseous tissue, the amount of the remaining tissues which it includes being very small, on account of the solidity of the bone ; but the latter contains in its cancelli a large quantity of blood-vessels, fat-cells, &c., which swell the proportion of the animal matter from 33J to 46 per cent. 299. The Lime of bones is for the most part in a state of Phosphate, especially among the higher animals ; the remainder is a Carbonate. In Human bones, the proportion of the latter seems to be about one- sixth or. one-seventh of the whole amount of bone-earth. In the bones of the lower animals, however, the proportion of Carbonate is greater ; and it is curious that in callus, exostosis, and other irregular osseous formations in the higher animals, the proportion of the Carbonate should be much greater than in the sound bone. In caries, however, the proportion of the Carbonate is less than usual. The composition of the Phosphate of Lime in Bones, is somewhat peculiar ; eight pro- portions of the base being united with three of the acid. According to Professor Graham, it is to be regarded as a compound of two tribasic * Edinburgh Medical and Surgical Journal, April, 1845. I I COMPOSITION OF BONE. 179 pnDSphates ; one atom of the neutral phosphate (in which one propor- tional of the ucid is united with two of lime and one of water), being united with two proportionals of the alkaline phosphate (in which one part of acid is united with three of the base), together with an atom of water, which is driven off by calcination. Besides these components, some Chemists assert that a small quantity of Fluoride of calcium is present in Bone ; but this is rather doubtful, since it has been shown by Dr. G. 0. Rees that the solvent action upon glass, which has been supposed to be characteristic of fluoric acid, may be imitated by phos- phoric acid in combination with water, which, if heated upon glass of inferior quality until it volatilizes, will act upon it with considerable energy. — Other saline matters, such as phosphate of magnesia, oxides of iron and manganese, and chloride of sodium, are found in bones in small amount. 300. The first development of Bone is usually preceded by the for- mation of a Cartilaginous structure, which occupies the place after- wards-to be taken