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"Ask now the beasts, and they shall tkach thee ; and the fowls of the air,
and they shall tell thee:

" Or speak to THE EARTH, AND IT SHALL TEACH THKE ; AND THE FISHES OF THE SEA
SHALL DECLARE UNTO THEE.

" Who K^owETH 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.

SlXlil.TARY or THK ROYAL SOCIKTY, VICE 1'RF.SIUK^T OF THE SOCIF.TY OF ARTS, MEMnKR OF THF.

SENATF. OF THE UNIVERSITY OF LONDON, AND HXAMINER IN PHYSIOLOGY A N IJ

COMPAR'VTIVE ANATOMY IN THAT UNIVERSITY, FELLOW OF THE ROYAL COLLEGE OF PIIVSIOIANS,

OONStlLTINC PHYSICIAN TO (JiaU'N CHARLOTTE'S LYING IN HOSPITAL,

AND TO THE NORTHERN DISPENSARY, ETC. ETC.

VOL I

THIRD EDITION
WITH NUMEROirS ADDITIONS AND EMENDATIONS

LONDON

WILLIAM PICKERING

1840

/3L
/B5S

u.s:
fit J

C. WHITTINGIIAM, TOdKS COURT, CHANCF.RY LANE.

TO HIS ROYAL HIGHNESS
PRINCE AUGUSTUS FREDERICK,

DUKE OF SUSSEX, K. G.

LATE PRESIDENT 0 1 ' 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 DEVOTi.D, HUMBLE SERVANT,

P. M. ROGET.

ADVERTISEMENT.

Since the publication of the first edition of this
Work, a great number of vahiable additions
have been made to our knowledge of Physiology
in its various departments. Tliese I have in-
serted in their proper places in the present
edition ; in which I have also made several
alterations necessary to adapt it to the present
advanced state of the science.

P. M. R.

Bernard Street, Russell Square,
August 1st, 1840.

PREFACE

TO THE FIRST EDITION

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 Treatises,
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 sup-
port in this arduous undertaking ; the progress
of which has throughout been seriously im-
peded by the various interruptions incident to
my profession, by long protracted anxieties
and afflictions, and by the almost overwhelming-
pressure of domestic calamity.

The o])ject of this treatise is to enforce the
great truths of Natural Theology, by adducing

PREFACE.

those evidences of the power, wisdom, {iiid
goodness of God, which are manifested in the
livino- creation. The scientific knowledge of
the phenomena of life, as they are exhibited
under the infinitely varied forms of organiza-
tion, constitutes what is usnally 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 from
it all those particulars of the natural history
both of animals and of plants, and all descrip-
tion of those structures, of which the relation
to final causes cannot be distinctly traced ;

PREFACE. XI

and have admitted only such facts as afford
manifest evidences of design. These facts I
have studied to arrange in that methodized
order, and to unite in those comprehensive
generalizations, which not only conduce to
their more ready acquisition and retention in
the memory, but tend also to enlarge our views
of their mutual connexions, and of their sub-
ordination to the general plan of creation.
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 Theo-
logy, published prior to the present series ; not
excepting even the unrivalled 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 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
lurnish, to contemplative minds, a rich fountain
of religious instruction. To render these bene-
fits generally accessible, I have confined myself
to such subjects as are adapted to every class

Xll PREFACE.

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 ante-
diluvian races ; the extent of the field which
remained, and which, with these few excep-
tions, embraces nearly the whole of the phy-
siology of the two kingdoms of nature, already
aftbrding ample occupation for a single la-
bourer.

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 requi-
site, either as bearing testimony to facts not
generally known, or as pointing out sources

rREFACE. Xlll

of more copious information. It may liowever
be proper to mention, that I liave more espe-
cially availed myself of the ample materials on
Comparative Anatomy and Physiology con-
tained in the works of Cuvier, Blumenbacli,
Cams, Home, Meckel, De Blainville, Ln-
treille, and St. Hilaire, and in the volumes of
the Philosophical Transactions, of the M6-
moires and Annales du Museum, and of the
Annales des Sciences Naturelles. I should
be uno;rateful vrere I not also to acknowledge
the instruction I have derived from my attend-
ance on the lectures at the Royal 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 Uni-
versity of London .

I have likewise to return my tlianks 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

XIV PREFACE.

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
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 gene-
rally 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.

1 also take this opportunity of conveying
my best thanks to my friend and colleague,

* 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. XV

Mr. Cliildreii, of the British Museum, for liis
kind assistance in revising the sheets while the
work was printing, and for liis many valua])le
suggestions during its progress through the
press.

A catalogue of the wood engravings has
been subjoined ; and also a tabular view of the
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 whicli the present is one, is
published under the following circumstances:

The Right Honourable and Revereno Francis
Henry, Earl of Bridgewater, died in the month of
February, 1829 ; and by his last Will and Testament, bearing-
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 divi-
dends 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 O/i the Power, Wisdom, and
Goodness of God, as manifested in the Creation ; illustrating
such work by all reasonable arguments, as for instance the
variety and formation of God's creatures in the animal,
vegetable, and mineral kingdoms ; 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 dis-
coveries ancient and modern, in arts, sciences, and the tvhole
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.

The late President of the Royal Society, Davies 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 concurrence
of a nobleman immediately connected with the deceased,
Mr. Davies Gilbert appointed the following eight gentlemen
VOL. I. b

XVIU

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.

REGIUS PROFESSOR OP MEDICINE IN THE UNIVERSITY OF OXFORD.

ON THE ADAPTATION OF EXTERNAL NATURE TO THE
PHYSICAL CONDITION OF MAN.

THE REV. WILLIAM WPIEWELL, 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. E.R.S. L. & E.

THE HAND: ITS MECHANISM AND VITAL ENDOWMENTS
AS EVINCING DESIGN.

PETER MARK ROGET, M.D.

FELLOW AND SECRETARY OP THE ROYAL SOCIETY.

ON ANIMAL AND VEGETABLE PHYSIOLOGY, CONSIDERED
WITH REFERENCE TO NATURAL THEOLOGY.

THE REV. WILLIAM BUCKLAND, D.D. F.R.S.

CANON OF CHRIST CHURCH, AND PROFESSOR OF GEOLOGY 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.

CONTENTS

OF THE FIRST VOLUME.

INTRODUCTION.

Page

Chapter I. — Final Causes 1

II. — The Functions of Life 28

PART I.— THE MECHANICAL FUNCTIONS.

Chapter I. — Organic Mechanism 50

§ 1 . Organization in general 50

2. Vegetable Organization 55

3. Developement of Vegetables 72

4. Animal Organization 83

5. Muscular Power 112

Chapter II. — The Mechanical Functions in Zoophytes 131

§ 1. General Observations 131

2. Sponges 1 35

3. Polypifera 146

4. Infusoria 1 66

5. Acalepbae 175

6. Echinodermata 181

Chapter III. — Mollusca 193

§ 1 . Mollusca in general 193

2. Acephala 195

3. Gasteropoda 204

4. Structure and formation of the Shells of Mollusca 206

5. Pteropoda 230

6. Cephalopoda 231

Chapter IV,— Articulata 240

§ 1 . Articulated animals in general 240

2. Annelida 242

3. Arachnida 252

4. Crustacea 257

XX CONTENtS.

Page

Chapter V. — Insects 265

§ 1 . Aptera 265

2. Insecta alata 268

3. Developement of Insects 270

4. Aquatic Larvae 277

5. Terrestrial Larvoe 278

6. Imago, or perfect Insect 283

7. Aquatic Insects 299

8. Progressive motion of Insects on land 302

9. Flight of Insects 308

Chapter VI. — Vertebrata 322

§ 1. Vertebrated Animals in general 322

2. Structure and Composition of the Osseous Fabric 326

3. Formation and Developement of Bone 335

4. Skeleton of the Vertebrata 345

Chapter VII. — Fishes 363

Chapter VIII.— Reptilia 387

§ 1. Terrestrial Vertebrata in general 387

2. Batrachia 388

3. Ophidia 398

4. Sauria 406

5. Chelonia 411

Chapter IX. — Mammalia 423

§ 1 . Mammalia in general 423

2. Cetacea 427

3. Amphibia 431

4. Mammiferous Quadrupeds in general 432

5. Ruminantia 442

6. Solipeda 456

7. Pachydermata 458

8. Rodentia 462

9. Insectivora 464

10. Carnivora 467

1 1 . Quadrumana 470

12. Man 473

Chapter X. — Vertebrata capable of flying 481

§ 1. Vertebrata without feathers, formed for flying .... 481

2. Birds 489

LIST OF ENGRAVINGS.

VOLUME I.

Fig. Page

1 Rotifer redivivus, (from Muller) 53

2 Vibrio fritici, (Bauer) 53

3 Simple vegetable cells, (Slack) 58

4 Fucus vesiculosus, transverse section, (De Candolle) .... 58

5 Ditto, longitudinal section, (id.) 58

6 Compressed cells of vegetables, (Slack) 58

7 Hexagonal and elongated cells, (id.) 58

8 Elongated cells, (id.) 58

9 Fibrous cells, (id.) 58

10 Reticulated cells, (id.) 58

1 2 Junction of cells to form a tube 64

1 3 Beaded vessels 64

14 Spiral vessels, or Tracheae 64

1 5 Annular vessels 64

16 Punctuated vessels 64

17 Transitions of vessels from one class to another 64

18 Woody fibres 64

19 Nervures of a leaf 64

20 Cells composing the cuticle, (De Candolle) 69

21 Stomata magnified, (Amici) 69

22 Arrangement of stomata in Cuticle, (De Candolle) 69

23 Roots terminated by spongioles, (id.) 69

24 Cells composing a spongiole, (id.) 69

25 Animal cellular substance 86

26 Blood vessels 90

27 Section of blood vessel, with the valves open 90

28 Ditto, with the valves closed 90

28* Perpendicular section of the layers of the skin, (Breschet) 101

XXU LIST OF ENGRAVINGS.

Fig. Page

29 Striated surface of the scale of the Cyprinus alburnus,

(Heisinger) 1 05

30 Ditto of the Perca Jluviatilis, (Carus) 105

31 Imbricated arrangement of the scales of fishes (Heisinger). . 105

32 Section of the bulbs of hair, magnified 106

33 Quill of Porcupine, (F. Cuvier) 109

34 Transverse section of the same, (id.) 109

35 Longitudinal section of the root of ditto, (id.) 109

36 Capsule of bulb of ditto laid open, (id.) 109

36* Groups of vibrating cilia in action, (Farre) 116

37 Muscle in a state of relaxation 120

38 The same muscle contracted 120

39 Diagram illustrating the action of obhque muscles 120

40 Semi-penniform muscle 120

41 Penniform muscle 120

42 Complex muscle 120

43 Tendon of muscle 120

44 Trapezius muscle 120

45 Muscular structure of the Ear-drum, (Home) 125

46 Orbicular muscle of the Eye-lids, (Albinus) 125

47 Muscular structure of the Iris, (Home) 125

48 Muscular fibres of a sucking disk 125

49 Longitudinal muscular fibres of a blood- vessel . . , 127

50 Transverse muscular fibres of ditto 127

51 Muscular fibres of the human stomach, (Cooper) 127

52 Muscular fibres of the Heart, (id.) 127

53 Magnified view of a Sponge, (Grant) 136

54 Spicula in the texture of a Sponge, (id.) 136

55 Gemmule of a Sponge, (id.) 136

56 Lobularia. Alcyoniu7n jyelasgica, {J)eter\i\\e) 147

57 Detached polype of ditto, (id.) 147

58 Zoanthus, (Actinia sociata), (Ellis) 147

59 Hydra viridis, (Trembley) 147

60 Sertularia pelasgica, (Deterville) 149

6 1 Tubipora musica, (EUis) 149

62 Section and polypes of ditto, magnified, (id.) 149

63 Flustra carbasea, (id.) 149

64 Cells of ditto, magnified, (id.) 149

65 Corallium rubruni, (id. ) 150

66 Polypes of ditto, magnified, (id.) 150

LIST OF ENGRAVINGS. XXllI

Fig. Page

67 Section of Gorgonia Btiareus, (id.) 150

68 Isis hippuris, (id.) 1 50

69 Polype of Flustra carbasea, (Grant) 155

70 Tentaculum of ditto, magnified, (id.) 155

71 Penjiatula phosphnrea, (Ellis) 157

72 Magnified view of the polypes of ditto, (id) 157

73 to 76 Mode of progression of tlie Hydra viridis, (Trembley) 160

77 Vorticella cyathina, (Muller) 165

78 Am ceba dijiuens, (id.) 170

79 Volvox (jlobator, (id.) 170

80 Bracldonus urceolaris, (id.) 172

81 Medusa pulmo, (Macri) 175

82 Beroe ovatus, (Bruguiere) 177

83 Beroe pileus, (id.) 177

84 Velella limbosa, (Guerin) 177

85 Physalia atlantica, (id.) 177

86 Actinia rufa, (original) 180

87 Ditto expanded, (original) 180

88 Asterias serrulata, (Bruguiere) 182

89 Asterias regularis, (id.) 182

90 Echinus Ananchites ovata, (id.) 182

91 Clypeaster rosaceus, (id.) 182

92 Ophiura lacertosa, (id.) 1 82

93 Euryale muricatum, (id.) 182

94 Pentacrinus eurojJCBus, (Thomson) 182

95 Ambulacra, and feet of Asterias, viewed from the under side

(Reaumur) 183

96 Ditto, viewed from the upper side, (id.) 183

97 Vesicles appended to the feet of the Asterias 183

98 Polygonal pieces composing the test of the Echinus 185

99 Structure of a detached piece of ditto 185

1 00 Spine of the Cidaris, (Cams) 185

101 Shell of Unio batava, (Goldfuss) 195

102 Adductor muscle of Oyster, (Hunterian Museum) 196

103 Shell of Pholas Candida, with abductor muscle, (Osier). . 198

104 Foot of Cardium edule, (Reaumur) 199

105 Planorbus cornutus, (Cuvier) 204

106 Magnified view of the striae on the surface of Mother of

Pearl, (Herschel) 208

107 Directions of the fibres in the component strata of shells . 210

\X1V LIST OF ENGRAVINGS.

Fig. Page

108 Shell of Achatina zebra, (De Blainville) 217

109 Longitudinal section of ditto, (id.) , 217

110 Shell of Pterocerus scoi-pio, at an early stage of growth,

(id.) 220

111 Shell of the same when completely formed, (id.) 220

112 Shell of CyprcEa exanthema at an early period of growth,

(id.) 220

1 1 3 Shell of the same animal, when completed, (id.) 220

114 Transverse section of the shell of the Cyprcea exanthema,

(Hunterian Museum) 22 1

1 15 Shell of Conus 224

116 Longitudinal section of the same, (original) 224

117 Transverse section of the same, (Bruguiere) 224

118 Inner surface of the Epiphragma of the Turbo, (De Blain-

ville) 22(5

1 19 Outer surface of the same, (id.) 226

120 Clio borealis, (Cuvier) 231

121 Sepia loligo, (De Blainville) 232

122 Suckers of the same, (id.) 232

123 Bone or internal shell of the same, (id.) 232

123* Suckers of the Octopus, (original) 233

124 Shell of Sinrula australis, (De Blainville) 237

125 Longitudinal section of the same, (id.) 237

126 Shell of Nautilus pompllius, (id.) 237

127 Longitudinal section of the same (id.) 237

128 Pontobdella muricata, (Bruguiere) 243

129 Nereis, (id) 243

130 Erpobdella vulgaris (Lam.) Hirudo hyalina 243

131 Diagram illustrating the rings and muscles of Annelida,

(original) 243

1 32 Gordius aquations 247

1 33 Serpula ojjercularia 247

1 34 Terebella concliilega, (De Blainville) 247

135 Arenicola piscatorum, or Lumbricus marinus 247

136 Aranca diadema, (Roesel) 253

137 Divisions of the limb of a Crustaceous animal 258

138 Mandible and palpus of My sis Fabricii, (Bruguiere) .... 258

139 to 141 Feet-jaws belonging to the first, second, and third

pairs, (id.) 258

142 True foot belonging to the first pair, (id.) 258

LIST OF ENGRAVINGS. XXV

Fig. I^age

143 Julus terrestris 268

144 Muscles of the trunk of the Melolontha vulgaris, (Straus

Durckhehn) 269

145 Eggs of Bombyx rnori 273

146 Larva of the same 27 3

147 Pupa of the same 273

148 Imago of the same 273

148* A Caterpillar of the Fhalena striaria, (Hubner) 281

B The same in a rigid position, (Lyonet) 281

149 Calosoma Sycophanta, (Kirby and Spence) 286

150 Analysis of skeleton of the same, (Carus) 287

151 Hind view of the segment of the head in the same, (id.). . 287

152 Suckers on the foot of the Musca vomitoria, expanded;

magnified view, (Bauer) 297

153 Cushions on the foot of the Cimbex lutea, magnified, (id.) 297

154 Suckers on the under side of the foot of a male Dytiscus

marginalis, (id.) 297

155 Cushions and sucker of the Acridium biguttulum, Latr.

(id.) 297

156 Dytiscus marginalis, upper side, (Roesel) 300

157 Lower side of the same insect, (id.) 300

158 Notonecta glauca, (RcEsel) 301

1 58* Fore leg of Gryllotalpa, (Kidd) 307

159 Wing of Gryllus riasutus. Orthoptera 313

\Q0 Vf lag o{ Libellula grandis. Neuroptera 313

161 Wing of Ichneumon per suasorius. Hymenoptera 313

1 62 Wing of Tipula oleracea. Diptera 313

163 Sting of Anthophora retusa, (original) 315

1 64 Separate scales of the wing of Hesperia Sloanus, (original) 318

165 Arrangement of the scales in the wing of the same 318

172 Longitudinal section of the thigh-bone to show the can-

cellated structure, (Cheselden) 333

1 73 Longitudinal section of the humerus, (id.) 333

174 Ossification of the parietal bone, (id.) 338

175 Early stage of ossification of the bones of the skull,

(Cloquet) 338

176 The same in the adult, showing the sutures 338

177 Dorsal vertebra, human 346

1 78 Junction of vertebrae forming the spinal column 346

179 Longitudinal section of the same, showing the spinal canal 346

XXVI LIST Ol ENGRAVINGS.

Fig. Page

180 Elements of Structure of the vertebra, (Carus) 351

181 Skeleton of Hog, (Pander and D'Alton) 353

182 Sternum, clavicle, and scapula ; human , 353

184 Skeleton of Cyprinus carpio, (Bonnaterre) 366

185 Diagram illustrating the progressive motion of Fishes. . . . 367
185* Path of the centre of gravity of a fish in swimming .... 369

186 Front view of the vertebra of a Cod, {Gadus morrhua) . . 370

1 87 Side view of the same 370

188 Vertical and longitudinal section of a part of the spinal

column in the same 370

189 A similar section, showing the gradation of structure. . . . 370

190 Similar section in the Squalus centrina, (Carus) 370

191 Bones of the shoulder of the Lophiiis piscatorius, (id.) . . 376

192 Pectoral fin of the Raia clavata, (id.) 376

193 Belt of bones of the shoulder of a Ray, (id.) 377

194 Muscular system of Cyprinus alburnus, (id.) 379

195 Air bladder of Cyprinus carpio, (Blasius) 382

1 96 Eggs of the Frog 389

197 Side view of the Tadpole magnified, (Rusconi) 389

198 Upper view of the same, (id.) 389

199 Adult Frog 389

200 Skeleton of Frog, (Cheselden) 393

201 Skeleton of the Viper 398

202 Ribs and spine of Boa constrictor (Home) 401

203 Bones of the foot of the same, (Mayer) 399

204 Muscles moving the claw of the same, (id.) 399

205 Rudimental bones of the foot of the Tortryx scytale, {k\.) 399

206 of the Tortryx corallinus, (id.) 399

207 of the Anguis fragilis, (id.) 399

208 of the Amphisbcena alba, (id.) 399

209 of the Coluber jmllutatus, (id.) 399

210 Chalcides pentadactylus, (Bonnaterre) 399

21 1 Under surface of the foot of the Lacerta gecko, magnified

four times, (Bauer) 410

212 Side view of a longitudinal section of the same, (id.) .... 410

213 Skeleton of Tortoise, (Carus) 413

214 Section of the thigh bone of the same, (id.) 413

215 Hind view of skull of Testudo my das, (id.) 417

216 Bones sustaining the fin of the Delphinus phocoena, (Pan-

der and D'Alton) 431

LIST Ol' ENGRAVINGS. XXVll

Fig. Page

217 Fore part of the Skeleton of an Ox with the Ligamentum

nuchcE, (original) 444

218 Skeleton of the Stag, (Cheselden) 449

218*A. Longitudinal section of the horn of an Ox, (original). . 455

B. Ditto, of an Antelope, (original) 455

c. Extremity of the same, (original) 455

219 Subcutaneous muscles of the Hedge-hog, relaxed, (Carus) 466

220 The same muscles contracted, and drawn over the body,

(Cuvier) 466

221 Skeleton of the Lion, (Pander and D'Alton) 468

222 Skeleton of Draco volans, (Tiedemann) 485

223 Skeleton of Vespertilio molossus, (Temmink) 487

224 Skeleton of the Swan, (Cheselden) 493

225 Longitudinal section of the cervical vertebra of the Ostrich,

(original) 496

226 Fibrils of the vane of a feather, magnified, (original). . . . 502

227 Edges of the fibres, magnified, (original) 502

228 Feather, showing its structure, (F. Cuvier) 507

229 Capsule, or Matrix of the feather, (id.) 507

230 View of the parts enclosed in the Capsule, when laid open,

(id.) 507

231 Section of the stem, while growing, exhibiting the series

of conical membranes, (id.) 507

233 Extensor muscles of the foot and toes of a bird, (Borelli) 520

234 Position of a bird in roosting, (id.) 520

XXVlll LIST OF KNGUAVIXGS.

VOLUME ir.

Fig. Page

239 Rotation, or partial circulation in the cells of the Cau-

linia fragilis, magnified, (Amici) 45

240 The same in the jointed hair of the Tradescantia virginica

(Slack) 45

241 Section of the Hydra viridis, magnified, (Trembley) ... 6Q

242 Hydra viridis seizing a worm, (id.) 68

243 The same after swallowing a minnow, (id.) 68

244 A Hydra which has swallowed another of its own species,

(id.) 68

245 Compound Hydra, with seven heads, (id.) . . . ., 68

246 Veretilla lutea, showing the communicating vessels of

Polypes, (Quoy et Gaimard) 74

247 Nutrient vessels of the Tcknia solium, (Chiaje) 74

248 Tcenia ylobosa, or Hydatid of the Hog, (Goeze) ...... 74

249 Horizontal section of the Rhizostoma Cuvieri, Peron,

(Eysenhardt) 78

250 Geronia hexaphylla, Peron, Medusa proboscidalis,

(Forskal) 78

251 Vascular network in margin of the disk of the Rhizos-

toma Cuvieri, (Eysenhardt) 78

252 Vertical Section of the Rhizostoma Cuvieri, (id.) 79

253 Transverse section of one of the arms of the same, (id.). . 79

254 Transverse section of the extremity of a tentaculum of the

same, (id.) 79

255 XeMcopArapa/wZa, highly magnified, (Ehrenberg) 85

256 Alimentary canal and caeca of the same, viewed sepa-

rately, (id.) S5

151 Vertical section of the Actinia coriacea, (Spix) 89

258 Digestive organs of the Asterias, (Tiedemann) 90

259 Stomachs of the Nais vermicularis, (Roesel) 91

260 Stomachs of the Hirudo medicbialis, (original) 92

261 Mouth of the same, showing the three semicircular teeth,

(original) 92

262 Tooth of the same, detached, (original) 92

263 Glossopora tuber culata i Hirudo complanata, Lin. (John-

son) 93

LIST OF ENGRAVINGS. XXIX

Fig. Page

264 The same seen from the under side, showing the digestive

organs, (id.) 93

295 Diagram showing the arrangement and connexions of the
organs of the vital functions in the Vertebrata, (ori-
ginal) 95

266 Spiral probosces of Papilio urticce, (Griffith) 102

267 Trophi of Locus ta viridissima, (Goldfuss) 109

268 Filaments composing the rostrum, or proboscis, of the

Cimex nigricomis, (Savigny) Ill

269 Sheath of the proboscis of the same insect, (id.) Ill

270 Toothed cartilage of the Helix pomatia, (Cuvier) 113

271 Mechanism for projecting and retracting the tongue of the

Woodpecker, (original) 119

272 Laminae of Whalebone descending from the palate of the

Balcena mysticetus, (Bonnaterre) 123

273 Teeth of the Delphinus phocoena, (Cloquet) 127

274 Skull of Tiger, (Cuvier) 130

275 Skull of Antelope, (Pander and D'Alton) 131

276 Skull of Rat, (id.) 132

277 Longitudinal section of simple tooth, (Rosseau) 134

278 Surface of a grinding tooth of a Horse, (Home) 134

279 Surface of the grinding tooth of a Sheep, (id.) 134

280 Longitudinal section of the incisor tooth of the Rodentia.. 134

281 Vertical section of the grinding tooth of the Elephant,

(Home) 138

282 Grinding tooth of the African Elephant, (id.) 138

283 Grinding tooth of the Asiatic Elephant, (id.) 138

284 Succession of teeth in the Crocodile, (Cams) 146

285 Venomous fang of the Coluber naia, (Smith) 148

286 Transverse section of the same, (id.) 148

287 The same tooth at an earlier period of growth, (id.) 148

288 The same, still less advanced in its growth, (id.) 148

289 Base of the former, (id.) ! 148

290 Base of the latter, (id.) 148

291 Transverse section of the young fang, about its middle, (id.) 148

292 A section, similar to the last, of another species of serpent,

(id.) 148

293 Squalus pristis. b. Under side of its snout, (Latham). . 149

294 Interior of the Stomach of a Lobster, (original) 150

295 Gastric teeth of Bullcea aperta, (Cuvier) 1 50

XXX LIST OF ENGRAVINGS.

Fig. Page

298 Gizzard of the Swan (Home) 151

299 Crop and gizzard of the Parrot, (id.) 100

300 Crop of the Pigeon, (id.) 160

301 Human Stomach, (id) 162

302 Interior of the stomach of the African Ostrich, (id.) .... 165

303 Gastric glands of the same, (id.) 165

304 Gastric glands of the American Ostrich, (id.) 165

305 Longitudinal section of the gastric glands of the Beaver,

(id.) 165

306 Stomach of Dormouse, (id.) 170

307 Stomach of Hyrax capensis, (Cuvier) 170

308 Stomach of Porcupine, (id.) 170

309 Stomach of Kaufjuroo, (id.) 170

310 Stomach of Delphinus phoccena, (id.) 170

311 Cardiac valve of the Horse, (Gurlt) 170

312 The four stomachs of a Sheep, (Carus) 172

313 Inner surface of the honey-comb stomach, (Home) 172

314 Inner surface of the many-plies stomach of an Ox, (id.). . 172

315 Interior cellular surface of the second stomach of the

Camel, (id.) 172

316 Spiral valve in the intestine of the Shark, (Blasius) .... 182

317 Digestive organs of the Mantis religiosa, (Marcel de

Serres) 187

318 Melolo7itha vulgaris, (Leon Dufour) 188

319 Cicindela campestris, (id.) 188

320 Portion of a hepatic vessel of the Melolontha, highly

magnified, (Straus Durckheim) 189

321 Alimentary canal of the Acrida aptera, (original) 190

322 Interior of the gizzard of the same magnified, (original). . 190

323 Row of large teeth in the same, still more magnified,

(original) 190

324 Profile of one of those teeth still more highly magnified,

(original) 190

325 Base of the same tooth seen from below, (original) 190

326 Alimentary canal of the Larva of the Sphinx Ligustri,

(original) 192

327 of the Pupa of the same, (original) 192

328 of the Imago of the same, (original) 192

329 of the Patella, (Cuvier) 194

330 Stomachs of the Pleurobranchus Peronii, (id.) 195

LIST OF ENGRAVINGS. XXXI

Fig. Page

331 Pyloric appendices in the Salmon, (id.) 196

332 Plan of double circulation 208

333 Detached Dorsal vessel of Melolontha vulgaris, (Straus

Durckheim) 219

334 The same with its ligamentous and muscular attachments,

(id.) 219

335 Side view of the anterior extremity of the same vessel,

(id.) 219

336 Section of the dorsal vessel to show its valves, (id.) .... 219

337 Circulation in the antenna of the Semblis viridis, (Cams) 222

338 Course of circulation in the same insect, (id.) 222

339 Dorsal vessel of the Caterpillar of the Sphinx ligustri, side

view, (original) 224

340 The same in the Chrysalis (original) 224

341 The same in the Moth, (original) , 224

342 The same viewed from above, (original) 224

343 Magnified lateral view of the anterior extremity of the

dorsal vessel, (original) 224

344 Magnified dorsal view of the same, (original) 224

345 Structure of the valves of the dorsal vessel, (original) . . 224

346 Heart and vessels of the Aranea domestica, (Treviranus) . 229
346*Circulation in the Planaria nigra, (Duges) 230

347 Course of circulation in the Erpobdella vulgaris (Morren) 232

348 Vessels in abdominal surface of the same, (id.) 232

349 Vascular dilatations, or hearts of the Lumbricus ter-

restris, (Morren) 235

350 Cavities and great vessels of the Heart 238

351 The Heart laid open to show its Valves 239

353 Plan of circulation in Reptiles 247

354 Branchial circulation in Maia Squinadn, (Audouin) .... 237

355 Organs of circulation in the Loligo sagittata, (id.) 243

356 Plan of circulation in Fishes 245

357 Plan of circulation in Batrachia 248

359 Plan of double, or warm-blooded circulation 252

360 Heart of the Dugong, (Home) 252

365 Valves of the Veins, (Cloquet) 262

366 Heart, branchial artery and gills of a fish, (Blasius) .... 273

367 Branchial apertures in the Squnlus glaucus, (Bonnaterre) 273

368 Branchial apertures in the Petromyzon marinus, (id.) .. 273

369 Internal structure of the branchiae of the same, (Home). . 273

XXXU LIST OF ENGRAVINGS.

Fig. Page

370 Stigmata in the abdominal surface of the Dijfiscus mar-

ginalis, (Leon Dufour) 28 1

371 Stigmata of Cera/rti?/x /<e?-os, (Fab.) magnified, (id.) .. 281

372 Longitudinal tracheae of Cara^MS awra^MS, (id.) 281

373 Air vesicles and tracheae of the Scolia hortorum, (Fab.)

highly magnified, (id.) 281

374 Respiratory apparatus of the Scorpio europceics, (Trevi-

ranus) 284

375 Internal structure of the lungs of the Turtle, (Bojanus). . 290

377 Air cells of the Ostrich, (Parisian Academicians) » 295

378 Lymphatic Absorbents 317

379 Passage of Nerves through a ganglion 324

380 Plexus of nerves 324

381 Varietiesof forms of antennae of Insects, (Goldfuss) .... 345

382 Vertical and longitudinal section of the right nostril in

man 357

383 Vertical transverse section of the same 358

384 Transverse section of the nostril of a Sheep, (Harwood) . . 359

385 Turbinated bones of the Seal, (id.) 359

386 Turbinated bones of the Turkey, (id.) 362

387 Nerves distributed to the bill of the Duck, (id.) 362

388 Nasal cavities of the Percajiuviatilis, (Cuvier) 367

389 Nasal cavity of the Raia batis or Skate, (Harwood) — . 367

390 Human ear, (Cloquet) 376

391 Posterior surface of the cavity of the tympanum, (id.) . . 380

392 Ossicula auditus, or small bones of the tympanum 380

393 The position of the latter in the tympanum 380

394 Magnified view of the labyrinth detached from the sur-

rounding parts, (Breschet) 381

395 Interior structure of the labyrinth, (id.) 382

396 Membranous labyrinth, with its nerves, (id.) 382

397 Cretaceous bodies in the labyrinth of the Bog, (id.) .... 382

398 Ditto in that of the Hare, (id.) 382

399 Organ of hearing in the Lobster, (Carus) 390

400 Groove in the sac of the former, (id.) 390

401 Organ of hearing in the Astacus fiuviatilis , (id.) 390

402 Interior view of the same, (id.) 390

403 Membranous labyrinth of the Lophius piscatori^is, (id). . 392

404 Organ of hearing in the Frog, (Bell) 394

405 Ear of the Turkey, (Carus) 394

LIST OF ENGKAVINGS. XXXIU

Pig. Page

406 Diagram illustrating one mode of obtaining images of ob-

jects, (original) 403

407 Simple Camera Obscura 404

408 Law of the refraction of a ray of light 406

409 Convergence of rays to a focus 407

410 Convergence by a double convex lens 409

411 Spherical aberration 410

412, 413, and 414 Variations of focal distance, consequent

upon variations of divergence of the incident rays .... 410

415 Horizontal section of right human eye magnified, (Home) 412

416 Straight and oblique muscles of the eye-ball 415

417 Lacrymal apparatus • 417

418 Eye of Helix pomatia, (Muller) 430

419 Stemmata of Caterpillar, (Marcel de Serres) 433

420 Eye of the Scorpio tunensis, (Muller) 433

421 Conglomerate eyes of Julus terrestris, (Kirby and

Spence) 433

422 External magnified view of the compound eye of the

Melolontha vulgaris, (Straus Durckheim) 435

423 Ditto of that of a Phalena 435

424 Section of the compound eye of the Libellula vultjata,

magnified, (Duges) 435

425 Highly magnified view of the outer margin of the pre-

ceding section, (id.) 437

426 Portion of the section of the eye of the Melolontha vul-

garis, (Muller) 437

427 Portion of the section of the eye of the Libellula,

(Duges) 437

428 Portion of the section of the eye of the Melolontha
vulgaris, (Straus Durckheim) 437

430 Interior of the eye of the Perca fiuviatilis, (Cuvier) .... 442

431 Fibres of the crystalline lens of the Cod, (Brewster) .... 444

432 Denticulated structure of these fibres, (id.) 444

433 Section of the eye of the Goose, (Home) 448

434 Nictitating membrane of a Bird, (Petit) 448

435 Muscles of the nictitating membrane, (id.) 448

438 Talitrus, (Latreille) 488

439 Nervous system of the Talitrus, (Audouin) 488

440 Nervous system of Cymothoa, Fab. (id.) 488

44 1 Nervous system of Maia squinado, (id.) 490

VOL. I. C

XXXIV LIST OF ENGRAVINGS.

F'g- Page

442 Nervous system of the Larva of the Sphinx ligustri,

(Newport) 492

443 Ditto of tlie Chrysalis of the same, (id.) 492

444 Ditto of the Imago of the same, (id.) 492

445 Nervous system of the Asterias, (Tiedemann) 494

446 Ditto of the Aplysia, (Cuvier) 494

447 of the Patella, (id.) 494

448 of the Sepia Octopus, (id.) 494

449 Brain and spinal cord of the Colurnba turtur, (id.) .... 496

450 Transverse section of the spinal cord in Man, (Solly). . . . 497

451 Brain and spinal marrow of the Trigla hjra, (Arsaky) . . 496

452 Brain of the Murcena co7iger, (Serres) 496

453 Perca fiuviatilis, (Cuvier) 496

454 Testudo mydas, (Cams) 496

455 Crocodile, (id.) 496

456 Lion, (Serres) 496

457 Lateral view of the brain of the Perch, (Cuvier) 496

458 of the Testudo mydas, (Carus) 496

459 of a section of the brain of the Dove, (id.) .... 496

460 of the Lion 496

461 Vertical section of the human brain, (Monro) 502

462 Progressive changes in the Manas 523

463 Vorticella 523

OUTLINE OF CUVIER'S CLASSIFICATION
OF ANIMALS;

WITH EXAMPLES OF ANIMALS BELONGING TO EACH DIVISION.

Bimana .
Quadiumana

Cheiroptera .

Insectivora .

Plantigrada .

Digitigrada .
Amphibia

Marsupialia .
Rodentia

Edentata

Pachydermata

Ruminantia .

Cetacea . .

Accipltres

Passeres

Scansores
Gallinse .
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.

Deer, Badger, Glutton.

Dog, Lion, Cat, Martin, Weasel, Otter.

Seal, Walrus.

Opossum, Kanguroo, Wombat.

Beaver, Rat, Squirrel, Porcupine, Hare.
y Sloth, Armadillo, Ant-eater, Pangolin, Or-
\ nithorhynchus.

Elephant, Hog, Rhinoceros, Tapir, Horse.
y Camel, Musk, Deer, Giraffe, Antelope,
\ Goat, Sheep, Ox.

Dolphin, Whale.

2. AvES.

Vulture, Eagle, Owl.
C Thrush, Sioallow, 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-Jish, Mullet.
( Salmon, Herring, Pike, Carp, Silurus, Cod,
\ Sole, Remora, Eel.

Pipe-Jish, Pegasus.

Sun-Jish, Trunk-Jish.

Lamprey, Shark, Ray, Sturgeon.

XXXVl

CLASSiriCATlON OF ANIMALS.

1. Cephalopoda

2. Pteropoda .

3. Gasteropoda

4. Acephala

5. Brachiopoda

6. Cirrhopoda .

Tubicola

Dorsibrancliia

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.

Serpnla, Sabella, Amphitrite.
Nereis, Aphrodite, Lob-worm.
Earth-xuorm, Leech, Nais, Hair-worm.

2. Crustacea.

Crab, Lobster, Praivn.

Squill, Phyllosoma.

Ga?nmarus, Sand-hopper.

Cyamus.

Wood-louse.

Monoculus.

3. Araciinida.

Spider, Tarantula, Scorpion.
Phalangium, Mite.

4. Insecta.

Centipede, Podura.
Beetle, Glow-worm.
Grasshopper, Locust.
Fire-fiy, Aphis.
Dragon-fiy , 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, Peimatula.

Brachionus, Vibrio, Proteus, Manas.

ANIMAL AND VEGETABLE
PHYSIOLOGY.

LNTRODUCTION.
Chapter I.

FINAL CAUSES.

1 o investigate the relations which connect Man
witli his Creator is the noblest exercise of human
reason. The Being who bestowed on him this
faculty must evidently have intended that he should
so exercise it, and that he should acquire, through
its means, some insight, however limited, into the
order and arrangements of creation ; some know-
ledge, however imperfect, of the divine attributes ;
and a distinct, though faint perception of the tran-
scendent glory with which those attributes are
encompassed. To Man have been revealed the
POWER, the WISDOM, and the goodness of God,
through the medium of the Book of Nature, in the
varied pages of which they are inscribed in inde-
lible 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

VOL. I. B

2 FINAL CAUSES.

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 mo-
mentous interest to mankind, must necessarily be
arduous, and has required for its successful prose-
cution 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 difficulties 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 comprehension of even a small part
of the system of the universe, is appalled by the
overwhelming consideration of the infinity that sur-
rounds us. The reflection continually presents itself
that the portion of creation we are here permitted
to behold, is as nothing when compared with the
immensity of space, which, on every side, spreads
far beyond the sphere of our vision, and which the
power of human imagination is utterly inadequate
to comprehend. Of the planetary system, which
includes this earth, our knowledge is almost
entirely confined to the mathematical laws which
regulate the motions of the bodies that 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?

IINAL CAUSES. 3

where the result of our inquiries respecting the
myriads of luminous 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 throughout
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 inex-
haustible variety of objects is contained ; what an
endless diversity of phenomena is presented ; what
wonderful changes are occurring in rapid and per-
petual succession ! Throughout the whole series of
terrestrial beings, what studied arrangements, what
preconcerted adaptations, what multiplied evidences
of intention, what signal proofs of beneficent design
exist to attract our notice, to excite our curiosity,
and to animate our inquiries. Splendid as are the
monuments of divine power and wisdom displayed
throughout the firmament, in objects fitted by their
stupendous magnitude to impress the imagination
and overpower us by their awful grandeur, not less
impressive, not 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 com-
prehension. The modern improvements of optical
science, which have expanded our prospects into
the more distant regions of the universe, have like-
wise 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

4 FINAL CAUSES.

exist worlds far removed from the cognizance of
every human sense, however assisted by the utmost
refinements of art ; worlds occupied by the elemen-
tary corpuscles of matter, composing, by their
various configurations, systems upon systems, and
comprising endless diversities of motions, of com-
plicated 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 inquiries,
we are sure to arrive at boundaries within which
our powers are circumscribed. Infinity meets us
in every direction, whether in the ascending or de-
scending scale of magnitude : and we feel the im-
potence 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
infinitely 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 rea-
soning by which we are conducted to the know-
ledge 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

FINAL CAUSES. O

which the mind has cognizance. The phenomena
of the universe may be viewed as connected with
one another either by the relation of cause and
effect, or by that of means and end; and accord-
ingly these two classes of relations give rise to dif-
ferent kinds of knowledge, each of which requires
to be investigated in a peculiar mode and by a dif-
ferent process of reasoning. The foundation of
both these kinds of knowledge 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 metaphorically 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 antecedents,
or causes : the unknown links by which these causes
are connected with their respective consequents, or
effects, being denominated the poiuers 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 characters of these forces admit
of exact definition, according to the qualities of the
corresponding effects they produce. It is by pur-
suing the method of philosophical induction, so
well explained 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 affording just
grounds for exultation at the achievements of the
human intellect.

6 FINAL CAUSES.

In the investigation of the powers which are con-
cerned in the phenomena of living beings we meet
with difficnlties incomparably greater than those
that attend the discovery of the physical forces
which actuate the parts of inanimate matter. The
elements of the inorganic world are few and simple ;
the combinations they present are, in most cases,
easily unravelled ; and the powers which effect
their union, their changes, and their motions, 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 observations of modern
astronomers have rendered it probable that the
influence extends to the remotest regions of space ?
The most undeviating regularity is exhibited in the
motions of those stupendous planetary masses,
which continually roll onwards in the orbits pre-
scribed by this all -pervading force. Even the
slighter perturbations occasioned by their mutual
influence are but direct results of the same general
law ; and are necessarily restrained within certain
limits, which they never can exceed, and by which
the permanence of the system is effectually secured.
All the terrestrial changes dependent 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 per-
ceive, is incapable of being disturbed by any exist-
ing cause.

FINAL CAUSKS. 7

Equally definite are the operations of the forces
of cohesion, of elasticity, or of whatever other me-
chanical powers of attraction or repulsion there
may be, which actuate, at insensible distances, the
particles of matter. We see liquids, in obedience
to these forces, collecting in spheroidal masses, or
assuming, at their contact with solids, certain cur-
vilinear forms, which are susceptible of precise
mathematical determination. In different circum-
stances, again, we behold these particles suddenly
changing their places, mashalling themselves in
symmetric order, and constructing by their union
solid crystals of determinate figure, having all their
angles and facets shaped with mathematical ex-
actness.

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 determinable as
the former. They are now known to be regulated
by the Law of dejinite proportions ; a law, the dis-
covery 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 Magnetism have been, in like
manner, reduced to laws of suflicient simplicity to
admit of the application of mathematical reasoning,
and to furnish the accurate results derived from
such application.

Thus to whatever department of physical science
our researches have extended, we every where meet
with the same regularity in the phenomena, the
same simplicity in the laws, and the same unifor-

FINAL CAUSES.

mity in the results. All is strictly detined, 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
imerring 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, exhibiting pecu-
liar movements, actuated by new and unknown
powers, and gifted with high and refined endow-
ments. In place of the simple combinations of
elements, and the definite properties of mineral
bodies, all organic structures, even the most mi-
nute, present exceedingly complicated arrange-
ments, and a prolonged succession of phenomena,
so varied and so anomalous, as to be utterly irre-
ducible to the known laws which govern inanimate
matter. Let us hasten, with fresh ardour, to ex-
plore this new world that here opens to our view.

Turning, then, from the examination of the pas-
sive objects of the material world, we now direct
our attention to the busy theatre of animated ex-
istence, where scenes of wonder and enchantment
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 peo-
pled by multitudinous races of sensitive beings,

FINAL CAUSES. 9

who have received from the bounteous hand of tlieir
Creator the gift of existence and the means of en-
joyment. Our curiosity is powerfully excited by
phenomena in which our own welfare is so inti-
mately concerned, as are all those that relate to
animal life; and we cannot but take a lively and
sympathetic interest in the history of beings in so
many respects analogous to ourselves ; like us, pos-
sessing powers of spontaneous action, impelled by
passions and desires, and endowed with capacities
of enjoyment 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, revel-
ling in the bliss of existence, and testifying by its
incessant 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 profound
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 end-
less .modifications of shape, still preserve their con-
formity 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

* Four-fiftbs of the insects at present known have been discovered
within the last ninety years: for in 1743, Ray estimated the total
number of species at 20,000 only. See liis work on " The Wisdom
of God as manifested in the Creation," p. 24.

10 FINAL CAUSES.

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-
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 compatible
with an adherence to the same general model of
design, and the same ultimate reference to the capa-
city 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 ani-
mals ! Of Fishes alone the varieties, as to confor-
mation and endowments, 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 divi-
sions of the animal scale ; some swimming in count-
less myriads near the surface ; some dwelling in the
inaccessible depths of the ocean : some attached to
shells, or other solid structures, the productions of
their own bodies, and which, in process of time,
form, by their accumulation, enormous submarine
mountains, rising often from unfathomable depths
to the surface. What sublime views of the mag-
nificence of creation have been disclosed by the
microscope, in the world of infinite minuteness,
peopled by countless multitudes of atomic beings
which animate almost every fluid in nature? Of
these, a vast variety of species has been discovered,
each animalcule being provided with appropriate
organs, endowed with spontaneous powers of mo-

FINAL CAUSES. 11

tion, and giving unequivocal signs of individual
vitality. The recent observations of Professor
Ehrenberg have brought to light the existence
of 3Ioimds, 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 com-
putation 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 mountain sum-
mits to the dark abysses of the deep ; if we pene-
trate 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 stagnant 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 iind life pro-
duced. It would almost seem as if Nature* had
been thus 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

* In order to avoid the too frequent, and consequently irreverent,
introduction of the Great Name of the Supreme Being into famiUar
discourse on the operations of his power, I have, throughout this
Treatise, followed the common usage of employing the term Nature
as a synonym, expressive of the same power, but veiling from our
feeble sight the too dazzling splendour of its glory.

12 FINAL CAUSES.

combinations, for their repetition in endless per-
petuity, and for their subordination to one har-
monious 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
our reasoning faculties, and abundant sources are
supplied of intellectual enjoyment. To discrimi-
nate the different characters of plants, amidst the
infinite diversity of shape, of colour, and of struc-
ture, which they offer to our observation, is the la-
borious, 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 differ-
ences of situation, sustains the modified life of each
individual plant, and which continues its species
in endless perpetuity. Wherever circumstances
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 con-
sequence of the germination of seeds which had
remained latent and inactive during the lapse of
perhaps many centuries. Islands formed by coral
reefs, Avhich 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 vet recent cinders and lava of the

FINAL CAUSE?. 13

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 afibrds 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 successive
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 gar-
den, 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 the traveller is often refreshed
by the appearance of a i'ew hardy plants, wdiich
find sufficient materials for their growth in these
arid regions : and in the realms of perpetual snow
which surromid the poles, the navigator is occa-
sionally startled at the prospect of fields of a scarlet
hue, the result of a wide expanse of microscopic
vegetation.*

But whatever charms the naturalist may find in
the occupations in which he is engaged, and how-

* The red snow, discovered in Baffin's Bay on the 17th of August,
1818, during the Northern Expedition under the command of Cap-
tain, now Sir John Ross, was found to owe its colour to minute
fungi, or microscopic mushrooms (Protococcus nivalis), which vege-
tate on the surface of snow, as their natural abode. (See Philoso-
phical Transactions for 1820, p. 16.5.) Another microscopic plant,
the Palmella nivalis, is met with on the granular snow, called Firn,
of the glaciers of Switzerland. Snow is likewise the natural habita-
tion of a minute insect, the Podura nivalis, which is met with in no
other place. (Mag, Nat. Hist. iv. 30.) Great numbers of infu-
sorial animalcules have also been seen in melted snow. (Silliman's
Journal, xviii. 50.)

14 FINAL CAUSES.

ever wide may be the field of his exertions, they
still are insufficient to satisfy the more enlarged
curiosity of a philosophic mind. The passive emo-
tion of astonishment, in which inferior intellects
are content to rest, serves but to awaken, in him
who has learned to think, a desire of further know-
ledge. Filled with an ardent spirit of inquiry, he
cannot but be impatient 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 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 se-
veral actions that take place within it. But, over-
whelmed 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 la-
bours wdll be fruitless, unless, previously to any
attempt at theory, he takes a careful and accurate
account of all the circumstances attending the his-
tory and conditions of life, from the dawn of its
existence to its appointed close. On tracing living
beings to their origin, he learns that every indi-
vidual vegetable and animal takes its rise from an
atom of imperceptible minuteness, and gradually
increases in bulk by successive accretions of new
matter, derived from foreign sources, and, by some
refined, but unknown process, transmuted into its
own substance. Then, following the progressive

FINAL CAUSES. 15

developement of the organs, he observes them un-
dergoing various modifications, as they are assum-
ing new forms, which characterise certain definite
epochs in the general growth of the system. In a
great number of instances, especially among the
lower orders of animals, he witnesses the same indi-
vidual being acting, in its time, a variety of dif-
ferent parts ; often re-appearing 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 maturity
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 arrangements 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 exercising the functions of life,
that the materials which compose its fabric should
undergo a certain 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 proportion
may be maintained between the supply and the
consumption of these materials. In the course of
a certain time, however, even under the most fa-
vourable circumstances, this equilibrium begins to
fail ; the energies of the system decline, and the
processes of nutrition are insufficient to repair the
waste in the substance of the body. The fluids are

16 FINAL CAUSES.

dissipated faster than they can he renewed ; the
channels through which they circulate are more
and more obstructed, and at length cease to be per-
vious ; 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 duration. Even
when exempt from external interference, 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 ac-
cidental causes which may hasten or precipitate
their destruction, long before the period of natural
decay. How striking is the contrast, on those oc-
casions, between the scene w^e have just beheld of
an animal in the full vigour of its powers, either
rapidly bounding across the plain, or gliding be-
neath 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 contem-
plate without amazement so complete and instanta-
neous a change ; so sudden and awful a catastrophe?
Shall we not be animated by an eager desire to
penetrate so great a mystery, and resolve the many

FINAL CAUSES. 17

questions which so striking a phenomenon will na-
turally 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
delighted the eye by its beauty, and producing this
sudden and irretrievable extinction of the powers
of life? How comes it that all those mighty ener-
gies 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 fabric ? What potent spell has
been dissolved, which could retain in combination
for so long a period the multifarious elements of
that complex 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 mechanical powers have been
employed in its construction? What refined che-
mistry has been exerted in assimilating new par-
ticles of matter to those previously organized, and
in appropriating them to the nourishment of the
parts with which they became identified? By
what transcendent power, above all, did this assem-
blage of material particles first become animated
by the breath of life; and from what elevated
source did they derive those higher energies, appa-
rently 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 intelligence? Shall we ever com-

VOL. I. c

18 FINAL CAUSES.

prebend the nature of that subtle and pervading
principle, by the agency of which all these won-
derful 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 fabric, 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 cultiva-
tion of the science of physiology. But before
engaging in this arduous study, we ought previously
to inquire into the methods of reasoning by which
it is to be conducted.

The object of Physiology is, by the diligent ex-
amination of the phenomena of life, to ascertain
the laws which regulate those phenomena, both as
they apply to the individual beings endowed with
life, and also as they relate to the various assem-
blages that constitute the species, 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 before
observed, of two kinds, each founded upon rela-
tions of a different class. The first, which depend
upon the simple relation of cause and effect, are
concerned merely with the natural powers of mat-
ter. They are the laws that regulate the succession
of phenomena purely physical in all their stages.
These phenomena consist in changes among mate-
rial particles, which are either of a mechanical or
chemical nature ; or in the affections of imponder-

FINAL CAUSES. 19

able physical agents, such as heat, light, electricity,
and magnetism ; and they include also the pheno-
mena which take place in organized bodies, and
which are referable to the operation of certain
physical powers, appertaining to particular struc-
tures, such as muscular contraction and nervous
irritation ; phenomena which, as we shall after-
wards find, are not reducible to any of the former
laws, but are apparently peculiar to the living state.
The second class of laws comprises those which
are founded on the relation of means to an end ;
and which are usually denominated Jinal causes.
They involve the operations of mind, in conjunction
with those of matter. They pre-suppose intention
or design ; a supposition which implies intelligence,
thought, motives, volition, — particular purposes to
be answered, requiring the agency of powers and
of instruments adapted to the production of the in-
tended effects : — the knowledge of the properties
of matter, the selection 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 good.

The study of these final causes is, in some mea-
sure, forced upon our attention by even the most
superficial survey of nature. It is impossible 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

20 FINAL CAUSES.

with their history. Microscopic 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
successive expansion and developement of 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 fibre,
resulting from this developement, contributes its
share in the production of certain definite effects,
which we constantly see taking place around us, as
well as experience in our own persons. But these
effects, though so familiar to us, are not on that
account the less involved in mystery, or the less
replete with wonder. To say that they are the re-
sults of chance conveys no information ; and is
equivalent to the assertion that they are wholly
without a cause. Every one who is accustomed to
reflect upon the operations of his own mind must
feel that such a conclusion is contrary to the con-
stitution 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 estab-
lished and invariable order of sequence among the
changes which take place in the universe.

But though it be granted that all the phenomena
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 unknown to us, but
that their discovery is wholly beyond the reach of
our faculties. The argument is specious only be-
cause it is true in one particular sense ; but that

FINAL CAUSES. Hi

sense is a very limited one. Those who urge it, do
not seem to be aware that its general application, in
that very same sense, would skake the foundation
of every kind of knowledge, even that which we
regard as built on the most solid basis. Of causa-
tion, it is agreed that we know nothing ; all that we
do know is, that one event succeeds another with un-
deviating 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 external objects, in their undergo-
ing certain changes, and in their possessing certain
physical properties, rests on a different foundation,
namely, the evidence of our senses; for it is the
result of inferences which the mind is, by the con-
stitution of its frame, necessarily led to form. We
may trace to a similar origin the persuasion, irre-
sistibly 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 gra-
vitation, or the subtle sources of electricity and
magnetism. We nevertheless 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 impressions made upon our senses, of
which impressions 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

22 FINAL CAUSES.

centered in myself; but that there exist other in-
tellects similar to my own ? Undoubtedly no other
than the observation that certain effects are pro-
duced, which the experience I have had of the
operations of my own mind leads me, by an irre-
sistible analogy, to ascribe to a similar agency,
emanating from other beings ; beings, however, of
whose actual intellectual presence I cannot be con-
scious, whose nature I cannot fathom, whose essence
I cannot understand. I can judge of the opera-
tions of other minds only in as far as those opera-
tions accord with what has passed in my own. I
cannot divine processes of thought to which mine
have borne no resemblance ; I cannot appreciate
motives of which I have never felt the influence,
nor comprehend 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 diffi-
culty of applying this scale of measurement will,
of course, increase in proportion to the difference
between the objects compared; and although we
may conceive that there are powers and intelli-
gences infinitely surpassing our own, the concep-
tions we can form of such superior essences must
necessarily be indefinite and obscure, 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

FINAL CAUSES. 23

expressed. When, on the other hand, the objects
contemplated are more within the range of our
mental vision ; when, for instance, they are pheno-
mena that we can assimilate to our own voluntary
acts, and in which we can clearly trace the con-
nexion between means and end, then does our
recognition of the agency of intellect become most
distinct, and our conviction of its real and indepen-
dent 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.*

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 between
His works and those of man ; and strive to gather
some faint notions of the divine intelligence by
applying the only standard of admeasurement
which we possess, and are permitted to employ,
namely, that derived from the operations of human
intellect. Our interpretations of the designs of the
Creator must here be obtained through the medium
of human views ; and our judgment of His benevo-
lence can be formed only by reference to our own
affections, and by their accordance with those fer-

* The view here taken is, of course, Hmited to Natural Theology ;
that being the express and exclusive object of these Treatises.

24 FINAL CAUSES.

vent 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 un-
known 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 mechanist, possessed of in-
telligence, actuated by a motive, and guided by
intention. Farther, if we had a previous expe-
rience of the operation of similar kinds of me-
chanism, we could not doubt that the effect we saw
produced was the one intended by the artificer.
Thus, if in an unexplored country, we saw, moving
upon the waters of a lake, the trunk of a tree,
carved into the shape of a boat, we should imme-
diately conclude that this form had been given to
it for the purpose 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 hesi-
tate to conclude that the whole was the work of
human hands, and the product of human intelli-
gence 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 con-
trivances, and our ideas of the skill of the artificer
would rise in proportion to the excellence of the
apparatus, and the ingenuity displayed in its adap-
tation to circumstances.

Let us suppose that in another part of this lake

FINAL CAUSF.S. 25

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, ex-
tending 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 forming it of
this shape, and providing it m ith 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 me-
dium? 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 propitious, and
by means of which it is wafted along the surface of
the sea, but which it quickly retracts in unfavour-
able circumstances, is not her design similar to
that of the human artificer, when he equips his
bark with sails, and provides the requisite machi-
nery for their being hoisted or furled with ease and
expedition ?

The maker of an hydraulic engine places valves

* Such as the Notonecta glauca, Lin., or water boatman, and
the Dytiscus marginalis, or water beetle.

26 FINAL CAUSES.

in particular parts of its pipes and cisterns, 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 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 unknown
to the Power who created the torpedo, and who
armed it with an energetic galvanic battery, con-
structed 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 cir-
cular window for the light to enter, and spread out
on the opposite side a canvass to receive the picture ?
Has no thought been exercised in darkening the
walls of this camera obscura, and thus preventing

FINAL CAUSES. 27

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 de-
fects, unavoidable in all optical instruments of
human workmanship, are remedied. Of this nature
are those which render the organ achromatic, which
correct the spherical aberration, 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 protec-
tion, the preservation, 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 suffi-
cient 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 ob-
servations have extended. The truths they teach
not only acquire confirmation by the corroborating
tendency of each additional fact of the same de-
scription, but the multitude of these facts is so great,
that the general conclusion 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

28 FINAL CAUSES.

point, on which they concentrate their united
powers of illumination.

The more we extend our knowledge of the ope-
rations of creative power, as manifested in the
structure and economy of organized beings, the
better we become quahfied to appreciate the inten-
tions with which the several arrangements and con-
structions have been devised, the art with which
they have been accomplished, and the grand com-
prehensive plan of which they form a part. By
knowing the general tendencies of analogous for-
mations, we can sometimes recognise designs that
are but faintly indicated, and trace the links which
connect them with more general laws. By ren-
dering ourselves familiar with the hand- writing
where the characters are clearly legible, we gra-
dually learn to decypher the more obscure passages,
and are enabled to follow the continuity of the nar-
rative through chapters which would otherwise ap-
pear mutilated and defaced. Hence the utility of
comprehending in our studies the whole range of
the organized creation, with a view to the discovery
of final causes, and obtaining 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 FUNCTIONS OF LIFE. 29

the individual welfare of the animal, embracing the
whole sphere of its sensitive existence, 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
continuance of the race to which each animal be-
longs. The third includes all those arrangements
which have been resorted to in order to accommo-
date the system to the consequences that result
from a power of indefinite increase in the numbers
of each species. The fourth class relates to that
systematic economy 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 inquiry.

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 sen-
sitive enjoyment. To this all the arrangements of
its system, and all the energies of its vital powers
ultimately tend. Of what value to its possessor
would be mere vegetative life, unless accompanied
by the faculty of sensation, and unless the sensa-
tions thence arising were attended with pleasure?
It is only by reasoning analogically from the feel-
ings 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 feel-
ing are most distinct, we find that they result from
a peculiar organization, denominated the nervous

.30 THE FUNCTIONS OF LIFE.

substance, and Qonsisting of a soft, pulpy matter, of
a greyish-white colour, exhibiting traces of a fibrous
structure. This substance has been termed neurine.
The name of brain is given to a particular mass of
neurine placed in the interior of the body, where it
is carefully protected from injury.

The sensations, for exciting which the brain is
the material instrument, are the result of certain
impressions made on particular parts of the body,
and conveyed to that organ by the medium of fila-
ments, composed of a similar substance, and termed
nerves. In this way, then, it has been provided
that a communication 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 affec-
tions, both of sensation and of intellect. A consi-
derable 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 provided to
receive and collect them, so as not only to give rise
to varied sensations, but also to convey a know-
ledge 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 surrounding
objects, and of deriving from them various sensa-
tions of pleasure and of pain, without 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

THE rUNCTlONS OF LIFE. 31

merely passive, and the sport of every contingent
agency, would have been not merely useless, but
even baneful endowments. The same beneficent
power, which has conferred these gifts, has con-
joined that of voluntar}^ 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 sen-
sation, 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 employed 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 sensa-
tion 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 mobility requisite
for the different actions which the animal is to per-
form : and, secondly, the provision 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 protection, and other
mechanical conditions of the organs of the body.
It will be convenient to comprehend these under
one general head, considering them as composing

32 THE FUNCTIONS OF LIFE.

the Mechanical Functions of the animal economy.
They will engage a considerable share of our atten-
tion 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 continual 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 inquiry, and furnishing abundant
evidence of the w ise and beneficent operations of
nature. These may be comprehended under a sepa-
rate class bearing the general title of Nutritive Func-
tions. 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,
perception, and voluntary motion, which are the
ultimate ends of animal existence, and w hich 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 mechanical con-
struction, adapted to those objects. To the contri-
vances of the mechanist must be added a refined
hydraulic apparatus for the conveyance of fluids,

THE FUNCTIONS OF LIFE. t)t)

and for the regulation of their movements ; and
with these must be conjoined the skilful combina-
tions of the laboratory, by which the powers of a
most subtle chemistry are exercised in effecting all
tlie transmutations required by this complex system
of operations. As far as they involve mechanical
principles, these objects again arrange themselves
under the mechanical functions : and I shall ac-
cordingly 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 accomplish these peculiar com-
binations; and neither the organs v\ith 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 substances 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 or-
ganized beings. This important office is consigned
to the vegetable kingdom. Receiving the simple
food furnished by nature, which consists chiefly of
water, air, and carbonic acid, together with a small
proportion of other substances, plants convert these
aliments into products, which not only maintain
their own vitality, but serve the further purpose of
supporting the life of animals. Thus was the crea-
tion and continuance of the vegetable kingdom a

VOL. 1. D

34 THE FUNCTIONS OI' LI IE.

necessary 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 tinal causes,
and will in this Treatise be reviewed with special
reference to this important object.

Nutrition, both in the vegetable and animal sys-
tems, comprises a very extended series of operations.
In the former it includes the absorption of the crude
materials from the surrounding 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 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-
ment of the whole plant.

Still more various and complicated are the cor-
responding functions in animals. Their objects
may be arranged under the following general heads ;
each, again, admitting of further subdivision. The
first end to be accomplished is to animalize the
food ; that is, to convert it into a matter having the
chemical properties of the animal substances with
which it is to be afterwards incorporated. The
entire change thus effected is termed Assimilalioii,
of which Digestion forms a principal part. The
second object is to collect and distribute this pre^
pared nutriment, which is the blood, to the diffe-
rent organs, or wherever it may be wanted. The
necessary motions for these purposes are given to

THE FUNCTIONS OF LIFE. .'lo

tlie 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 che-
mical action of oxygen : a purpose which is an-
swered by the function of Respiration. The fourth
stage of nutrition relates to the more immediate
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 constantly tending
to definite ends. Most of the parts of which the body
consists undergo continual and progressive changes
in their dimensions, figure, arrangement, and com-
position. The materials which have been united
together and fashioned into the several organs,
are themselves successively removed and replaced
by others, which again are, in their turn, discarded,
and new materials substituted, though v/ithout any
perceptible change of external form. Perpetual
mutation appears to constitute the fundamental law
of living nature ; and it has been further decreed
by the power which gave the first impulse of ani-
mation 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 conse-

.'56 THi: FUNCTIONS OF LIFE.

quence of the law of mutation ; and the same
causes that originally effected the developement
and growth of the system, and maintained it in the
vigour of its maturity, by continuing to operate, are
certain to lead to the demolition of the fabric which
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 renovations.

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 employed to give origin
and birth to the offspring. The process itself, by
which the germs of living beings originate, is veiled
in the most impienetrable 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
economy 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 setting up
of temporary structures, which are required only at
an eaily stage of growth, and which are afterwards

THE FUNCTIONS OF LIFE. .37

iLiiioved to give place to more periiuuient 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 circumstances.
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 espe-
cially, flow from this law of indetinite production.
As animals are ultimately dependent on the vege-
table 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 Mdiich vegetation is spread, a time must
arrive when the number of animals thus continually
increasing is exactly such as the amount of food
produced by the earth will maintain. When this
limit has been attained, no further increase could
take place in their number, except by resorting to
the expedient which we find actually adopted,
namely, that of employing the substance of one
animal for the nourishment of others. Thus the
identical combinations of elements, effected by the
powers of vegetation, are transferred in succession
from one living being to another, and become sub-
servient to the maintenance of a great number of
different animals before they finally, by the process
of decomposition, revert to their original inorganic
state.

" See dying vegetables life sustain.
Sec life dissolving vesretate again ;

38 THE rUNCTIONS OF LIFE.

All forms that perish other forms supply,

By turns we catch the vital breath and die." — Pope.

Hence lias 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 animals.
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 sub-
stances, which might otherwise have filled the air
with noxious exhalations, and contaminated 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 locomo-
tion must be supplied for the pursuit of prey, and
powerful weapons for attack and destruction. But
nature has not left the weaker animals unprovided

* As specially appointed for the performance of this useful office
in the police of nature may be cited, among the larger beasts of prey,
the hysena and the jackall ; among birds, the crow and the vul-
ture ; 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. 39

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 enjoy
ment from the exercise of those powers and facul-
ties which are called forth by this state of constant
activity ; and when this consideration is combined,
as it ought to be, with that of the immense multi-
plication of life which is admissible upon this system
alone, we shall find ample reason for acknowledg-
ing the wisdom and the benevolent intentions of
the Creator, who, for the sake of a vastly superior
good, has permitted the existence of a minor evil.

From this system of hostilities there must also
arise new relations among the different races of
animals. It aftbrds a ready and effectual means
of preserving the proper balance between different
races. Each separate species of animals, far from
being isolated and independent, performs the part
assigned to it in the system of nature, and, how-
ever apparently insignificant, may have a sensible
influence on the rest of the animal creation. Man,
above all other animals, has effected a most im-
portant change in the condition of a multitude of
other races, in every region where his numbers

40 THE FUNCTIONS OF LIFE.

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 characteristic
and predominant feature of her productions. It is
only when the object to be attained is dependent
upon certain detinite conditions, excluding the pos-
sibility of modification, that these conditions are
uniformly and strictly adhered to. 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 may seem. Even when the purpose to
be answered is identical, the means which are
employed are infinitely diversified in different in-
stances, as if a design had existed of displaying to
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 admis-
sible permutations in the order of their union.

Some wise purpose, though dimly perceptible to
our imperfect understandings, is no doubt answered
by this great law of organic formation, the Imv
of variety. That it is not blindly or indiscriminately
followed, is apparent from its being circumscribed
within certain limits, and controlled by another
law, which we have next to consider — that of con-
Jormily to a definite type.

THE FUNCTIONS OF LIFE. 41

The most superficial survey of nature is suffi-
cient to show that there prevail certain general
resemblances among great multitudes of species,
which lead us to class them into more or less com-
prehensive groups. Thus in the animal kingdom,
Quadrupeds, Birds, Fishes, Reptiles, Shell-fish,
and Insects, compose natural assemblages or classes,
and each of these is readily divisible into subordi-
nate 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 conducted 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 sepa-
rate copies, differing, indeed, as to particulars, but
agreeing as to general characters. The same ob-
servation 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 pur-
pose is usually accomplished by similar 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 pre-
sented to us, from our general knowledge of the
characters and economy of the tribe or family, on
the type of which it has been modelled. Thus the
discovery of a mutilated portion of the skeleton of a
fossil animal, conveys to the physiologist, who is

42 THE FUNCTIONS OF LIFE.

conversant with the details of comparative anatomy,
a knowledge 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.*

Not only does this tendency to conform to parti-
cular types obtain in all organic formations, but
further inquiry leads to the conclusion that the de-
viations from these standard forms, far from being
arbitrary, are themselves referable to definite laws.
The regulating principle of the variations is subor-
dinate to higher views, and has reference to the
respective objects and destination of each particular
species in the general system of created beings.
Nature, as far as we can discern, appears, in con-
formity 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 ac-
commodated to this general design. Hence arises
the necessary and reciprocal dependence of each
organ and of each function on every other; and hence
are deduced what have been termed the laws of 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 cor-
responding variations in other parts, and extends its
influence in modifying, in a greater or less degree,
the whole fabric. It is in comparative anatomy as
in mechanics, where any alteration made in the

* See Cuvier's " Discours sur les revolutions de la surface du
globe," p. 47, prefixed to the first volume of his " Ossemens Fos-
siles."

THE FUxVCTIONS OF LIFE. 43

position of one part of a system of bodies occasions
a change in the centres of gravity, of gyration, and
of oscillation ; 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 harmony
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 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 be-
tween adjacent types, and appear as links of con-
nexion 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 apper-
tain to each.

44 THE FUNCTIONS Ol LIFE.

All these ajiparent anomalies and gradations of
structure tend still farther to demonstrate the gene-
rality of the plans of nature, and the comprehen-
siveness 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
iill 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 liglit on the most in-
teresting questions in Geology as well as in Phy-
siology.

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 regu-
larly ascending to the most refined and compli-
cated organizations, till it reached its highest point
in man, who is unquestionably placed at the sum-
mit of the scale. Bonnet, in particular, cherished
with enthusiastic ardour the hypothesis that all
organic beings formed a continuous 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 applying it even to the productions of
the mineral world. But, divesting ourselves of
these hypothetical views and figurative images, we
find, on sober observation, that instead of one con-
tinuous series, we are presented with only detached

THE FUNCTIONS OF LIFE. 45

fra«?ments and interrupted portions of this imagi-
nary system : so that, if, for the sake of illustration,
we must employ a metaphor, the natural distribu-
tion 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 variety of facts,
have been adduced in favour of another hypothesis,
namely, that the real types or models of structure,
are more correctly represented by circular, or re-
currinar arrangements.* But as the discussion of
these and other topics relating to the plans and
designs of nature in the formation of organic beings
requires 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 formation of the diffe-
rent 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 origi-
nal creation of species has been successive, and
took place in the order of their relative complexity
of structure ; that the standard types have arisen
the one from the other; that each succeeding form
was an improvement upon the preceding, and fol-
lowed in a certain order of developement, according

* Mr. M'Leay is the author of this circular hypothesis, whicli
he lias developed in his " HorcB Entomohgicce."

46 THE FUNCTIONS OF LIFE.

to a regular plan traced by the great Author of the
universe for bestowing perfection on his works.
This gradation of structure was necessarily accom-
panied 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 ultimate
ends of the animal creation. Many apparent ano-
malies which are inexplicable upon any other
supposition, are reconcilable to this theory. The
develoj)ements of structure belonging to a parti-
cular type, being always prospective, are not com-
pleted in the inferior orders of the group formed
on that model, but remain more or less imper-
fect, although each organ always fully answers the
particular purpose of the individual animal. But
it sometimes happens that the imperfection of an
organ is so great, in consequence of its develope-
ment having proceeded to a very small extent, as
to render it wholly useless in that particular spe-
cies, although in a higher race of animals it fully
performs its proper function. Thus we shall find
that rudiments of feet are contained within the
bodies of various kinds of serpents, which can ob-
viously not be serviceable as organs of progression.
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 growtli is arrested, and they afterwards
wholly disappear. This imperfect or rudhnental
condition of an organ indicates its relation to other
species belonging to the same type, and demon-
strates the existence of a general plan in their for-
mation. I shall have occasion to mention several

THE rUNC'I'lONS OF LIFF,. 47

striking instances 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 purpose
foreign to its usual destination, or to qualify 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 auxiliary organs of progressive mo-
tion ; and in the Draco volans, or flying lizard, they
are extended outwards from the sides to serve as
wings. The teeth, usually intended for mastica-
tion, are in many animals enlarged in order to con-
stitute weapons of offence, as in the Elephant, the
Boar, the Nanvhal, and the Pristis. In like man-
ner, in the Crustacea, organs of the same general
structure are converted sometimes into jaws, some-
times 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 dis-
tinction between them.

In pursuing the ascending series of animal struc-
tures we meet also with instances of a contrary
change, yet still resulting from the continued ap-
plication 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 circum-
stances may even render it w holly useless. In such
cases we find that it is gradually discarded from
the system, becoming continually smaller, till it

48 THE FUNCTIONS OF LIFE.

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 arrangement 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 arrange-
ment is exhibited in the similar segments of annu-
lose and other articulated animals, and also in the
pieces which compose the spinal column ofverte-
brated animals. In these two latter classes, also, a
remarkable law of symmetry obtains in the forma-
tion of the two sides of the body, which exhibits
the lateral junction of similar but reversed struc-
tures. The violations of this law are extremely
rare ; yet some remarkable instances of anomalous
formations in this respect will hereafter be noticed.

In treating of the particular functions of the
animal and vegetable economy I shall follow a dif-
ferent order from that in which I have presented
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 Nu-
tritive or Vital functions both of vegetable and
animal structures : for as they involve the chemical
properties of organized substances, and are, there-
fore, of a more refined and intricate nature than the

THE FUNCTIONS OF LIFE. 4.0

preceding, I conceive they will be best nnderstood
after the general mechanism of the frame has been
explained. These studies will prepare us for the
consideration of living animals as sentient and
active beings, endowed by their bounteous Creator
with the exalted faculties of perception and of voli-
tion, 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 revealed 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.

VOL. I.

50

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 combinations of material
particles which constitute organizatio7i. All these
arrangements, both as respects the mechanical con-
figuration and the chemical constitution of the ele-
ments of which the organized body is composed,
even when apparently most simple, are, in reality,
complex and artificial in the highest possible de-
gree. 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 appear-
ance homogeneous, spherule. No one, unaccus-
tomed to explore the wonders of nature, would sus-
pect that so simple a body, which he might sup-
pose to be formed of a uniform material cast in a
mould, would disclose, when examined under a
powerful microscope, and with the skill of a Brew-

ORGANIC MECHANISM. 51

ster, the most refined and exquisite 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 an-
other 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 im-
portant 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 inscrut-
able, but surpassing all our powers of conception !
It is from the energies of life alone that organic
forms are produced. No fabric achieved by human
power ever approached in refinement the simplest
of nature's works. The utmost efforts of the inge-
nuity or skill of man in the construction of the
most delicate machinery is infinitely surpassed by
the most ordinary of the 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 unvivitied statues of Prometheus,
must remain for ever mere masses of insentient and
inert materials.

52 THE MRCHANICAL FUNCTIONS.

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 particles of a fluid, being
equally moveable in every direction, have no deter-
minate relative situations, and possess no character
of permanence. All organic and living structures,
therefore, 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 perfectly
dry state ; as is proved by the examples of vege-
table seeds, and also of many species of animal-
cules, and even of some of the more highly organized
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 imder
these circumstances is matter of common observa-
tion; but the revivification of animalcules is a more
curious phenomenon, for it takes place more ra-
pidly, and is more striking in its results. The
Rotifer vulgaris (Ehr.) or wheel -animalcule,*
(Fig. 1.) which was first observed by Lewenhoeck,
and was afterwards rendered celebrated by the ex-

* Vorticella rotatoria of Muller, and Furcularia rediviva of
Lamarck. This animalcule was first described by Eichhorn, in
1767, under the name of Wasserbar, or water bear. Corti, iu
1774, first observed the phenomenon of its reviviscence after desic-
cation. Spallanzani's researches on the same subject were made in

ORGANIC MECHANISM. • 53

periments made upon it by Spallaiizani, 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 func-
tions of life shall be completely suspended, yet
without the destruction of the vital principle ; 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 alternate
suspension and restoration of life may be repeated,
without apparent injury to the animalcule, 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, ap-
pears in the form of a fine powder: on being moist-
ened, it soon resumes its living and active state. f
The Gordius aquaticus, or hair worm, which inha-
bits stagnant pools, and which remains in a dry,

1776 ; and their accuracy has been since fully verified by Schultze,
who communicated his observations on this animalcule, which he
called the Macrobius Hufelandii, to the meeting of German natu-
ralists at Breslau. (See the his for 1834, p. 708 : and Dujardin,
Ann, Sc. Nat. serie 2, x, 181.)

t Bauer, Philosophical Transactions for 1823, p. 1,

54 THE MECHANICAL FUNCTIONS.

and apparently lifeless state when the pond is eva-
porated, will, in like manner, revive, in a very
short time, on being again immersed in water. The
same phenomenon is exhibited by the Filaria oculi,
a thread-like parasitic worm, infesting the cornea
of the eye of the horse.*

Both the composition of the fluid and the texture
of the solid parts of animal and vegetable bodies
are infinitely varied, according to the purposes they
are designed to serve in the economy. Scarcely
any part is perfectly homogeneous ; that is, com-
posed throughout of a single uniform material.
Few of the fluids are entirely limpid, and none are
perfectly simple in their composition ; for they ge-
nerally contain more or less of a gelatinous matter,
which, when very abundant, imparts to them vis-
cidity, constituting an approach to the solid state.
Many fluids contain minute masses of matter, gene-
rally 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.t We next perceive that
these globules have, in many instances, cohered, so
as to form solid masses ; or have united in lines so
as to constitute fibres. We find these fibres col-
lecting and adhering together in bundles ; or inter-
woven and agglutinated, composing various other
forms of texture; sometimes resembling a loose net-
work of filaments ; sometimes constituting laminae
or plates ; and, at other times, both plates and fila-

* De Blainville, Annales des Sciences Natu relies ; x. 104.

t Globules of this description are found not only in the blood,
and in milk, where they exist in great abundance, but also in the
lymph, the saliva, and even in the aqueous humour of the eye.

ORGANIC MECHANISM. 55

merits 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
diflerent degrees of complication, according to the
respective functions they are called upon to perform.
We shall now examine the several kinds of tex-
tures 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 compara-
tively passive condition, require for the perform-
ance of these functions mechanical constructions of
a very different kind from those which are neces-
sary to the sentient, the active, and the locomotive
animal. The organs essential to vegetables are
those which receive and elaborate the nutritive
fluids they require, those which are subservient to
reproduction, and also those composing the general
framework, Mhich must be superadded to the whole
for the purpose of giving mechanical support and
protection to these finer organizations. As plants
are destined 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 inju-
rious action of the elements, so we find these several
objects provided for by three descriptions of parts :
namely, first, the Roots, which fix plants in their

•56 THIi MECHANICAL FUNCTIONS.

situation ; secondly, 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 elementary
texture met with in vegetables is formed of exceed-
ingly minute vesicles, the coats of which consist of
transparent membranes of extreme tenuity.^ Fig. 3

* Schleiden has made the curious discovery that every vegetable
cell is the result of the developement of a very minute body, or
7iucleus, which he terms a cytoblust. Its shape is globular, or oval,
and sometimes flattened, or lenticular; it is apparently composed
of minute grains, the outlines of which are not very distinct; and it
generally contains a still more minute, but well defined, central par-
ticle, which he terms the nucleolus. The size of the nucleus varies
from the 10,COOth to the 500th of an inch. The cytoblast, or more
probably the nucleolus, is the first visible germ of the structure
which is to be formed; and the developement of which takes place
by the rising, from the surface of the cytoblast, of a minute trans-
parent vesicle, which on its first appearance may be compared to a
watch-glass set on a watch. This vesicle gradually enlarges, spread-
ing at first equally in all directions, and constituting a cell ; and
afterwards expanding more in particular parts, so as to acquire the
peculiar form which characterizes the texture to be fabricated : such
texture being always in the first instance composed of an aggrega-
tion of similar cells. In the progress of developement, these cells
undergo various subsequent modifications ; first, by extension in
particular directions ; secondly, by partial or general adhesions of
their membranes ; and thirdly, by depositions of other materials,
either in the substance of their coats, or within their cavities, or in
the intervening 'spaces. These materials are frequently deposited in
spiral lines on the membranes of the vesicles ; thus constituting the
spiral cells, and spiral vessels of plants, which are presently to be
noticed. This spiral formation appears to result from the rotatory
movement of a fluid along the walls of the cells, between them and
the central mass of gelatine.

During a great part of this process, the nucleus, with its nucle-

VEGIiTABLE ORGANIZATION. 57

is a highly magnified representation of the simplest
form of these vesicles.* But they generally ad-
here together more closely, composing by their
union a species of vegetable cellular tissue, which
may be regarded as the basis or essential compo-
nent material of every organ in the plant. This
cellular structure is represented in figures 4 and 5,
as it appears in the Fiicns vesiculosus ; the first being
a horizontal, and the second a vertical section of
that plant.t The size of these cells differs consi-
derably 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 fiom

olus, is retained within the cell to which it gave origin ; and, with
the aid of a good microscope, it may still be seen either loose within
the cavity of the cell, or adhering to its sides. But by the time the
cell has grown to its full size, the nucleus has generally disappeared,
having apparently dissolved itself in the fluid contents of the cell.
Moreover, the cell never shows any trace of spiral fibres, previously
to its having attained its complete growth, and before the absorption
of the cytoblast.

A new set of nuclei, again, are seen to form, either within the
cell, or in the intercellular spaces ; and they give rise to a train of
phenomena exactly similar to that already described ; and thus new
cells, furnishing the fabric of growth, are supplied, and their number
multiplied in continued succession.

The frequent presence of the nucleus which formed the vegetable
cells had already attracted the notice of botanists; and its import-
ance was specially recognised and pointed out by Mr. Robert
Brown, in his masterly researches on the OrcliidecE : but the merit
of the discovery of its being the germ of the vegetable cell is due to
Schleiden. See his memoirs in Miiller's Archiv. fiir Anatomie und
Physiologie, Part II. 1838; a translation of which memoir will be
found in R. Taylor's Scientific Memoirs, Vol. II. Part vj. p. 281.

* These cells are well represented in the engravings which illus-
trate Mr. Slack's memoir on the elementary tissue of plants, con-
tained in the 49th volume of the Transactions of the Society of Arts.

+ De Candolle, Organographie Vegetale. - •

58 THE MECHANICAL FUNCTIONS.

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 transformed
into other shapes, either by the mutual compres-
sion which they sustain from being crowded into a
limited space, or from unequal expansions in the
progress of their developement. From the first of
these causes they often acquire angles, assuming the
forms of irregular rhomboidal dodecahedrons, and
often of hexagonal prisms, like the cells of a honey-
comb ; and by the second, they are elongated into
cylinders, or slowly tapering cones, thus passing
by insensible gradations into the tubular form.
Figures 6, 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

VEGETABLE ORGANIZATION. 59

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.

The chemical composition of the vegetable tissue,
composing these primitive cells and tubes, appears
to be identical with starch, oxfecula. Like starch,
it is rendered blue by the action of iodine ; and it
possesses the same optical property of giving to the
rays of polarized light which traverse it a circular
rotation directed towards the right : it contains no
nitrogen. This primitive vegetable membrane pre-
sents universally the same composition and proper-
ties throughout the vegetable kingdom ; whether
examined in the lower orders of Fungi and Algae,
or in the higher class of plants.*

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 membra-
nous coats of the cells, which they keep in an
expanded state. (See Fig. 9.) When the mem-
branes are torn, the fibres, being detached, unroll
themselves, and being loosely scattered among the
neighbouring cells, give the appearance of fibrous
connexions among these cells, which did not origi-
nally exist. Simple membranous 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

* See Payen, Ann. Sc. Nnt. serie 2, xi. 21.

60 THE MECHANICAL I UNCTIONS.

or oval perforations, as shown in Fig. 10. Cells of
this description are said to be reticulated or spotted,
and these, together with cells having more regularly
formed spiral threads, are very abundantly met
with in plants belonging to the tribe of OrchidecB.

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 appeared
to him, wlien he examined the coats of the cells
with the microscope, to be pores and fissures. But
subsequent observations have rendered 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 botanists* ap-
pear to have satisfactorily established, the partitions
which separate them, however thin and delicate,
mvist consist of a double membrane, formed by the
adhesion of the coats of the two contiguous vesicles.
But as these coats can hardly be supposed to adhere
in every point, we may expect to find that spaces
are left in various parts between them ; and that
communications 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

* 111 parliculiir, Treviranus, Kieser, Link, Du Petit Thouars, Pol-
liiii, Ainici, Dulrochct, and De Candrllc.

VEGETABLE ORGANIZATION. 61

been supposed to perform, as will hereafter be seen,
an important part in the function of nutrition.

Fluids of various 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 com-
mon example, and which is distinguished by the
property of becoming blue when acted upon by
iodine.

The material which gives to the different parts
of plants, and more especially the leaves, their green
colour, is a peculiar resinous substance, termed
Chlorophyllite^^ or CliromuUte.'\ It sometimes oc-
curs as a semi-fluid gelatinous substance, having
no determinate figure : but its more usual form is
that of globular grains, intermixed with the colour-
less fluid of the cells. Mohl has discovered that
each of these globules contains one or more granules
of fecula, varying in size from the .3,600th to the
60,000th part of an inch in diameter, around which,
as on a nucleus, the chlorophyllite appears to have
been deposited ; for he found that the central gra-
nules acquire a blue tinge by the action of iodine,
while the same agent renders chlorophyllite brown. j.
In the autumn, when the leaves decay, the fecula
is found to have disappeared, and the chlorophyllite
to have acquired a yellow colour.

* Pelletier and Caventou, Ann. Ch. and Phys. ix. 194.

f De Candolle, ib. xxxviii. 422.

I Ann. Sc. Nat. serie 2. Bot. ix. 162,

62 THE MECHANICAL FUNCTIONS.

The cells of the ligneous portion of trees and
shrubs are farther encrusted with particles of a
more dense material, peculiar to vegetable organi-
zation, and termed Lignin. It contains a larger
proportion of carbon and hydrogen, and less oxy-
gen, than the primitive tissue of the simple cells
and vessels ; and is not, like the latter, coloured
blue by iodine.

It is this substance 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 here-
after to be described, surrounded by assemblages
of cells thus fortified, and the whole cohering in
bundles, so as to present greater resistance to force,
tending to displace them, in the longitudinal direc-
tion 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, consisting of mem-
branous tubes of considerable length, interspersed
throughout every part of the system. These tubes
exhibit different modifications of structure, more
especially with regard to the form of the fibres, or
other materials, which adhere to the inner surface
of their membranes ; and these modifications corre-
spond very exactly with those of the vesicles already
described as constituting the simpler forms of vege-
table tissue. There can be little doubt, indeed,
that the vessels of plants take their origin from
vesicles, which become elongated by the progress

VEGETABLE ORGANIZATION. 63

of developemeiit in one particular direction ; and
it is easy to conceive that where the extremities of
these elongated cells meet, 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 pas-
sage. 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.
Elongated cells are often met with applied to each
other endwise, as if preparatory to their coalescence
into tubes. Sometimes the tapering ends of fusi-
form 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 application to each
other in the same line, being separated only by
membranes passing transversely ; 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
continuous, 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 headed
vessels* Traces of the membranous partitions some-
times remain where their obliteration has been only
partial, leaving transverse fibres. The conical ter-
minations occasionally observable in the vessels of
plants also indicate their cellular origin.!

* Mirbel gave them the name of " Vaisseaux en chapelet."
t This theory of the derivation of vessels from cells was first
advanced by Treviranus, and has been fully confirmed by subse-
quent observers, and particularly by Schleiden.

64

THE MErHANICAL FUNCTIONS.

The membrane constituting tlie tube is sometimes
simple, like those of the simple cells : but it fre-

12 13

15 16 1/

quently 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 deno-
minated trachece or spiral vessels; or, from their
being found very generally to contain air, they are
often called air tubes* Their diameter is in general
between the 1000th and the 300th part of an inch.
These spiral, or air vessels, pervade extensively the
vegetable system. The threads they contain are fre-
quently 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 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 pervious, by

* Schleiden observes, however, that spiral vessels and spiral cells
occur in the living plant quite as frequently filled with sap, in the
young vegetating portions, as with air in the older organs which
have attained their full dimensions.

VEGETABLE ORGANIZATION. 05

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 regu-
lar intervals, and constituting what are called
aiimdar vessels (Fig. 15). They are generally 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, ajipears, from its transparency, as if
spotted or perforated in various places (Fig. 16).
Every intermediate stage may occasionally be
seen in the ti^ansition from one of these forms to
the other, in consequence of the various kinds of
convolution, of branchings, or of transverse junc-
tions 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
different portions of the same vessel, when followed
by the eye throughout a great extent of its length.
Thus, 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 m others, again, form rings : these may after-
wards, by a closer junction, present a reticulated
appearance, or a series of transverse lines, which,
becoming smaller and smaller, are at length mere

* Vessels are sometimes met with which appear to be formed
simply by the coils of a spiral fibre in close juxtaposition, and unat-
tached to any external envelope, or connecting membrane.
VOL. I. F

66 THE MECHANICAL FUNCTIONS.

points, arranged in circular rows around the cylin-
drical surface of the vessel.*

What are called the ivoody fibres originate, like
all other parts of plants, in cells. These are gene-
rally 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 internal fibres, although they receive,
in the progress of their developement, large addi-
tions of solid matter. These fibres are generally
collected together into bundles or layers, and are
accompanied by cells and vessels of various descrip-
tions, 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. -f Their assemblage thus constitutes a
kind of framework for the whole system, which

* Many distinguished botanists, such as Rudolphi, Link, Trevi-
ranus, and Dutrochet, consider these spots as being produced not
by the deficiency of the internal coating, but by the addition of
granular bodies. See De Candolle's Organographie Vegetale, torn.
i. p. 5Q.

t By drying different specimens of wood in a stove. Count Rum-
ford 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 vegetation, constitutes
only two-fifths of the whole bulk : and that ordinary dry wood con-
tains above one-fourth of its weight of water. Thomson's Annals
of Philosophy, i. 388.

VEGETABLE ORGANIZATION, (37

may be regarded as the skeleton of the plant.
Tims, what are called the Jihres of leaves (Fig. 19),
are principally composed of these woody fibres, dis-
tributed in the manner best adapted to support the
expansion of the soft and pulpy substance of those
important 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 pro-
bably contribute to impart the requisite degree of
buoyancy.

There are also contained, in the interior of vege-
tables, certain organs, denominated Glands, which
are composed of closely compacted cells, and which
perform the function of secretion, that is, the con-
version of the nutritious juices into particular pro-
ducts required for various purposes in the economy
of the plant.

The external parts of a living plant require pro-
tection against the injurious effects of the atmos-
phere, 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 interposing 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,

68 THE MECHANICAL FUNCTIONS.

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 represent-
ing the cuticle with its series of cells.

Oval orifices, or stomata, as they have been
termed, are discoverable on almost every part of
the surface of the cuticle, but more especially in
those that have a green colour.! They are placed
at nearly equal distances from one another, 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 general 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 con-
ceives these orifices are opened and closed, J 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

* Annales des Sciences Naturelles, ii. 211.

f Fig. 22 is a magnified representation of the appearance in the
cuticle of the Lycopodimn denticulatum, taken in the central part
of the lower surface of the leaf, from De CandoUe. Fig 2 1 is a
still more magnified view of the stomata in the leaf of the Lilium
cundiduvi, from Amici.

X Annales des Sciences Naturelles, ii. 215.

VEGETABLE ORGANIZATION.

fj.9

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 par-
tially immersed, stomata are met with in those
parts exclusively which are above the water. The
leaves of the Ranu7iculus aquaticus, 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 circum-
stances, and in particular exigences.

The principal organs through which the fluids
that serve for nourishment are received into the
system of plants, are those situated at the extremi-
ties of the roots, where they are termed, from their
peculiar texture, spongioles* Of the functions of

* Fig. 23 exhibits the termination of a root of a willow in a
spongiole ; the arrangement of the cells composing which is shown
in Fig. 24, from De CandoUe.

70 THE MECHANICAL FUNCTIONS.

spongioles in absorbing fluids I shall have occasion
to speak when treating of nutrition : but as the
roots exercise a mechanical as mcU as a nutrient
office, we should here consider them in the light of
organs adapted to procure for the plant a perma-
nent 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 perform 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 exert the
most effectual resistance to the force of the winds,
which, acting upon so large a surface as that pre-
sented 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 requi-
site 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 of vegetable life.
The stems of the grasses are hollow tubes ; their
most solid parts, which frequently consist of a thin
layer of silex, occupying 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

VEGETABLE ORGANIZATION. 71

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 exactly the
form which is best calculated to give stability to
the trunk. Evidence of express mechanical de-
sign is likewise afforded by the manner in which
the trunk is subdivided into its branches, spreading
out in all directions, manifestly with a \iew to pro-
cure 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 irregular 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 alburnum. Nothing can exceed the ele-
gance of those forms which are presented in every
part of the vegetable kingdom, whether they be
considered with reference to their direct utility for
the support of individual life, and the continuance
of the species, or whether they be viewed as com-
ponent parts of that beauty which is spread over
the scenery of nature, and is so delightfully re-
freshing 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

* Galileo, the most profound philosopher of his age, when inter-
rogated 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.

72 THE MECHANICAL FUNCTIONS.

more forcibly will our hearts be impressed with the
truth of the divine appeal, that " Even Solomon

IN ALL HIS GLORY WAS NOT ARRAYED LIKE ONE OF
THESE."

§ 3. Dev elopement of Vegetables.

Further proofs of design may be collected from an
examination into the modes in which these struc-
tures, so admirably adapted to their objects, have
been gradually formed. Confining our attention
to vascular plants, in which the process of deve-
lopement has been studied with the greatest atten-
tion and success, we find that Nature has pursued
two different plans in conducting their growth.*
In the greater number, 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 addi-
tions made internally; a plan which is followed in
all Endogenous 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 tro-
pical 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 sim-
plest of these two kinds of vegetable developement.

* The tribe of Filices, or ferns, the structure of which is vascu-
lar, constitute an exception to this rule : as they differ in their mode
of developement, both from exogenous and endogenous plants.

DEVELOPEMENT OF VEGETABLES. 73

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 ex-
ternal cylindric layer of wood ; but the texture of
the interior becomes gradually softer and more
porous as it comes nearer to the centre; though
with regard to its essential character 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 ordi-
nary exogenous trees.

The first stage of its growth consists in the ap-
pearance of a circle of leaves, which shoot upwards
from the neck of the plant, and attain, during the
first year, a certain size. The following year, an-
other circle of leaves arises ; but they grow from
the interior of the former circle, which they force
outwards as their vegetation advances, and as lig-
neous 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 distension, and remain 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 immedi-
ately directed upwards; so that they each rise in
succession by distinct stages, always proceeding

74 THE MECHANICAL FUIsX^TlONS.

from the interior ; a mode of developement which
has been compared by De Candolle to the drawing
out of the sUding tubes of a telescope. The whole
stem, whatever height it may attain, never increases
its diameter after its outward layer has been con-
solidated. 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 existence, consisting of a cir-
cular 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 successive knots which are seen in the
stems of other endogenous plants, as may be ob-
served in growing corn, and also in various grasses,
have a similar origin.

The structure of exogenous trees is more com-
plicated : for, when fully grown, they are composed
of two principal parts, the ivoocl and the bark. The
woody portion exhibits a further division into the
pith, Avhich occupies the centre, and consists of
large vesicles, not cohering very closely, but form-
ing a light and spongy texture, readily permeable
to liquids and to air ; the harder wood, which sur-
rounds the pith in concentric rings, or layers ; and
the softer wood, or aJburnmn, which is also disposed
in concentric layers on the outer side of the former.
Each of these concentric layers of wood and of al-
burnum may be farther distinguished into an inner
and an outer portion ; the former being of less den-
sity than the latter, and consisting of a lighter cel-
lular tissue : while the outer portion is composed of
the denser woody fibres resulting from the union
of numerous vessels with a cellular envelope. The

DEV ELOPEMENT OF VEGETABLES. 75

bark is formed by concentric layers of cortical sub-
stance, of which the innermost are denominated
the Liber; and the whole is surrounded by an outer
zone of cellular 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 Medul-
lary Rays, radiating from the pith to the circum-
ference. They are composed chiefly of large cells,
extending transversely, or in the direction of the
diameter of the tree, and composing 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 minute-
ness, and totally imperceptible by any optical
means of which we have the command. As soon
as it becomes visible, and its structure can be dis-
tinguished, 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 possible compass.
The portion destined to form the stem is gradually
expanded both in breadth and height, but princi-
pally the latter, so that it rises as it grows, during
a certain period, until the fibres acquire the soli-
dity and strength necessary not only for their own
support, but also for sustaining the parts which are
to be farther added. In trees this process gene-
rally 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

76 THE MECHANICAL FUNCTIONS.

unrestrained. On the second year, a fresh impulse
being given to vegetation, a new growth commences
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 supplied
with nourishment, their growth, which is restrained
laterally, is now directed upwards, and there is no
further enlargement of their diameter. From the
same cause the pith cannot increase in size ; and is
even found to diminish by the pressure of the sur-
rounding wood. Thus the vertical elongation of
the entire stem continues 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, cor-
responding 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 con-
stitutes the liber. All these new depositions 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

DEVELOPEMENT OF VEGETABLES. 11

pressure, they acquire increasing rigidity ; and, the
same cause continuing 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 consoli-
dated layers of cortical envelope which form the
epidermis. But the epidermis, which is continually
splitting by the expansion of the parts it encloses,
itself soon decays, and is constantly succeeded by
fresh layers, produced by the same process of con-
solidation in the subjacent cortical substance.

During the third, and each succeeding year, the
same process is repeated ; new layers of cellular
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 nourishment. 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 dilatation is prevented. But the
tree still continues to enlarge its trunk by the
annual accessions of vigorous and expansible albur-
num, and to take its station among its kindred in-
habitants of the forest ; till, arriving at maturity,
its majestic form towers above all the junior or less
vigorous trees.*

* It is contended by Dr. Darwin and other writers on vegetable
pliysiology that each annual shoot should be regarded as a collec-

78 THE MECHANICAL FUNCTIONS.

The developement of eacli branch takes place in
the same manner, and by the same kind of 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 sym-
metry is established by nature in the developement
of all the parts of plants. The leaves, in particular,
are frequently observed to arise in a circle, or sym-
metrically round the parent stem ; forming what is
termed a whorl, or, in botanical language, a verti-
cillated 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 branch from which they grow. When
these intervals correspond to the semi-circumfe-
rence 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

tion of individual buds, each bud being a distinct individual plant,
and the vi'hole tree an aggregation of such individuals.

These speculations have been carried to their extreme limit by
Schleiden, who regards every one of the cells, which compose the
vegetable tissue, as possessing an independent life; and the plant
itself as an aggregate of fully individualized independent beings,
namely, the cells themselves. See Taylor's Scientific Memoirs, II.
281. Mirbel adopts the same views; for he says, speaking of the
vegetable cells, " II ne parait pas que jamais il s'etablisse entre elles
une veritable liaison organique. Ce sont autant d' individus vivants,
jouissant chacun de la propriete de croitre, de se multiplier, de se
modifier dans de certaines limites, travaillant en commun a I'edifica-
tion de la plante, dont ils deviennent eux-memes les materiaux
constituants. La plante est done un etre collectif." Ann. Sc. Nat.
serie 2. Bot. xi. 325.

DEVELOPEMENT (XF VEGETAIJLES. 79

wood of the pine, liowever 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 primitive direc-
tion 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 gene-
rally twisted throughout their whole length ; a dis-
position evidently conducive to the purpose of their
formation, namely, 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 tendency
to a spiral growth might be owing to the influence
of the sun's light, acting successively on the 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 fac-
ing 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 press-
ing forward to obtain the beneficial action of that
agent. The leaves, whose functions in a more
especial manner require its operation, will always

80 THE MECHANICAL FUNCTIONS.

be found turned towards the light. The branches
of a tree, which liave naturally a tendency to rise
vertically, have this tendency modified by the su-
perior attraction of the light, when it can reach
them only laterally. Thus while those on the upper
part spread out in full luxuriance in 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, how-
ever, 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 verification of the theory would be
obtained were it found that plants transported from
the southern to the northern hemispheres had this
spiral 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 hemis-
phere, 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 subject ; so that the hypothesis
is untenable.

DEVELOPEMENT OF VEGETABLES. 81

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 increase 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 developement of buds, as are the branches
of the stem ; but they arise merely from the addi-
tional deposits taking different directions. 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 circulating system of the plant itself.
De Candolle asserts that they generally grow from
certain points on the surface of the bark, which ap-
pear as dark spots, and which he terms Lenticellce.^
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, con-
formably with their general 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.

* Ann. Sc. Nat. vii. i., and Organograpliie, i. 94. Professor
Hugo Mohl, on the other hand, contendc, that this opinion is unsup-
ported by fact. Ibid. serie2, Bot. x. 33 and 46.
VOL. I. G

82 THE MECHANICAL FUNCTIONS.

Nature has supplied various plants with certain
appendages to the above mentioned structures, 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 stipules, which protect the
nascent leaves ; and the bractete, 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,
particularly that of defending the plant from mo-
lestation by animals. The sting of the nettle is of
this class ; and its structure bears a striking analogy,
as we shall have 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 j^rotection from the injurious
effects of cold winds on the tender surface : or it
may have a relation to the deposition of moisture ;
or, as it may be farther conjectured, 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 disturbed by the processes of vegetation.

In the smaller parts of plants, as in the general
fabric of the whole, we find, on examination, the
most admirable provision made, according to the
particular circumstances of the case, for the me-
chanical objects of cohesion, support and defence.
Thus the substance of the leaf, of which the

* The finer hairs, and filaments of down, are composed of elon-
gated cells, either single, or several conjoined end to end.

DEVELOPEMENT OF VEGETABLES. 83

functions require that a large surface should be
expanded to the air and light, is spread out in a
thin layer upon a framework of fibres, like rays,
connected by a network 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 accomplishment,
in conformity with the law of diversity 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 intelligence.

§ 4. Animal Organization.

The structures adapted to the purposes of vegetable
life, which are limited to nutrition and reproduction,
would be quite insufiicient 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 vinion of solid and fluid

84 THE MECHANICAL FUNCTIONS.

parts. Every animal texture appears to be formed
from matter that was originally in a fluid state ;
the particles of which they are composed having
been brought together and afterwards concreting
by a process, which may, by a metaphor borrowed
from physical science, be termed animal crystal-
lization. Many of those animals, indeed, which
occupy the lowest rank in the series, such as
Medusa, approach nearly to the fluid state; appear-
ing like a soft and transparent jelly, which, by
spontaneous decomposition after death, or by the
application of heat, is resolved almost wholly into
a limpid watery fluid,* More accurate exami-
nation, however, will show that it is in reality not
homogeneous, but that it consists of a large pro-
portion 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
artificial 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 exist-
ence. As we rise in the scale of animals, we
approximate to the condition of the more advanced

* 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 Quay and Gaimard, Annales des Sciences Natu-
relles, torn. i. p. 245.

ANIMAL ORGANIZATION. 85

states of developement 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 their
labours have hitherto been fruitless. Fanciful
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 ardour, 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 diversified in their chemical composition.
Neither has the microscope enabled us to resolve
the jiroblem : for although it has been alleged by
many observers that the ultimate elements of every
animal structure consists of minute globules, 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 animal fluids, there can be no
doubt ; and that these globules, by cohering, com-
pose 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

86 THE MECHANICAL FUNCTIONS.

tendons, and the cellular texture : for the most
recent, and apparently most accurate microscopical
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 extremely thin
laminge, or plates, variously connected together by
25 fibres, and by other plates, which cross

them in different directions, leaving
cavities or cells. (Fig. 25). These cells,
or rather intervening spaces, communi-
cate freely with one another; and, in
fact, may be considered as one common
cavity, subdivided by an infinite num-
ber of partitions into minute compartments. Hence
the cellular texture is throughout readily permeable
to fluids of all kinds, and retains these fluids in
the manner, and on the same principle, as a
sponge.

* See the Appendix to Dr. Hodgkin and Dr. Fisher's translation
of Edwards's work on the Influence of Physical Agents on Life,
p. 440.

The theory of Schleiden respecting the origin of vegetable cells
from a vesicular developement of nuclei, has been found by Schwann
to extend to animal structures also ; and a remarkable similarity is
traced by him in the phenomena of the early developement of both
these classes of organized beings. The conclusion which he deduces
liom his microscopical investigations is, that all the animal tissues,

ANIMAL ORGANIZATION. 87

The cellular texture is not only the element, or
essential material employed by nature in the con-
struction 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 cellular texture is the universal
cement employed to bind together all the solid
structures. Its properties are admirably adapted
to the mechanical purposes which are required in
different 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 interposed
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 inter-
texture closer and more condensed.

Besides being flexible and extensible, the cellular

whether cellular, muscular, cartilaginous, or nervous, have their
origin from vesicles, which themselves arise from nuclei, by a mode
of developement which, in its principle, is similar in all cases ; and
that it is only through the modifications which the vesicles subse-
quently undergo that the particular tissues are formed and acquire
their peculiar properties. Mikroskopische Untersuchungen iiber
die Uebereinstimmung in der Struktur und dem Wachsthum der
Thiere und Pflanzen, von Dr. Th. Schwann. Berlin, 1839,

88 THE MECHANICAL FUNCTIONS.

texture is also highly elastic; a property which is
exceedingly advantageous in the construction of
the frame. Not only the displacement 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 interpose 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 oblite-
rate all the cellular interstices, and become imper-
vious to fluids. Though equally flexible and elastic
with the original texture of which it is formed, the
membrane has acquired, by this consolidation,
greater strength and tirmness ; properties which
adapt it to a great number of important purposes.*

Membranes are extensively employed to connect
distant organs, and often serve to determine the
direction and extent of their relative motions. They
furnish strong coverings for the investment, the
support, and the protection of all the important

* With a view of ascertaining the actual strength of membranes,
Scarpa stretched a portion of peritoneum, (which is a very thin mem-
brane Hning the abdominal cavity), over a hoop, and placing weights
upon its surface, found that it did not give way till it was loaded
with fifteen pounds.

ANIMAL ORGANIZATION. 89

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 membranes, 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 contained
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 neighbour-
hood of joints, closed cavities of the same descrip-
tion, 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
extensively employed as receptacles for retaining
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 accommodate themselves to the
variable bulk of their contents.

90

THE MECHANICAL FUNCTIONS.

Ill the second place, we find membranes com-
posing tubes of various descriptions for conducting
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 con-
stant direction. For preventing the retrograde
motions of the fluids passing along these canals, re-
course is had to the beautiful contrivance of valves.
The inner membrane of the vessel is employed to
construct these valves ; for which purpose it is ex-
tended 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. Whenever the fluid is impelled in a direc-
tion 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

ANIMAL ORGANIZATION. 9J

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 per-
haps none more striking and more palpable, than
this admirable mechanism of the valves.

As we ascend from the simpler to the more com-
plicated systems of organization, adapted to a
greater range of faculties, we find greater diversity
in the mechanical means employed for carrying on
the functions of life. Textures of greater strength
than can be constructed by membranes alone be-
come necessary for the security, the support, and
the defence of important organs ; and more espe-
cially for the execution of extensive movements.
For obtaining these advantages a peculiar species
of fibres is provided, formed of a much denser sub-
stance than even the most consolidated forms of
celkdar texture. The animal product termed albu-
me?i possesses a much stronger cohesive power than
gelatin, which is the basis of membrane. The
addition of albumen, therefore, 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 composed, a close texture is formed, which is
exceedingly tough and unyielding. These fihtous
textures, as tliey are termed, while they retain the
flexibility of membranes, greatly surpass them in
strength ; but, being at the same time incapable of

* 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
l»y the arrow. Jn Fig. 28, they are seen closing the passage by the
y)ressure of the retrograde current.

92 THE MECHANICAL FUNCTIONS.

extension, they are necessarily devoid 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 Jibro its
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 instance, with the membranes surrounding the
brain of quadrupeds, and which form two partitions,
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 retain-
ing them in their relative situations. The joints
are surrounded by similar bandages, known by the
name of Capsular Ligaments.

In following the series of animal structures in
the order of their increasing density, we find the
proportion of albumen which enters into their com-
position 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 inex-
tensible cords, constituting the ordinary Ligaments,
and the Tendons. 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 tough-

ANIMAL ORGANIZATION. 93

ness. In a word, they possess all the qualities that
can be desired in a rope. It will hardly be cre-
dited 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 in-
creased 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
disposed, 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 struc-
tures, which are strictly inextensible ligatures,
render them applicable to purposes of connexion
where motion is to be restrained. Many cases,
however, occur, in which a substance is wanted,
uniting great compactness and strength with a con-
siderable 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 ligmnents of animals,
which are very generally employed for the support
of heavy parts that require being suspended. An
instance occurs in quadrupeds, in that strong liga-
ment which, as we shall find, 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 nuclice, as it is termed, is capable of
great extension, and by its elasticity reacts with

94 THE MECHANICAL I UNCTIONS.

considerable 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. Many birds are provided with strong
elastic ligaments connecting the vertebree of the
neck with those of the back ; ligaments 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 tex-
ture in this animal has an unusual degree of elas-
ticity.*

Another example of an elastic ligament occurs in
that which connects the two shells of bivalve mol-
lusca (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 exten-
sively in the fabric of insects. t

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

* Hunter on the Blood, &c. p. 112.

t Chabrier, Memoires du Musee, torn. vi. p. 416.

ANIMAL ORGANIZATION. 95

in which a solid basis is required for the support of
softer or more flexible parts, and where the mecha-
nical properties that are wanted are firmness, con-
joined 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 presents no obvious fibres ;
but when cut with a sharp knife, it appears as 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 em-
ploys cartilage to supply the place of bone, when
rigidity is required to be given to the fabric. In
an extensive order of fishes, including the Shark,
the Sturgeon, and the Ray, we find the whole
skeleton constructed of cartilage. In the fabric of
very young quadrupeds cartilage is substituted for
bone ; and in the adult animal, various organs,
such as the external ears, the eye-lids, the nostrils,
and different parts of the apparatus of the throat
and windpipe, are composed of flexible cartilage,
which gives them a determinate shape and suffi-
cient 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 di-
minishes those jars which are incident to the frame
in all its violent actions.

96 THE MECHANICAL FUNCTIONS.

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 constituents. But sub-
stances of still greater hardness, 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 frame-
work of the body, and for constructing levers of
various kinds, to be employed in the more ener-
getic movements of the higher animals. For all
these purposes it was necessary to superadd a ma-
terial endowed with stronger cohesive powers, and
capable by its dense concretion of forming solid
and inflexible organs. The substances which na-
ture has selected for this office are the salts of lime.
Sometimes the Carbonate, and sometimes the Phos-
phate of lime is employed for forming these hard
and unyielding structures ; and often both these
calcareous substances are united together in dif-
ferent proportions in the same solid fabric. When
the carbonate of lime predominates, or is the sole
earthy ingredient, it constitutes 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 com-
poses the different forms of Bone. I shall have
occasion to describe the formation and properties
of each of these structures in the sequel.

Txie protection of the delicate structure of the
fabric from the injurious influence of external
agents is an object of great importance in the ani-
mal economy, and is one which nature has shown

ANIMAL ORGANIZATION. 97

extreme solicitude to secure. For this purpose she
has provided the Integuments, under which designa-
tion 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 varieties, that the same general
plan has been followed in their construction, and
that each particular formation is the result of a
combination of the same elementary structures.
These elements compose three principal layers ; of
which the most important, and that which gene-
rally constitutes 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
it and the corium there is an intermediate layer
denominated the Corpus Mucosum.

The corium is generally of considerable thick-
ness, and is composed of strong and tough fibres,
closely compacted together, enclosing glandular
structures of different kinds, and pervaded by innu-
merable ramifications of blood-vessels, of nerves,
and of absorbent vessels. It is endowed with great
flexibility, and is capable of being considerably
stretched ; properties which fit it for readily accom-
modating 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 regains 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 texture, which, according to the

VOL. I. H

.08 TIfE MECHANICAL FUNCTIONS.

particular intentions of its formation, sometimes
binds it tightly over these parts, and on other occa-
sions allows of a free and extensive motion. This
latter property is remarkably exemplified in the
Racoon, an animal whose skin hangs loosely on the
limbs, and encloses the body like a wide elastic
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 Mol-
lusca, this muscular apparatus is inseparably
blended with the integument, and composes a pe-
culiar structure, termed the rnanlle.

The outer portion of the corium is formed of a
great number of conical processes, or papillcB, as
they are termed, projecting somewhat obliquely
from the surface, and disposed in regular series of
parallel rows, with intervening furrows. They
chiefly contain the terminations of the nervous
filaments distributed to the skin, and receive also a
small blood vessel. This outer layer of the corium,
which is termed the corpus paplllare, is, as we shall
afterwards find, the principal seat of the sense of
touch.

The Corpus 3Tucosum, which immediately covers
the papillae, and is every where moulded on their

ANIMAL ORGANIZATION. 99

surfaces, thus furnishing them with a protecting
investing membrane, constitutes a thin layer of a
soft gelatinous consistence, intermixed with minute
scales ; these scales are coloured by a material
termed ihe pigmentum, from which the skin derives
its peculiar hue.*

The corpus mucosum is covered externally by
the epidermis, into which it is itself gradually con-
verted, in proportion as the latter is, in process of
time, worn away ; its place being supplied by a
fresh production of the same material. For this
purpose a secreting apparatus is provided, consist-
ing of a vast number of minute glands, termed by
Breschett the blenuogeiioiisola/ids, which are lodged
in the deepest part of the corium. The albuminous
matter which they prepare is conveyed by ducts,
one proceeding from each gland, which traverse
the whole thickness of the corium, till they reach
its outer surface, when they open at the bottom of
the furrows, between the rows of papillae. Here
also terminate another set of canals, much shorter
than the former, inasmuch as they proceed from
organs situated immediately below the corpus pa-
pillare, and consequently close to the outer surface
of the corium. These latter organs, which are also
of a glandular nature, are denominated by Breschet
the chromatogeuuus glands, because they prepare
the pigment already spoken of, as mixing itself
with the albuminous fluid to form together the
corpus mucosum. J

* Being black in the Negro race, it was termed by Malpighi, its
discoverer, the Pigmentum nigrum.

t Nouvelles Recherches sur la structure de la Peau. Paris, 1835.
X On raising the epidermis from the corium, a portion of the layer

100 THE MECHANICAL FUNCTIONS.

The Epidermis consists of a thin expansion of
albuminous matter, arranged in layers, but appa-
rently homogeneous in its texture and composition.
On accurate examination of portions which have
been long macerated in water, they are found to con-
tain a great number of minute scales, the product of
the chromatogenous glands already described ; and
which have been agglutinated together into a solid
layer by means of the albuminous matter secreted
by the blennogenous glands. The epidermis is
perforated at regular intervals by ducts* arising
from a third set of glands, situated in the most in-
terior portion of the corium, the office of which is
to secrete the perspiration, and which may accord-
ingly be termed the sudorific glands.-\ This fluid,
which, under ordinary circumstances, escapes in
the form of invisible vapour, but is, at other times,
deposited in a liquid state, is conveyed from these
glands by the above-mentioned ducts, which pur-
sue a spiral course through the whole thickness of
the corium, corpus mucosum, and epidermis, till
they open obliquely at the surface of the latter. J

The extreme branches of the lymphatic absorb-
ents also extend into the substance of the epider-
mis, but nowhere penetrate to its external surface.

of corpus mucosum adheres to the former, while the remaining por-
tion which surrounds the bases of the papillae remains attached to
them, constituting a reticulated layer perforated by the papillee. It
was this appearance which led Malpighi, the discoverer of this part
of the skin, to consider it as a real network, and to give it the name
of Rete Mucosum, by which it is still frequently known.

* The ducts are termed by Breschet, Canaux sudoriferes, ou hi-
drophores.

t These, together with the ducts, constitute the appareil diapno-
yene of Breschet,

ANIMAL ORGANIZATION.

101

From this circumstance, and also from the obli-
quity of the terminations of the sudorific ducts,
which produces the effect of valves, the epidermis
is impervious to fluids, although capable of im-
bibing moisture hygrometrically, and of slowly
transmitting a portion to the subjacent textures.
Its thickness varies exceedingly in diflerent parts;
being adapted to the kind of protection it has to
afford against pressure, friction, or other causes of
injury. As it is not nourished by vessels, its outer
layer is liable to become dry and unfit for use : and
accordingly a separation of this outward layer ge-
nerally takes place from time to time, the loss being
speedily repaired by a fresh growth iVom the sur-
face in contact with the skin. This process is often
performed periodically, as is most remarkably ex-
emplified in serpents.

Special provisions are made for preserving the
cuticle in a healthy condition ; and more parti-
cularly for defending it from the injurious action
of the surrounding element. These sometimes con-

ription of the minute structure of the skin will
be better understood by reference to
the annexed representation of a per-
pendicular section made through all
the layers of the skin : (Fig. 28*),
where D is the derm or corium ;
M, the corpus mucosum, gradually
passing into E, the epidermis; P,
the papillfe ; B, the blennogenous ;
C, the chromatogenous ; and S, the
sudorific glands, with their respec-
tive ducts : and L, the lymphatic
absorbents, not reaching the sur-
face.

102 THE MEI HANICAL 1 UNCTIONS.

sist of a supply of oily fluid, prepared in small
cavities wliich 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 in 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 glu-
tinous secretion, prepared in this manner, and con-
tinually poured out on the surface, through ducts,
the orifices of which are easily seen with the
naked eye, disposed in a line on each side of the
body.

The Chameleon has long been celebrated for the
changes of colour it exhibits at different times, and
which credulous observers asserted the animal could
effect at pleasure, so as to assume that of adjoining
objects. It is, however, an undoubted fact that
frequent and remarkable changes of colour take
place in the skin, from causes not easily determin-
able ; and which has been attempted to be explained
on different hypotheses more or less plausible, but
in reality devoid of all foundation in truth. But
the recent and accurate researches of Milne Ed-
wards appear at length to have solved tlie enigma,
by the discovery of a curious apparatus of follicles,
or minute bladders, arranged in a layer underneath
the true skin, and containing a fluid pigment, of
a dark bottle-green colour. From the outer end
of each of these follicles, there proceed several
tubes, which divide and ramify into very minute
branches rising towards the epidermis, and pene-

ANIMAL ORGANIZATION. 103

trating through a more superficial layer of yellow,
white, or grey pigment, which lies immediately
underneath the epidermis, in the situation usually
occupied by the corpus mucosum. In the ordinary
state of the Chameleon, the colour of the skin is
determined by this yellow pigment; the whole of
the dark pigment being collected in the subcuta-
neous follicles, which are concealed underneath
the true skin. When, on the other hand, these
follicles contract, or are compressed by the action
of the skin or neighbouring muscles, the dark pig-
ment they contain is propelled into the tubes and
their ramifications, and becomes visible by its in-
termixture with the yellow and grey pigment,
which, if the former be very abundant, it may over-
power and wholly obliterate. The apparent colour
of the skin, therefore, is determined by the propor-
tion in which the two pigments become visible, and
the degree in which the one predominates over the
other.*

Connected with the skin, and more particularly
with the cuticle, are structures of very various
forms, intended for giving additional protection,
occasionally contributing their aid in progressive
motion, and sometimes fashioned into weapons of
otlence. 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 composition, differing only in their
degrees of hardness and condensation. Horn is
formed of the same material as hair ; as are also

*• Ann, Sc. Nat. serie 2. i. 46.

104 THE MECHANICAL FUNCTIONS.

the nails, the hoofs, and the claws of quadrupeds,
and the scales of fishes, reptiles, and other ani-
mals.* The integuments of insects, and especially
their more solid and horny coverings, contain, how-
ever, as will hereafter be noticed, a peculiar che-
mical principle termed JEntomoli/ie.

All these parts seem to be but remotely connected
with the vital actions of the system with which they
are associated ; and it is doubtful how far they are
to be considered as appertaining to the living por-
tion 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 or glandular structure,
variously arranged to suit the particular circum-
stances 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 the apparatus by
which the nutrient particles are selected, combined
and elaborated into the materials of the intended
structure. The original form, situation, and dispo-
sition of this vascular pulp determine the future
figure and extent of growth of the production which
is to arise from it. The materials which compose
it are deposited 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

* The scales of the greater number of fossil fishes contain a large
quantity of phosphate of lime ; and may therefore be regarded as
having the character of bone.

ANIMAL ORGANIZATION.

105

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, are no longer suscep-
tible of being nourished, and from the time of their
deposition, undergo no further change, except from
the action of external agents. t 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 apparatus

* The laminated structure of the scales of fishes is easily distin-
guished by applying to them a higli 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 Jiuviatilis. The imbricated
arrangements 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.

f Dr. Mandl takes a very different view of the growth of the
scales of fishes, considering it as effected by an internal nutrition.
Comptes RenduSjX. 338, and Edin. Phil. Journal, xxviij. 126 and 274.

106

THE MECHANICAL FUNCTIONS.

employed in its construction, in an imaginary sec-
tion of the root, on a magnified scale. Every hair
takes its rise from a minute vascular pulp, (f,) of
an oval shape, which is implanted below the corium,
or true skin (d).* This pidp 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, composes
the hulb of the hair.
The bulb itself is con-
tained in a small cell
formed by condensed
membranes (s), to which
it has no attachment
excepting at the lower
part (v), where the vessels and nerves of the pulp
are passing into it. The hair, growing by depo-
sitions from the inside of the capsule which forms
the outer part (o), of the shaft, and from the out-
side 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 coarse of this canal is
generally oblique. In the Elephant, where the
thickness and density of the hide present consider-
able obstacles to the passage of the hairs through
it, we may discover, on minute examination, many
hairs which have only penetrated a certain way,

* In the above figure, e is a section of the epidermis, or cuticle ;
the dotted part, r, represents the situation of the subjacent corpus
rnucosuni, and d, the derm, or corium.

ANIMAL OUGANIZATION. 107

(as shown at b), without ever succeeding in reach-
ing the surface.

An opinion has been very commonly entertained
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 integument, which,
stretching as the hair increases in length, forms
over it a very fine external tunic. But later obser-
vations have shown that this is not the case, and
that there is simply an adhesion of the edge of the
cuticle to the origin of the hair, without any ac-
companying prolongation ; so that if the whole
bulb be destroyed, and its pulp absorbed, the hair
may be detached by the slightest force.

From this account it will be seen that a hair is,
in its origin, tubular ; the inner part being occupied
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 sup-
plying nutriment become exhausted. The first
deficiency in its nourishment appears in the cessa-
tion of the deposit of colouring matter, and the
hair in consequence 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.

108 THE MECHANICAL FUNCTIONS.

Sometimes, instead of being cylindrical, the lila-
inents 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, fineness, tena-
city, rigidity, and disposition to curl. All these va-
rieties may be traced to corresponding differences in
the form, and the relative actions 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 extended uses than that of
merely covering the body, and which are even sup-
plied with nerves, converting them into instruments
of a sense of touch.

In the quills of the Porcupine a still more com-
plicated organization has been detected. Fig. 33
shows a quill with its bulbous root, detached from
the body ; and Fig. 34, a transverse section magni-
fied. The bulb itself is contained in a distinct cell,
shown at (a), Fig. 35, which represents a longi-
tudinal section of these organs. This cell contains
a portion of fat in which the numerous vessels sup-
plying 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 substance on the outside of
the sheath. This upper cell communicates below

* See F, Cuvier's Memoir on the Formation of the Quills of the
Porcupine, in the Nouvelle.s Aunales du Museum, i. 429.

ANIMAL ORGANIZATION.

109

with another cavity (c), 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 compo-
sition. 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 membrane (i), and
deposited in successive laminse. 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 pro-
cesses, is moulded to their shape, and concretes
into laminae which converge from the circumference
of the cylinder towards the centre. The section
(Fig. 34) shows these converging laminae, which,
being of a dark colour, give to the surface of the
quill the appearance of being grooved ; this, how-
ever, is merely an optical illusion, occasioned by
the dark laminae being seen through the transparent

110 THE MECHANICAL FUNCTIONS.

exterior covering ; as may readily be detected by
viewing the surfoce with a magnifying glass.*
After a certain period of the growth of the quill,
the pulp ceases to supply the materials for forming
the spongy substance which occupies the interior
of the quill. But although it no longer secretes, it
still retains its place ; and the capsule continuing
to deposit horn, the quill becomes 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 varia-
tions in the bulk and the action of the pulp, accom-
panied with changes in that of the capsule, are
sufficient to account for every diversity in the form
and condition 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 especially of the
bones, from unequal pressure, ought not to be over-
looked. This object is promoted by the interposi-
tion of a layer oi fat, which is another animal
substance entitled to be enumerated 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 animals the temperature of the

* 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.

t These two constituent principles possess very different degrees
of cohesion ; elain being liquid, and stearin nearly solid, at the usual
temperature : and the consistence of the compound will, therefore,

ANIMAL ORGANIZATION. Ill

body is always sufficient to preserve this compound
substance in a fluid state : but it is prevented from
being diftused through the cellular texture by being
contained in separate vesicles of extreme minute-
ness,* and through which it cannot transude.
Hence the whole mass of the fat, which is formed
of an aggregation of these vesicles, has not the
appearance of being fluid, but seems to be com-
posed of small grains united by membranous
investments into larger masses; a structure pecu-
liarly adapted to the purposes of a soft cushion, as
it retains only a small share of elasticity; and, while
it at the same time possesses the absolute incom-
pressibility of fluids, it admits readily, but only to
a certain limited extent, of displacement by ex-
ternal pressure.

These mechanical properties render it a peculiarly
serviceable material for filling up the spaces left
between adjacent organs, whose forms are not per-
fectly adapted to each other ; and especially those
whose functions require them to be in frequent or
continual motion. This is exemplified more par-
ticularly in the heart and the great vessels connected
with it ; between which we accordingly find a large
quantity of fat interposed. Diff'erent parts of the
alimentary tube are, in like manner, protected
from friction and from mechanical strains, which
might have occurred from mutual displacement, by
cushions of fat, interposed between their folds, and

depend altogether on the proportions hi which they are united. Thus
a ready expedient has been provided for varying the mechanical pro-
perties of fat, according as circumstances required.

* Dr. Munro estimated their diameter at between the 800th and
600th of an inch. But their size varies in different animals.

112 THE MECHANICAL FUNCTIONS.

which adapt, themselves to their various changes of
form and to their different movements. The ball
of the eye obtains a similar protection by resting
on a mass of fat, which occupies the remaining
space at the bottom of the orbit, and facilitates the
quick and sudden movements required of that
organ. The same design is apparent in the gra-
nules of fat which occur in the cavities of most of
the joints, and which being interposed between the
terminal cartilages, limit their extent of contact,
diminish their friction, and by equalizing the pres-
sures facilitate their relative motion.

In warm-blooded animals, the fat which forms a
layer underneath the skin is of great use in pre-
venting the rapid dissipation of warmth, being itself
a very slow conductor of heat. All the Cetacea
derive considerable protection from cold, by the
great thickness of this subcutaneous layer.

§ 5. 3Iuscula}' 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, however numerous
may be its parts, however various the forms of its
wheels, its levers, or its pulleys, and however com-
plicated may be their connexions, resolves itself
into a series of intermediate instruments for trans-
ferring motion from the source of power, or the

MUSCULAR POWER. 11.3

point where its action is impressed, to the parts
which are designed ultimately to receive the action
of the force employed. It is an established prin-
ciple in physics, that mere machinery is incapable
of generating mechanical force ; and that such force
must always be originally derived from an extra-
neous 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 m motion, and to continue
its movements when they are once begun. Nor is
the case essentially different when the source of
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 en-
gine, 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 regarded as impelling powers,
are, in reality, but intermediate agents in the dis-
tribution 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 machinery
in containing within itself a principle of motion not
referable, as far as we can perceive, 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 substance of the body seems to be equally
endowed with this contractile property, although
exhibiting no distinct appearance of a fibrous struc-

VOL. I. I

114 THE MECHANICAL FUNCTIONS.

ture. This is the case with all the lower Zoophytes,
such as the Lifusoria, Polypi, 3Iedusce, 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 microscopic
organs, resembling short hairs, termed cilia, which
the animal, by some unknown power, causes to
vibrate with great rapidity. These organs are
also met with in many parts of animals belonging
to the higher classes. Wherever they exist, they
perform, as will hereafter be shown, very important
functions; either serving as instruments of pro-
gressive motion, or determining currents of fluids
which contribute to respiration, to the supply of
food, or to internal circulation.

Cilia are long slender filaments of a conical, and
sometimes slightly flattened form, broad at the
base, and tapering to an invisible point. Their
length varies, in different animals, from the 13,000th
to the 500th part of an inch. They are arranged
in regular order ; often in rows, and sometimes in
circular, or spiral lines ; and are generally set close
together in the same row. Being transparent, and
for the most part colourless, they are not easily
seen, especially as during the life of the animal
their vibrations are too rapid and incessant to be
followed by the quickest eye, even when assisted
by powerful microscopes : so that they can, in
general, be detected only at those times when, in
consequence of the failing vigour of the animal,
their motions have become comparatively languid.*

* Milne Edwards states, however, that he has repeatedly seen tlie
ciha on various polypi, when they were at rest. Ann. Sc. Nat. 2"^*
ser. vi. 22.

MUSCULAR POWER. 115

But a never failing indication of their presence is
obtained from the visible currents in the adjacent
fluid, which is driven along the surface when they
are in full activity. When the animal is at rest,
and attached to any object which prevents it from
moving in the fluid, these currents are apparent
wherever cilia are present, whether on the outer
surface, or in the internal cavities of the body.
But when the animal is detached, and at liberty to
move, the reaction of the fluid on which the cilia
strike, propels the body in the opposite direction,
and gives it a rapid progressive movement. The
motion of the individual cilia appears to be that
of simple flexion in one direction, and a return
to the original position. Some naturalists, how-
ever, have represented it as being circular, each
cilium describing a cone which has its apex at the
base.-j"

On viewing a part which is fringed with cilia,
they frequently appear as if they were in progres-
sive motion along the edge of the part, an ap-
pearance which, when the cilia surround the margin
of a circular disk, gives rise to the illusion of its
having a rotatory motion ; for the disk then looks
exactly like a toothed wheel incessantly revolving
on its centre in one constant direction ; and the
deception is aided by the fluids being also carried
round in a continual vortex. This remarkable
phenomenon has excited much curiosity ; for the
slightest reflection is sufficient to convince us that
the continued revolution round an axis of any part
or appendage to the body is inconsistent with any

f Piirkinje, A^alentin, and Ehrenberg.

IIG THE MECHANICAL FUNCTIONS.

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. But the difficulty is soon explained when
we examine with more attention the construction
of the apparent teeth of this wheel ; and find that
each tooth is, in fact, composed of a group of cilia
^^« in different states of extension (as

Ob ^

,^ \i u shown in fig. 36*) ; and that their
ikf real motion is not in the direction

of the plane of the disk, but at
right angles to it. Each group contains cilia in
different states of elongation and contraction, com-
posing together a pyramidal figure, the apex of
which appears to be continually advancing, not
from the real progressive motion of the longest of
the cilia, but from each, in turn, becoming by its
elongation the highest of the group, as it stretches
out to its full extent, and afterwards folds itself up
so as to take its place among the shortest. As
these changes take place in each cilium, in regular
succession, the pyramid appears to advance, like
a succession of waves, which being instinctively
followed by the eye, produce the illusion of a pro-
gressive translation of the same object. t

The action of the cilia is frequently continued
for some time after the apparent death of the
animal ; and is even persisted in in very small
parts that have been separated from the body ; a
fact which shows that these motions may be per-

t See Dr. Fane's paper on Ciliobranchiate Polypi, Phil. Trans.
1834,410,

MUSCULAR POWIiR. 117

fectly independent of the will. Thus we find that
if any one of the ciliated ten taenia of a polype be cut
off, its cilia will continue to vibrate, and will propel
it forwards in the fluid for a considerable time, as
if it possessed individual volition. Yet in most
instances these ciliary motions appear to be of a
voluntary character, suddenly ceasing and being
renewed, without other obvious cause than the will
of the animal.

It has been conjectured that the cilia are tubular
organs, which are distended and protruded by the
injection of water from elastic canals placed along
their base, through which fluid is impelled by suc-
cessive undulations. Ehrenberg states that the
cilia of the infusoria are bulbous at the root ; and
supposes that they are moved by small muscles
attached to the bulb-t Dr. Sharpey, who has
given an excellent account of these organs as they
occur in the diflerent tribes of animals, is of opinion,
from a review of the whole of the phenomena, that
they are best explained on the supposition of mus-
cular act ion. J

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 sub-
stance : but as the organization becomes more
refined, these fibres are collected into bundles, and
compose what are properly called muscles. Mus-
cular fibres are attached at their extremities to the
parts intended to be moved. In the lower animals,
these attachments are principally to the skin, or
other external parts, which are subservient to the

t Ann. Sc. Nat. serie 2, i. 222.

X Dr. Todd's Cyclop, of Anat. and Physiol, article Cilia, i. 606.

no THE MECHANICAL FUNCTIONS.

purposes of progressive motion. In the higher
classes, the solid parts, or skeleton, being disposed
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 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 that which
constitutes a muscle, is therefore capable of over-
coming great resistances, or of raising enormous
weights. Those muscles, which, by means of their
nerves, as will hereafter 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 con-
tractions 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, i" has been esta-
blished by nature as the primary moving power of
the animal machine. This agent is resorted to on

* 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.

t 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 Hedy-
sarum gyrans, which have been fancifully compared to the move-

MUSCULAR POWER. 119

all occasions where considerable 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 exe-
cuted, the human mechanist avails himself of some
constant moving 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 pro-
duced.

Although the power be the same, yet the mode
of its application is exceedingly diversified ; and
the comparison of these diversities is the more in-
teresting, inasmuch as there are few of the animal
iunctions in w hich the ends to be answered are so
definite, and the operation of the expedients em-
ployed is so plain and intelligible. For while the
intricate chemical processes of the living system
generally elude our research, and the higher facul-
ties of sensation and perception are dependent on
still more recondite and mysterious powers of na-

ments 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 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 resemblance 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 vegetable
kingdom.

l-2()

THE MECHANICAL FUNCTIONS.

tiire, tlie inecliaiiical functions are eflectcd by the
simpler properties of matter, and allow iis 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 diminish
in length.* It is on this account, more especially.

38

43

that a knowledge of anatomy is so necessary to the
painter and the scul^Dtor. In every movement and
attitude of the body, some particular sets of muscles
are in action, and consequently tense and pro-
minent, 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 uncon-
nected, have occasioned too great a separation and

* 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.

MUSCULAR POWER. 121

(lisplacemeiU, and have impeded their co-operatioii
in one common eft'ect. Nature has guarded against
this evil by collecting 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 com-
pleted.

That we may be the better able to appreciate the
excellence of the plans adopted in the mechanism
of the animal frame, let us inquire what arrange-
ments would occur to us, prior to an acquaintance
with those actually adopted, as the most advan-
tageous dispositions of the muscular power. It is
evident, that the simplest mode would be that of
extending the fibres of the muscle in a straight
line between the points intended 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 adjust-
ments. Straight muscles, accordingly, are cm-
ployed chiefly for the movements of the minuter
parts of the apparatus 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

J 22 THE MECHANICAL FUNCTIONS.

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 attended
with a loss of force corresponding to the degree of
obliquity ; but there are, at the same time, advan-
tages 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 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 between the direction of the two equal
forces. Thus, in order to bring a bone at p. Fig.
39, down to the point q, the two muscles a and b,
extending from the fixed points m and n, may be
employed ; for as they exert forces in the directions
p M and p N, there will result a force in the inter-
mediate direction p o : and the effect desired will
be accomplished more quickly, and with a smaller
extent of contraction 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 obliquely,
that the ribs are brought in closer approximation
every time that the chest is elevated in breathing.
Thus carefully does nature dispose the muscular
fibres so as to obviate the necessity of their being

* See A paper by Dr. Monro, in the Transactions of the Royal
Society of l:!dinbur{j;h, vol. iii. p. 250.

MUSCULAR POWER. 123

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-
liquity of the action of muscular fibres, is applied,
not only to the entire muscle, but also to the in-
ternal arrangement of its fibres. Thus, we gene-
rally find that, in a flat muscle, its upper and
under surfaces are covered by a 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 directions on each side ;
like the fibres proceeding from the shaft of a pen.
A muscle thus constructed has accordingly been
termed a. pennijorm 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 (thejlexor pollicis pedis lon<j;us).
The arrangement first described. Fig. 40, forms the
semi-peunij'orm 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. 12) occurs, for example, in the

124 J'Hi: MECHANICAL FUNCTIONS.

soheus, or large muscle, which raises the heel, and
forms the thickest part of the calf of the leg.

It very commonly happens in the animal Irame,
as it does in other machines, that the presence of
the moving agent in the place where its action is
wanted, would be exceedingly inconvenient. The
usual plan adopted for transferring 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 tendons, 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 encumbered
with all the muscles which are necessary for the
movements of the fingers, it never could have per-
formed its office as a delicate mechanical instru-
ment. These muscles, accordingly, 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 intervention
the united power of all the fibres of the muscle
may be obtained, and concentrated upon any par-
ticular point. In this respect, likewise, they re-
semble 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 prin-
cipal use of tendons is that a diflerent direction
may, by their means, be given to the moving power,
without altering its position. Many instances occur
of their application in this manner, by their being
made to pass round corners of bones, and along
grooves or channels, expressly formed for their
transmission, and producing the effect of pidlies.

MUSCULAR POWER. 125

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 parti-
cular 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 exemplified in the muscle of the back,
called the trapezius, represented in Fig. 44. In
many instances, the fibres radiate in all directions
from a common centre : this is the case with the
delicate 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 cir-
cular 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 arrange-
ment 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

* Home, Phil. Trans, for 1800, p. 1.

126 THE Ml'XHANICAL FUNCTIONS.

the central aperture, or pupil, the size of which
they diminish when they contract; while on the
contrary, the radiating fibres, acting on the inner
circle, and drawing it nearer to the outer circum-
ference, which is fixed, lessen the breadth of the
ring, and consequently enlarge the circular aperture.

A similar combination of radiating and circular
fibres is employed in the construction of flat, or
slightly concave muscular disks, which are thus
rendered capable of exerting a strong force of ad-
hesion to the surfaces to which they are applied.
In these organs the circular fibres are placed at the
circumference, and the radiating fibres in the inte-
rior of the sucker, (see Fig. 48) ; so that, while the
margin of the disk is closely applied to the object,
the force resulting from the contraction of the cir-
cular fibres is exerted to remove the central por-
tions 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 surround-
ing canals of various kinds, such as the blood ves-
sels, 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 provided another layer
of fibres, disposed longitudinally, as shown in Fig.
49 ; the circular fibres being seen in Fig. 50. The
action of the longitudinal fibres is evidently to

MUSCULAR POWRR.

127

shorten the canal ; while that of the circular fibres,
by the yielding of the coats, and the partial re-
action of the contents of the vessel, has a tendency

to extend it. The Ascidia, which is a species of
marine worm, is an example of an animal whose
skin contains a multitude both of straight and cir-
cular fibres, by which all its movements are readily
performed. Many instances occur in the cylin-
drical 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 external one has lately been discovered, which
is composed of oblique or spiral fibres, crossing one
another in opposite directions, and greatly facili-
tating the varied movements of the animal.*

A variety of still more complicated arrangements
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, conse-
quently, unite the effects of both longitudinal and
circular fibres ; and, when combined with either of

Cams, Tabulse Anat. Conip. fol. Tab. I. Fig. 6.

128 THE MECHANICAL FUNCTIONS.

these, they serve to modify and regulate the actions
of each organ in a great variety of ways. 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 different in each, the
two layers cross or decussate, producing the effect,
and procuring the advantages of a combination of
oblique muscles already explained. Thus beauti-
fully is the arrangement of the muscular fibres of
the heart calculated to produce the rapid and com-
plete expulsion of its contained blood, with the
smallest amount of contraction in the individual
fibres.

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 execu-
tion of the smaller motions, requiring the most ac-
curate 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 consi-
derable power and velocity, while the minuter ap-
proximations to the exact positions are eftected 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
powerful machinery, so as rapidly to direct it to
that part of the heavens, where the object he wishes
to view is situated, a more nice mechanism is em-
ployed to direct the instrument accurately to the

MUSCULAR POWER. 129

exact point ; and, again, another is provided for
making the proper focal adjustments. Many pa-
rallel cases occur in the mechanism of the animal
frame ; one set of powerful muscles being employed
for the larger movements, and another set provided
for the accurate regulation of the more delicate in-
flexions 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 ef-
fecting each particular purpose are characterised
by the most admirable simplicity. In this respect,
also, as well as in all others, we cannot fail to re-
cognise their infinite superiority over every corre-
sponding 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." Pope.

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 resist-
ance to be overcome. With regard to the direct
force, therefore, it is evident that they must act at
a great mechanical disadvantage ; and this disad-
vantage is still farther increased by the obliquity
of the action with reference to the direction of the
motion. But the contractile power, which is inhe-
rent in the muscular fibre, is so enormous, as amply
to afford these losses, great as they necessarily are;
while, on the other hand, full compensation is made

VOL. I. K

130 THE MECHANICAL FUNCTIONS.

by the greater freedom and velocity of motion
thereby obtained. Strength is sacrificed without
scruple to beauty of form or convenience of pur-
pose ; 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 ob-
jects proposed, whether it be exerted in laborious
efforts of the limbs and trunk ; whether employed
in balancing the frame, or urging it into quick pro-
gression ; or whether it be applied to direct the
delicate evolutions of the fingers, the rapid move-
ments of the organs of speech, or the more exqui-
site adjustments of the eye, or of the internal ear.
Amidst the endless combinations of machinery
exhibited in different parts of the animal kingdom,
although the mode of application be diversified in
ten thousand ways, the original power is still the
same in kind, and is regulated by the same phy-
sical laws ; and similar instruments are employed
in effecting this infinite variety of purposes, by the
all-wise and omnipotent Architect of animated
creation.

131

Chapter II.

THE MECHANICAL FUNCTIONS IN ZOOPHYTES.

§ 1 . General Observations.

The mechanism of an organized being is designed
to fulfil various important objects. These we may
distinguish into two classes ; the one having refer-
ence to its internal welfare, the other to its rela-
tions 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 impressions; and they must at
the same time be so constructed as to admit of all
the internal movements, which the performance of
their functions renders necessary. They must, in
the second place, be madecapableof exerting upon
external matter the actions which conduce to their
well being ; and in order to enlarge their sphere of
action, they must have the power of transferring
the whole body from one place to another; or, in
other words, of effecting its progressive motio7i.

The objects included in the first of these branches
of the mechanical functions are answered 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 lo-
comotion, more especially, constitutes the most
general and palpable feature of distinction between
these two classes of beings. A plant, during the

132 THE MECHANICAL FUNCTIONS.

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
embedded, 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 condi-
tion 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 spontaneously changing their
situation, according to their several wants and ne-
cessities, 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 com-
plex 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 animals,
according to the particular model on which they
are constructed, and the relations which their or-
ganization bears to the element assigned as their
residence. All the mechanical circumstances in
their economy, indeed, are so closely linked toge-
ther, as scarcely to admit of being considered sepa-
rately. Thus we find, in one animal, a variety of
mechanical effects accomplished by one and the
same instrument; while, in others, they are each

ZOOPHYTES. 13.3

produced by a separate and distinct organ. In
some, the leading principle of the construction is
simplicity ; in others, the most elaborate mechanism
is displayed ; but the means have constant refer-
ence to the design, and are ever varied in exact
conformity with the change of purpose. The rela-
tive advantages of each plan of structure appear to
have been carefully estimated, and studiously ba-
lanced. Each quality has been bestowed in dif-
ferent degrees of perfection ; so that in following
the series of gradation among the successive tribes
of animals, we occasionally meet with favoured
species, endowed with great superiority in some
particular faculty. Some animals excel in swift-
ness; others in strength. Some are formed to dive
into the recesses of the deep ; others to flutter in the
light regions of air ; while, in many of the inferior
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 im-
portance. That construction of limbs which fa-
vours certain movements will necessarily interfere
with the ready performance of others, and must
preclude the developement of the organs which
would be necessary 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 perforating
wood. Some are provided with organs for pene-
trating 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

|:}4 THE MliCilANlCAL FUNCTIONS.

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 functions
which will form the first part of this treatise, I shall
keep in view, as the leading object of inquiry, the
faculty o^ progressive motion, noticing its different
degrees of perfection 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 mecha-
nical construction of animals constantly inhabiting
a watery element is more simple than the construc-
tion of those which live on land, and which are en-
compassed 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 gra-
vity, 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 cohesion of substance which are re-
quired in terrestrial animals.

The animals which occupy the lower divisions of
the scale can exist only in a liquid element. Their
forms present many analogies with vegetables ;
and hence they have been denominated Zoophytes,
that is, animated plants : but as it is now well
ascertained that they possess the essential charac-
ters of animals, the term of Phytozoa, or plant-like
animals, which has been given to them by some

SPONGES. 135

modern writers, would appear to be a more appro-
private designation. It is, however, 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. Sponges.

Among Zoophytes, the lowest station in the scale
of organization is occupied by the animals which
fabricate 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 dimi-
nutive, and of a firmer texture, as we approach the
Polar circles. They adhere to, and spread over
the surface of rocks and marine animals, to which
they are so firmly attached that they cannot be
removed without lacerating their bodies. In their
general appearance they resemble many kinds of
plants; but in their internal organization they differ
entirely from every vegetable production ; being
composed of a soft flesh, intermixed with a tissue
of tubular fibres ; and the whole being interwoven
together into a curious and complicated net- work.
The substance of which this solid portion, or basis,
is formed, is composed partly of horn, and partly
of siliceous or calcareous 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 protection to the entire fabric.

13() THE MECHANICAL FUNCTIONS.

The material of which the fleshy portion is com-
posed is of so tender and gelatinous a nature that the
slightest pressure is sufhcient to tear it asunder, and
allow the fluid parts to escape ; and the whole soon
melts away into a thin gelatinous liquid. When ex-
amined with the microscope, the soft flesh is seen
to contain a great number of minute grains, dis-
seminated through a transparent jelly. Every part
of the surface of a living sponge (as may be seen
in Fig. 53) presents to the eye two kinds of ori-
iices; the larger, having a rounded shape, and
generally raised margins, which form projecting

55

papillae ; the smaller, being much more numerous,
and exceedingly 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 gela-
tinous substance is endowed with a considerable
power of contractility : it was generally believed
that it shrunk from the touch, and that visible tre-
mulous 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 beyond the time of Aristotle ;
and they have been handed down by succeeding

SPONGES. 137

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 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 dilata-
tion and contraction in the round apertures visible
on the surface of sponges. This statement, so con-
fidently 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 examination, the
hasty assertion of Ellis into his " Elenchus Zoophy-
torum ;" 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 established fact in natural
history. The more accurate researches of Dr.
Grant on these subjects have at length dispelled the
prevailing illusion, and have clearly proved that the
sponge does not possess, in any sensible degree,

* Vol. Iv. p. 284.

138 THIi MECHANICAL FUNCTIONS.

that power of contraction which had, for so many
ages, been ascribed to it.*

Dr. Grant has also shown the trne 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. He gives
the following account of his discovery of the fluid
currents ; " I put a small branch of the Spongia
coalita, with some sea- water, into a watch-glass,
under the microscope, and, on reflecting 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 fountain, vomiting forth, from a circular
cavity, an impetuous torrent of liquid matter, and
hiuling 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 direction, or
diminish, in the slightest degree, the rapidity of its

* 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.

SPONGES. 139

course. I continued to watch the same oritice, 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 observed by Dr. Grant in a great
variety of species. They take place only from
those parts which are under water, and immedi-
ately 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 dis-
charge of a constant stream of water from the
interior of the body ; carrying away particles which
separate from the sides of the canals, and which are
not only seen, under the microscope, constantly issu-
ing from these orifices, but may even be perceived
by the naked eye, propelled occasionally in larger
masses.* Hence they are termed fcecal orifices.

For the supply of these constant streams, it is
evident that a large quantity of water must be con-
tinually 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

* 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 tlie surface, the motions of
which will render the currents very sensible to the eye. Fig. 53
exhibits these phenomena.

140 THE MECHANICAL FUNCTIONS.

through the numerous channels of communication
which pervade the substance of the body, it is col-
lected into wider passages, terminating in the fecal
orifices above described, and is finally discharged.
The mechanism by which these currents are pro-
duced is involved in much obscurity. The analogy
of other zoophytes would lead us to ascribe them
to the action of cilia, projecting from the sides of
the canals through which the streams pass; but
these cilia have hitherto 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 stRicture,
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 throughout
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 com-
pression, and prevent the entrance of foreign bodies.
Some of these spicula are delineated in Fig. 54:
but their forms, though 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 com-
pletely stationary as that of plants, yet such is not

SPONGES. 141

the condition of the earlier, and more transitory
stages of their developement. Nature, ever soli-
citous to provide for the multiplication of each race
of beings, and for their dissemination over the habi-
table globe, has always provided 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 endowed 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 and extension
of locomotive powers, they are enabled, in their
turn, to go in quest of food. But in the tribes of
animals 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 loco-
motion have been conferred, apparently 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 cha-
racters, but, after a certain period, giving birth to an immense
number of animated globules, which, for a time, move briskly in the
fluid, like infusory animalcules, and then congregate together, and

142 THE MECHANICAL FUNCTIONS.

The parts of the Spougia pmiicea, 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 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 pro-
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, V7hich have
been recently studied by M. Gaillon, and which he has established
into a natural family, denominated Nemazoaria, seem, in their
progressive developements, to possess alternately the characters of
vegetables and of animals, and may perhaps be regarded as con-
necting links between the two great kingdoms of living nature.
(See " Appercu d'Histoire Naturelle, et Observations sur les limites
qui separent le Regne Vegetal du Regne Animal. Par B. Gaillon,
Boulogne, 1833.")

* 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. 143

gression througli 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 cilia, which are
in constant and rapid vibration. These ciha are
spread over about two thirds of the surface of the
body, leaving the narrower portion, which has a
whiter and more pellucid appearance, uncovered.
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 direction backwards, or towards
the tail ; but when they are floating in the water,
the same action propels them forwards in the op-
posite direction, that is, with the broad ciliated
extremity foremost. They thus advance, without
appearing to have any definite object, by a slow
gliding motion, totally unlike the zig-zag course
of animalcules in search of prey. Yet they appear
to have a consciousness of impressions made upon
them ; for on meeting any obstacle, or striking
against each other, they retard a little the motion
of their cilia, wheel for a few seconds round the
spot, and then, renewing the vibrations, proceed in
their former course.

In about two or three days after these gemmules
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 gradually to expand itself
laterally, so as to form a broad base of attachment.

144 THE MECHANICAL FUNCTIONS.

While this is going on, the ciHa are still kept in
rapid motion on the upper part, scattering the
opaque particles, which may happen to be in the
fluid, to a certain 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
granules, like the flesh of the parent sponge; and
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 spicula, which were at first
small, confined to the central part, and not exceed-
ing twenty in number, now become much larger
and more numerous ; and some of them shoot into
the thin homogeneous margin. It is a remarkable
circumstance that the spicula make their appear-
ance completely formed, as if by a sudden act of
crystallization, and never afterwards increase their
dimensions.

When two gem mules, 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 con-
tinue 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

SPONGES. 145

find an analogy between the constitution of zoo-
phytes and that of plants.

In the course of a few weeks, the spicula are
assembled in groups, similar to those of the parent
sponge ; assuming circular arrangements, and pre-
senting,distinct openings at the points they enclose.
The young animal now rapidly spreads and en-
larges in every direction, becoming more convex,
and at the same time more opaque, and more com-
pact 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 re-
markable for their inertness, and for the privation
of all active powers ; and it has been conferred
evidently for the purpose of their being widely
disseminated over the globe. Had not this appa-
ratus 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 distance 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 old to corresponding latitudes of the new
world.

VOL. [.

140 THE MECHANICAL FUNCTTONS.

§ 3. Polypifera.

The next step in the organic series introduces ns
to the extensive family of Polypifera. The transi-
tion from the structure of the sponge to that of the
polypus may be thus conceived. Suppose the ab-
sorbing 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 pro-
vided 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 object that comes within
their reach, and of conveying it to the central ori-
fice, which performs the ofiice of a mouth. Each
tube, thus furnished with a circle of radiating fila-
ments, or ientacula, as they are called, is deno-
minated a Polype* The animal structure thus
composed has received the name of Lobularia (Fig.
56), and is the genus among this tribe that ap-
proaches the nearest in its character to the sponge,
which it resembles in the nature of its internal
texture. Each of the polypes with which its sur-
face 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

* For the sake of greater distinctness I shall employ the term
polype to denote the single tube with its tentacula ; and shall desig-
nate by the Latin term polypus the entire animal mass composed of
an aggregation of these polypes. Polypifera, the name of the
order, expresses animals bearing polypes.

POLYPI.

147

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 Zoanthus of Cuvier* (Fig. 58) :
at other times they are altogether isolated, as in the
Hydra viridis (Fig. 59). i

57

Polypi form a very extensive order of zoophytes,
abounding in every part of the ocean, but growing
in greatest luxuriance in the warmer 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 cal-
careous matter. The tentacula, which may be
compared to arms, vary in number and in length
in different species of polypi, and sometimes, in-
stead 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 usually tubular ; and

* The Hydra sociata of Gnielin ; the Actinia sociata of ElHs.
t In tliis figure two hydrae are seen attaclied to the stem of a
plant.

148 THE MECHANICAL FUNCTIONS.

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 pro-
pulsion of a fluid into their interior, derived from
the general cavity of the bodj^ ; 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 com-
posed of slender radiating petals. We find, indeed,
that as the organs of zoophytes become more deve-
loped, the affinities which these lower departments
of the animal kingdom retain with plants, are more
marked and more predominant. In the construc-
tion of zoophytes, nature seems still to keep in
view the models of vegetable forms, the characters
of which, while effecting 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 verti ciliated
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 Cu-
vier has designated them.

Like the animals of the sponge tribe, Polypi are

* See page 78.

POLYPI.

149

for the most part attached to some shell or base,
which may be either of a horny or calcareous na-
ture. The form of this shell admits of almost infi-
nite variety. In some it constitutes the external
surface of the animal, and encloses the flesh in a
general sheath, leaving only openings at the extre-
mities of the tubes for the expansion 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 the case in the Sertu-
laria. (Fig. 60.) At other times
the tubes are placed parallel to
each other, like the pipes of an
organ, with transverse partitions
at regular intervals : 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.

In some species the horny base is fashioned into
a number of cells, each of which serves for the pro-
tection of its respective polype. These cells are
generally placed at the extremity of the branches,

150

THE MECHANICAL FUNCTIONS.

presenting the greatest similitude to flowers. The
Fhistra (Fig. 63) is composed of minute and almost
microscopic cells, spread over a flat membranous
substance, resembling, in the flexibility of its tex-
ture and its mode of subdivision, the leaves of plants.
These cells are arranged in rows, with great regu-
larity, like those of a honey-comb, as is seen in the
magnified view of them. Fig. 64.

In other tribes the base of support is internal,
constituting a kind of skeleton or axis; the poly-
pous mouths being spread at intervals over the sur-
face 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 stems. The
flesh contains granules of calcareous matter, which,
in the dried specimens, adhere to the surface of the
stems. Fig. 65 is a branch of the Corallium rnh-
rum, of which Fig. QQ is a magnified portion, show-
ing the appearance of the polypes in their expanded

and contracted states. The way in which the po-
lypes are embedded in the flesh is seen in Fig. 67,
w hich represents a section of the GorgoniaBriareus.
In many cases the polypes are lodged in cup-like
depressions in the surface of the calcareous axis,
which affords them some degree of protection. In

POLYPI. 151

3Iadrepores these ilepressions are crossed by radi-
ating plates, adapted to the form and number of
the tentacula. In Millepores 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 network.

The material of which this axis, to which the
polypes are attached, is composed, is of various
kinds. Sometimes it is horny, flexible, and elas-
tic, corresponding in its nature to animal mem-
brane : at other times it is hard and calcareous,
being composed principally of carbonate of lime,
with a small quantity of the phosphate ; the pro-
portion of this latter ingredient varying in diflerent
species. In all cases the particles of calcareous
matter are united together by some portion of
animal substance, which may be obtained by dis-
solving out the former by an acid. We always find
the materials arranged in concentric layers, indi-
cating 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 disposed alternately, compos-
ing a jointed structure, which some have fancied
might be considered as making an approach to an
articulated skeleton ; for it is capable of consider-
able flexion, and readily yields to the impulse of
the waves, without the risk of being broken. This
is the case with the Isis hippuris, commonly known
hy the name of joitited coral. (Fig. 68.) There is,
in short, hardly any possible combination of these
parts which does not occasionally occur amidst the

152 THE MECHANICAL FUNCTIONS.

infinite diversities of condition displayed in this
department of the animal creation.

These structures are generally attached to sub-
marine 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 number
of instances, these fixed zoophytes are multiplied,
like the sponge, by the detachment of gemmules,
or imperfectly formed portions of their soft sub-
stance. 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 exhibit 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 super-
structure ; and, after the foundation has thus been
laid, they proceed in their upward growth, deposit-
ing a calcareous or horny axis in successive layers,
until it has acquired the requisite thickness ; * and
they then gradually assume the forms characteristic
of the particular species to which they belong. The

* The materials thus deposited have been usually considered as
being permanent structures, not capable of removal or modification,
and not possessing vital properties ; such properties being supposed
to belono- exclusively to the animated flesh with which these struc-
tures are associated ; but the recent inquiries of Milne Edwards afford
reason for the belief that, in some cases at least, the horny and even
calcareous parts of zoophytes are not wholly inorganic, that they

POLYPI. l-j;3

polypes themselves are not developed till after the
formation of the root and stem ; their growth being
in this respect analogous to that of the leaves and
flowers of a plant : but this analogy must be under-
stood with reference only to the mode of develope-
ment, for it in no respect extends to the functions
of those vegetable structures.

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 w ith its narrow
end foremost, while the opposite broad end, whicli
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 sta-
tionary, all the while revolving rapidly round their
longer axis, with their broad end uppermost : they
often bound forwards, either in straight lines, or
describing circles, with no other apparent object
than to keep themselves afloat, until they shall
arrive at a favourable spot for flxing their perma-
nent abode, and proceeding in their further deve-
lopement. The time of their remaining in this
free and moving state varies according to circum-

partlcipate, during the life of the animal which has formed them, in
the vitality of the soft parts, and that they are even capable of
growth by a kind of intussusception ; that is, by the introduction of
fresh materials through the pores of the substance itself. Ann. Sc.
Nat. serie 2. vj, 25, and x, 321, 334.

* Edinburgh Philosophical Journal, XVII. 107 and 337.

154 THE MECHANICAL FUNCTIONS.

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 exhibits 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 clearing the
space immediately surrounding the gemmule. It
remains for three days in this attitude, without un-
dergoing any perceptible change of form, and witli-
out 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 transparent,
and the whole gradually assumes the form of a cell,
surrounded by a delicate white opaque line, which
is the rudiment of the calcareous wall of the future
cell. Towards the base of this rudimental cell, the
gelatinous substance in the interior may be per-
ceived to become more consistent and opaque at a
particular point ; from this dull spot within the
cell, short straight tentacula begin to bud, extend-
ing upwards in the direction of the future aperture.
The gelatinous spot, from which the tentacula ori-
ginated, 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

POLYPI.

155

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 protruding from the
aperture of the first cell, the upper part of that cell
has already extended outwards to form the rudi-
ment of a second: and so on, in succession, till the
whole structure is completed.

The tentacula of polypi are exquisitely sensible,
and are frequently seen, either singly or conjointly,
bending their extremities towards the mouth, wlien
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 pro-
duced by the motions of the tentacula themselves ;
but are invariably the effects of the rapid vibra-
tions of the cilia placed on the
tentacula. In the polypes of the
Fliistra carbasea (Fig. 69), the
tentacula have each a single row
of cilia, extending along both the
lateral margins, from their base
to their termination.* Each po-
lype has usually twenty-two ten-
tacula ; 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,

* 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 gemmulcs of the same polypus, also greatly magnified.

156 THE MECHANICAL FUNCTIONS.

and the branches ol" an ordinary specimen present
about ten square inches of surface, we may esti-
mate tliat an ordinary specimen of this zoophyte
presents more than 18,000 polypes, 396,000 ten-
tacula, and 39,600,000 ciha. But other species
certainly contain more than ten times these num-
bers.!

The currents determined by the vibrations of
these cilia ascend on one side of the tentaculum
and descend on the other. (Fig. 70.) If any one
of the tentacula be cut oft', its cilia will continue to
vibrate, and will propel the fragment forward in
the fluid for a considerable time, as if it had become
itself an individual animal.

A question arises with regard to the constitution
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 the same
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 individual
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 number of naturalists have
adopted this latter view, regarding each portion, so
provided with a distinct circle of tentacula, as a
separate animal, associated with its neighbours in
the construction of a common habitation, and con-

t Dr. Grant has calculated that are about 400,000,000 cilia on
a single Flustra foliacea. Transactions of the Zoological Society
of London, vol. i. p. 11.

PENNATULA.

157

tributing its quota to 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 M'hole mass be regarded as one in-
dividual animal, or as an aggregate of distinct
individuals. But the question becomes of some
importance when applied to detached zoophytes,
such as PemiatuIcB, which are formed of a multi-
tude of polypes connected with a common 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 set of 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 fea-
ther. Some of these polypes are
seen magnified in Fig. 72. Immense numbers of
these curious animals are met with in different
parts of the ocean. If they possessed in any degree
the power of locomotion, which many naturalists
have ascribed to them, we should be able to ascer-
tain whether all their movements are conducted by
a common volition, or whether they are performed
independently of one another. It has often, indeed,
been asserted, that pennatulae swim through the

158 THE MECHANICAL FUNCTIONS.

water by their own spontaneous movements, con-
sisting either in the waving up and down of the
lateral branches, or in the simtdtaneous impulses
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 concludes from his observations that penna-
tulae are not in reality possessed of any such loco-
motive faculty ; but that they are carried to and
fro in the ocean, like the gulf weed, without the
slightest voluntary power of directing 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 sup-
posed to supply the means of progressive motion.

It is only when the contractile flesh of the
polypus is released from the restraint which the
solid axis imposes on its movements, that the animal
becomes capable of any distinct power of locomo-
tion. Such is the condition of the animals belong-
ing to the genus Hydra, of which the Hydra viridis,
or fresh water polype (Fig. 59, p. 147), 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 carrying
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

HYDRA. 159

as the head, and has for a mouth the aperture of
the tube, which is furnished at its margin with a
single row of tentacula. It thus corresponds to the
general definition of a polypus, and exemplifies its
most simple form.

The whole body may, on the one hand, be con-
siderably 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, capable of being extended
to a great length, and of being bent in all direc-
tions. In this way, they can quickly surround and
grasp any small object which they may happen to
touch ; and whenever irritated, they instantly re-
tract, so as hardly to be visible without the aid of
a magnifier. Each tentaculum may be moved
independently of the rest, at the pleasure of the
animal. The remainder of the body tapers gra-
dually from the head to the other extremity, be-
coming very slender, and having at its termination
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 animal,
the examination of which has contributed to throw
great light on the natural Iiistory of polypiferoiis

160 THE MECHANICAL FUNCTIONS.

animals.* While observing some aquatic plants,
Mhich 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 vegetables : but further observation con-
vinced him that they were endowed with powers of
spontaneous 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 direction 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

* Memoires pour seivir a I'Histoire d'un genre de Polypes d'eau
douce, a bras en forme de cornes. Par A. Trembley, 1744.

HYDRA. 101

but slow : for the hydra often pauses iu the middle
of a step, as if deliberating whether it should pro-
ceed : 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 consists of a succession of somersets:
the hydra, 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. Their favourite position
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 in the fluid, thereby receiving a
degree of buoyancy 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

VOL. I. M

162 THi: MECHANICAL FUNCTIONS.

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 hydro-
static 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 assistance 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 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 organiza-
tion, enabled to perform these actions? To this
question, however, no satisfactory 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

HYDRA. 163

composing the principal bulk of the body.* These
grains, when pressed out of the flesh into water, are
scattered indiscriminately ; 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 poly-
pus : nor is there the least indication of the forma-
tion 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 movements appear to be go-
verned 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 inequali-
ties of the surface of the bodies to which it is ap-
plied ; and perhaps this cause alone occasions it to
adhere 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 exceedingly mi-
nute disks, interspersed over every part of the sur-
face, constituting so many suckers, and resembling,
though on a very diminutive scale, the sucking ap-
paratus on the arms of the cuttle-fish.

The minute structure of this tribe of polypi has
been much elucidated by the recent discoveries of

* To this peculiar gelatinous substance, which composes the chief
part of the substance of polypi, and of the lower order of infusory
animalcules, Dujardin has given the name of Sarcode. Ann. Sc.
Nat. serie 2, i v. 367.

164 THE MECHANICAL FUNCTIONS.

Corda respecting that of the Hydra fusca, a species
nearly alhed to the hydra viridis. By the applica-
tion of high magnifying powers he found the sub-
stance of the body of this animal to consist of an
assemblage of vesicles, disposed in layers: those
which form the outermost layer having a more com-
pact arrangement than the rest ; and the inner-
most, or the surface of the stomach, being studded
with numerous vesicles projecting into its cavity.
Some of these have, at their apex, a minute oritice;
others are entirely closed. The tentacula, like the
rest of the body, is composed of cells; but are
hollow tubes, containing in their interior a spiral
fibre, which traverses their whole length, and pro-
bably serve as the agent of their rapid contractions.
Their surface is devoid of cilia; but, instead of
them, are thickly beset with short bristles. The
tentacula are also provided at particular parts,
which are raised like warts all round their circum-
ference, with firm and sharp spines, concealed, in
their ordinary state, in a cell, or sheath, beneath the
surface, but capable of being protruded by the ex-
pansion of a small vesicle which lies underneath
their root. The remarkable prehensile power of
tentacula thus armed on every side is, therefore, no
longer a matter of surprise ; and it affords a ready
explanation of the fatal injury sustained by all soft
animals that have once been encircled by these for-
midable weapons. From the suddenness of the
death of the victim, indeed, Corda conjectures that,
in addition to the mechanical injury inflicted by the
spines, they instil into the wounds some poisonous
liquid, like the fangs of venomous serpents. (Acta
Acad. Cses. Leop.-Carol. Naturae Curios, xviij.)
The Zoanthus (Fig. -38) belongs to a tribe of

HYDRA. 165

larger polypi, which are generally met with as-
sembled in clusters ; on which account it is termed
by Ellis the Actinia sociata, or cluster-animal flower.
It consists of a globular body, with a mouth sur-
rounded by one or two rows of tentacula ; termi-
nating below in a firm and fleshy tube, which ad-
heres 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).* Cur-
rents are, as usual, excited by the vibrations of the
cilia ; which in the simpler species, such as the
Vorticella cyathina, here deline-
ated, are the efficient instruments
of progressive motion. When at-
tached by its foot, the vorticella
advances in search of food, by
the extension of the foot-stalk
into a straight line : but quickly
retreats from danger, by suddenly
^ti^a^ throwing it into spiral folds.

Many of the species of vorticellse are so exceed-
ingly diminutive as to be imperceptible without
the aid of the microscope. They conduct us, there-
fore, by a natural gradation, to the next order we
have to notice, which is composed wholly of micro-
scopic animals.

* They also difter from Polypi in having a distinct intestinal
canal, with numerous stomachs. The whole genus is now included
by Ehrenberg in his class of Polygastric Infusoria.

f^iL

166 THE MECHANICAL FUNCTIONS.

* § 4. Infusoria.

Infusory animalcules, or Infusoria, were so named
by Muller, a Danish naturalist, from the circum-
stance of their swarming in all infusions of vege-
table 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 existence, and of the curious
phenomena they present : yet even the best instru-
ments afford us but little insight into their real
organization and physical conditions. On this ac-
count it is extremely difficult to assign their true
place in the scale of animals. By most systematic
writers they have been regarded as occuj^ying the
very lowest rank in the series, and as exemplifying
the simplest of all possible conditions to which ani-
mal 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 expressed doubts whe-
ther they really belong to the animal kingdom, and
whether they should not rather be considered as
the elementary molecules of organic beings, sepa-
rated from each other by the effects of chemical
decomposition, and retaining the power of sponta-
neous, 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 apti-

INFUSORIA. 167

tilde, or capability of uniting into living masses,
and constituting, therefore, the essential elements
of all organization. According to this view, all vege-
tables or animals in existence would be mere aggre-
gations of infusory animalcules, which gradually
accumulate by continual additions to their num-
bers 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 appear-
ance of simplicity, as well as by the brilliant style
and glowing imagination of its author, has had
many zealous partizans. The new world, which
was disclosed to the wondering eyes of naturalists
by the microscope, abounding in objects and in
phenomena of which no conception 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 principle in the arrange-
ment of the atoms of organic bodies. During the
greater part of the last century, infusory animal-
cules were the subject of frequent and laborious
microscopical research, and gave rise to endless
conjecture and speculation as to their origin, their
vitality, and their functions in the economy of na-
ture. Notwithstanding their minuteness, consider-
able differences 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 magnifying
powers, are homogeneous globules of living matter,

\C)H THE MECHANICAL FUNCTIONS.

without organization, but endowed with the single
attribute of voluntary motion : and even this pro-
perty 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 tenno is pos-
sessed of internal cavities for the reception and the
digestion of its food ; and who has rendered it pro-
bable that their organization is not much less com-
plex than that of the larger species of infusoria, in
which he has succeeded in tracing a muscular, a
nervous, and even a vascular system.

Those animalcules, whose form can be at all dis-
tinguished, exhibit a great diversity of shapes, and
variety of modes of progressive motion. Many, as
the CycUdimn, have the appearance of a thin oval
pellicle, smoothly gliding in all directions through

* His first researches on the structure and classification of infu-
soria were published at Berlin in the year 1830, in the Abhandlan-
gen dcr Koniglichen Akademie der Wissenschaften zu Berlin, for
the year 1830. His last great work, in folio, accompanied by an
atlas of 64 coloured plates, recently published, is entitled, Die
Infiisionsthierchcn ah Vollkommene Organismen, ein blick ein das
tiefere organische Ichen der Natur. Von D. Christian Gottfried
Ehrenberg zu Berlin. Leipsig. 1838. Ehrenberg divides the
Infusoria into the two following classes, namely, 1. Polygastricu,
characterized by the possession of numerous stomachs, into which
the food passes from the mouth, which is single and surrounded by
vibrating cilia ; and by having generally from one to four organs of
a bright red colour, which are supposed to be eyes. 2. Rotatoria,
which have ciliated organs of a circular form, creating by their
vibrations rotatory currents in the surrounding fluid: one longitu-
dinal, and several transverse vessels ; bands of muscular fibres ; a
distinct nervous system, with eyes, generally of a red colour; and
well defined organs for digestion, consisting of a mouth, jaws, gene-
rally furnished with teeth for mastication, an intestinal tube, with
pouches or dilatations corresponding to stomachs, or cseca.

INFUSORIA. 169

the fluid: some, as the Volvox, are globular;
others, as the Bodo* are shaped like a pear, taper-
ing at one end, and often terminating in a slender
tail, so as to resemble a tadpole. In many, and
particularly in the Enchlanis macrura, this tail is
of great length ; in some, as the Diglena, Euchlanis,
and Furculuria,^ it is forked ; in others, as in all the
Vorticellce, it takes spiral turns, like a corkscrew ;
and in many of the Rotatoria it can be drawn out
and folded in like a telescope. The Euchlanis lyn-
ceus, and many of the Peridiiiia have 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 undulatory mode of swimming.j: Some, as the
Gonium, have an angular, others, as the Colpoda, a
waving outline, and many of the Rotatoria present
the likeness of a bell or funnel. In many species,
as in the Baccillaria, Galionella, Naviciila, Synedra^
&c., the soft portion of the body is protected by a
hard covering, or shield, consisting of silica, and
caj)able of withstanding the effects of fire, as well
as most other disorganizing agents.

Forms still more irregular are exhibited by other
infusoria. Of these the most singular is the Anurba
dijflmns, (Ehr.) better known by the name of Pro-
teus (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

* Ccrcaria (Muller). f Furcocera, Lamarck.

X Animalcules referable to this genus are met with in great num-
bers in blighted wheat, (Fig. 2, page 53) in sour paste, and in vine-
gar 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.

170

THE MECHANICAL FUNCTIONS.

mass of soft gelly, higiily 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

78

watch its motions for any time, we see some parts
shooting out, as if suddenly 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 metamorphoses as rapid as they are irregular
and capricious.

The Volvox glohator (Fig. 79) is found in prodi-
gious numbers at the surface of many stagnant
pools. Its figure is perfectly spherical ; and its
movements consist in a continual and rapid rota-
tion round its axis, frequently remaining all the
while in the same spot. Another species, the Vol-
vox conjiictor, moves by turning alternately to the
right and to the left.*

The progressive movements of infusory animal-
cules are of two kinds, the one consisting in a

* The later observations of Ehrenberg have led him to the conclu-
sion that the globular mass, hitherto considered as a single animal-
cule, is in reality an aggregate of a multitude of more minute
individuals constituting a particular species of monad, and which
are retained within a vesicular envelope. This enveloping membrane,

INFUSORIA. 171

smooth and equable gliding through the fluid, pro-
duced 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 obstacles, while swimming toge-
ther in myriads in a single drop.

The great agent in the movements of the animal
frame being the muscular fibre, it was 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, however, that the actual
presence of contractile fibres could be recognized.
But this problem has at length been solved by the
discoveries of Ehrenberg, who, in his observations
of the larger and more highly organized species
belonging to the order of Rotatoria, 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

after being stretched to the utmost by the growth and multiphcation
of the monads within it, at length bursts and releases the included
smaller groupes of monads, which are found to be themselves
invested by their proper membrane, and which, in process of time,
undergo similar changes.

172 THE MECHANICAL FUNCTIONS.

these are muscular organs, and that they are tlie
real agents by which the motions witnessed are
effected.

These Rotatoria, or wheel animalcules, are so
named from their being provided
with an apparatus for creating a
perpetual eddy, or circular current,
in the surrounding fluid. The re-
markable organs, by which this
effect is produced, are generally two
in number, as seen in the Brachi-
onus urceolaris, (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 polypes. They
consist of circular disks, the margins of which are
fringed with rows of cilia, bearing a great resem-
blance to a crown wheel, which, on the principle
already explained, (p. 116), appears to be inces-
santly revolving, and generally in one constant
direction ; giving to the fluid a rotatory impulse,
which carries it round in a continual vortex.

Insignificant as the infusoria may appear, when
we attend only to the diminutive size of the indi-
viduals, yet their number is so inconceivably great,
(far surpassing, indeed, that of all other organized
beings,) as in some measure to compensate for this
minuteness, and to constitute them objects worthy
of regard as occupying an important part in the
economy of nature, and even as exerting a sensible
influence on the changes which have taken place
in our globe. Such, indeed, is the conclusion to
which we are irresistibly led by the remarkable
discovery, recently made by Ehrenberg, that large

WHEEL ANIMALCULES. 173

masses of earth consist wholly of the fossil remains
of animals of this class, the soft bodies of which
had been, during life, protected by a dense and
hard covering, or shield. These shields, which,
being formed of silica, were capable of resisting
the agency of an intense heat, have been preserved
in immense numbers, without change either of
form or of composition, during all the geological
revolutions which have taken place since the exist-
ence of their living inhabitants. They have been
found in different situations, in various parts of
Europe, in the Mauritius, and the United States
of America, forming, in their looser state, whole
beds of siUceous soil, well known by the name of
tiipoli, or polishing earth, and which is extensively
used in the arts ; and constituting, in their more
compact form, whole masses of mountains.* Ves-
tiges of these animalcules may be also discovered
in the white siliceous material with which nodules
of flint, found in chalk-beds, are encrusted ; and
are also rendered apparent in the flint itself, when
properly prepared for microscopic examination.
These facts render it probable that the siliceous
stones met with in these situations, derive their
origin from the agglutination of the remains of vast

* The size of each individual animalcule which composes the
Polirschiefer, or tiipoli of Biliii, in Bohemia, is estimated by
Ehrenberg' at about the 288th part of a line, or the 3456th of an
inch: and as this substance is slaty and without cavities, the ani-
malcules lie closely compressed. A cubic line would, therefore,
contain nearly 24,000,000; and a cubic foot above 41, 278,000,000
animalcules; of which 187,000,000 are in weight only equal to a
grain. Those of the Raseneisen, or bog-iron ore, from Berlin, are
of considerably smaller dimensions, being only the 12, 000th of an
inch in diameter; so that 1000,000,000 would be contained in a
cubic line, and a billion in a cubic inch.

174 THE MECHANICAL FUNCTIONS.

multitudes of infusoria of this description.* The
fossil remains of several species of microscopic
plants, in the form of a siliceous powder, have also
been found in great abundance in various places. t
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 con-
sider them as compound beings, the individual
animals are often so minute 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 as-
sumed as the criterion of complication or refine-
ment 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.

* See Turpln, Ann. Sc. Nat. serie 2, VII. 129.

f Those discovered by Mr. Binney, under the surface of a re-
claimed peat bog, in Biytonlear, near Gainsborough, have the
appearance of minute rectangular crystals, and are considered by
Mr. J. E. Bowman as the indestructible remains of myriads of
microscopic confervae, nearly allied to, if not identical with, para-
sitic species now existing.

MEDUSA.

175

§ 5. Acalepho'.

Floating masses of living gelatinous matter are
met with in every part of the ocean ; often in vast
numbers, and of various forms ; and having but
little the appearance of belonging to the animal
kingdom. They compose the order Acalephce, 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 Zoo-
phytes which are not per-
manently fixed to the spot
where they were first de-
veloped. They are ahnost 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 of
a mantle, extending loosely downwards from the
circumference ; together with a central pedicle
descending from the lower surface, like the stalk of
a mushroom, and terminating below in several
fringed laminse, or processes, which have sometimes
been denominated tentacula.

17() THE MECHANICAL FUNCTIONS.

The whole substance of the body of these medusae
is semi-transparent and gelatinous, without any
distinct librous structure ; yet it has considerable
elasticity, and possesses also some degree of con-
tractile power. The animal is seen alternately to
raise and depress the margin of its hemispherical
body, and to flap with the fringed membrane, or
mantle, which descends from it, in a manner some-
what similar to the opening and shutting of a
parasol. This pulsatory movement is performed
about fifteen times in every minute, with great
regularity : and by the reaction of the water, the
animal is sustained at the surface ; or by striking
the water obliquely, it may even perform a slow
lateral movement. Medusae descend in the water
by simply contracting their dimensions in every
direction : and 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 the
ocean.* In some parts of the Greenland seas they
swarm to such an extent that they give a visible
tinije to the colour of the waves for hundreds of

* The luminous property of sea water, or its phosphorescence, as
it is sometimes called, generally arises from the presence of minute
medusae, which are met with in greatest numbers at the surface,
being specifically lighter than the surrounding fluid. Humboldt
ascribes this phenomenon principally to the Mammaria scintillans.
It is also frequently produced by certain species of Infusoria ; more
especially the following : Prorocenteum micans ; Peridinium Mich-
aelis, micans, fusus, furca, and acuminatum ; and Synchceta
baltica. Ehrenberg.

BKKOE.

177

miles. The total number of* these animals dis-
persed over that space surpasses the utmost stretch
of the imagination. In these situations a cubic
foot of water, taken indiscriminately, was found by
Mr. Scoresby to contain above 100,000 of these
diminitive medusae.

Belonging- to the tribe of Medusariae is a singular
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, extend down its sides,
like those of a melon ; and along each of these is
attached a set of little membranes, extended hori-
zontally, and supported 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 pro-
cesses, which come out from the sides of the body,
and from these a great number of still finer fila-

VOL. I.

N

178 THE MECHANICAL FUNCTIONS.

ments, 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 prehension.* It was observed by
Otto Fabricius, that when a Beroe is cut into many
pieces, each piece continues 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 Velelia, provision is made for the mecha-
nical support of the soft gelatinous mass, by means
of an internal cartilage. In the former, this car-
tilage is of a circular form ; in the latter (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, denominated the

* See a description of the Beroe pileiis, by Dr. Grant, in the
Transactions of the Zoological Society of London, vol. i. p. 9, and
also a Memoir on the Beroidece, by R. P. Lesson, Ann. Sc. Nat.
serie 2, v. 235.

ACALEPH^. 179

Physalida, or Hydrostatic Acalephce. They have
obtained this latter appellation from their being
rendered buoyant by means of vesicles filled with
air, which enable them to float without the neces-
sity of using any exertion for that purpose. The
Physalia, or Portuguese Man -of- War, as it is called,
(Fig. 85.) is furnished 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 distance
from land. In calm weather they float on the sur-
face of the sea, rearing their purple crests, and
appearing at first like large air bubbles, but distin-
guishable 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 tro-
pical 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 Medusae, 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 included under

180 THE MECHANICAL FUNCTIONS.

that order. But they exhibit a much greater de-
velopement in their organization; having very dis-
tinct muscular fibres, endowed 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
those of the polypus. Fig. 86 exhibits an Actinia
in its 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 composite flowers ; and accordingly the par-
ticular species are named from these resemblances,
the sea-anemone, the sea-marygold, the sea-carnation^
the sun-Jtoiver, daisy, &c. The tentacula are co-
nical muscular tubes, perforated at their extremity
by a minute orifice, which the animal can close at
pleasure, and through which the sea-water is ad-
mitted into cavities in the interior of the body
which form a circular channel round the mouth,
connecting the basis of all the tentacula. They
may be distended simultaneously by means of water
injected into them from this channel. Actiniae 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 wea-
ther is fine, and the sea calm, it is very amusing to
watch the rapid expansions and retractions of their

ACALEPHiE. 181

many coloured tentacula, while they are moving in
search of food : to observe 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 confined to the particular spots
where we see them fixed ; for they are capable,
when disturbed, of seeking, by a slow progressive
motion, a more secure abode. Reaumur has mi-
nutely examined the arrangements of their mus-
cular 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 Echmodermata, a class which comprehends the
families of the Asterida, the Ec/iinida, the Holo-
thurida, and the Crmoidea, together with other
tribes of less note.

These animals, both in their general form, and
in the arrangement of their internal organs, 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 proceed 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-cal-

* Memoires tie TAcademie des Sciences, 1740, p. 490.

102

THE MECHANICAL FUNCTIONS.

careous covering, there is also provided, for the
support of the softer parts, a kind of internal ske-
leton, or jointed framework. The organs in the

interior of the body are farther supported by mem-
branous 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 supported by
a series of calcareous pieces, resembling those
which compose the spinal column of vertebrated
animals, and forming an articulated axis, con-
structed with the evident design of combining great
strength with a proper degree of flexibility. Car-
tilaginous 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 muse ular fibres by which its motions

ASTERIAS.

183

are effected are not easily distinguished. Calca-
reous grains, of 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 particu-
larly large around the mouth of the animal, which
opens at the centre of the under side. These cal-
careous masses have a crystalline arrangement, and
exhibit, 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 groove

96

termed, by Linneus, the ambulacrum, or avenue, a
name which it has received from its fancied resem-
blance to a walk between rows of trees : for each
groove contains a quadruple row of perforations,
like pin holes, through which small fleshy cylin-
drical 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 their expanded state, the Asterias
is capable of effecting a slow progressive motion ;
so that these processes may be regarded as corre-

184 THE MECHANICAL FUNCTIONS.

spending to feet, being levers for the advance of
the body. Each of these feet is terminated by a
concave disk, which, when applied to any flat sur-
face, acts as a sucker, on the principles already
adverted to.* Reaumur counted 304 of these feet
in each of the five rays of the star-fish, making
lo20 in all. I Each foot consists of a tube, closed
at the outer end, and the stem of which, after pass-
ing 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 por-
tion 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 distance. By the retreat of the fluid into its
reservoir, 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
process.^ From the shortness of these feet, not-
withstanding their great number, the advance which

* Page 126.

+ Memoires de I'Acadeniie des Sciences, 1710, p. 487.

I The mechanism by which the feet are protruded and retracted is
ilhistrated by the diagram, Fig. 97, which exhibits the bladders
connected with them, in different states of distention and contrac-
tion. Fig. 96 shows the upper side of the ambulacra, and of the
bladders connected with the feet. These bladders are supplied with
fluid by a set of canals quite distinct, according to Tiedemann, from
the vessels which circulate the nutritious juices, and which will be
hereafter described.

In addition to these larger tubes, constituting the feet, 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
also will be noticed more particularly hereafter.

ECHINUS.

185

this animal can make in any particular direction is
excessively slow.

Besides this movement of creeping, the Asterias
is capable of bending or unbending each of its rays,
and of bringing them nearer to each other ; actions,
however, which it can perform but very slowly,
and not to an extent sufficient to accomplish its
removal from one place to another, but which may
enable it to adapt its form to the passages through
which it may be creeping.

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 employed in its
construction, instead of forming isolated grains, is
accumulated and extended into polygonal plates

(Fig. 98), the edges of
which are dovetailed
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. Am-
bulacra 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 containing,
like those of the Asterias, a double row of per-
forations.*

* An architecture of a still more curious description is exhibited in
the calcareous framework 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.

18(3 THE MECHANICAL FUNCTIONS.

On the outer spherical surface of the external
crust, there are formed a great number of calca-
reous tubercles, arranged with beautiful regularity
and symmetry in double lines, passing, like meri-
dian circles, from the upper to the lower pole of
the sphere. Each appears, when magnified, to be
a smooth and solid ball, projecting from the surface
of one of the polygonal plates of the crust. These
balls serve for the support of the spines,* which
have grooves or sockets at their base, allowing of
their accurate application to the spherical surface
of the tubercles. They thus constitute ball-and-
socket joints, allowing of free motion in all direc-
tions. 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 surfaces 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

* It has been ascertained by Mr. Haidinger, that the structure of
these spines is crystalline, and that their cleavage presents the exact
rhomboidal angles characteristic of carbonate of lime. See his
translation of Mohs's Mineralogy, vol, ii. p. 91.

ECHINUS. 187

limited rotatory motion. Tliose which grow from
the sides are more slender, 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, ar-
ranged 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 re-
volving 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 particles ; 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
progress 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 sur-
face. In this situation the long dorsal bristles come
into play, preventing the sand from closing com-
pletely, and preserving a small round hole for the
admission of water to the mouth and respiratory
organs.*

AVhenever, in following the series of organic
structures, new forms are met with, we always find
them accompanied by corresponding modifications
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,

* The account here given is taken from Mr. Osier's papers in the
Philosophical Transactions for 1826, p. 347.

188 THE MECHANICAL FUNCTIONS.

of removing or modifying the substances that have
at any time been deposited, and suffered to harden.
Hence the structures composed of these substances
remain unchanged during the life-time of the ani-
mal, although they may continue to receive addi-
tions 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 conse-
quences of this law of increase. As these conse-
quences are important in their relation to the con-
ditions 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 alto-
gether upon the relative situations of the soft se-
creting 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, and push it
back for a space equal to its own thickness. In
process of time, the addition of numerous layers
having led to successive encroachments 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 ex-
pansion, 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,

ECHINUS. 189

since their relative proportions and situations may
be preserved unaltered. But this cannot happen
when the new materials are to be deposited on the
internal surface of a membrane, or a shell, which
encloses the soft parts : for the additions thus made
to the thickness 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 dimen-
sions : and if the process of deposition were to go
on, the space occupied by the soft organs would at
last be entirely tilled up with solid matter, and the
cavity be obliterated. Accordingly it is necessary,
whenever cells, intended for the lodgment of soft
organs, are to be constructed of hard materials, that
the foundation of these cells should be laid, and
their construction begun, on a scale of the same
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 deposited, 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 at-
tained its full expansion ; for were it at first built
of a smaller size, proportioned to that of the young
polype, it would prevent all further growth.

The globular shell of the Echinus, which is ex-
ternal 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 deviat-

1.90 THE MECHANICAL FUNCTIONS.

ing 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 perfect junction, without leaving any
intervening spaces. Thus has she provided for the
enlargement of the whole structure, by admitting of
additions being made to the margins of each of the
separate polygonal pieces ; fresh layers of cal-
careous substance being deposited on the under
side, and on the edges of each, in proportion as the
expansion of the contents of the shell causes their
separation. That such a succession of deposits has
taken place, may easily be seen, by minutely exa-
mining 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 probably 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 Echinoder-
mata the gradations which nature has observed in
passing from one model of construction to another
of a totally different aspect, through every inter-
mediate form. What shapes can be more diversi-
fied, 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 Pentacrinns; (Fig. 94) and yet we find

ECHINUS. 191

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 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, how-
ever, preserving its flatness. This stage is attained
in the Scutella, and the Clypeaster. (Fig. 90.) We
next find that, in the Spatangus, the thickness in-
creases ; 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 ^c//m?<5. (Fig. 91.) If we
might be permitted to conjecture the objects of all
these changes, which occur in this continuous gra-
dation, we might not unreasonably suppose them
to be the concentration of the internal organs into
one compact mass, and the retrenchment of all the
external appendages. It is also curious to observe,
how, amidst all these modifications, the double rows
of perforations, which constitute the ambulacra, re-
tain their situations, diverging in five equidistant
lines from one of the extremities of the axis, and
winding round to the other.

Returning to the Asterias, we can trace changes
equally gradual, though in an opposite sense, in
another series, which presents a striking contrast

102 THF. MECHANICAL FUNCTIONS.

with the former. Here, instead of the retrench-
ment of the appendages, we find them greatly de-
veloped, and amplified in every possible 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 retire within a central
disk, to which the slender rays, now bereft of feet,
become mere appendages. 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 be-
come branched, subdividing by simple bifurcations,
as in the Euryale palmiferum (Fig. 93) ; next into
minuter ramifications, as in the Caput MeduscB^
where the thousands of filaments have the appear-
ance of a tangled web, which defies all attempts at
unravelling.

The steps are but short from the Caput Medusae
to the Crmoidea, or lily-shaped tribe, (of which,
Fig. 94, representing the Pentacrinns europcDus, is
an example) ; for they consist chiefly in the addi-
tion 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
resemblance to a liliaceous plant.

]m

Chapter 111

:vi()llusca.

^ 1 . MoUusca in gejteral.

The series of animal structures, arranged according
to their mechanical functions, conducts us next to the
MoUusca ; an assemblage of beings which was first
recognised as constituting one of the primary divi-
sions of the animal kingdom by Cuvier, the greatest
naturalist of modern times. A vast multitude of
species, possessing in common many remarkable
physiological characters, are comprehended in this
extensive class. In all, as their name imports, the
body is of soft consistence ; and it is enclosed more
or less completely in a muscular envelope, called
the mantle, composed of a layer of contractile fibres,
which are interwoven with the soft and elastic
integument. 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 occasional
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 occasionally elongated
to form tubes, often of considerable length, for the
purpose of conducting water into the interior of the
body.

MoUusca, with the exception of a few among the
higher orders, are but imperfectly furnished with

VOL, I. o

194 THE MECHANICAL FUNCTIONS.

organs of locomotion. The greater number, indeed,
are formed for an existence as completely sta-
tionary as the Zoophytes attached to a fixed base.
The Oyster, the Muscle, and the Limpet, for ex-
ample, are usually adherent to rocks at the bottom
of the sea, and are consequently dependent for
their nourishment on the supplies of food casually
brought within their reach by the waves and cur-
rents of the ocean. This permanent attachment
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 com-
mencement of their separate 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 pre-
serve 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 unfitted
for the construction of an internal skeleton. Nature
has ordained that the purposes of Mechanical sup-
port and protection shall be answered by the for-
mation 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 may be either univalve,
bivalve, or multivalve, according as they consist of
one, two, or more pieces. Univalve shells have

* 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 ciha
which we found to be instrumental in the progression of the infusory
animalcules, and of the young of polypi.

MOLLUSCA ACEPHALA.

195

generally more or less of a spiral form, and are
then called turbmated shells. In a few, the cavity
of the shell is divided by transverse partitions into
numerous compartments. Some MoUusca have in-
ternal shells for the defence and support of par-
ticular 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.

§ 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 articulation varies much
in different species. The hinge is secured by a sub-
stance of great strength.
It is seen in Fig. 101,
which shows the valves
of the Unio batava, with
the connecting ligament.
This ligament is com-
posed of two kinds of
texture : the one, which
is always external, is
strictlyligamentous; that
is, perfectly inelastic : the
other has more of the

196 THE MECHANICAL FUNCTIONS.

properties of cartilage, being liighly 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 carti-
lage is always situated within the ligament ; some-
times 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 nourish-
ment and respiration. But as a security against
danger, it was necessary to furnish the animal with
the means of rapidly closing the shell, and retain-
ing the valves in a closed state. These actions
being only occasional, yet requiring considerable
force, are effected by a muscular power ; for which
purpose sometimes one, sometimes two, or even a
greater number of strong muscles are placed be-
tween the valves, their fibres passing directly across
from the inner surface of the one to that of the
102 other, and firmly at-

tached to both. — They
are named from their
ofiice of bringing the
valves towards each
other, the adductor mns-
chs. Fig. 102, wliich re-

MOLLUSCA ACEl'HALA. 197

presents the section of an oyster, shows the situa-
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 adductor muscle
might, it is evident, be called into play to coun-
teract that action ; but this would require a con-
stant muscular exertion and a great expenditure,
therefore, of vital force. Nature has also shown a
solicitude to economize muscular power, whenever
a substitute can 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 Ugamentum michce,
and being placed on the side of the muscle next to
the hinge, allows the valves to separate to the pro-
per distance only.* When the animal dies, the
muscular force ceases ; but the ligament, wdth
which the muscle is associated, retaining its elas-
ticity, allows the shell to open, but only to a certain
extent ; and accordingly, this is the state in which
we tind bivalve shells that are cast upon the shore,
after the soft flesh of the animal has decayed and
been washed out, provided the cartilage and the
ligament of the hinge are still preserved.

The P kolas is an exception to this rule ; for
instead of its valves being united, as usual, by an

* 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.

198

THE MECHANICAL FUNCTIONS.

elastic ligament, they are connected chiefly by
means of muscles. This departure from the ordi-
nary structure is probably occasioned by a new
condition introduced into the economy of the ani-
mal 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 P kolas ccmdida extremely
expanded, in order to show the
hinge, together with the liga-
ment (l); the long and thin pro-
cess of shell (p), to the ends of
which, on each side, a pair of
fan-shaped muscles, more parti-
cularly employed in boring, are attached ; and the
two adductor muscles (a, a), which retain the valves
in contact independently of the ligaments.*

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, by 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 suddenly closing
its shell, and thereby expelling the contained
water, with a degree of force, which, by the re-
action of the fluid in the opposite direction, gives

* 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 ACEPHALA. 199

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 respiration, 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 after-
wards 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 moUusca are provided with an
instrument shaped like a leg and foot, which they

employ extensively for pro-
gressive motion. Its form
in the Cardium, or cockle, is
seen in Fig. 104. This organ
is composed of a mass of
muscular fibres, interwoven
together in a very complex
manner, and which may be compared to the mus-
cular structure of the human tongue : the effect in
both is the same, namely, the conferring a power
of motion in all possible ways; thus it may be
readily protruded, retracted, or inflected at every
point. The Solen, 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

* Journal de Physique, xxviii. 244.

200 THE MECHANICAL FUNCTIONS.

the organ is formed of a spongy texture, capable of
receiving a considerable quantity of water, M'bich
the animal has the power of injecting into it, and of
thus increasing its dimensions.

The foot of the Mytilus edulis, or common muscle,
can be protruded 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 to-
wards it ; and by repeating the operation succes-
sively 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 employs its foot for
this purpose : first elongating it, directing its point
downwards, and insinuating 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 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 downwards.
When the animal is moderately active, these move-
ments are repeated two or three times in a minute.
The apparent progress at first is 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 moi'e rapidl}^
sinking visibly at every stroke, till nothing but the
extremity of the tube can be perceived above the

MOLLUSCA ACEl'HALA. 201

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 truncala, 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 ex-
cluded, on sand, in a glass of sea-water, he found
that they buried themselves immediately.

By a process exactly the inverse of this, that is,
by doubling up the foot, and pushing with it down-
wards against the sand below, the shell 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 Solcu 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 flexi-
bility of this organ to execute various other move-

* Philud. Trdiis. foi 18'26, p. 349.

202 THE MECHANICAL FUNCTIONS.

meats, of which, from the habitual inactivity of
animals of this class we should scarcely have sup-
posed 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 ex-
tending 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, in
addition, with a singular apparatus for withstanding
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 buffeting 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 com-
posed 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 certain 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

MOLLUSCA ACEPHALA. 203

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 tena-
city, the foot is protruded, and the thread it contains
is applied to the object 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 applied
in different directions around the shell. Sometimes
the attempt fails in consequence of some imperfec-
tion in the thread ; but the animal, as if aware of
the importance of ascertaining the strength of each
thread, on which its safety depends, tries everyone
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 exceedingly
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 disturbed in its operations, are
more slender than those which are constructed at
its leisure. Reaumur, to whom we are indebted
for these interesting observations, states also that

"201 THK MECHANICAL FUNCTIONS.

the marine muscles possess the artof forming these
threads from the earHest periods of their existence ;
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 manufac-
tured into gloves, and other articles, which resemble
silk.

§ 3. Gasleropoda.

The Mollusca which inhabit univalve or turbinated
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 surl;ice 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). In some
species it is fashioned into
a projecting ridge, which
cuts its way, like a plough-
share, along the surface on
which it moves. The bands of muscular fibres,
which compose the principal 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

* Memoires de I'Academie des Sciences: 1711, p. 118 to 123.
Poll conceived that these threads are dried ninscidar fibres; an opi-
iiion which has been adopted by De Blainville.

GASTKROPODA. 20 5

produce a visible undulatory 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 corresponding portion of the
animal is dragged along, so that the body advances
by a slow and uniform gliding motion. The opera-
tion 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 advance in
the direction of the motion of the snail, but with
much greater velocity, present a curious and inter-
esting spectacle.

A mucilaginous secretion generally exudes from
the surface of the disk, and tends to increase con-
siderably 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 adhe-
sion is greatly favoured by the conical form of the
shell, which, having a circular base, enables the
muscles of the disk, by their efforts to create a
vacuum underneath it, to command the whole hy-
drostatic pressure of the superincumbent water, as
well as of the atmosphere above the water. Be-
sides the muscular bands contained in the sub-
stance 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 di-
rections.

The foot of the JBuccinum nudatum, 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 cpiantity of water. It
may, by this means, be distended to a size even

206 THE MECHANICAL FUNCTIONS.

greater than the shell itself; so that the opening
which it forms in the sand is large enough to re-
ceive 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 pur-
pose 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 me-
chanism is matter of familiar observation in the
tentacula, or horns, of the snail and of the slug,
which are terrestrial mollusca belonging to this
order. The former of these has a turbinated shell
of the ordinary structure : the latter, though ex-
tremely 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 carti-
lage, giving support to some of the vital organs.

>§ 4. Structure and Formation of the Shells
of Mollusca.

The structure and formation of the shells of mollus-
cous animals is a subject of much interest in com-
parative physiology, as presenting many beautiful
illustrations of the laws by which the inorganic
parts of the living system are increased in their
dimensions.

* Osier, Pliil. Trans, for 1826, p. 352.

STRUCTURE OF SHELLS. 207

All shells are composed of two portions, the one
consisting of particles of carbonate of lime, the
other having the character of an animal substance,
and corresponding in its chemical properties either
to albumen or to gelatine. The mode in which
these two constituent parts are united, as well as
the nature of the animal portion, differs much in
different kinds of shell ; and it is chiefly in refer-
ence to these circumstances that 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 membranous
substance deposited in layers, which may be sepa-
rated from one another, either by mechanical divi-
sion with a sharp instrument, or by the slow actions
of air, water, or other decomposing 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, silvery,

208 THE MECHANICAL FUNCTIONS.

or iridescent appearance.* This appearance is
caused by the peculiar thinness, transparency, 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 be-
lieved to be a peculiar substance, and was dignified
with the appellation 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,
i(^)g 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 ef-
^x^iy^'^tN^^^ feet of the parallel grooves con-
^^^^Aj^^y sequent upon the regularity of
^^^^ °* arrangement in the successive

deposits of shell. | The appearance of these
grooves or striae when highly magnified is shown in
Fig. 106.J; This iridescent property may be com-
municated to shell lac, sealing wax, gum Arabic,

* Examples of this nacreous structure, as it is termed, occur in
the shells of the Haliotis, or Sea-ear, and of the Anodon, or fresli
water muscle.

t Philosophical Transactions for 1814, p. 397.

X See also a paper on this subject by Sir John Herschel in the
Edinburgh Philosophical Journal, ii. 114, from which the annexed
figure is taken.

STRUCTURE OF SHELLS. 209

balsam of Toln, or fusible metal, by taking an ac-
curate 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 dis-
posed 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 Cyprcea and the Volute are
examples of i)orcellaneous shells.

In shells of this kind, the carbonate of lime as-
sumes more or less of a crystalline arrangement ;
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 directions. Each layer is composed
of very thin plates, marked by oblique lines, which
show the direction of the crystalline fibres. The
direction 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

* When these shells decay and fall to pieces, they separate into
numerous thin scales of a pearly lustre. The fine scales thus ob-
tained 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.)

VOL. L P

210 THI2 MECHANICAL FUNCTIONS.

direction of the spire : while, on the contrary, those
J07 of the intermediate plate form con-
centric rings round the cone pa-
rallel to its base. Thus the fibres
of each layer are at right angles
to those of the layer which is con-
tiguous 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 direc-
tion they may be applied.* We here find that a prin-
ciple, which has only of late years been recognised
and applied to the building of ships, namely, that
of the diagonal arrangement of the framework, 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 perpendicular to
the surface, and the prisms are generally hexagonal.
This structure is observable in the Teredo gigautea
from Sumatra,! and also in many bivalves, such as
those belonging to the genera Avicula and Pinna.

* The lines indicating the direction of the fibres are shown in the
diagram, Fig. 107, which represents 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 appa-
rently homogeneous calcareous material, D. Of this latter substance
I shall afterwards have occasion to speak.

t 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.)

STRUCTURE OF SHELLS. 211

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 effervescence. 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.*

The difference between the textures of these two
kinds of shell is farther illustrated by the impres-
siop made upon them by tire. Porcellaneous shells,
when exposed to a red heat, give out neither smell
nor smoke : they lose, indeed, their colour, but re-
tain 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 corre-
sponding modifications of their mechanical proper-
ties. The toughness of the fibrous basis of mem-
branous shells imparts to them greater strength than
is possessed by the porcellaneous shells, which, in
consequence of the tenuity and uniform intermix-
ture 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 beautifully variegated with brilliant

* Poll has given a minute and elaborate description of the ap-
pearances of these fragments of membrane, when seen under the
microscope. See his folio work on the Testacea of the Two Sicilies.

212 THE MECHANICAL FUNCTIONS.

colours, and presenting altogether a close resem-
blance to porcelain, 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 intermediate 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 epidermis:* which,
however, is frequently rubbed off by friction.

The process employed by nature for the formation
and enlargement of the shells of the mollusca was
very imperfectly understood prior to the investiga-
tions of Reaumur, who may be considered as having
laid the first solid foundations of the theory of this
branch of comparative physiology, t His experi-
mental 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 ma-
terials constituting each layer, so added, are fur-
nished by the organized 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 in-
stance, be removed, which can be done without
injury to the animal, since it adheres to the flesh

* This membrane has been termed the Periostrac^tm.

t Mem. de I'Acad. des Sc. 1709, p. 367, and 1716, p. 30.3.

FORMATION OF SHELLS. 21^

only in one point, there is formed, in the course of
twenty-four hours, a fine pelUcle, resembling a spi-
der'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 evi-
dently no share in the formation of the new shell,
of which the materials have been supplied ex-
clusively by the mantle. This Reaumur proved by
introducing through the aperture a piece of leather
underneath the broken edges, all round their cir-
cumference, 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 glutinous ;
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, resembling porcelain ; or they are
deposited in a bed of membranous texture, having
already the properties 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

214 THE MECHANICAL FUNCTIONS.

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 consolidated,
is capable of afterwards undergoing any altera-
tion 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 inconvenience. We
should at the same time regard these cases in the
light of exceptions to the ordinary rule that a
portion of shell once formed remains ever after un-
changed, 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 inor-
ganic body, forming 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 cal-
careous matter takes place, and a pearly deposit is
produced. The worn edges of shells, and the frac-
tures, and other accidents which befall them, are

* Philos. Transactions for 1833, p. 796, et seq.

FORMATION OF SHELLS. 215

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 impregnated 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 surface 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 integument, and the subja-
cent corium, which is incorporated with the mantle,
and may be regarded as forming one aud the same
organ, t

The shape of the shell depends altogether on
the extent and particular form and position of the
secreting organ. The animal, on its exclusion from
the egg, has already a small portion of shell formed ;
and the simplest case is that in which this rudi-
ment 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

* Mr. Gray considers the external membrane of the sliell, or
epidermis, as formed by the outer edge of the plates of animal sub-
stance, which have scarcely any calcareous matter in their compo-
sition, and which are soldered together into a membranous coat.

t 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 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.)

"216 THE MECHANICAL FUNCTIONS.

edge of the shell, where it then forms a new layer
of shell; which new layer is parti}' applied to the
inner or concave surface of the original shell, and
partly extends a little way beyond its circum-
ference. The same happens with the succeeding
layers, each of which, being broader than the pre-
ceding, projects in a circle beyond it ; and the
whole series of these conical layers, of increasing
diameters, forms a compound cone, of which the
outer surface exhibits transverse lines, showing the
successive additions made to the shell in the pro-
gress of its increase. The Patella, 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 continual
deflection thence arising necessarily gives the shell
a spiral form, the coils of which are all in the same
plane ; constituting what is called a discoid shell.
This is the case in the Planorbis (Fig. 105, p.
204), the Spirula, and the Nautilus. Most com-
monly, however, as in the Buccinum, and Ac/iatina
(Fig. 108), the deposit of shell takes place late-
rally, and more on one side than on the other ;
hence the coils produced descend as they advance,
giving rise to a curve, which is continually chang-
ing its plane, being converted from a spiral into a
helix, a term of Geometry borrowed from the Latin
name of the common snail, which, as is well known,
has a shell of this form. These are called turbi-
nated shells. Fig. 108, which represents the shell

FORMATION OF SHELLS. 217

of the Achatina zebra, and of which Fig. 109 shows
a longitudinal section, may serve as an example of

a turbinated shell. The axis of revolution is termed
the Columella, and the turns of the spiral are de-
nominated whorls. In consequence of the situation
of the heart and great blood-vessels relatively to
the shell, the left side of the mantle is usually
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, how-
ever, 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 by its colour and appearance

* 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.

218 THE MECHANICAL FUNCTIONS.

from that which is formed subsequently. The suc-
ceeding turns made by the shell in the progress of
its growth, enlarging in diameter as they descend
from the apex, form by degrees a wider base.*
During the growth of the animal, as the body ex-
tends 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 Conus,
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 por-
tions of the spiral turns shoot downwards, so as to
form a pointed process ; thus the whole is still a
cone, but reversed from the former, the parts last
formed being the outer surface of the cone, and tiie
circumference of the apparent base, or flat surface ;
while the central portion of this base is the part
which was first formed.

* Mr. Moseley has shown that the increase in tlie size of the
whorls, and also of the distance between adjacent whorls, takes
place according to a geometrical progression ; and that, as conse-
quences of the observance of this law, the form of the curves gene-
rated is in all cases that of a logarithmic spiral, and the angle at
which the curve intersects the radius vector is constant. He has
verified the accuracy of this law by actual admeasurement in a great
number of cases, both of turbinated and of discoid shells, and has
deduced from it some interesting conclusions relative to the increase
in the powers of vitality as the animal advances in growth. See
Philosophical Transactions for 1838, p. 351.

FORMATION OF SHELLS. 219

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 pro-
ductive of corresponding inequalities in the surface
of the shell, such as transverse lines, or sttice.
Whenever an exuberance of materials has occa-
sioned a sudden expansion of growth, M'hich has
again soon subsided, a projecting ridge is pro-
duced at the place where the margin of the mantle
was situated at the time this happened. This en-
largement 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 par-
ticular parts of the mantle, which become in conse-
quence rapidly enlarged, shooting out into long
slender processes. Every part of the surface of
these processes has the power of secreting and
forming shell, so that the portion of shell they con-
struct, being consolidated around each fleshy pro-
cess, must necessarily have at first the shape of a
tube closed at the extremity. As fresh deposits are
made by the secreting surface, 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 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

220

THE MECHANICAL FUNCTIONS.

which several shells exhibit, and which have arisen
periodically during their growth from their outer
surface. In the Murex these processes are often
exceedingly numerous, and occur at regular inter-
vals, frequently shooting out into various anomalous
forms. In many shells of the genus Stromhus the
spines are of great length, and are arranged round
the circumference of the base, being at first tubu-
lar, 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 exist-
ence, when considerable changes take place in the
disposition of the mantle, and in its powers of secre-
tion. Often we find it suddenly expanding into a
broad surface, and adding to the shell what may
be termed a large lip. Sometimes no sooner has
this been accomplished 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 adja-

FORMATION OF SHELLS. 221

cent 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 recog-
nised as belonging to the same tribe of mollusca.
This is remarkably the case with the shell of the
CyprcBa, 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 narrow, and the edges of the aperture
are marked by indentations, moulded on corres-
ponding processes of the mantle.* But in this in-
stance 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, a
dense and highly polished porcellaneous shell, beau-
tifully variegated with coloured spots, which corre-
jj^^ spond exactly with the coloured

parts of the mantle that deposits
them. The additional plate, thus
deposited, completely envelopes
the original shell, giving it a
new covering, and disguising its
former character. A transverse
section (Fig. 1 14) at once shows

* Similar changes occur in the shells of the Ovula (spindles),
Erato (tear-shells), and Marginella, (dates). Gray, Phil. Trans,
for 1833, p. 792.

222 THE MECHANICAL FUNCTIONS.

the real steps by which these changes have taken
place.*

Changes equally remarkable are observed to
occur in the interior of the shell at diflferent stages
of its growth. On the inner surface of the 3Iitra,
the Volute, and other shells of a similar kind, there
is deposited a layer of a hard semi-transparent cal-
careous material, having a vitreous appearanccl'
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 much elongated, or
Inrrited, as it is called, \ this deposit 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 re-
maining in its original state, would have been ex-
tremely liable to be broken off 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 sur-
face thus withdrawn. That portion of the shell,

* According to Bruguiere, there is reason to believe that the ani-
mal of the Cijprcea, 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 succes-
sion several shells. De Blainville, however, considers it impossible
that the living animal can ever quit its shell. Malacologie, p. 94.

t This is the substance represented at d, Fig. 107, p, 210.

X As in the genera Turritella, Terebra, Cerithiuin, and Fas-
ciolaria.

FORMATION OF SHELLS. 223

which is thus abandoned, being very thin and brittle,
and having no support internally, soon breaks off,
leaving what is termed a decollated shell ; examples
of this occur in the Cerithium decollatinu, the ]3n-
Ihmis decollatus, &c. The young of the genus
Magilus has a very thin shell of a crystalline tex-
ture ; but when it has attained its full size, and has
formed for itself a lodgment in a coral, it fills up
the cavity of the shell with a glassy deposit, leav-
ing 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 original
shell is quite buried in this vitreous substance.

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 be-
comes, in the progress of its enlargement, exces-
sively cramped. In order to obtain more space,
and at the same time to 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 suffi-
cient strength to give full protection to the animal.
That this change has actually been effected is very
distinctly seen in the Conus (Fig. 115) by exa-
mining a vertical section of that shell, as is repre-
sented in Fig. 11(3. All the inner partitions of the
cavity thus laid open are found to be extremely
thin and transparent, and to consist only of the
innermost lamina of the original shell ; as will
appear on tracing them up to that outer portion of

224 THE MECHANICAL FUNCTIONS.

the section (b b), which lies on each side of the
proper apex of the shell, and which forms the ap-

117

parent base. The lines on this part of the section
indicate the thickness which each successive whorl
had originally, and when it was itself the outer-
most 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 mol-
luscous animals, respiring by means of pulmonary
organs. In the young shell of this tribe, the parti-
tions 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

* 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
tlie shell of the Cyprsea, Fig. 114; p. 221.

FORMATION OF SHELLS. 225

when the mouth of the spire is advancing over the
irregular surface of the preceding whorls. Thus we
often tind 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 destroying portions of
shell previously deposited seems to exist ; and
each successive whorl is moulded upon 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 fool's cap, has this faculty in a
remarkable degree ; and it is also met with occa-
sionally in Siphonarice and Patellce. 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 ex-
posed to injury is that which is situated at the mouth
of the shell. With a view to its protection, it con-
structs, in many instances, a separate plate of shell,
adapted to the aperture, and denominated an Oper-
culum. 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 successive
layers on the internal surface, sometimes in an an-
nular, and sometimes in a spiral form.* Fig. 118

* The curves exhibited in the operculum are logarithmic spirals,
like those of divided shells. See the paper of Mr. Moseley above
referred to in the note at page 218.

VOL. I. Q

226 THE MECHANICAL FUNCTIONS.

exhibits the lines which appear on
the inner side of the operculum of
the Turbo, and which indicate the
succession of deposits by which it
has been formed. The appearance
of the outer side is shown in Fig.
WJ. If an operculum were to be
constructed of a considerable size,
and were connected 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 remarked by Adanson, a connecting link be-
tween the univalve and the bivalve testacea. A
Clausimn 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 Epiphragma
is a partition of a membranous or calcareous nature,
which has no adhesion to any part of the animal
that formed it, and is constructed merely for tempo-
rary use. It is employed for closing the aperture
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 the Helix
pomatia will construct this covering, when circum-
stances occur to urge its completion. On the
approach of winter, the animal prepares itself for
passing that season in a state of torpidity ; first,
by choosing a safe retreat ; and next, by retiring
completely within its shell, and then barricading its
entrance by constructing an epiphragma. This

FORMATION OF SHELLS. 227

operation is described by Mr. T. Bell, who repeat-
edly watched it, as being conducted in the following
manner.* A large quantity of extremely viscid
mucus is poured out on tlie under surface of the
foot, to which a layer of earth or dried leaves readily
adheres: this is turned on one side; and a fresh
secretion being thrown out, the layer of earth
agglutinated together by the mucus is left. The
animal then takes another layer of earth on the
bottom of the foot, turns it also to the part where he
intends to form the wall of his habitation, and leaves
it in the same manner ; repeating the process till
the cavity is sutficiently large, and thus making the
sides smooth, even, and compact. In constructing
the arch, or dome of the form, a similar plan is
followed, the foot collecting on its under surface a
quantity of earth ; and the animal, turning it up-
wards, leaves it by throwing out fresh mucus ; and
this is repeated until a perfect roof is formed. In
about an hour, or even less, after this roof is covered
in, the whole surface of the collar of the mantle in-
stantaneously pours out a considerable quantity of
a calcareous secretion, which is at first as fluid as
thick cream, but very soon acquires the consist-
ence of bird-lime, and is exceedingly adhesive and
tenacious : and in about an hour after it is poured
out, it has become perfectly solid, thus forming a
second barrier, 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
mixture, it will immediately set about constructing
a covering for its protection against the cold ; and

* Zoological Journal, i. 94.

2*28 THE MECHANICAL FUNCTIONS.

it works with siicli 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 quan-
tity of a mucous tiuid, 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 this process of construction has to be re-
newed, 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 corresponding
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 addi-
tion of colouring particles secreted at particular
periods by the mantle.

* Gray, ibid. p. 214.

t 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.

FORMATION OF SHELLS. 229

The shells of Oysters and other acephalous mol-
lusca which adhere to rocks, are often moulded,
durmg their growth, to the surfaces to which they
are applied. The mantle, being exceedingly flexi-
ble, 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 irregular surface on which it grew.

As the enlargement of the shell proceeds, it is
necessary that the muscle, which closes the valves,
and is attached to their inner surface, should be
gradually removed to a greater distance from the
hinge, so that it may preserve its relative situation
with regard to the whole shell, and retain undimi-
nished 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 jjrogress of the removal may gene-
rally 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 been
accordingly comprised in the order of Miiltivalves,

* Defrance, Annates des Sciences Naturelles, ii. 16.

230 THE MECHANICAL FUNCTIONS.

which also comprehends Cuvier's order of Cirrho-
poda, including the several kinds of Barnacles (the
genus Lepas of Linnseus), which are furnished with
a great number of jointed filaments, or cirrJii, and
form an intermediate link of connexion between
the Mollnsca and the Articidala* 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 Mollnsca belonging to the two orders which
have now passed under our review, namely, the
Acepliala 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 neces-
sary to adopt a different arrangement with regard
to the gills, and to place them externally ; and we
then find them spreading out like a pair of wings,
on each side of the neck. Since this general clos-
ing of the mantle precludes, also, the formation of
any organ of progressive motion corresponding to a
foot, advantage is taken of the projection of the
gills to employ them as oars for the purpose of en-
abling the animal to swim through the water.

* The Cirrhopoda are now by most naturalists removed from the
class Mollnsca, and transferred to that of Crustacea. Mr. J. V.
Thompson has discovered that they undergo distinct metamorphoses.
Zoological Researches, 1830.

MOLLUSCA CEPHALOPODA. 231

MoUusca of this description are found in great
abundance in the colder regions of the ocean sur-
rounding both the north and south
poles ; and other species are also
met with, though in smaller num-
bers, in the tropical seas. The
Clio borealis, of which Fig. 1*20 is
a representation, is the most per-
fect 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
perform 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 sug-
gested the title of Pterojmda, given by Cuvier to
this order of Mollusca. The Cymbulia, which also
belongs to this order, is found on the coast of Pro-
vence, where it is called " le papillon de mer,"
from the resemblance of its large lateral oars to a
butterfly's wings.

§ i). Cephalopodd.

Following the progress of organic developement,
we now arrive at a highly interesting family of
Mollusca, denominated the Cephalopoda, and dis-
tinguished above all the preceding orders by being
endowed with a much more elaborate organization,
and a far wider range of faculties. The Cephalo-
poda have been so named from the position of cer-
tain organs of progressive motion, which are situated
on the head^' and like the tentacula of the Polypus,

232

THE MECHANICAL 1 UiNCTlONS.

surround the opening of the mouth. (See Fig. 121.)
These feet, or arms, or lentacnla, if we choose so to

call them, are long, slender, and flexible processes,
exceedingly irritable, and contractile in every part,
and provided with numerous muscles, which are
capable of moving and twisting them in all direc-
tions with extraordinary quickness and precision.
They are thus capable of being employed as instru-
ments, not only of progressive motion, but also of pre-
hension. For this latter purpose they are in many
species peculiarly well adapted, because, being per-
fectly flexible as well as highly muscular, they twine
with ease round an object of any shape, and grasp
it with prodigious force. In addition to these pro-
perties they derive a remarkable power of adhesion
to the surfaces of bodies from their being furnished
along the whole of their inner sides with numerous
suckers, or acetabula. 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 mechanism is more artificial than the
simple construction already described (p. 120): for
when the surface of the disk is fully expanded, as

MOLLUSCA CEPHALOPODA.

233

shown ill Fig. 1 23* b, we tind 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, 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 pro-
ducing rarefaction, or the tendency to a vacuum,
the power of adhesion is considerably augmented.*

* The description I have here given is the result of my own ex-
amination of a large Octopus, which I had lately an opportunity 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 some-
what flattened by the operation of dividing it.

234 THE MECHANICAL FUNCTIONS.

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 re-
markable 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 denomi-
nated Polypus by Aristotle, has eight arms of equal
length, and contains in its interior two very small
rudimental shells, formed by the inner surface of
the mantle. This shell becomes much more dis-
tinct 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 composed wholly of carbonate of lime :
it is well known by the name of the cuttlejish bone.
Its structure is extremely curious, and deserves
particular attention, as establishing the universality
of the principles which regulate the formation of
shells, whether internal or external, and from which
structures differing much in their outward appear-
ance 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 mate-
rial, passing perpendicularly between the adjacent
surfaces. This shell is not adherent to any internal

MOLLUSCA CEPHALOPODA. 235

part of the animal 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 relation.
It, no doubt, is of use in giving mechanical support
to the soft substance of the body, and especially to
the surrounding muscular flesh ; and thus probably
contributes to the high energy which the animal
displays in all its movements. It has been re-
garded as an internal skeleton ; but it certainly has
no pretensions to such a designation ; for, although
enveloped 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 accounts it
must be considered as a true shell, and classed
among the productions of the integuments. It
differs, indeed, altogether from bony structures,
which are composed of a different kind of material,
and formed on principles of growth totally dis-
similar.*

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 ex-
tremity of the body which is opposite to the head.
They have been regarded as the rudiments of tine
fins, which are organs developed in fishes, and

* 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. Several cartilaginous
structures also exist in the interior of the body of the cephalopoda,
which are considered by Meckel and Carus as indicating an approach
to the formation of an internal skeleton, analogous to that of verte-
brated animals.

236 THE MECHANICAL FUNCTIONS.

Mhich are supported by slender bones, called rays;
but no structure of this kind exists in the fins of tiie
Cephalopoda.

In swimming, the organs principally employed
by cuttle-fish for giving an eff^ective impulse to 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 foremost, 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 downwards, 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 accompany-
ing 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, dur-
ing violent agitations of the sea ; and accordingly
we find that it is only the extremities of these long
tentacula that are provided with suckers, while the
short ones have them along their whole length.

The other genera of Cephalopodous Mollusca

MOLLUSCA CEPHALOPODA. 237

are, like the Sepisp, provided with tentacula at-
tached to the head. The shells of these animals
are often found to contain partitions dividing them
into a number of chambers ; hence they have been
termed camerated, or mu/tilocular, or polytlialamous
shells ; and sometimes the greater part of the shell
is contained within the body. The Spirula (Fig.
124) has an internal shell of this description, of
which the cellular structure and numerous par-
titions are rendered visible by making a section
through it (Fig. 125). The Nautilus Poynpilius is
the inhabitant of an external polythalamous shell,
(Fig. 126), the section of which is represented in
Fig. 127: but the shell of the Paper Nautilus, or
Argonaut, is unilocular, or monothalamous, being
undivided by partitions.

The shell of the Argonaut is exceedingly thin,
and almost pellucid, probably for the sake of light-
ness, 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 air, while it is thus floating
on the waters, nature 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
assiduous navigator does not neglect to ply its

238 THE MECHANICAL FUNCTIONS.

tentacula as oars on either side, to direct, as well
as accelerate 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 tentacida within its shell,
by which means it renders itself specifically heavier
than the water, sinks immediately into more tran-
quil regions beneath the surface.* The Pearly
Nautilus (N. Pompilius) is provided with a similar
sailing apparatus.

The formation of external polythalamous shells

* The liabits of the Arg;onaut or Ocythoe, as it is sometimes
called, are still very imperfectly known. Doubts have been enter-
tained whether the shell it occupies 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 ; for it is now ascertained that the animal
does not adhere to it in any part. It appears, however, to be satis-
factorily determined by the recent inquiries of Professor Owen, that
the shell is really formed by the Argonaut.

There exists an extensive series of very minute, and even
microscopic polythalamous shells, which were considered by M.
D'Orbigny as having been constructed by Mollusca belonging to
the family of Cephalopoda (Ann. Sc. Nat. VII., 96): but the
more recent inquiries of M. Dujardin have shown that the supposed
analogy is without foundation, and that the inhabitants of these
shells really belong, not to the class Mollusca, but to that of
Zoophyta.

Professor Ehrenberg has lately discovered that limestone and
chalk contain a vast multitude of microscopic, and hitherto un-
known nautilites, or polythalamic shells, in size from the 3456th to
the 288th of an inch, so that each cubic inch of chalk contains
frequently a number of these shells, far exceeding a million. Many,
and probably all the chalk rocks of Europe are the products of
microscopic animalcules, some of which have formed calcareous,
and others siliceous coverings, which, at subsequent periods, have
been transformed into chalk and flint. See PoggendorfF's Annalen,
1839, No. 7: and Edinburgh New Philosophical Journal, xxviii,
161.

MOLLUSCA CEPHALOPODA. 239

presents very curious phenomena. The animal, at
certain periods of its growth, tinding itself cramped
in the narrow part of the spire, withdraws from it
that portion of the mantle which had been lodged
there, a vacant space being left. The surface of
this part of the mantle immediately begins to
secrete calcareous matter, which is deposited in
the form of a partition, stretching across the area
of the cavity. As the animal proceeds to increase
in size, and to occupy a wider portion of the ex-
ternal shell, the same necessity soon recurs, and
the same expedient is resorted to. It again with-
draws its mantle from the narrower into the wider
part of the shell ; and then forms a second par-
tition, at a little distance from the first, corres-
ponding to the space left by the receding of the
mantle. This process is repeated at regular in-
tervals, and produces the multitude of chambers
contained in polythalamous shells, of which the
living animal occupies only the largest, or that
which continues open ; all the other chambers
probably contain air only.* The partitions are in
general perforated either in the centre or at one
side, for the purpose of giving passage to a tube,
which extends as far as the apex of the shell. This
tube is often surrounded, either entirely or par-
tially, by shell, which forms what is denominated
the syphon or siphuncle ; portions of which are seen
in the section. Fig. 127.

* This structure is exhibited in a great variety of 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 often have undu-
lating surfaces.

240 THE MECHANICAL FUNCTIONS.

It has been conjectured that this hydrostatic
apparatus is provided for the express purpose of
enabling the animal to descend or rise rapidly in
the water; the former of which it might effect by
injecting fluid into this extensible tube, so as to
compress into a smaller bulk the air contained in
each chamber, and thus increase the specific gra-
vity of the whole mass ; and the latter, by the
contrary action, of withdrawing fluid from the
tube.*

Chapter IV.

ARTICULATA.

^ I. Articulated Animals in ge7ieral.

From the Cephalopoda, the transition is easy to
the lowest order of vertebrated animals. But pre-
viously to pursuing the analogies which connect
these two divisions of the animal kingdom, we have
to pass in review a very extensive series of animal
forms, constructed on a peculiar system, and oc-
cupying, as well as the Mollusca, a place inter-
mediate 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

* See Dr. Buckland's Bridgewater Treatise.

ARTICULATA. 241

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 apparatus for motion,
and endowed with extensive capacities for using
and applying it in various ways. While Nature
has preserved, in the construction of their vital
organs, the simplicity which marks the primitive
modes of organization, and has adhered to a defi-
nite model in the formation of the different parts of
the system, she has displayed the most boundless
variety in the forms and combinations of the me-
chanical instruments of prehension and of pro-
gression .

All the tribes of Zoophytes, and by far the greater
number of Mollusca, are limited, by the constitution
of their system, to an aquatic existence. 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 structures. A further design, also,
soon becomes manifest ; and instruments are given
for elevating the body above the ground, and for
traversing with rapidity the light and scarcely
resisting air. 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.

VOL. I.

242 THE MECHANICAL FUNCTIONS.

^ 2. Annelida.

The lowest division of articulated animals compre-
hends those which have a vermiform shape, and
which compose the class of Annelida, or Annulose
animals ; of which the earth-worm 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 developement, through which nature
appears to pass in attaining the higher and more
perfect conformations.

It may be remarked that, in effecting the tran-
sition from Zoophytes to the new model of con-
struction here presented, nature seems to have
wholly abandoned that radiated disposition of parts
and those star-like forms, so characteristic 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 symmetry, 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 platie. As a
direct consequence 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 reflected
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

ANNELIDA.

24.-5

lower surface, and there was no side which could
be exclusively denominated 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 modifications
of the integuments themselves. They consist of a
framework, 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 Lumbriciis, or
earth-worm; in the Pontohdella (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,
however, we find all these rings compressed into
the form of a flat oval disk. This is the case in
the Erpobdella, of which Fig. 130 is an enlarged
representation.

244 THE MECHANICAL FUNCTIONS.

In general, the first of the segments into which
the body is divided, contains the principal organs
of sense, and is sufficiently distinct from those
which follow^ to entitle it to the appellation of the
head; while the lengthened prolongation of the
opposite extremity, when such a form is present,
may be denominated the tail.

The rings which encircle the body are connected
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 irritability, and contracting
with great force. The fibres composing each band
are all parallel to one another, and have seldom
any tendinous attachments ; being generally in-
serted directly on the parts they are destined to
move. Thus the adjacent margins of the rings of
worms (as shown in the diagiam, Fig. 131) are
connected together by muscular bands of this des-
cription, passing transversely from the one to the
other, immediately under the skin, and parallel to
the 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 anticipated.
The lower set, when they contract, bring the rings
nearer to one another at that lower part ; and when
the whole series occupying that situation are
exerted in concert, they raise the body in the form
of an arch. An opposite curvature will be produced

ANNELIDA. 245

by the contraction of the upper bands, which by
raising both ends of the body bend the back down-
M'ards. In proportion as the bands, situated on
either side, act in concert, while the others are
relaxed, the body will be bent laterally towards
that side. When all the four muscular bands con-
tract together equally, their joint effect will be to
bring the rings near to each other, and to contract
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 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 set(S, 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. These hairs, both in the anterior
and posterior segments, are directed towards the
centre of the animal ; while those on the middle
segments are perpendicular.* We almost con-
stantly find, in animals belonging to the order of

* As an instance of the extraordinary multiplicity of species ex-
isting in every department of living nature, 1 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.

246 THE MECHANICAL FUNCTIONS.

Annelida^ some provision of this kind, often con-
sisting of tufts of hair regularly disposed in rows
on each side of the under surface. In the Nereis
(Fig. 129), 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 effecting which, as they
are used under w^ater, very little support is neces-
sary.* 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 description
advances along the ground is very simple. It first
protrudes the head by the elongation of the fore-
most segments of the body, while the other segments
cling to the earth by means of the rings, and also
of the bristles and other appendages to the integu-
ments. 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
segments are necessarily elongated : these are next
contracted ; and so on, in succession, till the whole

* Home; Lectures, &c. vol. i. p. 115.

ANNELIDA.

247

is brought forwards to the head : after which the
same series of actions is repeated, beginning with
the advance of the head. 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 animals
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, or even
tentacula, have been given to this simplest of ver-
miform animals. t

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 alone, the Serpula
(Fig. 133), forms for itself an external shell, which

* See Home ; Lectures on Comparative Anatomy, vol. i. p. 11 4.

t Jacobson suggests that the Gordius may be an ag-gregation of
numerous individual animals enclosed in a longitudinal tubular
envelope. (Ann. Sc. Nat. serie 2, i. 320.)

248 THE MKCHANICAL FUNCTIONS.

is shaped into a spiral tube. Others, as the Sabella
and the Terebella, accomplish 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 Tere-
bella conchilega. These coverings, however, com-
posed as they are of extraneous materials, and not
being organic productions of the animals them-
selves, are structures wholly foreign to their systems.
These inhabitants of tubes, the Tubicolcc 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 motion
by the Lumbricus marimts of Linnaeus ( Arenicola
piscatornni of Lamarck), is very remarkable. * 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 regular cone. Each ring being so much smaller
than the one behind it as to admit of being received
within it, the whole head, when completely 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

* See the account given by Mr. Osier, Philosophical Transac-
tions for 1 826, p. 342.

ANNELIDA, 249

cement, which exudes from its skin, and which
unites the particles 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 ad-
vances ; 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, belonging to
a tribe of marine worms, which from the peculiar
circumstances of their situation, inhabiting parts of
the shore nearly midway between 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, 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

•250 THE MECHANICAL FUNCTIONS.

the surface. Bending the head alternately from
side to side, while it continues to apply the mate-
rials 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 inter-
esting. For the purpose of fixing the different
fragments compactly, it presses them into their
places with the erected scales, at the same time re-
tracting 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 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 througli 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 accom-
plished, 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 summer 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. 1*29), proceeding
from the margin of the mouth and other parts of

ANNELIDA, 251

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 dis-
tances through the sand ; first extending the an-
terior 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*

Facilities for progression are also given by the
addition of tubercles, arranged in pairs along the
under side of the body, which serve the purposes
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 specially provided
for that purpose, serve as levers for efi^ecting pro-
gressive 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 appa-
ratus for suction at the two extremities of the body,
which are formed into disks for that purpose. By
fixing alternately the one and the other, and con-
tracting or elongating the body as the occasion
requires, the leech can move at pleasure either for-
wards or backwards. Thus, while the tail is fixed,
the head may be advanced by lengthening the

* Osier, Phil. Tmns. fur 1826, p. 342.

252 THE MECHANICAL FUNCTIONS.

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. <l

Most of the parasitic animals which inhabit the
interior cavities of the body, and especially the ali-
mentary canal, correspond in external form, as well
as in many circumstances of internal conformation,
to the Annelida. They compose an order denomi-
nated the Enlozoa. As they, in general, remain
fixed to the part from which they draw their nou-
rishment, their powers of progressive motion, if
they possess any, are very limited.

§ 3. Arachnida.

In passing from the Annelida to the Aiac/mida, an
order which comprehends all the species of Spiders,
together with anin)als allied to them in conforma-
tion, we find that a considerable advance has been
made in the degree of developement. The frame-
work of the body is more consolidated, and the
instruments provided for progressive motion are
shaped into longer and more perfect levers, are
united by a more refined system of articulation,
and are moved by more distinct 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 framework
of the Annelida, are here consolidated so as to form
two principal divisions of the body ; the one in

^

ARACHNIDA. 253

front, whicli 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 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 ce-
phalo-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 suc-
ceeding ones the leg and the tarsus ; and each foot
is terminated by two, or sometimes 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 prehen-
sion, 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 their outer skin
several times, and at regular periods. In the earlier

254 THE MECHANICAL FUNCTIONS.

stages of their existence, although they have the
general form of the mature 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 commence-
ment 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 suspended,
being firmly braced by their articulations, enable
the muscles to act with great mechanical advantage
in accelerating the progression of the body. Hence
these animals are enabled to run with great swift-
ness, and to spring in all directions and from con-
siderable distances on their prey; powers which
are necessary to those tribes that live altogether by
the chase.

Progression on a plane surface is effected, in
these animals, by the alternate advance of two sets
of feet ; the one set consisting of the first and third
feet on the right side, and the second and fourth on
the left ; the body resting on the other four, which,
on the former set having been brought to the
ground, are, in their turn, raised and brought for-
wards : so that four feet are always in action, and
four at rest, at the same moment.*

The greater number of species, however, as is
well known, are provided with a curious apparatus
for spinning threads, and for constructing webs to

* Muller Diss. Inauo". Physiol, de Phoronomia animalium. Bonn.
1822.

ARACHNIDA. 255

entangle flies and other small insects. Every
species of spider weaves its web in a manner pecu-
liar 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 them-
selves, 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 acquainted with the event,
and, by the assistance of the same thread, passes
quickly to the spot.

Some species have the power of conveying them-
selves 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 sup-
posed that during their flight they 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.
Spiders apply this power of manufacturing threads
in various ways, not only for entangling and bind-
ing their victims, but occasionally also for effecting
their escape from situations of peculiar difficulty.*

* A remarkable instance of a manoeuvre of this kind is thus re-
lated by Mr. T. Bell ; " I insulated a common house-spider, (Ara-
nea domestica) , by placing it on a little platform, supported by a
stick with a weight at the bottom, in the middle of a rummer of
water. The platform was about half an inch above the surface,
which was nearly even with the top of the glass. It presently
made its escape, as was anticipated, by suffering a thread to be
wafted to the edge of the glass ; but, supposing that it might have

25fi THE MECHANICAL FUNCTIONS.

The Natural History of the spider is in many
points of view highly interesting, not only from the
great extent to which the organic developement 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 construc-
tion 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 topic, without being led too far from the
object I have at present more particularly in view,
namely the developement of organization with refe-
rence to the organs of progressive motion.

been assisted by the water being so nearly on the same level, I
poured some of it away, and placed the spider as before. It de-
scended by the stick, till it reached the water, and examined with its
two anterior feet all round ; but finding no way to escape, it returned
to the platform, and for some time prepared itself by forming a web,
with which it loosely enveloped the abdomen, by means of its hinder
legs. It then descended, without the least hesitation, into the
water, to the bottom ; when I observed the whole of the abdomen
covered with a web containing a bubble of air, which I presume was
intended for respiration, as it evidently included the spiracles. The
spider, enveloped in this httle diving-bell, endeavoured on every side
to make its escape, but in vain, on account of the slipperiness of the
glass ; and after remaining at the bottom of the water for thirteen
minutes, it returned, apparently much exhausted ; for it immediately
coiled itself closely under the little platform, and remained afterwards
without motion. This property of forming for itself a reservoir of air,
by means of which it is preserved under water, is somewhat analogous
to the interesting habit of the Argyroneta, although it serves for a
different purpose. In the present case, it is doubtless intended to
enable the animal to cross the water in safety." Zoological Journal,
i. 283.

CKUSTACEA. 257

§ 4. Crustacea.

The plan which Nature appears to have commenced
in the construction of the Arachnida, is farther
advanced in that of the Crustacea. The portions
into which the external framework of the body
was divided in the former are still farther consoli-
dated in the latter : they are composed of denser
materials, and endowed wath 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
carbonate of lime ; they are articulated with great
accuracy, 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 attached to cartilaginous
plates, which, for the purpose of receiving the
muscles, are made to project into the interior of the
upper portion of the limb, being themselves im-
movably connected with the lower portion. By
this expedient, not only is the employment of a
tendon dispensed with, but a larger surface is pre-
sented for the attachment of the muscles, which,
by acting also upon a longer lever, obtain great
mechanical advantage. It would be superfluous to
occupy more time in explaining the minutiae of
structure in these joints, because the simple inspec-
tion of the limbs of a crab or lobster will give clearer
ideas of this mechanism than can be conveyed by
VOL. I. s

258

THE MECHANICAL FUNCTIONS.

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 five 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 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 com-
posed 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 some-
times by a pincer, or claw.

New organs, not met with among the Arachnida,
are here for the first time developed, namely, the
Antetince, 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 conveying perceptions of mere touch.
The antennae consist of slender filaments, composed

CRUS^TACEA. 25i)

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 structure ;
and many of these parts are employed in various
uses besides those of mastication ; such as the seiz-
ing 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 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 previously 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 the specimen we are examining
properly belongs. It is apparently with a view to
evade this difficulty that a term has been invented
which shall include them all, namely, that oi feet-
jaivs. These transitions are illustrated by the an-
nexed figures of several of these members in the
Mysis Fahricii; Fig. 138 being that of a mandible,

260 THE MECHANICAL FUNCTIONS.

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 constituent element
of the fabric is converted by 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 Crus-
tacea, the foremost pair of true feet are also modi-
fied 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 the 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 serrated.

The large portion of shell which is consolidated
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 transverse plane.
This is the cause of the greater facility the Crab
finds in walking side-ways, which it can do with
great quickness when urged by a sense of danger.

* The labours ofSavigny, Auduoin 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 articulated
animals, whether belonging to the classes of arachnida, Crustacea,
myriapoda, or winged insects.

CRUSTArr.A. 261

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, acting 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 composi-
tion : they contain, however, some phosphate of
lime, in addition to the carbonate. The calcareous
particles are deposited on a membrane of consider-
able firnniess; and they together compose a dense,
but thin and fragile structure, which, in order to
distinguish it from the shells 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 necessary 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 arc elfected, has been verv

262 THE MECHANICAL FUNCTIONS.

satisfactorily investigated by Reaumur. Tlie ten-
dency in the body and in the Hmbs to expand
during growth is restrained by the limited dimen-
sions of the shell, which resists the efforts to enlarge
it. But as this force of expansion goes on increas-
ing, it is at length productive of much uneasiness
to the animal, which is, in consequence, prompted
to make a violent eftbrt 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 withdraws
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 antennae ; 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 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 opera-
tion is not unfrequently fatal. Even when success-
fully 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, pre-
parations had been making for forming a new one.
The membrane which lined the shell had been
acquiring greater density, and had already col-
lected a quantity of liquid materials proper for the
consolidation of the new shell. These materials

CitUSTACEA. '263

are mixed with a large proportion of colouring
matter, of a bright scarlet hue, giving it the ap-
pearance 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 ingredient
commences, and is rapidly completed ; while an
abundant supply of fresh matter is added to in-
crease 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 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 interior
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, previous 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

264 THE MECHANICAL FUNCTIONS.

Reaumur, that this new growth takes place more
readily at particular parts of the limb, and espe-
cially 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 spontaneous effort, breaks oti"
the whole limb at its junction with the trunk, which
is the point where the growth more speedily com-
mences. The wound soon becomes covered with a
delicate 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 vigo-
rous effort of expansion which afterwards takes
place. As it grows, the membrane is pushed for-
wards, 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 preceded it. Usually,
however, it does not attain the same size ; a cir-
cumstance which accounts for our frequently meet-
ing with lobsters and crabs which have one claw
much smaller than the other. In the course of the
subsequent castings, this disparity gradually dis-
appears. A similar 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 im-
planted in each articulation a certain number of

CRUSTACEA. 265

embryo limbs, ready to be developed as occasion
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 had been removed. But this hypo-
thesis is overturned by the fact that if the animal
loses only part of the limb, it is the deficient por-
tion alone, and not the whole limb which is rege-
nerated. 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, how-
ever, 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 imagi-
nation can devise.

Chapter V.

INSECTS.

§ 1. Apt era.

Apterous, or wingless insects, form the next term
in the series of articulated animals.* Closely allied
in their organization to many of the preceding
families, they differ from them in being essentially

* The Linnean order of Aptera has, more recently, been broken
up and distributed among other orders of insects, according to their

266 THE MECFfANICAL FUNCTIONS.

formed for a terrestrial, instead of an aquatic life.
Most of the lower tribes of this order are parasitic,
that is, derive their nourishment 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
Pediculi, or lice, of Ricini, or ticks, of Pulices, or
fleas ; together with the Podura, or spring-tail ;
the Lepisma, 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 remark-
able circumstances attending their mechanical
conformation.

The Pulex, or flea, 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, with-
out 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 astonish us
in beings of so diminutive a size, and afford a
striking proof of the exquisite mechanism pervad-
ing even the lowest orders of the animal creation.

natural affinities. Latreille establishes four orders of wingless insects;
namely, Myriopoda, Thysanoura, Parasitica, and Sugentia. Cuvier,
Regne Animal, 2nd edition, 1829.

* Accordingly Lamarck, in the last edition of his " Histoire
Naturelle des Animaux sans vertebres," considers the order Aptera
as formed by the genus Pulex alone. Tom. iv. p. 7.

APTEROUS INSKCTS. 267

The Podiua leaps into the air by a mechanical
contrivance of another kind \ employing for this
purpose the tail, which is very long, and forked at
the end. In its ordinary state this 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 appen-
dages, placed in a double row along the under
side of the body, and acting like springs. There
are eight pair of these members, corresponding
in situation and structure to the false feet of the
Crustacea, and, like them, terminating in jointed
filaments.

The Julus and the Scolopendra, which compose
the family of the Myriapoda, 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 full complement of

268 THE iMECHANlCAL FUNCTIONS.

segments, witli their accompanying legs. The
Julus terresiris, for example,
^^^ (Fig. 143) has, at its entrance

into the world, only eight seg-
ments and six feet ; but ac-
quires in the course of its de-
velopement, 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. Tnsecta alata.

Our attention is uow 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 extensive class of
the whole animal kingdom. Nature has lavished
her choicest gifts of animal powers, as far as they
are compatible with the diminutive 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 in-
jurious impressions from without, with the most
extensive powers of locomotion ; and which also
admits of the fullest exercise of all those faculties
of active enjoyment which are characteristic of
animal life. She has provided for the first of these
objects by enclosing the softer organs in dense and
horny coverings, which perform the office of an
external skeleton, sustaining and protecting the

WINGED INSECTS. 209

viscera, and furnishing extensive surfaces of attach-
ment to the muscles, from the action of which all
the varied movements of the system are derived.

The muscular system of perfect insects is exceed-
ingly complex. Lyonet has described and deli-
neated an immense number of muscular bands in
the caterpillar of the Cossus, and the plates he has
given have been copied in a variety 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 cock-
chaffer, the anatomy of which has been minutely
investigated by that distinguished entomologist.

These muscles are represented in Fig. 144, which
has been carefully 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 framework 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, encircling
the body; but variously united and consolidated,

270 THE MECHANICAL FUNCTIONS.

for the manifest purpose of obtaining greater me-
chanical strength and elasticity than if they had
remained detached pieces, joined only by mem-
branous 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 progression, has been
shortened by a reduction in the number of its
segments, and rendered more compact. The seg-
ments 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 unyielding in order to support their
action.

Nature has farther provided insects with instru-
ments adapted to different kinds of external actions.
They consist of articulated levers, variously com-
bined together, and forming legs, claws, pincers,
oars, palpi, and, lastly, wings, calculated for exe-
cuting every variety of prehension, of progression,
or whatever other action their wants and neces-
sities require.

§ 3. Developement 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 attained
only by the animal being subjected to a long series
of modifications, and passing through many inter-

WINGED INSECTS. 271

mediate stages of developement. The power of
flight is never conferred upon the insect in the
earlier periods of its existence: for before its struc-
ture 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 considerable as to justify the term of
metamorphoses, which has been generally given to
them.* But transient is the state of perfection in
every thing that relates to animal existence. When
the insect has, by a slow developement, reached
this ultimate elaboration of its organs, its life is
hastening to a close ; and the period of its perfect
state is generally the shortest of its whole exist-
ence.

The history of the successive stages of the de-
velopement of insects opens a highly interesting
field of philosophical inquiry. For a certain period
of the early life of these animals, the growth of all
the parts appears to proceed equably and uni-
formly: 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 developement is con-

* 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 Lerncpa among parasitic worms.

272 THE MECHANICAL FUNCTIONS.

ducted. The researches of modern entomologists
have led to the conclusion that the framework, or
skeleton of insects, is always formed by the union
of a certain determinate number of parts, or ele-
ments, originally distinct from one another, but
M'hich are variously joined and soldered together
in the progress of growth : frequently exhibiting a
great disproportion in the comparative expansion
of different parts. The enlargement of any one
part, however, exercises a certain influence on all
the neighbouring parts, and thus are the foun-
dations laid of all the endless diversities which
characterize the several species belonging to each
tribe and family.

In the progress of developement, we may recog-
nize 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
compose the thorax are greatly enlarged, in order
to support the more recently developed organs of
progressive motion, they are also more consoli-
dated, and rendered stronger by the approximation
of several pieces which were before less closely
united. The posterior segments, having no such
appendages 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
Lepidopteray or butterfly and moth tribes.* The

* The four periods of the existence of the Bombyx mori, or
the moth of the silk-worm, are shown in the annexed engravings ;

DEVELOPEMENT OF INSECTS.

•273

egg, which is deposited by the butterfly, gives birth
to a caterpillar ; an animal, which, in outward

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 appear-
ance, and the mechanical structure of a worm.
The same elongated cylindric shape, the same
annular structure of the denser parts of its integu-
ment, the same arrangements of longitudinal and
oblique muscles connecting these rings, the same
apparatus of short feet, with claws, for facilitating
progression ; in short, all the circumstances most
characteristic of the vermiform type are equally
exemplified in the diflerent tribes of caterpillars, as
in the proper Annelida.

But these vermiform insects have this peculiarity,
that they contain in their interior the rudiments of
all the organs of the perfect insect. These organs,
however, are concealed from view by a great num-
ber of membranous coverings, which successively
invest one another, like the coats of an onion, and

Fig. 145 are the eggs; Fig. 146, the Larva, or caterpillar; Fig.

147, the Pupa or chrysalis; and Fig, 148, the Imago, or perfect
insect.

VOL. I. T

274 THi: MECHANICAL FUNCTIONS.

are thrown off, one after another, as the internal
parts are gradually developed. 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 rendered
necessaiy in consequence of the greater compact-
ness of the integuments of insects, as compared
with those of the annelida. Tn proportion 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 internal organs, and
must be thrown off to make way for the further
growth of the insect. In the mean time, a new
skin has been preparing underneath, 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.

These successive peelings of the skin are but so
many steps in preparation for a more important
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, presenting no appear-
ance of external members, and retaining but feeble

DEVELOPEMENT OF INSECTS. 27-5

indications of life. In this condition it remains for
a certain period : its internal system continuing in
secret the further 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 spor-
tive inhabitants of air ; and expanding its newly
acquired 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 gratification,
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 rela-
tions, a mere worm. The imago, or perfect state,
on the other hand, exhibits strong analogies with
the crustaceous tribes, not only in the general form
of the body, but also in the consolidated 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 cir-
cumstances 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, subservient to a more expanded
range of existence, and entitling the beings on
which they have been conferred to the most ele-
vated rank among the lesser inhabitants of the
globe.

276 THK MKCHANICAL FUNCTIONS.

The meclianical 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 frequently 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 them-
selves, and of which we meet with no example in
other parts of the animal kingdom.

In attempting to delineate a sketch of the move-
ments 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 entomologists.

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 Hemiptera and
Orthoptera, for example, come out of the egg with
nearly the same form as that which they have in

AQUATIC LARViE. 277

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.

§ i. Aquatic Larvce.

Many insects, which, when fully developed, are the
most perfectly constructed for flying, are, when in
the state of larvae, altogether aquatic 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 LibeUula, or dragon-
fly, a singular artifice has been resorted to for
giving an impulse to the body, without the help of
external members. It is that of the alternate ab-
sorption 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
direction, on 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 presents us
with so perfect an example, for the purpose of pro-
pelling ships, instead of the ordinary mode of steam
navigation.

278 THE MECHANICAL 1 T'NCTIONS.

Some larvae, such as that of the Stratiomys, col-
lect a bubble of air, which they retain within a
tuft of hair at the extremity of the tail, evidently
with a view of diminishing the specific gravity of
the body, and thus giving greater efficacy to the
muscular actions which they employ in their pro-
gression 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 tiuid 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 larvas of the Ephemera, though furnished
with legs, do not use them for this purpose, 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 considerable distance from
the sides, and flattened at the ends, to serve as
oars. The larvse of the Hydrophilns are also admi-
rably formed for swimming ; and they not only
dart forwards with surprising velocity, but also turn
in all directions with the utmost facility.

§ o. Terrestrial Larvce.

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

' Page 160.

TERRESTRIAL LARV.E. 279

annular segments of the body are moved, are ex-
ceedingly numerous, and beautifully 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 entomologists, and still oft'ers much that
remains to be explored. The researches of Lyonet,
already alluded to, on the anatomy of the larva of
the Bombyx Cossus* of which he has published an
elaborate description, accompanied by admirable
engravings, will ever remain a splendid monument
of patience and ingenuity in overcoming the diffi-
culties which impede this kind of inquiry. In the
body and the limbs of this caterpillar, Lyonet
counted above 4000 separate muscular bands, all
arranged with the most perfect symmetry, and
adapted with wonderful precision to the perform-
ance of the required effects.

In these larvse, as in the simpler forms of the
Annelida, progression is often accomplished solely
by the alternate contraction and extension of the
annular segments, aided, in many cases, by short
hairs, and frequently, also, by a slimy secretion
which exudes from their bodies. 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 unbending of a bow. By an artifice of the
same kind, some larvae, unfolding with a violent
effort the curvatures of their bodies, contrive to leap
to considerable distances.

* CossKS iiyniperda, Fcibiicius.

280 THl«: iMEClIANlCAL lUNCTIONS.

Some larvae avail themselves of their jaws in
order to tix the head, and drag the rest of the body
towards it. In this manner the larvae of the Ce-
ramhyx, or Capricorn beetle, advance along the wind-
ing passages which they have themselves excavated;
liolding by the jaws, and dragging themselves for-
wards. These movements are assisted by the re-
sistance afforded by short tubercles which project
from different parts of the back, and under surface
of the body ; so that these insects 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 ob-
serve them furnished with spines, or hooks, which
are moved by appropriate muscles, and 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 men-
tioned, and the more regularly formed feet or legs.
Those which are regarded as spurious legs, or pro-
legs, as they have been termed, occupy an inter-
mediate 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
segments of the thorax ; and are formed of those
portions articulated to each other, corresponding to

TERRESTRIAL LARV^. 281

the three principal joints of the imago. The true
legs are generally protected by horny integuments;
but the coverings of the prolegs are wholly mem-
branous. The spurious legs are frequently termi-
nated 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 advance
is regulated by many circumstances, independent
of the mere possession of legs ; for some caterpillars
move but slowly, while others can run very nimbly.
The following is the order in which the legs are
usually moved ; namely, the anterior and the pos-
terior leg on the same side are advanced at the
same moment, together witli 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 furnished witli
prolegs, close to the head. The hind extremity
being then fixed by means of the prolegs situated
at that part, the body is again extended into a

282 THE MECHANICAL FUNCTIONS.

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 the slender branch of a tree,
and extending the body, as if it were a rigid cylin-
der, at an acute angle, so as exactly to resemble
one of the twigs from the same branch, and thus
escape the observation of their enemies. This atti-
tude, which they persist in assuming for hours and
days together, is shown in Fig. 148* b.

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 cab-
bage 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 hard-
ness by the action of the air.

Other caterpillars, which feed on trees, and have
often occasion to descend from one branch to an-
other, 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

STKl'CTUKE OF INSECTS. '283

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 them-
selves to drop.

§ 6. Imago, or Perfect Insect.

The process which nature has followed in the de-
velopenient of the structure of insects, has for its
object the gradual hardening and consolidation of
texture, and the union and concentration of organs:
for we find that the segments which were at a dis-
tance from one another in the larva, are approxi-
mated in the perfect insect, and often closely tied
together by ligaments; and in other cases, adjoin-
ing segments cohere so as to form but a single
piece. Thus the number of separately moveable
parts composing the solid fabric is considerably
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 larvae, and are formed
of several layers, in which the component 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 contain but a small proportion of
this material : and whenever a calcareous ingredient
enters into their composition, it is in the form of

284 THE MECHANICAL FUNCTIONS.

phosphate of lime. 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 pe-
culiar nature; it has been termed Chitine by M.
Odier,* and Entomoline by M. Lassaigne.t This
substance is found in large quantity in the wings
and elytra of coleopterous insects. It is remark-
able for not liquefying, as horn does, by the action
of heat; and accordingly the integuments of insects,
even after having been subjected to a red heat, and
reduced to a cinder, are found to retain their ori-
ginal form.j:

With this substance there is blended a quantity
of colouring matter, which has usually a dull brown
or black hue. But the colour of the external sur-
face is generally owing to another portion of this
matter, which is spread over it like a varnish, and
being soluble in alcohol and in ether, may be re-
moved by means of these agents. The colours
which are displayed by insects, and which arise
from the presence of this latter substance, are often
very brilliant ; and, as is the case with many other
classes of animals, the intensity of the tints is
heightened by the action of light. The elytra of

* Annales de Chimie, torn. 76.

t See the work of Straus Durckheitn, p. 33,

X M. Odier had concluded from his experiments that no nitrogen
enters into the composition of this substance. That this conchision
has been too hastily adopted has been proved by Mr, Children, who,
by pursuing another mode of analysis, found that the chitine of can-
tharides contains not less than nine or ten per cent, of nitrogen.
See Zoological Journal, i. Ill — 1 15,

STRUCTURE OF INSECTS. 285

tropical insects display a gorgeous metallic lustre,
depending on the reflexion of the prismatic colours;
and the same variegated hues adorn the scales of
the butterflies 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 per-
ceived 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 dif-
ferent, for they are formed of the same substance
as the integuments, namely entomoline. The pur-
poses served by the hairs are not always obvious.
In many cases they seem intended to protect the
integuments from the water, which they repel from
their surfaces. They 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: by
their elasticity, also, they help to break the force
of falls from heights.

The divisions of the body are frequently marked
by deep incisions ; whence has originated the term
insect, expressive of this separation into sections.
It is, however, a character which they possess in
common with all articulated animals, the typical
form of which consists, as we have seen, of a series
of rings, or segments, joined endwise in the direc-

* It is a singular fact, that almost all the insects which are found
in the South of Africa are furnished with a large quantity of liair.
This is particularly the case with the Buprestidce and Melalun-
thidce.

286

THE MECHANICAL FUNCTIONS.

tion 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 com-
posed of several segments.
I have here given in illus-
tration, the annexed fig-
ures, showing the succes-
sive portions into which
the solid framework, or
skeleton, of one of the
beetle tribe, the Calosoma
si/cophanta* may be separated. The entire insect,
which presents the most perfect specimen of a
complete skeleton in this class of animals, is re-
presented 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 essentially annular, and that
it resembles in this respect 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
different insects, and is in general proportionably

Carabus sycophanta. Linn.

STRUCTURE OF INSECTS.

287

larger than it is in the larva state. Its integument,
which, from analogy with vertebrated animals, has
been called the skull, or cranium^ (c, Fig. l.')0), is

usually the hardest part of the general crust.
Although it may appear, on a superficial examina-
tion, to consist of a single undivided piece, yet, on
tracing its gradual formation, it is found to be in
reality composed of several of the segments of the
larva united together. Audouin and Carus dis-

288 THE MECHANICAL FUNCTIONS.

tinguish three component segments in the cranium
of insects ; while Straus Durckheim considers it
as formed by the consohdation of no less than six
segments of the vermiform larva. According to
this theory, the same elements, which in the tho-
racic segments are developed into feet, are here
employed to form parts having other destinations.
From the segment adjacent to the thorax the an-
tennae 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 articulated joints
called the labial palpi (l).

The mode in which the head is connected with
the trunk varies much in different insects. Some-
times it is united by a broad basis of attachment,
forming a joint between the adjacent surfaces:
but usually it is only appended by a narrow fila-
ment, 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
Hymenoptera and Diptera, the head moves upon a
pivot, so as to admit of its being turned completely
round.

The trunk, or lliorax, is composed, as shown
in the figure, of three segments, termed respect-
ively the Prothorax (x) ; the Mesotkorax (y) ; and
the Metathorax (z).* The first of these, the pro-

* 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 segment is

STRUCTURE OF INSECTS. 289

thorax, carries the first pair of legs ; the second,
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 are joined together at the sides of
the trunk ; these again admit of further subdi-
vision ; but for the names and descriptions of the
smaller pieces I must refer the reader to works on
Entomology. 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 satis-
factorily established the general proposition that,
amidst the endless diversity of forms exhibited by
the skeleton of insects, they are invariably com-
posed of the same number of elements, disposed in

termed by Straus Durckheim and other French writers, the Corselet.
Mr. Kirby calls it the Manitrunk, and restricts the term Prothorux
to its upper portion. The united second and third segments are
the Thorax of Straus Durckheim, the Tronc alifere of Chabrier,
and the Alitrunk of Kirby.

VOL. I. U

290 THE MECHANICAL FUNCTIONS.

the same relative situations and order of arrange-
ment ; and that the only source of difference is a
variation in the proportional developement of these
elements. He has also observed that the great
expansion of one part is generally attended by a
corresponding diminution 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 the
viscera subservient to nutrition, respiration, and
reproduction. The number of these abdominal
segments is very various in different genera of
insects. Sometimes there appear to be but three
or four ; while, in other cases, there are six or
seven, or even a greater number. In the Calosoma
(Fig. 150, b), the abdomen has six complete, fol-
lowed 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 segment has a ligamentous con-
nexion 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 construction 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 mus-

STRUCTURE OF INSECTS. 291

cles they enclose : for since the surfaces which
receive, as well as those which are received, are
segments of spheroids, this structure admits of a
twisting motion ; and also the latter segment 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 liga-
mentous band, which runs the whole length of the
abdomen. Great advantage results from this divi-
sion of the circles, allowing of the upper and lower
portions of the abdominal covering being at one
time separated, and at another brought nearer toge-
ther : for thus the cavity is capable of being en-
larged 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 im-
perfect 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 tail of which terminates in a kind of
forceps, or pincers.

The junction of the abdomen with the trunk is
effected in various ways. In all the Coleopiera, 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 a groove all
round, or an incision, as it is technically termed.

292 THE MECHANICAL FUNCTIONS.

In the Hymeiioptera, this incision 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 condition ; all
the spurious legs having disappeared during its
metamorphosis. We have seen that in the Myria-
poda^ the result of developement is an increase in
the number both of segments and of legs ; the
reason of which is that, being terrestrial 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 concentrated, and all super-
fluous parts must be retrenched, and discarded from
the fabric. The multiplication of organs, which, in
the former case, indicated the progress of a higher
developement, would in the latter have been the
source of imperfection. As long as the insect re-
mains 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

* For the details of this structure I must refer to writers on ento-
mology, and in particular to Kirby and Spence's " Introduction to
Entomology," vol. iii. p. 701.

STRUCTURE OF INSECTS. 293

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 hori-
zontal, 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 correspond
to the hip bone of quadrupeds, is a broad, but very
short truncated cone. The mode of its articulation
with the trunk is very various : 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 tro-
chanter (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 arti-
culation of this portion with the haunch is always
effected by a hinge-joint. Joints of this description,
when formed, as they are in insects, by the apposi-
tion of two tubular pieces, are constructed in tlie
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 these
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

294 THE MECHANICAL FLNCTIONS.

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 depres-
sions are so adjusted to one another, that the joint
cannot be dislocated without 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 combi-
nation of both, a capability in the thigh of executing
a circular motion, in a manner almost as perfect as
if it had revolved in a spherical socket. The prin-
ciple of this compovmd motion is the same as that
employed 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, whatever may be the
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.*

* In many insects, the apex of the tibia is furnished with a pair
of spurs, termed calcaria, which articulate with it, and support the
hmb, projecting from it at an acute angle : in some instances, these
spurs are pectinated, and are employed as instruments for burrowing.
In the Hymcnoptera, one of these spurs, hollowed out on the inner

STRUCTURE OF INSECTS. 295

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 application over all the ine-
qualities 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 support so as
to ensure the stability of their bodies 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. 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.

The last joint of the tarsus is generally termi-
nated by a double claw, which contributes to fasten
the foot, under a variety of circumstances, both of

side, is placed on the anterior tibia ; the hollow edge corresponding
with a similar notch in the first joint of the tarsus ; and through
these the insect draws its antennse in order to clean them. In some
species of Tenthredo, and some of the Trichloptera, the tarsus is
furnished, in the middle of its length, with similar organs. In the
Cimbices, the calcaria have a cushion at their apex.

296 THE MECHANICAL FUNCTIONS.

action and of repose. By means of feet thus
armed, the insect can ascend or descend the per-
pendicular sides of a rough body with the greatest
ease ; but it is scarcely able to advance a single
step upon glass, or other polished surfaces, 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 affording the greatest possible
security against displacement.

Many insects are provided with cushions at the
extremity of the feet, evidently for the purpose of
breaking the force of falls, and preventing the jar
which the frame would otherwise have to sustain.
These cushions are variously formed ; sometimes
having the appearance of contractile vesicles, de-
nuded of hair, and inserted at the base of the claws
and between them ;* at other times covered with
dense velvety 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 unnecessary, because the light-
ness of their bodies sufficiently secures them from
any danger arising from falls. f

* These vesicles, which are termed pulvilli, often project con-
siderably beyond the claws. The most extraordinary expansion of
the membrane covering the underside of the joints of the tarsi occurs
in the Cleridce.

t Several exceptions, however, occur to this general rule: thus

STRUCTURE OF INSIiCTS.

•297

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 exceed-
ingly 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 imder 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 the 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 which are serrated, so as to fit
them for the closest application to any kind of

surface. In the Tabcmiis, or horse-fly, each foot is
furnished with three suckers. In the Cimbejc lutea,
or yellow saw-fly, there are four, of which one is
placed on the under surface of each of the four

the DynastidcB, which are among the largest of the Coleoptera,
have no such cushions : while the small Longicornua are provided
with them. In the Harpalidce, which do not fly, cushions exist only
in the male insects.

298 THE MKCHAMCAL FUNCTIONS.

first joints of the toes (Fig. 153); and all the six
feet are provided with these suckers. In the Dy-
tiscus, suckers are furnished 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 higuttulum, which is a species of grass-
hopper, has one large oval sucker, under 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 sub-
stance ; which, in order to increase the elasticity of
the whole structure, is looser in its texture towards
the circumference.*

The mode in which 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 appli-
cation 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 :

* Philosophical Transactions for 1826, p. 324.

STRUCTURE OF INSECTS. 299

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 moistened with spirit
of wine, this apparent repulsion no longer takes
place; and the insect immediately sinks and is
drowned.

§ 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 bodies
becomes permeated by air, which forms altogether
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 con-
tained 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, there-
fore, 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
(luestion is to carry the body forwards nearly in a
horizontal direction.

300 THE MECHANICAL FUNCTIONS.

In insects destined to move in water, sometimes
all the legs, but occasionally only one pair, are
lengthened and expanded into broad triangular
surfaces, capable of acting as oars ; and these
surfaces are farther 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 impulses are given with great regu-
larity, all the feet striking the water at the same
moment.

Of all the coleopterous insects, the Dytiscus, or
water-beetle (of which Fig. 156 represents the
upper, and Fig. 1 57 the under side), is the one best
constructed for swimming : its body having a flat-
tened 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 very near the under surface. The pos-
terior legs, which act as powerful oars, are attached

AQUATIC INSECTS. 301

to very large haunches, for the purpose of contain-
ing the thick muscular bands which are inserted
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 re-
quired to be moveable : and accordingly they are
firmly united to the thorax ; a structure which ren-
ders 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.

Among the Hemiptera, the Notonecta, or water-
boatman (Fig. 158,) is remarkable for always swim-
ming 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 equili-
brium is maintained. It is evident that, under
these circumstances, if it were placed in the water
with its legs undermost, it would unavoidably tilt
over, and resume its usual position. Its long legs,
extending at riglit angles to the body, present a
striking resemblance to the oars of a boat ; they
act, indeed, in the same manner, and on the same
principles.

302 THF. MECHANICAL FUNCTIONS.

§ 8. Progressive Motion of Insects on Land.

The actions of the limbs of insects in walking are
quite different from what they are in swimming,
and are very similar to those of the caterpillar, in
which we have seen that the motions of the ante-
rior 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 con-
sequence of their positions relatively to the trunk,
the anterior legs are advanced by the extension,
and the posterior legs by the flexion of the corre-
sponding joints. When the feet have fixed them-
selves 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. At other times
the order in which the fore and hind legs are
moved is similar to that which takes place in the
trot of quadrupeds : the left fore leg and the right
hind leg being advanced together : after which, the
right fore and left hind legs are advanced ; and last
of all, the middle legs, either simultaneously, or the
one after the other.

The action of leaping is performed by the sudden
extension of all the joints of the limb, which are
previously folded as close as possible. The joints

progrp:s.sive motion of insects, 303

principally concerned in this action are those of
the thigh and tibia, as they furnish the longest and
most powerful levers. Preparatory 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 position 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 accu-
mulated impulse, to a considerable height in the
air. The leaps of insects being generally forwards,
all the legs do not participate equally in the effect;
for the fore legs contril)ute 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, necessarily swell that division of the limb
to an 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
generally, indeed, infer the particular kind of pro-
gressive 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

304 THE MECHANICAL FUNCTIONS.

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 progression is impeded. The great prolongation
of the posterior legs is generally accompanied by
the power of jumping; imless, 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 equiva-
lent to a man of ordinary stature vaulting through
the air the length of a 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. t

The insects belonging to the genus Elater are
provided with a peculiar mechanism for the special
purpose of accomplishing a singular mode of leap-
ing, independently of any action of the legs. The
legs of this insect are so short, that, when it is laid
on its back, it cannot turn itself, being unable to

* This is particularly exemplified in the Sayra, the Chalcis, and
the Leucopsis, all which insects have the femora enormously deve-
loped and the tibise much curved, and are thereby incapacitated
from jumping.

t De Gear, iii. 178, quoted by Kirby and Spence.

PROGRESSIVE MOTION OF INSECTS. 305

reach with its feet the plane on which it is lying,
and procure a fulcrnm 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 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 prosternum,
or pectoral portion of the prothorax, is also extended
backwards, and terminates in an elastic spine,
which is received into a cavity in the mesothorax.
While the insect is lying on its back, with the pro-
thorax bent backwards on the mesothorax, so as to
form together with the head and abdomen a kind
of arch, convex upwards, and the spine brought
forwards out of its socket, the muscles which tend
to bring these two segments of the thorax into a
straight line are thrown into strong action, and the
prothorax, together with the base of the elytra, are
made to strike with violence against the plane on
which the insect is lying: the force of the blow
being augmented by the sudden re-entrance of the
spine into its socket, with the eft'ect of a spring. The
elater is thrown upwards by the reaction of the
plane of support : but as the impulse is given to it
at a point anterior to the centre of gravity, a rota-
tory force is communicated, by which the body,
while in the air, is turned over, and falls with its
feet on the ground. It retains itself in this position
by suddenly clinging with its legs extended, and
thus guards against a rebound, which might again

VOL. 1, X

306 THE MECHANICAL FUNCTIONS.

throw it upwards ; for it frequently falls from 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 in-
tended to burrow in the earth : and of these the
Gryllo-talpa, or mole cricket, presents a remarkable
example. A minute account of the anatomy of this
insect has been given by Dr. Kidd,t from which it
appears that being destined, like the mole, to live
beneath the surface 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 resistance the
opposing medium. By being endowed 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 retrograde movements, it is pro-
vided 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

* See Burmeister's Manual of Entomology, by Shuckard, § 269.
He regards the spine as merely regulating the direction, but giving
no elastic force to the movements.

t Phil. Trans, for 1825, p. 203.

PROGRESSIVE MOTION OF INSECTS. 307

Fig. 158*) are the burrowing implements, and they

158*^,*^^. ^^^ admirably calculated

for their peculiar office,
both in their shape and in
the mode of articulation
of their several divisions,
which bear a considerable analogy to the corre-
sponding 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.*

Examples without number might be given of
similar mechanical adaptations of the limbs of in-
sects to the performance of particular actions re-
quired by the peculiar circumstances of their con-
dition : such as the construction of the anterior legs
of the Scaritidce, which, like those just described,
are excellent implements for burrowing ; of those
of the Mantis, admirably formed for prehension ;
and of the hind legs of the Gyrimis, expressly
adapted to their singular rotatory mode of swim-
ming : but the limits of this work necessarily re-
strain me from entering into so many details of
minute structure, and expatiating in so immense a
field of entomological inquiry.

* For a more particular description of tlie mechanism of this
instrument, I must refer the reader to the paper above quoted.

308 THE MECHANICAL FUNCTIONS.

§ 9. Flight of Insects.

If the excellence of a mechanic art were 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 con-
trived the construction of an automaton, capable,
by the operation of an internal force, of sustaining
itself in the air, in opposition to gravity, for even a
ie\Y minutes ; and far less of performing in that ele-
ment the evolutions which we daily witness even in
the lowest of the insect tribes. To the ultimate
attainment of this facult}^ it would appear that all
the transformations they undergo in external appear-
ance, and all the developements of their internal
mechanism, are expressly directed. Wings are
added to the frame only in the last stage of its com-
pletion ; after it has disencumbered itself of every
ponderous material that could be spared, after it
has been condensed into a small compass, and after
it has been perforated in all directions by air-tubes,
giving lightness and buoyancy to every part. Cu-
riously folded up in the pupa, the wings there
attain their full dimensions, ready to expand when-
ever the bandages which surround them are re-
moved. No sooner is the insect emancipated from
its confinement, than these organs, which are com-
posed of duplicatures 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.

FLIGHT OF INSE( rS. 300

Their moisture rapidly evaporates, leaving the deH-
cate film dry and firm, so as to be ready for imme-
diate action. Fibres, or nervines, as they are called,
are generally apparent on their surface, and form
a delicate network for the support of this fine mem-
brane ; 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 con-
joining lightness with strength ; and many entomo-
logists 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 ali-
Irunk, constitute the most solid portion of the ske-
leton, and are frequently strengthened by ridges,
and other mechanical contrivances for support.
The superior extremities of these supports, which
have been compared to the clavicles, or furcular
bones of birds, are always curved inwards. This
part of the trunk requires to be alternately dihited
and contracted during flight ; and hence the several
pieces of which its dorsal portion is conqjoscd are
loosely connected together by ligaments. t

* The wings of many insects present no nervures. This is the
case with all the smaller Hynienoptera,

t See Chabrier's " Essai sur le Vol des Insectes," Memoires clu
Museum d'Histoire Naturelle; vi. 410, vii. '297, and viii. 47 and
349. See also Zoological Journal; i. 391.

310 THE MECHANICAL FUNCTH)NS.

The shape of the wings is more or less triangular.
They are moved by numerous muscles, which oc-
cupy a large space in the interior of the trunk, and
consist of various kinds of flexors, extensors, re-
tractors, levators, and depressors; the whole forming
a very complicated assemblage of moving powers.
The largest, and consequently 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
renewed in very rapid succession ; for, indeed, un-
less 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
membranous wings of insects, as appears from the
observations of Straus, are weaker at the parts far-
thest from the trunk ; and consequently they bend
on being raised ; and presenting, when thus bent,
a smaller surface, the resistance which the air op-
poses to their ascent is less than that which it op-
poses to their descent, in which latter action they
are expanded to the utmost.

The simple ascent and descent of the wings would
be sufficient, without any other movement being
imparted to them, to carry forwards the 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 created by 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

FLIGHT OF INSECTS. 311

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 gene-
ral resultant of all these successive impulses is a
force directed forwards and upwards : the first part
of this force produces the horizontal progression of
the insect ; while the second operates in counter-
acting 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 situa-
tion of the centre of gravity, so that the reaction of
the air on the wings is exerted in a different direc-
tion to what it was before ; and the motion of the
body is modified accordingly.

By exerting with the wings a force just sufficient
to balance that of gravity, insects can poise them-
selves in the air, and hover for a length of time
over the same spot, without rising or falling, ad-
vancing or retreating; and the body may, all the
while, be kept either in the horizontal, or in the
erect position. In the latter case, the motions are
similar to those which take place in ordinary Hying,
only they are more feebly exerted, since all that is
required is to sustain the weight of the body with-
out urging it to a greater speed. LibellnUe, Sphinges,
and a great number of Diptera, exhibit this kind of
action : among the latter the Eristalis, or common
drone-fly, is most remarkable for its power of re-
maining long in the same fixed position.

The number, form, and structure of the wings of

312 THE MECHANICAL FUNCTIONS.

insects have furnished entomologists with very con-
venient characters for their classification : on these
are founded the orders of Coleoptera, Ortlioptera,
Rhipiptera, Hemiplera, Neuioptera, Hi/menoptera,
Diplera, and Lepidoptera. To enter into any
detail in a field of such vast extent as is presented
by the infinitely diversified mechanism of the insect
creation, would, it is obvious, 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 probably com-
prehends the largest number of genera of insects,
the lower pair of wings (w, Fig. 150, p. 287) are
light and membranous, and of a texture exceedingly
fine and delicate. They are of great extent com-
pared with the size of the body, when fully ex-
panded ; 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
in 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 im-
pressions to which they might otherwise 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 motionless during the flight
of the insect. They probably, however, contribute
to direct the course of flight, by variously modifying
the resistance of the air.*

* The Elytra of insects have been regarded by Oken as corre-
sponding to the bivalve shells of the Molluaca; a notion founded,
on a very fanciful and strained analogy.

FLIGHT OF INSECTS.

313

111 the Orthoptera, (Fig. 159), the coverings of
the wings, or tegiiiina, instead of being of a horny
texture, are soft and flexible, or semi-membranous.
The wings themselves, being broader than their
coverings, are, when not in use, folded longitudi-
nally, 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 mesothorax, 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 longitudinally.

In the Hemiptera, the tegmina, or as they are
here called, the hemi-elytra, are coriaceous towards
their base, but membranous towards their extre-
mity, 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 move-
ments accompany those of the wings.

* The Strepsiptera of Kirby. See Transactions of the Linnean
Society, XL 86.

314 THE MECHANICAL FUNCTIONS.

Insects having four thin membranous and trans-
parent wings are arranged under two orders ;
namely, the JSeuroptera (Fig. 160), in which the
lesser nervures form an interlacement of fibres,
crossing one another nearly at right angles, like net-
work, or lace; and the Hi/menoptera (Fig. 161), in
which they are disposed like the ramifications of
arteries or veins, diverging at acute angles from
the main trunks. The insects belonging to these
two orders enjoy extensive powers of flight. Libel-
lul(B, and ^shnce, which are included 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 great-
est 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.
J9ee5, which are hymenopterous insects, have often
been observed to fly to great distances from their
hive in search of food. The humble bee occasion-
ally adopts a very peculiar mode of flight, describ-
ing, in its aerial course, segments of circles, alter-
nately to 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 their comparative size.-f

* The Agrion, which belongs to the same family, when reposing,
closes its wings vertically over the body.

t I have been favoured by Mr. George Newport with the follow-
ing account of the structure of the sting of the Wild Bee, (Antho-
phora retusa, Kirby) which he has lately carefully examined, and
from whose drawings of the dissected parts the annexed figures

FLIGHT OF INSECTS.

315

Although the greater number of insects have four
wings, there are many, such as the common house

(163) have been engraved. " The sting of this bee (a) is formed of
two portions placed laterally together, but capable of being sepa-
rated. 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 semicir-
cular dilatation, apparently 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 tendon,
to which the muscles are attached.
Jt is between these plates, when
approximated, that the poison flows
from the orifice of the somewhat
dilated extremity 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 contain-
ing it ; for it is conveyed into this
bladder by means of a long convo-
luted vessel (c), which receives it from the secreting organs (s).
These org^ans consist of two somewhat dilated vessels resembliuir
ccRca, 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. Swammerdara has delineated these parts as cseca 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 Anthopliora
retusa."

316 THE MECHANICAL FUNCTIONS.

fly, and the gnat, which have only two. These
compose the order Diptera (Fig. 162). In these
insects we meet with two organs, consisting of
cylindrical filaments, terminated by a clubbed ex-
tremity ; 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 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, sphinges,
and moths, are all comprehended in the order Le-
pidoptera, 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 ex-
amined with the microscope, their construction and
arrangement appear to be exceedingly beautiful,
being marked with parallel and equidistant striae,
often crossed by still finer lines, the distinct visi-
bility of which, in many kinds of scales, as those of
Pontia Brassicw, or cabbage butterfly, and the
3Iorp/io Menelaus of America, constitutes a good
criterion of the excellence of the instrument. Tlie
beautiful colours which these scales possess may

FLIGHT OF INSECTS. 317

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 thinness of the transparent plate of which they
consist ; for I have observed in several detached
scales, that the colours they exhibit by transmitted
light are the complementary colours to those which
they display when seen by reflected light.

The forms of these scales are exceedingly diver-
sified, 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 the antennae in some
species are more or less covered with these scales.*
Fig. 164 exhibits some of the more usual shapes
as they appear when viewed with high magnifying
powers.

Each scale is inserted by a short pedicle into
a small tubular sheath, which is itself implanted
into the membrane of the wing, or root; each
overlaps 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. t This imbricated arrangement, together
with the marks that are left on the membrane of

* In the posthumous work of Lyonct, which has lately appeared,
nearly the whole of six quarto plates are crowded with the delinea-
tions of the different forms of the scales found in the Bombyx
Cossus.

t The scales on the abdominal rinses of the Lepisma are of two
kinds; one set being arranged in rows, as usual, and the others,
which are shaped like a heart, being inserted by a pedicle projecting
from the middle of the base between and over the former, so as to
fasten each firmly in its place.

118

THE MECHANICAL FUNCTIONS.

the wing where the scales have been rubbed off,
are shown in Fig. 165, which is a faithful delinea-
tion of the appearance of the wing of the Hesperia

Sloani, 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 ento-
mologists, ocelli, or eyes, which arise from those
parts being naturally destitute of scales. The num-
ber of these scales necessary to cover the surface
of the wings must, from their minuteness, be
exceedingly great. The moth of the silk worm
{Bomhyx mori. Fig. 148), which has but a small
wing, contains, according to Leweidioeck, more
than two hundred thousand of these scales in each
wing.*

* The account of an elaborate examination of the scales of the
Lepidoptera, by Bernard Deschamps, is given in the Ann. Sc. Nat.
serie 2. iii. 111.

FLIGHT OF INSECTS. 319

These scales doubtless contribute to the protec-
tion of the wing ; but they at the same time add
considerably to their weight, and impede the velo-
city of their action. This inconvenience appears
to have been in a great measure compensated 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 unsteadiness, 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 pursued, not only by naturalists, but
also by birds, that are eagerly seeking to catch
them. It is astonishing to what a distance the silk
Avorm moths will fly : some have been known to
travel more than a hundred miles in a short time.
The Apatura Iris (Fab.) 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 connecting the wings together,
and keeping them more steady during flight. It
consists of a horny process, which projects out-
wards from the base of the upper margin of the
lower wing, and is received into a hook in the
under edge of the upper wing. Sometimes two or
three bristles are substituted for the horny process.
By this mechanism all the wings are locked toge-
ther, and brought into action at the same time.f

* Notable exceptions occur to this rule, in Sphinges, Sesice, and
Egerim, which are all insects of very rapid flight.

t DeGeer, Memoires pour servir a I'Histoiredes Insectes, i. i73,

320 THE MECHANICAL FUNCTIONS.

Insects of the Sphinx tribe are also provided with
a kind of rudder 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 occasion to
notice a remarkable instance of the force and per-
manence of muscular contraction in those cater-
pillars 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
Tlallica, 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. Linnaeus has computed that the Me-
lolontha, or chaffer, is, in proportion to its bulk,

and Esprit Gioina, Trans. Linnean Soc. i. 135. The same object
is eftected in the Ichneumon, (which belongs to the order Hymen-
optera,) by the catching of the nervure, which forms the posterior
margin of the upper wing, into a series of hooks projecting from the
anterior margin of the lower wing. De Geer counted more than forty
of these hooks on each of the lower wings of a large ichneumon.
Ibid. i. 562. Mr. Ashton has discovered that the wings of the
Homoptera are locked together in a similar manner.
* See Fig. 148*, p. 281.

FLIC.HT OF INSPXTS. 321

more than six times stronger than the horse ; and
has asserted that if the same proportional strength
as is possessed by the Liicanns, 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 mythology.

But while we must admit that all these facts
indicate a remarkable degree of energy in the con-
tractile power of the muscular fibres of insects, we
should at the same time recollect that the dimi-
nutive size of the beings which display those
powers is itself the source of a mechanical advan-
tage not possessed by larger animals. The efficacy
of all mechanical arrangements must ultimately
depend on a due proportion between 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 dimen-
sions 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 mate-
rials increases more rapidly than the strength.
How 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 in-

VOL. I. Y

322 THE MF.CHANK AL FUNCTIONS.

creased in its dimensions until the force of gravity
becomes superior to the cohesion of its own par-
ticles ; and consequently any structure, like a
vegetable or animal body, composed of a combina-
tion 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 employ-
ing 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 neces-
sarily restricted within narrow limits, and the latter
expedient would entirely alter the relative propor-
tions of the parts, and would require a complete
change in the plan of their construction. In pass-
ing 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 procedure of nature in
the execution of this more enlarged and compre-
hensive scheme of animal organization.

Chapter VI.

VERTEBRATA.

§ 1. Vertehrated 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.

VERTEBRATED ANIMALS. 323

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 organization ; re-
sorting to new models of structure, on a scale of
greater magnitude than before ; devising 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 con-
structions is seen in the vast series of Vertehrated
Animals^ which comprises a well-marked division
of Zoology, comprehending 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 assemblage
of beings. Whatever may be the size or external
form of these animals, whatever the activity or
sluggishness of their movements, whether inha-
bitants 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 framework, or skeleton, which sustains and
protects their fabric. The Quadruped, the Bird,
the Tortoise, the Serpent, and the Fish, however

.324 THE MECHANICAL FUNCTIONS.

they may differ in subordinate details of organi-
zation, are yet constructed on one uniform prin-
ciple, and appear like varied copies from the same
original model. In no instance do they present
structures which are altogether isolated, or can be
regarded as the results of separate and independent
formations.

In proceeding from the contemplation of the
structures of articulated to those of vertebrated
animals, we appear to pass by a rapid excursive
flight, from one great continent to another, sepa-
rated by an immense gulf, containing no inter-
mediate islands from which Me might gather
indications of these tracts of land having been
originally connected. At the very first sight, in-
deed, the general fabrics of these two descriptions
of animals appear to have been constructed on
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 sur-
faces for the attachment of the organs of motion.
But in the Vertebrata, the solid framework 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
defence is entrusted solely to the integuments, and
their appendages. Such is the general character
of the arrangements which nature has here adopted ;
from which, however, she has occasionally deviated
with respect to some important organs of extremely

VERTEBRATED AM.^JALS. 325

delicate texture, and which require to be shielded
from the slightest pressure. This occurs with re-
gard to the brain, and the spinal marrow, which
we shall presently find are specially guarded by a
bony structure, enclosing them on every side, and
forming a solid case for their protection. The
mass of bone, thus provided to defend the brain,
gives also the opportunity of lodging safely the
delicate apparatus subservient 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 improvement than could be attained in
the lower orders.

There is also another advantage, of considerable
moment, which results from the internal situation
of the skeleton, namely, that it admits of an in-
definite extension by growth, without interfering
with tlie 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 ope-
ration is generally required to be performed a great
number of times, before the animal can acquire the
size it is destined to attain. Hence the frequent
moultings of the caterpillar ; hence the repeated
castings of the shells of the Crustacea ; and hence ,
also the successive metamorphoses of the insect.

326 THE MECHANICAL FUNCTIONS.

Nothing of this kind takes place among the Verte-
brata; where all the organs are developed in regular
and harmonious succession, without the slightest
mutual interference, and without those vicissitudes
of action and of torpidity, which we witness in the
chequered existence of the insect.

§ 2. Structure and Composition of the
Osseous Fabric.

The process employed for the formation and exten-
sion of the solid framework of the Vertebrata
differs totally from that which we have seen exem-
plified in the growth of shells, and of the hard
coverings of insects and 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 ver-
tebrata, the skeleton is composed o^Xvvxq bones; that
is, of solid pieces, which, although they are dense
calcareous structures, 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 mem-
brane, 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.*

* De Blainville regards the hard coverings of insects, together
with the shells of the Crustacea, as structures derived altogether from

STRUCTURE OF BONE. 327

In their chemical composition, likewise, bones
are strikingly contrasted with the calcareous pro-
ducts of the Mollusca ; for in the former, the earthy
portion consists almost wholly of phosphate of lime ;
a material, which appears to have been selected for
this purpose from its forming much harder com-
pounds with animal membrane than the carbonate.
Wherever great strength and rigidity are required,
this is the material resorted to 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 m hich
the proportion of phosphate of lime is the greatest,
when compared with that of the animal substance
which cements them together ; the force of mutual
cohesion among its own particles being much
greater than that imparted by the cementing in-
gredient. The internal bony portions of the ear,
where, in order perfectly to transmit the sonorous

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 contrary, 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 framework 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 denomination of Dermo- skeleton, and to the latter
that of N euro-skeleton. (See his Tabula? Anatomiam Compara-
tivam ilhistrantes, edited by Thienemann). Analogies have also
been imagined to exist between the external and internal situa-
tions of the woody fibres of plants belonging respectively to the
endogenous and exogenous 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. Carolina} Natur. Curios, xvi. 219.)

.'V28 Ttii: MECHANICAL FUNCTIONS.

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 tem-
poral bone of the Whale and the Cachalot, where
the great size of the organ gives us advantages in
examining them, have the density and hardness of
a stone. The bony portions of the teeth, likewise,
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. It is scarcely necessary to point
out the obvious intentions which are ftdfilled by
this peculiarity of structure, conferring extraordi-
nary hardness on a part of which the appropriate
office is that of breaking down hard bodies sub-
jected to its 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 brittleness, and
been productive of the worst consequences to parts
exposed to sudden and violent concussions. It is
in order to guard against this evil that an elastic
animal matter is enq^loyed as the basis of the struc-
ture, acting as a strong cement interposed between
the calcareous particles.

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, during the

STItUCTUKE OF BONE. 329

destruction of the animal membrane. The oil con-
tained in the cavities exudes, and, taking fire, is
soon totally consumed. 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
weight, and to have become exceedingly brittle ;
this, as already mentioned, being the natural pro-
perty 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 breaking 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 sufiiciently diluted to prevent its
injuring the animal membrane, but yet sufficiently
powerful to dissolve the phosphate and carbonate
of lime. Diluted nitric, or muriatic acids may be
used for this purpose ; and they 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 appearance, indicating the presence
of a small quantity of carbonate of lime, which
always exists in bones, intermixed with the phos-
phate. The phosphate may be recovered from its
solution in the acid by precipitation with a pure
alkali, such as a solution of ammonia. This pre-
cipitate is readily dissolved, without etFervescence,
by nitric, muriatic, or acetic acids. A small quan-
tity of sulphuric acid may also be detected in the

330 THK MECHANICAL FUNCTIONS.

fluid by the addition of nitrate 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 mus-
cular or fibrous structures, and being, like them,
resolvible into gelatin and albumen by long boiling
in ' ater. This substance has sometimes, but erro-
neously, been considered as identical with carti-
lage ; for it has neither the whiteness, nor the elas-
ticity, nor the texture of cartilage ; nor is it at all
similar to that substance in its chemical composi-
tion ; for while cartilage is formed almost wholly of
albumen, the animal basis of bone is almost en-
tirely resolvible into gelatin.

Thus may a bone be analysed into its two con-
stituent parts : by the process first described we
obtain its earth deprived of its animal constituent ;
by the second, we obtain its membranous 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 homogeneous and imorganized matter,
are scarcely compatible with one another, are skil-
fully united.

The mechanical structure of bone is no less
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 de-
posited, partakes of the reticular structure belong-

STRUCTURE OF BONE. 3.31

ing to ordinary cellular texture ; and a bone, when
minutely examined, exhibits also the same appear-
ance 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 mace-
rated in water. The interior of most bones, in the
higher classes of animals, presents distinctly the
appearance of irregular cavities, resulting from the
partial separation 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

* The great vascularity of bones is a remarkable feature in their
structure. Breschet has found that bones are traversed by numerous
veins of considerable size, which communicate largely together. The
existence of a multitude of very minute canals in the solid substance
of bones was discovered by Havers, about the end of the seventeenth
century. The form and arrangement of these canals have been in-
vestigated by Mr. Howship, who found that they compose a system
of nearly cylindrical cavities, passing in a longitudinal direction,
and freely communicating, by collateral branches, both with one
another, and also with the central medullary, or spongy cavities of
the bone. Their diameter is, on an average, about the 200th part of
an inch : but it varies from the 100th to the 400th. They are
every where lined with a vascular membrane of great tenuity, and
are tilled with an oily substance, or marrow. (Medico-Chirurgical
Trans, vi. 263; and vii. 387.)

Purkinje has discovered in bones the constant presence of a great

332 THE MECHANICAL FUNCTIONS,

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 object is either extensive pro-
tection, or the provision of broad surfaces for the
attachment of muscles, we find the osseous struc-
ture 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 locomo-
tion, the bones are modelled into lengthened cylin-
ders, generally somewhat expanded at the extre-
mities, for greater convenience of mutual connexion.
In the form, the structure, and the arrangement
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, we discern most
palpable manifestations of profound design, and
the most exquisite refinements of mechanic skill.
All the scientific principles of architecture and of

number of minute opaque calcareous corpuscles, of an oblong shape,
and only about two or three times larger than the globules of the
blood. They are interspersed between the osseous fibres ; and are
arranged with some degree of regularity in rows surrounding the
canals of Havers, with the sides of which they are connected by very
slender filaments, radiating in every direction from the surface of the
corpuscles. Mr. Smee has found that these corpuscles are hollow,
and that their cavities occasionally communicate with those of the
canals. Proceedings of the Royal Society, Jan. 23, 1840. Miescher
considers the spongy substance in the interior of bones as being
formed by the canals of Havers, when much dilated. See Ann.
Sc. Nat. serie 2, xi. 33.

STRUCTURE OF BONE.

333

dynamics are more or less exemplified in the con-
struction 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 in various parts of the osseous systems of
vertebrated 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 con-
dition by which mechanical advan-
tage could be gained. In the more
perfect developements of struc-
tures, such as those which obtain
in the higher orders of mammalia,
and also in the class of birds, all
the long bones are hollow cylin-
ders ; and their cavity is largest in
the middle of their length. This
is shown in Fig. 172, which repre-
sents a longitudinal section of a
human thigh bone, and in Fig. 1 73,
which is a similar section of the
humerus, or bone of the arm. The
v/allsof these bonesconsist of adense
and compact substance, formed by
the close cohesion of the osseous
plates. These walls are of greater
thickness in the middle of the
shank, or shaft of the column, and become thinner
as we follow them towards either end. This gra-
dual diminution in the thickness of the walls arises

.334 THli MECHANICAL FUNCTIONS.

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 cancelU. The plates, proceeding from each
side obliquely inwards, at length meet each other
in the axis of the cylinder, 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 mechanical
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 materials could not possibly have
been disposed in a manner better calculated for
such resistance than when in the form of a tube,
or hollow cylinder.* To this mechanical principle
I have already had occasion to advert, when speak-
ing 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
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.

* An elaborate mathematical demonstration of this proposition
was long ago given by Dr. Porterfield, in a paper contained in the
first volume of Medical Essays and Observations, published by a
Society in Edinburgh, p. 95.

t Pages 70 and 71.

STRUCTURE OF BONE. 335

§ 3. Formation and JDevelopemenl of Bone.

But it is not enough to contemplate the purposes
so admirably answered by these arrangements. Our
curiosity cannot but be powerfully excited to learn
what processes and refined series of means are
employed by nature to raise and to perfect all
these artificially contrived structures. It fortu-
nately happens that in this instance we are per-
mitted 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 developement of the harder parts, than in
the quicker growth of more yielding and expansible
textures. The peculiar material, also, of which
bone is formed, is easily distinguished by its hard-
ness, its whiteness, and its opacity from the softer
and more transparent animal substance with which
it is intermixed. Hence we are allowed an oppor-
tunity of observing the earliest stages of its deposi-
tion, and of accurately following the subsequent
changes it undergoes.

The parts of the embryo animal, which are des-
tined 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 distinguished from the
semi-fluid portions which surround them. In pro-
cess of time, when the vascular circulation of the
blood has been established, and the newly formed
vessels are extending their branches over every
part of the nascent organization, the arteries which

336 THE MECHANICAL FUNCTIONS.

are appropriated to the task of forming the bones,
are spread over 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 another, of a
simpler and softer nature, namely cartilage. In
every case, then, cartilage is first formed, and
becomes visible by its greater opacity when com-
pared 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 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 elas-
tic cartilage may be employed with great advantage
as its substitute : for a hard and unyielding struc-
ture 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 building.
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

OSSIFICATION. 337

of the edifice proceeds. The way is cleared at first
by the absorption of the central part of the carti-
lage, and a few particles of ossific matter are depo-
sited in its room. While this process is going on,
greater activity is displayed 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 we trace the branches to a certain extent,
although we cannot pursue them to their minuter
ramifications. They now assume more active func-
tions, 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 con»
tinuous 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 parallel
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 prin-
cipally in its length ; the spicula becoming 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 par-
ticles at both their extremities. In like manner,
the ring increases in thickness, not by the depo-
sition of phosphate of lime between the original

VOL. I. z

338

THE MECHANICAL FUNCTIONS.

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
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

176 171 IT'S

human skull, in an early stage of its ossification,
and shows very distinctly the radiating fibres. In
the cubical, and more irregularly shaped bones,
the process is, doubtless, conducted with the same
order and regularity, although it cannot be so
readily followed by the eye.

The same process is repeated in different parts
of the bone, wherever nature has, in conformity
with determinate laws of developement, appointed
particular centres of ossification. The bone con-
tinues to extend from each of these centres, pro-
ceeding gradually towards the circumference, or
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, however, this peculiarity, that
the cartilage is progressively absorbed to make

OSSIFICATION. 339

room for the deposits of bony substance. When
the bone is long, separate points of ossification
appear in the extremities, before the central por-
tions are ossified ; and the ends, thus formed into
bone, are afterwards united to the shaft, so that the
whole shall form a continuous bony mass. In the
flat 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-operation.

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, requires this protec-
tion 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 quickness for the exigencies of the occa-
sion, if it had originated from a single point. There-
fore 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 expedited 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 ex-
posed 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 prescribed
line, they have become so much expanded as to
have lost the power of coalescing with the fibres
which have originated from other centres, and are

340 THE MECHANICAL FUNCTIONS.

proceeding in a contrary direction. Yet the arte-
ries still continuing to deposit ossific matter, each
set of tibres insinuate themselves between those of
the opposite set, for some little distance, and until
their further progress is stopped by the increasing
resistance they encounter. The consequence is
that the edges of the bones, which have thus met,
are irregularly jagged, like the teeth of a saw, pre-
senting externally 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 dif-
ferent centres of ossification is not indiscriminate,
but is found to be regulated by definite laws, and
to have certain relations to the general plan of con-
formation originally established. Each distinct
bone is formed from a certain number of ossific
centres, which altogether constitute a system apper-
taining to that bone only, 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 belonging to other groups. .The groups
themselves are not arbitrary, but are pre-established
parts of the original design. Circumstances occa-
sionally, indeed, arise, which may overrule this
inherent tendency to preserve the line of separation
between two bones ; and we then find them coa-
lescing to form a single piece. Such unions are
technically called anchi/loses.

Were this the whole of what takes place in the
formation of a bone, the process would not, perhaps,

OSSIFICATION. 341

dift'er 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, form-
ing one layer after another, in union with a corre-
sponding deposit of animal membrane. But the
subsequent changes which occur show that the con-
stitution 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 influ-
ence. 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 under-
going continual alterations of shape, and even re-
newals 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 proportions
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, how-
ever, there is no stretching of elastic parts; for all
the osseous fibres and laminae are rigid and un-
yielding, 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 re-
moval of such parts of the young bone as had
occupied the situations where vacuities are found to

342 THE MECHANICAL FUNCTIONS.

exist in the old bone. We find, for instance, that
in the early state of a bone there are no internal
cavities, but the whole may be considered as a
uniform solid mass.* At a certain stage of ossifi-
cation cells are excavated by the action of the
absorbent vessels, which carrj^ away portions of
bony matter lying in the axis of the cylindrical, or
in the middle layer of the flat bones, t Their place
is supplied by an oily matter, which constitutes 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 sur-
round it; until by the removal of all the layers
situated nearer to the centre, it becomes the inner-
most layer, and is itself destined in its turn to
disappear, leaving the new bone without a single
particle which had entered into the composition of
the original structure.

It has been found that by mixing certain colour-
ing substances with the food of animals the bones
will soon become deeply tinged by them. This
fact was discovered accidentally by Mr. Belchier,

* In this general statement I do not take into account the canals
of Havers, formerly described, as they are so minute as to be disco-
verable only by the aid of the microscope : but they exist not only
in bones, but also in the cartilages which precede ossitication.

-j- The bones of the lower classes of vetebrated 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.

OSSIFICATION. 343

who gives the following account of the circum-
stances that led him to notice it.* Happening to
be dining with a calico printer on a leg of fresh
pork, he was surprised to observe that the bones,
instead of being white as usual, were of a deep red
colour; and on enquiring into the circumstances,
he learned that the pig had been fed upon the
refuse of the dyeing-vats, which contained a large
quantity of the colouring substance of madder. So
curious an effect naturally attracted the attention
of physiologists ; and many experiments were un-
dertaken with a view to ascertain the time required
to produce this change, and to determine whether
the effect was permanent, or only temporary.
The red tinge was found to be communicated much
more quickly to the bones of growing animals 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 tlie
rose colour. The dye was more intense in the solid
parts of those bones which were nearest to the
centre of circulation, while in bones of equal soli-
dity, 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 animal had
been fed upon madder. When this diet was dis-
continued, the colour became gradually more faint,
till it entirely dissappeared.t

* Phil. Trans, for 1736, vol. xxxix. 287 and 289. It appears
that this effect of madder had been noticed by Antoine Mizaud, a
Physician in Paris, as long ago as the year 1572.

t These experiments by no means prove, as was once supposed,
that the substance of the bone is renewed with every change of hue ;

344 THE MECHANICAL FUNCTIONS.

When madder was alternately given and withheld
during certain regular periods, in a young animal,
Duhamel found that the bones which had been
growing during the whole of that time, were com-
posed of alternate layers of a red and white colour ;*
a result which, as Flourens has lately shown, is
owing to the particles of madder, which were
mingled with the circulating blood, adhering to the
phosphate of lime while it was deposited to form
each new layer of bone : and the reddened layer
thus formed being afterwards covered over and
concealed by the next superposed layer, which
being deposited at a later period, when no madder
was supplied, had the white colour natural to bone.
So rapid is the colorific operation of madder, that
Flourens obtained, in the course of five hours, a
perceptible tinge from only six grains of madder,
given to a young pigeon during a single meal-t

but merely that the colouring particles of madder, when present in
the blood, readily attach themselves to the particles of phosphate of
lime which are deposited in the bones. Mr. Gibson had inferred,
from some experiments which he made, that the colouring matter so
deposited is again removed from the bones, in consequence of its
affinity with the serum of the blood, when that fluid is restored to
its usual state by the discontinuance of the supply of madder.
CMemoirs. of the Lit. and Phil. Soc. of Manchester. Second series,
i. 146.) But the recent experiments of Mr. Paget throw much
doubt on the correctness of this theory. (London Medical Gazette,
%\v. 277.)

* Mem. Acad. Sc. 1739.

f Comptes Rendus, x. 143 and 305.

SPINO-CEREBRAL AXIS. 345

§ 4. Skeleton of the Vertebrata.

The purposes to be answered by the Skeleton, in
vertebrated animals, resolve themselves into the
three following ; first, to afford mechanical support
to the body generally, and also to different portions
of the body ; secondly, to provide a solid basis for
the attachments of the muscles which are to effect
their movements ; and thirdly, to give protection to
the vital organs, and more particularly to the cen-
tral parts of the nervous system. Of these the
last is the circumstance that has the greatest influ-
ence in determining the principles on which the
osseous framework has been constructed. In the
nervous 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 important 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 contradis-
tinction to the analogous parts of the 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 di-
visions of the animal kingdom. In the embryo
state of the vertebrata the central parts of that
system consist of two separate filaments, running

.346

THE MECHANICAL FUNCTIONS.

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 pri-
mary type of the skeleton is determined by the
peculiar form of this, the central organ of the ner-
vous system.

In laying the foundations of the skeleton, then,
the tirst object is to provide for the security 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 pressure. It is this
part of the skeleton, accordingly, of which the
rudiments appear the earliest in the embryo animal.
Tliese rings form a column, extending in a longi-
tudinal 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 Articulala,
are encased. When ossified, these several rings
are termed vertebrcs ; and the entire column which
they compose is the Spine. Fig. 177 shows the

form of one of the vertebrae of the back in the
human skeleton. Fig. 178 is a side view of four

VERTEBRAL COLUMN. 347

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 tlie constancy
with w hich the spinal column is found in all ani-
mals of this type, and from the uniformity of the
plan on which, amidst endless variations, it is
modelled, it has been chosen as the distinctive cha-
racter of this great assemblage of animals, which
have accordingly been denominated the Vertebrata,
or Vertehrated Animals.

The spine is of great importance in its mecha-
nical 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 continuous framework : 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 tixed points of attach-
ment 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 framework 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 mecha-
nism 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 marrow it encloses shall,
amidst all these motions, remain secure from pres-
sure ; for so delicate is its structure, that the least
degree of compression would at once interrupt its

,348 THE MECHANICAL FUNCTIONS.

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 intervals, on each side, throughout its whole
length.

No where has mechanical skill been more con-
spicuously displayed than in the construction of
a fabric capable of fulfilling these opposite, and
apparently incompatible functions. The principal
difficulty was to combine great strength with suffi-
cient flexibility. This we find accomplished, 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 tightly braced by con-
necting ligaments, is allowed 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 degree 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 17a and 179), composed of
a peculiar substance, intermediate in its texture to
ligament and cartilage, and possessing in a remark-
able degree the qualities of toughness and adhesion,
united with compressibility 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 con-
cussions incident to violent action : for we find that,
however the spine may be bent, no chasm is left by

VERTEBRAL COLUMN. 34.0

the flexions of the vertebrae upon one another, nor
is the continuity of the column in the smallest
degree interrupted.

The motions of the vertebree upon each other
are farther regulated by the mode in which their
articular processes, which are the pieces that pro-
ject obliquely on each side, play upon each other.
These processes, which are seen at a, a, in the pre-
ceding figures (177 and 17{3), are of great use in
preventing the sudden displacement of the vertebree ;
for this effect cannot be produced by any force
short of that which would occasion fracture. Any
one who will try to dislocate, by sheer force, the
spine of a hare or rabbit, will find reason to admire
the art with which its bones have been locked toge-
ther, 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, corre-
sponding to each other, are left in the sides of each
of the arches, forming apertures for the secure pas-
sage of the nerves as they issue from the spinal
marrow. All these circumstances are visible in the
figures, particularly in the section. Fig. 1 79, where
c, c, is the canal for the spinal marrow, and in
which the apertures just mentioned are distinctly
seen, at o, o.

.350 THE MECHANICAL FUNCTIONS.

In order to give an advantageous purchase to the
muscles which are attached to the spine, each ver-
tebra has, besides the parts above described, a pro-
jecting piece of bone, extending upwards from the
crown of the arch, and denominated 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 pro-
jecting pieces, which are denominated the transverse
processes (i), 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 different tribes of animals, according to the par-
ticular mechanical circumstances of the system,
and to the particular intentions of their forma
tion.

As it is in the spinal column more especially that
speculative naturalists have sought for illustrations
of their favourite doctrine of a strict unity of plan
in the conformation of the skeleton in all verte-
brated animals, it may be proper here to notice
the principles assumed by Geoffroy St. Hilaire as
regulating the formation of this part of the osseous
fabric* In common with all bones, the vertebrae
take their rise from certain determinate points, or
centres of ossitication, where, at first, detached
pieces of bone are formed, destined to unite toge-
ther so as to compose the entire bone. The num-
ber of elementary pieces which enter into the com-
position of a vertebra has been differently estimated

* Memoires du Museum, ix. 79 and 89.

STRUCTURE OF VERTEBRA.

351

by different physiologists ; but
the following are more parti-
cularly recognised as being en-
titled to that character. They
are represented in their relative
situations in Fig. 180. The first
is the part which forms the nu-
cleus, 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 em-
brace the spinal marrow which is situated 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 re-
garded as the key stone, for the protection of the
spinal marrow. Then come the two transverse pro-
cesses (t, 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 have been considered as the ele-
ments that are most essential, and most constantly
present in the composition of the vertebree. 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 ; w hich
surfaces, in some of the lower orders of the verte-
brata, become articular. There is frequently, also,
a developement of processes (i), forming arches
and spines at the lower surface of the vertebra^, or

352 THE MECHANICAL FUNCTIONS.

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 ob-
served 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
present, and more variable in their shape. They
form articular surfaces for the purpose of being
connected with the surfaces of corresponding pro-
cesses in the contiguous vertebra. Of these there
are four (a, a, a, a) belonging to each vertebra, two
in front, and two behind. These, however, do not
properly rank among the primary elements of the
vertebrae, because we tind them, in different in-
stances, occupying different positions, and formed
sometimes by extensions of the bodies, and at other
times of the leaves. In following them through
the several tribes of animals, we observe them
shifting their places, in various ways, and not even
preserving 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 an-
other, and a third posterior to these. In the Onti-
tliorJiynclmSy while the latter retains its situation in
the middle, the other surfaces have separated from
each other, and have travelled 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 processes.

In addition to these, accessory bones are often

STRUCTURE OF VERTEBRiE. 353

developed to suit particular occasions. Thus in
Fishes, we see that one or two additional pieces (i)
are affixed to the ends of each spinous process. In
many cases, instead of being thus placed in a line
with these processes, they appear at a little dis-
tance, having, according to this theory, wandered
from their proper situations ; they are then found
between the spinous processes, and receive the
name of interspinoiis hones.

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 ac-
cessory pieces, as is found to be the case in some
reptiles ; but, in other instances, they undergo a
gradual change of position, as we follow them back-
wards 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 pro-
cesses. When these are much developed, they
necessarily obstruct the flexion of the vertebrae in
the directions in which they are situated : when
they are small, no such hinderance arises, and the
spine is free to move in all directions. Thus, when
we see the spinous processes much enlarged, while
the transverse processes 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 is the condition of the spine of fishes,
where this latter kind of motion is the one princi-
pally wanted. In Dolphins and other Cetacea, on
the contrarj^ where the actions are required to be

VOL. I. A A

354 THE MECHANICAL FUNCTIONS.

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, has evidently
a relation to some particular circumstance in the
living habits of the animal, and is subordinate
to the general plan of its economy. According as
each of these elements of ossification receives dif-
ferent 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 difficulty identified :
on some occasions, as we have just seen in the spi-
nous bones of fishes, its accessory structures are
multiplied, as if continued efforts were made by the
system to repeat the same structures. Amidst all
these modifications, the parts that preserve the
greatest constancy of form are those which are of
most importance, and which are therefore consi-
dered as 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 ; decreasing
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

CRANIUM. 355

become stinted in their growth and ossification,
until we find the terminal pieces generally remain-
ing 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 illus-
trates 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 finally ceases. In the
present instance, the consequences of this law are
highly advantageous, since it provides for the flexi-
bility of the tail, and qualifies it for being applied
to a great variety of useful purposes, as we find
more especially exemplified in the ^^e/e.s, or spider
monkey, and in the Kanguroo,

Next in importance to the spine is the cranium,
or osseous covering of the brain ; together with the
bones of the face which protect the organs of the
finer senses. A diligent investigation 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 developements of vertebrae ; much al-
tered, indeed, in shape, in consequence of the new
conditions to which they have been subjected ; but
still possessing all the essential elements of ver-
tebrae. In the embryo condition of these organs,
and while the brain is yet undeveloped, the bony
circles which enclose it certainly resemble ver-
tebrae ; but in proportion as the brain becomes ex-
panded, the similarity diminishes; for the rapid
growth of the brain in the higher orders of animals
is necessarily attended with an equally sudden ex-
pansion of the bones of the skull. Hence their

.*356 THE MECHANICAL FUNCTIONS.

several elements are thrown into unusual positions,
and being variously distorted and disfigured, can
hardly be recognized under the strange disguises
they assume.

The extensive researches that have been recently
made in this branch of comparative anatomy, have
supplied many facts which have been adduced by
the supporters of the hypothesis that the bony
coverings of the brain are the result of the deve-
lopement of three vertebrae. According to this
theory, the first of these supposed cranial vertehrce,
beginning our enumeration from the neck, is the
origin of the occipital bone, of which the lower
part, or that which immediately supports the cere-
bellum, corresponds to the body of the vertebra ;
the two lateral 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 spi-
nous process. The third cranial vertebra is con-
stituted by the anterior half of the sphenoid bone,
which is its body, and the frontal bones, which are
its leaves. This theory, which originated with
Dumeril,* was fondly nurtured by the poetic mind
of Goethe,t was extended by Oken, and adopted
by Spix, Meckel, Carus, and other German phy-
siologists. It has been farther applied to the bones
of the face, by Geoffroy St. Hilaire, who conceives

* Considerations generales sur I'analogie entre tous les os et les
muscles du tronc des animaux. Magazin Encyclopedique. 1808.
f Zur Naturwissenschaft Uberhaupt, &c. 1817-24.

SKELETON OF VEUTEBRATA. 357

them to be likewise developements of several other
supposed cranial vertebrae;! but the analogies by
which the hypothesis is supported become more
feeble and confused as we recede from the middle
of the spinal column.

AH the other parts of the skeleton may be re-
garded 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 in-
stances, 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 grada-
tions of evolution, it will be best to consider them
first in their more perfectly developed forms, as
they are presented in the higher classes of quadru-
peds. In the following descriptions, the skeleton
of the Hog (Fig. 101) 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 pro-
cesses ; where, in general, they are allowed a slight
degree of motion. Their primary use is to defend
the vital organs situated in the region of the chest,
or thorax (namely, the heart and the lungs) ; but
they are subservient also to the function of respira-
tion, by the alternate movements which are given
to them by their muscles. The two parts of which
they are composed often form an angle by their

\ III this theory of St. Hilaire the number of cranidl vertebrtTe is
seven, each composed of nine elementary pieces.

358

THE MECHANICAL FUNCTIONS.

junction, 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 intervention of
cartilages, which, from their similarity of form to
the ribs, appear as continuations of them, and
which complete the semicircle. These cartilages,
which have been termed sterno-costal appendicesy
often become ossified, either wholly or in part.

The sternum has been considered as being formed
of nine elementary pieces, proceeding from sepa-
rate centres of ossification. Two of these occupy
the end which is nearest to the head ; four are
lateral ; and two are situated at the opposite ex-
tremity ; one only being central, and surrounded
by the rest. Speculative anatomists have laboured
to trace the developement of these several elemen-
tary parts in the different classes of animals, from
the rudimental states of this bone as it occurs in
Fishes, to its greatly expanded conditions in the

SKELETON OF VEKTEBRATA. 359

Tortoise and the Bird, which appear to exhibit the
most opposite proportions of these elements : but
Cuvier regards this theory as untenable.

Last in the order of constancy come the bones of
the extremities. As we ascend in the scale of ani-
mals we may observe the prevalence of a tendency
to the concentration of organs, and consequently to
the diminution of their number. While in animals of
the inferior orders, which are possessed of extremi-
ties, we find a considerable number of legs; in all
the animals comprised in the class of true insects
nature has limited the number to six ; and in the
Vertebrata 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 Quadrupeds a
striking correspondence in the bones of the fore
and the hind extremities. Both the one and the
other are connected with the spine by the inter-
medium of large and broad bones, which are in-
tended to serve as a basis for their more secure
attachment, and for giving, at the same time, ex-
tensive and advantageous purchase to the n)uscles,
which are to move the limbs. The two bones by
which the anterior extremity is connected with the
trunk are the blade-hone, or Scapula (b), which sends
out a process called the coracoid-bone ; and the
collarbone, or the Clavicle,* which extends from the
scapula to the sternum. The corresponding con-

* 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 car-
tilage ; c, the clavicle : b, the scapula ; a, the acromion ; k, the cora-
coid process ; and g, the glenoid cavity for the articulation of the
humerus.

PA)0 THE MECHANICAL FUNCTIONS.

necting 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 number
of sepamte yertebrae, which have afterwards become
consolidated into a single bone, and which bear the
marks of having been compressed from behind for-
wards 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 cocci/gis. (q),
or more properly the coccygeal vertebrce : when they
are sufficiently numerous to compose a tail, they
come under the denomination of caudal vertebrce.
The three 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 ace-
tabulum, 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 foreleg it is termed the humerus (b), in the hind
leg, the Jemur (f.) The next division contains two
bones, placed parallel to each other ; they are in
the formerj the radius (r), and the ulma (u) ; in the
latter, the tibia (t), and Jibula (f). These are fol-
lowed by a number of small, rounded or cubical
bones, collected together in a group, which consti-
tutes the Carpus (w), in the fore leg, and the 2\ir-
us (t), in the hind leg. Next come a set of long

SKELETON OF VERTEBRATA. 361

cylindrical bones, composing 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
enveloped 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 ungual bone,
there is usually attached either a nail, a claw, or a
hoof Small detached bones are 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 putella, or
knee pan (p), is the largest of these, and is pretty
constantly present. Smaller bones of this descrip-
tion are met with on the joints of the fingers, and
are termed sesamoid bones.

On comparing these divisions of the limbs of
(piadrupeds 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 arti-
culated and of vertebrated animals, however strong
may be the contrast which they offer in all the
essential featuresof their conformation. The rings
which constitute the skeleton of the insect, and
which enclose its principal nervous cords, have
been supposed to have an analogy with the circles

3G2 THE MECHANICAL FUNCTIONS,

of bone which constitute tlie primary forms of the
Tertebrae, and which contain the spinal cord; but
the rings of insects include the viscera, whereas the
spinal cord alone is contained within the vertebral
circles. Vertebrated and articulated animals agree,
also, in having the head placed at one extremity,
distinct from the trunk, and containing the princi-
pal organs of the senses.

An approximation is apparently made towards
an internal skeleton in thecephalopodous Mollusca ;
where we find a central body, cartilaginous 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 car-
tilaginous spine of the fish called the Mifxine, or
Gastrobranclms, which does not enclose the spinal
marrow, but only admits it to pass along a groove
in its upper edge. But the search after remote
analogies of this kind has been carried much beyond
the boundaries of rational inquiry ; and by enlisting
the imagination in the pursuit, has too often led
naturalists far away from the path of truth, and the
reality of nature. All that can with safety be said
is that there exist many perceptible links of con-
nexion among all the classes of created beings^
even in those apparently the most remote from one
another ; so as to render it clear to 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.

363

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 compre-
hending an immense number of species, which are
all inhabitants of the water, which exhibit an end-
less variety of forms, and open to the physiologist a
wide field of interesting research. We cannot fail
to perceive, on the most cursory glance, the beau-
tiful 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 re-
duced 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 the reflux of
the water collected behind. With a view to dimi-
nish friction as much as possible, the surface of the
body has been rendered smooth, and the skin im-
pregnated with oil, which defends it from injurious
impressions, and at the same time prevents the
water from j^enetrating into its substance.

364 THE MECHANICAL FUNCTIONS.

The body of a fish is nearly of the same specific
gravity as the water it inhabits ; and the effect
of gravity is therefore ahnost wholly counterba-
lanced 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 neces-
sary 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 exclu-
sively for progression ; and the powerful hydrostatic
pressure, which supports the body on all sides,
supersedes the necessity of that cohesive rigidity
of frame, which is essential to the safety of terres-
trial animals. Hence we find that in one whole
tribe of fishes, the skeleton is composed merely of
cartilage ; and, in all, it exhibits much less of the
osseous character than in the higher classes. The
framework of the skeleton, even of osseous fishes,
has not the compactness possessed by that of
quadrupeds or reptiles : the pieces which compose
it are joined together less firmly ; many of them.

STRUCTURE OF FISHES. 365

indeed, remain in an imperfectly ossified condition,
their elementary pieces being detached from one
another, 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 semitransparent
bone. Compared with the quantity of gelatin
Mhich enters into their composition, the bones of
fishes contain but a small proportion of earthy
ingredient ; a circumstance which explains the
pellucidity of the mass, and the readiness with
wJiich the osseous fibres it contains can be distin-
guished. Another consequence of the want of
density in the bones of fishes is, that their articula-
tions are less regular and perfect than the corres-
ponding joints of terrestrial animals ; for it is
evident that where the parts are soft and flexible,
joints are scarcely required.

In the osseous fishes, the bony structures are
more finished ; and they even may 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 in-
stance, while some of the bones have acquired a
great hardness, others remain wholly and perma-
nently 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,

366

THE MECHANICAL FUNCTIONS.

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

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 employed
for this purpose being the tail ; for the fins, as we
shall presently find, are merely auxiliary organs,
serving chiefly to balance the body, while it re-
ceives its propulsion from the tail. A fish moves
in the water on the same principle as a boat is
impelled in paddling ; for the action of the tail on
the water is lateral, like that of an oar, which it
resembles in the vertical position of its plane ; and
the effect is transferred by the resistance of the
water to the body whence the impulse originates.

* Cuvi'er, sur les Poissons. Tom. i. p. 218.

SWIMMING OF FISHES. 367

Let us suppose, for example, that the tail has been
brought into the position, represented in Fig. 185 ;
that is, slightly inclined to the right. When it is
in this situation, the muscles on the left side, which
tend to bring the tail in a right line with the body,
being suddenly thrown into action, cause the tail
to press against the water in a direction r p, per-
pendicular to its surface, and by the reaction of
that fluid in the opposite direction p r, transfer
to the whole body of the fish an impulse in this
latter direction, causing its centre of gravity, c, to
move onwards in the line c b, parallel to p r. The
impulse thus given is not destroyed by the further
flexion of the tail towards the left side, for two
reasons. The first is, that the water
against which the tail has been
pressing, in moving from r to m,
having by that action been set in
motion in the direction r p, con-
tinues its motion in the same direc-
tion, and while thus accompanying
the tail, presents no resistance, or
scarcely any, to it while it is bent
from M to L, so as to occupy the
position directed by the dotted outline.* The
second reason is, that the muscles on the left side

* In the above explanation, 1 have not thought it necessary to
take into account the effect resulting from the action of the tail on
the water when it is first bent from a straight position into that with
which I begun the description, lest I should have rendered it too
complex. It is true, indeed, that the resistance arising from this
action must tend to make the body of the fish recede to a certain
extent : but then this effect is immediately afterwards much more
than compensated by the forward impulse which is created by the
return of tiie tail to its straight position ; during v/hich return, it is

368 THE MECHANICAL FUNCTIONS.

of the fish having expended their principid force
in the extension of the tail from ii to m, that is, in
bringing it to a straight line with the body, exert a
weaker force in bending it from m to l, and conse-
quently the reaction of the water is in the same
proportion more feeble. When, again, the tail has
arrived at the position l, a similar energetic action
of the muscles on the right side of the fish, creates
a powerful resistance in the water, which now acts
in the direction k l, and consequently gives an
impulse to the whole fish in the same direction a ;
and this reaction of the water is the stronger, inas-
much as it was previously moving, together with
the tail, in the opposite direction k l. Thus do the
alternate movements of the tail to the right and to
the left give to the fish alternate impulses in the
directions cb and ca; and these impulses being
repeated in quick succession, the body moves for-

opposed by the more resisting mass of water previously set in motion
by the first lateral stroke of the tail.

In the popular view given in the text, I have also purposely
omitted to notice some mechanical circumstances, which, in a more
accurate mathematical analysis of the problem, would claim atten-
tion as having a sensible influence on the results. It is to be con-
sidered, in the first place, that a motion of rotation round the centre
of gravity will be given to the fish by every impulse, the direction
of which does not pass through that centre The muscular action by
which the tail is first bent from m to R, gives, by the reaction of the fluid,
an impulse in the contrary direction, and hence a rotatory force is
created, which turns the snout round c, towards b ; and the contrary
effect is produced by the opposite motion of the tail from r to l,
whereby the snout is turned back towards a. It is next to be
observed, that the first action of the tail when changing its position
is more energetic than during the remainder of the stroke, when its
velocity gradually diminishes till it is brought to rest, preparatory
to its reversing its motion. As the angle which the tail makes with

SWIMMING OF FISHES.

369

wards in the diagonal c d, intermediate between
the directions of the two forces. By bending the
whole body almost in a circle, and then suddenly
straightening 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 on which progression is to
be effected, we find that every part of the me-
chanism 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

the axis of the fish is, at the same time, continually changing, the
direction of the projectile force changes likewise at every instant.
The combined effect of all these changes, both of
direction and of intensity in the impelling forces,
renders the problem of determining the exact form
of the curve described by the centre of gravity
of the fish during its progression, exceedingly in-
tricate.

Mr. Lubbock, who has pointed out to me the
necessity of taking all these circumstances into
account, conceives that this curve must have some
resemblance to that delineated in Fig. 185*, com-
mencing from G, the position of the centre of
gravity, before the fish begins to move ; receding to
the point marked 1, by the first flexion of the tail
to the right; then advancing to 2, by its return to
the straight position ; again receding to 3, by the
next flexion to the left : and so on, through a series
of points of contrary flexure 1,2, 3, 4, 5, &c. But,
in consequence of the velocity acquired, the motion
will not be exactly in the direction of the residtant
of the pressure ; and the fish will not, in fact, have
resumed the straight or uncurved position at these
points of contrary flexure, but rather sooner.
The theory of Barthez, which has been adopted by subsequent
writers, appears to me to be incorrect.
VOL. I. B B

.370

THE MECHANICAL FUNCTIONS.

which give it the lateral flexions that have been
just described. For this purpose all the 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
prolonged 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 mo-
bility has been given to that organ. The first
peculiarity we meet M-ith in the structure of the
spine of fishes is the mode in which the vertebrae
are connected together. The bodies of each verte-
bra, as may be seen in Figures 186 and 187, are

/ ^^^^
187

hollowed out, both before and behind, (considering
the spinal column as extended 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

SKELETON OF I ISHES. 371

of a double cone, is formed by the junction of the
margins of these conical hollows. These cavities
are distinctly seen laid open in Fig. 188, which
represents a vertical section of three adjacent
vertebrse 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 vertebras upon
one another by the application 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 multi-
plied in the whole series, the effect which results
is considerable. The cavity itself is filled with
a gelatinous, but incompressible fluid substance,
which constitutes a spherical pivot for all the mo-
tions of the joint.

This singular kind of articulation would appear
framed with a view to allow of motion in all direc-
tions. Here, however, the motions are restricted
by the extension of the spinous processes (s, s, in
the preceding figures), which in fishes are of great
length ; so that they effectually prevent all flexions
either upwards or downwards, and limit it to those
from side to side. It is precisely these latter kind
of motions 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 ap-
pearance, (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 num-

.372 THE MECHANICAL FUNCTIONS.

ber of vertebrse is very various in different fishes :
in some they are multiplied exceedingly, as in the
shark, where there are more than two hundred.

In the spine of fishes, we may trace a regular
gradation of developement from the simplest and
almost rudimental condition in which it exists in
the Myxine and the Lampreij, 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 mol-
luscous, 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 framework of the Vermes, that doubts have
often arisen in the minds of naturalists whether
that animal ought not properly to be ranked among
this latter class. Its pretensions to be included
amons the Vertebrata are, indeed, but slender and
equivocal ; 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 cartilagi-
nous substance, exhibiting scarcely any trace of
division into separate rings, but appearing as if it
were formed of a continuous hollow cylinder of
invertebral substance, usurping the place of the
vertebrse, which it is the usual office of that sub-
stance 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 nervous 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 pro-
tection. The nervous matter here consists merely

SKELETON OF IISHES. 373

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 re-
quires minute attention to detect. The partial
protection thus afforded to so important an organ
is not greater than that given by the cartilaginous
lamina of the cuttle-tish, which in form, texture,
and situation is somewhat analogous to the spine
of the myxine.

As we ascend from this rudimental condition of
the spine, we find it, in the lamprey, more distinctly
divided into rounded portions, appearing like beads
strung together. These rudimental bodies of ver-
tebrae have not yet completed the cup-like hollows
on their two ends, but are shaped like rings, being-
perforated in the centre, so as still to form a con-
tinuous canal throughout the whole column.

Proceeding to more advanced developements, we
find, in the Sturgeon and other cartilaginous fishes,
a greater condensation of substance produced 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.* Frequently the sides do
not quite meet, and the leaves, which are developed
from the upper surfaces of the vertebrae, now form
arches over the spinal cord, and are united above
by spinous processes. Yet the whole skeleton in

* A small aperture still remains, establishing a communication
between tbe cavities the whole length of the spine. This is supposed
to be designed to obviate the compression of the fluid in the different
cells or cavities during the motions of the spine. The vertical sec-
tions, Fig. 189 and 190, of two contiguous vertebrae in different
fishes, will convey an idea of this gradation of developement.

374 THE MECHANICAL FUNCTIONS.

these 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 iso-
lated ; affording to the speculative physiologist 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.

It is here, more especially, that we obtain evi-
dence in favour of the theory that the cranial bones
are derived from vertebrae analogous to those of the
spine. The occipital bone, in particular, corre-
sponds to a spinal vertebra in all its essential ele-
ments. In many fishes, the body of this bone,
being lengthened out to form the posterior part of
the basis of the skull, becomes the basilar portion.
We find, on its posterior surface, 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 spinous 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 pro-
cesses of the vertebrae, 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 completely fill this cavity ; for a space is still

SKELETON OF FISHES. 375

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 particular pur-
poses relating to sensation, to the prehension and
deglutition of food, and also to aquatic respiration.

The rest of the skeleton of fishes is extremely
simple. In many, as in the Ray and Tetrodon,
there are no ribs. When these bones exist, they
are articulated with the extremities of the trans-
verse processes of the vertebrae, of which they ap-
pear to be merely continuations, or appendices.
There is generally no sternum to which they can
be attached below : in a liew 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 corre-
spond 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 tolerable pre-
cision the analogous bones composing the anterior
extremities of a quadruped ; namely, the scapula,
clavicle, humerus, ulna, and radius.* These two

* Those anatomists who are fond of pursuing the theory of ana-
logies, maintain that all these bones are merely developements of
certain ribs, proceeding from the spine in its anterior parts. A sinii-

.370

THE MECHANICAL FUNCTIONS.

latter bones are very distinctly marked in the
Lophius piscaforins, or Angler, as may be seen in
Fig. 191, where b is the scapula; c, the clavicle; u,
the ulna ; and it, the radius. The carpus may also

be recognised in a chain of small bones, w, inter-
posed between the radius and the Phalanges, z.
In the Rai/ these phalanges 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
skeleton, and at other times connected with the
spine : in most cases, however, it is suspended from
the cranium ; a fact which has been cited in fur-
ther corroboration of the analogy which the cranial
bones are supposed to have to vertebrae.

lar origin has been assigned to the pieces of bone to which the
ventral fins are attached : but it is difficult to reconcile this 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 analogous to the bones
of the hinder extremities in the mammalia, they must be in a condi-
tion of very imperfect developement.

STRUCTURE OF FISHES. 377

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 struc-
ture is an approach to
that which obtains in
many reptiles, and indi-
cates a further step in
the progress of developement. 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 ho 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 emanci-
pated from the restraints to which they would have
been subjected had they been fixed to a sacrum,
or to any particular part of the spine ; we find
them, accordingly, often placed considerably for-
wards ; and in some instances, as in the Siihbra-
chieni, 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 ves-
tige of ventral fins, nor are any pelvic bones pro-
vided 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

378 THE MECHANICAL FUNCTIONS.

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 sub-
divided 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 admit
of a greater degree of flexibility. These rays as-
sume branched forms from the further subdivision
of their parts ; and when, for the purpose of adding
strength to the fin, it becomes necessary to mul-
tiply the points of support, intermediate bones are
developed, serving as the basis of the rays. Con-
venience 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 iuter-
spinous hones; and when a still greater length of os-
seous 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 bone, and the ray
which terminates it, to a considerable 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 ver-
tical plane, and next to the tail, are, in like manner,
supported by rays, having the same parallel, or
fan-like arrangement 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 animals of
the higher classes. Those which appear imme-

MUSCULAR SYSTEM OF FISHES.

379

diately 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
moving which are seen at c* o is the operculum, or
flap, which covers the gills ; and n, the nasal cavi-
ties, 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 cords is dispensed with.*

The actions of the muscles are easily understood
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 par-
ticipating in this motion. These muscles occupy

* 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 Anthony Carlisle's account of this
structure in the Philosophical Transactions for 1806.

380 THE MECHANICAL FUNCTIONS.

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 velo-
city of action which results from this obliquity has
already been pointed out.

Those fins which are in pairs are capable of four
motions; namely, those of flexion and extension,
and also those of expanding and closing the rays,
for each of M'hich 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
JEsox, 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 in-
strumental in effecting progressive motion. The
great agent by which the fish is impelled forwards
is the tail : the fins, which correspond to the extre-
mities 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,

FINS OF FISHKS. ,381

while the fish darts forwards in its course. The
fins which are in paiis (tliat is the pectoral and the
ventral fins,) by their alternate flexions and exten-
sions, act. like oars ; while they are capable, at the
same time, 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 con-
trary. All these auxiliary instruments are chiefly
serviceable in modifying the direction, and adjust-
ing the variations of force derived from the impulse
of the tail. They are employed also in suddenly
checking or stopping the motion, or 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 off'ence.

Independently of these external instruments of
progression, most fishes are provided with internal
means of changing their situation in the water.
The structure by which this effect is accomplished
is one of the most remarkable instances that is met
with of an express contrivance for a specific pur-
pose, and of the employment 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 mus-
cular exertions were made, it must necessarily de-
scend in that fluid. If, on the contrary, it were
specifically lighter, it would as necessarily rise to
the surface. But if the animal had the power
of altering at pleasure its specific gravity, it
would then possess the means of rising or sinking,
without calling into action either the fins or the

382

THE MECHANICAL FUNCTIONS.

tail. Such is precisely the object of a pecuUar
mechanism, which nature has provided in the
interior of the body of the iish. A large bladder,
tilled 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-bta(hler, or the swimmino-bladfler, and in
the cod-tish it is called the somid. 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 surrounding 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 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 design
than the placing of this hydrostatic apparatus,
acting upon philosophical principles, in the interior

* There is great variety in the form and structure of the air-
bladder in different fishes. Sometimes it contains a large glandular
body of a pecuhar 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.

SWIMMING BLADDER OF FISHES. .*58.3

of the organization, for a purpose so definite and
unequivocal !

In several tribes of fishes there is a canal (c d),
called the ductus pueuntaficus, establishing a com-
munication between this bladder and the stomach,
or the gullet (o); so that by compressing the blad-
der, 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 grovelling at the bottom, lying on its
back, and can never afterwards rise to the surface.
On the other hand, it occasionally happens 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 retained 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 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 fiat shape, forming
the family of Pleurouectes. They are chiefly inha-
bitants of sand-banks, or other situations where
they are comparatively stationary, seldom moving
to a distance, or rising much in the water ; and
when they do so, it is with manifest effort ; for their
asrent must be accomplished entirely by the con-
tinued beating and flapping of the water with their

.384 THE MECHANICAL FUNCTIONS.

expanded pectoral fins. It is only the larger fish
of this form, such as Rays, which have very volu-
minous 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 corresponding
movements of a bird when it rises vertically in the
air. Those fishes which swim rapidly, and fre-
quently 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 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 mus-
cles, 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 toge-

SWIIVmiNG 15 LADDER OF FISHES. 385

ther, like a tesselated pavement, and reminding us
of the arrangements we have seen adopted in the
calcareous coverings of the Echiniclu.

Some of the cartilaginous fishes are, in like
manner, protected by calcareous plates, appended
to the integuments. There is a row of plates of
this kind, of a quadrangular sliape, which pass along
the middle of the back in the Sturgeon : and the
whole body of the Ostraciou is covered with osseous
scales. All these 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.*

* The different forms of the scales atford valuable indications of
the natural families of fishes ; and their importance in this respect
has been ably pointed out by M. Agassiz, who, assuming them as
the basis of his system of classification, divides fishes into the four
following orders.

1. Placoides; so named on account of the irregularity of the
solid parts of their integuments, which consist of masses of bone,
having the character of enamel, sometimes of considerable size ;
but in other cases, being merely small points, as in the rings of
Rays, and the shagrined integuments of Sharks.

2. Ganoides, characterized by the angular form of the scales,
which are composed of two substances, namely, a horny material,
and a superposed layer of enamel. Many families, both recent and
fossil, as the Scleroder/nata, Gymnodonta, Goniodonta, Silures,
and Sturiones, are included in this order.

3. Ctenoides, in which the scales are formed of numerous
plates pectinated on their posterior edge ; the pectinations being
superposed on each other, so that the margin of the one projects
beyond that which is above it ; and thus the whole surface is ren-
dered rough to the touch. This structure is particularly remarkable
in the Chenodonta and Pleuronectes: and is met with also in the
Percoides, Polyacanthes, Scieuoides, Sparoides, Scorpionoides, and
Aulostomata, which, accordingly are referred to this order,

4. Cycloides, where the scales are formed of simple plates, with
a smooth border ; such as the Scomhcroides, Gadoides, Sahnones,
Clupece, and Cyprini.

VOL. I. C C

386 THE MECHANICAL FUNCTIONS.

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 lightest, the body turns over, the
stomach becoming the uppermost part ; and the
fish floats on its back, without having the power of
directing itself during this state of forced disten-
sion. 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 beset, 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.*

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 ra-
pidity as a bird flies through the air.f Although

* See the account given by Lieutenant DaldorfF in the Trans-
actions of the Linnean Society, III. 62. I shall have occasion to
notice, in the 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.

t Lacepede computes the velocity of the Salmon at eight metres
per second, which is about three miles in a minute, or 180 miles an
hour ! Some error must surely have crept into a calculation pro-
ductive of so extraordinary a result.

DIODONS AND TETRODONS. 387

this may partly be accounted for by the size of
their muscles, aad 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 desultory, as is the case with
Eels, and other fishes of the same elongated con-
struction.* Most fishes, however, move with the
utmost rapidity, and with scarcely any visible
effort; and perform long journeys without apparent
fatigue. Sharks often follow ships across the
Atlantic, not only outstripping them in their swift-
est sailing, but playing round them on every side,
just as if the vessels were at rest.

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 progression, and in
the mechanical advantages of their formation. The
greater number are quadrupeds ; 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 ad-

* Carlisle, Phil. Trans, for 1806, p. 9.

3i]H THE MECHANICAL FUNCTIONS.

vancing; 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 occu-
pies a place where he stands alone, being distin-
guished by the exclusive faculty of permanently
sustaining himself on the lower extremities.

In reviewing the developements and the mecha-
nical functions exhibited by so great a diversity of
structures, I shall commence with an examination
of those amphibious reptiles which appear to form
an intermediate link in the chain connecting the
strictly aquatic, with the terrestrial vertebrated ani-
mals: then, taking up this latter series, I shall
consider the more simple conformation and less
perfect motions of terrestrial animals destitute of
limbs ; and gradually ascend to those in which the
support and progression of the body is effected by
extremities, more and more artificially formed :
concluding with the human structure, which termi-
nates this extensive series.

§ 2. Batrachia.

The order of Batrachia, or Amphibious Reptiles,
constitutes the first step in the transition from
aquatic to terrestrial vertebrata. It is more par-
ticularly the function of respiration that requires to
be modified in consequence of the change of ele-
ment in which the animal is to reside ; and as if it
had been necessary, conformably with the laws of
animal creation, that this change should not be

UlIl^TlLIA.

389

abruptly made, we find that Batrachian Reptiles,
with which this series commences, are constructed
at first on the model of fishes ; breathing the atmo-
spheric 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 resemble in every part of their me-
chanical conformation. The tadpole, which is the
young of the frog, is at first scarcely distinguish-
able 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 vertebrae,
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 tadpole
contains within its organization the germs of a

390 . THE MECHANICAL FUNCTIONS.

higher developement. Preparations are silently
making for a change of habitation, for the animal's
emerging from the waters, for the reception of
atmospheric air into new cavities, for the acquisi-
tion of limbs, suited to new modes of progression ;
in a word, for a terrestrial life, and for all the attri-
butes and powers which belong to quadrupeds.
The succession of forms, which these metamor-
phoses present, are in themselves exceedingly
curious, and bear a remarkable analogy to the
progress of the transformations 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 interesting ; and occurring, as we here
tind them, among a tribe of animals allied to the
more perfect 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 appearance 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 surrounding fluid. These, how-
ever, are mere temporary organs, for they serve the
purposes of respiration only until the proper gills
are formed, and they then shrink and become

DEVELOPKMENT OF THE BATKACHIA. . .*39 1

obliterated. The true gills, or hranchice^ are con-
tained 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 constitu-
tion, 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 em-
bryo 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 atmospheric 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 mouth-
ful of air, which is received into its newly de-
veloped lungs, and afterwards discharging it in the
form of a small bubble. When the necessary in-
ternal changes are at length completed, prepara-
tions 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 removed. The gills are
by this lime shrunk, and ra])idly disappear, their

392 . THE MECHANICAL FUNCTIONS.

function being superseded 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, however, 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 derived from a thin extension of the integu-
ments.*

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 circum-
stances of that animal, — this total reversal of its
wants, of its habits, of its functions, and of its very
constitution. 1 shall have occasion to notice seve-
ral of these transitions when reviewing 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 correct idea of these
relations it will be necessary to notice the leading
peculiarities of the skeletons of this tribe of animals.

* Duges distinguishes the following six periods in the life of the
frog, after its exclusion from the egg : the first being characterized
by a lengthened body, and the presence of external temporary
branchiae : the second, by the absence of these branchiae, and by
the globular form of the body : the third, by the developement of
the hind legs : the fourth, by the protrusion of the fore legs, and
the gradual disappearance of the tail: the fifth, by the total loss of
tail and branchiae, the body being as yet but imperfectly developed :
the sixth, and last, by the body and limbs having attained their full
dimensions. — Recherches sur I'Osteologie et la Myologie des Ba-
traciens a leurs diffcrens ages, p. 79.

SKELETON OF THE BATRA(!H1A.

393

Tlie 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 coccygis. It was
evidently the intention of nature to consolidate the
framework of the trunk, in which flexibility was
not required for progressive motion ; the perform-
ance of that function being transferred to the hind
extremities, which are exceedingly large in pro-
portion to the rest of the body. There is a tendency
in every part of the skeleton to develope itself in a
transverse direction, while the trunk is shortened
as much as possible.

The mode in which the vertebrae are articulated
together differs w idely 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 projecting ball, which is

394 THE MECHANICAL FUNCTIONS.

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 watch-
ing the progress of ossification, while this change is
taking place, Dutrochet found that the gelatinous
ball, on which both the adjacent vertebrae play in
the tadpole, becomes gradually more solid, and is
converted into cartilage. This cartilage afterwards
becomes united by its anterior surface to the ver-
tebra 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 con-
tinuing to play within the cup-like hollow of the
vertebra which is behind it. The cartilaginous
coccygeal vertebrae of the tadpole 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 coccygis, which being joined
to the sacrum at an angle, gives rise to the strange
deformity observable 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 portion 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

SKELETON OF THE BATRACHIA. 395

bones is suj^posed to receive 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 vertebrae,
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 on the root of each leaf of the
occipital bone ; w hile 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 some of
the vertebrae : they may be regarded as rudimental
ribs.*

The pelvis consists of two slender and elongated
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 consists of a
single bone only, must be considered as a sacrum.
The two pubic and ischiatic bones are exceedingly
small, but still contribute to form the acetabulum,

* 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.

rJOfi TMF': MKCHANICAL FUNCTIONS.

or cavity for the reception of the thigh bone, at the
liinder extremity of the slender ])ones above men-
tioned. 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 re-
spect a more advanced stage of developement than
that of fishes.

The connexion of the bones of the anterior ex-
tremities 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 is large, and consider-
ably 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
Older of reptiles evidently approximates. There
are five toes in the foot, with sometimes the rudi-
ment of a sixth : the anterior extremity has only
four toes, which are without claws.

The necessity of employing the same instruments
for progression in the water and on land, is pro-
bably 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 conse-
quence, less capable of exerting a force sufficient
to raise and support the weight of the body in
w^alking ; and hence this animal is exceedingly
awkward in its attempt to walk. On a short level

PROGRESSIVE MOTION IN BATRACHIA. 397

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 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 permanent part
of the structure. The rudimental ribs are exceed-
ingly small, and the sternum continues cartila-
ginous. The pelvis has no osseous connexion with
the spine, but is merely suspended to it by liga-

* It is singular that the frog, though so low in the scale of
vertebrated animals, should bear a strikingresemblance to the 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.

.'398 THE MECHANICAL FUNCTIONS.

ments. The land salamanders have a ronnded tail,
but the aquatic species, or Tritons, have it com-
pressed vertically ; thus retaining the fish-like form
of the tadpole, and the same radiated disposition of
the muscles.

§ 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, how-
ever, traced obscure rudiments of pelvic bones in
the Aiiguis fragilis, the Anguis veyitralis, and the
Typhlops crocotatiis, and is of opinion that they
exist much more generally in this order of reptiles,

SERPENTS.

.399

than has been commonly imagined. Some ser-
pents, as the Boa, Python, Tortryx, and Eryx,
have claws, which may be considered as rudi-
ments of feet, visible externally. In others, as the
Anguis, Typhlops, and Amphishcenu, they exist
concealed under the skin. In others, he has dis-
covered cartilaginous filaments, which, he con-
ceives, 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,

* Some of these rudimental parts are repi'esented in the following
figures. Fig. 203 exhibits the claw of the Boa constrictor, placed

203

207

209

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, respec-
tively. Those of the Amphisbcena alba. Fig. 208, and the Coluber
p'dlatus, 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.)

400 THE MECHANICAL FUNCTIONS.

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. The food assigned
to them is living prey, which they must attack and
vanquish before they can convert it into nourish-
ment. 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 defenceless object of its attack,
and twining round its body, so as to compress its
chest, and put a stop to its respiration. Veno-
mous serpents, on the other hand, coil themselves
into the smallest possible space, and suddenly dart-
ing 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 prehen-
sion and of progressive motion, it is necessary 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 num-
ber of small pieces; secondly, by the great freedom
of their articulations ; and thirdly, by the peculiar
mobility and connexions of the ribs.

Numerous as are the vertebras 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 {Crotaliis
horridus) there are about two hundred vertebrae ;
and above three hundred have been counted in

SERPENTS.

401

the spine of the Coluber natrix. These vertebrae

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
Soa cojistrictor, 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 ef-
fectually 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 its
assuming every degree and variety of curvature.
The presence of a sternum, restraining the motions
of the ribs, would have impeded all these move-
ments, and would have also been an insurmountable
bar to the dilatation of the stomach, which is ren-

VOL. I. D D

402 THE MECHANICAL FUNCTIONS.

tiered 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 back-
wards, 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 ordi-
nary mode of articulation would not have afforded.
The spinous processes, 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 hmit 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 ca-
pable, it will, at first view, be difficult to conceive
how these simple actions can be rendered subser-
vient to the purposes of progression on land ; and
yet experience teaches us that few animals advance
with more celerity on the surface 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 obser-
vation, 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

SERPENTS. 403

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 motion. In some
species the integument is formed into annular
plates, reminding us of the structures so prevalent
among worms and myriapode animals. Each scale
is connected with a particular set of muscular
fibres, capable of raising or depressing it, so that
in this way it is converted into a kind of toe ; and
thus the body rests on the ground by numerous
fixed points of support.

This support is farther strengthened by the con-
nexion 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 instruments of
progressive motion was first noticed by 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 suc-
cession, like the feet of a caterpillar. Sir Everard
Home, to whom Sir Joseph Banks pointed out this
circumstance, 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 peculiar, and
has evidently been employed with reference to this
particular function of the ribs, which here stand in
place of the anterior and posterior extremities, pos-

* Philos. Trans, for 1812, p. 163.

404 THE MECHANICAL FUNCTIONS.

sessed by most vertebrated animals, and charac-
terising the type of their osseous fabric. In the
ordinary structure, the head of each rib has a
convex surface, which plays either upon the body
of a single vertebra witli 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
upon a convex protuberance of the vertebra. This
structure is attended witli the advantage of pre-
venting the ribs from interfering with the motions
of the vertebrae upon one another. At 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 Ophio-
saurus 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, on the upper surface of one of
the scuta, or broad scales on the lower side of the
body.. These scuta, which are thus connected with
the ends of the ribs, and which are moved by means
of short muscles, may be compared to hoofs, while
the ribs themselves may be considered as perform-
ing 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

PROGRESSIVE MOTION IN SERPENTS. 405

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 for-
wards, by which, as levers, they effect this species
of progression. These muscles are disposed in re-
gular laj^ers ; 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 projected forwards. By a suc-
cession of these alternate movements, assisted by
the actions of the ribs, serpents are enabled to glide
onwards with considerable rapidity, and without
attracting observation. But where greater expe-
dition is necessary, they employ a more hurried
kind of pace, although one which exposes them
more to immediate 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 body being

406 THE MECHANICAL FUNCTIONS.

propelled by bringing it from a curved to a straight
line.

There is yet a third kind of motion, which ser-
pents occasionally resort to when springing upon
their prey, or when desirous of making a sudden
escape from danger. They coil themselves into a
spiral, by contracting all the muscles on one side
of the body, and then, suddenly throwing into vio-
lent 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 sub-
stitution of a different kind of mechanism, 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 rapidly across the plain, and of
executing leaps with a vigour and agility which
astonish the beholder, and which, in ages of igno-
rance and superstition, were easily ascribed to super-
natural agency.

§ 4. Sauria.

The instruments of progressive motion are more
efficient in the class of Saurian reptiles, which in-
cludes all the Lizard tribes, than in the Batrachia.
Several links of connexion with the preceding class
may still be noticed, marking the progress of de-
velopement, as we follow the ascending series of
animals. Rudiments of the bones of the extre-

SAURIAN REPTILES. 407

niities, and also of the sternum, make their appear-
ance 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 Laccrta lumhricoldes of Linnaeus, or
the JBipes cmialiculntus 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 Lacerla
bipes (Linn.), or Sheltopusic of Pallas, has, on the
other hand, a pair of hind feet only, but extremely
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 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 considerable
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 distinguish-
able from the abdomen ; and a distinct sacrum is
interposed between the lumbar and the caudal
vertebrae.

A further approach to the higher classes is ob-
servable in the number of cervical vertebra?, which

408 THE MECHANICAL FUNCTIONS.

is almost constantly seven ; as we shall find it to
be in the Mammalia. The articulations 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
structure in the Batrachia. The caudal vertebrae
are always numerous ; and the tail is compressed
verticall}^ which is the form most favourable for
progression in water. They are remarkable also
for having inferior spinous processes attached to
the bodies by cartilages; a structure analogous to
that which we have seen in fishes.

The number of ribs differs in difierent species of
Sauria : they are always articulated to the extre-
mities of the transverse processes of the vertebrae,
of which they appear to be continuations. Pro-
cesses 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. They present impediments to the flexions of
the neck ; whence arises the difficulty which the
Crocodile has in bending the neck while turning
round upon the animal he is pursuing. In the
thorax, the ribs are connected with a broad ster-
num ; but there are other ribs, both before and
behind, which have no such termination, and there-
fore 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

SAURIAN REPTILES. 40.9

and slender bones extend forwards from the pubic
bones, on the under side of the body, apparently
for the purpose of supporting the abdominal vis-
cera.* The bones of the extremities are very per-
fectly formed, approaching in their shape and ar-
rangement very nearly to the corresponding parts
of the skeleton of the higher orders of quadrupeds.
The toes are usually provided with membranes
spread between 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 mus-
cular power is bestowed upon it as an instrument
of aquatic progression, producing, by its lateral
flexions, a horizontal movement of the body. Cro-
codiles 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. 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

* They appear to be analogous to the marsupial bones peculiar to
a family of Mammalia, comprising the Ojwssum, Kanguroo, Pha-
lanijer, Koala, Phusculome, &c.

410

THE MECHANICAL FUNCTIONS.

toe, (represented in Fig. 211), there are as many as
sixteen transverse slits, leading to the same number
of cavities, or sacs : these open forwards, and their

external edge is serrated,
212 211 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 ten-
dons of tliese 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 accurately
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 of the Remora.^
By its means the animal is enabled to walk securely
on 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 on the suspension derived from a suc-
cession of rapid and momentary adhesions.

Although the Sauria are better formed for pro-

Philosophical Transactions for 1816, p. 151, and 323.

CHELONIAN REPTILES. 411

gressive motion than any of the other orders of
reptiles, yet the greater shortness and oblique posi-
tion of their limbs, compared with those of mam-
miferous quadrupeds, oblige them in general 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 Chameleon walks with great slowness,
and apparent difficulty ; and we have seen that,
in consequence of the structure of the bones of its
neck, the Crocodile, though capable of swift motion
in a straight line, is unable to turn itself round
quickly. The general type of these reptiles, having
reference to an amphibious life, has not attained
that exclusive adaptation to a terrestrial existence,
which we find in the higher orders of the Mammalia.
But before proceeding to consider these, we have
to notice a singular group of animals, whose con-
formation appears to be exceedingly anomalous,
and as if it interrupted the regularity of the as-
cending series, of which it seems to be a collateral
ramification.

1^ 5. Chelonia.

The order of Chelotiian Reptiles, which comprises
all the tribes of Tortoises and Turtles, appears to
constitute an exception to the general laws of con-
formation which prevail among Vertebrated Ani-
mals : for instead of presenting a skeleton wholly

412 THE MECHANICAL FUNCTIONS.

internal, the trunk of the body is found to be en-
closed 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 de-
fence provided for animals very low in the scale of
organization, such as the Echinus, the Crustacea,
and the bivalve Mollusca. Yet the substance
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 formation of the Che-
Ionia is security ; and for the attainment of this
object she has constructed a vaulted and impene-
trable roof, capable of resisting enormous pressures
from without, and proof against any ordinary mea-
sures 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 remark-
able inversion they appear to have undergone,
when compared with the usual arrangement. 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

CHELONIAN REPTILES.

41.'{

parts is really formed by an extension of the
spinous processes of the vertebrae 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-costal appendices, which
become ossified. Thus no new element has been
created ; but advantage has been taken of those

already existing in the general type of the verte-
brata, to modify their forms, by giving them dif-
ferent degrees of relative developement, and con-
verting them, by these transformations, into a me-
chanism of a very different kind, and subservient
to other objects than those to which they are
usually applied.

The first step taken to secure the relative immo-

414 THE MECHANICAL FUNCTIONS.

bility 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 together, 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 pre-
served, as usual, above the bodies of these coalesced
vertebrae, and is formed by their united leaves ;
the arches being completed by the spinous pro-
cesses. 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 cara-
pace. 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
another along their whole length, are inserted
below, between the bodies of every two adjoin-
ing vertebrae ; 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
designed to be very moveable, we find them at-
tached 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 between the
bodies of two vertebrae. 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,

CHELONIAN REPTILES. 415

indeed, that a great number of the peculiarities
which distinguish the conformation 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, cartila-
ginous ; though occasionally we find them partially
ossified. In the tortoise, however, their ossification
is not only complete, but has been expanded la-
terally, 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, oblite-
rated ; and the bony fibres are continuous through-
out 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 plas-
tron, 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 recognise the
nine elements of which the sternum is supposed to

410 THE MECHANICAL FUNCTIONS.

be formed; 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, in-
deed, they are widely different from the same parts
as they are presented in the skeletons of other ani-
mals ; yet in number and in relative situations they
preserve that constancy and uniformity so charac-
teristic of the harmony which pervades all animal
structures.

It is to be noticed, also, that as the plates, which
form this investing case, are bony structures, 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, origin-
ally a production of the integument. 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 vertebrae are joined by the ball and socket
articulation common to all the existing species of
reptiles. t The articulation of the head with the
neck is effected in the same manner ; but it is

* 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 respect-
ing the form of the latter from the mere inspection 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 Ichthyosaurus.

CHELONIAN HEFTILES.

417

interesting to remark that tlie occipital condyle,
which is situated at the lower margin of the great
aperture, though presenting a single convex surface,

yet has that surface evidently
divided into three parts ; the
two upper portions being la-
teral, and the lower portion
in the middle. These three
articular surfaces are seen im-
mediately below the central
aperture, f, in Fig, 215, which exhibits the skull of
the Testudo mydds, viewed from behind. Although
closely approximated, a faint line of denmrcation,
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 obliteration of the lower articular surface,
and the transfer of the two lateral surfaces to the
condyloid processes, arising from the developement
of the leaves of the occipital bone.

The singular conformation of the bones of the
head in the turtle has been considered as affording
additional evidence in support of the theory that
these bones were originally vertebrae. The brain
of this animal is exceedingly small ; and yet the
skull, when viewed from above, presents an appear-
ance of great breadth, as if it enclosed a cavity of
large dimensions. But if we look upon it from
behind, 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

VOL. I. E E

418 THE MECHANICAL FUNCTIONS.

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 tendency to lateral expansion, which
exists in the spinous processes of the dorsal verte-
brae, prevails also among those which contribute to
form the skull. The parietal bones, which repre-
sent 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 develope-
ment, forming first a crest on the upper part of the
real cranium, and then separating to the right and
left, and expanding horizontally into the upper roof
(p, p) already mentioned, 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 en-
titled. 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 suffi-
ciently capacious to give shelter to the head when-
ever occasion may require. The analogy of the
spine of the occipital bone with that of a vertebra,
is confirmed 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 pro-
cesses and ribs in the Chelonia gives rise to a sin-

CHELONIA^ REPTILES. 419

gular inversion in the position of the scapuhx ; for
it is here placed on the inside of the ribs and
sternum ; that is, between the carapace and plas-
tron.* The humerus is remarkably curved, espe-
cially 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 compressed kind of
hand.

The pelvis, like the scapula and clavicle, is en-
closed within the bony shell which protects the
trunk. The sacrum is moveable upon the last
dorsal vertebra ; and the coccygeal vertebrEE 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.'l" 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 feet. The impulse which they give,
being lateral and oblique, renders them more effi-
cacious for progression in the water than on land :
this circumstance, in conjunction with the consti-
tutional torpor of the animal, sufficiently accounts

* The anomalous situation of these bones, and the strangely dis-
guised forms which their several parts assume, render it very diffi-
cult to recognise in the skeleton the several pieces which correspond
to the normal type of the scapula, acromion, coracoid bone, and
clavicle ; and anatomists are not yet agreed as to the proper desig-
nations vvhich are applicable to these bones in the Chelonia.

t The cylindrical bones of the tortoise are solid throughout, and
have no cavity for containing marrow, as in the more highly de-
veloped bones of the mammalia. This is seen in the section of the
femur. Fig. 214.

420 THE MECHANTCAL FUNCTIONS.

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 ske-
leton. 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 ex-
ceedingly flattened, and approximate 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 hu-
merus is of great advantage to the tortoise in assist-
ing it to turn itself, when, by any accident, it has
been laid on its back.

Considerable difterences may be noticed in the
structure of the several species of Chelonia, accord-
ing to the diversity of their habits. Tortoises, which
live on land, require more complete protection by
means of their shell than turtles, or Emydcs, which
dwell in the water; hence the convexity of their
carapace, the solidity of its ossification, its im-
moveable connexion with the plastron, and the
complete shelter it aftbrds to the head and limbs.
Turtles, on the other hand, receiving support from
the element in M^hich they reside, require less
provision to be made for these objects. Their

CHELONIAN REPTILES. 421

carapace is smaller, has a more flattened form,
and cannot afford protection to the head and limbs.
These latter organs are proportionally larger, pre-
sent a greater developement of the radius and ulna,
and are compressed into a flat expanded surface.
Previously to the retraction of the head and limbs
M'ithin 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 vertebrata, 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 continued 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, se-
parate from each other, and resume their usual
form ; the intervening spaces between two adjacent
ribs being here filled up by membrane. The plas-
tron is united with the carapace by membrane
likewise; and the sternum, instead of forming one
broad plate of bone, has the intervals between its
imperfectly developed elements 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 arc bony in tlie

422 THE MECHANICAL TUNCTIONS.

tortoise are replaced by simple cartilage or mem-
brane.

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 pro-
vision made for diminishing the specific gravity of
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 fulfil-
ment of one condition, we are certain to meet
with a compensation in the structure of some
other part, and in the 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 sur-
face 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 instance 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.

42:?

Chapter IX.

MAMMALIA.

§ 1 . Mammalia m 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 verte-
brated animals which dwell in the water, to those
which inhabit the land. The class of 3Iammifera^
or Mammalia, comprehends all the animals which
possess a spinal column, breathe air by means of
hings, and are also warm-blooded and viviparous ;
conditions which render it necessary that they
should possess organs, called mammcB, endowed
with the power of preparing milk for the nourish-
ment of their young ; a peculiarity from which the
name of the class is derived. But they are not ex-
clusively land animals ; for among the mammalia
must be ranked several amphibious and aquatic
tribes, such as the Seal, the Walrus, the Porpus,
the Dolphin, the Narwhal, the Cachalot, and the
Whale ; animals which, however widely they differ
in their habits and external conformation from ter-
restrial quadrupeds, possess, in common with the
liitter, all the essential characters of internal struc-
ture and of functions above enumerated. These
characters belong also to the human species, which
must consequently, in its zoological relations, be

4*24 THE MECHANICAL FUNCTIONS.

ranked as n genua of the class mammalia. So
numerous, indeed, are the analogies which connect
the natural families of this class with our own race,
that we must ever feel a deep interest in the accu-
rate 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 en-
larged our knowledge of the structm-e, 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 gene-
rally in the inferior departments of the animal crea-
tion, 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 exhibit more variety in
their habits and modes of existence, than the one
we are now examining. 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, dis-
playing the savage energies of their nature, in
practising all the arts of ferocious and sanguinary
destruction. Others, intent 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

MAMMALIA. 425

like manner, a great diversity of construction,
adapted to the particular nature of that subsistence,
whether it be herbage, or the leaves of trees, or
fruits, or seeds, or the coarse tibres 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 pro-
vided with instruments of defence, or with means
of flight. Each has its respective sphere of opera-
tion ; and to each its appropriate soil, habitation,
climate, and element have been assigned.

It is easy to conceive that all these various cir-
cumstances must lead to great diversities in the
apparatus for mastication and for digestion, in the
organization of the senses, in the construction 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 multiform 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 car-
nivorous or the herbivorous quadruped, in the inha-
bitant of the land or of the water, of the frigid or
of the torrid zone, or in animals of the most dimi-
nutive 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

426 THE MECHANICAL FUNCTIONS.

seven vertebrsp. Whatever be the length or short-
ness of the neck, whether it be compressed into a
small space, as in the Elephant and the 31ole,
whether it be lengthened to allow the head to
reach the ground, as in the Horse and the Ox, or
whether it be excessively prolonged, to allow the
animal to reach the tops of trees, as in the Carne-
lopard, still this same constant number is preserved
in the vertebrae which it contains. When the neck,
is long, each individual 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 pro-
cesses, 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, hov/ever, retains the marks of having
originally consisted of separate vertebrae ; and still,
in this extreme case, the number of primary pieces
is constantly seven. t

* In the Cachalot, the whole of these seven vertebree are usually
anchylosed into one bone.

t The Brady pus 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 vertebrae. 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

CETACEA. 427

§ 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 earl 3^,
or rudimental stage of developement. Here, as
before, we have to seek these first elements 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 develope-
ment proceeding with more rapidity, and raising
structures of greater dimensions in aquatic than in

also from their having small bony appendices, articulated with them
by a regular joint at their extremities, and corresponding exactly,
both in shape and situation, to the ribs, of which they may, in fact,
be considered as 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 contirmed the observations of these
anatomists, but has farther discovered, that similar rudimental ribs
are attached also to the eighth vertebra. Trans. Zool. Soc. i. 113.
The Bradypus torquatus, which has been said to possess eight cer-
vical vertebrse, 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 vertebrse. Instances
have occurred of supernumerary cervical processes, or ribs in the
human skeleton. (See Edinburgh Medical and Surgical Journal,
xl. 304.) De Blaiaville, however, maintains that these animals
present real exceptions to the rule. See Comptes Rendus, ix. 762.

428 THE MECHANICAL FUNCTIONS.

terrestrial animals. The order Cetacea comprises
by far the largest 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 convidsions
of a former world, certain it is that, at the present
day, the Whales of the northern seas are the most
colossal of the living animal structures existing on
the surface of this planet.

A cursory survey of the organization of the tribes
belonging to this semi-amphibious family, will im-
press 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
nnited with the head without an intervening neck ;
the same fin-like shape of the external instruments
of motion; and the same enormous expansion and
prolongation of the tail, vv^hich is here also, as in
fishes, the chief agent in progression. With all
this agreement in external characters, their internal
economy is conducted on a totally different plan ;
for although constantly inhabiting the ocean, their
vital organs 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 respi-
ration compels them to rise, at short intervals, to
the snrface of the water ; and this air, with which
they fill their lungs in respiration, gives their
bodies the buoyant force which is required to facili-
tate their ascent, and supersedes the necessity of

CETACEA. 429

a s\\ iimiiiiig-biadder, an organ wliirh 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 of the spi-
racle, or orifice of the nostrils, which is placed
there, to the surface of the water.*

Another peculiarity of conformation, in which
the cetacea differ from fishes, and which has also
an obvious relation to their peculiar mode of breath-
ing, 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 spinal
column, and its immediate dependencies, the 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 substance called Spermaceti is lodged in cells, formed of
a cartilaginous material, situated on the upper part of the head of
the Cacholof.

4.30 THE MECHANICAL FUNCTIONS.

the object of the conformation is here, as in 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 attachment to these muscles, the trans-
verse processes of the dorsal and lumbar vertebrae
are expanded both in length and breadth ; and
being situated horizontally, they ofter no impedi-
ment 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 extremities,
so there is no enlargement of any of the vertebrae
corresponding to a sacrum : and the caudal verte-
brae are uninterrupted continuations 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, enclosing and giving protection to a
large artery. Although the bones of the legs do
not exist, 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, apparently corresponding to pelvic
bones, and giving a firm attachment to certain
muscles.

A similar adherence to the law of uniformity in
the plan of construction of all the animals belonging

* These leaves being formed of cartilage, are generally lost
when the bones are macerated for the purpose of preparing the
skeleton.

CETACKA.

431

to the same class, is strikingly shown in the con-
formation of the bones of the
anterior extremities of the Ce-
tacea ; for although they present,
externally, no resemblance to the
leg and foot of a quadruped, be-
ing fashioned into fin-like mem-
bers, with a flat oval surface for
I striking the water, yet when the
bones are stripped of the thick
integument which covers them,
and conceals their real form, we
find them (as may be seen in
Fig. 210) exhibiting the same
divisions into carpal and meta-
carpal 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.

§ 3. Amphibia.

In the small tribe, denominated by Cuvier Am-
phibia, and consisting of the Phoca, or Seal, and
the Trichecus, or Walrus, we perceive that an
advance is made towards a fuller developement 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

432 THE I\IC( HANICAL FUNCTIONS.

depressed, they perform tlie same olFice as the tail
of the cetacea, and propel the animal forwards:
but when employed separately, they are more
qualified to act as oars. The Walrus has feet still
more developed, and distinctly divided into toes,
which are disposed so as to strike backwards
against the water.

1^ 4. Mammiferous Quadrupeds in general.

From the imperfectly developed aquatic and am-
phibious tribes we gradually ascend to the more
finished structures of mammiferous quadrupeds,
which are expressly fitted for progression on land.
In these the powers of developement, 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 concen-
trating their energies, so as ultimately to attain the
extent of power and harmony of action, which are
displajed in the higher orders of warm-blooded
quadrupeds.

It is to these favoured tribes that we must look
for examples of the most complete developement
of the skeleton, and the most advantageous dispo-
sition of mechanic force. We have seen that rep-
tiles, from the comparative shortness of their limbs,
and the torpidity of their muscular powers, are but
ill adapted for rapid progression. All the more
pei'fectly formed quadrupeds of the class mam-
malia, having the trunk of the body raised high

MAMMIFEROUS QUADRUPEDS. 4.33

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, whicli
they must do by acting in opposition to the force of
gravity; and they must, secondly, give the body
an impulse forwards. Let us consider more par-
ticularly the relations which the structures bear to
each of these two functions.

The limbs of quadrupeds constitute four columns
of support to the trunk, which is placed horizon-
tally 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 extre-
mities 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 be-
cause 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 support a weight is the
greater in proportion as it approaches to the per-
pendicular 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 extremity ; and, consequently, if they had
been disposed 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 connects the spine

VOL. I. F F

434 THE MECHANICAL FUNCTIONS.

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 must be turned forwards
in its whole length from the heel to the extremi-
ties of the toes. On comparing the positions of
the corresponding divisions of the anterior and
posterior extremities, we observe that they incline,
when bent, in opposite directions ; for in the
former we find, in following 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 449.)
The weight of the body, in consequence of this
alternate direction of the angles at the successive
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 ex-
tensors 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 su-
perincumbent 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

MAMMIFEROUS QUADRUPEDS. 435

power than those of the hinder legs to retain them
in their proper relative positions.

The most complete instance of a vertical ar-
rangement of the bones of the extremities is seen
in the Elephant ; where in order to sustain the
enormous w^eight of the body, the limbs are shaped
into four massive columns, of which the several
bones are disposed nearly in perpendicular lines.
By this means the body is supported 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
disorder, 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 perpendicularly 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
quadrupeds 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 on the ground.

The conformation of the hind extremities, which,
as we have seen, is not so well calculated for the
simple support of the trunk, is, on the other hand,
better adapted to give it those impulses which are

436 THE MECHANICAL FUNCTIONS.

to effect its progressive movements. The nature of
those movements, and the order in which they suc-
ceed each other, are different according to the
peculiar mode of progression which the animal
practises, the degree of speed it is desirous of ex-
erting, and the particular end it has in view. The
paces of a quadruped 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 quadran-
gular base formed by the feet while they rest on
the ground, the first effort to advance which the
quadruped makes, propels the centre of gravity
forwards. This it accomplishes by pressing one of
its hind legs against the ground ; which leg, being
thus fixed by the resistance it there meets with,
becomes the fulcrum 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 hind foot is
raised from the ground, brought forwards, and laid
down close to the fore foot of the same side. This
fore 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 sup-
port, the stability of the body is constantly pre-

PROGRESSIVE MOTION IN QU.VDUUPEDS. 437

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 ver-
tical 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 re-
mains 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 tri-
angle, and the body is consequently still supported
on these three feet. The fore foot may now be ad-
vanced, without endangering the stability of the
body ; and by the time this foot is set down, and
has thereby formed a new quadrilateral base 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 for-
wards by the other hind foot, which now comes
into action, and repeats on the other side the same
succession of actions, which are attended 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

4.*{8 THE IMECHANICAL FUNCTIONS.

be raised from the ground, the other three feet are
always so placed as to form a stable base of
support.

In quick walking, it often happens that quadru-
peds 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 advance 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 considered as a succession of
short leaps made by each set of feet taken dia-
gonally ; 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 por-
tion of the time, the body is resting upon the two
feet placed diagonally with regard to each other.
The undulations 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

PROGRESSIVE MOTION IN QUADRUPEDS. 439

together ; so that the impulsions given are directed
much more laterally than in any other pace, and
the body is thrown into a strong undulatory motion
from side to side.

Another kind of pace is the bound, which is
often practised by deer, and is performed by strik-
ing 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 uni-
formly 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 execute
progressive movements, by the variations she has
introduced into the comparative lengths of the
several parts of the trunk, and the size and mobi-
lity of the extremities. Of all the large animals,
the Lion has been constructed with the finest pro-
portions 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 exhibits strongly the
lateral undulations of the trunk.

Quadrupeds having a very long, or a very mas-
sive body, or whose limbs are short and nearly of
equal height, are incapable of advancing 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 walking pace. It excels princi-
pally in the vigour and extent of its bounds ; for
which it is admirably qualified by the prodigious
power of its muscles, enabling it to spring forwards

440 THE MECHANICAL FUNCTIONS.

upon its victim wdth 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 extensors of
the hind extremities, than on the form of the body.
Great length and muscularity in the hind legs are
generally attended with considerable power of leap-
ing. This is exemplified in the Jerboa and the Kan-
guroo, animals, which, from the disproportionate
shortness of their fore legs, are totally incapacitated
from walking ; and for the same reason, they can-
not 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 ani-
mals are capable, for a long continuance, of taking
leaps of nine feet distance, 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 con-
sidered 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 jfiTareand the Rabbit furnish other instances
of an extraordinary length of the hinder legs de-
priving the animal of the power of walking, and

PROGRESSIVE MOTION IN QUADRUPEDS. 441

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 hops or gallops w ith
the hinder ; but this disadvantage is amply com-
pensated 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, otherwise they would
be in danger of oversetting, as happens occasionally
to the Agouti and the Guitiea 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 muscular powers
are probably consequences of the singular mode in
which, as I shall afterwards have occasion to notice,
its arteries are distributed.

The Camelopard, likewise, has the fore legs much
longer than the hinder. The object of this con-
formation 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 considerable inclination
to the whole column, for the purpose of distributing
more equally the weight 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

44*2 THE MECHANICAL FUNCTIONS.

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 modifi-
cations in the mode of progression of the animal.
The ordinary pace of the camelopard is the amble;
but it has also a slower walking pace, and occasion-
ally a gallop : in performing which last movement,
the hind feet are brought more forwards than the
fore feet, so that at that moment the fore and hind
limbs cross one another like the strokes of the
letter X. In the amble, its undulation is so con-
siderable 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. Rurrmiantia.

In following the series of Mammalia in the order
which best exhibits their successive stages of de-
velopement, I shall commence with those whose
digestive 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 Rummants,
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, constitute the peaceful

RUMINANT QUADRUPEDS. 443

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 constitute the limbs,
is adapted. The anterior extremities having only
to support the weight of the fore part of the trunk,
and to assist in progressive motion, have a less
complicated arrangement of joints, and exhibit
many of those consolidations of the bones, which
tend to simplify the structure, and contribute to its
strength .

But though never incited by the calls of appetite
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 Antelope, and the Deer, animals which
exhibit the 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 animals than
in other tribes of mammalia, so that they are always
ready for instantly commencing their flight, and
springing forwards on the slightest notice of danger.
(See Fig. 218, page 449.)

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

444

THE MECHANICAL FUNCTIONS.

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 sufficient sur-
face 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 ligament is employed to
sustain the weight of the head. This, which is
termed the ligamentum nuclue, and is represented
at N, in Fig. 217, is formed of a great number of

bands, which connect the hinder part of the cra-
nium, at the ridge of the occipital bone, and all the
spinous processes of the neck, with those of the
back ; the separate slips from each being succes-
sively joined 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 considerable as-
sistance to the muscles in raisino^ it. In the Deer

RUMINANT QUADRUPEDS. 445

and tlie 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 particularly 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 compressed
laterally, in order to allow room for the unrestrained
motions of the anterior extremity ; and the sternum
projects so as to resemble the keel of a ship. The
bones of the anterior extremity are not joined to
the rest of the skeleton by means of any bone cor-
responding 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 down-
wards, and has a deep hemispherical cavity for the
lodgement of the round head of the thigh bone.
The lengthened 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 aj)pear, on a superficial view, to

446 THi: MECHANICAL FUNCTIONS.

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 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 distance from
the top, where a separation may still be observed
in the processes which project from that end, form-
ing 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
separate.

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 con-
stituting 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 carnivorous mammalia, but
of a single bone only, termed the ccumon hone. In

RUMINANT QUADRUPEDS. 447

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
ossific deposition, which fills up the interval be-
tween them, and leaves behind no trace of suture.*
In proportion 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 inspection 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 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

* The observations which establish this fact are detailed by G.
St. Hilaire, in a paper in the " Memoires du Museum," x. 173.

448 THE MECHANICAL FUNCTIONS.

situation to the first phalanges of the fingers and
toes. These are followed by a second and third set
of phalanges ; the last of which terminate in hoofs.
All ruminant 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 con-
ditions of their existence, while she has still pre-
served their relations to the primitive type of the
class to which they belong. Thus has she be-
stowed upon them the slender and elegant forms,
so pleasing to the eye, which characterize the
fleetest races, and has provided for the agile, yet
firm and secure movements w hich they are to exer'
cise in various ways in eluding the observation, 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 retirement into unfre-
quented forests, or mountains of difficult access,
crossing their rugged surfaces in all directions,
clambering their precipitous acclivities, and fear-
lessly bounding over intervening abysses, from
point to point, till the place of safety is attained on
some rocky eminence. From this secure station
the Alpine Chamois looks down upon its pursuers,
and defies their further efforts at capture or moles-
tation. The astonishing feats of agility practised
by this animal, and by which the most experienced
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 per-

RUMINANT QUADRUPEDS.

449

pendicular precipice of twenty or thirty feet in
height, without sustaining the slightest injury.
How the ligaments that bind the joints can resist

VOL. I.

G G

450 THE MECHANK AL FUNCTIONS.

the violent strains and concussions 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 formidable
enemies, she has not left them altogether 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 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 weapons 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 that
was lost. Previously to the formation of this struc-
ture, those branches of the artery, termed the caro-
tid, 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 accompa-
nied by general heat and redness, like a part in a

RUMINANT QUADUUl'EIXS. 451

state of high inflammation.* Presently the skin is
elevated by the growth of a tubercle from the sub-
jacent bone: this tubercle is at first a cartilage,
and after it has attained a certain size, becomes
ossified, and grows like other osseous structures,
first shooting into the form of a lengthened cylinder,
and then dividing into branches. 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 deli-
cate investment of hair, its velvet coat. The blood-
vessels of the proper membrane of the antler, or
periosteum, still continuing to supply it with the
materials required for its growth and consolidation,
deposit so great an abundance of bony matter, that
its enlargement 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 kingdom 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 depo-
sition of osseous substance still continues at its base,
around the trunks of the arteries which are pro-
ceeding along the investing membrane of the bone
for the purpose of conveying nourishment. The
accumulation of this substance raises a ring, called
the buri\ round that part of the antler; and by en-
croaching on the arteries themselves, it gradually
diminishes their capacity of conveying blood, and

* These phenomena are connected with periodical changes in the
constitution relating; to the reproductive functions.

4')'2 THK MECHANICAL FUNCTIONS.

they at lengtli become entirely obliterated. The
bone, no longer receiving a superabundant nourish-
ment, ceases to grow ; the integuments, which co-
vered it, decay, and becoming dry and shrivelled,
are torn by 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-
cientl}^ nourished by its own interior vessels, con-
tinues in a living state, and preserves its connexion
with the system. But at length the arteries, whe-
ther from the effect of the progressive 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 actively employed in scooping out a groove
of separation 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 surroimding 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 phe-
nomena in its growth and decay as its predecessor,
only that its developement is usually carried far-

RUMINANT QUADRUPEDS. 453

tlier; 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 extre-
mities of the branches present broad expansions of
bone, which the antlers of an earlier growth had
not exhibited.

The short bony processes which extend in a per-
pendicular direction on the head of the Camelopard,
are analogous, in some of the circumstances of their
formation, to the antlers of the deer, being of an
osseous nature, and continuous 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
integuments, which retain, at the extremities, a
tuft of hair. The developemeut 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, on attaining which the growth is ar-
rested, and never proceeds farther. The arteries
cease to deposit superabundant nourishment, but
continue to maintain an exact equilibrium between
the expenditure and the supply ; so that the horns
of the camelopard are never shed, and remain per-
manent bony structures.

A further modification of this process occurs 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 tubercles may be seen
in the young calf, proceeding from the bones of

454 THi: IMJX'HAMCAL FUNCTIONS.

the forelicad : the skin covering these tubercles,
unlike that which precedes 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
protection to the subjacent bony layers which are
forming underneath it. The highly vascular mem-
brane, from which these new structures chiefly
arise, appears to have different powers of produc-
tion at its two surfaces ; for while the inner sur-
face is forming the osseous portion of the horn, and
supplying the phosphate of lime required for the
construction of its plates and fibres, the exterior
surface is adding successive layers of horny sub-
stance to the inner side of those portions which
had been before deposited. These two operations,
Mhich 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 is an organic
structure, which continues to be nourished by ves-
sels forming part of the general system : the horn
is a mere excretion, which appears to be destitute
of vessels, and is, consequently, removed from the
influence of the living powers. Thus the growth of
horn is somewhat analagous to that of shell ; for the
layers which compose it are deposited in succes-
sion ; each new layer is agglutinated to the inner
surface of the preceding ; and each has the shape
of a hollow cone, occupying the part towards the
apex of the former cone, and extending farther to-

RUMINANT QUADRUPEDS.

455

M'ards 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 ex-
tremity 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 hap-
pens in that of shells, there sometimes occur irre-
gularities, or periodical intermissions and increase
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 re-
gularity as the tiles of a house. The tendency in
these horns to assume a spiral form is explicable

456 THE mkchanu:al functions.

on the same principles as those which regnhite 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 com-
pacted together, and exhibit 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 integu-
ment covering the nose, to which they adhere
without having any connexion with the subjacent
bones. They have a pyramidal shape, and are
composed of parallel fibres, resembling hairs,
agglutinated together into a solid mass by a ma-
terial which acts as a cement. This fibrous struc-
ture 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 magnify-
ing glass, a great number of orifices are seen,
marking the empty spaces that intervene between
the hairs ; and if the section be made in a longitu-
dinal 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 quadrupeds,
including the Horse, the Ass, the Qmigga, the
Zebra, &c. which are very nearly allied in their
conformation to the ruminant tribe. To combine

MAMMALIA SOLU'EDA. 457

fltetiiess 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
l)halanges, which are in the latter articulated with
the cannon bone, there is here only a single meta-
carpal bone. The three phalanges, of which that
single finger consists, bear the names of the pastern,
the coronet, and the cojfiu bone; and the hoof, of
course, is single likewise ; there is also a small
bone, connected with the last, and called the shut-
tle 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,
OY 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 un-
usually short. The muscles, which extend the
joint, and throw the thigh backwards in kicking,
are particidarly powerful. This is the natural de-
fensive 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,

* This process has been tenned the processus recurvatics fc-

inons.

458 THE MKCHANU AL FUNCTIONS.

that they might not interfere with the motions of
the neck. In the vertebrie 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.

Fkom the lioise we pass by a natural transition to
the Pachydennala, a small group of animals inte-
resting 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 on vegetables,
they do not ruminate, nor are they cloven-footed.
They are for the most part huge and unwieldy ani-
mals, with thick integuments, rendered tough by a
large mass of condensed cellular 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 Elephunt, the colossal giant of quadrupeds.
The many peculiarities which are observable in
the conformation of this animal have all an ob-
vious relation to the circumstances of its condition.
Formed for feeding on a great variety of vegetable
substances, and more especially on the tender
shoots of trees, fruits, and grains, as well as on
herbage, and succulent roots, its organs of mastica-
tion are powerful, and its teeth of great size. The
whole of this apparatus requires an immense de-
velopement of bone to render it efficient ; so that

MAIM iM ALIA PACHYDKKMATA. 459

tlie 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 poMer. Nature has accordingly aban-
doned this form of structure, and has at once cur-
tailed the neck, bringing the head close to the
trunk of the body, and supporting it by means of
short, but powerful muscles, which are not im-
planted in any particular 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 enlarged
by an immense expansion given to its interior cel-
lular structure, and thus the muscles are attached
to a considerable extent of bone, instead of being
affixed to a single process, which would have in-
curred great risk of being broken off by their action.
These large cells are constructed with a view to
combine strength with lightness ; the plates which
form their sides being disposed in a radiated
manner towards the circumference, and arranged
with great regularity ; and the cells themselves,
instead of containing 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 nuchae
also comes in aid of the muscular power, being here
of vast size and strength.

4(>0 THK IMECHAMCAL FUNtTIONS.

The head being limited in its range of motion by
its approximation to the trunk, the month 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 purpose a peculiar
organ, the proboscis, has been constructed : it con-
sists 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 radia-
ting from the interior towards the circumference,
others situated more obliquely, and a third set run-
ning longitudinally, and forming an exterior layer;
but tliey are all variously interlaced together so as
to compose a very complicated arrangement. The
extremity of the proboscis, which is endowed with
great sensibility, is furnished 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
lias not been necessary to deviate from the ordinary
laws of developement by the creation of a new
organ ; the same end being accomplished 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 lengthened and so moveable as very
much to resemble, though on a small scale, the
proboscis of the elephant. This latter organ, then,

MAMMALIA FACH Y 1)EUM/\TA. 4(> 1

may he considered as merely an elon^^ation of the
nostrils, which have heen drawn ont to suit a special
purpose, very different from the function to which
that part is usually sul)servient.*

While tleetness and elasticity are the results of
the mechanical conformation of the horse, solidity
and strength are the objects chiefly 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 ar-
ranged 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 iibula 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 externally,
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 exa-
mination of all the varieties of forms and structures
which occur in the mechanism of the different
tribes of mammalia, in reference to the purposes
they are intended to serve, and to the peculiar cir-
cumstances of the animal to which they belong. I

* A defective developement of the bones of the nasal cavity, wliiie
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 re-
sembling the trunk of the tapir or of the elephant. (See Geoffrey
St. Hilaire.)

462 THE MECHANICAL FUNCTIONS.

must necessarily pass over a multitude of instances
of express adaptation, which are suited only to par-
ticular cases, and are, consequently, of minor im-
portance as regards the general plans of organiza-
tion. In the sort of bird's-eye view that I am
taking of the endless 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 ex-
amined employ the anterior extremities for the
purposes of progression only, they are destitute 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, appli-
cable 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 imper-
fect clavicles are too short to connect the scapula
with the sternum ; the rest of the space being eked
out by cartilage, and by ligaments : 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 carni-
vorous tribes, which make considerable use of their
fore paws in striking and seizing their prey, have
clavicles of this description. Those quadrupeds

RODENTIA. 4C/.1

which have to execute still more complex actions
with their fore feet, have perfect clavicles, extend-
ing from the shoulder to the chest, and connecting
the bones of the anterior extremity with the general
framework 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 require 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 Mole, the Ant-eater,
and the Hedge-hog are all provided with these
bones, which, by keeping the shoulders at the
same constant distance from the trunk, and afford-
ing a tirm axis for the rotatory motions of the limb,
materially assist them in the performance of these
actions.

The beaver presents a singular moditication 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.

404 THE MECHANICAL FUNCTIONS.

§ 9. Insectivora.

In the tribe of Insectivorous quadrupeds we meet
with several races which present singular confor-
mations. In none are these anomalies more re-
markable than in the 31ole, 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 back-
wards for scooping the soil, while the feet are em-
ployed to throw it out with great quickness. 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 resist-
ances 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 extraor-
dinary power. The scapula is a long and slender
bone, more resembling a humerus in its shape than
an ordinary scapula : the humerus, on the con-
trary, is thick and square, and the clavicle is short
and broad. The radius and the ulna are distinct
from each other ; the hand is very large and ex-
panded; the palms being turned outwards and
backwards, and its lower margin being fashioned

INSECTIVOROUS MAMMALIA. 465

into a sharp cutting edge. The carpal bones and
the phalanges of the fingers are very much com-
pressed ; but they are furnished 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 extremity 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 principal force.
The head terminates in front by a pointed nose,
which is armed at its extremity with a small bone,
intended to assist in penetrating through the ground.

While all this attention has been paid to the de-
velopement 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 reduced to a slender sacrum ;
and it is thrown far back from the abdomen, to
which it could give no effectual protection. Hence
the animal, when above ground, walks very awk-
wardly, and is unable to advance but by an irre-
gular and vacillating pace.

The only quadrupeds which resemble the mole in
the perfect adaptation of their structure to the pur-
poses of burrowing, are the Wombat and the Koala,
which are among the many extraordinary animals
inhabiting the continent of Australia. Their hind
legs are constructed in a manner very much re-
sembling the human fore-arm.*

* See Home, Lectures, &c. i. 134.
VOL. I. H H

466 THE MECHANICAL FUNCTIONS.

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 Hedgehogs which belong to
the family of insectivorons quadrupeds, are fur-
nished with a similar kind of defensive armour.
For the purpose of erecting these bristles, when
the animal is irritated or alarmed, there is provided
a peculiar set of muscular bands, which forms part
of the usual subcutaneous layer, termed the pamii-
culus carnosns. In the hedgehog 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 integuments ; and capable, by the contraction
of different portions of its fibres, of carrying the
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 "reat ba«.

CARNIVOROUS MAMMALIA. 467

§ 10. Carnivora.

The type of the Mammalia may be considered as
having attained its full developement in the carni-
vorous tribes, which comprehend the larger beasts
of prey. As their food is animal, they require a
less complicated apparatus for digestion than herbi-
vorous quadrupeds, possess greater activity and
strength, and enjoy a greater range of sensitive and
intellectual faculties. In accordance with these
conditions we may notice the greater expansion of
their brain, the superior acuteness of their senses,
and their enormous muscular power. The trunk
of the body is lighter than that of vegetable feeders,
especially in the abdominal region, and is com-
pressed 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 required
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 transverse processes, while the second

* 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 devolope-
ment of their spinous processes.

4G8

THF, MECHANICAL FUNCTIONS.

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 skeleton
of these animals is constructed with reference to

their predatory nature. The sudden springs with
which they pounce upon their prey must impart to
the whole osseous frame the most violent concus-
sion. 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.

CARNIVOROUS MAMiMALIA. 469

The lore feet rest on the ground by means of tlie
second of the three joints of which each toe is com-
posed. The hist phahmges are raised at right
angles to the former, for the purpose of supporting
the claws in an erect position. It has been consi-
dered of such importance to preserve these formid-
able instruments constantly sharp, and in a condi-
tion fitted for immediate use, that an express
contrivance has been resorted to for this purpose. It
consists in a sheath, within which the claws, when
not employed, 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 in-
flicting 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 com-
pact in its form, and its processes are more strongly
developed : the fibula is a separate bone from the
tibia. The muscles, in general, are more divided
into portions, and are thus capable of greater di-

* In consequence of a different arrangement of the ligaments of
the claws in the fore and hind feet, those of the former are capable
of being more completely retracted than those of the latter. 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 tlie tail.

470 THE MECHANICAL FUNCTIONS.

versity of action, at the same time that they have
greater power than those of herbivorous quadru-
peds. The articular surfaces are of greater extent,
and are hibricated with a more copious supply of
synovia ; their ligaments are more delicate and
more numerous ; and thejoints, in general, adapted
to a greater variety of movements. All these pro-
visions are evidently directed to confer great free-
dom and facility of motion, and to enlarge the
sphere of action of the body generally, as well as
of the limbs.

§ 11. Quadrumana.

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 Elephant, which has live. To these suc-
ceed another series, where nails, or claws, are sub-
stituted for hoofs, as is the case with all the Carni-
vora, which, standing on the extremities of their
toes, have been termed Dioitio-rades. 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 con-
sequence able to make great use of their fore paws.
These conduct us to the family of the Quadrumana,

MAMMALIA QUADRUMANA. 471

coiiipreliending the Monkey and the Lemur tribes,
which are characterized 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 naturally
inhabitants of the forest, and their conformation 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 enemies ; retain-
ing their positions in perfect security on the moving
branches, or sportively swinging by their extremi-
ties in the air. Both the feet and the hands are
formed for this species of prehension ; 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 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 atti-
tude of the animal, supported chielly by the cer-
vical vertebrae. The greater developement of the
brain, and more especially of its posterior lobes,
creates a necessity for an extension of the occipital

472 THE MECHANICAL FUNCTIONS.

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 articu-
lation 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 although 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 Ateles,
or spider monkey of America, is moved by power-
ful muscles, and is an organ of great flexibility
and strength. Monkeys possess a distinct clavicle,
a lengthened humerus and femur, a radius and
ulna moveable upon each other, and a hand nearly
approaching to the human construction. 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 produce
is increased in the same proportion. The fibula is
here a bone of more importance than in quadru-
peds ; 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 con-
verting the leg into an arm, as we have already

MAMMALIA QUADRUMANA. 473

seen that the foot itself is transformed 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 walk-
ing 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 im-
perfection ; for it is evidently designed to assist
the animal in climbing trees, which is its most
usual action ; the oblique position of the foot en-
abling 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
no large projection, as in other orders of mam-
malia ; nor are the muscles which are inserted into
the heel particularly powerful : they hardly, in-
deed, can be said to compose a calf as in the
human leo;.

-'o"

§ 12. Blan.

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 phy-
siological constitution place him incontestably at
the summit of the scale of terrestrial beings. Con-
sidered zoologically, indeed, the human species
must rank among the Mammalia, and it even

474 THE MKCHANUAL FUNCTIONS.

makes a near approach to the Quadrumana ; yet
there exist many peculiarities of structure, which
entitle Man to be placed in a separate order, where
disclaiming any close alliance with inferior crea-
tures, 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
to superiority can be founded ; for in each of these
endowments man is excelled in turn by particular
races of the lower animals ; but the chief perfection
of his frame consists in its general adaptation to an
incomparably greater variety of objects, 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 harmony subsisting among all its
parts, and pervading the whole system of its func-
tions. 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 in-
tellectual, 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 forma-
tion. This will best appear when we come to exa-
mine the organs which are subservient to the sen-
sitive 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.

THE HUMAN I'lJAIMIC. 475

Man presents the only instance among the mam-
malia of a conformation by which the erect posture
can be permanently maintained, and in which the
office of supporting the trunk of the body is con-
signed exclusively to the lower extremities. To
this intention the form and arrangement of all the
parts of the osseous fabric, and the position and
adjustments of the organs of sense have a well
marked reference.

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 con-
trary, the thorax is flattened anteriorly, and extends
more in width than in depth ; thus throwing out
the shoulders, and allowing an extensive range of
motion to the arms.

The lower limbs are qualified to be the efficient
instruments 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 employed almost
solely for that mode of progression. The Quad-
rumana, which come nearer to the human form
than any of the other tribes, have the lower limbs
comparatively weak. In almost all other quadru-
peds 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.

476 THE MECHANICy\L FUNCTIONS.

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
on the broad expansion of the iliac bones ; in qua-
drupeds, these bones, having no such weight to
support, are much narrower.

The base on which the whole body is supported
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 direc-
tions, the one longitudinal, the other transverse,
constituting a double arch. This construction,
besides conferring strength and elasticity, provides
room for the convenient passage 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 protection of the various nerves and blood
vessels distributed to all these parts. The con-
cavity of the foot adapts it, also, to retain a firmer
hold of the inequalities of the ground on which we
tread. The muscles which raise the heel, and
compose the calf of the leg, are of great size and
strength, and derive a considerable increase of
power from the projection of the bone of the heel,
into which their united tendons are inserted. In
all these respects the human structure possesses

THE HUATAN FRAME. 477

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 developement 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 muscles 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 M'hich graze, and
in which the mouth and eyes must frequently be
directed downwards for the purpose 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 of the head in the ordi-
nary horizontal attitude of quadrupeds.

" Pronaque cum spectant animalia ceetera terram,

Os homini sublime dedit, CKlumque tueri

Jussit, et erectos ad sidera tollere vultus." — Ovid.

The space comprehended by the two feet is ex-
tremely narrow, when compared with the extended
base on which the quadruped is supported : hence
the stability of the body must be considerably less.
The statue of an elephant placed on a level surface,
would stand without danger of oversetting ; but the
statue of a man resting on the feet, in the usual atti-

478 THE MECHANICAL FUNCTIONS.

tilde 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
moment to pass beyond the base, no muscular effort
which could then be made would avail to prevent
the bod}^ 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 compara-
tive repose : in man it requires nearly as great an
expenditure of muscular power as the act of walk-
ing. Soldiers on parade experience more fatigue
by remaining in the attitude of standing, than they
would by marching during 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 alter-
nately from one foot to the other. The action of
standing consists, in fact, of a series of small and
imperceptible motions, by which the centre of
gravity is perpetually shifted from one part of the
base to another ; 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 accom-
])lish them successfully. It is one among those arts

THE HUMAN FRAME. 479

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 tranferring the whole weight of the body 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 unsup-
ported, 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 prepared for ; in which case the
body, having acquired 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 transferred
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 souiewhat complicated in their form. This un-

480 THE MECHANICAL FUNCTIONS.

dulation of the body from one foot to the other
would scarcely ever be performed 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 a person blindfolded can-
not 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 quadru-
mana, the fore extremities more or less contribute
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 exe-
cuting an infinite variety of movements and of
actions, requiring either strength, delicacy, or pre-
cision, the human arm and hand, considered in
their mechanism alone, are structures of unrivalled
excellence ; and, when viewed in relation 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, fortu-
nately, the task has devolved upon one far more
able than myself to do it justice.

401

Chapter X.

VERTRBRATA CAPABLE OF FLYING.

§ 1. Vertehrata without Feathers, formed for JJi/'utg.

Feav 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 re-
quisite for the performance of the higher functions
of life; yet Nature has achieved all this, not only
in endless tribes of the more diminutive inverte-
brate animals, but also in the more solid and mas-
sive organizations which are modelled on the ver-
tebrate type. These objects have been accom-
plished, in all cases, without the employment of
any other than the ordinary elements of those orga-
nizations; modified, indeed, to suit the particular
purpose in view ; but yet essentially the same,
and regulated by the same laws of developement
which prevail throughout the whole animal system.
The adaptation of these elements to the construc-
tion of an apparatus of so refined a nature, as that
which is required for flying, implies the deepest
foresight, the most extensive plan, and the most
artificial combination of means. The foundations
for these peculiar forms of mechanism are laid in
the primeval constitution of the embryo ; and a
long and curious series of preparatory changes
take place before the completion of the finished

VOL. 1. II

482 THE MECHANICAL FUNCTIONS.

Structures. Of this we have already had a remark-
able example 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, pre-
sent us with the highest refinement of mechanical
conformation, which can be attained by the deve-
lopement of a vertebrate 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 sensible, and
hardly ever attracts notice ; but it increases in pro-
portion 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, secondly, be
sufficient muscular power to give these instruments
a very great velocity. Both these advantages are
found combined in the anterior extremities of birds,
and no animals belonging to any other class pos-
sess them in the same perfection. No quadruped,
except the Bat, has sufficient muscular power in
its limbs, however aided by an expansion of sur-
face, 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

POWER OF FLYING. 483

possible wings could be so artificially adapted to
the body as to receive the full force of the actions
of the limbs, however these actions might be com-
bined, they would fall very far short of the exer-
tion 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 Exocetns, or flying-
fish, the pectoral fins have been enormously ex-
panded, 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 scarcely ever rises more than
five or six feet above the surface of the water.

A species of lizard, called the Draco Volans, 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 ex-
tremities. By the aid of these moveable flaps, the
animal is able to descend 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
imperfect organs. The construction of these anoma-
lous members is highly curious in a physiological
point of view; as showing how Nature, in effecting
a new purpose, is inclined to resort to the modifica-
tion of structures already established as constituent
parts of the frame, in preference to creating new
organs, or such as have no prototype in the model
of its formation. Frequent illustrations of this law,
indeed, are afforded by the comparative examina-
tion of the anatomy of the organs of progressive
motion. The ribs, in particular, are often the

484 THE MECHANICAL FUNCTIONS,

subject of this conversion to uses very different
from their ordinary function, which is that of as-
sisting in respiration. Tims we have seen that in
the Tortoise they are expanded to form the cara-
pace ; uniting with corresponding dilatations of the
sternum, and sterno-costal appendages, in com-
posing a general osseous encasement to the body.
In Serpents, again, the ribs are employed as organs
of progressive motion ; performing the functions of
legs, and having, affixed to their extremities, the
abdominal scuta, by way of feet. The cervical ribs
of the Cobva de Capello, or hooded snake of the
East Indies, are employed for the mechanical pur-
pose of supporting an expansion of the skin of the
neck, which forms a kind of hood, capable of being
raised or depressed at the pleasure of the animal.*
These ribs are entirely unconnected with the respi-
ration of the serpent.

In the Draco volans, which was to be furnished
with instruments for assisting it in its distant leaps
through the air, it is again the ribs which are re-
sorted to for furnishing the basis of such an appa-
ratus. 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

* Phil. Trans, for 1804, p. 346,

FLYING LIZAKD. 485

pleasure of tlie animal, during its short aerial ex-
cursions.

Among the mammalia we meet with a few spe-
cies, which have a broad membrane, formed of a

222

duplicature of the skin, extended like a cloak from
the fore to the hind extremities, and enabling the
animal to flutter in the air, and to break its fall
during its descent from the branches of trees.
Structures of this kind are possessed by the Sciurus
volans, or flying squirrel, and also by some other
species of the same genus. They are seen on a
still larger scale in the Lemur volajis, or Galeopi-
tliecus. The resistance which these broad expan-
sions 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

486 THE MECHANICAL FUNCTIONS.

heights ; but these appendages to the body are mere
parachutes, not wings ; and none of the animals
which 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 skeleton (f, Fig.
223) which correspond to the phalanges of the
fingers, are extended to an enormous length ; and
the pectoral muscles, which move the anterior ex-
tremities, are of extraordinary size and power. In
the larger species, each wing is at least two feet in
length. The fine membrane, which is spread be-
tween 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 en-
closes 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 and
sudden impulses which are requisite for flying ;
and thus, although its structure is totally different
from that of birds, it yet performs fully the office of
a real w ing. 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

BAT.

487

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 clavicles
(c) are of enormous size and length, being larger
than either the scapula or the sternum, and remark-
ably curved in their shape. The sternum is much
developed, extending laterally, and having a pro-
tecting crest along the middle of its lower surface.

The humerus (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 accordingly 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

4HH THE MECHANICAL FUNCTIONS.

walking, in consequence of the centre of gravity
being between the wings. On a level plane, in-
deed, the bat can advance only by a kind of crawl-
ing 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 v, ith the rest of the skeleton : the pubic
bones are lengthened backwards, and are joined
merely at a small point. The Vi^hole 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
bat, 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 downwards. It is probably with the in-
tention of facilitating this action that the toes are
turned completely backwards; and that they are of
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 sus-
tained by the tail, in those species which have the
spine prolonged to form one.

Bats are also provided with another instrument
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.

BIRDS. 489

§ 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 peculiar 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 to the organs of progressive
motion, a more developed type is adopted ; still
preserving a conformity 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 exclu-
sively to support the wing, retain the same divi-
sions, and are composed of the usual elements; 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 ne-
cessity 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 dimen-
sions ; and the form of the beak, which is drawn to
a point, and cuts the opposing air, tends to facili-
tate the progress of the bird in its flight.

In the conformation of the body, also, every cir-

490 THE MECHANICAL FUNCTIONS.

cumstaiice Avhich could contribute to give it light-
ness has been sedulously provided. The general
size of birds is considerably smaller than quadru-
peds 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 has been given to the osseous sub-
stance,* in order that the greatest degree of strength
might be procured with the same weight of solid
materials; and the mechanical advantage derived
from their being disposed in the circumference,
rather than in central masses, has been obtained to
ihe 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 exceedingly 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 like-
wise been made hollow ; and instead of their cavities
being filled with marrow, they contain only air.f
Thus the whole skeleton is rendered remarkably
light : that, for instance, of the Pelicmms onocro-
tahis, 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 tw enty-five pounds.J The cavities

* 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.

t 111 the Bat there is no provision of this kind for hghtening the
bones; and we find them containing marrow, as in other mammalia,
and not air.

t The dry skeleton of the Carrion Crow {Corvus corone) weighs
only twenty-five grains. Jaquemin, An. Sc. Nat. seiie 2, ii. 278.

BIRDS. 491

in the bones communicate with large air cells,
which are distributed in various parts of the body,
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. This air, being contained in the in-
terior of the body, which preserves a very elevated
temperature, must be constantly in a state of greater
rarefaction than the cooler external air; a condition
which must contribute, in some slight degree, to
render the whole bodv lighter than it would other-
wise 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 converted 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 difterence of temperature between the air
included in its body and the external atmosphere,
is perfectly insignificant. Any one who will take
the trouble to calculate the real diminution of weight
arising from this cause, under the most favourable
cirumstances, will find that, even in the case of the
largest bird, it can never amount to more than a
few grains.

The conditions in which birds are placed with
regard to the density of the surrounding medium,

41)2 THK MECHANICAL FUNCTIONS.

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 conforma-
tion. These two classes of vertebrata, accordingly,
are remarkably contrasted with respect to the struc-
ture 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 circulation 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 contrary, the ribs, and the viscera which they
protect, 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 circum-
stances are very apparent in the skeleton of the
Swan, represented in Fig. 224. In a fish, pro-
gressive motion is effected principally by the move-
ments 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 immoveable in the trunk, and
is capable of extensive motion only in the neck.

In order that the body may be exactly balanced
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 resistance of the air causes it
to be supported by the wings. When the bird is

BIRDS. 49'J

resting on its legs, tlie centre of gravity imist, in
like manner, be brought immediately over the base

of support formed by the toes : it becomes neces-
sary, therefore, to provide means for shifting the
centre of gravity from one place to another, accord-
ing to 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 motions of the
head and neck ; m hich last is, for that purpose,

494 THE MECHANICAL FUNCTIONS.

rendered exceedingly long and flexible. The num-
ber of cervical vertebrae is generally very consider-
able ; in the mammalia, as we have seen, there are
always seven, but in many birds there are more
than twice that number. In the swan (Fig. 224),
there are twenty-three; and they are joined to-
gether by articulations, generally allowing free
motion in all directions ; that is, laterally, as well
as forwards and backwards. This unusual degree
of mobility is conferred by a peculiar mechanism,
which is not met with in the other classes of verte-
brated animals. A cartilage is interposed between
each of the vertebrae, 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 intervening cartilage.*

It is to be observed, however, that in conse-
quence of the positions of the oblique processes,
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 back-
wards : hence, in a state of repose, the neck natu-
rally 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

* See Mr. Henry Earle's paper on this subject in the Philoso-
phical Transactions for 1823, p. 277.

BIRDS. 495

form which has just been described ; and thus the
equilibrium is, under all circumstances, preserved by
movements remarkable for their elegance and grace.
The great mobility of the neck enables the bird
to employ its beak as an organ of prehension for
taking its food ; an object which was the more ne-
cessary, in consequence of the conversion 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. Another advantage arising from the
length and mobility of the neck is, that it facili-
tates the application of the head to every part of
the surface of the body. Birds require this power
in order that they may be enabled to adjust their
plumage, whenever it has by any accident become
ruffled. In aquatic birds, it is necessary that every
feather should be constantly anointed with an oily
secretion, which preserves it from being M^etted,
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 occi-
pital bone being received into a hemispherical de-
pression 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

490 THE MECHANICAL FUNCTIONS.

leaves of the first cranial vertebra, and assimilat-
ing 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
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 mar-
row passes down along the canal formed by the
arches of the vertebrae, and that any pressure ap-
plied to its tender substance would instantly para-
lyze the whole body, and speedily put an end to
life. Some extraordinary provision was therefore
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 accomplished in the
'^25 ms^ simplest and most effec-

tual manner by enlarging
the diameter of the canal
at the upper and lower
part of each vertebra,
while at the middle it re-
mains of the usual size ;
so that the shape of the
cavity, as is well seen in
Fig. 225, which shows a

BIRDS. 497

longitudinal section of one of the cervical vertebrae
of the Ostrich, resembles that of an hour glass.
Thus a wide space is left at the junction of each
successive vertebra, allowing 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 mar-
row. Mr. Earle found* 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 enclosed 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 ver-
tebra3 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 accordingly
the whole of this portion of the spine, together witli
the sacrum, is consolidated into one piece. All the
processes are largely developed, and pass obliquely
from one vertebra to the next, mutually locking
them together; and in order most effectually to
preclude the possibility of any flexion, the spinous
processes, and sometimes even the bodies of the
dorsal vertebrae are immoveably soldered together
by ossific matter, 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

* In the paper already quoted, p. 278,
VOL. I. K K

498 THE MECHANICAL FUNCTIONS.

being anchylosed together for this purpose ; so
that they form a bone of great length. The coccy-
geal vertebrae (q) are also numerous ; but they are
compressed into a small space, and enjoy great lati-
tude of motion, being subservient to the movements
of the tail.

The ribs are numerous, and of considerable
strength : they send out processes, which are di-
rected 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 protection 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 continuations 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 furnish extensive at-
tachment to the large pectoral muscles employed
to move the wings, and which, taken together, are
generally heavier than the rest of the body. Con-
sidered with reference to all the other muscles, and
to the weight of the body itself, these pectoral
muscles are of enormous strength. The flap of a
swan's wing is capable of breaking a man's leg ;
and a similar blow from an eagle has been known
to be instantly flital. The bat is the only instance,
among the mammalia, where the sternum presents
this pecuViar car in a fed, or keel-like shape ; and the

WINGS OF BIRDS. 499

purpose is evidently the same as in the bird. Not-
withstanding 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 elements entering into
its composition, though delevoped in very different
proportions.

The scapula is generally a small and slender
bone. The coracoid bone (k) is largely developed,
and assumes much of the appearance of a clavicle.*
But the real clavicles (c) are united below, where
they join the fore part of the sternum, forming a
single bone, which, from its forked shape, has been
denominated the fur cular 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 being, at the same time, very elastic,
it tends to restore them to their proper situations,
after they have been disturbed by any violent
impulse.

The wing of a bird does not, at first view, present
much analogy with the fore extremity of a quadru-
ped ; 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

* Many have considered this bone as being the clavicle, and have
regarded the furcular bone as a new bone, or supplementary clavicle;
but all the analogies of position and of developement are in favour
of the views stated in the text.

500 THE MECHANICAL FUNCTIONS.

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 sur-
face upon the scapula ; thus admitting of the com-
plete 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 quadru-
peds, the movements are made from side to side,
instead of their consisting of flexion and extension ;
this variation from the usual structure being for the
purpose of folding down the joints of the wing, and
bringing them close to the body. The metacarpus
(m) consists originally 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 pro-
cess a small pointed bone is connected, correspond-
ing to a rudimental thumb (t). There are generally
two fingers, of which the first exhibits traces of
having been originally two bones : the inner finger
consists of two or three long phalanges, and the
outer one of a single phalanx : there is sometimes
also a rudimental bone corresponding to a little

WINGS OF BIRDS. 501

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 ; and they also furnish the
warmest possible covering to the body. This latter
property results from the great multitude of minute
detached fibrils of which they are principally com-
posed : as is more especially observable in the down
which protects the chest of aquatic birds where
such provision is particularly needed.

The horny materials 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
advantageous mariner 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
filaments, so arranged as to oppose a much greater
resistance to a force striking perpendicularly against
their surface, than to one which is directed late-
rally ; that is, in the plane of the stem. They
derive this power of resistance 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

502

THE MECHANICAL FUNCTIONS.

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 laminated
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 connexion must be
eifected by some mechanism invisible to the unas-
sisted 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 laminae are brought
within a certain distance. The fibrils of a feather
from the wing of a goose are represented magnified
at a, a, b, b. Fig. 226, as they arise from the two

sides of the edges of each lamina. They are ex-
ceedingly numerous, above a thousand being con-
tained 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

FEATHERS OF BIRDS. 503

those marked b, b, proceeding from the other side
of the lamina, or that nearest the root of the featlier,
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 en-
tangled 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 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 ex-
amining with a good microscope their cut edges,
while they are gently separated from one another.
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 repeated over every part of the
feather, and constitutes a closely reticulated surface
of great extent, admirably calculated to prevent the
passage of the air through it, and to create by its
motion that degree of resistance which it is intended
the wing should encounter.* In feathers not in-
tended for flight, as in those of the ostrich, the
fibrils are altogether wanting : in those of the pea-

* 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 observa-
tions 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.

504 THE MECHANICAL FUNCTIONS.

cock 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 adapted to
the mechanical object which it is to answer, cannot
be contemplated without the deepest feeling of ad-
miration, 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 dis-
playing such minute and curious workmanship.
It is only very recently 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 ap-
parently 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 filaments, col-
lected in tufts, and resembling in their arrange-
ment 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 rudiments of the organs

FEATHERS OF BIRDS. 505

that afterwards form the feathers, of which this
down, serving the purpose of a first garment, hastily
spread over the young bird, is bnt the precursor ;
for the tufts generally soon fall off and disappear,
except in the rapacious tribes, as the Eagle and the
Vulture, where they remain attached to the feathers
for a considerable time.

While this temporary protection is given to the
integument, extensive preparations are making
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 rudiments are
receiving the necessary supply of nutrient juices,
and of vessels for their circulation, 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 proceed-
ing 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 elongated 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
enlarged vessels, which are supplying it with
nourishment. 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 of
the several parts of the feather being successively
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

506 THE MKCHANICAL FUNCTIONS.

another ; and while the outer covering of the stem
is growing from one membrane, the interior spongy
tissue is forming in other places, in various stages
of softness or consolidation : so that the whole com-
poses a system of operations, which may be said
to resemble in its complication at least, although
on a microscopic scale, an extensive manufactory.
Hence will be readily understood how great must
be the difficulty of tracing all the steps of these
multifarious processes, 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
proceeds ; its different parts being produced suc-
cessively in proportion as they are wanted, and
their form and structure undergoing frequent varia-
tion in the course of their developement.

The most elaborate, and apparently accurate re-
searches 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 observa-
tions. 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 purpose we need only
examine a common feather, such as that repre-
sented in Fig. 228, where s is 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

* Memoires du Museum, xiii. 327 ; and Annales des Sciences
Naturclles, ix. 113,

FEATHERS OF BIRDS.

507

termed the vane of the feather (v.) The lines from
which these laminae arise, approach one another at

228

229

230

231,

the lower part of the stem, till they meet at a
point, where the longitudinal groove terminates,
and where there is a small orifice (o), leading to
the interior of the quill. From this part the trans-
parent 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 matrLu, is represented in
Fig. 229, when it has attained the cylindric form
already described ; of which a is the apex, or coni-
cal part that rises above the cuticle, and b the
base, by which it is attached to the corium, or true

508 THE MECHANICAL FUNCTIONS.

skin. A white line is seen running longitudinally
the whole length of the cylinder, and another, ex-
actly 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. 2'30, 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 con-
solidated 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 beginning to take place.

The laminse and their fibrils, the assemblage of
which constitutes the vane of the feather, are the
parts which are first formed ; and their construc-
tion is eftected 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 wdth which we are acquainted. Instead
of growing from a base, like hairs, and other pro-
ductions of the integuments, by successive deposi-
tions 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 cavi-
ties. The next object of our curiosity, then, is to
learn the way in which these moulds are con-
structed ; and on careful examination they appear
to be formed by two striated membranes, the exte-
rior one (e) enveloping the other (i), or interior
membrane. These membranes are separated by a
series of partitions, which commence at the edges

FEATHERS OF BIRDS. 509

of the longitudinal white band, seen in Fig. 229,
and wind obliquely upwards till they reach the
opposite longitudinal band already described, w^iere
they join a longitudinal partition which occupies
a line answering to that posterior band. Thus they
leave between them narrow spaces, which consti-
tute 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 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
laminae 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 lamince,
is to construct the stem of the feather, and unite
the laminae to its sides. For this purpose the an-
terior 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 sub-
stance, 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 tlie sec-

olO THE MECHANICAL FUNCTIONS.

tion, Fig. 231 ; the points of the cones being di-
rected upwards, and their intervals being occupied
by the spongy substance in different stages of con-
solidation, and more perfected in proportion as
they are situated nearer the apex of the stem.

While the construction of the feather, in its dif-
ferent stages, is thus advancing from below, those
parts which are completely formed are rising above
the surface of the skin, still enveloped in the cap-
sule 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, allow-
ing 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 supe-
rior orifice (o), Fig. 228 ; where the laminae, 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 deposit-
ing a horny covering to the feather : all that re-
mains of its substance is a thin membrane, which
adheres to the outside of the tubular part, or bar-
rel 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 sub-
stance, but forms a transparent horny material

FEATHERS OF BIRDS. 511

over the 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, constitute 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 process of time, this also
decays, and the whole feather is cast off, prepara-
tory to the formation of another, which in due sea-
son is to replace it. All the feathers are in general,
moulted annually, or even at shorter periods ; and
the same complicated process is again begun and
completed, 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 fore-
sight, which are implied in all this long and com-
plicated 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 instruments of flight. A
temporary investment of down is in readiness to
shelter the tender chicken from the rude impres-
sions 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 portion 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 essential 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

512 THE MECHANICAL FUNCTIONS.

quill, which is prolonged for the purpose of con-
verting 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 scaffi)ldings which had been set up as
temporary structures ; the membranes, 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 res-
pecting the structure and actions of the wings of
birds. If we attend to the mode of their articula-
tion with the scapula, w^e find it producing a motion
oblique with regard to the axis of the body, so that
the stroke which they give to the air is directed
both downwards and backwards ; and the bird,
while moving forwards, is at the same time sup-
ported in opposition to the force of gravity. The
different portions of the wing are likewise so dis-
posed as to be contracted and folded together when
the wing is drawai 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

FLIGHT OF BIRDS. 513

oar in rowing is a similar operation, performed
with the same intention, and deriving its name
from this resemblance. Independently of this cir-
cumstance, however, the force exerted by the wing
in its descent is considerably greater than that with
which it rises ; and consequently the resistance and
reaction of the air is greatest in the opposite direc-
tion to the descent of the wing, which strikes
obliquely backwards, as well as downwards: and
the effect of the muscular action is, in consequence
of that resistance and reaction, to raise the bird,
and, at the same time, propel it forwards.*

As the inclination of the wing is chiefly back-

* Erroneous views have frequently been entertained respecting tlie
theory of the progressive and other movements of animals, from
mistaking the reaction of the ground, or medium against which
pressure is made, for a moving or propelling force, analogous to
that arising from elasticity : whereas it is merely a force, which,
being equal and opposed to that pressure, only destroys or neutralizes
its effect ; and thus enables the moving power from which the pres-
sure is derived, to exert its full efficiency in exactly the opposite
direction. This error, which will be found to vitiate many of the
reasonings of Borelli, was clearly pointed out by Barthez, in his
work entitled " Nouvelle Mechanique des Mouvemens de I'Homme
et des Animaux," published in 1798.

M. Navier has undertaken a very long and elaborate mathematical
investigation of the dynamic conditions which obtain during the
flight of birds, and the swimming of fishes, in the Memoires de
I'Academie Royale des Sciences de I'lnstitut. de France, for 1832 ;
Tome xi. p. Ixi — cxiii ; under the title of " Rapport sur un Memoire
de M. Chabrier, concernant les moyens de voyager dans I'air, et de
s'y diriger ; contenant una nouvelle theorie des mouvemens pro-
gressifs." He asserts that a bird in flying folds its wings but little
while raising them to give the effective stroke; and ascribes the
effect of the movement of the wings to the greater velocity with
which they are made to descend, compared with that of their eleva-
tion. His calculations lead him to the startling conclusion, that a
swallow, while simply sustaining itself in the air without either rising
or advancing, must cause its wings to make 23 strokes in each
VOL. I. L L

•514 THE MRCHANKAL FUNCTIONS.

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 con-
sequently better formed for horizontal progressive
motion, which is what they chiefly practise in pur-
suing their prey, than for a rapid perpendicular
ascent. Those birds, on the contrary, which rise
to great heights in a direction nearly vertical, such
as the Quail and the Lark, have the wings so dis-
posed as so 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, contributes to the ascent of the body, on

second of time, or 1380 beats per minute ! — a conclusion, of which
the simplest observation of the actual motion of the wings is suffi-
cient to show the fallacy.

The following is among the most curious of the results of Navier's
analytical researches. Assuming that a labourer when employed in
turning a winch during eight hours a day, produces a mechanic
effect equivalent to the raising of a weight of 6 kilogrammes,
(or 13 avoirdupois pounds), to the height of one metre, (or 3.28
feet), in each second of time, which is about 43 pounds raised to
the height of one English foot) ; and assuming the weight of the
man so working to be 70 kilogrammes, (or 154 pounds), this me-
chanic force, if employed to raise himself, would carry him to a
height of 0.086 metres (or 3.366 inches) in each second of time.
But the action exerted during each second by a bird, when only ho-
vering in the air, is sufficient to raise its own weight to a height of 8
metres, (or 313 inches), which is 93 times greater than the height to
which the corresponding exertion of a man would enable him to
attain. It follows as a consequence, that if the human structure
were such as to enable a man to employ his muscular power in the
way best calculated to raise his body in the air, and that also he had
the means of expending the whole of the force which he exerts during
an ordinary day's labour of eight hours, in as short a space of time as
he pleased, the concentrated effect of this labour would be no more
than to sustain him in the air during five minutes. Ibid. p. Ixxii.

FLIGHT OF BIRDS. 515

the same principle that a kite is carried up into the
air when retained in an oblique position. This
circumstance is particularly observable in the ascent
of birds of prey, whose wings have a great obliquity,
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 assist-
ance in the steerage of the animal. In many birds,
as in the Woodpecker, the tail is much employed
as a support to the body in climbing trees. The
caudal vertebrae are often numerous, but are short
and compressed together: they are remarkable for
the great developement of their transverse pro-
cesses, 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 tlie
wing ; for a great expenditure of muscular power
is constantly going on while they support them-
selves in the air. Occasional rest is necessary to
them as w ell as to other animals ; and means are
accordingly provided by nature for their mecha-
nical 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 atmosphere, the
offices of sustaining and of moving the body along
the ground must be entrusted wholly to the hind

•516 THE MECHANICAL FUNCTIONS.

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
advanced so as to be brought underneath that
centre. But as the bones of the posterior extremity
have their origin from tlie remote part of the pelvis,
which is elongated backwards, at a considerable
distance from the wings, it became 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 accordance with the vertebrate 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
forwards, 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 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
GrallcB, or wading birds : the fibula is exceedingly
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

FEKT OF HIRDS. 517

wholly disappeared. The long bone which suc-
ceeds to the tibia, though considered by some ana-
tomists as the tarsus, is properly the metatarsal
bone, and in the Grallae is of great length. At its
lower end it has three articulations, shaped like
pullies, for the attachment 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,
M^hich have therefore been termed the n/toiial bones.
This remarkable numerical relation among the
several bones of the toes exists quite independently
of their length.

There is one whole order of birds which are par-
ticularly 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 Scatisores, 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 ani-
mals to suit the purposes required in the bird, had
purposely omitted one of the toes, which are usually

518 THE MECHANICAL FUNCTIONS.

five in number. But instances occur of birds, in
Avliich we may trace the rudiment of a fifth toe
In'gh upon the metatarsus, and upon its inner side.
The spur of the cock may be regarded as having
this origin. What confirms this view of the sub-
ject is that in those birds which have only three
toes, namely, in the JEmu, the Cassowary^ and the
Rhea, it is again the inner toe which disappears,
leaving only the three outer toes, namely, those
which have respectively three, four, and five pha-
langes. The 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. The last bone of the outer toe of the
ostrich is very small, and being usually lost in pre-
paring the skeleton, has been overlooked by natu-
ralists ; but Dr. Grant has ascertained, by the
careful dissection of a recent specimen, the exist-
ence of this fifth phalanx.

A bird is capable of shifting the position of the
centre of gravity of its body, according as circum-
stances 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

FEET OF BIRDS. 519

of gravity is thus transferred alternately from one
foot to the other : hence, in walking, the head of a
bird is in constant motion ; whilst the duck and
other birds, whose legs are ver}^ short, have a
waddling gait. It may be observed that the more
perfectly predaceous birds are not the best formed
for w^alking ; 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 main-
tain this attitude with greater ease and security
than if they rested upon both feet. The true expla-
nation 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 inter-
vening joints, so that whenever these joints are
bent, as shown in Fig. 234, those tendons are put
upon the stretch, and mechanically, or without
any action of the muscles, tend to close the foot.
When the bird is on its perch, this effect is pro-
duced 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

520

THE MECHANICAL FUNCTIONS.

would have done by resting upon both ; because
in the latter case, the weight of the body, being

divided between them, does not stretch the ten-
dons sufficiently. In this position the bird not
only sleeps in perfect security, but resists the im-
pulse 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 stabi-
lity in two ways, first, it adds to the weight of
the body, which is the force that stretches the ten-
dons, and causes them to grasp the bough ; and,
secondly, it also lowers the centre of gravity.

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

FEET OF BIRDS. 521

state of extension without any muscular effort. The
mechanism is such as to withstand the effect of the
ordinary oscillations of the body, wdien the bird is
reposing ; but it is easily unlocked by a voluntary
muscular exertion, 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 dis-
tinguish the different orders and families. Some
of the more remarkable deviations from what may
be considered as the standard conformation, 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 of birds, and
in many of its peculiarities of structure it makes
some approach to that which characterizes the
quadruped. Though this bird is provided with
wings, it was evidently 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 sternum
is flat, and does not present the keel-like projection
which is so remarkable in that of all other birds.

* This mechanism is noticed by Dr. Macartney in the Transac-
tions of tlie Royal Irish Academy, vol. xiii. p. 20, and is more fully
described in Rees's Cyclopajdia, Art. Bird. He observes that botlt
Cuvier and Dumeril have committed an error in referring this pecu-
liarity of structure to the knee instead of the tarsal joint.

o22 THE MECHANICAL FUNCTIONS.

The clavicles do not reach the sternum, nor even
meet at the anterior part of the chest to form the
furciilar hone; 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 structure ; 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 elaborate
apparatus of crotchets and fibres, which are uni-
versally 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 considered physiologically, as a
species of degeneracy in the structure of feathers.

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, received 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 altogether changed. As penguins
are intended for a maritime life, all their extremities
are formed for swimming. Their legs are exceed-
ingly 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

MUSCULAR POWER IN BIRDS. 523

posture which gives them a strange and grotesque
appearance.

I have ah^eady alhided 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 con-
tinual exposure to wet. But birds of a truly
aquatic nature, have their toes webbed, that is,
united by a membrane ; a mechanism which qua-
lifies them to act as oars, and indeed gives them a
great advantage over all 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 presents a very small surface to the opposing
water. Their plumage is so constructed as to pre-
vent 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 unwearied 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 liigher degree of irri-
tability, dependent probably on the great extent of
tlieir respiratory functions, than is possessed by

524 THE MECHANICAL FUNCTIONS.

any other class of animals. Birds, in their annual
migrations, are known to traverse whole seas and
continents in the course of a very few days. Many
continue for weeks constantly on the wing : Frigate-
birds (Pelicanus aquilus, Linn.) are seen hovering
over the sea, in search of their prey, at the greatest
distances from land, without ever taking rest : and
their lives may almost be said to be passed in one
continued flight.

END OF VOL. I.

C. WHITTINGIIAM, TOOKS COURT, CHANCERY LANE.

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