V
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LIBEAEY
.11 THE X
Theological Seminar
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PRINCETON, N. J. ' ,
Case DW,
Shelf Section
Book Na, r
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Just Published, Parts I. to IV., each containing Eighty quarto
pages, price 3s. 6d. each, to be continued Monthly : of
A NEW DICTIONARY
ENGLISH LANGUAGE
By CHARLES RICHARDSON.
REVIEWS & CRITICAL NOTICES.
* " Mr. Pickering has just put forth a New Dictionary of the English
Language, which, whether we regard its extraordinary cheapness, or the extra-
ordinary labour and ability by which it is characterised, bids fair to rival all
similar publications. The work is to be completed in Thirty Parts, each Part
to contain Eighty 4to pages, with three columns of Diamond type upon each
page ; the meaning of each word is illustrated by a greater number of passages
from standard English writers than is. to be found in any similar work ; and the
reading necessary for the supply of this immense body, must have been the labour
of years. A part of this Dictionary appeared, we find, in the Encyclopedia Me-
tropolitana, and was spoken of by the Quarterly and other reviews, as the greatest
lexicographical achievement of the age. In its complete form it will be, to judge
from the sample before us, a work of unrivalled ability, labour and utility." —
Old England.
" The compiler, who has already approved his ability for this work by what
he has contributed of it to the Encyclopedia Metropolitana, justly observes,
that Dr. Johnson did not execute his own project, and that the desideratum of a
Dictionary to ' exhibit, first, the natural and primitive signification of words,
then give the consequential, and then the metaphorical meaning, and the
quotations to be arranged according to the ages of the authors,' is, at the distance
of nearly ninety years, still more to be desiderated now, than in 1747, when the
learned lexicographer made his proposition to Lord Chesterfield. Mr. Richardson
derives considerable aid from Home Tooke's philological labours ; and from the
part before us, we would anticipate a useful and interesting work." — Literary
Gazette. '
" The arrangement is founded upon the plan which Dr. Johnson put forth as
the proper mode of proceeding with his great undertaking, though he did not,
in the execution, adhere to his own scheme. The task which our great philo-
loger left unfulfilled has been performed by Mr. Richardson, with a patient
labour in research and collection, which Johnson, we suspect, never possessed,
and with means at his disposal, by the resuscitation of our ancient writers, which
Johnson certainly never had. Judging from the specimen before us, the result
will be to present the world with the most complete Dictionary that ever was
published, as regards the etymology and primitive meaning of the words, the
successive growth of their secondary significations, the gradual advance and
changes of the language, the vast body of quotations from all authors, whether
ancient or modern, and, in consequence, the skeleton history of the English
language which it indirectly presents ; it will, in short, be a work indispensable
to every one who is curious in his mother tongue, and without which no library
can be considered complete. Though we have limited our praise to the specimen
before us, this was scarcely needed. The Dictionary has already appeared as
the Lexicon in the Encyclopedia Metropolitana, where it excited considerable
attention, and drew forth much praise. But in its independent form it will be
increased by upwards of a third, be subject to a careful revision, be enriched by
the author's additional knowledge, and simplified by his increased experience;
thus combining, as it were, the freshness of a novelty with the mechanical
advantage attendant upon a new edition." — Spectator.
" It would be impossible to speak of the value of this work within the short
space of a literary notice ; but thus much we can assure our readers, that in its
plan it is novel, and more comprehensive than any of its predecessors'; that the
quotations from the earliest poets, chroniclers, divines, &c. arranged in chrono-
logical order, in illustration of different words, supply an admirable view of the
progress of the English tongue ; that reference is made to chapter and verse for
every quotation given ; that it is cheap ; and that the publisher engages to de-
liver all parts beyond thirty free of expense. No library should be without it." —
Christian Remembrancer.
"This laborious work, of which the two first Parts are before us, is understood
to be completed in the manuscript ; the subscriber, therefore, incurs no risk of
disappointment from the non-accomplishment of the design. Of the care and
diligence bestowed in getting up the New Dictionary we are prepared to speak
in the highest praise. The paper is good, the type remarkably clear, the size
convenient, in every respect becoming a work of national importance. The
radical word with its derivatives, is placed at the head of the meaning, of the
etymological derivation and of the quotations, by which their usages are illus-
trated. These quotations are selected and digested in the chronological order of
the writers appealed to, so that one, with common sagacity, may trace the
changes through which a word has passed down to its modern acceptation. The
primitive signification is thus made to give a strength and clearness to our own
perception of the word. We remember when it was the custom to characterize a
dull heavy work by the remark, " I would as soon read a Dictionary through."
We may now say, without drawing upon the truth, that we have a Dictionary
surpassing in entertainment and knowledge most books. The deep research and
extensive reading which have amassed this wealth of quotations, make us
acquainted with stores of thought, hitherto buried in the dust of time, or acces-
sible only to the favoured few. The divines, the poets, the dramatists, the philo-
sophers, the historians, who have helped to build up the noble fabric of our
language, are made in short but appropriate sentences, to give us their own
literary portraits ; and, if style be an index to character, and expression to
thought, we have here a fine opportunity of comparing age with age, not only in
its literary, but also in its intellectual features. We add, that no deeper stain
could be marked upon our national reputation, than that such a work, so grand
in its design, and so perfect in its execution, should meet with indifference, or
even with partial success." — Gloucestershire Chronicle.
" The Fourth Division [Ency. Met.] is so much like an ordinary Encyclo-
paedia in its scheme and contents, that it would not detain us a single moment
were it not for the English Dictionary which is incorporated with it. It is an
undertaking of immense labour ; and notwithstanding all the aid which may be
derived from Johnson and other lexicographers, it cannot fail to prove an Her-
culean task. If the compiler persevere, and finish as he has begun, we have no
doubt the English Dictionary will, soon be called for in a separate form." — British
Critic, Oct. 1818.
" This is certainly one of the most interesting parts of the volume before us ;
we mean as to the Lexicon .- it is apparently executed with care ; possesses a con-
siderable degree of novelty in the arrangement of the radicals and derivatives ;
and is rendered both amusing and instructive by the number of appropriate quo-
tations from the earliest poets, chroniclers, and historians, down to the latest and
most approved writers in the English language, with the exception of all living
authors. The citations afford a very pleasing illustration of the progressive
changes in the language, and the almost directly opposite signification which
we now attach to some words, when compared with the import which they were
at first intended to convey. We make one extract from an example taken at
random, to manifest the nature of the arrangement of this instructive part of the
work. We regret that it has not been kept distinct. " — Monthly Review, June, 1819.
" We are inclined to consider the English language as having attained that
fulness of maturity which leaves no wish for increase, but only anxiety for pre-
servation, As helps to this, we have the various acceptations, in which every
word has been used by approved writers, collected by Mr. Richardson, in a
Dictionary, such as, perhaps, no other language could ever boast : and we have a
new guide for the theory and use of languages, exemplifying his (Home Tooke's)
principles, by applying them to our own tongue." — Quarter!// Review for March,
1827.
Alluding to the portions published in the Encyclopaedia Metropolitana, the
Reviewer of Dr. Webster observes —
" Let the valuable contributions to an improved Dictionary by Mr. Richardson,
in which he lias embodied many of the principles of Tooke, be compared with
the corresponding articles in the Dictionary of Dr. Johnson, and it will be seen
how much lexicography owes to the Diversions of Purley." — Westminster Review,
Jan. 1831.
WILLIAM PICKERING, PUBLISHER, CHANCERY LANE, LONDON.
THE BRIDGEWATER TREATISES
ON THE POWER WISDOM AND GOODNESS OF GOD
AS MANIFESTED IN THE CREATION
TREATISE V
ANIMAL AND VEGETABLE PHYSIOLOGY CONSIDERED
WITH REFERENCE TO NATURAL THEOLOGY
BY PETER MARK ROGET, M.D.
SEC. R. S. liTC.
IN TWO VOLUMES
VOL I
[SECOND EDITIOiN J
" Ask now the beasts, and they shall teach thee; and the fowls of the air,
and they shall tell thee:
" Or speak to the earth, and it shall teach thee; and the fishes of the sea
shall declare unto thee.
" Who knoweth not in all these that the hand of the Lord hath wrought
this." Job, xii. 7, 8, 9.
ANIMAL AND VEGETABLE PHYSIOLOGY
CONSIDERED WITH REFERENCE TO
NATURAL THEOLOGY
BY
PETER MARK ROGET, M. D.
SECRETARY TO THE ROYAL SOCIETY, FULLERIAN PROFESSOR OF PHYSIOLOGY IN THE ROYAL
INSTITUTION OF GREAT BRITAIN, VICE PRESIDENT OF THE SOCIETY OF ARTS,
FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS, CONSULTING PHYSICIAN TO THE QUEEN
CHARLOTTE'S LYINC-IN HOSPITAL, AND TO THE NORTHERN
DISPENSARY, ETC. ETC.
VOL I
LONDON
WILLIAM PICKERING
1834
C. WHUTINGHAM, TOOKS COURT, CHANCERY LANK.
TO HIS ROYAL HIGHNESS
PRINCE AUGUSTUS FREDERICK,
DUKE OF SUSSEX, K. G.
PRESIDENT OF THE ROYAL SOCIETY,
&C. &c. &C. &c.
THIS TREATISE
IS, WITH PERMISSION, HUMBLY DEDICATED,
AS A TRIBUTE OF PROFOUND RESPECT AND GRATITUDE
FOR THE BENEFITS RESULTING TO
SCIENCE
AND ITS CULTIVATORS,
FROM HIS ILLUSTRIOUS PATRONAGE,
BY HIS DEVOTED, HUMBLE SERVANT,
P. M. ROGET.
PREFACE.
I probably never should have ventured to
engage in the composition and publication of
a work like the present, had not that task
been assigned me by my nomination as one of
the writers of the series of Bridgewater Trea-
tises, and had I not deeply felt the honour
done me by that appointment, as well as the
importance of the duty which it imposed.
The hope, in which I have indulged, that my
labours might eventually be useful, has been
my chief support in this arduous undertaking ;
the progress of which has throughout been
seriously impeded by the various interruptions
incident to my profession, by long protracted
anxieties and afflictions, and by the almost
overwhelming pressure of domestic calamity.
The object of this treatise is to enforce the
Vlll PREFACE.
great truths of Natural Theology, by adducing
those evidences of the power, wisdom, and
goodness of God, which are manifested in the
living creation. The scientific knowledge of
the phenomena of life, as they are exhibited
under the infinitely varied forms of organiza-
tion, constitutes what is usually termed Phy-
siology, a science of vast and almost bound-
less extent, since it comprehends within its
range all the animal and vegetable beings on
the globe. This ample field of inquiry has, of
late years, been cultivated with extraordinary
diligence and success by the naturalists of
every country ; and from their collective la-
bours there has now been amassed an immense
store of facts, and a rich harvest of valuable
discoveries. But in the execution of my task
this exuberance of materials was rather a
source of difficulty ; for it created the necessity
of more careful selection and of a more ex-
tended plan.
In conformity with the original purpose of
the work, which I have all along endeavoured
to keep steadily in view, I have excluded
PREFACE, rx
from it all those particulars of the natural
history both of animals and of plants, and all
description of those structures, of which the
relation to final causes cannot be distinctly
traced ; and have admitted only such facts as
afford manifest evidences of design. These
facts I have studied to arrange in that me-
thodized order, and to unite in those compre-
hensive generalizations, which not only con-
duce to their more ready acquisition and re-
tention in the memory, but tend also to enlarge
our views of their mutual connexions, and of
their subordination to the general plan of crea-
tion. My endeavours have been directed to
give to the subject that unity of design, and
that scientific form, which are generally
wanting in books professedly treating of
Natural Theology, published prior to the
present series ; not excepting even the un-
rivalled and immortal work of Paley. By
furnishing those general principles, on which
all accurate and extensive knowledge must
substantially be founded, I am not without a
hope that this compendium may prove a
vol. i. b
PREFACE.
useful introduction to the study of Natural
History ; the pursuit of which will be found
not only to supply inexhaustible sources of
intellectual gratification, but also to furnish, to
contemplative minds, a rich fountain of re-
ligious instruction. To render these benefits
generally accessible, I have confined myself to
such subjects as are adapted to every class of
readers ; and, avoiding all unnecessary ex-
tension of the field of inquiry, have wholly
abstained from entering* into historical ac-
counts of the progress of discovery ; content-
ing myself with an exposition of the present
state of the science. I have also scrupulously
refrained from treading in the paths, which
have been prescribed to the other authors of
these treatises ; and have accordingly omitted
all consideration of the hand, the voice, the
chemical theory of digestion, the habits and
instincts of animals, and the structures of
antediluvian races ; the extent of the field
which remained, and which, with these few
exceptions, embraces nearly the whole of the
physiology of the two kingdoms of nature,
PREFACE. XI
already affording ample occupation for a single
labourer.
The catalogue of authors whose works have
furnished me with the principal facts detailed
in these volumes, is too long for insertion in
this place. I have not encumbered the pages
of the work by continual citations of authori-
ties ; but have given references to them only
when they appeared to be particularly re-
quisite, either as bearing testimony to facts
not generally known, or as pointing out
sources of more copious information. It may
however be proper to mention, that I have
more especially availed myself of the ample
materials on Comparative Anatomy and
Physiology contained in the works of Cuvier,
Blumenbach, Cams, Home, Meckel, De
Blainville, Latreille, and St. Hilaire, and in the
volumes of the Philosophical Transactions, of
the Memoires and Annales du Museum, and
of the Annales des Sciences Naturelles. I
should be ungrateful were I not also to ac-
knowledge the instruction I have derived from
my attendance on the lectures at the Royal
Xll PREFACE.
College of Surgeons, delivered successively,
during many years, by the late Sir Everard
Home, Sir Astley Cooper, Mr. Lawrence,
Mr. Brodie, Mr. Green, and Sir Charles
Bell ; and also from those of Professor Grant,
at the University of London.
I have likewise to return my thanks for the
liberal manner in which the Board of Curators
of the Hunterian Museum gave me permission
to take such drawings of the preparations it
contains, as I might want for the illustration
of this work ; and to Mr. Clift, the conserva-
tor, and Mr. Owen, the assistant conservator
of the museum, for their obliging assistance
on this occasion. Mere verbal description can
never convey distinct ideas of the form and
structure of parts, unless aided by figures ;
and these I have accordingly introduced very
extensively in the course of the work.*
Being compelled, from the nature of my
* All the wood engravings have been executed by Mr.
Byfield, and the drawings for them were, for the most part, made
by Miss Catlow, whose assistance on this occasion has been
most valuable to me.
PREFACE. Xlll
subject, and in order to avoid tedious and
fatiguing circumlocution, to employ many
terms of science, I have been careful to ex-
plain the meaning of each when first intro-
duced : but as it might frequently happen that,
on a subsequent occurrence, their signification
may have been forgotten, the reader will
generally find in the index, which I have,
with this view, made very copious, a reference
to the passage where the term is explained.
I beg, in this place, to express my deep
sense of the obligation conferred on me by
Mr. Davies Gilbert, the late president of the
Royal Society, to whose kindness I owe my
being appointed to write this treatise.
I also take this opportunity of conveying
my best thanks to my friend and colleague,
Mr. Children, of the British Museum, for his
kind assistance in revising the sheets while
the work was printing, and for his many valu-
able suggestions during its progress through
the press.
A catalogue of the wood engravings has
been subjoined ; and also a tabular view of the
XIV PREFACE.
classification of animals adopted by Cuvier in
his " Regne Animal," with familiar examples
of animals included under each division ; both
of which I conceived might prove useful for
purposes of reference. The latter table is
reprinted from that which I have given in my
" Introductory Lecture on Human and Com-
parative Physiology," published in 1826, with
only such alterations as were required to make
it correspond with the second and improved
edition of Cuvier's work.
Bernard Street, Russell Square.
May 1, 1834.
NOTICE.
The series of Treatises, of which the present is one, is
published under the following circumstances :
The Right Honourable and Reverend Francis
Henry, Earl of Bridgewater, died in the month of
February, 1829 ; and by his last Will and Testament, bear-
ing date the 25th of February, 1825, he directed certain
Trustees therein named to invest in the public funds the
sum of Eight thousand pounds sterling ; this sum, with
the accruing dividends thereon, to be held at the disposal
of the President, for the time being, of the Royal Society
of London, to be paid to the person or persons nominated
by him. The Testator further directed, that the person or
persons selected by the said President should be appointed
to write, print, and publish one thousand copies of a work
On the Power, Wisdom, and Goodness of God, as mani-
fested in the Creation ; illustrating such work by all reason-
able arguments, as for instance the variety and formation of
God's creatures in the animal, vegetable, and mineral king-
doms ; the effect of digestion, and thereby of conversion •
the construction of the hand of man, and an infinite variety
of other arguments; as also by discoveries ancient and
modern, in arts, sciences, and the whole extent of literature.
He desired, moreover, that the profits arising from the sale
of the works so published should be paid to the authors of
the works.
XVI v
The late President of the Royal Society, Da vies Gilbert,
Esq. requested the assistance of his Grace the Archbishop
of Canterbury and of the Bishop of London, in determining
upon the best mode of carrying into effect the intentions of
the Testator. Acting with their advice, and with the con-
currence of a nobleman immediately connected with the
deceased, Mr. Davies Gilbert appointed the following eight
gentlemen to write separate Treatises on the different
branches of the subject as here stated :
THE REV. THOMAS CHALMERS, D.D.
PROFESSOR OF DIVINITY IN THE UNIVERSITY OF EDINBURGH.
ON THE POWER, WISDOM, AND GOODNESS OF GOD
AS MANIFESTED IN THE ADAPTATION
OF EXTERNAL NATURE TO THE MORAL AND
INTELLECTUAL CONSTITUTION OF MAN.
JOHN KIDD, M. D. F. R. S.
REC1US PROFESSOR OF MEDICINE IN THE UNIVERSITY OF OXFORD.
ON THE ADAPTATION OF EXTERNAL NATURE TO THE
PHYSICAL CONDITION OF MAN.
THE REV. WILLIAM WHEWELL, M.A. F.R.S.
FELLOW OF TRINITY COLLEGE, CAMBRIDGE.
ASTRONOMY AND GENERAL PHYSICS CONSIDERED WITH
REFERENCE TO NATURAL THEOLOGY.
SIR CHARLES BELL, K. G. H. F.R.S. L. & E.
THE HAND : ITS MECHANISM AND VITAL ENDOWMENTS
AS EVINCING DESIGN.
PETER MARK ROGET, M. D.
FELLOW OF AND SECRETARY TO THE ROYAL SOCIETY.
ON ANIMAL AND VEGETABLE PHYSIOLOGY.
XV11
THE REV. WILLIAM BUCKLAND, D. D. F. R. S.
CANON OF CHRIST CHURCH, AND PROFESSOR OF CEOLOGY IN THE
UNIVERSITY OF OXFORD.
ON GEOLOGY AND MINERALOGY.
THE REV. WILLIAM KIRBY, M. A. F. R. S.
ON THE HISTORY, HABITS, AND INSTINCTS OF ANIMALS.
WILLIAM PROUT, M.D. F.R.S.
CHEMISTRY, METEOROLOGY, AND THE FUNCTION OF
DIGESTION, CONSIDERED WITH REFERENCE TO
NATURAL THEOLOGY.
His Royal Highness the Duke of Sussex, Presi-
dent of the Royal Society, having desired that no unneces-
sary delay should take place in the publication of the
above mentioned treatises, they will appear at short inter-
vals, as they are ready for publication.
CONTENTS
OF THE FIRST VOLUME.
INTRODUCTION.
Page
Chapter I. — Final Causes 1
II. — The Functions or Life 34
PART I.— -THE MECHANICAL FUNCTIONS.
Chapter I. — Organic Mechanism 59
§ 1. Organization in general 59
2. Vegetable Organization 65
3. Developement of Vegetables 82
4. Animal Organization 96
5. Muscular Power 124
Chapter II. — The Mechanical Functions in Zoo-
phytes 142
§ 1 . General Observations 142
2. Porifera, or Sponges 147
3. Polypifera 161
4. Infusoria 183
5. Acalepha 192
6. Echinodermata 199
Chapter III. — Mollusca , 213
§ 1. Mollusca in general 213
2. Acephala 217
XX CONTENTS.
Page
§ 3. Gasteropoda 227
4. Structure and formation of the Shells of Mol-
lusca 230
5. Pteropoda 257
6. Cephalopoda 258
Chapter IV. — Articulata 268
§ 1 . Articulated animals in general « 268
2. Annelida 269
3. Arachnida 282
4. Crustacea 286
Chapter V. — Insects 296
§ 1. Aptera 296
2. Insecta alata 299
3. Developement of Insects 302
4. Aquatic Larvae 309
5. Terrestrial Larvae 311
6. Imago, or perfect Insect 317
7. Aquatic Insects 335
8. Progressive motion of Insects on land 338
9. Flight of Insects 344
Chapter VI. — Vertebrata 361
§ 1. Vertebrated Animals in general 361
2. Structure and Composition of the Osseous
Fabric 365
3. Formation and Developement of Bone 375
4. Skeleton of the Vertebrata 386
Chapter VII.— Fishes 408
Chapter VIII. — Reptilia 435
§ 1 . Terrestrial Vertebrata in general 435
2. Batrachia 436
3. Ophidia 447
4. Sauria 457
5. Chelonia 463
CONTENTS. XXI
Page
Chapter IX. — Mammalia 477
§ 1. Mammalia in general 477
2. Cetacea 482
3. Amphibia 487
4. Mammiferous Quadrupeds in general 487
5. Ruminantia 499
6. Solipeda 516
7. Pachydermata 518
8. Rodentia 523
9. Insectivora 525
10. Carnivora 528
1 1 . Quadrumana 532
12. Man 536
Chapter X. — Vertebrata capable of Flying 545
§ 1. Vertebrata without feathers, formed for flying. . 545
2. Birds 554
LIST OF ENGRAVINGS.
VOLUME I.
Pig. Page
1 Rotifer redivivus, (from Muller) 62
2 Vibrio tritici, (Bauer) 62
3 Simple vegetable cells, (Slack) 67
4 Fucus vesiculosuSy transverse section, (De Candolle) . . 67
5 Ditto, longitudinal section, (id.) 67
6 Compressed cells of vegetables, (Slack) 67
7 Hexagonal and elongated cells, (id.) 67
8 Elongated cells, (id.) 67
9 Fibrous cells, (id.) 67
10 Reticulated cells, (id.) 67
12 Junction of cells to form a tube 73
1 3 Beaded vessels 73
14 Spiral vessels, or Tracheae 73
15 Annular vessels 73
16 Punctuated vessels 73
17 Transitions of vessels from one class to another 73
18 Woody fibres 73
19 Nervures of a leaf 73
20 Cells composing the cuticle, (De Candolle) 79
21 Stomata magnified, (Amici) 79
22 Arrangement of stomata in cuticle, (De Candolle) .... 79
23 Roots terminated by spongioles, (id.) 79
24 Cells composing a spongiole, (id.) 79
25 Animal cellular substance 99
26 Blood vessel 103
27 Section of blood vessel, with the valves open 103
28 Ditto, with the valves closed 103
29 Striated surface of the scale of the Cyprinus alburnus,
(Heisinger) 116
LIST OF ENGRAVINGS. XX111
Fig. Page
30 Ditto of the Perca fluviatilis, (Carus) 116
3 1 Imbricated arrangement of the scales of fishes (Heisinger) 116
32 Section of the bulbs of hair, magnified 117
33 Quill of Porcupine, (F. Cuvier) 121
34 Transverse section of the same, (id.) 121
35 Longitudinal section of the root of ditto, (id.) 121
36 Capsule of bulb of ditto laid open, (id.) 121
37 Muscle in a state of relaxation 129
38 The same muscle contracted 129
39 Diagram illustrating the action of oblique muscles .... 129
40 Semi-penniform muscle 129
41 Penniform muscle 129
42 Complex muscle 129
43 Tendon of muscle 129
44 Trapezius muscle 129
45 Muscular structure of the Ear-drum, (Home) 136
46 Orbicular muscle of the Eye-lids, (Albinus) 136
47 Muscular structure of the Iris, (Home) 136
48 Muscular fibres of a sucking disk 136
49 Longitudinal muscular fibres of a blood-vessel 137
50 Transverse muscular fibres of ditto 137
51 Muscular fibres of the human stomach, (Cooper) .... 137
52 Muscular fibres of the Heart, (id.) 137
53 Magnified view of a Sponge, (Grant) 149
54 Spicula in the texture of a Sponge, (id.) 149
55 Gemmule of a Sponge, (id.) 149
56 Lobularia. Alcyonium pelasgica, (Deterville) 162
57 Detached polype of ditto, (id.) 162
58 Zoanthus, (Actinia sociata), (Ellis) 162
59 Hydra viridis, (Trembley) 162
60 Sertularia pelasgica, (Deterville) 165
61 Tubipora musica, (Ellis) 165
62 Section and polypes of ditto, magnified, (id.) 165
63 Flustra carbacea, (id.) l£>5
64 Cells of ditto, magnified, (id.) 165
65 Corallium rubrum, (id.) 166
66 Polypes of ditto, magnified, (id.) 166
67 Section of Gorgonia Briareus, (id.) 166
XXLV LIST OF ENGRAVINGS.
Fig. Page
68 Isis hippuris, (id.) 166
69 Polype of Flustra carbasea, (Grant) 172
70 Tentaculum of ditto, magnified, (id.) 172
71 Pennatula phospkorea, (Ellis) 174
72 Magnified view of the polypes of ditto, (id.) 174
73 to 76 Mode of progression of the Hydra viridis, (Trembley) 178
77 Vorticella cyathina, (Muller) 183
78 Proteus diffluens, (id.) . 187
79 Volvox globator, (id.) 187
80 Brachionus urceolaris, (id.) 189
81 Medusa pulmo, (Macri) 192
82 Beroe ovatus, (Bruguiere) 194
83 Beroe pileus (id.) 194
84 Velella limbosa, (Guerin) 194
85 Physalia atlantica, (id.) 194
86 Actinia rufa, (original) ... 198
87 Ditto expanded, (original) 198
88 Asterias serrulata (Bruguiere) 199
89 Asterias regularis, (id.) 199
90 Echinus Ananchites ovata, (id.) 199
91 Clypeaster rosaceus, (id.) 199
92 Ophiura lacertosa, (id.) 199
93 Euryale muricatum, (id.) 199
94 Pentacrinus europceus, (Thomson) 199
95 Ambulacra, and feet of Asterias, viewed from the under
side, (Reaumur) 201
96 Ditto, viewed from the upper side, (id.) 201
97 Vesicles appended to the feet of the Asterias 201
98 Polygonal pieces composing the test of the Echinus . . . 204
99 Structure of a detached piece of ditto 204
100 Spine of the Cidaris, (Carus) 204
101 Shell of Unio batava, (Goldfuss) 217
102 Adductor muscle of Oyster, (Hunterian Museum). ... 218
103 Shell of Pholas Candida, with abductor muscle, (Osier) 220
104 Foot of Cardium edule, (Reaumur) 221
105 Planorbus cornutus (Cuvier) 227
106 Magnified view of the striae on the surface of Mother
of Pearl, (Herschel) 232
LIST OF ENGRAVINGS. XXV
Fig. Page
107 Directions of the fibres in the component strata of
shells 234
108 Shell of Achatina zebra, (De Blainville) 242
1 09 Longitudinal section of ditto, (id.) 242
110 Shell of Pterocerus scorpio, at an early stage of growth ,
(id.) 246
111 Shell of the same when completely formed, (id.) 246
112 Shell of Cyprcea exanthema at an early period of
growth, (id.) 246
113 Shell of the same animal, when completed, (id.) .... 246
114 Transverse section of the shell of the Cyprcea exan-
thema, (Hunterian Museum) 248
115 Shell of Conus 250
116 Longitudinal section of the same, (original) . 250
117 Transverse section of the same, (Bruguiere) 250
118 Inner surface of the Epiphragma of the Turbo, (De
Blainville) 253
1 19 Outer surface of the same, (id.) 253
120 Clio borealis, (Cuvier) 258
121 Sepia loligo, (De Blainville) 259
122 Suckers of the same (id.) 259
123 Bone or internal shell of the same, (id.) 259
123* Suckers of the Octopus, (original) 260
124 Shell of Spirula australis, (De Blainville) 265
125 Longitudinal section of the same (id.) 265
126 Shell of Nautilus pompilius (id.) 265
127 Longitudinal section of the same (id.) 265
128 Pontobdella muricata, (Bruguiere) 27 1
129 Nereis, (id.) 271
130 Erpobdella vulgaris (Lam.) Hirudo hyalina 271
131 Diagram illustrating the rings and muscles of Annelida,
(original) 271
1 32 Gordius aquaticus 276
133 Serpula opercularia 276
134 Terebella conchilega, (De Blainville) 276
135 Arenicola piscatorum, or Lumbricus marinus 276
1 36 Aranea diadema, (Rcesel) 283
137 Divisions of the limb of a Crustaceous animal 287
VOL. I. C
XXVI LIST OF ENGRAVINGS.
Fig. Page
138 Mandible and palpus of My sis Fabricii, (Bruguiere) 287
139 to 141 Feet-jaws belonging to the first, second, and
third pairs, (id.) 287
142 True foot, belonging to the first pair, (id.) 287
143 Julus terrestris 299
144 Muscles of the trunk, of the Melolontha vulgaris,
(Straus Durckheim) 300
145 Eggs of Bombyx mori 305
146 Larva of the same 305
147 Pupa of the same 305
148 Imago of the same 305
148* a Caterpillar of the Phalena striaria, (Hubner) .... 315
b The same in a rigid position, (Lyonet) 315
149 Calosoma Sycophanta, (Kirby and Spence) 320
150 Analysis of skeleton of the same, (Carus) 321
151 Hind view of the segment of the head in the same, (id.) 321
152 Suckers on the foot of the Musca vomitoria, expanded ;
magnified view, (Bauer) 333
153 Cushions on the footof the Cimbexlutea, magnified, (id.) 333
154 Suckers on the under side of the foot of a male Dytis-
cus marginalis, (id.) 333
155 Cushions and sucker of the Acridium biguttulum,
Latr. (id.) 333
156 Dytiscus marginalis, upper side, (Roesel) 336
157 Lower side of the same insect, (id.) 336
158 Notonecta glauca, (F.oesel) 337
158* Fore leg of Gryllotalpa, (Kidd) 343
159 Wing of Gryllus nasutus. Orthoptera 350
160 Wing of Libellula grandis. Neuroptera 350
161 Wing of Ichneumon per suasorius. Hymenoptera .. 350
162 Wing of Tipula oleracea. Diptera 350
163 Sting of Anthophora retusa, (original) 352
164 Separate scales of the wing of Hesperia Sloanus,
. (original) 355
165 Arrangement of the scales in the wing of the same . . 355
172 Longitudinal section of the thigh-bone to show the
cancellated structure, (Cheselden) 373
173 Longitudinal section of the humerus, (id.) 373
LIST OF ENGRAVINGS. XXV11
Fig- Page
174 Ossification of the parietal bone, (id.) 379
175 Early stage of ossification of the bones of the skull,
(Cloquet) 379
176 The same in the adult, showing the sutures 379
177 Dorsal vertebra, human 383
178 Junction of vertebroe forming the spinal column .... 388
179 Longitudinal section of the same, showing the spinal
canal ,. 388
180 Elements of structure of a vertebra, (Carus) 393
181 Skeleton of Hog, (Pander and D'Alton) 402
182 Sternum, clavicle, and scapula ; human 402
184 Skeleton of Cyprinus carpio, (Bonnaterre) 411
185 Diagram illustrating the progressive motion of Fishes 412
186 Front view of the vertebra of a Cod, (Gadus morrhua) 414
187 Side view of the same 414
188 Vertical and longitudinal section of a part of the spinal
column in the same 414
189 A similar section, showing the gradation of structure . 414
190 Similar section in the Squalus centrina, (Carus) . 414
191 Bones of the shoulder of the Lophius piscatorius, (id.) 422
192 Pectoral fin of the Raia clavata, (id.) 422
193 Belt of bones of the shoulder of a Ray, (id.) 423
194 Muscular system of Cyprinus albumus, (id.) 425
195 Air bladder of Cyprinus carpio, (Blasius) 429
196 Eggs of the Frog 437
197 Side view of Tadpole magnified, (Rusconi) 437
198 Upper view of the same, (id.) 437
199 Adult Frog 437
200 Skeleton of Frog, (Cheselden) 441
201 Skeleton of the Viper 447
202 Ribs and spine of Boa constrictor, (Home) 450
203 Bones of the foot of the same, (Mayer) 448
204 Muscles moving the claw of the same, (id.) 448
205 Rudimental bones of the foot of the Tortryx scytale,(id.) 448
206 . of the Tortrix corallinus, (id.) 448
207 of the Anguisfragilis, (id.) 448
208 of the Amphisbcena alba, (id.) 448
209 of the Coluber pullutatus, (id.) 448
XXV111 LIST OF ENGRAVINGS.
Fig. Page
210 Chalcides pentadactylus, (Bonnaterre) 448
211 Under surface of the foot of the Lacerta gecko, mag-
nified four times, (Bauer) 461
212 Side view of a longitudinal section of the same, (id.) 461
213 Skeleton of Tortoise, (Cams) 465
214 Section of the thigh bone of the same (id.) 465
215 Hind view of skull of Testudo my das, (id.) 469
216 Bones sustaining the fin of the Delphinus phoccena,
(Pander and D'Alton) 486
217 Fore part of the Skeleton of an Ox with the Ligamen-
tum nuchce, (original) 502
218 Skeleton of the Stag, (Cheselden) 507
218*a. Xongitudinal section of the horn of an Ox, (original) 515
b. Ditto, of an Antelope, (original) 515
c. Extremity of the same, (original) 515
219 Subcutaneous muscles of the Hedge-hog, relaxed,
(Carus) 528
220 The same muscles contracted, and drawn over the body,
(Cuvier) 528
221 Skeleton of the Lion, (Pander and D'Alton) 530
222 Skeleton of Draco volans, (Tiedemann) 550
223 Skeleton of Vespertilio molossus, (Temmink) 551
224 Skeleton of the Swan, (Cheselden) 559
225 Lateral section of the cervical vertebra of the Ostrich,
(original) 563
226 Fibrils of the vane of a feather, magnified, (original) 570
227 Edges of the fibres, magnified, (original) 570
228 Feather, showing its structure, (F. Cuvier) 575
229 Capsule, or Matrix of the feather, (id.) 575
230 View of the parts enclosed in the Capsule, when laid
open, (id.) 575
231 Section of the stem, while growing, exhibiting the
series of conical membranes, (id.) 575
233 Extensor muscles of the foot and toes of a bird, (Borelli) 589
234 Position of a bird in roosting, (id.) 589
LIST OF ENGRAVINGS. XXIX
VOLUME II.
Fig. Page
239 Cyclosis, or partial circulation in the cells of the Cau-
linia fragilis, magnified, (Amici) 50
240 The same in the jointed hair of the Tradescantia vir-
ginica, (Slack) 50
241 Section of the Hydra vividis, magnified, (Trembley) .. 74
242 Hydra vividis seizing a worm, (id.) 76
243 The same after swallowing a minnow, (id.) 76
244 A Hydra which has swallowed another of its own
species, (id.) 76
245 Compound Hydra, with seven heads, (id.) 76
246 Veretilla lutea, showing the communicating vessels of
the Polypes, (Quoy et Gaimard) 83
247 Nutrient vessels of the Taenia solium (Chiaje) 83
248 Tcenia globosa, or Hydatid of the Hog, (Goeze) 83
249 Horizontal section of the Rhizostoma Cuvieri, Peron,
(Eysenhardt) 88
250 Geronia hexaphylla, Peron, Medusa proboscidalis ,
(Forskal) 88
251 Vascular net-work in margin of the disk of the Rhizos-
toma Cuvieri, (Eysenhardt) 88
252 Vertical Section of the Rhizostoma Cuvieri, (id.). ... 89
253 Transverse section of one of the arms of the same, (id.) 89
254 Transverse section of the extremity of a tentaculum of
the same, (id.) 89
255 Leucophra patula, highly magnified, (Ehrenberg) ... 96
256 Alimentary canal and caeca of the same, viewed sepa-
rately, (id.) 96
257 Vertical section of the Actinia coriacea, (Spix) 99
258 Digestive organs of the Asterias, (Tiedemann) 100
259 Stomachs of the Nais vermicularis, (Roesel) 102
260 Stomachs of the Hirudo medicinalis, (original) 103
261 Mouth of the same, showing the three semicircular
teeth, (original) 103
262 Tooth of the same, detached, (original) 103
XXX LIST OF ENGRAVINGS.
Fig. Page
263 Glossopora tuberculata ; Hirudo complanata, Lin.
(Johnson) 1 04
264 The same seen from the under side, showing the diges-
tive organs, (id.) 1 04
265 Diagram showing the arrangement and connexions of
the organs of the vital functions in Vertebrata,
(original) 1 06
266 Spiral probosces of Papilio urticce, (Griffith) 114
267 Trophi of Locusta viridissima, (Goldfuss) 122
268 Filaments composing the rostrum, or proboscis, of the
Cimex nigricornis, (Savigny) 125
269 Sheath of the proboscis of the same insect, (id.) .... 125
270 Toothed cartilage of the Helix pomatia, (Cuvier) .... 126
271 Mechanism for projecting and retracting the tongue of
the Woodpecker, (original) 132
272 Laminae of Whalebone descending from the palate of
the Balcena mysticetus, (Bonnaterre) 137
273 Teeth of the Delphinus phoccena (Cloquet) 142
274 Skull of Tiger, (Cuvier) 146
275 Skull of Antelope, (Pander and D' Alton) 147
276 Skull of Rat, (id.) 148
277 Longitudinal section of simple tooth, (Rousseau) .... 151
278 Surface of the grinding tooth of a Horse, (Home) .... 151
279 Surface of the grinding tooth of a Sheep, (id.) 151
280 Longitudinal section of the incisor tooth of the Rodentia 151
281 Vertical section of the grinding tooth of the Elephant,
(Home) 154
282 Grinding tooth of the African Elephant, (id.) 154
283 Grinding tooth of the Asiatic Elephant, (id.) 154
284 Succession of teeth in the Crocodile, (Carus) 163
285 Venomous fang of the Coluber naia, (Smith) 165
286 Transverse section of the same, (id.) 165
287 The same tooth at an earlier period of growth, (id.) . . 165
288 The same, still less advanced in its growth, (id.) .... 165
289 Base of the former, (id.) 165
290 Base of the latter, (id.) 1 65
291 Transverse section of the young fang, about its
middle, (id.) 165
LIST OF ENGRAVINGS. XXXI
Fig- Page
292 A section, similar to the last, of another species of ser-
pent, (id.) 165
293 Squalus pristis. b. Under side of its snout, (Latham) 166
294 Interior of the Stomach of a Lobster, (original) 167
295 Gastric teeth of Bulloea aperta, (Cuvier) 168
298 Gizzard of the Swan, (Home) . 169
299 Crop and gizzard of the Parrot, (id.) 179
300 Crop of the Pigeon, (id.) , 179
301 Human stomach, (id.) =,.... 182
302 Interior of the stomach of the African Ostrich, (id.). . 185
303 Gastric glands of the same, (id.). > . . 185
304 Gastric glands of the American Ostrich, (id.) ....... 185
305 Longitudinal section of the gastric glands of the Beaver,
(id.) .. 185
306 Stomach of Dormouse, (id.) 191
307 Stomach of Hyrax capensis, (Cuvier) 191
308 Stomach of Porcupine, (id.) 191
309 Stomach of Kanguroo, (id) 191
310 Stomach of Delphinus phoccena, (id.) 191
31 1 Cardiac valve of the Horse, (Gurlt) 192
312 The four stomachs of a Sheep, (Carus) 194
313 Inner surface of the honey-comb stomach, (Home) .. 194
314 Inner surfaceof the many-plies stomach of an Ox, (id.) 194
315 Interior cellular surface of the second stomach of the
Camel, (id.) 194
316 Spiral valve in the intestine of the Shark, (Blasius) . . 205
317 Digestive organs of the Mantis religiosa, (Marcel de
Serres) 211
318 Melolontha vulgaris, (Leon Dufour) 213
319 Cicindela campestris, (id.) 213
320 Portion of a hepatic vessel of the Melolontha, highly
magnified, (Straus Durckheim) 214
321 Alimentary canal of the Acrida aptera, (original) .... 214
322 Interior of the gizzard of the same magnified, (original) 214
323 Row of large teeth in the same, still more magnified,
(original) .' 214
324 Profile of one of those teeth still more highly magnified,
(original) 214
XXX11 LIST OF ENGRAVINGS.
Fig. Page
325 Base of the same tooth seen from below, (original) 214
326 Alimentary canal of the Larva of the Sphinx Ligustri,
(original) 217
327 of the Pupa of the same, (original) 217
328 of the Imago of the same, (original) 217
329 of the Patella, (Cuvier) 220
330 Stomachs of the Pleurobranchus Peronii, (id.) 220
331 Pyloric appendices in the Salmon, (id.) 222
333 Detached Dorsal vessel of Melolontha vulgaris, (Straus
Durckheim) 237
334 The same with its ligamentous and muscular attach-
ments, (id.) 237
335 Side view of the anterior extremity of the same vessel,
(id.) . , , . 237
336 Section of the dorsal vessel to show its valves, (id.) . . 237
337 Circulation in the antenna of the Semblis viridis,
(Carus) 242
338 Course of circulation in the same insect, (id.) 242
339 Dorsal vessel of the Caterpillar of the Sphinx ligustri,
side view, (original) 245
340 The same in the Chrysalis, (original) 245
341 The same in the Moth, (original) 245
342 The same viewed from above, (original) 245
343 Magnified lateral view of the anterior extremity of the
dorsal vessel, (original) 245
344 Magnified dorsal view of the same, (original) 245
345 Structure of the valves of the dorsal vessel, (original) . 245
346 Heart and vessels of the Aranea domestica (Treviranus) 249
346*Circulation in the Planaria nigra, (Duges) 250
347 Course of circulation in the Erpobdellavtdgaris(Morren) 253
348 Vessels in abdominal surface of the same, (id.) 253
349 Vascular dilatations, or hearts of the Lumbricus ter-
restris, (Morren) 255
350 Cavities and great vessels of the Heart 259
351 The Heart laid open to show its Valves 260
352 Plan of simple circulation 262
353 Plan of double circulation 266
354 Branchial circulation in Maia Squinado, (Audouin) . . 269
LIST OF ENGRAVINGS. XXXlll
Fig. Page
355 Organs of circulation in the Loligo sagittata, (id.) . . . 271
356 Plan of circulation in Fishes 272
357 Plan of circulation in Batrachia 274
359 Plan of double, or warm-blooded circulation 278
360 Heart of the Dugong, (Home) 279
365 Valves of the Veins, (Cloquet) 288
366 Heart, branchial artery and gills of a fish, (Blasius) . . 302
367 Branchial apertures in the Squalus glaucus, (Bonna-
terre) 302
368 Branchial apertures in the Petromyzon marinus, (id.) 302
369 Internal structure of the branchiae of the same, (Home) 302
370 Stigmata in the abdominal surface of the Dytiscus
marginalis, (Leon Dufour) 311
371 Stigmata of Cerambyx heros, (Fab.) magnified, (id.) 311
372 Longitudinal tracheae of Carabus auratus, (id.) 311
373 Air vesicles and tracheae of the Scolia hortorum, (Fab.)
highly magnified, (id.) 311
374 Respiratory apparatus of the Scorpio europceus, (Tre-
viranus) 315
375 Internal structure of the lungs of the Turtle, (Bojanus) 322
377 Air cells of the Ostrich, (Parisian Academicians) .... 328
378 Lymphatic Absorbents 352
379 Passage of Nerves through a ganglion 359
380 Plexus of nerves 359
381 Varieties of forms of antennae of Insects, (Goldfuss). . 384
382 Vertical and longitudinal section of the right nostril in
man 400
383 Vertical transverse section of the same 401
384 Transverse section of the nostril of a Sheep, (Harwood) 402
385 Turbinated bones of the Seal, (id.) 402
386 Turbinated bones of the Turkey, (id.) 405
387 Nerves distributed to the bill of the Duck, (id.) 406
388 Nasal cavities of the Percafiuviatilis, (Cuvier) .... 410
389 Nasal cavity of the Raia batis or Skate, (Harwood) . . 410
390 Human ear, (Cloquet) 421
391 Posterior surface of the cavity of the tympanum, (id ) 425
392 Ossicula auditus, or small bones of the tympanum . . 425
393 The position of the latter in the tympanum 425
XXXI V LIST OF ENGRAVINGS.
Fig. Page
394 Magnified view of the labyrinth detached from the sur-
rounding parts, (Breschet) 427
395 Interior structure of the labyrinth, (id.) 428
396 Membranous labyrinth, with its nerves, (id.) 428
397 Cretaceous bodies in the labyrinth of the Dog, (id.) . . 428
398 Ditto in that of the Hare, (id.) 428
399 Organ of hearing in the Lobster, (Carus) 435
400 Groove in the sac of the former, (id.) 435
401 Organ of hearing in the Astacus fluviatilis, (id.) .... 435
402 Interior view of the same, (id.) 435
403 Membranous labyrinth of the Lophius piscatoriits, (id.) 438
404 Organ of hearing in the Frog, (Bell) 439
405 Ear of the Turkey, (Carus) 439
406 Diagram illustrating one mode of obtaining images of
objects, (original) 450
407 Simple Camera Obscura 451
408 Law of the refraction of a ray of light 454
409 Convergence of rays to a focus 455
410 Convergence by a double convex lens 457
41 1 Spherical aberration 458
412, 413, and 414 Variations of focal distance, consequent
upon variations of divergence of the incident rays . . 459
415 Horizontal section of right human eye magnified, (Home) 461
416 Straight and oblique muscles of the eye-ball 464
41 7 Lacrymal apparatus 467
418 Eye of Helix pomatia. (Muller) 481
419 Stemmata of Caterpillar, (Marcel de Serres) 484
420 Eye of the Scorpio tunensis, (Muller) 484
421 Conglomerate eyes of Julus terrestris, (Kirby and
Spence) 484
422 External magnified view of the compound eye of the
Melolontha vulgaris, (Straus Durckheim) 487
423 Ditto of that of a Phalena 487
424 Section of the compound eye of the Libellula vulgata,
magnified, (Duges) 487
425 His:hlv magnified view of the outer margin of the pre-
ceding section, (id.) 488
426 Portion of the section of the eye of the Melolontha
vulgaris, (Muller) 488
LIST OF ENGRAVINGS. XXXV
Fig. Page
427 Portion of the section of the eye of the Libellula,
(Duges) 488
428 Portion of the section of the eye of the Melolontha
vulgaris, (Straus Durckheim) . 488
430 Interior of the eye of the Perca fluviatilis, (Cuvier). . 495
431 Fibres of the crystalline lens of the Cod, (Brewster) . . 496
432 Denticulated structure of these fibres, (id.) 496
433 Section of the eye of the Goose, (Home) 501
434 Nictitating membrane of a Bird, (Petit) 501
435 Muscles of the nictitating membrane, (id.) 501
438 Talitrus, (Latreille) 542
439 Nervous system of the Talitrus, (Audouin) 543
440 Nervous system of Cymothoa, Fab. (id.) 543
441 Nervous system of Maia squinado, (id.) 545
442 Nervous system of the Larva of the Sphinx ligustri,
(Newport) 547
443 Ditto of the Chrysalis of the same, (id.) 547
444 Ditto of the Imago of the same, (id.) 547
445 Nervous system of the Asterias, (Tiedemann) 550
446 Ditto of the Aplysia, (Cuvier) 550
447 of the Patella, (id.) 550
448 of the Sepia Octopus, (id.) 550
449 Brain and spinal marrow of the Columba turtur, (id.) 552
450 Transverse section of the spinal marrow of the Cypri-
nus carpio 552
451 Brain and spinal marrow of the Trigla lyra, (Arsaky) 552
452 Brain of the Murcena conger, (Serres) 552
453 Perca fluviatilis, (Cuvier) 552
454 Testudo my das, (Carus) 552
455 Crocodile, (id.) 552
456 Lion, (Serres) 552
457 Lateral view of the brain of the Perch, (Cuvier) .... 552
458 of the Testudo mydas, (Carus) 552
459 of a section of the brain of the Dove, (id.) . . 552
460 of the Lion 552
461 Vertical section of the human brain, (Monro) 560
462 Progressive changes in the Monas 584
463 Vorticella 584
OUTLINE OF CUVIER'S CLASSIFICATION
OF ANIMALS;
WITH EXAMPLES OF ANIMALS BELONGING TO EACH DIVISION.
Bimana .
Quadrumana
Cheiroptera
Insectivora .
Plantigrada
Digitigrada
Amphibia .
Marsupialia
Rodentia
Edentata
Pachydermata
Ruminantia
Cetacea . .
Accipitres
Passeres
Scansores
Gallinae .
Grallae .
Palmipedes
Chelonia
Sauria
Ophidia .
Batrachia
Acanthopterygii
Malacopterygii
Lophobranchi .
Plectognathi .
Chondropterygii
I. VERTEBRATA.
1. Mammalia.
Man.
Monkey, Ape, Lemur.
Bat, Colugo.
Hedge-hog, Shrew, Mole.
Bear, Badger, Glutton.
Dog, Lion, Cat, Martin, Weasel, Otter.
Seal, Walrus.
Opossum, Kanguroo, Wombat.
Beaver, Rat, Squirrel, Porcupine, Hare.
Sloth, Armadillo, Ant-eater, Pangolin,
Ornithorhynchus.
Elephant, Hog, Rhinoceros, Tapir, Horse.
Camel, Musk, Deer, Giraffe, Antelope,
Goat, Sheep, Ox.
Dolphin, Whale.
2. Aves.
Vulture, Eagle, Owl.
Thrush, Swalloiu, Lark, Crow, Sparrow,
Wren.
Woodpecker, Cuckoo, Toucan, Parrot.
Peacock, Pheasant, Grous, Pigeon.
Plover, Stork, Snipe, Ibis, Flamingo.
Auk, Grebe, Gull, Pelican, Swan, Duck.
3. Reptilia.
Tortoise, Turtle, Emys.
Crocodile, Lizard, Gecko, Chameleon.
Serpents, Boa, Viper.
Frog, Salamander, Newt, Proteus, Siren.
4. Pisces.
Perch, Mackerel, Sword-fish, Mullet.
Salmon, Herring, Pike, Carp, Silurus,
Cod, Sole, Remora, Eel.
Pike-fish, Pegasus.
Sun-fish, Trunk-fish.
Lamprey, Shark, Ray, Sturgeon.
CLASSIFICATION OF ANIMALS.
xxxvn
1. Cephalopoda
2. Pteropoda .
3. Gasteropoda
4. Acephala
5. Brachiopoda
6. Cirrhopoda .
Tubicola
Dorsibranchia .
Abranchia .
1. Malacostraca.
Decapoda .
Stomapoda .
Amphipoda
Lsemodipoda
Isopoda .
2. Entomostraca
Pulmonalia
Trachealia
Aptera .
Coleoptera .
Orthoptera .
Hemiptera .
Neuroptera .
Hymenoptera
Lepidoptera
Rhipiptera .
Diptera . .
1. Echinodermata
2. Entozoa . .
3. Acalephse
4. Polypi
5. Infusoria
II. MOLLUSCA.
Cuttle-fish, Calamary, Nautilus.
Clio, Hyalcea.
Slug, Snail, Limpet, Whelk.
Oyster, Muscle, Ascidia.
Lingula, Terebratula.
Barnacle.
III. ARTICULATA.
1. Annelida.
Serpula, Sabella, Amphitrite.
Nereis, Aphrodite, Lob-worm.
Earth-worm, Leech, Nais, Hair-worm.
2. Crustacea.
Crab, Lobster, Prawn.
Squill, Phyllosoma.
Gammarus, Sand-hopper.
Cyamus.
Wood-louse.
Monoculus.
3. Arachnida.
Spider, Tarantula, Scorpion.
Phalangium, Mite.
4. Insecta.
Centipede, Podura.
Beetle, Glow-worm.
Grasshopper, Locust.
Fire-fly, Aphis.
Dragon-fly , Ephemera.
Bee, Wasp, Ant.
Butterfly, Moth.
Xenos, Sty lops.
Gnat, House-fly.
IV. ZOOPHYTA.
Star-fish, Urchin.
Fluke, Hydatid, Tape-worm.
Actinia, Medusa.
Hydra, Coral, Madrepore, Pennatula.
Brachiomts, Vibrio, Proteus, Monas.
ANIMAL AND VEGETABLE
PHYSIOLOGY.
INTRODUCTION.
Chapter I.
FINAL CAUSES.
1 o investigate the relations which connect Man
with his Creator is the noblest exercise of human
reason. The Being who bestowed on him this
faculty cannot but have intended that he should
so exercise it, and that he should acquire,
through its means, some insight, however li-
mited, into the order and arrangements of
creation ; some knowledge, however imperfect,
of the divine attributes ; and a distinct, though
faint, perception of the transcendent glory with
which those attributes are encompassed. To
Man have been revealed the power, the wisdom,
and the goodness of God, through the medium
vol. i. B
FINAL CAUSES,
of the Book of Nature, in the varied pages of
which they are inscribed in indelible characters.
On Man has been conferred the high privilege
of interpreting these characters, and of deriving
from their contemplation those ideas of grandeur
and sublimity, and those emotions of admiration
and of gratitude, which elevate and refine the
soul, and transport it into regions of a purer and
more exalted being.
A study which embraces so extensive a range
of objects, and which involves questions of such
momentous interest to mankind, must necessarily
be arduous, and requires for its successful pro-
secution the strenuous exertions of the human
intellect, and the combined labours of different
classes of philosophers, during many ages. The
magnitude of the task is increased by the very
success of those previous efforts : for the diffi-
culties augment as the objects multiply, and the
eminence, on which the accumulated knowledge
of centuries has placed us, only discloses a wider
horizon, and the prospect of more fertile regions
of inquiry ; till at length the mind, conscious of
the inadequacy of its own powers to the compre-
hension of even a small part of the system of the
universe, is appalled by the overwhelming con-
sideration of the infinity that surrounds us. The
reflection continually presents itself that the
portion of creation we are here permitted to
behold is as nothing when compared with the
FINAL CAUSES-. 3
immensity of space, which, on every side,
spreads far beyond the sphere of our vision, and
which the power of human imagination is inade-
quate to comprehend. Of the planetary system,
which includes this earth, our knowledge is
almost entirely limited to the mathematical laws
that regulate the motions of the bodies which
compose it, and to the celestial mechanism which
modern analysis has discovered to be that most
admirably calculated to preserve their harmony
and maintain their stability. Still less have we
the means of penetrating into the remoter
regions of the heavens, where the result of our
investigations respecting the myriads of lumi-
nous bodies they contain amounts to little more
than the knowledge of their existence, of their
countless numbers, and of the immeasurable
distances at which they are dispersed through-
out the boundless realms of space.
Measured on the vast scale of the universe,
the globe we inhabit appears but as an atom ;
and yet, within the compass of this atom, what
an inexhaustible variety of objects is contained ;
what an endless diversity of phenomena is pre-
sented ; what wonderful changes are occurring
in rapid and perpetual succession ! Throughout
the whole series of terrestrial beings, what stu-
died arrangements, what preconcerted adapta-
tions, what multiplied evidences of intention,
what signal proofs of beneficent design exist to
4 FINAL CAUSES.
attract our notice, to excite our curiosity, and to
animate our inquiries. Splendid as are the mo-
numents of divine power and wisdom displayed
throughout the firmament, in objects fitted by
their stupendous magnitude to impress the ima-
gination and overpower us by their awful gran-
deur, not less impressive, nor less replete with
wonder, are the manifestations of those attributes
in the minuter portions of nature, which are
more on a level with our senses, and more within
the reach of our comprehension. The modern
improvements of optical science, which have
expanded our prospects into the more distant
regions of the universe, have likewise brought
within our range of vision the more diminutive
objects of creation, and have revealed to us
many of the secrets of their structure and ar-
rangement. But, farther, our reason tells us
that, from the infinite divisibility of space, there
still exist worlds far removed from the cogni-
zance of every human sense, however assisted
by the utmost refinements of art ; worlds occu-
pied by the elementary corpuscles of matter,
composing, by their various configurations, sys-
tems upon systems, and comprising endless
diversities of motions, of complicated changes,
and of widely extended series of causes and
effects, destined for ever to remain invisible to
human eyes, and inscrutable to human science.
Thus, in whatever field we pursue our in-
FINAL CAUSES. O
quiries, we are sure to arrive at boundaries
within which our powers are circumscribed.
Infinity meets us in every direction, whether in
the ascending or descending scale of magnitude ;
and we feel the impotence of our utmost
efforts to fathom the depths of creation, or to
form any adequate conception of that Supreme
and Dominant Intelligence, which comprehends
the whole chain of being extending from that
which is infinitely small to that which is infi-
nitely great.
It is incumbent on us, before engaging in a
study of such vast importance, and extending
over so wide a field as that which lies before us,
to examine with attention the nature of those
processes of reasoning, by which we are con-
ducted to the knowledge of the peculiar class
of truths we are seeking. Such a preliminary
inquiry is the more necessary, inasmuch as the
investigation of these truths is beset with many
formidable difficulties, and liable to various
sources of fallacy, which are not met with in the
study of other departments of philosophy.
The proper objects of all human knowledge
are the relations that exist among the phenomena
of which the mind has cognizance. The pheno-
mena of the universe may be viewed as con-
nected with one another either by the relation of
cause and effect, or by that of means &xi&end; and
accordingly these two classes of relations give
(f FINAL CAUSES.
rise to different kinds of knowledge, each of
which requires to be investigated in a peculiar
mode and by a different process of reasoning.
The foundation of both these kinds of know-
ledge is, indeed, the same ; namely, the constant
uniformity which takes place in the succession
of events, and which, when traced in particular
classes of phenomena, constitutes what we me-
taphorically term the Laivs of Nature. It is the
province of philosophy, strictly so called, to
discover the circumstances or laws which regu-
late this uniformity, and to arrange the observed
changes according to their invariable antece-
dents, or causes: the unknown links by which
these causes are connected with their respective
consequents, or effects, being denominated the
powers of Nature. With reference to phenomena
which are purely mechanical, that is, to changes
which consist in the sensible motions of material
bodies, these powers are denominated forces;
and the intensities, the operations, and the cha-
racters of these forces admit of exact definition,
according to the qualities of the corresponding
effects they produce. It is by pursuing the
method of philosophical induction, so well ex-
plained by Bacon, that the physical sciences,
which the misdirected efforts of former ages had
failed to advance, have, within the last two
centuries, been carried to a height of perfection
FINAL CAUSES. 7
affording just grounds for exultation at the
achievements of the human intellect.
In the investigation of the powers which are
concerned in the phenomena of living beings
we meet with difficulties incomparably greater
than those that attend the discovery of the
physical forces by which the parts of inanimate
matter are actuated. The elements of the inor-
ganic world are few and simple ; the combina-
tions they present are, in most cases, easily un-
ravelled ; and the powers which actuate their
motions, or effect their union and their changes,
are reducible to a small number of general laws,
of which the results may, for the most part, be an-
ticipated and exactly determined by calculation.
What law, for instance, can be more simple than
that of gravitation, to which all material bodies,
whatever be their size, figure, or other properties,
and whatever be their relative positions, are
equally subjected; and of which the observa-
tions of modern astronomers have rendered it
probable that the influence extends to the
remotest regions of space? The most undevi-
ating regularity is exhibited in the motions of
those stupendous planetary masses, which con-
tinually roll onwards in the orbits prescribed
by this all-pervading force. Even the slighter
perturbations occasioned by their mutual influ-
ence are but direct results of the same general
8 FINAL CAUSES.
law, and are necessarily restrained within certain
limits, which they never can exceed, and by
which the permanence of the system is effectu-
ally secured. All the terrestrial changes de-
pendent on these motions partake of the same
constancy. The same periodic order governs
the succession of day and night, the rise and
fall of the tides, and the return of the seasons :
which order, as far as we can perceive, is inca-
pable of being disturbed by any existing cause.
Equally definite are the operations of the
forces of cohesion, of elasticity, or of whatever
other mechanical powers of attraction or repulsion
there may be, which actuate, at insensible dis-
tances, the particles of matter. We see liquids,
in obedience to these forces, collecting in spheroi-
dal masses, or assuming, at their contact with
solids, certain curvilinear forms, which are sus-
ceptible of precise mathematical determination.
In different circumstances, again, we behold
these particles suddenly changing their places,
marshalling themselves in symmetric order, and
constructing by their union solid crystals of de-
terminate figure, having all their angles and
facets shaped with mathematical exactness.
The forces by which dissimilar particles are
united into a chemical compound have been
termed Chemical Affinities; and the operation of
these peculiar forces is as definite and determi-
nable as the former. They are now known to be
FINAL CAUSES. 9
regulated by the law of definite proportions ;
a law, the discovery of which has conferred on
Chemistry the same character of precision which
appertains to the exact sciences, and which it
had never before attained. The phenomena of
Light, of Heat, of Electricity, and of Magnet-
ism have been, in like manner, reduced to laws
of sufficient simplicity to admit of the applica-
tion of mathematical reasoning, and to furnish
the accurate results derived from such applica-
tion.
Thus to whatever department of physical
science our researches have extended, we every
where meet with the same regularity in the phe-
nomena, the same simplicity in the laws, and
the same uniformity in the results. All is
strictly defined, and subjected to rigid rule : all
is subordinate to one pervading principle of
order. The Great Creator of the universe has
exercised in its construction the severest and
most refined geometry, has traced with unerring
precision the boundaries of all its parts, and has
prescribed to each element and each power its
respective sphere and limit.
Far different is the aspect of living Nature.
The spectacle here offered to our view is every
where characterised by boundless variety, by
inscrutable complexity, by perpetual mutation.
Our attention is solicited to a vast multiplicity of
objects, curious and intricate in their mechanism,
10 FINAL CAUSES.
exhibiting peculiar movements, actuated by new
and unknown powers, and gifted with high and
refined endowments. In place of the simple
combinations of elements, and the simple pro-
perties of mineral bodies, all organic structures,
even the most minute, present exceedingly com-
plicated arrangements, and a prolonged succes-
sion of phenomena, so varied and so anomalous,
as to be utterly irreducible to the known laws
which govern inanimate matter. Let us hasten,
with fresh ardour, to explore this new world that
here opens to our view.
Turning, then, from the examination of the
passive objects of the material world, we now di-
rect our attention to the busy theatre of animated
existence, where scenes of wonder and enchant-
ment are displayed in endless variety around us ;
where life in its ever-changing forms meets the
eye in every region to which our researches can
extend ; and where every element and every
clime is peopled by multitudinous races of sensi-
tive beings, who have received from the boun-
teous hand of their Creator the gift of existence
and the means of enjoyment. Our curiosity is
powerfully excited by phenomena in which our
own welfare is so intimately concerned, as are
all those that relate to animal life ; and we can-
not but take a lively and sympathetic interest
in the history of beings in many respects so ana-
logous to ourselves ; like us, possessing powers
FINAL CAUSES. 1 1
of spontaneous action, impelled by passions and
desires, and endowed with capacities of enjoy-
ment and of suffering. Can there be a more
gratifying spectacle than to see an animal in the
full vigour of health, and the free exercise of its
powers, disporting in its native element, revelling
in the bliss of existence, and testifying by its in-
cessant gambols the exuberance of its joy ?
We cannot take even a cursory survey of the
host of living beings profusely spread over every
portion of the globe without a feeling of pro-
found astonishment at the inconceivable variety
of forms and constructions to which animation
has been imparted by creative power. What
can be more calculated to excite our wonder
than the diversity exhibited among insects, all
of which, amidst endless modifications of shape,
still preserve their conformity to one general
plan of construction? The number of distinct
species of insects already known and described
cannot be estimated at less than 100,000 ; and
every day is adding to the catalogue.* Of the
comparatively large animals which live on land,
how splendid is the field of observation that lies
open to the naturalist ! What variety is con-
spicuous in the tribes of Quadrupeds and of Rep-
* Four-fifths of the insects at present known have been dis-
covered within the last ninety years : for in 1743, Ray estimated
the total number of species at 20,000 only. See his work on
" The Wisdom of God as manifested in the Creation," p. 24.
12 FINAL CAUSES.
tiles; and what endless diversity exists in their
habits, pursuits, and characters ! How extensive
is the study of Birds alone ; and how ingeniously,
if we may so express it, has nature interwoven in
their construction every possible variation com-
patible with an adherence to the same general
model of design, and the same ultimate reference
to the capacity for motion through the light
element of air. What profusion of being is
displayed in the wide expanse of the ocean,
through which are scattered such various and
such unknown multitudes of animals ! Of Fishes
alone the varieties, as to conformation and en-
dowments, are endless. Still more curious and
anomalous, both in their external form, and
their internal economy, are the numerous orders
of living beings that occupy the lower divisions
of the animal scale ; some swimming in countless
myriads near the surface ; some dwelling in the
inaccessible depths of the ocean : some attached
to shells, or other solid structures, the produc-
tions of their own bodies, and which, in process
of time, form, by their accumulation, enormous
submarine mountains, rising often from un-
fathomable depths to the surface. What sublime
views of the magnificence of creation have been
disclosed by the microscope, in the world of
infinite minuteness, peopled by countless multi-
tudes of atomic beings which animate almost
every fluid in nature ? Of these, a vast variety
FINAL CAUSES. 1 .')
of species has been discovered, each animalcule
being provided with appropriate organs, endowed
with spontaneous powers of motion, and giving
unequivocal signs of individual vitality. The
recent observations of Professor Ehrenberg have
brought to light the existence of Monads, which
are not larger than the 24,000th part of an inch,
and which are so thickly crowded in the fluid
as to leave intervals not greater than their own
diameter. Hence he has made the computation
that each cubic line, which is nearly the bulk of
a single drop, contains 500,000,000 of these
monads, a number which almost equals that of
all the human beings existing on the surface of
the earth.
Thus, if we review every region of the globe,
from the scorching sands of the equator to the
icy realms of the poles, or from the lofty moun-
tain summits to the dark abysses of the deep ; if
we penetrate into the shades of the forest, or into
the caverns and secret recesses of the earth ;
nay, if we take up the minutest portion of stag-
nant water, we still meet with life in some new
and unexpected form, yet ever adapted to the
circumstances of its situation. Wherever life
can be sustained, we find life produced. It
would almost seem as if Nature* had been thus
* In order to avoid the too frequent, and consequently irre-
verent, introduction of the Great Name of the Supreme Being
into familiar discourse on the operations of his power, I have,
14 FINAL CAUSES.
lavish and sportive in her productions with the
intent to demonstrate to Man the fertility of her
resources, and the inexhaustible fund from
which she has so prodigally drawn forth the
means requisite for the maintenance of all these
diversified combinations, for their repetition in
endless perpetuity, and for their subordination
to one harmonious scheme of general good.
The vegetable world is no less prolific in
wonders than the animal. In this, as in all
other parts of creation, ample scope is found
for the exercise of the reasoning faculties, and
abundant sources are supplied of intellectual
enjoyment. To discriminate the different cha-
racters of plants, amidst the infinite diversity
of shape, of colour, and of structure, which they
offer to our observation, is the laborious, yet
fascinating, occupation of the Botanist. Here,
also, we are lost in admiration at the never-
ending variety of forms successively displayed to
view in the innumerable species which compose
this kingdom of nature, and at the energy of
that vegetative power, which, amidst such great
differences of situation, sustains the modified life
of each individual plant, and which continues its
species in endless perpetuity. Wherever circum-
throughout this Treatise, followed the common usage of employ-
ing the term Nature as a synonym, expressive of the same
power, but veiling from our feeble sight the too dazzling splen-
dour of its glory.
FINAL CAUSES. 15
stances are compatible with vegetable existence,
we there find plants arise. It is well known
that, in all places where vegetation has been
established, the germs are so intermingled with
the soil, that whenever the earth is turned up,
even from considerable depths, and exposed to
the air, plants are soon observed to spring, as if
they had been recently sown, in consequence of
the germination of seeds which had remained
latent and inactive during the lapse of perhaps
many centuries. Islands formed by coral reefs,
which have risen above the level of the sea,
become, in a short time, covered with verdure.
From the materials of the most sterile rock, and
even from the yet recent cinders and lava of the
volcano, Nature prepares the way for vegetable
existence. The slightest crevice or inequality is
sufficient to arrest the invisible germs that are
always floating in the air, and affords the means
of sustenance to diminutive races of lichens and
mosses. These soon overspread the surface, and
are followed, in the course of a few years, by suc-
cessive tribes of plants of gradually increasing
size and strength ; till at length the island, or
other favoured spot, is converted into a natural
and luxuriant garden, of which the productions,
rising from grasses to shrubs and trees, present
all the varieties of the fertile meadow, the tangled
thicket, and the widely spreading forest. Even
in the desert plains of the torrid zone, the eye of
16 FINAL CAUSES.
the traveller is often refreshed by the appear-
ance of a few hardy plants, which find sufficient
materials for their growth in these arid regions :
and in the realms of perpetual snow which sur-
round the poles, the navigator is occasionally
startled at the prospect of fields of a scarlet hue,
the result of a wide expanse of microscopic vege-
tation.*
But whatever charms the naturalist may find
in the occupations in which he is engaged, and
however wide may be the field of his exertions,
they still are insufficient to satisfy the more
enlarged curiosity of a philosophic mind. The
passive emotion of astonishment, in which in-
ferior intellects are content to rest, serves but to
awaken, in him who has learned to think, a
desire of further knowledge. Filled with an
ardent spirit of inquiry, he cannot but be impa-
tient under the feeling that, while Nature has
placed before his eyes this splendid spectacle of
animation, she has thrown a dense veil over the
interior machinery of life, and has concealed from
his view the springs by which she sets the whole
in motion. With the hope of discovering her
* The red snow, discovered in Baffin's Bay on the 17th of
August, 1818, during the Northern Expedition, under the com-
mand of Captain Ross, was found to owe its colour to minute
fungi, or microscopic mushrooms, which vegetate on the surface
of snow, as their natural abode. See Philosophical Transactions
for 1820, p. 165.
FINAL CAUSES. 17
proceedings, he hastens to explore the several
parts which compose the organized fabric, to
examine in minute detail the anatomy of its
structure, and to ascertain the nature of the
several actions that take place within it. But,
overwhelmed by the multiplicity of objects, and
lost amidst the complication of phenomena, he
soon becomes dismayed by the magnitude and
arduous nature of the investigation. He finds
that his labours will be of no avail, unless,
previously to any attempt at theory, he takes a
careful and accurate account of all the circum-
stances attending the history and conditions of
life, from the dawn of its existence to its ap-
pointed close. On tracing living beings to their
origin, he learns that every individual vegetable
and animal takes its rise from an atom of imper-
ceptible minuteness, and gradually increases in
bulk by successive accretions of new matter,
derived from foreign sources, and, by some re-
fined, but unknown process, transmuted into its
own substance. Then, following the progressive
developement of the organs, he observes them
undergoing various modifications, as they are
assuming new forms, which characterise certain
definite epochs in the general growth of the
system. In a great number of instances, espe-
cially among the lower orders of animals, he
witnesses the same individual being acting, in
its time, a variety of different parts ; often re-ap-
vol. i. c
18 FINAL CAUSES.
pearing on the stage of life with new organs, new
faculties, and new conditions of existence, and
undergoing metamorphoses as complete as any
that have been depicted in the fables of antiquity.
The period at length arrives when the animal,
having completed its growth, attains the matu-
rity of its being, and acquires the full possession
of its powers. Every organ in succession has
received its entire developement, and has united
its energies with those which had been before
perfected. Yet, however complete the arrange-
ments that have thus been established, it is still
necessary, in order to preserve the whole system
in a state in which it may be capable of exer-
cising the functions of life, that the materials
which compose its fabric should undergo a cer-
tain slow, but constant renovation ; and the
same circle of actions and reactions, which have
brought it to its state of perfection, must con-
tinue to be repeated, in order that a due propor-
tion may be maintained between the consump-
tion and the supply of these materials. In the
course of a certain time, however, even under
the most favourable circumstances, this equili-
brium begins to fail ; the energies of the system
decline, and the processes of nutrition are in-
sufficient to repair the waste in the substance of
the body. The fluids are dissipated faster than
they can be renewed ; the channels through
which they circulate are more and more ob-
FINAL CAUSES. 10
structed, and at length cease to be pervious;
and the solids gradually become hard and rigid.
As in a machine of which the wheels are worn,
and the springs have lost their elastic force, so
in the animal body, at an advanced age, the
slightest additional impediment that occurs will
stop the movements of the whole system : and,
when once stopped, their renewal is impossible.
Nature has thus assigned to every living being
a certain period as the utmost extent of its dura-
tion. Even when exempt from external inter-
ference, all are doomed to perish, sooner or
later, by the slow but unerring operation of the
same internal causes which originally effected
their developement and growth, and which are
inseparably interwoven with the conditions of
their existence.
Numerous, however, are the extraneous and
accidental causes which may hasten or precipi-
tate their destruction, long before the period of
natural decay. How striking is the contrast, on
those occasions, between the scene we have just
beheld of an animal in the full vigour of its
powers, either rapidly bounding across the plain,
or gliding beneath the wave, or soaring in the
elevated regions of air, and the spectacle of the
same animal lying, the next moment, extended at
our feet, bereft at once of activity and of sense —
of all the faculties and powers that constitute
life. Can we contemplate without amazement
20 FINAL CAUSES.
so complete and instantaneous a change; so
sudden and awful a catastrophe ? Must we not
be animated by an eager desire to penetrate so
great a mystery, and resolve the many questions
which so striking a phenomenon must naturally
suggest ? What, we are led to ask, is the nature
of this extraordinary revolution, extending over
the whole of that frame which had so long de-
lighted the eye by its beauty, and producing
this sudden and irretrievable extinction of the
powers of life? How comes it that all those
mighty energies which the animal had so lately
displayed, and which had called forth our
admiration, perhaps even excited our envy, are
at once and for ever annihilated? What was
the bond, thus suddenly dissevered, which held
together the various parts of that compound
frame? What potent spell has been dissolved,
which could retain in combination for so long a
period the multifarious elements of that exqui-
site organization ; and from the control of which
being now released, these elements hasten to
resume their wonted attractions, and entering
into new forms of combination, are scattered into
dust, or dissipated in air, leaving no trace of
their former union ? What mechanism has been
employed in its construction ? What refined
chemistry has been exerted in assimilating new
particles of matter to those previously organized,
and in appropriating them to the nourishment of
FINAL CAUSES. 21
the parts with which they became identified?
By what transcendent power, above all, did this
assemblage of material particles first become
animated by the breath of life ; and from what
elevated source did they derive those higher
energies, apparently so foreign to their inherent
properties, and investing these once lifeless and
inert materials with the exalted attributes of
activity, of sensation, of perception, of intelli-
gence ? Shall we ever comprehend the nature
of this subtle and pervading principle, by the
agency of which all these wonderful phenomena
of life are produced, and which, combining into
one harmonious system so many heterogeneous
and jarring elements, has led to the formation
of this exquisite frame, this elaborate machine,
this miraculous assemblage of faculties ?
The discovery of a clue, if any such can be
found, to the mazes of this perplexing labyrinth
can be hoped for only from the successful cul-
tivation of the science of physiology. But be-
fore engaging in this arduous study, we ought
previously to inquire into the methods of reason-
ing by which it is to be conducted.
The object of physiology is, by the diligent
examination of the phenomena of life, to ascer-
tain the laws which regulate those phenomena,
both as they apply to the individual beings en-
dowed with life, and also as they relate to the
various assemblages that constitute the species,
22 FINAL CAUSES.
the genera, the families, the orders, and the classes
of those beings ; and, lastly, as they concern the
whole collective union of the organized world.
These peculiar laws, which it is the province
of physiology to investigate, are, as I have be-
fore observed, of two kinds, each founded upon
relations of a different class. The first, which
depend upon the simple relation of cause and
effect, are concerned merely with the natural
powers of matter. They are the laws that re-
gulate the succession of phenomena purely phy-
sical in all their stages. These phenomena con-
sist in changes among material particles, which
are either of a mechanical or chemical nature ;
or in the affections of imponderable physical
agents, such as heat, light, electricity, and mag-
netism ; and they include also the phenomena
that take place in organized bodies, and which
are referable to the operation of certain physi-
cal powers, appertaining to particular structures,
such as muscular contraction and nervous irri-
tation ; phenomena which, as we shall after-
wards find, are not reducible to any of the
former laws, but are peculiar to the living state.
The second class of laws comprise those which
are founded on the relation of means to an end ;
vand which are usually denominated final causes.
They involve the operations of mind, in conjunc-
tion with those of matter. They pre-suppose
intention or design; a supposition which implies
FINAL CAUSES. 23
intelligence, thought, motives, volition, — parti-
cular purposes to be answered, requiring the
agency of powers and of instruments adapted
to the production of the intended effects : — the
knowledge of the properties of matter, the selec-
tion and choice of particular means, and the
power of employing them in an effective manner.
These purposes may themselves be subservient
to more general objects, and these objects again
may be subordinate to remoter ends ; so that
the whole shall comprehend a systematic plan
of operations, conducive, on the most enlarged
views, to ultimate and general utility.
The study of these final causes is, in some
measure, forced upon our attention by even the
most superficial survey of nature. It is impos-
sible not to recognise the character of intention,
which is so indelibly impressed upon every part
of the structure both of vegetable and animal
beings, and which marks the whole series of
phenomena connected with their history. Mi-
croscopic observations teach us that the embryo
of an organic being contains, at a certain period
of its formation, the rudiments of the future
vegetable or animal structure, into which it is
gradually transformed by the slow and succes-
sive expansion and developement of all its parts.
The subsequent processes of nutrition do nothing
more than fill up the outlines already sketched
on the living canvass. Every organ, nay every
24 FINAL CAUSES.
fibre, resulting from this developement, contri-
butes its share in the production of certain defi-
nite effects, which we constantly witness taking
place around us, as well as experience in our
own persons. But these effects, though so fami-
liar to us, are not on that account the less invol-
ved in mystery, or the less replete with wonder.
To say that they are the results of chance conveys
no information ; and is equivalent to the asser-
tion that they are wholly without a cause. Every
one who is accustomed to reflect upon the opera-
tions of his own mind must feel that such a con-
clusion is contrary to the constitution of human
thought; for if we are to reason at all, we can
reason only upon the principle that for every
effect there must exist a corresponding cause ;
or, in other words, that there is an established
and invariable order of sequence among the
changes which take place in the universe.
But though it be granted that all the pheno-
mena we behold are the effects of certain causes,
it might still be alleged, as a bar to all further
reasoning, that these causes are not only utterly
unknown to us, but that their discovery is wholly
beyond the reach of our faculties. The argu-
ment is specious only because it is true in one
particular sense, and that a very limited one.
Those who urge it, do not seem to be aware that
its general application, in that very same sense.
FINAL CAUSES. 25
would shake the foundation of every kind of
knowledge, even that which we regard as built
upon the most solid basis. Of causation, it is
agreed that we know nothing ; all that we do
know is, that one event succeeds another with
undeviating constancy. Now by probing this
subject to the bottom, we shall find that, in
rigid strictness, we have no certain knowledge
of the existence of any thing, save that of the
sensations and ideas which are actually passing
in our minds, and of which we are necessarily
conscious. Our belief in the existence of ex-
ternal objects, in their undergoing certain
changes, and in their possessing certain physical
properties, rests on a different foundation, name-
ly, the evidence of our senses ; for it is the
result of inferences which the mind is, by the
constitution of its frame, necessarily led to form.
We may trace to a similar origin the persuasion,
irresistibly forced upon us, that there exist not
only other material objects beside our own
bodies, but also other intellectual beings beside
ourselves. We can neither see nor feel those
extraneous intellects, any more than we can
see or feel the cause of gravitation, or the subtle
sources of electricity or magnetism. We never-
theless believe in the reality both of the one and
of the other ; but it is only because we infer
their existence from particular trains of impres-
26 FINAL CAUSES.
sions made upon our senses, of which impres-
sions alone our knowledge can, in metaphysical
strictness, be termed certain.
Upon what evidence do I conclude that I am
not a solitary being in the universe ; that all is
not centered in myself; but that there exist other
intellects similar to my own? Undoubtedly no
other than the observation that certain effects
are produced, which the experience I have had
of the operations of my own mind lead me, by
an irresistible analogy, to ascribe to a similar
agency, emanating from other beings ; beings,
however, of whose actual intellectual presence I
cannot be conscious, whose nature I cannot
fathom, whose essence I cannot understand. I
can judge of the operations of other minds only
in as far as those operations accord with what
has passed in my own. I cannot divine pro-
cesses of thought to which mine have borne no
resemblance ; I cannot appreciate motives of
which I have never felt the influence, nor compre-
hend the force of passions never yet awakened
in my breast : neither can I picture to myself
feelings to which no sympathetic chord within
me has ever vibrated.
Our own intelligence, our own views, and our
own affections, then, furnish the only elements
by which it is possible for us to estimate the
analogous powers and attributes of other minds.
The difficulty of applying this scale of measure-
FINAL CAUSES. 27
ment will, of course, increase in proportion to the
difference between the objects compared ; and
although we may conceive that there are powers
and intelligences infinitely surpassing our own,
the conceptions we can form of such superior
essences must necessarily be indefinite and ob-
scure, and must partake of the same kind of
imperfection as our notions of the distances of
the heavenly bodies, however familiar we may
be with the units of the scale by which those
distances are capable of being expressed. When,
on the other hand, the objects contemplated are
more within the range of our mental vision ;
when, for instance, they are phenomena that we
can assimilate to our own voluntary acts, and in
which we can clearly trace the connexion be-
tween means and end, then does our recognition
of the agency of intellect become most distinct,
and our conviction of its real and independent
existence become most intimate and assured.
Such is the kind of evidence on which rests our
belief of the existence of our fellow men. Such,
also, is the foundation of our assurance that
there exists a Mighty Intellect, who has planned
and executed the stupendous works of creation,
with a skill surpassing our utmost conceptions ;
by powers to which we can assign no limit, and
the object of whose will is universal good.*
* The view here taken is, of course, limited to Natural Theology ;
that being the express and exclusive object of these Treatises.
23 FINAL CAUSES.
It will argue no undue presumption, therefore,
if, in our earnest endeavours to form just ideas of
the attributes of the Deity from the examination
of nature, we are led to institute comparisons be-
tween His works and those of man ; and strive
to gather some faint notions of the divine intelli-
gence by applying the only standard of admea-
surement which we possess, and are permitted to
employ, namely, that derived from the operations
of human intellect. Oar interpretations of the
designs of the Creator must here be obtained
through the medium of human views ; and our
judgment of His benevolence can be formed
only by reference to our own affections, and by
their accordance with those fervent aspirations
after good, which the Author of our being has
deeply interwoven with our frame.
The evidence of design and contrivance in the
works of nature carries with it the greatest force
whenever we can trace a coincidence between
them and the products of human art. If in any
unknown region of the earth we chanced to
discover a piece of machinery, of which the
purpose was manifest, we should not fail to
ascribe it to the workmanship of some me-
chanist, possessed of intelligence, actuated by
a motive, and guided by intention. Farther, if
we had a previous experience of the operation of
similar kinds of mechanism, we could not doubt
that the effect we saw produced was the one
FINAL CAUSES. 2.9
intended by the artificer. Thus, if in an unex-
plored country, we saw, moving upon the waters
of a lake, the trunk of a tree, carved into the
shape of a boat, we should immediately conclude
that this form had been given to it for the pur-
pose of enabling it to float. If we found it also
provided with paddles at its sides, we should
infer, from our previous knowledge of the effects
of such instruments, that they were intended to
give motion to this boat, and we should not
hesitate to conclude that the whole was the work
of human hands, and the product of human
intelligence and design. If, in addition, we
found this boat furnished with a rudder and with
sails, we should at once understand the object of
these contrivances, and our ideas of the skill of
the artificer would rise in proportion to the ex-
cellence of the apparatus, and the ingenuity dis-
played in its adaptation to circumstances.
Let us suppose that in another part of this
lake we found an insect,* shaped like the boat,
and moving through the water by successive
impulses given to that medium by the action of
levers, extending from its sides, and shaped like
paddles, having the same kind of movement, and
producing the same effects. Could we resist the
persuasion that the Artificer of this insect, when
* Such as the Notonecta r/lauca, Lin., or water boatman, and
the Dytiscus marginalis, or water beetle.
.30 FINAL CAUSES.
forming it of this shape, and providing it with
these paddles, had the same mechanical objects
in view ? Shall we not be confirmed in this idea
on finding that these paddles are constructed
with joints, which admit of no other motion than
that of striking against the water, and of thus
urging forwards the animal in its passage
through that dense and resisting medium ? Many
aquatic animals are furnished with tails which
evidently act as rudders, directing the course
of their progressive motion through the fluid.
Who can doubt but that the same intention and
the same mechanical principles which guide the
practice of the ship-builder, are here applied in
a manner still more refined, and with a master's
hand? If Nature has furnished the nautilus
with an expansible membrane, which the animal
is able to spread before the breeze, when propi-
tious, and by means of which it is wafted along
the surface of the sea, but which it quickly
retracts in unfavourable circumstances, is not her
design similar to that of the human artificer,
when he equips his bark with sails, and provides
the requisite machinery for their being hoisted
or furled with ease and expedition ?
The maker of an hydraulic engine places
valves in particular parts of its pipes and cis-
terns, with a view to prevent the retrograde
motion of the fluids which are to pass through
them. Can the valves of the veins, or of the
FINAL CAUSES. 31
lymphatics, or of the heart have a different
object ; and are they not the result of deliberate
and express contrivance in the great Mechanist
of the living frame ?
The knowledge of the laws of electricity, in its
different forms, is one of the latest results which
science has revealed to man. Could these laws,
and their various combinations, have been un-
known to the Power who created the torpedo,
and who armed it with an energetic galvanic
battery, constructed upon the most refined
scientific principles, for the manifest purpose
of enabling the animal to strike terror into its
enemies, and paralyse their efforts to assail it.
Does not the optician, who designedly places his
convex lens at the proper distance in a darkened
box, for the purpose of obtaining vivid pictures
of the external scene, evince his knowledge of
the laws of light, of the properties of refracting
media, and of the refined combinations of those
media by which each pencil is brought to a
separate focus, and adjusted to form an image of
remote objects ? Does it not, in like manner,
argue the most profound knowledge and foresight
in the divine Artist, who has so admirably hung
the crystalline lens of the eye in the axis of a
spherical case, in the fore part of which He has
made a circular window for the light to enter,
and spread out on the opposite side a canvass to
receive the picture ? Has no thought been exer-
•32 FINAL CAUSES.
cised in darkening the walls of this camera
obscura, and thus preventing all reflection of the
scattered rays, which might interfere with the
distinctness of the image ?
But we farther observe in the eye many
exquisite refinements of construction, by which
various defects, unavoidable in all optical instru-
ments of human workmanship, are remedied.
Of this nature are those which render the organ
achromatic, which correct the spherical aberra-
tion, and which provide for the adjustment of
its refracting powers to the different distances of
the objects viewed ; not to speak of all the
external apparatus for the protection, the pre-
servation, and the movements of the eye-ball,
and for contributing in every way to the proper
performance of its office. Are not all these
irrefragable proofs of the continuity of the same
design ; and are they not calculated still farther
to exalt our ideas of the Divine Intelligence,
of the elaborate perfection impressed upon His
works, and of the comprehensive views of His
providence ?
These facts, if they stood alone, would be suf-
ficient to lead us irresistibly to this conclusion :
but evidence of a similar kind may be collected
in abundance from every part of living nature to
which our attention can be directed, or to which
our observations have extended. The truths
they teach not only acquire confirmation by the
FINAL CAUSES. 33
corroborating tendency of each additional fact
of the same description, but the multitude of
these facts is so great, that the general conclu-
sion to which they lead must be considered as
indubitable. For the argument, as it has been
justly remarked, is cumulative; that obtained
from one source being strengthened by that
derived from another ; and all tending to the
same conclusion, like rays converging to the
same point, on which they concentrate their
united powers of illumination.
The more we. extend our knowledge of the
operations of creative power, as manifested in the
structure and economy of organized beings, the
better we become qualified to appreciate the in-
tentions with which the several arrangements
and constructions have been devised, the art with
which they have been accomplished, and the
grand comprehensive plan of which they form a
part. By knowing the general tendencies of ana-
logous formations, we can sometimes recognise
designs that are but faintly indicated, and trace
the links which connect them with more general
laws. By rendering ourselves familiar with the
hand-writing where the characters are clearly
legible, we gradually learn to decypher the more
obscure passages, and are enabled to follow the
continuity of the narrative through chapters which
would otherwise appear mutilated and defaced.
Hence the utility of comprehending in our studies
VOL. I. D
34 FINAL CAUSES.
the whole range of the organized creation, with a
view to the discovery of final causes, and obtain-
ing adequate ideas of the power, the wisdom, and
the goodness of God.
Chapter II.
THE FUNCTIONS OF LIFE.
The intentions of the Deity in the creation of
the animal kingdom, as far as we are competent
to discern or comprehend them, are referable to
the following classes of objects. The first relates
to the individual welfare of the animal, em-
bracing the whole sphere of its sensitive exis-
tence, and the means of maintaining the vitality
upon which that existence is dependent. The
second comprises the provisions which have been
made for repairing the chasms resulting, in the
present circumstances of the globe, from the
continual destruction of life, by ensuring the
multiplication of the species, and the continuity
of the race to which each animal belongs. The
third includes all those arrangements which have
been resorted to in order to accommodate the
system to the consequences that follow from an
indefinite increase in the numbers of each species.
The fourth class relates to that systematic eco-
THE FUNCTIONS OF LIFE. 35
nomy in the plans of organization by which all
the former objects are most effectually secured.
I shall offer some observations on each of these
general heads of enquiry.
With reference to the welfare of the individual
animal, it is evident that in the brute creation,
the great end to be answered is the attainment
of sensitive enjoyment. To this all the arrange-
ments of the system, and all the energies of its
vital powers must ultimately tend. Of what
value would be mere vegetative life to the being
in whom it resides, unless it were accompanied
by the faculty of sensation, and unless the sen-
sations thence arising were attended with plea-
sure ? It is only by reasoning analogically from
the feelings we have ourselves experienced that
we ascribe similar feelings to other sentient
beings, and that we infer their existence from
the phenomena which they present. Wherever
these indications of feeling are most distinct, we
find that they result from a particular organiza-
tion, and from the affections of a peculiar part
of that organization denominated the nervous
substance. The name of brain is given to a
particular mass of this substance placed in the
interior of the body, where it is carefully pro-
tected from injury.
The sensations, for exciting which the brain is
the material instrument, or immediate organ, are
the result of certain impressions made on par-
36 THE FUNCTIONS OF LIFE.
ticular parts of the body, and conveyed to that
organ by the medium of filaments, composed of
a similar substance, and termed nerves. In this
way, then, it has been provided that a communi-
cation shall be established between the sentient
principle and the external objects, by which its
activity is to be excited, and on which it is to be
dependent for the elements of all its affections,
both of sensation and of intellect. A consider-
able portion of this treatise will be occupied
with the developement of the series of means
by which impressions from external objects are
made on the appropriate organs that are pro-
vided to receive and collect them, so as not only
to give rise to varied sensations, but also to con-
vey a knowledge of the existence and different
qualities of the objects which produce them.
This latter faculty is termed Perception.
But in the formation of animals it was not
the intention of Providence to endow them with
the mere capacity of being affected by sur-
rounding objects, and of deriving from them
various sensations of pleasure and of pain, with-
out granting them the power of controlling these
effects, and of acting on those objects in return.
The faculties of sensation and perception, in
beings destined to be merely passive, and the
sport of every contingent agency, would have
been not merely useless, but even baneful en-
dowments. The same beneficent power, which
THE FUNCTIONS OF LIFE. 37
has conferred these gifts, has conjoined that of
voluntary motion, by which the animal may
not only obtain possession of such objects as
minister to its gratification, and reject those
which are useless or hurtful, but may also move
from place to place, and enlarge the sphere of
its perceptions and of its power. The same
mass of nervous substance which, under the
name of brain, we have recognised as the organ
of sensation, is also, as will afterwards be shown,
the organ of volition ; and the medium, by
which the commands of the will are transmitted
from the brain to the mechanical apparatus em-
ployed for motion, is again certain filaments of
nerves ; but these nervous filaments are distinct
from those which are subservient to sensation.
Next in importance, then, to the organs of
sensation and perception, are those of Voluntary
Motion. They comprise two kinds of objects;
first, the establishment of a certain mechanism,
having the cohesion, the strength, and the mo-
bility requisite for the different actions which the
animal is to perform ; and, secondly, the provi-
sion of a power, or agent, which shall be capable
of supplying the mechanical force for setting this
machinery in motion. With these objects must
be combined various subsidiary arrangements
relating to the connexions, the support, the pro-
tection, and other mechanical conditions of the
organs of the body. It will be convenient to
38 THE FUNCTIONS OF LIFE.
comprehend these under one general head, con-
sidering them as composing the Mechanical
Functions of the animal economy. They will
engage a considerable share of our attention in
this work, as affording the clearest and most
palpable proofs of contrivance and design.
From the peculiar conditions of the living
body, not only with regard to the mechanical
properties of its various parts, and the powers
by which their movements are effected, but also
with regard to the chemical laws which regulate
the combinations of elements composing the
substance of the body, there is required, as will
be more fully explained in the sequel, a con-
tinual renovation of that substance. For this
purpose new materials are perpetually wanted,
and must be as regularly supplied. Hence
arises a new class of functions, comprising a
great extent of operations, opening a wide field
of curious and interesting enquiry, and fur-
nishing abundant evidence of the wise and bene-
ficent operations of nature. These may be com-
prehended under a separate class bearing the
general title of Nutritive Functions. They are
often, also, spoken of under the designation of
the Vital Functions, from their more immediate
relation to the continuance of vitality, that is, of
mere vegetative life, as distinguished from the
exercise of the higher faculties of sensation, per-
ception, and voluntary motion, which are the
THE FUNCTIONS OF LIFE. 39
ultimate ends of animal existence, and which
are emphatically termed the Animal Functions.
The vital as well as the animal functions
require for the execution of their various objects
certain instruments of an appropriate mecha-
nical construction, adapted to those objects. To
the contrivances of the mechanist must be added
a refined hydraulic apparatus for the conveyance
of fluids, and for the regulation of their move-
ments ; and with these must be conjoined the
skilful combinations of the laboratory, by which
the powers of the most subtle chemistry are
exercised in effecting all the transmutations re-
quired by this elaborate system of operations.
As far as they involve mechanical principles,
these objects again arrange themselves under
the mechanical functions : and I shall accordingly
include them under that head, when giving an
account of this branch of the subject.
There is another, and a most important conse-
quence flowing from the peculiar chemical
condition of the materials of which animal
structures are composed. The mode in which
their elements are combined is so complex as
to require a long and elaborate process to ac-
complish that combination ; and neither the
organs with which animals are furnished, nor the
powers with which those organs are endowed,
are adequate to the conversion of the materials
furnished by the inorganic world into the sub-
40 THE FUNCTIONS OF LIFE.
stances required for the construction of their
bodies, and the maintenance of their powers.
These inorganic elements must have passed
through intermediate stages of combination, and
must have been previously elaborated by other
organized beings. This important office is con-
signed to the vegetable kingdom. Receiving
the simple food furnished by nature, which con-
sists chiefly of water, air, and carbonic acid,
together with a small proportion of other sub-
stances, plants convert these aliments into pro-
ducts, which not only maintain their own vita-
lity, but serve the further purpose of supporting
the life of animals. Thus was the creation and
continuance of the vegetable kingdom a neces-
sary step towards the existence of the animal
world ; as well as a link in the great chain of
being, formed and sustained by Almighty power.
The Physiology of Vegetables presents many
topics of great interest with relation to final
causes, and will in this Treatise be reviewed
with special reference to this important object.
Nutrition, both in the vegetable and animal
systems, comprises a very extended series of
operations. In the former it includes the ab-
sorption of the crude materials from the sur-
rounding elements, — their transmission to organs
where they are aerated, that is, subjected to the
chemical action of the air ; — their circulation in
the different parts of the plant,— their further
THE FUNCTIONS OF LIFE. 41
elaboration in particular vessels and receptacles
— their deposition of solid materials — and their
conversion into peculiar products, as well as
into the substances which compose the several
organs ; — and, finally, the growth and develope-
inent of the whole plant.
Still more various and complicated are the
corresponding functions in animals. Their ob-
jects may be arranged under the following
general heads ; each, again, admitting of further
subdivision. The first end to be accomplished
is to annualize the food ; that is, to convert it into
a matter having the chemical properties of the
animal substances with which it is to be after-
wards incorporated. The entire change thus
effected is termed Assimilation, of which Diges-
tion forms a principal part. The second object
is to collect and distribute this prepared nutri-
ment, which is the blood, to the different organs,
or wherever it may be wanted. The necessary
motions for these purposes are given to the blood
by the organs of Circulation, consisting of the
Heart, which impels it through a system of pipes
called Arteries, and receives it back again by
means of another set of tubes called Veins. In
the third place it is necessary that the circulating
blood should continually undergo purification by
the chemical action of oxygen : a purpose which
is answered by the function of Respiration. The
fourth stage of nutrition relates to the more im-
42 THE FUNCTIONS OF LIFE.
mediate application of this purified material to
the wants of the system, to the extension of the
organs, to the reparation of their losses, and to
the restoration of their exhausted powers.
Life, then, consists of a continued series of
actions and reactions, ever varying, yet con-
stantly tending to definite ends. Most of the
parts of which the body consists undergo con-
tinual and progressive changes in their dimen-
sions, figure, arrangement, and composition.
The materials which have been united together
and fashioned into the several organs, are them-
selves successively removed and replaced by
others, which again are, in their turn, discarded,
and new materials substituted, though without any
perceptible change of external form. Perpetual
mutation appears to constitute the fundamental
law of living nature ; and it has been farther
decreed by the power which gave the first im-
pulse of animation to this organized fabric, that
its movements and its powers shall be limited in
their duration, and that, even when they are not
destroyed by extraneous causes, after continuing
for a certain period, they shall come to a close.
The law of Mortality, to which all the beings
that have received the gift of life are subjected,
is a necessary consequence of the law of muta-
tion ; and the same causes that originally effected
the developement and growth of the system, and
maintained it in the vigour of its maturity, by
THE FUNCTIONS OF LIFE. 43
continuing to operate, are certain to lead to the
demolition of the fabric they had raised, and to
the exhaustion and final extinction of its powers.
The individual dies ; but it is only to give place
to other beings, alike in nature and in form,
equally partaking of the blessings of existence,
and destined, after having, in their turn, given
rise to a new race of successors, to run through
the same perpetual cycle of changes and reno-
vations.
Thus the continuance and multiplication of
each species may be assigned as the second of
the great ends which are to be accomplished in
the system of living nature. A portion of the
vital power of the parent is for this purpose em-
ployed to give origin and birth to the offspring.
The process itself, by which the germs of living
beings originate, is veiled in the most impene-
trable mystery. But we are permitted to trace
many of the subsequent steps in the gradual
developement both of vegetable and animal
organizations ; and certainly no part of the eco-
nomy of animated nature is more calculated to
impress us with exalted ideas of the immensity of
the scheme of Providence, and the vigilant care
with which the most distant consequences have
been anticipated, than the history of the early
periods of their existence. Nothing can be more
admirable than the progressive architecture of
the frame ; nothing more beautiful than the
44 THE FUNCTIONS OF LIFE.
setting up of temporary structures, which are
required only at an early stage of growth, and
which are afterwards removed to give place to
more permanent and finished organs.
The utmost solicitude has been shown in every
part of living nature to secure the perpetuity of
the race, by the establishment of laws, of which
the operation is certain in all contingent circum-
stances. It has also been manifestly the object
of various provisions to diffuse the races as widely
as possible over a great surface of the habitable
globe.
We are next to advert to the important conse-
quences which, in the animal kingdom more
especially, flow from this law of indefinite pro-
duction. As animals are ultimately dependent
on the vegetable kingdom for the materials of
their subsistence, and as the quantity of these
materials is, in a state of nature, necessarily
limited by the extent of surface over which vege-
tation is spread, a time must arrive when the
number of animals thus continually increasing
is exactly such as the amount of food pro-
duced by the earth will maintain. When this
limit has been attained, no further increase
can take place in their number, except by re-
sorting to the expedient which we find actually
adopted, namely, that of employing the sub-
stance of one animal for the nourishment of
others. Thus the identical combinations of ele-
THE FUNCTIONS OF LIFE. 45
ments, effected by the powers of vegetation, are
transferred in succession from one living being
to another, and become subservient to the main-
tenance of a great number of different animals
before they finally, by the process of decomposi-
tion, revert to their original inorganic state.
" See dying vegetables life sustain,
See life dissolving vegetate again ;
All forms that perish other forms supply,
By turns we catch the vital breath and die." — Pope.
Hence has the ordinance been issued to a large
portion of the animal world that they are to main-
tain themselves by preying upon other animals ;
either consuming their substance when already
dead, or depriving them of life in order to prolong
their own. Such is the command given to the
countless hosts of living beings which people the
vast expanse of ocean; to the unnumbered tribes
of insects which every spot of earth discloses ; to
the greater number of the feathered race ; and
also to a more restricted order of terrestrial ani-
mals. To many has the commission been given
to ravage and to slaughter by open violence ;
others are taught more insidious, though no less
certain arts of destruction ; and some appear
to be created chiefly for the purpose of quickly
clearing the earth of all decomposing animal or
vegetable materials, which might otherwise have
46* THE FUNCTIONS OF LIFE.
filled the air with noxious exhalations, and con-
taminated the sources of vitality.*
This new law of animal existence must neces-
sarily introduce new conditions of organization
and of functions. Structures adapted to rapid
locomotion must be supplied for the pursuit of
prey, and powerful weapons for attack and des-
truction. But nature has not left the weaker
animals unprovided with the means of repulse,
of defence, or of escape ; and for these purposes
various expedients, either of force, of swiftness,
or of stratagem, have been resorted to in different
cases.
That a large portion of evil is the direct con-
sequence of this system of extensive warfare, it
is in vain to deny. But although our sensibility
may revolt at the wide scene of carnage which
is so generally presented to our view, our more
sober judgment should place in the other scale
the great preponderating amount of gratification
which is also its result. We must take into
account the vast accession that accrues to the
mass of animal enjoyment from the exercise of
those powers and faculties which are called forth
by this state of constant activity ; and when this
* As specially appointed for the performance of this useful
task may be cited, among the larger beasts of prey, the hyaena,
the jackal], the crow, and the vulture: among marine animals,
the Crustacea, and numerous mollusca ; and among- the lower
orders innumerable tribes of insects, such as ants, flesh flies, &c.
THE FUNCTIONS OF LIFE. 47
consideration is combined, as it ought to be, with
that of the immense multiplication of life which
is admissible upon this system alone, we shall
find ample reason for acknowledging the wisdom
and the benevolent intentions of the Creator, who,
for the sake of a vastly superior good, has per-
mitted the existence of a minor evil.
From this system of hostilities there must also
arise new relations among the different races of
animals. It affords a ready and effectual means
of preserving the proper balance between differ-
ent races. Each separate species of animals, far
from being isolated and independent, performs
the part assigned to it in the system of nature,
and, however apparently insignificant, may have
a sensible influence on the rest of the animal
creation. Man, above all other animals, has ef-
fected a most important change in the condition
of a multitude of other races, in every region
where his numbers have multiplied, where the
arts of civilization have enlarged his dominion,
and where science has armed him with still more
extensive power.
In every department of nature it cannot fail
to strike us that boundless variety is a charac-
teristic and predominant feature of her produc-
tions. It is only when the object to be attained
is dependent upon certain definite conditions, ex-
cluding the possibility of modification, that these
conditions are uniformly and strictly adhered to.
48 THK FUNCTIONS OF LIFE.
But wherever that absolute necessity does not
exist, and there is afforded scope for deviation,
there we are certain to find introduced all those
modifications which the occasion admits of. Not
only is this tendency to variety exemplified in
the general appearance and form of the body,
but it also prevails in each individual organ,
however minute and insignificant that organ
ma)7 seem. Even when the purpose to be an-
swered is identical, the means which are employ-
ed are infinitely diversified in different instances,
as if a design had existed of displaying 10 the
astonished eyes of mortals the unbounded re-
sources of creative power. While the elements
of structure are the same, there is presented to
us in succession every possible combination of
organs, as if it had been the object to exhaust
all the admissible permutations in the order of
their union.
Some wise purpose, though dimly perceptible
to our imperfect understandings, is no doubt an-
swered by this great law of organic formation, the
law of variety. That it is not blindly or indis-
criminately followed, is apparent from its being
circumscribed within certain limits, and controlled
by another law, which we have next to consider —
that of conformity to a definite type.
The most superficial survey of nature is suffi-
cient to show that there prevail certain general
resemblances among great multitudes of species,
THE FUNCTIONS OF LIFE. 4!)
which lead us to class them into more or less
comprehensive groups. Thus in the animal
kingdom, Quadrupeds, Birds, Fishes, Reptiles,
Shell-fish, and Insects, compose natural assem-
blages or classes, and each of these is readily
divisible into subordinate groups or families.
Now it results from a closer examination of the
structure and economy of plants and animals,
that the formation of all the individual species
comprehended in the same class, has been con-
ducted in conformity with a certain ideal model,
or type, as it is called. Of this general type all
the existing forms appear as so many separate
copies, differing, indeed, as to particulars, but
agreeing as to general characters. The same
observation applies to the families, the genera,
and other subordinate groups of living beings.
The more extensive our acquaintance is with
the anatomy and physiology of both plants and
animals, the more striking do these analogies
appear ; so that amidst endless diversity in the
details of structures and of processes, the same
general purpose is usually accomplished by simi-
lar organs and in similar modes. So firmly is
this principle established, that we may venture
with confidence to predict many circumstances
relating to an unknown animal, of which only
a few fragments are presented to us, from our
general knowledge of the characters and econo-
my of the tribe or family, on the type of which it
VOL. I. E
•50 THE FUNCTIONS OF LIFE.
has been modelled. Thus the discovery of a mu-
tilated portion of the skeleton of a fossil animal,
conveys to the physiologist, who is conversant
with the details of comparative anatomy, a know-
ledge of the general structure and habits of that
animal, though all other traces of its existence
may have been swept away, amidst the primeval
revolutions of the globe.*
Xot only does this tendency to conform to
particular types obtain in all organic formations,
but further inquiry leads to the conclusion that
the deviations from these standard forms, far
from being arbitrary, are themselves referable to
definite laws. The regulating principle of the
variations is subordinate to higher views, and
has reference to the respective objects and des-
tination of each particular species in the general
system of created beings. Nature, as far as we
can discern, appears, in conformity with these
intentions, first to have laid down certain great
plans of functions to which she has adapted the
structure of the organs; the minor objects and
more subordinate functions being accommodated
to this general design. Hence arises the neces-
sary and reciprocal dependence of each organ
and of each function on every other ; and hence
are deduced what have been termed the laics of
* See Cuvier's " Discours sur les revolutions de la surface
du globe," p. 47, prefixed to the first volume of his " Ossemens
Fossiles."
THE FUNCTIONS OF LIFE. 51
the co-existence of organic forms. By attention
to these laws we may often explain how each
variation that is observed in any one organ,
common to a natural group of animals, entails
certain necessary and corresponding variations
in other parts, and extends its influence in modi-
fying, in a greater or less degree, the whole fabric.
It is in comparative anatomy as in mechanics,
where any alteration made in the position of one
part of a system of bodies occasions a change in
the centres of gravity, of gyration, and of oscilla-
tion ; and evolves new mechanical forces and
conditions of equilibrium, which render new
adjustments in other parts necessary, in order
to restore the equipoise, and preserve the har-
mony of their movements.
We may conclude from these inquiries that
the numerous classes or assemblages of beings,
which science has formed, are by no means
arbitrary creations of the human mind, invented
merely with a view to facilitate the study and
to recognise the identity of species, or calculated
only to supply the imperfections of our memory;
but that they have a real foundation in nature.
To regard any of the beings in the creation as
isolated from the rest, would be to take a very
narrow and a false view of their condition ; for
all are connected by mutual relations. Even
among the leading types which represent the
great divisions of the animal kingdom we may
52 THE FUNCTIONS OF LIFE.
trace several points of resemblance, which show
them to be parts of one general plan, and to
have emanated from the same Creator. In the
progress of discovery we are continually meeting
with species which occupy intermediate places
between adjacent types, and appear as links
of connexion in the chain of being. It often
happens, as I shall hereafter have occasion to
point out, that throughout an extensive series of
organic forms, the steps of gradation by which
one type passes into another, are so numerous
and so regular, as to preclude the possibility of
drawing a decided line of demarcation between
those that properly appertain to each.
All these apparent anomalies and gradations
of structure tend still farther to demonstrate the
generality of the plans of nature, and the com-
prehensiveness of her design, which embraces the
whole series of animated beings. These views
are strongly corroborated by the discoveries
that are continually being made of species now
no longer in existence, but which, in former
ages of the world, helped to fill up many of the
chasms which now interrupt the continuity of
that series. This knowledge has been revealed
to us by the examination of their fossil remains,
those monuments of former epochs, which have
thrown such important light on the most inte-
resting questions in Geology as well as in Phy-
siology.
THE FUNCTIONS OF LIFE. 58
The notion has long prevailed that the beings
composing the vegetable and animal kingdoms,
might, if we were thoroughly acquainted with
their structure and economy, be arranged in a
linear series, commencing with the simplest, and
regularly ascending to the most refined and com-
plicated organizations, till it reached its highest
point in man, who is unquestionably placed at
the summit of the scale. Bonnet, in particular,
cherished with enthusiastic ardour the hypo-
thesis that all organic beings formed a con-
tinuous gradation, each member of which, like
the successive links of a chain, was connected
with that which preceded, and with that which
followed it ; and he pursued this idea by ap-
plying it even to the productions of the mineral
world. But, divesting ourselves of these hypo-
thetical views and figurative images, we find, on
sober observation, that instead of one continuous
series, we are presented with only detached frag-
ments and interrupted portions of this imaginary
system : so that, if, for the sake of illustration, we
must employ a metaphor, the natural distribution
of animals would appear to be represented, not
by a chain, but by complicated net- work, where
several parallel series are joined by transverse
and oblique lines of connexion. A multitude of
facts, however, tend to show that the real types
or models of structure, are more correctly re-
presented by circular, or recurring arrange-
54 THE FUNCTIONS OF LIFE.
ments.* But as the discussion of these and
other topics relating to the plans and designs of
nature in the formation of organic beings re-
quires a previous acquaintance with the details
of comparative anatomy and physiology, I shall
defer all further observations respecting them
till I have finished the review I propose to take
of the several structures and functions of the
animal and vegetable economy. There are,
however, some views that have been entertained
respecting the procedure of nature in the forma-
tion of the different races of animals, which it
will be proper to notice in this place, as they
will occasionally be referred to when the facts
that more particularly illustrate and support
them come to be noticed.
An hypothesis has been advanced that the
original creation of species has been successive,
and took place in the order of their relative com-
plexity of structure ; that the standard types
have arisen the one from the other ; that each
succeeding form was an improvement upon the
preceding, and followed in a certain order of
developement, according to a regular plan
traced by the great Author of the universe for
bestowing perfection on his works. This grada-
* Mr. M'Leay is the author of this ingenious theory, which
he has developed in his " Horce Entomologies," and which
appears to be verified to a gTeat extent by the modern disco-
veries in comparative anatomy.
THE FUNCTIONS OF LIFE. 55
tion of structure was necessarily accompanied
by a gradation of faculties : the object of each
change of type being to attain higher objects,
and to advance a further step towards the ulti-
mate ends of the animal creation. Many appa-
rent anomalies which are inexplicable upon any
other supposition, are easily reconcilable to this
theory. The developements of structure be-
longing to a particular type being always pro-
spective, are not completed in the inferior orders
of the group formed upon that model, but
remain more or less imperfect, although each
organ always fully answers the particular pur-
pose of the individual animal. But it sometimes
happens that the imperfection of an organ is so
great, in consequence of its developement having
proceeded to a very small extent, as to render
it wholly useless in that particular species, al-
though in a higher race of animals it fully per-
forms its proper function. Thus we shall find
that rudiments of feet are contained within the
bodies of various kinds of serpents, which can
obviously not be serviceable as organs of pro-
gression. In the young of the whale, before its
birth, there is found in the lower jaw, a row of
small teeth, which do not rise above the gums,
and can, therefore, be of no use as instruments of
mastication. Their further growth is arrested,
and they are afterwards obliterated. This im-
perfect or rudimental condition of an organ indi-
50 THE FUNCTIONS OF LIFE.
cates its relation to other species belonging to
the same type, and demonstrates the existence
of a general plan in their formation. I shall
have occasion to mention several striking in-
stances of this kind, both in the animal and
vegetable kingdom.
In following the transitions from one model of
structure to another, we often observe that a par-
ticular organ has been very greatly enlarged, or
otherwise modified to suit some particular pur-
pose, foreign to its usual destination, or to qua-
lify it for performing some new office, rendered
necessary by the particular circumstances in
which the animal is placed. Thus the ribs,
which in quadrupeds are usually employed for
respiration, are in serpents converted into auxi-
liary organs of progressive motion : and in the
Draco volans, or flying lizard, they are extended
outwards from the sides to serve as wings. The
teeth, usually intended for mastication, are in
many animals enlarged in order to constitute
weapons of offence, as in the Elephant, the Boa?;
the Narwhal, and the Pristis. In like manner,
in the Crustacea, organs of the same general
structure are converted sometimes into jaws,
sometimes into feelers (or palpi), and sometimes
into feet ; and the transition from the one to the
other is so gradual that it is difficult to draw a
proper distinction between them.
In pursuing the ascending series of animal
structures we meet also with instances of a con-
THE FUNCTIONS OF LIFE. 57
trary change, yet still resulting from the conti-
nued application of the same principle. An
organ which has served an important purpose
in one animal, may be of less use in another,
occupying a higher station in the scale, and the
change of circumstances may even render it
wholly useless. In such cases we find that it is
gradually discarded from the system, becoming
continually smaller, till it disappears altogether.
We may often, however, perceive some traces
of its existence, but only in a rudimental state,
and as if ready to be developed, when the
occasion may demand it.
In the greater number of organic structures
we may trace a tendency to the repetition of
certain organs, or parts, and the regular arrange-
ment of these similar portions either round a
central axis, or in a longitudinal series. The
former is apparent in the verticillated organs of
plants, and in the radiated forms of zoophytes.
The linear arrangement is exhibited in the si-
milar segments of annulose and other articulated
animals, and also in the pieces which compose
the spinal column of vertebrated animals. In
these two latter classes, also, a remarkable law
of symmetry obtains in the formation of the two
sides of the body, which exhibits the lateral
junction of similar but reversed structures. The
violations of this law are extremely rare ; yet
some remarkable instances of anomalous forma-
tions in this respect will hereafter be noticed.
58 THE FUNCTIONS OF LIFE.
In treating of the particular functions of the
animal and vegetable economy I shall follow a
different order from that in which I have pre-
sented them in the preceding sketch. As the
Mechanical functions depend upon the simpler
properties of matter and the well known laws of
mechanism, I think it best to commence with the
examination of these. Our attention will next
be directed to the highly interesting subjects
which relate to the Nutritive or Vital functions
both of vegetable and animal structures : for as
they involve the chemical properties of organized
substances, and are, therefore, of a more refined
and intricate nature than the preceding, I
conceive they will be best understood after the
general mechanism of the frame has been ex-
plained. These studies will prepare us for the
consideration of living animals as sentient and
active beings, endowed by their bounteous Crea-
tor with the exalted faculties of perception and
of volition, which alone give value to existence,
and which raise them so far above the level of
the vegetable world. I shall lastly give a very
brief account of the reproductive functions, and
of the phenomena of animal developement, in
which the discoveries of modern times have re-
vealed to us so considerable a portion of those
extensive plans which an all-wise providence
has beneficently devised for the general welfare
of animated beings.
59
PART I.
THE MECHANICAL FUNCTIONS.
Chapter I.
ORGANIC MECHANISM.
§ 1 . Organization in General.
Life, which consists of a continued series of
actions directed to particular purposes, cannot
be carried on but by the instrumentality of those
peculiar and elaborate structures and combina-
tions of material particles which constitute or-
ganization. All these arrangements, both as
respects the mechanical configuration and the
chemical constitution of the elements of which
the organized body is composed, even when
apparently most simple, are, in reality, complex
and artificial in the highest possible degree.
Let us take as a specimen the crystalline lens,
or hard central part, of the eye of a cod fish,
which is a perfectly transparent, and to all
appearance homogeneous, spherule. No one,
unaccustomed to explore the wonders of nature,
would suspect that so simple a body, which he
(JO THE MECHANICAL FUNCTIONS.
might suppose to be formed of a uniform mate-
rial cast in a mould, would disclose, when exa-
mined under a powerful microscope, and with the
skill of a Brewster, the most refined and exqui-
site conformation. Yet, as I shall have occasion
to specify more in detail in its proper place, this
little spherical body, scarcely larger than a pea,
is composed of upwards of five millions of fibres,
which lock into one another by means of more
than sixty-two thousand five hundred millions of
teeth. If such be the complication of a portion
only of the eye of that animal, how intricate
must be the structure of the other parts of the
same organ, having equally important offices !
What exquisite elaboration must those textures
have received, whose functions are still more
refined ! What marvellous workmanship must
have been exercised in the organization of the
nerves and of the brain, those subtle instruments
of the higher animal faculties, and of which
even the modes of action are to us not merely
inscrutable, but surpassing all our powers of
conception !
It is from the energies of life alone that or-
ganic forms are produced. No fabric achieved
by human power ever approached in refinement
the simplest of nature's works. The utmost
efforts of the ingenuity or skill of man in the
construction of the most delicate machinery is
infinitely surpassed by the most ordinary of the
ORGANIC MECHANISM. f>l
mechanisms which are presented to our view
in living bodies. However successful may be
human artists in their attempts to contrive
automata, which shall exactly imitate different
animal movements, there will always be wanting
that internal principle of action, derived from a
higher source than mechanism can supply, and
without which these highly wrought works of
man, like the unvivified statues of Prometheus,
must remain for ever mere masses of insentient
and inert materials.
As the living functions imply the mechanical
action and re-action of parts which cohere in
some definite order of arrangement, so as to
preserve that determinate form to which they
constantly tend to return on being displaced,
it is impossible to conceive that a mere fluid
can exercise these functions ; because the par-
ticles of a fluid, being equally moveable in
every direction, have no determinate relative
situations, and possess no character of perma-
nence. All organic and living structures, there-
fore, must be composed of solid as well as fluid
parts ; although the proportion between these is,
in different cases, almost infinitely varied. A
dormant vitality may, indeed, exist in a system
of organs which have been brought into a per-
fectly dry state ; as is proved by the examples
of vegetable seeds, and also of many species of
animalcules, and even of some of the more
02 THE MECHANICAL FUNCTIONS.
highly developed Annelida, or worms, which
may be kept in a dry state for an indefinite
length of time, and, when moistened with water,
resume their activity, as if restored to life. The
germination of seeds under these circumstances
is matter of common observation ; but the revi-
vification of animalcules is a more curious phe-
nomenon, for it takes place more rapidly, and is
more striking in its results. The Rotifer redi-
vivus, or wheel animalcule,* (Fig. 1.) which was
first observed by Lewenhoeck, and was after-
wards rendered celebrated by the experiments
made upon it by Spallanzani, can live only in
water, and is commonly found in that which has
remained stagnant for some time in the gutters
of houses. But it may be deprived of this fluid,
and reduced to perfect dryness, so that all the
functions of life shall be completely suspended,
yet without the destruction of the vital principle :
* Vorticella rotatoria of Gmelin, and Furcularia of Lamarck.
ORGANIC MECHANISM. 63
for this atom of dust, after remaining for years
in a dry state, may be revived in a few minutes
by being again supplied with water. This alter-
nate suspension and restoration of life may be
repeated, without apparent injury to the animal-
cule, for a great number of times. Similar
phenomena are presented by the Vibrio tritici,
(Fig. 2.) or the animalcule, resembling an eel in
its shape, which infests diseased wheat, and
which, when dried, appears in the form of a fine
powder : on being moistened, it soon resumes its
living and active state.* The Gordius aquaticus,
or hair worm, which inhabits stagnant pools, and
which remains in a dry, and apparently lifeless
state when the pond is evaporated, will, in like
manner, revive, in a very short time, on being
again immersed in water. The same pheno-
menon is exhibited by the Filaria, a thread-like
parasitic worm, infesting the cornea of the eye
of the horse. t
Both the composition of the fluid, and the
texture of the solid parts of animal and vege-
table bodies are infinitely varied, according to
the purposes they are designed to serve in the
economy. Scarcely any part is perfectly homo-
geneous ; that is, composed throughout of a single
uniform material. Few of the fluids are entirely
* See a paper on this subject by Mr. Bauer, Philosophical
Transactions for 1823, p. 1.
t De Blainville, Annates des Sciences Naturelles; x. 104.
04 THE MECHANICAL FUNCTIONS.
limpid, and none are perfectly simple in their
composition ; for they generally contain more or
less of a gelatinous matter, which, when very
abundant, imparts to them viscidity, constituting
an approach to the solid state. Many fluids
contain minute masses of matter, generally
having a globular shape, which can be seen
only by means of the microscope, and which
float in the surrounding liquid, and often thicken
it in a very sensible manner.* We next per-
ceive that these globules have, in many in-
stances, cohered, so as to form solid masses ; or
have united in lines, so as to constitute fibres.
We find these fibres collecting and adhering
together in bundles ; or interwoven and aggluti-
nated, composing various other forms of texture ;
sometimes resembling a loose net-work of fila-
ments ; sometimes constituting laminae or plates ;
and, at other times, both plates and filaments
combining to form an irregular spongy fabric.
These various tissues, again, may themselves be
regarded as the constituent materials of which
the several organs of the body are constructed,
with different degrees of complication, according
to the respective functions which they are called
upon to perform.
We shall now examine the several kinds of
* Globules of this description have been found in the lymph,
the saliva, and even in the aqueous humour of the eye.
VEGETABLE ORGANIZATION. <>">
texture in relation to these functions, in the order
of their increasing complexity; beginning with
those of vegetables, which are apparently the
simplest of all.
§ *2. Vegetable Organization.
Plants, being limited in their economy to the
functions of nutrition and reproduction, and
being fixed to the same spot, and therefore in a
comparatively passive condition, require for the
performance of these functions mechanical con-
structions of a very different kind from those
which are necessary to the sentient, the active,
and the locomotive animal. The organs essential
to vegetables are those which receive and ela-
borate the nutritive fluids they require, those
which are subservient to reproduction, and also
those composing the general framework, which
must be superadded to the whole for the purpose
of giving mechanical support and protection to
these finer organizations. As plants are des-
tined to be permanently attached to the soil,
and yet require the action both of air and of
light ; and, as they must also be defended from
the injurious action of the elements, so we find
these several objects provided for by three des-
criptions of parts : namely, first, the Roots,
which fix plants in their situation ; secondly,
VOL. I. F
60 THE MECHANICAL FUNCTIONS.
the Stems, which support them in the proper
position, or raise them to the requisite height
above the ground ; together with the branches,
which are merely subdivisions of the stem ; and
thirdly, the external coverings, which correspond
in their office to the integuments, or skins of
animals.
The simplest and apparently the most ele-
mentary texture met with in vegetables is formed
of exceedingly minute vesicles, the coats of
which consist of transparent membranes of ex-
treme tenuity. Fig. 3 is a highly magnified
representation of the simplest form of these ve-
sicles.* But they generally adhere together more
closely, composing by their union a species of
vegetable cellular tissue, which may be regarded
as the basis or essential component material of
every organ in the plant. This cellular structure
is represented in figures 4 and 5, as it appears
in the Fucus vesiculosus ; the first being a hori-
zontal, and the second a vertical section of that
plant. t The size of these cells differs consider-
ably in different instances. Kieser states that
the diameter of each individual cell varies from
the 330th to the 55th part of an inch ; so that
* These cells are well represented in the engravings which
illustrate Mr. Slack's memoir on the elementary tissue of plants,
contained in the 49th volume of the Transactions of the Society
of Arts.
f De Candolle, Organographie Vegetale.
VEGETABLE ORGANIZATION.
07
from .3,000 to 100,000 cells would be contained
in an extent of surface equal to a square inch.
But they are occasionally met with of different
sizes, from even the 1000th part of an inch to
the 30th.
In their original state, these vesicles have an
oval or globular form ; but they are soon trans-
formed into other shapes, either by the mutual
compression which they sustain from being-
crowded into a limited space, or from unequal
expansions in the progress of their develope-
ment. From the first of these causes they often
acquire angles, assuming the forms of irregular
rhomboid al dodecahedrons, and often of hexa-
gonal prisms, like the cells of a honeycomb ; and
by the second, they are elongated into cylinders,
08 THE MECHANICAL FUNCTIONS.
or slowly tapering cones, thus passing by insen-
sible gradations into the tubular form. Figures
0, 7, and 8, are representations of some of these
different states of transition from the one to
the other. These various modifications of the
same elementary texture have been distinguished
into several classes of cells, and dignified by
separate technical denominations, which I shall
not stop to specify, as it does not appear that
they have as yet thrown any light on vegetable
physiology.
Many of the cells are fortified by the addition
of elastic threads, generally disposed in a spiral
course, and adhering to the inner surfaces of the
membranous coats of the cells, which they keep
in an expanded state. (See Fig. 9.) When the
membranes are torn, the fibres, being detached,
unrol themselves, and being loosely scattered
among the neighbouring cells, give the appear-
ance of fibrous connexions among these cells,
which did not originally exist. Simple mem-
branous cells, containing no internal threads,
are often found intermixed with these fibrous
cells. In many of the cells, again, the original
spiral threads appear to have coalesced by their
edges ; thus presenting a more uniform surface,
excepting that a few interstices are left, where
the pellucid membrane, having no internal
lining, presents the appearance of transverse
fissures or oval perforations, as shown in Fig. 10.
VEGETABLE ORGANIZATION. 69
Cells of this description are said to be reticu-
lated or spotted, and, together with those having
more regularly formed spiral threads, are very
abundantly met with in plants belonging to the
tribe of Orchidece.
It has been much disputed whether the cells
of the vegetable texture are closed on all sides,
or whether they communicate with one another.
Mirbel has given us delineations of what ap-
peared to him, when he examined the coats of
the cells with the microscope, to be pores and
fissures. But subsequent observations have ren-
dered it probable that these appearances arise
merely from darker portions of the membranes,
where opaque particles have been deposited
in their substance. Fluids gain access into these
cells by transuding through the membranes
which form their sides, and not by any apertures
capable of being detected by the highest powers
of the microscope.
If all the cells consist of separate vesicles,
as the concurring observations of modern bo-
tanists* appear to have satisfactorily established,
the partitions which separate them, however thin
and delicate, must consist of a double membrane,
formed by the adhesion of the coats of the two
contiguous vesicles. But as these coats can
* In particular, Treviranus, Kieser, Link, Du Petit Thouars,
Pollini, Amici, Dutrochet, and De Candolle.
70 THE MECHANICAL FUNCTIONS.
hardly be supposed to adhere in every point, we
may expect to find that spaces have been left
in various parts between them ; and that com-
munications exist to a certain extent between
all these spaces ; so as to compose what may
be regarded as one large cavity. These have
been denominated the intercellular spaces ; and
they have been supposed to perform, as will
hereafter be seen, an important part in the func-
tion of Nutrition.
Fluids of different kinds occupy both the cells
and the intercellular spaces. The contents of
some is the simple watery sap ; that of others
consists of peculiar liquids, the products of
vegetable secretion : and very frequently they
contain merely air. In many of the cells there
are found small opaque and detached particles
of the substance termed by chemists, Fecula, of
which starch is the most common example. In
several parts, and more especially in the leaves,
and in the petals of flowers, the material which
gives them their peculiar colour is contained in
the cells in the form of minute globules. De
Candolle has given it the name of C/ironwle*
The cells of the ligneous portion of trees and
shrubs are farther encrusted with particles of a
more dense material, peculiar to vegetable organ-
ization, and termed Lignin. It is this substance
* Organographie, lorn 1, p. 19.
VEGETABLE ORGANIZATION. 71
which principally contributes to the density and
mechanical strength of what are called the Woody
Fibres, which consist of collections of fusiform, or
tapering vessels, hereafter to be described, sur-
rounded by assemblages of cells thus fortified,
and the whole cohering in bundles, so as to
present greater resistance to forces, tending to
displace them, in the longitudinal direction than
in any other.
Most of the plants which are included in the
Linnean class of Cryptogamia have a structure
exclusively composed of cells, as has been
already shown in the Fucus vesiculosus. But the
greater number of other plants have, in addition
to these cells, numerous ducts or vessels, con-
sisting of membranous tubes of considerable
length, interspersed throughout every part of the
system. These tubes exhibit different modifi-
cations of structure, more especially with regard
to the form of the fibres, or other materials,
which adhere to the inner surface of their mem-
branes ; and these modifications correspond very
exactly with those of the vesicles already de-
scribed as constituting the simpler forms of
vegetable tissue. There can be little doubt,
indeed, that the vessels of plants take their
origin from vesicles, which become elongated by
the progress of developement in one particular
direction ; and it is easy to conceive that where
the extremities of these elongated cells meet,
t'l THE MECHANICAL FUNCTIONS.
the partitions which separate their cavities may
become obliterated at the points of junction,
so as to unite them into one continuous tube
with an uninterrupted interior passage. This
view of the formation of the vessels of plants is
confirmed by the gradation which may be traced
among these various kinds of structures. Elon-
gated cells are often met with applied to each
other endwise, as if preparatory to their coales-
cence into tubes. Sometimes the tapering ends
of fusiform cells are joined laterally (as seen in
Fig. 12), so that the partitions which divide their
cavities are oblique. At other times their ends
are broader, and admit of their more direct ap-
plication to each other in the same line, being
separated only by membranes passing trans-
versely ; in which case they present, under the
microscope, the appearance of a necklace of
beads (Fig. 13). When, by the destruction of
these partitions, their cavities become conti-
nuous, the tubes they form exhibit a series of
contractions at certain intervals, marking their
origin from separate cells. In this state they
have received the names of moniliform, jointed
or beaded vessels* Traces of the membranous
partitions sometimes remain where their oblite-
ration has been only partial, leaving transverse
fibres. The conical terminations occasionally
* Mirbel gave them the name of" Vaisseaux en chapelet."
VEGETABLE ORGANIZATION,
73
observable in the vessels of plants also indicate
their cellular origin.*
i
The membrane constituting the tube is some-
times simple, like those of the simple cells : but
it frequently contains fibres, or other internal
coatings, corresponding to those met with in the
more compound cells. The vessels in which the
internal fibres run in a spiral direction (Fig. 14),
are denominated tracheae, or spiral vessels; or,
from their being found very constantly to contain
air, they are often called air tubes. Their
diameter is generally between the 1000th and the
300th part of an inch. These spiral, or air ves-
sels, pervade extensively the vegetable system.
The threads they contain are frequently double,
treble, quadruple, or even still more numerous :
they are of great length, and when the external
membrane of the vessel is divided, they may
* This theory of the derivation of vessels from cells was first
advanced bv Treviranus.
74 THE MECHANICAL FUNCTIONS.
easily be drawn out and uncoiled, their elas-
ticity enabling them to retain their spiral shape.
The object of this structure appears to be that
of keeping the cavity of the tube always per-
vious, by presenting resistance to any external
force tending to compress and close it.*
In many instances the inner fibres of the tube,
instead of forming a continuous spiral, appear in
the shape of rings, succeeding one another at
regular intervals, and constituting what are
called annular vessels (Fig. 15). They are gene-
rally larger than the spiral vessels. In other
cases, as was first observed by Hedwig, the
adjacent coils are found to be closely coherent
throughout the greatest part of their course ;
leaving, however, occasional intervals, where the
external membrane, being unprotected, appears,
from its transparency, as if spotted or perforated
in various places (Fig. 16). Every intermediate
stage may occasionally be seen in the transition
from one of these forms to the other, in conse-
quence of the various kinds of convolution, of
branchings, or of transverse junctions of fibres, as
well as the greater or less extent of their lateral
adhesions. All these varieties are met with,
not only in different vessels, but; as was observed
by Moldenhawer and Kieser, even in the dif-
* Vessels are sometimes met with which appear to be formed
simply by the coils of a spiral fibre in close juxtaposition, and
unattached to any external envelope, or connecting membrane.
VEGETABLE ORGANIZATION. 75
ferent portions of the same vessel, when followed
by the eye throughout a great extent of its length.
Tims, in the course of the same tube, (as seen in
Fig. 17), we find parts exhibiting spiral fibres,
which, in other parts, bifurcate and again unite;
and in others, again, form rings : these may
afterwards, by a closer junction, present a reti-
culated appearance, or a series of transverse
lines, which, becoming smaller and smaller, are
at length mere points, arranged in circular rows
around the cylindrical surface of the vessel.*
What are called the ivoody fibres originate,
like all other parts of plants, in cells. These
are generally fusiform, that is, of the shape of a
double cone, very greatly elongated, and placed
close and parallel to one another, with the
narrow extremities of one set wedged in between
those of another set (Fig. 18). Their coats are
more firm and elastic than those of ordinary
vessels, but do not appear to contain any in-
ternal fibres, although they receive, in the pro-
gress of their developement, large additions of
solid matter. These fibres are generally col-
lected together into bundles or layers, and are
accompanied by cells and vessels of various
* Many distinguished botanists, such as Rudolphi, Link,
Treviranus, and Dutrochet, consider these spots as being pro-
duced not by the deficiency of the internal coating, but by the
addition of granular bodies. See De Candolle's Organographie
Vegetale, torn. i. p. 56.
76 THE MECHANICAL FUNCTIONS.
descriptions, and in different stages of transition.
The density of the woody fibres increases in
proportion as these incrustations are formed, till
they have become nearly impervious ; and have
acquired a degree of rigidity peculiarly fitting
them for the office of giving mechanical support
to the fabric of the plant.* Their assemblage
thus constitutes a kind of frame- work for the
whole system, which may be regarded as the
skeleton of the plant. Thus, what are called the
fibres of leaves (Fig. 19), are principally com-
posed of these woody fibres, distributed in the
manner best adapted to support the expansion
of the soft and pulpy substance of those impor-
tant organs.
Besides the minute cavities of the cellular
tissue, there occur, in various parts of a plant,
much larger spaces, apparently serving the
purpose of reservoirs of particular fluids ; but
sometimes containing only air. Large air cells
are, in particular, met with very commonly in
aquatic plants, where they probably contribute
to impart the requisite degree of buoyancy.
There are also contained, in the interior of
* By drying different specimens of wood in a stove, Count
Rumford was led to the conclusion that the specific gravity of
the solid matter which constitutes timber is nearly the same in
all trees. He found that the woody part of oak, in full vegeta-
tion, constitutes only two-fifths of the whole bulk : and that
ordinary dry wood contains above one-fourth of its weight of
water. Thomson's Annals of Philosophy, i. 388.
VEGETABLE ORGANIZATION. 77
vegetables, certain organs, denominated Glands,
which are composed of closely compacted cells,
and which perform the function of secretion, that
is, the conversion of the nutritious juices into
particular products required for various purposes
in the economy of the plant.
The external parts of a living plant require
protection against the injurious effects of the
atmosphere, and of the moisture it deposits.
For this purpose there is provided a membrane,
termed the Cuticle, which is spread over the
whole surface, investing the leaves and flowers,
as well as the stem and branches, and inter-
posing a barrier to the action of fluids, or other
extraneous bodies, on the living organs. The
cuticle is formed originally by the condensation
of a layer of cellular tissue, of which the cells,
being consolidated by exposure to the air, and
by compression, compose a thin but impervious
pellicle. Amici has distinctly shown, by means
of his powerful microscope, the cellular structure
of the cuticle, and also that the layer of cells of
which it consists is independent of the subjacent
cellular tissue.* Fig. 20 is intended to show this
circumstance, the shaded part representing the
cuticle with its series of cells.
Oval orifices, or stomata, as they have been
termed, are discoverable on almost every part of
* Annales des Sciences Naturelles, ii. 211.
78 THE MECHANICAL FUNCTIONS.
the surface of the cuticle, but more especially
in those that have a green colour.* They are
placed at nearly equal distances from one ano-
ther, and are particularly numerous in the cuticle
of the leaves, where they occupy the intervals
between the fibres. These orifices conduct into
the interior of the plant, probably into the gene-
ral cavity of the intercellular spaces. It is
evident, from the functions they perform, that
they must occasionally open and close ; but the
minuteness of their size precludes any accurate
observation as to the nature of the apparatus
provided for the performance of these motions.
Amici describes their margins as formed by
two cells, by the movements of which, combined
perhaps with those of the adjoining cells, he
conceives these orifices are opened and closed. t
Great variety, however, is observable in the
structure of the stomata in different species of
plants.
Many plants have no stomata, either on the
cuticle of the leaves, or on that of the stem.
This is the case with such aquatic plants as are
habitually immersed in water. In those that
are only partially immersed, stomata are met
* Fig. 22 is a magnified representation of the appearance
in the cuticle of the Lycojwdium denticulatam, taken in the
central part of the lower surface of the leaf, from De Candolle.
Fig. 21 is a still more magnified view of the stomata in the leaf
of the Lilium candidum, from Amici.
t Annales des Sciences Naturelles, ii. 21.5.
VEGETABLE ORGANIZATION.
79
with in those parts exclusively which are above
the water. The leaves of the Ranunculus aqua-
ticus, when made to grow in the air, acquire
stomata, but lose them entirely when growing
under water. Stomata are wanting in all plants
whose structure is wholly cellular.
Botanists are far from being agreed as to the
precise functions which the stomata perform.
Their usual office undoubtedly is to exhale
water ; but they probably also absorb air under
certain circumstances, and in particular exi-
The principal organs through which the fluids
that serve for nourishment are received into the
system of plants, are those situated at the ex-
tremities of the roots, where they are termed,
from their peculiar texture, spongioles* Of the
* Fig. 23 exhibits the termination of a root of a willow in a
spongiole ; the arrangement of the cells composing which is
shown in Fiff. 24, from De Candolle.
80 THE MECHANICAL FUNCTIONS.
functions of spongioles in absorbing fluids I shall
have occasion to speak when treating of nutri-
tion : but as the roots exercise a mechanical
as well as a nutrient office, we should here con-
sider them in the light of organs adapted to
procure to the plant a permanent attachment
to the soil, upon which it is wholly dependent
for its supply of nourishment. It is scarcely
necessary to point out how effectually they per-
form this office. Our admiration cannot fail to
be excited when we contemplate the manner in
which a large tree is chained to the earth by its
powerful and widely spreading roots. By the
firm hold which they take of the ground, they
procure the most effectual resistance to the force
of the winds, which, acting upon so large a sur-
face as that presented by the branches covered
with dense foliage, must possess an immense
mechanical power.
The principal seat of the vitality of a plant is
the part which intervenes between the root and
the stem. Injuries to this part are always fatal
to the life of the plant.
As the roots penetrate downwards into the
earth to different distances in order to procure
the requisite nourishment, so the stem grows
upwards for the purpose of obtaining for the
leaves and flowers an ample supply of air, and
the influence of a brighter light, both of which
are of the highest importance to the maintenance
VEGETABLE ORGANIZATION. 81
of vegetable life. The stems of the grasses are
hollow tubes ; their most solid parts, which
frequently consist of a thin layer of silex, occu-
pying the surface of the cylinder. Of all the
possible modes of disposing a given quantity
of materials in the construction of a column,
it is mathematically demonstrable that this is
the most effective for obtaining the greatest
possible degree of strength.*
The graceful continuous curve with which the
stem of a tree rises from the ground, is the
form which is best calculated to give stability
to the trunk. Evidence of express mechanical
design is likewise afforded by the manner in
which the trunk is subdivided into its branches,
spreading out in all directions, manifestly with a
view to procure for the leaves the greatest extent
of surface, and thus enable them to receive the
fullest action of both light and air. The branches,
also, are so constructed as to yield to the irre-
gular impulses of the wind, and again, by their
elasticity, to return to their natural positions,
and by these alternate inflexions on opposite
sides, to promote the motion of the sap in the
vessels and cellular texture of the liber and
* Galileo, the most profound philosopher of his age, when
interrogated by the inquisition as to his belief in a Supreme
Being, replied, pointing to a straw on the floor of his dungeon,
that from the structure of that object alone he would infer with
certainty the existence of an intelligent Creator.
VOL. I. G
82 THE MECHANICAL FUNCTIONS.
alburnum. Nothing can exceed the elegance of
those forms which are presented in every part of
the vegetable kingdom, whether they be consi-
dered with reference to their direct utility for the
support of individual life, and the continuance
of the species, or whether they be viewed as
component parts of that beauty which is spread
over the scenery of nature, and is so delightfully
refreshing to the eye of every beholder alive
to its fascinating charms. How enchanting are
all the varieties of flowers, that decorate in
gay profusion every part of the garden of
creation ; and into which the farther we carry
our philosophic scrutiny, the more forcibly will
our hearts be impressed with the truth of the
divine appeal that " Even Solomon in all his
GLORV WAS NOT ARRAYED LIKE ONE OF THESE. "
§ .3. Developement of Vegetables.
Further proofs of design may be collected from
an examination into the modes in which these
structures, so admirably adapted to their objects,
have been gradually formed. Confining our
attention to vascular plants, in which the pro-
cess of developement has been studied with the
greatest attention and success, we find that
Nature has pursued two different plans in
DEVELOPEMENT OF VEGETABLES. 83
conducting their growth.* In the greater num-
ber, the successive additions to the substance
of the stem are made on the exterior side of the
parts from which they proceed. This mode is
adopted in what are called Exogenous plants. In
others, the growth is the result of additions made
internally ; a plan which is followed in all En-
dogenous plants. The Oak, the Elm, the Beech,
the Pine, and all the trees of these northern
regions, belong to the first of these divisions.
The Palm tribe, such as the Date, the Cocoa-
nut tree, and, indeed, a large proportion of the
trees of tropical climates, together with the
sugar-cane, the bamboo, and all gramineous and
liliaceous plants, belong to the latter. We shall
first inquire into the endogenous mode of
growth, as being the simplest of these two kinds
of vegetable developement.
A Palm tree may be taken as an example of
the mode of growth in endogenous plants. The
stem of this tree is usually perfectly cylindrical,
attains a great height, and bears on its summit a
tuft of leaves. It is composed of an extremely
dense external cylindric layer of wood ; but
the texture of the interior becomes gradually
* The tribe of Filices, or ferns, the structure of which is
vascular, constitute an exception to this rule : as they differ in
their mode of developement, both from exogenous and endoge-
nous plants.
84 THE MECHANICAL FUNCTIONS.
softer and more porous as it comes nearer to the
centre ; though with regard to its essential cha-
racter it appears to be uniform in every part,
having neither medullary rays, nor true outward
bark, nor any central pith ; in all which respects
it differs totally from the ordinary exogenous
trees.
The first stage of its growth consists in the
appearance of a circle of leaves, which shoot
upwards from the neck of the plant, and attain,
during the first year, a certain size. The follow-
ing year, another circle of leaves arises; but they
grow from the interior of the former circle, which
they force outwards as their vegetation advances,
and as ligneous matter is deposited within them.
Thus each succeeding year brings with it a fresh
crop of leaves, intermixed with ligneous matter,
which leaves, exerting an outward pressure,
stretch out the preceding layers that enclose
them ; until the latter, acquiring greater density,
no longer admit of further distention, and re-
main permanently fixed. This happens first to
the outermost layer, which is the oldest : then
each succeeding layer becomes consolidated in
its turn. As soon as the outer layer has become
too hard to yield to the pressure from within, the
growth of the inner layers is immediately directed
upwards ; so that they each rise in succession by
distinct stages, always proceeding from the in-
terior ; a mode of developement which has been
DEVELOPEMENT OF VEGETABLES. 85
compared by De Candolle to the drawing out
of the sliding tubes of a telescope. The whole
stem, whatever height it may attain, never in-
creases its diameter after its outward layer has
been consolidated. A circle of leaves annually
sprouts from the margin of the new layer of
wood ; these, when they fall off in autumn, leave
on the stem certain traces of their former exist-
ence, consisting of a circular impression round
the stem. The age of the tree may accordingly
be estimated by the number of these circles, or
knots, which appear along its stem. The suc-
cessive knots which are seen in the stems of
other endogenous plants, as may be observed in
growing corn, and also in various grasses, may
be traced to a similar origin.
The structure of exogenous trees is more com-
plicated : for, when fully grown, they are com-
posed of two principal parts, the wood and the
bark. The woody portion exhibits a further
division into the pith, which occupies the centre,
and consists of large vesicles, not cohering very
closely, but forming a light and spongy texture,
readily permeable to liquids and to air ; the
harder wood, which surrounds the pith in con-
centric rings, or layers ; and the softer wood,
or alburnum, which is also disposed in concentric
layers on the outer side of the former. Each of
these concentric layers of wood and of alburnum
may be farther distinguished into an inner and
86 THE MECHANICAL FUNCTIONS.
an outer portion ; the former being of less
density than the latter, and consisting of a
lighter cellular tissue : while the outer portion
is composed of the denser woody fibres resulting
from the union of numerous vessels with a cel-
lular envelope. The bark is formed by con-
centric layers of cortical substance, of which the
innermost are denominated the Liber; and the
whole is surrounded by an outer zone of cel-
lular tissue, termed the cellular envelope. The
exterior surface of this envelope is called the
Epidermis.
All these concentric zones may be readily dis-
tinguished in a horizontal section of the stem ;
which also presents a number of lines called
Medullary Rays, radiating from the pith to the
circumference. They are composed chiefly of
large cells, extending transversely, or in the
direction of the diameter of the tree, and com-
posing by their union continuous vertical planes
the whole length of the trunk.
Every vegetable stem, and also every branch
which arises from it, is developed from a germ,
or bud, which is originally of inconceivable
minuteness, and totally imperceptible by any
optical means of which we have the command.
As soon as it becomes visible, and its structure
can be distinguished, it is found to contain within
itself many of the parts which are to arise from
it, in miniature, and folded up in the smallest
DEVELOPEMENT OF VEGETABLES. 87
possible compass. The portion destined to form
the stem is gradually expanded both in breadth
and height, but principally the latter, so that it
rises as it grows, during a certain period, until
the fibres acquire the solidity and strength ne-
cessary not oidy for their own support, but also
for sustaining the parts which are to be farther
added. In trees this process generally occupies
one whole season ; during which the growth of
the first layer of wood, with its central pith, and
its covering of a layer of bark, is free and unre-
strained. On the second year, a fresh impulse
being given to vegetation, a new growth com-
mences from the upper end of the original stem,
as if it were the developement of a new bud :
and at the same time a layer of cellular tissue is
formed by the deposition of new materials on the
outside of the former wood, and between it and
the bark. This is followed by a second layer
of wood, enveloping the new layer of cellular
tissue.
The effect of this new growth is to compress
the layer of wood which had been formed during
the first year, and to impede its further extension
in breadth. But as its fibres, consisting of vessels
and cells, are not yet consolidated, and admit of
still greater expansion as long as they are sup-
plied with nourishment, their growth, which is
restrained laterally, is now directed upwards,
and there is no further enlargement of their
88 THE MECHANICAL FUNCTIONS.
diameter. From the same cause the pith cannot
increase in size ; and is even found to diminish
by the pressure of the surrounding wood. Thus
the vertical elongation of the entire stem con-
tinues during the whole of the second year, and
the trunk becomes sufficiently strengthened by
the addition of the second layer on its outside
to bear this increase of its height.
While this process is going on in the wood,
corresponding changes take place in the bark,
and a new layer is added on its inner surface,
or that which is contiguous to the wood. This
layer constitutes the liber. All these new depo-
sitions must of course tend to stretch the outer
portions of the bark, which had been first formed,
and which yield to this pressure to a certain
extent ; but, becoming themselves consolidated
by the effects of the same pressure, they acquire
increasing rigidity ; and, the same cause con-
tinuing to operate, they at length give way,
in various places, forming those deep cracks,
which are observable in the bark of old trees,
and which give so rugged an appearance to
their surface. The cuticle has, long before this,
peeled off, and has been succeeded by the con-
solidated layers of cortical envelope which form
the epidermis. But the epidermis, which is con-
tinually splitting by the expansion of the parts
it encloses, itself soon decays, and is constantly
succeeded by fresh layers, produced by the
DEVELOPEMENT OF VEGETABLES. 89
same process of consolidation in the subjacent
cortical substance.
During the third, and each succeeding year,
the same process is repeated ; new layers of cel-
lular texture and of woody fibres are deposited
around those of the preceding year's growth, and
a new internal coating is given to the liber of the
bark. The compressing power continues to be
exerted on the internal layers of wood, directing
their growth vertically, while they are capable
of elongation, and can be supplied with nourish-
ment. In time, however, by continued pressure,
and accumulating depositions of solid matter,
the vessels and the cells become less and less
pervious to fluids ; till at length all further dila-
tation is prevented. But the tree still continues
to enlarge its trunk by the annual accessions of
vigorous and expansible alburnum, and to take
its station among its kindred inhabitants of the
forest ; till, arriving at maturity, its majestic
form towers above all the junior or less vigorous
trees.*
The developement of each branch takes place
in the same manner, and by the same kind of
* It is contended by Dr. Darwin and other writers on vege-
table physiology that each annual shoot should be regarded as
a collection of individual buds, each bud being a distinct indivi-
dual plant, and the whole tree an aggregation of such individuals.
I shall have occasion to revert to this question when I come to
consider the subject of vegetable nutrition.
90 THE MECHANICAL FUNCTIONS.
process, as that of the trunk. The buds from
which they originate, spring from the angle
formed by the stalk which supports a leaf, and
which is termed by botanists the axilla of that
leaf. A law of symmetry is established by na-
ture in the developement of all the parts of
plants. The leaves, in particular, are frequently
observed to arise in a circle, or symmetrically
round the parent stem ; forming what is termed
a whorl) or, in botanical language, a verticillated
arrangement. In other cases they are found to
have their origins at equal intervals of a spiral
line, which may be conceived to be drawn along
the stem, or the branch from which they grow.
When these intervals correspond to the semi-
circumference of the stem, the leaves alternate
with one another on its opposite sides.
The stems of most plants, even those which
are perfectly erect, exhibit a tendency to a spiral
growth. This is observable in the fibres of the
wood of the pine, however straight may be the
direction of the whole trunk. This tendency is
shown even in the epidermis of the cherry tree,
for it may be stripped off with more facility in
a spiral direction than in any other. The pri-
mitive direction of the leaves of endogenous
plants is a spiral one. It is particularly marked,
also, in the stems of creepers and of parasitic
plants, which are generally twisted throughout
their whole length ; a disposition evidently con-
ducive to the purpose of their formation, namely,
DEVELOPEMENT OF VEGETABLES. 91
that of laying hold of the objects with which
they come in contact, and of twining round them
in search both of nourishment and of support.
The twisted stems of the hop and of ivy show
this structure in a remarkable degree ; and the
purpose for which this tendency was given can-
not be mistaken.
A conjecture has been offered that this ten-
dency to a spiral growth might be owing to the
influence of the sun's light, acting successively
on different sides of the plant, in the course of
its diurnal motion. In these northern latitudes
the direction of that motion is from east to west ;
or, to an observer facing the south, from left to
right. That light has a powerful influence in
determining the direction of the growth of all
the parts of the plant which are above ground,
is manifest to every one who has observed the
habits of vegetables. If a growing plant be
placed in a situation where the light reaches it
only on one side, it will always, by degrees, turn
itself to that side, as if eagerly pressing forward
to obtain the beneficial action of that agent.
The leaves, whose functions in a more especial
manner require its operation, will always be
found turned towards the light. The branches
of a tree, which have naturally a tendency to rise
vertically, have this tendency modified by the
superior attraction of the light, when it can
reach them only laterally. Thus while those on
the upper part spread out in full luxuriance in
92 THE MECHANICAL FUNCTIONS.
all directions, those below them are obliged to
expand more in a lateral direction : and this is
still more the case with the lowest branches,
which shoot out horizontally to a considerable
distance before they turn upwards, and present
their leaves to the light. Often, however, from
the deficiency of this necessary agent, their
growth is much stinted, or entirely prevented.
The operation of this cause is extensively seen
in the interior of a dense forest.
It may be objected to the theory of the spiral
growth being the result of the sun's motion, that
were it so, the direction of the spiral would
always be the same, that is, ascending from left
to right with reference to the axis. But this is
not found to be the case, for the direction of the
turns, though generally constant in the same
plant, is far from being the same in all. Dr.
Wollaston ingeniously suggested that a verifica-
tion of the theory would be obtained were it
found that plants transported from the southern
to the northern hemispheres, would have this
direction reversed ; for it is evident that the
motion of the sun's light in the two hemispheres
is in opposite directions ; being, in the southern
hemisphere, from right to left, to a spectator
facing the meridian position of the sun, which
in those regions is to the north. But, the facts
are not in accordance with this view of the sub-
ject ; so that we may consider the hypothesis
as untenable.
DEVELOPEMENT OF VEGETABLES. 93
The roots differ considerably from the stems
both in their structure, and in their mode of
growth. They exhibit, indeed, the appearance
of medullary rays and of concentric layers, but
they are destitute of any central pith, and they
have no tracheae ; neither does their surface
present any appearance of stomata. They in-
crease in thickness in the same way as the stem
increases. This law obtains both in exogenous
and endogenous plants : they do not, however,
grow in length by the elongation of any of their
parts, but simply by additions made to their
extremities. Their ramifications are not the
result of the dev elopement of buds, as are the
branches of the stem ; but they arise merely
from the additional deposits taking different di-
rections. Almost every part of the surface of the
stem or branches may shoot forth roots if they
are covered with earth, and properly moistened,
and if they are supplied with sap from the cir-
culating system of the plant itself. It is ob-
served, however, that they generally grow from
certain points on the surface of the bark, which
appear as dark spots, and are termed Lenticellie.*
Great variety exists in the form and disposition
of roots in different families of plants, according
to the particular purposes they are intended to
serve, conformably with their general functions
* This name was given to them by De Candolle, Annales des
Sciences Naturelles, vii. 1. and Organographie, i. 94.
94 THE MECHANICAL FUNCTIONS.
of absorption and of mechanical support. Both
these purposes are promoted by their sending
out from their sides numerous fibrils, or lesser
roots, which increase their firm hold upon the
soil, as well as multiply the channels for the
introduction of nourishment.
Nature has supplied various plants with cer-
tain appendages to the above mentioned struc-
tures, the uses of which are for the most part
sufficiently obvious. Of this description are the
tendrils, which assist in fixing and procuring
support to the stems of the weaker plants ; the
stipuhe, which protect the nascent leaves ; and
the bractece, which perform a similar office to the
blossom. The different kinds of hairs, of down,*
of thorns, and prickles, which are found on the
surface of different plants, have various uses ;
some of which are easily understood, particu-
larly that of defending the plant from molesta-
tion by animals. The sting of the nettle is of
this class ; and its structure bears a striking
analogy, as we shall afterwards have occasion to
notice, to that of the poisonous fangs of serpents.
The purposes answered by the down, which
covers a great number of plants, are not very
obvious. It perhaps serves as a protection from
the injurious effects of cold winds on the tender
surface : or it may have a relation to the depo-
* The finer hairs, and filaments of down, are composed of
elongated cells, either single, or several conjoined end to end.
DEVELOPEMENT OF VEGETABLES. 95
sition of moisture ; or, as it may be farther con-
jectured, the number of points which are thus
presented to the air may be designed to convey
electricity from the atmosphere, or to restore the
electric equilibrium, which may have been dis-
turbed by the processes of vegetation.
In the smaller parts of plants, as in the ge-
neral fabric of the whole, we find, on examina-
tion, the most admirable provision made, ac-
cording to the particular circumstances of the
case, for the mechanical objects of cohesion,
support and defence. Thus the substance of
the leaf, of which the functions require that a
large surface should be expanded to the air and
light, is spread out in a thin layer upon a frame
work of fibres, like rays, connected by a net-
work of smaller fibrils, and constituting what is
often called the skeleton of the leaf.
In all these vegetable structures, while the
objects appear to be the same, the utmost variety
is displayed in the means for their accomplish-
ment, in obedience, as it were, to the law of diver-
sity which, as has been already observed, seems
to be a leading principle in all the productions
of nature. It is more probable, however, judging
from that portion of the works of creation, which
we are competent to understand, that a specific
design has regulated each existing variation of
form, although that design may in general be
placed beyond the limited sphere of our intelli-
gence.
90 THE MECHANICAL FUNCTIONS.
§ 4. Animal Organization.
The structures adapted to the purposes of vege-
table life, which are limited to nutrition and
reproduction, would be quite insufficient for the
exercise of the more active functions and higher
energies of animal existence. The power of
locomotion, with which animals are to be invested,
must alone introduce essential differences in
their organization, and must require a union of
strength and flexibility in the parts intended
for extensive motion, and for being acted upon
by powerful moving forces.
The animal, as well as the vegetable fabric is
necessarily composed of a union of solid and
fluid parts. Every animal texture appears to be
formed from matter that was originally in a
fluid state ; the particles of which they are com-
posed having been brought together and after-
wards concreting by a process, which may, by a
metaphor borrowed from physical science, be
termed animal crystallization. Many of those
animals, indeed, which occupy the lowest rank
in the series, such as Medusa, approach nearly
to the fluid state ; appearing like a soft and
transparent jelly, which, by spontaneous decom-
position after death, or by the application of
ANIMAL ORGANIZATION. 97
heat, is resolved almost wholly into a limpid
watery fluid.* More accurate examination, how-
ever, will show that it is in reality not homoge-
neous, but that it consists of a large proportion
of water, retained in a kind of spongy texture,
the individual fibres of which, from their extreme
fineness and uniformity of distribution, can with
difficulty be detected. Thus even those animal
fabrics, which on a superficial view appear most
simple, are in reality formed by an extremely ar-
tificial and complex arrangement of parts. The
progress of developement is continually tending
to solidify the structure of the body. In this
respect the lower orders of the animal kingdom,
even when arrived at maturity, resemble the
conditions of the higher classes at the earliest
stages of their existence. As we rise in the
scale of animals, we approximate to the con-
dition of the more advanced states of develope-
ment which are exhibited in the highest class.
Great efforts have been made by physiologists
to discover the particular structure which might
be considered as the simplest element of all the
animal textures ; the raw material, as it were,
with which the whole fabric is wrought : but
* Thus a Medusa, weighing twenty or thirty pounds, will, by
this sort of general liquefaction, be found reduced to only a few
grains of solid matter. Peron, Annales du Musee, torn. xv.
p. 43. See also a memoir by Quoij and Gaimard, Annales des
Sciences Naturelles, torn. i. p. 245.
VOL. I. H
98 THE MECHANICAL FUNCTIONS.
their labours have hitherto been fruitless. Fan-
ciful hypotheses in abundance might be adduced
on this favourite topic of speculation ; but they
have led to no useful or satisfactory result.
Haller, who pursued the inquiry with great ar-
dour, came to the conclusion that there existed
what he calls the simple or primordial fibre,
which he represents as bearing to anatomy the
same relation that a line does to geometry.
Chemical analysis alone is sufficient to overturn
all these hypotheses of the uniformity of the
proximate elementary materials of the animal
organs : for they are found to be extremely di-
versified in their chemical composition. Neither
has the microscope enabled us to resolve the
problem : for although it has been alleged by
many observers that the ultimate elements of
every animal structure consists of minute glo-
bules, little confidence is to be placed in these
results obtained by the employment of high
magnifying powers, which are open to so many
sources of fallacy. That globules exist in great
numbers, not only in the blood, but in all ani-
mal fluids, there can be no doubt ; and that
these globules, by cohering, compose many of
the solids, is also extremely probable. But it is
very doubtful whether they are essential to the
composition of other parts, such as the fibres of
the muscles, the nerves, the ligaments, the ten-
dons, and the cellular texture : for the most
ANIMAL ORGANIZATION. 99
recent, and apparently most accurate microscop-
ical observations tend to show that no globular
structure exists in any of these textures.*
The element which we can recognise without
difficulty as composing the greater portion of
animal structures, is that which is known by the
name of the cellular texture. Although bearing
the same designation as the elementary material
of the vegetable fabric, it differs widely from
it in its structure and mechanical properties. It
is not, like that of plants, composed of a union
of vesicles ; but is formed of a congeries of ex-
tremely thin liminae, or plates, variously con-
nected together by fibres, and by other plates,
25 which cross them in different direc-
tions, leaving cavities or cells. (Fig.
25). These cells, or rather interven-
ing spaces, communicate freely with
one another ; and, in fact, may be
considered as one common cavity,
subdivided by an infinite number of partitions
into minute compartments. Hence the cellular
texture is throughout readily permeable to fluids
of all kinds, and retains these fluids in the man-
ner, and on the same principle, as a sponge.
The cellular texture is not only the element,
or essential material employed by nature in the
* See the Appendix to Dr. Hodgkin and Dr. Fisher's transla-
tion of Edwards's work on the Influence of Physical Agents on
Life, p, 440.
100 THE MECHANICAL FUNCTIONS.
construction of all the parts of the animal
fabric ; but, in its simplest form, it constitutes
the general medium of connexion between
adjacent organs, and also between the several
parts of the same organ. Like the mortar
which unites the stones of a building, the cel-
lular texture is the universal cement employed
to bind together all the solid structures. Its
properties are admirably adapted to the me-
chanical purposes which are required in dif-
ferent parts of the frame : and these properties
are variously modified and adjusted to suit the
particular exigencies of the case. When, for
instance, different parts require to be moveable
upon each other, the cellular substance inter-
posed between them has its state of condensation
adapted to the degree of motion required. That
which connects the muscles, or surrounds the
joints, and all other parts concerned in extensive
action, has a looser texture, being formed of
broad and extensible plates, with few lateral
adhesions, and leaving large interstices ; while
in the more quiescent organs, the plates of the
cellular substance are thin and small, the fibres
short and slender, and their intertexture closer
and more condensed.
Besides being flexible and extensible, the cel-
lular texture is also highly elastic, a property
which is exceedingly advantageous in the con-
struction of the frame. Not only the displace-
ANIMAL ORGANIZATION. 101
ment of parts is resisted by this elasticity, but,
when displaced, they tend to return to their
natural position. This property performs a
more important part in the mechanism of the
animal than of the vegetable system ; as might,
indeed, have been anticipated from the more
active and energetic movements required by the
functions of the former.
The cellular texture, in its simple form, admits
of the ready transmission of fluids through it;
but it is necessary, on many occasions, to inter-
pose a barrier to their passage. Such barriers
are provided in membranes, which are merely
modifications of the same material, spread out
into a continuous sheet of a closer texture, after
the surfaces of the plates have been brought to
cohere so as to obliterate all the cellular in-
terstices, and become impervious to fluids.
Though equally flexible and elastic with the ori-
ginal texture of which it is formed, the mem-
brane has acquired, by this consolidation, greater
strength and firmness, properties which adapt
it to a great number of important purposes.*
Membranes are extensively employed to con-
nect distant organs, and often serve to determine
the direction and extent of their relative motions.
* With a view of ascertaining the actual strength of mem-
branes, Scarpa stretched a portion of peritoneum, (which is a very
thin membrane lining the abdominal cavity), over a hoop, and
placing weights upon its surface, found it did not give way till it
was loaded with fifteen pounds.
102 THE MECHANICAL FUNCTIONS.
They furnish strong coverings for the invest-
ment, the support, and the protection of all the
important organs of the body. What Paley
has termed the package of the organs is effected
principally by their intervention. Membranes
are also employed to line the interior of all the
large cavities of the body, as those of the chest,
and of the abdomen, or lower part of the trunk
containing the organs of digestion. These mem-
branes, after lining the sides of their respective
cavities, are reflected back upon the organs
which are enclosed in those cavities, so as to
furnish them with an external covering. Their
inner sides present every where a smooth and
polished surface, over which the organs con-
tained in the cavity may glide without injury.
In all these cases, a thin fluid, called serum,
is provided, which moistens and lubricates the
surfaces that are in contact with one another,
and obviates the injury that would otherwise
arise from friction. From this circumstance,
the linings of these cavities have been termed
serous membranes. In the neighbourhood of
joints, closed cavities of the same description,
but of smaller size, are met with, for the obvious
purpose of facilitating motion ; and here also
friction is prevented by a highly lubricating fluid,
termed synovia, which is poured out between the
surfaces of the membrane lining the cavities.
Membranes, being impermeable to fluids, are
ANIMAL ORGANIZATION.
103
extensively employed as receptacles for retain-
ing them ; forming, in the first place, sacs, or
pouches of various kinds for that purpose. The
ink-bag of the cuttle fish, the gall-bladder, and
even the stomach itself, are examples of this
kind of structure. The coats of these sacs,
being very extensible and elastic, readily ac-
commodate themselves to the variable bulk of
their contents.
In the second place, we find membranes com-
posing tubes of various descriptions for con-
ducting fluids. Thus, in the higher classes of
animals, the whole of the body is traversed by
innumerable canals, conveying different kinds
of fluids. These canals, when uniting into
trunks, or subdividing into branches, are called
Vessels (Fig. 26).
The fluids contained in vessels are never
stagnant, but are almost always carried forwards
in one constant direction. For preventing the
retrograde motions of the fluids passing along
these canals, recourse is had to the beautiful
contrivance of valves. The inner membrane of
104 THE MECHANICAL FUNCTIONS.
the vessel is employed to construct these valves ;
for which purpose it is extended into a fold,
having the shape of a crescent, and fixed by its
convex edge to the sides of the vessel, while the
other edge floats loosely in its cavity. When-
ever the fluid is impelled in a direction contrary
to its proper course, it raises the loose edge of
the valve, which, being applied to the opposite
side of the canal, effectually closes the passage.
On the contrary, it presents no obstacle to the
natural flow of the contents of the vessel, both
edges being then closely applied to the same
side. Frequently two, or even three valves are
used at the same part, their edges being made
to meet in the middle of the passage, like the
floodgates, or locks of a canal.* Among the
numberless instances of express contrivance
which are met with in the examination of the
fabric of animals, there is perhaps none more
striking and more palpable, than this admirable
mechanism of the valves.
As we ascend from the simpler to the more
complicated systems of organization, adapted
to a greater range of faculties, we find greater
diversity in the mechanical means employed
* Fig. 27, representing the section of a vessel, is intended to
show the position of the valves when applied to the sides of the
vessel, by the stream moving onwards in the direction pointed
out by the arrow. In Fig. 28, they are seen closing the passage
by the retrograde pressure of the current.
ANIMAL ORGANIZATION. 105
for carrying on the functions of life. Textures
of greater strength than can be constructed by
membranes alone become necessary for the
security, the support, and the defence of im-
portant organs ; and more especially for the
execution of extensive movements. For ob-
taining these advantages a peculiar species of
fibres is provided, formed of a much denser
substance than even the most consolidated
forms of cellular texture. The animal product
termed albumen possesses a much stronger
cohesive power than gelatin, which is the basis
of membrane. The addition of albumen, there-
fore, procures the quality required : and the
fibres which are produced by its combination
with gelatin are opaque, and of a glistening
white colour. By interlacing fibres thus com-
posed, a close texture is formed, which is ex-
ceedingly tough and unyielding. These fibrous
textures, as they are termed, while they retain
the flexibility of membranes, greatly surpass
them in strength ; but, being at the same time
incapable of extension, they are necessarily de-
void of elasticity. Hence they are adapted to
form external tunics for the investment of such
organs as are not intended to vary in their size.
Occasionally these fibrous capsules, as they are
called, send down processes into the interior of
those organs, for the purpose of giving them
mechanical support. This is the case, for in-
106 THE MECHANICAL FUNCTIONS.
stance, with the membranes surrounding the
brain of quadrupeds, and which form two par-
titions, the one vertical, the other horizontal ;
both being firmly stretched in their respective
positions, and serving to divide the pressure. In
other cases these sheets of fibrous membrane
are employed as bandages, tightly bracing the
muscles, and retaining them in their relative
situations. The joints are surrounded by similar
bandages, known by the name of Capsular Li-
gaments.
In following the series of animal structures in
the order of their increasing density, we find the
proportion of albumen which enters into their
composition becoming greater, while that of the
gelatin and mucilage diminishes. When the
product is more uniform in its composition it is
in general less elastic than when it consists of a
more complex combination of ingredients. A
great preponderance of albumen tends also to
diminish the elasticity. Thus the densest kinds
of fibrous texture present, instead of thin and
broad expansions of elastic membrane, the thick
and elongated form of inextensible cords, con-
stituting the ordinary Ligaments, and the Ten-
dons. These structures resist with great power
any force calculated to extend them : a property
which of course excludes elasticity, but, when
united with flexibility, implies great toughness.
In a word, they possess all the qualities that can
ANIMAL ORGANIZATION. 107
be desired in a rope. It will hardly be credited
how great a force is required to stretch, or rather
rend asunder a ligament ; for it will not yield in
any sensible degree until the force is increased
so enormously as at once to dissever the whole
contexture of its fibres. Nothing can be more
artificially contrived than the interweaving of
the fibres of ligaments ; for they are not only dis-
posed, as in a rope, in bundles placed side by
side, and apparently parallel to each other : but,
on careful examination, they are found to be tied
together by oblique fibres curiously interlaced,
in a way that no art can imitate. It is only after
long maceration in water, that this complicated
and beautiful structure can be unravelled.
The mechanical properties of these fibrous
structures, which are strictly inextensible liga-
tures, render them applicable to purposes of
connexion where motion is to be restrained.
Many cases, however, occur in which a sub-
stance is wanted, uniting great compactness and
strength with a considerable degree of elastic
power. For this purpose a different texture is
fabricated, consisting of twisted fibres, which
impart this required elasticity. Such is the
structure of the elastic ligaments of animals,
which are very generally employed for the
support of heavy parts that require being sus-
pended. An instance occurs in quadrupeds, in
that strong ligament which, as we shall find,
108 THE MECHANICAL FUNCTIONS.
passes along the back and neck to be fixed to
the head, and to support its weight when the
animal stoops to graze. This, the ligamentum
nuc/ue, as it is termed, is capable of great exten-
sion, and by its elasticity reacts with consider-
able force in recovering its natural length, after
it has been stretched. This ligament is par-
ticularly strong in the Camel, whose neck is of
great length.* Another example of an elastic
ligament occurs in that which connects the two
shells of bivalve mollusca (as those of the oyster
and muscle), and which keeps them open when
the animal exerts no force to close them. The
claws of the Lion, and other animals of the cat
tribe, are retracted within their sheaths by means
of two strong elastic ligaments. Structures of
this kind are employed very extensively in the
fabric of insects. |"
* Many birds are provided with strong elastic ligaments con-
necting the vertebrae of the neck with those of the back ; liga-
ments of the same kind are also employed for retaining the wings
close to the body, where they are not used in flying : and a
similar provision is made in the wings of Bats. The weight of
the bulky organs of digestion in herbivorous quadrupeds require
some permanent support of this kind ; and this is furnished by
a broad, elastic, fibrous band, extended across the lower part of
the abdomen. It is particularly strong in the Elephant, which
remains more constantly in the horizontal position than most
quadrupeds: and it has been remarked that the general cellular
texture in this animal has an unusual degree of elasticity. —
Hunter on the Blood, &c. p. 112.
f Chabrier, Memoires du Musee, torn. vi. p. 416.
ANIMAL ORGANIZATION. 109
The animal substance which comes next in
the order of density is Cartilage. The purposes
for which this kind of structure is employed are
those in which a solid basis is required for the
support of softer or more flexible parts, and
where the mechanical properties that are wanted
are firmness, conjoined with some degree of
elasticity. Cartilage (or gristle) is composed of
a finer and more uniform material than any of
the preceding structures. It consists almost
wholly of albumen, with a slight proportion of
calcareous matter. Unlike membrane in any of
its forms, it contains no fibres ; but, on being
cut with a sharp knife, presents the appearances
of a dense homogeneous substance of a white
pearly hue. Its surface is smooth, and where
it is exposed to friction, as in the joints, is often
highly polished.
In all the inferior tribes of animals, Nature
employs cartilage to supply the place of bone,
when rigidity is required to be given to the
fabric. In an extensive order of fishes, in-
cluding the Shark, the Sturgeon, and the Ray,
we find the whole skeleton constructed of car-
tilage. In the fabric of very young quadrupeds
cartilage is substituted for bone ; and in the
adult animal, various organs, such as the exter-
nal ears, the eye-lids, the nostrils, and different
parts of the apparatus of the throat and wind-
pipe, are composed of flexible cartilage, which
1 10 THE MECHANICAL FUNCTIONS.
gives them a determinate shape and firmness.
In all these cases bone, which, besides being
three times as heavy, is devoid of elasticity, and
liable to fracture, would have been much less
suitable. Cartilage is often employed as an
intermedium for connecting different bones, as
for instance, between the ribs and the sternum,
or breast-bone ; whereby, besides the advantage
of greater lightness, the pliancy of the material
diminishes those jars which are incident to the
frame in all its violent actions.
In the construction of cartilage, nature seems
to have attained the utmost degree of density
which could be given to an internal texture
composed merely of the usual animal consti-
tuents. But substances of still greater hard-
ness, united with perfect rigidity, are wanted,
in numberless instances, for giving effectual
protection to soft and delicate structures, for
supplying a firm basis to the framework of the
body, and for constructing levers of various
kinds, to be employed in the more energetic
movements of the higher animals. For all these
purposes it was necessary to superadd a mate-
rial endowed with stronger cohesive powers, and
capable by its dense concretion of forming solid
and inflexible organs. The substances which
nature has selected for this office are the salts of
lime. Sometimes the Carbonate, and sometimes
ANIMAL ORGANIZATION. Ill
the Phosphate of lime is employed for forming
these hard and unyielding- structures ; and often
both these calcareous substances are united to-
gether in different proportions in the same solid
fabric. When the carbonate of lime predomi-
nates, or is the sole earthy ingredient, it consti-
tutes Shell: when there is a greater proportion
of the phosphate, it is called a Crust, as is the
case with the coverings of the Lobster and the
Crab : when the earthy matter consists almost
wholly of phosphate of lime, it composes the dif-
ferent forms of Bone. I shall have occasion to
describe the formation and properties of each of
these structures in the sequel.
The protection of the delicate structure of the
fabric from the injurious influence of external
agents is an object of great importance in the
animal economy, and is one which nature has
shown extreme solicitude to secure. For this
purpose she has provided the integuments, under
which designation are included not merely the
skin, but also all the parts that are immediately
connected with it, and are formed and nourished
by the same vessels. No parts of the animal
structure present greater diversity in their form
and outward appearance than the integuments ;
yet it is easy to discover, amidst all these va-
rieties, that the same general plan has been fol-
lowed in their construction, and that each par-
1 12 THE MECHANICAL FUNCTIONS.
ticular formation is the result of a combination
of the same elementary structures. Of these
elements the most important, and that which
generally composes the chief bulk of the skin,
is the Corium, or true skin. The outermost layer
is termed the Epidermis, Cuticle, or scarf-skin ;
and between these there is often found an inter-
mediate layer denominated the Mete Mucomm,
or the Pigmentum.
The corium is generally of considerable thick-
ness, and is composed of strong and tough fibres,
closely compacted together, and pervaded by
innumerable ramifications of blood-vessels of
every kind. It is endowed with great flexibility,
and is capable of being considerably extended ;
properties which fit it for readily accommo-
dating itself to all the movements of the body
and limbs, and to the variable bulk of the parts
it covers. Being also very elastic, it soon re-
gains its natural form and dimensions when
left to itself after being stretched. The skin
is connected with the subjacent muscles and
other parts by a large quantity of cellular tex-
ture, which, according to the particular inten-
tions of its formation, sometimes binds it tightly
over these parts, and on other occasions allows
of a free and extensive motion. This latter
property is remarkably exemplified in the Ra-
coon, an animal whose skin hangs loosely on the
limbs, and encloses the body like a wide elastic
ANIMAL ORGANIZATION. 113
garment ; so that, however firmly a person may
attempt to grasp the animal by the neck, it can
easily turn its head completely round, and bite
the fingers that are holding it. In like manner
the skin of the Frog is attached to the body only
at a few places, and may be readily stripped off.
A thin layer of muscular fibres is often found
lying immediately underneath the skin, and is
provided for the purpose of moving it over the
subjacent parts. In animals that roll themselves
into a ball, as the Hedge-hog, these muscles are
of great size and importance. We shall see that
in the Mollusca, this muscular apparatus is inse-
parably blended with the integument, and com-
poses a peculiar structure, termed the mantle.
Immediately covering the corium is the Rete
Mucosnm, which is a very thin layer of soft
animal matter, composed of a net-work of delicate
fibres, and containing more or less of the material
from which the colour of the skin is derived.
The Epidermis is a membrane of a very
peculiar nature, consisting of a thin expansion of
albuminous matter, apparently homogeneous in
its texture and composition. It is impervious to
fluids, although capable of imbibing moisture,
and of slowly transmitting a portion to the sub-
jacent textures. Its thickness varies exceedingly
in different parts ; being adapted to the kind of
protection it has to afford against pressure,
friction, or other causes of injury. As it is not
VOL. I. I
114 THE MECHANICAL FUNCTIONS.
nourished by vessels, its outer layer is liable to
become dry and unfit for use : and accordingly
a separation of this outward layer generally
takes place from time to time, the loss being
speedily repaired by a fresh growth from the
surface in contact with the skin. This process
is often performed periodically, as is most re-
markably exemplified in serpents.
Special provisions are made for preserving
the cuticle in a healthy condition ; and more
particularly for defending it from the injurious
action of the surrounding element. These some-
times consist of a supply of oily fluid, prepared
in small cavities which are situated in the skin
itself, and have minute ducts opening upon the
surface. These cavities, termed sebaceous follicles,
are generally interspersed in great numbers on
different parts of the body, abounding more
especially in those places where folds occur, and
where there is the greatest friction. In Fishes,
Mollusca, and other aquatic animals, the skin is
at all times defended from the action of the
water by a viscid or glutinous secretion, pre-
pared in this manner, and continually poured
out on the surface, through ducts, the orifices of
which are easily seen with the naked eye, dis-
posed in a line on each side of the body.
Connected with the skin, and more particu-
larly with the cuticle, are structures of very
various forms, intended for giving additional
protection, occasionally contributing their aid in
ANIMAL ORGANIZATION. 1 1 f>
progressive motion, and sometimes fashioned
into weapons of offence. In this class should be
included all the varieties of hair, such as wool,
fur, feathers, bristles, quills, and spines, as well
as the more ordinary kinds of hair. All these
resemble the cuticle in their chemical compo-
sition, differing only in their degrees of hardness
and condensation. Horn is formed of the same
material as hair ; as are also the nails, the hoofs,
and the claws of quadrupeds, and the scales of
fishes, reptiles, and other animals. The integu-
ments of insects, and especially their more solid
and horny coverings, contain, however, as will
hereafter be noticed, a peculiar chemical prin-
ciple termed Entomoline.
All these parts seem to be but remotely con-
nected with the vital actions of the system with
which they are associated ; and it is doubtful
how far they are to be considered as apper-
taining to the living portion of the body, or as
mere extraneous appendages. Yet, however they
may differ in their forms, uses, and external
appearance, they all are produced by the same
kind of vascular structure, variously arranged to
suit the particular circumstances in each case :
and the mode of their developement and growth
is essentially the same in all.
An extremely delicate and finely organized
pulp, composed partly of a congeries of minute
vessels, and partly of a gelatinous substance, in
which these vessels are embedded, constitutes
116
THE MECHANICAL FUNCTIONS.
the apparatus by which the nutrient particles
are selected, combined and elaborated into the
materials of the intended structure. The original
form, situation, and disposition of this vascular
pulp, determines the future figure and extent of
growth of the production which is to arise from
it. The materials which compose it are depo-
sited sometimes in masses, as in the scales of the
Crocodile ; more generally in layers, as in hoofs
and nails, and also in the scales of fishes ;*
and occasionally in filaments, as in hair ; which
latter, again, are often agglutinated together by
a strong cement, uniting them into a hard
and solid structure, of which the horn of the
Rhinoceros is a remarkable example. In all
cases, the portions thus successively produced,
* The laminated structure of the scales of fishes is easily dis-
tinguished by applying to them a high magnifying power. As
the breadth of each new layer is greater than the last, its edges
project farther, the whole surface having that concentric striated
appearance which renders it an interesting object for microscopic
examination. Fig. 29 exhibits the striated surface of the scale of
the Cyprinus Alburnus, and Fig. 30 that of the Perca fluviatilis.
The imbricated arrangement of these scales, resembling that of
the tiles on the roof of a house, is shown in Fig. 31. All these
figures represent the objects highly magnified.
ANIMAL ORGANIZATION.
117
are no longer susceptible of being nourished,
and from the moment of their deposition, un-
dergo no further change, except from the action
of external agents. By the continual additions
which are made to them at their base, or root,
where the vessels deposit fresh materials, they
gradually increase in size, protrude through the
skin, and continue to grow by the same process,
as long as these vessels continue in activity.
The nature of this process is well exemplified
in the growth of hair. Fig. 32 shows the appa-
ratus employed in its construction, in an imagi-
nary section of the root, on a magnified scale.
Every hair takes its rise from a minute vascular
pulp, (p,) of an oval shape, which is implanted
below the corium, or true skin (d)*. This pulp is
invested by a sheath
or capsule (c), which,
together with the con-
tained pulp, and the
root of the hair that
grows from it, com-
poses the bulb of the
hair. The bulb itself
is contained in a small
cell formed by con-
densed membranes
(s), to which it has no
* In the above figure e is a section of the epidermis, or cuticle ;
the dotted part, r, represents the situation of the subjacent rete
mucosum, and d, the derm, or corium.
1 Itf THE MECHANICAL FUNCTIONS.
attachment excepting at the lower part (v), where
the vessels and nerves of the pulp are passing
into it. The hair, growing by depositions from
the inside of the capsule, which forms the outer
part (o), of the shaft, and from the outside of the
pulp, which forms its inner or central part (i), is
forced upwards till it has pierced the skin : in
the course of its passage a canal is formed for it
in the skin itself, continuous with that which
encloses the bulb ; and the course of this canal
is generally oblique. In the Elephant, where
the thickness and density of the hide, present
considerable obstacles to the passage of the hairs
through it, we may discover, on minute exami-
nation, many hairs which have only penetrated
a certain way, (as shown at b), without ever suc-
ceeding in reaching the surface.
An opinion has been very commonly enter-
tained that each hair, on its protruding from
underneath the cuticle (e), at the point q, carries
up along with it a portion of this outward integu-
ment, which, stretching as the hair increases in
length, forms over it a very fine external tunic.
But later observations have shown that this is
not the case, and that there is simply an adhe-
sion of the edge of the cuticle to the origin of the
hair, without any accompanying prolongation ;
so that if the whole bulb be destroyed, and its
pulp absorbed, the hair may be detached by the
slightest force.
ANIMAL ORGANIZATION. 119
From this account it will be seen that a hair is,
in its origin, tubular ; the inner part being occu-
pied by the pulp. But as the pulp extends only
to that portion of the hair which is in a state of
growth, it never rises above the surface of the
skin ; and the cavity in the axis of the hair is
either gradually obliterated, or is filled with a
dry pith, or light spongy substance, probably
containing air. After a certain period, the bulb
diminishes in size, from the collapse of the
vessels, whose powers of supplying nutriment
become exhausted. The first deficiency in its
nourishment appears in the cessation of the
deposit of colouring matter, and the hair in con-
sequence becomes grey. After a time, the
vessels becoming quite impervious, the bulb
shrivels, the hair is detached, and the canal
which its root occupied in the skin becomes
obliterated.
The hair of different animals, and sometimes
even of different parts of the same animal, varies
in shape, texture, and mechanical properties.
Sometimes, instead of being cylindrical, the fila-
ments are more or less flattened, striated, deeply
grooved, or even beaded. Instead of being solid,
they may even be tubular : and they exhibit
also the greatest diversity in their length, fine-
ness, tenacity, rigidity, and disposition to curl.
All these varieties may be traced to corresponding
differences in the form, and the relative actions
120 THE MECHANICAL FUNCTIONS.
of the component parts of the bulb, namely, the
pulp and its capsule.*
The structure of the organs by which hairs
are formed is not easily distinguished, in the
ordinary kinds of hair, on account of their
minuteness : it is readily seen, however, in the
large whiskers of the feline species, and also
of the Seal, which are subservient to more ex-
tended uses than that of merely covering the
body, and which are even supplied with nerves,
converting them into instruments of a sense of
touch.
In the quills of the Porcupine a still more
complicated organization has been detected.
Fig. 33 shows a quill with its bulbous root, de-
tached from the body ; and Fig. 34, a transverse
section magnified. The bulb itself is contained
in a distinct cell, shown at (a), Fig. 35, which
represents a longitudinal section of these organs.
This cell contains a portion of fat in which the
numerous vessels supplying its pulp and capsule
are embedded. The bulb is itself surrounded
by an outer sheath (s), into the cavity of which,
(b), there opens a duct (d), proceeding from a
small cell or follicle (f), lodged in the cellular sub-
stance on the outside of the sheath. This upper
cell communicates below with another cavity (c),
* See F. Cuvier's Memoir on the Formation of the Quills of
the Porcupine, in the Nouvelles Annales du Museum, i. 429.
ANIMAL ORGANIZATION.
121
containing an unctuous matter. During the
formation of the quill this unctuous matter is
supplied through that channel, and probably
enters as an ingredient in its composition. The
capsule of the pulp consists of two membranes,
the one enveloping the other. Fig. 36' shows
the bulb laid open by dividing the membranes
and turning them aside. The horny portion
of the quill is secreted by the internal mem-
brane (i), and deposited in successive laminae.
The external membrane is seen at o. The
pulp itself, seen at p, is still more curiously
organized ; its surface being fluted, or formed
into longitudinal processes. The horny matter,
being deposited on these processes, is moulded
to their shape, and concretes into laminae which
converge from the circumference of the cylin-
der towards the centre. The section (Fig. 34)
122 THE MECHANICAL FUNCTIONS.
shows these converging laminae, which being
of a dark colour, give to the surface of the quill
the appearance of being grooved ; this, however,
is merely an optical illusion, occasioned by the
dark laminae being seen through the transparent
exterior covering ; as may readily be detected
by viewing the surface with a magnifying glass.*
After a certain period of the growth of the quill,
the pulp ceases to supply the materials for form-
ing the spongy substance which occupies the
interior of the quill. But although it no longer
secretes, it still retains its place ; and the cap-
sule continuing to deposit horn, the quill be-
comes a hollow tube of considerable diameter.
When it has attained a certain size, the pulp
begins to shrink, and the diameter of the tube
diminishes ; so that it exhibits a tapering form
at both ends. Thus mere variations in the bulk
and the action of the pulp, accompanied with
changes in that of the capsule, are sufficient to
account for every diversity in the form and con-
dition of the resulting structures.
Among the mechanical uses of the integument,
that of serving as a cushion for relieving the
more prominent parts of the frame, and especi-
ally of the bones, from unequal pressure, ought
not to be overlooked. This object is promoted
* It is observed by F. Cuvier, that this striated appearance is
peculiar to the quills of porcupines of the old world. Those from
America have no such arrangement of laminae.
ANIMAL ORGANIZATION. 123
by the interposition of a layer of fat, which is
another animal substance entitled to be enume-
rated among the elements of its structure. It
consists of an oily fluid, composed, according to
the analysis of Chevreuil, of two constituent
principles, which he has distinguished by the
terms stearin and elain* In warm blooded ani-
mals the temperature of the body is always suf-
ficient to preserve this compound substance in a
fluid form : but it is prevented from being dif-
fused through the cellular texture by being con-
tained in separate vesicles of extreme minute-
ness, f Hence the whole mass of the fat, which
is thus formed of an aggregation of these vesi-
cles, has not the appearance of being fluid, but
seems to be composed of small grains united by
membranous investments into larger masses ; a
structure peculiarly adapted to the purposes of
a soft cushion, retaining only a small share of
elasticity, and yielding only in a certain limited
degree to pressure.
* These two constituent principles possess very different de-
grees of cohesion; elain being liquid, and stearin nearly solid,
at the usual temperature : and the consistence of the compound
will, therefore, depend altogether on the proportions in which
they are united. Thus a ready expedient has been provided for
varying the mechanical properties of fat, according as circum-
stances required.
t Dr. Monro estimated their diameter at between the 800th
and 600th of an inch. But their size varies in different animals.
124 THE MECHANICAL FUNCTIONS.
§ 5. 31uscular Power.
In Machines contrived by human skill the chief
art consists in devising expedients for regulating
and directing the given moving power, so that
it may bear, in the proper degree, and in the
proper order, upon some assigned objects, and
produce some particular effect. The whole of
the apparatus employed with this intention, how-
ever numerous may be its parts, however various
the forms of its wheels, its levers, or its pulleys,
and however complicated may be their con-
nexions, resolves itself into a series of interme-
diate instruments for the transference of motion
from the source of power, or the point where its
action is impressed, to the parts which are de-
signed ultimately to receive the action of the
force employed. It is an established principle
in physics, that mere machinery is incapable of
generating mechanical force ; and that such
force must always be originally derived from
an extraneous source. Some impulse from
without, whether it be the pressure of the wind,
the fall of a stream of water, or the action of
men or horses, or any other kind of foreign
agency, must be resorted to, both to set the
engine in motion, and to continue its move-
ments when they are once begun. Nor is the
case essentially different when the source of
MUSCULAR POWER. 125
motion apparently resides in some internal part
of the machine itself ; in a watch, for instance,
which is actuated by the main spring ; or in a
steam engine, which is set in motion by the
elastic vapour contained in its cylinder : the
spring in the one case, and the vapour in the
other, although they may in one sense be re-
garded as impelling powers, are, in reality, but
intermediate agents in the distribution of a
force originating from other sources. In the
watch, the force may be traced to the hand
which coiled the spring : in the steam-engine,
to the fire, which has imparted elasticity to the
vapour.
The living body differs from inorganic ma-
chinery in containing within itself a principle
of motion not referable, as far as we can per-
ceive, to any of the primary forces which exist
in the inanimate world. This principle has
been termed contractility. In animals of the
simplest construction, every part of the sub-
stance of the body seems to be equally endowed
with this contractile property, although ex-
hibiting no distinct appearance of a fibrous
structure. This is the case with all the lower
Zoophytes, such as the Infusoria, Polypi, Me-
dusa, and the simpler kinds of Entozoa.
Among Polypi and Infusoria we meet with a
singular mode of acting upon the surrounding
fluid by means of very minute and generally
l'ifcl THE MECHANICAL FUNCTIONS.
microscopic filaments, termed cilia, which the
animal, by some unknown power, causes to vi-
brate with great rapidity. Occasionally these
organs are found even in animals belonging to
the higher classes. Wherever they are met
with they perform, as will hereafter be shown,
very important functions ; sometimes assisting
in respiration, at other times contributing to the
supply of food, and very generally serving as
instruments of progressive motion.
In animals placed a little higher in the
scale, we begin to trace the formation of fibres,
which at first are irregularly scattered through
the soft substance : but as the organization be-
comes more refined, these fibres are collected
into bundles, and compose what are properly
called muscles. Muscular fibres are attached at
their extremities to the parts intended to be
moved. In the lower animals these attach-
ments are principally to the skin, or other
external parts, which are subservient to the
purposes of progressive motion. In the higher
classes, the solid parts, or skeleton, being dis-
posed more in the centre of the system, the
muscles are applied to them in the interior of
the body, and are more distinctly separated
into masses, each having its proper function in
the movements of the frame.
The peculiar property which characterises
the muscular fibre is that of suddenly shortening
MUSCULAR POWER. 127
itself, so as to bring its two ends, and the parts
to which those ends are attached, nearer to
one another. This contraction is performed
with astonishing quickness and force, and the
accumulated effect of a large collection of these
fibres, such as constitutes a muscle, is therefore
capable of overcoming great resistances, or
of raising enormous weights. Those muscles,
which, by means of their nerves, as will here-
after be noticed, are subservient to voluntary
motion, are excited into action by an exertion
of the will of the animal. There are, however, a
great number of other muscles, the contractions
of which are involuntary, that is, are produced
by other causes than the will.*
Muscular contractility, of which there exists
no trace in the vegetable kingdom,']" has been
established by nature as the primary moving-
power of the animal machine. This agent is
resorted to on all occasions where considerable
* These two classes of muscles do not differ in their outward
appearance : but Dr. Hodgkin has lately pointed out a curious
difference in the microscopic structure of the fibres of some of
the involuntary muscles. See Appendix to his Translation of
Edwards on the influence of Physical Agents on Life, p. 443.
f The principal instances, which have been adduced in
support of the opinion that muscularity occasionally exists in
vegetable structures, are the alternate movements of the leaflets
of the Hedysarum gyrans, which have been fancifully com-
pared to the movements of the ribs in respiration ; the quick
motions of the stamina of the Berberis, Opuntia, and many
plants of the genera Carduus, and Centaurea; the closing of the
128 THE MECHANICAL FUNCTIONS.
mechanical force is wanted ; just as in a great
manufactory, where an immense quantity of
machinery is to be set in motion, and a great
variety of work is to be executed, the human
mechanist avails himself of some constant mov-
ing force, such as that derived from a fall of
water, or from the expansion of steam. The
laws of inorganic matter furnish no force which
could conveniently have been applied in the
animal body for that purpose ; but muscular
power, from its high intensity, is adequate to
every object, and has been accurately adjusted,
by the most refined application of the laws of
mechanism, to all the degrees and kinds of
effects intended to be produced.
Although the power be the same, yet the
mode of its application is exceedingly diversi-
fied ; and the comparison of these diversities
is the more interesting, inasmuch as there are
few of the animal functions in which the ends
to be answered are so definite, and the opera-
tion of the expedients employed is so plain and
intelligible. For while the intricate chemical
leaves of the Dioncea muscipula ; and the shrinking of those
of the Mimosa pudica, or sensitive plant. On a superficial
view, it must be acknowledged that these motions bear a re-
semblance to the effects of muscular contractility ; but I believe
that naturalists are now generally agreed that there is no real
analogy between these phenomena, and that there is no sub-
stantial evidence for the existence of that property in the vege-
table kingdom.
MUSCULAR POWER.
129
processes of the living system generally elude
our research, and the higher faculties of sensa-
tion and perception are dependent on still more
recondite and mysterious powers of nature, the
mechanical functions are effected by the simpler
properties of matter, and allow us a clearer
insight into the wonderful art which has been
exerted in their accomplishment.
Muscles, during their contraction, increase in
thickness in the same proportion as they dimi-
nish in length.* It is on this account, more
especially, that a knowledge of anatomy is so
necessary to the painter and the sculptor. In
every movement and attitude of the body, some
particular sets of muscles are in action, and con-
* This is illustrated by the annexed figures, 37 and 38, the
former showing the relaxed and elongated, and the latter the
contracted and swollen state of the same muscle.
VOL. I,
K
130 THE MECHANICAL FUNCTIONS.
sequently tense and prominent, while others are
relaxed and flattened ; differences which it is
requisite that the artist should faithfully express,
in order to give a correct representation of the
living figure.
The dilatation of the muscular fibres in thick-
ness, which accompanies their contraction in
length, would, if these fibres had been loose
and unconnected, have occasioned too great a
separation and displacement, and have im-
peded their co-operation in one common effect.
Nature has guarded against this evil by col-
lecting a certain number of the elementary
fibrils, and tying them together with threads of
cellular substance ; thus forming them into a
larger fibre ; and again packing a number of
these fibres into larger bundles : always sur-
rounding each packet with a web of cellular
tissue ; which thus forms a separate investment
for each. This plan of successive reunion into
larger and larger assemblages is carried on
through several gradations of size, till the entire
muscle is completed.
That we may be the better able to appreciate
the excellence of the plans adopted in the me-
chanism of the animal frame, let us inquire what
arrangements would occur to us, prior to an ac-
quaintance with those actually adopted, as the
most advantageous dispositions of the muscular
power. It is evident, that the simplest mode
MUSCULAR POWER. 131
would be that of extending the fibres of the
muscle in a straight line between the points in-
tended to be brought nearer to each other. This
direct application of the power, however, is
seldom compatible with convenience, unless the
parts to be moved are of very small size, and
require very delicate adjustments. Straight
muscles, accordingly, are employed chiefly for
the movements of the minuter parts of the ap-
paratus belonging to the senses, such as the
eye, and the ear, and also that of the voice.
In insects, when the hard case, or skeleton, is
wholly external, this direct application of the
moving force is also very generally employed.
The shells of the bivalve Mollusca, as of the
Oyster and the Cardium, are closed by one or
two straight muscles, the fibres of which pass
immediately from the inner surface of the one
to that of the other.
In the greater number of cases it is more con-
venient to place the muscle in a situation which
causes it to act obliquely with respect to the
direction of the motion produced in the part to
which it is attached. This will, of course, be at-
tended with a loss of force corresponding to the
degree of obliquity ; but there are, at the same
time, advantages gained, not only in point of
velocity of motion, but also in the effect being
produced by a smaller extent of contraction in
the fibres of the muscle. Oblique muscles are
1.32 THE MECHANICAL FUNCTIONS.
frequently employed in pairs, and are made to
act on opposite sides of the line of the intended
motion, which is, in this case, the diagonal be-
tween the direction of the two equal forces.
Thus, in order to bring a bone at p, Fig. 39,
down to the point o, the two muscles a and
b, extending from the fixed points m and x,
may be employed ; for as they exert forces in
the directions p m and p n, there will result
a force in the intermediate direction p o : and
the effect desired will be accomplished more
quickly, and with a smaller extent of contrac-
tion in the muscles producing it, than if the
same power had been applied by means of a
straight muscle in the direction p o.* It is by
means of two sets of muscles, acting thus ob-
liquely, that the ribs are brought in closer ap-
proximation every time that the chest is ele-
vated in breathing. Thus carefully does nature
dispose the muscular fibres so as to obviate the
necessity of their being contracted beyond a
certain extent : and thus does she economize, as
much as possible, the expenditure of muscular
power, wherever there is a constant call for its
exertion.
The principle which I have just explained,
whereby certain advantages result from the ob-
* See a paper by Dr. Monro, in the Transactions of the Royal
Society of Edinburgh. Vol. hi. p. 250.
MUSCULAR POWER. 133
liquity of the action of muscular fibres, is ap-
plied, not only to the entire muscle, but also to
the internal arrangement of its fibres. Thus, we
generally find that, in a flat muscle, its upper
and under surfaces are covered by a thin sheet of
fibrous texture, or thin expansion of ligament or
tendon ; and that the muscular fibres which are
attached to them are directed obliquely from the
one to the other, in the manner represented by
the section, Fig. 40. There is frequently a
middle tendinous layer interposed between those
that are on the surface (as shown in Fig. 41), in
which case the muscular fibres pass obliquely
from the former to the latter, but in different di-
rections on each side ; like the fibres proceeding
from the shaft of a pen. A muscle thus con-
structed has accordingly been termed a penni-
form muscle ; as is exemplified in the straight
muscle inserted into the knee-pan (the rectus
extensor cruris), and also in the muscle which
bends the great toe (the flexor pollicis pedis
longus). The arrangement first described, Fig.
40, forms the semi- pe uniform muscle ; an instance
of which occurs in the muscle of the leg, which
is termed the semi-membranosus. Frequently the
structure is rendered still more complex, by
the interposition of several tendinous layers
among the fleshy fibres. This arrangement,
which constitutes a complex muscle, (as shown in
Fig. 42) occurs, for example, in the solceus, or
134 THE MECHANICAL FUNCTIONS.
large muscle, which raises the heel, and forms
the thickest part of the calf of the leg.
It very commonly happens in the animal
frame, as it does in other machines, that the
presence of the moving agent in the place where
its action is wanted, would be exceedingly in-
convenient. The usual plan adopted for trans-
ferring the effect of the moving power to a
distant point is the employment of a rope, or
strap. Such is precisely the office of the ten-
dons, which are long straps, attached at one end
to the muscle, and at the other to the bone,
or other part intended to be moved. (See Fig.
43). If the hand, for instance, had been en-
cumbered with all the muscles which are neces-
sary for the movements of the fingers, it never
could have performed its office as a delicate
mechanical instrument. These muscles, accord-
ingly, are disposed high up on the arm, and
their tendons are made to pass along the wrist to
the joints of the fingers which are to be moved.
The employment of tendons is accompanied
with this further advantage, that by their inter-
vention the united power of all the fibres of the
muscle may be obtained, and concentrated upon
any particular point. In this respect, likewise,
they resemble a rope, at which a great number
of men are pulling at the same moment, and
whose combined strength is thus brought into
action. Another principal use of tendons is that
MUSCULAR POWER. 135
a different direction may, by their means, be
given to the moving power, without altering its
position. Many instances occur of their appli-
cation in this manner, by their being made to
pass round corners of bones, and along grooves,
or channels, expressly formed for their trans-
mission, and producing the effect of pullies.
In a great number of muscles, the fibres,
instead of running parallel to one another, are
made either to converge, or to diverge, in order
to suit particular kinds of movements ; and we
frequently find that different portions of the
same muscle have the power of contracting
independently of the rest, so as to be capable of
producing very various effects, according as they
act separately or in combination. This is exem-
plified in the muscle of the back, called the
trapezius, represented in Fig. 44. In many in-
stances, the fibres radiate in all directions from
a common centre : this is the case with the deli-
cate muscle of the ear-drum, as shown in Fig.
45. In that of the Elephant, which is about
an inch and a half in diameter, these radiating
fibres are very conspicuous, even to the naked
eye : and they are also visible in the membrane
of the human ear, when viewed with a good
microscope.*
At other times, the muscular fibres run in a
* Home Phil. Trans, for 1800, p. 1.
136 THE MECHANICAL FUNCTIONS.
circular direction, forming what is called an
orbicular, or sphincter muscle, of which an example
occurs in that which surrounds and closes the
eye. (Fig. 46.) Very frequently these two last
modes of arrangement are united in some part,
as appears to be the case in the membrane of
the eye, called the Iris. (Fig. 47.) The circular
fibres of the iris surround the central aperture,
or pupil, the size of which they diminish when
they contract ; while on the contrary, the radi-
ating fibres, acting on the inner circle, and
drawing it nearer to the outer circumference,
which is fixed, lessen the breadth of the ring,
and consequently enlarge the circular aperture.
A similar combination of radiating and cir-
cular fibres is employed in the construction of
flat, or slightly concave muscular disks, which
are thus rendered capable of exerting a strong
force of adhesion to the surfaces to which they
are applied. In these organs the circular fibres
are placed at the circumference, and the radi-
ating fibres in the interior of the sucker, (see
Fig. 48) ; so that, while the margin of the disk
is closely applied to the object, the force result-
MUSCULAR POWER. J 37
ing from the contraction of the circular fibres
is exerted to remove the central portions from
the surface of attachment, and thereby tends to
create a vacuum underneath the disk ; the two
surfaces remain, therefore, strongly attached by
the atmospheric pressure, which acts on their
outer sides. An apparatus of this kind, as we
shall afterwards find, is met with very frequently
among the lower orders of the animal kingdom.
Another kind of circular disposition of fibres
is that which occurs in the muscular coats sur-
rounding canals of various kinds, such as the
blood vessels, and the alimentary tube. Their
action tends to contract the diameter of the canal,
and to exert pressure on its contents. In these
cases, there is generally at the same time pro-
vided another layer of fibres, disposed longitudi-
nally, as shown in Fig. 49 ; the circular fibres
being seen in Fig. 50. The action of the longi-
tudinal fibres is evidently to shorten the canal ;
while that of the circular fibres, by the yielding
of the coats, and the partial reaction of the con-
tents of the vessel, has a tendency to extend it.
138 THE MECHANICAL FUNCTIONS.
The Ascidia, which is a species of marine worm,
is an example of an animal whose skin contains
a union of straight and circular fibres, by which
all its movements are readily performed. Many
instances occur in the cylindrical envelopes of
animals, of the combination of a third series of
fibres, passing obliquely, with those which have
transverse and longitudinal directions. In the
muscular skin of the Leech, for example, besides
two internal layers of longitudinal fibres, an ex-
ternal one has lately been discovered, which is
composed of oblique or spiral fibres, crossing
one another in opposite directions, and greatly
facilitating the varied movements of the animal.*
A variety of still more complicated arrange-
ments may be traced in the fibres of those
muscles which invest hollow sacs, or receptacles,
such as the stomach, (Fig. 51,) and the heart,
(Fig. 52). We find, in the substance of these
organs, sets of fibres, which pass in a spiral
direction, and which, consequently, unite the
effects of both longitudinal and circular fibres ;
and, when combined with either of these, they
serve to modify and regulate the actions of each
organ in a great variety of ways.|
* Carus, Tabulae Anat. Comp. fol. Tab. I. Fig. 6.
f The muscular fibres of the heart are disposed in two layers ;
each set passing in a spiral course from the basis, or broad part,
to the point or apex ; but the direction of the turns being dif-
ferent in each, the two layers cross or decussate, producing the
MUSCULAR POWER. 139
The infinite mechanical skill, with which the
moving power has been applied to the purposes
to be accomplished, is displayed not only in the
larger organs, where great force is to be exerted,
but also, in a still more conspicuous manner, in
the execution of the smaller motions, requiring
the most accurate regulation, and the nicest
adjustments. We cannot but be struck with the
accordance which may often, in these instances,
be traced with human contrivances, where the
greater motions are rapidly executed by one set
of agents, acting with considerable power and
velocity, while the minuter approximations to
the exact positions are effected by a distinct part
of the apparatus, capable of more delicate action,
though with a smaller force. Thus, while the
astronomer brings his telescope round by power-
ful machinery, so as to direct it to that part of
the heavens, where the object he wishes to view
is situated, a more nice mechanism is employed
to direct the instrument accurately to the exact
point; and, again, another is provided for making
the proper focal adjustments. Many parallel
cases occur in the mechanism of the animal
frame ; one set of powerful muscles being em-
effect, and procuring the advantages of a combination of oblique
muscles already explained. Thus beautifully is the arrangement
of the muscular fibres of the heart calculated to produce the
rapid and complete expulsion of its contained blood, with the
smallest amount of contraction in the individual fibres.
140 THE MECHANICAL FUNCTIONS.
ployed for the larger movements, and another set
provided for the accurate regulation of the more
delicate inflexions and nicer positions. This we
shall find exemplified in the movements of the
fingers, and of many of the organs of the finer
senses.
In general, however, we may observe that the
mechanical expedients devised by Nature for
effecting each particular purpose are charac-
terised by the most admirable simplicity. In
this respect, also, as well as in all others, we
cannot fail to recognise their infinite superiority-
over every corresponding invention of man.
"In human works, though labour'd on with pain,
A thousand movements scarce one purpose gain ;
In God's, one single can its ends produce,
Yet serves to second too some other use." Pupe.
We may generally observe, in the mechanism
of the joints, that the muscles are made to act,
either directly or by means of their tendons, at
a point much nearer to the axis of motion than
the resistance to be overcome. With regard to
the direct force, therefore, it is evident that
they must act with a great mechanical disad-
vantage ; and this disadvantage is still farther
increased by the obliquity of the action with re-
ference to the direction of the motion. But the
contractile power, which is inherent in the mus-
cular fibre, is so enormous, as amply to afford
MUSCULAR POWER. 141
these losses, great as they necessarily are ; while,
on the other hand, full compensation is made
by the greater freedom and velocity of motion
thereby obtained. Strength is sacrificed with-
out scruple to beauty of form or convenience of
purpose ; and that disposition of the force is
always adopted, from which, on the whole, the
greatest practical benefit results. Every where
do we find the wisest adaptation of muscular
power to the objects proposed, whether it be
exerted in laborious efforts of the limbs and
trunk ; whether employed in balancing the
frame, or urging it into quick progression ; or
whether it be applied to direct the delicate evo-
lutions of the fingers, the rapid movements of the
organs of speech, or the more exquisite adjust-
ments of the eye, or of the internal ear. Amidst
the endless combinations of machinery exhi-
bited in different parts of the animal kingdom,
although the mode of application be diversified
in ten thousand ways, the original power is still
of the same kind, and is regulated by the same
physical laws ; and similar instruments are em-
ployed in effecting this infinite variety of pur-
poses, by the all-wise and omnipotent Architect
of animated creation.
142
Chapter II.
THE MECHANICAL FUNCTIONS IN ZOOPHYTES.
§ 1. General Observations.
The mechanism of an organized being is de-
signed to fulfil various important objects. These
we may distinguish into two classes ; the one
having reference to its internal welfare, the
other to its relations with external bodies. The
different parts of its system must, in the first
place, be mechanically united and supported, as
well as protected from injurious external impres-
sions ; and they must at the same time be so
constructed as to admit of all the internal move-
ments, which the performance of their functions
renders necessary. They must, in the second
place, be made capable of exerting upon exter-
nal matter the actions which conduce to their
well being ; and in order to enlarge their sphere
of action, they must have the power of transfer-
ring the whole body from one place to another ;
or, in other words, of effecting its progressive
motion.
The objects included in the first of these
ZOOPHYTES. 143
branches of the mechanical functions are an-
swered by the organization both of the vegetable
and the animal systems : but those of the latter
belong exclusively to the functions of animal
life. The power of locomotion, more especially,
constitutes the most general and palpable fea-
ture of distinction between these two classes of
beings. A plant, during the whole period of its
existence, is fixed to the spot where it was first
produced, and is dependent for the continuance
of its life on local circumstances ; such as the
nature of the soil in which its roots are embed-
ded, and the qualities of the air and water in its
immediate vicinity. It is exposed to the action
of the surrounding elements, and affected by
their vicissitudes, without the means of retreat,
and without the power of reaction. With respect
to all external agents, indeed, vegetables may
be regarded as passive beings. Very different
are the condition and destination of animals.
Excepting a few among the lower orders of the
creation, such as Zoophytes and Mollusca, all
animals are gifted with the power of sponta-
neously changing their situation, according to
their several wants and necessities, and are thus
enabled to seek and to choose those objects
which are salutary, and to avoid or reject those
which are injurious. Nature has, for these
purposes, furnished them with a more complex
144 THE MECHANICAL FUNCTIONS.
organization and more varied powers, adapted to
a greater diversity of pursuits, and to a higher
and more expanded sphere of existence.
The power of progressive motion is enjoyed in
very different degrees by different races of ani-
mals, according to the particular model on
which they are constructed, and the relations
which their organization bears to the element
assigned as their residence. All the mechanical
circumstances in their economy, indeed, are so
closely linked together, as scarcely to admit of
being considered separately. Thus we find, in
one animal, a variety of mechanical effects ac-
complished by one and the same instrument ;
while, in others, they are each produced by a
separate and distinct organ. In some, the lead-
ing principle of the construction is simplicity ;
in others, the most elaborate mechanism is dis-
played. But the means have constant reference
to the design, and are ever varied in exact
conformity with the change of purpose. The
relative advantages of each plan of structure
appear to have been carefully estimated, and
studiously balanced. Each quality has been
bestowed in different degrees of perfection ; so
that in following the series of gradation among
the successive tribes of animals, we occasion-
ally meet with favoured species, endowed with
great superiority in some particular faculty.
Some animals excel in swiftness ; others in
ZOOPHYTES. 145
strength. Some are qualified to dive into the
recesses of the deep ; others to flutter in the
light regions of air ; while, in many of the infe-
rior ranks, we find all these objects renounced
for the more certain advantage of security,
which the softer texture of the organs renders
one of paramount importance. That construc-
tion of limbs which favours certain movements
will necessarily interfere with the ready per-
formance of others, and must preclude the deve-
lopement of the organs which would be neces-
sary for facilitating them. Different kinds of
prey require dexterity in particular actions for
their pursuit and seizure. The animal is, in
one case, formed for climbing trees ; in another,
for burrowing in the earth : in a third, for per-
forating wood. Some are provided with organs
for penetrating into the bodies of other animals ;
others with the means of ensnaring their captives;
while others, again, instil into the veins of their
victims a deadly poison. Hence it is necessary,
in studying the organization of animals, to bestow
particular attention on the habits and mode of
life for which each respective tribe and species
has been destined.
In the examination of the mechanical func-
tions which will form the first part of this
treatise, I shall keep in view, as the leading
object of inquiry, the faculty of progressive
motion, noticing its different degrees of per-
VOL. I. L
140 THE MECHANICAL FUNCTIONS.
fection as we follow the ascending series of
animals ; but adverting, also, occasionally, to
the other topics which belong to this class of
functions.
It may be observed in general, that the me-
chanical construction of animals which con-
stantly inhabit a watery element is more simple
than the construction of those which live on
land, and are encompassed by a lighter medium.
Differing but little in their specific gravity from
the fluid in which they are immersed, aquatic
animals are necessarily supported, on all sides,
by a powerful hydrostatic pressure, which nearly
balances the force of gravity, and counteracts the
tendency of their bodies to descend in the fluid.
Many of the obstacles to progressive motion are
thus removed ; and there is no necessity for the
compactness of frame, and the rigidity and co-
hesion of substance which are required in ter-
restrial animals.
The animals which occupy the lower divisions
of the scale can exist only in a liquid element.
Their forms present many analogies with vege-
tables ; and hence they have been denominated
Zoophytes, that is, animated plants : but as it is
now well ascertained that they possess the essen-
tial characters of animals, the term of Phytozoa,
or plant-like animals, which has been given to
them by some modern writers, would appear to
be a more appropriate designation. It is, how-
SPONGES. 147
ever, scarcely worth while, at the present day,
to change a name so generally received as that
of Zoophytes, and the application of which is
not likely to lead to any misunderstanding.
§ 2. P or if era, or Sponges.
Among Zoophytes, the lowest station in the scale
of organization is occupied by the tribes of Po-
rifera, the name given by Dr. Grant to the ani-
mals which form the various species of sponge,
and which are met with in such multitudes on
every rocky coast of the ocean, from the shores
of Greenland to those of Australia. Sponges
grow to a larger size within the tropics, and are
found to be more diminutive, and of a firmer
texture, as we approach the Polar circles. Dr.
Grant observes* that they are met with equally
in places covered perpetually by the sea, as in
those which are left dry at every recess of the
tide. They adhere to, and spread over the sur-
face of rocks and marine animals, to which they
are so firmly attached that they cannot be re-
moved without lacerating and injuring their
bodies. " Although they thrive best," he far-
ther remarks, " in the sheltered cavities of rocks,
they come to maturity in situations exposed to
* Edinburgh Philosophical Journal, vol. xiii. p. 94.
148 THE MECHANICAL FUNCTIONS.
the unbroken fury of the surge. They cover the
nakedness of cliffs and boulders ; they line with
a variegated and downy fleece the walls of sub-
marine caves, or hang in living stalactites from
the roof."
In their general appearance they resemble
many kinds of plants, but in their internal or-
ganization they differ entirely from every vege-
table production ; being composed of a soft flesh,
intermixed with a tissue of fibres, some of which
are solid, others tubular ; and the whole being
interwoven together into a curious and compli-
cated net-work. The substance of which this
solid portion, or basis, is formed, is composed
partly of horn, and partly of siliceous or calca-
reous matter. It has been termed the axis of the
Zoophyte; and as it supports the softer substance
of the animal, it may be regarded as performing
the office of a skeleton, giving form and protec-
tion to the entire fabric.
The material of which the fleshy portion is
composed is of so tender and gelatinous a nature
that the slightest pressure is sufficient to tear it
asunder, and allow the fluid parts to escape ;
and the whole soon melts away into a thin oily
liqnid. When examined with the microscope,
the soft flesh is seen to contain a great number
of minute grains, disseminated through a trans-
parent jelly. Every part of the surface of a
living sponge (as may be seen in Fig. 53) pre-
SPONGES.
149
sents to the eye two kinds of orifices ; the larger,
having a rounded shape, and generally raised
55
margins, which form projecting papilla?; the
smaller, being much more numerous, and ex-
ceedingly minute, and constituting what are
termed the pores of the sponge.
It was, for a long time, the received opinion
among naturalists that this superficial layer of
gelatinous substance is endowed with a consi-
derable power of contractility : it was generally
believed that it shrunk from the touch, and that
visible tremulous motions could be excited in it
by punctures with sharp instruments, or other
modes of irritation. These notions are of very
ancient date, for they may be traced even be-
yond the time of Aristotle ; and they have been
handed down by succeeding naturalists, and
echoed from the one to the other, so as to have
gained admission, without being questioned, in
all the recent systematic works on Zoology.
The alleged spontaneous palpitation of the
150 THE MECHANICAL FUNCTIONS.
flesh, occurring in particular parts, had its origin
in the views taken of the nature of sponges by
Marsigli, an Italian naturalist, who, in the year
1771, announced that he had seen movements of
dilatation and contraction in the round apertures
visible on the surface of sponges. This state-
ment, so confidently advanced, seems to have
made a strong impression on Ellis, who, while
pursuing a similar train of observations, came to
persuade himself that he could see, not only the
movements described by Marsigli, but also the
passage of water to and fro, through the same
apertures. He communicated this account to
the Royal Society in 1765 ; it was published
in its Transactions,* and will ever remain an
instructive proof of the degree in which our very
perceptions may be influenced by preconceived
views, and by the force of the imagination.
Pallas immediately admitted, without examina-
tion, the hasty assertion of Ellis, into his " Elen-
chus Zoophytorum ,•" whence it was copied by
succeeding authors, and the error became at
length so widely disseminated, that for more
than half a century it was received as an es-
tablished fact in natural history. The more
accurate researches of Dr. Grant on these sub-
jects have at length dispelled the prevailing
* Vol. lv. p. 284.
SPONGES. 151
illusion, and have clearly proved that the sponge
does not possess, in any sensible degree, that
power of contraction which had, for so many
ages, been ascribed to it.*
Dr. Grant has also shown the true nature of
the currents of fluid issuing at different points
from the surface of these animals, as well as the
absence of all visible movements in the orifices
which give exit to the fluid. Never did he find,
in his experiments, the slightest appearance of
contraction produced in any part of the sponge,
by puncturing, lacerating, burning, or otherwise
injuring its texture, or by the application of
corrosive chemical agents. Of his discovery of
the fluid currents, he gives the following inte-
resting account : " I put a small branch of the
Spongia coalita, with some sea-water, into a
watch-glass, under the microscope, and, on re-
flecting the light of a candle through the fluid,
I soon perceived that there was some intestine
motion in the opaque particles floating through
the water. On moving the watch-glass, so as to
bring one of the apertures on the side of the
sponge fully into view, I beheld, for the first
time, the splendid spectacle of this living foun-
* See his papers on this subject in the Edinburgh Philosophical
Journal, vol. xiii. p. 95 and 333, from which most of the facts
mentioned in the above account are taken.
152 THE MECHANICAL FUNCTIONS.
tain, vomiting forth, from a circular cavity, an
impetuous torrent of liquid matter, and hurling
along, in rapid succession, opaque masses, which
it strewed everywhere around. The beauty and
novelty of such a scene in the animal kingdom,
long arrested my attention, but after twenty-five
minutes of constant observation, I was obliged
to withdraw my eye from fatigue, without having
seen the torrent for one instant change its direc-
tion, or diminish, in the slightest degree, the
rapidity of its course. I continued to watch the
same orifice, at short intervals, for five hours,
sometimes observing it for a quarter of an hour
at a time, but still the stream rolled on with a
constant and equal velocity." About the end of
this time, however, the current became languid,
and, in the course of another hour, it ceased
entirely. Similar currents were afterwards ob-
served by Dr. Grant in a great variety of species.
They take place only from those parts which are
under water, and immediately cease when the
same parts are uncovered, or when the animal
dies.
It thus appears that the round apertures in
the surface of a living sponge are destined for
the discharge of a constant stream of water from
the interior of the body ; carrying away par-
ticles, which separate from the sides of the
canals, and which are not only seen, under the
SPONGES. 153
microscope, constantly issuing from these orifices,
but may even be perceived by the naked eye,
propelled occasionally in larger masses.*
For the supply of these constant streams, it is
evident that a large quantity of water must be
continually received into the body of the sponge.
It is by the myriads of minute pores, which exist
in every part of the surface, that this water
enters, conveying with it the materials necessary
for the subsistence of the animal. These pores
conduct the fluid into the interior, where, after
percolating through the numerous channels of
communication which pervade the substance of
the body, it is collected into wider passages,
terminating in the fecal orifices above described,
and is finally discharged. The mechanism by
which these currents are produced is involved
in much obscurity. There can be no doubt
that they are occasioned by some internal move-
ments ; and the analogy of other zoophytes
would lead us to ascribe them to the action of
fibrils, or cilia, as they are termed, projecting
from the sides of the canals through which the
streams pass ; but these cilia have hitherto
* The currents issuing from the larger orifices are best seen
by placing the living animal in a shallow vessel of sea water,
and strewing a little powdered chalk on the surface, the motions
of which will render the currents very sensible to the eye. Fig.
53 exhibits these phenomena.
154 THE MECHANICAL FUNCTIONS.
eluded observation, even with the highest powers
of the microscope.
The organization of sponges is as regular and
determinate as that of any other animal struc-
ture, and presents as systematic an arrangement
of parts. In some species, such as the common
sponge, the basis is horny and elastic, and com-
posed of cylindric tubes, which open into each
other, and thus form continuous canals through-
out the whole mass. Others have a kind of
skeleton, composed of a tissue of needle-shaped
crystals of carbonate of lime, or of silex. These
hard and sharp-pointed fibres, or spicula, are
disposed around the internal canals of the sponge,
in the order best calculated to defend them from
compression, and from the entrance of foreign
bodies. Some of these spicula are delineated in
Fig. 54 : but their forms, although constant in
each species, admit of considerable diversity in
the different kinds of sponge.
Although sponges, in common with the greater
number of zoophytes, are permanently attached
to rocks, and other solid bodies in the ocean,
and are consequently destined to an existence
as completely stationary as that of plants, yet
such is not the condition of the earlier, and more
transitory stages of their developement. Nature,
ever solicitous to provide for the multiplication
of each race of beings, and for their dissemina-
tion over the habitable globe, has always pro-
SPONGES. 155
vided effectual means for the accomplishment of
these important ends. The seeds of plants are
either scattered in the immediate neighbourhood
of the parent, and take root in the adjacent soil,
or are carried to more distant situations by the
wind or other agents. In the animal kingdom,
the young offspring of those races which are en-
dowed with a wide range of activity, are reared
on the spot where they were produced, either by
the fostering care of the parent, or by means of
the nourishment with which they are surrounded
in the egg, and there remain until the period
when, by the acquisition or extension of locomo-
tive powers, they are enabled, in their turn, to
go in quest of food. But in the tribes of ani-
mals at present under our consideration, this
order is reversed. It is the parent that is
chained to the same spot from an early period
of its growth, and it is on the young that active
powers of locomotion have been conferred, appa-
rently for the sole purpose of seeking for itself a
proper habitation at some distance from the
place of its birth ; and when once it has made
this selection, it there fixes itself unalterably for
the remaining term of its existence.*
* Phenomena still more curious are presented by a tribe of
natural productions, resembling aquatic plants in all their external
characters, but, after a certain period, giving birth to an im-
mense number of animated globules, which, for a time, move
briskly in the fluid, like infusory animalcules, and then congre-
156 THE MECHANICAL FUNCTIONS.
The parts of the Spongia panicea, which are
naturally transparent, contain at certain seasons
a multitude of opaque yellow spots, visible to
the naked eye, and which, when examined by
means of a microscope, are found to consist of
groups of ova, or more properly gemmules,']' since
we cannot discover that they are furnished with
any envelope. In the course of a few months
these gemmules enlarge in size, each assuming
an oval or pear-like shape, and are then seen
projecting from the sides of the internal canals
of the parent, to which they adhere by their
narrow extremities. In process of time, they
become detached, one after the other, and are
swept along by the currents of fluid, which are
rapidly passing out of the larger orifices. Fig.
55 represents one of these gemmules detached
gate together, and arrange themselves in linear juxtaposition, as
if by a kind of organic crystallization, thereby forming the
stems and branched filaments of these apparent plants. These
singular productions, which have been recently studied by M.
Gaillon, and which he has established into a natural family,
denominated Nemazoaria, seem, in their progressive develope-
ments, to possess alternately the characters of vegetables and of
animals, and may perhaps be regarded as connecting links between
the two great kingdoms of living nature. (See " Appercu d'His-
toire Naturelle, et Observations sur les limites qui separent le
Regne Vegetal du Regne Animal. Par B. Gaillon. Boulogne,
1833.")
f Gemmule is a term derived from the Latin word gemma, a
bud ; and its meaning, as applied to zoophytes, is that of a
young animal, not contained within an envelope, or egg.
SPONGES. 157
from the parent sponge. When thus set at
liberty, they do not sink by their gravity to the
bottom of the water, as would have happened
had they been devoid of life ; but they continue
to swim, by their own spontaneous motions, for
two or three days after their separation from the
parent. In their progression through the fluid
they are observed always to carry their rounded
broad extremity forwards. On examining this
part with the microscope, we find that it is
covered with short filaments, or cilia, which are
in constant and rapid vibration. These cilia
are spread over about two thirds of the surface
of the body, leaving the narrower portion, which
has a whiter and more pellucid appearance, un-
covered. They are very minute transparent
filaments, broadest at their base, and tapering
to invisible points at their extremities : they
strike the water by a rapid succession of in-
flexions, apparently made without any regular
order, but conspiring to give an impulse in a
particular direction. When the body is attached
by its tail, or narrow end, to some fixed object,
the motion of the cilia on the fore part of the body
determines a current of fluid to pass in a direc-
tion backwards, or towards the tail ; but when
they are floating in the water, the same action
propels them forwards in the opposite direction,
that is, with the broad ciliated extremity fore-
153 THE MECHANICAL FUNCTIONS.
most. They thus advance, without appearing to
have any definite object, by a slow gliding
motion, totally unlike the zig-zag course of ani-
malcules in search of prey. Yet they appear to
have a consciousness of impressions made on
them ; for on striking against each other, or
meeting any obstacle, they retard a little the
motion of their cilia, wheel for a few seconds
round the spot, and then, renewing the vibra-
tions, proceed in their former course.
In about two or three days after these gem-
mules have quitted the body of the parent, they
are observed to fix themselves on the sides or
bottom of the vessel in which they are contained ;
and some of them are found spread out, like a
thin circular membrane, on the surface of the
water. In the former case, they adhere firmly
by their narrow extremity, which is seen gra-
dually to expand itself laterally, so as to form a
broad base of attachment. While this is going
on, the cilia are still kept in rapid motion on
the upper part, scattering the opaque particles,
which may happen to be in the fluid, to a cer-
tain distance around. But these motions soon
become languid, and, in the course of a few hours,
cease ; and the cilia, being no longer wanted,
disappear. The gemmule then presents the
appearance of a flattened disk, containing gra-
nules, like the flesh of the parent sponge ; and
\
SPONGES. 15.9
also several spicula interspersed through the
central part. In less than twenty-four hours,
a transparent colourless margin has extended
round the whole gemmule, and continues to
surround it during its future growth. The spi-
cula, which were at first small, confined to the
central part, and not exceeding twenty in num-
ber, now become much larger and more nume-
rous ; and some of them shoot into the thin ho-
mogeneous margin. It is a remarkable circum-
stance that the spicula make their appearance
completely formed, as if by a sudden act of
crystallization, and never afterwards increase
their dimensions.
When two gemmules, in the course of their
spreading on the surface of a watch-glass, come
into contact with each other, their clear margins
unite without the least interruption ; they thicken
and produce spicula : in a few days we can
detect no line of distinction between them, and
they continue to grow as one animal. The same
thing happens, according to the observation of
Cavolini, to adult sponges, which, on coming
into mutual contact, grow together and form an
inseparable union. In this species of animal
grafting we again find an analogy between the
constitution of zoophytes and that of plants.
In the course of a few weeks, the spicula are
assembled in groups, similar to those of the
16*0 THE MECHANICAL FUNCTIONS.
parent sponge ; assuming circular arrangements,
and presenting distinct openings at the points
they enclose. The young animal now rapidly
spreads and enlarges in every direction, be-
coming more convex, and at the same time more
opaque, and more compact in its texture ; and
before it has attained the tenth of an inch in
diameter, it presents, through the microscope,
a miniature representation of its parent.
Thus has a power of spontaneous motion been
given to what may be regarded as the embryo
condition of animals, which are afterwards so
remarkable for their inertness, and for the pri-
vation of all active powers ; and it has been con-
ferred evidently for the purpose of their being
widely disseminated over the globe. Had not
this apparatus of moving cilia been provided to
the gemmules of such species as hang vertically
from the roofs of caves, they would have sunk
to the bottom of the water and been crushed
or buried among the moving sand, instead of
supporting themselves while carried to a dis-
tance by the waves and tides of the ocean.
Many species which abound in the Red Sea
and Indian Ocean have, in this way, been
gradually transported, by the Gulf stream, from
the shores of the east to corresponding latitudes
of the new world.
POLYPI, 101
§ 3. Polypi/era*
The next step in the organic series introduces
us to the extensive family of Polypi/era. The
transition from the structure of the sponge to that
of the polypus may be thus conceived. Suppose
the absorbing orifices of the former to be enlarged,
and their number to be at the same time reduced :
and let these orifices be drawn out into tubes,
and provided with vibratory cilia ; in addition to
which, let there be placed around their margin
a circular row of larger filaments, extremely
flexible, and capable of twining round any ob-
ject that comes within their reach, and of con-
veying it to the central orifice, which performs
the office of a mouth. Each tube, thus furnished
with a circle of radiating filaments, or tentacula,
as they are called, is denominated a Polype.*
The animal structure thus composed has re-
ceived the name of Lobularia (Fig. 56), and is
the genus among this tribe that approaches the
nearest in its character to the sponge, which it
resembles in the nature of its internal texture.
* For the sake of greater distinctness I shall employ the term
polype to denote the single tube with its tentacula; and shall
designate by the Latin term polypus the entire animal mass com-
posed of an aggregation of these polypes. Polypi/era, the name
of the order, expresses animals bearing polypes.
VOL. I. M
162
THE MECHANICAL FUNCTIONS*
Each of the polypes with which its surface is
studded has eight serrated tentacula. Fig. 57
represents one of these polypes detached. Po-
lypes may thus be united in immense numbers
into one mass, having mutual organic connexion.
In other cases they may form smaller clusters,
or be even totally unconnected. Sometimes the
detached polypes are still disposed to assemble
in groups, as is the case with the Zoanthiis of
Cuvier* (Fig. 58) : at other times they are alto-
gether isolated, as in the Hydra viridis (Fig. 59).|
Polypi form a very extensive order of zoo-
phytes, abounding in every part of the ocean,
but growing in greatest luxuriance in the warmer
* The Hydra sociata of Gmelin; the Actinia sociata of Ellis,
t In this figure two hydrse are seen attached to the stem of a
plant.
POLYPI. 163
regions of the globe. Their flesh exhibits the
same granular appearance as that of the sponge,
but it is generally firmer, and often intermingled
with masses of calcareous matter. The tenta-
cula, which may be compared to arms, vary in
number and in length in different species of
polypi, and sometimes, instead of a single row,
each of the mouths has two or more series of
tentacula placed around it. They are formed
of a prolongation of the soft substance of the
polypus, and are sometimes tubular ; and their
cavities are then continuous with that of the
general internal cavity into which the several
mouths open. Besides being flexible in every
direction, the tentacula are also capable of being-
lengthened or shortened at the pleasure of the
animal. Their elongation is produced by the
propulsion of a fluid into their interior, derived
from the general cavity of the body ; and their
retraction is effected by the return of the same
fluid.
The whole arrangement of the tentacula on
the margin of the projecting mouths bears a
striking resemblance to a flower, especially to
those which, like the daisy, or china-aster, have
the corolla composed of slender radiating petals.
We find, indeed, that as the organs of zoophytes
become more developed, the affinities which these
lower departments of the animal kingdom retain
with plants, are more marked and more predomi-
164 THE MECHANICAL FUNCTIONS.
nant. In the construction of zoophytes, nature
seems still to keep in view the models of vege-
table forms, the characters of which, while effect-
ing the transition from one kingdom to the other,
she continues to impress on her productions.
Zoophytes, both in their outward form, and in
the disposition of their internal organs, preserve
the symmetrical arrangement round a common
centre so generally exhibited in plants, and
especially in flowers, and in the verticillated
leaves and branches.* Hence the radiated or
star-like forms which predominate in most of
the animals composing this class : and hence
they have obtained the title of Radiata, by
which Cuvier has designated them.
Like the animals of the sponge tribe, Polypi
are for the most part attached to some inorganic
shell or base, which may be either of a horny or
calcareous nature. The form of this shell admits
of almost infinite variety. In some it constitutes
the external surface of the animal, and encloses
the flesh in a general sheath, leaving only open-
ings at the extremities of the tubes for the ex-
pansion of each set of tentacula surrounding the
respective mouths. Sometimes these tubes are
joined together endwise, like the branches of a
tree, leaving lateral apertures for the protrusion
of the tentacula of each separate polype : this is
* See page 90.
POLYPI.
105
the case in the Sertularia. (Fig. 00.) At other
times the tubes are placed
parallel to each other, like the
pipes of an organ, with trans-
verse partitions at regular in-
tervals : such is the structure
of the Tubipora musica, as
shown in Fig. 61. In Fig. 62,
a portion of the tubes is seen
highly magnified, and laid
open, to show the polypes in their interior.
61
62
63
64
In some species the horny base is fashioned
into a number of cells, each of which serves for
the protection of its respective polype. These
cells are generally placed at the extremity of
the branches, presenting the greatest similitude
to flowers. The Flustra (Fig. 63) is composed
of minute and almost microscopic cells, spread
over a flat membraneous substance, resembling,
in the flexibility of its texture and its mode of
subdivision, the leaves of plants. These cells are
arranged in rows, with great regularity, like those
(36
THE MECHANICAL FUNCTIONS.
of a honey-comb, as is seen in the magnified
view of them, Fig. 64.
In other tribes the inorganic base of support
is internal, constituting a kind of skeleton or
axis ; the polypous mouths being spread at in-
tervals over the surface of the fleshy layer which
covers this skeleton. This is the case with the
Gorgonia, Antipathes, and Coral, which exhibit
still closer resemblances to the branched forms
of vegetable steins. The flesh contains granules
of calcareous matter, which, in the dried speci-
mens, adhere to the surface of the stems. Fig.
65 is a branch of the Corallium ruhrum, of
4W=)
which Fig. 66 is a magnified portion, showing
the appearance of the polypes in their expanded
and contracted states. The way in which the
polypes are embedded in the flesh is seen in
Fig. 67, which represents a section of the Gor-
gonia Uriareus.
In many cases the polypes are lodged in cup-
like depressions in the surface of the calca-
reous axis, which affords them some degree of
protection. In Madrepores these depressions
POLYPI. 107
are crossed by radiating plates, adapted to the
form and number of the tentacula. In Mille-
pores the cells are closer and more minute, and
exhibit none of these star-like radiations. In
some species the plates have more of a parallel
arrangement; and in others they form a net-
work.
The material of which this axis, to which the
polypes are attached, is composed, is of various
kinds. Sometimes it is horny, flexible, and
elastic, corresponding in its nature to animal
membrane : at other times it is hard and calca-
reous, being composed principally of carbonate
of lime, with a small quantity of the phosphate ;
the proportion of this latter ingredient varying
in different species. In all cases the particles
of calcareous matter are united together by some
portion of animal substance, which may be ob-
tained by dissolving out the former by an acid.
We always find the materials arranged in con-
centric layers, indicating that their deposition
has been successive ; and the surface is marked
by longitudinal lines, corresponding to the figure
of the animal covering of flesh. Sometimes the
stem consists of horny and calcareous parts dis-
posed alternately, composing a jointed structure,
which some have fancied might be considered
as making an approach to an articulated skele-
ton ; for it is capable of considerable flexion,
and readily yields to the impulse of the waves,
108 THE MECHANICAL FUNCTIONS.
without the risk of being broken. This is the
case with the Isis hippuris, commonly known by
the name of jointed coral. (Fig. G8.) There is,
in short, hardly any possible combination of these
parts which does not occasionally occur amidst
the infinite diversities of condition displayed in
this department of the animal creation.
These structures are generally attached to
submarine rocks by an expansion of the base
into a kind of foot, or root, which has a strong
power of adhesion. In this respect, therefore, as
in so many others, these animals preserve an
analogy with plants.
It has been ascertained that, in a great num-
ber of instances, these fixed zoophytes are mul-
tiplied, like the sponge, by the detachment of
gemmules, or imperfectly formed portions of
their soft substance. These gemmules require
to undergo the same kind of metamorphosis in
order to bring them to their perfect state ; and
when newly detached from the parent, they ex-
hibit the same singular spontaneous motions,
buoying themselves in the water, and swimming
in various directions, by the rapid vibrations of
their cilia, till they find a place favourable to
their growth. On becoming fixed, they spread
out to form a base for the future superstructure ;
and, after the foundation has thus been laid,
they proceed in their upward growth, depositing
a calcareous or horny axis in successive layers,
POLYPI. 169
until it has acquired the requisite thickness ;
and they then gradually assume the forms cha-
racteristic of the particular species to which
they belong. The materials thus deposited are
permanent structures, not capable of modification
or removal, and not possessing any vital pro-
perties ; for these properties belong exclusively
to the animated flesh with which these structures
are associated. The polypes themselves are not
developed till after the formation of the root and
stem ; their growth being in this respect analo-
gous to that of the leaves and flowers of a plant.
The gemmules of the Flustra carbasea may
be selected in illustration of these phenomena.
These have been observed by Dr. Grant,* to swim
about in the water as soon as they have escaped
from the cells of the parent ; each moving with
its narrow end foremost, while the opposite broad
end, which is covered with cilia, expands into a
flat circular zone. These gemmules are very
irritable, and are frequently seen to contract the
circular margin of their broad extremity, and,
while swimming, to stop suddenly in their course.
They swim with a gentle gliding motion ; at
other times they appear stationary, all the while
revolving rapidly round their longer axis, with
their broad end uppermost : they often bound
forwards, either in straight lines, or describing
* Edinburgh Philosophical Journal, XVII. 107 and 337.
170 THE MECHANICAL FUNCTIONS.
circles, with no other apparent object than to
keep themselves afloat, until they shall arrive
at a favourable spot for fixing their permanent
abode, and proceeding in their further develope-
ment. The time of their remaining in this free
and moving state varies according to circum-
stances, from a few hours to about three days.
When about to fix, the slightest agitation of the
water causes them to desist, and to recommence
their gliding motions, which they continue for
some time longer. If, when any of these gem-
mules has begun to fix, it be again disturbed,
and separated from the surface to which it had
become attached, it generally remains free, and
perishes. During the process of fixing, it exhi-
bits no peculiar appearance or change of form ;
it simply lies on its side ; and the cilia continue
to vibrate over the whole surface, producing a
constant current in the water, apparently for the
purpose of cleaning the space immediately sur-
rounding the gemmule. It remains for three
days in this attitude, without undergoing any
perceptible change of form, and without relaxing
the vibrations of its cilia. At the end of this
time, the cilia cease to move, and shortly after
disappear : then the gemmule begins to swell,
the surrounding margin becomes more trans-
parent, and the whole gradually assumes the
form of a cell, surrounded by a delicate white
opaque line, which is the rudiment of the calca-
POLYPI. 171
reous wall of the future cell. Towards the base
of this rudimental cell, the gelatinous substance
in the interior may be perceived to become more
consistent and opaque at a particular point ;
from this dull spot within the cell, short straight
tentacula begin to bud, extending upwards in
the direction of the future aperture. The gela-
tinous spot, from which the tentacula originated,
assumes the vermiform appearance of the body
of a polype ; and we may distinctly perceive
the bundles of fibres which connect its head with
the base of the cell. The structure of the polype
is perfected by the addition of a closed capsule ;
and when it is first detected protruding from the
cell, it possesses all the parts of an adult polype,
and vibrates the cilia of its tentacula with as
much regularity and velocity as at any future
period. Before the polype is capable of protrud-
ing from the aperture of the first cell, the upper
part of the cell has already extended outwards
to form the rudiment of a second : and so on, in
succession, till the whole structure is completed.
The tentacula of polypi are exquisitely sen-
sible, and are frequently seen, either singly or
altogether, bending their extremities towards
the mouth, when any minute floating body
comes in contact with them. When a polype
is expanded, a constant current of water is
observed to take place, directed towards the
mouth. These currents are never produced by
the motions of the tentacula themselves ; but
1 72 THE MECHANICAL FUNCTIONS.
are invariably the effects of the rapid vibrations
of the cilia placed on the ten-
tacnla. In the polypes of the
Flustra carbasea, (Fig. 69),
the tentacula have each a
single row of cilia, extending
along both the lateral margins,
from their base to their ter-
mination.* Each polype has
usually twenty-two tentacula ;
and there are about fifty cilia on each side of a
tentaculum, making 2200 cilia on each polype.
As there are above 1800 cells in each square
inch of surface, and the branches of an ordinary
specimen present about ten square inches of sur-
face, we may estimate that an ordinary specimen
of this zoophyte presents more than 18,000 po-
lypes, 396,000 tentacula, and 39,600,000 cilia.
But other species certainly contain more than
ten times these numbers. t
The vibrations of these cilia are far too rapid
to be followed by the quickest eye, even when
assisted by the most powerful microscope, and
can be detected only at the times when they
have become comparatively languid, by the di-
* A portion of one of these tentacula is represented, highly mag-
nified, in Fig. 70. The lower figure (g) is the delineation of one
of the gemmules of the same polypus, also greatly magnified.
f Dr. Grant has calculated that there are about 400,000,000
cilia on a single Flustra foliacea. Transactions of the Zoolo-
gical Society of London, vol. i. p. 11.
POLYPI. 173
minished vigour of the animal : their motions may
then be seen, ascending on one side of the ten-
taculum and descending on the other. (Fig. 70.)
All the cilia appear to commence and to cease
their motions at the same moment. The con-
stancy with which they continue would seem
to exclude the possibility of their being the
result of volition ; and they are, therefore, more
probably determined by some unknown phy-
sical cause, dependent, however, on the life of
the animal. But so retentive are they of the
power of motion, whatever may be its cause,
that if any one of the tentacula be cut off, its
cilia will continue to vibrate, and will propel
it forward in the fluid for a considerable time,
as if it had become itself an individual animal.
A question arises with regard to the con-
stitution of these zoophytes, similar to that
which has been proposed with regard to trees,
namely, what limits should be assigned to
their individuality? Is the whole mass, which
appears to grow from one root, and which
consists of multitudes of branches, proceeding
from a common stem, to be considered as one
individual animal, or is it an assemblage or
aggregation of smaller individuals : each indi-
vidual being characterised by having a single
mouth, with its accompanying tentacula, and
yet the whole being animated by a common
principle of life and growth ? The greater num-
ber of naturalists have adopted this latter view,
174
THE MECHANICAL FUNCTIONS.
regarding each portion, so provided with a dis-
tinct circle of ten taenia, as a separate animal,
associated with its neighbours in the construc-
tion of a common habitation, and contributing its
quota Jtp the general nourishment of this animal
republic. As the determination of this question
involves the consideration of the function of
nutrition, I shall postpone its further discussion
to a future part of this treatise. As far, indeed,
as regards the mechanical condition of animals
which are so completely stationary, it matters
little whether the whole mass be regarded as
one individual animal, or as an aggregate of
distinct individuals. But the question becomes
of some importance when applied to detached
zoophytes, such as Pcnnatula, which are formed
of a multitude of polypes connected with a com-
mon stem, but which float at liberty in the sea.
The Pennatula (Fig. 71) has been termed the
sea pen, from the circumstance
of its calcareous axis, or stem,
having a double setof branches,
extending in the same plane
from both the sides, like the
vane of a quill, and of its series
of polypes being set along one
edge of each branch, like the
filaments which arise from the
fibres of the feather. Some of
these polypes are seen magnified in Fig. 7*2.
Immense numbers of these curious animals are
PENNATULA. 175
met with in different parts of the ocean. If
they possessed in any degree the power of loco-
motion, which many naturalists have ascribed
to them, we should be able to ascertain whether
all their movements are conducted by a common
volition, or whether they are performed inde-
pendently of one another. It has often, indeed,
been asserted, that pennatulse swim through the
water by their own spontaneous movements,
consisting either in the waving up and down of
the lateral branches, or in the simultaneous im-
pulses of the tentacula of all the polypes. Cuvier
even represents the polypes of the pennatula as
having the power of keeping time, while they
are waving the mass through the water, as if
they were all actuated by a single undivided
volition. But Dr. Grant, who has watched the
motions of these animals with great care, is led
by his observations to the conclusion that penna-
tulae are not in reality possessed of any such
locomotive faculty ; but that they are carried
to and fro in the ocean, like the gulf weed,
without the slightest voluntary power of direct-
ing their course. Whatever may be the result
of the combined movements of the tentacula, the
arms are certainly incapable of those inflexions
which have been supposed to supply the means
of progressive motion.
It is only when the contractile flesh of the
polypus is released from the restraint which
176 THE MECHANICAL FUNCTIONS.
the solid axis imposes on its movements, that
the animal becomes capable of any distinct
power of locomotion. Such is the condition
of the animals belonging to the genus Hydra,
of which the Hydra viridis, or fresh water
polype (Fig. 59, p. 162), may be taken as the
type. This singular animal presents us with
perhaps the simplest kind of structure that exists
in the animal kingdom. It would almost seem
as if Nature had formed it with the design of
exhibiting to us the resources of vitality in carry-
ing on the functions of animal life without the
aid of the complicated apparatus which she has
bestowed upon the higher orders of the creation.
The Hydra consists merely of a fleshy tube,
open at both ends, one of which, being more
dilated, may be regarded as the head, and has
for a mouth the aperture of the tube, which is
furnished at its margin with a single row of ten-
tacula. It thus corresponds to the general defi-
nition of a polypus, and exemplifies its most
simple form.
The whole body may, on the one hand, be
considerably elongated, and on the other, so
much retracted, as to appear a mere globule ;
and these movements are the effect of a voluntary
power in the animal directed to specific ends.
The number of tentacula varies from six to
twelve : they are slender tubular filaments, ca-
pable of being extended to a great length, and
HYDRA. 177
of being bent in all directions. In this way,
they can quickly surround and grasp any small
object which they may happen to touch ; and
whenever irritated, they instantly retract, so as
hardly to be visible without the aid of a mag-
nifier. Each tentaculum may be moved inde-
pendently of the rest, at the pleasure of the
animal. The remainder of the body tapers
gradually from the head to the other extremity,
becoming very slender, and having at its termi-
nation a flat surface, which has been termed the
foot ; for although every portion of the surface
has the power of adhering to the bodies to which
it is applied, it is principally by this extremity
that the animal chooses to attach itself to the
sides or bottom of the vessel in which it is kept.
No trace of the existence of cilia is to be met
with on any part of the surface of these animals.
It is to Mr. Trembley of Geneva that we are
indebted for the discovery of this singular ani-
mal, the examination of which has contributed
to throw great light on the natural history of
polypiferous animals.* While observing some
aquatic plants, which he had collected and
put into water, his attention was called to the
appearance of filaments adhering to them,
which he at first conceived to be parasitic vege-
* Memoires pour servir a l'Histoire d'un genre de Polypes
d'eau douce, a bras en forme de cornes. Par A. Trembley, 1744.
VOL. I. N
178 THE MECHANICAL FUNCTIONS.
tables : but further observation convinced him
that they were endowed with powers of spon-
taneous motion, and that they preyed upon small
insects : and he, therefore, could no longer
doubt their animal nature. He found that they
always placed themselves on the side of the
glass next to the light ; and by watching their
changes of position, he discovered the mode in
which they effect their progressive motions. If
the hydra be standing in the erect position, its
foot being applied to the bottom of the glass
(Fig. 73), it slowly bends the body in the direc-
tion in which it intends to advance, till its head
touches the vessel, as shown in Fig. 74. It then
adheres to the surface by the mouth, or by one
or two of its tentacula, and, detaching the foot,
bends the body into a curve, at the same time
slightly retracting it, so that the foot is brought
near the head (Fig. 75). The foot is then again
fixed, preparatory to a new step, which it takes
by detaching the head and projecting it forwards
as before (Fig. 76).
The progress made by these successive efforts
is but slow: for the hydra often pauses in the
middle of a step, as if deliberating whether it
HYDRA. 179
should proceed : so that the traversing a distance
of seven or eight inches is to these animals a
very good day's journey, even in summer. But
a mode of travelling rather more expeditious
than this is occasionally resorted to. It con-
sists of a succession of somersets : the hvdra,
while adhering firmly by the mouth, detaches
its foot, and, making it describe a semi-
circle, throws it over its head, and places it
foremost in the line of progression. Having
attained this situation, the foot is then fixed,
and a similar semi-revolution is performed by
the head, the body continuing all the while
elongated.
By these and other manoeuvres these animals
contrive to walk with equal facility in any
direction, either on the bottom or sides of the
vessel, or along the stems of aquatic plants, to
which they are most frequently found attached.
The position in which they appear to take most
delight, is that of remaining suspended from the
surface of the water by means of the foot alone ;
and this they effect in the following manner.
When the flat surface of the foot is exposed for
a short time to the air, above the surface of the
water, it becomes dry, and in this state exerts
a repulsive action on the liquid : so that when
dragged below the level of the surface by the
weight of the body, it still remains uncovered,
and occupies the bottom of a cup-shaped hollow
180 THE MECHANICAL FUNCTIONS.
in the fluid, thereby receiving a degree of buoy-
ancy sufficient to suspend it at the surface. The
principle is the same as that by which a dry
needle is supported on water in the boat-like
hollow which is formed by the cohesive force of
the liquid, if care be taken to lay the needle
down very gently on the surface. If, while the
hydra is floating in this manner, suspended by
the extremity of the foot, a drop of water be
made to fall upon that part, so as to wet it, this
hydrostatic power will be destroyed, and the
animal will immediately sink to the bottom.
While in this state of suspension from the
surface, the hydra is capable of performing
several curious evolutions, and with the assist-
ance of the tentacula, by which it lays hold of
objects within its reach, is able to cross over
from one side of the vessel to the other. It does
not appear that these animals ever employ the
tentacula as instruments for swimming ; but they
frequently use them as cables, or anchors, to
enable them to retain their positions in security,
however violently the water may be agitated.
Great use is also made of the tentacula as organs
of prehension for seizing and detaining their
living prey, and for conveying it to the mouth,
where it is quickly swallowed. On the other
hand, when alarmed, or exposed to irritation,
the hydra suddenly shrinks, by the gradual
contraction of all the tentacula, and of the body
also, into a small globule, which might easily
HYDRA. 181
escape notice, unless its previous situation were
accurately observed.
It might be asked by what power is this
animal, occupying so low a place in the scale
of organization, enabled to perform these ac-
tions ? To this question, however, no satisfac-
tory answer has yet been given. The substance
of the hydra, when examined by the microscope,
appears to be nearly homogeneous, except that a
number of grains are intermixed with the pulpy
and gelatinous matter composing the principal
bulk of the body. These grains, when pressed
out of the flesh into water, are scattered indis-
criminately ; and appear to have been united
in the living animal, by means of this glutinous
material.
No perceptible fibres, either muscular, or of
any other kind, can be detected in the flesh of
the polypus : nor is there the least indication
of the formation of transverse rings, similar to
those which exist in worms, and which, in these
latter animals, contribute to progressive motion.
Every portion of the substance of the body is
equally irritable and contractile, and its move-
ments appear to be governed by some voluntary
power belonging to the animal, and directed to
the attainment of certain ends. The softness
and pliancy which it possesses allow of its being
closely fitted to all the inequalities of the sur-
face of the bodies to which it is applied ; and
perhaps this cause alone occasions it to adhere
182 THE MECHANICAL FUNCTIONS.
with great force to these bodies, without the aid
of any glutinous fluid. A conjecture, which has
much appearance of probability, has been
offered, that this power of adhesion is derived
from the presence of a great number of exceed-
ingly minute disks, interspersed over every part
of the surface, constituting so many suckers,
and resembling, though on a very diminutive
scale, the sucking apparatus on the arms of the
cuttle-fish.
The Zoanthus (Fig. 58) belongs to a tribe of
larger polypi, which are generally met with
assembled in clusters ; on which account it is
termed by Ellis the Actinia sociata, or cluster-
animal flower. It consists of a globular body,
having a mouth surrounded by one or two rows
of tentacula ; and connected below with a firm
and fleshy tube, which adheres strongly to the
rocks at the bottom of the sea ; so that it remains
permanently fixed in the same place.
The genus Vorticella is formed by a small
tribe of animals, which, although they have
been usually included under the present order,
differ from Polypi in having no tentacula, but
only cilia, surrounding the margin of a bell-
shaped body, which is mounted upon a long
and slender foot-stalk (Fig. 77).* Currents are
* They also differ from Polypi in having a distinct intestinal
canal, with numerous stomachs.
INFUSORIA. 183
as usual, excited by the vibrations of the cilia ;
which in the simpler species, such as the
77 Vorticella cyathina, here deli-
neated, are the efficient in-
struments of progressive mo-
tion. When attached by its
foot, the vorticella advances
in search of food, by the ex-
tension of the foot-stalk into
a straight line ; but quickly
retreats from danger, by suddenly throwing
it into spiral folds. Many of the species of
vorticellae are so exceedingly diminutive as to
be imperceptible without the aid of the micro-
scope. They conduct us, therefore, by a natural
gradation, to the next order we have to notice,
and which is composed wholly of microscopic
animals.
§ 4. Infusoria.
The Infusory animalcules, or Infusoria, were so
named by Muller, a Danish naturalist, from the
circumstance of their swarming in all infusions
of vegetable or animal substances which have
been kept for a sufficient time. They are, in
general, far too minute to be perceptible to the
naked eye : it is to the microscope alone,
therefore, that we owe our knowledge of their
134 THE MECHANICAL FUNCTIONS.
existence, and of the curious phenomena they
present : yet even the hest instruments afford us
but little insight into their real organization and
physical conditions. On this account it is ex-
tremely difficult to assign their true place in the
scale of animals. By most systematic writers
they have been regarded as occupying the very
lowest rank in the series, and as exemplifying
the simplest of all possible conditions to which
animal life can be reduced. Monads, which are
the smallest of visible animalcules, have been
spoken of as constituting " the ultimate term of
animality ;" and some writers have even ex-
pressed doubts whether they really belong to the
animal kingdom, and whether they should not
rather be considered as the elementary molecules
of organic beings, separated from each other by
the effects of chemical decomposition, and re-
taining the power of spontaneous, but irregular
and indeterminate motion. It was conceived that
all material particles belong to the one or the
other of two classes ; the first, wholly inert, and
insusceptible of being organized ; the second,
endowed with a principle of organic aptitude, or
capability of uniting into living masses, and con-
stituting, therefore, the essential elements of all
organization. According to this view, all vege-
tables or animals in existence would be mere
aggregations of infusory animalcules, which gra-
dually accumulate by continual additions to
INFUSORIA. 185
their numbers, derived from organic matter
in the food : so that the body of man himself
would be nothing more than a vast congregation
of monads !
This bold and fanciful hypothesis, devised by
Buffon, and recommended by its seductive ap-
pearance of simplicity, as well as by the glow-
ing style and brilliant imagination of its author,
has had many zealous partisans. The new
world, which was disclosed to the wondering
eyes of naturalists by the microscope, abounding
in objects and in phenomena of which no con-
ception could have been formed previously to
the invention of that instrument, was peculiarly
calculated to excite curiosity, and to inspire the
hope of its revealing the secret of the living prin-
ciple in the arrangement of the atoms of organic
bodies. During the greater part of the last cen-
tury, infusory animalcules were the subject of
frequent and laborious microscopical research,
and gave rise to endless conjecture and specula-
tion as to their origin, their vitality, and their
functions in the economy of nature. Notwith-
standing their minuteness, considerable differ-
ences of organization were perceived to exist
among them : but many naturalists still clung to
the idea that monads, the most diminutive of the
tribe, and whose very presence can be detected
only by the application of the highest magni-
fying powers, are homogeneous globules of living
186 THE MECHANICAL FUNCTIONS.
matter, without organization, but endowed with
the single attribute of voluntary motion : and
even this property was denied to them by some
authors.
All these fanciful dreams have been dispelled
by the important discoveries of Ehrenberg, who
has recently found that even the Monas termo is
possessed of internal cavities for the reception
and the digestion of its food ; and who has ren-
dered it probable that their organization is
equally complex with that of the larger species
of infusoria, such as the Rotifera, in which he
has succeeded in distinguishing traces of a mus-
cular, a nervous, and even a vascular system.
Those animalcules, whose form can be at all
distinguished, exhibit a great diversity of shapes,
and variety of modes of progressive motion.
Many, as the Cyclidium, have the appearance of
a thin oval pellicle, smoothly gliding in all di-
rections through the fluid : some, as the Volvox,
are globular ; others, as the Cercaria, are shaped
like a pear, tapering at one end, and often ter-
minating in a slender tail, so as to resemble a
tadpole. In many, this tail is of great length ; in
some, as the Furcocerca, it is forked ; in others,
it takes spiral turns, like a corkscrew. The
Kerona has processes like horns. The shape of
the Vibrio is cylindrical, and more or less
pointed at one or both ends, like an eel, or a
serpent, which animals it also resembles in its
INFUSORIA.
187
undulatory mode of swimming.* Some, as the
Gonium, have an angular, others, as the Kol-
poda, a waving outline. Some, as the Urceolaria,
present the likeness of a bell or funnel, and ap-
pear to be analogous to the Vorticella, in which
genus they should probably be included.
Forms still more irregular are exhibited by
other infusoria. Of these the most singular is
the Proteus (Fig. 78), which cannot, indeed, be
said to have any determinate shape, for it seldom
remains the same for two minutes together. It
looks like a mass of soft gelly, highly irritable
and contractile in every part ; at one time wholly
shrunk into a ball, at another stretched out into
a lengthened ribbon ; and again, at another
moment, perhaps, we find it doubled upon itself
like a leech. If we watch its motions for any
time, we see some parts shooting out, as if sud-
* Animalcules referable to this genus are met with in great
numbers in blighted wheat, (Fig. 2, page 62) in sour paste, and
in vinegar which has lost the whole of its alcohol. In this last
fluid they sometimes attain so large a size as to be visible to the
naked eye.
188 THE MECHANICAL FUNCTIONS.
denly inflated, and branching forth into star-like
radiations, or assuming various grotesque shapes,
while other parts will, in like manner, be as
quickly contracted. Thus the whole figure may,
in an instant, be completely changed, by meta-
morphoses as rapid as they are irregular and
capricious.
The Volvox giobator (Fig. 79) is found in pro-
digious numbers at the surface of many stagnant
pools. Its figure is perfectly spherical ; and
its movements consist in a continual and rapid
rotation round its axis, frequently remaining all
the while in the same spot. Another species,
the Volvox conflict or, moves by turning alter-
nately to the right and to the left.
The progressive movements of infusory ani-
malcules are of two kinds, the one consisting in
a smooth and equable gliding through the fluid,
produced apparently by the vibrations of cilia,
which are set on various parts of the body, and
often seem to cover the whole surface : the other,
more rapid and energetic, when the animalcule
darts forward in a particular direction, as if in
pursuit of prey, and proceeds by sudden and
irregular starts, like a vivacious insect or fish.
The voluntary nature of their motions is evident
from the dexterity they display in avoiding ob-
stacles, while swimming together in myriads in
a single drop.
The great agent in the movements of the
animal frame being the muscular fibre, it was
WHEEL ANIMALCULES.
1 89
natural to suppose that a texture analogous to
that of muscles might exist in these latter genera
of infusoria. It was not till very recently, how-
ever, that the actual presence of contractile fibres
could be recognised. But this problem has at
length been solved by the discoveries of Ehren-
berg, who, in his observations of the larger and
more highly organized species belonging to the
order of Rotifera, has, with a magnifying power
of 380, distinctly seen muscular bands running
in pairs between the two layers of transparent
membrane which envelope the body. When the
animalcule throws itself into its violent lateral
contortions, these fibrous bands are observed to
become broader and thicker, as well as shorter, ■
on the side towards which the contractions take
place. There can, therefore, be no doubt that
these are muscular organs, and that they are the
real agents by which the motions witnessed are
effected.
These Rotifera, or wheel animalcules, are so
R named from their being provided
with an apparatus for creating a
perpetual eddy, or circular cur-
rent, in the surrounding fluid.
The remarkable organs, by which
this effect is produced, are gene-
rally two in number,(Fig.80,R,R)
and are situated on the head, but
do not surround the opening of
the mouth, as is the case with the tentacula of
190 THE MECHANICAL FUNCTIONS.
polypes. They consist of circular disks, the
margins of which are fringed with rows of cilia,
bearing a great resemblance to a crown wheel.
This wheel appears to be incessantly revolving,
and generally in one constant direction ; giving
to the fluid a rotatory impulse, which carries
it round in a continual vortex. The constancy
of this motion would seem to indicate that it is
related to some function of vital importance,
such as respiration. But even considered as
a mechanical action, which is the view we have
now to take of it, this phenomenon is of a nature
to excite much curiosity ; for the continued re-
volution round an axis of any part or appendage
to the body, is quite inconsistent with any
notion we can form of the solid organic attach-
ment of such appendage ; and we can have no
conception of organization extending through
the medium of a fluid, or of any substance,
which, like a fluid, admits of the continual
displacement of its parts. Mr. Dutrochet has
offered an ingenious solution of this difficulty.
He suggests that the revolution of the wheels of
the Rotifera may not be real, but apparent only.*
The indented margin of each wheel being com-
posed of a material so exceedingly flexible as to
be capable of assuming quickly all kinds of cur-
vatures, may be conceived to be thrown into
undulations, which follow one another round the
* The same opinion was advanced long ago by Vicq d'Azyr.
WHEEL ANIMALCULES. 191
circumference ; each part, in succession, be-
coming alternately convex and concave, and
thus producing the appearance of the actual
advance of the portions that are raised ; while
their real motions are only those of elevation and
depression, by the alternate elongation and con-
traction of their perpendicular fibres.
Besides possessing extensive powers of loco-
motion, the infusoria manifest in several of
the vital functions, as we shall hereafter find, a
degree of complication, which appears to entitle
them to a higher station in the animal scale,
than that which most naturalists have assigned
to them. They are certainly superior to the
sponges or the polypi, doomed by nature to be
permanently fixed, like plants, to the same spot ;
and of which, if we consider them as compound
beings, the individual animals are often so mi-
nute as to be scarcely visible without the aid of
the microscope. Mere size, indeed, is of all the
circumstances attendant on organized beings,
that which should least be assumed as the crite-
rion of complication or refinement of structure.
An object is great or small, only in relation to
the standard of our own limited and imperfect
senses ; but with reference to the operations of
creative power, all such distinctions must vanish.
There is not, as far as we have the means of
judging, in the colossal fabric of the elephant, any
structure more complicated than exists in the
minutest insect that crawls unheeded at our feet.
192
THE MECHANICAL FUNCTIONS.
§ 5. Acalephce.
Floating masses of living gelatinous matter
are met with in every part of the ocean ; often
in vast numbers, and of various forms ; and hav-
ing but little the appearance of belonging to the
animal kingdom . They compose the order Aca-
lephce, of which the
Medusa (Fig. 81) may
be taken as the type.
They appear, from
their organization, to
be raised but a single
step above polypi ; and
in point of activity and
locomotive powers, they
rank among the lowest
of those Zoophytes
which are not permanently fixed to the spot
where they were first developed. They are
almost wholly passive beings, floating on the
surface of the sea, or remaining at a small depth
below it, carried to and fro by the motion of
every tide and current, and destined to be the
unresisting prey of innumerable tribes of animals
which people every part of the ocean.
The usual form of a Medusa is that of a hemis-
phere, with a marginal membrane, like the fold
MEDUSA. ID.)
of a mantle, extending loosely downwards from
the circumference ; together with a central pe-
dicle descending from the lower surface, like the
stalk of a mushroom, and terminating below in
several fringed laminae, or processes, which have
sometimes been denominated tentacula.
The whole substance of the body of these
medusas is semi-transparent and gelatinous, with-
out any distinct fibrous structure ; yet it has
considerable elasticity, and possesses also some
degree of contractile power. The animal is
seen alternately to raise and depress the mar-
gin of its hemispherical body, and to flap with
the fringed membrane, or mantle, which descends
from it, in a manner somewhat similar to the
opening and shutting of a parasol. This pulsa-
tory movement is performed about fifteen times
in every minute, with great regularity : and by
the reaction of the water, the animal is sus-
tained at the surface ; or by striking the water
obliquely, it may even perform a slow lateral
movement. They descend in the water by
simply contracting their dimensions in every
direction. Sometimes, in order to sink more
quickly, they turn themselves over, so that their
convex part is undermost.
Medusae are met with of very various sizes ;
the larger abound in the seas around our coast ;
but immense numbers of the more minute and
often microscopic species occur in every part of
vol. i. o
194
THE MECHANICAL FUNCTIONS.
the ocean.* In some parts of the Greenland
seas they swarm to such an extent that they
give a visible tinge to the colour of the waves for
hundreds of miles. The total number of these
animals dispersed over that space surpasses the
utmost stretch of the imagination. In these
situations a cubic foot of water, taken indiscri-
minately, was found by Mr. Scoresby to contain
above 100,000 of these diminutive medusae.
Belonging to the tribe of Medusariae is a sin-
gular genus, denominated the Bero'e, (Fig. 82
and 83,) which is remarkable for its organs of
progressive motion. Its body is either globular,
or oblong, and it swims with its axis in a vertical
position. Eight longitudinal bands or ridges,
which have been sometimes compared to ribs,
* The luminous property of sea water, or its phosphorescence,
as it is sometimes called, generally arises from the presence of
minute medusa?, which are met with in greatest numbers at the
surface, being specifically lighter than the surrounding fluid.
BEROE. 19-5
extend down its sides, like those of a melon ;
and along each of these is attached a set of little
membranes, extended horizontally, and sup-
ported on radiating fibres ; so that they bear a
pretty exact resemblance to the fin of a fish.
Their action is not unlike that of the wings of a
bird ; for they are made to flap up and down,
striking the water vertically, and communicating
an ascending impulse to the body. This animal
is also provided with two very long and slender
processes, which come out from the sides of the
body, and from these a great number of still
finer filaments, or cilia, proceed : the whole
apparatus is highly sensitive and irritable, and
on the slightest touch the filaments are thrown
into spiral coils, and retract rapidly within the
body. They thus act the part of tentacula, or
delicate organs both of touch and of prehen-
sion.* It was observed by Fabricius, that when
a Beroe is cut into many pieces, each piece con-
tinues to live, and to swim about by the action
of the cilia, which still continue their vibratory
motions.
In two other genera of Acalephae, the Porpita
and the Velella, provision is made for the me-
chanical support of the soft gelatinous mass, by
means of an internal cartilage. In the former,
this cartilage is of a circular form ; in the latter,
* See a description of the Beroe jiilens, by Dr. Grant, in the
Transactions of the Zoological Society of London, vol. i. p. 9.
196 THE MECHANICAL FUNCTIONS.
(Fig. 84), it is oval, and bears upon its upper
edge a thin pellucid membrane of a triangular
shape, which extends the whole length of the
upper surface of the body. As this membrane
is connected with the cartilage at its middle part
only, while its edges are loose and floating, it is
peculiarly adapted, when above the surface of
the water, to catch the wind and act as a sail.
Such, indeed, appears to be the purpose for
which it was given to the animal ; enabling it to
steer its course by means of the loose edges, and
also of the tentacula, which extend from the
lower side of the body, and act as a rudder,
while the sail is impelled by the wind.
A construction still more artificial is provided
in another family of the same order, denomi-
nated the Physalida, or Hydrostatic Acalephce.
They have attained this latter appellation from
their being rendered buoyant by means of
vesicles filled with air, which enable them to
float without the necessity of using any exertion
for that purpose. The Pkysalia, or Portuguese
Man-of-War, as it is called, (Fig. 85,) is fur-
nished with a large air-bladder, of an oval
shape, placed on the upper part of the body ;
and also with a membrane of a beautiful purple
colour, which, as in the Velella, serves as a sail.
These Zoophytes are met with in great numbers
in the Atlantic Ocean, and more especially in
its warmest regions, and at a considerable dis-
ACTINIA. 197
tance from land. In calm weather they float
on the surface of the sea, rearing their purple
crests, and appearing at first like large air
bubbles, but distinguishable by the vivid hues
of the tentacula which hang down beneath them.
Nothing can exceed the beauty of the spectacle
presented by a numerous fleet of these animals,
quietly sailing in the tropical seas. Whenever
the surface is ruffled by the slightest wind, they
suddenly absorb the air from their vesicles, and
becoming thus specifically heavier than the
water, immediately disappear, by diving into the
still depths of the ocean. By what process
they effect these changes of absorption and of
reproduction of air yet remains to be discovered.
Other genera, as the Physsophora, have several
of these air-bladders ; but in other respects
resemble the ordinary Medusa?, in having no
membranous crest.
The ActinicB are a tribe of Zoophytes, which,
from the general resemblance of their forms to
those of Polypi, are by most naturalists in-
cluded under that order. But they exhibit a
much greater developement in their organiza-
tion ; having very distinct muscular fibres, en-
dowed with strong powers of contraction. Their
digestive organs, also, as I shall have afterwards
occasion more fully to notice, are constructed
upon a more complicated plan than in the
polypus. Fig. 86 exhibits an Actinia in its
IT/8 THE MECHANICAL FUNCTIONS.
contracted state. When their tentacula, which
surround the mouth, and are very numerous, are
fully expanded, (as shown in Fig. 87,) these
animals present a striking analogy of form to
many of the compound flowers; and accordingly
the particular species are named from these re-
semblances, the sea-anemone, the sea-marygohl,
the sea- carnation, the san-floiuer, daisy, &c. Ac-
tiniae are seen in great numbers on many shores,
adhering by their flat surfaces to rocks, and
being generally permanently fixed to their abode.
When the weather is fine, and the sea calm, it
is very amusing to watch the rapid expansions
and retractions of their many coloured tentacula,
while they are moving in search of food : to ob-
serve the quickness with which they seize on
whatever prey comes within their reach, and to
notice the suddenness with which they collapse
into a round contracted mass, on receiving the
slightest injury.
Yet these animals are not of necessity con-
fined to the particular spots where we see them
fixed ; for they are capable, when disturbed, of
seeking, by a slow progressive motion, a more
ECHINODERMATA.
199
secure abode. Reaumur has minutely examined
the arrangements of their muscular fibres, and
has described the actions by which they either
attach themselves to the surfaces of rocks, or
effect their sluggish movements.*
§ 6. Echinodermata.
Ascending in the scale of organization we come
to the Echinodermata, a class which compre-
hends the families of the Asterida, the Echinida,
the Holothurida, and the Crinoidea, together
with other tribes of less note.
These animals, both in their general form,
and in the arrangement of their internal organs,
* Memoires de l'Academie des Sciences, 1710, p. 490.
200 THE MECHANICAL FUNCTIONS.
retain, in a very marked manner, the radiated
disposition so characteristic of Zoophytes : for
we find all their parts symmetrically arranged
either in lines, or in compartments, which pro-
ceed from a common centre, or axis, and which
are repeated, in regular succession, all round the
circumference (See Fig. 88 to 94). Besides an
external horny, or semi-calcareous covering, there
is also provided, for the support of the softer parts,
a kind of internal skeleton, or jointed frame- work.
The organs in the interior of the body are farther
supported by membranous walls, which impart
mechanical firmness to the fabric.
The Asterias, or star-fish (Fig. 88), is so named
from its star-like form ; and the number of rays
composing the star is generally five. Besides
the tough coriaceous integument, which protects
the mass of the body, each ray is farther sup-
ported by a series of calcareous pieces, resem-
bling those which compose the spinal column of
vertebrated animals, and forming an articulated
axis, constructed with the evident design of com-
bining the greatest strength with a proper degree
of flexibility. Cartilaginous plates are also added
for the more special support of the integument.
This integument itself is irritable, and has the
power of changing its form, although the mus-
cular fibres by which its motions are effected are
not easily distinguished. Calcareous grains, of
ASTERIAS.
201
a solid consistence, are thickly interspersed
throughout its texture ; and these, in various parts
of the body, both in the upper and the under side,
often project from the surface in the form of
spines or prickles. They are particularly large
around the mouth of the animal, which opens at
the centre of the under side. These calcareous
masses have a crystalline arrangement, and ex-
hibit on fracture the exact oblique angles cha-
racteristic of the primitive rhomboid of carbonate
of lime.
The under side of each ray (Fig. 95) has a
96 h&^a&ms&^l 97
groove termed, by Linneus, the ambulacrum, or
avenue, a name which it has received from its
fancied resemblance to a walk between rows of
trees : for each groove contains a quadruple row
of perforations, like pin holes, through which
small fleshy cylindrical processes pass. These
processes extend but a short distance from the
surface ; but they admit of being elongated, or
retracted, at the pleasure of the animal, by a
very curious mechanism, which I shall presently
describe. By bending them on either side, in
202 THE MECHANICAL FUNCTIONS.
their expanded state, the Asterias is capable of
effecting a slow progressive motion ; so that these
processes may be regarded as corresponding to
feet, being levers for the advance of the body.
This, it may be remarked, is the first time that
we meet with organs of that description in our
progress through the animal kingdom. Each of
these feet is terminated by a concave disk, which
when applied to any flat surface acts as a sucker,
on the principles already adverted to.* Reau-
mur counted 304 of these feet in each of the five
rays of the star-fish, making 1520 in all.f Each
foot consists of a tube, closed at the outer end,
and the stem of which, after passing through
the aperture in the integument, is dilated into a
bag or reservoir of fluid ; as is shown in Fig. 97.
By the contraction of this reservoir, the fluid it
contains is propelled into the outer portion of the
tube, which protrudes by being thus distended ;
the foot fixes itself by means of its terminal
fleshy disk to the point it touches, and then, by
retracting, draws the body along for a short dis-
tance. By the retreat of the fluid into its reser-
voir, the foot is again detached, and ready to be
moved forwards ; and is thus made instrumental
in taking another step, by a repetition of the same
* Page 137.
f Memoires de l'Academie des Sciences, 1710, p. 487.
ECHINUS. 203
process.* From the shortness of these feet,
notwithstanding their great number, the advance
which this animal can make in any particular
direction is excessively slow.
Besides this movement of creeping, the Asterias
is capable of bending and unbending each of its
rays ; actions, however, which it can perform
but very slowly, and not to an extent sufficient
to accomplish its removal from one place to ano-
ther.!
The skeleton of the Echinus or sea urchin,
(Fig. 91), is still more artificially framed than
that of the Asterias. It has a spheroidal form,
like that of an orange ; the calcareous material
* The mechanism by which the feet are protruded and re-
tracted is illustrated by the diagram, Fig. 97, which exhibits the
bladders connected with them, in different states of distention
and contraction. Fig. 96 shows the upper side of the ambu-
lacra, and of the bladders connected with the feet. Dr. Grant,
from some observations which he made on the structure of the
cilia of the Beroe pileus, is led to suspect that the rapid vibra-
tions of these singular organs in the lowest animals may depend
on the undulations of water conveyed through elastic tubes along
their bases, in a manner resembling the injection of the tubular
tentacula of Actiniae and Asterise. If this conjecture were veri-
fied, he remarks, one of the most remarkable phenomena of
animal motion, though one of the most frequent, would lose
much of its present marvellous character.
f In addition to these larger tubes, there exists also a smaller
set, which pierce the skin in different places, and are channels for
the absorption of the water used in respiration. These I shall
have occasion to notice more particularly hereafter.
204
THE MECHANICAL FUNCTIONS.
employed in its construction, instead of forming
isolated grains, is accumulated and extended
into polygonal plates
(Fig. 98), the edges
of which are dove-
tailed into each other.
The form of each
piece is that of a
lengthened hexagon ;
and the whole are regularly arranged in rows,
like a mosaic or tesselated pavement. Ambu-
lacra are also seen on the surface of the shell,
passing vertically down the sides of the sphere,
similar to the meridians of a globe ; and con-
taining, like those of the Asterias, a double row
of perforations.*
On the outer spherical surface of the ex-
ternal crust, there are formed a great number
of calcareous tubercles, arranged with beautiful
regularity and symmetry in double lines, pass-
ing, like meridian circles, from the upper to the
lower pole of the sphere. Each appears, when
magnified, to be a smooth and solid ball, pro-
jecting from the surface of one of the polygonal
plates of the crust. These balls serve for the
* An architecture of a still more curious description is exhi-
bited in the calcareous frame-work which has been provided for
the support of the teeth, and other organs of mastication, with
which this animal is furnished. The structure of these organs
will be noticed when treating of that function.
ECHINUS. 205
support of the spines,* which have grooves or
sockets at their base, allowing of their accurate
application to the spherical surface of the tu-
bercles. They thus constitute ball-and-socket
joints, allowing of free motion in all directions.
Each joint is connected with the plate on which
it turns, by means of the integument, which
acts the part of a capsular ligament ; and sets
of radiating muscular fibres are provided for
effecting the movements of the spines. By
employing these spines as levers, the Echinus
advances with great facility along plane sur-
faces at the bottom of the sea. This animal
is also aided in its progressive motion by the
employment of suckers, which are placed at the
end of the slender tubes, protruding from the
pores of the ambulacra, and analogous to those
of the Asterias.
The Spatangus, a genus belonging to this order,
buries itself in the sand by the action of its
spines, which on its under surface are short,
thick, and expanded at the ends, like the handle
of a spoon, with the convexity downwards ; and
which have a limited rotatory motion. Those
which grow from the sides are more slender,
* It has been ascertained by Mr. Haidinger, that the struc-
ture of these spines is crystalline, and that their cleavage pre-
sents the exact rhomboidal angles characteristic of carbonate
of lime. See his translation of Mohs's Mineralogy, vol. ii.
p. 91.
206 THE MECHANICAL FUNCTIONS.
and taper towards the extremities, and when
not in use they fall flat upon the body with
their points directed backwards. Besides these,
there are a few longer bristles, arranged in a
crescent on the back, and converging till their
points meet, but capable of being erected to
a perpendicular position. The animal, when
placed on sand, commences its operations by
revolving the lower spines, thus soon creating
a hollow quicksand, into which it sinks by its
own weight so far as to enable the lowest of
the lateral spines to cooperate with them, by
scattering and throwing up the loosened par-
ticles ; while these, at the same time, contribute,
by their reaction, still farther to depress the
body. As the animal sinks, a greater number
of spines are brought into action, and its pro-
gress becomes more rapid ; while the sand,
which had been pushed aside, flows back, and
covers the body, when it has sunk below the
level of the surface. In this situation the long
dorsal bristles come into play, preventing the
sand from closing completely, and preserving a
small round hole for the admission of water to
the mouth and respiratory organs.*
Whenever, in following the series of organic
structures, new forms are met with, we always
* The account here given is taken from Mr. Osier's papers in
the Philosophical Transactions for 1826, p. 347.
ECHINUS. 207
find them accompanied by corresponding modi-
fications in the processes of developement. The
organization of the animals belonging to the
lowest division of the series is not sufficiently
perfect to afford the means, which are supplied
in the higher animals, of removing or modifying
the substances that have at any time been de-
posited, and suffered to harden. Hence the
structures composed of these substances remain
unchanged during the life-time of the animal,
although they may continue to receive additions
of new layers of the same material, deposited
on their surface by the soft parts in contact with
them ; for it is through the medium of the soft
parts alone that these materials are supplied.
All the solid structures of zoophytes are formed
by this process, and they are subjected to all
the consequences of this law of increase. As
these consequences are important in their rela-
tion to the conditions of growth, and to the forms
which result, it will be necessary to direct our
attention to them more particularly.
The influence, which this mode of increase
by superficial depositions may have, in changing
the form of the original structure, will depend
altogether upon the relative situations of the soft
secreting organ, and the hard part on which
it is to deposit new layers : for, as every new
layer must occupy the situation of the soft organ
which has formed it, it must displace the latter,
208 THK MECHANICAL FUNCTIONS.
and push it back for a space equal to its own
thickness. In process of time, the addition of nu-
merous layers having led to successive encroach-
ments of the solid substance, the latter will have
been displaced to an extent which must sooner
or later become sensible. If the soft organs have
sufficient room for their expansion, as is the
case when they are external to the hard axis
of the zoophyte, the growth of that axis may go
on without impediment ; and no change need
take place in the general figure of the parts,
since their relative proportions and situations
may be preserved unaltered. But this cannot
happen when the new materials are to be de-
posited on the internal surface of a membrane,
or a shell, which completely encloses the soft
parts : for the additions thus made to the thick-
ness of the layer must encroach upon the space
within ; and, that space being limited, the soft
parts contained in it will not merely cease to
grow, but will be actually contracted in their
dimensions : and if the process of deposition
were to go on, the space occupied by the soft
organs would at last be entirely filled up with
solid matter, and the cavity be obliterated. Ac-
cordingly it is necessary, whenever cells, in-
tended for the lodgement of soft organs, are to
be constructed of hard materials, that the foun-
dation of these cells should be laid, and their
construction begun, upon a scale of the same
ECHINUS. 200
size as that which they are intended to have
at all future periods; because, as we have just
seen, after the innermost layer has been depo-
sited, they admit not of any future enlargement
of their cavity. Thus we find that, in the case
of polypes which are lodged in cells, the walls
of these cells must be completed before the soft
polypous portion has attained its full expansion ;
for were it at first built of a smaller size, propor-
tioned to that of the young polype, it would
prevent all further growth.
The globular shell of the Echinus, which is
external to the soft parts that nourish it, and
which yet grows from a very minute sphere
to one of large dimensions, keeping pace with
the gradual expansion of the internal organs,
might appear to be an exception to the general
law. Nature has, however, accomplished her
purpose without deviating from her usual plan ;
first, by dividing the shell of the Echinus into
a great number of small pieces ; and secondly,
by giving to each piece the polygonal form,
which is best adapted to their mutual and per-
fect junction, without leaving any intervening
spaces. Thus has she provided for the enlarge-
ment of the whole structure, by admitting of
additions being made to the margins of each
of the separate polygonal pieces ; fresh layers
of calcareous substance being deposited on the
under side, and on the edges of each, in pro-
VOL. i. p
210 THE MECHANICAL FUNCTIONS.
portion as the expansion of the contents of the
shell causes their separation. That such a suc-
cession of deposits has taken place, may easily
be seen, by minutely examining the texture
of the plates, which will be found marked by
concentric polygonal lines. (Fig. 99.)
The spines of the Echinus must be formed by
the successive deposition of layers on their outer
surface, as appears from the examination of
their structure, when a longitudinal section of
them has been made. The lines exhibiting the
succession of layers are seen in Fig. 100, which
represents such a section. Hence they are pro-
bably deposited by the membrane which covers
them during the whole period of their growth.
There is probably no series of animals that ex-
emplify in so marked a manner as the Echino-
dermata, the gradations which nature has ob-
served in passing from one model of construction
to another of a totally different aspect, through
every intermediate form. What shapes can be
more diversified, and apparently irreducible to a
common standard, than those of the star-like
Asterias, (Fig. 88) of the globular Echinus, (Fig.
91) and of the lily-shaped Pentacrinus ; (Fig. 94)
and yet we find these passing the one into the
other by the most gradual transitions ? Setting
out from the star with five slender rays, which
is the standard form of the Asterias ; we find the
rays, in succeeding species, assuming gradually
GRADATION IN THE ECHINODERMATA. 211
a greater breadth at their base, and their sides
joining at more obtuse angles : the star-like
form is gradually effaced, and the outline is
rather a pentagon, with its sides curved inwards
(Fig. 89). We soon perceive this curvature
giving place to a straight line, so that the shape
becomes an exact pentagon. The next change
effected is in the angles of this pentagon, which
by degrees are lost in a general rounded outline ;
still, however, preserving its flatness. This
stage is attained in the Scutella, and the Cly-
peaster. (Fig. 90.) We next find that, in the
Spatangus, the thickness increases ; though at
first with an oval outline, and with several
changes in the situation of the mouth of the
animal. At length, after passing through many
intermediate steps, we arrive at the perfectly
circular and spheroidal Echinus. (Fig. 91 .) If we
might be permitted to conjecture the objects of
all these changes, which occur in this continuous
gradation, we might not unreasonably suppose
them to be the concentration of the internal
organs into one compact mass, and the retrench-
ment of all the external appendages. It is also
curious to observe, how, amidst all these modifi-
cations, the double rows of perforations, which
constitute the ambulacra, retain their situations,
diverging in five equidistant lines from one of
the extremities of the axis, and winding round
to the other.
212 THE MECHANICAL FUNCTIONS.
Returning to the Asterias, we can trace changes
equally gradual, though in an opposite sense, in
another series, which presents a striking con-
trast with the former. Here, instead of the re-
trenchment of the appendages, we find them
greatly developed, and amplified in every pos-
sible degree. The rays of the Asterias become
narrower, while their length is at the same time
increased ; the vital organs, and also the tubular
feet, are gradually withdrawn from them, and re-
tire within a central disk, to which the slender
rays, now bereft of feet, become mere appen-
dages. Such is the condition of the Ophiura.
(Fig. 92.) By the prolongation and tapering of
these rays to slender filaments, they acquire a
greater prehensile power, and twine with ease
round their prey. We next find their number
augmented ; it is at first doubled, then tripled,
and at length indefinitely augmented. They
also become branched, subdividing by simple
bifurcations, as in the Euryale palmiferum (Fig.
93) ; next into minuter ramifications, as in the
Caput Medusce, where the thousands of filaments
have the appearance of a tangled web, which
defies all attempts at unravelling.
The steps are but short from the Caput Me-
dusae to the Crinoidea, or lily-shaped tribe, (of
which, Fig. 94, representing the Pentacrinus
europaus, is an example) ; for they consist chiefly
MOLLUSCA. 213
in the addition of a jointed stalk, which is made
to proceed downwards from the centre of the
whole assemblage of rays, and which is to serve
as a common stem for sustaining the whole mass ;
while the branches themselves are carried up,
and folded inwards. The lower joint of the foot-
stalk is a little expanded, in order to procure a
more extensive base of support ; and the whole
structure thus presents a remarkable resem-
blance to a liliaceous plant.
Chapter III.
MOLLUSCA.
§ 1 . Molhisca in general.
The series of animal structures, arranged ac-
cording to their mechanical functions, conducts
us next to the Molhisca; an assemblage of beings
which was first recognised as constituting one
of the primary divisions of the animal kingdom
by Cuvier, the greatest naturalist of modern
times. A vast multitude of species, possessing
in common many remarkable physiological cha-
racters are comprehended in this extensive class.
In all, as their name imports, the body is of soft
214 THE MECHANICAL FUNCTIONS.
consistence ; and it is enclosed more or less com-
pletely in a muscular envelope, called the mantle,
composed of a layer of contractile fibres, which
are interwoven with the soft and elastic integu-
ment. Openings are left in this mantle for the
admission of the external fluid to the mouth and
to the respiratory organs, and also for the occa-
sional protrusion of the head and the foot, when
these organs exist. But a large proportion of
the animals of this class are acephalous, that is,
destitute of a head, and the mantle is then occa-
sionally elongated to form tubes, often of con-
siderable length, for the purpose of conducting
water into the interior of the body.
Mollusca, with the exception of a few among
the higher orders, are but imperfectly furnished
with organs of locomotion. The greater number,
indeed, are formed for an existence as completely
stationary as the Zoophytes attached to a fixed
base. The Oyster, the Muscle, and the Limpet,
for example, are usually adherent to rocks at the
bottom of the sea, and are consequently depend-
ent for their nourishment on the supplies of food
casually brought within their reach by the waves
and currents of the ocean. This permanent at-
tachment to the solid body on which they fix
their abode, does not, however, take place till
they have arrived at a certain period of their
growth : for at the commencement of their sepa-
MOLLUSCA. 215
rate existence, that is, immediately after they
are hatched, they are free to move in the water,
and to roam in search of a habitation. In this
respect, therefore, they preserve an analogy with
the gemmules of sponges, and of polypi, which
exercise locomotive powers only in the early
stages of their developement.*
The organization of the Mollusca being un-
fitted for the construction of an internal skeleton,
Nature has ordained that the purposes of mecha-
nical support and protection shall be answered
by the formation of hard calcareous coverings, or
shells, the result of a peculiar process of animal
production. These shells are formed either of
one piece, or of several ; the separate pieces, in
either case, being termed valves; so that shells
* This analogy is strengthened by the circumstance that the
movements of many of these animals, in the first periods of their
existence, are effected by the same mechanism of vibratory cilia
which we found to be instrumental in the progression of the infu-
sory animalcules, and of the young of polypi. On observing the
first evolution of the ova of the Buccinum undatum, Dr. Grant
found them to consist of groups of spherical gelatinous bodies,
which soon become covered on one side with a transparent enve-
lope, the rudiment of the future shell ; while, on the other side,
the gelatinous matter is extended outwards, so as to form the
margin of an internal cavity, of which the entrance is surrounded
with vibratory cilia, and in the interior of which a revolution of
particles is seen, indicating a constant current of fluid. The vi-
brations of these cilia are perceived long before the pulsations of
the heart, and even before any appearance of that organ is visible ;
216' THE MECHANICAL FUNCTIONS.
may be either univalve, bivalve, or multiiahc,
according as they consist of one, two, or more
pieces. Univalve shells have generally more or
less of a spiral form, and are then called turbin-
ated shells. In a few, the cavity of the shell is
divided by transverse partitions into numerous
compartments. Some Mollusca have internal
shells for the defence and support of particular
organs ; and others have shells which are partly
external, and partly internal. As respects their
shape, colour, and appearance, shells admit of
infinite diversity; yet, as will presently be shown,
all are composed of the same kind of material ;
and their production and increase are regulated
by the same uniform laws.
they are, indeed, the first indications of life in the embryo. The
cilia are in activity even before the animal is hatched ; for while
confined within the egg, it is seen almost continually revolving
round its centre ; a motion which appears destined to bring a
constant supply and renewal of sea water into the interior of the
organization, in order to perfect the formation of the shell before
the animal is, as it were, launched into the ocean. Possibly,
also, the continued friction of the cilia against the interior of the
egg may tend to abrade it, and open a passage for the young
animal. No sooner has the animal effected its escape, than
it darts rapidly forwards by the motion of its cilia. The same
appearances have also been observed by Dr. Grant in the young
of different Mollusca, such as the Doris, Eolis, &c. which have
no shell.— Edin. Journal of Science, vol. vii.
MOLLUSCA ACEPHALA.
217
§ 2. Acephala.
The Mollusca which inhabit bivalve shells, such
as the Oyster, the Muscle, and the Cockle, are all
acephalous. The two valves of the shell are
united at the back by a hinge joint, often very
artificially constructed, having teeth that lock
into each other : and the mechanism of this arti-
culation varies much in different species. The
hinge is secured by a substance of great strength.
It is seen in Fig. 101,
which shows the valves
of the XJnio batava, with
the connecting liga-
ment. This ligament is
composed of two kinds
of texture : the one,
which is always exter-
nal, is strictly liga-
mentous ; that is, per-
fectly inelastic : the
other has more of the
properties of cartilage, being highly elastic, and
formed of parallel series of condensed transverse
fibres, directed from the hinge of one valve to the
similar part of the other, and having generally
a deep black colour, and a pearly lustre. The
cartilage is always situated within the ligament ;
218 THE MECHANICAL FUNCTIONS.
sometimes in immediate contact, and forming
with it one and the same mass ; at other times,
placed at a distance, in a triangular cavity,
amongst the teeth of the hinge. The closing of
the valves produces, in all cases, a compression
of the cartilage, the elasticity of which tends,
therefore, to separate the valves from each other ;
that is, to open the shell.
During the life of the animal, the usual and
natural state of its shell is that of being kept
open for a little distance, so as to allow of the
ingress and egress of the water necessary for its
nourishment and respiration. But as a security
against danger, it was necessary to furnish the
animal with the means of rapidly closing the
shell, and retaining the valves in a closed state.
These actions being only occasional, yet requir-
ing considerable force, are effected by a mus-
cular power; for which purpose sometimes one,
sometimes two, or even a greater number of
strong muscles are placed between the valves,
their fibres passing directly across from the inner
surface of the one to that of the other, and firmly
attached to both. —
They are named,
from their office of
bringing the valves
towards each other,
the adductor muscles.
Fig. 102, which re-
presents the section of an oyster, shows the situa-
MOLLUSCA ACEPHALA. 219
tion of the hinge (l), the adductor muscle (a), and
the transverse direction of its fibres, with respect
to the valves. When these muscles are not in
action, the elasticity of the cartilage attached to
the hinge is sufficient to separate the valves ;
but as they were not intended to open beyond a
certain extent, it was necessary to provide some
limitation to the action of the cartilage. The ad-
ductor muscle might, it is evident, be called into
play to counteract that action ; but this would re-
quire a constant muscular exertion, and a great
expenditure, therefore, of vital force. Nature
has always shown a solicitude to economize mus-
cular power, whenever a substitute could be had,
and such a substitute she has here provided, by
uniting with the muscle an elastic ligament, of a
peculiar construction. It has a texture similar to
that of the ligamentum nuch(e, and being placed
on the side of the muscle next to the hinge,
allows the valves to separate to the proper dis-
tance only.* When the animal dies, the mus-
cular force ceases, but the ligament, with which
the muscle is associated, retaining its elasticity,
allows the shell to open, but only to a certain
extent ; and accordingly, this is the state in
which we find bivalve shells that are cast upon
the shore, after the soft flesh of the animal has
* This remarkable structure was first described by Dr. Leach,
in a paper read before the Royal Academy of Paris. Bulletin
des Sciences, 1818, p. 14. See also Gray, in Zoological Journal,
i. 219.
220
THE MECHANICAL FUNCTIONS.
103
decayed and been washed out, provided the car-
tilage and the ligament of the hinge are still
preserved.*
The simple actions of opening and closing
the valves are capable of being converted into
a means of retreating from
danger, or of removing to a
more commodious situation, in
the case of those bivalves which
are not actually attached to
rocks or other fixed bodies.
Diquemare long ago observed
that even the oyster has some
power of locomotion, by sud-
denly closing its shell, and thereby expelling the
* The Pholas is an exception to this rule ; for instead of its
valves being united, as usual, by an elastic ligament, they are
connected chiefly by means of muscles. This departure from
the ordinary structure is probably occasioned by a new condition
introduced into the economy of the animal in consequence of its
being fitted for excavating passages through hard rocks. It is
furnished, for this purpose, with a complicated boring apparatus
moved by many muscles, and requiring great freedom of action.
Fig. 103 represents the shell of the Pholas Candida extremely
expanded, in order to show the hinge, together with the liga-
ment (l) ; the long and thin process of shell (p), to the ends of
which, on each side, a pair of fan-shaped muscles, more particu-
larly employed in boring, are attached ; and the two adductor
muscles (a, a), which retain the valves in contact independently
of the ligaments. For a full description of this apparatus, I
must refer to a paper by Mr. Osier, on burrowing and boring
marine animals, contained in the Phil. Trans, for 1826, p. 342,
from which the above figure has been taken.
MOLLUSCA A( EP1IALA. 221
contained water, with a degree of force, which,
by the reaction of the fluid in the opposite direc-
tion, gives a sensible impulse to the heavy mass.
He notices the singular fact that Oysters, which
are attached to rocks occasionally left dry by the
retreat of the tide, always retain within their
shells a quantity of water sufficient for respira-
tion, and that they keep the valves closed till the
return of the tide : whereas those Oysters which
are taken from greater depths, where the water
never leaves them, and are afterwards removed
to situations where they are exposed to these
vicissitudes, of which they have had no previous
experience, improvidently open their shells after
the sea has left them, and by allowing the water
to escape, soon perish.*
Many bivalve mollusca are provided with an
instrument shaped like a leg and foot, which
they employ extensively
for progressive motion.
Its form in the Cardmm,
or cockle, is seen in Fig.
104. This organ is com-
posed of a mass of mus-
cular fibres, interwoven together in a very com-
plex manner, and which may be compared to
the muscular structure of the human tongue :
the effect in both is the same, namely, the con-
* Journal de Physique, xxviii. 244.
222 THE MECHANICAL FUNCTIONS.
ferring a power of motion in all possible ways ;
thus it may be readily protruded, retracted, or
inflected at every point. The So/en, or razor-
shell fish, has a foot of a cylindrical shape,
tapering at the end, and much more resembling
in its form a tongue than a foot. In some bivalves
the dilatation of the foot is effected by a curious
hydraulic mechanism : the interior of the organ
is formed of a spongy texture, capable of re-
ceiving a considerable quantity of water, which
the animal has the power of injecting into it,
and of thus increasing its dimensions.
The foot of the Mytilus eclulis, or common
muscle, can be advanced to the distance of two
inches from the shell, and applied to any fixed
body within that range. By attaching the point
to such body, and retracting the foot, this animal
drags its shell towards it ; and by repeating the
operation successively on other points of the
fixed object, continues slowly to advance.
This instrument is of great use to such shell-
fish as conceal themselves in the mud or sand,
which its structure is then peculiarly adapted
for scooping out. The Cardium continually em-
ploys its foot for this purpose : first elongating
it, directing its point downwards, and insinu-
ating it deep into the sand ; and next, turning
up the end, and forming it into a hook, by
which, from the resistance of the sand, it is
MOLLUSCA ACEPHALA. 2*23
fixed in its position, and then the muscles which
usually retract it are thrown into action, and the
whole shell is alternately raised and depressed,
moving on the foot as on a fulcrum. The effect
of these exertions is to drag the shell down-
wards. When the animal is moderately active
these movements are repeated two or three
times in a minute. The apparent progress is
at first but small ; the shell, which was raised on
its edge at the middle of the stroke, falling back
on its side at the end of it ; but when the shell is
buried so far as to be supported on its edge, it
advances more rapidly, sinking visibly at every
stroke, till nothing but the extremity of the tube
can be perceived above the sand. Mr. Osier,
who has given us this account,* observes that
the instinct, which directs the animal thus to
procure a shelter, operates at the earliest period
of its existence. The Mya truncate, when fully
grown, will not attempt to burrow ; but on
placing two young ones, which were scarcely
more than a line in length, and apparently but
just excluded, on sand, in a glass of sea-water,
he found that they buried themselves imme-
diately.
By a process exactly the inverse of this, that
is, by doubling up the foot, and pushing with
it downwards against the sand below, the shell
* Philos. Trans, for 1826, p. 349.
224 THE MECHANICAL FUNCTIONS.
may be again made to rise by the same kind of
efforts which before protruded the foot. By this
process of burrowing the animal is enabled
quickly to retreat when danger presses : and
when this is past, it can, with equal facility,
emerge from its hiding place.
The Cardium can also advance at the bottom
of the sea along the surface of the soft earth,
pressing backwards with its foot, as a boatman
impels his boat onwards, by pushing with his
pole against the ground, in a contrary direction.
It is likewise by a similar expedient that the
Soleu forces its way through the sand, expanding
the end of its foot into the form of a club. The
course of these locomotive bivalves may readily
be traced on the sand by the furrows which they
plough up in their progress.
These, as well as many other of the bivalve
mollusca, are enabled by the great size and
flexibility of this organ to execute various other
movements, of which, from the habitual inacti-
vity of animals of this class we should scarcely
have supposed them capable. The Tellina is
remarkable for the quickness and agility with
which it can spring to considerable distances by
first folding the foot into a small compass, and
then suddenly extending it ; while the shell is
at the same time closed with a loud snap.
The Pinna, or Marine Muscle, when inhabiting
the shores of tempestuous seas, is furnished,
MOLLUSCA ACEPHALA. 22-3
in addition, with a singular apparatus for with-
standing the fury of the surge, and securing
itself from dangerous collisions, which might
easily destroy the brittle texture of its shell.
The object of this apparatus is to prepare a
great number of threads, which are fastened at
various points to the adjacent rocks, and then
tightly drawn by the animal ; just as a ship is
moored in a convenient station to avoid the buf-
feting of the storm. The foot of this bivalve is
cylindrical, and has, connected with its base, a
round tendon of nearly the same length as itself,
the office of which is to retain all the threads
in firm adhesion with it, and concentrate their
power on one point. The threads themselves are
composed of a glutinous matter, prepared by a
particular organ. They are not spun by being
drawn out of the body like the threads of the
silk-worm, or of the spider, but they are cast in
a mould, where they harden, and acquire a cer-
tain consistence before they are employed. This
mould is curiously constructed ; there is a deep
groove which passes along the foot from the root
of the tendon to its other extremity; and the sides
of this groove are formed so as to fold and close
over it, thereby converting it into a canal. The
glutinous secretion, which is poured into this
canal, dries into a solid thread ; and when it has
acquired sufficient tenacity, the foot is protruded,
and the thread it contains is applied to the object
VOL. I. Q
226 THE MECHANICAL FUNCTIONS.
to which it is to be fixed ; its extremity being
carefully attached to the solid surface of that
object. The canal of the foot is then opened
along its whole length, and the thread, which
adheres by its other extremity to the large tendon
at the base of the foot, is disengaged from the
canal. Lastly, the foot is retracted, and the same
operation is repeated.
Thread after thread is thus formed, and ap-
plied in different directions around the shell.
Sometimes the attempt fails in consequence of
some imperfection in the thread ; but the ani-
mal, as if aware of the importance of ascer-
taining the strength of each thread, on which its
safety depends, tries every one of them as soon
as it has been fixed, by swinging itself round, so
as to put it fully on the stretch : an action which
probably also assists in elongating the thread.
When once the threads have been fixed, the
animal does not appear to have the power of
cutting or breaking them off. The liquid
matter out of which they are formed is so ex-
ceedingly glutinous as to attach itself firmly to
the smoothest bodies. It is but slowly produced,
for it appears that no Pinna is capable of forming
more than four, or at most five threads in the
course of a day and night. The threads which
are formed in haste, when the animal is dis-
turbed in its operations, are more slender than
those which are constructed at its leisure. Reau-
MOLLUSCA GASTEROPODA.
"I'll
mur, to whom we are indebted for these inte-
resting observations, states also that the ma-
rine muscles possess the art of forming these
threads from the earliest periods of their ex-
istence ; for he saw them practising it, when the
shells in which they were inclosed were not
larger than a millet seed.* In Sicily, and other
parts of the Mediterranean, these threads have
been manufactured into gloves, and other arti-
cles, which resemble silk.
§ 3. Gasteropoda.
The Mollusca which inhabit univalve or turbin-
ated shells belong to the order of Gasteropoda,
and have a more highly developed organization
than the Acephala. The part which performs
the office of a foot is a broad expansion of
fleshy substance, occupying nearly the whole
under surface of the animal, and forming a flat
disk, capable of being applied to the plane
along which it moves.
This is seen in the
Planorbis (Fig. 105, d).
J In some species it is
fashioned into a pro-
* Memoires de l'Academie des Sciences : 171 1, p. 1 18 to 123.
Poli conceived that these threads are dried muscular fibres ; an
opinion which has been adopted by Blainville.
228 THE MECHANICAL FUNCTIONS.
jecting ridge, which cuts its way, like a plough-
share, along the surface on which it moves.
The bands of muscular fibres, which compose
the principle part of its structure, are short,
and are interlaced together in a very intricate
arrangement. All the columns of their fibres
terminate at the surface of the disk ; so that
when the animal is crawling, their successive
actions produce a visible modulatory motion of
that surface. The effect of these actions is that
different parts of the plane on which it moves
are laid hold of in succession, and each corres-
ponding portion of the animal is dragged along,
so that the body advances by a slow and uni-
form gliding motion. The operation of this
mechanism may easily be seen in a snail, by
making it crawl on a pane of glass, and viewing
the movement of its disk from the other side of
the glass : the regular undulations which ad-
vance in the direction of the motion of the snail,
but with twice the velocity, present a curious
and interesting spectacle.
A mucilaginous secretion generally exudes
from the surface of the disk, and tends to in-
crease considerably its power of adhesion, both
when the animal is crawling, and also when it
fixes itself on any surface. In the Patella, or
limpet, this adhesion is greatly favoured by the
conical form of the shell, which, having a cir-
cular base, enables the muscles of the disk, by
GASTEROPODA. 229
their efforts to create a vacuum underneath it,
to command the whole hydrostatic pressure of
the superincumbent water, as well as of the
atmosphere above the water. Besides the mus-
cular bands contained in the substance of the
foot, other sets of fibres are provided for the
purpose of protruding or of retracting the whole
member, and of moving it in different directions.
The foot of the Buccinum undatum, or Whelk,
is capable of great dilatation by means of four
tubes, which open from the surface near the
gullet, and convey into it a large quantity of
water. It may, by this means, be distended to
a size even greater than the shell itself; so that
the opening which it forms in the sand is large
enough to receive the shell, when the latter is
drawn down by the contraction of the muscles
which are attached to the foot.* The foot of the
Scyllcea is grooved, for the purpose of enabling
the animal to lay hold of the stems and branches
of marine plants, and advance along them by a
gliding motion.
The head is generally furnished with tubular
tentacula, which the animal protrudes for the
purpose of feeling its way as it advances, and
which are quickly retracted, by the reversion
of the tube, when they are touched or irritated.
This mechanism is matter of familiar observa-
* Osier, Phil. Trans, for 1826, p. 352.
230 THE MECHANICAL FUNCTIONS.
tion in the tentacula, or horns, of the snail and
of the slug, which are terrestrial mollusca be-
longing to this order. The former of these has
a turbinated shell of the ordinary structure : the
latter, though extremely similar in its internal
structure to the snail, is destitute of any external
shell ; but is furnished, instead of it, with a small
internal plate of cartilage, giving support to some
of the vital organs.
§ 4. Structure and Format ion of the Shells of
Mollusca.
The structure and formation of the shells of mol-
luscous animals is a subject of much interest in
comparative physiology, as presenting many
beautiful illustrations of the laws by which the
inorganic parts of the living system are increased
in their dimensions.
All shells are composed of two portions, the
one consisting of particles of carbonate of lime,
the other having the character of an animal sub-
stance, and corresponding in its chemical pro-
perties either to albumen or to gelatine. The
mode in which these two constituent parts are
united, as well as the nature of the animal por-
tion, differ much in different kinds of shell; and it
is chiefly in reference to these circumstances that
STRUCTURE OF SHELLS. 231
shells have been divided into two classes, namely,
the membranous and the porcellaneous shells.
In shells belonging to the first of these classes,
the carbonate of lime is united with a mem-
branous substance deposited in layers, which
may be separated from one another, either by
mechanical division with a sharp instrument, or
by the slow actions of air, water, or other de-
composing chemical agents. The shells of the
limpet, of the oyster, and of almost all the larger
bivalve mollusca which reside in the ocean are
of this kind. They are usually covered with a
thick outer skin, or epidermis ; and their texture
is of a coarser grain than that of other shells.
If a shell of this description be immersed in
an acid capable of dissolving carbonate of lime,
such as the muriatic or nitric acids properly
diluted, at first a brisk effervescence is produced,
but this soon slackens, and the carbonate of
lime contained in the shell is slowly dissolved ;
the membranous layers being left entire, and
sufficiently coherent to retain the figure of the
shell, but, having lost the earthy material which
gave them hardness, they assume their natural
form of soft and flexible plates.
Many membranous shells exhibit, on several
parts of their internal surface, a glistening, sil-
very, or iridescent appearance.* This appear-
* Examples of this nacreous structure, as it is termed, occur
in the shells of the Haliotis, or Sea-ear, and of the Anodon, or
fresh water muscle.
232 THE MECHANICAL FUNCTIONS.
ance is caused by the peculiar thinness, transpa-
rency, and regularity of arrangement of the outer
layers of the membrane, which, in conjunction
with the particles of carbonate of lime, enter into
the formation of that part of the surface of the
shell. The surface, which has thus acquired a
pearly lustre, was formerly believed to be a pe-
culiar substance, and was dignified with the ap-
pellation of mother of pearl, from the notion that
was entertained of its being the material of which
pearls are formed. It is true, indeed, that pearls
are actually composed of the same materials, and
have the same laminated structure as the mem-
branous shells ; being formed by very thin con-
centric plates of membrane and carbonate of
lime, disposed alternately, and often surrounding
a central body, or nucleus :
but Sir David Brewster has
satisfactorily shown that the
iridescent colours exhibited
by these surfaces are wholly
the effect of the parallel
Wt grooves consequent upon the
^SjSgS^ regularity of arrangement in
the successive deposits of
shell.* The appearance of these grooves or striae
when highly magnified is shown in Fig. 106.|
* Philosophical Transactions for 1814, p. 397.
t See also a paper on this subject by Herschel in the Edin-
burgh Philosophical Journal, ii. 114, from which the annexed
figure is taken.
STRUCTURE OF SHELLS. 23.3
This iridescent property may be communicated
to shell lac, sealing wax, gum Arabic, balsam of
Tolu, or fusible metal, by taking an accurate cast
or impression of the surface of mother of pearl
with any one of these substances.*
Porcellaneous shells have a more uniform and
compact texture than those of the former class.
The animal matter which unites the carbonate
of lime is less in quantity, and not so evidently
disposed in layers; but it is more equally blended
with the earthy particles, with respect to which
it appears to perform the office of a cement,
binding them strongly together, although it has
of itself but little cohesive strength. The Cypreea
and the Volute are examples of porcellaneous
shells.
In shells of this kind the carbonate of lime
assumes more or less of a crystalline arrange-
ment ; the minute crystals being sometimes in
the form of rhombs, and sometimes in that of
prisms. In the former case they are composed
of three distinct layers, as may be seen by
making sections of any of the spiral univalve
shells, or simply by breaking them in various
* When these shells decay and fall to pieces, they separate
into numerous thin scales of a pearly lustre. The fine scales
thus obtained from the Placuna, or window oyster, are employed
by the Chinese in their water-colour drawings to produce the effect
of silver. Some of this powder has been brought to England
and used for this purpose. (Gray; Phil. Trans, for 1833, p. 794.)
234 THE MECHANICAL FUNCTIONS.
directions. Each layer is composed of very thin
plates, marked by oblique lines,
which show the direction of the
crystalline fibres.* The direc-
tion of the layers and fibres is
also rendered manifest by the
planes of cleavage, when they
are broken into fragments. The
plates of the outer and inner
layers are always directed from
the apex of the cone to its base, so as to follow
the direction of the spire : while, on the contrary,
those of the intermediate plate form concentric
rings round the cone parallel to its base. Thus
the fibres of each layer are at right angles to
those of the layer which is contiguous to it ;
an arrangement admirably calculated for giving
strength to the shell, by opposing a considerable
cohesive resistance to all forces tending to break
it, in whatever direction they may be applied.*
We here find that a principle, which has only of
late years been recognised and applied to the
* These lines are shown in the diagram, Fig. 107, which re-
presents a longitudinal section of a shell of this kind. A is the
outer layer, of which the fibres pass obliquely downwards. B is
the middle layer, having fibres placed at right angles with the
former. C is the third, or inner layer, the fibres of which have
a direction similar to the outer layer. Within this layer there is
frequently found a deposit of a hard, transparent, and apparently
homogeneous calcareous material, D. Of this latter substance I
shall afterwards have occasion to speak.
STRUCTURE OF SHELLS. '235
building of ships, namely, that of the diagonal
arrangement of the frame-work, and the oblique
position of the timbers, is identical with that
which, from the beginning of creation, has been
acted upon by nature in the construction of
shells.
When the form of the crystals is prismatic,
the fibres are short, their direction is perpen-
dicular to the surface, and the prisms are gene-
rally hexagonal. This structure is observable
in the Teredo gigantea from Sumatra,* and also
in many bivalves, such as those belonging to the
genera Avicula and Pinna.
When porcellaneous shells are subjected to
the solvent action of acids, the animal matter in
their composition offering but little resistance,
there is a considerable and long continued effer-
vescence. The solution of the carbonate of lime
proceeds rapidly, in consequence of the speedy
disintegration of the animal substance, which is
broken up, and partly dissolved. The remainder
is reduced to minute fragments, which subside
in the form of flakes or scales to the bottom of
the fluid. Poli has given a minute and elaborate
description of the appearances of these fragments
of membrane, when seen under the microscope, f
* In this shell the crystalline appearance is so perfect, that
when some fragments were sent to England, they were mistaken
for a mineral production. (Home ; Lectures, i. 53.)
t See his folio work on the Testacea of the Two Sicilies.
230 THE MECHANICAL FUNCTIONS.
The difference between the textures of these
two kinds of shell is further illustrated by the
impression made upon them by fire. Porcel-
laneous shells, when exposed to a red heat, give
out neither smell nor smoke : they lose, indeed,
their colour, but retain their figure unaltered.
Membranous shells, on the contrary, emit a
strong fetid odour, and become black ; after
which the plates separate, and the structure falls
to pieces.
This variety in the composition and structure
of different kinds of shell is accompanied by
corresponding modifications of their mechanical
properties. The toughness of the fibrous basis
of membranous shells imparts to them greater
strength than is possessed by the porcellaneous
shells, which, in consequence of the tenuity and
uniform intermixture of the animal cement with
the calcareous particles, present a harder and
more transparent, but at the same time more
brittle compound. It is these qualities, together
with their smooth enamelled surface, often beau-
tifully variegated with brilliant colours, and pre-
senting altogether a close resemblance to porce-
lain, that have procured them the name they bear.
When the transparency and brittleness of these
shells are very great, they have been considered
as forming another class, and they have been
termed Vitreous shells, from their making a nearer
approach to glass. Some shells present interme-
FORMATION OF SHELLS. 237
diate textures between the membranous and the
porcellaneous.
All those surfaces of the shell on its outer side
which are not in contact with any part of the
animal, are originally covered with an epider-
mis :* which, however, is frequently rubbed off
by friction.
The process employed by nature for the for-
mation and enlargement of the shells of the
mollusca was very imperfectly understood prior
to the investigations of Reaumur, who may be
considered as having laid the first solid founda-
tions of the theory of this branch of comparative
physiology. t His experimental inquiries have
fully established the two following general facts :
first, that the growth of a shell is simply the
result of successive additions made to its surface ;
and secondly, that the materials constituting
each layer, so added, are furnished by the or-
ganized fleshy substance, which he termed the
skin of the animal, but which is now known by
the name of the mantle, and not by any vessels
or other kind of organization belonging to the
shell itself.
If a portion of the shell of a living snail, for
instance, be removed, which can be done without
injury to the animal, since it adheres to the flesh
* This membrane has been termed the Periostracum.
f Memoires de l'Aeademie des Sciences, 1709, p. 367, and
1716, p. 303.
238 THE MECHANICAL FUNCTIONS.
only in one point, there is formed, in the course
of twenty-four hours, a fine pellicle, resembling a
spider's web, which is extended across the vacant
space, and constitutes the first stratum of the
new shell. This web, in a few days, is found to
have increased in thickness, by the addition of
other layers to its inner surface; and this process
goes on until, in about ten or twelve days, the
new portion of shell has acquired nearly the same
thickness as that which it has replaced. Its
situation, however, is not exactly the same, for it
is beneath the level of the adjacent parts of the
shell. The fractured edges of the latter remain
unaltered, and have evidently no share in the
formation of the new shell, of which the materials
have been supplied exclusively by the mantle.
This Reaumur proved by introducing through
the aperture a piece of leather underneath the
broken edges, all round their circumference, so
as to lie between the old shell and the mantle :
the result was that no shell was formed on the
outside of the leather; while, on the other hand,
its inner side was lined with shell.
The calcareous matter which exudes from the
mantle in this process is at first fluid and gluti-
nous ; but it soon hardens, and consolidates into
the dense substance of the shell. The particles
of carbonate of lime are either agglutinated
together by a liquid animal cement, which unites
them into a dense and hard substance, resem-
FORMATION OF SHELLS. 239
bling porcelain ; or they are deposited in a bed
of membranous texture, having already the pro-
perties of a solid and elastic plate. This explains
the laminated structure possessed by many shells
of this class, such as that of the oyster, of which
the layers are easily separable, being merely
agglutinated together like the component leaves
of a sheet of pasteboard.
It has long been the prevailing opinion among
naturalists that no portion of a shell which has
been once deposited, and has become consoli-
dated, is capable of afterwards undergoing any
alteration by the powers of the animal that
formed it. Very conclusive evidence has, in my
opinion, been adduced against the truth of this
theory, by Mr. Gray.* From a variety of facts,
it appears certain that on some occasions the
molluscous animal effects the removal of large
portions of its shell, when they interfere with its
own growth, or are otherwise productive of in-
convenience. We should at the same time re-
gard these cases in the light of exceptions to the
ordinary rule that a portion of shell once formed
remains ever after unchanged, while it continues
to be connected with the animal which produced
it. In a general way, indeed, we may consider
the connexion between the animal and the shell
as mechanical, rather than vital ; and the shell
itself as an extraneous inorganic body, forming
* Philos. Transactions for 1833, p. 796, et seq.
240 THE MECHANICAL FUNCTIONS.
no part of the living system : for whatever share
of vitality it may have possessed at the moment
of its deposition, all trace of that property is
soon lost. Accordingly we find that the holes
made in shells by parasitic worms are never
filled up, nor the apertures of the cavities so
made covered over, unless the living flesh of the
animal be wounded ; in which case an exudation
of calcareous matter takes place, and a pearly
deposit is produced. The worn edges of shells,
and the fractures, and other accidents which
befall them, are never repaired, except as far
as such repairs can be made by the addition
of materials from the secreting surfaces of the
mantle. It is found that shells may be impreg-
nated with poisonous metallic salts, such as those
of copper, without any detriment to the animals
they enclose.
The power of secreting the materials of shell
does not usually extend to the whole of the sur-
face of the mantle, but is generally confined to
the parts near the margin, composing what is
termed the collar. The calcareous substance is
always poured out underneath the epidermis,*
that is, between this outermost layer of integu-
* Mr. Gray considers the external membrane of the shell, or
epidermis, as formed by the outer edge of the plates of animal
substance, which have scarcely any calcareous matter in their
composition, and which are soldered together into a membranous
coat.
FORMATION OF SHELLS. 24 I
ment, and the subjacent corium, which is incor-
porated with the mantle, and may be regarded
as forming one and the same organ.*
The shape of the shell depends altogether on
the extent and particular form and position of
the secreting organ. The animal, on its exclu-
sion from the egg, has already a small portion of
shell formed ; and the simplest case is that in
which this rudiment of shell is a concave disk.
We may conceive the animal, covered by its
mantle, to expand the border of this organ, and
extend it beyond the edge of the shell, where it
then forms a new layer of shell ; and this new
layer, being applied to the inner or concave sur-
face of the original shell, will, of course, extend
a little way beyond its circumference. The
same happens with the succeeding layers, each
of which being larger than the one which has
preceded it, projects in a circle beyond it ; and
the whole series of these conical layers, of in-
creasing diameters, forms a compound cone, of
which the outer surface exhibits transverse lines,
showing the successive additions made to the
* A secreting power is also, in some instances, possessed by
the foot, as is exemplified in some of the Gasteropoda, where it
forms an operculum, or calcareous covering to the mouth of the
shell. Mr. Gray also ascertained that in the Cymbia, the Oliva,
and the Ancillaria, shell is deposited, and most probably secreted
by the upper surface of the foot, which is very large, and not by
the mantle, which is small, and does not extend beyond the edge
of the mouth. (Phil. Trans, for 1833, p. 805.)
VOL. I. R
242 THE MECHANICAL FUNCTIONS.
shell in the progress of its increase. The Pa-
tella, or limpet, is an example of this form of
structure.
But in by far the greater number of mollusca
which inhabit univalve shells, the formation and
deposition of the earthy material does not, as in
the preceding instance, proceed equally on all
sides. If the increase take place in front only,
that is, in the fore part of the mantle, the conti-
nual deflection thence arising necessarily gives
the shell a spiral form, the coils being simply in
one plane. This is the case in the Planorbis, (Fig.
105, p. 227), the Spirula, and the Nautilus. Most
commonly, however, as in the Buccinum, and
Achatina, (Fig. 108), the deposit of shell takes
place laterally, and more on one side than on the
other ; hence the coils produced descend as they
advance, giving rise to a curve, which is continu-
ally changing its plane, being converted from a
spiral into a helix, a term of Geometry borrowed
from the Latin name of the common snail, which,
FORMATION OF SHELLS. 243
as is well known, has a shell of this form. Fig.
108, which represents the shell of the Achatina
zebra, and of which Fig. 109 shows a longitudi-
nal section, may serve as an example of a shell
of this kind. The axis of revolution is termed
the Columella, and the turns of the spiral are de-
nominated whorls. In consequence of the situa-
tion of the heart and great blood-vessels rela-
tively to the shell, the left side of the mantle is
more active than the right side, so that the lateral
turns are made in the contrary direction, that is,
towards the right.* There are a few species,
however, where, in consequence of the heart
being placed on the right side, the turns of the
spiral are made to the left. Such shells have
been termed sinistral, or reversed shells : but this
left-handed convolution seldom occurs among
the shells of land or fresh-water mollusca.
It results from this mode of formation that the
apex both of the simple and of the spiral cone is
the part which was formed the earliest, and which
protected the young animal at the moment of its
exclusion from the egg. This portion may gene-
rally be distinguished tby its colour and appear-
ance from that which is formed subsequently.
The succeeding turns made by the shell in the
progress of its growth, enlarging in diameter as
* The terms right and left have reference to the position of the
animal when resting on its foot ; the head being of course in front.
See Gray, Zoological Journal, i. 207.
244 THE MECHANICAL FUNCTIONS.
they descend from the apex, form by degrees a
wider base. During the growth of the animal,
as the body extends towards the mouth of the
shell, its posterior end often quits the first turn
of the spire, and occupies a situation different
from that which it had originally. In these
cases the cavity at the apex of the spire is filled
up with solid calcareous matter of a hardness not
inferior to that of marble.
Such is the general form of turbinated shells.
It sometimes happens, however, as in the Conns,
that the upper surface of the spiral scarcely des-
cends below the level of the original portion of
the shell, which, in the former disposition of its
parts, would have been the apex : while the
lower portions of the spiral turns shoot down-
wards, so as to form a pointed process ; thus the
whole is still a cone, but reversed from the
former, the part last formed being the outer sur-
face of the cone, and the circumference of the
apparent base, or flat surface, of which the cen-
tral part is the one first formed.
Various causes may occur to disturb the regu-
larity of the process of deposition, by which the
shell is enlarged in its dimensions ; at one time
accelerating, and at another retarding, or totally
arresting its growth. These irregularities are
productive of corresponding inequalities in the
surface of the shell, such as transverse lines, or
stria. Whenever an exuberance of materials
FORMATION OF SHELLS. 245
has led to a sudden expansion of growth, which
has again soon subsided, a projecting ridge is
produced in the direction of the margin of the
mantle at the time this happens. This change
generally recurs at regular periods, so that these
ridges, or ribs, as they are often called, succeed
one another at equal distances along the course
of the spiral turns.
It not unfrequently happens that, at different
periods, a sudden developement takes place in
particular parts of the mantle, which become in
consequence rapidly enlarged, shooting out into
long slender processes. Every part of the sur-
face of these processes has the power of secreting
and forming shell, so that the portion of shell
they construct, being consolidated around each
fleshy process, must necessarily have at first the
shape of a tube closed at the extremity. As
fresh deposits are made by the secreting sur-
face, which are in the interior of the tube, the
internal space is gradually filled up by these
deposits ; the process of the mantle retiring to
make way for their advance towards the axis of
the tube. In the course of time, every part of
the cavity is obliterated, the process of the shell
becoming entirely solid. Such is the origin of
the many curious projecting cones or spines
which several shells exhibit, and which have
arisen periodically during their growth from
their outer surface. In the Murex these pro-
240
THE MECHANICAL FUNCTIONS.
cesses are often exceedingly numerous, and occur
at regular intervals, frequently shooting out into
various anomalous forms. In many shells of
the genus S trombus these spines are of great
length, and are arranged round the circumfe-
rence of the base, being at first tubular, and
afterwards solid, according to the period of
growth. This is exemplified in the Pterocera
scorpio (Lamarck) of which Fig. 110 shows the
early, and Fig. Ill the later period of growth.
A limit has been assigned by nature to the
growth of molluscous animals, and to the shells
which they form ; and there is a certain epoch
of their existence, when considerable changes
take place in the disposition of the mantle,
and in its powers of secretion. Often we find
it suddenly expanding into a broad surface,
adding to the shell what may be termed a large
lip. Sometimes no sooner has this been accom-
FORMATION OF SHELLS. 247
plished than the same part again shrinks, and
the mantle retires a little way within the shell,
still continuing to deposit calcareous layers,
which give greater thickness to the adjacent
part of the shell ; and at the same time narrow
its aperture, and materially alter its general
shape and aspect. Thus it happens that the
shells of the young and of the old individuals of
the same species are very different, and would
not be recognised as belonging to the same tribe
of mollusca. This is remarkably the case with
the shell of the Cyprcea, or Cowrie, which, in the
early stage of its growth (Fig. 112), has the
ordinary form of an oblong turbinated shell :
but, from the process just described taking place
at a certain period, the mouth of the shell (as
shown in Fig. 113), becomes exceedingly nar-
row, and the edges of the aperture are marked
by indentations, moulded on corresponding pro-
cesses of the mantle.* But in this instance the
change does not stop here ; for both edges of the
mantle next take a wider expansion, turning
over the outer surface of the shell, and passing
on till they meet at the upper convex part, or
back of the shell, forming what has been termed
the dorsal line. They deposit, as they proceed,
* Similar changes occur in the shells of the Ovula (spindles),
Erato (tear-shells), and Marginella, (dates). Gray, Phil.
Trans, for 1833, p. 792.
*248 THE MECHANICAL FUNCTIONS.
a dense and highly polished porcellaneous shell,
beautifully variegated with coloured spots, which
correspond exactly with the coloured parts of
the mantle that deposits them. This new plate
completely envelopes the ori-
ginal shell, giving it a new
covering, and disguising its
former character. A trans-
verse section (Fig. 114) at
once shows the real steps by
which these changes have
taken place.*
Changes equally remarkable are observed to
occur' in the interior of the shell at different
stages of its growth. On the inner surface of
the Mitra, the Volute, and other shells of a similar
kind, there is deposited a layer of a hard semi-
transparent calcareous material, having a vitreous
appearance. t The thickness of the layer, which
thus lines the cavity of the shell, is greater as it
approaches the apex ; and where the spire is
* According to Bruguiere, there is reason to believe that the
animal of the Cyprcea, after having completed its shell, in the
manner above described, still continuing to grow, and being
incommoded for want of space, quits its shell altogether, and
sets about forming a new one, better suited to its enlarged
dimensions. It is stated also that the same individual is even
capable of forming in succession several shells. Blainville,
however, considers it impossible that the living animal can ever
quit its shell. Malacologie, p. 94.
f This is the substance represented at v>, Fig. 107. p. 234.
FORMATION OF SHELLS. 249
much elongated, or tttrrited, as it is called,* this
deposition entirely fills the upper part, which,
in the early condition of the shell, was a hollow
space with thin sides. The purpose answered
by this deposit is evidently to give solidity and
strength to a part which, by remaining in its
original state, would have been extremely liable
to be broken oft* by the action of the sea.
In other cases a different expedient is adopted.
The animal, instead of fortifying the interior of
the apex by a lining of hard shell, suddenly
withdraws its body from that part, and builds a
new wall or partition across the cavity, so as to
protect the surface thus withdrawn. That por-
tion of the shell, which is thus abandoned, being
very thin and brittle, and having no support in-
ternally, soon breaks off, leaving what is termed
a decollated shell ; examples of this occur in the
Cerithium decollatum, the Bidimus decollates,
&c. The young of the genus Magilus has a
very thin shell of a crystalline texture ; but
when it has attained its full size, and has formed
for. itself a lodgement in a coral, it fills up the
cavity of the shell with a glassy deposit, leaving
only a small conical space for its body ; and it
continues to accumulate layers of this material,
so as to maintain its body at a level with the top
of the coral to which it is attached, until the
* As in the genera Turritella, Terebra, Cerithium, and Fas-
ciolaria.
250
THE MECHANICAL FUNCTIONS.
original shell is quite buried in this vitreous sub-
stance.
The forms of the Cone and Olive shells are such
as to allow but a small space for the convolutions
of the body of the animal, which accordingly
becomes, in the progress of its enlargement, ex-
cessively cramped. In order to obtain more
space, and at the same time lighten the shell,
the whole of the two exterior layers of the inner
whorls of the shell are removed, leaving only
the interior layer, which is consequently very
thin when compared with the other whorl, that
envelopes the whole, and which, retaining its
original thickness, is of sufficient strength to
give full protection to the animal. That this
change has actually been effected is very dis-
tinctly seen in the Conns (Fig. 115) by examining
a vertical section of that shell, as is represented
in Fig. 116. All the inner partitions of the
117
cavity thus laid open are found to be extremely
FORMATION OF SHELLS. 251
thin and transparent, and to consist only of the
innermost lamina of the original shell ; as will
appear on tracing them np to that outer portion
of the section (b b), which lies on each side of
the proper apex of the shell, and which forms
the apparent base. The lines on this part of
the section indicate the thickness which each
successive whorl had originally, and when it
was itself the outermost whorl. The section
also shows the vitreous deposit which lines the
upper parts of the cavity, and which completely
fills up the smaller turns of the spire, near the
apex.*
There are, indeed, instances among shells of
the total removal of the interior whorls. This is
found to occur in that of the genus Auricula,
which are molluscous animals, respiring by means
of pulmonary organs. In the young shell of this
tribe, the partitions which separate the cavities
of the whorls are incomplete, and twine parallel
to each other ; but they wholly disappear as the
animal approaches to maturity. In other cases,
the animal is found to remove exterior portions
of shell formerly deposited, when they lie in the
way of its further growth, and when the mouth
of the spire is advancing over the irregular sur-
* Fig. 117, which is a transverse section of the same shell,
shows the spiral convolutions, and the comparative thinness of the
inner portions. It also forms a striking contrast with a similar
section of the shell of the Cyprrea, Fig. 114; p. 248.
252 THE MECHANICAL FUNCTIONS.
face of the preceding whorls. Thus we often
find that the ridges, ribs, or processes which had
been deposited on the surface of the shells of
the Triton, Murex, &c. are removed to make
way for the succeeding turn of the spire. In
other cases, however, no such power of destroy-
ing portions of shell previously deposited seems
to exist ; and each successive whorl is moulded
irpon the one which it covers.
It may also be observed, that some mollusca
have the means of excavating the shells of other
animals on which they may choose to fix, for
the purpose of forming a convenient lodgement
for themselves. The Pileopsis, or fools cap, has
this faculty in a remarkable degree ; and it is
also met with occasionally in SiphonaritB and
Patella. The common Patella, or limpet of our
own coasts, often, indeed, forms for itself, by
some unknown process, a deep cavity out of a
calcareous rock.
When the animal which inhabits a spiral shell
retires within it, the only part of its body that is
exposed to injury is that which is situated at the
mouth of the shell. With a view to its protec-
tion, it constructs, in many instances, a separate
plate of shell, adapted to the aperture, and deno-
minated an Operculum. This piece is constructed
by a process similar to that by which the rest of
the shell is formed ; that is, by the deposition of
FORMATION OF SHELLS. 253
successive layers on the internal surface, some-
times in an annular, and sometimes in a spiral
form. Fig. 118 exhibits the lines
which appear on the inner side
of the operculum of the Turbo,
and which indicate the succes-
sion of deposits by which it has
been formed. If an operculum
were to be constructed of a con-
siderable size, and were con-
nected to the shell itself by a regular hinge, it
would be entitled to be considered as a distinct
valve. Here, therefore, we perceive, as was re-
marked by Adanson, a connecting link between
the univalve and the bivalve testacea. A Clau-
sium is another kind of covering, serving also for
protection, and consisting of a thin spiral plate
of shell, attached to the columella by an elastic
spring, by which the plate is retracted when the
animal retires into its shell. It thus corresponds
exactly in its office to a door, opening and closing
the entrance as occasion requires. An Epi-
phragma is a partition of a membranous or cal-
careous nature, constructed merely for tempo-
rary use. It is employed for closing the aper-
ture of the shell during certain periods only,
such as the winter season, or a long continued
drought.
It is remarkable in how short a time this
254 THE MECHANICAL FUNCTIONS.
species of Helix will construct this covering,
when circumstances occur to urge its completion.
On the approach of winter, the animal prepares
itself for passing that season in a state of tor-
pidity ; first, by choosing a safe retreat ; and
next, by retiring completely within its shell, and
then barricading its entrance by constructing
the epiphragma just described, and of which the
outer surface is represented in Fig. 1 19. Having
formed this first barrier, the animal afterwards
constructs a second, of a membranous nature,
situated more internally than the first, and at a
little distance from it. If at any other season,
while the snail is in full vigour, the experiment
be made of surrounding it with a freezing mix-
ture, it will immediately set about constructing
a covering for its protection against the cold ;
and it works with such diligence, that in the
course of an hour or two, it will have completed
its task, and formed an entire epiphragma.*
When the genial warmth of returning spring-
has penetrated into the abode of the snail, the
animal prepares for emerging from its prison,
by secreting a small quantity of a mucous fluid,
which loosens the adhesion that had taken place
between the epiphragma and the sides of the
aperture ; and the former is, by the pressure of
the foot of the snail, thrown off. The whole of
* Gray, Zoological Journal, i. 214.
FORMATION OF SHELLS. 255
this process of construction has to be renewed,
on every occasion when another covering is
required.*
One great use of these coverings is to prevent
evaporation from the surface of the body of the
animal. It is thus that Snails, Bulimi, &c. may
be preserved for months, and even years in a
torpid, but living state, ready to be restored to
the active functions of life, when sufficient water
is supplied.!
The enlargement of bivalve shells is conducted
on the same principles as that of univalves; the
augmentation of bulk taking place principally at
the outer margin of each valve, and correspond-
ing with the growth of the included animal.
The order of succession in which the layers are
deposited is clearly indicated by the lines on
the surface, which frequently appear of different
hues from the addition of colouring particles se-
creted at particular periods by the mantle.
The shells of Oysters and other acephalous
mollusca which adhere to rocks, are often
moulded, during their growth, to the surfaces to
which they are applied. The mantle, being ex-
* An epiphragma differs from true shells in having no adhe-
sion in any part to the animal which formed it.
f A remarkable instance of this apparent reviviscence of snails,
which had lain for many years in a dormant state in a cabinet
of shells, and which crawled out on being accidentally put into
warm water, is recorded in the Philosophical Transactions for
1774, p. 432.
250 THE MECHANICAL FUNCTIONS.
ceedingly flexible, accommodates itself to all the
inequalities it meets with, and depositing each
successive layer of shell equally on every part,
the figure of the surface is assumed, not only by
the valve in contact with it, but also by the other
valve, which is formed by the opposite surface of
the mantle,* and which during its formation was
immediately superposed on the thin edge of the
other valve, while it was deflected by the irregu-
lar surface on which it grew. As the enlargement
of the shell proceeds, it was necessary that the
muscle, which closes the valves, and is attached
to their inner surface, should be gradually re-
moved to a greater distance from the hinge, so
that it may preserve its relative situation with re-
gard to the whole shell, and retain undiminished
its power of acting upon the valves. For this
purpose its adhesions are gradually transferred,
by some unknown process, along the surface of
the valves ; and the progress of the removal
may generally be distinctly traced by the marks
which are left in the shell at the places before
occupied by the attachments of the muscular
fibres. The same process takes place when
there are two or three muscles instead of one.
A few genera of Mollusca, such as the Pholas,
have, in addition to the two principal valves,
small supplementary pieces of shell. They have
* Defiance, Annates des Sciences Naturelles, ii. 16.
MOLLUSCA PTEUOPODA. 257
been accordingly comprised in the order of Mul-
tivalves, which also comprehends Cuvier's order
of Cirrhopoda, including the several kinds of
Barnacles (the genus Lepas of Linnaeus), which
are furnished with a great number of jointed
filaments, or cirrhi, and form an intermediate
link of connexion between the Mollusca and the
Articulata. But the limits of this treatise will
not allow me to dwell on the endless diversities
of structure which this subject presents.
§ 5. Pteropoda.
In the Mollusca belonging to the two orders
which have now passed under our review, namely,
the Acephala and Gasteropoda, the mantle, while
it folds over the principal viscera of the body,
leaves apertures for the admission of water to the
gills, or organs of respiration. But there exist a
few genera having the sac formed by the mantle
closed on every side ; a structure which renders
it necessary to adopt a different arrangement
with regard to the gills, and to place them ex-
ternally, and we then find them spreading out
like a pair of wings, on each side of the neck.
Since this general closing of the mantle pre-
cludes, also, the formation of any organ of pro-
gressive motion corresponding to a foot, advan-
tage is taken of the projection of the gills to
vol. i. s
258 THE MECHANICAL FUNCTIONS.
employ them as oars for the purpose of enabling
the animal to swim through the water.
Mollusca of this description are found in great
abundance in the colder regions of the ocean
surrounding both the north and
south poles ; and other species
are also met with, though in
smaller numbers, in the tro-
pical seas. The Clio borealis,
of which Fig. 120 is a repre-
sentation, is the most perfect
specimen of this form of con-
struction. It swarms in the
Arctic seas, and constitutes the principal food of
the whale. The position of its gills, which per-
form the office of oars or feet, at the same time
that they resemble in their shape and action the
wings of an insect, are characters which have
suggested the title of Pteropoda, given by Cuvier
to this order of Mollusca.
§ 6. Cephalopoda.
Following the progress of organic develope-
ment, we now arrive at a highly interesting family
of Mollusca, denominated the Cephalopoda, and
distinguished above all the preceding orders by
being endowed with a much more elaborate
organization, and a far wider range of faculties.
MOLLUSCA CEPHALOPODA.
259
The Cephalopoda have been so named from the
position of certain organs of progressive motion,
which are situated on the head, and like the ten-
tacula of the Polypus, surround the opening of
the mouth. (See Fig. 121.) These feet, or arms,
or tentacula, if we choose so to call them, are long,
slender, and flexible processes, exceedingly irri-
table, and contractile in every part, and provided
with numerous muscles, which are capable of
moving and twisting them in all directions with
extraordinary quickness and precision. They
are thus capable of being employed as instru-
ments, not only of progressive motion, but also
of prehension. For this latter purpose they are
in many species peculiarly well adapted, because,
being perfectly flexible as well as highly muscu-
lar, they twine with ease round an object of any
shape, and grasp it with prodigious force. In
addition to these properties they derive a re-
markable power of adhesion to the surfaces of
bodies from their being furnished with mime-
260
THE MECHANICAL FUNCTIONS.
rous suckers all along their inner sides. Each
of these suckers, as shown separately in Fig.
122, is usually supported on a narrow neck, or
pedicle, and strengthened at its circumference
by a ring of cartilage. Their internal mecha-
nism is more artificial than the simple construc-
tion already described (p. 137) : for when the
surface of the disk is fully expanded, as shown
in Fig. 123* b, we find that it is formed of a
great number of long slender pieces, resembling-
teeth closely set together, and extending from
the inner margin of the cartilaginous ring, in
the form of converging radii, to within a short
distance of the centre, where they leave a circular
aperture. In the flattened state of the sucker,
this aperture is filled by the projecting part of a
softer substance, which forms an interior portion,
capable of being detached from the flat circle
of teeth, when the sucker is in action, and of
leaving an intervening cavity. The form of this
cavity is exhibited in Fig. c, which represents a
perpendicular section of the whole organ, and
where the central portion, or principal mass of
the sucker is drawn away from the circular disk.
MOLLUSCA CEPHALOPODA. 201
the inner margin of which appears like a row of
teeth. It is evident that by this mechanism,
which combines the properties of an accurate
valve, with an extensive cavity for producing
rarefaction, or the tendency to a vacuum, the
power of adhesion is considerably augmented.*
So great is the force with which the tentacula
of the cuttle-fish adhere to bodies by means of
this apparatus, that while their muscular fibres
continue contracted, it is easier to tear away the
substance of the limb, than to release it from its
attachments. Even in the dead animal I have
found that the suckers retain considerable power
of adhesion to any smooth surface to which they
may be applied.
Our attention must first be directed to the
remarkable family of Sepice, which comprehends
three principal genera, namely, the Octopus, the
Loligo, or Calamary, (depicted in Fig. 121),
and the common Sepia, or Cuttle-fish. The first
of these, the Octopus, which was the animal
denominated Polypus by Aristotle, has eight
arms of equal length, and contains in its interior
* The description I have here given is the result of my own
examination of a large Octopus, which I had lately an opportu-
nity of dissecting : and the annexed figures 123*, a, b, c, are
copied from drawings I made on that occasion, a represents
the sucker in its usual form when not in action : b shows the
sucking surface fully expanded : and c is a section of the whole,
which had become somewhat flattened by the operation of di-
viding; it.
2(J2 THE MECHANICAL FUNCTIONS.
two very small rudimental shells, formed by the
inner surface of the mantle. This shell becomes
much more distinct in the Loligo, where it is
cartilaginous, and shaped like the blade of a
sword. (Fig. 123.) The internal shell of the
common Sepia is large and broad, and com-
posed wholly of carbonate of lime : it is well
known by the name of the cuttle-fish bone. Its
structure is extremely curious ; and deserves
particular attention, as establishing the univer-
sality of the principles which regulate the forma-
tion of shells, whether internal or external, and
from which structures differing much in their
outward appearance may result. It is composed
of an immense number of thin calcareous plates,
arranged parallel to one another and connected
by thousands of minute hollow pillars of the
same calcareous material, passing perpendicu-
larly between the adjacent surfaces. This shell
is not adherent to any internal part of the ani-
mal which has produced it ; but is enclosed in
a capsule, and appears like a foreign body,
impacted in the midst of organs, with which
at first sight, it would appear to have no re-
lation. It, no doubt, is of use in giving me-
chanical support to the soft substance of the
body, and especially to the surrounding mus-
cular flesh ; and thus probably contributes to
the high energy which the animal displays in
all its movements. It has been regarded as an
MOLLUSCA CEPHALOPODA. *2o'.'J
internal skeleton ; but it certainly has no pre-
tensions to such a designation ; for, although en-
veloped by the mantle, it is still formed by that
organ ; and the material of which it is composed
is still carbonate of lime. On both these ac-
counts it must be considered as a true shell, and
classed among the productions of the integu-
ments. It differs, indeed, altogether from bony
structures, which are composed of a different
kind of material, and formed on principles of
growth totally dissimilar.*
Besides tentacula, the Sepia is also furnished
with a pair of fleshy fins, extending along the
two sides of the body. The Loligo has similar
organs of a smaller size, and situated only at
the extremity of the body which is opposite to
the head. They have been regarded as the
rudiments of true fins, which are organs de-
veloped in fishes, and which are supported by
slender bones, called rays ; but no structure of
this kind exists in the fins of the Cephalopoda.
In swimming, the organs principally employed
by cuttle-fish for giving an effective impulse to
* Some analogies have, indeed, been attempted to be traced
between the cartilaginous lamina of the Loligo, and the spinal
column of the lowest order of cartilaginous fishes : these I shall
have occasion to point out in the sequel. Solid cartilaginous
structures also exist in the interior of the body of the cepha-
lopoda, which are considered by some naturalists as indicating
an approach to the formation of an internal skeleton, analogous
to that of vertebrated animals.
'21)4 THE MECHANICAL FUNCTIONS.
the water, are the tentacula. These they employ
as oars, striking with them from behind forwards ;
so that their effect is to propel the hinder part of
the body, which is thus made to advance fore-
most, the head following in the rear. They also
use these organs as feet for moving along the
bottom of the sea. In their progress, under these
circumstances, the head is always turned down-
wards, and the body upwards, so that the animal
may be considered as literally walking upon its
head. The necessity of this position for the feet
arises probably from the close investment of the
mantle over the body ; for although the mantle
leaves an aperture in the neck for the entrance
of water to the respiratory organs, yet, in other
respects, it forms a sac, closed in every part,
except where the head, neck, and accompanying
tentacula protrude.
In the Calamary, as well as in the common
Sepia, two of the arms are much longer than
the rest, and terminate in a thick cylindrical
portion, covered with numerous suckers, which
may not unaptly be compared to a hand. These
processes are employed by cuttle-fish as anchors
for the purpose of fixing themselves firmly to
rocks, during violent agitations of the sea ; and
accordingly we find that it is only the extre-
mities of these long tentacula that are provided
with suckers, while the short ones have them
along, their whole length.
MOLLUSCA CEPHALOPODA.
265
The other genera of Cephalopodous Mollusca
are, like the Sepiae, provided with tentacula at-
tached to the head. They comprehend animals
differing exceedingly in their size : some being
very large, but a great number very minute, and
even microscopic* The shells of these animals
are often found to contain partitions dividing
them into a number of chambers ; hence they
have been termed earner ated, or multilocular, or
polythalamous shells. The Spirula (Fig. 1 24) is
a shell of this description, of which the cellular
structure and numerous partitions are rendered
visible by making a section through it : (Fig.
125). Some, however, as the Argonaut, or
Paper Nautilus, have shells undivided by par-
titions ; and are accordingly termed unilocular,
or monothalamous. The shell of the Argonaut
is exceedingly thin, and almost pellucid, pro-
bably for the sake of lightness, for it is intended
to be used as a boat. For the purpose of enabling
the animal to avail itself of the impulses of the
* A particular account has been given of the shells of these
microscopic cephalopoda by M. D'Orbigny, in the Annales des
Sciences Naturelles ; vii. 96.
2b'b' THE MECHANICAL FUNCTIONS.
air, while it is thus floating on the waters, na-
ture has furnished it with a thin membrane,
which she has attached to two of the tentacula ;
so that it can be spread out like a sail to catch
the light winds which waft the animal forwards
on its course. While its diminutive bark is thus
scudding on the surface of the deep, the assidu-
ous navigator does not neglect to ply its tentacula
as oars on either side, to direct, as well as ac-
celerate its motion. No sooner does the breeze
freshen, and the sea become ruffled, than the
animal hastens to take down its sail, and quickly
withdrawing its tentacula within its shell, ren-
ders itself specifically heavier than the water,
and sinks immediately into more tranquil regions
beneath the surface.*
The common Nautilus, which is provided with
a similar sailing apparatus, is an inhabitant of
a polythalamous shell (Fig. 126), of which Fig.
127 represents the section. The formation of
this, as well as of other shells of this description,
presents very curious phenomena. The animal
at certain periods of its growth, finding itself
cramped in the narrow part of the spire, draws
* It must be confessed, however, that the habits of the Argo-
naut are still very imperfectly known. Considerable doubts are
entertained whether the shell it inhabits is formed by the animal
itself, or whether it is the production of some other, but unknown
species of Mollusca, and is merely taken possession of by the
Argonaut as a convenient habitation, which it can quit and enter
again at pleasure.
MOLLUSCA CEPHALOPODA. 267
up that portion of the mantle which occupied it,
thus leaving a vacant space. The surface of the
mantle which has receded immediately begins to
secrete calcareous matter, which is deposited in
the form of a partition, stretching completely
across the area of the cavity. As the animal
proceeds to increase in size, and to occupy a
wider portion of the external shell, the same
necessity soon recurs, and the same expedient is
again resorted to. It withdraws its mantle from
the narrower into the wider part of the shell ;
and then forms a second partition, at a little dis-
tance from the first, corresponding to the space
left by the receding of the mantle. This process
is repeated at regular intervals, and produces
the multitude of chambers contained in poly-
thalamous shells, of which the living animal oc-
cupies only the largest, or that which continues
open.* The partitions are in general perforated
either in the centre or at one side, for the pur-
pose of giving passage to a tube, which extends
to the apex of the shell. This tube is often sur-
rounded, either entirely or partially, by shell,
which forms what is denominated the syphon;
portions of which are seen in the section Fig. 1 27.
* This structure is extremely prevalent in fossil shells : some
of which are spiral, such as the Cornu Ammonis, while others
are straight cones, such as the Bacculite and Orthoceratite .
In most of these the partitions are very numerous, and have
undulating surfaces.
•208
Chapter IV.
ARTICULATA.
§ 1. Articulated Animals in general.
From the Cephalopoda, the transition is easy to
the lowest order of vertebrated animals. But
previously to pursuing the analogies which con-
nect these two divisions of the animal kingdom,
we have to pass in review a very extensive series
of animal forms, constructed upon a peculiar
system, and occupying, as well as the Mollusca,
a place intermediate between Zoophytes and the
more highly organized classes.
We have seen that even in those Zoophytes
which are distinguished from the rest by a more
elaborate conformation of organs, the powers of
progressive motion are always extremely limited.
Nor are the Mollusca in general more highly
favoured with respect to the degree in which
they enjoy this faculty. But the greater number
of the animals composing the series we are now
to examine are provided with a complete appa-
ratus for motion, and endowed with extensive
capacities for using and applying it in various
ways. While Nature has preserved in the con-
struction of their vital organs the simplicity which
marks the primitive modes of organization, and
ARTICULATA. 269
has adhered to a definite model in the formation
of the different parts of the system, she has no-
where displayed more boundless variety in the
combinations of the forms which she has im-
pressed upon the mechanical instruments, both
of prehension and of progression.
All the tribes of Zoophytes, and by far the
greater number of Mollusca, are limited, by the
constitution of their system, to an aquatic exist-
ence. But in following the series of Articulated
animals, we very soon emerge from the waters,
and find structures adapted to progression on
land. For this we see that preparation is early
made in the developement of the nascent struc-
tures. A further design, also, soon becomes ma-
nifest ; and instruments are given for elevating
the body above the ground, and for traversing
with rapidity the light and scarcely resisting
atmosphere. This prospective design may be
traced in the whole system of insects ; every
part of which is framed with reference to the
properties of the medium through which these
movements are to be performed.
§ *2. Annelida.
The lowest division of articulated animals com-
prehends those which have a vermiform shape,
and which compose the class of Annelida, or
Annulose animals ; of which the earth-worm
270 THE MECHANICAL FUNCTIONS.
may be taken as the type, and most familiar
example. In the series of structures which
constitute this division of the animal kingdom,
we may trace remarkable gradations of de-
velopement, through which nature appears to
pass in attaining the higher and more perfect
conformations.
It may be remarked that, in effecting the
transition from Zoophytes to the new model of
construction here presented, nature seems to
have wholly abandoned that radiated disposition
of parts, and those star-like forms, so charac-
teristic of the beings which are placed on the
confines of the animal kingdom, and which
still retain an analogy with vegetable structures.
She now adopts a more regular law of sym-
metry, by which all the parts are referable to
one longitudinal axis, and also to a vertical
plane passing through that axis, and which has
been termed the mesial plane. As a direct con-
sequence of this law, we shall find that in the
forms which are hereafter to pass under our
review, as far as the external organs and general
outline of the body are concerned, all that exists
on one side is an exact counterpart, like a re-
flected image, of what is found on the other side.
While in the Star-fish, and Echinus, nothing
in point of situation was definite, excepting the
upper and the lower surface, and there was no
side which could be exclusively denominated
ANNELIDA.
271
either the right or the left side, and no end which
could be properly said to be the front, or the
back, in Articulated, as well as in Vertebrated
animals, all these distinctions are clearly marked
and easily defined.
In all the Annelida the firmest parts of the
body, or those which give mechanical support to
the rest, are external, and may be regarded either
as appendages to the integuments, or as modifi-
cations of the integuments themselves. They
consist of a frame-work, composed of a series of
horny bands or rings ; their assemblage having
more or less of a lengthened cylindric shape,
and constituting a kind of external skeleton,
which encloses all the other organs. This is
exemplified in the Lumbricus, or earth-worm ;
in the Pontobdella (Fig. 128), which is a species
of leech ; and in the Nereis (Fig. 129). These
rings give rise to the division of the body into as
many different segments. In some cases, how-
ever, we find all these rings compressed into
272 THE MECHANICAL FUNCTIONS.
the form of a flat oval disk. This is the case in
the Erpobdella, of which Fig. 130 is an enlarged
representation.
In general, the first of the segments into
which the body is divided, contains the prin-
cipal organs of sense, and is sufficiently distinct
from those which follow to entitle it to the ap-
pellation of the head; while the lengthened pro-
longation of the opposite extremity, when such
a form is present, may be denominated the
tail.
The rings which encircle the body are con-
nected laterally by a looser and more flexible
portion of integument, and also by layers of
muscular fibres, curiously collected into bands.
The muscular flesh of insects, and other animals
of this class, differs much from that of the larger
animals, being soft and gelatinous in its texture,
though endowed with a high degree of irritabi-
lity, and contracting with great force. The
fibres composing each band are all parallel to
one another, and have seldom any tendinous
attachments ; being generally inserted directly
on the parts they are destined to move. Thus
the adjacent margins of the rings of worms
(as shown in the diagram, Fig. 131) are con-
nected together by these muscular bands, which
pass transversely from the one to the other,
immediately under the skin, and parallel to the
]
ANNELIDA. 273
axis of the body. There are generally four
distinct bands provided ; two running along the
back, and two along the lower part of the
body.
The effects which result from the action of
these muscles are such as might easily be anti-
cipated. The lower set must, when contracting,
bring the rings nearer to one another at that
lower part ; and when the whole series occupy-
ing that situation are exerted in concert, they
raise the body in the form of an arch. An op-
posite curvature will be produced by the con-
traction of the upper bands, which by raising
both ends of the body bend the back downwards.
In proportion as the two bands, situated on each
side, act in concert, while the others are relaxed,
the body will be bent laterally towards that side.
When all the four muscular bands contract
together equally, their joint effect will be to
bring the rings near to each other, and to con-
tract the length of the worm ; the skin being at
the same time wrinkled, and swelled out between
the rings.
Other muscular bands, also attached to the
rings, pass from the one to the other in oblique
directions. By means of these muscles the
rings may be made to recede at some points,
while they approach at others ; so that the body
may be either twisted laterally on its axis, or
VOL. I. t
274 THE MECHANICAL FUNCTIONS.
wholly elongated, according as the actions of
these oblique muscles are partially or generally
exerted.
The skin on the surface of the earth-worm is
furnished, at the parts where it covers the rings,
with very minute bristles, called Seta, by means
of which the animal is enabled to fix those parts
on the ground, while the other portions of its
body are in motion. Both in the anterior and
posterior segments, these hairs are directed to-
wards the centre of the animal ; while those on
the middle segments are perpendicular.* We
almost constantly find, in animals belonging to
the order of Annelida, some provision of this
kind, often consisting of tufts of hair regularly
disposed in rows on each side of the under sur-
face. In the Nereis (Fig. 1*29), a genus of sea-
worms, there are often above a hundred pair
of little tufts of strong bristles : and between
these we find tentacula to prevent the animal
from running against any thing by which it
might be injured. They also raise the body
from the ground, for which purpose, as they
* As an instance of the extraordinary multiplicity of species
existing in every department of living nature, I may here notice,
that of the common earth-worm, apparently so uniform in its
shape, Savigny has lately, by a closer examination, been able to
distinguish no less than twenty-two different species, among
those found in the neighbourhood of Paris alone.
ANNELIDA. 275
are used under water, very little support is ne-
cessary.* Sometimes the whole body is covered
with hair ; at other times, these appendages are
in the form of hooks, which, of course, give
greater power of clinging to the objects on which
they fasten. In some, again, they assume more
the nature of feet, of which they exercise, during
progression, all the functions ; being furnished
with several sets of muscles for adjusting and
strengthening their actions.
The mode by which an animal of this de-
scription advances along the ground is very
simple. It first protrudes the head by the elon-
gation of the foremost segments of the body,
while the others cling to the earth by means
of the rings, and also of the bristles and other
appendages to the integuments. The head is
then applied to the ground, and made the fixed
point, and the segments next to it, which had
been elongated, are now contracted by the
action of their longitudinal muscles ; in doing
which, equal portions of the succeeding seg-
ments are necessarily elongated : these are next
contracted ; and so on, in succession, till the
whole is brought forwards to the head : after
which the same series of actions is repeated,
beginning with the advance of the head.
* Home; Lectures, &c. vol. i. p. 115.
270
THE MECHANICAL FUNCTIONS.
Worms often reverse this motion, and are thus
enabled to move backwards, or with the tail
foremost.*
Great variety exists in the forms of the ani-
mals referable to the type of Annelida. The
Gordius, or hair-worm (Fig. 132), is that which
exhibits the greatest developement in length,
compared with the breadth of the body. It has
the form of a very long and slender thread : the
annular structure being indicated only by very
slight transverse folds of the integuments. No
external members, nor even tentacula, have
been given to this simplest of vermiform ani-
mals.
Many of the animals of this class, being soft
and defenceless, are obliged to consult their
safety by retreating into holes and recesses, or
by burrowing in the sand or mud. One genus
* See Home ; Lectures on Comparative Anatomy, vol. i.
p. 114.
ANNELIDA. 277
only, the Serpula (Fig. 133), forms for itself an
external shell, which is shaped into a spiral tube.
Others, as the Sabella and the Terebella, accom-
plish the same object by collecting grains of
sand, or fragments of decayed shells, or other
substances, which they agglutinate together by
means of a viscid exudation, so as to form a
firm defensive covering, like a coat of mail.
Fig. 134 shows this rude architecture in the
Terebella conchilega. These coverings, however,
composed as they are of extraneous materials,
and not being organic productions of the animals
themselves, are structures wholly foreign to their
systems. These inhabitants of tubes, the Tu-
bicolcB of Cuvier, are generally furnished with
tentacula, issuing from the head, which, when
the rest of the body has retired within the tube,
is the only part exposed.
The expedient resorted to for progressive mo-
tion by the Lumbricus marinus of Linnaeus
(Arenicola piscatorum of Lamarck), is very re-
markable.* This worm, depicted in Fig. 135,
swarms on all sandy shores, and is dug up in
great numbers as bait by the fishermen. It
bores its way through the sand by means of
the peculiar construction of the rings of its head,
which, when elongated, has the shape of a re-
* See the account given by Mr. Osier, Philosophical Trans-
actions for 1826, p. 342.
278 THE MECHANICAL FUNCTIONS.
gular cone. As each ring is so much smaller
than the one behind it as to admit of being
received within it, the whole head, when com-
pletely retracted, presents a flat surface. When
this disk is applied to the sand, the animal, by
gradually projecting the cone, and successively
dilating the rings of which it is composed, opens
for itself a passage through the sand, and then
secures the sides of the passage from falling in
by applying to them a glutinous cement, which
exudes from its skin, and which unites the par-
ticles of sand into a kind of wall, or coating.
This covering does not adhere to the body, but
forms a detached coherent tube, within which
the animal moves with perfect freedom, and
which it leaves behind it as it progressively
advances ; so that the passage is kept pervious
throughout its whole length by means of this
lining, which may be compared to the brick-
work of the shaft of a mine, or tunnel.
An apparatus of a more complex description
is provided in the Terebella conchilega, belong-
ing to a tribe of marine worms, which from the
peculiar circumstances of their situation, inha-
biting parts of the shore nearly midway be-
tween high and low water, are obliged often to
prolong their tubes to a great length through
the sand ; for, in consequence of the frequent
shifting of the sands in storms, these animals
are sometimes buried to a considerable depth,
ANNELIDA. 279
and at others have several inches of their tubes
exposed. In the one case, they must work their
way speedily to the surface ; in the other, they
must dive deeper below it. The manoeuvres of
the Terebella are best observed by taking it out
of its tube, and placing it under water upon sand.
It is then seen to unfold all the coils of its body,
to extend its tentacula in every direction, often
to a length exceeding an inch and a half, and to
catch, by their means, small fragments of shells,
and the larger particles of sand. These it drags
towards its head, carrying them behind the
scales which project from the anterior and lower
part of the head, where they are immediately
cemented by the glutinous matter which exudes
from that part of the surface. Bending the head
alternately from side to side, while it continues
to apply the materials of its tube, the terebella
has very soon formed a complete collar, which it
sedulously employs itself to lengthen at every
part of the circumference with an activity and
perseverance highly interesting. For the pur-
pose of fixing the different fragments compactly,
it presses them into their places with the erected
scales, at the same time retracting its body.
Hence the fragments, being raised by the scales,
are generally fixed by their posterior edges, and
thus overlaying each other, often give the tube
an imbricated appearance.
Having formed a tube of half an inch, or an
280 THE MECHANICAL FUNCTIONS.
inch in length, the terebella proceeds to burrow ;
for which purpose it directs its head against the
sand, and contracting some of the posterior rings,
effects a slight extension of the head, which thus
slowly makes its way through the mass before it,
availing itself of the materials which it meets
with in its course, and so continues to advance
till the whole tube is completed. After this has
been accomplished, the animal turns itself within
the tube, so that its head is next to the surface,
ready to receive the water which brings it food,
and is instrumental in its respiration. In sum-
mer, the whole task is completed in four or five
hours ; but in cold weather, when the worm is
more sluggish, and the gluten is secreted more
scantily, its progress is considerably slower.
Tentacula of various kinds are also met with
in several of the more active and vivacious kinds
of annelida, such as the Nereis (Fig. 129), pro-
ceeding from the margin of the mouth and other
parts of the head. This animal swims with
great facility by rapid, undulating inflexions of
its body ; and by practising a similar succession
of movements in the loose sand at the bottom
of the water, it quickly buries itself, and even
travels to considerable distances through the
sand, first extending the anterior rings, and then
bringing up the posterior part of the body ; its
progress being also much assisted by the action
of its numerous bristly feet.*
* Oder, Phil. Trans, tor 1826, p. 342.
ANNELIDA. 281
Facilities for progression are also given by the
addition of tubercles, arranged in pairs along
the under side of the body, which serve the pur-
poses of feet, and are often furnished with bristles
or hooks. In the Amphitrite, and many other
genera, tufts of hair occupy the place of feet
on each side, and being moved by muscles spe-
cially provided for that purpose, serve as levers
for effecting progressive motion.
We find the same object accomplished by very
different means in other animals of this class.
The leech, for instance, having the rings which
encircle its body very numerous and close to
each other, could not well have advanced by the
ordinary modes of vermiform progression. As a
substitute, accordingly, it has been furnished
with an apparatus for suction at the two extremi-
ties of the body, which are formed into disks for
that purpose. By fixing alternately the one
and the other, and contracting or elongating the
body as the occasion requires, the leech can
move at pleasure either forwards or backwards.
Thus, while the tail is fixed, the head may be
advanced by lengthening the whole body, and
when the head is fixed, the hinder sucker can
be brought forwards by the contraction of the
body, and applied to the ground near to the
head, and preparation may thus be made for
taking another step.
Most of the parasitic animals which inhabit
the interior cavities of the body, and especially
282 THE MECHANICAL FUNCTIONS.
the alimentary canal, correspond in external
form, as well as in many circumstances of inter-
nal conformation, to the Annelida. They com-
pose an order denominated the Entozoa.
§ 3. Arachnida.
In passing from the Annelida to the Arachnida,
an order which comprehends all the species of
spiders, together with animals allied to them in
conformation, we find that a considerable advance
has been made in the progress of developement.
The frame-work of the body is more consoli-
dated, and the instruments provided for pro-
gressive motion are shaped into longer and more
perfect levers, are united by a more refined sys-
tem of articulation, and are moved by more dis-
tinct and more powerful muscles ; so that the
body is elevated from the ground, and enjoys a
greater range of action, and a wider sphere of
perception.
The rings, which always compose the frame-
work of the Annelida, are here consolidated so
as to form two principal divisions of the body ;
the one in front, which is termed the Cephalo-
thorax, contains the organs of sensation, and of
mastication, and also the principal reservoir of
circulating fluids ; the other, which is behind,
and contains the organs of digestion, is termed
ARACHNIDA. 283
the abdomen. In the spider (Fig. 136, where
c is the cephalo-thorax,
and a the abdomen) these
two portions of the body
are separated by a deep
groove, which leaves only
a slender pedicle, or tube
of communication be-
tween them. There are usually in the male
four pair of legs, constantly articulated with the
cephalo-thorax ; but the female is furnished with
an additional pair, to enable her to carry her
eggs. For the purpose of obtaining an extensive
base of support, the feet of the spider are spread
out in diverging rays, so as to include a very
wide circle. They are divided into several joints,
those next to the body being termed the haunches,
and the succeeding ones the leg, and the tarsus,
and each foot is terminated by two, or some-
times three hooks. Besides these, there are other
members, resembling feet, which are placed in
front of the head, and have affixed to them either
a moveable hook, or pincers, which are employed
as organs of prehension, and of offence. Through
the larger branches of these a canal passes, which
opens near the point, and conducts a poisonous
fluid into the wounds inflicted by this formidable
weapon.
In common with all articulated animals,
Spiders, in the progress of their growth, cast
284 THE MECHANICAL FUNCTIONS.
their outer skin several times, and at regular
periods. In the earlier stages of their existence,
although they have the general form of the ma-
ture insect, yet they have a smaller number of
legs ; the last pair not making their appearance
till after the spider has attained a certain size.
We may here trace the commencement of that
system of metamorphosis, which, as we shall
afterwards find, is carried to so great a length in
winged insects.
Spiders are endowed with extensive powers of
progressive motion, and display great activity
and energy in all their movements. The long
and elastic limbs on which the body is sus-
pended, being firmly braced by their articu-
lations, enable the muscles to act with great
mechanical advantage in accelerating the pro-
gression of the body. Hence these animals are
enabled to run with great swiftness, and to spring
from considerable distances on their prey; powers
which were necessary to those tribes that live
altogether by the chase. The greater number
of species, however, as is well known, are pro-
vided with a curious apparatus for spinning
threads, and for constructing webs to entangle
flies and other small insects. Every species of
spider weaves its web in a manner peculiar to
itself: and, besides the principal web, they often
construct in the neighbourhood a smaller one,
in the form of a cell, in which they conceal
ARACHNIDA. 285
themselves, and lie in ambush for their prey.
Between this cell and the principal web they
extend a thread of communication, and by the
vibrations into which it is thrown, on the contact
of any solid body, the spider is immediately ac-
quainted with the event, and passes quickly to
the spot, by the assistance of the same thread.
Some species have the power of conveying
themselves to considerable distances through the
air by means of threads which they dart out,
and which are borne onwards by the wind,
while the spider is clinging to the end of the
thread which is next to it. In this manner these
spiders are often carried up to a great height in
the air : and it has been supposed that during
their flight they often seize upon gnats and other
flies ; because the mutilated remains of these
insects are often seen adhering to the threads :
this point, however, is still open to much doubt.
The Natural History of the spider is in many
points of view highly interesting, not only from
the great extent to which the organic develope-
ment is carried, and the energy with which all
the functions of animal life are performed ; but
also with reference to the wonderful instincts
displayed in the construction of its web, in the
surprise and destruction of its victims, and in
the zealous guardianship of its young. It would
be impossible, in so brief an outline as the one
I am now tracing, to enlarge upon so fertile a
286 THE MECHANICAL FUNCTIONS.
topic, without being led too far from the object
I have at present more particularly in view,
namely the developeraent of organization with
reference to the organs of progressive motion.
§ 4. Crustacea.
The plan which Nature appears to have com-
menced in the construction of the Arachnida, is
farther pursued in that of the Crustacea. The
portions into which the external frame-work of
the body was divided in the former, are still
farther consolidated in the latter : they are
composed of denser materials, and endowed
with greater rigidity; thus not only offering more
resistance to external forces, but also giving a
firmer purchase to the muscles which are the
moving powers. The limbs, as well as the
whole body, are encased in tubes of solid car-
bonate of lime ; they are articulated with great
care, and almost always compose hinge joints.
The muscles, by which these solid levers are
moved, are lodged in the interior, and their
fibres either pass directly from one point to
another across the joint, or else they are at-
tached to cartilaginous plates, which, for the
purpose of receiving the muscles, are made to
project into the interior of the upper portion of
CRUSTACEA.
287
the limb, being themselves immovably connected
with the lower portion. By this expedient, not
only is the employment of a tendon dispensed
with, but a larger surface is presented for the
attachment of the muscles, which, by acting
also upon a longer lever, obtain great mecha-
nical advantage. It would be superfluous to
occupy more time in explaining the minutiae of
structure in these joints, because the simple in-
spection of the limbs of a crab or lobster will
give clearer ideas of this mechanism than can
be conveyed by any laboured description. I
shall, therefore, only give a brief sketch of the
principal constituent parts of these external
members of the Crustacea.
The number of pairs of legs is either three or
four : each leg is divided into live pieces. The
piece (h, Fig. 137), next the trunk, is termed
the haunch, to which is united the trochanter
(t) ; after which come, in succession, the femur
288 THE MECHANICAL FUNCTIONS.
or thigh (f) ; two portions of the leg (l) ; and
the tarsus (p). The haunch is usually short,
being interposed merely as a base for increasing
the extent of motion of the pieces which follow ;
and sometimes it is itself composed of more than
one piece. The leg is usually divided by a joint
into two pieces. The tarsus is terminated by a
single or double hook, and sometimes by a pincer,
or claw.
New organs, not met with among the Arach-
nida, are here for the first time developed,
namely, the Antennce, of which there is one on
each side of the head. They are denominated,
in popular language, the feelers ; although it is
more than probable that they perform some
function of higher importance than that of con-
veying perceptions of mere touch. The antennae
consist of slender filaments, composed of a great
number of pieces articulated together : and they
are infinitely diversified in their form in the
different genera and species, both of Crustacea
and of Insects.
The jaws, and other parts connected with the
mouth, present a great complication of struc-
ture ; and many of these parts are employed in
various uses besides those of mastication : such
as the seizing of objects, turning them in various
ways for examination, and, according to their
suitableness as articles of food, conveying them
into the mouth. These organs are called the
CRUSTACEA. 28.9
Palpi, and sometimes the false feet. They
always exist in pairs, and take their rise from
the lower lip, or some adjacent part of the head.
The portions of which each is composed are
articulated together and moved by muscles in
the same manner as the ordinary or proper feet.
It is worthy of notice, however, that sometimes
the foremost pairs of palpi are shaped more like
jaws, and actually perform the office proper to
jaws, of compressing and dividing the food pre-
viously to its introduction into the mouth : these
auxiliary jaws are then called mandibles. In other
instances, we see them assuming every variety
of intermediate form between that of mandibles
and of false feet, so that it is often difficult,
amidst these gradual transitions of structure, to
decide to which of these two kinds of organs a
specimen we meet with properly belongs. It is
apparently with a view to evade this difficulty
that a term has been invented which shall in-
clude them all, namely, that of feet-jaws. These
transitions are illustrated by the annexed figures
of several of these members in the 31ysis Fa-
bricii ; Fig. 138 being that of a mandible, with
its feeler, or palpus; Figures 139, 140, and 141,
representing the first, second, and third pair of
feet- jaws ; and Fig. 142, the first pair of true
feet. It would thus seem as if the same con-
stituent element of the fabric is converted by
vol. i. u
290 THE MECHANICAL FUNCTIONS.
nature into the one or other of these organs,
according as best suits the exigencies of each
particular case.*
In the Lobster, the Crab, and many other
Crustacea, the foremost pair of true feet are
also modified to suit a particular purpose ; the
pincers which terminate them being expanded
into a claw, and constituting a powerful organ
of prehension, and a formidable weapon of
offence. It resembles a finger and thumb in
its power of grasping and strongly compressing
any object on which it seizes ; and to enable
it to do this with more effect, the inner edges
of both parts of the claw are notched or
serrated.
The large portion of shell which is consoli-
dated into one piece, and covers the upper part
of the body, is termed the shield, or carapace.
The tail of the crab is very short, and is united
with the body, appearing as if it had been folded
under it. The feet-jaws are particularly large,
but short : the articulations of the feet are such
as to allow of scarcely any motion but in a trans-
verse plane. This is the cause of the greater
* The labours of Savigny, Audouiu and Latreille appear to
have established a complete analogy in the respective component
parts, not only of the feet, feet-jaws, jaws and mandibles, but
also of the palpi and other appendices attached to the head, in
all the articulated animals, whether belonging to the classes of
arachnida, Crustacea, myriapoda, or winged insects.
CRUSTACEA. 291
facility the Crab finds in walking side-ways,
which it can do with great quickness when
urged by a sense of danger. The Lobster, on
the contrary, is better formed for swimming
than for walking. The hinder part of its body
is divided into segments, which play upon each
other by a remarkable kind of mechanism, the
margins of each portion overlapping the suc-
ceeding segment, and partly enclosing it. The
tail is the principal agent used in swimming,
and the whole force of the muscles is bestowed
upon its movements. As it strikes the water
from behind forwards, the lobster can only swim
backwards ; and it is assisted in this action by
five pair of false feet, which are attached to the
under side of the body, behind the true feet, and
which terminate in a fin-shaped expansion, act-
ing like an oar. The extremity of the tail is still
more expressly formed for giving effect to the
stroke, being terminated by a number of flat
scales, which, when expanded, present a broad
surface to the water.
The calcareous coverings of these Crustacea
are analogous to shell both in structure and
composition : they contain, however, some phos-
phate of lime, in addition to the carbonate. The
calcareous particles are deposited on a membrane
of considerable firmness ; and they together com-
pose a dense, but thin and fragile structure,
which, in order to distinguish it from the shells
292 THE MECHANICAL FUNCTIONS.
of the Mollusca, has been denominated a crust.
A solid structure of this kind, as we have already
seen, does not admit of increase by the extension
of its own parts : so that in order to allow of the
growth of the parts which it encloses, it is neces-
sary that it be cast off, and exchanged for a new
shell of larger dimensions.
The process by which this periodical casting
and renewal of the shell are effected, has been
very satisfactorily investigated by Reaumur.
The tendency in the body and in the limbs to
expand during growth is restrained by the
limited dimensions of the shell, which resists
the efforts to enlarge its diameter. But this
force of expansion goes on increasing, till at
length it is productive of much uneasiness to the
animal, which is, in consequence, prompted to
make a violent effort to relieve itself: by this
means it generally succeeds in bursting the shell ;
and then, by dint of repeated struggles, extricates
its body and its limbs. The lobster first with-
draws its claws, and then its feet, as if it were
pulling them out of a pair of boots : the head
next throws off* its case, together with its anten-
nae ; and the two eyes are disengaged from their
horny pedicles. In this operation, not only the
complex apparatus of the jaws, but even the
horny cuticle and teeth of the stomach, are all
cast off along with the shell : and, last of all, the
tail is extricated. But the whole process is not
CRUSTACEA. 293
accomplished without long continued efforts.
Sometimes the legs are lacerated or torn off, in
the attempt to withdraw them from the shell ;
and in the younger Crustacea the operation is
not unfrequently fatal. Even when successfully
accomplished it leaves the animal in a most
languid state : the limbs, being soft and pliant,
are scarcely able to drag the body along. They
are not, however, left altogether without defence.
For some time before the old shell was cast off,
preparations had been making for forming a new
one. The membrane which lined the shell had
been acquiring greater density, and had already
collected a quantity of liquid materials proper
for the consolidation of the new shell. These
materials are mixed with a large proportion of
colouring matter, of a bright scarlet hue, giving
it the appearance of red blood, though it differs
totally from blood in all its other properties. As
soon as the shell is cast off, this membrane, by
the pressure from within, is suddenly expanded,
and by the rapid growth of the soft parts, soon
acquires a much larger size than the former shell.
Then the process of hardening the calcareous in-
gredient commences, and is rapidly completed ;
while an abundant supply of fresh matter is
added to increase the strength of the solid walls
which are thus constructing for the support of
the animal. Reaumur estimates that the lobster
gains, during each change of its covering, an
204 THE MECHANICAL FUNCTIONS.
increase of one-fifth of its former dimensions.
When the animal has attained its full size, no
operation of this kind is required, and the same
shell is permanently retained.
A provision appears to be made, in the inte-
rior of the animal, for the supply of the large
quantity of calcareous matter required for the
construction of the shell at the proper time. A
magazine of carbonate of lime is collected, pre-
vious to each change of shell, in the form of two
rounded masses, one on each side of the stomach.
In the Crab these balls have received the absurd
name of crab's eyes; and during the formation of
the shell they disappear.
It is well known that when an animal of this
class has been deprived of one of the claws, that
part is in a short time replaced by a new claw,
which grows from the stump of the one which
had been lost. It appears from the investigations
of Reaumur, that this new growth takes place
more readily at particular parts of the limb, and
especially at the joints; and the animal seems
to be aware of the greater facility with which a
renewal of the claw can be effected at these
parts ; for if it chance to receive an injury at
the extremity of the limb, it often, by a spon-
taneous effort, breaks off the whole limb at its
junction with the trunk, which is the point
where the growth more speedily commences.
The wound soon becomes covered with a delicate
CRUSTACEA. 295
white membrane, which presents at first a convex
surface : this gradually rises to a point, and is
found on examination to conceal the rudiment
of a new claw. At first this new claw enlarges
but slowly, as if collecting strength for the more
vigorous effort of expansion which afterwards
takes place. As it grows, the membrane is
pushed forwards, becoming thinner in proportion
as it is stretched ; till at length it gives way,
and the soft claw is exposed to view. The claw
now enlarges rapidly, and in a few days more
acquires a shell as hard as that which had pre-
ceded it. Usually, however, it does not attain
the same size ; a circumstance which accounts
for our frequently meeting with lobsters and
crabs which have one claw much smaller than
the other. In the course of the subsequent
castings, this disparity gradually disappears.
The same power of restoration is found to reside
in the legs, the antennae, and the jaws.
We must naturally be curious to learn, if pos-
sible, from what source these astonishing powers
of regeneration are derived. Reaumur hazarded
the conjecture, that there might be originally
implanted in each articulation a certain number
of embryo limbs, ready to be developed as occa-
sion might require ; somewhat in the way in
which the rudiments of the secondary teeth
remain concealed in the jaw, in preparation for
replacing the first set when these have been re-
290 THE MECHANICAL FUNCTIONS.
moved. But this hypothesis is overturned by
the fact that if the animal loses only part of the
limb, it is the deficient portion alone, and not the
whole limb which is regenerated. The sprouting
of the new claw bears a strong analogy to the
budding of a plant ; both having their origin
from an imperceptible atom, or germ, which is
either formed on the occasion, or had pre-existed
in the organization. We are, however, totally
destitute of the means of deciding which of these
alternatives is nearest to the truth. It is but too
probable that the agents which can effect such
wonderful operations will ever baffle our most
scrutinizing inquiries, and that they are of too
refined an order to come within the reach of the
most subtle conjectures that human imagination
can devise.
Chapter V.
INSECTS.
§ 1. Aptera.
Apteuous, or wingless insects form the next term
in the series of articulated animals. Closely
allied in their organization to many of the pre-
ceding families, they differ from them in being
essentially formed for a terrestrial, instead of an
aquatic, life. Most of the lower tribes of this
APTEROUS INSECTS. 297
order are parasitic, that is, derive their nourish-
ment from the juices of other animals, the skin
of which they infest and penetrate, and into
which they insert tubes for suction. The various
tribes of Acari, or mites, of Pedieuli, or lice, of
Ricini, or ticks, of Puliccs, or fleas ; together with
the Podura, or spring- tail ; the Lepismat and
the family of Myriapoda, or millepedes, are
comprehended in this order. I shall be obliged
to pass over these tribes very cursorily, noticing
only a few of the more remarkable circumstances
attending their mechanical conformation.
The Pulex is the only apterous insect which
undergoes complete metamorphoses in the course
of its developement. In the first stage of its
existence, it has the form of a long worm, without
feet, frequently rolling itself into a spiral coil.
It consists of thirteen segments, having tufts of
hair growing upon each. In its mature state it
has six articulated legs, the hindmost of which
are of great size, for the purpose of enabling the
insect to take those prodigious leaps which as-
tonish us in beings of so diminutive a size, and
afford a striking proof of the exquisite mecha-
nism pervading even the lowest orders of the
animal creation.
The Podura leaps into the air by a mecha-
nical contrivance of another kind ; employing
for this purpose the tail, which is very long, and
forked at the end. In its ordinary state this
298 THE MECHANICAL FUNCTIONS.
organ is kept folded under the abdomen, where
it is concealed in a groove. The pieces of
which it is composed are articulated together in
such a manner as to admit of their being rapidly
unbent by the action of its muscles, the whole
mechanism conspiring to produce the effect of a
powerful spring, by which the body is propelled
forwards to a considerable distance. In some
species, this flexible tail has a flattened form,
for the purpose of enabling the insect to leap
from the surface of water, an action which it
performs with apparently as much ease as if it
sprung from a solid resisting plane.
The Lepisma leaps by means of moveable
appendages, placed in a double row along the
under side of the body, and acting like springs.
There are eight pair of these members, cor-
responding in situation and structure to the
false feet of the Crustacea, and, like them, ter-
minating in jointed filaments.
The Julus and the Scolopendra, which com-
pose the family of the Myriapocla, so called from
the immense number of their feet, undergo, to a
certain extent, a kind of metamorphosis in the
progress of their developement. When first
hatched they have often no feet whatever, and
resemble the simpler kinds of worms. Legs at
length make their appearance; but they arise in
succession, and it is not until the later periods
of their growth that these animals acquire their
WINGED INSECTS. 299
full complement of segments, with their accom-
panying legs. The Julus terrestris, for example,
(Fig. 143) has, at its entrance
into the world, only eight
^^^mfef segments and six feet ; but
acquires in the course of its
developement, fifty segments and about two
hundred feet. The anterior legs are directed
obliquely forwards, and the rest more or less
backwards. The mandibles have the form of
small feet ; as we have seen is frequently the
case in crustaceous animals.
^ 2. Insecta (data.
Our attention is now to be directed to the more
highly developed Insects, which have been
formed with a view to progression through the
air. On these, which compose the most exten-
sive class of the whole animal kingdom, Nature
has lavished her choicest gifts of animal powers,
as far as they are compatible with the diminu-
tive scale to which she has restricted herself in
their formation. The model she has chosen for
their construction is that which combines the
greatest security against injurious impressions
from without, with the most extensive powers of
locomotion ; and which also admits of the fullest
300
THE MECHANICAL FUNCTIONS.
exercise of all those faculties of active enjoyment
which are characteristic of animal life. She has
provided for the first of these objects by en-
closing the softer organs in dense and horny
coverings, which perform the office of an ex-
ternal skeleton, sustaining and protecting the
viscera, and furnishing extensive surfaces of
attachment to the muscles, from the action of
which all the varied movements of the system
are derived.
The muscular system of perfect insects is ex-
ceedingly complex. Lyonet has described and
delineated an immense number of muscular
bands in the caterpillar of the Cossus, and the
plates he has given have been copied in a va-
riety of books in illustration of this part of the
structure of insects. The recent work of Straus
Durckheim affords an equally striking example
of admirable arrangement in the muscles of the
Melolontha vulgaris, or cockchaffer, the ana-
tomy of which has been minutely investigated by
WINGED INSECTS. ,301
that distinguished entomologist. These muscles
are represented in Fig. 144, which has been care-
fully reduced from his beautifully executed plates.
The largest mass of muscular fibres is that marked
a, constituting the muscles which depress the
wings, and which are of enormous size and
strength.
On examining the different structures which
compose the solid frame-work of insects, we find
them conforming in every instance to the general
type of Annulose animals, inasmuch as they con-
sist of thickened portions of integument, encir-
cling the body ; but variously united and con-
solidated, for the manifest purpose of obtaining
greater mechanical strength and elasticity than
if they had remained detached pieces, joined only
by membranous connexions. A long flexible
body, such as that possessed by the Myriapoda,
could not easily have been transported through
the air ; for every bend would have created a
resistance, and have impeded its advance during
flight. Hence the body of the insect, which is
to be ultimately adapted to this mode of pro-
gression, has been shortened by a reduction in
the number of its segments, and rendered more
simple and compact. The segments destined to
support the wings have been expanded for the
purpose of lodging the powerful muscles which
are to move them ; and rendered dense and un-
yielding in order to support their action.
30*2 THE MECHANICAL FUNCTIONS.
Nature has farther provided insects with in-
struments adapted to different kinds of external
actions. They consist of articulated levers, va-
riously combined together, and forming legs,
claws, pincers, oars, palpi, and, lastly, wings,
calculated for executing every variety of pre-
hension, of progression, or whatever other action
their wants and necessities require.
<§ .3. Dev elopement of Insects.
It would appear as if the final accomplishment
of objects so numerous, so widely different, and
so liable to mutual interference, could be at-
tained only by the animal being subjected to a
long series of modifications, and passing through
many intermediate stages of developement. The
power of flight is never conferred upon the in-
sect in the earlier periods of its existence : for
before its structure can obtain the lightness
which fits it for rising in the air, and before it
can acquire instruments capable of acting upon
so light an element, it has to go through several
preparatory changes, some of which are so con-
siderable as to justify the term of metamorphoses,
which has been generally given to them.* But
* Transformations quite as remarkable occur in several tribes
of animals belonging to other classes: such as those of the Frog
among reptiles, and of the Lerncea among parasitic worms.
WINGED INSECTS. .'K)3
transient is the state of perfection in every thing
that relates to animal existence. When the in-
sect has, by a slow developement, reached this
ultimate elaboration of its organs, its life is
hastening to a close ; and the period of its per-
fect state is generally the shortest of its whole
existence.
The history of the successive stages of the de-
velopement of insects opens a highly interesting
field of philosophical inquiry. For a certain pe-
riod of the early life of these animals, the growth
of all the parts appears to proceed equably and
uniformly : but at subsequent epochs, some parts
acquire a great and sudden increase of size, and
others that were in a rudimental condition become
highly developed, and constitute what appear to
be new forms of organs, although their elements
were in existence from a much earlier period.
The modifications which the harder and more
solid structures of insects exhibit in the progress
of these changes, are particularly remarkable, as
illustrating the principles on which the develope-
ment is conducted. The researches of modern
entomologists have led to the conclusion that the
frame-work, or skeleton of insects, is always
formed by the union of a certain determinate
number of parts, or elements, originally distinct
from one another, but which are variously joined
and soldered together in the progress of growth :
frequently exhibiting a great disproportion in
304 THE MECHANICAL FUNCTIONS.
the comparative expansion of different parts.
The enlargement of any one part, however, ex-
ercises a certain influence on all the neighbour-
ing parts, and thus are the foundations laid of
all the endless diversities which characterize the
several species belonging to each tribe and
family.
In the progress of developement, we may re-
cognize two principles, which, though apparently
opposite to each other, concur and harmonize in
their operation : these are expansion and concen-
tration. Thus while those segments of body
which follow the head are greatly enlarged, in
order to support the more recently developed
organs of progressive motion, they are also more
consolidated, and rendered stronger by the union
of several pieces which were before separate.
The posterior segments, having no such appen-
dages to support, are less dilated, and the whole
body is much shortened by the approximation
of the segments, which in this way compose the
abdomen, or hinder division of the insect.
The progress of the metamorphoses of insects
is most strikingly displayed in the history of the
Lepidopterous, or butterfly and moth tribe.*
* The four periods of the existence of the Bombyx mori, or
the moth of the silk-worm, are shown in the annexed engravings;
Fig. 145 are the eggs ; Fig. 146, the Larva, or caterpillar ; Fig.
147, the Pupa, or chrysalis; and Fig. 148, the Imago, or per-
fect insect.
DEVELOPEMENT OF INSECTS.
305
The egg, which is deposited by the butterfly, gives
birth to a caterpillar; an animal, which, in out-
ward shape, bears not the slightest resemblance
to its parent, or to the form it is itself afterwards
to assume. It has, in fact, both the external
appearance, and the mechanical structure of a
worm. The same elongated cylindric shape, the
same annular structure of the denser parts of its
integument, the same arrangements of longitudi-
nal and oblique muscles connecting these rings,
the same apparatus of short feet, with claws, or
bristles, or tufts of hairs, for facilitating pro-
gression ; in short, all the circumstances most
characteristic of the vermiform type are equally
exemplified in the different tribes of caterpillars,
as in the proper Annelida.
But these vermiform insects have this pecu-
liarity, that they contain in their interior the
rudiments of all the organs of the perfect insect.
These organs, however, are concealed from view
vol. i. x
306 THE MECHANICAL FUNCTIONS.
by a great number of membraneous coverings,
which successively invest one another, like the
coats of an onion, and are thrown off, one after
another, as the internal parts are gradually de-
veloped. These external investments, which
hide the real form of the future animal, have
been compared to a mask ; so that the insect,
while wearing this disguise, has been termed
larva, which is the Latin name for a mask.
This operose mode of developement is ren-
dered necessary in consequence of the greater
compactness of the integuments of insects, as
compared with those of the annelida. In pro-
portion as they acquire density, they are less
capable of being farther stretched, and at length
arrive at the limit of their possible growth. Then
it is that they obstruct the dilatation of the in-
ternal organs, and must be thrown off to make
way for the farther growth of the insect. In the
mean time, a new skin has been preparing under-
neath, moulded on a larger model, and admitting
of greater extension than the one which preceded
it. This new skin, at first, readily yields to the
distending force from within, and a new impulse
is given to the powers of developement ; until,
becoming itself too rigid to be farther stretched,
it must, in its turn, be cast off in order to give
place to another skin. Such is the process
which is repeated periodically, for a great number
of times, before the larva has attained its full size.
DEVELOPEMENT OF INSECTS. 307
These successive peelings of the skin are but
so many steps in preparation for a more impor-
tant change. A time comes when the whole of
the coverings of the body are at once cast off,
and the insect assumes the form of a pupa, or
chrysalis; being wrapt as in a shroud, present-
ing no appearance of external members, and
retaining but feeble indications of life. In this
condition it remains for a certain period : its
internal system continuing in secret the farther
consolidation of the organs ; until the period
arrives when it is qualified to emerge into the
world, by bursting asunder the fetters which had
confined it, and to commence a new career of
existence. The worm, which so lately crawled
with a slow and tedious pace along the surface
of the ground, now ranks among the sportive
inhabitants of air ; and expanding its newly ac-
quired wings, launches forward into the element
on which its powers can be freely exerted, and
which is to waft it to the objects of its gratifica-
tion, and to new scenes of pleasure and delight.
Thus do the earlier stages of the developement
of insects exhibit a recurrence of those structures
which are found in the lowest department of this
series of animals. The larva, or infantile stage
of the life of an insect, is, in all its mechanical
relations, a mere worm. The imago, or perfect
state, on the other hand, exhibits strong analogies
with the crustaceous tribes, not only in the
308 THE MECHANICAL FUNCTIONS.
general form of the body, but also in the consoli-
dated texture of its organs, (especially of those
which compose its skeleton) and in the possession
of rigid levers, shaped into articulated limbs,
and furnished with large and powerful muscles,
from all which circumstances great freedom and
extent of motion are derived. To this elaborate
frame, nature has added wings, those refined
instruments of a higher order of movements, sub-
servient to a more expanded range of existence,
and entitling the beings on which they have
been conferred to the most elevated rank among
the lesser inhabitants of the globe.
The mechanical functions of insects scarcely
admit of being reduced to general principles, in
consequence of the great diversity of forms, of
habits and of actions, that is met with among the
innumerable host of beings which rank under
this widely extended department of the animal
creation. In these minute creatures may be
discovered all the mechanical instruments and
apparatus required for the execution of those
varied movements which we witness in the larger
animals, and which, though almost peculiar to the
different classes of those animals, are here fre-
quently united in the same individual. Insects
swim, dive, creep, walk, run, leap, or fly with as
much facility as fishes, reptiles, quadrupeds, or
birds. But besides these, a great number have
also movements peculiar to themselves, and of
I
PROGRESSIVE MOTION IN INSECTS. 309
which we meet with no example in other parts
of the animal kingdom.
In attempting to delineate a sketch of the
movements of insects, and of the mechanism by
which they are performed, I am compelled, by
the great extent of the subject, to confine myself
to very general views ; and must refer such of
my readers as are desirous of fuller information
on this subject to the works of professed ento-
mologists.
The mechanical conditions of an insect in its
several states of larva, pupa, and imago, are so
widely different, that it will be necessary to con-
sider each separately. In many tribes, however,
the difference between the larva and the perfect
insect is much less considerable than in others.
Those belonging to the orders of Hcmiptera and
Orlkoptera, for example, come out of the egg
with nearly the same form as that which they
have in the mature state ; excepting that they
are without wings, these organs being added in
the progress of their growth, and. constituting,
when acquired, their perfect or imago condition.
§ 4. Aquatic Larvce.
Many insects, which, when fully developed, are
the most perfectly constructed for flying, are,
when in the state of larvae, altogether aquatic
310 THE MECHANICAL FUNCTIONS
animals. Some of them are destitute of feet, or
other external instruments of motion, swimming
only by means of the alternate inflections of the
body from side to side, in the same manner as
the Nais, and the Leech. Sometimes these
actions are performed by abrupt strokes, giving
rise to an irregular zig-zag course : this is the
case with the larva of the gnat, and with many
others which have no feet. In the structure of
the larva of the Libellula, or dragon-fly, a sin-
gular artifice has been resorted to for giving an
impulse to the body, without the help of external
members. It is that of the alternate absorption
of water into a cavity in the hinder part of the
body, and its sudden ejection from that cavity, so
that the animal is impelled in a contrary direc-
tion, upon the same principle that a rocket rises
in the air by the reaction of that fluid. It has
at various times been proposed to apply the
power of steam to the production of an effect
exactly similar to that of which Nature here pre-
sents us with so perfect an example, for the pur-
pose of propelling ships, instead of the ordinary
mode of steam navigation.
Some larvae, such as that of the Stratiomys,
collect a bubble of air, which they retain within
a tuft of hair at the extremity of the tail, evi-
dently with a view of diminishing the specific
gravity of the body, and thus giving greater
efficacy to the muscular actions which they
AQUATIC LARViE. 311
employ in their progression through the water.
Another use is also made of these tufts of hair ;
for by repelling the water, they allow of the
insect's suspending itself from the surface of the
fluid in the manner already noticed in giving
the history of the evolutions of the hydra.*
The impulse given by the lateral inflections of
the body are in many cases assisted by short
legs ; but the larvae of the Ephemera, though
furnished with legs, do not use them for this pur-
pose, and swim simply by the action of the tail.
Those of the Dytiscus are furnished with a pair
of very long members, projecting to a consider-
able distance from the sides, and flattened at the
ends, to serve as oars. The larvae of the Hydro-
philus are also admirably formed for swimming ;
and they not only dart forwards with surprising
velocity, but also turn in all directions with the
utmost facility.
§ 5. Terrestrial Larva.
The movements of larvae that are not aquatic
are perfectly analogous to those of the Annelida,
which they much resemble in their outward
form and mechanical structure. The muscles
by which the annular segments of the body are
moved, are exceedingly numerous, and beauti-
* Page 179.
312 THE MECHANICAL FUNCTIONS.
fully arranged with reference to the motions
they are intended to effect. The investigation
of the structure of these minute organs has long
exercised the talents of the most skilful entomo-
logists, and still offers much that remains to be
explored. The researches of Lyonet, already
alluded to, on the anatomy of the larva of the
Hornby x Cossus* of which he has published an
elaborate description, accompanied by admi-
rable engravings, will ever remain a splendid
monument of patience and ingenuity in over-
coming the difficulties which impede this kind
of inquiry. In the body and the limbs of this
caterpillar, Lyonet counted above 4000 sepa-
rate muscular bands, all arranged with the most
perfect symmetry, and adapted with wonderful
precision to the performance of the required
effects.
In these larvae, as in the simpler forms of
the Annelida, progression is often accomplished
solely by the alternate contraction and exten-
sion of the annular segments, aided, in many
cases, by short hairs, and frequently, also, by
a slimy secretion which exudes from their bo-
dies. Many larvae which are destitute of feet,
move onwards by first coiling the body into a
circle, making the head and the tail meet, and
then springing forwards by a sudden extension
of the back, producing an effect like the un-
* Cossus ligniperda. Fabricius.
TERRESTRIAL LARViE. 313
bending of a bow. By an artifice of the same
kind, some larvae contrive to leap to a consider-
able distance, by the violent effort which they
make in unfolding the curvatures of their bodies.
Some larvae avail themselves of their jaws in
order to fix the head, and drag the rest of the
body towards it. In this manner do the larvae of
the Cerambyx, or Capricorn beetle, advance along
the winding passages which they have themselves
excavated; holding by the jaws, and dragging
themselves forwards. These movements are as-
sisted by the resistance afforded by short tuber-
cles which project from different parts of the back,
and under surface of the body ; so that these in-
sects advance in the passage by an act similar to
that by which a chimney-sweeper, exerting the
powerful pressure of his elbows, shoulders, and
knees, manages to climb up a chimney.
For the purpose of enabling insects to take
stronger hold of the surfaces they pass over, we
often observe them furnished with spines, or
hooks, which are moved by appropriate muscles;
and they occupy different situations on the body.
Modifications without end occur with regard to
these and other external parts subservient, in
various degrees, to progressive motion. Every
possible gradation is also seen between the short
tubercles already mentioned, and the more re-
gularly formed feet or legs. Those which are
regarded as spurious legs, or prolegs, as they
314 THE MECHANICAL FUNCTIONS.
have been called, occupy an intermediate place
between these two extremes. They consist of
fleshy and retractile tubercles, and are often
very numerous ; while the number of the true
legs, as they are called, is limited to six. These
last are the representatives of the legs of the
future perfect insect ; for they are attached to
the three first segments of the thorax ; and are
formed of those portions articulated to each
other, corresponding to the three principal joints
of the imago. The true legs are generally pro-
tected by horny scales ; but the coverings of the
prolegs are wholly membranous. The office of
these spurious legs is merely to serve as props
to support the body while the insect is walking,
and to prevent its hinder part from trailing on
the ground. They are frequently terminated by
single or double hooks ; and also by a marginal
coronet of recurved spines. These hooks, or
spines, enable the insect to cling firmly to
smooth surfaces ; and also to grasp the most
slender twig, which could not have been laid
hold of by legs of the usual construction.
The speed with which these larvae can ad-
vance is regulated by many circumstances, in-
dependently of the mere possession of legs ; for
some caterpillars move slowly, while others can
run very nimbly. The following is the order in
which the legs are usually moved ; namely, the
anterior and the posterior leg on the same side
TERRESTRIAL LARVAE. 315
are advanced at the same moment, together with
the intermediate one on the other side ; and this
takes place alternately on both sides.
There is one tribe of caterpillars, called Sur-
veyors, or Geometers, (Fig. 148*, a) which walk
by first fixing the fore feet, and then doubling
the body into a vertical arch ; this action brings
up the hind part of the caterpillar, which is fur-
nished with prolegs, close to the head. The hind
extremity being then fixed by means of the pro-
legs situated at that part, the body is again ex-
tended into a straight line; and this process being
repeated, the caterpillar advances by a succession
of paces, as if it were measuring the distance, by
converting its body into a pair of compasses. At
the same time that they employ this process, they
farther provide for their security by spinning a
thread, which they fasten to different points of
the ground as they go along. The great force
exerted by the muscles of many caterpillars is
exemplified by their often fixing themselves to
an object, and extending the body to a distance,
as if it were a rigid cylinder : this attitude is
shown in Fig. 148* b.
316 THE MECHANICAL FUNCTIONS.
Many other species of caterpillar practise the
same art of spinning fine silken threads, which
especially assist them in their progression over
smooth surfaces, and also in descending from a
height through the air. The caterpillar of the
cabbage butterfly is thus enabled to climb up
and down a pane of glass, for which purpose it
fixes the threads which it spins in a zig-zag line,
forming so many steps of a rope ladder. The
material of which these threads are made is a
glutinous secretion, which, on being deposited
on glass, adheres firmly to it, and very soon
acquires consistence and hardness by the action
of the air.
Other caterpillars, which feed on trees, and
have often occasion to descend from one branch
to another, send out a rope made with the same
material, which they can prolong indefinitely ;
and thus either suspend themselves at pleasure
in the air, or let themselves down to the ground.
They continue, while walking, to spin a thread
as they advance, so that they can always easily
retrace their steps, by gathering up the clue
they have left, and reascend to the height from
which they had allowed themselves to drop.
STRUCTURE OF INSECTS. 317
§ 6. Imago, or Perfect Insect.
The process which nature has followed in the
developement of the structure of insects, has for
its object the gradual hardening and consolida-
tion of texture, and the union and concentration
of organs : for we find that the segments which
were at a distance from one another in the larva,
are approximated in the perfect insect, and
often closely tied together by ligaments ; and in
other cases, adjoining segments cohere so as to
form but a single piece. Thus the number of
separate parts composing the solid fabric is con-
siderably diminished. Other segments, again,
fold inwardly, forming internal processes, and
adding to the extent and complication of the
skeleton.
The integuments of perfect insects, being de-
signed to be permanent structures, are thicker
and more rigid than those of their larva?, and
are formed of several layers, in which the com-
ponent parts of the integuments of the larger
animals may readily be distinguished. Their
rigidity does not, like that of shells, arise from
the presence of carbonate of lime ; for they con-
tain but a small proportion of this material : and
whatever calcareous ingredient enters into their
composition is in the form of phosphate of lime.
318 THE MECHANICAL FUNCTIONS.
In external appearance their texture approaches
nearer to that of horn than to any other animal
product ; yet in their chemical composition they
differ from all the usual forms of albuminous
matter. The substance to which they owe their
characteristic properties is of a very peculiar
nature ; it has been termed Ckitine by M. Odier,*
and Entomoline by M. Lassaigncj" This sub-
stance is found in large quantity in the wings
and elytra of coleopterous insects. It is re-
markable for not liquefying, as horn does, by
the action of heat ; and accordingly the integu-
ments of insects, even after having been sub-
jected to a red heat, and reduced to a cinder,
are found to retain their original form.|
With this substance there is blended a quan-
tity of colouring matter, which has usually a dull
brown or black hue. But the colour of the ex-
ternal surface is generally owing to another por-
tion of this matter, which is spread over it like
a varnish, and being soluble in alcohol and in
ether, may be removed by means of these agents.
The colours which are displayed by insects, and
* Annales de Chimie, torn. 76.
f See the work of Straus Durckheim, p. 33.
X M. Odier bad concluded from his experiments that no
nitrogen enters into the composition of this substance. That
this conclusion has been too hastily adopted has been proved by
Mr. Children, who, by pursuing another mode of analysis, found
that the chitine of cantharides contains not less than nine or ten
per cent, of nitrogen. See Zoological Journal, i. Ill — 115.
STHUCTURE OF INSECTS. 'AID
which arise from the presence of this latter sub-
stance, are often very brilliant, and, as is the
case with many other classes of animals, the in-
tensity of the tints is heightened by the action
of light. The elytra of tropical insects display
a gorgeous metallic lustre, depending on the re-
flexion of the prismatic colours ; and the same
variegated hues adorn the scales of the butter-
flies of those regions.
Hair grows in various parts of the surface of
insects. Where the integument is membranous
and transparent, these hairs may be distinctly
perceived to originate from enlarged roots, or
bulbs, and to pass out through apertures in the
skin ; as is the case with the hair of the larger
animals. Their chemical composition, however,
is very different, for they are formed of the same
substance as the integuments, namely entomo-
line. The purposes served by the hairs are not
always obvious. In many cases they seem in-
tended to protect the integuments from the water,
which they repel from their surfaces. They also
tend to prevent injury arising from friction ; and
are accordingly found to be more abundant in
those parts, as the joints, which are liable to rub
much against one another.
The divisions of the body are frequently
marked by deep incisions ; whence has origin-
ated the term insect, expressive of this separation
into sections. It is, however, a character which
320
THE MECHANICAL FUNCTIONS.
they possess in common with all articulated ani-
mals, the typical form of which consists, as we
have seen, of a series of rings, or segments,
joined endwise in the direction of a longitudinal
axis. The principal portions into which the
body is divided are the
head, the trunk, and
the abdomen : each of
which is composed of
several segments. I
have here given in il-
lustration, the annexed
figures, showing the
successive portions into
which the solid frame-
work, or skeleton, of
one of the beetle tribe,
the Calosoma sycophanfa* may be separated.
The entire insect, which presents the most per-
fect specimen of a complete skeleton in this class
of animals, is represented in Fig. 149 ; and the
several detached segments, on an enlarged scale,
in Fig. 150. The head (c), as seen in the latter
figure, may be regarded as being composed of
three segments ; the trunk (x, y, z), of three ; and
the abdomen (b), of nine. Fig. 151, is a view of
the head separated from the trunk, and seen from
behind, in order to show that its form is essen-
Carabits sycophant a. Linn.
STRUCTURE OF INSECTS.
321
tially annular, and that it resembles in this re-
spect the rings of which the thorax consists, and
to which it forms a natural sequel.
The head contains the brain, or principal en-
largement of the nervous system, and the organs
of sensation and of mastication. Its size, as com-
pared with the rest of the body, varies much in
VOL. I. Y
32*2 THE MECHANICAL FUNCTIONS.
different insects, and is in general proportionably
larger than it is in the larva state. Its integu-
ment, which, from analogy with vertebrated ani-
mals, has been called the skull, or cranium,
(c, Fig. 150), is usually the hardest part of the
general crust. Although it may appear, on a
superficial examination, to consist of a single un-
divided piece, yet, on tracing its gradual forma-
tion, it is found to be in reality composed of a
union of several of the segments of the larva.
Audouin and Carus distinguish three component
segments in the cranium of insects; while Straus
Durckheim considers it as formed by the con-
solidation of no less than six segments of the
vermiform larva. According to this theory, the
same elements which in the thoracic segments
are developed into feet, are here employed to
form parts having other destinations. From the
segment adjacent to the thorax the antennae are
supposed to be developed. The two anterior
segments belong properly to the face ; the one
giving origin to the mandibles (m), to the
maxillae, or proper jaws, (j), and also to the palpi
(p); the other, producing the processes called the
labial palpi (l).
The mode in which the head is connected
with the trunk varies much in different insects.
Sometimes it is united by a broad basis of at-
tachment, forming a joint between the adjacent
surfaces : but usually it is only appended by a
STRUCTURE OF INSECTS. 323
narrow filament, or neck ; so that the articulation
is effected by ligament alone. Occasionally it
is placed at the end of a long pedicle, which
removes it to a considerable distance from the
trunk. In the Hymcnoptera and Diptera, the
head moves upon a pivot, so as to admit of its
being turned completely round.
The trunk, or Thorax, is composed, as shown
in the figure, of three segments, termed respect-
ively the Prothorax (x) ; the Mesothorax (y) ;
and the Metathorax (z).* The first of these, the
prothorax, carries the first pair of legs ; the se-
cond, or mesothorax, gives origin to the second
pair of legs, and also to the first pair of wings,
or to the elytra (e), as in the example before
us ; and the third, or metathorax, supports
the third pair of legs, and the second pair of
wings (w). These two last segments are closely
united together, but the original distinction
into two portions is marked by a transverse line.
Each of these three segments is divisible into an
upper, a lower, and two lateral portions, which
* In these denominations I have followed the nomenclature of
Victor Audouin (Annales des Sciences Naturelles, torn. i. p. 119),
as being the simplest and the clearest : but other entomologists
have applied the same terms to different parts. The first seg-
ment is termed by Straus Durckheim and other French writers,
the Corselet. Mr. Kirby calls it the Manitrunk, and restricts
the term Prothorax to its upper portion. The united second and
third segments are the Thorax of Straus Durckheim, the Tronc
alifere of Chabrier, and the Alitrank of Kirby.
324 THE MECHANICAL FUNCTIONS.
are joined together at the sides of the trunk ;
these again admit of further subdivision ; but
for the names and descriptions of these smaller
pieces I must refer the reader to works on Ento-
mology. The parts of the thorax to which the
wings are attached indicate the situation of the
centre of gravity of the whole insect ; a point,
which being in the line of the resultant of all the
forces concerned in the great movements of the
body, requires to be sustained by the moving
powers under all circumstances either of action
or repose.
Victor Audouin, who has made extensive re-
searches on the comparative forms of all these
parts in a great variety of insects, appears to have
satisfactorily established the general proposition
that, amidst the endless diversity of forms exhi-
bited by the skeleton of insects, they are invari-
ably composed of the same number of elements,
disposed in the same relative situations and order
of arrangement ; and that the only source of dif-
ference is a variation in the proportional deve-
lopement of these elements. He has also ob-
served that the great expansion of one part is
generally attended by a corresponding diminu-
tion of others.
The third division of the body is termed the
Abdomen (b) ; it is composed of all the remaining
segments, which join to form a cavity enclosing
STRUCTURE OF INSECTS. 325
the viscera subservient to nutrition, respiration,
and reproduction. The number of these abdo-
minal segments is very various in different ge-
nera of insects. Sometimes there appear to be
but three or four; while, in other cases, there
are twelve, or even a greater number. In the
Calosoma (Fig. 150, b), the abdomen has six
complete, followed by three imperfect segments.
Not being intended to carry any of the organs
of progressive motion, they retain the form of
simple hoops, which is the primitive type of
the segments of annulose animals. Each seg-
ment has a ligamentous connexion with the next,
which is often so close, as hardly to admit of
any motion between them; but in other instances
it is more lax, and allows of the abdomen being
flexible. In the former case, which is the con-
struction in all the Coleoptera, or beetles, the
rings have an imbricated arrangement ; that is,
each overlaps the next, often to the extent of
two-thirds of its breadth : so that they present a
succession of spheroidal hoops, capable of being
drawn out, to a certain extent, like the tubes of
a telescope. This very artificial construction is
manifestly designed to allow of a great variety of
movements, determined by the position of the
muscles they enclose : for since the surfaces
which receive, as well *as those which are re-
ceived, are segments of spheroids, this structure
320 THE MECHANICAL FUNCTIONS.
admits of a twisting motion ; and the latter seg-
ment may be pushed more or less into the cavity
of the former, either generally, or on one side.
Each segment, besides being separate from
the rest, is farther divided into an upper, or
dorsal, and a lower, or ventral portion ; each
portion having the form of a semicircle, or rather
of an arch of a circle. These are connected at
the sides by a ligamentous band, which runs the
whole length of the abdomen. Great advantage
results from this division of the circles, allowing
of the upper and lower portions of the abdominal
covering being at one time separated, and at
another brought nearer together ; for thus the
cavity is capable of being enlarged or contracted
in its dimensions, and adapted to the variable
bulk of its contents. It is deserving of notice
that, during the process of transformation, some
of the abdominal segments, which are present
in the larva, disappear entirely, or leave only
imperfect traces of their former existence.
Sometimes the posterior segments become so
exceedingly contracted in their diameter as to
give rise to the appearance of a tail : this is
exemplified in the Panorpa.
The junction of the abdomen with the trunk
is effected in various ways. In all the Coleoptera,
it is united by the whole margin of its base,
without having a narrower part : in other tribes
there is a visible diminution of diameter, forming
STRUCTURE OF INSECTS. 327
a groove all round, or an incision, as it is tech-
nically termed. In the Hymenoptera, this inci-
sion is so deep as to leave only a narrow pedicle,
like a neck, connecting these two divisions of
the body. In some this pedicle is short; in
others, long : in the former case, an exceedingly
refined mechanism is resorted to for effecting
the necessary movements in a part so bulky,
compared with the narrowness of the surface of
attachment.*
Insects in their perfect state have constantly
six legs, which are the developements of the six
proper legs of the same animal in its larva con-
dition ; all the spurious legs having disappeared
during its metamorphosis. We have seen that
in the Myriapoda, the result of developement is
an increase in the number both of segments and
of legs; the reason of which is that, being terres-
trial animals, a lengthened form was more useful
and accordant with their destination ; but in
winged insects, where the object is to procure
the means of flight, the organs require to be con-
centrated, and all superfluous parts must be re-
trenched, and discarded from the fabric. The
multiplication of organs, which, in the former
case, indicated the progress of a higher develope-
ment, would in the latter have been the source
* For the details of this structure I must refer to writers on
entomology, and in particular to Kirby and Spence's " Introduc-
tion to Entomology," vol. iii. p. 701.
328 THE MECHANICAL FUNCTIONS.
of imperfection. As long as the insect remains
in its larva stage, its condition is analogous to
that of the myriapode : but in the more elevated
state of its existence, its structure is subject to
new conditions, and regulated by new laws.
While the number of members is thus reduced,
ample compensation is given by their increased
activity and power, derived from their augmented
length, and the more distinct lever-like forms of
the pieces which compose them.
These pieces (see Fig. 150) are named, from
their supposed analogy to the divisions of the
limbs of the higher orders of vertebrated animals,
the haunch (h), the trochanter (t), the femur
(f), the tibia (s), and the tarsus (r). In general
the femur (or thigh) has nearly a horizontal, and
the tibia (or leg) a vertical position, while the
whole tarsus (or foot) is applied to the ground.
The haunch (h), which is supposed to corres-
pond to the hip bone of quadrupeds, is a broad,
but very short truncated cone. The mode of its
articulation with the trunk admits of great variety ;
sometimes it is united by a ball and socket joint,
as in the Curculio and Cerambyx ; and it then
has, of course, great freedom of motion : at other
times the joint is of the hinge kind, as in the
Melolontha. The trochanter (t), and the femur
(f), though in reality distinct pieces, are usually
so firmly united as to compose only one division
of the limb. The articulation of this portion
STRUCTURE OF INSECTS. 329
with the haunch is always effected by a hinge-
joint. Joints of this description, when formed,
as they are in insects, by the apposition of two
tubular pieces, are constructed in the following
manner. One of the tubes has, at the end to be
articulated, two tubercles, which project from
the margin, and are applied to the adjacent end
of the other tube, at two opposite points of its
circumference ; the line which passes through
those two points being the axis of motion. On
the side where the flexion is intended to be
made, both tubes are deeply notched, in order
to admit of their being bent upon one another at
a very acute angle ; and the space left by these
notches is filled up by a pliant membrane, which
performs the office of a ligament. These articular
tubercles and depressions are so adjusted to one
another, that the joint cannot be dislocated with-
out the fracture of some of its parts. As the
different axes of motion in the successive joints
are not coincident, but inclined at different
angles to one another, the extent of motion in
the whole limb is very greatly increased.
Thus in the cases where the articulation of the
haunch with the trunk is a hinge joint, the
axes of this joint, and of the next, are placed
at right angles to each other ; so that there
results, from the combination of both, a capa-
bility in the thigh of executing a circular mo-
tion, in a manner almost as perfect as if it had
.'i;>0 THE MECHANICAL FUNCTIONS.
revolved in a spherical socket. The principle of
this compound motion is the same as that em-
ployed on ship-board for the mariner's compass,
and other instruments which require to be kept
steady during the motion of the ship. For this
purpose what are called gimbals are used, the
parts of which have two axes of rotation, at right
angles to each other, so as to enable the compass
to take its proper horizontal position, indepen-
dently of any inclination of the ship.
The tibia, or shank (s), is joined at an acute
angle with the femur ; and is frequently either
beset with spines, or else notched or serrated.
The tarsus, or foot (r), is the last division of
the limb : it is divided into several joints, which
have been supposed to represent those of the
toes of quadrupeds. The joints are generally
of the hinge kind, but some are met with of a
more rounded form, and approaching to that
of the ball and socket. The whole structure
is most admirably adapted to its exact appli-
cation over all the inequalities of the surfaces
on which the insect treads. But as the habits
and modes of life of this numerous class are
exceedingly diversified, so the form of the feet
admits of greater variety than that of any other
part of the limb.
The feet of insects diverge, and spread over a
wide surface ; thus extending the base of sup-
port so as to ensure the stability of their bodies
STRUCTURE OF INSECTS. 331
in the most perfect manner. When the legs are
very long, as in the Tipula* the body seems,
indeed, more to be suspended than supported by
them ; contrary to what obtains in quadrupeds,
where the feet are more immediately underneath
the points at which they are connected with the
trunk.
The last joint of the tarsus is generally ter-
minated by a claw, which is sometimes single
and sometimes double, and which contributes to
fasten the foot, under a variety of circumstances,
both of action and of repose. By means of feet
thus armed, the insect can ascend or descend
the perpendicular sides of a rough body with the
greatest ease ; but it is scarcely able to advance
a single step upon glass, or other polished sur-
faces, even when horizontal. The hooks at the
ends of the anterior pair of feet are directed
backwards, those of the middle pair inwards,
and of the posterior pair forwards ; thus afford-
ing the greatest possible security against dis-
placement.
Many insects are provided with cushions at
the extremity of the feet, evidently for the pur-
pose of breaking the force of falls, and prevent-
ing the jar which the frame would otherwise
have to sustain. These cushions are formed of
* It has been conjectured that the object in furnishing this
insect with legs of so great a length is that of enabling it to
walk among blades of grass.
332 THE MECHANICAL FUNCTIONS.
dense velvetty tufts of hair, lining the underside
of the tarsi, but leaving the claw uncovered ; and
the filaments, by insinuating themselves among
the irregularities of the surfaces to which they
are applied, produce a considerable degree of
adhesion. Cushions are met with chiefly in large
insects which suddenly alight on the ground
after having leaped from a considerable height :
in the smaller species they appear to be unne-
cessary, because the lightness of their bodies
sufficiently secures them from any danger arising
from falls.
Some insects are furnished with a still more
refined and effectual apparatus for adhesion, and
one which even enables them to suspend them-
selves in an inverted position from the under
surfaces of bodies. It consists of suckers, the
arrangement and construction of which are ex-
ceedingly beautiful ; and of which the common
house-fly presents us with an example. In this
insect that part of the last joint of the tarsus
which is immediately under the root of the claw,
has two suckers appended to it by a narrow
funnel-shaped neck, moveable by muscles in all
directions. These suckers are shown in Fig.
152, which represents the under side of the foot
of Musca vomitoria, or blue-bottle fly, with the
suckers expanded. The sucking part of the
apparatus consists of a membrane, capable of
contraction and extension, and the edges of
STRUCTURE OF INSECTS.
333
which are serrated, so as to fit them for the
closest application to any kind of surface. In
the Tabanus, or horse-fly, each foot is furnished
with three suckers. In the Cimbex hitea, or
yellow saw-fly, there are four, of which one is
placed upon the under surface of each of the
four first joints of the toes (Fig. 153) ; and all
the six feet are provided with these suckers.
In the Dytiscas marginalis, suckers are fur-
nished to the feet of the male insect only. The
three first joints of the feet of the fore-legs of
that insect have the form of a shield, the under
surface of which is covered with suckers having
long tubular necks ; there is one of these suckers
very large, another of a smaller size, and a
great number of others exceedingly small. A
few of the latter kind are represented highly
magnified in Fig. 154. In the second pair of
feet, the corresponding joints are proportionally
much narrower, and are covered on their under
surface with a multitude of very minute suckers.
The Acridium biguttulum, which is a species of
grasshopper, has one large oval sucker, under
334 THE MECHANICAL FUNCTIONS.
the last joint of the foot, immediately between
the claws. On the under surface of the first
joint are three pair of globular cushions, and
another pair under the second joint. Fig. 155
shows these parts. The cushions are filled
with an elastic fibrous substance ; which, in
order to increase the elasticity of the whole
structure, is looser in its texture towards the
circumference.*
The mode in w^hich these suckers operate
may be distinctly seen, by observing with a
magnifying glass the actions of a large blue-
bottle fly in the inside of a glass tumbler. A
fly will, by the application of this apparatus,
remain suspended from the ceiling for any
length of time without the least exertion ; for
the weight of the body pulling against the
suckers serves but to strengthen their adhesion :
hence we find flies preferring the ceiling to the
floor, as a place of rest.
Insects which, like the gnat, walk much upon
the surface of water, have at the ends of their
feet a brush of fine hair, the dry points of which
appear to repel the fluid, and prevent the leg
from being wetted. If these brushes be moist-
ened with spirit of wine, this apparent repulsion
no longer takes place ; and the insect imme-
diately sinks and is drowned.
* Philosophical Transactions for 1826, p. 324.
STRUCTURE OF INSECTS. 335
§ 7. Aquatic Insects.
Although many insects are inhabitants of water
while in their larva state, few continue to reside
in that element after they have undergone all
their metamorphoses. When they have attained
the imago state, indeed, every part of their bo-
dies becomes permeated by air, which forms alto-
gether a large portion of their bulk, and gives
to the insect, when it is immersed in water, a
strong buoyant force. As the largest volume of
air is contained in the abdomen, this part is
comparatively lighter than either the trunk or
head ; and the natural position of the insect in
the fluid is oblique to the horizon, the head
being depressed, and the abdomen elevated.
Any force impelling the body forwards in the
direction of its axis tends, therefore, to make it
also descend. The effect of this downward force
is counteracted by the sustaining pressure of the
water, which is directed vertically upwards : so
that the real operation of the force in question is
to carry the body forwards nearly in a horizontal
direction.
In insects destined to move in water, some-
times all the legs, but occasionally only one
pair, are lengthened and expanded into broad
triangular surfaces, capable of acting as oars :
33G THE MECHANICAL FUNCTIONS.
and these surfaces are further extended by the
addition of marginal fringes of hair, so disposed
as to project and act upon the water every time
the impulse is given, but to bend down when
the leg is again drawn up, preparatory to the
succeeding stroke ; thus imitating the action
which is called feathering an oar. The im-
pulses are given with great regularity, all the
feet striking the water at the same moment.
1
**n
Of all the coleopterous insects, the Dytiscus,
or water-beetle (of which Fig. 156 represents
the upper, and Fig. 157 the under side), is the
one best constructed for swimming : its body
having a flattened form, very much resembling
a boat, narrower before than behind, and its
surface presenting no projecting parts. The
upper surface in particular is extremely smooth,
to enable it to glide under the water with the
least possible friction. Its centre of gravity is
placed very near the under surface. The poste-
rior legs, which act as powerful oars, are attached
AQUATIC INSECTS. 337
to very large haunches, for the purpose of con-
taining the thick muscular bands which are in-
serted into the trochanter, and by which these
joints are moved with great power. As the
motion of these oars is to be performed in a
plane nearly parallel to the axis of the body,
the haunches are not required to be moveable:
and accordingly they are firmly united to the
thorax ; a structure which renders the motion of
the other joints more regular and uniform. When
the Dytiscus wishes to rise, it need only desist
from all action, and abandon itself to the buoyant
force of the fluid, which quickly carries it to the
surface.
The Notonecta, or water-boatman (Fig. 158),
is remarkable for always swimming on its back,
a peculiarity depending on
the form of its body, which is
semi-cylindrical, with the legs
affixed to the flat surface ; so
that, when lying on its back
in the fluid, the centre of gravity is below the
centre of the whole figure, or the metacentre, as
it is termed, and the equilibrium is maintained.
It is evident that, under these circumstances, if
it were placed in the water with its legs under-
most, it would unavoidably tilt over, and resume
its usual position. Its long legs extending at
right angles to the body, present a striking re~
vol. i. z
.338 THE MECHANICAL FUNCTIONS.
semblance to the oars of a boat ; and they act,
indeed, in the same manner, and on the same
principles.
§ 8. Progressive Motion of Insects on Land.
The actions of the limbs of insects in walking
are quite different from what they are in swim-
ming, and are very similar to those of the cater-
pillar, in which we have seen that the motions of
the anterior and posterior legs on one side are
combined with that of the middle one on the
other side ; and the two sets of legs are moved
alternately. In consequence of their relative
positions with the trunk, the anterior legs are
advanced by the extension, and the posterior
legs by the flexion of the corresponding joints.
When the feet have fixed themselves on the
ground, the contrary actions take place, and the
body is brought forwards. During this period
the legs which compose the other set are called
into play, and are advanced ; and the same
succession of actions takes place with these as
with the former. This can easily be seen when
the insect walks very leisurely ; but in a more
quickened pace, the succession of actions is too
rapid to be followed by the eye.
The action of leaping is performed by the
PROGRESSIVE MOTION OF INSECTS. :>39
sudden extension of all the joints of the limb,
which are previously folded as close as possible.
The joints principally concerned in this action
are those of the thigh and tibia, as they furnish
the longest and most powerful levers. Prepara-
tory to the effort, the tibia is brought down as
close as possible to the ground, by bending it
over the tarsus ; and the thigh also is bent upon
the tibia, so as to form with it a very acute
angle. In order to enable it to take this posi-
tion with most advantage, we find in many of
the Coleoptera, that the thigh has a longitudinal
groove for the reception of the tibia, with a row
of spines on each side of the groove. While the
limb is in this bent position, the extensor muscles
are violently exerted, and by producing a sudden
unbending of this apparatus of folded springs,
they project the whole body, by the accumu-
lated impulse, to a considerable height in the
air. The leaps of insects being generally for-
wards, all the legs do not participate equally in
the effect; for the fore legs contribute much
less to it than the hind legs, and are more useful
in modifying the direction of the leap, than in
adding to its force. The power of leaping is
derived principally from the great size and
strength of the extensor muscles of the legs,
which, being contained within the femur, neces-
sarily swell that division of the limb to an
340 THE MECHANICAL FUNCTIONS.
unusual thickness ; and in order to procure
sufficient velocity of action, both the femur and
tibia are much elongated. Thus the locust, which
is so constructed, leaps with ease to a distance
two hundred times the length of its own body.
We may in general, indeed, infer the particular
kind of progressive motion for which the insect
is intended by observing the comparative length
of the different pairs of legs. When they are
of equal size, the pace is uniform ; swiftest in
those that have the longest legs ; slowest, when
they are short. When the anterior legs are
much longer than the posterior, the power of
prehension may be increased, but that of pro-
gression is impeded. The great prolongation of
the posterior legs is generally accompanied by
the power of jumping ; unless, indeed, they are
at the same time much bent, for such curvature
disqualifies them from acting advantageously as
levers.
Many insects have the extremity of the tibia
armed with a coronet of spines, which assist in
fixing this point against the plane from which
they intend to spring, and which give to the
limb a steady fulcrum. The Cicada spumaria
has been known to leap to a distance of five or
six feet ; which is two hundred and fifty times
its own length : this, if the same proportions
were observed, is equivalent to a man of ordinary
stature vaulting through the air the length of a
PROGRESSIVE MOTION OF INSECTS. 341
quarter of a mile. When the same insect is laid
on glass, on which the spines cannot fasten, it is
unable to leap farther than six inches.*
The insects belonging to the genus Elater
are provided with a peculiar mechanism for the
special purpose of accomplishing a singular
mode of leaping, independently of any action
of the legs. The legs of this insect are so short,
that, whea it is laid on its back, it cannot turn
itself, being unable to reach with its feet the
plane on which it is lying, and procure a fulcrum
for the action of its muscles. It is apparently
with the design of remedying this inconvenience,
that nature has bestowed on this tribe of insects
the faculty of springing into the air, and making
a somerset, so as to light on the feet ; an effect
which is accomplished by an exceedingly curious
mechanism. The prothorax is prolonged beyond
the length it usually has in other Coleoptera,
and it is articulated with the mesothorax, on the
dorsal side, by two lateral tubercles, which form
a hinge joint, limiting its motions to a vertical
plane. The sternum, or pectoral portion of the
prothorax, is also extended backwards, and ter-
minates in an elastic spine, which is received
into a cavity in the mesothorax, and which,
while the insect is lying on its back, with the
prothorax bent upon the mesothorax, recoils with
* De Geer, iii. 178, quoted by Kirby and Spence.
342 THE MECHANICAL FUNCTIONS.
the force of a spring, and communicates to the
body an impulse which carries it upwards to a
considerable height. If the elater should fail in
its first attempts to recover its feet, it repeats its
leaps till it succeeds. We find no example of a
similar structure in any other part of the animal
kingdom.
The express adaptation of structure to the
mode of life designed for each species of insect
is nowhere more strongly marked than in those
which are intended to burrow in the earth : and
of these the Gryllo-talpa, or mole cricket, pre-
sents a remarkable example. A minute account
of the anatomy of this insect has been given by
Dr. Kidd,* from which it appears that being
destined, like the mole, to live beneath the sur-
face of the earth, and to excavate for itself a
passage through the soil, it is furnished with
limbs peculiarly calculated for burrowing ; with
a skin which, being covered with a fine down,
effectually prevents the adhesion of the moist
earth through which it moves ; and with a form
of body enabling it to penetrate with least re-
sistance the opposing medium. By being en-
dowed with the power of moving as easily in a
backward as in a forward direction, it is enabled
quickly to retreat in the narrow channel it has
excavated ; and as a safeguard in these retro-
* Phil. Trans, for 1825, p. 203.
PROGRESSIVE MOTION OF INSECTS. 343
grade movements, it is provided with a pair of
posterior appendages, which are supplied with
large nerves, and may be regarded as serving
the purpose of caudal antennae.
The fore-legs, (one of which is represented in
Fig. 158*) are the burrowing implements, and
they are admirably cal-
culated for their pecu-
liar office, both in their
shape and in the mode
of articulation of their
several divisions, which bear a considerable ana-
logy to the corresponding member of the mole.
Dr. Kidd observes, that, compared with the
other legs, and with the general size of the
animal, they are as if the brawny hand and
arm of a robust dwarf were set on the body of a
delicate infant ; and the indications of strength
which their structure manifests, fully answer to
their extraordinary size. For a more particular
description of the mechanism of this instrument
I must refer the reader to the paper above
quoted.
344 THE MECHANICAL FUNCTIONS.
§9. Flight of Insects.
If the excellence of a mechanic art be mea-
sured by the difficulties to be surmounted in the
attainment of its object, none surely would rank
higher than that which has accomplished the
flight of a living animal. No human skill has
yet contrived the construction of an automaton,
capable, by the operation of an internal force, of
sustaining itself in the air, in opposition to gra-
vity, for even a few minutes ; and far less of
performing in that element the evolutions which
we daily witness even in the lowest of the insect
tribes. To the ultimate attainment of this fa-
culty it would appear that all the transforma-
tions they undergo in external appearance, and
all the developements of their internal me-
chanism, are expressly directed. Wings are
added to the frame only in the last stage of its
completion ; after it has disencumbered itself of
every ponderous material that could be spared,
after it has been condensed into a small com-
pass, and after it has been perforated in all
directions by air- tubes, giving lightness and
buoyancy to every part. Curiously folded up
in the pupa, the wings there attain their full
dimensions, ready to expand whenever the ban-
dages which surround them are removed. No
FLIGHT Or INSECTS. 343
sooner is the insect emancipated from its con-
finement, than these organs, which are composed
of duplicatnres of a dense, but exceedingly fine
membrane, identical in its composition with the
general integuments, begin to separate from the
sides of the body, and to unfold all their parts.
Their moisture rapidly evaporates, leaving the
delicate film dry and firm, so as to be ready for
immediate action. The fibres, or nervures, as
they are called, form a delicate net-work for the
support of this fine membrane ; like the frame
of the arms of a windmill, which supports the
canvass spread over them. The microscope
shows that these fibres are tubular, and contain
air ; a structure the most effectual for conjoining
lightness with strength ; and many entomologists
are of opinion that the insect has the power,
during the act of flying, of directing air into the
nervures, so as to dilate them to the utmost, and
render them quite tense and rigid.
In the great majority of insects the wings are
four in number ; of which the first pair are, as
we have seen, affixed to the mesothorax, and the
second to the metathorax. These two segments
of the thorax, composing what has been termed
the alitrunk, constitute the most solid portion of
the skeleton, and are frequently strengthened by
ridges, and other mechanical contrivances for
support. The superior extremities of these sup-
ports, which have been compared to the clavicles,
346 THE MECHANICAL FUNCTIONS.
or furcular bones of birds, are always curved in-
wards. This part of the trunk requires to be
alternately dilated and contracted during flight ;
and hence the several pieces of which its dorsal
portion is composed are loosely connected to-
gether by ligaments.*
The shape of the wings is more or less trian-
gular. They are moved by numerous muscles,
which occupy a large space in the interior of the
trunk, and consist of various kinds of flexors,
extensors, retractors, levators, and depressors ;
the whole forming a very complicated assem-
blage of moving powers. The largest, and con-
sequently most powerful of these muscles, are
those which depress, or bring down the wings.
They form a large mass, marked a in Fig. 144.
All these muscles exert great force in their
contractions, which are capable of being re-
newed in very rapid succession ; for, indeed,
unless they had this power, even so light a
body as that of an insect could not have been
sustained for a moment in so rare a medium as
the atmosphere ; far less raised to any height by
its resistance.
The simple ascent and descent of the wings
would be sufficient, without any other movement
being imparted to them, to carry forwards the
* See Chabrier's " Essai sur le Vol des Insectes," Memoires
du Museum d'Histoire Naturelle ; vi. 410, vii. 297, and viii. 47
and 349. See also Zoological Journal; i. 391.
FLIGHT OF INSECTS. 347
body of the insect in the air. The action in
which the muscles exert the greatest force is in
striking the air during the descent of the wing ;
an impulse in the opposite direction being the
result of the reaction of the air. The axis of
motion of the wings is a line inclined at a small
angle to the axis of the body, and directed from
before backwards, outwards, and downwards ;
and they move in a plane, which is not vertical,
but inclined forwards. The angle which the
plane of the wing forms with the horizon varies
continually in the different positions of the wing,
but the general resultant of all these successive
impulses is a force directed forwards and up-
wards ; the first part of this force produces the
horizontal progression of the insect, while the
second operates in counteracting the force of
gravity, and during the advance of the insect,
either maintains it at the same height, or enables
it to ascend.
When the insect wishes to turn, or to pursue
an oblique course, it effects its purpose very
easily by striking the air with more force on one
side than on the other ; or, by employing certain
muscles which bend the body to one side, it
shifts the situation of the centre of gravity, so
that the reaction of the air on the wings is
exerted in a different direction to what it was
before ; and the motion of the body is modified
accordingly.
348 THE MECHANICAL FUNCTIONS.
By exerting with the wings a force just suffi-
cient to balance that of gravity, insects can poise
themselves in the air, and hover for a length
of time over the same spot, without rising or
falling, advancing or retreating ; and the body
may, all the while, be kept either in the hori-
zontal, or in the erect position. In the latter
case the motions are similar to those which take
place in ordinary flying, only they are more
feebly exerted, since all that is required is to
sustain the weight of the body without urging it
to a greater speed. Libellulce, Sphinxes, and a
great number of Diptera, exhibit this kind of
action : among the latter the Stratiomys is most
remarkable for its power of remaining long in
the same fixed position.
The number, form, and structure of the wings
of insects have furnished entomologists with very
convenient characters for their classification : on
these are founded the orders of Coleoptera, Or-
t/wptera, Rhipiptera, Hemiptera, Neuroptera,
Hymenoptera, Diptera, and Lepidoptera. To
enter into any detail in a field of such vast extent
as is presented by the infinitely diversified me-
chanism of the insect creation, would, it is ob-
vious, far exceed the proper limits of this treatise.
I must therefore confine myself to a few leading
points in their structure and modes of progression .
In the Coleoptera, an order which comprehends
FLIGHT OF INSECTS. 349
by far the largest number of genera of insects,
the lower pair of wings (w, Fig. 150, p. .321)
are light and membranous, and of a texture
exceedingly fine and delicate. They are of great
extent compared with the size of the body, when
fully expanded ; and are curiously folded when
not in use. For the protection of these delicate
organs, the parts which correspond to the upper
pair of wings of other insects, are here converted
into thick, opaque, and hard plates (e), adapted
to cover the folded membranous wings when the
insect is not flying, and thus securing them from
injurious impressions to which they might other-
wise be exposed from heat, moisture, or the
contact of external bodies. These wing-cases,
or elytra as they are termed, are never themselves
employed as wings, but remain raised and motion-
less during the flight of the insect. They pro-
bably, however, contribute to direct the course
of flight, by variously modifying the resistance
of the air.*
In the Orthoptera, (Fig. 159), the coverings of
the wings, or tegmina, instead of being of a
horny texture, are soft and flexible, or semi-
membranous. The wings themselves, being
* The Elytra of insects have been regarded by Oken as cor-
responding to the bivalve shells of the Mollusca, a notion which
seems to be founded upon a fanciful and strained analogy.
350
THE MECHANICAL FUNCTIONS.
broader than their coverings, are, when not in
use, folded longitudinally, like a fan.
In the new order of Rhipiptera of Latreille,*
which includes only two genera, the tegmina are
160
159
162
anomalous both in their situation and shape ;
being fixed at the base of the anterior legs, very
long and narrow, and apparently incapable of
protecting the wings. The wings themselves are
of ample extent ; forming, when expanded, a
quadrant of a circle, with five or six nervures
radiating from their base, and folded longitu-
dinally.
In the Hemiptera, the tegmina, or as they
are here called, the hemi-elytra, are coriaceous
towards their base, but membraneous towards
* The Strepsiptei-a of Kirby. See Transactions of the Lin-
nsean Society, XI. 86.
FLIGHT OF INSECTS. .351
their extremity, and the true wings are folded
transversely, so as to cross one another. These
hemi-elytra are employed to strike the air in
flight, and their movements accompany those of
the wings.
Insects having four thin membranous and
transparent wings are arranged under two orders;
namely, the Neuroptera (Fig. 169), in which the
lesser nervures form an interlacement of fibres,
crossing one another nearly at right angles, like
net-work, or lace ; and the Hymenoptera (Fig.
101), in which they are disposed like the rami-
fications of arteries or veins, diverging at acute
angles from the main trunks. The insects be-
longing to these two orders enjoy extensive powers
of flight. Libellulce, and JEshnce, which are in-
cluded in the first of these orders, never close their
wings, but, when they are not flying, keep them
constantly expanded, and ready for instant action.
They fly with the greatest ease in all directions,
sideways, or backwards, as well as forwards; and
can instantly change their course without being-
obliged to turn their bodies : hence they possess
great advantages both in chasing other insects,
and in evading the pursuit of birds. Bees, which
are hymenopterous insects, have often been ob-
served to fly to great distances from their hive
in search of food. The humble bee adopts a
very peculiar mode of flight, describing, in its
aerial course, segments of circles, alternately to
352
THE MECHANICAL FUNCTIONS.
the right and to the left. The velocity with
which these insects move through the air in
general much exceeds that of a bird, if estimated
with reference to the comparative size of these
animals.*
* I have been favoured by Mr. George Newport with the fol-
lowing- account of the structure of the sting of the Wild Bee,
(Anthophora retusa, Kirby) which he has lately carefully exa-
mined, and from whose drawings of the dissected parts the
annexed figures (163) have been engraved. " The sting of this
bee (a) is formed of two portions placed laterally together, but
capable of being separated. The point (p) is directed a little
upwards, and is a little curved :
the barbs, (seen still more highly
magnified at q), are about six in
number, and are placed on the
under surface, and their points
directed backwards. At the
base of the sting, (e), there is
a semicircular dilatation, appa-
rently intended to prevent the
instrument from being thrust
too far out of the sheath (shown
separately at v), in which it
moves : it has also a long ten-
don, to which the muscles are
attached. It is between these
plates, when approximated, that
the poison flows from the orifice
of the somewhat dilated extre-
mity of the poison duct, (d),
which comes from the anterior
part of the poison bag (b). This
bag is of an oval shape, and
is not the organ which secretes
the poison, but merely a receptacle for containing it; for it is
FLIGHT OF INSECTS. 35o
Although the greater number of insects have
four wings, there are many, such as the common
house fly, and the gnat, which have only two.
These compose the order Dipt era (Fig. 162).
In these insects we meet with two organs, con-
sisting of cylindrical filaments, terminated by a
clubbed extremity ; one arising from each side
of the thorax (as seen in the above figure), in
the situation in which the second pair of wings
originate in those insects which have four wings.
They are named the halteres, or poisers, from
their supposed use in balancing the body, or
adjusting with exactness the centre of gravity
when the insect is flying. Whatever may be
their real utility, they may still be regarded as
rudiments of a second pair of wings ; and they
afford, therefore, when thus viewed, a striking
instance of the operation of the tendency which
conveyed into this bladder by means of a long convoluted
vessel (c), which receives it from the secreting organs (s). These
organs consist of two somewhat dilated vessels resembling cceca,
but which have each a slender secretory vessel extending from
them. The sting moves in a tubular sheath (v), which is open
at its base and along its upper surface, as far as the part where
the sting is prevented from being thrust out any farther. The
muscles, which move the sheath, are distinct from those of the
sting, and are attached to an elongated and curved part on each
side of its base, and to an arched and moveable part which is
apparently articulated with it. Swammerdam has delineated
these parts as ceeca in his dissection of the common hive bee, but
has not noticed the secretory vessels. The sting of the hive bee
resembles that of the Anthophora retusa."
VOL. I. A A
354 THE MECHANICAL FUNCTIONS.
prevails universally in the animal kingdom, and
modifies the structure of each individual part
so as to preserve its conformity to one general
type.
The innumerable tribes of butterflies, sphinxes,
and moths, are all comprehended in the order
Lepidoptera, and are distinguished by having
wings covered with minute plumes, or scales.
These scales are attached so slightly to the
membrane of the wing as to come off when
touched with the fingers, to which they adhere
like fine dust. When examined with the mi-
croscope, their construction and arrangement
appear to be exceedingly beautiful, being
marked with parallel and equidistant striae,
often crossed by still finer lines, the distinct
visibility of which, in many kinds of scales, as
those of Pontia brassica, or cabbage butterfly,
and the JSIorpho Menelaus of America, consti-
tutes a good criterion of the excellence of the
instrument. The beautiful colours which these
scales possess may perhaps generally be owing
to the presence of some colouring material : but
the more delicate hues are probably the result
of the optical effect of the striae on the surface ;
and in some cases they result from the thin-
ness of the transparent plate of which they con-
sist ; for I have observed in several detached
scales that the colours they exhibit by trans-
mitted light are the complementary colours to
FLIGHT OF INSECTS.
355
those which they display when seen by reflected
light.
The forms of these scales are exceedingly
diversified, not only in different species, but
also in different parts of the wings and body
of the same insect ; for the surface of the body,
generally, as well as the limbs, and even in some
species the antennae are more or less covered
with these scales.* Fig. 164 exhibits some of
164
165
4 » fc • % ti
* * * « * > \ \ * », «>
the more usual shapes as they appear when
viewed with high magnifying powers.
Each scale is inserted into the membrane of
the wing1 by a short pedicle, or root, and over-
* In the posthumous work of Lyonet, which has lately ap-
peared, nearly the whole of six quarto plates are crowded with
the delineations of the different forms of the scales found in the
Bombyx Cossus.
356 THE MECHANICAL FUNCTIONS.
laps the adjoining scales ; and the whole are
disposed in rows, with more or less regularity ;
one row covering the next, like tiles on the
roof of a house.* This imbricated arrangement,
together with the marks that are left on the
membrane of the wing where the scales have
been rubbed off, are shown in Fig. 165, which
is a faithful delineation of the appearance of the
wing of the Hesperia Sloanus, seen through a
powerful microscope. The membrane of the
wing itself, when stripped of its scales, is as
perfectly transparent as that of the bee, and
is, in like manner, supported by diverging
nervures. Many butterflies exhibit, in some
parts of the wing, smooth pearly spots, called
by entomologists, ocelli, or eyes, which arise
from those parts being naturally destitute of
scales. The number of these scales necessary
to cover the surface of the wings must, from
their minuteness, be exceedingly great. The
moth of the silk worm {JBombyx mori, Fig. 148),
which has but a small wing, contains, according
to Lewenhoeck, more than two hundred thou-
sand of these scales in each wing.
These scales doubtless contribute to the pro-
tection of the wing ; but they at the same time
* The scales on the wing of the Lepisma are of two kinds ;
one set being arranged in rows, as usual, and the others, which
are of a different shape, being inserted between and over the
former, so as to fasten each firmly in its place.
FLIGHT OF INSECTS. .'357
add considerably to their weight, and impede
the velocity of their action. This inconvenience
appears to have been in a great measure com-
pensated by the greater size of the wings, and by
the extent of the surface with which they strike
the air. Still, however, it is sufficiently obvious
that insects of this order fly with less rapidity
and steadiness than most others. But this un-
steadiness, again, is turned to good account ; for
the butterfly, by its irregular and apparently
capricious movements, alternately dipping and
rising in the air, so as to describe a series of zig-
zag lines, more easily eludes capture when pur-
sued, not only by naturalists, but also by birds,
that are eagerly seeking to secure them. It is
astonishing to what a distance the silk worm
moths will fly : some have been known to travel
more than a hundred miles in a short time. The
Papilio Iris often rises to so great a height in
the air as to be quite invisible.
A mechanical contrivance is adopted in many
of the Lepidoptera for keeping their wings
steady during flight, consisting of a hook,
covered with hair and scales, attached to the
under side of the upper wings, near their base,
and connected also, by means of bristles, to the
base of the lower wing : by this attachment all
the wings are locked together, and brought into
action at the same time. Insects of the Sphinx
tribe are also provided with a kind of rudder
358 THE MECHANICAL FUNCTIONS.
formed by the expansion of the tail, enabling
them to steer their course with more certainty.
The Lepidoptera in general fly with the body
nearly upright, contrary to the habits of most
other winged insects, whose bodies, while flying,
are nearly in a horizontal position.
The feats of agility and strength exhibited by
insects have often been the theme of admiration
with writers on natural history ; and have been
considered as affording incontrovertible proofs of
the enormous power with which their muscles
must be endowed. We have already had occa-
sion to notice a remarkable instance of the force
and permanence of muscular contraction in those
caterpillars which frequently remain for hours
together in a fixed attitude, with their bodies
extended from a twig, to which they cling by
their hind feet alone.* Ants will carry loads
which are forty or fifty times heavier than their
own bodies ; and the distances to which many
species, such as the Elater, the Locust, the
Lepisma, and above all the Pulex, are capable
of leaping, compared with the size of the insects
themselves, appear still more astonishing. Lin-
neus has computed that the MelolontJia, or chaf-
fer, is, in proportion to its bulk, more than six
times stronger than the horse ; and has asserted
that if the same proportional strength as is pos-
* See Fig. 148*, p. 315.
FLIGHT OF INSECTS. 359
sessed by the Lucanus, or stag-beetle, had been
given to the Elephant, that animal would have
been capable of tearing up by the roots the
largest trees, and of hurling huge rocks against
his assailants, like the Giants of ancient mytho-
logy.
But while we must admit that all these facts
indicate a remarkable degree of energy in the
contractile power of the muscular fibres of in-
sects, we should at the same time recollect that
the diminutive size of the beings which display
those powers is itself the source of a mechani-
cal advantage not possessed by larger animals.
The efficacy of all mechanical arrangements
must ultimately depend on a due proportion be-
tween the moving and the resisting forces : hence
mechanism of every kind must be adjusted with
reference, not merely to the relative, but to the
absolute dimensions of the structures themselves.
This will be evident when we consider that the
forces which are called into action are resisted
by the cohesion of the particles composing the
solid parts of the machine ; and this cohesion,
being not a variable, but a constant and definite
force, must necessarily limit the dimensions of
every mechanical structure, whether intended
for stability or for action. An edifice, raised
beyond a certain magnitude, will not support
itself, because the weight of the materials in-
creases more rapidly than the strength. How
.360 THE MECHANICAL FUNCTIONS.
often has it been found that a machine, which
works admirably in a small model, will totally
fail in its performance when constructed on a
larger scale? Any lever, of whatever form, may
be increased in its dimensions until the force of
gravity becomes superior to the cohesion of its
own particles ; and consequently any structure,
like a vegetable or animal body, composed of a
combination of levers, would, if its size were to
exceed a certain limit, fall to pieces merely by
its own weight. This can be prevented either
by employing materials of greater cohesive
strength, or by increasing, at the points where
the strains are greatest, the thickness of the
parts compared with their length : but the
choice of materials is necessarily restricted within
narrow limits, and the latter expedient would
entirely alter the relative proportions of the
parts, and would require a complete change
in the plan of their construction. In passing
from the smaller to the larger animals, we find,
accordingly, that new models are adopted, a
new order of architecture introduced, and new
laws of developement observed. We have,
next, then, to direct our attention to the pro-
cedure of nature in the execution of this more
enlarged and comprehensive scheme of animal
organization.
361
Chapter VI.
VERTEBRATA.
§ 1 . Vertebrated Animals in general.
If it be pleasing to trace the footsteps of nature
in constructions so infinitely varied as those of
the lower animals, and to follow the gradations
of ascent from the zoophyte to the winged insect,
which exhibits the greatest perfection compatible
with the restricted dimensions of that class of
beings, still more interesting must be the study
of those more elaborate efforts of creative power,
which are displayed on a wider field in the
higher orders of the animal kingdom. In the
various tribes of beings which are now to come
before us, we find nature proceeding to display
more refined developements in her system of or-
ganization ; resorting to new models of structure,
on a scale of greater magnitude than before ; de-
vising new plans of economy, calculated for more
extended periods of duration ; and adopting new
arrangements of organs, fitted for the exercise of
a higher order of faculties. The result of these
more elaborate constructions is seen in the vast
series of Vertebrated Animals, which comprises
362 THE MECHANICAL FUNCTIONS.
a well-marked division of Zoology, comprehend-
ing all the larger species that exist on the globe,
in whatever climate or element they may be
found ; and including man himself, placed, as he
unquestionably is, at the summit of the scale ; —
the undisputed Lord of the Creation.
A remarkable affinity of structure prevails
throughout the whole of this extensive assem-
blage of beings. Whatever may be the size or
external form of these animals, whatever the
activity or sluggishness of their movements,
whether they be inhabitants of the land, the
waters, or the air, a striking similitude may be
traced both in the disposition of their vital organs,
and in the construction of the solid frame- work,
or skeleton, which sustains and protects their
fabric. The Quadruped, the Bird, the Tortoise,
the Serpent, and the Fish, however they may
differ in subordinate details of organization, are
yet constructed upon one uniform principle, and
appear like varied copies from the same original
model. In no instance do they present struc-
tures which are altogether isolated, or can be
regarded as the results of separate and inde-
pendent formations.
In proceeding from the contemplation of the
structures of articulated to those of vertebrated
animals, we appear to pass by a rapid excur-
sive flight, from one great continent to ano-
ther, separated by an immense gulf, contain-
VERTEBRATED ANIMALS. 3G3
ing no intermediate islands from which we
might gather indications of these tracts of land
having been originally connected. At the very
first sight, indeed, the general fabrics of these
two descriptions of animals appear to have been
constructed upon opposite principles ; for in the
one, as we have already seen, the softer parts
are internal, and are enclosed in a solid crust, or
shell, or horny covering, answering at once the
purposes of protection and mechanical support,
and furnishing extensive surfaces for the attach-
ment of the organs of motion. But in the Verte-
brata, the solid frame work which serves these
purposes occupies, for the most part, an internal
situation, constituting a true jointed skeleton,
which is surrounded by the softer organs, and to
which the muscles, destined to move their several
parts, are attached. The office of external de-
fence is entrusted solely to the integuments, and
their different appendages. Such is the general
character of the arrangements which nature has
here adopted ; from which, however, she has oc-
casionally deviated with respect to some import-
ant organs of extremely delicate texture, and
which require to be shielded from the slightest
pressure. This occurs with regard to the brain,
and the spinal marrow, which we shall pre-
sently find are specially guarded by a bony
structure, enclosing them on every side, and
forming an impenetrable case for their pro-
304 THE MECHANICAL FUNCTIONS.
tection. The solid mass of bone, thus provided
to defend the brain, gives also the opportunity
of lodging safely the delicate apparatus subser-
vient to the finer senses, namely, those of sight,
of hearing, and of smell. The security which
these organs derive from this protection allows
of their being carried to a higher degree of im-
provement than could be attained in the lower
orders.
There is also another advantage, of consider-
able moment, which results from the internal
situation of the skeleton, namely, that it admits
of an indefinite extension by growth, without in-
terfering with the corresponding enlargement of
the softer organs ; for we have seen that in all the
instances in which this arrangement is reversed,
that is, whenever the enclosing surfaces become
solid, and can no longer yield to the dilatation of
the contained organs, no alternative remains but
that of breaking up the exterior case, and wholly
casting it off, to make room for the further
growth of the animal ; after which operation, it
has to be replaced by another covering of larger
dimensions. This operation is generally re-
quired to be performed a great number of times,
before the animal can acquire the size it is
destined to attain. Hence the perpetual moult-
ings of the caterpillar ; hence the repeated
castings of the shells of the Crustacea ; and
hence also the successive metamorphoses of the
STRUCTURE OF BONE. 30-5
insect. Nothing of this kind takes place among
the Vertebrata ; where all the organs are deve-
loped in regular and harmonious succession,
without the slightest mutual interference, and
without those vicissitudes of action and of tor-
pidity, which we witness in the chequered ex-
istence of the insect.
§ 2. Structure and Composition of the
Osseous Fabric.
The process employed for the formation and ex-
tension of the solid frame work of the Verte-
brata differs totally from that which we have
seen exemplified in the growth of shells, or of
the hard coverings of insects and of crustaceous
animals. These latter structures, and the modes
adopted for their increase, are suited only to
animals in which the functions of the economy
have not reached that perfection to which they
are carried in the higher classes. In the more
elaborate system of the vertebrata, the skeleton
is composed of true bones ; that is, of solid pieces,
which, although they are dense calcareous struc-
tures, yet continue organized during the whole
period of developement, and form as much a
part of the living system as any other organ of
the body. We have formerly seen that the
366 THE MECHANICAL FUNCTIONS.
membrane, in which the calcareous matter of
the shell is deposited, should properly be classed
among the integuments ; being analogous to
them not only in being situated externally, but
also in their structure and in their function. It
is not so with bone, which is essentially an
internal structure.*
In their chemical composition, likewise, bones
are strikingly contrasted with the calcareous
products of the Mollusca ; for in the former, the
earthy portion consists almost wholly of phos-
phate of lime ; a material, which appears to have
* De Blainville regards the hard coverings of insects, together
with the shells of the Crustacea, as structures derived altogether
from the integuments, and as perfectly analogous, in this respect,
to the scales, hoofs, or other horny productions of the skin in
vertebrated animals. Geoffroy St. Hilaire contends, on the con-
trary, that the former constitute the true skeleton of the lower
classes, and that a perfect analogy may be traced between the
rings, which are the essential constituents of the frame-work of
annulose animals, and the vertebrae, which enclose the spinal
cord of the higher classes. Professor Carus appears, in his
system of organic formations, to have kept in view both these
analogies; giving to the former class of structures the denomina-
tion of Der mo -skeleton, and to the latter that of N euro- skeleton.
(See his Tabulae Anatomiam Comparativam illustrantes, edited
by Thienemann). Analogies have also been imagined to exist
between the external and internal situations of the woody fibres
of plants belonging respectively to the endogenous and exoge-
nous classes, and that of the corresponding relative situations of
the skeletons of invertebrated and vertebrated animals. (See a
Memoir by Dumortier, in the Nova Acta Physico-Medica Acad.
Caesar. Leopold. Carolines Natur. Curios, xvi. 219).
CHEMICAL COMPOSITION OF BONE. 367
been selected for this purpose from its forming
much harder compounds with animal membrane
than the carbonate. Wherever great strength
and rigidity are required, this is the material
depended upon for imparting these qualities; and
it has accordingly been employed for the osseous
structures, which are among the most elaborate
results of organization. The densest and hardest
of these structures are those in which the pro-
portion of phosphate of lime is the greatest,
when compared with that of the animal sub-
stance which cements them together; the force
of mutual cohesion among its own particles
being much greater than that imparted by the
cementing ingredient. The internal bony por-
tions of the ear, where, in order perfectly to
transmit the sonorous vibrations, the greatest
solidity is required, are the densest parts of the
skeleton ; and phosphate of lime enters most
largely into the composition of these bones.
The tympanic portions of the temporal bone of
the Whale and the Cachalot, where the great
size of the organ gives us advantages in ex-
amining them, are as dense and as hard as
marble. The bony portions of the teeth, like-
wise, afford instances of very hard calcareous
formations ; but the enamel, which consists
almost wholly of phosphate of lime, is harder
still, and resembles the siliceous stones, being,
like flint, capable of striking fire with steel.
3(J8 THE MECHANICAL FUNCTIONS.
It is scarcely necessary to point out the obvious
intentions which are fulfilled by this peculi-
arity of structure, conferring extraordinary hard-
ness on a part of which the appropriate office
is that of breaking down hard bodies subjected
to their mechanical action. But this extreme
degree of crystalline hardness would be ill suited
to other parts of the frame. In ordinary bones,
absolute rigidity is not the quality which is
alone wanted; for, in general, the hardest bodies
are also the most fragile. An excess of rigidity,
therefore, would have been attended with brittle*
ness, and been productive of the worst conse-
quences to parts exposed to sudden and violent
concussions. It is in order to guard against this
evil that an elastic animal matter is employed as
the basis of the structure, acting as a strong
cement interposed between the calcareous par-
ticles.
This composition of bone is rendered evident
by subjecting it to certain chemical processes.
On exposure to heat, we find it first becoming
black, from the developement of the charcoal
attendant upon the destruction of the animal
membrane. The oil contained in the cavities
exudes, and, taking fire, is soon totally con-
sumed. The bone then recovers its whiteness,
and undergoes no further change by the action
of the fire. If it be now examined, it will
be found to have lost nearly half its original
CHEMICAL COMPOSITION OF BONE. 309
weight, and to have become exceedingly brittle ;
this, as already mentioned, being the natural
property of phosphate of lime, when deprived of
its animal cement. We may perceive, on the
surface of a bone so treated, a number of minute
crevices, showing where this animal substance
had been situated in its original state. On break-
ing the bone across, we may also discover the size
and shape of the cavities which contained the
marrow, or oily fluid above-mentioned.
It is easy to reverse this process by steeping
the bone in an acid sufficiently diluted to pre-
vent its injuring the animal membrane, but yet
sufficiently powerful to dissolve the phosphate
and carbonate of lime. Diluted nitric, or mu-
riatic acids may be used for this purpose, and
will, in this way, gradually separate the earthy
particles from the membranous portion of the
bone. During the action of the acid, a few
bubbles of carbonic acid gas make their ap-
pearance, indicating the presence of a small
quantity of carbonate of lime, which always
exists in bones, intermixed with the phosphate.
The phosphate may be recovered from its solu-
tion in the acid by precipitation with a pure
alkali, such as a solution of ammonia. This
precipitate is readily dissolved, without effer-
vescence, by nitric, muriatic, or acetic acids.
A small quantity of sulphuric acid may also be
detected in the fluid by the addition of nitrate
VOL. I. B B
370 THE MECHANICAL FUNCTIONS.
of barytes. Iron, in small quantity, is also found
in the composition of human bones.
The substance which remains, after the earth
has been thus abstracted, retains the exact figure
and dimensions of the original bone, but has
lost all its other mechanical properties. It is
soft, flexible, and elastic ; resembling in every
respect the muscular or fibrous structures, and
being, like them, resolvable into gelatin and
albumen by long boiling in water. This sub-
stance has sometimes, but erroneously, been
considered as identical with cartilage ; for it has
neither the whiteness, nor the elasticity, nor the
texture of cartilage ; nor is it at all similar to
that substance in its chemical composition ; for
while cartilage is formed almost wholly of albu-
men, the animal basis of bone is almost entirely
resolvable into gelatin.
Thus may a bone be analysed into its two
constituent parts : by the process first described
we obtain its earth deprived of its animal con-
stituent ; by the second, we obtain its mem-
branous basis free from earth. The first of
these gives it hardness ; the second, tenacity :
and thus, by the intimate combination of these
elements, two qualities, which, in masses of ho-
mogeneous and unorganized matter, are scarcely
compatible with one another, are skilfully
united.
The mechanical structure of bone is no less
STRUCTURE OF BONE. 371
worthy of admiration, as evincing the skill with
which every part is adapted to its destined uses.
The animal membrane, which, as we have seen,
is the bed in which the calcareous phosphate is
deposited, partakes of the reticular structure
belonging to ordinary cellular texture ; and a
bone, when minutely examined, exhibits also the
same appearance of plates intermixed with fibres.
In the outer compact portion, indeed, the fibrous
arrangement of the particles is not so easily
distinguished : but it may be detected in young
bones while they are becoming ossified : and
also in bones which have been long exposed to
the weather, or long macerated in water. The
interior of most bones, in the higher classes of
animals, presents distinctly the appearance of
irregular cavities, resulting from the partial sepa-
ration of the plates, and their mutual crossings,
and fibrous connexions.
The different mechanical purposes for which
bones are employed in the animal economy
require them to be of different forms. Where
a part is intended to have compactness and
strength, with a very limited degree of motion,
it is divided into a great number of small pieces,
united together by ligaments ; and the separate
bones are short and compressed, approaching
more or less to a cubical shape. Of such is the
column of the spine composed, as also the joints
of the wrist and ankle. Where the principal
372 THE MECHANICAL FUNCTIONS.
object is either extensive protection, or the pro-
vision of broad surfaces for the attachment of
muscles, we find the osseous structure expanded
into flat plates ; as is exemplified in the bones
of the skull, in the shoulder blade, and still
more remarkably in the bony shield which sur-
rounds the body of the tortoise. On the other
hand, where a system of levers is wanted, as in
the limbs, which have to sustain the weight of
the trunk, and to confer extensive powers of
locomotion, the bones are modelled into length-
ened cylinders, generally somewhat expanded at
the extremities, for greater convenience of mu-
tual connexion.
In the form, the structure, and the arrange-
ment of these levers, which allow of the regular
and accurate application of the moving power,
and are calculated, in circumstances so various,
to give effectual support to the fabric, and also
to execute a great diversity of movements, Ave
discern most palpable manifestations of pro-
found design, and the most exquisite refinements
of mechanic skill. All the scientific principles
of architecture and of dynamics are more or less
exemplified in the construction of this part of
the animal fabric. Levers of various kinds are
most artificially combined in the formation of
the fins of fishes, the wings of birds, and the
limbs of quadrupeds. The power of the arch in
resisting superincumbent pressure is exhibited
STRUCTURE OF BONK.
373
in various parts of the osseous systems of verte-
brated animals; such as the human foot, the
spine, the pelvis, and more especially in the
vaulted roof of the skull, and in the carapace,
or upper shell, of the tortoise.
The construction of these levers evinces that
a minute attention has been bestowed on every
condition by which mechanical advantage could
be gained. In the more perfect dev elopements
of structures, such as those which obtain in the
higher orders of mammalia, and also in the class
of birds, all the long bones are
hollow cylinders ; and their ca-
vity is largest in the middle of
their length. This is shown in
Fig. 172, which represents a
longitudinal section of a human
thigh bone, and in Fig. 173,
which is a similar section of the
humerus, or bone of the arm.
The walls of these bones consist
of a dense and compact sub-
stance, formed by the close co-
hesion of the osseous plates.
These walls are of greater thick-
ness in the middle of the shank,
or shaft of the column, and be-
come thinner as we follow them
towards either of the ends. This
gradual diminution in the thickness of the walls
374 THE MECHANICAL FUNCTIONS.
arises from the continual separation of the
plates, which bend inwards, and crossing each
other, leave a multitude of irregular spaces or
cells, which are termed cancelli. The plates, pro-
ceeding from each side obliquely inwards, at
length meet each other in the axis of the cy-
linder, so as to close the middle cavity near the
extremities of the bone, where this spongy, or
cancellated structure is found to occupy its whole
diameter.
Now if we consider that the principal me-
chanical property required in every cylindrical
lever is rigidity, and more especially the power
of resisting forces applied transversely, that is,
tending to break the cylinder across, we shall
soon perceive, that a given quantity of ma-
terials could not possibly have been disposed in
a manner better calculated for such resistance
than when in the form of a tube, or hollow cy-
linder.* To this mechanical principle I have
already had occasion to advert, when speaking
of the hollow stems of vegetables, which derive
their chief strength from their possessing this
form ; t and we now find it again applied in the
structure of bones, which by having been made
* An elaborate mathematical demonstration of this proposi-
tion was long ago given by Dr. Porterfield, in a paper contained
in the first volume of Medical Essays and Observations, pub-
lished by a Society in Edinburgh, p. 95,
i P. 81.
OSSIFICATION. 375
hollow, are rendered considerably stronger than
if the same materials had been collected into a
solid cylinder of the same length. We may
farther remark, that as it is in the middle of
the shaft that the strain is greatest, so it is here
that the cavity is largest, and the resistance
most effectual.
§ 3. Formation and Developement of Bone.
But it is not enough to contemplate the pur-
poses so admirably answered by these arrange-
ments. Our curiosity cannot but be powerfully
excited to learn what processes and refined se-
ries of means are employed by nature to raise
and to perfect all these artificially contrived
structures. It fortunately happens that in this
instance we are permitted to penetrate a little
farther than usual into the secrets of organic
evolution : for the succession of changes can be
better followed by the eye in the slow develope-
ment of the harder parts, than in the quicker
growth of more yielding and expansible tex-
tures. The peculiar material, also, of which bone
is formed, is easily distinguished by its hardness,
its whiteness, and its opacity from the softer and
more transparent animal substance with which
it is intermixed. Hence we are allowed an op-
376 THE MECHANICAL FUNCTIONS.
portunity of observing the earliest stages of its
deposition, and of accurately following the sub-
sequent changes it undergoes.
The parts of the embryo animal, which are
destined to become bone, partake of the soft
and gelatinous consistence, which, at that early
period, characterizes all the textures of the
body ; and they can hardly, indeed, be dis-
tinguished from the semi-fluid portions which
surround them. In process of time, when the
vascular circulation of the blood has been estab-
lished, and the newly formed arteries have ex-
tended their branches over every part of the
nascent organization, those vessels which are
appropriated to the task of forming the bones,
arrive at the pulpy masses where their work is
to commence. As sculptors, before working upon
the marble, first execute a model of a coarser and
more plastic material, so the first business of
these arteries is to prepare a model of the future
bone, constructed, not with the same material
of which it is afterwards to consist, but with ano-
ther, of a simpler and softer nature, namely car-
tilage. In every case, then, cartilage is first
formed, and becomes visible by its greater opacity
when compared with the adjacent jelly. It is an
exact representation, in miniature, of the bone,
which is, in due course, to take its place. It is
evident that until the other parts of the fabric
have proceeded so far in their developement as
OSSIFICATION. 377
to have acquired a certain degree of solidity and
firmness, and to bear, as well as to require, the
support of more massive and rigid structures,
this flexible and elastic cartilage may be em-
ployed with great advantage as its substitute :
for a hard and unyielding structure would, in
the early stages of its formation, have even been
injurious. But in proportion as the fabric is
enlarged, the necessity for mechanical support
increases, and further provision must be made
for resistance to external violence.
When, at length, all is prepared for the con-
struction of the bone, the next step to be taken
is the removal of the cartilage, which had been
erected as the scaffolding for the intended build-
ing. But in taking down this scaffolding, the
whole must not be removed at once ; each part
must be carried away, piece by piece, while the
operation of fixing in their position the beams
and pillars of the edifice proceeds. The way is
cleared at first by the absorption of the central
part of the cartilage, and a few particles of
ossific matter are deposited in its room. While
this process is going on, greater activity is dis-
played in the arteries ; they rapidly enlarge in
diameter, so as to admit the colouring globules
of the blood ; and they thus become visible to
the eye, which can now follow their course
without difficulty. From being at first red
points, they soon spread out into lines, of which
378 THE MECHANICAL FUNCTIONS.
we trace the branches to a certain extent, al-
though we cannot pursue them to their minuter
ramifications. They now assume more active
functions, and hasten to execute their task by
depositing granules of calcareous phosphate :
these are laid down, particle by particle, in a
certain determinate order, and in regular lines,
so as to form continuous fibres. When a great
number of these delicate fibres are gathered
together, and connected by other fibres, which
shoot in various directions across them, a texture
composed of an assemblage of long spicula, and
thin plates, is constituted.
In the cylindrical bones, the spicula prevail,
and they are arranged longitudinally, and pa-
rallel to one another, and to the axis of the bone.
They first constitute a ring in the middle of its
length : this ring enlarges in all its dimensions,
but principally in its length ; the spicula be-
coming larger, not by the stretching of their
parts, in consequence of the insinuation of fresh
materials between those already deposited, but
by the addition of new particles at both their
extremities. In like manner, the ring increases
in thickness, not by the deposition of phosphate
of lime between the original layers, but by the
application of fresh layers on the outside of
those already existing.
In the flat bones, the process of ossification is
very similar to what I have just described ; only
OSSIFICATION. 379
the fibres have a radiated arrangement, shooting-
out from the spot where the first deposit took
place, as from a common centre. This is seen
in Fig. 174, which represents the parietal bone of
the human skull, in an early stage of its ossifi-
cation, and shows very distinctly the radiating
fibres. In the cubical, and more irregularly
shaped bones, the process is, doubtless, con-
ducted with the same order and regularity, al-
though it cannot so readily be followed by the
eye.
The same process is repeated in different
parts of the bone, wherever nature has, in con-
formity with determinate laws of developement,
appointed particular centres of ossification. The
bone continues to extend from each of these cen-
tres, proceeding gradually towards the circum-
ference, or the remoter parts of the cartilage, on
which the ossific materials are moulded, and by
the form of which that of the future bone is
regulated. The process of ossification has, how-
380 THE MECHANICAL FUNCTIONS.
ever, this peculiarity, that the cartilage is pro-
gressively absorbed to make room for the depo-
sits of bony substance. When the bone is long,
separate points of ossification appear in the ex-
tremities, before the central portions are ossified ;
and the ends, thus formed into bone, are after-
wards united to the shaft, so that the whole
shall form a continuous bony mass. In the fiat
bones, also, if the surface be extensive, an addi-
tional number of arteries are engaged to perform
the work, which is begun from several auxiliary
centres of ossification, and the completion of
which is materially accelerated by their co-ope-
ration.
This mode of increase often gives rise to a
curious result, of which a striking example is
presented in the bones of the skull. The brain,
which these bones are designed to protect, re-
quires this protection at a very early period of
life. The growth of so large a surface of bone,
as would be required for covering the brain,
could not have proceeded with sufficient quick-
ness for the exigencies of the occasion, if it had
originated from a single point. Therefore it is
that, besides being commenced at a very early
age, the process goes on from a great number of
separate points at the same time. The ossifica-
tion is evidently hurried on, in order to complete
the roofing in of the edifice by the time at which
the animal is to be ushered into the world, and
OSSIFICATION. 381
exposed to dangers from the contact of external
bodies. The divergent fibres shoot out rapidly,
coalescing with those in their immediate neigh-
bourhood, which co-operate to form an extensive
bony plate. When they have reached the pre-
scribed line, they have become so much ex-
panded as to have lost the power of coalescing
with the fibres which have originated from other
centres, and are proceeding in a contrary direc-
tion. Yet the arteries still continuing to deposit
ossific matter, each set of fibres insinuate them-
selves between those of the opposite set, for some
little distance, and until their further progress
is stopped by the increasing resistance they en-
counter. The consequence is that the edges of
the bones, which have thus met, are irregularly
jagged, like the teeth of a saw, presenting exter-
nally the zig-zag line of junction which is called
a suture. This is seen in Figures 175 and 176,
the former of which represents the upper side of
the skull of an infant ; and the latter, the same
bones when completely ossified.
The union of bony fibres proceeding from
different centres of ossification is not indiscri-
minate, but is found to be regulated by definite
laws, and to have certain relations to the gene-
ral plan of conformation originally established.
Each distinct bone is formed from a certain
number of ossific centres, which altogether con-
stitute a system appertaining to that bone only,
382 THE MECHANICAL FUNCTIONS.
and not extending to the adjacent bones. These
pieces unite together, as if by a natural affinity ;
and they refuse to unite with the bony fibres
proceeding from neighbouring centres, and be-
longing to other groups. The groups themselves
are not arbitrary, but are pre-established parts of
the original design. Circumstances occasionally,
indeed, arise, which may overrule this inherent
tendency to preserve the line of separation be-
tween two bones ; and we then find them coa-
lescing to form a single piece. Such unions are
technically called anchyloses.
Were this the whole of what takes place in
the formation of a bone, the process would not,
perhaps, differ very materially from that by
which a shell is produced ; for a shell, as we
have seen, is the result of successive depositions
of calcareous matter, forming one layer after
another, in union with a corresponding deposit
of animal membrane. But the subsequent
changes which occur, show that the constitution
of bone is totally dissimilar to that of shell : for
no portion of the shell that is once formed, and
has not been removed, is subject to any further
alteration. It is a dead, though perhaps not
wholly inorganic mass ; appended, indeed, to
the living system, but placed beyond the sphere
of its influence. But a bone continues, during
the whole of life, to be an integrant part of the
system, partaking of its changes, modified by
its powers, and undergoing continual alterations
OSSIFICATION. 383
of shape, and even renewals of its substance, by
the actions of the living vessels.
The form, which had at first been rudely
sketched, slowly advances towards perfection in
the course of growth ; and the general propor-
tions of the parts are still preserved ; the finished
bone exhibiting prominences and depressions in
the same relative situation as at first ; and not
only having similar internal cavities, but being
frequently excavated in parts which had before
been solid. During all these gradual alterations
of shape, however, there is no stretching of
elastic parts ; for all the osseous fibres and
laminae are rigid and unyielding, and in this
respect retain an analogy with shell. The
changes thus observed can have been effected in
no other way than by the actual removal of such
parts of the young bone as had occupied the
situations where vacuities are found to exist in
the old bone. We find, for instance, that in the
early state of a bone there are no internal cavi-
ties, but the whole is a uniform solid mass. At
a certain stage of ossification cells are excavated
by the action of the absorbent vessels, which
carry away portions of bony matter lying in the
axis of the cylindrical, or in the middle layer of
the flat bones.* Their place is supplied by an
* The bones of the lower classes of vertebrated animals, as of
Fishes and Reptiles, seldom reach this stage of ossification, but
remain solid throughout ; corresponding to the bones of the
higher classes at the early periods of their developement.
384 THE MECHANICAL FUNCTIONS.
oily matter, which is the marrow. As the growth
proceeds, while new layers are deposited on the
outside of the bone, and at the ends of the long
fibres, the internal layers near the centre are
removed by the absorbent vessels, so that the
cavity is farther enlarged. In this manner the
outermost layer of the young bone gradually
changes its relative situation, becoming more
and more deeply buried by the new layers which
are successively deposited, and which cover and
surround it ; until by the removal of all the
layers situated nearer to the centre, it becomes
the innermost layer ; and is itself destined in its
turn to disappear, leaving the new bone without
a single particle which had entered into the com-
position of the original structure.
It has been found that by mixing certain
colouring substances with the food of animals
the bones will soon become deeply tinged by
them. This fact was discovered accidentally by
Mr. Belchier, who gives the following account
of the circumstances that led him to notice it.*
Happening to be dining with a calico printer on
a leg of fresh pork, he was surprised to ob-
serve that the bones, instead of being white as
usual, were of a deep red colour ; and on inquir-
ing into the circumstances, he learned that the
* Philosophical Transactions for 1736, vol. xxxix. 287 and
289.
OSSIFICATION. ,38o
pig had been fed upon the refuse of the dyeing-
vats, which contained a large quantity of the
colouring substance of madder. So curious a
fact naturally attracted much attention among
physiologists; and many experiments were under-
taken with a view to ascertain the time required
to produce this change, and to determine whether
the effect Mas permanent, or only temporary.
The red tinge was found to be communicated
much more quickly to the bones of growing ani-
mals than to those which had already attained
their full size. Thus the bones of a young pigeon
were tinged of a rose colour in twenty- four hours,
and of a deep scarlet in three days ; while in the
adult bird, fifteen days were required merely to
produce the rose colour. The dye was more in-
tense in the solid parts of those bones which were
nearest to the centre of circulation, while in bones
of equal solidity, but more remote from the heart,
the tinge was fainter. The bone was of a deeper
dye in proportion to the length of time the ani-
mal had been fed upon madder. When this diet
was discontinued, the colour became gradually
more faint, till it entirely disappeared.*
* These experiments by no means prove, as was once supposed,
that the substance of the bone is renewed with every change of
hue ; but merely that the colouring particles of madder, when
present in the blood, readily attach themselves to the phosphate
of lime in the bones, and are as quickly washed out again by the
circulating fluid, when restored to its usual state, (See a paper
VOL. I. C C
:W> THE MECHANICAL FUNCTIONS.
§ 4. Skeleton of the Vertebrata.
The purposes to be answered by the Skeleton,
in vertebrated animals, resolve themselves into
the three following ; first, the affording mecha-
nical support to the body generally, and also to
different portions of the body ; secondly, the
providing a solid basis for the attachments of
the muscles which are to effect their movements;
and thirdly, the giving protection to the vital
organs, and more particularly to the central parts
of the nervous system. Of these the last is the
circumstance that has the greatest influence in
determining the principles on which the osseous
frame- work has been constructed. In the ner-
vous system of all the animals coming under the
denomination of vertebrata, the spinal marrow,
together with the brain, (which may, indeed, be
considered as the anterior extremity of the spinal
marrow, only much enlarged by an additional
mass of nervous substance,) are the most import-
ant parts of that system, and the organs which
stand most in need of protection from every kind
of injury. These two portions of the nervous
system, when viewed as composing a single organ,
have been denominated the spino- cerebral axis,
in contradistinction to the analogous parts of the
by Mr. Gibson, in the Memoirs of the Lit. and Phil. Soc. of
Manchester. Second series, i. 146.)
SPINO-CEREBRAL AXIS. 387
nervous system of articulated animals: for amidst
great differences of structure and of functions,
an analogy is still retained among the several
forms of the nervous system, characterising these
two great divisions of the animal kingdom. In
the embryo state of the vertebrata the central
parts of that system consist of two separate fila-
ments, running parallel to each other the whole
length of the body : but in process of time these
two filaments unite, and constitute a single spinal
cord : and the primary type of the skeleton is
determined by the peculiar form of this, the
central organ of the nervous system.
In laying the foundations of the skeleton,
then, the first object is to provide for the secu-
rity of the spinal cord ; and this is accomplished
by enclosing it within a series of cartilaginous
rings, which are destined to shield it during its
growth, and by their subsequent ossification, to
protect it most effectually from all injurious pres-
sure. It is this part of the skeleton, accordingly,
of which the rudiments appear the earliest in the
embryo animal. These rings form a column,
extending in a longitudinal direction along the
trunk ; retracing to us the series of horny rings,
in which the bodies of worms, of insects, and
indeed of all the Articulata, are encased. When
ossified, these several rings are termed vertebra;
and the entire column which they compose is
the Spine. Fig. 177 shows the form of one of
:*88 THE MECHANICAL FUNCTIONS.
the vertebrae of the back in the human skeleton.
Fig. 178 is a side view of four vertebrae joined
together, and Fig. 179 is a vertical section of
the same part of the spine, showing the canal
formed by the rings. From the constancy with
which the spinal column is found in all animals
of this type, and from the uniformity of the plan
on which, amidst endless variations, it is model-
led, it has been chosen as the distinctive character
of this great assemblage of animals, which have
accordingly been denominated the Vertebrata, or
Vertebrated Animals.
Nor is the spine of less importance when
viewed in its mechanical relations to the rest of
the skeleton. It is the great central beam of
the fabric, establishing points of union between
all its parts, and combining them into one con-
tinuous frame-work : it is the general axis of all
their motions, or the common fulcrum on which
the principal bones of the extremities are made
to turn . it furnishes fixed points of attachment
VERTEBRAL COLUMN. .'389
to all the large muscles which act upon these
bones as levers, and also to those which move
the trunk itself.
If this column had been perfectly rigid, the
whole frame-work would have been exposed to
inconvenience, and even danger, amidst the
shocks it must encounter during all the quick
and sudden movements of the body. Not only
must its mechanism be framed to sustain these
shocks, but also to accommodate itself to various
kinds of flexions, and twistings of the trunk.
While these objects are provided for, care must
at the same time be taken that the spinal mar-
row it encloses shall, amidst all these motions,
remain secure from pressure ; for so delicate is
its structure that the least degree of compression
would at once interrupt its functions, and lead
to the most fatal consequences. A safe passage
is likewise to be afforded to the nerves, which
issue from the spinal marrow, at certain inter-
vals, on each side, throughout its whole length.
No where has mechanical art been more con-
spicuously displayed than in the construction of
a fabric capable of fulfilling these opposite, and
apparently incompatible functions. The prin-
cipal difficulty was to combine great strength
with sufficient flexibility. This we find accom-
plished, first, by the division of the column into
a great number of pieces, each of which being
locked in with the two adjoining pieces, and
.'390 THE MECHANICAL FUNCTIONS.
tightly braced by connecting ligaments, is al-
lowed but a very small degree of flexion at the
point of junction. This slight flexion at each
single joint, however, by becoming multiplied
along the series, amounts to a considerable de-
gree of motion in the whole column.
The broad basis of each bone is connected
with the next, not by a joint, but by a plate of
equal breadth (m, m, Figures 178 and 179), com-
posed of a peculiar substance, intermediate in
its texture to ligament and cartilage, and pos-
sessing in a remarkable degree the qualities of
toughness and adhesion, united with compres-
sibility and elasticity. By yielding for a certain
extent to a force tending to bend it to either side,
it diminishes the quantity of motion which would
otherwise have been required in each individual
joint ; and by acting, at the same time, as a
spring, it softens all the jars and concussions
incident to violent action : for we find that how-
ever the spine may be bent, no chasm is left by
the flexions of the vertebras upon one another,
nor is the continuity of the column in the
smallest degree interrupted.
The motions of the vertebrae upon each other
are farther regulated by the mode in which their
articular processes, which are the pieces that
project obliquely on each side, play upon each
other. These processes, which are seen at a, a,
in the preceding figures (177 and 178), are of
VERTEBRAL COLUMN. .'3.91
great use in preventing the sudden displacement
of the vertebrae ; for this effect cannot be pro-
duced by any force short of that which would
occasion fracture. Any one who will try to dis-
locate, by sheer force, the spine of a hare or
rabbit will rind reason to admire the art with
which its bones have been locked together, and
the skill displayed in combining great flexibility
with such powerful resistance to every effort that
can be made to separate them.
For the purpose of allowing a passage to the
spinal marrow, the bodies of the vertebrae
(b, Fig. 177 and 178), are hollowed out behind,
into a groove, over which a broad plate of bone is
thrown from the sides of the vertebrae, like the
arch of a bridge. The succession of arches,
when the vertebrae are joined together, forms a
continuous canal, which is occupied by the
spinal marrow. Notches, corresponding to each
other, are left in the sides of each of the arches,
forming apertures for the secure passage of the
nerves as they issue from the spinal marrow.
All these circumstances are visible in the figures,
particularly in the section, Fig. 179, where c, c,
is the canal for the spinal marrow, and in which
the apertures just mentioned are distinctly seen,
at o, o.
In order to give an advantageous purchase to
the muscles which are attached to the spine,
each vertebra has, besides the parts above des-
392 THE MECHANICAL FUNCTIONS.
cribed, a projecting piece of bone, extending
upwards from the crown of the arch, and deno-
minated the spinous process (s, s). The sharp
ridge that runs along the middle of the back of
a quadruped, is formed by the continued series
of these processes There are also, on the sides
of the vertebrae, two other projecting pieces,
which are denominated the transverse processes
(t), and which serve as levers for bending the
column laterally, that is, either to the right or to
the left. All these component parts of the spine
are subject to considerable modifications, in dif-
ferent tribes of animals, according to the par-
ticular mechanical circumstances of the system,
and to the particular intentions of their forma-
tion.
There is scarcely any part of the osseous
fabric of which the variations better illustrate
the strict unity of plan, and the beautiful law of
gradation observed by nature in all her opera-
tions, than the spine. In studying the various
modifications which this part of the skeleton
undergoes, it will be useful to bear in mind the
principles, which appear to regulate its forma-
tion, and which GeofFroy St. Hilaire has de-
duced by following the history of its early
growth, and noticing the order in which its
several parts are developed.* In common with
* Memoires du Museum, ix. 79 and 89.
STRUCTURE OF VERTEBRJE.
393
all bones, the vertebrae take their rise from cer-
tain determinate points, or centres of ossifica-
tion, where, at first, detached pieces of bone are
formed, destined to unite together so as to com-
pose the entire bone. An accurate knowledge
of the general forms and relative situations of
these elementary pieces is of much importance,
because we find that particular circumstances
determine the developement of some of these
parts much earlier, and to a greater extent than
other parts; and thus lead to great differences in
the shapes and proportions of various bones, at
different periods of their growth, although their
origin and composition are essentially the same.
The number of elements which enter into the
composition of a vertebra has been differently
estimated by different phy-
siologists ; but the following
are certainly entitled to that
character. They are repre-
sented in their relative situa-
tions in Fig. 180. The first
is the part which forms the
nucleus, or body (b) of the
vertebra ; and its ossification
begins at the centre. Next
in importance are the two
bony plates, or leaves, as they may be called
(l, l), which proceed from the sides of the body,
and embrace the spinal marrow which is situated
3.94 THE MECHANICAL FUNCTIONS.
between them. The fourth essential element is
the spinous process (s), which unites the two
leaves, and thus completes the superior arch,
of which it may be regarded as the key stone,
for the protection of the spinal marrow. Then
come the two transverse processes (r, t), which
extend outwards from the sides, and with which
the arches of bone, constituting the ribs (r, r),
are generally connected. These are the six
parts which may be considered as the elements
that are most essential, and most constantly
present in the composition of the vertebrae. But
some other parts may also be noticed as of very
frequent occurrence : such are the bony plates
which cover the two flat portions of the bodies of
the vertebrae, forming the surfaces immediately
contiguous to the intervertebral ligament ; which
surfaces, in some of the lower orders of the
vertebrata, become articular. There is fre-
quently, also, a developement of processes .(f),
forming arches and spines at the lower surface
of the vertebrae, or the one opposite to that
which gives rise to the superior arches already
mentioned. This structure is very generally
met with in Fishes, and it is observed also in
the Cetacea. The arches thus formed enclose
a large artery, which is the continuation of the
aorta, or the main artery running along the
back, immediately under the spinal column.
There are still other processes, less constantly
STRUCTURE OF VERTEBRAE. 395
present, and more variable in their shape. They
form articular surfaces for the purpose of being
connected with the surfaces of corresponding
processes in the contiguous vertebra. Of these
there are four (a, a, a, a) belonging to each ver-
tebra, two in front, and two behind. These,
however, should not be included among the
primary elements of the vertebrae, because we
find them, in different instances, occupying
different positions, and formed sometimes by
extensions of the bodies, and at other times of
the leaves. In following them through the se-
veral tribes of animals, we observe them shifting
their places, in various ways, and not even pre-
serving any constancy in their number. They
are wholly absent in Fishes : in the Crocodile,
and* other Reptiles, they approximate so as to
form three articular surfaces, namely, two close
to one another, and a third posterior to these.
In the Omithorhynchus, while the latter retains
its situation in the middle, the other surfaces
have separated from each other, and have tra-
velled outwards, taking their stations upon the
leaves. In the Mammalia, the middle surface
has wholly disappeared, and the outer surfaces
have risen into what are termed the oblique pro-
cesses.
In addition to these, accessory bones are often
developed to suit particular occasions. Thus in
Fishes, we see that one or two additional pieces
3.96 THE MECHANICAL FUNCTIONS.
(i) are affixed to the ends of each spinous pro-
cess. In many cases, instead of being thus
placed in a line with these processes, they appear
at a little distance, as if they had slipped from
their proper situations ; they are then found
between the spinous processes, and receive the
name of interspinous bones.
The spinous processes have a tendency, when
their developement proceeds, to divide into two
branches, and this bifurcation frequently takes
place also in the interspinous bones. The trans-
verse processes likewise occasionally develope
accessory pieces, as is found to be the case in
some reptiles ; but in other instances they un-
dergo a gradual change of position, as we follow
them backwards along the spinal column, where
they descend towards the abdominal region.
The flexibility of particular portions of the
spinal column is regulated by the size and form
of its processes. When these are much deve-
loped, they necessarily obstruct the flexion of
the vertebrae in the directions in which they are
situated : when they are small, no such hind-
rance arises, and the spine is free to move in all
directions. Thus, when we see the spinous pro-
cesses much enlarged, while the transverse pro-
cesses are small, we may infer that the spine is
incapable of any bending in that direction ; but
that it has the power of free lateral flexion. This
STRUCTURE OF THE SPINE. 3,97
is the condition of the spine of fishes, where this
latter kind of motion is the one principally
wanted. In Dolphins, and other Cetacea, on the
contrary, where the actions are required to be
vertically upwards and downwards, the spinous
processes are small, and the transverse processes
very long and broad.
Every instance of variation in the forms of
these important parts of the osseous system, will,
in like manner, be found to have a relation to
some particular circumstance in the living habits
of the animal, and to be subordinate to the
general plan of its economy. But in order to
understand the mode in which nature has effected
these changes, it is necessary to study the ele-
ments of each part of the osseous system ; for
these constitute the alphabet by which the com-
binations she presents to us become legible, and
by which their origin and progress are unfolded
to our comprehension. According as each of
these elements of ossification receives different
degrees of developement, so the different bones
they compose acquire their particular shapes and
relative dimensions. Sometimes, indeed, we find
that one or other of these elements has disap-
peared ; or at least we can discover no trace
of its developement ; in other cases, we see it
exceedingly expanded, and appearing under
forms of greater complication, so as to be with
398 THE MECHANICAL FUNCTIONS.
difficulty identified : on some occasions, as we
have just seen in the spinous bones of fishes,
its accessory structures are multiplied, as if con-
tinued efforts were made by the system to repeat
the same structures. Amidst all these modifica-
tions, the parts that preserve the greatest con-
stancy of form are those which are of most im-
portance, and which are constituent parts of the
primordial type of the class to which the animal
belongs.
The spinal column is generally prolonged at
its posterior extremity into a series of vertebrae,
which are sometimes exceedingly numerous ; de-
creasing in their size as they extend backwards,
and having continually smaller processes, the
one disappearing after the other, till all of them
are lost, and nothing remains in those at the
extremity of the series but the cylindrical bodies
of the vertebrae. Even these become stinted in
their growth and ossification, until we find
the terminal pieces generally remaining in the
state of cartilage. Such is the structure of the
osseous support of the tail, as seen in many
quadrupeds in its most developed forms. It
illustrates the law, that when in any system
there occurs a frequent repetition of the same
structure, the evolution, in the latest of those
repetitions, becomes less perfect, and ends by
being abortive. In the present instance, the
consequences of this law are highly advanta-
CRANIUM. 399
geous, since it provides for the flexibility of the
tail, and qualifies it for being applied to a great
variety of useful purposes, as we find more espe-
cially exemplified in the Ateles, or spider mon-
key, and in the Kangnroo.
Next in importance to the spine is the cra-
nium, or osseous covering of the brain ; together
with the bones of the face which protect the
organs of the finer senses. An accurate inves-
tigation of the mode in which these bones are
formed has led many modern anatomists to the
opinion that they were originally parts of the
spinal column, and that they are in fact deve-
lopements of vertebrae, much altered, indeed, in
shape, in consequence of the new conditions to
which they have been subjected ; but still pos-
sessing all the essential elements of vertebrae.
In the embryo condition of these organs, and
while the brain is yet undeveloped, the resem-
blance of the bony circles which enclose it to
vertebrae is certainly very striking ; but in pro-
portion as the brain becomes expanded, the si-
milarity diminishes ; for the rapid growth of the
brain in the higher orders of animals is neces-
sarily attended with an equally sudden expan-
sion of the bones of the skull. Hence their
several elements are thrown into unusual posi-
tions, and being variously distorted and disfi-
gured, can hardly be recognised under the
strange disguises they assume.
400 THE MECHANICAL FUNCTIONS.
The extensive researches that have been re-
cently made in this branch of comparative ana-
tomy, have supplied many facts which tend to
support the hypothesis that the bony coverings
of the brain are the result of the developement of
three vertebrae. According to this theory, the
first of these supposed cranial vertebra, beginning
our enumeration from the neck, is the origin of
the occipital bone, of which the lower part, or that
which immediately supports the cerebellum, cor-
responds to the body of the vertebra; the two late-
ral portions, to the leaves; and the upper flat
plate to the spinous process. The body of the
second cranial vertebra becomes, in process of
time, the posterior half of the sphenoid bone,
which lies in the middle of the basis of the skull;
the temporal bones being formed by its leaves,
and the parietal bones by the lateral halves of its
spinous process. The third cranial vertebra is
constituted by the anterior half of the sphenoid
bone, which is its body, and the frontal bones,
which are its leaves. This theory, which origi-
nated with Dumeril, and was extended by Oken,
has been farther applied to the bones of the face,
by Geoffroy St. Hilaire, who conceives them to be
likewise developements of several other supposed
cranial vertebrae ;* but the analogies by which
the hypothesis is supported become more feeble
* In this theory of St. Hilaire the number of cranial vertebree
is seven, each composed of nine elementary pieces.
SKELETON OF VERTEBRATA. 401
and confused as we recede from the middle of
the spinal column.
All the other parts of the skeleton may be
regarded as accessory to the spine ; and they are
far from exhibiting the same constancy either in
form or number, as the vertebral column. In
some instances, as in serpents, these accessory
parts are altogether wanting ; in others, they
exist only in rudimental states ; and it is but in
a few that they can be considered as having
reached their full developement. In order to
obtain a standard of comparison by which to
estimate all their gradations of evolution, it will
be best to consider them first in their more
perfectly developed forms, as they are pre-
sented in the higher classes of quadrupeds.
In the following descriptions, the skeleton of the
Hog (Fig. 181) will be taken for the purpose of
reference.
The ribs consist of arches of bone, affixed at
their upper ends to the bodies of the vertebrae,
and also, by a separate articulation, to their
transverse processes ; where, in general, they
are allowed a slight degree of motion. Their
primary use is to defend the vital organs situ-
ated in the region of the chest, or thorax (namely,
the heart and the lungs) ; but they are subser-
vient also to the function of respiration, by the
alternate movements which are given to them
by their muscles. The two parts of which they
VOL. I. D D
402
THE MECHANICAL FUNCTIONS.
are composed often form an angle by their junc-
tion, and at this angle a process occasionally
extends, for the purpose of forming connexions
with the neighbouring ribs.
The ribs are connected in front with the
breast bone, or sternum (s), often by the inter-
vention of cartilages, which, from their simi-
larity of form to the ribs, appear as continua-
tions of them, and are provided apparently to
eke out the remainder of the semicircle. These
cartilages, which have been termed the sterno-
costal appendices, often become ossified, either
wholly or in part.
The sternum is formed of nine elementary
pieces, each proceeding from a separate centre
of ossification. Two of these occupy the end
which is nearest to the head ; four are lateral ;
and two are situated at the opposite extremity ;
SKELETON OF VERTEBRATA. 403
one only being central, and surrounded by the
rest. Few subjects in comparative osteology are
more curious and instructive than to trace the
developement of these several elementary parts
in the different classes of animals, from the ru-
dimental states of this bone as it occurs in
Fishes, to its greatly expanded conditions in the
Tortoise and the Bird, which exhibit the most
opposite proportions of these elements.
Last in the order of constancy come the bones
of the extremities. As we ascend in the scale of
animals we may observe the prevalence of a ten-
dency to the concentration of organs, and con-
sequently to the diminution of their number.
While in animals of the inferior orders, which
are possessed of extremities, we find a con-
siderable number of legs ; in all the animals
comprised in the class of true insects nature
has limited the number to six ; and in the Ver-
tebrata it never exceeds four. As in insects
we observed that all the legs are divided into the
same number of parts ; so we find among Qua-
drupeds a stinking correspondence in the bones
of the fore and the hind extremities. Both the
one and the other are connected with the spine
by the intermedium of large and broad bones,
which are intended to serve as a basis for their
more secure attachment, and for giving, at the
same time, extensive and advantageous purchase
to the muscles, which are to move the limbs.
404 THE MECHANICAL FUNCTIONS.
The two bones by which the anterior extremity
is connected with the trunk are the blade-bone,
or Scapula (b), which sends out a process called
the coracoid bone ; and the collar-bone, or the
Clavicle* which extends from the scapula to the
sternum. The corresponding connecting bones
of the posterior extremity are three in number,
and constitute, together with the part of the spine
to which they are attached, what is called the
Pelvis (p). The part of the spine which is thus
included in the pelvis, is termed the Sacrum.
In its complete state of ossification it is a single
bone ; but it was originally composed of a num-
ber of separate vertebra;, which have afterwards
become consolidated into a single bone, and
which bear the marks of having been compressed
from behind forwards during their growth, so
that they could only expand laterally. The
vertebrae which succeed to these, and which are
not consolidated with the sacrum, compose what
is called the os coccygis, (q), or more properly the
coccygeal vertebra : when they are sufficiently
numerous to compose a tail, they come under
the denomination of caudal vertebra. The three
* This bone does not exist in the skeleton of the hog; but its
form and connexions with the sternum and scapula in the
human skeleton are shown in Fig. 182, where s is the sternum;
x, the xiphoid carriage; c, the clavicle ; b, the scapula; a, the
acromion ; k, the coracoid process ; and g, the glenoid cavity
for the articulation of the humerus.
SKELETON OF VERTEBRATA. 405
bones of the pelvis, are the ilium, the ischium,
and the pubis. They all concur in the formation
of a large cup-like cavity, called the acetabulum,
which receives the round head of the thigh bone
(f), and constitutes generally the largest joint in
the body.
A single bone composes the first division of
each limb, both in the fore and hind extremities.
In the fore leg it is termed the humerus (h), in the
hind leg, the femur (f). The next division con-
tains two bones, placed parallel to each other ;
they are in the former, the radius (r), and the
ulna (u); in the latter, the tibia (t), and fibula (f).
These are followed by a number of small,
rounded or cubical bones, collected together in
a group, which constitutes the Carpus (w), in
the fore leg, and the Tarsus (t), in the hind leg.
Next come a set of long cylindrical bones, com-
posing the metacarpus (m), in the former, and the
metatarsus (m), in the latter case. In the most
complete forms of developement these are always
five in number in each limb ; they are placed
generally parallel to each other, but are enve-
loped in one common covering of integument.
The Phalanges, or toes (z), are cylindrical bones,
continued in a line from each of the former :
they are generally three in number in each toe.
To the last joint, which is often termed the un-
gual bone, there is usually attached either a nail,
a claw, or a hoof. Small detached bones are
406 THE MECHANICAL FUNCTIONS.
frequently found at the exterior part of the angles
which they form by their junction, serving the
purpose of giving a more advantageous position
to the tendons of the muscles which extend
those joints. The patella, or knee pan (p), is
the largest of these, and is pretty constantly
present. Smaller bones of this description are
met with on the joints of the fingers, and are
termed sesamoid bones.
On comparing these divisions of the limbs of
quadrupeds with those of insects we cannot fail
to perceive that there exists between them a
marked analogy : and that naturalists were not
led away by mere fancy when they applied to
the latter the same names as those borne by the
former. This, however, is not the only instance
of analogy that may be discovered between the
structures of articulated and of vertebrated ani-
mals, however strong may be the contrast which
they offer in all the essential features of their
conformation. The rings which compose the
skeleton of the insect, and which enclose its prin-
cipal nervous chords, have been supposed to have
an analogy with the circles of bone which consti-
tute the primary forms of the vertebrae, and which
contain the spinal chord ; although in the former
case, it is true, other viscera are included within
the arches, whereas none are contained in the
latter. They agree, also, in having the head
placed at one extremity, distinct from the trunk,
SKELETON OF VERTEBUATA. 407
and containing the principal organs of the senses.
Further correspondences have been likewise
traced in the minuter anatomy of these parts,
which it would here occupy too much space to
examine in detail.
An approximation is apparently made towards
an internal skeleton in the cephalopodous Mol-
lusca ; where we find a central body, cartila-
ginous in some species, calcareous in others.
In the Loligo, it has a long and slender shape,
and is pointed at the end, like the blade of a
sword ; it bears, as we shall hereafter notice,
some resemblance to the cartilaginous spine of
the fish called the Myxine, or Gastrobranchus,
which does not enclose the spinal marrow, but
only admits it to pass along a groove in its upper
edge.
All these multiplied instances, when weighed
together, and united in a comprehensive view,
are sufficient to prove, that there exist very per-
ceptible links of connexion among all the classes
of created beings, even in those apparently the
most remote from one another. They render it
clear to the discerning eye of the philosophic
naturalist, that all the races of animated beings
are members of one family, and the offspring of
the same provident parent, who has matured all
his plans on a deeply premeditated system, and
who dispenses all his gifts with the most salutary
regard to the general welfare of his creatures.
408
Chapter VII.
FISHES.
In reviewing the series of animals which compose
each great division of this kingdom of nature,
we constantly find that the simplest structures
and modes of progression are those belonging to
the aquatic tribes. Among vertebrated animals,
the lowest rank is occupied by Fishes, a class
comprehending an immense number of species,
which are all inhabitants of the water, which
exhibit an endless variety of forms, and open to
the physiologist a wide field of interesting re-
search. We cannot fail to perceive, on the most
cursory glance, the beautiful adaptation of the
form and structure of all these animals to the
properties of the element in which they are
destined to reside. In order that the fish might
glide through the fluid with the least resistance,
all its vital organs have been collected into a
small compass, and the body has been reduced
into the shape of a compact oval, compressed
laterally, and tapering to a thin edge, both before
and behind, for the purpose of readily cleaving
the water as the fish darts forward, and also of
obviating the retardation which might arise from
FISHES. 409
the reflux of the water collected behind. With
a view to diminish friction as much as possible,
the surface of the body has been rendered smooth,
and the skin impregnated with oil, which defends
it from injurious impressions, and at the same
time prevents the water from penetrating into its
substance.
The body of a fish is nearly of the same spe-
cific gravity as the water it inhabits ; and the
effect of gravity is therefore almost wholly coun-
terbalanced by the buoyant force of that fluid ;
for the weight of a mass of water, equal in bulk
to the body itself, is the exact measure of this
buoyant force. If this weight were precisely
the same as that of the fish, the animal would
be able to remain suspended in any part of the
fluid without the necessity of employing any
voluntary motion or exertion for that purpose ;
but as the body of a fish is generally a little
heavier than the fluid medium, especially if it
be fresh water, it is necessary for the animal to
give its body some degree of motion, in order to
prevent its sinking.
In land quadrupeds, the limbs have to perform
the double office of supporting the body, and of
effecting at the same time its locomotion ; but as
nearly the whole of the weight of a fish is already
sustained by the element in which it is immersed,
its instruments of motion may be employed
exclusively for progression ; and the powerful
410 THE MECHANICAL FUNCTIONS.
hydrostatic pressure, which supports the body
on all sides, supersedes the necessity of that co-
hesive rigidity of frame, which is essential to the
safety of terrestrial animals. Hence we find
that in one whole tribe of fishes, the skeleton is
composed merely of cartilage ; and, in all, it ex-
hibits much less of the osseous character than in
the higher classes. The frame-work of the ske-
leton, even of osseous fishes, has not the com-
pactness possessed by that of quadrupeds or
reptiles : the pieces which compose it are joined
together less firmly ; many of them, indeed,
remain in an imperfectly ossified condition, their
elementary pieces being detached from one ano-
ther, as if the usual process of consolidation had
been arrested at an early stage. The texture of
the bones of cartilaginous fishes corresponds to
this primeval condition ; for it is composed merely
of granules of calcareous phosphate, interspersed
amidst the cartilaginous substance in detached
masses, or presenting the appearance of coarse
fibres, thinly scattered through the semitrans-
parent bone. Compared with the quantity of
gelatin which enters into their composition, the
bones of fishes contain but a small proportion of
earthy ingredient ; a circumstance which ex-
plains the pellucidity of the mass, and the readi-
ness with which the osseous fibres it contains
can be distinguished. Another consequence of
the want of density in the bones of fishes is, that
FISHES.
411
their articulations are less regular and perfect
than the corresponding joints of terrestrial ani-
mals ; for it is evident that where the parts are
soft and flexible, joints are not required.
In the osseous fishes, the bony structures are
more finished ; and they even arrive at a degree
of hardness equal to that of the higher classes.
But this developement is not uniform in all the
bones ; in the head of the pike, for instance,
while some of the bones have acquired a great
hardness, others remain wholly and permanently
in a cartilaginous condition. The bones of fishes,
however advanced in their ossification, never
reach that stage of the process in which cavities
are formed ; thus there is no space for marrow,
nor even for the cellular or cancellated structure
which we have noticed in the more perfect bones.*
The general disposition of the bones which com-
pose the entire skeleton will be understood from
Fig. 184, which represents that of the Cyprinus
* Cuvier, sur les Poissons. Tom. i. p. 218.
412 THE MECHANICAL FUNCTIONS.
carpio, or carp. The muscular flesh of fishes is
likewise softer than that of the higher classes ;
and the cellular substance more attenuated and
more gelatinous ; so that the membranes which
it forms are of a looser and more pulpy texture.
Progressive motion in fishes is effected by the
simplest means, the principal instrument em-
ployed for this purpose being the tail ; for the
fins, as we shall presently find, are merely auxi-
liary organs, serving chiefly to balance the body,
while it receives its propulsion from the tail. A
fish moves in the water upon the same principle
as a boat is impelled in paddling ; for the action
of the tail upon the water is lateral, like that
of an oar, which it resembles in the vertical
position of its plane ; and the effect is transferred
d by the resistance of the water to
/ | \ the body where the impulse ori-
a/ \b ginates. Let us suppose, for
\ 1 / example, that the tail is slightly
X>K inclined to the right, as shown
If in Fig. 185. If, in this situa-
V'- '\R tion, the muscles on the left side,
V ™ tending to bring the tail in a
v/'m n£ right line with the body, are sud-
denly thrown into action, the resistance of the
water, by reacting against the broad surface of the
tail in the direction p r, perpendicularly to that
surface, will cause the muscular action to give
the whole body an impulse in that direction ; and
PROGRESSIVE MOTION IN FISHES. 413
the centre of gravity, c, will move onwards in
the direction c b, parallel to p r. This impulse
is not destroyed by the further flexion of the tail
towards the left side, because the principal force
exerted by the muscles has already been ex-
pended in the motion from r to m, in bringing it
to a straight line with the body ; and the force
which carries it on to l is much weaker, and
therefore occasions a more feeble reaction.
When the tail has arrived at the position l, in-
dicated by the dotted outline, a similar action of
the muscles on the right side will create a resist-
ance and an impulse in the direction of k l, and
a motion of the whole body in the same direction,
c a. These impulses being repeated in quick
succession, the fish moves forwards in the diago-
nal c d, intermediate between the directions of
the two forces. By bending the whole body
almost in a circle, and then suddenly straighten-
ing it, fishes are often able to leap to the top of
a high cataract, in ascending against the stream
of a river.
Such being the plan upon which progression
is to be effected, we find that every part of the
mechanism of the fish is calculated to promote
its execution. The principal muscular strength
is bestowed upon the movements of the tail ; and
the largest assemblage of muscles consists of
those which give it the lateral flexions that have
been just described. For this purpose all the
414
THE MECHANICAL FUNCTIONS.
important viscera are placed forwards, and
crowded towards the head. No room is allowed
for a neck ; and the abdomen may be almost
regarded as continuous with the head, there
being properly no intervening thorax ; for the
respiratory organs are situated rather beneath
than behind the head. All this has been done
with a view to leave ample scope for the pro-
longed expansion of the coccygeal vertebrae, and
of their muscles, which compose more than half
the bulk of the animal.
Having seen how all impediments to the free
motion of the tail have been carefully removed,
let us next inquire into the mechanism by which
mobility has been given to that organ. The
first peculiarity we meet with in the structure of
the spine of fishes is the mode in which the
vertebra? are connected together. The bodies of
each vertebra, as may be seen in Figures 186
and 187. are hollowed out, both before and
SKELETON OF FISHES. 415
behind, (considering the spinal column as ex-
tended horizontally), so as to form cup-like
hollows ; by which means, where the concave
surfaces of two adjacent vertebrae are applied to
one another, a cavity, having the shape of a
double cone, is formed by the junction of the
margins of these conical hollows. These cavi-
ties are distinctly seen laid open in Fig. 188,
which represents a vertical section of three
adjacent vertebrae of a cod. The edges that are
in contact, are united all round by an elastic
ligament, which readily yields to the bending of
the vertebrae upon one another by the applica-
tion of any force to one side of the spine, and
restores it to its former state when the force has
ceased to act. The extent of motion in each
joint is but small ; but being multiplied in the
whole series, the resulting effect is considerable.
The cavity itself is filled with a gelatinous, but
incompressible fluid substance, which constitutes
a spherical pivot for all the motions of the joint.
This singular kind of articulation would ap-
pear framed with a view to allow of motion in all
directions. Here, however, the motions are
restricted by the extension of the spinous pro-
cesses (s, s, in the preceding figures), which in
fishes are of great length ; so that they effec-
tually prevent all flexions either upwards or
downwards, and limit it to those from side to
side. It is precisely these latter kind of motions
41G THE MECHANICAL FUNCTIONS.
which are wanted in the fish, for striking the
water laterally, with the broad vertical surface of
the tail. Processes of a similar form and appear-
ance, (f, f), and which impede any flexion
downwards, are generally also met with in the
lower surface of the spine, and more especially
in the hinder portion of the column. These are
the inferior spinous processes, and, like the
superior, they also form an arch, through which
there passes the continuation of the abdominal
aorta, or great artery which proceeds down the
back. The number of vertebrae is very various
in different fishes : in some they are multiplied
exceedingly, as in the shark, where there are
more than two hundred.
There are few parts of the structure of animals
that exhibit more remarkable instances of the
law of gradation than the spine of fishes, in
which we may trace a regular progress of deve-
lopement from the simplest and almost rudi-
mental condition in which it exists in the
Myxine and the Lamprey, to that of the most
perfect of the osseous tribes. Its condition, in
the former of these animals, presents a close
analogy with some structures that are met
with in the molluscous, and even in annulose
animals. So near is the resemblance of the
spinal column of the myxine, more especially, to
the annular condition of the frame-work of the
Vermes, that doubts have often arisen in the
STRUCTURE OF FISHES. 417
minds of naturalists whether that animal ought
not properly to be ranked among this latter
class. Its pretensions to be included among the
Vertebrata are, indeed, but slender and equi-
vocal ; for, in place of a series of bones com-
posing the vertebral column, it has merely a soft
and flexible tube of a homogeneous and cartila-
ginous substance, exhibiting scarcely any trace
of division into separate rings, but appearing as
if it were formed of a continuous hollow cylinder
of intervertebral substance, usurping the place of
the vertebrae, which it is the usual office of that
substance to connect together, and having in its
axis a continuous canal filled with gelatinous
fluid. This, however, is not the channel intended
for containing the spinal marrow, for that ner-
vous cord is on the outside of this column. The
cartilage, indeed, sends out no processes to bend
round the spinal marrow, and forms no canal for
its passage and protection. The nervous matter
here consists merely of two slender cords, which
run parallel to one another in a groove on the
upper part of the spinal column ; and these
cords are covered only by a thin membrane, the
presence of which it requires very minute atten-
tion to detect. The partial protection thus af-
forded to so important an organ is not greater
than that given by the cartilaginous lamina of
the cuttle-fish, which in form, texture, and
VOL. I. E E
418 THE MECHANICAL FUNCTIONS.
situation is very analogous to the spine of the
myxine.
As we ascend from this rudiment al condition
of the spine, we find it, in the lamprey, more
distinctly divided into rounded portions, appear-
ing like beads strung together. These rudi-
mental bodies of vertebrae have not yet com-
pleted the cup-like hollows on their two ends,
but are shaped like rings, being perforated in the
centre, so as still to form a continuous canal
throughout the whole column.
Proceeding to more advanced developements,
we find, in the Sturgeon and other cartilaginous
fishes, a greater condensation of substance pro-
duced by the deposition of granules of osseous
matter; the central canal becomes divided into
lozenge-shaped compartments by the closing in
of the sides of the body of each vertebra.* Fre-
quently the sides do not quite meet, and the
leaves, which are developed from the upper
surfaces of the vertebra?, now form arches over
the spinal cord, and are united above by spinous
processes. Yet the whole skeleton in these
* A small aperture still remains, establishing a communication
between the cavities the whole length of the spine. This is sup-
posed to be designed to obviate the compression of the fluid in
the different cells or cavities during the motions of the spine.
The vertical sections, Fig. 189 and 190, of two contiguous ver-
tebrae in different fishes, will convey an idea of this gradation of
developement.
STRUCTURE OF FISHES. 419
fishes remains in the incipient stage of ossifi-
cation, being more or less cartilaginous ; and
where the ossific process has begun, it has not
advanced the length of producing union between
the pieces formed from the separate centres of
ossification. Where they meet without uniting,
they form no sutures, but overlap one another.
Thus the bony structures are detached, and often
completely isolated ; affording to the physiolo-
gist an opportunity of studying the earlier stages
of this interesting process, and marking with
distinctness the number of the elements of each
bone, and the relative situations of their centres.
This knowledge is more especially of importance
towards understanding the formation and con-
nexions of the bones of the head, which are
very numerous and complicated ; and the inves-
tigation of which has been prosecuted with ex-
traordinary diligence by Geoffroy St. Hilaire,
and other continental zootomists.
It is here, more especially, that we obtain the
clearest evidence of the derivation of the cranial
bones from vertebrae analogous to those of the
spine. The occipital bone, in particular, corre-
sponds to a spinal vertebra in all its essential
elements. In many fishes, the body of this
bone, being lengthened out to form the posterior
part of the basis of the skull, becomes the basi-
lar portion. We find, on its posterior surface,
420 THE MECHANICAL FUNCTIONS.
the same cup-like cavity as in the true vertebrae ;
and it is joined to the next vertebra in the same
manner as the spinal vertebrae are joined to each
other. Its crest has the exact shape of a spi-
nous process. In front, the basilar bone is united
to the sphenoid bone, which, with the vaulted roof
that springs from the sides of both these bones,
like the leaves and spinous processes of the ver-
tebrae, form together a long cranial cavity. This
cavity is placed in a direct line with the spinal
canal, and contains the nervous tubercles which
constitute the brain. Yet the brain does not com-
pletely fill this cavity ; for a space is still left,
which is occupied by a pulpy substance. In like
manner, the accordance of the other cranial bones
with vertebrae has been attempted to be traced ;
but in proportion as we recede from the central
parts of the spine, this correspondence is less
distinct, in consequence of the various degrees
of developement which these several elements
have received, in order to adapt them to particu-
lar purposes relating to sensation, to the pre-
hension and deglutition of food, and also to
aquatic respiration. It is impossible, however,
without exceeding the limits within which I
must here confine myself to enter into the details
of structure which would be requisite in order to
render this subject sufficiently intelligible.
The rest of the skeleton of fishes is extremely
simple. In many, as in the Ray and Tetrodoti,
STRUCTURE OF FISHES. 421
there are no ribs. When these bones exist,
they are articulated with the extremities of the
transverse processes of the vertebrae, of which
they appear to be merely continuations, or ap-
pendices. There is generally no sternum to
which they can be attached below : in a few
fishes only, such as the Herring and the Dory,
we find rudiments of this bone, consisting of a
few pieces placed in a line on the lower part of
the trunk.*
The parts of the skeleton of fishes, which cor-
respond to the arms and legs of quadrupeds, are
the pectoral and ventral fins (marked respectively
by the letters p and v, in Fig. 184). The former
are met with, with but few exceptions, in all
fishes ; and they consist of a series of osseous
pieces, in which we may often recognise with to-
lerable precision the analogous bones composing
the anterior extremities of a quadruped ; such as
the scapula, clavicle, humerus, ulna, and radius. f
These two latter bones are very distinctly marked
* The bony arches arising from the skull, which support the
branchiae, or gills, have been considered as the bones correspond-
ing to the ribs of terrestrial quadrupeds ; and if this view were
taken of them, it would tend to confirm the analogy of the cra-
nial bones to the spinal vertebrae.
f Those anatomists who are fond of pursuing the theory of
analogies, maintain that all these bones are merely developements
of certain ribs, proceeding from the spine in its anterior parts.
A similar origin has been assigned to the pieces of bone to which
the ventral fins are attached : but it is difficult to reconcile this
422
THE MECHANICAL FUNCTIONS.
in the Lophius piscatoriiis, or Angler, as may be
seen in Fig. 191, where e is the scapula; c, the
clavicle ; u, the ulna ; and r, the radius. The
carpus may also be recognised in a chain of
small bones, w, interposed between the radius
and the Phalanges, z. In the Rat/ these pha-
langes are very numerous, and each is divided
into several pieces by regular articulations :
these are shown in Fig. 192 : they are arranged
close to one another in one plane, and form an
effectual base of support to the integument which
covers them. The scapula, according to Cuvier,
is sometimes detached from the rest of the ske-
leton, and at other times connected with the
spine : in most cases, however, it is suspended
theory with the fact that these bones do not proceed from the
spine, and are quite detached from the rest of the skeleton. It
is evident, therefore, that if they are to be considered as analo-
gous to the bones of the hinder extremities in the mammalia,
they must be in a condition of very imperfect developement.
STRUCTURE OF FISHES. 423
from the cranium ; a fact which may be cited
in further corroboration of the analogy which
the cranial bones have to vertebrae.
In the ray and the shark tribes, both the ante-
rior and posterior extremities are supported by
arches of bones, forming a sort of belt. This
structure is an ap-
proach to that which
obtains in many rep-
tiles, and indicates a
further step in the regular progress of develope-
ment. This belt in the Ray is shown in Fig. 193.
In examining that part of the skeleton of fishes
which corresponds to the posterior extremity, we
observe the total absence of both femur and
tibia ; but the bones of the toes are attached to
a set of small bones, which appear to act the
part of a pelvis, but which, in consequence of
their not being connected with the spine, have
no determinate situation, and are found at various
distances from the head in different fishes.
They appear emancijiated from the restraints
to which they would have been subjected had
they been fixed to a sacrum, or to any parti-
cular part of the spine : we find them, accord-
ingly, often placed considerably forwards ; and
in some instances, as in the Subbrachieni, even
anteriorly to the pectoral fins, which are the
true arms of the animal. But in one whole
order of fishes, the Apodes, there is not even a
424 THE MECHANICAL FUNCTIONS.
vestige of ventral fins, nor are any pelvic bones
provided for their support. This is the case with
the Eel, the Gymnotus, &c. In a few species
there is also a total absence of pectoral, as well
as ventral fins.
The dorsal fins are supported by a series of
slender bones (d Fig. 184), which are joined to
the spinous processes of the vertebrae, and are
formed from distinct centres of ossification.
These rays, as they are called, are sometimes
destined to grow to so considerable a length, as
to require being subdivided into many pieces,
in order to lessen the danger of fracture, to
which a very long filament of bone would have
been exposed, and also to allow of a greater de-
gree of flexibility. These rays assume branched
forms from the further subdivision of their parts ;
and when, for the purpose of adding strength to
the fin, it becomes necessary to multiply the points
of support, intermediate bones are developed,
serving as the basis of the rays. Convenience
requires that they should be detached from the
ends of the spinous processes, which is their
usual position, and placed between them : when
in this situation, they bear the name of inter -
spinous bones; and when a still greater length
of osseous support is wanted, new centres of
ossification are developed at their extremities,
giving rise to a series of additional pieces, joined
end to end, and carrying out the interspinous
MUSCULAR SYSTEM OF FISHES. 425
bone, and the ray which terminates it, to a con-
siderable distance. This structure is distinctly
seen in the small dorsal fins of the Mackerel.
The anal fins, which are situated on the lower
side of the body, in the vertical plane, and next
to the tail, are, in like manner, supported by
rays, having the same parallel, or fan-like ar-
rangement as the preceding. The caudal fin, or
terminal expansion of the tail, has also a similar
structure.
The muscles of fishes compose a large portion
of the bulk of the body ; but they are arranged
in a less complex manner than those of the ani-
mals of the higher classes. Those which appear
immediately underneath the integuments are
shown in Fig. 194, where m, m, are the great
lateral muscles, producing the flexion of the
body and tail : d is the dorsal fin, which is
raised by the muscle d ; p, the pectoral fin,
expanded by the muscle p : v, the ventral fin,
moved by the muscles situated at v : a, the anal
fin, in like manner moved by muscles at its
base a : and c, the caudal fin, the muscles for
426 THE MECHANICAL FUNCTIONS.
moving which are seen at c: o is the opercu-
lum, or flap, which covers the gills ; and n, the
nasal cavities, or organs of smell. The form of
the body, and disposition of the skeleton, allow
of their being inserted immediately on the parts
which they are intended to approximate. Hence
the use of long tendinous chords is dispensed
with.*
The actions of the muscles are easily under-
stood from the nature of their insertions. In
general, the direction of the fibres is in some
degree oblique, with reference to the motion
performed. Two series of muscles are provided
for the movements of the tail, which consist
almost exclusively of lateral flexion, the whole
spine in some degree participating in this motion.
These muscles occupy the upper and lower
portions of the trunk ; their limits being strongly
marked by a line running longitudinally the
whole length of the body on each side. The
inclination of their fibres is somewhat different
in each. The advantage in point of velocity of
action which results from this obliquity has al-
ready been pointed out.
Those fins which are in pairs are capable of
four motions ; namely, those of flexion and ex-
* Between the layers of flesh, however, there occur slender
semi-transparent tendons, which give attachment to a series of
short muscular fibres, passing nearly at right angles between the
surfaces of the adjoining plates. See Sir A. Carlisle's account
of this structure in the Philosophical Transactions for 1806.
SWIMMING IN FISHES. 427
tension, and also those of expanding and closing
the rays, for each of which motions appropriate
muscles are provided ; and indeed each ray is
furnished with a distinct muscular apparatus for
its separate motion ; and these smaller muscles
regulate with great nicety all the movements of
the fins, expanding or closing them like a fan,
according as their action is to be strengthened
or relaxed. This feathering of the fin, as it
may be called, takes place in most fishes, and
is particularly observable in the tail of the Esox,
or pike tribe. Each ray of these fins, indeed, is
furnished with a distinct muscular apparatus,
for its separate motion.
Whatever analogy may exist in the structure
of the fins of fishes and the feet of quadrupeds,
there is none in the manner in which they are
instrumental in effecting progressive motion.
The great agent by which the fish is impelled
forwards is the tail : the fins, which correspond
to the extremities of land animals, are useful
chiefly for the purposes of turning, stopping, or
inclining the body, and for retaining it in its
proper position. The single fins, or those which
are situated in a vertical plane, passing through
the axis of the body (the mesial plane), prevent
the rolling of the body, while the fish darts for-
wards in its course. The fins which are in pairs
(that is the pectoral and the ventral fins), by
their alternate flexions and extensions, act like
oars ; while they are capable, at the same time,
428 THE MECHANICAL FUNCTIONS.
of expanding and of closing the rays, like the
opening and shutting of a fan, according as their
action is required to be effective, or the contrary.
All these auxiliary instruments are chiefly ser-
viceable in modifying the direction, and ad-
justing the variations of force derived from the
impulse of the tail. They are employed also in
suddenly checking or stopping the motion, and
giving it a more rapid acceleration. But still
the tail is the most powerful of the instruments
for progression, being at once a vigorous oar, an
accurate rudder, and a formidable weapon of
offence.
Independently of these external instruments
of progression, most fishes are provided with in-
ternal means of changing their situation in the
water. The structure by which this effect is
accomplished is one of the most remarkable in-
stances that is met with of an express con-
trivance for a specific purpose, and of the em-
ployment of an agency of a class different from
that of the mechanical powers usually resorted
to for effecting the same object. We have seen
that if the body of a fish were heavier than an
equal bulk of water, and if no muscular exertions
were made, it must necessarily descend in that
fluid. If, on the contrary, it were specifically
lighter, it would as necessarily rise to the surface.
Were the animal to acquire the power of altering
at pleasure its specific gravity, it would then pos-
SWIMMING BLADDER OF FISHES. 429
sess the means of rising or sinking, without
calling into action either the fins or the tail.
Such is precisely the object of a peculiar me-
chanism, which nature has provided in the
interior of the body of the fish. A large blad-
der, filled with air, has been placed immediately
under the spine, in the middle of the back, and
above the centre of gravity. This is known by
the name of the air-bladder, or the swimming-
bladder, and in the cod-fish it is called the sound.
It frequently, as in the Carp, consists of two
bladders (a, b, Fig. 195) joined endwise, and
communicating with each other by a narrow
neck.* When distended with air, it renders the
whole fish specifically lighter than the surround-
ing water ; and the fish is thus buoyed up, and
remains at the surface without any effort of its
own. On compressing the bladder, by the action
of the surrounding muscles, the included air is
* There is great variety in the form and structure of the air-
bladder in different fishes. Sometimes it contains a large glan-
dular body of a peculiar structure, which has been conjectured
to be an apparatus for secreting air from the blood : but this is
by no means very generally met with.
430 THE MECHANICAL FUNCTIONS.
condensed, the specific gravity of the whole body
is increased, and the fish sinks to the bottom. On
relaxing the same muscles, the air recovers its
former dimensions, and the fish is again rendered
buoyant. Can there be stronger evidence of de-
sign than the placing of this hydrostatic appa-
ratus, acting upon philosophical principles, in the
interior of the organization, for a purpose so
definite and unequivocal !
In several tribes of fishes there is a canal
(c d) establishing a communication between
this bladder and the stomach, or the gullet (o) ;
so that by compressing the bladder, a quantity
of air may be forced out, and a very sudden
increase of specific gravity produced ; followed,
of course, by a quick descent. When, by any
accident, the air-bladder has been opened, or
has burst, so that all the air has escaped, the
fish is seen to grovel at. the bottom, lying on its
back, and can never afterwards rise to the sur-
face. On the other hand, it occasionally hap-
pens that a fish which has remained too long at
the surface of the sea, exposed to the scorching-
rays of a tropical sun, suddenly finds itself re-
tained against its will at the surface, because
the bladder has become over-distended by the
heat, and resists all the efforts which the animal
can make to compress it. It thus continues
floating, until the coolness of the night has
again condensed the air in the bladder to its
SWIMMING BLADDER OF FISHES. 431
former bulk, and restored the power of de-
scending.
Some tribes of fish are totally unprovided
with an air-bladder. This is the case with the
flounder, the sole, and other genera of a flat
shape, forming the family of Pleuronectes. They
are chiefly inhabitants of sand-banks, or other
situations where they are comparatively sta-
tionary, seldom moving to a distance, or rising
much in the water ; and when they do so, it is
with manifest effort, for their ascent must be
accomplished entirely by the continued beating
and flapping of the water with their expanded
pectoral fins. It is only the larger fish of this
form, such as Rays, which have very voluminous
and powerful pectoral fins for striking the water
downwards with considerable force, that can rise
with facility, without the assistance of an air-
bladder. In these, the lateral fins, which are
enormous expansions of the pectoral fins, may
be compared to wings, their vertical action on
the water being similar in effect to the cor-
responding movements of a bird, when it rises
vertically in the air. Those fishes which swim
rapidly, and frequently ascend and descend in
the water, are in general provided with the
largest air-bladders.
In studying the varieties presented by the
forms of the fins in different tribes of fishes, we
find the same constant relation preserved with
432 THE MECHANICAL FUNCTIONS.
the particular situations and circumstances in
which they are placed. The dorsal fins, which
are more especially useful for steadying the
body, are longest in those fishes which inhabit
the most stormy seas. The most voracious
tribes, which incessantly pursue their prey, are
furnished with most powerful muscles, and
possess the greatest means of rapid progression.
On the other hand, many of the more pacific,
and weaker species are studiously guarded
by a dense and hard integument, serving as a
shield against the attacks of enemies, and often
armed with sharp points, which are sufficient to
repel the most daring assailant. The Balistes
is covered with scales of singular hardness,
closely set together, and frequently having
rough edges. The Ostracion, or Trunk-fish,
instead of these scales, is provided with a kind
of coat of mail, composed of osseous plates,
curiously joined together, like a tesselated pave-
ment, and reminding us of the arrangements we
have seen adopted in the calcareous coverings of
the Echinida.
Some of the cartilaginous fishes are, in like
manner, protected by calcareous plates, ap-
pended to the integuments. There is a row of
plates of this kind, of a quadrangular shape,
which pass along the middle of the back in
the Sturgeon : and the whole body of the Ostra-
cion is covered with osseous scales. All these
DIODONS AND TETRODONS. 433
have no immediate relation to the skeleton, but
are apparently remnants of inferior types, of
which one of the prevailing characters is the
external situation of the protecting organs.
Diodons and Tetrodons are remarkable for
being provided with the means of suddenly
assuming a globular form by swallowing air,
which, passing into the crop, or first stomach,
blows up the whole animal like a balloon. The
abdominal region being thus rendered the light-
est, the body turns over, the stomach becoming
the uppermost part ; and the fish floats upon its
back, without having the power of directing it-
self during this state of forced distension. But
it is while lying thus bloated and passive at the
mercy of the waves, that this animal is really
most secure ; for the numerous spines, with
which the surface of the body is universally be-
set, are raised and erected by the stretching out
of the skin, thus presenting an armed front to
the enemy, on whatever side he may venture to
begin the attack.
There is a numerous family of fishes, found
in the seas of India, so constructed as to be able
to crawl on land to some distance from the
shore. One of these, the Perca scandens, is even
capable of climbing on the trees which grow on
the coast.*
* See the account given by Lieutenant Daldorff in the Linneau
Transactions, III. 62. I shall have occasion to notice, in the
VOL. I. F F
434 THE MECHANICAL FUNCTIONS.
If we consider the density of the medium
which fishes have to traverse, the velocity with
which they move will appear surprising. They
dart through the water with apparently as much
ease and rapidity as a bird flies through the air.
Although this may partly be accounted for by
the size of their muscles, and the advantageous
mode of their insertion, yet these advantages
would avail but little, were it not for the sudden
manner in which their power is exerted. Where
the great length and flexibility of the spine tend
to impair the force with which the tail strikes
the water, the resulting motion is slow and de-
sultory, as is the case with eels, and other fishes
of the same elongated construction.* Most
fishes, however, move with the utmost rapidity,
and with scarcely any visible effort ; and per-
form long journeys without apparent fatigue.
The Salmon has been known to travel at the
rate of sixteen miles an hour for many days
together. Sharks often follow ships across the
Atlantic, not only outstripping them in their
swiftest sailing, but playing round them on every
side, just as if the vessels were at rest.
sequel, the remarkable conformation of the respiratory organs of
these and other fishes, which enables them to live for a time out
of their natural element.
* Carlisle, Phil. Trans, for 1806, p. 9.
43^
Chapter VIII.
REPTILIA.
§ 1 . Terrestrial Vertebrata in general.
The numerous tribes of vertebrated animals
which are strictly terrestrial, or destined to move
on land, differ widely in their modes of pro-
gression, and in the mechanical advantages of
their formation. The greater number are qua-
drupeds ; some formed for climbing trees ; others,
for burrowing in the earth ; some for treading
on sandy plains ; some for scaling precipices.
A few seem scarcely capable of advancing ;
others outstrip the winds in fleetness. Some
families of reptiles are entirely destitute of any
external organs of motion, the whole trunk of
the body resting on the ground ; while man oc-
cupies a place where he stands alone, being
distinguished by the exclusive faculty of per-
manently sustaining himself on the lower extre-
mities.
In reviewing the developements and the me-
chanical functions exhibited by so great a diver-
sity of structures, I shall commence with an ex-
amination of those amphibious reptiles which
436 THE MECHANICAL FUNCTIONS.
appear to form an intermediate link in the chain
connecting the strictly aquatic, with the terres-
trial vertebrated animals : then, taking up this
latter series, I shall consider the more simple
conformation, and less perfect motions of terres-
trial animals destitute of limbs ; and gradually
ascend to those in which the support and pro-
gression of the body is effected by extremities,
more and more artificially formed : concluding
with the human structure, which terminates this
extensive series.
§ 2. Batrachia.
The order of Batrachia, or Amphibious Rep-
tiles, constitutes the first step in the transition
from aquatic to terrestrial vertebrata. It is more
particularly the function of respiration that re-
quires to be modified in consequence of the
change of element in which the animal is to re-
side ; and as if it had been necessary, conformably
to the laws of animal creation, that this change
should not be abruptly made, we find that Ba-
trachian reptiles, with which this series com-
mences, are constructed at first on the model of
fishes ; breathing the atmospheric air contained
in the water by means of gills, and moving
through the fluid by the same instruments of
progression as fishes, which indeed they exactly
BATRACHIA.
437
resemble in every part of their mechanical con-
formation. The tadpole, which is the young of
the frog, is at first not distinguishable in any
circumstance of its internal skeleton, or in the
disposition of its vital organs, from the class of
fishes. The head, indeed, is enlarged ; but the
body immediately tapers to form a lengthened
tail, by the prolongation of the spinal column,
which presents a numerous series of coccygeal
vertebras, furnished with a vertical expansion of
membrane to serve as a caudal fin, and with
appropriate muscles for executing all the motions
required in swimming. The appearance of the
tadpole in its early stage of developement is seen
in Fig. 197 and 198, the former being a side,
and the latter an upper view of that animal.
Yet with all this apparent conformity to the
structure of a strictly aquatic animal, the tad-
pole contains within its organization the germs
of a higher developement. Preparations are
silently making for a change of habitation, for
the animal's emerging from the waters, for the
438 THE MECHANICAL FUNCTIONS.
reception of atmospheric air into new cavities,
for the acquisition of limbs suited to new modes
of progression ; in a word, for a terrestrial life,
and for all the attributes and powers which
belong to quadrupeds. The succession of forms,
which these metamorphoses present, are in them-
selves exceedingly curious, and bear a remark-
able analogy to the progress of the trans-
formations of those insects, which in the first
stages of their existence are aquatic. To the
philosophic inquirer into the marvellous plans of
creation, the series of changes which mark these
singular transitions cannot fail to be deeply in-
teresting ; and occurring, as we here find them,
among a tribe of animals allied to the more per-
fect forms of organization, they afford us a better
opportunity of exploring the secrets of their
developement by tracing them from the earlier
stages of this complicated process, so full of
mystery and of wonder.
The egg of the frog (Fig. 196) is a round
mass of transparent nutritive gelly, in the centre
of which appears a small black globule. By
degrees this shapeless globule exhibits the ap-
pearance of a head and tail ; and in this form it
emerges from its prison, and moves briskly in
the water. From the sides of the neck there
grow out feathery tufts (Fig. 198, b, b), which
float loosely, and without protection, in the sur-
rounding fluid. These, however, are mere tern-
DEVELOPEMENT OF THE BATRACHIA. 439
porary organs, for they serve the purposes of
respiration only until the proper gills are formed,
and they then shrink and become obliterated.
The true gills, or branchice, are contained within
the body, are four in number on each side, and
are constructed on a plan very similar to those of
fishes. Retaining this aquatic constitution, the
tadpole rapidly increases in size and in activity
for several weeks. In the mean time the legs,
of which no trace was at first apparent, have
commenced their growth. The hind legs are
the first to make their appearance, showing their
embryo forms within the transparent coverings
of the hinder part of the trunk, just at the origin
of the tail. These are soon succeeded by the
fore legs, which exactly follow the hind legs in
all the stages of their developement, until they
have acquired their due proportion to the size of
the trunk. The animal, at this period, wears a
very ambiguous appearance, partaking of the
forms both of the frog and of the lizard, and
swimming both by the inflections of the tail, and
the irregular impulses given by the feet. This
interval is also employed by this amphibious
being, in acquiring the faculty of respiring at-
mospheric air. We observe it rising every now
and then to the surface, and cultivating its
acquaintance with that element, into which it is
soon to be raised ; occasionally taking in a
mouthful of air, which is received into its newly
440 THE MECHANICAL FUNCTIONS.
developed lungs, and afterwards discharging it
in the form of a small bubble. When the neces-
sary internal changes are at length completed,
preparations are made for getting rid of the tail,
which is now a useless member, and which,
ceasing to be nourished, diminishes by degrees,
leaving only a short stump, which is soon re-
moved. The gills are by this time shrunk, and
rapidly disappear, their function being super-
seded by the lungs, which have been called into
play ; and the animal now emerges from the
water and begins a new mode of existence, having
become a perfect frog (Fig. 199). It still, how-
ever, retains its aquatic habits, and swims with
great ease in the water by means of its hind
feet, which are very long and muscular, and of
which the toes are furnished with a broad web de-
rived from a thin extension of the integuments.
No less curious are the changes which take
place in all the other organs, for the purpose of
effecting the transformations rendered necessary
by this entire alteration in all the external cir-
cumstances of that animal, — this total reversal
of its wants, of its habits, of its functions, and of
its very constitution. I shall have occasion to
notice several of these transitions when review-
ing the other functions of the animal economy :
but at present our concern is chiefly with the
structure of the frame in its mechanical relations
to progressive motion. In order to form a cor-
rect idea of these relations it will be necessary
.SKELETON OF THE BATRACHIA.
441
to notice the leading peculiarities of the skele-
tons of this tribe of animals.
The skeleton of the adult frog is shown in Fig.
200 ; from which it will be seen that the spinal
column is comparatively much shorter than that
of fishes, or indeed of any other class of animals ;
for it consists of only eight vertebrae, exclusive
of those which have united to form the os coc-
cygis. It was evidently the intention of nature
to consolidate the frame-work of the trunk, in
which flexibility was not required for progressive
motion ; the performance of that function being
transferred to the hind extremities, which are
exceedingly large in proportion to the rest of the
body. There is a tendency in every part of the
skeleton to develope itself in a transverse direc-
tion, while the trunk is shortened as much as
possible.
442 THE MECHANICAL FUNCTIONS.
The mode in which the vertebrae are articu-
lated together differs widely from what we have
seen in fishes, and approaches to the structure of
the higher classes of vertebrata. The body of
each vertebra, instead of having at its posterior
surface a cup-like cavity, terminates by a pro-
jecting ball, which is received into the cavity in
the anterior surface of the next vertebra, so as
to compose a true ball and socket joint, capable,
when other circumstances permit, of a rotatory
motion. But the vertebrae of the tadpole, as we
have seen, are constructed on the model of those
of a fish ; that is, have cup-like cavities on both
their surfaces, which play on balls of soft elastic
matter interposed between them. We should
naturally be curious to learn the mode in which
the transition from this structure to that of the
frog is accomplished. By carefully watching
the progress of ossification, while this change is
taking place, Dutrochet found that the gelatinous
ball, on which both the adjacent vertebras play
in the tadpole, becomes gradually more solid,
and is converted into cartilage. This cartilage
afterwards becomes united by its anterior surface
to the vertebra which is in front of it ; and the
whole then becomes ossified, so as to compose
only one bone, its posterior surface remaining
distinct, and continuing to play within the cup-
like hollow of the vertebra which is behind it.
The cartilaginous coccygeal vertebrae of the tad-
SKELETON OF THE BATRACHIA. 443
pole are lost long before there is time for their
being ossified ; but those nearest to the body are
consolidated into one long and straight os coccy-
gis, which being joined to the sacrum at an
angle, gives rise to the strange deformity observ-
able at that part of the back of a frog ; for it
here looks as if it had been broken. The spinal
cavity is at the same time obliterated, that por-
tion of the spinal marrow which had passed
through it in the aquatic life of the animal being
now withdrawn.
The theory of the spinal origin of the cranial
bones receives considerable support from their
structure and relative position in the skeleton
of the frog. The cavity for the lodgement of
the brain, which is enclosed by these vertebras,
is perfectly continuous in the same line with
the spinal canal, which, indeed, it scarcely
exceeds in its diameter. The bones of the face
are, at the same time, expanded laterally, so
as to bear no proportion to the cranial cavity.
The head plays on the vertebral column by two
lateral articular surfaces, formed upon the root
of each leaf of the occipital bone ; while its
body, or basilar portion, is scarcely connected
with the first cervical vertebra, and has no
articular surface.
In place of ribs, we find only small slender
detached bones, or rather cartilages, affixed to
the extremities of the transverse processes of
444 THE MECHANICAL FUNCTIONS.
some of the vertebrae : they may be regarded as
rudimental ribs.*
The pelvis consists of two slender and elon-
gated iliac bones, which are extended backwards,
and which, at their anterior extremities, merely
touch the points of the transverse processes of
the last vertebra of the back. This vertebra is
much broader than the rest, and although it con-
sists but of a single bone, must be considered
as a sacrum. The two pubic and ischiatic bones
are exceedingly small, but still contribute to
form the acetabulum, or cavity for the reception
of the thigh bone, at the hinder extremity of the
slender bones above mentioned. This is the
simplest possible form to which the pelvis can be
reduced, while it preserves its attachments to the
spine. It presents in this respect a more ad-
vanced stage of developement than that of fishes.
The connexion of the bones of the anterior
extremities with the spine is analogous to that
which takes place in Rays and Sharks : there
being an osseous belt formed by the scapula,
clavicle, and coracoid bone, with the latter of
which the humerus is connected. The sternum
* The plan of reproduction in these animals requires that the
ovary, or organ which contains the eggs, should be capable of
enormous dilatation, in order to contain the immense bulk to
which these eggs are expanded, previously to their being brought
forth. It was probably in order to make room for this dilated
ovary that the ribs have not been developed.
PROGRESSIVE MOTION IN BATRACHIA. 445
is large, and considerably developed ; making
some slight approach to the expansion it receives
in the Chelonia. The radius and ulna are united
into one bone : the bones of the arm and leg in
general resemble in their figure and connexions
those of the higher orders of Mammalia, to the
type of which this order of reptiles evidently
approximates. There are five toes in the foot,
with sometimes the rudiment of a sixth : the
anterior extremity has only four toes, which are
without claws.
The necessity of employing the same instru-
ments for progression in the water and on land,
is probably the cause which prevents their having
the form best adapted for either function. The
hind feet of the frog, being well constructed for
striking the water backwards in swimming, are,
in consequence, less capable of exerting a force
sufficient to raise and support the weight of the
body in walking ; and hence this animal is
exceedingly awkward in its attempt to walk.
On a short level plane it can proceed only by
leaps ; an action which the length and great
muscularity of the hind legs particularly fit it
for performing. The toad, on the other hand,
whose hind legs are short and feeble, walks
better, but does not jump or swim so well as the
frog.* The Hyla, or tree-frog, has the extremities
* It is singular that the frog, though so low in the scale of
vertebrated animals, should bear a striking resemblance to the
446* THE MECHANICAL FUNCTIONS.
of each of its toes expanded into a fleshy tubercle,
approaching in the form of its concave surface
to that of a sucker, and by the aid of which it
fastens itself readily to the branches of trees,
which it chiefly inhabits, and along which it runs
with great agility.
The Salamander is an animal of the same class
as the frog, undergoing the same metamorphoses
from the tadpole state. It differs much, however,
in respect to the developement of particular parts
of the skeleton. The anterior extremities of the
salamander make their appearance earlier than
the hind legs, and the tail remains as a perma-
nent part of the structure. The rudimental ribs
are exceedingly small, and the sternum continues
cartilaginous. The pelvis has no osseous con-
nexion with the spine, but is merely suspended
to it by ligaments. The land salamanders have
a rounded tail, but the aquatic species, or Tritons,
have it compressed vertically ; thus retaining
the fish-like form of the tadpole, and the same
radiated disposition of the muscles.
human conformation in its organs of progressive motion. This
arises from the exertions which it makes in swimming being
similar to those of man in walking, in as far as they both result
from the strong action of the extensors of the feet. Hence we
find a distinct calf in the legs of both, produced by the swelling
of similar muscles. The muscles of the thigh present, also, many
analogies with those of man ; particularly in the presence of the
long muscle called the sartorius, the use of which is to turn the
foot outwards, both in stepping and in swimming.
J
SERPENTS. 447
§ 3. Ophidia.
In the class of serpents we see exemplified the
greatest possible state of simplicity to which a
vertebrated skeleton can be reduced ; for, as may
be seen in Fig. 201, which shows the skeleton
of a viper, it consists merely of a lengthened
spinal column, with a head but little developed,
and a series of ribs ; but apparently destitute of
limbs, and of the bones which usually connect
those limbs with the trunk ; there being neither
sternum, nor scapula, nor pelvis. Professor
Mayer has, however, traced obscure rudiments
of pelvic bones in the Anguis frag His, the Anguis
ventralis, and the Typhlops crocotatns, and is of
opinion that they exist much more generally in
this order of reptiles, than has been commonly
imagined. Some serpents, as the Boa, Python,
Tortryx and Eryx, have claws, which may be
considered as rudiments of feet, visible exter-
nally. In others, as the Anguis, Typhlops, and
Amphisbvena, they exist concealed under the
448
THE MECHANICAL FUNCTIONS.
skin. In others, he has discovered cartilaginous
filaments, which he conceives to correspond to
these parts.*
In the conformation of the skull and bones of
the face, Serpents present strong analogies with
batrachian reptiles, and also with fishes, one tribe
of which, namely, the apodous or anguiliform
fishes, they greatly resemble by the length and
flexibility of the spine. These peculiarities of
conformation may be traced in a great measure
to the mode of life for which they are destined.
* Some of these rudimental parts are represented in the fol-
lowing figures. Fig. 203 exhibits the claw of the Boa constrictor,
203
209
placed at the termination of a series of bones, representing very
imperfectly the bones of the lower extremities. Fig. 204 shows
the muscles attached to these small bones. The three following
figures, 205, 206, and 207, represent the claws and rudimental
bones of the Tortrix scytale, Tortrix corallinus, and Anguis
fragilis, respectively. Those of the Amphisbcena alba, Fig. 208,
and the Coluber pullatus, Fig. 209, are still less developed.
The Chalcides, or snake lizard, which has four minute feet, is
represented in Fig. 210. (Ann. des Sc. Nat. vii. 170.)
SERPENTS. 449
The food assigned to them is living prey, which
they must attack and vanquish before they can
convert it into nourishment. The usual mode
in which the Boa seizes and destroys its victims
is by coiling the hinder part of its body round
the trunk or branch of a tree, keeping the head
and anterior half of the body disengaged ; and
then, by a sudden spring, fastening upon the de-
fenceless object of its attack, and twining round
its body, so as to compress its chest, and put a
stop to its respiration. Venomous serpents, on
the other hand, coil themselves into the smallest
possible space, and suddenly darting upon the
unsuspecting or fascinated straggler, inflict the
quickly fatal wound.
It is evident, from these considerations, that,
in the absence of all external instruments of
prehension and of progressive motion, it is neces-
sary that the spine should be rendered extremely
flexible, so as to adapt itself to a great variety of
movements. This extraordinary flexibility is
given, first, by the subdivision of the spinal
column into a great number of small pieces ; se-
condly, by the great freedom of their articu-
lations ; and thirdly, by the peculiar mobility
and connexions of the ribs.
Numerous as are the vertebrae of the Eel, the
spine of which consists of above a hundred pieces,
that of serpents is in general formed of a still
greater number. In the rattle-snake (Crotalus
VOL. I. GO
4">o
THE MECHANICAL FUNCTIONS.
horridus) there are about two hundred vertebrae ;
and above three hundred have been counted in
the spine of the Coluber natrix. These vertebra;
are all united by ball and
socket joints, as in the
adult batrachia ; the pos-
terior rounded eminence
of each vertebra being re-
ceived into the anterior
surface of the next. Fig.
202 is a view of this por-
tion of the skeleton in the
Boa constrictor, showing
also the articulation of the
ribs with the vertebrae.
While provision has thus been made for extent
of motion, extraordinary care has at the same
time been bestowed upon the security of the
joints. Thus we find them effectually protected
from dislocation by the locking in, above and
below, of the articular processes, and by the
close investment of the capsular ligaments. The
direction of the surfaces of these processes, and
the shape and length of the spinous processes,
are such as to allow of free lateral flexion, but to
limit the vertical and longitudinal motions ; and
whatever degree of freedom of motion may exist
between the adjoining vertebrae, that motion
being multiplied along the column, the flexibility
of the whole becomes very great, and admits of
SERPENTS. 451
its assuming every degree and variety of curva-
ture. The presence of a sternum, restraining
the motions of the ribs, would have impeded all
these movements, and would have also been an
insurmountable bar to the dilatation of the sto-
mach, which is rendered necessary by the habit
of the serpent of gorging its prey entire.
The mode in which the Boa exerts a powerful
pressure on the bodies of the animals it has
seized, and which it has encircled within its
folds, required the ribs to be moveable laterally,
as well as backwards, in order to yield to the force
thus exerted. The broad convex surfaces on
which they play give them, in this respect, an
advantage, which the ordinary mode of articula-
tion would not have afforded. The spinous pro-
cesses, in this tribe of serpents, are short and
widely separated, so as to allow of flexion in
every direction. In the Rattle-snake, on the
other hand, their length and oblique position are
such as to limit the upward bending of the spinal
column, although, in other respects, its motion is
not restricted. The vertebrae at the end of the tail
are furnished with broad transverse processes for
the attachment of the first joints of the rattle.
But of whatever variety of flexions we may
suppose the lengthened body of a serpent to be
capable, it will, at first view, be difficult to con-
ceive how these simple actions can be rendered
subservient to the purposes of progression on
452 THE MECHANICAL FUNCTIONS.
land ; and yet experience teaches us that few
animals advance with more celerity on the sur-
face of the ground, or dart upon their prey with
greater promptitude and precision. They raise
themselves without difficulty to the tops of the
highest trees, and escape to their hiding places
with a quickness which eludes observation, and
baffles the efforts of their pursuers.
The solution of this enigma is to be sought for
partly in the structure of the skin, which, in
almost every species, is covered with numerous
scales ; and partly in the peculiar conformation
of the ribs. The edges of the scales form rough
projections, which are directed backwards, so as
to catch the surfaces of the bodies to which they
are applied, and to prevent any retrograde mo-
tion. In some species, the integument is formed
into annular plates, reminding us of the struc-
tures so prevalent among worms and myriapode
animals. Each scale is connected with a parti-
cular set of muscular fibres, capable of raising
or depressing it, so that in this way it is con-
verted into a kind of toe ; and thus the body
rests upon the ground by numerous fixed points
of support.
This support is farther strengthened by the
connexion of the ribs with the abdominal scuta,
or the scales on the under side of the body. The
mode in which the ribs become auxiliary instru-
ments of progressive motion was first noticed by
PROGRESSIVE MOTION IN SERPENTS. 453
Sir Joseph Banks.* Whilst he was watching
the movements of a Coluber of unusual size,
which was exhibited in London, and was
moving briskly along the carpet, he thought he
saw the ribs come forward in succession, like
the feet of a caterpillar. Sir Everard Home, to
whom Sir Joseph Banks pointed out this cir-
cumstance, verified the fact by applying his hand
below the serpent, and he then distinctly felt
the ends of the ribs moving upon the palm, as
the animal passed over it. The mode in which
the ribs are articulated with the spine is pecu-
liar, and has evidently been employed with re-
ference to this particular function of the ribs,
which here stand in place of the anterior and
posterior extremities, possessed by most verte-
brated animals, and characterising the type of
their osseous fabric. In the ordinary structure,
the head of each rib has a convex surface, which
plays either on the body of a single vertebra
with which it is connected, or upon the two
bodies of adjacent vertebrae ; but in serpents the
extremity of the head of the rib has two slightly
concave articular surfaces, which play on a
convex protuberance of the vertebra. This
structure is attended with the advantage of
preventing the ribs from interfering with the
motions of the vertebrae upon one another. At
* Philos. Trans, for 1812, p. 163.
4-54 THE MECHANICAL FUNCTIONS.
their lower ends, the ribs of one side have no
connexion with those of the other ; nor are they
joined to any bone analogous to a sternum ;
for, except in the Ophiosaurus and the Blind-
worm (Anguis fragilis), there is no vestige
either of a sternum or scapula, in any animal
of this class. Each rib terminates in a slender
cartilage, tapering to a point, which rests, for its
whole length, upon the upper surface of one
of the scuta, or broad scales on the lower side of
the body. These scuta, which are thus con-
nected with the ends of the ribs, and which are
moved by means of short muscles, may be com-
pared to hoofs, while the ribs themselves may
be considered as performing the office of legs.
The ribs move in pairs ; and the scutum under
each pair, being carried along with it in all its
motions, and laying hold of the ground by its
projecting edge, becomes a fixed point for the
advance of the body. This motion, Sir E. Home
observes, is beautifully seen when a snake is
climbing over an angle to get upon a flat surface.
When the animal is moving on a plane, it alters
its shape from a circular or oval form, to one
that approaches to a triangle, of which the sur-
face applied to the ground forms the base. Five
sets of muscles are provided for the purpose of
giving to the ribs the motions backwards and
forwards, by which, as levers, they effect this
species of progression. These muscles are dis-
PROGRESSIVE MOTION IN SERPENTS. 455
posed in regular layers ; some passing over one
or two ribs to be attached to the succeeding rib.
In all snakes the ribs are continued backwards,
much beyond the region occupied by the lungs ;
and although the anterior set are subservient
to respiration, as well as to progressive motion,
it is evident that all those posterior to the lungs
must be employed solely for the latter of these
purposes.
It is easy to understand how the serpent can
slowly advance, by this creeping, or vermicular
motion, consisting in reality of a succession of
very short steps. But its progress is accelerated
by the curvatures into which it throws its body ;
the fore part being fixed, and the hind part
brought near to it ; then, by a reverse process,
the hind part being fixed, and the head pro-
jected forwards. By an alternation of these
movements, assisted by the actions of the ribs,
serpents are enabled to glide onwards with con-
siderable rapidity, and without attracting obser-
vation. But where greater expedition is neces-
sary, they employ a more hurried kind of pace,
although one which exposes them more to im-
mediate view. The body, instead of being bent
from side to side, is raised in one great arch,
of which the two extremities alone touch the
ground ; and these being alternately employed
as points of support, are made successively to
approach and to separate from each other ; the
4oG THE MECHANICAL FUNCTIONS
body being propelled by bringing it from a
curved to a straight line.
There is yet a third kind of motion, which
serpents occasionally resort to when springing
upon their prey, or when desirous of making a
sudden escape from danger. They coil them-
selves into a spiral, by contracting all the
muscles on one side of the body, and then,
suddenly throwing into violent action all the
muscles on the opposite side, the whole body is
propelled, as if by the release and unwinding of
a powerful spring, with an impulse which raises
it to some height from the ground, and projects
it to a considerable distance.
Thus these animals, to which Nature has
denied all external members, are yet capable,
by the substitution of a different kind of me-
chanism, still constructed from the elements
belonging to the primitive type of vertebrated
animals, of silently gliding along the surface of
the earth, of creeping up trees, of striding ra-
pidly across the plain, and of executing leaps
with a vigour and agility which astonish the
beholder, and which, in ages of ignorance and
superstition, were easily ascribed to supernatural
agency.
SAURIAN REPTILES. 457
§ 4. Sauria.
The conformation of those parts of the frame
which are subservient to progressive motion
becomes more perfect in the class of Saurian
reptiles, which includes all the Lizard tribes.
Several links of connexion with the preceding
class may still be noticed, marking the progress
of developement, as we follow the ascending-
series of animals. Rudiments of the bones of
the extremities, and also of the sternum, make
their appearance very visibly in the Ophiosaurus,
and in the blind worm (Anguis fragilis). The
Siren lacertina has two diminutive fore feet,
placed close to the head. The Lacerta lumbri-
coides of Linneus, or the Bipes canaliculatus of
Lacepede, which is found in Mexico, and of
which a specimen is preserved in the collection
at Paris, has a pair of very short feet, also placed
near the head, and divided into four toes, with
the rudiment of a fifth. The Lacerta bipes
(Linn.), or Sheltopusic of Pallas, has, on the
other hand, a pair of hind feet only, but ex-
tremely small, together with rudiments of a
scapula and clavicle, concealed under the skin.
Next in order must be placed the Chalcides,
or Snake-lizard (Fig. 210), and the Lacerta
458 THE MECHANICAL FUNCTIONS.
seps, animals frequently met with in the South
of France, and which have four minute feet,
totally inefficient for the support of the body*
and only remotely useful in contributing to its
progressive undulations.
Ascending from these, we may form a series of
reptiles, in which the developement of the limbs
becomes more and more extended, till we arrive
at Crocodiles, in which they attain a consider-
able degree of perfection. As a consequence of
this greater developement of the skeleton, we
find the trunk divisible into separate regions.
We now, for the first time, meet with a distinct
neck, separating the head from the thorax, which
is itself distinguishable from the abdomen; and a
distinct sacrum is interposed between the lumbar
and the caudal vertebrae.
A further approach to the higher classes is
observable in the number of cervical vertebra?,
which is almost constantly seven ; as we shall
find it to be in the Mammalia. The articula-
tions of the vertebrae are similar to those of
serpents, inasmuch as they consist of ball and
socket joints. In that of the occipital bone with
the first vertebra of the neck, we find that
nature again reverts to the simpler form of a
single condyle, projecting from the body of the
occipital bone, instead of lateral condyles pro-
ceeding from its leaves, as we noticed was the
SAURIAN REPTILES. 459
structure in the Batrachia. The caudal verte-
bras are always numerous, and the tail is com-
pressed vertically, which is the form most
favourable for progression in water. They are
remarkable also for having inferior spinous pro-
cesses attached to the bodies by cartilages ; a
structure analogous to that which we have seen
in fishes.
The number of ribs differs in different species
of Sauria : they are always articulated to the ex-
tremities of the transverse processes of the ver-
tebras, of which they appear to be continuations.
Processes of this description also occur in the
neck, attached to the transverse processes of the
cervical vertebrae ; and these have been regarded
as cervical ribs. Their presence are impedi-
ments to the flexions of the neck ; whence
arises the difficulty which the Crocodile appears
to have in bending the neck while turning round
upon the animal he is pursuing. In the thorax,
the ribs are connected with a broad sternum ;
but there are other ribs, both before and behind,
which have no such termination, and therefore
bear the name of false ribs.
The pelvis consists chiefly of the iliac bones,
which, as in the Batrachia, pass backwards to
form the articular cavity for the thigh bone.
Two small and slender bones extend forwards
from the pubic bones, on the under side of the
460 THE MECHANICAL FUNCTIONS.
body, apparently for the purpose of supporting
the abdominal viscera.* The bones of the ex-
tremities are very perfectly formed, approaching
in their shape and arrangement very nearly to
the corresponding parts of the skeleton of the
higher orders of quadrupeds. The toes are
usually provided with membranes spread be-
tween them, to assist in swimming. The form
of the tail, which is generally compressed ver-
tically, like that of fishes, though perhaps not to
an equal degree, is another indication of their
being formed for an aquatic life ; for where the
tail has this shape, we always find that the chief
muscular power is bestowed upon it as an in-
strument of aquatic progression, producing, by
its lateral flexions, a horizontal movement of the
body. Crocodiles and Alligators, for instance,
which have this conformation, are comparatively
weak when on land ; and as soon as they have
seized their prey, their efforts are always directed
to drag it into the water ; knowing that when
they are in their own element, they can readily
master its struggles, and destroy it by drowning.
In the Gecko tribe, we find a particular me-
chanism provided for effecting the adhesion of
the feet to the objects to which they are applied.
* They appear to be analogous to the marsupial bones peculiar
to a family of Mammalia.
FEET OF THE GECKO.
401
It is somewhat analogous to that employed in
the case of the house-fly, already mentioned.
Each foot has five toes ; all, except the thumb,
terminated by a sharp curved claw. On the
under surface of each toe, (represented in Fig.
211), there are as many as sixteen transverse
slits, leading to the same number of cavities, or
sacs : these open for-
wards, and their exter-
nal edge is serrated,
appearing like the teeth
of a small -toothed comb.
A section of the foot,
showing these cavities,
is seen in Fig. 212. All
these parts, together
with the cavities, are
covered or lined with
cuticle. Below them
are large muscles, which draw down the claw ;
and from the tendons of these muscles arise two
sets of smaller muscles, situated so as to be put
upon the stretch, when the former are in action.
By the contractions of these muscles, the orifices
of the cavities, or sacs, to which they belong,
are opened, and the serrated edges applied ac-
curately to the surfaces with which the feet are
in contact. Sir Everard Home, in his account
of this structure, compares it to the sucking disk
462 THE MECHANICAL FUNCTIONS.
of the Remora* By its means the animal is
enabled to walk securely upon the smoothest
surfaces, even in opposition to the tendency of
gravity. It can run very quickly along the
walls or ceiling of a building, in situations where
it cannot be supported by the feet, but must
depend altogether upon the suspension derived
from a succession of rapid and momentary adhe-
sions.
Although the Sauria are better formed for
progressive motion than any of the other orders
of reptiles, yet the greater shortness and oblique
position of their limbs, compared with those of
mammiferous quadrupeds, obliges them in gene-
ral to rest the weight of the trunk of the body
on the ground, when they are not actually
moving. None of these reptiles have any other
kind of pace than that of walking, or jumping ;
being incapable of performing either a trot or a
gallop, in consequence of the obliquity of the
plane in which their limbs move. The Chame-
lion walks with great slowness, and apparent
difficulty ; and we have seen that, in conse-
quence of the structure of the bones of its
neck, the Crocodile, though capable of swift mo-
tion in a straight line, is unable to turn itself
round quickly. The general type of these rep-
tiles, having reference to an amphibious life, has
* Philosophical Transactions for 1816, p. 151, and 323.
CHELOMAN REPTILES. 463
not attained that exclusive adaptation to a ter-
restrial existence, which we find in the higher
orders of the Mammalia. But before proceed-
ing to consider these, we have to notice a sin-
gular group of animals, whose conformation
appears to be exceedingly anomalous, and as if
it interrupted the regularity of the ascending
series, of which it seems to be a collateral rami-
fication.
§ 5. Chelonia.
The order of Chelonian Reptiles, which com-
prises all the tribes of Tortoises and Turtles,
appears to constitute an exception to the general
laws of conformation, which prevail among Ver-
tebrated Animals : for instead of presenting a
skeleton wholly internal, the trunk of the body
is found to be enclosed on every side in a bony
case, which leaves openings only for the head,
the tail, and the fore and hind extremities.
That portion of this osseous expansion which
covers the back is termed the carapace; and
the flat plate which defends the lower part of
the body is termed the plastron. It is a form of
structure which reminds us of the defence pro-
vided for animals very low in the scale of or-
ganization, such as the Echinus, the Crustacea,
and the bivalve Mollusca. Yet the substance
464 THE MECHANICAL FUNCTIONS.
which forms these strong bucklers, both above
and below, is a real osseous structure, developed
in the same manner as other bones, subject to
all the changes, and having all the properties
of these structures. The great purpose which
nature seems to have had in view in the forma-
tion of the Chelonia is security ; and for the
attainment of this object she has constructed a
vaulted and impenetrable roof, capable of resist-
ing enormous pressures from without, and proof
against any ordinary measures of assault. It is
to the animal a strong castle, into which he can
retire on the least alarm, and defy the efforts of
his enemies to dislodge or annoy him.
These considerations supply us with a key to
many of those apparent anomalies, which cannot
fail to strike us in viewing the dispositions of
the parts of the skeleton (Fig. 213), and the re-
markable inversion they appear to have under-
gone, when compared with the usual arrange-
ment. We find, however, on a more attentive
examination, that all the bones composing the
skeleton in other vertebrated animals exist also
in the tortoise ; and that the bony case which
envelopes all the other parts is really formed by
an extension of the spinous processes of the ver-
tebrae and ribs on the one side, and of the usual
pieces which compose the sternum on the other.
The upper and lower plates thus formed are
united at their edges by expansions of the sterno-
CHELONIAN REPTILES.
465
costal appendices, which become ossified. Tims
no new element has been created ; but advantage
has been taken of those already existing in the
general type of the vertebrata, to modify their
forms, by giving them different degrees of relative
developement, and converting them, by these
transformations, into a mechanism of a very dif-
ferent kind, and subservient to other objects
than those to which they are usually applied.
It is scarcely possible to have stronger proofs, if
such were wanting, of the unity of plan which
has regulated the formation of all animal struc-
VOL. I. H H
406 THE MECHANICAL FUNCTIONS.
tures, than those afforded by the skeleton of the
tortoise.
The first step taken to secure the relative
immobility of the trunk, is to unite in one rigid
bony column all its vertebrae, and to allow of
motion only in those of the neck, and of the tail.
The former, accordingly, are all anchylosed to-
gether, leaving, indeed, traces of their original
forms as separate vertebrae, but exhibiting no
sutures at the place of junction. The canal for
the spinal marrow is preserved, as usual, above
the bodies of these coalesced vertebrae, and is
formed by their united leaves ; the arches being
completed by the spinous processes. But these
processes do not terminate in a crest as usual ;
they are farther expanded in a lateral direction,
forming flat pieces along the back, which are
united to one another by sutures, and which are
also joined to the expanded ribs, so as to form the
continuous plane surface of the carapace. The
transverse processes of the vertebrae are well
marked ; but, though firmly united to the ribs,
do not give rise to them ; for the ribs, which are
flattened and expanded, so as to touch one ano-
ther along their whole length, are inserted below,
between the bodies of every two adjoining ver-
tebrae ; while above, they are united by suture
with the plates of the spinous processes. This
change in the situation of the ribs is the con-
sequence of the change in their office. When
CHELONIAN REPTILES. 4(17
designed to be very moveable, we find them
attached either to the extremities of the transverse
processes, or to the articular surfaces of a single
vertebra ; but where solidity and security are
to be provided, they are always inserted be-
tween the bodies of two vertebra?. This we shall
find to be the case also in birds, where the bones
of the thorax are required to be immoveable.
It is remarkable, indeed, that a great number of
the peculiarities which distinguish the conforma-
tion of the chelonia from that of other reptiles,
indicate an approach to the structure of birds ;
as if nature had intended this small group of
animals to be an intermediate link of gradation
to that new and important type of animals
destined for a very different mode of existence.
The sterno-costal appendages, which connect
the ribs to the sternum, are, in most animals,
cartilaginous ; though occasionally we find them
partially ossified. In the tortoise, however, their
ossification is not only complete, but has been
expanded laterally, so as to form a continuous
surface with the extremities of the ribs and with
the edges of the plastron, and completely to fill
up the vacancy between them ; constituting a
dense and solid wall, which entirely closes the
sides of the general bony case. So strong is the
tendency to ossification in all these pieces, that
the sutures at first formed between them are
often, in process of time, obliterated ; and the
468 THE MECHANICAL FUNCTIONS.
bony fibres are continuous throughout a great
extent of surface.
The most remarkable metamorphosis in the
osseous system of this new type is that which
occurs in the sternum. So expanded are all its
parts, that it is difficult to recognise this bone
under the disguised form in which it constitutes
the plastron, or broad plate, which, as we have
seen, covers the whole of the under side of the
body. Yet, by a careful examination of its
structure, both in the young animal, and also in
the adult, when the sutures are not obliterated,
we may easily recognise the nine elements of
the sternum ; namely, the one in the middle and
fore part, and the four pair of lateral pieces ;
each having been formed from its respective
centre of ossification. In form and relative
proportion, indeed, they are widely different
from the same parts as they are presented in the
skeletons of other animals ; yet in number and
in relative situations they preserve that con-
stancy and uniformity so characteristic of the
beautiful harmony which pervades all animal
structures.
It is to be noticed, also, that as the plates,
which form this investing case, are bony struc-
tures, they could not with any safety have been
exposed to the action of the atmosphere. Hence
we find them covered throughout with a thin
horny plate, originally a production of the inte-
CHELONIAN REPTILES. 469
gument. It is this substance which is commonly
known by the name of tortoise shell*
The immobility of the trunk is compensated,
as far as regards the safety of the head, by the
great flexibility of the neck ; which is composed
of seven vertebrae, unencumbered by processes,
and capable of taking a double curvature like
the letter S, when the head is to be retracted
within the carapace. These vertebra? are joined
by the ball and socket articulation common to
all the existing species of reptiles. t The articu-
lation of the head with the neck is effected in
the same manner; but it is interesting to Remark
that the occipital condyle, which is situated at
the lower margin of the great aperture, though
presenting a single convex surface, yet has that
215 £-— — !kt-^ surface evidently divided
into three parts ; the two
upper portions being late-
ral, and the lower portion
in the middle. These three
articular surfaces are seen
immediately below the central aperture, f, in Fig.
* It should be observed, that the divisions of these plates,
which appear externally, bear no relation to the sutures which
separate the subjacent bones, so that it is not possible to draw
inferences respecting the form of the latter from the mere inspec-
tion of the external shell.
f The expression of this fact is thus qualified, because it does
not apply to many fossil or extinct species, such as the Ichthyo-
saurus.
470 THE MECHANICAL FUNCTIONS.
2 1 5, which exhibits the skull of the Testudo my das,
viewed from behind. Although closely approxi-
mated, a faint line of demarcation, which divides
their surface, indicates an incipient tendency to
separate. We shall find that in the further steps
of developement which occur in the higher classes,
this separation actually takes place by the obli-
teration of the lower articular surface, and the
transfer of the two lateral surfaces to the con-
dyloid processes, arising from the developement
of the leaves of the occipital bone.
The singular conformation of the bones of the
head in, the turtle affords fresh evidence in sup-
port of the theory that these bones were origi-
nally vertebrae. The brain of this animal is
exceedingly small ; and yet the skull, when
viewed from above, presents an appearance of
great breadth, as if it enclosed a cavity of large
dimensions. But if we look upon it from be-
hind, as is shown in Fig. 215, we soon discover
that the real cavity in which the brain is lodged,
and to which the aperture at f leads, is very
small, only just admitting the end of the finger,
and that the broad plates of bone, p, p, which
form the upper surface of the skull, have no
relation to this cavity, and are merely extended
over the temporal muscles, which are of very
large size, occupying the whole of the spaces,
s, s ; which spaces are completely surrounded
by these bones. It would appear that the same
CHELONIAN REPTILES. 471
tendency to lateral expansion, which exists in
the spinous processes of the dorsal vertebrae,
prevails also among those which contribute to
form the skull. The parietal bones, which re-
present the spinous processes of the second
cranial vertebra, after having performed their
primary office of protecting the hemispheres of
the brain by closing over them, still proceed in
their developement, forming first a crest on the
upper part of the real cranium, and then sepa-
rating to the right and left, and expanding hori-
zontally into the upper roof (p, p) already men-
tioned, for the protection of the temporal muscles.
This great breadth of the head in the turtle gives
the animal an aspect of superior intelligence, to
which character, from the really diminutive size
of its brain, it is in no respect entitled. As the
turtle is unable to withdraw its head within the
carapace, such extraordinary protection appears
to have been necessary ; for it is not met with
in the tortoise, which has a carapace sufficiently
capacious to give shelter to the head whenever
occasion may require. The analogy of the spine
of the occipital bone with that of a vertebra is
farther shown by this bone extending backwards
to a considerable length, exactly in the manner
of the spinous processes of the cervical vertebrae
in other animals.
This arrangement of #the expanded spinous
processes and ribs in the Chelonia gives rise to a
472 THE MECHANICAL FUNCTIONS.
singular inversion in the position of the scapula;
for it is here placed on the inside of the ribs and
sternum ; that is, between the carapace and
plastron.* The humerus is remarkably curved,
especially in the tortoise, where it has the form
nearly of a semi-circle. The radius and ulna
are distinct from each other : the carpus and
phalanges are short and stunted, forming a com-
pressed kind of hand.
The pelvis, like the scapula and clavicle, is
enclosed within the bony shell which protects
the trunk. The sacrum is moveable upon the
last dorsal vertebra ; and the coccygeal vertebras
are continued from it, forming a short tail. The
femur is short and powerful, and somewhat bent,
but less so than the humerus ; and the rest of
the bones of the hind extremity are similar to
those of the fore leg.t All the feet are joined
obliquely to the limbs which support them,
giving the animal an apparent awkwardness
of gait, as if it were obliged to walk upon club
* The anomalous situation of these bones, and the strangely
disguised forms which their several parts assume, render it very
difficult to recognise in the skeleton the several pieces which
correspond to the normal type of the scapula, acromion, cora-
coid bone, and clavicle ; and anatomists are not yet agreed as
to the proper designations which are applicable to these bones
in the Chelonia.
f The cylindrical bones of the tortoise are solid throughout,
and have no cavity for containing marrow, as in the more highly
developed bones of the mammalia. This is seen in the section
of the femur, Fig. 214.
CHELONIAN REPTILES. 473
feet. The impulse which they give, being lateral
and oblique, renders them more efficacious for
progression in the water than on land : this cir-
cumstance, in conjunction with the constitutional
torpor of the animal, sufficiently accounts for the
excessive, and indeed proverbial tardiness of its
movements.
Security appears still to be the object aimed
at in the mechanism of all the other parts of the
skeleton. The articulations at the shoulders and
the hips are such as facilitate the complete
retraction of the limbs within the carapace.
After the head has been drawn in by the double,
or serpentine flexion of the neck, the knees are
brought together, and the whole limb withdrawn
within the shell, the fore legs folding completely
over the head, so as to cover and protect it most
effectually. For this purpose, the carpus and
metacarpus are exceedingly flattened, and ap-
proximate to the fin-like form, which we shall
presently see exemplified in the cetaceous tribes.
The phalanges are also large and lengthened,
forming a kind of oval hand, or rather paddle,
the functions of which it is well calculated to
perform. The curvature of the humerus is of
great advantage to the tortoise in assisting it to
turn itself, when, by any accident, it has been
laid on its back.
Considerable differences may be noticed in
the structure of the several species of Chelonia,
474 THE MECHANICAL FUNCTIONS.
according to the diversity of their habits. Tor-
toises which live on land, require more complete
protection by means of their shell than turtles,
or Emydes, which dwell in the water ; hence
the convexity of their carapace, the solidity of
its ossification, its immoveable connexion with
the plastron, and the complete shelter it affords
to the head and limbs. Turtles, on the other
hand, receiving support from the element in
which they reside, require less provision to be
made for these objects. Their carapace is
smaller, has a more flattened form, and cannot
afford protection to the head and limbs. These
latter organs are proportionally larger, present
a greater developement of the radius and ulna,
and are compressed into a flat expanded sur-
face. Previously to the retraction of the head
and limbs within the shell, the air is expelled
from the large cavities of the lungs, by the
vigorous actions of the abdominal muscles, which
exist in these animals as well as in all the verte-
brata, although here they are covered by the
bones, and compress the lungs by pushing the
abdominal viscera against them. This sudden
expulsion of air is the cause of the long con-
tinued hissing sound, which the tortoise emits
while preparing to retreat into its strong hold.
The ribs, though they at first assume the form
of broad plates, immoveably united to the spine,
when they have proceeded a certain distance,
CHELONIAN REPTILES. 475
separate from each other, and resume their usual
form; the intervening spaces between two adja-
cent ribs being here filled up by membrane.
The plastron is united with the carapace by
membrane likewise ; and the sternum, instead
of forming one broad plate of bone, has the in-
tervals between its imperfectly developed ele-
ments also membranous. All this renders the
whole shell less compact, more flexible, and
weaker ; but the movements of the animal are
quicker and more energetic.
These characteristic differences between the
aquatic Chelonia and those that live on land are
still more strongly marked in the genus Trionyx,
or soft tortoise ; which is destitute of scales, and
in which many of the pieces that are bony in
the tortoise are replaced by simple cartilage or
membrane.
The enormous weight of the shell of the turtle
would be a serious impediment to the motion of
this animal in the water, were there not some
provision made for diminishing the specific gra-
vity in the body. This purpose is answered by
the great capacity of the lungs, which, when
inflated with air, nearly fill the thorax, and
give great buoyancy to the whole mass. Thus,
wherever there exists a supposed inconvenience,
dependent on the fulfilment of one condition,
we are certain to meet with a compensation
in the structure of some other part, and in the
476 THE MECHANICAL FUNCTIONS.
mode of executing some other function. An
express provision for giving buoyancy has been
made in the construction of the shell of a species
of tortoise inhabiting the coasts of the Seychelle
Islands. The under surface of the shell, instead
of being gently concave, as in land tortoises, has
a deep circular concavity in the centre, above
four inches in depth, which, when the animal
goes into the water, retains a large volume of
air, buoying up the whole mass while it remains
in that element.* The greater size of turtles,
when compared with tortoises, is a further in-
stance of the superior facility with which organic
growth proceeds in aquatic than in land animals
formed on the same model of construction.
Home's Lectures, vi. 37.
477
Chapter IX.
MAMMALIA.
$ 1 . Mammalia in general.
The singular animals, so remarkable for their
anomalous shapes, their torpid vitality, and their
amphibious constitution, which have lately oc-
cupied our attention, appear placed by nature
as forms of transition, in the passage from those
vertebrated animals which dwell in the water,
to those which inhabit the land. The class
of Mammifera, or Mammalia, comprehends all
the animals which possess a spinal column,
breathe air by means of lungs, and are also
warm blooded and viviparous ; conditions which
render it necessary that they should possess
organs, called mammas, endowed with the power
of preparing milk for the nourishment of their
young ; a peculiarity from which the name of
the class is derived. But they are not exclu-
sively land animals ; for among the mammalia
must be ranked several amphibious and aquatic
tribes, such as the Seal, the Walrus, the Porpus,
478 THE MECHANICAL FUNCTIONS.
the Dolphin, the Narwhal, the Cachalot, and the
Whale ; animals which, however widely they
differ in their habits and external confor-
mation from terrestrial quadrupeds, possess, in
common with the latter, all the essential cha-
racters of internal structure and of functions
above enumerated. These characters belong
also to the human species, which must conse-
quently, in its zoological relations, be ranked as
a genus of the class mammalia. So numerous,
indeed, are the analogies which connect the na-
tural families of this class with our own race, that
we must ever feel a deep interest in the accurate
investigation of their comparative anatomy and
physiology ; and it has been found, accordingly,
that the progress, which has, of late years, been
made in this branch of science, has materially
enlarged our knowledge of the structure, the
functions, and the physical history of Man ;
subjects with which our welfare has obviously
the closest and most intimate relation.
The principle of analogy, which prevails so
generally in the inferior departments of the ani-
mal creation, may be also traced in the class
mammalia ; for we always find its influence
more conspicuous in proportion as the objects
comprehended in the natural series of beings are
more numerous and more diversified. Scarcely
any of the great natural assemblages of animals
MAMMALIA. 479
exhibit more variety in their habits and modes
of existence, than the one we are now exa-
mining. Each race has its peculiar destination
with regard to the kind of food by which it is
nourished, and the means by which that food
is obtained. The carnivorous tribes wage war
with the larger animals, whom they either
spring upon unawares, or openly pursue and
overpower, displaying the savage energies of
their nature, in practising all the arts of fero-
cious and sanguinary destruction. Others, in-
tent on meaner prey, resort to divers stratagems
for its possession ; some are designed to feed
chiefly on the mollusca, and others swallow
insects only. The numerous tribes which are
formed to subsist on vegetable food exhibit, in
like manner, a great diversity of construction,
adapted to the particular nature of that subsist-
ence, whether it be herbage, or the leaves of
trees, or fruits, or seeds, or the coarse fibres of
wood and bark. While all are gifted with
powers to obtain the nourishment they require,
those that have not been armed with weapons
of attack, are still provided with instruments of
defence, or with means of flight. Each has its
respective sphere of operation ; and to each its
appropriate soil, habitation, climate, and element
have been assigned.
It is easy to conceive that all these various
480 THE MECHANICAL FUNCTIONS.
circumstances must lead to great diversities in
the apparatus for mastication and for digestion,
in the organization of the senses, in the con-
struction of the instruments of locomotion and of
prehension, and in the general form of the body
to which these various parts are to be adapted.
Yet, amidst all these variations, we may perceive
the same laws of analogy connecting the whole
into one series, and assimilating all these multi-
form structures to one common standard. The
same organ, however modified in its shape and
size, however stinted in one, or developed in
another, is ever found in its appropriate place,
and retains the same connexions with adjacent
organs, whether we seek it in the carnivorous or
the herbivorous quadruped, in the inhabitant of
the land or of the water, of the frigid or of the
torrid zone, or in animals of the most diminutive
or most colossal statures.
As an example, we may take the vertebrae of
the neck. It is a universal law, that this part of
the spinal column shall, in every animal of the
class mammalia, consist of neither more nor less
than seven vertebrae. Whatever be the length
or shortness of the neck, whether it be com-
pressed into a small space, as in the Elephant
and the Mole, whether it be lengthened to allow
the head to reach the ground, as in the Horse
and the Ox, or whether it be excessively pro-
longed, to allow the animal to reach the tops of
M A MM ALT A. 481
trees, as in the Camelopard, still this same con-
stant number is preserved in the vertebrae which
it contains. When the neck is long, each indi-
vidual vertebra must necessarily be lengthened
in the same proportion. Thus in the Camelopard,
the vertebrae of the neck consist of seven very-
long tubes, joined together endwise, with scarcely
any developement of spinous processes, lest they
should impede the bending of the neck. The
greatest contrast to this structure is met with in
the Dolphin, and other Cetacea, which present
externally no appearance whatever of a neck,
but whose skeleton exhibits cervical vertebrae,
closely compressed together, and exceedingly
thin, and most of them united together ;* every
bone, thus formed, however, retains the marks
of having originally consisted of separate verte-
brae ; and still, in this extreme case, the number
of primary pieces is constantly seven. f
* In the Cachalot, the whole of these seven vertebrae are
usually anchylosed into one bone.
f The Bradypus tridactylus, or three-toed sloth, was, till very
lately, thought to constitute a notable exception to this law,
being described as having nine, instead of seven, cervical ver-
tebrae. It is now found, however, that the two last of these
vertebrae, which appeared to be supernumerary, ought properly
to be classed among the dorsal vertebrae, of which they possess
the distinctive characters, not only from the form and size of
their transverse processes, but also from their having small bony
appendices, articulated with them by a regular joint at their
extremities, and corresponding exactly, both in shape and situa-
tion, to the ribs, of which they may, in fact, be considered a?
VOL. I. I T
482 THE MECHANICAL FUNCTIONS.
§ 2. Cetacea.
Remarkable exemplifications of the law of uni-
formity of organic structure are furnished by the
family of the Cetacea, which includes the Whale,
the Cachalot, the Dolphin, and the Porpus, and
exhibits the most elementary forms of the type
of the mammalia, of which they represent the
early, or rudimental stage of developement.
Here, as before, we have to seek these first ele-
ments among the inhabitants of the water ; for
whenever, in our progress through the animal
kingdom, we enter upon a new division, aquatic
tribes are always found to compose the lowest
links of the ascending chain. Here, also, we
observe organic developement proceeding with
more rapidity, and raising structures of greater
dimensions in aquatic than in terrestrial animals.
The order Cetacea comprises by far the largest
rudiments. These small bones have been observed, both by
Meckel and by Cuvier, attached to the ninth vertebra: and
Mr. T. Bell has recently not only confirmed the observations of
these anatomists, but has farther discovered, that similar rudi-
mental ribs are attached also to the eighth vertebra. (See Philo-
sophical Magazine, third series, iii. 376). The Bradypus tor-
quatus, which has been said to possess eight cervical vertebrae,
will, perhaps, on closer examination, be hereafter found not to
deviate, any more than the three-toed sloth, from the normal
type, as regards the number of these vertebrae. Instances have
occurred of supernumerary cervical processes, or ribs in the human
skeleton. (See Edinburgh Medical andSurgicalJournal, xl. 304.)
CETACEA. 483
animals which inhabit the globe. Whatever
may have been the magnitude of those huge
monsters which once moved in the bosom of the
primeval ocean, or stalked with gigantic strides
across antediluvian plains, and whose scattered
remains bear fearful testimony of the convulsions
of a former world, certain it is that, at the pre-
sent day, the Whales of the northern seas are the
most colossal of the living animal structures ex-
isting on the surface of this planet.
A cursory survey of the organization of the
tribes belonging to this semi-amphibious family,
will impress us with the resemblance they bear
to fishes ; for they present the same oval outline
of the body ; the same compact form of the trunk,
which is united with the head without an inter-
vening neck ; the same tin-like shape of the ex-
ternal instruments of motion; and the same enor-
mous expansion and prolongation of the tail,
which is here also, as in fishes, the chief agent in
progression. With all this agreement in exter-
nal characters, their internal economy is con-
ducted upon a totally different plan ; for although
constantly inhabiting the ocean, their vital or-
gans are so constructed as to admit of their
breathing only the air of the atmosphere ; and
the consequences which flow from this difference
are of great importance. The necessity of aerial
respiration compels them to rise, at short inter-
vals, to the surface of the water; and this air,
with which they fill their lungs in respiration,
484 THE MECHANICAL FUNCTIONS.
gives their bodies the buoyant force which is
required to facilitate their ascent, and supersedes
the necessity of a swimming-bladder, an organ
which is so useful to fishes.
With the intent of diminishing still farther
their specific gravity, nature has provided that a
large quantity of oily fluid shall be collected
under the skin ; a provision which answers also
the purpose of preserving the vital warmth of
the body. A great accumulation of this lighter
substance is formed on the upper part of the
head, apparently with a view to facilitate the
elevation to the surface of the spiracle, or ori-
fice of the nostrils, which is placed there.*
Another peculiarity of conformation, in which
the cetacea differ from fishes, and which has also
an obvious relation to their peculiar mode of
breathing, is in the form of the tail, which, instead
of being compressed laterally, and inflected from
side to side, as in fishes, is flattened horizontally,
and strikes the water in a vertical direction ;
thereby giving the body a powerful impulsion,
either towards the surface, when the animal is
constrained to rise, or downwards, when, by
diving, it hastens to escape from danger.
All the essential and permanent parts of the
skeleton of vertebrated animals, that is, the spi-
nal column, and its immediate dependencies, the
* The substance called Spermaceti is lodged in cells, formed
of a cartilaginous substance, situated on the upper part oftf-he
head of the Cachalot.
CETACEA. 485
skull, the caudal prolongation, and the ribs, are
found in that of the Cetacea. The thorax is
carried very much forwards, especially in the
whale, and the neck is so short as to be scarcely
recognisable ; for the object of the conformation
is here, as in that of fishes, to allow free scope
for the movements of the tail, and ample space
for the lodgement of its muscles. For the purpose
of giving greater power and more extensive at-
tachment to these muscles, the transverse pro-
cesses of the dorsal and lumbar vertebrae are ex-
panded both in length and breadth ; and being
situated horizontally, they offer no impediment to
the vertical flexure of the spine. For the same
reason the ribs are continued in a line with the
transverse processes, and articulated with their
extremities ; thus giving still further breadth to
the trunk.
As there is a total absence of hinder ex-
tremities, so there is no enlargement of any
of the vertebras corresponding to a sacrum ; and
the caudal vertebrae are uninterrupted continua-
tions of those of the trunk. They develope,
however, parts which are met with only among
fishes and reptiles ; namely arches, composed
of inferior leaves* and spinous processes, en-
closing and giving protection to a large artery.
Although the bones of the legs do not exist,
* These leaves being formed of cartilage, are generally lost
when the bones are macerated for the purpose of preparing the
skeleton.
48G
THE MECHANICAL FUNCTIONS.
yet there are found, in the hinder and lower part
of the trunk, concealed in the flesh, and quite
detached from the spine, two small bones, appa-
rently corresponding to pelvic bones, for the
presence of which no more probable reason can
be assigned than the tendency to preserve an
analogy with the more developed structures of
the same type.
A similar adherence to the law of uniformity
in the plan of construction of all the animals be-
longing to the same class, is strikingly shown in
the conformation of the bones
of the anterior extremities of
the Cetacea ; for although they
present, externally, no resem-
blance to the leg and foot of
a quadruped, being fashioned
into fin-like members, with a
flat oval surface for striking
the water, vet when the bones
are stripped of the thick in-
tegument which covers them,
and conceals their real form,
we find them (as may be
seen in Fig. 216) exhibiting
the same divisions into carpal
and metacarpal bones, and
phalanges of fingers, as exist
in the most highly developed organization, not
merely of a Quadruped, but also of a Monkey,
and even of Man.
AMPHIBIA. 487
§ 3. Amphibia.
In the small tribe, denominated by Cuvier
Amphibia, and consisting of the Phoca, or Seal,
and the Trichecus, or Walrus, we perceive tl\at
an advance is made towards a fuller develope-
ment of the limbs; these animals having a
distinct neck and pelvis, and both hind and
fore extremities. In the Seal, the hind legs are
drawn out posteriorly to a considerable length,
and placed parallel to each other : when united
and alternately raised and depressed, they per-
form the same office as the tail of the cetacea,
and propel the animal forwards : but when em-
ployed separately, they are more qualified to act
as oars. The Walrus has feet still more deve-
loped, and distinctly divided into toes, which are
disposed so as to strike backwards against the
water.
§ 4. Mammiferous Quadrupeds in general.
From the imperfectly developed aquatic and
amphibious tribes we gradually ascend to the
more finished structures of mammiferous quad-
rupeds, which are expressly fitted for progression
on land. In these the powers of developement,
488 THE .MECHANICAL FUNCTIONS.
not being expended in the mere effort of giving
expansion to the several textures, and of swelling
the bulk of the frame, sometimes to inordinate
dimensions, are employed rather in reducing
the elements of the organization into compact
forms, and in concentrating their energies, so as
ultimately to attain the extent of power and
harmony of action, which are displayed in the
higher orders of warm-blooded quadrupeds.
It is to these favoured tribes that we must
look for examples of the most complete develope-
ment of the skeleton, and the most advantageous
disposition of mechanic force. We have seen
that reptiles, from the comparative shortness of
their limbs, and the torpidity of their muscular
powers, are but ill adapted for rapid progression.
All the more perfectly formed quadrupeds of the
class mammalia, having the trunk of the body
raised high upon the limbs, possess great range
of motion, and can traverse with fewer steps a
given space.
The office of the limbs, as far as they are con-
cerned in progressive motion, is two-fold. They
have, first, to sustain the weight of the body,
which they must do by acting in opposition to
the force of gravity ; and they must, secondly,
give the body an impulse forwards. Let us con-
sider more particularly the relations which the
structures bear to each of these two functions.
The limbs of quadrupeds constitute four
MAMMIFEROUS QUADRUPEDS. 489
columns of support to the trunk, which is placed
horizontally above them ; but the whole weight
of the body, together with that of the head and
neck, does not bear equally upon them ; the
fore extremities almost always sustain the greater
part of that weight, both because the fore part
of the trunk is itself heavier than the hind part,
and because it is loaded with the additional
weight of the head and neck. Hence, in the
usual attitude of standing, the pieces of which
the fore limbs are composed are required to be
placed more in a straight line than those of the
hinder limb ; for the power of a column to sup-
port a weight is the greater in proportion as it.
approaches to the perpendicular position. The
hind limbs are composed of exactly the same
number of divisions ; but the separate portions
are usually longer than those of the fore ex-
tremity ; and, consequently, if they had been dis-
posed vertically in a straight line, they would
have elevated the hinder part of the trunk to too
great a height compared with the fore part.
This is obviated by their forming alternate
angles with one another. As the pelvis con-
nects the spine with the joint of the hip, and
even extends farther backwards, the thigh bone
must necessarily be brought forwards ; then the
tibia and fibula, which compose the bones of the
leg must be carried backwards to their junction
with the bones of the foot ; and again, the foot
490 THE MECHANICAL FUNCTIONS.
must be turned forwards in its whole length from
the heel to the extremities of the toes. On com-
paring the positions of the corresponding divisions
of the anterior and posterior extremities, we ob-
serve that they incline, when bent, in opposite
directions ; for in the former we find, in fol-
lowing the series of bones from the spine, that
the scapula proceeds forwards, the humerus
backwards, the radius and ulna again forwards,
and the fore foot backwards ; positions which are
exactly the reverse of the corresponding bones
of the hind limb. (See Fig. 218, page 507.)
The weight of the body, in consequence of
this alternate direction of the angles at the suc-
cessive joints, must always tend, while the
quadruped is on its legs, to bend each limb ; a
tendency which is required to be counteracted
by the actions of the muscles which are situated
on the external side of each of those angles.
These muscles are the extensors of the joints ;
that is, the muscles which tend to bring their
parts into a straight line. It is, in fact, by this
muscular action, much more than by simple
rigidity, that the limb supports the super-
incumbent weight of the body. It is evident
that greater muscular force is necessary for this
purpose when the joints are bent, than when
they are already extended ; and the portions of
the fore legs, being naturally in this condition,
require less power than those of the hinder legs
to retain them in their proper relative positions.
MAMMIFEROUS QUADRUPEDS. 491
The most complete instance of a vertical
arrangement of the bones of the extremities is
seen in the Elephant ; where in order to sustain
the enormous weight of the body, the limbs are
shaped into four massive columns, of which the
several bones are disposed nearly in perpen-
dicular lines. By this means the body is sup-
ported with scarcely any muscular effort ; and
the attitude of standing is, in this animal, a state
of such complete repose, that it often sleeps in
that position. The elephant which was kept
some years ago at the Menagerie at Paris,
although much enfeebled by a lingering dis-
order, was never seen to lie down till the day
on which he died. When he was in the last
stage of debility, what seemed to give him most
distress was the effort requisite to support his
head ; and in order to relieve the muscles of the
neck which were strained in that exertion, he
was in the habit of extending his trunk per-
pendicularly to the ground, by contracting all
the muscular fibres which run transversely in
that organ, and of thus forming a vertical prop
for the head. But in almost all other quadru-
peds the mere act of standing, though a state of
comparative rest, implies, for the reasons already
given, a degree of muscular exertion ; and they
can enjoy complete repose only by letting the
body recline upon the ground.
The conformation of the hind extremities,
which, as we have seen, is not so well calculated
492 THE MECHANICAL FUNCTIONS.
for the simple support of the trunk, is, on the
other hand, better adapted to give it those im-
pulses which are to effect its progressive move-
ments. The nature of those movements, and the
order in which they succeed each other, are dif-
ferent according to the peculiar mode of pro-
gression which the animal practises, the degree
of speed it is desirous of exerting, and the par-
ticular end it has in view. The paces of a qua-
druped usually distinguished, are the walk, the
trot, the gallop, the amble, and the bound.
In slow walking, only one foot is raised from
the ground at the same moment, so that three
points of support always exist for sustaining the
weight of the body. If the centre of gravity be
situated, as it generally is, nearly over the middle
of the quadrangular base formed by the feet,
while they rest upon the ground, the first effort
to advance which the quadruped makes, propels
the centre of gravity forwards. This it accom-
plishes by pressing one of its hind legs against
the ground ; which leg, being thus fixed by the
resistance it there meets with, becomes the ful-
crum of the first movements. The extensor
muscles of the limb are now exerted in giving
the body an impulse forwards. As soon as this
impulse has been given, the muscles which had
been in action are relaxed ; and the leg is raised
from the ground, brought forwards, and laid down
close to the fore foot of the same side. This fore
PROGRESSIVE MOTION IN QUADRUPEDS. 493
foot is next raised and advanced ; and then the
same succession of actions takes place with the
hind and the fore foot of the other side.
An attentive examination of the conditions of
these successive positions will show that, amidst
all the changes which take place in the points of
support, the stability of the body is constantly pre-
served. It is an elementary proposition in me-
chanics that all that is necessary for ensuring the
support of a body on any given base, is that the
vertical line drawn from the centre of gravity
shall fall within that base. When the animal is
standing, the feet form a quadrilateral base, and
the centre of gravity is in a vertical line passing
either through the centre of the base, or, as, for
the reasons already mentioned, more frequently
happens, through a point a little in front of the
exact centre. At the time when the hind foot
which began the action is raised from the ground,
the centre of gravity, having been, by that action,
impelled forwards, still remains above the base
formed by the other three feet, and which is now
reduced to a triangle. That hind foot being set
down, while the corresponding fore foot is raised,
a new triangular base is formed by the same
hind foot, together with the two of the other side,
which have not yet been raised. The centre of
gravity is still situated above this new triangle,
and the body is consequently still supported on
these three feet. The fore foot may now be ad-
4i>4 THE MECHANICAL FUNCTIONS.
vanced, without endangering the stability of the
body ; and by the time this foot is set down, and
has thereby formed a new quadrilateral basis
with the other feet, the centre of gravity has
arrived above the centre of this new base. But
at this moment the centre of gravity is again
urged forwards by the other hind foot, which now
comes into action, and repeats on the other side
the same succession of actions, which are at-
tended with the same consequences as before.
Thus, during its whole progress, the animal is
never for an instant in danger of falling : for
whichever of the feet may be raised from the
ground, the other three feet are always so placed
as to form a stable base of support.
In cmick walking, it often happens that qua-
drupeds raise their fore foot, on either side, a little
before the hind foot comes to the ground. This
is shown by the impression made by the latter
being in the same spot, or even rather in ad-
vance of the impression made by the former.
But the time, during which the body is thus
supported only by two feet, is so short as not
sensibly to influence the results.
In consequence of the obliquity of the alternate
impulses given to the centre of gravity by the
successive actions of both the hind legs, a slight
degree of undulation is occasioned ; but these
undulations are only lateral. A trot may be con-
sidered as a succession of short leaps made by
PROGRESSIVE MOTION IN QUADRUPEDS. 495
each set of feet taken diagonally ; that is, by the
right fore foot, and the left hind foot ; or, vice
versa, the one set being raised together a short
time before the others have reached the ground :
so that during that minute interval of time, all
the feet are in the air at the same moment ; and
during the remaining portion of the time, the
body is resting upon the two feet placed diago-
nally with regard to each other. The undula-
tions are here chiefly vertical, instead of lateral,
as they are in the walking pace.
A gallop is a continued succession of longer
leaps made by the two hind feet in conjunction.
In this case, the centre of gravity is lifted higher
from the ground, and is projected in a wide arch,
and with great velocity.
In the amble, both the legs on one side are
raised together ; so that the impulsions given are
directed much more laterally than in any other
pace, and the body is thrown into a strong undu-
latory motion from side to side.
Another kind of pace is the bound, which is
often practised by deer, and is performed by
striking the ground with all the legs at the same
moment. It consists, therefore, like the gallop,
of a series of leaps ; but their direction is more
uniformly upwards, from the concurrence of all
the legs in the same action.
Nature has purposely endowed different tribes
of animals with very different capacities to exe-
4fK> THE MECHANICAL FUNCTIONS.
cute progressive movements, by the variations she
has introduced into the comparative lengths of
the several parts of the trunk, and the size and
mobility of the extremities. Of all the large
animals, the Lion has been constructed with the
finest proportions for conferring both strength
and activity. The mass of his body is supported
more by the fore than by the hind extremities.
In walking, the Lion takes long strides, and exhi-
bits strongly the lateral undulations of the trunk.
Quadrupeds having a very long, or a very-
massive body, or whose limbs are short, and
nearly of equal height, are incapable of ad-
vancing by a gallop, or at least cannot sustain
this pace without a painful effort, and never but
for a short time. The Tiger, which has a longer
body than the Lion, gallops with less facility ;
and runs chiefly by an acceleration of its walk-
ing pace. It excels principally in the vigour
and extent of its bounds ; for which it is admir-
ably qualified by the prodigious power of its mus-
cles, enabling it to spring forwards upon its victim
with an impetus which nothing can resist.
The speed with which a quadruped is capable
of advancing depends more on the disposition of
the muscles and the extent of the articulations,
and more especially on the power of the ex-
tensors of the hind extremities, than on the form
of the body. Great length and muscularity in
the hind legs are generally attended with con-
PROGRESSIVE MOTION IN QUADRUPEDS. 487
siderable power of leaping. This is exemplified
in the Jerboa and the Kanguroo, animals, which,
from the disproportionate shortness of their
fore legs, are totally incapacitated from walk-
ing ; and for the same reason, they cannot run
with any degree of swiftness. It is only in
climbing up a steep acclivity that the jerboa is
enabled to employ all its limbs : in a descent,
on the contrary, it uses only its fore legs, the
hinder being dragged after them. But, when
pursued, these animals are capable, for a long
continuance, of taking leaps of nine feet dis-
tance, and of repeating these leaps so quickly,
that the Cossacks, though mounted on the
swiftest horses, are unable to overtake them.
The Kanguroo, in almost all his movements,
brings into action his powerful tail, which is fur-
nished with very strong muscles, and may be
considered as constituting a fifth limb. It is of
great assistance to the animal in taking leaps ;
and, during its repose, contributes, together with
the hind feet, to support the weight of the body,
as on a tripod, and to leave at liberty the fore
legs, which may then be employed as arms.
The Hare and the Rabbit furnish other in-
stances of an extraordinary length of the hinder
legs depriving the animal of the power of walk-
ing, and obliging it to move forwards only by
a succession of leaps. The hare may be said,
indeed, to walk with its fore legs only, while it
VOL. I. K K
498 THE MECHANICAL FUNCTIONS.
hops or gallops with the hinder ; but this dis-
advantage is amply compensated by its amazing
swiftness when running at full speed.
Animals like the hare, in which, from the
great length of the hinder limbs, the posterior
half of the body is higher than the anterior, run
much better up an acclivity than on level ground.
In a descent, on the contrary, they are obliged
to pursue an oblique and zig-zag course, other-
wise they would be in danger of oversetting, as
happens occasionally to the Agouti and the
Guinea pig, when these animals attempt to run
down hill.
The Sloth, which is formed for clinging with
great tenacity to the boughs of trees, presents a
remarkable contrast to the animals we have just
noticed ; its fore legs being much longer than the
hinder, and its movements being proverbially
slow. The peculiar modifications of its mus-
cular powers are probably consequences of the
singular mode in which, as I shall afterwards
have occasion to notice, its arteries are dis-
tributed.
The Camelopard, likewise, has the fore legs
much longer than the hinder. The object of
this conformation was probably to elevate the
anterior part of the spine, so as to raise the head
as much as possible ; and also to give a con-
siderable inclination to the whole column, for the
purpose of distributing more equally the weight
PROGRESSIVE MOTION IN QUADRUPEDS. 499
of the head and of the very long neck upon
all the legs ; for the length of the neck is fully
equal to that of the trunk. It is evident that if
the body had been placed in the usual horizontal
position, the anterior extremities would have
had to support the whole of the enormous weight
of this neck and head. This peculiarity of
structure, however, introduces considerable mo-
difications in the mode of progression of the
animal. The ordinary pace of the camelo-
pard is the amble ; but it has also a slower
walking pace, and occasionally a gallop. In
the amble, its undulation is so considerable as to
give it the appearance of being lame. A similar
kind of limping gait, arising from the same
cause, namely, the disproportionate elevation of
the fore part of the spine, has been observed in
the Hycena.
§ 5. Ruminautia.
In following the series of Mammalia in the order
which best exhibits their successive stages of de-
velopement, I shall commence with those whose
disgestive apparatus is formed to extract nourish-
ment exclusively from the vegetable kingdom.
The first assemblage that presents itself to our
notice is the remarkable family of Ruminants,
500 THE MECHANICAL FUNCTIONS.
which feed principally on herbage. Wherever
the earth is clothed with vegetation, it requires
neither skill nor exertion on their part to seek
and to devour the rich repast which is profusely
spread under their feet. To remove from one
pasture to another, to browse, and to repose, con-
stitute the peaceful employments of their lives,
and satisfy the chief conditions of their existence.
To these purposes the whole conformation of their
skeleton, and especially of those parts which con-
stitute the limbs, is adapted. The anterior ex-
tremities having only to support the weight of
the fore part of the trunk, and to assist in pro-
gressive motion, have a less complicated arrange-
ment of joints, and exhibit many of those conso-
lidations of the bones, which tend to simplify the
structure, and contribute to its strength.
But though never incited by the calls of appe-
tite to engage in sanguinary warfare, they are
yet liable to the assaults of many ferocious and
well armed adversaries, and are often unprovided
with any adequate means of defence ; their only
resource, therefore, is to avoid the dangers of
the encounter by a rapid and precipitate flight.
To confer this power appears to have been the
object aimed at by nature in every part of the
conformation of these animals. It is among the
ruminant tribes that the fleetest of quadrupeds
are to be found, such as the Gazelle, the Ante-
lope, and the Deer, animals which exhibit the
RUMINANT QUADRUPEDS. 501
highest perfection of structure belonging to this
type. We may observe that the parts com-
posing the hind legs are longer, and inclined to
one another at angles more acute in these ani-
mals than in other tribes of mammalia, so that
they are always ready for instantly commencing
their flight, and springing forwards on the slight-
est notice of danger. (See Fig. 218, page 507.)
As it was necessary, from the situation of their
food, that their heads should reach the ground
in grazing, we find that the neck has been much
elongated, that the muscles which raise the head
have been enlarged and strengthened, and that
the spinous processes of the back and neck have
been much expanded in order to allow of suffi-
cient surface for the attachments of these muscles.
The effort requisite to raise, and even support the
head is very considerable ; as will appear when
we reflect that its weight acts by means of an
extremely long lever ; for such is the mechanical
office of the elongated neck. But in order to
economize the muscular power, an elastic liga-
ment is employed to sustain the weight of the
head. This, which is termed the ligamentum
nucha, and is represented at n, in Fig. 217, is
formed of a great number of bands, which con-
nect the hinder part of the cranium, at the ridge
of the occipital bone, and all the spinous pro-
cesses of the neck, with those of the back ; the
separate slips from each being successively joined
502
THE MECHANICAL FUNCTIONS.
together, and composing a ligament of great
length and power It differs in its structure
from ordinary ligaments, being highly elastic ;
so that it yields to the extension of the neck
when the animal lowers its head, and gives con
siderable assistance to the muscles in raising it.
In the Deer and the Ox, which toss their heads
with force, and especially in the males, which
are armed with antlers or horns, the muscles
performing these motions are remarkably strong,
and the spinous processes of the back particu-
larly prominent. In the loins, on the contrary,
we find the transverse processes more enlarged,
for the purpose of giving a powerful mechanical
purchase to the muscles which are inserted into
them.
The chest of ruminant quadrupeds is com-
pressed laterally, in order to allow room for the
RUMINANT QUADRUPEDS. 503
unrestrained motions of the anterior extremity ;
and the sternum projects so as to resemble the
keel of a ship. The bones of the anterior extre-
mity are not joined to the rest of the skeleton by
means of any bone corresponding to a clavicle ;
but they are connected with the spine and ribs
only by ligaments and muscles ; so that the fore
part of the trunk is, in fact, suspended between
the limbs by its muscular attachments alone.
This is not the case with the hind extremities ;
for their bones commence with the pelvis, which
proceeds backwards from the sacrum, but with
a considerable inclination downwards, and has
a deep hemispherical cavity for the lodgement of
the round head of the thigh bone. Thel ength-
ened forms of the iliac bones, and also of the
scapula, provide for the application of muscles
of considerable length, which are consequently
capable of communicating to the parts they move
a greater velocity than could have been effected
by muscles of equal strength, but with shorter
fibres.
Both the humerus in front, and the femur
behind, are so short as to appear, on a super-
ficial view, to form part of the trunk ; being
entirely enveloped and concealed by the large
muscles connecting them with the body. The
heads of the two humeri, in consequence of the
absence of the clavicle, are brought very near
504 THE MECHANICAL FUNCTIONS.
each other ; so as to occupy a situation as nearly
as possible underneath the weight which the
limb has to support.
The radius and ulna, which are the two bones
of the fore arm, although completely separate at
an early period of growth, soon unite to form
but one bone. This union begins at their lower
end, and proceeds upwards to within a short dis-
tance from the top, where a separation may still
be observed in the processes which project from
that end, forming for some way down a distinct
suture. This union of the two bones must, of
course, preclude all rotatory motion ; but it is
calculated to give the joint great security :
and this appears to have been the main object
in the conformation of the whole limb. The
same process of consolidation takes place in the
hind leg, between the tibia and the fibula, which
are so completely united, as to afford scarcely
any trace of their having been originally se-
parate.
The carpus and the tarsus are both of very
limited extent, and consist of a smaller number
of pieces than usually occur in these joints.
The consolidation of parts is most conspicuous
in the succeeding division of the limb, namely,
that constituting the metacarpus in the anterior,
and the metatarsus in the hind extremity. In
either case, we find it consisting, not of five
bones, as in the more highly organized carni-
RUMINANT QUADRUPEDS. 505
vorous mammalia, but of a single bone only,
termed the cannon bone. In the early periods of
ossification, however, they each consisted of two
slender bones, lying close and parallel to each
other ; but afterwards united by an ossiflc depo-
sition, which fills up the interval between them,
and leaves behind no trace of suture.* In pro-
portion as the young animal acquires strength,
the union of these two bones becomes still more
intimate, by the absorption of the partition which
separated their cavities ; so that ultimately they
constitute but one cylinder, with a single central
cavity, which is occupied by marrow.
The cannon bone is much elongated, both in
the fore and hind extremity ; so that the carpus
and tarsus, which are the commencements of the
real feet, are raised considerably above the
ground. It is a common mistake, arising from
the height of these joints, and the names they
bear in ordinary language, to consider them as
the knees of the animal. The slightest inspec-
tion of the skeleton will be sufficient to show
that what is called the knee in the fore leg is
properly the wrist ; and in the hind leg, the part
so misnamed is really the heel. Thus the foot,
especially in the posterior extremity, is of great
length ; a structure which is evidently intended
* The observations which establish this fact are detailed by G.
St. Hilaire, in a paper in the " Memoires du Museum," x. 173.
506 THE MECHANICAL FUNCTIONS.
to give greater velocity to the actions of the
muscles, while it at the same time ensures the
utmost steadiness and security of motion.
At the lower extremity of the cannon bone
there are two articular surfaces, indicating the
originally separate ends of its two component
bones. They are for the articulation of the two
following bones, which are also very long, and
which correspond in situation to the first pha-
langes of the fingers and toes. These are fol-
lowed by a second and third set of phalanges ;
the last of which terminate in hoofs. All rumi-
nant quadrupeds have thus a double hoof; a
character which is peculiar to this family.
Thus, then, has Nature moulded the organs of
progressive motion in this remarkable tribe of
animals to accommodate them to the peculiar
conditions of their existence, while she has still
preserved their relations to the primitive type of
the class to which they belong. Thus has she
bestowed upon them the slender and elegant
forms, so pleasing to the eye, which characterise
the fleetest races, and has provided for the agile,
yet firm and secure movements which they are
to exercise in various ways in eluding the obser-
vation, and escaping from the pursuit of their
stronger and more sagacious foes. This purpose
they effect, at one time by rapid flight across
extensive tracts of country ; at another, by re-
tirement into unfrequented forests, or mountains
RUMINANT QUADRUPEDS.
507
of difficult access, crossing their rugged surfaces
in all directions, clambering their precipitous
508 THE MECHANICAL FUNCTIONS.
acclivities, and fearlessly bounding over inter-
vening abysses, from point to point, till the
place of safety is attained on some rocky emi-
nence. From this secure station the Alpine
Chamois looks down upon its pursuers, and de-
fies their further efforts at capture or molesta-
tion. The astonishing feats of agility practised
by this animal, and by which the most expe-
rienced hunters are perpetually baffled in their
attempts to approach it, sufficiently attest the
perfection of its organization in reference to all
these objects. The chamois has often been seen
to leap down a perpendicular precipice of twenty
or thirty feet in height, without sustaining the
slightest injury. How the ligaments that bind
the joints can resist the violent strains and con-
cussions they must be exposed to in these quick
and jarring efforts, is truly wonderful.
While Nature has provided these animals
with the means of safety from their more for-
midable enemies, she has not left them alto-
gether without defence against their more equal
rivals in the field. It is on the head that she
has implanted those powerful arms which are
sometimes wielded with deadly effect in their
mutual combats. Even when not furnished
with horns, the animal instinctively strikes with
its forehead, where the frontal bone has been
expanded and fortified, apparently with a view
to this mode of attack. Thus, the ram butts with
RUMINANT QUADRUPEDS. 509
its head without reference to the horns, which
are coiled so as to be turned away from the
object to be struck. In the Deer and the Ox
tribes, however, the horns are formidable wea-
pons of offence : and it will be interesting to
inquire into the nature of these organs, and the
phenomena of their production.
The antlers of the male Stag are osseous struc-
tures, supported on short and solid tubercles of
the frontal bone : after remaining nearly a year,
they are cast off, and soon replaced by a newly
formed antler, which is of larger size than the
one which was lost. Previously to the forma-
tion of this structure, those branches of the
artery, termed the carotid, which supply blood to
the frontal bone, are observed very rapidly to
dilate, and to throb with unusual force ; and all
the blood-vessels of the skin of the part where
the antler is to arise, soon become distended
with blood, an effect which is accompanied by
general heat and redness, like a part in a state
of high inflammation.* Presently the skin is
elevated by the growth of a tubercle from the
subjacent bone : this tubercle is at first a carti-
lage, and after it has attained a certain size,
becomes ossified, and grows like other osseous
structures, first shooting into the form of a length-
ened cylinder, and then dividing into branches.
* These phenomena are connected with periodical changes in
the constitution relating to the reproductive functions.
510 THE MECHANICAL FUNCTIONS.
It is followed in its elongation by the skin, which
during the whole time that the antler is growing
is extended over it in every part, forming what
is called, from the delicate investment of hair,
its velvet coat. The blood-vessels of the proper
membrane of the antler, or periosteum, still con-
tinuing to supply it with the materials required
for its growth and consolidation, deposit so great
an abundance of bony matter, that its enlarge-
ment is exceedingly rapid. The whole antler,
which often weighs nearly thirty pounds, has
been known to be completely formed in ten
weeks from the time of its first appearance.
There is no other instance in the animal king-
dom of so rapid a growth ; which is the more
remarkable from its occurring in a small part of
the system, and in a bony structure.
After the antler has attained its full size, a
deposition of osseous substance still continues at
its base, around the trunks of the arteries which
are proceeding along the investing membrane of
the bone for the purpose of conveying nourish-
ment. The accumulation of this substance raises
a ring, called the burr, round that part of the
antler; and by encroaching on the arteries
themselves, it gradually diminishes their capa-
city of conveying blood, and they at length be-
come entirely obliterated. The bone, no longer
receiving a superabundant nourishment, ceases to
grow ; the integuments, which covered it, decay,
and becoming dry and shrivelled, are torn by
RUMINANT QUADRUPEDS. 511
rubbing against trees, and peel off in long
shreds, leaving the antler exposed, which, by
the continued effects of the same kind of friction,
soon acquires a polished surface.
During many months, the antler being suffi-
ciently nourished by its own interior vessels,
continues in a living state, and preserves its
connexion with the system. But at length the
arteries, whether from the effect of the progres-
sive deposition of osseous matter, or from some
change in the balance of the vital powers, shrink
and become by degrees obliterated. The antler
dies in consequence, and although it continues
to adhere to the skull, it is only as a foreign
body, and it is not long destined to remain thus
attached ; for the absorbent vessels are now ac-
tively employed in scooping out a groove of se-
paration between the living and the decayed
substance, at the place where the base of the
antler is contiguous to the frontal bone. As
soon as this has proceeded to a sufficient depth,
the adhesion ceases, and the slightest concussion
occasions the fall of the whole structure. After
the separation of the antler, the eminence of the
frontal bone on which it stood is left rough and
uneven like that of a fractured part : but the
surrounding integuments soon close over, and
cover it completely ; until the period arrives
when it is to be replaced by a new antler, which
exhibits the same succession of phenomena in
its growth and decay as its predecessor, only that
512 THE MECHANICAL FUNCTIONS.
its developement is usually carried farther ; the
new stem being both thicker and longer, and
the branches wider and more numerous. The
antler of each successive year has, consequently,
a different form from that of the preceding ; and
when the animal has attained a certain age, the
extremities of the branches present broad ex-
pansions of bone, which the antlers of an earlier
growth had never exhibited.
The short bony processes which extend in a
perpendicular direction on the head of the
Camelopard, are analogous, in some of the cir-
cumstances of their formation, to the antlers of
the deer, being of an osseous nature, and con-
tinuous with the frontal bone: but in other
respects they are very different; for instead of
being annually shed, they remain through life,
and continue to be covered with the integu-
ments, which retain, at the extremities, a tuft of
hair. The developement of these processes in
the young animal takes place in the same man-
ner as that of an antler, but it reaches only to a
certain point, upon attaining which the growth
is arrested, and never proceeds farther. The
arteries cease to deposit superabundant nourish-
ment, but continue to maintain an exact equili-
brium between the expenditure and the supply ;
so that the horns of the camelopard are never
shed, and remain permanent bony structures.
A further modification of this process occurs
RUMINANT QUADRUPEDS. 513
in the construction of the horns of the Ox and
of the Sheep ; for in these the bony processes
arising from the frontal bones are invested with
a covering composed of horn, the nature of
which is totally different from bone. Two tu-
bercles may be seen in the young calf, proceed-
ing from the bones of the forehead : the skin
covering these tubercles, unlike that which pre-
cedes the antlers of the deer, is unusually thick
and hard. As the skull expands, this portion
of integument becomes more and more callous ;
till it is converted, by the action of the subjacent
vessels, into a solid, hard, elastic, and insensible
fibrous substance, fitted to give effectual protec-
tion to the subjacent bony layers which are
forming underneath it. The highly vascular
membrane, from which these new structures
chiefly arise, appears to have different powers of
production at its two surfaces ; for while the inner
surface is forming the osseous portion of the horn,
and supplying the phosphate of lime required for
the construction of its plates and fibres, the ex-
terior surface is adding successive layers of horny
substance to the inner side of those portions which
had been before deposited. These two opera-
tions, which offer a remarkable contrast, both as
to the mode of their performance, and as to the
nature of the resulting products, are carried on
at the same time, and by the same organ, but
on different sides. The bony basis of the horn
VOL. I. L L
514 THE MECHANICAL FUNCTIONS.
is an organic structure, which continues to be
nourished by vessels forming part of the general
system : the horn is a mere excretion, which
appears to be destitute of vessels, and is, conse-
quently, removed from the influence of the living
powers. Thus the growth of horn is somewhat
analogous to that of shell ; for the layers which
compose it are deposited in succession ; each
new layer is agglutinated to the inner surface of
the preceding ; and each has the shape of a hol-
low cone, occupying the part towards the apex
of the former cone, and extending farther to-
wards the base. Hence a longitudinal section of
the whole presents the appearance represented
in the annexed figures (218*), where a is the
section of the horn of an Ox, and b, a similar
section of the horn of an Antelope. C is a
magnified view of the extremity of the latter,
together with a portion of the bone (d), which
occupies the axis of the horn.
In this process of the formation of horn, as
happens in that of shells, there sometimes occur
irregularities, or periodical intermissions and in-
crease of action in the secreting organs, giving
rise to transverse grooves, or ridges. These may
be seen in the horns of the Goat, in which the
fibres are short, and laid one over another with
the same regularity as the tiles of a house. The
tendency in these horns to assume a spiral form
is explicable on the same principles as those
RUMINANT QUADRUPEDS.
515
which regulate the growth of turbinated shells.
The horns of the Ox and of the Antelope tribes
are formed of longer and more continuous fibres,
which are closely compacted together, and ex-
hibit very distinctly the series of hollow cones of
which they are composed.
The horns of the Rhinoceros, both of the one
and the two horned species, grow from the inte-
gument covering the nose, to which they adhere
without having any connexion with the subja-
cent bones. They have a pyramidal shape,
and are composed of parallel fibres, resembling
516 THE MECHANICAL FUNCTIONS.
hairs, agglutinated together into a solid mass
by a material which acts as a cement. This
fibrous structure is most distinctly seen at the
base of the horn, where the ends of the fibres
project, like those of a brush, from the surface.
When these horns are sawn transversely, and
examined with a magnifying glass, a great num-
ber of orifices are seen, marking the empty spaces
that intervene between the hairs; and if the sec-
tion be made in a longitudinal direction, the same
spaces give rise to the appearance of parallel
grooves. These horns are not deciduous, like
those of the stag : but continue to adhere to the
skin, and to grow from the root, in proportion as
they are worn at the extremity.
§ 6. Solipeda.
The Solipeda form a natural family of quadru-
peds, including the Horse, the Ass, the Quagga,
the Zebra, &c. which are very nearly allied in
their conformation to the ruminant tribe. To
combine fleetness with strength has been the
obvious design of nature in the construction of
these animals. We find, accordingly, that the
consolidation of the bones of the foot is carried
still farther than in the ruminant tribe ; for in
place of the two parallel phalanges, which are
in the latter articulated with the cannon bone,
MAMMALIA SOLIPEDA. 517
there is here only a single metatarsal bone. The
three phalanges, of which that single ringer con-
sists, bear the names of the pastern, the coronet,
and the coffin bone ; and the hoof, of course, is
single likewise ; there is also a small bone, con-
nected with the last, and called the shuttle bone.
To the cannon bone are joined, behind, and on
the side, two much shorter and very slender
bones, which are rudiments of the other metacar-
pal bones. They have been termed the styloid,
or splint bones ; and are generally united by ossi-
fication with the cannon bone. The scapula of
the horse is very narrow, and placed very nearly
in a straight line with the humerus ; which latter
bone is very short, and scarcely descends below
the line of the chest. The thigh-bone is also
unusually short. The muscles, which extend the
joint, and throw the thigh backwards in kicking,
are particularly powerful. This is the natural
defensive action of the horse ; and its force is in-
creased by a particular process with which the
bone is furnished, and which has the form of a
strong curved spine, situated on the outside, and
opposite to the lesser trochanter,* giving to the
muscles the advantage of a long lever. The cer-
vical vertebrae have only short spinous processes,
that they might not interfere with the motions of
* This process has been termed the processus rccurvatus fe-
moris.
518 THE MECHANICAL FUNCTIONS.
the neck. In the vertebrae of the back, on the
other hand, these processes are remarkably long,
especially at the part where the shoulder rests ;
their projection constituting what is called the
Withers.
§ 7. Pachydermata.
From the horse we pass by a natural transition
to the Pachydermata, a small group of animals
interesting by their peculiarities, and by their
being remnants of a very extensive tribe, which
formerly inhabited the earth, but have now almost
entirely disappeared. Although they feed upon
grass, they do not ruminate, nor are they cloven-
footed. They are for the most part huge and
unwieldy animals, with thick integuments, ren-
dered tough by a large mass of condensed cellu-
lar substance, which forms the chief defensive
armour of those that are destitute of either tusk,
proboscis, or nasal horn.
The most remarkable genus of this family is
the Elephant, the colossal giant of quadrupeds.
The many peculiarities which are observable in
the conformation of this animal have all an
obvious relation to the circumstances of its con-
dition. Formed for feeding on a great variety of
vegetable substances, and more especially on
the tender shoots of trees, fruits, and grains, as
MAMMALIA PACH YDERMATA. 519
well as on herbage, and succulent roots, its
organs of mastication are powerful, and its teeth
of great size. The whole of this apparatus re-
quires an immense developement of bone to
render it efficient ; so that the head, with its
huge tusks and grinders, is of enormous weight.
Had this ponderous head been suspended at the
end of a neck of such length as to admit of
its being carried to the ground, as is the case
in grazing animals, it would have destroyed the
balance of the body, and would have required
greater force to raise and retain it in a horizontal
position than could have been given by any
degree of muscular power. Nature has accord-
ingly abandoned this form of structure, and
has at once curtailed the neck, bringing the
head close to the trunk of the body, and sup-
porting it by means of short, but powerful
muscles, which are not implanted in any par-
ticular point of the skull, as they are in other
quadrupeds, where the occipital bone forms
a crest or ridge for that purpose ; but the
general surface of the cranium has been en-
larged by an immense expansion given to its
interior cellular structure, and thus the muscles
are attached to a considerable extent of bone,
instead of being affixed to a single process,
which would have incurred great risk of being
broken off by their action. These large cells
are constructed with a view to combine strength
520 THE MECHANICAL FUNCTIONS.
with lightness ; the plates which form their sides
being disposed in a radiated manner towards the
circumference, and arranged with great regu-
larity ; and the cells themselves, instead of con-
taining marrow, are filled with air, by means of
communications with the Eustachian tubes,
which open into the nostrils : thus a great
extent of surface is given to the skull, without
any addition to its weight. The ligamentum
nucha? also comes in aid of the muscular power,
being here of vast size and strength.
The head being limited in its range of motion
by its approximation to the trunk, the mouth
cannot be applied directly to seize the food : and
some means were therefore to be provided for
bringing the food to the mouth. For this pur-
pose a new organ, the proboscis, has been con-
structed : it consists of a cylinder, perfectly
flexible, and of a length sufficient to reach the
ground, when the elephant is standing. The
animal has the power of moving it in all possible
directions by means of a prodigious number of
muscular fibres, which are collected in small
bands, some passing transversely, and radiating
from the interior towards the circumference,
others situated more obliquely, and a third set
running longitudinally, and forming an exterior
layer ; but they are all variously interlaced
together so as to compose a very complicated
arrangement. The extremity of the proboscis,
MAMMALIA PACHYDERM ATA. 5W21
which is endowed with great sensibility, is fur-
nished with an appendix, resembling a finger,
most of the functions of which, indeed, it is
capable of performing.
For the formation of this admirable member
it has not been necessary to deviate from the
ordinary laws of developement by the creation
of a new organ ; the same end being accom-
plished by the extension of a structure already
belonging to the type of mammiferous animals.
In several of the pachydermata the nostrils are
already considerably advanced, so as to form a
moveable snout : this is observable in a certain
degree in the Hog ; it is still more remarkably
seen in the Tapir, which has a snout so length-
ened and so moveable as very much to re-
semble, though on a small scale, the proboscis
of the elephant. This latter organ, then, may
be considered as merely an elongation of the
nostrils, which have been drawn out to suit a
special purpose, very different from the function
to which that part is usually subservient.*
While fleetness and elasticity are the results
of the mechanical conformation of the horse,
solidity and strength are the objects chiefly
* A defective developement of the bones of the nasal cavity,
while the natural growth of the soft parts has continued, has
often, in the case of the human foetus, given rise to a monstrosity
very much resembling the trunk of the tapir or of the elephant
(See Geoffroy St. Hilaire.)
522 THE MECHANICAL FUNCTIONS.
aimed at in the construction of the Pachydermata.
The limbs have a great weight to sustain, in
consequence of the huge size of the body ; and
hence the several bones -which compose the
pillars for its support are arranged nearly in
vertical lines. The joints of the elbow and knee
are placed low from the body ; the ulna in the
fore legs, and the fibula in the hinder, are fully
developed, and are distinct from the radius and
the tibia. The number of the toes, instead of
being reduced to one. as in the horse, or to two,
as in ruminants, is here increased to five : though,
in consequence of their being very short, and
of the skin which covers and surrounds them
being very thick, they hardly appear exter-
nally, and are distinctly recognised only in the
skeleton .
It would carry me far beyond the limits of the
present work, were I to engage in a detailed
examination of all the varieties of forms and
structures which occur in the mechanism of the
different tribes of mammalia, m reference to the
purposes they are intended to serve, and to the
peculiar circumstances of the animal to which
they belong. I must necessarily pass over a
multitude of instances of express adaptation,
which are suited only to particular cases, and
are, consequently, of minor importance as regards
the general plans of organization. In the sort
of birds-eye view that I am taking of the end-
MAMMALIA RODENTIA. 523
less modifications of structure which have been
executed in conformity with those plans, I am
only able particularly to notice such as are most
remarkable.
§ 8. Rodent ia.
As the tribes of mammalia we have hitherto
examined employ the anterior extremities for
the purposes of progression only, they are desti-
tute of a clavicle. In most of those which follow,
and where a greater developement of the limb
confers more extensive and more varied powers
of motion, applicable to a greater range of objects,
this bone is found. In the greater number,
however, it is merely in a rudimental state; that
is, developed only to a certain extent, one portion
being bony, and the rest cartilaginous ; as if the
ossification had been arrested at an early stage.
These imperfect clavicles are too short to connect
the scapula with the sternum ; the rest of the
space being eked out by cartilage, and by liga-
ments : but still they are of great use in affording
points of attachment to the muscles of the limb,
and giving them the advantage of acting by a
rigid lever. The carnivorous tribes, which make
considerable use of their fore paws in striking
and seizing their prey, have clavicles of this
description. Those quadrupeds which have to
•524 THE MECHANICAL FUNCTIONS.
execute still more complex actions with their
fore feet, have perfect clavicles, extending from
the shoulder to the chest, and connecting the
bones of the anterior extremity with the general
frame-work of the skeleton. This is the case
in a large proportion of the family of Rodentia,
such as the Squirrel, which employs its paws
for holding objects ; and the Beaver, which
likewise makes great use of its fore feet, which
might almost be termed hands, in building its
habitation.* Animals that dwell in trees, and re-
quire to grasp with force the branches in moving
along them, such as the Sloth, have also distinct
clavicles. Animals which rake or dig the ground,
as the JSLole, the Ant-eater, and the Hedge-hog
are all provided with these bones, which, by
keeping the shoulders at the same constant dis-
tance from the trunk, and affording a firm axis
for the rotatory motions of the limb, materially
assist them in the performance of these actions.
* The beaver presents a singular modification in the structure
of the tail, which is expanded into a flattened oval disk, covered
by a skin beset with scales ; and which is used by the animal as
a paddle for supporting itself on the water, or for quickly diving
to the bottom. There does not appear to be any truth in the
opinion commonly entertained, that the Beaver employs its tail
as a trowel for plastering the mud walls of its dwelling.
INSECTIVOROUS MAMMALIA. 525
§ 9. Insectivora.
In the tribe of Insectivorous quadrupeds we meet
with several races which present singular con-
formations. In none are these anomalies more
remarkable than in the 3Iole, an animal which
nature has formed for subterranean residence,
and whose limbs are constructed with a view to
the rapid excavation of passages under ground.
The hands of the mole, for its fore paws almost
deserve that appellation, are turned upwards and
backwards for scooping the soil, while the feet
are employed to throw it out with great quick-
ness. These mining operations are aided by the
motions of the head, which is lifted with great
power, so as to loosen the ground above, and
overcome the resistances that may be opposed to
the progress of the animal. That no impediment
might be offered to these motions of the head,
the spinous processes of the cervical vertebrae
have not been suffered to extend upwards.
Large muscles are provided for bending the
head backwards upon the neck ; and they are
assisted by a cervical ligament of great strength,
which is generally in part ossified. The muscles
of the fore extremities are also of extraordinary
power. The scapula is a long and slender bone,
more resembling a humerus in its shape than an
526 THE MECHANICAL FUNCTIONS.
ordinary scapula : the humerus, on the contrary,
is thick and square, and the clavicle is short
and broad. The radius and the ulna are dis-
tinct from each other ; the hand is very large
and expanded ; the palms being turned out-
wards and backwards, and its lower margin
being fashioned into a sharp cutting edge. The
carpal bones and the phalanges of the fingers
are very much compressed ; but they are fur-
nished with large nails, which compose more
than half the hands ; and they are expressly
constructed for digging, being long, broad, and
sharp at the extremities. The sternum has a
large middle crest, and is prolonged at its ex-
tremity into a sharp process, having the figure
of a ploughshare, thus affording an extensive
surface of attachment for the large pectoral
muscles, from which the limb derives its prin-
cipal force. The head terminates in front by a
pointed nose, which is armed at its extremity
with a small bone, intended to assist in pene-
trating through the ground.
While all this attention has been paid to the
developement of the anterior part of the body
to which these instruments specially contrived
for burrowing are affixed, the hinder part is
comparatively feeble, and appears stinted in its
growth, and curtailed of its fair proportions.
The pelvis is exceedingly diminutive, being re-
duced to a slender sacrum ; and it is thrown far
INSECTIVOROUS MAMMALIA. 527
back from the abdomen, to which it could give
no effectual protection. Hence the animal,
when above ground, walks very awkwardly, and
is unable to advance but by an irregular and
vacillating pace.*
We have seen that there is a tribe of fishes
armed externally with sharp spines, which they
are capable of erecting when in danger of attack.
The Porcupine and the Hedgehog, which belong
to the family of insectivorous quadrupeds, are
furnished with a similar kind of defensive armour.
For the purpose of erecting these bristles, when
the animal is irritated or alarmed, there is pro-
vided a peculiar set of muscular bands, which
forms part of the usual subcutaneous layer,
termed the panniculitis carnosus. In the hedge-
hog these muscles are very complicated, and
give the animal the power of rolling itself into
a ball. A minute description of these muscles
has been given by Cuvier, who found that the
whole body is enveloped in a large muscular
bag, or mantle, lying immediately under the in-
teguments ; and capable, by the contraction of
different portions of its fibres, of carrying the
* The only quadrupeds which resemble the mole in the perfect
adaptation of their structure to the purposes of burrowing, are
the Wombat and the Koala, which are among the many extra-
ordinary animals inhabiting the continent of Australia. Their
hind legs are constructed in a manner very much resembling the
human fore-arm. (See Home, Lectures, &c. i. 134.)
528 THE MECHANICAL FUNCTIONS.
skin over a great extent of surface. In the usual
state of the animal, this broad muscle appears on
the back (as represented in Fig. 219), contracted
into a thick oval disk, of which the fibres are
much accumulated at the circumference. From
the edges of this disk there pass down auxiliary
muscles towards the lower parts of the body ; the
action of which muscles tends to draw the skin
downwards, and to coil it over the head and paws,
in the manner shown in Fig. 220, like the closing
of the mouth of a great bag.
§ 10. Carnivora.
The type of the Mammalia may be considered
as having attained its full developement in the
carnivorous tribes, which comprehend the larger
beasts of prey. As their food is animal, they
require a less complicated apparatus for digestion
than herbivorous quadrupeds, possess greater
activity and strength, and enjoy a greater range
of sensitive and intellectual faculties. In ac-
cordance with these conditions we may notice
CARNIVOROUS MAMMALIA. 529
the greater expansion of their brain, the supe-
rior acuteness of their senses, and their enor-
mous muscular power. The trunk of the body
is lighter than that of vegetable feeders, espe-
cially in the abdominal region, and is compressed
laterally : the spine is more pliant and elastic,*
the limbs have greater freedom of motion, the
extremities are more subdivided, and they are
armed with formidable weapons of offence and
destruction. Great mechanical power was re-
quired for raising the head, not only on account
of the force to be exerted in tearing flesh, but
also that these animals might be enabled to
carry away their prey in their mouths. Hence
we find that in the Lion, of which the skeleton
is represented in its relations to the outline of
the body, in Fig. 221, the first vertebra of the
neck, or atlas, has very widely expanded trans-
verse processes, while the second vertebra has a
largely developed spinous process, for supplying
levers for the muscles which have to perform
these and other actions in which the head is
concerned.
The whole of the remaining part of the ske-
leton of these animals is constructed with re-
ference to their predatory nature. The sudden
* The suppleness of the spine might at once be inferred, on
the simple inspection of the skeleton, from the circumstance that
the vertebrae of the neck and loins have a comparatively small
developement of their spinous processes.
VOL. I. M M
530 THE MECHANICAL FUNCTIONS.
springs with which they pounce upon their
prey must impart to the whole osseous frame the
most violent concussion. The first stroke with
which they attempt the destruction of their
victims is given with the fore leg : so that had
the limb been rigidly connected with the sternum
by means of an entire clavicle, its motions would
have been too limited, and danger of fracture
would have been incurred. The scapula is
broad, and the humerus of great length, com-
pared with the same bones in ruminants ; and
the latter has besides a large surface for its
articulation with the former of these bones,
thus allowing of a great range of motion : the
radius and ulna are perfectly distinct, and play
extensively on each other.
The fore feet rest on the ground by means of
the second of the three joints of which each toe
CARNIVOROUS MAMMALIA. 531
is composed. The last phalanges are raised at
right angles to the former, for the purpose of
supporting the claws in an erect position. It
has been considered of such importance to pre-
serve these formidable instruments constantly
sharp, and in a condition fitted for immediate
use, that an express contrivance has been re-
sorted to for this purpose. It consists in a
sheath, within which the claws, when not em-
ployed, are kept retracted, by means of an
elastic ligament, which constantly tends to with-
draw them within the sheath : and they are at
the same time so connected with the tendons of
the flexor muscles of the toes, that the moment
these muscles are thrown into action, which is
the case when the animal aims a stroke with its
paw, the claws are instantly drawn out, and
combine in inflicting the severest lacerations.*
Connected with the superior strength of the
hind extremities, we find the pelvis extending
farther backwards, and more in a perpendicular
line with the femur. This latter bone is longer
and more slender than in the horse, but it is
more compact in its form, and its processes are
more strongly developed : the fibula is a sepa-
rate bone from the tibia. The muscles, in
* There exists, concealed in the tuft of hair, at the extremity
of the lion's tail, a small conical and slightly curved claw, which
is attached to the skin only, and not to the last caudal vertebra :
its use is probably to increase the effect of blows given with the tail.
532 THE MECHANICAL FUNCTIONS.
general, are more divided into portions, and are
thus capable of greater diversity of action, at the
same time that they have greater power than
those of herbivorous quadrupeds. The articular
surfaces are of greater extent, and are lubricated
with a more copious supply of synovia ; their
ligaments are more delicate and more numerous ;
and the joints, in general, adapted to a greater
variety of movements. All these provisions are
evidently directed to confer great freedom and
facility of motion, and to enlarge the sphere of
action of the body generally, as well as of the
limbs.
§11. Quaclrumana.
We may trace in the series of quadrupeds which
have come under our review a gradual increase
in the developement of the hind feet ; beginning
from the horse, which is single hoofed, or
solipede; next to which rank the cloven-footed
ruminants, a tribe which includes the Camel,
whose foot is widely expanded for the purpose
of treading securely on sand ; then come the
Rhinoceros, which has three hoofed toes; the
Hippopotamus, which has four, and the Ele-
phant, which has five. To these succeed ano-
ther series, where nails, or claws, are substituted
for hoofs, as is the case with all the Caniivora,
which, standing on the extremities of their toes,
MAMMALIA QUADRUMANA. 533
have been termed Digitigrades. Then follow
the Plantigrade quadrupeds, such as the Bear,
the Badger, the Hedgehog, and the Mole, which
rest with the whole foot on the ground, and are
in consequence able to make great use of their
fore paws. These conduct us to the family of
the Quadrumana, comprehending the Monkey
and the Lemur tribes, which are characterised
by having the inner toe quite distinct from the
others, like the human thumb, and which appear,
therefore, as if they had four hands.
The Quadrumana present the nearest approxi-
mation to the human structure : they are natu-
rally inhabitants of the forest, and their confor-
mation is adapted to the actions of climbing upon
trees, of grasping the branches, and of springing
from the one to the other, with precision and
agility. It is here that they are at home ; it is
here that they gather the food which is most suited
to their nature ; it is here that they engage in
successful combats with serpents and other ene-
mies ; retaining their positions in perfect security
on the moving branches, or sportively swinging
by their extremities in the air. Both the feet
and the hands are formed for this species of pre-
hension ; and many are farther provided with a
strongly prehensile tail, which is an instrument
admirably adapted to all these purposes. Hence
the attitude most natural to these animals is
neither the horizontal one of quadrupeds, nor the
Q.'34 THE MECHANICAL FUNCTIONS.
erect posture of man, but an intermediate or
semi-erect position.
This view of the living habits of the quadru-
mana will afford the key to most of the peculia-
rities of structure they present to our observation.
The head, being no longer suspended at the end
of a horizontal, or recurved neck, is, in the usual
attitude of the animal, supported chiefly by the
cervical vertebra?. The greater developement of
the brain, and more especially of its posterior
lobes, creates a necessity for an extension of the
occipital bone in that direction ; a portion of the
weight to be sustained by the atlas is accordingly
thrown behind the centre of motion, which is at
its articulation with the latter bone; and this
weight tends, therefore, to balance that of the
anterior part of the head. Hence there is no
need of the strong cervical ligament, which is so
universally met with in quadrupeds ; and al-
though this ligament exists in the monkey, it is
very slender, and of no very great extent.
Great mobility has been conferred on the
spine by the form of its articulations ; and the
caudal vertebrae are generally greatly multiplied
to form a tail of considerable length, which in the
A teles, or spider monkey of America, is moved by
powerful muscles, and is an organ of great flexi-
bility and strength. Monkeys possess a distinct
clavicle, a lengthened humerus and femur, a
radius and ulna moveable upon each other, and
MAMMALIA QUADRUMANA. 535
a hand nearly approaching to the human con-
struction. But the thumb is less developed, and
its muscles are much weaker than in man.
The bones of the pelvis, as well as those of
the leg, are elongated, for the purpose of giving
greater length to the muscles which are to move
their several parts ; by this means, although the
force with which they act may be somewhat les-
sened, yet the velocity of the motion they pro-
duce is increased in the same proportion. The
fibula is here a bone of more importance than in
quadrupeds ; for it performs a motion of rotation
round the tibia, analogous to that of the radius
upon the ulna, giving a great extent of action to
the foot, and converting the leg into an arm, as
we have already seen that the foot itself is trans-
formed into a hand. A small inclination is given
to the articulation of the tarsus with these last
mentioned bones, which imparts a degree of
twist to the feet, throwing the sole inwards, and
causing the monkey while walking to rest chiefly
on its outer edge. This seeming defect gives a
slight appearance of awkwardness to the gait :
it is not, however, to be viewed as an imperfec-
tion ; for it is evidently designed to assist the
animal in climbing trees, which is its most usual
action ; the oblique position of the foot enabling
it most effectually to lay hold of the branches.
Monkeys are evidently not formed to excel in
swiftness ; for the heel, in these animals, presents
536 THE MECHANICAL FUNCTIONS.
no large projection, as in other orders of mam-
malia ; nor are the muscles which are inserted
into the heel particularly powerful : they hardly,
indeed, can be said to compose a calf as in the
human leg.
§ 12. Man.
The series of structures modelled on the charac-
teristic type of the Mammalia, after having ex-
hibited the successive developement of all its
elements, attains the highest perfection in the
human fabric : for even independently of those
prerogatives of intellect and of sensibility, by
which Man is so far exalted above the level of
the brute creation, both his physical structure
and his physiological constitution place him in-
contestably at the summit of the scale of terres-
trial beings. Considered zoologically, indeed,
the human species must rank among the Mam-
malia, and it even makes a near approach to
the Quadrumana ; yet there exist many peculia-
rities of structure, which entitle Man to be placed
in a separate order, where disclaiming any close
alliance with inferior creatures, he proudly
stands alone, towering far above them all.
It is not, however, on a pre-eminence in any
single physical quality or function that this title
THE HUMAN FRAME. 537
to superiority can be founded ; for in each of these
endowments man is excelled in turn by particu-
lar races of the lower animals ; but the chief per-
fection of his frame consists in its general adap-
tation to an incomparably greater variety of ob-
jects, and an infinitely more expanded sphere
of action . As the beauty of an edifice depends
not on the elaborate finishing of any one portion,
but results from the general suitableness of the
whole to the purposes for which it was constructed,
so the excellence of the human fabric is to be
estimated by the exquisite proportion and har-
mony subsisting among all its parts, and per-
vading the whole system of its functions. The
design of its structure and economy embraces
widely different, and far higher aims than those
contemplated in the organization of any of the
inferior animals. Destined to possess an intel-
lectual, a social, and a moral existence, Man has
had every part of his organization modified with
an express relation to these great objects of his
formation. This will best appear when we come
to examine the organs which are subservient to
the sensitive and active faculties ; but even
here, where our views must, for the present, be
limited to the mechanical circumstances of his
structure, the proofs are sufficiently numerous to
warrant this general conclusion.
Man presents the only instance among the
mammalia of a conformation by which the erect
538 THE MECHANICAL FUNCTIONS.
posture can be permanently maintained, and in
which the office of supporting the trunk of the
body is consigned exclusively to the lower extre-
mities, To this intention the form and arrange-
ment of all the parts of the osseous fabric, and
the position and adjustments of the organs of
sense have a well marked reference.* The
lower limbs are qualified to be the efficient in-
struments of progression by their greater length
and muscularity, compared with the generality
of quadrupeds. The only exceptions to this rule
occur in those mammalia which are constructed
expressly for leaping, such as the Kanguroo and
Jerboa, where, however, the hind legs are em-
ployed almost solely for that mode of progression.
The Quadrumana, which come nearer to the
human form than any of the other tribes, have the
lower limbs comparatively weak. In almost all
other quadrupeds the disproportion is still greater,
the thigh being short, and almost concealed by
the muscles of the trunk, and the remainder of
the limb being slender, and not surrounded by
any considerable mass of muscles.
* In most quadrupeds, as we have seen, the thorax is deep in
the direction from the sternum to the spine, but is compressed
laterally, for the evident purpose of bringing the fore limbs
nearer to each other, that they might more effectually support
the anterior part of the trunk. In Man, on the contrary, the tho-
rax is flattened anteriorly, and extends more in width than in
depth ; thus throwing out the shoulders, and allowing aii exten-
sive range of motion to the arms.
THE HUMAN FRAME. 539
The articular surfaces of the knee joint are
broader, and admit of greater extent of motion
in man than in quadrupeds : hence the leg can
be brought into the same line with the thigh,
and form with it a straight and firm column of
support to the trunk ; and the long neck of the
thigh bone allows of more complete rotation.
The widely spread basin of the pelvis effectually
sustains the weight of the digestive organs, and
they rest more particularly upon the broad ex-
pansion of the iliac bones ; in quadrupeds, these
bones, having no such weight to support, are
much narrower.
The base, on which the whole body is sup-
ported in the erect position, is constituted by the
toes, and by the heel, the bone of which projects
backwards at right angles to the leg. Between
these points the sole of the foot has a concavity
in two directions, the one longitudinal, the other
transverse, constituting a double arch. This
construction, besides conferring strength and
elasticity, provides room for the convenient pas-
sage of the tendons of the toes, which proceed
downwards from the larger muscles of the leg ;
and also for the lodgement of smaller muscles
affixed to each individual joint, and for the pro-
tection of the various nerves and blood vessels
distributed to all these parts. The concavity of
the foot adapts it, also, to retain a firmer hold of
the inequalities of the ground on which we
540 THE MECHANICAL FUNCTIONS.
tread. The muscles which raise the heel, and
which compose the calf of the leg, are of great
size and strength, and derive a considerable in-
crease of power from the projection of the bone
of the heel, into which their united tendons are
inserted. In all these respects the human struc-
ture possesses decided advantages over that of
the monkey, with reference to the specific objects
of its formation.
It is impossible to doubt that nature intended
man to assume the erect attitude, when we
advert to the mode in which the head is placed
on the spinal column. The enormous develope-
ment of the brain, and of the bones which invest
it, increases so considerably the weight of that
part of the head, which is situated behind its
articulation, with the vertebrae of the neck, that
the balance of the whole is much more equal
than it is in the monkey, where the weight of
the fore part greatly preponderates. The mus-
cles which bend the head back upon the neck,
and retain it in its natural position, are there-
fore not required to be so powerful as they must
be in quadrupeds, especially in those which
graze, and in which the mouth and eyes must
frequently be directed downwards, for the pur-
pose of procuring food. In man this attitude
would, if continued, be extremely fatiguing, from
the weakness of those muscles, and the absence
of that strong ligament which sustains the weight
THE HUMAN FRAME. 541
of the head in the ordinary horizontal attitude of
quadrupeds.
" Pronaque cum spectant animalia esetera terrain,
Os homini sublime dedit, cselumque tueri
Jussit, et erectos ad sidera tollere vultus." — Ovid.
The space comprehended by the two feet is
extremely narrow, when compared with the ex-
tended base on which the quadruped is sup-
ported : hence the stability of the body must be
considerably less. The statue of an elephant
placed upon a level surface, would stand without
danger of oversetting ; but the statue of a man
resting on the feet, in the usual attitude of
standing, would be thrown down by a very small
impulse. It is evident, indeed, that in the living
body, if the centre of gravity were at any mo-
ment to pass beyond the base, no muscular
effort which could then be made would avail to
prevent the body from falling. But the actions
of the muscles are continually exerted to prevent
the yielding of the joints under the weight of the
body, which tends to bend them. In quadrupeds
less exertion is requisite for that purpose ; and
standing is in them, as we have seen, a posture
of comparative repose : in man it requires nearly
as great an expenditure of muscular power as
the act of walking. Soldiers on parade expe-
rience more fatigue by remaining in the attitude
of standing, than they would by marching during
542 THE MECHANICAL FUNCTIONS.
an equal time. Strictly speaking, indeed, it is
impossible for even the strongest man to remain
on his legs, in precisely the same position, for
any considerable length of time. The muscles in
action soon become fatigued, and require to be
relieved by varying the points of support, so as to
bring other muscles into play. Hence the weight
of the body is transferred alternately from one foot
to the other. The action of standing consists, in
fact, of a series of small and imperceptible mo-
tions, by which the centre of gravity is perpe-
tually shifted from one part of the base to ano-
ther ; the tendency to fall to any one side being
quickly counteracted by an insensible movement
in a contrary direction. Long habit has rendered
us unconscious of these exertions, which we are,
nevertheless, continually making ; but a child
learning to walk finds it difficult to accomplish
them successfully. It is one among those arts
which he has to acquire, and which costs him in
the apprenticeship many painful efforts, and
many discouraging falls. But whenever nature
is the teacher, the scholar makes rapid progress
in learning ; and no sooner have the muscles
acquired the necessary strength, than the child
becomes an adept in balancing its body in various
attitudes, and in a very short time is unconscious
that these actions require exertion.
In walking, the first effort that is made consists
in transferring the whole weight of the body
PROGRESSIVE MOTION IN MAN. 543
upon one foot, with a view to fix it on the ground ;
and then the other foot, being at liberty, is
brought forwards. By this action the centre of
gravity is made to advance, till it passes beyond
the base of the foot : in this situation the body,
being unsupported, falls through a certain space,
and would continue its descent, were it not that
it is received on the other foot, which, by this
time, has been set upon the ground. This falling
of the body would, if not immediately checked,
become very sensible ; as happens when, on
walking inattentively, the foot we had advanced
comes down to a lower level than we were pre-
pared for ; in which case the body, having ac-
quired a certain velocity by its greater descent,
receives a sudden shock when that velocity is
checked, and thus a disagreeable jar is given to
the whole frame.
While the weight of the body is thus trans-
ferred alternately from one foot to the other, the
centre of gravity not only rises and falls, so as
to describe at every step a small arch, but also
vibrates from side to side, so that the series of
curves it describes are somewhat complicated in
their form. This undulation of the body from
one foot to the other would scarcely ever be per-
formed with perfect equality on both sides, if we
trusted wholly to the sensations communicated
by the muscles, and if we were not guided by
the sense of sight, or some other substitute. Thus
544 THE MECHANICAL FUNCTIONS.
a person blindfolded cannot walk far in a straight
line ; for, even on a level plane, he will incline
unconsciously either to the right or to the left.
In all quadrupeds, and even also in the quad-
rumana, the fore extremities more or less contri-
bute to the support and progression of the body :
it is only in man that they are wholly exempted
from these offices, and are at liberty to be applied
to other purposes, and employed as instruments
of prehension and of touch. In the power of
executing an infinite variety of movements and
of actions, requiring either strength, delicacy, or
precision, the human arm and hand, considered
in their mechanism alone, are structures of un-
rivalled excellence ; and, when viewed in rela-
tion to the intellectual energies to which they
are subservient, plainly reveal to us the divine
source, from which have emanated this exquisite
workmanship, and these admirable adjustments,
so fitted to excite in our breasts the deepest
veneration, and to fill us with never ceasing
wonder.
To specify all the details of express contrivance
in the mechanical conformation of the hand
would alone fill a separate treatise : but I must
refrain from pursuing this interesting subject, as,
fortunately, the task has devolved upon one far
more able than myself to do it justice.
>45
Chapter X.
VERTEBRATA CAPABLE OF FLYING.
§ 1 . Vertebrata without Feathers, formed for flying.
Few problems in mechanic art present greater
practical difficulties than that of raising from
the ground, and of sustaining and moving rapidly
through the air an animal body, composed as it
must be of many ponderous organs, which are
requisite for the performance of the higher func-
tions of life ; yet Nature has achieved all this,
not only in endless tribes of the more diminutive
invertebrate animals, but also in the more solid
and massive organizations which are modelled
on the vertebrate type. These objects have been
accomplished, in all cases, without the employ-
ment of any other than the ordinary elements of
those organizations ; modified, indeed, to suit the
particular purpose in view ; but yet essentially
the same, and regulated by the same laws of de-
velopement which prevail throughout the whole
animal system. The adaptation of these ele-
ments to the construction of an apparatus of so
refined a nature, as that which is required for
flying, implies the deepest foresight, the most
VOL. i. N n
5-46 THE MECHANICAL FUNCTIONS.
extensive plan, and the most artificial combina-
tion of means. The foundations for these pecu-
liar forms of mechanism are laid in the primeval
constitution of the embryo; and a long and curious
series of preparatory changes must take place
before the completion of the finished structures.
Of this we have already had a remarkable ex-
ample in the metamorphoses of insects, which
exhibit, in their last stage of developement, the
highest degree of perfection compatible with the
articulate type. Birds, in like manner, present
us with the highest refinement of mechanical
conformation, which can be attained by the de-
velopement of a vertebrated structure.
The power of flying is derived altogether from
the resistance which the air opposes to bodies
moving through it, or acting upon it by mecha-
nical impulse. In the ordinary movements of
our own bodies, this resistance is scarcely sen-
sible, and hardly ever attracts notice ; but it
increases in proportion to the surface which acts
upon the air, and still more according to the
velocity of the moving body ; for the increase is
not merely in the simple ratio of the velocity,
but as its square, or perhaps even a higher
power. In order that an animal may be able
to fly, therefore, two principal conditions are
required: there must, first, be a considerable
extent of surface in the wings, or instruments
which act upon the air ; and there must, se-
POWER OF FLYING. 547
condly, be sufficient muscular power to give
these instruments a very great velocity. Both
these advantages are found combined in the an-
terior extremities of birds, and no animals be-
longing to any other class possess them in the
same perfection. No quadruped, except the Bat,
has sufficient muscular power in its limbs, how-
ever aided by an expansion of surface, to strike
the air with the force requisite for flight. No
refinement of mechanic ingenuity has ever
placed the Daedalian art of flying within the
reach of human power ; for even if the lightest
possible wings could be so artificially adapted to
the body as to receive the full force of the ac-
tions of the limbs, however these actions might
be combined, they would fall very far short of
the exertion necessary for raising the body from
the ground.
Examples, however, occur in every one of
the classes of vertebrated animals, where an
approach is made to this faculty. In the Exo-
cetus, or flying-fish, the pectoral fins have been
enormously expanded, evidently for the purpose
of enabling the animal to leap out of the water,
and support itself for a short interval in the air ;
but its utmost efforts are inadequate to sustain
it beyond a few moments in that element, and
it can never rise to more than five or six feet
above the surface of the water.
A species of lizard, called the Draco Volaus,
548 THE MECHANICAL FUNCTIONS.
has a singularly constructed apparatus, which
appears like two wings, affixed to the sides of
the back, and quite independent of either the
fore, or the hind extremities. By the aid of
these moveable flaps, the animal is able to de-
scend from the tops of trees, or flutter lightly
from branch to branch ; but this is the utmost
that it can accomplish by means of these imper-
fect organs. The construction of these anoma-
lous members is highly curious in a physiolo-
gical point of view ; as showing how Nature, in
effecting a new purpose, is inclined to resort to
the modification of structures already established
as constituent parts of the frame, in preference
to creating new organs, or such as have no pro-
totype in the model of its formation. Frequent
proofs of this law, indeed, are afforded by the
comparative examination of the anatomy of the
organs of progressive motion. The ribs, in par-
ticular, are often the subject of these conversions
to uses very different from their ordinary func-
tion, which is that of assisting in respiration.
Thus we have seen that in the Tortoise they are
expanded to form the carapace ; uniting with
corresponding dilatations of the sternum, and
sterno-costal appendages, in composing a ge-
neral osseous encasement to the body. In Ser-
pents, again, the ribs are employed as organs of
progressive motion ; performing the functions of
legs, and having affixed to their extremities the
FLYING LIZARD. 549
abdominal scuta, by way of feet. The cervical
ribs of the Cobra de Capello, or hooded snake of
the East Indies, are employed for the mecha-
nical purpose of supporting an expansion of the
skin of the neck, which forms a kind of hood,
capable of being raised or depressed at the plea-
sure of the animal.* These ribs are entirely un-
connected with the respiration of the serpent.
In the Draco volans, which was to be fur-
nished with instruments for assisting it in its
distant leaps through the air, it is again the ribs
which are resorted to for furnishing the basis of
such an apparatus. On each side of the dorsal
vertebrae, as is seen in the skeleton of this animal
(Fig. 222), the eight posterior ribs on each side,
instead of having the usual curvature inwards,
and instead of being continued round to encircle
the body, are extended outwards and elongated,
and are covered with a thin cuticle, derived from
the common integuments. The ordinary muscles
which move the ribs still remain, but with
greatly increased power, and serve to flap these
strangely formed wings at the pleasure of the
animal, during its short aerial excursions.
Among the mammalia we meet with a few
species, which have a broad membrane, formed
of a duplicative of the skin, extended like a
cloak from the fore to the hind extremities, and
* Phil. Trans, for 1804, p. 346.
550
THE MECHANICAL FUNCTIONS.
enabling the animal to nutter in the air, and
to break its fall during its descent from the
222
branches of trees. Structures of this kind are
possessed by the Sciurus volans, or flying squir-
rel, and also by some other species of the same
genus. They are seen on a still larger scale in the
Lemur volans, or Galeopithecns. The resistance
which these broad expansions of skin oppose to
the air, when the limbs are spread out, enables
the animal to descend in perfect safety through
that medium from very considerable heights ;
but these appendages to the body are mere para-
chutes, not wings ; and none of the animals which
BAT.
551
possess them can, by their means, and with the
utmost efforts which their muscles are capable of
exerting, ever rise from the ground, or even sus-
pend themselves for a moment in the air.
The only quadruped that can properly be said
to be endowed with the power of flying is the
Bat. In this animal the portions of the skele-
ton (f, Fig. 223) which correspond to the pha-
langes of the fingers, are extended to an enor-
mous length ; and the pectoral muscles, which
move the anterior extremities, are of extraordi-
nary size and power. In the larger species,
each wing is at least two feet in length. The
fine membrane, which is spread between these
lengthened fingers, has its origin in the sides of
the neck, and reaches all along the body to the
extremities of the hinder legs, which it encloses
in its folds. Thus, not only is the surface, by
which it acts upon the air, sufficiently extensive,
but the muscular power, by which its motions
are effected, is adequate to give it those quick
552 THE MECHANICAL FUNCTIONS.
and sudden impulses which are requisite for
flying ; and thus, although its structure is to-
tally different from that of birds, it yet performs
fully the office of a real wing. The bat flies
with perfect ease, even while carrying along
with it one or two of its young : it is not,
however, fitted for very long flights.
The conformation of the skeleton is adapted to
this new and important function. The chest is
broad and capacious, to admit of free respiration
while the animal is flying, and to afford ample
space for the attachment of the large muscles
which have become necessary. The scapulae
(s) are large, and of a singular form, and they
are kept at a considerable distance asunder by
the expanded chest : their coracoid processes
are also large, and extend in the direction of the
sternum. The clavicle (c) are of enormous size
and length, being larger than either the scapula
or the sternum, and remarkably curved in their
shape. The sternum is much developed, ex-
tending laterally, and having a projecting crest
along the middle of its lower surface. The hu-
merus (h) is strong, but short; apparently in
order to avoid the danger of its being snapped
asunder by the violent actions of the pectoral
muscles, had it been longer. As the leading
object of the structure is to give power to the
wing, there was no necessity for the rotatory
motion of the bones of the fore-arm ; and ac-
BAT. 553
cordingly we find them consolidated into one (r) ;
or rather no part of the ulna is developed, except
the process of the olecranon, or elbow, which has
become soldered to the radius.
These advantages in the construction of the
fore extremities are obtained at the expense of
the hinder, which are too feeble to support the
weight of the body in the upright position
required for walking, in consequence of the
centre of gravity being between the wings. On
a level plane, indeed, the bat can advance only
by a kind of crawling or hopping motion. The
whole anterior half of the trunk is much more
fully developed than the posterior half, which
appears as if its growth had been arrested. The
pelvis (p) is of diminutive size, compared with
the rest of the skeleton : the pubic bones are
lengthened backwards, and are joined merely
at a small point. The whole posterior limb
is short, the femur (f ) comparatively long, and
the fibula is a very slender bone, yet quite
distinct from the tibia (t). The slight degree
of motion which is thus allowed between them
is useful to the animal, in enabling the feet to
lay hold of cornices, or other projecting parts
of the roofs of buildings, on which the animal
fastens itself, and hangs with the head down-
wards. It is probably with the intention of
facilitating this action that the toes are turned
completely backwards ; and that they are of
554 THE MECHANICAL FUNCTIONS.
a curved shape, and generally armed with sharp
claws. A bony appendix (a) projects outwards
from the heel, for the purpose of supporting the
hinder prolongation of the membrane, which
often extends between the hind feet, and is
farther sustained by the tail, in those species
which have the spine prolonged to form one.
Bats are also provided with another instru-
ment for suspending themselves to projecting
objects, formed by the thumb (b), which is,
apparently for this express purpose, detached
from the fingers that support the wing, and is
terminated by a strong claw, which projects,
even when the wings are folded, and is useful
in progression, by serving as a point of support.
§ 2. Birds.
It is in Birds alone that we find the most perfect
adaptation of structure to the purposes of rapid
and extensive flight : in them the frame of the
skeleton, the figure, position, and structure of
the wings, the size of the muscles, the pecu-
liar nature of their irritability, and even the
outward form of the body have all a direct and
beautiful relation to the properties of the element
in which Nature has intended them to move.
In their formation a new, and in as far as relates
BIRDS. 555
to the organs of progressive motion, a more de-
veloped type is adopted ; still preserving a con-
formity with the general plan of the vertebral
organization, and with the general laws of its
developement.
The skeleton of birds has the same constituent
parts as that of other vertebrated classes : the
bones of the anterior extremity, though destined
exclusively to support the wing, retain the same
divisions, and are composed of the usual ele-
ments ; and the general form of the body is that
best calculated to glide through the air with the
least resistance. As birds swallow their food
entire, there is no necessity for any part of the
bulky apparatus of hard and solid teeth, large
muscles and heavy jaws, which are required by
most quadrupeds : hence the head admits of
being greatly reduced in its dimensions ; and
the form of the beak, which is drawn to a point,
and cuts the opposing air, tends to facilitate the
progress of the bird in its flight.
In the conformation of the body, also, every
circumstance which could contribute to give it
lightness has been sedulously provided. The
general size of birds is considerably smaller
than quadrupeds of corresponding habits. No
where has Nature attempted to endow a huge
ponderous animal, like the fabled Pegasus,
with the power of flight. Great condensation
556 THE MECHANICAL FUNCTIONS.
has been given to the osseous substance,* in
order that the greatest degree of strength might
be procured with the same weight of solid ma-
terials ; and the mechanical advantage derived
from their being disposed in the circumference,
rather than in central masses, has been obtained
to the utmost extent. The horny material, of
which the stems of the feathers are constructed,
are, in like manner, formed into hollow cylinders,
which, compared with their weight, are exceed-
ingly strong. A similar shape has been given
to the cylindrical bones, which are fashioned
into tubes with dense but thin sides : most of the
other bones have likewise been made hollow ;
and instead of their cavities being filled with
marrow, they contain only air.| Thus the whole
skeleton is rendered remarkably light : that, for
instance, of the Pelicanus onocrotalus, or white
Pelican, which is five feet in length, was found
by the Parisian Academicians to weigh only
twenty -three ounces, while the entire bird
weighed nearly twenty-five pounds. The cavities
in the bones communicate with large air cells,
which are distributed in various parts of the body,
* Ossification not only proceeds more rapidly, but is also
carried to a greater extent in this class of animals than in any
other; as a proof of which, the tendons, especially those of the
muscles of the legs, are frequently ossified.
f In the Bat there is no provision of this kind for lightening
the bones; and we find them containing marrow, as in other
mammalia, and not air.
BIRDS. 557
and which contribute still farther to diminish its
specific gravity ; and by means of canals which
open into the air passages of the lungs, this air
finds a ready outlet when it becomes rarefied by
the ascent of the bird into the higher regions of
the atmosphere.*
The conditions in which birds are placed with
regard to the density of the surrounding me-
dium, as well as their mode of progression, are
so opposite to those of fishes, that we should
expect to find great corresponding differences in
their conformation . These two classes of verte-
brata, accordingly, are remarkably contrasted
* This air, being contained in the interior of the body, which
preserves a very elevated temperature, must be constantly in a
state of greater rarefaction than the cooler external air; a con-
dition which must contribute, in some slight degree, to render
the whole body lighter than it would otherwise have been. It
appears to me, however, that considerably greater importance
has been attached to this circumstance than it really possesses.
Many have gone so far as to represent the condition of a bird as
approaching to that of a balloon, filled with a lighter gas than
atmospheric air ; and have been lavish in their expressions of
admiration at the beauty of a contrivance, which thus con-
verted a living structure into an aerostatic machine. A little
sober consideration will suffice to show that the amount of the
supposed advantages resulting to the bird from the diminution
of weight, occasioned by the difference of temperature between
the air included in its body and the external atmosphere, is per-
fectly insignificant. Any one who will take the trouble to calcu-
late the real diminution of weight arising from this cause, under
the most favourable circumstances, will find that, even in the case
of the largest bird, it can never amount to more than a few
grains.
5-58 THE MECHANICAL FUNCTIONS.
with respect to the structure of their skeletons.
In fishes we have seen that the chest and all the
viscera are carried as far forwards as possible ;
the respiratory organs and the centre of circula-
tion being close to the head, the neck having
disappeared, and the trunk being continued into
the lengthened tail, in which the chief bulk of
the muscles are situated. In birds, on the con-
trary, the ribs, and the viscera which they pro-
tect, are placed as far back along the spinal
column as possible ; and a long and flexible neck
extends from the trunk to the head, which is
thus carried considerably forwards. These cir-
cumstances are very apparent in the skeleton of
the Swan, represented in Fig. 224. In a fish,
progressive motion is effected principally by the
movements of the tail, which impels the body
alternately from side to side : in a bird, the only
instruments of motion are the wings, which are
affixed to the fore part of the trunk, and are
moved by muscles situated in that region. In
the fish, the spine is flexible nearly throughout
its whole extent ; in the bird, it is rigid and im-
moveable in the trunk, and is capable of exten-
sive motion only in the neck.
In order that the body may be exactly ba-
lanced while the bird is flying, its centre of
gravity must be brought precisely under the line
connecting the articulations of the wings with
the trunk, for it is at these points that the re-
BIRDS. 559
sistance of the air causes it to be supported by
the wings. When the bird is resting on its legs,
the centre of gravity must, in like manner, be
brought immediately over the base of support
formed by the toes : it becomes necessary, there-
fore, to provide means for shifting the centre of
gravity from one place to another, according to
5G0 THE MECHANICAL FUNCTIONS.
circumstances, and to adjust its position with
considerable nicety ; otherwise there would be
danger of the equilibrium being destroyed, and
the body oversetting. The principal means of
effecting these adjustments consist in the mo-
tions of the head and neck ; which last is, for
that purpose, rendered exceedingly long and
flexible. The number of cervical vertebrae is
generally very considerable ; in the mammalia,
as we have seen, there are always seven, but in
many birds there are more than twice that num-
ber. In the swan (Fig. 224), there are twenty-
three ; and they are joined together by articu-
lations, generally allowing free motion in all di-
rections ; that is, laterally, as well as forwards
and backwards. This unusual degree of mobi-
lity is conferred by a peculiar mechanism,
which is not met with in the other classes of
vertebrated animals. A cartilage is interposed
between each of the vertebras, to the surfaces of
which these cartilages are curiously adapted,
being enclosed between folds of the membrane
lining the joint ; so that each joint is in reality
double, consisting of two cavities, with an inter-
vening cartilage.*
It is to be observed, however, that in conse-
quence of the positions of the oblique processes,
* See Mr. H. Earle's paper on this subject in the Philoso-
phical Transactions for 1823, p. 277.
BIRDS. 561
the upper vertebrae of the neck bend with more
facility forwards than backwards ; while those
in the lower half of the neck bend more readily
backwards : hence, in a state of repose, the
neck naturally assumes a double curvature, like
that of the letter S, as is well seen in the graceful
form of the swan's neck. By extending the
neck in a straight line, the bird can, while flying,
carry forwards the centre of gravity, so as to
bring it under the wings ; and when resting on its
feet, or floating on the water, it can transfer that
centre backwards, so as to bring it towards the
middle of the body, by merely bending back the
neck into the curved form which has just been
described ; and thus the equilibrium is, under
all circumstances, preserved by movements re-
markable for their elegance and grace.
The great mobility of the neck enables the
bird to employ its beak as an organ of prehen-
sion for taking its food ; an object which was
the more necessary, in consequence of the con-
version of the fore extremities into wings, of
which the structure is incompatible with any
prehensile power, such as is often possessed by
the anterior extremity of a quadruped. Ano-
ther advantage arising from the length and
mobility of the neck is, that it facilitates the ap-
plication of the head to every part of the sur-
face of the body. Birds require this power in
order that they may be enabled to adjust their
vol. i. o o
5(32 THE MECHANICAL FUNCTIONS.
plumage, whenever it has by any accident be-
come ruffled. In aquatic birds, it is necessary
that every feather should be constantly anointed
with an oily secretion, which preserves it from
being wetted, and which is copiously provided
for that purpose by glands situated near the tail.
The flexibility of the neck alone would have
been insufficient for enabling the bird to bring
its bill in contact with every feather, in order to
distribute this fluid equally over them ; and
there is, accordingly, a further provision made for
the accomplishment of this object in the mode of
articulation of the head with the neck. We have
seen that in fishes, and in most reptiles, this
articulation consists of a ball and socket joint ; a
rounded tubercle of the occipital bone being re-
ceived into a hemispherical depression in the
first vertebra of the neck. In the mammalia the
plan is changed, and there are two articular
surfaces, one on each side of the spinal canal,
formed on processes corresponding to the leaves
of the first cranial vertebra, and assimilating it
more to a hinge joint. In birds, however, where,
as we have just seen, the most extensive lateral
motions are required, the plan of the ball and
socket joint is again resorted to; and the occi-
pital bone is made to turn upon the atlas by a
single pivot. So great is the freedom of motion in
this joint, that the bird can readily turn its head
completely back upon its neck, on either side.
As spinous, or transverse processes of any length
IURDS.
503
would have interfered with the flexions of the
neck, we find scarcely a trace of these processes
in the cervical vertebrae of birds. But another,
and a still more important consideration was to
be attended to in the construction of this part of
the spine. It must be recollected that the spinal
marrow passes down along the canal formed by
the arches of the vertebrae, and that any pressure
applied to its tender substance would instantly
paralyze the whole body, and speedily put an end
to life. Some extraordinary provision was there-
fore required to be made, in order to guard against
the possibility of this accident occurring during
the many violent contortions into which the
column is liable to be thrown. This is accom-
plished in the simplest and most effectual manner
by enlarging the diameter of the canal at the
upper and lower part of each vertebra, while at the
middle it remains of the
usual size ; so that the
shape of the cavity, as
is well seen in Fig. 225,
which shows a vertical
section of one of the
cervical vertebras of the
Ostrich, resembles that
of an hourglass.* Thus
a wide space is left at
the junction of each successive vertebra, allowing
* For the specimen from which this engraving was made, I
am indebted to the kindness of Mr. Owen.
225
564 THE MECHANICAL FUNCTIONS.
of very considerable flexion, without reducing
the diameter of the canal beyond that of the
narrow portion, and therefore without producing
compression of the spinal marrow. Mr. Earle
foundf that vertebrae united in this manner may
be bent backwards to a right angle, and laterally
to half a right angle, without injury to the en-
closed nervous substance. The design of this
structure is farther evident from its not existing
in the dorsal and lumbar portions of the spine,
which admit of no motion whatever, and where
there is no variation in the diameter of the spinal
canal.
A plan entirely different is followed in the
vertebrae of the back and loins. For the purpose
of ensuring the proper actions of the wings, the
great object here is to prevent motion, and to
give all possible strength and security ; and ac-
cordingly the whole of this portion of the spine,
together with the sacrum, is consolidated into
one piece. All the processes are largely deve-
loped, and pass obliquely from one vertebra to
the next, mutually locking them together; and
in order most effectually to preclude the possi-
bility of any flexion, the spinous processes, and
sometimes even the bodies of the dorsal vertebrae
are immoveably soldered together by ossific mat-
ter, so as to form one continuous bone.
The sacrum (v, Fig. 224) consists of the union
of a great number of vertebrae ; as many as twenty
f In the paper already quoted, p. 278.
BIRDS. 565
being anchylosed together for this purpose ; so
that they form a bone of great length. The
coccygeal vertebrae (q) are also numerous ; but
are compressed into a small space, and enjoy
great latitude of motion,^ being subservient to the
movements of the tail.
The ribs are numerous, and of considerable
strength : they send out processes, which are
directed backwards, passing over the next rib
before they terminate, and giving very effectual
support to the walls of the chest. The ribs are
continued along the abdomen, and afford pro-
tection to the viscera in that cavity ; and some
arise even from the sacrum, and from the iliac
bones. Those which are in front are united to
the sternum (s) by means of sternal appendices,
which are ossified, and appear as the continua-
tions of the ribs, or as if the ribs were jointed
in the middle.
The sternum is of enormous size, extending
over a considerable part of the abdomen, and
having a large perpendicular crest descending,
like the keel of a ship, from its lower surface.
The object of this great developement is to fur-
nish extensive attachment to the large pectoral
muscles employed to move the wings, and which,
taken together, are generally heavier than the
rest of the body. Considered with reference to
all the other muscles, and to the weight of the
body itself, these pectoral muscles are of enor-
mous strength. The flap of a swan's wing is
566 THE MECHANICAL FUNCTIONS.
capable of breaking a man's leg ; and a similar
blow from an eagle has been known to be
instantly fatal. The bat is the only instance,
among the mammalia, where the sternum pre-
sents this peculiar caiinated, or keel -like
shape ; and the purpose is evidently the same
as in the bird.*
The scapula is generally a small and slender
bone. The coracoid bone (k) is largely deve-
loped, and assumes much of the appearance of
a clavicle. t But the real clavicles (c) are united
below, where they join the fore part of the
sternum, appearing as one bone, which, from its
forked shape, has been denominated the jktrcular
bone. In the fowl, it is commonly known by the
name of the merry -thought. This bone, placed
at the origin of the wings, and stretching from
the one to the other, is of great importance as
constituting a firm basis for their support, and
for securing their steadiness of action ; and be-
ing, at the same time, very elastic, it tends to
restore them to their proper situations, after they
have been disturbed by any violent impulse.
* Notwithstanding the great modification which the sternum
has received in the bird, when compared with its form in the
tortoise and the quadruped, we may still trace the same nine ele-
ments entering into its composition, though developed in very
different proportions.
f Many have considered this bone as being the clavicle, and
have regarded the furcular bone as a new bone, or supplemen-
tary clavicle ; but all the analogies of position and of develope-
ment are in favour of the views stated in the text.
WING OF BIRDS. 567
The wing of a bird does not, at first view,
present much analogy with the fore extremity of
a quadruped ; but, on a closer examination, we
find it to contain all the principal bones of the
latter, though somewhat altered in shape, and
still more changed in their functions. Yet still
the same unity of plan, and perfect harmony of
execution may be discerned in the mechanism
of this refined instrument of a higher mode of
progression.
The head of the humerus (h) has a compressed
form ; and in order to obtain great extent of
motion, it is made to play by a very small
cylindrical surface upon the scapula ; thus ad-
mitting of the complete descent of the wing,
unobstructed by any opposing process, but at
the same time limiting its motion to one plane.
It is connected below, by broad attachments, to
the radius and ulna ; forming with them a hinge
joint. These latter bones are separate, and of
great length, but so firmly united together by
ligament as scarcely to have any motion on one
another. The carpus (w) consists of two bones
only, the one articulated with the radius, the
other with the ulna. They move together as one
piece ; but, contrary to what takes place in
quadrupeds, the movements are made from side
to side, instead of their consisting of flexion and
extension ; this variation from the usual struc-
ture being for the purpose of folding down the
joints of the wing, and bringing them close to
o(J8 THE MECHANICAL FUNCTIONS.
the body. The metacarpus (m) consists origi-
nally of two bones, which soon become united
into one at the upper part. On the radial side
it has a process, derived perhaps from a third
metacarpal bone, which is anchylosed at a still
earlier period of ossification ; and to this process
a small pointed bone is connected, correspond-
ing to a rudimental thumb (t). There are gene-
rally two fingers, of which the first exhibits
traces of having been originally two bones : the
inner finger consists of two or three long pha-
langes, and the outer one of a single phalanx :
there is sometimes also a rudimental bone cor-
responding to a little finger. The degree of
developement of these bones varies in different
tribes of birds.
Feathers are attached to all these divisions of
the limb, namely, to the humerus, the fore arm,
the hand, and occasionally to the single phalanx
of the thumb. The structure of feathers is
calculated in an eminent degree to combine the
qualities of lightness and of strength, which we
elsewhere rarely find united. The horny mate-
rials of which the stem of the quill is made are
tough, pliant, and elastic ; and, as we have
already seen, are disposed in the most advan-
tageous manner for resisting flexion by being
formed into a hollow cylinder. But the vane of
the feather is still more artificially constructed ;
being composed of a number of flat threads, or
FEATHERS OF BIRDS. 509
filaments, so arranged as to oppose a much
greater resistance to a force striking perpen-
dicularly against their surface, than to one
which is directed laterally ; that is, in the plane
of the stem. They derive this power of resist-
ance from their flattened shape, which allows
them to bend less easily in the direction of their
flat surfaces than in any other ; in the same
way that a slip of card cannot easily be bent by
a force acting in its own plane, though it easily
yields to one at right angles to it. Now it is
exactly in the direction in which they do not
bend that the filaments of the feather have to
encounter the resistance and impulse of the air.
It is here that strength is wanted, and it is here
that strength has been bestowed.
On examining the assemblage of these lami-
nated filaments still more minutely, we find that
they appear to adhere to one another. As we
cannot perceive that they are united by any
glutinous matter, it is evident that their con-
nexion must be effected by some mechanism
invisible to the unassisted eye. By the aid of
the microscope the mystery is unravelled, and
we discover the presence of a number of minute
fibrils, arranged along the margin of the laminae,
and fitted to catch upon and clasp one another,
whenever the laminaB are brought within a cer-
tain distance. The fibrils of a feather from the
wing of a goose are represented magnified at
570
THE MECHANICAL FUNCTIONS.
a, a, b, b, Fig. 226, as they arise from the two
sides of the edges of each lamina. They are
exceedingly numerous, above a thousand being
contained in the space of an inch ; and they are
of two kinds, each kind having a different form
and curvature. Those marked a, a, which arise
from the side next to the extremity of the feather,
are branched or tufted, and bend downwards ;
while those marked b, b, proceeding from the
other side of the lamina, or that nearest the root
of the feather, are shorter and firmer, and do
not divide into branches, but are hooked at the
extremities, and are directed upwards. When
the two laminae are brought close to one another,
the long, curved fibrils of the one being carried
over the short and straight fibrils of the other,
both sets become entangled together ; their
crooked ends fastening into one another, just as
the latch of a door falls into the cavity of the
catch, which is fixed in the door-post to receive
FEATHERS OF BIRDS. 571
it. The way in which this takes place will be
readily perceived by making a section of the
vane of a feather across the laminae, and exa-
mining with a good microscope their cut edges,
while they are gently separated from one ano-
ther. The appearance they then present is
exhibited in Fig. 227, which shows distinctly
the form, direction, and relative positions of each
set of fibrils, and the manner in which they lay
hold of one another. This mechanism is re-
peated over every part of the feather, and con-
stitutes a closely reticulated surface of great
extent, admirably calculated to prevent the pas-
sage of the air through it, and to create by its
motion that degree of resistance which it is in-
tended the wing should encounter.* In feathers
not intended for flight, as in those of the ostrich,
the fibrils are altogether wanting : in those of
the peacock's tail, the fibrils, though large, have
not the construction which fits them for clasping
those of the contiguous lamina ; and in other
instances they do so very imperfectly.
A construction so refined and artificial as the
one I have been describing, and so perfectly
* A very clear account of the mechanism described in the
text is given by Paley, in the 12th chapter of his " Natural
Theology." Many of the minuter details I have supplied from
my own observations with the microscope. The branched form
of the upper fibrils, and the reticulated structure of the laminae
themselves, when viewed with a high magnifying power, are
particularly beautiful microscopic objects.
572 THE MECHANICAL FUNCTIONS.
adapted to the mechanical object which it is to
answer, cannot be contemplated without the
deepest feeling of admiration, and without the
most eager curiosity to gain an insight into the
elaborate processes, which, we cannot doubt, are
employed by nature in the formation of a fabric
so highly finished, and displaying such minute
and curious workmanship. It is only very re-
cently that we have been admitted to a close
inspection of the complicated machinery, which
is put in action in this branch of what may be
called organic architecture; and certainly none
is more fitted to call forth our profoundest wonder
at the comprehensiveness of the vast scheme of
divine providence, which extends its care equally
to the perfect construction of the minutest and
apparently most insignificant portions of the
organized frame, whether it be the down of a
thistle, the scales of a moth, or the fibrils of a
feather, as well as to the completion of the larger
and more important organs of vitality.
Every bird, on quitting the egg, is found to
be covered on all parts, except the under side,
with a kind of down, consisting of minute fila-
ments, collected in tufts, and resembling in their
arrangement the fibres of a camel-hair pencil.
Each tuft contains about ten or twelve filaments,
growing from the upper ends of bulbous roots
implanted in the skin, and which are the rudi-
ments of the organs that afterwards form the
FEATHERS OF BIRDS. 573
feathers, of which this down, serving the purpose
of a first garment, hastily spread over the young
bird, is but the precursor ; for the tufts generally
soon fall off and disappear, except in the rapa-
cious tribes, as the Eagle and the Vulture, where
they remain attached to the feathers for a consi-
derable time.
While this temporary protection is given to
the integument, extensive preparations are mak-
ing underneath for furnishing a more effective
raiment, adapted to the future wants of the bird.
The apparatus by which the feathers are to be
formed is gradually constructing ; and its rudi-
ments are receiving the necessary supply of
nutrient juices, and of vessels for their circula-
tion, together with their usual complement of
nerves and absorbents. When first visible, this
organ has the form of a very minute cone,
attached by a filament proceeding from its base
to one of the papillae of the skin, and establishing
its connexion with the living system. In the
course of a few days, this cone has become elon-
gated into a cylinder, with a pointed extremity,
while its base is united to the skin by a more
distinct bond of connexion, formed by the en-
larged vessels, which are supplying it with nour-
ishment. It is in the interior of this cylinder
that all the parts of the feather are constructed ;
their earliest rudiments being formed at the upper
part, or apex of this organ ; and the materials
074 THE MECHANICAL FUNCTIONS.
of the several parts of the feather being succes-
sively deposited and fashioned into their proper
shapes in different places : for while the first
laminae are constructing in one portion of the
cylinder, the next are only just beginning to be
formed in another; and while the outer covering
of the stem is growing from one membrane, the
interior spongy tissue is deposited in other places,
in various stages of softness or consolidation : so
that the whole composes a system of operations,
which may be said to resemble in its complication
at least, although on a microscopic scale, an ex-
tensive manufactory. Hence will be readily
understood how great must be the difficulty of
tracing all the steps of these multifarious pro-
cesses, which are carried on in so small a space :
and this difficulty is much increased from the
circumstance that the organ in which they take
place is itself only developed as the work pro-
ceeds ; its different parts being produced succes-
sively in proportion as they are wanted, and their
form and structure undergoing frequent variation
in the course of their developement.
The most elaborate, and apparently accurate
researches on this intricate subject, are those
lately undertaken by M. Frederick Cuvier, from
whose memoir* I have selected the following
abridged statement of the principal results of his
* Memoires du Museum, xiii. 327 ; and Annales des Sciences
Naturelles, ix. 113.
FEATHERS OF BIRDS.
575
observations. It will be necessary in order to
obtain a clear idea of the several steps of the
process to be described, to advert to the structure
of a feather in its finished state. For this pur-
pose we need only examine a common feather,
such as that represented in Fig. 228, where s is
228
229
230
231.
the posterior surface of the solid stem, which, it
will be perceived, is divided into two parts by a
longitudinal groove, and from either side of which
proceed a series of laminae, composing, with their
fibrils, what is termed the vane of the feather
(v). The lines from which these laminae arise,
576 THE MECHANICAL FUNCTIONS.
approach one another at the lower part of the
stem, till they meet at a point, where the longi-
tudinal groove terminates, and where there is a
small orifice (o), leading to the interior of the
quill. From this part the transparent tubular
portion of the quill (t) commences ; and at its
lower extremity (l) there exists a second, or lower
orifice.
The entire organ which forms the feather, and
which may be termed its matrix, is represented
in Fig. 229, when it has attained the cylindric
form already described ; of which a is the apex,
or conical part that rises above the cuticle, and
b the base, by which it is attached to the corium,
or true skin. A white line is seen running lon-
gitudinally the whole length of the cylinder, and
another, exactly similar to it, is met with on the
opposite side : the one corresponds in situation
to the front, and the other to the back of the
stem of the future feather. On laying open the
matrix longitudinally, as is shown in Fig. 230, it
is found to be composed of a sheath or capsule,
and of a central pulpy mass, termed the bulb.
The capsule consists of several membranous
layers (c, e, s, i), which are more consolidated
near the apex, and become gradually softer and
more delicate, as we trace them towards the base
of the matrix, where their formation is only be-
ginning to take place.
The laminae and their fibrils, the assemblage
FEATHERS OF BIRDS. 577
of which constitutes the vane of the feather, are
the parts which are first formed ; and their con-
struction is effected in the space between the
outer capsule (c), and the central bulb (b), in a
mode which is exceedingly remarkable, and
different from that of the formation of any other
organic product with which we are acquainted.
Instead of growing from a base, like hairs, and
other productions of the integuments, by suc-
cessive depositions of layers, the materials which
are to compose the laminae are cast in moulds,
where they harden, and acquire the exact shape
of the recipient cavities. The next object of our
curiosity, then, is to learn the way in which
these moulds are constructed ; and on careful
examination they appear to be formed by two
striated membranes, the exterior one (e) enve-
loping the other (i), or interior membrane.
These membranes are separated by a series of
partitions, which commence at the edges of the
longitudinal white band, seen in Fig. 229, and
wind obliquely upwards till they reach the
opposite longitudinal band already described,
where they join a longitudinal partition which
occupies a line answering to that posterior band.
Thus they leave between them narrow spaces,
which constitute so many compartments for the
deposition, as in a mould, of the material of
each lamina. The course of these channels,
and their junction at the back of the matrix is
vol. i. p p
578 THE MECHANICAL FUNCTIONS.
seen at s, Fig. 230. It is exceedingly probable,
though from the minuteness of the parts it is
scarcely possible to obtain ocular demonstration
of the fact, that the fibrils of the laminoe are
constructed in a similar manner, by being
moulded in still more minute compartments,
formed by transverse membranous partitions.
The proper office of the bulb, after it has sup-
plied the materials for the formation of the
lamina?, is to construct the stem of the feather,
and unite the laminae to its sides. For this pur-
pose the anterior portion of the bulb deposits on
its surface a plate of horny substance, while
another plate is formed by the posterior part in
the interior of the bulb. Thus the bulb becomes
divided into two portions ; one anterior, and the
other posterior. The former of these, after
having finished the external plate, proceeds to
form the spongy substance, which is to connect
the two plates, and the posterior portion of the
bulb embraces the inner plate, and gradually
folds it inwards till its sides meet at the middle
groove along the back of the stem. The anterior
part of the bulb, during the process of filling up
the stem, exhibits a series of conical shaped
membranes, as is seen in the section, Fig. 231 ;
the points of the cones being directed upwards,
and their intervals being occupied by the spongy
substance in different stages of consolidation,
and more perfected in proportion as they are
situated nearer the apex of the stem.
FEATHERS OF BIRDS. f>79
While the construction of the feather, in its
different stages, is thus advancing from below,
those parts which are completely formed are
rising above the surface of the skin, still enve-
loped in the capsule which originally protected
them, but the upper portions of which, from the
action of the air, and the obliteration of the
vessels that nourished them, now decaying,
shrivel and fall off in shreds, allowing the
successive portions of the feather to come forth,
and the laminae to unfold themselves as they rise
and assume their proper shapes. This succes-
sive evolution proceeds until the principal parts
of the stem and of the vane are completed ; and
then a different kind of action takes place. The
posterior part of the bulb now contracts itself,
and bringing the edges of that surface of the
stem closer together, at length unites them at the
superior orifice (o), Fig. 228 ; where the 1 ami nee,
which follow these lines, also terminate. Having
thus performed the office assigned to it, it
ceases to be nourished, and is incapable any
longer of depositing a horny covering to the
feather : all that remains of its substance is a
thin membrane, which adheres to the outside of
the tubular part, or barrel of the quill, and which
must be scraped off before the latter can be
used as a pen. The tubular part is the product
of the anterior part of the bulb, which now
ceases to deposit the spongy substance, but
forms a transparent horny material over the
580 THE MECHANICAL FUNCTIONS.
whole of its external surface ; but as it retires
towards the root, it leaves a succession of very
thin pellucid membranes, in the form of cones,
which, when dried, form what is termed the pith
of the quill. The last remnant of the bulb is
seen in the slender ligament which passes
through the lower orifice, and preserves the
attachment of the feather to the skin. In pro-
cess of time, this also decays, and the whole
feather is cast off, preparatory to the formation
of another, which in due season is to replace it.
All the feathers are, in general, moulted annu-
ally, or even at shorter periods; and the same
complicated process is again begun and com-
pleted, by a new matrix produced for the occa-
sion, every time a new feather is to be formed.
It is impossible, on reviewing these curious
facts, not to be struck with the admirable art and
foresight, which are implied in all this long and
complicated series of operations. While the
bird was yet nourished by the fluids of the egg,
the ground had already been prepared for its
future plumage, and for the formation of in-
struments of flight. A temporary investment of
down is in readiness to shelter the tender chicken
from the rude impressions of the air, and an
apparatus is preparing for the construction of
the most refined instruments for clothing and
for motion : first the scaffolding, as it may be
called, is erected, by the help of which each por-
WING OF BIRDS. 581
tion is built up in succession, and in proper
order. Nature's next care is to construct the
vane, which is the part of the feather most es-
sential to its office : and then to form the shaft,
to which the vane is to be affixed, and from
which it receives its support : lastly, she forms
the barrel of the quill, which is prolonged for the
purpose of converting it into a lever of sufficient
length for the mechanical office it has to perform.
In proportion as each structure is finished, she
neglects not to remove the scaffolding which had
been set up as a temporary structure ; the mem-
branes, with all their partitions, are carried away,
the vascular pulp of the bulb is absorbed, and
its place supplied by air, thus securing the
utmost lightness, without any diminution of
strength. Is it possible for any rational mind,
after meditating upon these facts, to arrive at the
persuasion that they are all the mere results of
chance ?
Several circumstances remain to be noticed
respecting the structure and actions of the wings
of birds. If we attend to the mode of their arti-
culation with the scapula, we find it producing
a motion oblique with regard to the axis of the
body, so that the stroke wdiich they give to the
air is directed both downwards and backwards ;
and the bird, while moving forwards, is at the
same time supported in opposition to the force of
58*2 THE MECHANICAL FUNCTIONS.
gravity. The different portions of the wing are
likewise so disposed as to be contracted and
folded together when the wing is drawn up, but
fully expanded when it descends in order to
strike the air. It is obvious that, without this
provision, a great part of the motion acquired by
the resistance of the air against the wing in its
descent would have been lost by a counteracting
resistance during its ascent. The disposition of
the great feathers is such that they strike the air
with their flat sides, but present only their edges
in rising : what is called feathering the oar in
rowing is a similar operation, performed with the
same intention, and deriving its name from this
resemblance.
As the inclination of the wing is chiefly back-
wards, the greatest part of the effect produced
by its action is to move the body forwards.
Birds of prey have a great obliquity of wing, and
are consequently better formed for horizontal
progressive motion, which is what they chiefly
practise in pursuing their prey, than for a rapid
perpendicular ascent. Those birds, on the con-
trary, which rise to great heights, in a direction
nearly vertical, such as the Quail and the Lark,
have the wings so disposed as to strike directly
downwards, without any obliquity whatsoever.
For the same reason, birds rise better against
the wind, which, acting upon the oblique surface
presented by the wings during their flexion, contri-
FLIGHT OF BIRDS. 583
butes to the ascent of the body, on the same
principle that a kite is carried up into the air
when retained in an oblique position. This cir-
cumstance is particularly observable in the ascent
of birds of prey, whose wings have a great obli-
quity, and, when fully expanded, present a very
large extent of surface.
The actions of the tail, which operates as a
rudder, are useful chiefly in directing the flight.
When the tail is short, this office is supplied by
the legs, which are in that case generally very
long ; and being raised high and extended back-
wards in a straight line, are of considerable
assistance in the steerage of the animal. In
many birds, as in the Wood-pecker, the tail is
much employed as a support to the body in
climbing trees. The caudal vertebra? are often
numerous, but are short and compressed toge-
ther : they are remarkable for the great deve-
lopement of their transverse processes, and for
having spinous processes both on their lower
and upper sides. The last vertebra, instead of
being cylindrical, has a broad carinated spine
for the insertion of large feathers.
Birds could not, of course, be always on the
wing ; for a great expenditure of muscular
power is constantly going on while they support
themselves in the air. Occasional rest is neces-
sary to them as well as to other animals, and
means are accordingly provided by nature for
•584 THE MECHANICAL FUNCTIONS.
their mechanical support and progressive motion
while on land.
The anterior extremities having been exclu-
sively appropriated to flight, and constructed
with reference to the properties of the atmos-
phere, the offices of sustaining and of moving
the body along the ground must be entrusted
wholly to the hind limbs. The centre of gravity,
before sustained by the wings, must now be
brought over the new basis of support formed
by the feet ; or rather, as it is placed far
forwards, the feet must be considerably ad-
vanced so as to be brought underneath that
centre. But as the bones of the posterior ex-
tremity have their origin from the remote part
of the pelvis, which is elongated backwards, at
a considerable distance from the wings, it be-
came necessary to lengthen some of their parts,
and to bend their joints at very acute angles.
We accordingly find that while nature, in the
formation of the limb, has preserved an accor-
dance with the vertebrated type, both as to the
number of pieces which compose it, and as to
their relative situations, she has deviated from
the model of quadrupeds in giving much greater
length to the division corresponding to the foot.
At the same time that the foot is brought for-
wards, the toes are lengthened, and made to
spread out so as to enclose a wide base, over
which the centre of gravity is situated. The
FEET OF BIRDS. 585
extent of this base is so considerable that a bird
can, in general, support itself with ease upon a
single foot, without danger of being overset by
the unavoidable vacillations of its body.
The femur is short compared with the tibia,
which is generally large, especially in the order
of Grallce, or wading birds : the fibula is ex-
ceedingly slender and always united, at its
lower part, with the tibia ; and there is a total
deficiency of tarsal bones, except in the Ostrich,
where rudiments of them may be traced. Already
we have seen, in ruminant quadrupeds, that these
bones have dwindled to a very small size : but
here they have wholly disappeared. The long-
bone which succeeds to the tibia, though con-
sidered by some anatomists as the tarsus, is pro-
perly the metatarsal bone, and in the Grallae is
of great length. At its lower end it has three
articulations, shaped like pullies, for the attach-
ment of the three toes : there is besides, in almost
all birds, a small rudiment of another metatarsal
bone, on which is situated the fourth toe. The
number of bones which compose each respective
toe appears to be regulated by a uniform law.
The innermost toe, which may be compared to a
thumb, consists invariably of two bones : that
which is next to it in the order of sequence has
always three ; that which follows has four ; and
the outermost toe has five bones : the claws in
every case being affixed to the last joints, which
586' THE MECHANICAL FUNCTIONS.
have therefore been termed the ungual bones.
This remarkable numercial relation among the
several bones of the toes exists quite indepen-
dently of their length.
There is one whole order of birds which are
particularly fitted for climbing and perching
upon trees, having the two middle toes parallel
to each other, and the inner and outer toes turned
back, so as to be opposed to them in their action.
They are thus enabled to grasp objects with the
greatest facility ; having, in fact, two thumbs,
which are opposable to the two fingers. They
have been termed Scansores, or Zygodactyli.
Almost all other birds have three toes before,
and one behind.
From this enumeration it would appear as if
Nature, in modifying the type of vertebrated
animals to suit the purposes required in the bird,
had purposely omitted one of the toes, which are
usually five in number. But instances occur of
birds, in which we may trace the rudiment of a
fifth toe high upon the metatarsus, and upon its
inner side. The spur of the cock may be re-
garded as having this origin. What confirms
this view of the subject is that in those birds
which have only three toes, namely, in the Emu,
the Cassoivary, and the Rhea, it is again the
inner toe which disappears, leaving only the
three outer toes, namely, those which have res-
pectively three, four, and five phalanges. The
FEET OF BIRDS. 587
Ostrich has only two toes, one having four, and
the other five phalanges ; here, again, it is the
innermost of the three former, that is, the one
having three phalanges, which has been sup-
pressed.*
A bird is capable of shifting the position of
the centre of gravity of its body, according as
circumstances require it, simply by advancing or
drawing back its head. While flying, the neck
is stretched forwards to the utmost, in order to
bring the centre of gravity immediately under
the origin of the wings, by which the body is
then suspended. When birds stand upon their
feet, they carry the head back as far as possible ;
so as to balance the body on the base of support.
When preparing to sleep, they bring the centre
of gravity still lower, by turning the head round
and placing it under the wing. These motions
of the head are again resorted to when the bird
walks ; and the centre of gravity is thus trans-
ferred alternately from one foot to the other :
hence, in walking, the head of a bird is in con-
stant motion ; whilst the duck and other birds,
whose legs are very short, have a waddling gait.
It may be observed that the more perfectly
predaceous birds are not the best formed for
* The last bone of the outer toe of the ostrich is very small,
and being usually lost in preparing the skeleton, has been over-
looked by naturalists ; but Dr. Grant has ascertained, by the
careful dissection of a recent specimen, the existence of this
fifth phalanx.
588 THE MECHANICAL FUNCTIONS.
walking ; because, were they to use their feet for
that purpose, their talons, which are required to
be kept sharp for seizing and tearing their prey,
would be blunted ; and accordingly the Eagle,
when moving along the ground, supports itself
partly by the motion of its wings.
In roosting, birds often support themselves on
their perch by means of one leg only, the other
being folded close to the body. They even
maintain this attitude with greater ease and se-
curity than if they rested upon both feet. The
true explanation of this curious fact was long
ago given by Borelli. On tracing the tendons
(t, t Fig. 233) of the muscles (m, m) which bend
the claws, and enable them to grasp an object,
we find them passing over the outer angles of
each of the intervening joints, so that whenever
these joints are bent, as shown in Fig. 234, those
tendons are put upon the stretch, and mechani-
cally, or without any action of the muscles, tend
to close the foot. When the bird is on its perch,
this effect is produced by the mere weight of the
body, which, of course, tends to bend all the
joints of the limb on which it rests ; so that the
greater that weight, the greater is the force with
which the toes grasp the perch. All this takes
place without muscular effort or volition on the
part of the bird. It remains in this position with
more security on one foot than it would have
done by resting upon both ; because in the latter
ROOSTING OF BIRDS.
5H9
case, the weight of the body, being divided be-
tween them, does not stretch the tendons suf-
ficiently. In this position the bird not only
sleeps in perfect security, but resists the impulse
of the wind and the shaking of the bough.
The great length of the toes of birds enables
them to stand steadily on one leg ; and in this
attitude many employ the other foot as a hand ;
especially Parrots, whose head is too heavy to be
readily brought to the ground. Some birds,
which frequent the banks of rivers, are in the
practice of holding a stone in one foot, while
they rest upon the other : this contributes to
increase their stability in two ways ; first, it
adds to the weight of the body, which is the
force that stretches the tendons, and causes
them to grasp the bough ; and, secondly, it also
lowers the centre of gravity.
590 THE MECHANICAL FUNCTIONS.
The Stork, and some other birds belonging to
the same order, which sleep standing on one
foot, have a curious mechanical contrivance for
locking the joint of the tarsus, and preserving
the leg in a state of extension without any
muscular effort. The mechanism is such as to
withstand the effect of the ordinary oscillations
of the body, when the bird is reposing ; but it is
easily unlocked by a voluntary muscular exer-
tion, when the limb is to be bent for progression.
On these occasions the ball of the metatarsal
bone is driven with some force into the socket of
the tibia.*
I must content myself with this general view
of the mechanism of birds ; as it would exceed
the limits within which I must confine myself,
to enter more fully into the peculiarities which
distinguish the different orders and families.
Some of the more remarkable deviations from
what may be considered as the standard confor-
mation, may, however, for a moment arrest our
attention.
The Ostrich, of all birds, presents the greatest
number of exceptions to the general rules
which appear to regulate the conformation
* This mechanism is noticed by Dr. Macartney in the Trans-
actions of the Royal Irish Academy, vol. xiii. p. 20, and is more
fully described in Rees's Cyclopaedia, Art. Bird. He observes
that both Cuvier and Dumeril have committed an error in re-
ferring this peculiarity of structure to the knee instead of the
tarsal joint.
STRUCTURE OF THE OSTRICH., 501
of birds, and in many of its peculiarities of
structure it makes some approach to that
which characterises the quadruped. Though
this bird is provided with wings, it was evi-
dently never intended that they should be
used for the purposes of flight. Hence the
chief muscular power has been bestowed on the
legs, which are remarkably thick and strong,
and well fitted for rapid progression. The ster-
num is flat, and does not present the keel-like
projection which is so remarkable in that of
all other birds. The clavicles do not reach the
sternum, nor even meet at the anterior part of
the chest to form the furcular bone ; for as the
wings are not employed in flying, the usual
office of that bone is not wanted. The form of
the pelvis is different from the ordinary struc-
ture ; for the pubic bones, which in all other
birds are separated by an interval, here unite as
they do in quadrupeds.
The feathers are unprovided with that elabo-
rate apparatus of crotchets and fibres, which are
universally met with in birds that fly. The
filaments of the ostrich's feathers, in consequence
of having none of these fibrils, hang loose and
detached from one another, forming the fine hair
or down, which, however ornamental as an
article of dress, must be viewed, when consi-
dered physiologically, as a species of degeneracy
in the structure of feathers.
5.92 THE MECHANICAL FUNCTIONS
The Penguin, in like manner, has a wing,
which is, by its shortness, totally unfitted for
raising the body in the air : it has, indeed, re-
ceived a very different destination, being formed
for swimming. In external form it resembles
the anterior extremity of the turtle ; but still we
find it constructed on the model of the wings of
birds ; as if nature had bound herself by a law
not to depart from the standard of organization,
although the purpose of the structure is alto-
gether changed. As penguins are intended for
a maritime life, all their extremities are formed
for swimming. Their legs are exceedingly
short, and placed far backwards ; so that these
birds are compelled, when resting on their feet
on the shore, to raise their bodies in a perpen-
dicular attitude, in order to place the centre of
gravity immediately above the base of support ;
a posture which gives them a strange and gro-
tesque appearance.
I have already alluded to the lengthened legs
and feet of the waders, the utility of which to
birds frequenting marshy places, and shallow
waters, is very obvious. Their legs are not
covered with feathers, which would have been
injured by continual exposure to wet. But
birds of a truly aquatic nature, have their toes
webbed, that is, united by a membrane ; a me-
chanism which qualifies them to act as oars, and
indeed gives them a great advantage over all
MUSCULAR POWER IN BIRDS. 503
artificial oars that have been constructed by
human ingenuity ; for as soon as the expanded
foot has impelled the water behind it, the toes
collapse ; and while it is drawn forward, it pre-
sents a very small surface to the opposing water.
Their plumage is so constructed as to prevent
the water from penetrating through it, and for
the purpose of preserving it in this condition,
these birds are provided with an oily fluid,
which they carefully spread over the whole
surface of their bodies. The swan, and many
other water-fowls, employ their wings as sails,
and are carried forwards on the water with
considerable velocity, by the mere impulse of
the wind.
Birds excel all other vertebrated animals in
the energy of their muscular powers. The
promptitude, the force, and the activity they
display in all their movements, and the un-
wearied vigour with which they persevere for
hours and days in the violent exertions required
for flight, far exceed those of any quadruped,
and imply a higher degree of irritability, de-
pendent probably on the great extent of their
respiratory functions, than is possessed by any
other class of animals.
END OF VOL. I.
VOL. I. Q Q
Erratum. — Page 356, note, line 1, for "wing," read " abdominal rings.
C. WHITTINGHAM, TOOKS COURT, CHANCERY LANE.
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