®i{e ^. ^. pm pfararg

*QH50l

SCIENCE.

C, 179").

r

So-
rt rs of
ud-'9 :

It...

*

#

-If
•If

ARCHIBALD A. CALDWELL, Salisbury
ROBERT D. DICKSON, Wilmington,
EDWARD B. DUDLEY,
JOHN H. DILLARD, Rockinqliam,
JOHN S. ERWIN, Burke,
SAMUEL HALL, Wilmingtuu,
JOHN D. HAWKINS, Franklin,
WILLIAM J. HAWKINS, "
HENRY C. LOGAN, Halifax, Va.
JAMES A. LONG, Randolph,
HORATIO L. POLK, Fayette Co. Tenn.
THEOPHILUS SCOTT, Raleigh,
HENRY A. SY'DNOR, Halifax, Va.
ROBERT STRANGE, Fayelleville.
JAMES F. TAYLOR, Ralcigli.

3

.St.

if-

*

.^

i^ssa^

"^

^•■^«

i

i.

This book must not be
taken from the Library
building.

.}/

' iy ^ *>«>

THE BRIDGEWATER TREATISES

ON THE POWER, WISDOM, AND GOODNESS OF GOD,
AS MANIFESTED IN THE CREATION.

TREATISE V.

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

BY PETER MARK ROGET, M. D.

SEC. K. S. ETC.

IN TWO VOLUMES.
VOL. I.

** Ask kow the beasts, and they shall teach thee; and the fowls of
the air, and thet shall tell thee:

" Or speak to the earth, and it shall teach thee; and THE FISHES OF

the sea shall declare tjnto thee.

" Who knoweth not in all these that the hand of the Lord hath
WROUGHT this?" Job, xu. 7, 8, 9.

ANIMAL

AND

VEGETABLE

PHYSIOLOGY,

CONSIDERED WITH REFERENCE

TO

IVATTRAIi THEOL.OGY.

BY

PETER MARK R 0 G E T, M. D.

SECRETARY TO THE ROYAL SOCIETY, FULLERIAN PROFESSOR OF PHYSIOLOGY IN THE ROYAL

INSTITUTION OF GREAT BRITAIN, VICE PRESIDENT OF THE SOCIETY OF ARTS,
FELLOW OF THE ROYAL COLLEGE OF PHYSICIANS, CONSULTING PHYSICIAN, 1*6 THE QUEEN
charlotte's lying-in HOSPITAL, AND TO THE NOKTHERN
DISPENSARY, ETC. ETC.

VOL. I.

PHILADELPHIA:
CAREY, LEA & BLANCHARD.

1S36.

GRIGGS 6l CO., PRINTERS.

TO HIS UOTAL HIGHNESS

PRINCE AUGUSTUS FREDERICK,

DUKE OF SUSSEX, K. G.

PHESIDEXT OF THE BOYAL SOCIETY,

&c. &c. &c. &c.
THIS TREATISE

IS, WITH PERMISSION, HUMBLY DEDICATED,

AS A TRIBUTE OF PROFOUND RESPECT AND GRATITUDK

FOR THE BENEFITS RESULTING TO

SCIENCE

AND ITS CULTIVATORS,
FROM HIS ILLUSTRIOUS PATRONAGE,

BV HIS DEVOTED HUMBLE SERVANT,

P. M. ROGET.

•^ 4k :

V

?^1990

PREFACE.

I PROBABLY never should have ventured to engage
in the composition and publication of a work like the
present, had not that task been assigned me by my
nomination as one of the writers of the series of
Bridgevvater 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 even-
tually be useful, has been my chief support in this
arduous undertaking; the progress of which has •
throughout been seriously impeded by the various in-
terruptions incident to my profession, by long pro-
tracted anxieties and afflictions, and by the almost
overwhelming pressure of domestic calamitv.

The object of this treatise is to enforce the great
truths of Natural Theology, by adducing those evi-
dences of the power, wisdom, and goodness of God,
which are manifested in the living creation. The
scientific knowledge of the phenomena of life, as they
are exhibited under the infinitely varied forms of or-

VIU PREFACE.

ganization, constitutes what is usually termed Physi-
ology, a science of vast and almost boundless extent,
since it comprehends within its range all the animal
and vegetable beings on the globe. This ample field
of inquiry has, of hite years, been cultivated with ex-
traordinary diligence and success by the naturalists
of every country ; and from their collective labours
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 witli the original purpose of the
work, which I have ail along endeavoured to keep
steadfastly in view\ I have excluded from it all those
particulars of the natural history both of animals
and of plants, and all description of those struc-
tures, of which the relation to final causes cannot
be distinctly traced; and have admitted only such
facts as afford manifest evidences of design. These
facts I have studied to arrange in that methodized or-
der, and to unite in those comprehensive generaliza-
tions, 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 subordination to the general plan of cre-
ation. My endeavours have been directed to give

PREFACE. ix

to the subject that unity of design, and that scientific
form, which are generally wanting in books profes-
sedly treating of Natural Theology, published prior
to the present series ; not excepting even the unri-
valled 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 grati-
fication, but also to furnish, to contemplative minds,
a rich fountain of religious instruction. To render
these benefits generally accessible, I have confined
myself to such subjects as are adapted to every class
of readers; and, avoiding all unnecessary extension of
the field of inquiry, have wholly abstained from en-
tering into historical accounts of the progress of dis-
covery; contenting 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 ha-
bits and instincts of animals, and the structures of an-
tediluvian races ; the extent of the field which re-
mained, and which, with these few exceptions, em-
braces nearly the whole of the physiology of the two
Vol. I. B

X I'KEFxVCE.

kingdoms of nature, already affording ample occupa-
tion for a single labourer.

The catalogue of authors whose works have fur-
nished 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 Vv^ork by con-
tinual citations of authorities ; but have given refer-
ences to them only when they appeared to be parti-
cularly requisite, either as bearing testimony to facts
not generally known;, or as pointing out sources of
more copious information. It may, how^ever, be pro-
per to mention, that I have more especially availed
myself of the ample materials on Comparative Anato-
my and Physiology contained in the works of Cuvier,
Blumenbach, Cams, Home, Meckel, De Blainville,
Latreille, and St. Hilaire, and in the volumes of the
Philosophical Transactions, of the Memoires and An-
nales du Museum, and of the Annales des Sciences
Naturelles. I should be ungrateful were I not also
to acknowledge the instruction I have derived from
my attendance 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,

I

at the University of London.

I have likewise to return my thanks for the liberal
manner in which the Board of Curators of the Hunte-

PREFACE. Xi

rian Museum gave mc permission to take such draw-
ings of the preparations it contains^ as I miglit want
for the illustration of this work; and to Mr. Clift, tlic
conservator, and Mr. Owen, the assistant conservator
of the museum, for their ohliging assistance on this
occasion. Mere verbal description can never con-
vey 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 circum-
locution, to employ many terms of science, I have
been careful to explain the meaning of each when first
introduced: but as it might frequently happen that,
en a subsequent occurrence, their signification may
have been forgotten, the reader will generally find in
the index, which I have, with this view, made very
copious, a reference to the passage where the term is
explained.

I beg, in this place, to express my deep sense of
the obligation conferred on me by Mr. Davies Gil-
bert, the late president of the Royal Society, to whose
kindness I owe my being appointed to write this
treatise.

* All the wood engravings have been executed by Mr. Byfield, and the
drawuigs for them were, for tlie most part, made by Miss Catlow, whose as-
sistance on this occasion has been most valuable to me.

XU PREFACE.

I also take this opportunity of conveying my best
thanks to my friend and colleague, Mr. Children, of
the British Museum, for his kind assistance, in re-
vising the sheets while the work was printing, and
for his many valuable suggestions during its progress
through the press.

A catalogue of the wood engravings has been sub-
joined; 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 use-
ful for purposes of reference. The latter table is re-
printed from that which I have given in my " Intro-
ductory Lecture on Human and Comparative Physio-
logy/' published in 1826, with only such alterations
as were required to make it correspond with the se-
cond and improved edition of Cuvier's w^ork.

NOTICE.

The series of Treatises, of which the present is one, is pub-
lished under the followino; circumstances:

The Right Honourable and Reverend Francis Henry,
Earl of Bridgewater, died in the month of February, 1829;
and by his last Will and Testament, 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 dividends thereon, to be
held at the disposal of the President, for the time being, of the
Royal Society of London, to be paid to the person or persons no-
minated by him. The Testator farther directed, that the person
or persons selected by the said President should be appointed to
write, print, and publish one thousand copies of a work On the
Power, Wisdom, and Goodness of God, as manifested in the
Creation; illustrating such loork 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 discove-
ries ancient and modern, in arts, sciences, and the whole extent
of literature. He desired, moreover, that the profits arising from
the sale of the works so publislied siiould 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 Canter-

XIV

bury and of the Bishop of London, in determining upon the best
mode of carrying into effect the intentions of the Testator. Act-
ing with their advice, and with the concurrence of a nobleman
immediately connected with the deceased, Mr. Davies Gilbert
appointed the following eight gentlemen to write separate Trea-
tises 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.

REGIDS PROFESSOR OF MEDICINE IN THE UNIVERSITY OF OXFORD.

ON THE ADAPTATION OF EXTERNAL NATURE TO THE PHYSICAL

CONDITION OF MAN.

THE REV. WILLIAM WHEWELL, M. A. F. R. S.

FELLOW OF TRINITY COLLEGE, CAMBRIDGE.

ASTRONOMY AND GENERAL PHYSICS CONSIDERED WITH
REFERE.NCE TO NATURAL THEOLOGY.

SIR CHARLES BELL, K. G. H. F. R. S. L. & E.

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

\

PETER MARK ROGET, M. D.

FELLOW OF AND SECRETARY TO THE ROYAL SOCIETY.

ON ANIMAL AND VEGETABLE PHYSIOLOGY.

XV

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

CANON OF CHRIST CHURCH, AND PROFESSOR OP 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 -yO NATURAL THEOLOGY.

His Royal Highness the Duke of Sussex, President of the
Royal Society, having desired that no unnecessary delay should
take place in the publication of the above mentioned treatises,
they will appear at short intervals, as they are ready for publica-
tion.

CO]^TE]\TS
OF THE FIRST VOLUME.

INTRODUCTION.

Chapter I. — Final Causes . . . . . .17

II. — The Functions of Life .... 39

PART I.— THE MECHANICAL FUNCTIONS.

Chapter I. — Organic Mechanism . . . . .56

§ 1. Org-anization in general .... 56

2. Veg-etable Org-anization . . . . .60

3. Development of Vegetables .... 71

4. Animal Organization . . . . .79

5. Muscular Power . • ... 97

Chapter II. — The Mechanical Functions in Zoophxtes . . 110

§ 1. General Observations . . . .110

2. Porifera, or Sponges ..... 113

3. Polypifera ...... 122

4. Infusoria . . . . . . .136

5. Acalepha ...... 142

6. Echinodermata ...... 147

Chapter III. — Mollusc a . . . . . .157

§ 1, Mollusca in general . . . . .157

2. Acephala ...... 159

3. Gasteropoda ...... 166

4. Structure and formation of the Shells of Mollusca . 168

5. Ptcropoda . . . . . .185

6. Cephalapoda ...... 186

Chapter IV. — Articulata ...... 193

§ 1. Articulated animals in general . . . 193

2. Annelida . . . . . . .194

3. Arachnida ...... 202

4. Crustacea ...... 204

Vol. I. C

XVI 11

CONTENTS.

Chapter V. — Insects .....
§ 1. Aptera .....

2. Insecta alata .....

3. Development of Insects . . . ,

4. Aquatic Larv?e ....

5. Terrestrial Larvae . . . ,

6. Imago, or perfect Insect

7. Aquatic Insects ....

8. Progressive motion of Insects on land

9. Flight of Insects . . . .

Chapter YI. — Vertebrata ....

§ 1. Vertebrated Animals in general .

2. Structure and composition of the Osseous Fabric

3. Formation and development of Bone

4. Skeleton of the Vertebrata

212
212
214
215

220
222
225
237
229
242

254
254
256
263
269

Chapter VII. — Fishes

284

Chapter VIII. — Reptilia

§ 1. Terrestrial Vertebrata in general

2. Batrachia

3. Ophidia . . . ,

4. Sauria

5. Chelonia

302
302
303
310
317
321

Chapter IX. — Mammalia ....

§ 1. Mammalia in general

2. Cetacea .....

3. Amphibia ....

4. Mammiferous Quadrupeds in general

5. Ruminantia ....

6. Solipeda .....

7. Pachvdermata ....

8. Rodentia .....

9. Insectivora ....

10. Carnivora .....

11. Quadrumana ....

12. Man . . . .

Chapter X. — Vertebrata capable of Flyixg

§ 1. Vertebrata without feathers, formed for flying
2. Birds .....

330
330

oob

rsrthf

345

356

357

360

3(

364

362

367
369

376
376
382

LIST OF ENGRAVINGS.

VOLUME I.

Fig. Page

1 Rotifer redivivus, (from Muller) .... 58

2 Vibrio iritici, (Bauer) . . . . . .58

3 Simple vegetable cells, (Slack) .... 61

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

5 Ditto, longitudinal section, (id.) . . . . 61

6 Compressed cells of vegetables, (Slack) . . .61

7 Hexagonal and elongated cells, (id.) ... 61

8 Elongated cells, (id.) . . . . . .61

9 Fibrous cells, (id.) ...... 61

10 Reticulated cells, (id.) . . . . . .61

12 Junction of cells to form a tube .... 6.5

13 Beaded vessels . . . . . .65

14 Spiral vessels, or Trachese ..... 65

15 Annular vessels . . . . . .65

16 Punctuated vessels ..... 65

17 Transitions of vessels from one class to another . . 65

18 Woody fibres ...... 65

19 NervLiresof a leaf . . . . . .65

20 Cells composing the cuticle, (De Candolle) . . 69

21 Stomata magnified, (Amici) . . . . .09

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 . . . . .82

26 Blood vessel /...... 84

27 Section of blood vessel, with the valves open . . .84

28 Ditto, with the valves closed .... 84

29 Striated surface of the scale of the Cyprinus albiirnus, (Hei-

singer) . . . qo

30 Ditto of thePerc«j?wm«a7/s, (Carus) . . . .92

31 Imbricated arrangement of the scales of fishes, (Ileisinger) 92

32 Section of the bulbs of hair, magnified . . .93

33 Quill of Porcupine, (F. Cuvier) .... 96

34 Transverse section of the same, (id.) . . . .96

XX

LIST OF ENGRAVINGS.

Fig.

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

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

37 Muscle in a state of relaxation

38 The same muscle contracted .

39 Diagram illustrating the action of oblique muscles

40 Semi-penniform muscle

41 Penniform muscle ....

42 Complex muscle ....

43 Tendon of muscle . .

44 Trapezius muscle ....

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

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

47 Muscular structure of the Iris, (Home)

48 Muscular fibres of a sucking disk

49 Longitudinal muscular fibres of a blood vessel

50 Transverse muscular fibres of ditto

51 Muscular fibres of the human stomach, (Cooper)

52 Muscular fibres of the heart, (id.)

53 Magnified view of a Sponge, (Grant)

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

55 Gemmule of a AS|po?2^e, (id.)

56 Lobularia. Alcyonium pelasgica, (Deterville)

57 Detached polype of ditto, (id.)

58 Zoanthus, (Actinia sociata,) (Ellis)
.59 Hydra viridis, (Trembley)

60 Sertularia pelasgica, (Deterville)

61 Tuhipora musica, (Ellis) .

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

63 Flustra carbasea, (id.)

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

65 Corallium ruhriim, (id.) .

66 Polypes of ditto, magnified, (id.)

67 Section of Gorgonia Briareus, (id.)

68 Isis hippuris, (id.) .

69 Polype of Flustra carbasea, (Grant)

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

71 Pennatula phosphor ea, (Ellis)

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

73 to 76 mode of progression of the Hydra viridis, (Trembley)

77 Vorticella cyathina, (Muller) .

78 Proteus dijjiuens, (id.)

79 Volwx globator, (id.)

80 Brachionus urceolaris, (id.)

Page
96
96
101
101
101
101
101
101
101
101
105
105
105
105
106
106
106
106
114
114
114
122
122
122
122
124
125
125
125
, 125
125
. 125
125
. 125
129
. 129
131
. 131
133
. 136
139
. 139
140

LIST OP ENGRAVINGS. XXi

Fig.

81 Medusa Pulmo, (Maori) . , . . . rS

82 Beroe ovatus, (Bruguiere) • • • . 144
SZ Beroe pileus, {vi\,) j^

84 Velella limbosa, (Guerin) .... 144

85 Phijsalia atlantica, (id.) . . . ^ .144

86 Actinia rvfa, (original) ..... \/^a

87 Ditto expanded, (original) .... 145

88 Asterias serrulata, (Bruguiere) .... 147

89 Asterias regularis, (id.) . . • . ^ -i/Vl

90 Echinus Ananchites ovata, (id.) .... 147

91 Clypeaster rosaceus, (id.) ..... 147

92 Ophiura lacertosa, (id.) ..... 147

93 Euryale muricatum, (id.) . . . ^ 24^

94 Pentacrinus europa^us, (Thomson) . . .147

95 Ambulacra, and feet o^ Asterias, viewed from the under side,

(Reaumur) ' 148

96 Ditto, viewed from the upper side, (id.) . . .148

97 Vesicles appended to the feet of the Asterias . .148

98 Polygonal pieces composing the test of the Echinus . . 150

99 Structure of a detached piece of ditto . . . ' 15()

100 Spine of the Cidaris, (Carus) . . . * . 150

101 Shell o^-Unio batava, (Goldfuss) . . . , ' j^g

102 Adductor muscle of Oyster, (Hunterian Museum) . . I'eo

103 Shell orPholas Candida, with abductor muscle, (Osier) . 161

104 Foot of Cardium edule, (Reaumur) . . . .162

105 Planorbus cornutus (Cuvier) ... * 166

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

Pearl, (Herschel) •• ... 169

107 Directions of the fibres in the component strata of shells . 170

108 Shell of Ac/i«fm«ze6r«, (DeBlainville) . . I'ya

109 Longitudinal section of ditto, (id.) . . , ' j-^g

110 Shell ofPterocerus scorpio, at an early stage of growth, (id.) ' 178

111 Shell of the same when completely formed, (id.) . . 173

112 Shell of Ci/proia exanthema at an early period of growth, (id.) ' 178

113 Shell of the same animal, when completed, (id.) . 179

114 Transverse section of the shell of the CyprcBa exanthema, '

(Hunterian Museum) . i-o

115 Shell of CoMMs . ..'.** jQj

110 Longitudinal section of the same, (original) . . .181

117 Transverse section of the same, (Brug°jiere) . * . * 181

118 Inner surface of the Epiphragma of the Helix pomatia, (De

^l^'"vil^e) • 183

xxu

LIST OF ENGRAVINGS.

Fig.

119 Outer surface of the same, (id.)

120 Clio borealis, (Cuvier)

121 Sepia loligo, (De Blainville)

122 Suckers of the same, (id.)

123 Suckers of the Octopus, (original) .

124 Shell of Spirilla australis, (De Blainville)

125 Longitudinal section of the same, (id.)

126 Shell of JSautilus pompilius (id.)

127 Longitudinal section of the same, (id.)

128 Pontobdella muricata, (Bruguiere)

129 Nereis, (id.) .

130 Erpobdella vulgaris (Lam.) Hirudo hyalina

131 Diagram illustrating the rings and muscles of Annelida,

(original) . . . ,

132 Gordius aquaticus ....

133 Serpula opercularia

134 Terebella conchilega, (De Blainville)

135 Arenicola piscatorum, or Lumbricus marinus

136 Aranea diadema, (Rcesel)

137 Divisions of the limb of a Crustaceous animal

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

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

pairs, (id.)

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

143 Julus terrestris .....

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

Durckheim) .....

145 Eggs of Bombyx mori .....

146 Larva of the same .....

147 Pupa of the same ......

148 Imago of the same .....
148* A Caterpillar of the Phalena striaria, (Hubner)

B The same in a rigid position, (Lyonet)

149 Calosoma Sycophanta, (Kirby and Spence)

150 Analysis of skeleton of the same, (Carus)

151 Hind vievi? of the segment of the head in the same, (id.) .

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

nified view, (Bauer) .....

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

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

marginalis, (id.) . . . . .

155 Cushions and sucker of the Acridium biguttulum, Latr. (id.)

Page

183
186

187
187
187
191
191
191
191
195
195
195

195
198
198
198
198
202
205
205

205
205
213

214
217
217
217
217
224
224
227
228
228

235
235

235
235

LIST OF ENGRAVINGS. xxiii

I^'g- Page

156 Dytiscus marginalis, upper side, (Roesel) . . . 2:37

157 Lower side of the same insect, (id.) . . . 287

158 Notonecta glauca, (Rccsel) . . . . . 238
158* Fore leg of Gryllotalpa, (Kidd) . . . 242

159 Wing of Gryllus nasutus. Orthoptera . . . 246
1(S0 Wmg 0^ Lihellula grandis. Neuroptera . . 246

161 Wing of Ichneumon persuasorius. Ilymenoptera . . 246

162 Wing of Tipula oleracea. Diptera . . . 246

163 Sting of Antliophora retusa, (original) . . . 248

164 Separate scales of the wing oT Hesperia Sloanus, (original) 250

165 Arrangement of the scales in the wing of the same . . 250

172 Longitudinal section of the thigli-bone to show the cancel-

lated structure, (Cheselden) .... 262

173 Longitudinal section of the humerus, (id.) . . 262

174 Ossification of the parietal bone, (id.) . . . 265

175 Early stage of ossification of the bones of the skull, (Cloquet) 265

176 The same in the adult, showing the sutures . . . 265

177 Dorsal vertebra, human ..... 271

178 Junction of vertebrse forming the spinal column . . 271

179 Longitudinal section of the same, showing the spinal canal 271

180 Elements of structure of a vertebra, (Cams) . . . 274

181 Skeleton of Hog, (Pander and D'Alton) . . .279

182 Sternum, clavicle, and scapula ; human . . . 279

184 Skeleton of Cyprinus carpio, (Bonnaterre) . . 286

185 Diagram illustrating the progressive motion of Fishes . 287

186 Front view of the vertebra of a Cod, (Gadus morrhua) . ' 288

187 Side view of the same ..... 288

188 Vertical and longitudinal section of a part of the spinal co-

lumn in the same ..... 288

189 A similar section, showing the gradation of structure . 288

190 Similar section in the Squalus centrina, (Carus) . . 288

191 Bones 0^ the shoulder of the Lophius piscatorius, (id.) . 293

192 Pectoral fin of the Raia davata, (id.) . . . 293

193 Belt of bones of the shoulder of a Ray, (id.) . . 294

194 Muscular system of Cyprinus alburnus, (id.) . . 295

195 Air bladder of Cyprinus carpio, (Blasius) . . 298

196 Eggs of the Frog . . , . . . ,303

197 Side view of Tadpole magnified, (Rusconi) . . 303

198 Upper view of the same, (id.) .... 303

199 Adult Frog ' 303

200 Skeleton of Frog, (Cheselden) .... 306

201 Skeleton of the Viper ..... 310

Xxiv LIST OF ENGRAVINGS.

Fig. I'^g^

202 Ribs and spine of Boa cons/ncfor, (Home) . . . 312

203 Bones of the foot of the same, (Mayer) . . .311

204 Muscles moving the claw of the same, (id.) . . . 311

205 Rudimental bones of the foot of the Tortryx scytale, (id.) 311

206 oHhe Tortrix corallinus, (\di.) . . • 311

207 of the Anguis fragilis, (id.) . . . 311

208 of the Amphisbcena alba, (id.) . . • 311

209 of the Coluber pullutatus, (\A.) . . . 311

210 Chalcides pentadacttjlus, (Bonnaterre) . . • 311

211 Under surface of the foot of the Lacerta gecko, magnified

four times, (Bauer) ..... 319

212 Side view of a longitudinal section of the same, (id.) . 319

213 Skeleton of the Tortoise, (Carus) . . . .322

214 Section of the thigh bone of the same, (id.) . . 322

215 Hind view of skull of Testudo mydas, (id.) . . . 325

216 Bones sustaining the fin of the Delphinus phoccena, (Pander

and D' Alton) 336

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

nucli(B, (original) ..... 346

218 Skeleton of the Stag, (Cheselden) . . .350
218* A. Longitudinal section of the horn of an Ox, (original) . 355

B. Ditto of an Antelope, (original) . . . 355

c. Extremity of the same, (original) . . . 355

219 Subcutaneous muscles of the Hedge-hog, relaxed, (Carus) 364

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

(Cuvier) . . . . . • .364

221 Skeleton of the Lion, (Pander and D' Alton) . . 365

222 Skeleton of Drflco ?;oZa?zs, (Tiedemann) . . . 379

223 Skeleton of Vespertilio Molossus, (Temmink) . . 380

224 Skeleton of the Swan, (Cheselden) .... 385

225 Lateral section of the cervical vertebra of the Ostrich, (ori-

ginal) ...... 388

226 Fibrils of the vane of a feather, magnified, (original) . 393

227 Edges of the fibres, magnified, (original) . . . 393

228 Feather, showing its structure, (F. Cuvier) . " . . 396

229 Capsule, or Matrix of the feather, (id.) . . .396
• 230 View of the parts enclosed in the Capsule, when laid open,

(id.) 396

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

conical membranes, (id.) .... 396

233 Extensor muscles of the foot and toes of a bird, (Borelli) . 405

234 Position of a bird in roosting, (id.) . . . 405

LIST OF ENGRAVINGS. XXV

VOLUME II.

Fig. Page

239 Cyclosis, or partial circulation in the cells of the Caulinia

fragilis, magnified, (Amici) . . . .42

240 The same in the jointed hair of the Tradcscantia virginica,

(Slack) ....... 42

241 Section of the Hydra vividis, magnified, (Trembley) . 58

242 Hydra vividis seizing a worm, (id.) . . . .59

243 The same after swallowing- a minnow, (id.) . .59

244 A Hydra which has swallowed another of its own species, (id.) 59

245 Compound Hydra, with seven heads, (id.) . . 59

246 Veretilla lutea, showing the communicating vessels of the Po-

lypes, (Quoy et Gaimard) .... 64

247 Nutrient vessels of the TcBiiia solium (Chiaje) . . 64

248 T(Enia globosa, or Hydatid of the Hog, (Goeze) . 64

249 Horizontal section of the Rhizostoma Cuvieri, Peron, (Ey-

senhardt) . . . . . . 67

250 Geronia HexaphyUa,I*cron, Medusa proboscidalis, (Forskal) 67

251 Vascular net-work in margin of the disk of the Rhizostoma

Cuvieri, (Eysenhardt) .... 67

252 Vertical section of the Rhizostoma Cuvieri, (id.) . . 68

253 Transverse section of one of the arms of the same, (id.) . 68

254 Transverse section of the extremity of a tentaculum of the

same, (id.) ...... 68

255 Leucophra patula, highly magnified, (Ehrenberg) . . 73

256 Alimentary canal and cffica of the same, viewed separately,

(id-) 73

257 Vertical section of the Acrmzrt conrtcea, (Spix) . . 75

258 Digestive organs of the Ai-^enas, (Tiedemann) . . 76

259 Siomsichs oHhe Na is vermicular is, (RcBsel) . . 77

260 Stomachs of the //«Vw(/o ?nefZicmaZi5, (original) . . 78

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

(original) ••-... 78

262 Tooth of the same, detached, (original) ... 78

263 Glassopora tuberculata; Hirudo complanata, Lin. (Johnson) 78

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

organs, (id.) 79

265 Diagram showing the arrangement and connexions of the or-

gans of the vital functions in Vertebrata, (original) . 81
Vol. I. D

XXVI LIST OF ENGRAVINGS.

Fig. Page

266 Spiral probosces of Papilio urticcc, (Griffith) . . 87

267 Trophi of Locusta viridissima, (Goldfuss) ,, . . 92

268 Filaments composing the rostrum, or proboscis, of the Cimex

nigricornis, (Savigny) . . . . .94

269 Sheath of the proboscis of the same insect, (id.) . . 94

270 Toothed cartilage of tlie f/eZia;_po7n«fra, (Cuvier) . . 95

271 Mechanism for projecting and retracting the tongue of the

Woodpecker, (original) .... 99

272 Laminse of Whalebone descending from the palate of the Ba-

IcBna mysticetus, (Bonnaterre) .... 102

273 Teeth of the Delphinus phoccena, (Cloquet) . . 106

274 Skull of Tiger, (Cuvier) 108

275 Skull of Antelope, (Pander and D'Alton) . . . 109

276 Skull of Rat, (id.) . . . . . .110

277 Longitudinal section of simple tooth, (Rousseau) . Ill

278 Surface of the grinding tooth of a Horse, (Home) . . Ill

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

280 Lonoitudinal section of the incisor tooth of the Rodentia . Ill

281 Vertical section of the grinding tooth of the Elephant, (Home) 114

282 Grinding tooth of the African Elephant, (id.) . . 114

283 Grinding tooth of the Asiatic Elephant, (id.) . . . 114

284 Succession of teeth in the Crocodile, (Carus) . . 120

285 Venomous fang of the Coluber naia, (Smith) . . 121

286 Transverse section of the same, (id.) . . . 121

287 The same tooth, at an earlier period of growth, (id.) . . 121

288 The same, still less advanced in its growth, (id.) . 121

289 Base of the former, (id.) ..... 121

290 Base of the latter, (id.) ..... 121

291 Transverse section of the young fang, about its middle, (id.) 121

292 A section, similar to the last, of another species of serpent, (id.) 121

293 Squalus pristus. b. Under side of its snout, (Latham) . 122

294 Interior of the Stomach of a Lobster, (original) . . 123

295 Gastric teeth of BuUcea aperta, (Cuvier) . . . 123

298 Gizzard of the Swan, (Home) .... 124

299 Crop and gizzard of the Parrot, (id.) v . . . 131

300 Crop of the Pigeon, (id.) . . . . .131

301 Human stomach, (id.) ..... 133

302 Interior of the stomach of the African Ostrich, (id.) . 135

303 Gastric glands of the same, (id.) .... 135

304 Gastric glands of the American Ostrich, (id.) . . 135

305 Longitudinal section of the gastric glands of the Beaver, (id.) 135

306 Stomach of Dormouse, (id.) ..... 139

LIST OP ENGRAVINGS. XXVii

^07 Stomach of Hi/rax capensis, (Cuvier) . . . ^39

308 Stomach of Porcupine, (id.) .... 139

309 Stomach of iTflw^^t/roo, (id.) . . . 109
'SIO Stomach of Delphinus phoccBna, (\^l.) . . ^ 239

311 Cardiac valve of the Horse, (Gurlt) . . . * 140

312 The four stomachs of a Sheep, (Carus) . . .141

313 Ini>er surface of the honey-comb stomach, (Home) . 141

314 Inner surface of the many-plies stomach of an Ox, (id.) . 141

315 Interior cellular surface of the second stomach of the Camel

<^'^'^ • • • . . . . ' 141

316 Spiral valve in the intestine of the Shark, (Blasius) . . 149

317 Digestive organs of the Mantis religiosa, (Marcel de Serres) 153

^1^ Melolontha vulgaris, (Leon Dufour) . . 154

^1^ Cicindela campestris, (id.) . . , J54

320 Portion of a hepatic vessel of the Melolontha, highly magni-

fied, (Straus Durckheim) .... 155

321 Alimentary canal of the .4cn^«cj9ferff, (original) . . 155

322 Interior of the gizzard of the same, magnified, (oriainal) . 155

323 Row of large teeth in the same, still more magnified,^ (original) 155

324 Profile of one of those teeth, still more highly magnified" (ori-

g^"al) j^

325 Base of the same tooth, seen from below, (original) . 155

326 Alimentary canal of the Larva of the Spkinx°ligustri, (orio-i-

nal) °. 157

P"^ ■ of the Pupa of the same, (original) . .157

328 of the Imagoof the same, (original) . . 157

329 of the Patella, (Cuvier) . . . ' . * 159

330 Stomachs of the PZeMroirancAiisPeromf, (id.) . . 159

331 Pyloric appendices in the Salmon, (id.) . . .160

333 Detached Dorsal vessel of Melolontha vulgaris, (Straus

Durckheim) . . . _ _ .172

334 The same, with its ligamentous and muscular attachments*

O^O ' 172

335 Side view of the anterior extremity of the same vessel, (id.) 172

336 Section of the dorsal vessel, to show its valves, (id.) . . 170

337 Circulation iit the antenna of the Semblis viridis, (Carus) * 17.5

338 Course of circulation in the same insect, (id.) . 175

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

view, (original) . ^ j^y

340 The same in the Chrysalis, (original) . . ' . 177

341 The same in the Moth, (original) . . * * 177

342 The same viewed from above, (original) . . * . 177

XXVlll

LIST OP ENGRAVINGS.

Fig.

343 Magnified lateral view of the anterior extremity of the dorsal

vessel, (original) . • . . .

344 Magnified dorsal view of the same, (original)

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

346 Heart and vessels of the Amnea dotnestica, (Treviranus)
346* Circulation in the Planaria nigra, (Duges)

347 Course of circulation in the Erpobdella vulgaris, (Morren) .

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

349 Vascular dilatations, or hearts of the Lumhricus terrestris,

(Morren) . . . . •

350 Cavities and great vessels of the Heart, .

351 The Heart laid open to show its Valves,

352 Plan of simple circulation,
' 353 Plan of double circulation,

354 Branchial circulation in Maia Squinado, (Audouin)

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

356 Plan of circulation in Fishes,

357 Plan of circulation in Batrachia,

359 Plan of double or warm-blooded circulation,

360 Heart of the Diigong, (Home)

365 Valves of the Veins, (Cloquet) .

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

367 Branchial apertures in the Squalus glaucus, (Bonnaterre)

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

369 Internal structure of the branchias of the same, (Home)

370 Stigmata in the abdominal surface of the Dytiscus margina-

lis, (Leon Dufour) . . . . •

371 Stigmata of the Ceramhyx heros, (Fab.) magnified, (id.)

372 Longitudinal trachese of Carabus auratus, (id.) .

373 Air vesicles and trachea? of the Scolia hortorum, (Fab.) high-

ly magnified, (id.) ....••

374 Respiratory apparatus of the Scorpio eiiropcBus, (Treviranus)

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

377 Air cells of the 0-s/ric/i, (Parisian Academicians)

378 Lymphatic Absorbents, . ■• ...

379 Passage of Nerves through a ganglion,

380 Plexus of Nerves, .....

381 Varieties of forms of antennae of Insects, (Goldfuss) .

382 Vertical and longitudinal section of the right nostril in man,

383 Vertical transverse section of the same, .

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

385 Turbinated bones of the »SeflZ, (id.)

Page

177
177
177
180
181
183
183

184

187

188

189

191

193

195

196

197

199

200

206

216

216

216

216

222
222
222

222
225
229
233
250
255
255
272
283
284
285
285

LIST OP ENGRAVINGS.

XXIX

^'^- Page

386 Turbinated bones of the T?<rA:fy, (id.) - . . 287

387 Nerves distributed to the bill of the Duck, (id.) - - 288

388 Nasal cavities of the Pero«/2<i;m/27i5, (Cuvier) - . 291

389 Nasal cavity of the Raia balls or Skate, (Harwood) - 291

390 Human ear, (Cloquet) - - _ _ _ 299

391 Posterior surface of the cavity of tlie tympanum, (id.) - 301

392 Ossicula auditus, or small bones of the tympanum, - - 301

393 The position of the latter in the tympanum, - - 301

394 Magnified view oT the labyrinth detached from the surround-

ing parts, (Breschet^ - - - . . 303

395 Interior structure of the labyrinth, (id.) - - - 304

396 Membranous labyrinth, witli its nerves, (id.) - . 304

397 Cretaceous bodies in the labyrinth of the Dog, (id.) - 304

398 Ditto in that of the //are, (id.) - - - - 304

399 Organ of hearing in the Lobster, (Cams) - - 308

400 Groove in the sac of the former, (id.) . _ . 308

401 Organ of hearing in the Astacusfliiviatilis, (id.) - 308

402 Interior view of the same, (id.) - - . . 308

403 Membranous labyrinth of the i«oyiiM5;)«scG?orms, (id.) - 310

404 Organ of hearing in the Frog, (Bell) - - - 311

405 Ear of the Turkey, (Carus) - - - . 31X

406 Diagram illustrating one mode of obtaining images of objects,

(original) - - - - _ - 319

407 Simple Camera Obscura - - - - - 320

408 Law of the refraction of a ray of light, ... 321

409 Convergence of rays to a fijcus, - - - _ 322

410 Convergence by a double conve.x lens, - - _ 324

411 Spherical Aberration, - - - . _ 324

412 Variations of focal distance consequent upon variations of di-

vergence of the incident rays, - - - _ 325

415 Horizontal section of right human eye, magnified, (Home) 326

416 Straight and oblique muscles of the eye-ball, - - 328

417 Lacrymal apparatus, ----.. 330

418 Eye of //eZix;jowaa'«, (Muller) - - - . 339

419 Stemmata of Caterpillar, (Marcel de Serres) - - 342

420 Eye of the -Scorpio fw^icnm, (Muller) - - . 340

421 Conglomerate eyes of Julus terrestris, (Kirby and Spencc) 342

422 External magnified view of the compound eye of the Melolon-

tha vulgaris, (Straus Durckhcim) - - - 344

423 Ditto of that of a P/ia?cn«, - - - - ;M4

424 Section of the compound eye of the Libellula vulgata, magni-

fied, (Duges) --.... 344

XXX LIST OF ENGRAVINGS.

Fig. Page

425 Highly magnified view of the outer margin of the preceding

section, (id.) ------ 344

426 Portion of the section of the eye of the Melolontha vulgaris,

(xMuller) - - - - - - 344

427 Portion of the section of tl)e eye of the Libellula, (Dnges) 344

428 Portion of the section of the eye of the Melolontha vulgaris,

(Straus Durckheim) ----- 344

430 Interior of the eye of the Percaj^iii;?V/H//s, (Cuvier) - 349

431 Fibres of the crystalline lens of the Cod, (Brewster) - 350

432 Denticulated structure of these fibres, (id.) - . - 350

433 Section of the eye of the Goose, (Home) - - 353

434 Nictitating membrane of a Bird, (Petit) - - - 353

435 Muscles of the nictitating membrane, (id.) - - 353

438 Talilrus, (Latreille) - - - - - 381

439 Nervous system of the Talitrus, (Audouin) - - 382

440 Nervous system of C^/mof/ioa, Fab,, (id.) - - - 382

441 Nervous system of the il/aia s^wma^^o, (id.) - - 383

442 Nervous system of the Larva of the Sphinx ligustri, (New-

port) 386

443 Ditto of the Chrysalis of the same, (id.) - - - 386

444 Ditto of the Imago of the same, (id.) - - - 386

445 Nervous system of the Asterias, (Tiedemann) - - 387

446 Ditto of the jl^Z?/sm, (Cuvier) - - - - 387

447 of the Patella, (id.) . ... 387

448 oHhQ Sepia Octopus, (\A.) - - - - 387

449 Brain and spinal marrow of the Cqlumba turtur, (id.) - 389

450 Transverse section of the spinal marrow of the Cyprinus car-

pio, - - - - - - - 389

451 Brain and spinal marrow of the Trigla lyra, (Arsaky) - 389

452 Brain of the MurcBua conger, (Serres) - , - 389

453 Percafuviatilis, (Cuvier) . - _ 389

454 Testudo mydas, (Carus) - - - - 389

455 Crocodile, (id.) - - - - - 389

456 Lion, (Serres) - . - _ . 339

457 Lateral view of the brain of the Perch, (Cuvier) - - 389

458 of the Testudo mydas, (Carus) - - _ 389

459 of a section of the brain of the Dove, (id.) - 389

460 of the Lion, - - - - - - 389

461 Vertical section of the human brain, (Monro) - - 393

462 Progressive changes in the Monas - - - - 410

463 Vorticella 410

OUTIilNE

OF

CUVIER'S CLASSIFICATION OF ANIMALS,

WITH

EXAMPLES OF AI^IMALS BELONGING TO EACH DIVISION.

Blmana

Quadrumana -

Cheiroptera -

Jnsectivora -

Plantigrada -

Digitig-rada -
Amphibia

Marsupialia -
Rodentia

Edentata

Pachydermata

Ruminantia -

Cetacea

Accipitres -

Passeres

Scansores

Gallinse

Grallce

Palmipedes -

Chelonia
Sauria
Ophidia
Batracliia

Acanthopteryg"ii
Malacopteiygii

L VERTEBRATA.

1. Mammalia.
Man.

Monkey^ Ape, Lemur.

Bat, Colugo.

Hedge-hog, Shrew, Mole.

Bear, Badger, Glutton.

Dog, Lion, Cat, 3Iarii7i, Weasel, Otter.

Seal, Walrus.

Opossum, Kanguroo, Wombat.

Beaver, Rat, Squin-el, Porcupine, Hare.
< Sloth, Armadillo, Ant-eater, Pangolin, Orni-
\ thorhyncus.

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

Dolphin, TVIiale.

2. AvES.

Vulture, Eagle, Owl.

Thrush, Swallow, Lark, Crow, Sparrow, Wren.

Woodpecker, Cuckoo, Toucan, Parrot.
Peacock, Pheasant, Grous, Pigeon.
Plover, Stork, Snipe, Jbis, 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, Swoi'd-fish, Mullet.
S Salmon, Herring, Pike, Carp, Silurus, Cod,
i Sole, Re mora, Eel.

XXXll

CLASSIFICATION OF ANIMALS.

Lophobranchi

Plectognathi

Chondropterygii

1. Cephalopoda

2. Pteropoda

3. Gasteropoda

4. Acephala

5. Brachiopoda

6. Cirrhopoda

Tubicola -
Dorsibranchia
Abranchia -

1. Malacostraca.
Decapoda
Stomapoda
Amphipoda ■
Lccmodipoda ■
Isopoda -

2. Entomostraca

Pulmonalia
Trachealia -

Aptera
Coleoptera -
Orthoptera -
Hemiptera -
Neuroptera
Hymenoptera
Lepidoptera
Rhipiplera -
Diptera

1. Echinodermata

2. Entozoa

3. Acalephae

4. Polypi -

5. Infusoria

Pike-Jish, Pegasus.
Sun-fish, Trunk-fish.
Lamprey^ Shark, Bay, Sturgeon.

II. MOLLUSCA.

Cuttlefish, Calamary, Nautilus.

Clio, Hyalaea.

Slug, Snail, Limpet, Whelk.

Oyster, Muscle, Ascidia.

Lingula, Terebratula.

Barnacle.

III. ARTICULATA.

1. Annelida,

Serpula, Sabella, Amphitrite.
Nereis, Aphrodite, Loh-worm.
Earth-worm, Leech, Nais, Hair-worm..

2. Crustacea.

Crab, Lobster, Prawn.

Squill, Phyllosoma.

Gammarus, Sand-hopper.

Cyamus.

Wood-louse.

Monoculus.

3. Arachnid A.

Spider, Tarantula, Scorpion..
Phalangium, Mite.

4. Insecta.

Centipede, Podura.
Beetle, Glow-worm..
Grasshopper, Locust.
Firefly, Aphis.
Dragon-fly, Ephemera.
Bee, Wasp, Jint.
Butterfly, Moth.
Xenos, Sty lops.
Gnat, House-fly.

IV. ZOOPHYTA.

Star-fish, Urchin.

Fluke, Hydatid, Tape-worm.

Actinia, Medusa.

Hydra, Coral, Madrepore, Pennatula.

Brachionus, Vibrio, Proteus, Monas.

«• C; State College

ANIMAL AND VEGETABLE

PHYSIOLOGY.

INTRODIJCTIOIV.

CHAPTER I.

FINAL CAUSES.

To investigate the relations which connect Man with his
Creator is the noblest exercise of human reason. The Be-
ing who bestowed on him this faculty cannot but have in-
tended that he should so exercise it, and that he should ac-
quire, 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 transcendent glory
with which those attributes are encompassed. To Man
have been revealed the power, the w^isdom, and the good-
ness of God, through the medium of the Book of Nature,
in the varied pages of which they are inscribed in indelible
characters. On Man has been conferred the high privilege
of interpreting these characters, and of deriving from their
contemplation those ideas of grandeur and sublimity, and
those emotions of admiration and of gratitude, which ele-
vate and refine the soul, and transport it into regions of a
purer and more exalted being.

Vol. I. 3

18 FINAL CAUSES.

A study which embraces so extensive a range of objects,
and which involves questions of such momentoue interest
to mankind, must necessarily be arduous, and requires for
its successful prosecution the strenuous exertions of the hu-
man 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 ef-
forts: 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 surrounds us. The reflection continually
presents itself that the portion of creation we are here per-
mitted to behold is as nothing, when compared with the
immensity of space, which, on every side, spreads far be-
yond the sphere of our vision, and, indeed, far beyond the
powers of human imagination. Of the planetary system,
which includes this earth, our knowledge is almost entirely
limited to the mathematical laws that regulate the motions
of the bodies which compose it, and to the celestial me-
chanism which patient investigation has at length discovered
to be that most admirably calculated to preserve their har-
mony and maintain their stability. Still less have we the
means of penetrating into the remoter regions of the heavens,
where the result of our investigations respecting the myriads
of luminous bodies they contain amounts to little more than
the knowledge of their existence, of their countless num-
bers, 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 inexhaustible variety of objects is con-
tained: what an endless diversity of phenomena is presented;
what wonderful changes are occurring in rapid and perpetual

PINAL CAUSES. 19

succession! Throughout the whole series of terrestrial be-
ings, what studied arrangements, what preconcerted adapta-
tions, what multiplied evidences of intention, what signal
proofs of beneficent design exist to attract our notice, to ex-
cite 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 stupen-
dous magnitude to impress the imagination and overpower
us by their awful grandeur, not less impressive, nor less re-
plete with wonder, are the manifestations of those attributes
in the minuter portions of nature, which are more on a
level with our senses, and more within the reach of our
comprehension. The modern improvements of optical
science, which have expanded our prospects into the more
distant regions of the universe, have likewise brought with-
in our range of vision the more diminutive objects of crea-
tion, and have revealed to us many of the secrets of their
structure and arrangement. But, farther, our reason tells
us that, from the infinite divisibility of space, there still exist
worlds far removed from the cognizance of every human
sense, however assisted by the utmost refinements of art;
worlds occupied by the elementary corpuscles of matter,
composing, by their various configurations, systems upon
systems, and comprising endless diversities of motions, of
complicated changes, and of widely extended series of
causes and effects, destined for ever to remain invisible to
human eyes, and inscrutable to human science.

Thus, in whatever field we pursue our inquiries, we are
sure to arrive at boundaries within which our powers are
circumscribed. Infinity meets us in every direction, whe-
ther in the ascending or descending scale of magnitude; and
we feel the impotence of our utmost efforts to fathom the
depths of creation, or to form any adequate conception of
that supreme and Dominant Intelligence, which compre-
hends the whole chain of being extending from that which
is infinitely small to that which is infinitely great.

It is incumbent on us, before ene;aging in a study of such

20 PINAL CAUSES.

vast importance, and extending over so wide a field as that
which lies before us, to examine with attention the nature
of those processes of reasoning, by which we are conducted
to the knowledge of the peculiar class of truths we are seek-
ing. Such a preliminary inquiry is the more necessary, in-
asmuch as the investigation of these truths is beset wdth
many formidable difficulties and liable to various sources of
fallacy, which are not met with in the study of other depart-
ments of philosophy.

The proper objects of all human knowledge are the rela-
tions that exist among the phenomena of 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, ac-
cordingly, these two classes of relations give rise to different
kinds of knowledge, each of which requires to be inves-
tigated in a peculiar mode and by a different process of rea-
soning. 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 call the Laws of Nature. It is the pro-
vince of philosophy, strictly so called, to discover the cir-
cumstances or laws which regulate 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 powers of Nature. With
reference to phenom.ena which are purely mechanical, that
is, to changes which consist in the sensible motions of ma-
terial bodies, these powers are denominated /orce.s; and the
intensities, the operations, and the characters of these forces
admit of exact definition, according to the qualities of the
corresponding efiects they produce. It is by pursuing the
method of philosophical induction, so well explained by Ba-
con, that the physical sciences, which the misdirected efTorts
of former ages had failed to advance, have, within the last

FINAL CAUSES. 21

two centuries, been carried to a height of perfection which
affords just grounds for exultation in the achievements of the
human intellect.

In the investigation of the powers which are concerned in
the phenomena of living beings, we meet with diflficulties in-
comparably greater than those that attend the discovery of
the physical forces by which the parts of inanimate matter
are actuated. The elements of the inorganic world are few
and simple; the combinations they present are in most cases
easily unravelled; and the powers which actuate their mo-
tions, or effect their union and their changes, are reducible
to a small number of general laws, of which the results may,
for the most part, be anticipated, and exactly determined by
calculation. What law, for instance, can be more simple
than that of gravitation, to which all material bodies, what-
ever 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 prescribed 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 per-
manence of the system is effectually secured. All the ter-
restrial changes dependent on these motions partake of the
same constancy. The same periodic order governs the suc-
cession 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 existing cause.

Equally definite are the operations of the forces of cohe-
sion, of elasticity, or of whatever other mechanical powers
of attraction or repulsion there may be, which actuate, at in-
sensible distances, the particles of matter. We see liquids,
in obedience to these forces, collecting in spheroidal masses,

22 FINAL CAUSES.

or assuming, at their contact with solids, certain curvilinear
forms, which are susceptible of precise mathematical deter-
mination. In different circumstances, again, we behold
these particles suddenly changing their places, marshalling
themselves in symmetric order, and constructing by their
union solid crystals of determinate figure, having all their
angles and facets shaped with mathematical exactness.

The forces by which dissimilar particles are united into
a chemical compound have been termed Chemical *Bffinities;
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 definite proportions; a law, the dis-
covery of which has conferred on Chemistry the same cha-
racter 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 sufficient 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 re-
searches have extended, we every where meet with the
same regularity in the phenomena, the same simplicity in
the laws, and the same uniformity in the results. All is
strictly defined, and subjected to rigid rule: all is subordi-
nate to one pervading principle of order. The great Creator
of the universe has exercised in its construction the severest
and most refined geometry, has traced with unerring precision
the boundaries of all its parts, and has prescribed to each
element and each power its respective sphere and limit.

Far different is the aspect of living Nature. The specta-
cle here offered to our view is every where characterized 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, exhibit-
ing peculiar movements, actuated by new and unknown
powers, and gifted with high and refined endowments. In
place of the simple combinations of elements, and the sim-

FINAL CAUSES. 23

pie properties of mineral bodies, all organic structures, even
the most minute, present exceedingly complicated arrange-
ments, and a prolonged succession of phenomena, so varied
and so anomalous, as to be utterly irreducible to the known
laws which govern inanimate matter. Let us hasten, with
fresh ardour, to explore this new world that here opens to
our view.

Turning, then, from the examination of the passive ob-
jects of the material world, we now direct our attention to
the busy theatre of animated existence, where scenes of won-
der 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 peopled by multitudinous
races of sensitive beings, who have received from the boun-
teous hand of their Creator the gift of existence and the means
of enjoyment. Our curiosity is powerfully excited by pheno-
mena in which our own welfare is so intimately concerned,
as are all those that relate to animal life; and we cannot but
take a lively and sympathetic interest in the history of be-
ings in many respects so 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, re-
velling in the bliss of existence, and testifying by its inces-
sant 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 inconceiva-
ble variety of forms and constructions to which animation
has been imparted by creative power. What can be more
calculated to excite our wonder than the diversity exhibited
among insects, all of which, amidst endless modifications of
shape, still preserve their conformity to one general plan of
construction ? The number of distinct species of insects

24 FINAL CAUSES.

already known and described cannot be estimated at less
tban 100,000; and every day is adding to the catalogue.*
Of the comparatively large animals which live on land, how
splendid is the field of observation that lies open to the na-
turalist! What variety is conspicuous in the tribes of Quad-
rupeds and of Reptiles; 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 capacity for motion through the light element of air.
What profusion of being is displayed in the wide expanse of
the ocean, through which are scattered such various and such
unknown multitudes of animals ! Of Fishes alone the varie-
ties, as to conformation 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 lowxr divisions of the animal
scale; some swimming in countless miyriads near the sur-
face; some dwelling in the inaccessible depths of the ocean:
some attached to shells, or other solid structures, the pro-
ductions of their own bodies, and which, in process of time,
form, by their accumulation, enormous submarine moun-
tains, rising often from unfathomable depths to the surface.
What sublime views of the magnificence of creation have
been disclosed by the microscope in the world of infinite
minuteness, peopled by countless multitudes of atomic be-
ings which animate almost every fluid in nature ? Of these,
a vast variety of species has been discovered, each animal-
cule being provided w^ith appropriate organs, endowed with
spontaneous powers of motion, and giving unequivocal signs
of individual vitality. The recent observations of Profes-

* Four-fifths 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 his work on " The Wisdom of God as manifested in the
Creation," p. 24.

FINAL CAUSES. 25

sor Ehrenberg have brought to light the existence of 71//?-
nadsy which are not larger than the 24,000th of an inch,
and which are so thickly crowded in the fluid as to leave
intervals not greater tlian their own diameter. Hence, he
has made the computation that each cubic line, which is
nearly the bulk of a single drop, contains 500,000,000 of
these monads, a number which equals that of all the human
beings existing on the surface of the globe.

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 summits to the dark abysses of
the deep; if we penetrate into the shades of the forest, or
into the caverns a^ secret recesses of the earth; nay, if wc
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. Where-
ver life can be sustained, we find life produced. 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 combinations,
for their repetition in endless perpetuity, and for their su-
bordination to one harmonious scheme of general grood.

The vegetable world is no less prolific in wonders than
the animal. In this, as in all other parts of creation, ample
scope is found for the exercise of the reasoning faculties;
and at the same time abundant sources are supplied of intel-
lectual enjoyment. To discriminate the different characters
of plants, amidst the infinite diversity of shape, of colour, and

• In order to avoid thee too frequent, and consequently irreverent, intro-
duction of ihe Great Name of the Supkkxk Ukixr into familiar discourse on
the operations of his power, I have, throughout this Treatise, followed the
common usag-e of employing- the term Nature as a synonym, expressive of
the same power, but veiling- fromoiu* feeble sight the too dazzling- spleudom*
of its glory.

Vol. I. 4

26 FINAL CAUSES.

of structure, 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
differences of situation, sustains the modified life of each in-
dividual plant, and which continues its species in endless
perpetuity. Wherever circum.stances are compatible with
vegetable existence, we there find plants arise. It is well
known that, in all places where vegetation has been estab-
lished, the germs are so* intermingled with the soil, that
whenever the earth is turned up, even from considerable
depths, and exposed to the air, plants are soon observed to
spring, as if they had been recently sown, in consequence of
the o-ermi nation of seeds which had remained latent and in-
active during the lapse of perhaps many centuries. Islands
formed by coral reefs, which have risen above the level of the
sea, become in a short time covered with verdure. From the
materials of the most steril rock, and even from the yet
recent cinders and lava of the volcano. Nature prepares the
way for vegetable existence. The slightest crevice or ine-
quality is sufficient to arrest the invisible germs that are al-
ways floating in the air, and affords the means of sustenance
to diminutive races of lichens and mosses. These soon
overspread the surface, and are followed, in the course of a
few years, by successive tribes of plants of gradually in-
creasing size and strength; till at length the island, or other
favoured spot, is converted into a natural and luxuriant
garden, of which the productions rising from grasses to
shrubs and trees, present all the varieties of the fertile mea-
dow, 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 few hardy-
plants, which find sufficient materials for their growth in
these arid regions: and in the realms of perpetual snow
which surround the poles, the navigator is occasionally

FINAL CAUSES. 27

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 oc-
cupations in which he is engaged, and however wide may
be the field of his exertions, they still are insufficient to
satisfy the more enlarged curiosity of a philosophic mind.
The passive emotion of astonishment, in which inferior in-
tellects are content to rest, serves but to awaken, in him
who has learned to think, a desire of farther knowledge.
Filled with an ardent spirit of inquiry, he cannot but be im-
patient under the feeling that, while Nature has placed be-
fore 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
it in motion. With the hope of discovering her proceed-
ings, he hastens to explore the several parts which compose
the organized fabric, to examine in minute detail the anato-
my of its structure, and to ascertain the nature of the seve-
ral actions that take place within it. But overwhelmed by
the multiplicity of objects, and lost amidst the compli-
cation of phenomena, he soon becomes dismayed by the
magnitude and arduous nature of the investigation. He
finds that his labours w^ill be of no avail, unless, previous-
ly to any attempt at theory, he takes a careful and accu-
rate account of all the circumstances attending the history
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 individual vegetable and animal takes its
rise from an atom of imperceptible minuteness, and gradual-
ly increases in bulk by successive accretions of new matter,
derived from foreign sources, and by some refined, but un-
known process, transmuted into its own substance. Then,

* The red snow, discovered in Baffin's Bay on the 17th of August, 1818,
during- the Northern Expedition, under the command of Captain Ross, was
found to owe its colour to minute fung-i, or microscopic mushrooms, which
veg-etate on the surface of snow, as their natural abode. See Phil. Trans, for
1820, p. 165.

28 FINAL CAUSES.

following the progressive development of the organs, he ob-
serves them undergoing various modifications, as they are
assuming new forms, which characterize 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 individual being acting, in
its time, a variety of different parts; often reappearing 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 an-
tiquity.

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 development, and has
united its energies with those which had been before per-
fected. Yet, however complete the arrangements that have
thus been established, it is still necessary, in order to pre-
serve the whole system in a state in which it may be capa-
ble of exercising the functions of life, th^t the materials
which compose its fabric should undergo a certain, slow, but
constant renovation; and the same circle of actions and re-
actions, which have brought it to its state of perfection, must
continue to be repeated, in order that a due proportion may
be maintained between the consumption and the supply of
these materials. In the course of a certain time, however,
even under the most favourable circumstances, this equili-
brium begins to fail: the energies of the system decline:
and the processes of nutrition are insufficient to repair the
waste in the substance of the body. The fluids are dissi-
pated faster than they can be renewed; the channels through
which they circulate are more and more obstructed, and at
length cease to be pervious: and the solids gradually become
hard and rigid. As in a machine of which the wheels are
worn, and the springs have lost their elastic force, so in the
animal bod}', at an advanced age, the slightest additional
impediment that occurs will stop the movements of the

FINAL CAUSES. 29

whole system: and. when once stopped, their renewal is im-
possible. 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
development and growth, and which are inseparably inter-
woven with the conditions of their existence.

Numerous, however, are the extraneous and accidental
causes that may hasten or precipitate their destruction, long
before the period of natural decay. How striking is the
contrast, on those occasions, between the scene we have just
beheld of an animal in the full vigour of its powers, either
rapidly bounding across the plain, or gliding beneath the
wave, or soaring in the elevated regions of air, and the spec-
tacle 0^ 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 instantaneous a
change; so sudden and awful a catastrophe? Must we not
be animated by an eager desire to penetrate so great a mys-
tery, and resolve the many questions which so striking a
phenomenon must naturally suggest? What, we are led to
ask, is the nature of this extraordinary revolution, extending
over the whole of that frame which had so long delighted
the eye by its beauty, and producing this sudden and irre-
trievable extinction of the powers of life? How comes it
that all those mighty energies which the animal had so
lately displayed, and which had called forth our admiration,
perhaps even excited our envy, are at once and for ever
annihilated? What was the bond, thus suddenly dissevered,
which held together the various parts of that compound
frame? What potent spell has been dissolved, which could
retain in combination for so long a period the multifarious
elements of that exquisite organization; and from the con-
trol of which being now released, these elements hasten to
resume their wonted attractions, and entering into new

30 FINAL CAUSES.

forms of combination, are scattered into dust, or dissipated
in air, leaving no trace of their former union? What me-
chanism has been employed in its construction? What re-
fined chemistry has been exerted in assimilating new parti-
cles of matter to those previously organized, and in appro-
priating them to the nourishment of the parts with which
they become identified? By what transcendent power,
above all, did this assemblage of material particles first be-
come animated by the breath of life; and from what elevated
source did they derive those higher energies, apparently so
foreign to their inherent properties, and investing these once
lifeless and inert materials with the exalted attributes of ac-
tivity, of sensation, of perception, of intelligence? Shall
we ever comprehend the nature of this subtle and pervading
principle, by the agency of which all these wonderful phe-
nomena of life are produced, and which, combining into one
harmonious system so many heterogeneous and jarring ele-
ments, has led to the formation of this exquisite frame, this
elaborate machine, this miraculous assemblage of faculties?

The discovery of a clew, if any such can be found, to the
mazes of this perplexing labyrinth can be hoped for only from
the successful cultivation 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 examination
of the phenomena of life, to ascertain the laws which re-
gulate those phenomena, both as they apply to the indivi-
dual beings endowed with life, and also as they relate to the
various assemblages 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 physio-
logy to investigate, are, as I have before observed, of two
kinds, each founded upon relations of a different class. The
first, which depend upon the simple relation of cause an'd ef-

FINAL CAUSES. 31

feet, are concerned merely with the natural powers of mat-
ter. They are the laws that regulate the succession of pheno-
mena purely physical in all their stages. These phenome-
na consist in changes among material particles, which are
either of a meclianical or chemical nature; or in the affec-
tions of imponderable pliysical agents, such as heat, light-,
electricity, and magnetism; and they include also the pheno-
mena that take place in organized bodies, and which are re-
ferrible to the operation of certain physical powers, apper-
taining to particular structures, 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 peculiar to the living state. The second class of laws
comprise those which are founded on the relation of means
to an end; and which are usually denominated Jifia I causes.
They involve the operations of mind, in conjunction with
those of matter. They presuppose intention or design; a
supposition which implies intelligence, thought, motives, vo-
lition,— particular purposes to be answered, requiring the
agency of powers and of instruments adapted to the produc-
tion of the intended efiects: — the knowledge of the proper-
ties 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 ge-
neral objects, and these objects again subordinate to remoter
ends: so that the whole shall comprehend a systematic plan
of operations, conducive, on the most enlarged views, to ul-
timate and general utility.

The study of these final causes is. In some measure, forced
upon our attention by even the most superficial survey of
nature. It is 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
with their history. Microscopic observations teach us
that the embryo of an organic being contains within it-
self the rudiments of the future vegetable or animal struc-

32 FINAL CAUSES.

ture, into which it is gradually transformed by the slow
and successive expansion and. development of all its parts.
The processes of nutrition do nothing more than fill up
the outlines already sketched on the living canvas. Every
organ, nay every fibre, resulting from this development,
contributes its share in the production of certain definite ef-
fects, which we constantly witness 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 in-
volved in mystery, or the less replete with wonder. To
say that they are the results of chance conveys no informa-
tion; 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 constitution of human thought;
for if we are to reason at all, we can reason only upon the
principle that for every effect there must exist a correspond-
ing cause; or, in other words, that there is an established
and invariable order of sequence among the changes which
take place in the universe.

But though it be granted that all the phenomena we be-
hold are the efiects of certain causes, it might still be alleged,
as a bar to all farther reasoning, that these causes are not
only utterly unknown to us, but that their discovery is
wholly beyond the reach of our faculties. The argument is
specious only because it is true in one particular sense, and
that a very limited one. Those who urge it, do not seem
to, be aware that its general application, in that very same
sense, would shake the foundation of every kind of know-
ledge, even that which we regard as built upon the most
solid basis. Of causation, it is agreed that we know nothing:
all that we do know is, that one event succeeds another with
undeviating constancy. Now, if we were to probe this sub-
ject to the bottom, we should 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 pass-
ing in our minds, and of which we are necessarily con-

FINAL CAUSE* 33

scious. Our belief in the existence of external objects, in
their undergoing 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 in-
ferences which the mind is, by the constitution of its frame,
necessarily led to form. We may trace to a similar origin
the persuasion, irresistibly forced upon us, that there exist
not only other material objects beside our own bodies, but
also other intellectual beings beside ourselves. We can
neither see nor feel those extraneous intellects, any more
than we can see or feel the cause of gravitation, or the sub-
tle sources of electricity or 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 im-
pressions alone our knowledge can, in metaph3-sical strict-
ness, be termed certain.

Upon what evidence do I conclude that I am not a solitary
being in the universe; that all is not centred in myself; but
that there exist other intellects similar to my own? Un-
doubtedly no other than the observation that certain effects
are produced, which the experience I have had of the ope-
rations of my own mind lead me, by an irresistible analogy,
to ascribe to a similar agency, emanating from other beings;
beings, however, of whose actual intellectual presence I can-
not be conscious, whose nature I cannot fathom, whose es-
sence I cannot understand. I can judge of the operations of
other minds only in as far as those operations accord with
what has passed in my own. I cannot divine processes of
thought to which mine have borne no resemblance, I can-
not appreciate motives of which I have never felt the in-
fluence, nor comprehend the force of passions, never yet
awakened in my breast: neither can I picture to myself fecl#
ings to which no sympathetic chord within me has ever vi-
brated.

Our own intelligence, our own views, and our own affec-
tions, then, furnish the only elements by which it is possible

Vol. I. 5

34 FINAL CAUSES.

for us to estimate the analogous powers and attributes of
other minds. The difficulty of applying this scale of mea-
surement will, of course, increase in proportion to the dif-
ference between the objects compared; and although we may
conceive that there are powers and intelligences infinitely
surpassing our own, the conceptions we can form of such
superior essences must necessarily be indefinite and 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 expressed. When, on
the other hand, the objects contemplated are more within
the range of our mental vision; when, for instance, they are
phenomena that we can assimilate to our own voluntary
acts, and in which we can clearly trace the connexion be-
tween means and end, then does our recognition of the
agency of intellect become most distinct, and our conviction
of its real and independent existence become most intimate
and assured.

Such is the kind of evidence on which rests our belief of
the existence of our fellow men. Such, also, is the founda-
tion of our assurance that there exists a mighty Intellect,
who has planned and executed the stupendous works of crea-
tion, 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 insti-
tute 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 wdiich we
^ssess, 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 view here taken is, of course, limited to Natural Theology ; that
being' the express and exclusive object of these Treatises.

FINAL CAUSES. 35

the medium of human views; and our judgment of his bene-
volence can be formed only by reference to our own affec-
tions, and by their accordance with those ardent aspirations
after good, which the Author of our being has deeplyinter-
woven 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 hu-
man art. If in any unknown region of the earth we chanced
to discover a piece of machinery, of which the purpose was
manifest, we should not fail to ascribe it to the workman-
ship of some mechanist, possessed of intelligence, actuated
,by a motive, and guided by intention. Farther, if we had
a previous experience of the operation of similar kinds of
mechanism, we could not doubt that the effect we saw pro-
duced 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 immediately 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 eff'ects of such in-
struments, that they were intended to give motion to this
boat, and we should not hesitate to conclude that the whole
was the work of human hands, and the product of human
intelligence and design. If, in addition, we found this boat
furnished with a rudder and with sails, we should at once
understand the object of these contrivances, and our ideas
of the skill of the artificer would rise in proportion to the
excellence of the apparatus, and the ingenuity displayed in
its adaptation to circumstances.

Let us suppose that in another part of this lake we found
an insect,* shaped like the boat, and moving through the
water by successive impulses given to that medium by the
action of levers, extending from its sides, and shaped like
paddles, having the same kind of movement, and producing

* Such as the Notoneda glanco, Lin., or water boatman, and the Dytiscus
marginalis, or water beetle.

36 FINAL CAUSES.

the same efifects. Could we resist the persuasion that the
Artificer of this insect, when forming it of this shape, and
providing it with these paddles, had the same mechanical
objects in view? Shall we not be confirmed in this idea
on finding that these paddles are constructed with joints,
that admit no other motion than that of striking against the
water, and of thus urging forwards the animal in its passage
through that dense and resisting medium? Many aquatic
animals are furnished with tails which evidently act as rud-
ders, 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 nau-
tilus 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 unfavourable circumstances, is
not her design similar to that of the human artificer, when
he equips his bark with sails, and provides the requisite
machinery for their being hoisted or furled with ease and
expedition?

The maker of an hydraulic engine places valves in par-
ticular parts of its pipes and cisterns, with a view to pre-
vent the retrograde motion of the fluids which are to pass
through them. Can the valves of the veins, or of the lym-
phatics, or of the heart have a difierent 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 difierent
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 paralyze their efforts to assail it.

Does not the optician, who designedly places his convex

FINAL CAUSES. 37

lens at the proper distance in a darkened box, for the pur-
pose of obtaining vivid pictures of the external scene, evince
his knowledge of the laws of light, of the properties of re-
fracting media, and of the refined combinations of those me-
dia 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 forepart of which He has made a circular win-
dow for the light to enter, and spread out on the opposite
side a canvas to receive the picture ? Has no thought been
exercised in darkening the walls of this camera obscura, and
thus preventing all reflection of the scattered rays, that
might interfere with the distinctness of the image ?

But we farther observe in the eye many exquisite refine-
ments of construction, by which various defects, unavoida-
ble in all optical instruments of human workmanship, are
remedied. Of this nature are those which render the orjian
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 protection, the preservation,
and the movements of the e3^e-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 sufficient to
lead us irresistibly to this conclusion: but evidence of a si-
milar kind may be collected in abundance from every part
of living nature to which our attention can be directed, or
to which our observations have extended. The truths they
teach not only acquire confirmation by the corroborating
tendency of each additional fact of the same description, but
the multitude of these facts is so great, that the general con-

3S FINAL CAUSES.

elusion to which they lead must be considered as indubita-
ble. For the argument, as it has been justly remarked, is
cumulative; that obtained from one source being strength-
ened by that derived from another; and all tending to the
same conclusion, like rays converging to the same point,
on which they concentrate their united powers of illumina-
tion. ^

The more we extend our knowledge of the operations of
creative power, as manifested in the structure and economy
of organized beings, the better we become qualified to ap-
preciate the intentions with which the several arrangements
and constructions have been devised, the aKt with which they
have been accomplished, and the grand comprehensive plan
of which they form a part. By knowing the general ten-
dencies of analogous formations, we can sometimes recognise
designs that are but faintly indicated, and trace the links
which connect them with more general laws. By render-
ing ourselves familiar with the handwriting where the cha-
racters are clearly legible, we gradually learn to decipher
the more obscure passages, and are enabled to follow the
continuity of the narrative through chapters that would other-
wise appear mutilated and defaced. Hence, the utility of
comprehending in our studies the whole range of the or-
ganized creation, with a view to the discovery of final
causes, and obtaining adequate ideas of the power, the wis-
dom, and the goodness of G od.

( 39 )

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 compre-
hend them, are referrible to the following classes of objects.
The first relates to the individual welfare of the animal, em-
bracing 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 that have
been made for repairing the chasms resulting, in the present
circumstances of the globe, from the continual destruction
of life, by ensuring the multiplication of the species, and the
continuity of the race to which each animal belongs. The
the third includes all those arrangements which have been
resorted to, in order to accommodate the system to the con-
sequences that follow from an indefinite increase in the
numbers of each species. The fourth class relates to tliat
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 in-
quiry.

With reference to the welfare of the individual animal,
it is evident that in the brute creation, the great end to be
answered is the attainment of sensitive enjoyment. To this
all the arrangements of the system, and all the energies of
its vital powers must ultimately tend. Of what value would
be mere vegetative life to the being in whom it resides, un-
less it were accompanied by the faculty of sensation, and
unless the sensations thence arising were attended with plea-
sure? It is only by reasoning analogically from the feelings

40 THE FUNCTIONS OP LIFE.

we have ourselves experienced that we ascribe similar feel-
ings to other sentient beings, and that we infer their existence
from the phenomena which they present. Wherever these
indications of feeling are most distinct, we find that they
result from a particular organization, and from the affections
of a peculiar part of that organization denominated the ner-
vous substance. The name of brain is given to a particular
mass of this substance placed in the interior of the body,
where it is carefully protected from injury.

The sensations, for exciting which the brain is the mate-
rial instrument, or immediate organ, are the result of cer-
tain impressions made on particular parts of the body, and
conveyed to that organ by the medium of filaments, com-
posed 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 ex-
ternal objects, by which its activity is to be excited, and on
which it is to be dependent for the elements of all its af-
fections, both of sensation and of intellect. A considerable
portion of this treatise will be occupied with the develop-
ment of the series of means by which impressions from ex-
ternal 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 knowledge of
the existence and different qualities of the objects that pro-
duce 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 sensations 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 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 voluntary motion, by which the animal may
not only obtain possession of such objects as minister to its

THE FUNCTIONS OP LIFE. 41

gratification, and reject those which are useless or hurtful,
but may always move from place to place, and enlarge the
sphere of its perceptions and of its power. The same mass
of nervous substance which, under the name of brain, we
have recognised as the organ of sensation, is also, as will af-
terwards 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 fila-
ments are distinct from those which are subservient to sen-
sation.

Next in importance, then, to the organs of sensation and
perception, are those of Voluntary Motion. They com-
prise two kinds of objects; first, the establishment of a cer-
tain mechanism having the cohesion, the strength, and the
mobility requisite for the different actions which the animal
is to perform; 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 ob-
jects must be combined various subsidiary arrangements re-
lating 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 the Mechanical Functions
of the animal economy. They will engage a considerable
share of our attention in this work, as afibrding 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 func-
tions, comprising a great extent of operations, opening a wide
Vol. I. 6

42 THE FUNCTIONS OF LIFE.

field of curious and interesting inquiry, and furnishing abun-
dant evidence of the wise and beneficent operations of na-
ture. These may be comprehended under a separate class
bearing the general title oi JVutritivc Functions. They are
often also, spoken of under the designation of the Vital
Ftinctions, from their more immediate relation to the con-
tinuance of vitality; that is, of mere vegetative life, as dis-
tinguished from the exercise of the hidier faculties of sen-
sation, perception, and voluntary motion, which are the ul-
timate ends of the animal existence, and which are empha-
tically 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 construction, adapted to those objects.
To the contrivances of the mechanist must be added a refined
hydraulic apparatus for the conve3^ance of fluids, and for the
resrulation of their movements; and with these must be con-
joined the skilful combinations of the laborator}^, by which
the powers of the most subtle chemistry are exercised in ef-
fecting all the transmutations required by this elaborate sys-
tem of operations. As far as they involve mechanical prin-
ciples, these objects again arrange themselves under the me-
chanical functions: and I shall accordingly include them
under that head, when giving an account of this branch of
the subject.

There is another, and a most important consequence that
flows from the peculiar chemical conditions 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 that purpose; and
neither tlie organs with which animals are furnished, nor
the powers with which those organs are endowed, are ade-
quate to tlie conversion of the materials furnished by the
inorganic world into the substances required for the con-
struction of their bodies, and the maintenance of their
powers. These inorganic elements must have passed
through intermediate stages of combination, and must
have been previously elaborated by other organized be-

THE FUNCTIONS OF LIFE. 43

ings. This important office is consigned to the vege-
table kingdom. Receiving the simple food furnished by
nature, which consists chiefly of water, air, and carbonic
acid, together with a small proportion of other subslan es,
plants convert these aliments into products, which not only
maintain their own vitality, but serve the farther purpose
of supporting the life of animals. Thus was the creation
and continuance of the vegetable kingdom a necessary step
towards the existence of the animal world; as well as a link
in the great chain of being, formed and sustained by Al-
mighty power. The physiology of Vegetables presents
many topics of great interest with relation to final causes,
and will in this Treatise be reviewed with special reference
to this important object.

Nutrition, both in the vegetable and animal sj'stems, com-
prises a very extended scries 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 farther elaboration in particular vessels and recep-
tacles—their deposition of solid materials — and their con-
version into peculiar products, as well as into the substances
which compose the several organs; — and, finally, the growth
and development of the whole plant. Still more various
and complicated are the corresponding functions in animals.
Their objects may be arranged under the following general
heads; each, again, admitting of farther 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
Jissimilation^ of which Digestion forms a principal part.
The second object is to collect and distribute this prepared
nutriment, which is the blood, to the different organs, or
wherever it may be wanted. The necessary motions for
these purposes are given to the blood by the organs of Cir-
culation, consisting of the Heart, which impels it /hrough

44 THE FUNCTIONS OP LIFE.

a system of pipes called Jirteries, 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 con-
tinually undergo purification by the chemical action of oxy-
gen: a purpose which is answered by the function of Res-
piratloii. The fourth stage of nutrition relates to the more
immediate application of this purified material to the wants
of the S3^stem, to the extention of the organs, to the repara-
tion of their losses, and to the restoration of their exhausted
powers.

Life, then, consists of a continued series of actions and re-
actions, ever varying, yet constantly tending to definite ends.
Most of the parts of which the body consists undergo con-
tinual and progressive changes in their dimensions, figure,
arrangement, and composition. The materials which have
been united together and fashioned into the several organs,
are them.selves successively removed and replaced by others,
which again are, in their turn, discarded, and new materials
substituted, though without any perceptible change of ex-
ternal form. Perpetual mutation appears to constitute the
fundamental law of living nature; and it has been farther
decreed by the power which gave the first 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 re-
ceived the gift of life are subjected, is a necessary conse-
quence of the law of mutation; and the same causes that
originally effected the development and growth of the sys-
tem, and maintained it in the vigour of its maturity, by con-
tinuing to operate, are certain to lead to the demolition of
the fabric they had raised, and to the exhaustion and final
extinction of its powers. The individual dies; but it is only
to give place to other beings, alike in nature and in form,
equally partaking of the blessings of existence, and destined,
after having, in their turn, given rise to a new race of sue-

THE FUNCTIONS OF LIFE. 45

cessors, 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 por-
tion of the vital power of the parent Is for this purpose em-
ployed to give origin and birth to the offspring. The pro-
cess itself, by which the germs of living beings originate, Is
veiled in the most Impenetrable mystery. But we are per-
mitted to trace many of the subsequent steps In the gradual
development 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 im-
mensity of the scheme of Providence, and the vigilant care
with which the most distant consequences have been antici-
pated, than the history of the early periods of their existence.
Nothing can be more admirable than the progressive archi-
tecture of the frame; nothing more beautiful than the setting
up of temporary structures, which are required only at an
early stage of growth, and which are afterwards removed to
give place to more permanent and finished organs.

The utmost solicitude has been shown in every part of
living nature to secure the perpetuity of the race, by the
establishment of laws, of which the operation is certain In
all contingent 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 consequences
which, in the animal kingdom more especially, flow from
this law of indefinite production. As animals are ultimately
dependent on the vegetable kingdom for the materials of
their subsistence, and as the quantity of these materials is, in
a state of nature, necessarily limited by the extent of surface
over which 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 main-
tain. When this limit has been attained, no farther in-

46 THE FUNCTIONS OF LIFE.

crease can 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 nourish-
ment of others. Thus the identical combinations of ele-
ments, effected by the powers of vegetation, are transformed
in succession from one living being into another, and be-
come subservient to the maintenance of a great number of
different animals before they finally, by the process of de-
composition, revert to their original inorganic state.

** See (lying" vegetables life sustain,
See life dissolving- veg-etate again;
All forms that perish other forms suppl)'-.
By turns we catch the vital breath and die." — Pope.

Hence has the ordinance been issued to a large portion of
the animal world that they are to maintain themselves by
preying upon other animals, either consuming their substance
when already dead, or depriving them of life in order to pro-
Ions: their own. Such is the command given to the count-
less hosts of living beings which people the vast expanse of
ocean; to the unnumbered tribes of insects which every spot
of earth discloses; to the greater number of the feathered
race; and also to a more restricted order of terrestrial ani-
mals. To many has the commission been given to ravage
and to slaughter by open violence; others are taught more
insidious, though no less certain arts of destruction; and some
appear to be created chiefly for the purpose of quickly clear-
ing the earth of all decomposing animal or vegetable mate-
rials, which might otherwise have filled the air with noxious
exhalations and contaminated the sources of vitality.*

This new law of animal existence must necessarily intro-
duce new conditions of organization and of functions. Struc-
tures adapted to rapid locomotion must be supplied for the

* As especially appointed for the performance of this useful task may be
cited, among- the larger beasts of prey, the hyaena, the jackal, the crow, and
the vulture: among- marine animals, the Crustacea, and mimerous mollusca;
and among the lower orders, imiumerable tribes of insects, such as ants,
flesh flies, he.

THE FUNCTIONS OF LIFE. 47

pursuit of prey, and powerful weapons for attack and de-
struction. But nature has not left the weaker animals un-
provided with the means of repulse, of defence, or of escape.
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 consequence 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 re-
sult. We must take into account the vast accession that
accrues to the mass of animal enjoyment from the exercise
of those powers and faculties which are called forth by this
state of constant activity; and when this 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 acknowledging 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 affords 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, however appa-
rently insignificant, may have a sensible influence on the
rest of the animal creation. Man, above all other animals,
has effected a most important change in the condition of the
multitude of other races, in every region where his numbers
have multiplied, where the arts of civilization have enlariied
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 charactesistic and predominant
feature of her productions. It is only when the object to

48 THE FUNCTIONS OF LIFE.

be attained is dependent upon certain definite conditions, ex-
cluding the possibility of modification, that these conditions
are uniformly and strictly adhered to. But wherever that
absolute necessity does not exist, and there is afibrded 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 ap-
pearance and form of the body, but it also prevails in each
individual organ, however minute and insignificant that or-
gan may seem. Even when the purpose to be answered
is identical, the means that are employed are infinitely di-
versified in different instances, as if a design had existed of
displaying to the astonished eyes of mortals the unbounded
resources of creative power. While the elements of struc-
ture are the same, there is presented to us in succession
every possible combination of organs, as if it had been the
object to exhaust all the admissible permutations in the order
of their union.

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

The most superficial survey of nature is sufficient to show
that there prevail certain general resemblances among great
multitudes of species, which lead us to class them into more
or less comprehensive groups. Thus in the animal kingdom,
quadrupeds, birds, fishes, reptiles, shell-fish, and insects,
compose natural assemblages or classes, and each of these
is readily divisible into subordinate groups or families. Now
it results from a closer examination of the structure and
economy of plants and animals, that the formation of all the
individual species comprehended in the same class, has been
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 separate copies, differing, indeed, as to

THE FUNCTIONS OF LIFE. 49

particulars, but agreeing as to general characters. The same
observation applies to the families, the genera, and other
subordinate groups of living beings.

The more extensive our acquaintance is with the anatomy
and physiology of both plants and animals, the more striking
do these analogies appear; so that amidst endless diversity
in the details of structures and of processes, the same general
purpose is usually accomplished by similar organs and in
similar modes. So firmly is this principle established, that
we may venture with confidence to predict many circum-
stances relating to an unknown animal, of which only a few
fragments are presented 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, gives
to the physiologist, who is conversant with the details of
comparative anatomy, a knowledge of the general structure
and habits of that animal, though all other traces of its exist-
ence may have been swept away, amidst the primeval revo-
lutions of the globe.*

Not only does this tendency to conform to particular types
obtain in all organic formations, but farther inquiry leads to
the conclusion that the deviations from these standard forms,
far from being arbitrary, are themselves referrible to par-
ticular laws. The regulating principle of the variations is
subordinate to higher views, and has reference to tlie respec-
tive objects and destination of each particular species in the
general system of created beings. Nature, as far as we can
discern, appears, in conformity with these intentions, first
to have laid down certain great plans of functions to which
she has adapted the structure of the organs; the minor ob-
jects and more subordinate functions being accommodated
to this general design. Hence arises the necessary and re-
ciprocal dependence of each organ and of each function on

* See Cuvier's "Discours sur les Revolutions de la Surface du Globe,*' p.
47, prefixed to the first volume of lii;^ Osscmens Fossiies.'*

Vol. I. 7

50 THE FUNCTIONS OF LIFE.

every other; and hence are deduced what have been termed
the laios 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 corresponding va-
riations in other parts, and extends its influence in modify-
ing, in a greater or less degree, the whole fabric. It is in
comparative anatomy as in mechanics, where any alteration
made in the position of one part of a system of bodies occa-
sions a change in the centres of gravity, of gyration, and of
oscillation; and evolves new mechanical forces and condi-
tions of equilibrium, which render new adjustments in other
parts necessary, in order to restore the equipoise, and pre-
serve 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, in-
vented merely with a view to facilitate the study and to
recognise the identity of species, or calculated only to sup-
ply 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 king-
dom 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 interme-
diate places between adjacent types, and appear as links of
connexion in the chain of being. It often happens, as I
shall hereafter have occasion to point out, that throughout
an extensive series of organic forms, the steps of gradation
by which one type passes into another, are so numerous and
so regular, as to preclude the possibility of drawing a de-
cided line of demarcation between those that properly ap-
pertain to each. "

THE FUNCTIONS OF LIFE. 51

All these apparent anomalies and gradations of structure
tend still farther to demonstrate the generality of the plans
of nature, and the comprehensiveness of her design, which
embraces the whole series of animated beings. These views
are strongly corroborated by the discoveries that are con-
tinually being made of species now no longer in existence,
but which, in former ages of the world, helped to fill up
many of the chasms which now interrupt the continuity of
that series. This knowledge has been revealed to us by the
examination of their fossil remains, those monuments of
former epochs, which have thrown such important light on
the most interesting 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 regularly ascending to the most refined and complicated
organizations, till it reached its highest point in man, who is
unquestionably placed at the summit of the scale. Bonnet,
in-particular, cherished with enthusiastic ardour the hypo-
thesis that all organic beings formed a 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 appljdng it
even to the productions of the mineral world. But, divest-
ing ourselves of these hypothetical views and figurative
images, we find, on sober observation, that instead of one
continuous series, we are presented with only detached frag-
ments and interrupted portions of this imaginary system: so
that, if, for the sake of illustration, we must employ a me-
taphor, the natural distribution of animals would appear to
be represented, not by a chain, but by complicated net-
work, where several parallel series are joined by transverse
and oblique lines of connexion. A multitude of facts, how-
ever, tend to show that the real types or models of struc-
ture, are more correctly represented by circular or recurring

52 THE FUNCTIONS OF LIFE.

arransements.* But as the discussion of these and other
topics relating to the plans and designs of nature in the for-
mation of organic beings requires a previous acquaintance
with the details of comparative anatomy and physiology, I
shall defer all farther observations respecting them till I
have finished the review T propose to take of the several
structures and functions of the animal and vegetable econo-
my. There are, however, some views that have been en-
tertained respecting the procedure of nature in the formation
of the different races of animals, which it will be proper to
notice in this place, as they will occasionally be referred to
when the facts that more particularly illustrate and support
them come to be noticed. -^

An hypothesis has been advanced that the original crea-
tion of species has been successive, and took place in the or-
der of their relative complexity of structure; that the stan-
dard types have arisen the one from the other; that each
succeed in g form was an improvement upon the preceding,
and followed in a certain order of development, according
to a regular plan traced by the great Author of the universe
for bestowing perfection on his works. This gradation- of
structure was necessarily accompanied by a gradation of fa-
culties: the object of each change of type being to attain
higher objects, and to advance a farther step towards the ul-
timate ends of the animal creation. Many apparent anoma-
lies which are inexplicable upon any other supposition, are
easily reconcileable to this theory. The developments of
structure belonging to a particular type being always pro- •
spective, are not completed in the inferior orders of the
group formed upon that model, but remain more or less
imperfect, although each organ always fully answers the par-
ticular purpose of the individual animal. But it sometimes
happens that the imperfection of an organ is so great, in con-
sequence of its development having proceeded to a very small

* Mr. M'Leay is the author of this ing-enious theory, which he has de-
veloped in his " Horx Entomologlca;" and which appears to be verified to a
great extent by the modern discoveries in comparative anatomy.

THE FUNCTIONS OF LIFE. 5SL

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 ser-
pents, which can obviously 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 farther growth is ar-
rested, and they are afterwards obliterated. This imperfect
or rudimentary 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 formation. I
shall have occasion to mention several striking instances of
this kind, both in the animal and vegetable kingdom.

In followino; the transitions from one model of structure
to another, we often observe that a particular organ has been
very greatly enlarged, or otherwise modified to suit some
particular purpose, foreign to its usual destination, or to qua-
lify it for performing some new office, rendered necessary
by the particular circumstances in which the animal is
placed. Thus, the ribs, which in quadrupeds are usually
employed for respiration, are in serpents converted into aux-
iliary organs of progressive motion: and in the Draco vo-
lans, or flying lizard, they are extended outwards from the
sides to serve as wings. The teeth, usually intended for
mastication, are in many animals enlarged in order to serve
as weapons of offence, as in the Elephant, the Boar, the
Narwal and the Pristis. In like manner, in the Crustacea,
organs of the same general structure are converted some-
times into jaws, sometimes into feelers, (or palpi,) and some-
times into feet; and the transition from the one to the other
is so gradual that it is difficult to draw a proper distinction
between them.

In pursuing the ascending series of animal structures we
meet also with instances of a contrary change, yet still re-
sulting from the continued application of the same principle.

54 THE FUNCTIONS OF LIFE.

f

An organ which has served an important purpose in one
animal, may be of less use in another, occupying a higher
station in the scale, and the change of circumstances may
even render it wholly useless. In such cases we find that
it is gradually discarded from the system, becoming conti-
nually smaller, till it disappears altogether. We may often,
however, perceive some traces of its existence, but only in
a rudimental state, and a^ 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 for-
mer is apparent in the verticillated organs of plants, and in
the radiated forms of zoophytes. The linear arrangement
is exhibited in the similar segments of annulose and other
articulated animals, and also in the pieces which compose
the spinal column of vertebrated animals. In these two
latter classes, also, a remarkable law of symmetry obtains
in the formation of the two sides of the body, which ex-
hibits the lateral junction of similar but reversed structures.
The violations of this law are extremely rare; yet some re-
markable 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 different order from that
in which I have presented them in the preceding sketch.
As the Mechanical functions depend upon the simpler pro- "
perties of matter and the well known laws of Mechanism, I
think it best to commence with the examination of these.
Our attention will next be directed to the highly interesting
subjects which relate to the Nutritive or Vital functions
both of vegetable and animal structures: for as they involve
the chemical properties of organized substances, and are,
therefore, of a more refined and intricate nature than the
preceding, I conceive they will be best understood after the
general mechanism of the frame has been explained. These

THE FUNCTIONS OF LIFE. 55

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 vo-
lition, 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 func-
tions and of the phenomena of animal development, in which
the discoveries of modern times have revealed to us so con-
siderable a portion of those extensive plans which an all-wise
Providence has beneficently devised for the general wel-
fare of animated beings.

PART 1.

THE MECHANICAL FUNCTIONS.

CHAPTER I.

ORGANIC MECHANISM.

§. 1. Organization in General.

Life, which consists of a continued series of actions, di-
rected to particular purposes, cannot be carried on but by
the instrumentality of those peculiar and elaborate structures
and combinations of material particles which constitute or-
ganization. All these arrangements, both as respects the
mechanical configuration and the chemical constitution of
the elements of which the organized body is composed,
even when apparently most simple, are, in reality, complex
and artificial in the highest possible degree. Let us take as
a specimen the crystalline lens, or hard central part, of the
eye of a codfish, which is a perfectly transparent, and to all
appearance homogeneous, spherule. No one, unaccustomed
to explore the wonders of nature, would suspect that so
simple a body, which he might suppose to be formed of a
uniform material cast in a mould, would disclose, when ex-
amined under a powerful microscope, and with the skill of
a Brewster, the most refined and exquisite conformation.
Yet, as I shall- have occasion to specify more in detail in its

ORGANIC MECHANISM. 57

proper place, this little spherical body, scarcely larger than
a pea, is composed of upwards of five millions of fibres, which
lock into one another by means of more than sixty-two
thousand five hundred millions of teeth. If such be the
complication of a portion only of the eye of that animal,
how intricate must be the structure of the other parts of the
same organ, having equally important offices! What ex-
quisite elaboration must those textures have received whose
functions are still more refined! What marvellous w^ork-
manship must have been exercised in the organization of the
nerves and of the brain, those subtle instruments of the hiirher
animal faculties, and of which even the modes of action are
to us not merely inscrutable, but surpassing all our powers
of conception!

It is from the energies of life alone that organic forms are
produced. No fabric achieved by human power ever ap-
proached in refinement the simplest of nature's works. The
utmost efforts of the ingenuity or skill of man in the con-
struction of the most delicate machinery is infinitely sur-
passed by the most ordinary of the mechanisms which are
presented to our view in living bodies. However success-
ful may be human artists in their attempts to contrive au-
tomata, which shall exactly imitate different animal move-
ments, there will always be wanting that internal principle
of action derived from a higher source than mechanism
can supply, and without which these highly wrought works
of man, like the unvivified statues of Prometheus, must re-
main for evermore masses of insentient and inert materials.

As the living functions imply the mechanical action and
reaction 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 im-
possible to conceive that a mere fluid can exercise these
functions; because the particles of a fluid, being equally
moveable in every direction, have no determinate relative
situations, and possess no character of permanence. All or-
ganic and living structures, therefore, must be composed of
solid as well as fluid partsj although the_'proportion between

Vol. I. 8 .

58

THE MECHANICAL FUNCTIONS.

these is, in different cases, almost infinitely varied. A dor-
mant 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 vegetable seeds, and also of many spe-
cies of animalcules, and even of some of the more highly
developed Jinnelida, or worms, which may he kept in a
dry state for an indefinite length of time, and when moist-
ened with water, resume their activity, as if restored to life.
The germination of seeds under these circumstances is matter
of common observation; but the revivification of animal-
cules is a more curious phenomenon, for it takes place more
rapidly, and is more striking in its results. The Rotifer
redivivus, or wheel animalcule,* (Fig. 1,) which was first
observed by Lewenhoeck, and was afterwards rendered ce-
lebrated by the experiments made upon it by Spallanzani..
can live only in water, and is commonly found in that which
has remained stagnant for some time in the gutters of houses.
But it may be deprived of this fluid, and reduced to perfect

dryness, so that all the functions of life shall be completely
suspended, yet without the destruction of the vital princi-
ple; for this atom of dust, after remaining for years in a dry
state, may be revived in a few minutes by being again sup-
plied with water. This alternate suspension and restora-
tion of life may be repeated, without apparent injury to tlie
animalcule, for a great number of times. Similar phenome-
na are presented by the Vibrio trilici, (Fig. 2,) or the ani-

* Vorlicella rotatoria of Ginclin, and Furcularia of Laniuik.

ORGANIC MECHANISM. 59

malcule resemblino; an eel in its shape, which infests dis-
eased wheat, and which, when dried, appears in the form
of a fine powder: on being moistened it soon resumes its
living and active state.* The Gordius aquaticiis, or hair
worm, which inhabits stagnant pools, and which remains in
a dry and apparently lifeless state when the pond is evapo-
rated, will, in like manner, revive, in a very short time, on
being again immersed in water. The same phenomenon is
exhibited by the Filaria, a thread-like parasitic worm, in-
festing the cornea of the eye of the horse.t

Both the composition of the fluid and the texture of the
solid parts of animal and vegetable bodies are infinitely va-
ried, according to the purposes they are designed to serve in
the^conomy. Scarcely any part is perfectly homogeneous;
that is, composed throughout of a single uniform material.
Few of the fluids are entirely limpid, and none are perfectly
simple in their composition; for they generally contain more
or less of a gelatinous matter, which, when very abundant,
imparts to them viscidity, constituting an approach to the so-
lid state. Many fluids contain minute masses of matter, ge-
nerally having a globular shape, which can be seen only by
means of the microscope, and which float in the surround-
ing liquid, and often thicken it in a very sensible manner.!
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 collectins:
and adhering together in bundles; or interwoven and agglu-
tinated, composing various other forms of texture; sometimes
resembling a loose net-work of filaments; sometimes consti-
tuting laminae or plates; and, at other times, both plates and
filaments combining to form an irregular spongy fabric.
These various tissues, again, may themselves be regarded as
the constituent materials of which the several organs of the
body are constructed, with difierent degrees of complication,

* See a paper on this subject by Mr. Bauer, Phil. Trans, for 1823, p. 1.
f Blauivillc, Annates des Sciences Naturelles; X. 104.
+ Globules of this description have been found in the lymph, the saliva,
and even in the aqueous humour of the eye.

60 THE MECHANICAL FUNCTIONS.

according to the respective functions which they are called
upon to perform.

We shall now examine the several kinds of texture in re-
lation to these functions, in the order of their increasing
complexity; heginning with those of vegetables, which are
apparently the simplest of all.

§ 2. Vegetable Organization,

Plants, being limited in their economy to the functions
of nutrition and reproduction, and being fixed to the same
spot, and therefore in a comparatively passive condition, re-
quire for the performance of these functions mechanical con-
structions of a very different kind from those which are ne-
cessary to the sentient, the active, and the locomotiv^ani-
mal. The organs that are 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 frame-work, which must be super-
added 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 injurious 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 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 teguments, or skin of animals.

The simplest and apparently the most elementary texture
met with in vegetables is formed of exceedingly minute ve-
sicles, the coats of which consist of transparent membranes
of extreme tenuity. Fig. 3 is a highly magnified represen-
tation of the simplest form of these vesicles.* But they ge-

* These cells are well represented in the engravings which illustrate Mr.

VEGETABLE ORGANIZATION.

61

nerally adhere together more closely, composing by their
union a species of vegetable cellular tissue, which may be
regarded as the basis or essential component material of
every organ in the plant. This cellular structure is repre-
sented in figures 4 and 5, as it appears in the Fucus vesicu-
losus; the first being a horizontal, and the second a vertical
section of that plant.* The size of these cells differs consi-
derably in different instances. Kieser states that the dia-
meter of each individual cell varies from the 330th to the
55th part of an inch; so that from 3,000 to 100,000 cells
would be contained in an extent of surface equal to a square
inch. But they are occasionally met with of different sizes,
from even the 1000th part of an inch to the 30th.

In their original state, these vesicles have an oval or glo-
bular form; but they are soon transformed into other shapes,
either by the mutual compression which they sustain from
being crowded into a limited space, or from unequal expan-
sions in the progress of their development. 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

Slack's memoir on the elementary tissue of plants, contained in the 49th vo-
lume of the Transactions of the Society of Arts.
* De Candollc, Org-anographie V^getale.

62 THE MECHANICAL FUNCTIONS.

the second, they are elongated into cylinders, or slowly ta-
pering 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 elemen-
tary texture have been distinguished into several classes of
cells, and dignified by separate technical denominations,
which I shall not stop to specify, as it does not appear that
they have as yet thrown any light on vegetable physio-
logy.

Many of the cells are fortified by the addition of elastic
threads, generally disposed in a spiral course, and adhering
to the inner surfaces of the membranous coats of the cells,
which they keep in an expanded state. (See Fig. 9.) When
the. membranes are torn, the fibres; being detached, unrol
themselves, and being loosely scattered among the neigh-
bouring cells, give the appearance of fibrous connexions
among these cells, which did not originally 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 coa-
lesced by their edges; thus presenting a more uniform sur-
face, excepting that a few interstices are left, where the pel-
lucid membrane, having no internal lining, presents the ap-
pearance of transverse fissures or oval perforations, (Fig. 10.)
Cells of this description are said to be reticulated or spot-
ted, and, together with those having more regularly formed
spiral threads, are very abundantly met with in plants be-
longino; to the tribe of Orchideae.

It has been much disputed whether the cells of the vege-
table texture are closed on all sides, or whether they com-
municate with one another. Mirbel has given us delinea-
tions of what appeared to him, when he examined the coats
of the cells with a microscope, to be pores and fissures. But
subsequent observations have rendered it probable that these
appearances arise merely from darker portions of the mem-
branes, where opaque particles have been deposited in their
substance. Fluids gain access into these cells by transuding ,

VEGETABLE ORGANIZATION. 63

through the membranes which form their sides, and not by
any apertures capable of being detected by the highest pow-
ers of the microscope.

If all the cells consist of separate vesicles, as the con-
curring observations of modern botanists* appear to have
satisfactorily established, the partitions which separate them,
however thin and delicate, must consist of a double mem-
brane, formed by the adhesion of the coats of the two con-
tiguous vesicles. But as these coats can hardly be supposed
to adhere in every point, we may expect to find that spaces
have been left in various parts between them; and that com-
munications exist to a certain extent between all these spaces;
so as to compose what may be regarded as one large cavity.
These have been denominated the intercellular spaces; and
they have been supposed to perform, as will hereafter be
seen, an important part in the functions of Nutrition.

Fluids of different kinds occupy both the cells and the
intercellular spaces. The contents of some is the simple
watery sap; that of others consists of peculiar liquids, the
products of vegetable secretion: and very frequently they
contain merely air. In many of the cells there are found
small opaque and detached particles of the substance termed
by chemists, Feciila, of which starch is the most common
example. In several parts, and more especially in the leaves,
and in the petals of flowers, the material which gives them
their peculiar colour is contained in the cells in the form of
minute globules. De Candolle has given it the name of
Chromule.\

The cells of the ligneous portion of trees and shrubs are
farther incrusted with particles of a more dense material,
peculiar to vegetable organization, and termed Ligniiie, It
is this substance which principally contributes to the den-
sity and mechanical strength of what are called tlie Woody
Fibres, which consist of collections of fasiform, or tapering
vessels, hereafter to be described, surrounded by assemblages

* In particular, Treviramis, Kioscr, Link, Du I'etit ThoiKU-s, I'oUiiii, Ainlci^
Dutrochet, and De Candolle.
f Organographie, Tom. 1, p. 19.

64 THE MECHANICAL FUNCTIONS.

of cells thus fortified, and the whole cohering in bundles, so
as to present greater resistance to forces tending to displace
them in the longitudinal direction than in any other.

Most of the plants which are included in the Linnean
class of Cryptogamia have a structure exclusively composed
of cells, as has been already shown in the Fucus vesiculosus.
But the greater number of other plants have, in addition to
these cells, numerous ducts or vessels, consisting of mem-
branous tubes of considerable length, interspersed through-
out 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 modifi-
cations correspond 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 ves-
sels of plants take their origin from vesicles, which become
elongated by the progress of development in one particular
direction; and it is easy to conceive that where the extre-
mities of these elongated cells meet, the partitions which se-
parate their cavities may become obliterated at the points of
junction, so as to unite them into one continuous tube with
an uninterrupted interior passage. This view of the forma-
tion of the vessels of plants is confirmed by the gradation
that 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 fusiform cells are joined
kterally (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 mem-
branes passing transversely; in which case they present, un-
der 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 exhi-
bit a series of contractions at certain intervals, marking their
origin from separate cells. In this state they have received

VEGETABLE ORGANIZATION.

65

the names of moniliform, Jointed or beaded vessels.*
Traces of the membranous partitions sometimes remain where
their obliteration has been only partial, leaving transverse
fibres. The conical terminations occasionally observable
in the vessels of plants also indicate their cellular origin, t

14 15

1

The membrane constituting the tube is sometimes simple,
like those of the simple cells: but it frequently contains
fibres, or other internal coatings, corresponding to those met
with in the more compound cells. The vessels in which the
internal fibres run in a spiral direction (Fig. 14,) are deno-
minated trachese, or spiral vessels; or, from their being found
very constantly to contain air, they are often called air tubes.
Their diameter is generally between the 1000th and the
300th part of an inch. These spiral, or air vessels, pervade
extensively the vegetable system. The threads they con-
tain are frequently double, treble, quadruple, or even still
more numerous: they are of great length, and when the ex-
ternal membrane of the vessel is divided, they may easily
be drawn out and uncoiled, their elasticity enabling them to
retain their spiral shape. The object of this structure ap-
pears to be that of keeping the cavity of the tube always
pervious, by presenting resistance to any external force tend-
ing to compress and close itij:

* Mirbel g'avc tlicm the name of" Vaisseaux en chapelet."

f This theory of the derivation of vessels from cells was first advanced by
Treviranus.

+ Vessels are sometimes met with which appear to be formed simply by
the coils of a spiral fibre in close juxtaposition, and unattached to any ex-
ternal envelope, or connecting" membrane.

Vol. I. 9

66 THE MECHANICAL FUNCTIONS.

In many instances the inner fibres of the tube, instead of
forming a continuous spiral, appear in the shape of rings,
succeeding one another at regular intervals, and constituting
what are called annular vessels (Fig. 15.) They are gene-
rally larger than the spiral vessels. In other cases, as was
first observed by Hedwig, the adjacent coils are found to be
closely coherent throughout the greatest part of their course;
leaving, however, occasional intervals, where the external
membrane, being unprotected, appears, from its transparency,
as if spotted or perforated in various places (Fig. 16.) Every
intermediate stage may occasionally be seen in the transi-
tion from one of these forms to the other, in consequence of
the various kinds of convolution, of branchings, or of trans-
verse junctions of fibres, as well as the greater or less extent
of their lateral adhesions. All these varieties are met with,
not only in different vessels, but, as was observed by Mol-
denhavver and Kieser, even in the dififerent 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
in others, again, form rings: these may afterwards, by a
closer junction, present a reticulated appearance, or a series
of transverse lines, which, becoming smaller and smaller,
are at length mere points, arranged in circular rows around
the cylindrical surface of the vessel.*

What are called the woody fibres have their origin, like all
other parts of plants, in cells. These are generally fusiform,
that is, of the shape of a double cone, very greatly elongated,
and placed close and parallel to one another, with the nar-
row extremities of one set wedged in between those of ano-
ther 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

* Many disting-uisbed botanists, such as Rudolpbi, Link, Treviranus, and
Dutrocbet, consider these spots as being produced not by the deficiency of
the internal coating, but by the addition of granular bodies. See De Can-
doUe's Organographie Vegetale, torn. 1, p. 56.

VEGETABLE ORGANIZATION. 67

tlieir development, large additions 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 incrusta-
tions are formed, till they have become nearly impervious;
and have acquired a degree of rigidity peculiarly fitting them
for the office of giving mechanical support to the fabric of
the plant.* Their assemblage thus constitutes a kind of
frame-work for the whole system, which may be regarded
as the skeleton of the plant. Thus, what are called the fibres
of leaves (Fig. 19,) are principally composed of these woody
fibres, distributed 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, appa-
rently serving the purpose of reservoirs of particular fluids;
but sometimes containing only air. Large air cells are^ in
particular, met with very commonly in aquatic plants, where
they probably contribute to impart the requisite degree of
buoyancy.

There are also contained, in the interior of vegetables,
certain organs, denominated Glands, which are composed
of closely compacted cells, and which perform the function
of secretion, that is, the conversion of the nutritious juices
into particular products required for various purposes in the
economy of the plant.

The external parts of a living plant require protection
against the injurious effects of the atmosphere, and of the
moisture it deposites. 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

* By dry'mg diflTerent specimens of wood in a stove, Count Rumford was led
to the conclusion that the specific gravity of the solid matter which consti-
tutes timber is nearly the same in all trees. He found that tlie woody part
of oak, in full vegetation, constitutes only two-fifths of the whole bulk: and
that ordinary dry wood contains above one-fourth of its weight of water.
Thomson's Annals of Philosophy, I. 388.

68 THE MECHANICAL FUNCTIONS.

the stem and branches, and interpos'ing a barrier to the ac-
tion of fluids, or other extraneous bodies, on the living or-
gans. The cuticle is formed originally by the condensation
of a layer of cellular tissue, of which the cells, being conso-
lidated by exposure to the air, and by compression, compose
a thin but impervious pellicle. Amici has distinctly shown,
by means of his powerful microscope, the cellular structure
of the cuticle, and also that the layer of cells of which it con-
sists is independent of the subjacent cellular tissue.* Fig.
20 is intended to show this circumstance, the shaded part
representing the cuticle with its series of cells.

Oval orifices, or stomata, as they have been termed, are
discoverable on almost every part of the surface of the cuti-
cle, 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 occasional-
ly open and close; but the minuteness of their size precludes
any accurate observation as to the nature of the apparatus
provided for the purpose of performing these motions. Ami-
ci describes their margins as formed by two cells, by the
movements of which, combined perhaps with those of the
adjoining cells, he conceives these orifices are opened and
closed.! Great variety, however, is observable in the struc-
ture of the stomata in different species of plants.

Many plants have no stomata, either on the cuticle of the
leaves, or on that of the stem. This is the case with such
aquatic plants as are habitually immersed in water. In those
that are only partially immersed, stomata are met with in
those parts exclusively which are above the water. The

• Annales des Sciences Naturelles, II. 211.

•j- Fig". 22 is a magnified representation of the appearance in the cuticle
of the Lycopodium denticulatum^ tixken in the central part of the lower sur-
face of the leaf, from De Candolle. Fig. 21 is a still more magnified view
of the stomata in the leaf of the Lilium ca?ididum, from Amici.

t Ibid. II. 215.

VEGETABLE orftANIZATION.

69

leaves of the Ranunculus aquaticus, when made to grow
in the air, acquire stomata, but lose them entirely when grow-
ing under water. Stomata are wanting in all plants whose
structure is wholly cellular.

Botanists are far from being agreed as to the precise func-
tions which the stomata perform. Their usual office un-
doubtedly is to exhale water; but they probably also absorb
air under certain circumstances, 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 extremities of the roots, where they are
termed, from their peculiar texture, spongioles.^ Of the
functions of spongioles in absorbing fluids I shall have occa-
sion to speak when treating of nutrition. But as the roots
exercise a mechanical as well as a nutrient office, we should
here consider them in the light of organs adapted to procure
to the plant a permanent attachment to the soil, upon which
it is wholly dependent for its supply of nourishment. It is
scarcely necessary to point out how effectually they 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 procure

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

70 THE MECHANICAL FUNCTIONS.

the most effectual resistance to the force of the winds, which,
acting upon so large a surface as that presented by the
branches covered with dense foliage, must possess an im-
mense 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 dif-
ferent distances in order to procure the requisite nourish-
ment, so the stem grows upwards for the purpose of obtain-
ing 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, oc-
cupying the surface of the cylinder. Of all the possible
modes of disposing a given quantity of materials in the con-
struction of a column, it is mathematically demonstrable that
this is the most eff'ective for obtaining the greatest possible
degree of strength.*

The graceful continuous curve with which the stem of a
tree rises from the ground, is the form which is best calcu-
lated to give stability to the trunk. Evidence of express
mechanical design is likewise afforded by the manner in
which the trunk is subdivided into its branches, spreading
out in all directions, manifestly with a view to procure for
the leaves the greatest extent of surface, and thus enable them
to receive the fullest action of both light and air. The
branches, also, are so constructed as to yield to the irregular
impulse 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. No-
thing can exceed the elegance of those forms which are pre-

* Galileo, the most profound philosopher of his age, when interrogated
by the inquisition as to his belief in a Supreme Being-, repUed, 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.

VEGETABLE ORGANIZATION. 71

sented 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 component parts of that beau-
ty which is spread over the scenery of nature, and is so de-
lightfully refreshing to the eye of every beholder alive to
its fascinating charms. How enchanting are all the varieties
of flowers, that decorate in gay profusion every part of the
garden of creation; and into which the farther we carry our
philosophic scrutiny, the more forcibly will our hearts be
impressed with the truth of the divine appeal that " Even
Solomon, in all his glory, was not arrayed like one

OF THESE."

§ 3. Development of Vegetables.

Farther proofs of design may be collected from an ex-
amination into the modes in which these structures, so
admirably adapted to their objects, have been gradually
formed. Confining our attention to vascular plants, in which
the process of development has been studied with the great-
est attention 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 ])lants. In others, the growth is the re-
sult of additions 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 resiions, belons
to the first of these divisions. The Palm tribe, such as the
Date, the Cocoa-nut tree, and, indeed, a large proportion of
the trees of tropical climates, together with the sugar-cane,
the bamboo, and all gramineous and liliaceous plants, belong
to the latter. We shall first inquire into the endogenous

• The tribe oii Filices, or ferns, the structure of which is vascular, consti-
tutes an exception to this rule: as they differ in their mode of development,
both from exogenous and endogenous plants.

72 THE MECHANICAL FUNCTIONS.

mode of growth, as being the simplest of these two kinds of
vegetable development.

A Palm tree may be taken as an example of the mode of
growth in endogenous plants. The stem of this tree is usu-
ally perfectly cylindrical, attains a great height, and bears on
its summit a tuft of leaves. It is composed of an extremely
dense external cylindric layer of wood; but the texture of
the interior becomes gradually softer and more porous as it
comes nearer to the centre; though wdth regard to its essen-
tial character it appears to be uniform in every part, having
neither medullary rays, nor true outward bark, nor any cen-
tral pith; in all which respects it differs totally from the or-
dinary exogenous trees.

The first stage of its growth consists in the appearance of
a circle of leaves, which shoot upwards from the neck of
the plant, and attain, during the first year, a certain size.
The following year, another circle of leaves arises; but they
grow from the interior of the former circle, which they
force outwards as their vegetation advances, and as ligneous
matter is deposited within them. Thus, each succeeding
year brings with it a fresh crop of leaves, intermixed with
lio-neous 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 farther dis-
tention, 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 immediately directed
upwards; so that they each rise in succession by distinct stages,
always proceeding from the interior; a mode of develop-
ment which has been compared by De Candolle to the draw-
ing out of the sliding tubes of a telescope. The whole stem,
whatever height it may attain, never increases its diameter
after its outward layer has been consolidated. A circle of
leaves annually sprouts from the margin of the new layer of
wood; these, when they fall off in autumn, leave on the stem
certain traces of their former existence, consisting of a cir-

DEVELOPMENT OF VEGETABLES. 73

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 observed in growing corn, and also in various grasses,
may be traced to a similar origin.

The structure of exogenous trees is more complicated:
for, when fully grown, they are composed of two principal
parts, the toood and the hark. The woody portion ex-
hibits a farther division into the j?;///?, which occupies the
centre, and consists of large vesicles, not cohering very
closely, but forming a light and spongy texture, readily per-
meable to liquids and to air; the harder luood, which sur-
rounds the pith in concentric rings, or layers; and the
softer wood, or alhiirnum, which is also disposed in con-
centric la)^ers on the outer side of the former. Each of these
concentric layers of wood and of alburnum may be farther
distinguished into an inner and an outer portion; the former
being of less density than the latter, and consisting of a
lighter cellular tissue: while the outer portion is composed of
the denser woody fibres resulting from the union of numerous
vessels with a cellular envelope. The bark is formed b}^ con-
centric layers of cortical substance, of which the innermost
are denominated the Liber; and the whole is surrounded
by an outer zone of cellular tissue, termed the cellular en-
velope. Of this envelope the exterior surface is called the
Epidermis.

All these concentric zones may be readily distinguished in
a horizontal section of the stem; which also presents a num-
ber of lines called Medullary Rays, radiating from the pith
to the circumference. They are composed chiefly of large
cells, extending transversely, or in the direction of the di-
ameter of the tree, and composing by their union conti-
nuous 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 origi-
nally of inconceivable minuteness, and totally imperceptible
by any optical means of which we have the command. As

Vol. I. 10

74 THE MECHANICAL FUNCTIONS.

soon as it becomes visible, and its structure can be distin-
guished, it is found to contain within itself the parts which
are to arise from it, in miniature, and folded up in the small-
est possible compass. The portion destined to form the
stem is gradually expanded both in breadth and height, but
principally the latter; so that it rises as it grows, during a
certain period, until the fibres acquire the solidity and
strength necessary not only for their own support, but also
for sustaining the parts which are to be farther added. In
trees this process generally occupies one whole season;
during which the growth of the first layer of wood, with its
central pith, and its covering of a layer of bark, is free and
unrestrained. On the second year, a fresh impulse being
given to vegetation, a new growth commences from the up-
per end of the original stem, as if it were the development
of a new bud: and at the sam^e time a layer of cellular tis-
sue 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 farther 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 late-
rally, is now directed upwards, and there is no farther en-
largement of their diameter. From the same cause the pith
cannot increase in size; and is even found to diminish by
the pressure of the surrounding wood. Thus, the vertical
elongation of the entire stem continues during the whole of
the second year, and the trunk becomes sufliciently strength-
ened 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 w^ood, correspond-
ing changes take place in the bark, and a new layer is add-
ed on its inner surface, or that which is contiguous to the
wood. This layer constitutes the liber. All these new de-
positions must of course tend to stretch the outer portions

DEVELOPMENT OF VEGETABLES. 75

of the bark, which had been first formed, and which yield
to this pressure to a certain extent; but, becoming tlicm-
selves consolidated by the effects of the same pressure, they
acquire increasing rigidity; and, the same cause continuing
to operate, they at length give way, in various places, form-
ing 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 consolidated layers of corti-
cal envelope which form the epidermis. But the epider-
mis, which is continually splitting by the expansion of the
parts it encloses, itself soon decays, and is constantly suc-
ceeded by fresh layers, produced by the same process of
consolidation in the subjacent cortical substance.

During the third and each succeeding year, the same pro-
cess 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 ver-
tically, while they are capable of elongation, and can be sup-
plied 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 farther dilatation is prevented. But the tree
still continues to enlarge its trunk by the annual accessions
of vigorous and expansible alburnum, and to take its station
among its kindred inhalntants of the forest; till, arriving at
maturity, its majestic form towers above all the junior or
less vigorous trees. ^'

The development of each 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

* It is contended by Dr. Uarwin and other writers on vegetable physiology''

that each annual shoot should be rcg-arded as a collection of individual buds,

each bud being a distinct individual plant, and the whole tree an aggregation

V of such individuals. I shall have occasion to revert to this question when I

come to consider the subject of vegetable nutrition.

70 THE MECHANICAL FUNCTIONS.

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 development of all the
parts of plants. The leaves, in particular, are frequently
observed to arise in a circle, or symmetrically round the pa-
rent stem; forming what is termed a lohorl, or, in botanical
lano-uase, a verticillated arrangement. In other cases the}^
are found to have their origins at equal intervals of a spiral
line, which may be conceived to be drawn along the stem,
or the branch from which they grow. When these inter-
vals correspond to the semi-circumference of the stem, the
leaves alternate with one another on its opposite sides.

The stems of most plants, even those that are perfectly
erect, exhibit a tendency to a spiral growth. This is obser-
vable in the fibres of the wood of the pine, however straight
may be the direction of the whole trunk. This tendency is
shown even in the epidermis of the cherry tree, for it may
be stripped off with more facility in a spiral direction than
in any other. The primitive direction of the leaves of en-
dogenous plants is a spiral one. It is particularly marked
also in the stems of creepers and of parasitic plants, which
are generally twisted throughout their whole length; a dis-
position evidently conducive to the purpose of their forma-
tion, 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
cannot be mistaken.

A conjecture has been offered that this tendency to a spi-
ral growth might be the effect of the influence of the sun's
light acting successively on different sides of the plant, in
the course of its diurnal motion. In these northern latitudes
the direction of that motion is from east to west; or, to an
observer facing the south, from left to right. That light has
a powerful influence in determining the direction of the
growth of all the parts of the plant which are above ground,
is manifest to every one who has observed the habits of ve-
getables. If a growing plant be placed in a situation where

DEVELOPMENT OF VEGETABLES. 77

the light reaches it only on one side, it will always, by de-
grees, turn itself to that side, as if eagerly pressing forward
to obtain the beneficial action of that agent. The leaves,
whose functions in a more especial manner require its ope-
ration, will always be found turned towards the light. The
branches of a tree, which have naturally a tendency to rise
vertically, have this tendency modified by the superior at-
traction of the light, when it can reach them only laterally.
Thus, while those on the upper part spread out in full luxu-
riance in all directions, those below them are obliged to ex-
pand more in a lateral direction: and this is still more the
case with the lowest branches, which shoot out horizontal-
ly to a considerable distance before they turn upwards, and
present their leaves to the light. Often, however, from
the deficiency of this necessary agent, their growth is much
stinted, or entirely prevented. The operation of this cause
is extensively seen in the interior of a dense forest.

It may be objected to the theory of the spiral growth be-
ing the result of the sun's motion, that were it so, the direc-
tion of the spiral would always be the same, that is, ascend-
ing 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 be-
ing the same in all. Dr. Wollaston ingeniously suggested
that a verification of the theory would be obtained, were it
found that plants transported from tlie southern to the nor-
thern hemispheres, would have this direction reversed,- for
it is evident that the motion of the sun's lisht in the two
hemispheres is in opposite directions; being, in the southern
hemisphere, from right to left, to a spectator facing the me
ridian 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 we may consider the hypothesis as
untenable.

The roots dificr considerably from the stems both in their
structure, and in their mode of growth. They exhibit, in-
deed, the appearance of medullary rays and of concentric lay-
ers, but they are destitute of any central pith, and they have
no tracheae; neither does their surface present any appear-

78 THE MECHANICAL FUNCTIONS.

ance of stomata. They increase in thickness in the same
way as the stem increases. This law obtains both in exo-
genous 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 development of buds, as are the
branches of the stem; but they arise merely from the addi-
tional deposites 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. It is observed, however, that they gene-
rally grow from certain points on the surface of the bark,
which appear as dark spots, and are termed Lenticellss.^'
Great variety exists in the form, and disposition of roots in
different families of plants, according to the particular pur-
poses they are intended to serve, conform.ably to their ge-
neral 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 chan-
nels for the introduction of nourishment.

Nature has supplied various plants with certain appen-
dages to the above mentioned structures, the use of which
are for the most part sufficiently obvious. Of this descrip-
tion are the tendrils, which assist in fixing and procuring
support to the stems of the weaker plants; the stipidde,
which protect the nascent leaves; and the hractese, which
perform a similar office to the blossom. The different kinds
of hairs, of down,t 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 defend-
ing the plant from molestation by animals. The sting of
the nettle is of this class; and its structure bears a striking

* This name was g^iven to them by De Candolle, Annates des Sciences
Naturelles, VII. 1, and Organograpliie, I. 94.

f The finer hairs, and filaments of down, are composed of elongated cells,
either single, or several conjplned end to end.

DEVELOPMENT OF VEGETABLES. 79

analogy, as we shall afterwards have occasion to notice, to
that of the poisonous fangs of serpents.

The purposes answered by the down, which covers a great
number of plants, are not very obvious. It perhaps serves
as a protection from the injurious effects of cold winds on
the tender surface: or it may have a relation to the deposi-
tion 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 dis-
turbed 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 pro-
vision made, according to the particular circumstances of the
case, for the mechanical objects of cohesion, support, and de-
fence. Thus, the substance, of the leaf, of which the func-
tions require that a large surface be expanded to the air and
light, is spread out in a thin layer upon a frame-work of
fibres, like rays, connected by a net-work of smaller fibrils,
and constituting what is often called the skeleton of the leaf.

In all these vegetable structures, while the objects appear
to be the same, the utmost variety is displayed in the means
for their accomplishment, in obedience, as it were, to 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 utterly be-
yond 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 insufficient for the exercise of the more active func-
tions 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

so THE MECHANICAL FUNCTIONS.

must require a union of strength and flexibility in the parts
intended for extensive motion, and for being acted upon by
poweiful moving forces.

The animal, as well as the vegetable fabric is necessarily
composed of a union of solid and fluid parts. Every animal
texture appears to be formed from matter that was original-
ly 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 physi-
cal science, be termed animal crystallization. Many of those
animals, indeed, which occupy the lowest rank in the se-
ries, such as Medusae, approach nearly to the fluid state; ap-
pearing like a soft and transparent jelly, which by spontane-
ous decomposition after death, or by the application of heat,
is resolved almost wholly into a limpid watery fluid.* More
accurate examination, however, w^ill show that it is in reali-
ty not homogeneous, but that it consists of a large propor-
tion of water, retained in a kind of spongy texture, the indi-
vidual fibres of which, from their extreme fineness and
uniformity of distribution, can with difliculty 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 development is continually tending to solidify the struc-
ture of the body. In this respect the lower orders of the
animal kingdom, even when arrived at maturity, resemble
the conditions of the higher classes at the earliest stages of
their existence. As we rise in the scale of animals, we ap-
proximate to the condition of the more advanced states of
development which are exhibited in the highest class.

Great efibrts 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 mate-
rial, as it were, with which the whole fabric is wrought:

* 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 Quoy and
Gaimardf Annales des Sciences Naturelles, torn. I. p. 245.

ANIMAL ORGANIZATION. 81

but their labours have hitherto been fruitless. Fanciful hy-
potheses in abundance might be adduced on this favourite
topic of speculation; but they have led to no useful or satis-
factory 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 ge-
ometry. Chemical analysis alone is sufficient to overturn
all these hypotheses of the uniformity of the proximate ele-
mentary 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 pro-
blem: for although it has been alleged by manj^ observers
that the ultimate elements of every animal structure con-
sists of minute globules, little confidence is to be placed in
these results obtained by the employment of high magnify-
ing 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, compose many of the solids, is
also extremely 'probable. But it is very doubtful whether
they are essential to the composition of other parts, such as
the fibres of the muscles, the nerves, the ligaments, the ten-
dons, and the cellular texture: for the most recent, and ap-
parently most accurate microscopical observations tend to
show that no globular structure exists in any of these tex-
tures.*

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. Al-
though bearing the same designation as the elementary mate-
rial 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 laminae, or plates, variously con-

* See the Appendix to Dr. Hodgkln and Dr. Fisher's translation of Ed-
ward's work on the Influence of Physical Agents on Life, p. 440.

Vol. I. 11

S2 THE MECHANICAL FUNCTIONS.

nected together by fibres, and by other plates which cross
25 them in different directionSjleaving cavities or

cells. (Fig. 25.) These cells, or rather inter-
vening spaces, communicate freely with one
another; and, in fact, may be considered as
one common cavity, subdivided by an infinite
number of partitions into minute compart-
ments. Hence the cellular texture is through-
out readily permeable to fluids of all kinds, and retains these
fluids in the manner, and on the same principle, as a sponge.
The cellular texture is not only the element, or essential
material employed by nature in the construction of all the
parts of the animal fabric; but, in its simplest form, it con-
stitutes the general medium of connexion between adjacent
oro-ans, 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 ad-
mirably adapted to the mechanical purposes which are re-
quired in difierent parts of the frame: and these properties
are variously modified and adjusted to suit the particular
exigencies of the case. When, for instance, difierent parts
require to be moveable upon each other, the cellular sub-
stance interposed between them has its state of condensation
adapted to the degree of motion required. That which con-
nects 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 ad-
hesions, and leaving large interstices; while the more quies-
cent organs, the plates of the cellular substance, are thin and
small, the fibres short and slender, and their intertexture
closer and more condensed.

Besides being flexible and extensible, the cellular texture
is also highly elastic, a property which is exceedingly advan-
tageous in the construction of the frame. Not only the dis-
placement of parts is resisted by their elasticity, but, when
displaced, they tend to return to their natural position. This
property performs a more important part in the mechanism

ANIMAL ORGANIZATION. 83

of the animal than of the vegetable system: as mlglit, 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 con-
tinuous sheet of a closer texture, after the surfaces of the
plates have been brought to cohere so as to obliterate all the
cellular interstices, and become impervious to fluids. Though
equally flexible and elastic with the original texture of
which it is formed, the membrane has acquired, by this con-
solidation, greater strength and firmness, 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 ex-
tent of their relative motions. They furnish strong cover-
ings for the investment, the support, and the protection of
all the important organs of the body. What Paley has
termed ihe package of the organs is efiected 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, af-
ter 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 coverino;. Their in-
ner sides present every where a smooth and polished sur-
face, 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 sur-
faces that are in contact with one another, and obviates the
injury that would otherwise arise from friction. From this

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

84

THE MECHANICAL FUNCTIONS.

circumstance, the linings of these cavities have been termed
serous mevibranes. In the neighbourhood of joints, closed
cavities of the same description, but of smaller size, are met
v^^ith, 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 pur-
pose. The ink-bag of the cuttle fish, the gall-bladder, and
even the stomach itself, are examples of this kind of struc-
ture. The coats of these sacs, being very extensible and
elastic, readily accommodate themselves to the variable bulk
of their contents.

In the second place, we find membranes composing 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 constant direction.
For preventing the retrograde motions of the fluids passing
along these canals, recourse is had to the beautiful con-
trivance of valves. The inner membrane of the vessel is
employed to construct these valves; for which purpose it is
extended into a fold having the shape of a crescent; fixed by
its convex edge to the sides of the vessel, while the other
edge floats loosely in its cavity. Whenever the fluid is im-

ANIMAL ORGANIZATION. ( 85

pelled in a direction contrary to its proper course, it raises
the loose edge of the valve, which, being applied to the op-
posite 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 ap-
plied 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 flood-gates, or
locks of a canal.* Among the numberless instances of ex-
press contrivance which are met with in the examination of
the fabric of animals, there is perhaps none more striking
and more palpable, than this admirable mechanism of the
valves.

As we ascend from the simpler to the more complicated
systems of organization, adapted to a greater range of facul-
ties, we find greater diversity in the mechanical means em-
ployed for carrying on the functions of life. Textures of
greater strength than can be constructed by membranes
alone become necessary for the security, the support, and
the defence of important organs; and more especially for the
execution of extensive movements. For obtaining: these
advantages a peculiar species of fibres is provided, formed
of a much denser substance than even the most consolidated
forms of cellular texture. The animal product termed albu-
men 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 that
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. Th^sQ Jibrous textures, as they are
termed, while they retain the flexibility of membranes,
greatly surpass them in strength; but, being at the same time
incapable of extension, they are necessarily devoid of elasti-

* Fig. 27, representing" the section of a vessel, is Intended to show the po-
sition of the valves when applied lo the sides of the vessel, by the stream
nnoving- onwards in the direction pointed out by the arrow. In Fig". 28, they
are seen closing the passage by the retrograde pressure of tlie current.

86 THE MECHANICAL FUNCTIONS.

city. Hence, they are adapted to form external tunics for
the investment of such organs as are not intended to vary in
their size. Occasionally, these fibrous capsules, as they are
called, send down processes into the interior of those organs,
for the purpose of giving them mechanical support. This
is the case, for 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 pres-
sure. In other cases these sheets of fibrous membrane are
employed as bandages, tightly bracing the muscles, and re-
taining them in their relative situations. The joints are sur-
rounded by similar bandages, known by the name of Cap-
sular Ligaments.

In following the series of animal structures in the order
of their increasing density, we find the proportion of albu-
men which enters into their composition becoming greater,
while that of the gelatin and mucilage diminishes. When
the product is more uniform in its composition it is in gene-
ral less elastic than when it consists of a more complex com-
bination of ingredients. A great preponderance of albumen
tends also to diminish the elasticity. Thus, the densest
kinds of fibrous texture present, instead of thin and broad
expansions of elastic membrane, the thick and elongated
form of inextensible cords, constituting the ordinary Liga-
vients, and the Tendons. These structures resist with
great power any force calculated to extend them: a proper-
ty which of course excludes elasticity, but, when united
with flexibility, implies great toughness. In a word, they
possess all the qualities that can be desired in a rope. It
will hardly be credited how great a force is required to
stretch, or rather rend asunder a ligament; for it will not
yield in any sensible degree until the force is increased so
enormously as at once to dissever the whole contexture of
its fibres. Nothing can be more artificially contrived than
the interweaving of the fibres of ligaments; for they are not
only disposed, as in a rope, in bundles placed side by side,
and apparently parallel to each other: but, on careful exami-

ANIMAL ORGANIZATION. 87

nation, they are found to be tied together by oblique fibres
curiously interlaced, in a way that no art can imitate. It is
only after long maceration in water, that this complicated
and beautiful structure can be unravelled.

The mechanical properties of these fibrous structures,
which are strictly inextensible ligatures, render them appli-
cable to purposes of connexion where motion is to be re-
strained. Many cases, however, occur in which a substance,
is wanted, uniting great compactness and strength with a
considerable degree of elastic power. For this purpose a
different texture is fabricated, consisting of twisted fibres,
which impart this required elasticity. Such is the structure
of the elastic ligaments of animals, which are very gene-
rally employed for the support of heavy parts that require
being suspended. An instance occurs in quadrupeds, in that
strong ligament which passes along the back and neck to be
fixed to the head, and to support its weight when the ani-
mal stoops to graze. This, the ligamentum nitchde, as it is
termed, is capable of great extension, and by its elasticity
reacts with considerable force in recovering its natural length,
after it has been stretched. This ligament is particularly
strong in the Camel, whose neck is of great length.* Ano-
ther example of an elastic ligament occurs in that which con-
nects the two shells of bivalve mollusca (as those of the oy-
ster and muscle,) and which keeps them open when the ani-
mal 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

* Many birds are provided with strong- elastic lig-aments connecting- the
vertebrae of the neck with those of the back; ligaments of the same kind are
also employed for retaining the wing-s close to the body, where they are not
used in flying-: and a similar provision is made in the wing-s of bats. The
weight of the bulky org-ans of digestion in herbivorous quadrupeds require
some permanent support of this kind; and this is furnished by a broad, elas-
tic fibrous band extended across the lower part of the abdomen. It is par-
ticularly strong in the elephant, which remains more constantly in the liori-
zontal position than most quadrupeds: and it has been remarked that the g-e-
neral cellular texture in this animal has an unusual degree of elasticity. —
Hunter on the Blood, &c. p. 112.

S8 THE MECHANICAL FUNCTIONS.

of this kind are employed very extensively in the fabric of
insects.*

The animal substance which comes next in the order of
density is Cartilage. The purposes for which this kind of
structure is employed are those in which a solid basis is re-
quired for the support of softer or more flexible parts, and
where the mechanical properties that are wanted are firmness,
conjoined with some degree of elasticity. Cartilage (or gris-
tle) is composed of a finer and more uniform material than
any of the preceding structures. It consists almost wholly
of albumen, with a slight proportion of calcareous matter.
Unlike membrane in any of its forms, it contains no fibres,
but, on being cut with a sharp knife, presents the appearances
of a dense homogeneous substance of a white pearly hue.
Its surface is smooth, and where it is exposed to friction, as
in the joints, is often highly polished.

In all the inferior tribes of animals Nature employs car-
tilage to supply the place of bone when ridigity 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 difierent parts of the apparatus
of the throat and windpipe, are composed of flexible carti-
lage, which gives them a determinate shape and firmness. In
all these cases bone, which besides being three times as hea-
vy, is devoid of elasticity, and liable to fracture, w^ould have
been much less suitable. Cartilage is often employed as an
intermedium for connecting difierent bones, as for instance,
between the ribs and the sternum, or breast bone; whereby,
besides the advantage of greater lightness, the pliancy of
the material diminishes those jars which are incident to the
frame in all its violent actions.

In the construction of cartilage, nature seems to have at-
tained the utmost degree of density which could be given
to an internal texture composed merely of the usual animal

* Chabrier, Memolres du Musee, torn. vi. p. 416.

ANIMAL ORGANIZATION. 89

constituents. But substances 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 framework of the body, and
for constructing levers of various kinds to be employed in
the more energetic movements of the higher animals. For
all these purposes it was necessary to superadd a material
endowed with stronger cohesive powers, and capable by its
dense concretion of forming solid and Inflexible organs.
The substances which nature has selected for this office are
the salts of lime. Sometimes the Carbonate, and sometimes
the Phosphate of lime is employed for forming these hard
and unyielding structures; and often both these calcareous
substances are united together in different proportions in the
same solid fabric. When the carbonate of lime predomi-
nates, 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 lob-
ster and the crab: when the earthy matter consists almost
wholly of phosphate of lime, it composes the different forms
oiBone. I shall have occasion to describe the formation
and properties of each of these structures in the sequel.

The protection of the delicate structure of the fabric from
the injurious influence of external agents is an object of
great imjDortance in the animal economy, and is one which
nature has shown extreme solicitude to secure. For this
purpose she has provided the integuments, under which
designation are included not merely the skin, but also all the
parts that are immediately connected with it, and are formed
and nourished by the same vessels. No parts of the animal
structure present greater diversity in their form and out-
ward appearance than the integuments; yet it is easy to dis-
cover, amidst all these varieties, that the same general plan
has been followed in their construction, and that each par-
ticular formation is the result of a combination of the same
elementary structures. Of these elements the most important,
and that which generally composes the chief bulk of the
skin, is the Corium, or true skin. The outermost layer is
Vol. I. 12

90 THE MECHANICAL FUNCTIONS.

termed the Epidermis, Cuticle, or scarf-skin; and between
these there is often found an intermediate layer denominated
the Rete Mucosum, or the Pigmentum.

The corium is generally of considerable thickness, and is
composed of strong and tough fibres, closely compacted to-
gether, and pervaded by innumerable ramifications of blood-
vessels of every kind. It is endowed with great flexibility,
and is capable of being considerably extended; properties
which fit it for readily accommodating itself to all the move-
ments 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 particular intentions of its formation, some-
times 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 ani-
mal in wdiich the skin hangs loosely on the limbs, and en-
closes 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
fino-ers 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 subja-
cent parts. In animals that roll themselves into a ball, as
the hedge-hog, these muscles are of great size and import-
ance. We shall see that in the mollusca, this muscular ap-
paratus is inseparably blended with the integument, and
composes a peculiar structure, termed the mantle. Imme-
mediately covering the corium is the Rete Mucosum, which
is a very thin layer of soft animal matter, composed of a net-
work of delicate fibres, and containing more or less of the
material from which the colour of the skin is derived.

The Epidermis is a membrane of a very peculiar nature,
consisting of a thin expansion of albuminous matter appa-

ANIMAL ORGANIZATION-. [)l

rently homogeneous in its texture and composition. It is
impervious to fluids, althougli capable of imbibinsr moisture,
and of slowly transmitting a portion to the subjacent tex-
tures. Its thickness varies exceedingly in different parts;
being adapted to the kind of protection it has to afford
against pressure, friction or other causes of injury. As it
is not nourished by vessels, its outer layer is liable to wear
away, or to become, by drying, unfit for use: and accord-
ingly a separation of this outward layer generally takes place
from time to time, the loss being speedily repaired by a
fresh growth from the surface in contact with the skin.
This process is often performed periodically, as is most re-
markably exemplified in serpents.

Special provisions are made for preserving the cuticle in
a healthy condition; and more particularly for defending it
from the injurious action of the surrounding element. These
sometimes consist of a supply of oily fluid, prepared in
small cavities that are situated in the skin itself, and have
minute ducts opening upon the surface. These cavities,
termed sebaceous follicles, are generally interspersed in
great numbers on different parts of the body, abounding
more especially in those places where folds occur, and where
there is the greatest friction. In fishes, mollusca, and other
aquatic animals, the skin is at all times defended from the
action of the water, by a viscid or glutinous secretion pre-
pared in this m.anner, and continually poured out on the sur-
face, through ducts, the orifices of which arc easily seen with
the naked eye, disposed in a line on each side of the body.
Connected v/ith 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 offence. In this class should bo 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 arc also the

92

THE MECPIANICAL FUNCTIONS,

nails, the hoofs, and the claws of quadrupeds, and the scales
of fishes, reptiles, and other animals. The integuments of
insects, and especially their more solid and horny coverings,
contain, however, as will hereafter be noticed, a peculiar
chemical principle termed Entornoline.

All these parts seem to be but remotely connected with
the vital actions of the system with which they are associ-
ated; and it is doubtful how far they are to be considered as
appertaining to the living portion of the body, or as mere
extraneous appendages. Yet, however they may difier in
their forms, uses, and external appearance, they all are pro-
duced by the same kind of vascular structure, variously ar-
ranged to suit the particular circumstances in each case: and
the mode of their development and growth is essentially the
same in all.

An extremely delicate and finely organized pulp, com-
posed 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 dis-
position of this vascular pulp, determines the future figure
and extent of growth of the production which is to arise
from it. The materials w^iich 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 fishes;* and occasionally in filaments, as in hair;

• The laminated structure of the scales of fishes is easily distinguished by-
applying to them a high magnifying power. As the breadth of each new
layer is greater than the last, its edges project farther, the whole surface

ANIMAL ORGANIZATION.

93

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 exam-
ple. In all cases, the portions thus successively produced,
are no longer susceptible of being nourished, and from the
moment of their deposition, undergo no farther change, ex-
cept from the action of external agents. By the continual
additions that are made to them at their base, or root, where
the vessels deposite 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 acti-
vity.

The nature of this process is well exemplified in the
growth of hair. Fig. 32 shows the apparatus employed in
its construction, in an imaginary section of the root, on a
magnified scale. Every hair takes its rise from a minute
vascular pulp, p, of an oval shape, which is implanted below
the corium, or true skin, p.* This pulp is invested by a

sheath or capsule, c, which,
together with the contained
pulp, and the root of the hair
that grows from it, composes
the bulb of the hair. The
bulb itself is contained in a
small cell formed by con-
densed membranes, s, to
which it has no attachment
excepting at the lower part, v,
where the vessels and nerves
of the pulp are passing into

having that concentric striated appearance which renders it an interesting* ob-
ject for microscopic cxaniination. Fig. 29 exhibits the striated surface of the
scale of the Cyprinxis alburnus, and Fig. 30 that of the Perca JluviatiUs.
The imbricated arrangement of these scales, resembling- that of the tiles on
the roof of a house, is shown in Fig. 31. All these figures represent the
objects highly magnified.

* In the above figure r. is a section of the Epidermis, or cuticle; the dotted
part, R, represents tiic situation of the subjacent retc mucosum, and d, the
derm, or corium.

94 THE MECHANICAL FUNCTIONS.

it. The hair, growing by depositions from the inside of the
capsule, which forms the outer part, o, of the shaft, and from
the outside of the pulp, w^hich forms the inner or central
part, I, is forced upwards till it has pierced the skin; and in
the course of its passage a canal is formed for it in the skin
itself, and continuous with that which encloses the bulb:
and the course of this canal is generally oblique. In the ele-
phant, where the thickness and density of the hide, present
considerable obstacles to the passage of the hairs through it,
we may discover, on minute examination, many hairs that
have only penetrated a certain way, as shown at b, without
ever succeeding in reaching 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
outer integument, which, stretching as the hair increases in
length, forms over it a very fine external tunic. But later
observations have shown that this is not the case, and that
there is simply an adhesion of the edge of the cuticle to the
origin of the hair, without any accompanying prolongation;
so that if the whole bulb be destroyed, and its pulp absorbed,
the hair may be detached by the slightest force.

From this account it will be seen that a hair is, in its ori-
gin, 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 sur-
face of the skin; and the cavity in the axis of the hair is
either gradually obliterated, or is filled with a dry pith, or
light spongy substance, probably containing air. After a
certain period, the bulb diminishes in size, from the collapse
of the vessels whose powers of supplying nutriment become
exhausted. The first deficiency in its nourishment appears
in the cessation of the deposite of colouring matter, and the
hair in consequence becomes gray. After a time, the ves-
sels becoming quite impervious, the bulb shrivels, the hair
is detached, and the canal which its root occupied in the
skin becomes obliterated.

ANIMAL ORGANIZATION. 95

The hair of difTcrcnt animals, and even of different parts
of the same animal, is very various in its shape, texture, and
mechanical properties. Sometimes, instead of being cylin-
drical, the filaments 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 great-
est diversity in their length, fineness, tenacity, rigidity, and
disposition to curl. All these varieties 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 b}^ 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 arc subservient to more extended uses than those of
merely covering the body, and which are even supplied
with nerves, converting them into instruments of a sense of
touch.

In the quills of the porcupine a still more complicated or-
ganization has been detected. Fig. 33 shows a quill with
its bulbous root, detached from the body; and Fig. 34, a
transverse section magnified. The bulb itself is contained
in a distinct cell, shown at a. Fig. 35, which represents a
longitudinal section of these organs. This cell contains a
portion of fat in which the numerous vessels supplying its
pulp and capsule are embedded. The bulb is itself sur-
rounded by an outer sheath, s, into the cavity of which, b,
there opens a duct, d, proceeding from a small cell or fol-
licle, F, lodged in the cellular substance on the outside of
the sheath. This upper cell communicates below with ano-
ther cavity, c, containing an unctuous matter. During the
formation of the quill this unctuous matter is supplied through
this channel, and probably enters as an ingredient in its
composition. The capsule of the pulp consists of two mem-

* See F. Cuvier's Memoir on the Formation of the Quills of the Porcu-
pine, in the Nouvelles Annulcii du Museum, I. 429.

96

ANIMAL ORGANIZATION.

branes, 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 laminae.
The external membrane is seen at o. The pulp itself, seen at
p, is still more curiously organized; its surface being fluted,
or formed into longitudinal processes. The horny matter,
being deposited on these processes, is moulded to their shape,
and concretes into laminae which converge from the circum-
ference of the cylinder towards the centre. The section
(Fig. 36) shows these converging laminae, which being of a
dark colour, give to the surface of the quill the appearance
of being grooved; this, however, is merely an optical illu-
sion occasioned by the dark laminae being seen through the
transparent exterior covering; as may readily be detected
by viewing the surface with a magnifying glass.* After a
certain period of the growth of the quill, the pulp ceases to
supply the materials for 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 deposite horn, the quill becomes a hollow tube of consi-
derable diameter. When it has attained a certain size, the

• It is observed by F. Cuvler, that this striated appearance is peculiar to
the quills of porcupines of the old world. Those from America have no
such arrang-ement of laminae.

MUSCULAR POWER. 97

pulp begins to shrink, and the diameter of the tube dimi-
nishes; so that it exhibits a tapering form at both ends. Thus,
mere variations in the bulk and the action of the pulp, ac-
companied 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 pres-
sure, ought not to be overlooked. This object is promoted
by the interposition of a layer oi fat, which is another ani-
mal substance entitled to be enumerated among the elements
of its structure. It consists of an oily fluid, composed, ac-
cording to the analysis of Chevreuil, of two constituent prin-
ciples, which he has distinguished by the terms .s/eflfr2?ie and
elaine. In warm-blooded animals the temperature of the
body is always sufficient to preserve this compound sub-
stance in a fluid form: but it is prevented from being dif-
fused through the cellular texture by being contained in
separate vesicles of extreme minuteness.* lience, the whole
mass of the fat, which is thus formed of an aggregation of
these vesicles, has not the appearance of being fluid, but
seems to be composed of small grains united by membranous
investments into larger masses; a structure peculiarly adapted
to the purposes of a soft cushion, retaining only a small
share of elasticity, and yielding only in a certain limited
degree to pressure.

§ 5. Muscular Power,

In Machines contrived by human skill the chief art con-
sists in devising expedients for regulating and directing the
giving moving power, so that it may bear, in the proper
degree, and in the proper order, upon some particular ob-
jects, and produce some particular effect. The whole of the
apparatus employed with this intention, however numerous

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

Vol. I. 13

98 THE MECHANICAL FUNCTIONS.

may be its parts, however various the forms of its v»^heels,
its levers, or its pulleys, and however complicated may be
their connexions, resolves itself into a series of intermediate
instruments for the transference of motion from the source
of power, or the point v.'here its action is impressed, to the
parts which are designed ultimately to receive the action of
the force employed. It is an established principle in physics,
that mere m.achinery is incapable of generating mechanical
force, and that such force must always be originally derived
from some extraneous source. Some impulse from without,
whether it be the pressure of the wind, the fall of a stream of
water, or the action of men or horses, or any other kind of fo-
reign agency, must be resorted to, both to set the engine in
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-engine, which is set in mo-
tion 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 distribution of a
force orioiinatingfrom 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 con-
taining within itself a principle of motion not referrible, as
far as we can perceive, to any of the primiary forces which
exist in the inanimate world. This principle has been
termed conlractility. In animals of the simplest construc-
tion, every part of the substance of the body seems to be
equally endowed with this contractile property, although ex-
hibiting no distinct appearance of a fibrous structure. This
is the case with all the lower zoophytes, such as the Infuso-
ria, Polypi, Medusas, and the simpler kinds of Eniozoa,

Among the Polypi and Infusoria we meet with a singular
mode of acting upon the surrounding fluid by means of very

MUSCULAR POWER. 99

minute and generally microscopic filaments, which the ani-
mal, by some unknown power, causes to vibrate with great
rapidity. Occasionally, these organs are found even in ani-
mals belonging to the higher classes. Wherever they arc
met with, they perform, as will hereafter be shown, very
important functions; sometimes assisting in respiration, at
other times contributing to the supply of food, and very ge-
nerally serving as instruments of progressive motion.

In animals placed a little higher in the scale, we begin to
trace the formation of fibres, which at first are irregularly
scattered through the soft substance: but as the organization
becomes more refined, these fibres are collected into l)undles,
and compose what are properly called m/iiscles. Muscular
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 pnrts, v/hich arc
subservient to the purposes of progressive motion. In the
higher classes, the solid parts, or skeleton, being disposed
more in the centre of the system, the niuscles 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 characterizes the muscular
fibre is that of suddenly shortening itself, so as to bring its
two ends, and the parts to which those ends are connected,
nearer to one another. This contraction is performed with
astonishing quickness and force, and the accumulated effect
of a large collection of these fibres, such as constitutes a mus-
cle, is therefore capable of overcoming great resistances, or
of raising enormous weights. Those muscles, which, by
means of their nerves, as will hereafter be noticed, are sub-
servient to voluntary motion, are excited into action by an
exertion of the will of the animal. There are, however, a
great number of other muscles, the contractions of which
are involuntary, that is, are produced by other causes than
the will.*

* These two classes of muscles do not differ in their outward appearance:
but Dr. Hodgkin has lately pointed out a curious difference in tJie micro-

100 THE MECHANICAL PUNCTIONS.

Muscular contractility, of which there exists no trace in
the vegetable kingdom,* has been established by nature as
the primary moving power of the animal machine. This
agent is resorted to on all occasions where considerable me-
chanical force is wanted; just as in a great manufactory,
where an immense quantity of machinery is to be set in mo-
tion, and a great variety of work is to be executed, the hu-
man mechanist avails himself of some constant moving force,
such as that of water, or steam. The laws of inorganic mat-
ter furnish no power that could conveniently have been ap-
plied 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 appli-
cation of the laws of mechanism, to all the degrees and kinds
of effects intended to be produced.

Although the power be the same, yet the mode of its ap-
plication is exceedingly diversified; and the comparison of
these diversities is the more interesting, inasmuch as there
are few of the animal functions in which the ends to be an-
swered are so definite, and the operation of the expedients
employed is so plain and intelligible. For while the intri-
cate chemical processes of the living system generally elude
our research, and the higher faculties of sensation and per-
ception are dependent on still more recondite and mysteri-
ous powers of nature, the mechanical functions are effected

scopic structure of the fibres of some of the involuntary muscles. See Ap-
pendix to his Translation of Edwards on the Influence of Physical Ag-ents in
Life, p. 443.

* The principal instances, which have been adduced in support of the
opinion that muscularity occasionally exists in vegetable structures, are the
alternate movements of the leaflets of the Hedysarum gyrans, which have
been fancifully compared to the movements of the ribs in respiration; the
quick motions of the stamina of the Berberis, Opuntia, and many plants of
the genera Carduus, and Centaurea; the closing of the leaves of the Dionaea
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 be-
tween these phenomena, and that there is no substantial evidence for tlie
existence of that property in the vegetable kingdom.

MUSCULAR POWER.

101

by the simpler properties of matter, and allow us a clearer
insight into the wonderful art which has been exerted in
their accomplishment.

Muscles, during their contraction, increase in thickness
in the same proportion as they diminish in length. "^ It is on
this account, more especially, that a knowledge of anatomy

37

38

43

40

is so necessary to the painter and the sculptor. In every
movement and attitude of the body, some particular sets of
muscles are in action, and consequently tense and prominent
while others are relaxed and flattened; differences which it
is requisite that the artist should faithfully express, in order
to give a correct representation of the living fio-ure.

The dilatation of the muscular fibres in thickness, which
accompanies their contraction in length, would, if these
fibres had been loose and unconnected, have occasioned too
great a separation and displacement, and have impeded their
co-operation in one common effect. Nature has guarded
against this evil by collecting a certain number of the ele-
mentary 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 surrounding each packet with a web of cellular tis-

* This is illustrated by the annexed figures, 37 and 38, the former show-
ing tlie relaxed and elongated, and the latter the contracted and swollen
state of the same muscle.

102 THE MECHANICAL FUNCTIONS.

sue; 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 en-
tire muscle is completed.

That we may be the better able to appreciate the excel-
lence of the plans adopted in the mechanism of the animal
frame, let us inquire what arrangements would occur to us,
prior to an acquaintance with those actually adopted, as the
most advantageous dispositions of the muscular power. It
is evident, that the simplest mode would be that of extend-
ing 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 compa-
tible with convenience, unless the parts to be moved are of
very small size, and require very delicate adjustments.
Straight muscles, accordingly, are employed chiefly for the
movements of the minuter parts of the 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 and the Cardhim, are closed by
one or two straight muscles, the fibres of which pass imme-
diately from the inner surface of the one to that of the other.

In the greater number of cases it is more convenient to
place the muscle in a situation which causes it to act ob-
liquely 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, advantages gained,
both in point of velocity of motion, and also in the effect
being produced by a smaller extent of contraction in the
fibres of the muscle. Oblique muscles are frequently em-
ployed in pairs, and are made to act on opposite sides of the
line of the intended motion, which is, in this case, the dia-
gonal 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

MUSCULAR POWER. 103

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 accom-
plished more quickly, and with a smaller extent of contrac-
tion in the muscles producing it, than if the same power had
been applied by means of a straight muscle in the direction
p o.* It is by means of two sets of muscles, acting thus ob-
liquely, that the ribs are brought in closer approximation
every time that the chest is elevated in breathing. Thu»
carefully does nature dispose the muscular fibres so as to
obviate the necessity of their being contracted beyond a cer-
tain extent: and thus does she economize, as much as possi-
ble, the expenditure of muscular power, wherever there is
a constant call for its exertion.

The principle which I have just explained, whereby cer-
tain advantages result from the obliquity of the action of
muscular fibres, is applied, not only to the entire muscle,
but also to the internal arrangement of its fibres. Thus,
we generally find that, in a flat muscle, its upper and
under surfaces are covered by a thin sheet of fibrous tex-
ture, or thin expansion of ligament or tendon; and that
the muscular fibres which are attached to them are direct-
ed obliquely from the one to the other, in the manner re-
presented 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 muscu-
lar fibres pass obliquely from the former to the latter, but in
different directions on each side; like the fibres proceedino-
from the shaft of a pen. A muscle thus constructed has ac-
cordingly been termed di penniform muscle; as is exempli-
fied in the straight muscle inserted into the knee-pan (the
rectus extensor c?'uris,) and also in the muscle which bends
the great toe (the flexor pollicis jjedis lo?igus.) The ar-
rangement first described. Fig. 40, forms the se?7ii-pcfi}ii-
form muscle; an instance of which occurs in the muscle of
the leg, which is termed the semimembranosus. Frequent-

• See a paper by Dr. Monro, iji the Transactions of the Koyal Society of
Edinburgh. Vol. iii. p. 250.

104 THE MECHANICAL FUNCTIONS.

ly the structure is rendered still more complex, by the in-
terposition of several tendinous layers among the fleshy
fibres. This arrangement, which constitutes a complex tuus-
cle, (as shown in Fig. 42) occurs, for example, in the Soiasus,
or large muscle, which raises the heel, and forms the thick-
est part of the calf of the leg.

It very commonly happens in the animal frame, as it
does in other machines, that the presence of the moving
agent in the spot where its action is w^anted, would be ex-
ceedingly inconvenient. The usual plan adopted for trans-
ferring the effect of the moving power to a distant point is
the employment of a rope, or strap. Such is precisely the
office of the 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 performed its office as a delicate mechani-
cal instrument. 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
farther advantage, that by their intervention the united pow-
er of all the fibres of the muscle may be obtained, and con-
centrated upon any particular point. In this respect, like-
wise, they resemble a rope, at which a great number of men
are pulling at the same moment, and whose combined strength
is thus brought into action. Another principal use of ten-
dons is that a different 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 trans-
mission, and producing the effect of pulleys.

In a great number of muscles, the fibres, instead of running-
parallel to one another, are made either to converge, or to
diverge, in order to suit particular kinds of movements: and

MUSCULAR POWER.

105

wc frequently find that different portions of the same mus-
cle have the. power of contracting independently of the rest,
so as to be capable of producing very various effects, accord-
ing as they act separately or in combination. This is exem-
plified in the muscle of the back, called the Trapezius, repre-
sented 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 circular di-
rection, forming what is called an orbicular, or sphincter
muscle, of which an example occurs in that which surrounds
and closes the eye. (Fig. 4G.) Very frequently these two
last modes of arrangement are united in some part, as ap-
pears to be the case in the membrane of the eye, called the
Iris. (Fig. 47.) The circular fibres of the iris surround the
central aperture, or pupil, the size of which they diminish
when they contract; while, on the contrary, the radiating
fibres, acting on the inner circle, and drawing it nearer to
the outer circumference, which is fixed, lessen the breadth
of the ring, and consequently enlarge the circular aperture.

47

43

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 adhesion to the surfaces on which they are
applied. In these organs the circular fibres are placed at

Vol. I.

Home Phil. Trans, for 1800, p. 1.
14

106

THE MECHANICAL FUNCTIONS.

the circumference, and the radiating fibres in the interior of
the sucker, (see Fig. 48;) so that, while the margin of the
disk is closely applied to the object, the force resulting from
the contraction of the circular fibres is exerted to remove
the central portions from the surface of attachment, and
thereby tends to create a vacuum underneath the disk; the
two surfaces remain, therefore, strongly attached by the at-
mospheric pressure, which acts on their outer sides. An ap-
paratus of this kind, as we shall afterwards find, is met with
very frequently among the lower orders of the animal king-
dom.

Another kind of circular disposition of fibres is that which
occurs in the muscular coats surrounding canals of various
kinds, such as the blood vessels and the alimentary tube.
Their action tends to contract the diameter of the canal, and
to exert pressure on its contents. In these cases, there is
2;enerally at the same time provided another layer of fibres,
disposed longitudinally, as shown in Fig. 49; the circu-
lar fibres being seen in Fig. 50. The action of the longi-
tudinal fibres is evidently to shorten the canal; while that
of the circular fibres, by the yielding and the partial re-
action of the contents of the vessel, has a tendency to
extend it. The Jiscidia, which is a species of marine worm,
is an example of an animal whose skin contains a union of
straight and circular fibres, by which all its movements are
readily performed. Many instances occur in the cylindrical
envelopes of animals, of the combination of a third series of
fibres, passing obliquely, with those which have transverse
and longitudinal directions. In the muscular skin of the
Leechy for example, besides two internal layers of longitudi-
nal fibres, an external one has lately been discovered, which
is composed of oblique or spiral fibres, crossing one another

MUSCULAR POWER. 107

in opposite directions, and greatly facilitating 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 receptjicles, such as the stomach, (Fig. 51,) and the
heart, (Fig. 52.) We find, in the substance of these organs,
sets of fibres, which pass in a spiral direction, and which,
consequently, unite the effects of both longitudinal and cir-
cular fibres; and, when combined with either of these, they
serve to modify and regulate the actions of each organ in a
great variety of ways.t

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 man-
ner, in the execution of the smaller motions, requiring the
most accurate regulation, and the nicest adjustments. We
cannot but be struck with the accordance which may often,
in these instances, be traced with human contrivances, when
the greater motions are rapidly executed by one set of
agents, acting with considerable power and velocity, while
the minuter approximations to the exact positions are eflfect-
ed by a distinct part of the apparatus, capable of more deli-
cate action, though with a smaller force. Thus, while the
astronomer brings his telescope round by powerful machi-
nery, so as to direct it to that part of the heavens, where
the object he wishes to view is situated, a more nice me-
chanism is employed to direct the instrument accurately to
the exact point; and, again, another is provided for making
the proper focal adjustments. Many parallel cases occur in

* Carus, Tabulae Anat. Comp. fol. Tab. I. Fig". 6.

•j- 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 combina-
tion of oblique muscles already explained. Thus beautifully is the arrange-
ment of tiie muscular fibres of the heart calculated to produce the rapid and
complete expulsion of its contained blood, with tlie smallest amount of con-
traction in the individual fibres.

108 THE MECHANICAL FUNCTIONS.

the mechanism of the animal frame; one set of powerful
muscles being employed for the larger movements, and ano-
ther set provided for the accurate regulation of the more
delicate inflections 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 efiecting each particular
purpose are characterized by the most admirable simplicity.
In this respect, also, as well as in all others, we cannot fail
to recognise their infinite superiority over every correspond-
ing invention of man.

"In human works, thoug-h labour'don 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 resistance to be overcome. With
regard to the direct force, therefore, it is evident that they
must act with a great mechanical disadvantage; and this dis-
advantage is still farther increased by the obliquity of the
action with reference to the direction of the motion. But
the contractile power, which is inherent in the muscular
fibre, is so enormous, as amply to afi'ord these losses, great
as they necessarily are; while, on the other hand, full com-
pensation is made by the greater freedom and velocity of
motion thereby obtained. Strength is sacrificed without
scruple to beauty of form or convenience of purpose; and
that disposition of the force is always adopted, from which,
on the whole, the greatest practical benefit results. Every
where do we find the wisest adaptation of muscular power
to the objects proposed, whether it be exerted in laborious
efforts of the limbs and trunk; whether employed in ba-
lancing the frame, or urging it into quick progression; or
whether it be applied to direct the delicate evolutions of

MUSCULAR POWER. 109

the fingers, the rapid movements of the organs of speech,
or the more exquisite adjustments of the eye, or of the in-
ternal ear. Amidst the endless combinations of machinery
exhibited in different parts of the animal kingdom, althou<rh
the mode of application be diversified in ten thousand ways,
the original power is still of the same kind, and is regulated
by the same physical laws; and similar instruments are em-
ployed in effecting this infinite variety of purposes, by the
all-wise and omnipotent Architect of animated creation.

( 110 )

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 reference to its internal
welfare, the other to its relations with external bodies. The
diflferent parts of its system must, in the first place, be me-
chanically 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 move-
ments, which the performance of their functions renders
necessary. They must, in the second place, be made capa-
ble of exerting upon external matter the actions which con-
duce 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 motion.
' 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 locomotion, more especially, constitutes the
most general and palpable feature of distinction between
these two classes of beings. A plant, during the whole pe-
riod 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 wa-

ZOOPHYTES. Ill

ter in its immediate vicinity. It is exposed so the action
of the surrounding elements, and affected by their vicissi-
tudes, without the means of retreat, and without the power
of reaction. With respect to all external agents, indeed, ve-
getables may be regarded as passive beings. Very different
are the condition and destination of animals. Excepting
a few among the lower orders of the creation, such as Zoo-
phytes and MoUusca, all animals are gifted with the power
of spontaneously changing their situation, according to their
several wants and necessities, and are thus enabled to seek
and to choose those objects which are salutary, and to avoid
or reject those which are injurious. Nature has, for these
purposes, furnished them with a more complex organization
and more varied powers, adapted to a greater diversity of
pursuits, and to a higher and more expanded sphere of ex-
istence.

The power of progressive motion is enjoyed in very dif-
ferent degrees by different races of animals, according to the
particular model on which they are constructed, and the re-
lations which their organization bears to the element assigned
as their residence. All the mechanical circumstances in
their economy, indeed, are so closely linked together, as
scarcely to admit of being considered separately. Thus, we
find, in one animal, a variety of mechanical effects accom-
plished by one and the same instrument; while, in others,
they are each 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 reference to the design, and are ever
varied in exact conformity with the change of purpose. The
relative advantages of each plan of structure appear to have
been carefully estimated, and studiously balanced. Each
quality has been bestowed in different degrees of perfec-
tion; so that in following the series of gradation among the
successive tribes of animals, we occasionally meet with fa-
voured species, endowed with great superiority in some
particular faculty. Some animals excel in swiftness; others
in strength. Some are qualified to dive into the recesses of
the deep; others to flutter in the light regions of air; while,

112 THE MECHANICAL FUNCTIONS.

in many of the inferior ranks, we find all these objects re-
nounced for the more certain advantage of security, which
the softer texture of the organs renders one of paramount
importance. That construction of limbs which favours cer-
tain movements will necessarily interfere with the ready
performance of others, and must preclude the development
of the organs which would be necessary for facilitating them.
Different kinds of prey require dexterity in particular ac-
tions 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 penetrating into the bodies of other
animals; others with the means of insnaring their captives;
while others, again, instil into the veins of their victims a
deadly poison. Hence it is necessary, in studying the or-
ganization 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 oi p7'0gressive ino-
tion, noticing its different degrees of perfection as we fol-
low the ascending series of animals; but adverting, also, oc-
casionally, to the other topics which belong to this class of
functions.

It may be observed in general, that the mechanical con-
struction of animals which constantly inhabit a watery ele-
ment is more simple than the construction of those which live
on land, and are encompassed by a lighter medium. Dif-
fering but little in their specific gravity from the fluid in
which they are immersed, aquatic animals are necessarily
supported, on all sides, by a powerful hydrostatic pressure,
which nearly balances the force of gravity, and counter-
acts the tendency of their bodies to descend in the fluid.
Many of the obstacles to progressive motion are thus re-
moved; and there is no necessity for the compactness of
frame, and the rigidity and cohesion of substance which are
required in terrestrial animals.

SPONGES. 113

The animals that 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 Zooj)hytes, that is, animated plants: but as it
is now well ascertained that they possess the essential cha-
racters of animals, the term of Phytozoa. or plant-like ani-
mals, which has been given to them by some modern writers,
would appear to be a more appropriate 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 misunder-
standing.

§ 2. Porifera, or Sponges.

Among Zoophytes, the lowest station in the scale of or-
ganization is occupied by the tribes of Porifera^ the name
given by Dr. Grant to the animals which form the various
species of sponge, and which are met with in such multi-
tudes 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 diminu-
tive, and of a firmer texture, as we approach the Polar cir-
cles. Dr. Grant observes* that they are met with equally
in places covered perpetually by the sea, as in those which
are left dry at every recess of the tide. They adhere to,
and spread over the surface of rocks and marine animals, to
which they are so firmly attached that they cannot be re-
moved without lacerating and injuring their bodies. "Al-
though they thrive best," he farther remarks, "in the shel-
tered cavities of rocks, they come to maturity in situations
exposed to the unbroken fury of the surge. They cover the
nakedness of cliffs and boulders; they line with a variegated
and downy fleece the walls of submarine caves, or hang in
living stalactites from the roof. "

In their general appearance they resemble many kinds of

* Edinburgh Philosophical Journal, vol. xiii. p. 94.
Vol. I. 15

114

THE MECHANICAL FUNCTIONS.

plants, but in their internal organization they differ entirely
from every vegetable production; being composed of a soft
flesh, intermixed with a tissue of fibres, some of which are
solid, others tubular; and the whole being interwoven to-
gether into a curious and complicated net-work. The sub-
stance of which this solid portion, or basis, is formed, is
composed partly of Jiorn, and partly of siliceous or calcare-
ous 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.

The material of which the fleshy portion is composed is
of so tender and gelatinous a nature that the slightest pres-
sure is sufficient to tear it asunder, and allow the fluid parts
to escape; and the whole soon melts away into a thin oily
liquid. When examined with the microscope the soft flesh
is seen to contain a great number of minute grains, dissemi-
nated through a transparent jelly. Every part of the sur-
face of a living sponge (as may be seen in Fig. 53) presents
to the eye two kinds of orifices; the larger having a rounded

55

shape, and generally raised margins, which form projecting
papillse; the smaller being much more numerous, and ex-
ceedingly minute, and constituting what are termed the
pores of the sponge.

It has, for a long time, been the received opinion among
naturalists that this superficial layer of gelatinous substance
was endowed with a considerable power of contractility: it
was generally believed that it shrunk from the touch, and
that visible tremulous motions coulcl be excited in it by

SPONGES. 115

punctures with sharp Instruments, or other modes of irrita-
tion. It is extraordinary that errors like these should have
crept into the writings of modern zoologists of the highest
authority, such as Lamarck, Bruguiere, Gmelin, Bosc, and
Lamouroux.* The notion that the sponge contracts when
touched is of very ancient date, for it may even be traced
beyond the time of Aristotle; and it has been handed down
by succeeding naturalists, and echoed from the one to the
other, so as to have gained admission, without being ques-
tioned, in all the recent systematic works on Zoology.

The alleged spontaneous palpitation of the flesh, occur-
ring in particular parts, had its origin in the views taken of
the nature of sponges, by Marsigli, an Italian naturalist, who
in the year 1771, announced that he had seen movements of
dilatation and contraction in the round apertures visible on
the surface of sponges. This statement, so confidently ad-
vanced, 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 movem.ents
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 whicli our very perceptions may be influ-
enced by preconceived views, and by the force of the ima-
gination. Pallas immediately admitted, without examina-
tion, the hasty assertion of Ellis, into his '•' Elenchiis
ZoophytoruTYif^ whence it was copied by succeeding au-
thors, and the error became at length so widely dissemi-
nated, that for more than half a century it was received as an
established fact in natural history. The elaborate and accu-
rate 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,

• This mistaken view was adopted by Cuvier in the first edition of his
"Regne Animal," T. iv. p. 88; but Dr. Grant's rectification of the error
is noticed in the second edition of that work. — T, iii. p. 322.

t Vol. Iv. p. 284.

116 THE MECHANICAL FUNCTIONS.

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

Dr. Grant has also shown the true nature of the currents
of fluid issuing at difTerent 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, lace-
rating, burning, or otherwise injuring its texture, or by the
application of corrosive chemical agents. Of his discovery
of the fluid currents, he gives the following interesting ac-
count: " 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 hurling along, in rapid succession, opaque masses, which
it strewed every where around. The beauty and novelty of
such a scene in the animal kingdom, long arrested my atten-
tion, 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 course.
I continued to watch the same orifice, at short intervals, for
five hours, sometimes observing it for a quarter of an hour
at a time, but still the stream rolled on w^ith a constant and
equal velocity." About the end of this time, however, the
current became languid, and, in the course of another hour,
it ceased entirely. Similar currents were afterwards ob-
served by Dr. Grant in a great variety of species. They
take place only from those parts that are under water, and

• See his papers on this subject in the Edinburgh Philosophical Journal,
vol. xiii. p. 95 and 333, fi"om which most of the facts mentioned in the above
account are taken.

SPONGES. 117

immediately cease when the same parts are uncovered, or
when the anima^^dies.

It thus appears that the round apertures in the surface of
a living sponge are destined for the discharge of a constant
stream of water from the interior of the body; carrying
away particles, which separate from the sides of the canals,
and which are not only seen, under the microscope, con-
stantly issuing from these orifices, but may even be per-
ceived by the naked eye, propelled occasionally in larger
masses.*

For the supply of these constant streams, it is evident that
a large quantity of water must be continually i-eceived inta
the body of the sponge. It is by the myriads of minute
pores, which exist in every part of the surface, that this
water enters, conveying with it the materials necessary for
the subsistence of the animal. These pores conduct the fluid
into the interior, where, after percolating through the nu-
merous channels of communication which pervade the sub-
stance of the body, it is collected into wider passages, ter-
minating in the fecal orifices above described, and is finally
discharged. The mechanism by which these currents are-
produced is involved in much obscurity. There can be no»
doubt that they are occasioned by some internal movements;
and the analogy of other zoophytes would lead us to ascribe
them to the action of fibrils, or cilia, as they are termed, pro-
jecting 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 structure, and presents as syste-
matic an arrangement of parts. In some species, such as the
common sponge, the basis is horny and elastic, and composed
of cylindric tubes, which open into each other, and thus form
continuous canals throughout the whole mass.

• The currents issuing from the larger orifices are best seen by placing- the
living animal in a shallow vessel of sea-water, and strewing a little powdered
chalk on the surface, the motions of which will render the currents very
sensible to the eye. Fig. 53 exhibits these phenomena.

118 THE MECHANICAL FUNCTIONS.

Others have a kind of skeleton, composed of a tissue of
needle-shaped crystals of carbonate of li^e, or of silex.
These hard and sharp-pointed fibres, or spicula, are disposed
around the internal canals of the sponge, in the order best
calculated to defend them from compression, and from the
entrance of foreign bodies. Some of these spicula are deli-
neated in Fig. 54: but their forms, although constant in each
species, admit of considerable diversity in the difierent kinds
of sponge.

Although sponges, in common with the greater number
of zoophytes, are permanently attached to rocks, and other
solid bodies in the ocean, and are consequently destined to
an existence as completely stationary as that of plants, yet
such is not the condition of the earlier, and more transitory
stages of their development. Nature, ever solicitous to pro-
vide for the multiplication of each race of beings, and for
. their dissemination over the habitable globe, has always pro-
^ vided effectual means for the accomplishment of these im-
portant 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 sur-
rounded in the egg, and there remain until the period when,
by the acquisition or 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 or-
der 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 the active powers of locomotion 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 un-
alterably for the remaining term of its existence."^

• Phenomena, which appear to bear some analogy with these, have been

SPONGES. 119

The parts of the Spongia panicea, which are naturally
transparent, contain at certain seasons a multitude of opaque
yellow spots, visible to the naked eye, and which, when ex-
^ amined by means of a microscope, are found to consist of
groups of ova, or more properly gemmules,^ since we can-
not 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 ra-
pidly 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 progression through
the fluid they are observed always to carry their rounded
broad extremity forwards. On examining this part with the
microscope, we find that it is covered with short fila-
ments, or cilia, which are in constant and rapid vibration.
These cilia are spread over about two-thirds of the surface
of the body, leaving the narrower portion, which has a whiter
and more pellucid appearance, uncovered. They are very
minute transparent filaments, broadest at their base, and ta-
pering to invisible points at their extremities: they strike

noticed in the veg-etable kingdom. The tribe of Zoocarpia, produce a kind
of fruit, which, when detached from the parent, appears to possess powers
of spontaneous motion, until the period of its taking root, and growing like
a vegetable structure. These singular productions, which seem, in their
progressive developments, to possess alternately the characters of vegetables
and of animals, may perhaps be regarded as connecting links between the
two great kingdoms of living nature.

f Gemmule is a term derived from the Latin word geinma, a bud: and its
meaning, as apphed to zoophytes, is that of a young animal, not contained
within an envelope, or egg.

120 THE MECHANICAL FUNCTIONS.

the water by a rapid succession of inflections, apparently
made without any regular order, but conspiring to give an
impulse in a particular direction. When the body is at-
tached 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 to-
wards the tail; but when they are floating in the water, the
same action propels them forwards in the opposite direction,
that is, with the broad ciliated extremity 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 on them; for on striking
against each other, or meeting any obstacle, they retard a
little the motion of their cilia, wheel for a few seconds
round the spot, and then, renewing the vibrations, proceed
in their former course.

In about two or three days after these gemmules have quit-
ted the body of the parent, they are observed to fix themselves
on the sides or bottom of the vessel in which they are con-
tained; and some of them are found spread out, like a thin cir-
cular 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. While this is going on, the cilia are still
kept in rapid motion on the upper part, scattering the opaque
particles which may happen to be in the fluid, to a 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 spiculae in-
terspersed 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 exceeding twenty
in number, now become much larger and more numerous;

SPONGES. 121

and some of them shoot into the thin homogeneous mnrcin.
It is a rcmarkahle circumstance that the spicula make their
appearance completely formed, as if by a sudden act of
crystallization, and never afterwards increase their dimen-
sions.

When two gemmules, in the course of their spreading on
the surface of a watch-glass, come into contact with each
other, their clear margins unite without the least interrup-
tion; they thicken and produce spicula: in a few days we
can detect no line of distinction between them, and they
continue to grow as one animal. The same thing happens,
according to the observation of Cavolini, to adult sponges,
which, on coming into mutual contact, grow together and
form an inseparable union. In this species of animal graft-
ing we again find an analogy between the constitution of
zoophytes and that of plants.

In the course of a few weeks, the spicula are assembled
in groups, similar to those of the parent sponge; assuming
circular arrangements, and presenting distinct openings at
the points they enclose. The young animal now rapidly
spreads and enlarges in every direction, 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 mi-
niature representation of its parent.

Thus has a power of spontaneous motion been given to
what may be regarded as the embryo condition of animals,
which are afterwards so remarkable for their inertness, and
for the privation of all active powers: and this has been con-
ferred evidently for the purpose of their being widely disse-
minated over the globe. Had not this apparatus of moving
cilia been provided to the gemmules of such species as
hang vertically from the roofs of caves, they would have
sunk to the bottom of the water and been crushed or buried
among the moving sand, instead of su])porting 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.
Vol. I. 16

122

THE MECHANICAL FUNCTIONS.

by the Gulf stream, from the shores of the east to corre-
sponding latitudes of the new world.

§ 3. Polypifera,

The next step in the organic series introduces us to the
extensive family of Polypifera. The transition from the
structure of the sponge to that of the polypus may be thus
conceived. Suppose the absorbing orifices of the former to
be enlarged, and their number to be at the same time re-
duced: and let these orifices be drawn out into tubes, and
provided with vibratory cilia; in addition to which, let
there be placed around their margin a circular row of larger
filaments, extremely flexible, and capable of twining round
any object that comes within their reach, and of conveying
it to the central orifice, which performs the office of a mouth.
Each tube, thus furnished with a circle of radiating fila-
ments, or tentacula, as they are called, is denominated a
Polype.^' The animal structure thus composed has received
the name of Lobularia (Fig. 5Q,) and is the genus among

• For the sake of greater distinctness I shall employ the term polype to
denote the sing-le tube with its tentacula; and shall designate by the Latin
term polypus the entire animal mass composed of an aggregation of these
polypes. rolypiferUy the name of the order, expresses animals bearing po-
lypes.

POLYPI. 123

this tribe that approaches the nearest in its character to the
sponge, which it resembles in the nature of its internal tex-
ture. Each of the polypes with which its surface is studded
has eight serrated tentacula. Fig. 57 represents one of these
polypes detached. Polypes may thus be united in immense
numbers in one mass, having mutual organic connexion. In
other cases they may form smaller clusters, or be even to-
tally unconnected. Sometimes the detached polypes are
still disposed to assemble in groups, as is the case with the
Zoanthus of Cuvier* (Fig. 5S:) at other times they are al-
together isolated, as in the Hydra viridis (Fig. 59.)

Polypi form a very extensive order of zoophytes, abound-
ing in every part of the ocean, but growing in greatest lux-
uriance in the warmer regions of the globe. Their flesh
exhibits the same granular appearance as that of the spon«rc,
but it is generally firmer, and often intermingled with masses
of calcareous matter. The tentacula, which may be com-
pared to arms, vary in number and in length in different
species of polypi, and sometimes, instead of a single row,
each of the mouths has two or more series of tentacula placed
around it. They are formed of a prolongation of the soft
substance of the polypus, and are sometimes tubular; and
their cavities are then continuous w^th that of the general
internal cavity into which the several mouths open. Be-
sides being flexible in every direction, the tentacula are also
capable of being lengthened or shortened at the pleasure of
the animal. Their elongation is produced by the propulsion
of a fluid into their interior, derived from the general cavity
of the body; and their retraction is effected by the return of
the same fluid.

The whole arrangement of the tentacula on the margin of
the projecting mouths bears a striking resemblance to a
flower, especially to those which, like the daisy, or china-
aster, have the corolla composed of slender radiating petals.
We find, indeed, that as the organs of zoophytes become
more developed, the alfmities which these lower departments

• The Hydra sociata of Gmclln; the Acl'mia sociata of Kills.

124 THE MECHANICAL FUNCTIONS.

of the animal kingdom retain with plants, are more marked
and more predominant. In the construction 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 gene-
rally exhibited in plants, and especially in flowers, and in
the verticillated leaves and branches.* Hence, the radiated
or star-like forms which predominate in most of the animals
cbmposing this class: and, hence, they have obtained the ti-
tle of Radiata, by which Cuvier has designated them.

Like the animals of the sponge tribe. Polypi are for the
most part attached to some inorganic shell or base, which
may be either of a horny or calcareous nature. The form
of this shell admits of almost infinite variety. In some it
constitutes the external surface of the animal, and encloses
the flesh in a general sheath, leaving only openings at the
extremities of the tubes for the expansion of each set of
tentacula surrounding the respective mouths. Sometimes
these 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 miisica, 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 inte-
rior. At other times the tubes are joined
together endwise, like the branches of a
tree, leaving lateral apertures for the pro-
trusion of the tentacula of each separate
polype: this is the case in the Seriularia.
(Fig. 60.)

In some species the horny base is fashioned into a num-
ber of cells, each of which serves for the protection of its
respective polype. These cells are generally placed at the

* See page 7Q.

POLYPI.

125

extremity of the branches, presenting the greatest similitucle
to flowers. The Flu^lra (Fig. 63) is composed of minute

61

62

63

64

and almost microscopic cells, spread over a flat membra-
neous substance, resembling, in the flexibility of its texture,
and its mode of subdivision, the leaves of plants. These
cells are arranged in rows, with great regularity, like those
of a honey-comb, as is seen in the magnified view of them,
Fig. 64.

In other tribes the inorganic base of support is internal,
constituting a kind of skeleton or axis; the polypous mouths
being spread at intervals over the surface of the fleshy lay-
er which covers this skeleton. This is the case with the
Gorgonia, jSntipathes, and the 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.
G5 is a branch of the Corallium rubum, of w^hich Fig. ^G

is a magnified portion, showing the appearance of the poly-
pes in their expanded and contracted states. The way in
which the polypes are embedded in the flesh is seen in Fig.
67, which represents a section of the Gorgonia Briareus,
In many cases the polypes arc lodged in cup-like depres-

126 THE MECHANICAL FUNCTIONS. *

sions in the surface of the calcareous axis, which affords them
some degree of protection. In Madrepores these depres-
sions are crossed by radiating plates, adapted to the form
and number of the tentacula. In Millej)ores the cells are
closer and more minute, and exhibit none of these star-like
radiations. In some species the plates have more of a pa-
rallel arrangement; and in others they form a net-work.

The material of which this axis, to which the polypes are
attached, is composed, is of various kinds. Sometimes it is
horny, flexible, and elastic, corresponding in its nature to
animal membrane: at other times it is hard and calcareous,
being composed principally of carbonate of lime, with a
small quantity of the phosphate; the proportion of this latter
ingredient varying in different species. In all cases the
particles of calcareous matter are united together by some
portion of animal substance which may be obtained by dis-
solving out the former by an acid. We always find the ma-
terials arranged in concentric layers, indicating that their
deposition has been successive; and the surface is marked
by longitudinal lines, corresponding to the figure of the ani-
mal covering of flesh. Sometimes the stem consists of horny
and calcareous parts disposed alternately, composing a jointed
structure, which some have fancied might be considered as
making an approach to an articulated skeleton; for it is ca-
pable of considerable flexion, and readily yields to the im-
pulse of the waves, without the risk of being broken. This
is the case with the his hipj)uris, commonly known by the
name oi jointed coral. (Fig. 68.) There is, in short, hardly
any possible combination of these parts which does not oc-
casionally occur amidst the infinite diversities of condition
displayed in this department of the animal creation.

These structures are generally attached to submarine rocks
by an expansion of the base into a kind of foot, or root,
which has a strong power of adhesion. In this respect,
therefore, as in so many others, these animals preserve an
analogy with plants.

It has been ascertained that, in a great number of instances,
these fixed zoophytes are multiplied, like the sponge, by the

POLYPI. 127

detachment of gemmulcs, or imperfectly formed portions of
their soft substance. These gcmmules require to undergo
the same kind of metamorphosis in order to bring them to
their perfect state; and when newly detached from the pa-
rent, they exhibit the same singular spontaneous motions,
buoying themselves in the water, and swimming in various
directions, by the rapid vibrations of their cHla, till they
find a place favourable to their growth. On becoming fixed,
they spread out to form a base for the future superstructure;
and, after the foundation has thus been laid, they proceed in
their upward growth, depositing a calcareous or horny axis
in successive layers, until it has acquired the requisite thick-
ness; and they then gradually assume the forms character-
istic of the particular species to which they belong. The
materials thus deposited are permanent structures, not ca-
pable of modification or removal, and not possessing any
vital properties; for these properties belong exclusively to
the animated flesh with which these structures are associated.
The polypes themselves are not developed till after the for-
mation of the root and stem; their growth being in this re-
spect analogous to that of the leaves and flowers of a plant.
The gemmules of the Flustra carbasea may be selected
in illustration of this phenomena. These have been ob-
served by Dr. Grant,* to swim about in the water as soon as
they have escaped from the cells of the parent; each moving
with its narrow end foremost, while the opposite broad
end, which is covered with cilia, expands into a flat circular
zone. These gemmules are very irritable, and are frequent-
ly seen to contract the circular margin of their broad ex-
tremity; and, while swimming, to stop suddenly in their
course. They swim with a gentle gliding motion: at other
times they appear stationary, all the while revolving rapidly
round their longer axis, with their broad end uppermost: they
often bound forwards, either in straight lines, or describing
circles, with no other apparent object than to keep them-
selves afloat, until they shall arrive at a favourable spot for

* Edinburgh Phllosoplncal Journal, XYII. 107 and 'oo7.

128 THE MECHANICAL FUNCTIONS.

fixing their permanent abode, and proceeding in their far-
ther development. The tune of their remaining in this free
and moving state varies according to circumstances, 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 gemmules has be-
gun to fix, it be again disturbed, and separated from the
surface to which it had become attached, it generally re-
mains 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 cleaning the space immediate-
ly surrounding the gemmule. It remains for three days in
this attitude, without undergoing any perceptible change
of form, and without relaxing the vibrations of its cilia. At
the end of this time, the cilia cease to move, and shortly af-
ter disappear: then the gemmule begins to swell, the sur-
rounding margin becomes more transparent, and the whole
gradually assumes the form of a cell, surrounded by a deli-
cate white opaque line, which is the rudiment of the calca-
reous wall of the future cell. Towards the base of this ru-
dimental cell, the gelatinous substance in the interior may
be perceived to become more consistent and opaque at a
particular point; from this dull spot within the cell, short
straight tentacula begin to bud, extending upwards in the di-
rection of the future aperture. The gelatinous spot, from
which the tentacula originated, assumes the vermiform ap-
pearance of the body of a polype; and we may distinctly
perceive the bundles of fibres which connect its head with
the base of the cell. The structure of the polype is per-
fected 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 tenta-
cula vvith as much regularity and velocity as at any future
period. Before the polype is capable of protruding from

POLYPI.

129

the aperture of the first cell, the upper part of the cell has
already extended outwards to form the rudiment of a second:
and so on, in succession, till the whole structure is completed.
The tentacula of polypi are exquisitely sensible, and are
frequently seen, either singly or altogether, bending their
extremities towards the mouth, when any minute floating
body comes in contact with them. When a polype is ex-
panded, a constant current of water is observed to take
place, directed towards the mouth. These currents are
never produced by the motions of the tentacula themselves;
but are invariably the effects of the rapid vibrations of the

cilia placed on the tentacula. In the
polypes of the Flustra carbasea,
(Fig. 69,) the tentacula have each a
single row of cilia, extending along
both the lateral margins, from their
base to their termination.* Each
polype has usually twenty-two ten-
tacula; and there are about fifty cilia
on each side of a tentaculum, makins:
2200 cilia on each polype. As there
are above 1800 cells in each square
inch of surface, and the branches of an ordinary specimen
present about ten square inches of surface, we may estimate
that an ordinary specimen of this zoophyte presents more
than 18,000 polypes, 396,000 tentacula, and 39,600,000 ci-
lia. But other species certainly contain more than ten times
these numbers.t

The vibrations of these cilia are far too rapid to be fol-
lowed by the quickest eye, even when assisted by the most
powerful microscope, and can be detected only at the times
when they have become comparatively languid, by the di-
minished vigour of the animal: their motions may then be

• 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.

f Dr. Grant has calculated that there are about 400,000,000 cilia on a sin-
gle Flustra foliacea. Transactions of the Zoological Society of London,
Vol. i. p. 11.

Vol. I. 17

130 THE MECHANICAL FUNCTIONS.

seen, ascending on one side of the tentaculum and descend-
ing on the other. (Fig. 70.) All the cilia appear to com-
mence and to cease their motions at the same moment. The
constancy with which they continue would seem to exclude
the possibility of their being the result of volition; and they
are, therefore, more probably determined by some unknown
ph)'sical cause, dependent, however, on the life of the ani-
mal. But so retentive are they of the power of motion-,
whatever may be its cause, that if any one of the tentacula
be cut off, its cilia will continue to vibrate, and will propel
it forward in the fluid for a considerable time, as if it had
become itself an individual animal.

A question arises with regard to the constitution of these
zoophytes, similar to that which has been proposed with re-
gard to trees, namely, what limits should be assigned to
their individuality? Is the whole mass, which appears ta
grow from one root, and which consists of multitudes of
branches, proceeding from a common stem, to be considered
as one individual animal, or is it an assemblage or aggrega-
tion of smaller individuals: each individual being characte-
rized by having a single mouth, with its accom.panying ten-
tacula, and yet the whole being animated by a common
principle of life and growth? The greater number of natu-
ralists 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 contributing its quota to the ge-
neral nourishment of this animal republic. As the deter-
mination of this question involves tlie consideration of the
function of nutrition, I shall postpone its farther discussion
to a future part of this treatise. As far, indeed, as regards
the mechanical condition of animals which are so complete-
ly stationary, it matters little, whether the whole mass be
regarded as one individual animal, or as an aggregate of dis-
tinct individuals. But the question becomes of some impor-
tance when applied to detached zoophytes, such as Fenna-
iiila, which are formed of a multitude of polypes connected
with a common stem, but w^hich float at liberty in the sea.

PENNATULA.

131

The Pennatula, (Fig. 71,) has been termed the sea pen,

from tlie circumstance of its calcare-
ous 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
feather. 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 natura-
lists have ascribed to them, we should be able to ascertain
wdiether 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 pen-
natulse swim through the water by their own spontaneous
movements, consisting either in the waving up and down of
the lateral branches, or in the simultaneous 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, who has watched the motions of these ani-
mals with great care, is led by his observations to the con-
clusion that pennatulae are not in reality possessed of an}'-
such locomotive faculty; but that they are carried to and fro
in the ocean, like the gulf weed, without the slightest vo-
luntary power of directing their course. Whatever may be
the result of the combined movements of the tentacula, the
arms are certainly incapable of those inflections which have
been supposed to supply the means of progressiv^e motion.

It is only when the contractile flesh of the polypus is re-
leased from the restraint which the solid axis imposes upon
its movements, that the animal becomes capable of any dis-
tinct power of locomotion. Such is the condition of the

132 THE MECHANICAL FUNCTIONS.

animals belonging to the genus Hydra, of which the Hydra
viridis, or fresh water polype, (Fig. 59, p. 122) may be taken
as the type. This singular animal presents us with perhaps
the simplest kind of structure that exists in the animal king-
dom. 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 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 considerably
elongated, and on the other, so much retracted, as to appear
a mere globule; and these movements are the efiect of a vo-
luntary power in the animal directed to specific ends. The
number of tentacula varies from six to twelve; they are slen-
der tubular filaments, capable of being extended to a great
length, and of being bent in all directions. In this way,
they can quickly surround and grasp any small object which
they may happen to touch; and whenever irritated they in-
stantly retract, so as hardly to be visible without the aid of
a magnifier. Each tentaculum may be moved independent-
ly of the rest, at the pleasure of the animal. The remainder
of the body tapers gradually from the head to the other ex-
tremity, becoming very slender, and having at its termina-
tion a flat surface, which has been termed the foot; for al-
though every portion of the surface has the power of ad-
hering 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 can 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

HYDRA. 1 33

which has contributed to throw great light on the natural
history of polypiferous animals.^ While observing some
a(juatic plants, which he had collected and put into water^
his attention was called to the aj)pearance of filaments ad-
hering to them, wiiich he had first conceived to be parasitic
vegetables: but farther observation convinced 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 al-
ways placed themselves on the side of the glass next to the
light; and by watching their changes of position, he disco-
vered the mode in which they effect their progressive mo-
tions. 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.)

75

76

The progress made by these successive efforts is but slow:
for the hydra often pauses in the midst of a step, as if de-
liberating whether it should proceed: so that the traversing
a distance of seven or eight inches is to these animals a very
good day's journey, even in summer. But a mode of tra-
velling rather more expeditious than this is occasionally re-
sorted to. It consists of a succession of somersets: the hy-

* Memolres pour servir a rHistoIro d'lin genre de Polypes d'eau douce,
a bras en forme dc cornes. Par A. Tremblev, 1744.

134 THE MECHANICAL FUNCTIONS.

dra, while adhering firmly by the mouth, detaches its foot,
and, making it describe a semicircle, 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 conti-
nuing all the while elongated.

By these and other manoeuvres these animals contrive to
walk with equal facility in any direction, either on the bot-
tom or sides of the vessel, or along the stems of aquatic
plants, to which they are most frequently found attached.
The position in which they appear to take most delight, is
that of remaining suspended from the surface of the water
by means of the foot alone: and this they effect in the fol-
io win o- manner. When the flat surface of the foot is ex-
posed 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 un-
covered, and occupies the bottom of a cup-shaped hollow
in the fluid, thereby receiving a degree of buoyancy suffi-
cient to suspend it at the surface. The principle is the same
as that by which a dry needle is supported on water in the
boat-like hollow which is formed by the cohesive force of
the liquid, if care be taken to lay the needle down very gen-
tly 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 hy-
drostatic power will be destroyed, and the animal will im-
mediately 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 an-
chors, to enable them to retain their positions in security,
however violently the water may be agitated. Great use is

HYDRA. 135

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,
vs^hen 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 organization, enabled to per-
form these actions? To this question, however, no satisfac-
tory answer has yet been given. The substance of the hy-
dra, when examined by the microscope, appears to be nearly
homogeneous, except that a number of grains are intermixed
with the pulpy and gelatinous matter composing the princi-
pal bulk of the body. These grains, when pressed out of
the flesh into water, are scattered indiscriminately; and ap-
pear to have been united in the living animal, by means of
this glutinous material.

No perceptible fibres, either muscular, or of any other
kind, can be detected in the flesh of the polypus: nor is
there the least indication of the formation of transverse
rings, similar to those which exist in worms, and which, in
these latter animals, contribute to progressive motion. Every
portion of the substance of the body is equally irritable and
contractile, and its movements appear to be governed by
some voluntary power belonging to the animal, and direct-
ed to the attainment of certain ends. The softness and pli-
ancy which it possesses allow of its being closely fitted to
all the inequalities of the surface of the bodies to which it
is applied; and perhaps this cause alone occasions it to ad-
here with great force to these bodies, without the aid of any
glutinous fluid. A conjecture, which has much appearance
of probability, has been ofi'ered, that this power of adhesion
is derived from the presence of a great numi)cr of exceed-
ingly minute disks, interlj^ersed over every part of the sur-
face, constituting so many suckers, and resembling, though
on a very diminutive scale, the sucking apparatus on the
arms of the cuttle-fish.

136 THE MECHANICAL FUNCTIONS.

The Zoantlius (Fig. 58) belongs to a tribe of larger poly-
pi, which are generally met with assembled in clusters; on
which account it is termed by Ellis the Actinia sociata, or
clustering animal flower. It consists of a globular body,
leaving a mouth surrounded by one or two rows of tentacu-
ja; and connected below with a firm and fleshy tube, which
adheres strongly to the rocks at the bottom of the sea, so
that it remains permanently fixed in the same place.

The genus Vorticella is formed by a small tribe of ani-
mals, which, although they have been usually included un-
der the present order, differ from polypi in having no tenta-
cula, but only cilia, surrounding the margin of a bell-shaped
body, which is mounted upon a long and slender foot-stalk
(Fig. 77.*) Currents are, as usual, excited by the vibra-
tions of the cilia; which in the simpler species, such as the

Vorticella cyathina, here delineated,
are the efficient instruments of progres-
sive motion. When attached by its foot-
stalk, 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 throwing it
into spiral folds. Many of the species of
vorticellae are so exceedingly diminutive
as to be imperceptible without the aid of
the microscope. They conduct us, therefore, by a natural
gradation, to the next order we have to notice, and which
is composed wholly of microscopic animals.

§ 4. Infusorna,

The Infusory animalcules, or Infusoria, were so named
by Muller, a Danish naturalist, from the circumstance of
their swarming in all infusions of vegetable or animal sub-
stances that 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

* They also dlffer'from polypi in having a distinct intestinal canal, with
numerous stomachs.

INFUSORIA. 137

knowledge of tlieir existence, and of the curious plicnomcna
they present: yet even the best instruments afford us but
little insight into their real organization and physical condi-
tions. On this account it is extremely diflicult to assi^rQ
their true place in the scale of animals. By most systema-
tic writers they have been regarded as occupying the very
lowest rank in the series, and as exemplifying the simplest
of all possible conditions to which animal life can ])e re-
duced. Monads, which are the smallest of visible animal-
cules have been spoken of as constituting *Uhe ultimate
term of animality;" and some writers have even expressed
doubts whether they really belong to the animal kingdom,
and whether they should not rather be considered as the ele-
mentary molecules of organic beings, separated from each
other by the effects of chemical decomposition, and retain-
ing the power of spontaneous, but irregular and indetermi-
nate 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 se-
cond, endowed with a principle of organic aptitude, or ca-
pability of uniting into living masses, and constituting, there-
fore, the essential elements of all organization. According
■ to this view, all vegetables or animals in existence would be
mere aggregations of infusory animalcules, which gradually
accumulate by continual additions to their numbers, de-
rived from organic matter in the food: so that the body of
man himself would be nothing more than a vast consreca-
tion of monads!

This bold and fanciful hypothesis, devised by Buffon,
and recommended by its seductive appearance of simplicity,
as well as by the glowing style and brilliant imagination of
its author, has had many zealous partisans. The new world,
which was disclosed to the wondering eyes of naturalists by
the microscope, abounding in objects and in phenomena of
which no 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 arrangement of the

Vol. I. IS . ■

138 THE MECHANICAL FUNCTIONS.

atoms of organic bodies. During the greater part of the last
century, infusory animalcules were the subject of frequent
and laborious microscopical research, and gave rise to end-
less conjecture and speculation as to their origin, their vi-
tality, and their functions in the economy of nature. Not-
withstanding their minuteness, considerable differences of
organization were perceived to exist among them: but many
naturalists still clung to the idea that monads, the most di-
minutive of the tribe, and whose very presence can be de-
tected only by the application of the highest magnifying
powers, are homogeneous globules of living matter, without
organization, but endowed with the single attribute of vo-
luntary motion: and even this property was denied to them
by some authors.

AH these fanciful dreams have been dispelled by the im-
portant discoveries of Ehrenberg, who has recently found
that even the Monas terrno is possessed of internal cavities
for the reception and the digestion of its food; and who has
rendered it probable that their organization is equally com-
plex with that of the larger species of infusoria, such as the
Rotifera, in which he has succeeded in distinguishing traces
of a muscular, a nervous, and even a vascular system.

Those animalcules, whose form can be at all distinguished,
exhibit a great diversity of shapes, and variety of modes of
progressive motion. Many, as the Cyclidium, have the ap-
pearance of a thin 6val pellicle, smoothly gliding in all di-
rections through the fluid: some, as the Volvox, are globular;
others, as the Cercaria, are shaped like a pear, tapering at
one end, and often terminating in a slender tail, so as to re-
semble a tadpole. In many, this tail is of great length; in
some, as the Furcocerca, it is forked; in others, it takes spi-
ral turns, like a corkscrew. The Kerona has processes like
horns. The shape of the Vibrio is cylindrical, and more
or less pointed at one or both ends, like an eel, or a serpent,
which animals it also resembles in its undulatory mode of
swimming.* Some, as the Gonium, have an angular, others,

* Animalcules refen-ible to this genus are met with in great numbers in
blighted wheat, (Fig. 2, p. 58,) in sour paste, and in vinegar which has lost

INFUSORIA.

139

as the Kolpoda, a waving outline. Some, as the Urceolaria
present the likeness of a boll or funnel, and appear to be
analogous to the Vorticella, in which genus they should pro-
bably be included.

Forms still more irregular are exhibited by other infuso-
ria. Of these the most singular is the Proteus (Fig. 78,)
which cannot, indeed, be said to have any determinate shape,

78

79

for it seldom remains the same for two minutes together. It
looks like a mass of soft jelly, highly irritable and contrac-
tile in every part; at one time wholly shrunk into a ball, at
another stretched out into a lengthened riband; and again,
at another moment, perhaps, we find it doubled upon itself
like a leech. If we watch its motions for any time, we see
some parts shooting out, as if suddenly inflated, and branch-
ing forth into star-like radiations, or assuming various gro-
tesque shapes, while other parts will, in like manner, be as
quickly contracted. Thus the whole figure may, in an in-
stant, be completely changed, by metamorphoses as rapid as
they are irregular and capricious.

The Volvox globator, (Fig. 79) is found in prodigious
numbers at the surface of many stagnant pools. Its figure
is perfectly spherical ; and its movements consist in a conti-
nual and rapid rotation, round its axis, frequently remaining
all the while in the same spot. Another species, the Volvox
conjlictor, moves by turning alternately to the right and to
the left.

The progressive movements of infusory animalcules are of
two kinds, the one consisting in a smooth and equable gliding

the whole of its alcohol. In this last fluid they sometimes attain so large a
size as to be visible to the naked eye.

140 THE MECHANICAL FUNCTIONS.

through the fluid, produced apparently by the vibrations of
cilia, which are set on various parts of the body, and often
seem to cover the whole surface: the other, more rapid and
energetic, when the animalcule darts forward in a particu-
lar direction, as if in pursuit of prey, and proceeds by sudden
and irregular starts, like a vivacious insect or fish. The vo-
luntary nature of their motions is evident from, the dexterity
they display in avoiding obstacles, while swimming together
in myriads in a single drop.

The great agent in the movements of the animal frame
being the muscular fibre, it was 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 recognised. 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 Rotifera, has, with a magnifying power of
380, distinctly seen muscular bands running in pairs between
the two layers of transparent membrane which envelope the
body. When the animalcule throws itself into its violent
lateral contortions, these fibrous bands are observed to be-
come broader and thicker, as well as shorter, on the side
towards w^hich the contractions take place. There can,
therefore, be no doubt that these are muscular organs, and
that they are the real agents by which the motions wit-
nessed are effected.

These Rotifera, or wheel animalcules, ar-e so named from
{liu^j^ R their being provided with an apparatus
for creating a perpetual eddy, or circu-
lar current in the surrounding fluid.
The remarkable organs, by which this
effect is produced, are generally two in
number, (Fig. 80, r, r,) and are situ-
ated 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

WHEEL ANIMALCULES. Ill

which are fringed with rows of cilia, bearing a great resem-
blance to a crown wheel. This wheel appears to be inces-
santly revolving, and generally in one constant direction;
giving to the fluid a rotary impulse, which carries it round
in a continual vortex. The constancy of this motion would
seem to indicate that it is related to some function of vital
importance, such as respiration. But even considered as
a mechanical action, which is the view we have now to take
of it, this phenomenon is of a nature to excite much curiosi-
ty; for the continued revolution round an axis of any part
or appendage to the body, is quite inconsistent with any no-
tion we can form of the solid organic attachment 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. M. Dutrochet has offered an ingenious solution
of this difficulty. He suggests that the revolution of the
wheels of the Rotifera may not be real, but apparent only.*
The indented margin of each wheel being composed of a
material so exceedingly flexible as to be capable of assuming
quickly all kinds of curvatures, may* be conceived to be
thrown into undulations, which follow one another round
the circumference; each part, in succession, becoming alter-
nately convex and concave, and thus producing the appear-
ance of the actual advance of the portions that are raised;
while their real motions are only those of elevation and de-
pression, by the elongation and contraction of their perpen-
dicular fibres.

Besides possessing extensive powers of locomotion, 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 polypi, doomed by
nature to be permanently fixed, like plants, to the same spot;
and of which, if we consider them as compound beings, the

• The same opinion was advanced long ago by Vicq. d'Azyr.

142

THE MECHANICAL FUNCTIONS.

individual animals are often so minute as to be scarcely vi-
sible without the aid of the microscope. Mere size, indeed,
is of all the circumstances attendant on organized beings, that
which should least be assumed as the criterion of complica-
tion or refinement of structure. An object is great or small,
only in relation to the standard of our own limited and
imperfect senses; but with reference to the operations of
creative power, all such distinctions must vanish. There
is not, as far as we have the means of judging, in the colossal
fabric of the elephant, any structure more complicated than
exists in the minutest insect that crawls unheeded at our
feet.

§ 5. *dcalepha.

Floating masses of living gelatinous matter are met with
in every part of the ocean; often in vast numbers and of va-
rious forms; and having but little the appearance of belong-
ing to the animal kingdom. They compose the order *^ca-

lepha, 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 loco-
motive powers, they rank
among the lowest of those Zoo-
phytes which are not perma-
nently fixed to the spot where
they were first developed.
They are almost wholly passive
beings, floating on the surface
of the sea, or remaining at a small depth below it, carried
to and fro by the motion of every tide and current, and
destined to be the unresisting prey of innumerable tribes
of animals which people every part of the ocean.

The usual form of a Medusa is that of a hemisphere, with

MEDUSA. 143

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 laminae, or processes, which have sometimes been
denominated tentacula.

The whole substance of the body of these medusai is se-
mi-transparent and gelatinous, without any distinct fibrous
structure; yet it has considerable elasticity, and possesses also
some degree of contractile power. The animal is seen al-
ternately to raise and depress the margin of its hemispheri-
cal body, and to flap with the fringed membrane or mantle,
which descends from it, in a manner somewhat similar to
the opening and shutting of a parasol. This pulsatory move-
ment is performed about fifteen times in every minute, with
great regularity: and by the reaction of the water, the ani-
mal is sustained at the surface; or by striking the water ob-
liquely, it may even perform a slow lateral movement. They
descend in the water by simply contracting their dimensions
in every direction. Sometimes, in order to sink more quick-
ly, they turn themselves over, so that their convex part is
undermost.

Medusae are met with of very various ^izes; 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
tinge to the colour of the waves for hundreds of miles. The
total number of these animals dispersed over that space sur-
passes the utmost stretch of the imagination. In these si-
tuations a cubic foot of water, taken indiscriminately, was
found by Mr. Scoresby to contain above 100,000 of these
diminutive medusae.

Belonging to the tribe of Medusaria is a singular genus,

• The luminous property of sea water, or its phosphorescence, as it is some-
times called, generally arises from the presence of minute medusec, which
are met with in greatest numbers at the surfiice, being spccifially lighter than
the surrounding fluid.

144

THE MECHANICAL FUNCTIONS.

denominated the Beroe, (Fig. 82 and 83,) which is remark-
able for its organs of progressive motion. Its body is either
globular, or oblong, and it swims with its aicis 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 horizontally, and support-
ed on radiating fibres; so that they bear a pretty exact re-
semblance 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 pro-
vided with two very long and slender processes, which come
out from the sides of the body, and from these a great num-
ber of still finer filaments, or cilia, proceed: the whole ap-
paratus 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 tenta-
cula, or delicate organs, both of touch and of prehension.* It
was observed by 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 vibrato-
ry motions.

In two other genera of Acalepha, the Porpita and the
Velella, provision is made for the mechanical support of the

* See a description of the Beroe pikusy Lam. by Dr. Grant, in the Trans-
actions of the Zoological Society of London, vol. i. p. 9.

VELELLA. 145

soft gelatinous mass, by means of an internal cartilage. In
the former, this cartilage 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 pur-
pose for which it was given to the animal ; enabling it to steer
its course by means of the loose edges, and also of the tenta-
cula, which extend from the lower side of the body, and act
as a rudder, wdiile the sail is impelled by the wind.

A construction still more artificial is provided in another
family of the same order, denominated the P/iysalida, or
Hydrostatic Acalepha. They have attained this latter ao-
pellation from their being' rendered buoyant by means of ve-
sicles filled with air, which enable them to float without the
necessity of using any exertion for that purpose. The Phy-
salia, or Portuguese Man-of-War, as it is called, (Fig. S5,)
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 thl?y float on the surface of the sea, rearing their
purple crests, and. appearing at first like large air bubbles,
but distinguishable by the vivid hues of the tentacula which
hang down beneath them. Nothing can exceed the beauty
of the spectacle presented by a numerous fleet of these ani-
mals, quietly sailing in the tropical seas. Whenever the
surface is ruffled by the slightest wind, they suddenly absorb
the air from their vesicles, and becoming tlius 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 Fhys-

VoL. I. 19

146

THE MECHANICAL FUNCTIONS.

sophora, have several of these air-bladders; but in other re-
spects resemble the ordinary Medusae, in having no mem-
branous crest.

The J^ctinix are a tribe of Zoophytes, v^hich, from the
general resemblance of their forms to those of Polypi, are
b}^ most naturalists included under that order. But they
exhibit a much greater development in their organization;
having very distinct muscular fibres, endowed with strong
powers of contraction. Their digestive organs, also, as I
shall have afterwards occasion more fully to notice, are con-
structed upon a more complicated plan than in 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
compound flowers; and accordingly the particular species
are named from these resemblances, the sea-anemone^ the
sea-mar y gold, the sea-carnation, the sun-flower, daisy,
Sic. Actiniae are seen in great numbers on many shores,
adhering by their flat surfaces to rocks, and beinjj generally
permanently fixed to their abode. When the weather is
fine, and the sea calm, it is very amusing to watch the rapid
expansions and retractions of their many coloured tentacula,
while they are moving in search of food: to 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 par-
ticular 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 minutely examined the

ECHINODERMATA.

147

arrangements of their muscular fibres, and has described the
action by which they either attach themselves to the sur-
faces of rocks, or effect their sluggish movements.*

§ G. Ecldnodermata.

Ascending in the scale of organization \vc come to the
Echinoderinata, a class which comprehends the families of
the Jislerida^ the Echinida, the Holothitrida, and the Cri-
noidea, together with other tribes of less note.

These animals, both in their general form, and in the ar-
rangement of their internal organs, retain, in a very marked
manner, the radiated disposition so characteristic of Zoo-
phytes: 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. SS to 94.)
Besides an external horny, or semi-calcareous covering,
there is also provided, for the support of the softer parts, a
kind of internal skeleton, or jointed frame-work. The or-
gans in the interior of the body are farther supported by

* M^molres de TAcacIemie des Sciences, 1710, p. 490.

148

THE MECHANICAL FUNCTIONS.

membranous walls, which impart mechanical firmness to the
fabric.

The Jisterias, or star-fish (Fig. SS,) is so named from its
star-like form; and the number of rays composing the star
is generally five. Besides the tough coriaceous integument,
which protects the mass of the body, each ray is farther sup-
ported by a series of calcareous pieces, resembling those
which compose the spinal column of vertebrated animals,and
forming an articulated axis, constructed with the evident de-
sign of combining the greatest strength with a proper degree
of flexibility. Cartilaginous plates are also added for the more
special support of the integument. This integument itself
is irritable, and has the power of changing its form, although
the muscular fibres by w^hich its motions are effected are not
easily distinguished. Calcareous grains, of a solid consist-
ence, are thickly interspersed throughout its texture; and
these, in various parts of the body, both in the upper and
the under side, often project from the surface in the form of
spines or prickles. They are particularly large around the
mouth of the animal, which opens at the centre of the under
side. These calcareous masses have a crystalline arrange-
ment, and exhibit in 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, termed,

95

9S

by Linneus, the ambulactnim, or avenue, a name which it
has received from its fancied resemblance to a walk between
rows of trees; for each groove contains a quadruple row of
perforations, like pin holes, through which small fleshy
cylindrical processes pass. These processes extend but a
short distance from the surface; but they admit of being

ECHINUS. 149

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 mo-
tion; so that these processes may be regarded as correspond-
ing to feet, being levers for the advance of the body. This,
it may be remarked, is the first time that we meet with or-
gans of that description in our progress through the animal
kingdom. Each of these feet is terminated by a concave
disk, which when applied to any flat surface acts as a sucker,
on the principles already adverted to.'' Reaumur counted 304
of these feet in each of the five rays of the star fish, making
1520 in all.t Each foot consists of a tube, closed at the
outer end, and the stem of which, after passing through the
aperture in the integument, is dilated into a bag or reservoir
of fluid; as is shown in Fig. 97. ^ By the contraction of this
reservoir, the fluid it contains is propelled into the outer
portion of the tube, which protrudes by being thus distend-
ed; 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, notwithstanding their great

• Page 105.

\ !Memoires de rAcadcmIc des Sciences, 1710, p. 487.

\ The mechanism by which the feet arc protruded and retracted is illus-
trated by the diagram. Fig". 97, which exhibits the bladders connected with
them, in different states of distention and contraction. Fig 96 shows the
upper side of the umbulacra, and of the bladders comiected with the feet.
Dr. Grant, from some observations which he made on the structure of the cilia
of the Beroe pileus, is led to suspect that the rapid vibrations of these sin-
gular organs in the lowest animals may depend on the undulations of water
conveyed through elastic tubes along their bases, in a manner resembling
the injection of the tubular tentacula of Actinix and Astcriae. If this con-
jecture were verified, he remarks, one of the most remarkable phenomena
of animal motion, though one of the most frequent, would lose much of its
present marvellous character.

150

THE MECHANICAL FUNCTIONS.

number, the advance which this animal can make in any
particular direction is excessively slow.

Besides this movement of creeping, the Asterias is capa-
ble of bending and unbending each of its rays: actions, how-
ever, which it can perform but very slowly, and not to an
extent sufficient to accomplish its removal from one place to
another.*

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 dove-tailed
into each other. The
form of each piece is that
of a lengthened hexagon;
and the whole are regular-
ly arranged in rows, like
a mosaic or tesselated pavement. Ambulacra are also seen
on the surface of the shell, passing vertically down the sides
of the sphere, similar to the meridians of a globe; and con-
taining, like those of the Asterias, a double row of perfora-
tions.t

On the outer spherical surface of the external crust, there
are formed a great number of calcareous tubercles, arranged
with beautiful regularity and symmetry in double lines,
passing, like meridian circles, fromi 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

* In adclltioa to these larger tubes, there exists also a smaller set, which
pierce the skin in different places, and are channels for the absorption of the
water used in respiration. These I shall have occasion to notice more parti-
cularly hereafter.

■\ An architecture of a still more curious description is exhibited in the
calcareous frame-work that has been provided for the support of the teeth,
and other organs of mastication, with which this animal is furnished. The
etructure of these organs will be noticed when treating of that function.

ECHINUS. 151

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 sphe-
rical surface of the tubercles. They thus constitute ball-and
socket joints, allowing of free motion in all directions. Each
joint is connected with the plate on which it turns, by means
of the integument, which acts the part of a capsular ligament;
and sets of radiating muscular fibres are provided for effect-
ing the movements of the spines. By employing these spines
as levers, the Echinus advances with great facility alono-
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, w^ith the convexity downwards; and
which have a limited rotatory motion. Those which grow
from the sides are more slender, and taper towards the ex-
tremities, and when not in use they f\dl flat upon the body
with their points directed backwards. Besides these, there
are a few longer bristles, arranged in a crescent on the back,
and converging till their points meet, but capable of being
erected to a perpendicular position. The animal, when
placed on sand, commences its operations by revolving the
lower spines, thus soon creating a hollow quicksand, into
which it sinks by its own weight so far as to enable the low-
est of the lateral spines to co-operate with them, by scatter-
ing and throwing up the loosened particles; while these, at
the same time, contribute, by their re-action, still fiirther to
depress the body. As the animal sinks, a greater number
of spines are brought into action, and its progress becomes

• It has been ascertained by Mr. Haiding-er, that the structure of these
spines is crystalline, and that their cleavag^e presents the exact rhomboidal
angles characteristic of carbonate of lime. See his Translation of Mohs's
Mineralog-y, vol. ii. p. 91.

152 THE MECHANICAL FUNCTIONS.

more rapid; while the sand, that had been pushed aside,
flows back, and covers the body, when it has sunk below
the level of the surface. In this situation the long dorsal
bristles come into play, preventing the sand from closing
completely, and preserving a small round hole for the ad-
mission of water to the mouth and respiratory organs.*

Whenever, in following the series of organic structures,
new forms are met with, we always find them accompanied
by corresponding modifications in the processes of develop-
ment. The organization of the animals belonging to the
lowest division of the series is not sufiiciently perfect to af-
ford the means, which are supplied in the higher animals,
of removing or modifying the substances that have at any
time been deposited, and sufiered to harden. Hence the
structures composed of these substances remain unchanged
during the life-time of the animal, although they may conti-
nue to receive additions of new layers of the same material,
deposited upon 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 struc-
tures of zoophytes are formed by this process, and they are
subjected to all the consequences of this law of increase. As
these consequences are important in their relation to the
conditions of growth, and to the forms which result, it will
be necessary to direct our attention to them more particu-
larly.

The influence which this mode of increase by superficial
depositions may have, in changing the form of the original
structure, will depend altogether upon the relative situations
of the soft secreting organ and the hard part on w^iich it is
to desposite 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

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

ECHINUS. 153

which must sooner or later become sensible. If the soft
organs have sufBcient room for their expansion, as is the
case when they are external to the hard axis of the zoophyte,
the growth of that axis may go on without impediment; and
no change need take place in the general figure of the parts,
since their relative proportions and situations may be pre-
served unaltered. But this cannot happen when the new
materials are to be deposited on the internal surface of a
membrane, or a shell, which completely 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
filled up with solid matter, and the cavity be obliterated.
Accordingly it is necessary, whenever cells, intended for
the lodgement of soft organs, are to be constructed of hard
materials, that the foundation of these cells should be laid,
and their construction begun, upon 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 polypoMS portion has attained its
full expansion; for were it at first built of a smaller size,
proportioned to that of the young polype, it would prevent
all farther growth.

The globular shell of the Echinus, which is external to
the soft parts that nourish it, and which yet grows from a
very minute sphere to one of large dim.ensions, kee])ing pace
with the gradual expansion of the internal organs, might ap-
pear to be an exception to the general law. Nature has,
however, accomplished her purpose without deviating from
her usual plan; first, by dividing the shell of the Echinus into
a great number of small pieces; and secondly, by giving to
each piece the polygonal form, which is best adapted to their

Vol. I. 20

154 THE MECHANICAL FUNCTIONS.

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 calcareous substance be^jng deposited on the under
side, and on the edges of each, in proportion as the expan-
sion of the contents of the shell causes their separation.
That such a succession of deposites has taken place, may easi-
ly be seen, by minutely examining the texture of the plates,
which will be found marked by concentric polygonal lines.
(Fig. 99.)

The spines of the Echinus must be formed by the succes-
sive 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 exemplify in
so marked a manner as the Echinodermata, the gradations
which nature has observed in passing from one model of
construction to another of a totally different aspect, through
every intermediate form. What shapes can be more diver-
sified, and apparently irreducible to a common standard,
than those of the star-like Asterias, (Fig. SS) of the globu-
lar Echinus, (Fig. 91,) and of the lily-shaped Pentacrinus;
(Fig. 94,) and yet we find these passing the one into the
other by the most gradual transitions? Setting out from
the star with five slender rays, which is the standard form
of the Asterias; we find the rays, in succeeding species, as-
suming 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

ECHINUS. 155

general rounded outline; still, however, preserving its flat-
ness. This stage is attained in the Scutella, and the C/y-
jnaster. (Fig. 90.) We next find that, in the Spatangus,
the thickness increases; though at first with an oval outline,
and with several changes in the situation of the mouth of the
animal. At length, after passing through many intermediate
steps, we arrive at the perfectly circular and spheroidal
Echinus. (Fig. 91.) If we might he permitted to conjec-
ture the objects of all these changes, which occur in this con-
tinuous gradation, we might not unreasonably suppose them
to be the concentration of the internal organs into one com-
pact mass, and the retrenchment of all the external appen-
dages. It is also curious to observe, how, amidst all these
modifications, the double rows of perforations, which con-
stitute the ambulacra, retain 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 with the former. Here, instead
of the retrenchment of the appendages, we find them great-
ly developed, 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 taper-
ing 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 dou-
bled, then tripled, and at lengtli indefinitely augmented.
They also become branched, subdividing by simple bifurca-
tions, as in the Euryale palmiferiim (Fig. 93;) next into
minuter ramifications, as in the Caput Medusic, where the
thousands of filaments have the appearance of a tangled web,
which defies all attempts at unravelling.

The steps are but short from the Comatula to the Crinoi-

156 THE MECHANICAL FUNCTIONS.

dea, or lily-shaped tribe, (of which, Fig. 94, representing
the Pentacrirms europxuSj is an example;) for they consist
chiefly in the addition of a jointed stalk, which is made to
proceed downwards from the centre of the whole assem-
blage 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 pre-
sents a remarkable resemblance to a liliaceous plant.

( 157 )

CHAPTER III.

MOLLUSCA.

§ 1. Molliisca in general.

The series of animal structures, arranged according to
their mechanical functions, conducts us next to the JMoUus-
ca; an assemblage of beings which was first recognised as
constituting one of the primary divisions of the animal king-
dom by Cuvier, the greatest naturalist of modern times. A
vast multitude of species, possessing in common many re-
markable 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 com-
pletely 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 pro-
trusion of the head and the foot, when these organs exist.
But a large proportion of the animals of this class arc ace-
phalous, 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.

Mollusca, with the exception of a few among the higher
orders, are but imperfectly furnished with organs of loco-
motion. The greater number, indeed, are formed for an
existence as completely stationary as the Zoophytes attaclied
to a fixed base. The Oyster, the Muscle, and the Limpet, for
example, arc usually adherent to rocks at the bottom of the
sea, and are consequently dependent for their nourishment

185 THE MECHANICAL FUNCTIONS.

on the supplies of food casually brought within their reach
by the waves and currents 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 commencement 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
preserve an analogy with the gemmules of sponges, and of
polypi, which exercise locomotive powers only in the ear-
ly stages of their devlopment.*

The organization of the Mollusca being unfitted for the
construction of an internal skeleton. Nature has ordained
that the purposes of mechanical support and protection shall
be answered by the formation of hard calcareous coverings,
or shells, the result of a peculiar process of animal production.
These shells are formed either of one piece, or of several;

* 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 mechanismof vibratory cilia which we found to be instrumental
in the progression of the infusory animalcules, and of the young of polypi.
On observing the first evolution of the ova of the Buccinum undaium. Dr.
Grant found them to consist of groups of spherical gelatinous bodies, which
soon become covered on one side with a transparent envelope, the rudiment
of the future shell; while, on the other side, the gelatinous matter is extend-
ed outwards, so as to form the margin of an internal cavity, of which the en-
trance is surrounded with vibratory cilia, and in the interior of which a revo-
lution of particles is seen, indicating a constant current of fluid. The vibra-
tions of these ciha are perceived long before the pulsations of the heart, and
even before any appearance of that organ is visible; th(jy are, indeed, the
first indications of life in the embryo. The cilia are in activity even before
the animal is hatched: for while confined within the q^^, it is seen almost
continually revolving around its centre; amotion which appears destined to
bring a constant supply and renewal of sea water into the interior of the or-
ganization, in order to perfect the formation of the shell before the animal
is, as it were, launched into the ocean. Possibly, also, the continued friction
of the cilia against the interior of the e^g may tend to abrade it, and open a
passage for the young animal. No sooner has the animal effected its escape,
than it darts rapidly forwards by the motion of its cilia. The same appear-
ances have also been observed by Dr. Grant in the young of different Mol-
lusca, such as the Dwis, Eolis, &c., which have no shell.— Edin. Journal of
Science, Vol. vii.

MOLLUSCA.

159

the separate pieces, in cither case, being termed valves; so
that shells may be either univalve, bivalve, or miiltivalve,
according as they consist of one, two, or more pieces. Uni-
valve shells have generally more or less of a spiral form,
and are then called turbinated shells. In a few, the cavity
of the shell is divided by transverse partitions into nume-
rous compartments. Some Mollusca have internal shells for
the defence and support of particular organs; and others
have shells which are partly external, and partly internal.
As respects their shape, colour, and appearance, shells ad-
mit 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. Jlcejihala.

The Mollusca which inhabit bivalve shells, such as the
Oyster, the Muscle, and the Cockle, are all acephalous.
The two valves of the shell are united at the back by a
hinge joint, often very artificially constructed, having teeth
that lock into each other: and the mechanism of this arti-
culation varies much in different species. The hino-e is se-
cured by a substance of great strength. It is seen in VW.

101, which shows the valves
of the Unio batava, with the
connecting ligament. This
ligament is composed of two
kinds of texture: the one,
which is always external, is
strictly ligamentous; that is,
perfectly inelastic: the other
has more of the properties of
cartilage, being highly elastic,
and formed of parallel series
of condensed transverse fibres,
101 directed from the hinire of one
valve to the similar part of the other, and having generally

160

THE MECHANICAL FUNCTIONS.

a deep black colour, and a pearly lustre. The cartilage is
always situated within the ligament, sometimes in immediate
contact, and forming with it one of 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.

Durins: 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 insfress and esiress of the water neces-
sary for its nourishment and respiration. But as a security
against danger, it was necessary to furnish the animal with
the means of rapidly closing the shell, and retaining the
valves in a closed state. These actions being only occa-
sional, yet requiring considerable force, are effected by
a muscular power: for which purpose sometimes one, some-
times two, or even a greater number, of strong muscles are
placed between the valves, their fibres passing directly
across from the inner surface of the one to that of the other,

and firmly attached to both.
— They are named, from
their office of bringing the
valves towards each other,
the adductor muscles. Fig.
102, which represents the sec-
tion of an oyster, shows the
situation of the hinge l, the
adductor muscles 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 intend-
ed to open beyond a certain extent, it was necessary to pro-
vide some limitation to the action of the cartilage. The ad-
ductor muscle might, it is evident, be called into play to
counteract that action; but this would require a constant
muscular exertion, and a great expenditure, therefore, of
vital force. Nature has always shown a solicitude to econo-

MOLLUSCA ACEPHALA.

161

103

mize muscular power, whenever a substitute could be had,
and such a substitute she has here provided, by uniting with
the muscle an elastic ligament, of a peculiar construction.
It has a texture similar to that of the ligamentum nuchw^
and being placed on the side of the muscle next to the hinge,
allows the valves to separate to the proper distance only.*
When the animal dies, the muscular force ceases, but the li-
gament with 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 find bivalve
shells that are cast upon the shore, after the soft flesh of the
animal has decayed and been washed out, provided the car-
tilage and the ligament of the hinge are still preserved.!

The simple actions of opening and closing the valves arc
capable of being converted into a means of retreating from

danger, or of removing to a more com-
modious situation, in the case of those
bivalves which are not actually attached
to rocks or other fixed bodies. Dique-
mare long; asro observed that even the
oyster has some power of locomotion, by
suddenly closing its shell, and thereby
expelling the contained water, with a de-
gree of force, which by the reaction of
the fluid in the opposite direction, gives
a sensible impulse to the heavy mass. He

t

* 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 Zoolog-ical Journal, I. 219.

\ The Pholas is an exception to this rule; for instead of its valves being-
united, as usual, by an elastic ligament, they are connected chiefly by means
of muscles. This departure from the ordinary structure is probably occa-
sioned by a new condition introduced into the economy of the animal in con-
- sequence of its being- fitted for excavating passages through liai-d rocks. It
is furnished, for this purpose, with a complicated boring apparatus moved
by many muscles, and requiring great freedom of action. Fig. 103 repre-
sents the shell of the Fholas Candida extremely expanded, in order to show
the hinge, together with the ligament, l; the long and thin process of shell,
p, to the ends of which, on each side, a pair of fan-shaped muscles, more par-
ticularly employed in boring, are attached^ and the two adductor muscles.
Vol. I. 21

162 THE MECHANICAL FUNCTIONS.

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 re-
turn of the tide: whereas those oysters which are taken from
greater depths, where the w^ater never leaves them, and are
afterwards removed to situations where they are exposed to
these vicissitudes, of which they have had no previous ex-
perience, improvidently open their shells after the sea has
left them, and by allowing the water to escape, soon perish.*
Many bivalve mollusca are provided with an instrument
shaped like a leg and foot, which they employ extensively

for progressive motion. Its form
in the Cardium, or cockle, is seen
in Fig. ] 04. This organ is com-
posed of a mass of muscular fibres,
interwoven together in a very com-
plex manner, and which may be
compared to the muscular structure
of the human tongue: the effect in both is the same, namely,
the 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 cylin-
drical shape, tapering at the end, and much more resembling
in its form a tongue than a foot. In some bivalves the dila-
tation of the foot is effected by a curious hydraulic mechan-
ism: the interior of the organ is formed of a spongy texture,
capable of receiving a considerable quantity of water, which
the animal has the powder of injecting into it, and of thus in-
creasins; its dimensions.

The foot of the Mytihis edid'is, or common muscle, can
be advanced to the distance of two inches from the shell,
and applied to any fixed body within that range. By at-

A A, which retain the valves in contact independently of the ligaments. For
a full description of this apparatus, I must refer to a paper by Mr. Osier, on
burrowing and boring marine animals, contained \n the Phil. Trans, for 1826,
p. 342, from which tlie above figure has been taken.
* Journal do Physique, xxviii. 244.

MOLLUSCA ACEPHALA. 163

taching the point to such hody, and retracting the foot, this
animal drags its shell towards it; and by repeating the ope-
ration successively on other points of the fixed object, con-
tinues 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 conti-
nually employs its foot for this purpose: first elongating it
and 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 re-
tract it are thrown into action, and the whole shell is alter-
nately raised and depressed, moving on the foot as on a ful-
crum. The effect of these exertions is to drag the shell
downwards. When the animal is moderately active these
movements are repeated two or three times in a minute.
The apparent progress is at first but small; the shell, which
was raised on its edge at the middle of the stroke, falling
back on its side at the end of it; but when the shell is bu-
ried so far as to be ^supported on its edge, it advances more
rapidly, sinking visibly at every stroke, till nothing but the
extremity of the tube can be perceived above the sand. JNIr.
Osier, who has given us this account,* observes that the in-
stinct, which directs the animal thus to procure a shelter,
operates at the earliest period of its existence. Tlie Mija
iruncata, when fully grown, will not attempt to burrow;
but on placing two young ones, which were scarcely more
than a line in length, and apparently but just excluded, on
sand, in a glass of sea-water, he found that they buried them-
selves immediately.

By a process exactly the inverse of this, that is, by dou-
bling up the foot, and pushing with it downwards 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

* Phllos. Trans, for 1826, p. 349.

164 THE MECHANICAL FUNCTIONS.

when clanger 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 Solen 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.

This, as well as many other of the bivalve mollusca, are
enabled by the great size and flexibility of this organ to
execute various other movements, of which, from the habit-
ual inactivity of animals of this class, we should scarcely have
supposed them capable. The Tellina is remarkable for the
quickness and agility with which it can spring to considera-
ble distances by first folding the foot into a small compass,
and then suddenly extending it; while the shell is at the
same time closed with a loud snap.

The Pinna, or Marine Muscle, when inhabiting the shores
of tempestuous seas, is furnished, in addition, with a singu-
lar apparatus for withstanding the fury of the surge, and se-
curing 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 adhe-
sion with it, and concentrate their powxr on one point. The
threads themselves are composed of a glutinous matter, pre-
pared 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, when they hard-
en, and acquire a certain consistence before they are em-

MOLLUSCA ACEPHALA. 165

ployed. This mould is curiously constructed; there is a
deep groove which passes along the foot from tlic 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 convert-
ing it into a canal. The glutinous secretion, which is poured
into this canal, dries into a solid thread; and when it has ac-
quired sufficient tenacity, 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 sur-
face of that object. The canal of ^thc 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 diffe-
rent directions around the shell. Sometimes the attempt
fails in consequence of some imperfection in the thread; but
the animal, as if aware of the importance of ascertainino- the
strength of each thread, on which its safety depends, tries
every one of them as soon as it has been fixed, by swingino"
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 ap-
pear to have the power of cutting or breaking them off. The
liquid matter out of which they are formed is so exceedino--
ly glutinous as to attach itself firmly to the smoothest bo-
dies. It is but slowly produced, for it appears that no Pin-
na is capable of forming more than four, or at most five
threads in the course of a day and night. The threads that
are formed in haste, when the animal is disturbed in its ope-
rations, are more slender than those tliat are constructed at
its leisure, Reaumur, to whom we are indebted for these
interesting observations, states, also, that the marine muscles
possess the art of forming these threads from the earliest pe-
riods of their existence: for he saw them practising it, when
the shells in whicli they were enclosed were not larger than
a millet seed.^ In Sicily, and other parts of the Mediter-

• Memoircs de I'Acadcmic des Sciences: 1711, p. 118 to 123. Poli con-

166

THE MECHANICAL FUNCTIONS.

ranean, these threads have been manufactured into gloves,
and other articles, which resemble silk.

§ 3. Gasteropoda,

The Mollusca which inhabit univalve or turbinated shells,
belong to the order of Gasteropoda, and have a more high-
ly developed organization than the Acephala. The part
which performs the office of a foot is a broad expansion of
fleshy substance, occupying nearly the whole under surface
of the animal, and forming a flat disk, capable of being

applied to the plane along
which it moves. This is
seen in the Planorbis (Fig.
105, D.) In some species it
is fashioned into a project-
ing ridge which cuts its way,
like a ploughshare, 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 atM;he surface of the disk; so that when the
animal is crawling their successive actions produce a visible
undulatory motion of that surface. The efiect of these ac-
tions is that different parts of the plane on which it moves
are laid hold of in succession, and each corresponding por-
tion of the animal is dragged along, so that the body ad-
vances 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 move-
ment of its disk from the other side of the glass: the regu-
lar undulations which advance in the direction of the mo-
tion of the snail, but with twice the velocity, present a cu-
rious and interesting spectacle.

A mucilaginous secretion generally exudes from the sur-
face of the disk, and tends to increase considerably its

ceived that these threads are dried muscular fibres; an opinion which has
been adgpted by Blahivillc.

GASTEROPODA. 167

power of adhesion, both when the animal is crawling, and
also when it fixes itself on any surface. In the Patella, or
limpet, this adhesion is greatly favoured by the conical form
of the shell, which having a circular base, enables the mus-
cles of the disk, by their efforts to create a vacuum under-
neath it, to command the whole hydrostatic pressure of the
superincumbent water, as well as of the atmosphere above
the water. Besides the muscular bands contained in the
substance of the foot, other sets of fibres are provided for the
purpose of protruding or of retracting the whole member,
and of moving it in different directions.

The foot of the Buccinmn xindatuvi, or Whelk, is capa-
ble of great dilatation by means of four tubes, which open
from the surface near the gullet, and convey into it a large
quantity of water. It may, by this means, be distended to
a size even greater than the shell itself; so that the opening
which it forms in the sand is large enough to receive the
shell, when the latter is drawn down by the contraction of
the muscles which are attached to the foot.* The foot of
the Scyllsea is grooved, for the purpose of enabling the ani-
mal 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 tcntacula,
which the animal protrudes for the purpose of feeling its
way as it advances, and which are quickly retracted, \i^ the
reversion of the tube, when they arc touched or irritated.
This mechanism is matter of familiar observation in the tcn-
tacula, or horns, of the snail and of the slug, which are ter-
restrial mollusca belonjiins to this order. The former of
these has a turbinated shell of the ordinary structure: the
latter, though extremely similar in its internal structure to
the snail, is destitute of any external shell; but is furnished,
instead of it, with a small internal plate of cartilage, giving
support to some of the vital organs.

• Osier, Phil. Trans, for 1826, p. 352.

16S THE MECHANICAL FUNCTIONS.

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

The structure and formation of the shells of molluscous
animals is a subject of much interest in comparative physi-
ology, as presenting many beautiful illustrations of the laws
by which the inorganic parts of the living system are in-
creased in their dimensions.

All shells are composed of two portions, the one consist-
ing 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, differ much in dif-
ferent kinds of shell; and it is chiefly in reference to these
circumstances that shells have been divided into two classes,
namely, the memhranous and jiorcellaneQUS shells.

In shells belonging to the first of these classes, the carbo-
nate of lime is united with a membranous substance deposit-
ed in layers, which may be separated from one another,
either by mechanical division with a sharp instrument, or
by the slow actions of air, water, or other decomposing che-
mical agents. The shells of the limpet, of the oyster, and
of almost all the larger bivalve mollusca which reside in the
oceaif are of this kind. They are usually covered with a
thick outer skin, or epidermis; and their texture is of a
coarser srain 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 ni-
tric acids properly diluted, at first a brisk efiervescence 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, or iridescent appear-

\ ••■^ ">,v ^

STRUCTURE OF SHELLS. ' IQO

ance.* 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 par-
ticles of carbonate of lime, enter into the formation of that
part of the surface of the shell. The surface, which has thus
acquired a pearly lustre, was formerly believed to be a pe-
culiar substance, and was dignified with the 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 mate-
rials, and have the same laminated structure as the mem-
branous shells; being formed by very thin concentric plates
of membrane and carbonate of lime, disposed alternately,
^^^^^^106 and often surrounding a central body,

pfi^': "'-^ or nucleus: but Sir David Brewster

has satisfactorily shown that the Iri-
descent colours exhibited by these
;|; surfaces are wholly the effect of the
parallel grooves consequent upon
-n^s^^ the regularity of arrangement in the
-'^'^ successive deposltes of shell. t The
appearance of these grooves or strise
w^hen highly magnified is shown in Fig. 106.:]: This Iride-
scent property may be communicated to shell lac, sealing
wax, gum Arabic, balsam of Tolu, or fusible metal, by taking
an accurate cast or impression of the surface of mother of
pearl with any one of these substances.§

Porcellaneous shells have a more uniform and compact
texture than those of the former class. The animal matter

* Examples of this nacreous structure, as it is termed, occur in the shells
of the ffalioiis, or Sea-ear, and of the ^iiodon, or fresh water muscle.

f Philosophical Transactions for 1814, p. 397.

^ See a paper on this subject by Herschel in the Edinburgh Philosophical
Journal, ii. 114.

§ When these shells decay and fall to pieces, they separate into numerous
thin scales of a pearly lustre. The fine scales thus obtained from tlxe Tia-
cuna, or window oyster, are employed by the Cliinese in their water-colour
drawings to produce the effect of silver. Some of this powder hiu> been
brought to England and used for this piu'pose. Sec Gra\-, Phil. Trans, for
1833.

Vol. I. > 22

■Jj^i^-^ . 1^^ '-^^ ^^i-'

170 THE MECHANICAL FUNCTIONS.

which unites the carbonate of lime is less in quantity and
not so evidently disposed in layers; but it is more equally
blended with the earthy particles, with respect to which it ap-
pears to perform the office of a cement, binding them strong-
ly together; although it has of itself but little cohesive
strength. The Cyprsea and the Volute are examples of por-
cellaneous shells.

In shells of this kind the carbonate of lime assumes more
or less of a crystalline arrangement; the minute crystals be-
ing 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 direction of
the spire: while, on the contrary, those
of the intermediate plate form concentric rings round the
cone parallel to its base. Thus the fibres of each layer are at
rio-ht andes to those of the layer which is contiguous to it;
an arrangement admirably calculated for giving strength to
the shell, by opposing a considerable cohesive resistance to
all forces tending to break it, in whatever direction they may
be applied. "^ We here find that a principle, which has only

* These lines 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 lay-
er there is frequently found a desposite of a hard, transparent, and apparent-
ly homogeneous calcareous material, D. Of this latter substance I shall af-
terwards have occasion to speak.

STRUCTURE OF SHELLS. 171

of late years been recognised and applied to the buildins; of
ships, namely, that of the diagonal arrangement of the frame-
work, and the oblique position of the timbers, is indentical
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 ob-
servable in the Teredo giganiea from Sumatra,* and also
in many bivalves, such as those belonging to the genera
Avictda and Pinna.

When porcellaneous shells are subjected to the solvent
action of acids, the animal matter in their composition offer-
ing but little resistance, there is a considerable and long con-
tinued effervescence. The solution of the carbonate of lime
proceeds rapidly, in consequence of the speedy disintegra-
tion of the animal substance, which is broken up, and partly
dissolved. The remainder is reduced to minute fragments,
which subside in the form of flakes or scales to the bottom
of the fluid. Poli has given a minute and elaborate descrip-
tion of the appearances of these fragments of membrane,
when seen under the microscope.!

The difference between the textures of these two kinds of
shell is farther illustrated by the impression made upon them
by fire. Porcellaneous shells, when exposed to a red heat,
give out neither smell nor smoke; they lose, indeed, their
colour, but retain their figure unaltered. JNIembranous shells,
on the contrary, emit a strong fetid odour, and become black;
after which the plates separate, and the structure falls to
pieces.

This variety in the composition and structure of different
kinds of shell is accompanied by corresponding modilica-
tions of their mechanical properties. The tougliness of the
fibrous basis of membranous shells, imparts to them greater

* In this shell the crystalline appearance is so perfect, that when some
frag-mcnts were sent to England they were mistaken for a minei-ai production.
Home; Lectures, I. 5o.

\ See hb folio work on the Testacca of the Two SiciUcs.

172 THE MECHANICAL FUNCTIONS.

strength than is possessed by the porcellaneous shells, which,
in consequence of the tenuity and uniform intermixture of
the animal cement with the calcareous particles, present a
harder and more transparent, but at the same time more
brittle compound. It is these qualities, together with their
smooth enamelled surface, often beautifully variegated with
brilliant 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 in-
termediate textures between the membranous and the por-
cellaneous.

All those surfaces of the shell on its outer side which are
not in contact with any part of the animal, are originally co-
vered with an epidermis:* which, however, is frequently
rubbed off by friction.

The process employed by nature for the formation and en-
largement of the shells of the mollusca was very imperfect-
ly understood prior to the investigations of Reaumur, who
may be considered as having laid the first solid foundations
of the theory of this branch of comparative physiology. t
His experimental inquiries have fully established the two
following general facts: first, that the grow^th of a shell is
simply the result of successive additions made to its surface,
and secondly, that the materials constituting each layer, so
added, are furnished by the 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 instance, be
removed, which can be done without injury to the animal,
since it adheres to the flesh only in one point, there is

* This membrane has been termed the Perlostracum.

f Memoires dc I'Academie des Sciences, 1709, p. 367, and 1716, p. 303.

FORMATION OF SHELLS. 173

formed, in the course of twenty-four hours, a fine pellicle,
resembling a spider's web, which is extended across the va-
cant space, and constitutes the Hrst stratum of the new shell.
This web, in a few days, is found to have increased in thick-
ness, 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 sam.e, for it is beneath the level of the adjacent
parts of the shell. The fractured edges of the latter remain
unaltered, and have evidently no share in the formation of
the new shell, of which the materials have been supplied
exclusively by the mantle. This Reaumur proved by in-
troducing through the aperture a piece of leather underneath
the broken edges, all round their circumference, so as to lie
between the old sliell 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 toge-
ther 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 easi-
ly separable, being merely agglutinated together like the
component leaves of a sheet of pasteboard.

It has long been the prevailing opinion among naturalists
that no portion of a shell which has been once deposited, and
has become consolidated, is capable of afterwards undergoing
any alteration by the powers of the animal that formed it.
Very conclusive evidence has, in my opinion, been adduced
against the truth of this theory, by ]\lr. Gray, in a paper
lately read to the Royal Society. From a variety of facts, it
appears certain that on some occasions the molluscous animal

174 THE MECHANICAL FUNCTIONS.

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 unchanged,
while it continues to be connected with the animal which
produced it. In a general way, indeed, we may consider
the connexion between the animal and the shell as m.echani-
cal, rather than vital; and the shell itself as an extraneous
inorganic body, formang 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 worm.s 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 deposite is produced.
The worn edges of shells, and the fractures, and other ac-
cidents which befall them, are never repaired, except as far
as such repairs can be made by the addition of materials from
the secreting surfaces of the mantle. It is found that shells
may be 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 subjacent corium,
which is incorporated with the mantle, and may be regarded
as forming one and the same organ. t

* Mr. Gray considers the external membrane of the shell, or epidermis, as
formed by the outer edge of the plates of animal substance, which have
scarcely any calcareous matter in their composition, and which are soldered
together into a membranous coat.

■{"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

FORMATION OF SHELLS. 175

The shape of the shell depends altogether on tlie 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. The simplest case is that
in which this rudiment of shell is a concave disk. We
may conceive the animal, covered by its mantle, to expand
the border of this organ, and extend it beyond the edge of
the shell, where it then forms a new layer of shell; and this
new layer, being applied to the inner or concave surface of
the original shell, will, of course, extend a little w^ay beyond
its circumference. The same happens with the succeeding
layers, each of whichbeing larger than the one which has pre-
ceded it, projects in a circle beyond it; and the whole scries
of these conical layers, of increasing diameters, forms a com-
pound cone, of which the outer surface exhibits transverse
lines, showing the successive additions made to the shell in
the progress 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 proceed equally on all sides, as happens
in the patella. If the increase take place in front only, that
is, in the fore part of the mantle, the continual deflexion
thence arising necessarily gives the shell a spiral form, the
coils being simply in one plane. This is the case in the
Planorbis. (Fig. 105) the Spirula,?iv\(\ the Nautilus. INIost,
commonly, however, as in the Bucclniiin, and Jichatina
(Fig. 108) the deposite of shell takes place laterall}^, 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 changing its plane, being converted from a spiral
to a helix, a term of Geometry borrowed from the Latin
name of common snail, which, as is well known, has a shell

calcareous covering- to the mouth of the shell. Mr. Gray also ascertained that
in the Cymbia, and Olivx, and the AnciUariae^ shell is deposited, and most
probably secreted by the upper surface of tlie foot, vvliich is very larg-e, and
not by the mantle, which is small, and does not extend beyond the cdg-e of
the mouth.

176

THE MECHANICAL FUNCTIONS.

of this form. Fig. 108, which represents the shell of the
t/ichaiina zebra,, and of which Fig. 109 shows a longitudinal
section, may serve as an example of a shell of this kind.
The axis of revolution is termed the Columella, and the
turns of the spiral are denominated lohorls. In consequence
of the situation of the heart and great blood vessels relative-

ly to the shell, the left side of the mantle Is more active than
the risht side, so that the lateral turns are made in the con-
trary direction, that is, towards the right.* There are a
few species, however, where, in consequence of the heart
being placed on the right side, the turns of the spiral are
made to the left. Such shells have been termed sinistral,
ov reversed shoVi'. but this left-handed convolution seldom
occurs amons; 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 generally be distinguished by its colour and appearance
from that which is afterwards formed. The succeeding 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
extends toward the mouth of the shell, its posterior end
often quits the first turn of the spire, and occupies a situa-

* The terms rig-ht and left have reference to the position of the animal
when resting- on its foot; the head being-, of course, in front. See Gray,
Zool. Journal, i. 207.

FORMATION OP SHELLS. 177

tion dlfierent from that vvliieh it Iiad 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 some-
times happens, however, as in the Conns , that the upper
surface of the spiral scarcely descends 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 part last formed being the outer surface
of the cone and the circumference of the apparent base, or
flat surface, of which the central part is the one first formed.

Various causes may occur to disturb the regularity of the
process of deposition, by which the shell is enlarged in its
dimensions: at one time accelerating, and at another retard-
ing, or totally arresting its growth. These irregularities are
productive of corresponding inequalities in the surface of
the shell, such as transverse lines, or striae. Whenever an
exuberance of materials has led to a sudden expansion of
growth, which has again soon subsided, a projecting ridge is
produced in the direction of the margin of the mantle at the
time this happens. This change generally recurs at regular
periods, so that these ridges, or ribs, as they are often called,
succeed one another at equal distances along the course of
the spiral turns.

It not unfrequently happens, that at diiTcrent periods, a
sudden development takes place in particular parts of the
mantle, which become in consequence 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 construct,
being consolidated around each fleshy process, must neces-
sarily have at first the shape of a tube closed at the extre-
mity. As fresh dcposites are made by the secreting surface,
which are in the interior of the tube, the internal space is

Vol. I. 23

178

THE MECHANICAL TUNCTIONS.

gradually filled up by these deposites; the process of the
mantle retiring to make way for their advance towards the
axis of the tube. In the course of time, every part of the
cavity is obliterated, the process of the shell becoming en-
tirely solid. Such is the origin of the many curious pro-
jecting cones or spines which several shells exhibit, and
which have arisen periodically during their growth from
their outer surface. In the Murex these processes are of-
ten exceedingly numerous, and occur at regular intervals,
frequently shooting out into various anomalous forms. In
many shells of the genus Strombus these spines are of great
length, and are arranged round the circumference of the
base, being at first tubular, and afterwards solid, according
to the period of growth. This is exemplified in the Piero-
cera 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 epocli of their existence, when consider-
able changes take place in the disposition of the mantle, and
in its powers of secretion. Often w^e find it suddenly ex-
panding into a broad surface, 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 deposite calcareous layers, which give greater thickness
to the adjacent part of the shell: and at the same time nar-

FORMATION OF SHELLS. 179

row its aperture, and materially alter its general shape and
aspect. Thus it happens that the shells of the young; and of
the old individuals of the same species are very different,
and would not be recognised as belonging to the same tribe
of mollusca. This is remarkably the case with the shell of
the Cypvcca, or Cowrie, which in the early stage of its
growth, (Fig, 112) has the ordinary form of an oblong tur-
binated 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 cor-
responding processes of the mantle.* But in this instance
the change does not stop here; for both edges of the mantle
next take a wider expansion, turning over the outer surface of
the shell, and passing on till they meet at the upper convex
part, or back of the shell, forming what has been termed the
dorsal line. They dcposite, as they proceed, a dense and high-
ly polished porcellaneous shell, beautifully variegated with
coloured spots, which correspond exactly with tlie coloured
1X4 parts of the mantle that deposites

them. This new plate of shell com-
pletely envelops the original shell,
giving it a new covering, and dis-
suisins: its former character. A
transverse section, (Fig. 114,) at once
shows the real steps by which these
changes have taken placet
Changes equally remarkable are observed to occur in the
interior of the shell at different stages of its growth. On

• Similar chang-es occur in the shells of the Ovula (spindles,) Erato (tear-
shells,) and Margmella^ (dates.) Gray, Phil. Trans, for 1833.

f According to Brug-uiere, there is reason to believe that the animal of the
Cy/Jra^a 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 cai)able of
forming in succession several shells. Blalnvillc, however, considers it im-
possible that the living animal can ever quit its shell. Malacologle, p. 94.

ISO THE MECHANICAL FUNCTIONS.

the inner surface of the Mitra, the Volute, and other
shells of a similar kind, there is deposited a layer of a
hard semi-transparent calcareous material, having a vitreous
appearance.* 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 turrited, as it is
called,! this deposition entirely fills the upper part, which,
in the early condition of the shell, was a hollow space with
thin sides. The purpose answered by this deposite is evident-
ly to give solidity and strength to a part which by remain-
ing in its original state would have been extremely 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 surface thus withdrawn. That portion
of the shell 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 Cerithiurn, decollatum, the Bulimus decolla-
tus, &c. The young of the genus Magilus has a very thin
shell of a crystalline texture; but when it has attained its
full size, and has formed for itself a lodgement in a coral,
it fills up the cavity of the shell with a glassy deposite,
leaving only a small conical space for its body; and it con-
tinues to accumulate layers of this material, so as to main-
tain 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 vi-
treous 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 becomes, in the
progress of its enlargement, excessively cramped. In or-
der to obtain more space, and at the same time lighten the

* This is the substance represented at d, Fig. 107, p. 170.

\ As iji the genera TurritdlUy Tercbra, Vcrithium, and Fa^ciolaria.

FORIMATION OF SHELLS.

181

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 envelops the whole, and which, re-
taining its original thickness, is of suilicicnt 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 examining a vertical section of that shell, as is represented
in Fig. 116. All the inner partitions of the cavity thus laid

115

117

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 the section b b, which lies on each side of the proper
apex of the shell, and which forms the apparent base. The
lines on this part of the section indicate the thickness which
each successive whorl had originally, and when it was itself
the outermost whorl. The section also shows the vitreous de-
posite 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 re-
moval of the interior whorls. This is found to occur in that
of the genus */luricida, which are molluscous animals, res-

* Fig". 117, which is a transveree section of the same shell, shows the spi-
ral convoUitions, and the comparative thinness of the inner portions. It also
forms a striking contrast with a similar section of the Cyprxa, Fig 11 -I-

182 THE MECHANICAL FUNCTIONS. .

piring by means of pulmonary organs. In the young shell
of this tribe, the partitions which separate the cavities of the
whorls are incomplete, and twine parallel to each other; but
they wholly disappear as the animal approaches to maturity.
In other cases, the animal is found to remove exterior por-
tions of shell formerly deposited, when they lie in the way
of its farther growth, and when the mouth of the spire is
advancing over the irregular surface of the preceding whorls.
Thus we often find that the ridges, ribs, or processes which
had been deposited on the surface of the shells of the Tri-
ton, Miirex, &c. are removed to make way for the succeed-
ing 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 conve-
nient lodgement for themselves. The Fileopsis (or fool's
cap) has this faculty in a remarkable degree; and it is also
met with occasionally in SlphonarHce and Patellar. The
common Patella^ or limpet of our own coasts, often, indeed,
forms for itself, by some unknown process, a deep cavity
out of a calcareous rock.

When the animal which inhabits a spiral shell retires
within it, the only part of its body that is exposed to injury
is that which is situated at the mouth of the shell. With
a view to its protection, it constructs, in many instances, a
separate plate of shell, adapted to the aperture, and denomi-
nated an Operculum. This piece is constructed by a pro-
cess similar to that by which the rest of the shell is formed;
that is, by the deposition of successive layers on the inter-
nal surface, sometimes in an annular, and sometimes in a spi-
ral form. If an operculum were to be constructed of a consi-
derable size, and were connected to the shell itself by a re-
gular hinge, it would be entitled to be considered as a dis-
tinct valve. Here, therefore, we perceive, as was remarked
by Adanson, a connecting link between the univalve and

FORMATION OP SHELLS.

183

118

the bivalve tcstacca. A Clausiinn is another kind of co-
vering, 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 re-
tires into its shell. It thus corresponds exactly in its office
to a door, opening and closing the entrance as occasion re-
quires. An Epiphragma is a partition
of a membranous or calcareous nature,
constructed merely for temporary use.
It is employed for closing the aperture
of the shell during certain periods only,
such as the winter season, or a long con-
tinued drought. Fig. 118 exhibits the
lines which appear on the inner side of
the epiphragma, of the Helix pomatia, or
garden snail, and which indicates the succession of depositcs
by which it has been formed.

It is remarkable in how short a time this species of Helix
will construct this covering, when circumstances occur to
urge its completion. On the approach of winter, the animal
prepares itself for passing that season in a state of torpidity,
first, by choosing a safe retreat; and next by retiring com-
pletely within its shell, and then barricading its entrance by
constructing the epiphragma just described, and of which
the outer surfiice is represented in Fig. 119. Having formed
this first barrier, the animal afterwards constructs a second,
of a membranous nature, situated more internally than the
first, and at a little distance from it. If at any other season,
while the snail is in full vigour, the experiment be made of
surrounding it with a freezing mixture, it will immediately
set about constructing a covering for its protection against
the cold; and it works with such diligence, that in the course
of an hour or two, it will have completed its task, and formed
an entire epiphragma.^ When the genial warmth of return-
ing spring has penetrated into the abode of the snail, the
animal prepares for emerging from ils prison, by secreting
a small quantity of a mucous fluid, which loosens the adhc-

* Gray, Zoolog-lcul Journal, i. 214.

184 THE MECHANICAL FUNCTIONS.

sioii 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 renewed, on every occasion when
another covering is required.*

One great use of these coverings is to prevent evaporation
from the surface of the body of the animal. It is thus that
Snails, Bulimi, &c. may be preserved for months, and even
years in a torpid, but living state, ready to be restored to
the active functions of life, when sufficient water is supplied.t

The enlargement of bivalve shells is conducted on the same
principles as that of univalves; the augnlentation 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 frequent-
ly appear of different hues from the addition of colouring
particles secreted at particular periods by the mantle.

The shells of Oysters and other acephalous mollusca which
adhere to rocks, are often moulded, during their growth, to
the surfaces to which they are applied. The mantle, being
exceedingly flexible, accommodates itself to all the inequali-
ties it meets with, and depositing each successive layer of
shell equally on every part, the figure of the surface is as-
sumed, 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,J and which during its formation was immediate-
ly 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 was necessary
that the muscle, which closes the valves, and is attached to

• An epiphragma differs from true shells in having no adhesion in any part
to the animal which formed it.

■f A remarkable instance of this apparent reviviscence of snails, which had
lain for many years i)i a dormant state in a cabinet of shells, and whicli
crawled out on being accidentally put into warm water, is recorded in the
Philosophical Transactions for 1774, p. 432.

4: Defrance, Annales des Sciences Naturellcs, ii. 16.

I\rOLLUSCA PTEROrODA. lS5

their inner surface, should he gradually removed to a great-
er distance from the hinge, so that it may preserve its rela-
tive situation with regard to the whole shell, and retain un-
diminished its power of acting upon the valves. For this
purpose its adhesions are gradually transferred, by some un-
known process, along the surface of the valves; and the pro-
gress of the removal may generally be distinctly traced by
the marks which are left in the shell at the places before oc-
cupied by the attachments of the muscular fibres. The same
process takes place when there are two or three muscles in-
stead of one.

A few genera of Mollusca, such as the Phohts, have, in
addition to the two principal valves, small supplementary
pieces of shell. They have been accordingly comprised in
the order of Midtivalves. which also comprehends Cuvier's
order of Cirrhopoda, including the several kinds of Barna-
cles, (the genus Lepas of Linnaeus,) which are furnished
wnth a great number of jointed fdaments, or c/rr/iz, and form
an intermediate link of connexion between the Mollusca
and the Jlrticidata. But the limits of this treatise will not
allow me to dwell on the endless diversities of structure
which this subject presents.

§ 5. Pteropoda.

In the Mollusca belonging to the two orders which have
now passed under our review, namely, the Jlcephala and
Gasteropoda, the mantle, while it folds over the principal
viscera of the body, leaves apertures for the admission of
water to the gills, or organs of respiration. But there exist
a few genera having the sac formed by the mantle closed
on every side; a structure which renders it necessary to
adopt a different arrangement with regard to the gills, and
to place them externally, and we then find them spreading
out like a pair of wings, on each side of the neck. Since
this general closing of the mantle precludes, also, the for-
mation of any organ of progressive motion corresponding
to a foot, advantage is taken of the projection of the gills to
Vol. I. 24

186 THE MECHANICAt FUNCTIONS.

employ them as oars for the purpose of enabling the animal
to swim through the water. '

Mollusca of this description are found in great abundance
in the colder regions of the ocean surrounding both the
Y2Q north and south poles; and other species

are also met with, though in smaller num-
bers in the tropical seas. The Clio borea-
lis, of which Fig. 120 is a representation,
is the most perfect specimen of this form
of construction. It swarms in the Arctic
seaSj and constitutes the princij^al 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 suggested the
title of Pterojjoda, given by Cuvier to this order of Mol-
lusca.

§ 6. Cej)halopodat

Following the progress of organic development, we now
arrive at a highly interesting family of Mollusca, denomi-
nated the Cephalopoda, and distinguished above all the pre-
ceding orders by being endowed with a much more elabo-
rate organization, and a far wider range of faculties. The
Cephalopoda 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, surround the
opening of the mouth. (See Fig. 121.) These feet, or arms,
or tentacula, if we choose so to call them, are long, slender,
and flexible processes exceedingly irritable, and contractile,
in every part, and provided with numerous muscles, which
are capable of moving and twisting them in all directions with
extraordinary quickness and precision. They are thus ca-
pable of being employed as instruments, not only of progres-
sive motion, but also of preliension. For this latter pur-
pose they are in many species peculiarly well adapted, be-
cause being perfectly flexible as well as highly muscular,

MOLLUSCA CEPHALOPODA.

187

they twine with ease round an object of any shape, and grasp
it with prodigious force. In addition to these properties
they derive a remarkable power of adhesion to the surfaces

of bodies from their being furnished with numerous suckers
all along their inner sides. Each of these suckers, as shown
separately in Fig. 122, is usually supported on a narrow
neck, or pedicle, and strengthened at its circumference by
a ring of cartilage. Their internal mechanism is more ar-
tificial than the simple construction already described, (p.

106;) for when the surface of the disk is fully expanded, as
shown in Fig. 123* b, we find that it is formed of a great
number of long slender pieces, resenibling teeth closely set
together, and extending from the inner margin of the cartila-
^^, ginous 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 in-
terior portion, capable of being detached from the flat cir-
cle of teeth, when the sucker is in action, and of leaving an

188 THE MECHANICAL FUNCTIONS.

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 mar-
gin of which appears like a row of teeth. It is evident that
by this mechanism, which combines the properties of an ac-
curate valve, with an extensive cavity for producing rare-
faction, or the tendency to a vacuum, the power of adhesion
is considerably augmented.*

So great is the force with which the tentacula of the cut-
tle-fish adhere to bodies by means of this apparatus, that
while their muscular fibres continue contracted, it is easier
to tear away the substance of the limb, than to release it from
its attachments. Even in the dead animal I have found that
the suckers retain considerable power of adhesion to any
smooth surface to which they may be applied.

Our attention must first be directed to the remarkable fa-
mily of Sepias, which comprehends three principal genera,
namely the Octopus, the Loligo, or Calamary, (depicted in
Fig. 121,) and the common Sepia, or Cuttle-fish. The first
of these, the Octopus, which was the animal denominated
Polypus by Aristotle, has eight arms of equal length, and
contains in its interior two very small rudimental shells,
formed by the inner surface of the mantle. This shell be-
comes much more distinct in the Loligo, where it is carti-
laginous, 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 ciittlc-Jish bone. Its structure is ex-
tremely curious; and deserves particular attention, as estab-
lishing the universality of the principles which regulate the
formation of shells, whether internal or external, and from

* The description I have here given is the result of my own examination
of a large Octopus, which I had lately an opportunity of dissecting: and the
annexed ^gures 123, » a, e, c, are copied from drawings I made on that oc-
casion. A represents the sucker in its usual form when not in action: b
shows the sucking surface fully expanded: and c is a section of the whole,
which had become somewhat flattened by the operation of dividing it.

♦ . MOLLUSC A CEPHALOPODA. 189

which structures dlfTering much in their outward appearance
may result. It is composed of an immense number of tliln
calcareous plates, arranged parallel to one another and con-
nected by thousands of minute hollow pillars of the same
calcareous material, passing perpendicularly between the ad-
jacent surfaces. This shell is not adherent to any internal
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 me-
chanical support to the soft substance of the body, and espe-
cially to the surrounding muscular flesh; and thus probably
contributes to the high energy which the animal displays
in all its movements. It has been regarded as an internal
skeleton; but it certainly has no pretensions to such a desig-
nation; for, although enveloped by the mantle, it is still
formed by that organ; and the material of which it is com-
posed 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, alto-
gether from bony structures, which are composed of a dif-
ferent kind of material, and formed on principles of growth
totally dissimilar.*

Besides tentacula, the Sepia is also furnished with a pair
of fleshy fins, extending along the two sides of the body.
The Loligo has similar organs of a smaller size, and situated
only at the extremity of the body which is opposite to the
head. They have been regarded as the rudiments of true
fms, which are organs, developed in fishes, and which are
supported by slender bones, called rays; but no structure of
this kind exists in the fins of the Cephalopoda.

In swimming, the organs principally employed by cuttle-

* Some analogies have, indeed, been attempted to be traced between the
cartilag-lnous himina of the Loligo, and the spinal column of the lowest order
of cartilaginous fishes: these I shall have occasion to point out in the sequel.
Solid cartilaginous structures also exist in the interior of the body of the ce-
phalopoda, which are considered by some natiu-idists as indicating an approach
to the formation of an internal skeleton, analogous to that of vcrtcbratcd ani-
mals.

190 THE MECHANICAL FUNCTIONS.

fish for giving an effective impulse to the water, are the ten-
tacula. 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 pro-
gress, 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 ne-
cessity of this position for the feet arises probably from the
close investment of the mantle over the body; for although
the mantle leaves an aperture in the neck for the entrance of
water to the respiratory organs, yet, in other respects, it
forms a sac, closed in every part, except where the head,
neck, and accompanying tentacula protrude.

In the Calamary, as well as in the common Sepia, two of
the arms are much longer than the rest, and terminate in a
thick cylindrical portion covered with numerous suckers,
which may not unaptly be compared to a hand. These pro-
cesses are employed by cuttle-fish as anchors for the purpose
of fixing themselves firmly to rocks, during violent agitations
of the sea; and accordingly we find that it is only the ex-
tremities 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 are, like the
Sepise, provided with tentacula attached to the head. They
comprehend animals differing exceedingly in their size:
some being very large, but a great number very minute, and
even microscopic* The shells of these animals are often
found to contain partitions dividing them into a number of
chambers; hence they have been termed ccnneraied, or mul-
tilocidar^ ov polythalamous shells. The Spirula (Fig. 124)
is a shell of this description, of which the cellular structure
and numerous partitions are rendered visible by making a

* A particular account has been given of the shells of these microscopic
cephalopoda by M. D'Orbig-ny, in the Annales des Sciences Naturelles; vii.
96.

MOLLUSCA CEPHALOPODA.

irn

section through it: (Fig. 125.) Some, however, as the Jlr-
gonaut, or Paper Nautilus, have shells undivided hy par-
titions; and are accordingly termed unilocular or mono-
thalamous. The shell of the Argonaut is exceedingly thin,
iiand almost pellucid, probahly for the sake of lightness, for
it is intended to be used as a boat. For the purpose of ena-
bling the animal to avail itself of the impulses of the air,
while it is thus floating on the waters, nature has furnished

126

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 forw^ards 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
tentacula as oars on either side, to direct, as w^ell as acce-
lerate its motion. No sooner does the breeze freshen, and
the sea become ruffled, than the animal hastens to take down
its sail, and quickly withdrawing its tentacula within its
shell, renders itself specifically heavier than the water, and
sinks immediately into more tranquil regions beneath the
surface.*

The common Nautilus, which is provided with a similar
sailing apparatus, is an inhabitant of a polythalamous shell,
(Fig. 126,) of which Fig. 127 represents the section. The
formation of this, as well as of other shells of this descrip-
tion, presents very curious phenomena. The animal at cer-
tain periods of its growth, finding itself cramped in the nar-

* It must be confessed, however, that the habits of the Arg-onaut are still
ver)^ imperfectly known. Considerable doubts are entertained whether the
shell it inhabits is formed by the animal itself, or whether it is the production
of some other, but unknown species of Mollusca, and is merely taken pos-
session of by the Arg-onaut as a convenient habitation, which it can quit and
enter again at pleasure.

192 THE MECHANICAL FUNCTIONS.

row part of the spire, draws up that portion of the mantle
which occupied it, thus leaving a vacant space. The sur-
face of the mantle which has receded, immediately begins
to secrete calcareous matter, which is deposited in the form
of a partition, stretching completely across the area of the
cavity. As the animal proceeds to increase in size, and to
occupy a wider portion of the external shell, the same ne-
cessity soon recurs, and the same expedient is again resort-
ed to. It withdraws its mantle from the narrower into the
wider part of the sheH; and then forms a second partition,
at a little distance from the first, corresponding to the space
left by the receding of the mantle. This process is repeat-
ed at regular intervals, and produces the multitude of cham-
bers contained in polythalamous shells, of which the living
animal occupies only the largest, or that which continues
open.* The partitions are in general perforated either in the
centre or at one side, for the purpose of giving passage to a
ligament, which preserves the attachment of the mantle to
the apex of the shell. This ligament is often surrounded
either entirely or partially by shell, which forms a tube,
denominated the syphon: and portions of which are seen in
the section Fig. 127.

* This structure is extremely prevalent in fossil shells: some of which are
spiral, such as the Cornu Ammonis, while others are straight cones, such as
the Bacculite and Orthoceratite. In most of tliese the partitions are very nu-
merous, and have undulating- surfaces.

( 193 )

0

CHAPTER IV.

ARTICULATA,

§ 1. *drticulatad Animals in general.

From the Cephalopoda, the transition is easy to the low-
est order of vertebrated animals. But previously to pur-
suing the analogies which connect these two divisions of the
animal kingdom, we have to pass in review a ^^vj exten-
sive series of animal forms, constructed upon a peculiar sys-
tem, and occupying, 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 arc
distinguished from the rest by a more elaborate conforma-
tion of organs, the powers of progressive motion are always
extremely limited. Nor are the JNIollusca in general more
highly favoured with respect to the degree in which they en-
joy 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 ca])acities 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 definite model
in the formation of the different parts of the system, she has
nowhere displayed more boundless variety in the combi-
nations of the forms whicli she has impressed upon the
mechanical instruments, both of prehension and of progres-
sion.

All the tribes of Zoophytes, and by far the greater num-
VoL. I. 25

194 THE MECHANICAL J'UNCTIONiJ.

•

ber of Mollusca, are limited by the constitution of tbeir
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 de-
velopment of the nascent structures. A farther design, also,
soon becomes manifest; and instruments are given for ele-
vating the body above tlie ground, and for traversing with
rapidit}^ the light and scarcely resisting atmosphere. This
prospective design may be traced in the whole system of in-
sects; every part of which is framed with reference to the
properties of the medium through which these movements
are to be performed.

§ 2. Annelida,

The lowest division of articulated animals comprehends
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 ex-
ample. In the series of structures which constitute this di-
vision of the animal kingdom, we may trace remarkable gra-
dations of development, through which nature appears to
pass in attaining the higher and more perfect conformations.

It may be remarked that, in effecting the transition from
Zoophytes to the new model of construction here presented,
nature seems to have wholly abandoned that radiated dispo-
sition of parts, and those star-like, forms, so characteristic
of the beings which are placed on the confines of the ani-
mal kingdom, and which still retain an analogy with vege-
table structures. She now adopts a more regular law of
symmetry; by which all the parts are referrible to one lon-
gitudinal axis, and also to a vertical plane passing through
that axis, and which has been termed the mesial plane. 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.

ANNELIDA.

195

While in the Star-fish, and Ecliinus, nothing; in point of si-
tuation was delinite, excepting tlie upper and the lower sur-
face, and there was no side which could be exclusively de-
nominated either the right or the left side, and no end that
could be properly said to be the front or the back, in Ar-
ticulated as well as in Vertebrated animals, all these distinc-
tions are clearly marked and easily defined.

In all the Jlnnelida 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 frame-work, composed of a series of horny bands
or rings: their assemblage having more orless of a lengthened
cylindric shape, and constituting a kind of external skeleton,
which encloses all the other organs. This is exemplified in
the earth-worm; in the Pont-obdella, (Fig. 128,) which is a
species of leech; and in the Nereis, (Fig. 120.) These rings

give rise to the division of the body into as many difierent
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 Erpohdella, of which Fig. 130 is an enlarged
representation.

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

196 THE MECHANICAL FUNCTIONS.

The rings which encircle the body are connected lateral-
ly 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 ano-
ther, and have seldom any tendinous attachments; being ge-
nerally inserted directly on the parts they are destined to
move. Thus, the adjacent margins of the rings of worms,
(as shown in the diagram. Fig. 131,) are connected together
by these muscular bands, which pass 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 must,
when contracting, bring the rings nearer to one another at
that lower part; and when the whole series occupying that
situation are exerted in concert, they will raise the body in
the form of an arch. An opposite curvature will be pro-
duced by the contraction of the uj^per bands; whereby the
back will be bent downwards, and both ends of the body
raised. In proportion as the two bands, situated on each side,
act in concert, while the others are relaxed, the body will
be bent laterally towards that side. When all the four mus-
cular bands contract 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 wrink-
led and swelled out between the rings.

Other muscular bands, attached to the rings, pass from the
one to the other in more 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 partial-
ly or generally exerted.

ANNELIDA. 197

The skin on the surface of the earth-worm is furnished
at the parts where it covers the rings, witli very minute bris-
tles, called Setx, by means of which the animal is enabled
to fix those parts on the ground, while the other portions of its
body are in motion. Both in the anterior and posterior seg-
ments, these hairs are directed towards the centre of the ani-
mal; while those on the middle segments are perpendicular.*
We almost constantly find, in animals belonging to the or-
der of Annelida, some provision of this kind. Often it con-
sists of tufts of hair regularly disposed in rows on each side
of the under surface. In the Nei^eis (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 pre-
vent the animal from running against any thing by which it
might be injured. They also raise the body from the ground;
for which purpose, as they are used under water, very little
support is necessary.! 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 as-
sume more the nature of feet, of which they exercise durinc;
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 foremost segments of the body,
while the others cling to the earth by means of tlie 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 tiicir longi-
tudinal muscles; in doing which, equal portions of tlie suc-

* As an instance of the extraordinary multiplicity of species existing in
every department of living- nature, I may here notice, that of the common
earth-worm, ajiparcntly so uniform in its shape, Savig-ny has lately, by a
closer examination, been able to disting-uish no less than twenty-two diflercnt
species amoni^ those found in the neighbourhood of Paris alone.

f Home; Lectures, &c. Vol i. p. 115.

198

THE MECHANICAL FUNCTIONS.

ceeding segments are necessarily elongated: these are next
contracted; and so on, in succession, till the whole is brought
forwards to the head: after which the same series of actions
is repeated, beginning with the advance of the head. Worms
often reverse this motion, and are thus enabled to move back-
wards, or with the tail foremost.*

Great variety exists in the forms of the animals referrible
to the type of Annelida. The Gordius, or hair-worm, (Fig.
] 32,) is that which exhibits the greatest development in
length compared with the breadth of the body. It has the
form of a very long and slender thread: the annular structure
being indicated only by very slight transverse folds of the
integuments. No external members, nor even tentacula,
have^been given to this simplest of vermiform animals.

135

Many of the animals of this class being soft and defence-
less, are obliged to consult their safety by retreating into
holes and recesses, or by burrowing in the sand or mud.
One genus only, the Serpula (Fig. 133,) forms for itself an
external shell, which is shaped into a spiral tube. Others,
as the Sabella and the Terebella, accomplish the same ob-
ject 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 defen-
sive covering, like a coat of mail. Fig. 134 shows this
rude architecture in the Terehella conchilega. These co-
verings, however, composed as they are of extraneous ma-

* See Home? Lectures on Coipparalive Anatomy, Vol. i. p. 114,

ANNELIDA. l.QO

terlals, and not being orjranin produnllons of Ihr animals
themselves, are structures wholly foreign to their systems.
These inhabitants of tubes, the Tubicolae of Cuvier, are gene-
rally 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
Lumbricns mariniis oi lumn^us {Jirenicola j)isc(tioriim 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 rin^-s
of its head, which, when elongated, has the shape of a re-
gular cone. As each ring is so much smaller than the one
behind it as to admit of being received within it, the whole
head, when completely retracted, presents a flat surface.
When this disk is applied to the sand, the animal, by gradu-
ally projecting the cone, and successively dilating the rings
of which it is composed, opens for itself a passage through the
sand, and then secures the sides of the passage from falling in
by applying to them a glutinous cement, which exudes from
its skin, and which unites the 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 advances: so that the passage is kept per-
vious 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 Terebellx conchilegx, belonging to a tribe of marine
worms, which from the peculiar circumstances of their situa-
tion, 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 tiic sands in storms, these animals are

♦ See the account given by Mr. Osier, Philosophical Ti-ansaclions fov
1826, p. 342.

200 THE MECHANICAL FUNCTIONS.

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 di-
rection, 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 an-
terior and lower part of the head, where they are immediate-
ly cemented by the glutinous matter which exudes from that
part of the surface. Bending the head alternately from side
to side, while it continues to apply the materials of its tube,
the terebella has very soon formed a complete collar, which
it sedulously employs itself to lengthen at every part of the
circumference with an activity and perseverance highly in-
teresting. For the purpose of fixing the different fragments
compactly, it presses them into their places with the erected
scales, at the same time retracting the 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 poste-
rior rings, effects a slight extension of the head, which thus
slowly makes its way through the mass before it, availing
itself of the materials which it meets with in its course, and
so continues to advance till the whole tube is completed.
After this has been accomplished, the animal turns itself
within the tube, so that its head is next to the surface, ready
to receive the water which brings it food, and is instrumental
in its respiration. In 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 considerabl}^ slower.

ANNELIDA. 201

Tentacula of various kinds arc also met witli in several of
the more active and vivacious kinds of Annelida, such as the
Nereis (Fig. 129,) proceeding from the margin of the mouth
and other parts of the head. This animal swims with great
facility by rapid, undulating inflections of its body; and by
practising a similar succession of movements in the loose sand
at the bottom of the water, it quickly buries itself, and even
travels to considerable distances through the sand, first ex-
tending the anterior rings, and then bringing up the poste-
rior 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 Jimphilrite^ 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 effecting progressive
motion.

We find the same object accomplished by very different
means in other animals of this class. The leech, for instance,
having the rings which encircle its body very numerous and
close to each other, could not well have advanced by the or-
dinary modes of vermiform progression. As a substitute,
accordingly, it has been furnished with an apparatus for suc-
tion at the two extremities of the body, which are formed
into disks for that purpose. By fixing alternately the one
and the other, and contracting or elongating the body as the
occasion requires, the leech can move at pleasure either for.
wards or backwards. Thus, while the tail is fixed, the head
may be advanced by lengthening the whole body, and when
the head is fixed, the hinder sucker can be brought forwards
by the contraction of the body, and applied to the ground
near to the head, and preparation may thus be made for
taking another step.

Most of the parasitic animals which inhabit the interior

• Osier, Phil. Trans. 1826, p. 342.
Vol. I. 26

202 . THE MECHANICAL FUNCTIONS.

cavities of the body, and especially the alimentary canal,
correspond in external form, as well as in many circum-
stances of internal conformation, to the Annelida. They
compose an order denominated the Entozoa.

§ 3. Arachnida.

In passing from the Annelida to the Arachnida^ an order
which comprehends all the species of spiders, together with
animals allied to them in conformation, we find that a conside-
rable advance has been made in the progress of development.
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 frame-work of the
Annelida, are here consolidated so as to form two principal di-
visions of the body, the one in front, termed the Cephalo-tho-
rax, which contains the organs of sensation, and of mastica-
tion, and also the principal reservoir of circulating fluids;
the other, which is behind, and contains the organs of diges-
tion, is termed the abdomen. In the spider (Fig. 136, where

c is the cephalo-thorax, and a
the abdomen) these two por-
tions of the body are separated
by a deep groove, which leaves
only a slender pedicle, or tube of
communication between them.
There are usually in the male
four pairs of legs, constantly articulated with the cephalo-tho-
rax; but the female is furnished with an additional pair to ena-
ble her to carry her eggs. For the purpose of obtaining an ex-
tensive 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

ARACHNIDA. 203

the haunches, and the succeeding 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 prehension, and of offence. Through the larger
branches of these a canal passes, which opens near the point,
and conducts a poisonous fluid into the wounds inflicted by
this formidable weapon.

In common with all articulated animals, spiders, in the
progress of their growth, cast off* their outer skin several
times, and at regular periods. In the earlier stages of their
existence, although they have the general form of the ma-
ture insect, yet they have a smaller number of legs: the last
pair not making their appearance till after the spider has at-
tained a certain size. We may here trace the commence-
ment of that system of metamorphosis, w^hich, as we shall af-
terwards find, is carried to so great a length in winged insects.

Spiders are endowed with extensive powers of progres-
sive 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 swiftness, and to
spring from a considerable distance on their pre}'; powers
which were necessary to those tribes that live altogether by
the chase. The greater number of species, however, as is
well known, are provided with a curious apparatus for
spinning threads, and for constructing webs to entangle flies
and other small insects. Every species of spider weaves
its web in a manner peculiar to itself: and, besides the prin-
cipal web, they often construct in the neighbourhood a
smaller one, in the form of a cell, in which they conceal
themselves, and lie in ambush for their prey. Between
this cell and the principal web they extend a thread of com-
munication, and by the vibrations into which this thread is
thrown, on the contact of any solid body, the spider is im-

204 THE MECHANICAL FUNCTIONS.

mediately acquainted with the event, and passes quickly to
the spot, by the assistance of the same thread.

Some species have the power of conveying themselves to
considerable distances through the air by means of threads
which they dart out, and which are borne onwards by the
wind, while the spider is clinging to the end of the thread
^which is next to it. In this manner these spiders are often
carried up to a great height in the air: and it has been sup-
posed that during their flight they often seize upon gnats and
other flies; because the mutilated remains of these insects are
often seen adhering to the threads: this point, however, is
still open to much doubt.

The Natural History of the spider is in many points of
view^, highly interesting, not only from the great extent to
which the organic development is carried, and the energy
with which all the functions of animal life are performed;
but also with reference to the wonderful instincts displayed
in the construction of its w^eb, in the surprise and destruc-
tion 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 development of or-
ganization with reference to the organs of progressive mo-
tion.

§ 4. Crustacea.

The plan which Nature appears to have commenced in
the construction of the Arachnida, is farther pursued in that
of the Crustacea. The portions into which the external
frame-work of the body was divided in the former, are still
farther consolidated in the latter: they are composed of
denser materials, and endowed with greater rigidity; thus
not only offering more resistance to external forces, but also
giving a firm.er purchase to the muscles which are the moving
powers. The lirnbs, as well as the whole body, are incased
in tubes of solid carbonate of lime: they are articulated with

CRUSTACEA.

205

great care, and almost always compose hinge joints. The
muscles, by which these solid levers arc moved, are lodged
in the interior, and their fd^res 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 intothe interior of the up-
per portion of the limb, being themselves immoveably con-
nected with the lower portion. By this expedient, not only
is the employment of a tendon dispensed with, but a larger
surface is presented for the attachment of the muscles, which
by acting also upon a longer lever, obtain great mechanical
advantage. It would be superfluous to occupy more time in
explaining the minutiae of structure in these joints, because
the simple inspection of the limbs of a crab or lobster will
give clearer ideas of this mechanism than can be conveyed
by any laboured description. We must content ourselves
with 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 ii, (Fig. 137,)

next the trunk, is termed the haunch, to which is united
the trochanter, t; after which comes in succession the fe-
mur or thigh, f; two portions of the leg, l; and the tarsus
p. The haunch is usually short, being interposed merely as
a base for increasing the extent of motion of the pieces
which follow; and sometimes it is itself composed of more

206 THB MECHANICAL FUNCTIONS.

than one piece. The leg is usually divided into two pieces,
by a joint. The tarsus is terminated by a single or double
hook, and sometimes by a pincer, or claw.

New organs, not met with among the Arachnida, are here
for the first time developed, namely, the Antennse^ of which
there is one on each side of the head. They are denomi-
nated, 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
of a great number of pieces articulated together: and they
are infinitel}^ 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, pre-
sent a great complication of structure; and many of these
parts are employed in various uses besides those of mastica-
tion; such as the seizing of objects, and turning them in va-
rious ways for examination; and, according to their suita-
bleness as articles of food, conveying them into the mouth.
These organs are called the Palpi, and sometimes ih^ false
feet. They always exist in pairs, and take their rise from
the lower lip, or some adjacent part of the head. The por-
tions 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 some-
times the foremost pairs of palpi are shaped more like jaws,
and actually perform the office proper to jaws, of compress-
ing 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 in-
termediate form between that of mandibles and of false feet,
so that it is often difficult, amidst these gradual transitions
of structure, to decide to which of these two kinds of organs
a specimen we meet with properly belongs. It is apparent-
ly with a view to evade this difficulty that a term has been
invented which shall include them all, namely, that oi feet-
jaws. These transitions are illustrated by the annexed
figures of several of these members in the Mysis Fabricii;

CRUSTACEA. 097

Fig. 138, being that of a mandible, with its feeler, or palpus;
Figures 139, 140, and 141, representing the fust, second,
and third pairs of feet-jaws; and Fig. 142, the first pair of
true feet. It would thus seem as if the same constituent ele-
ment 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 analogous Crus-
tacea, the foremost pair of true feet are also modified to suit
a particular purpose; the pincers which terminate them being
expanded into a claw, and constituting a powerful oro-an of
prehension, and a formidable weapon of offence. It resem-
bles a finger and thumb in its power of grasping and strongly
compressing any object on which it seizes; and, to enable it
to do this with more effect, the inner edges of both parts of
the claw are notched or serrated.

The large portion of shell which is 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 scarcclv
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 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 m.echanism, the margins of each
portion overlapping the succeeding segment, and partly en-
closing it. The tail is the principal agent used in swim-
ming, and the whole force of the muscles is bestowed upon
its movements. As it strikes the water from behind for-

• The labours ofSavigiiy, Audouin 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 at-
tached to the head, in all the articulated animals, whether belonging to the
closes of arachnlda, crustacca, myriapoda, or winged insects.

20S THE MECHANICAL FUNCTIONS.

wards, the lobster can only swim backwards; and it is as-
sisted in this action by five pair of false feet, which are at-
tached to the under side of the body, behind the true feet,
and w^hich terminate in a fin-shaped expansion, giving them
the effect of oars. The extremity of the tail is still more
expressly formed for giving effect to the stroke, being ter-
minated by a number of fiat scales, which, when expanded,
present a broad surface to the water.

The calcareous coverings of these Crustacea are analogous
to shell both in structure and composition. They contain,
however, some phosphate of lime, in addition to the carbo-
nate. The calcareous particles are deposited on a membrane
of considerable firmness; 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 ofi', and ex-
changed for a new shell of larger dimensions.

The process by w^hich this periodical casting and renewal
of the shell are effected, has been very satisfactorily investi-
gated by Reaumur. The tendency in the body and in the
limbs to expand during growth is restrained by the limited
dimensions of the shell, which resists the efforts to enlarge
its diameter. But this force of expansion goes on increasing,
till at length it is productive of much uneasiness to the ani-
mal, which is, in consequence, prompted to make a violent
effort to relieve itself; by this means it generally succeeds
in bursting the shell; and then, by dint of repeated struggles,
extricates its body and its limbs. The lobster first with-
draws its claws, and then its feet, as if it were pulling them
out of a pair of boots: the head next throws off its case, to-
gether with its antennas; 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

CRUSTACEA. 209

is not accomplished without long continued cfTorts. Some-
times the legs are lacerated or torn off, in the attempt to
withdraw them from the shell; and in the younger Crustacea
the operation is not unfrequently fatal. Even when success-
fully accom])lished, 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 altoge-
ther without defence. For some time before the old shell
was cast off, preparations had been making for forming a
new one. The membrane which lined the shell had been
acquiring greater density, and had already collected a quan-
tity of liquid materials proper for the consolidation of the
new shell. These materials are mixed with a large propor-
tion of colouring matter, of a bright scarlet hue, giving it
the appearance of red blood, thougli 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 sud-
denly 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 increase the strength of
the solid walls which are thus constructing for the support
of the animal. Reaumur estimates that the lobster gains,
during each change of its covering, an 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 ani-
mal, for the supply of the large quantity of calcareous mat-
ter required for the construction of the shell at the proper
time. A magazine of carbonate of lime is collected, pre-
vious to each change of shell, in the form of two rounded
masses, one on each side of the stomach. In the crab
these balls have received the absurd name of crab's eyes; and
during the formation of the shell they disap])car.

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

Vol. I. 27

210 THE MECHANICAL FUNCTIONS.

time replaced by a new claw, which grows from the stump
of the one which had been lost. It appears from the inves-
tigations of Reaumur, that this new growth takes place more
readily at particular parts of the limb, and especially at the
joints; and the animal seems to be aware of the greater fa-
cility with which a renewal of the claw can be effected at
these parts; for if it chance to receive an injury at the ex-
tremity of the limb, it often, by a spontaneous effort, breaks
off the whole limb at its junction with the trunk, which is
the point where the growth more speedily commences. The
wound soon becomes covered with a delicate white mem-
brane, which presents at first a convex surface: this gradu-
allv rises to a point, and is found on examination to conceal
the rudiment of a new claw. At first this new claw en-
larges but slowly, as if collecting strength for the more vigo-
rous effort of expansion which afterwards takes place. As it
o-rows, the membrane is pushed forwards, becoming thin-
ner in proportion as it is stretched; till at length it gives
wMy, 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 circumstance which accounts
for our frequently meeting with lobsters and crabs wdiich
have one claw much smaller than the other. In the course
of the subsequent castings, this disparity gradually disap-
pears. The same power of restoration is found to reside in
the legs, the antennse, and the jaws.

We must naturally be curious to learn, if possible, from
what source these astonishing powers of regeneration are de-
rived. Reaumur hazarded the conjecture, that there might be
orio^inally implanted in each articulation a certain number of
embryo limbs, ready to be developed as occasion might re-
quire; somewhat in the way in which the rudiments of the
secondary teeth remain concealed in the jaw, in preparation
for replacing the first set when these have been removed.
But this hypothesis is overturned by the fact that if the ani-
mal loses only part of the limb, it is the deficient portion
alone, and not the whole limb that is regenerated. The

CRUSTACEA. 211

sprouting of. the new claw bears a strong analogy to the
budding of a plant; both having their origin from an imper-
ceptible atom, or germ, which is either formed on the oc-
casion, or had pre-existed in the organization. We are,
however, totally destitute of the means of deciding which
of these alternatives is nearest to the truth. It is but too
probable that the agents which can effect such wonderful
operations will ever baffle our most scrutinizing inquiries,
and that they are of too refined an order to come within
the reach of the most subtle conjectures that human imagi-
nation can devise.

( 212 )

CHAPTER V.

INSECTS.

§ 1. Apt era.

Apterous, or wingless insects form the next term in the
series of articulated animals. Closely allied in their organi-
zation to many of the preceding families, they differ from
them in being essentially 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 oi JJcari, or mites, of Pediculi, or lice, oi Ricini, or
ticks, of Pulices^ or fleas: together with the Podura, or
spring-tail; the Lepisma, and the family of Myriopoda, or
millepedes, are comprehended in this order. I shall be
obliged to pass over these tribes very cursorily, noticing
only a few of the more remarkable circumstances attending
their mechanical conformation.

The Pulex is the only apterous insect that undergoes com-
plete metaphorphoses in the course of its development. In
the first stage of its existence, it has the form of a long
worm, without feet, frequently rolling itself into a spiral
coil. It consists of thirteen segments, having tufts of hair
growing upon each. In its mature state, it has six articu-
lated 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 af-
ford a striking proof of the exquisite mechanism j^ervading
even the lowest orders of the animal creation.

The Podiira leaps into the air by a mechanical contri-
vance of another kind; employing for this purpose the tail,
which is very long, and forked at the end. In its ordinary

APTERA. 213

state this organ is kept folded under the abdomen, where it
is concealed in a groove. The pieces of whicli it is com-
posed 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 in-
sect to leap from the surface of the water, an action which it
performs with apparently as much ease as if it sprung from
a solid resisting plane.

The Lapisma leaps by means of moveable appendages,
placed in a double row along the under side of the body, and
acting like springs. There are eight pairs of these members,
corresponding in situation and structure to the false feet of
the Crustacea, and, like them, terminating in jointed fda-
ments.

The Juhis and the Scolopendra, which compose the fa-
mily of the Myriapoda, so called from the immense num-
ber of their feet, undergo, to a certain extent, a kind of me-
taphorphosis in the progress of their development. When
first hatched they have often no feet whatever, and resem-
ble 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 segments, with their accompanying
legs. The Jidus ierrestris, for example, (Fig. 143) has, at
^ its entrance into the world, only

jiii0^(4^^^^^j^^^^^^ eight segments and six feet; but
^^^^^Hll!lliP\ acquires in the course of its deve-
lopment, 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 crusta-
ceous animals.

214

THE MECHANICAL FUNCTIONS.

§ 2. Insect a alata.

Our attention Is now to be directed to the more highly
developed Insects, which have been formed with a view to
progression through the air. On these, which compose the
most 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 injurious 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 viscera, and furnishing extensive surfaces of attachment
to the muscles, from the action of which all the varied move-
ments of the system are derived.

The muscular system of perfect insects is exceedingly
complex. Lyonet has described and delineated an immense
number of muscular bands in the caterpillar of the Cossus,
and the plates he has given have been copied in a variety
of books in illustration of this part of the structure of in-
sects. The recent work of Straus Durckheim affords an

equally striking example of admirable arrangement in the
muscles of the Melolontha vulgaris, or cockchaffer, the ana-

WINGED INSECTS. 215

tomy of which has l)ccn minutely invcstigntcd l)y that dis-
tinguished entomologist. These muscles are represented in
Fig. 144, which has heen carefully reduced from his beauti-
fully executed plates. The largest mass of muscular fibres
is that marked a, which depress the wings, and are of enor-
mous size and strength.

On examining the different structures which compose the
solid frame-work of insects, we find them conforming in every
instance to the general type of annulose animals, inasmuch
as they consist of thickened portions of integument, encir-
cling the body; but variously united and consolidated, for
the manifest purpose of obtaining greater mechanical strength
and elasticity than if they had remained detached pieces,
joined only by membranous connexions. A long flexible
body, such as that possessed by the Myriapoda, could not
easily have been transported through the air; for every bend
would have created a resistance, and have impeded its ad-
vance 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 simple and compact. The segments des-
tined to support tiie wings have been expanded for the pur-
pose of lodging the powerful muscles that are to move them;
and rendered dense and unyielding in order to sup])ort their
action.

Nature has farther provided insects with instruments
adapted to different kinds of external actions. They consist
of articulated levers, variouslv combined together, and form-
ing legs, claws, pincers, oars, palpi, and, lastly, wings, cal-
culated for executing every variety of prehension, of pro-
gression, or whatever other action their wants and necessities
require.

§ 3. Development of Insects.

It would appear as if the final accomplishment of objects
so numerous, so widely difierent, and so liable to mutual in-
terference, could be attained only by the animal being sub-

216 THE MECHANICAL FUNCTIONS.

jected to a long series of modifications, and passing through
many intermediate stages of development. The power of
flight is never conferred upon the insect in the earlier peri-
ods of its existence: for before its structure can obtain the
lightness which fits it for rising in the air, and before it can
acquire instruments capable of acting upon so light an ele-
ment, it has to go through several preparatory changes, some
of which are so considerable as to justify the term oi meta-
Tnoiyhoses, which has been generally given to them."^ But
transient is the state of perfection in every thing that re-
lates to animal existence. When the insect has by a slow
development 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 existence.

The history of the successive stages of development of in-
sects opens a highly interesting field of philosophical inqui-
ry. For a certain period of the early life of these animals,
the growth of all the parts appears to proceed equably and
uniformly: but at subsequent epochs, some parts acquire a
great and sudden increase of size, and others that were in a
rudimental condition become highly developed, and consti-
tute 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 particu-
larly remarkable, as illustrating the principles on which the
development is conducted. The researches of modern en-
tomologists have led to the conclusion that the frame-work,
or skeleton of insects, is always formed by the union of a
certain determinate number of parts, or elements, originally
distinct from one another, but v/hich are variously joined
and soldered together in the progress of growth: frequently
exhibiting a great disproportion in the comparative expan-
sion of different parts. The enlargement of any one part,
however, exercises a certain influence on all the neighbour-

* 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 Lemsea among parasitic worms.

DEVELOPMENT OP INSECTS.

o

17

ing parts, and thus are the foundations laid of all the endless
diversities which characterize the several species belonging
to each tribe and family.

In the progress of development, we may recognise two
principles, which, though apparently opposite to each other,
concur and harmonize in their operation: these are expaii-
sion and concentration. Thus, while those segments of body
which follow the head are greatly enlarged, in order to sup-
port the more recently developed organs of progressive mo-
tion, they are also more consolidated, and rendered stronger
by the union of several pieces which were before separate.
The hinder 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, com-
pose the abdomen, or hinder division of the insect.

The progress of the metamorphoses of insects is most
strikingly displayed in the history of the Lepidopterous, or
butterfly and moth tribe.* The 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 appearance, and the mechani-
cal structure, of a worm. The same elongated cylindric

* The four periods of the existence of the Bomhyx mor'i, or the moth of
the silk-worm, are shown in the annexed cng-raviiigs: Fig-. 145 are tlie ci,-gs;
Fig. 146, the Larva, or caterpillar; Fig-. 147, the Pupa, or chr^sidis; and
Fig. 148, the I?nagOy or perfect insect.

Vol. I. 28

218 THE MECHANICAL FUNCTIONS.

shape, the same annular structure of the denser parts of Its
integument, the same arrangements of longitudinal and ob-
lique muscles connecting these rings, the same apparatus of
short feet, with claws, or bristles, or tufts of hairs, for faci-
litating progression; in short, all the circumstances most
characteristic of the vermiformx type are equally exemplified
in the different tribes of caterpillars, as in the proper An-
nelida.

But these vermiform insects have this peculiarity, that
they contain in their interior the rudiments of all the or-
gans of the perfect insect. These organs, however, are con-
cealed from view by a great number of membranous cover-
ings, which successively invest one another, like the coats of
an onion, and are thrown off, one after another, as the inter-
nal parts are gradually developed. These external invest-
ments, 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 development is rendered necessary
in consequence of the greater compactness of the integu-
ments of insects, as compared with those of the annelida. In
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 di-
latation of the internal organs, and must be thrown off to
make way for the farther growth of the insect. In the
mean time a new skin has been preparing underneath,
moulded on a larger model, and admitting of greater exten-
sion 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 development: un-
til, 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

DEVELOPMENT OF INSECTS. 219

Steps in preparation for a more important change. A time
comes when the whole of the coverings of the hody are at
once cast off, and the insect assumes the form of :i pupa or
chrysalis; being wrapt as in a sliroiul, presenting no appear-
ance of external members, and retaining but feeble indica-
tions of life. In this condition it remains for a certain pe-
riod: its internal system continuing in secret the farther
consolidation of the organs; until the period arrives when
it is qualified to emerge into the world, by bursting asun-
der the fetters which had confined it, and to commence a
new career of existence. The worm, which so lately crawled
with a slow and tedious pace along the surface of the ground,
now ranks among the sportive inhabitants of air; and ex-
panding 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 development 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 me-
chanical relations, 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 circum-
stances 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 be-
ings on which they have been conferred to the most elevated
rank among the lesser inhabitants of the dobe.

The mechanical functions of insects scarcely admit of be-
ing reduced to general principles, in consequence of the
great diversity of forms, of habits, and of actions, that is met

220 THE MECHANICAL FUNCTIONS.

with among the innumcrahle hosts of beings which rank un-
der this widely extended department of the animal creation.
In these minute creatures may be discovered all the me-
chanical instruments and apparatus required for the execu-
tion of those varied motions which we witness in the larger
animals, and which, though almost peculiar to the different
classes of these 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 move-
ments peculiar to themselves, and of v/hich we meet with
no example in other parts of the animal kingdom.

In attempting to delineate a sketch of the movements of
insects, and of the m.echanism 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 consider each separately. In many tribes,
however, the difference between the larva and the perfect
insect is much less considerable than in others. Those be-
longing to the orders of Hemiptera and Orthoptera, for ex-
ample, come out of the egg with nearly the same form as
that which they have in the mature state; excepting that
they are without wings: these organs being added in the
progress of their growth, and constituting, when acquired,
their perfect or imago condition.
«

§ 4. K/lquatic Larvx.

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.

AQUATIC LARViE. 221

Somollmcs, these actions arc 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 tlie larva of the Libellula, or
dragon-fly, a singular artifice has been resorted to for giving
an impulse to the body, without the help of external mem-
bers. It is that of the alternate absorption of water into a
cavity in the hinder part of the body, and its sudden ejec-
tion from that cavity, so that the animal is impelled in a con-
trary direction, upon the same principle that a rocket rises
in the air by the reaction of that fluid. It has, at various
times, been proposed to apply the power of steam to the ])ro-
duction of an effect exactly similar to that of which Nature
here presents us with so pcriect an example, for the purpose
of propelling ships, instead of the ordinary mode of steam
navigation.

Some larvae, such as that of the Stratioinys, collect 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 effica-
cy 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 fluid, in the
manner already noticed, in giving the history of the evolu-
tions of the hydra.*

The impulse given by the lateral inflections of the body
are in many cases assisted by short legs; but the larvae of
the Ephcmer a, i\\ou^\ furnished with legs, do not use them
for this purpose, and swim simply by the action of the tail.
Those of the Dyiiscus 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 larvaj
of the Hy drop hi I us are also admirably formed for swim-
ming; and they not only dart forwards with surprising velo-
city, but also turn in all directions with the utmost facility.

• Page 133.

222 THE MECHANICAL FUNCTIONS.

§ 5» Terrestrial Larvae..

The movements of larvse that are not aquatic are perfectly
analogous to those of the Annelida, which they much resem-
ble in their outward form and mechanical structure. The
muscles by which the annular segments of the body are
moved, are exceedingly numerous, and beautifully arranged
with reference to the motions they are intended to effect.
The investigation of the structure of these minute organs
has Ions exercised the talents of the most skilful entomo-
logists, and still offers much that remains to be explored.
The researches of Lyonet, already alluded to, on the anato-
my of the larva of the Bomhyx Cossus,^ of which he has
published an elaborate description, accompanied by admi-
rable engravings, will ever remain a splendid monument of
patience and ingenuity in overcoming the difficulties which
impede this kind of inquiry. la the body and the limbs of
this caterpillar, Lyonet counted above 4000 separate muscu-
lar bands, all arranged with the most perfect symmetry, and
adapted, with wonderful precision, to the performance 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 segment, 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 larvse contrive to leap to a
considerable distance, by the violent efforts which they make
in unfolding the curvatures of their bodies.

Some larvss avail themselves of their jaws in order to fix
the head, and drag the rest of the body towards it. In this
manner do the larvse of the Capricorn beetles advance along

• C0SSU6 ligniperda. Fabricius.

TERRESTRIAL LAUVJE. 223

the winding passages which thc}^ have themselves excavated,
holding by the jaws, and dragging themselves forwards.
These movements are assisted by the resistance aflbrded 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, shoul-
ders, and knees, manages to climb up a chimney.

For the purpose of enabling insects to take stronger hold
of the surfaces they pass over, we often observe them fur-
nished with spines, or hooks, which are moved by appro-
priate muscles, and they occupy different situations on the
body. Modifications without end occur with regard to these
and other external parts, subservient in various degrees to
progressive motion. Every possible gradation is also seen
between the short tubercles already mentioned, and the more
regularly formed feet or legs. Those which are regarded as
spurious legs, or prolegs, as they have been called, occupy
an intermediate place between these two extremes. They
consist of fleshy and retractile tubercles, and are often very
numerous; while the number of the l?nie legs, as they are
called, is limited to six. These last are the representatives
of the legs of the future perfect insect; for they are attached
to the three first segments of tlie thorax; and are formed of
those portions articulated to each other, corresponding to the
three principal joints of the imago. The true legs are gene-
rally protected Ijy horny scales; but the coverings of the pro-
legs are wholly membranous. The office of these spurious
legs is merely to serve as props to support tlic body while
the insect is walking, and to prevent its hinder part from
trailing on the ground. They are frequently terminated by
single or double hooks; and also by a marginal coronet of
recurved spines. These hooks, or spines, enable the insect
to cling firmly to smooth surfaces; and also to grasp the most
slender twig, which could not have been laid hold of bv Icirs
of the usual construction.

The speed with which these larvae can advance is regu-
lated by many circumstances, independently of the mere

224 THE MECHANICAL FUNCTIONS.

possession of legs: for some caterpillars move slowly, while
others can run very nimbly. The following is the order in
which the legs are usually moved: namely, the anterior and
the posterior leg on the same side are advanced at the same
moment, tosether with the intermediate one on the other
side; and this takes place alternately on both sides.

There is one tribe of caterpillars called Surveyors, 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 with prolegs, close to the head. The hind ex-
tremity being then fixed by means of the prolegs situated at
that part, the body is again extended into a straight line;
and this process being repeated, the caterpillar advances
by a succession of paces, as if it were measuring the distance,
by converting its body into a pair of compasses. At the
same time that they employ this process, they farther pro-
vide for their security by spinning a thread, v/hich they
fasten to different points of the ground as they go along. ""

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 descend-
ing from a height through the air. The caterpillar of the
cabbage butterfly is thus enabled to climb up and down a
pane of glass, for which purpose it fixes the threads that it
spins in a zig-zag line, forming so many steps of a rope lad-
der. The material of which these threads are made is a glu-

* The great force exerted by the muscles of many caterpillars Is exempli-
fied by their often fixing- themselves to an object, and extending the body to
a distance, as if it were a rigid cylinder. This attitude is shown in Fig. 148* b.

TREATMENT OF LARViE. 225

tinous secretion, which, on heing 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 oc-
casion to descend from one branch to another, send out a
rope made with the same material, which they can prolong
indefinitely; and thus either suspend themselves at pleasure
in the air, or let themselves down to the ground. They
continue, while walking, to spin a thread as they advance, so
that they can always easily retrace their steps, by gathering
up the clew they have left, and reascend to the height from
which they have allowed themselves to drop.

§ 6. Imago, or Perfect Insect,

The process which nature has followed in the develop-
ment of the structure of insects, has for its object the gra-
dual hardening and consolidation of texture, and the union
and concentration of organs; for we find that the segments
which were at a distance from one another in the larva, are
approximated in the perfect insect, and often closely tied to-
gether by ligaments: and in other cases, adjoining segments
cohere so as to form but a single piece. Thus, the number of
separate parts composing the solid fabric is considerably di-
minished. Other segments, again, fold inwardly, forming
internal processes, and adding to the extent and complica-
tion of the skeleton.

The integuments of perfect insects, being designed 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 ani-
mals may readily be distinguished. Their rigidity does
not, like that of shells, arise from the presence of carbo-
nate of lime; for they contain but a small proportion of this
material: and whatever calcareous ingredient enters into
their composition is in the form of phosphate of lime. In
external appearance their texture approaches nearer to that

Vol. I. 29

-*«..

t«

226 THE MECHANICAL FUNCTIONS. ~

of horn than to any other animal product: yet in their che-
mical composition they differ from all the usual forms of al-
buminous matter. The substance to which they owe their
characteristic properties is of a very peculiar nature; it has
been termed Chiiine 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 original form.:}:

With this substance there is blended a quantity of colour-
ing matter, which has usually a dull brown or black hue.
But the colour of the external surface 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
removed 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
tropical insects display a gorgeous metallic lustre depending
on the reflection of the prismatic colours; and the same va-
riegated hues adorn the scales of 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 perceived to originate from enlarged
roots, or bulbs, and to pass out through apertures in the skin;
as is the case with the hair of the larger animals. Their
chemical composition, however, is very different, for they
are formed of the same substance as the integuments, name-

• Annales de Chimle, torn. 76.

I See the work of Straus Durckheim, p. 33.

t M. Odier had concluded from his experiments that no nitrog-en enters
into the composition of this substance. That this conclusion has been too
hastily adopted has been proved by Mr. Children, who, by pursuing" another
mode of analysis, found that the chitlne of cantharides contains not less than
nine or ten per cent, of nitrogen. See Zoological Journal, i. Ill — 115.

STRUCTURE OF INSECTS.

227

ly, entomoline. The purposes served by the hairs are not
always obvious. In many cases they seem intended to pro-
tect the integuments from the water, which they repel from
their surfaces. They also tend to prevent injury arising
from friction; and are found to be more abundant in those
parts, as the joints, vvhich are liable to rub much against one
another.

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 se-
ries of rings, or segments, joined endwise in the direction of
a longitudinal axis. The principal portions into which the

body is divided are the heady
the trunk, and the abdomen:
each of which is composed of
several segments. I have here
given, in illustration, the an-
nexed figures,showing the suc-
cessive portions into which
the solid frame-work, or ske-
leton, of one of the beetle
tribe, the Calosoma syco-
phanta,"* may be separated.
The entire insect, which pre-
sents the most perfect speci-
men of a complete skeleton in this class of animals, is repre-
sented 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.

■4>.

• Carnhus sycophanta. Linn.

22S

THE MECHANICAL FUNCTIONS.

The head contains the brain, or principal enlargement of
the nervous system, and the organs of sensation and of mas-
tication. Its size, as compared with the rest of the body,

varies much in different insects, and is in general propor-
tionably larger than it is in the larva state. Its integument,
which, from analogy with vertebrated animals, has been
called the skull, or cranmm, (c, Fig. 150,) is usually the
hardest part of the general crust. Although it may appear,
on a superficial examination, to consist of a single undivided
piece, yet, on tracing its gradual formation, it is found to be
in reality composed of a union of several of the segments of
the larva. Audouin and Carus distinguish three component

STRUCTURE OF INSECTS. Qoq

segments in the cranium of insects; while Straus Durckhcim
considers it as formed by the consolidation of no less than
six segments of the vermiform larva. According to this
theory, the same elements which in the thoracic segments
are developed into feet, are here employed to form parts
having other destinations. From the segment adjacent to
the thorax the antennae are supposed to be developed. The
two anterior segments belong properly to the face; the one
giving origin to the mandibles, (m,) to the maxillae, or proper
jaws, (j,) and also to the palpi, (p;) the other producing the
processes called the labial palpi, (l. )

The mode in which the head is connected with the trunk
varies much in different insects. Sometimes it is united by
a broad basis of attachment, forming a joint between the ad-
jacent surfaces: but usually it is only appended by a narrow
filament, or neck; so that the articulation is effected by liga-
ment alone. Occasionally, it is placed at the end of a long
pedicle, which removes it to a considerable distance from
the trunk. In the HymenojHera and Diptera^ the head
moves upon a pivot, so as to admit of its being turned com-
pletely round.

The trunk, or Thorax^ is composed, as shown in the figure,
of three segments, termed respectively the Prothorax (x;)
the Mesothorax (y;) and the Meiaihorax (z.*) The first
of these, the prothorax, carries the first pair of legs: the se-
cond, or mesothorax, gives origin to the second pair of legs,
and also to the first pair of wings, or to the Elytra (e,) as ia
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

* In these denominations I have followed the nomenclature of Victor Au-
douin (Annales des Sciences Naturelles, torn. i. p. 119,) as being- the sim-
plest and clearest: but other entomolog-ists have applied the same terms to
different parts. The first segment is termed by Straus Durckheim and
other French writers, the Corselet. Mr. Kirby calls it the Manitrunk, and
restricts the term Prothorax to its upper portion. The united second and
third se^ents are the Thorax of Straus Uurckheim, the Tronc alt/ere of
Chambrier, and the Mitrunk of Kirby.

230 THE MECHANICAL FUNCTIONS.

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 farther subdivision; but for the names and descriptions of
these 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 re-
sultant of all the forces concerned in the great movements
of the bod}^, requires to be sustained by the moving powers
under all circumstances either of action or repose.

Victor Audouin, who has made extensive researches on
the comparative forms of all these parts in a great variety
of insects, appears to have satisfactorily established the ge-
neral proposition that, amidst the endless diversity of forms
exhibited by the skeleton of insects, they are invariably
composed of the same number of elements, disposed in the
same relative situations and order of arrangement: and that
the only source of difference is a variation in the propor-
tional development 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 Mdomen
(b;) it is composed of all the remaining segments, which join
to form a cavity enclosing the viscera subservient to nutri-
tion, 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 twelve or even a greater
number. In the Calosoma (Fig. 150, b,) the abdomen has
six complete, followed by three imperfect segments. Not
being intended to carry any of the organs of progressive
motion, they retain the form of single hoops, which is the
primitive type of the segments of annulose animals. Each
segment has a ligamentous connexion with the next, which
is often so close, as hardly to admit of any motion between
them; but in other instances It is more lax, and allows of
the abdomen being flexible. In the former case, which is the

STRUCTURE OF INSECTS. 231

construction in all the Coleopiera, or heeilcs, the rings have
an imbricated arrangement; that is, each overlap the next,
often to the extent of two-thirds of its breadth: so that they
present a succession of spheroidal hoops, capable of beino-
drawn out, to a certain extent, like the tubes of a telescope.
This very artificial construction is manifestly designed to
allow of a great variety of movements, determined by the
position of the muscles they enclose: for since the surfaces
which receive, as well as those which are received, are seg-
ments of spheroids, this structure admits of a twistino- mo-
tion; and 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 far-
ther divided into an upper, or dorsal, and a lower, or ventral
portion; each portion having the form of a semicircle, or ra-
ther of an arch of a circle. These are connected at the sides
by a ligamentous band, which runs the whole length of the
abdomen. Great advantage results from this division of the
circles, allowing of the upper and lower portions of the ab-
dominal covering being at one time separated, and at ano-
ther brought nearer together; for thus the cavity is capable
of being enlarged or contracted in its dimensions, and adapt-
ed to the variable bulk of its contents. It is deservino- of
notice that, during the process of transformation, some of
the abdominal segments, which are present in the larva, dis-
appear entirely, or leave only imperfect traces of their for-
mer 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 Pa-
Tiorpa.

The junction of the abdomen with the trunk is effected
in various ways. In all the Coleoptera, it is united by the
whole margin of its base, without having a narrower part:
in other tribes there is a visible diminution of diameter, form-
ing a groove all round, or an incision^ as it is technically
termed. In the Ilymenoptera, this incision is so deep as to
leave only a narrow pedicle, like a neck, connecting these

232 THE MECHANICAL FUNCTIONS.

two divisions of the body. In some this pedicle is short, in
others long: in the former case, an exceedingly refined me-
chanism is resorted to for effecting the necessary movements
in a part so bulky compared with the narrowness of the sur-
face of attachment*

Insects in their perfect state have constantly six legs,
which are the developments 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 myriapoda, the result of development is an
increase in the number both of segments and of legs; the
reason of which is that, being terrestrial animals, a length-
ened form was more useful and accordant with their desti-
nation; but in winged insects, where the object is to procure
the means of flight, the organs require to be concentrated,
and all superfluous parts must be retrenched and discarded
from the fabric. The multiplication of organs, which, in the
former case, indicated the progress of a higher development,
would in the latter have been the source of imperfection.
As long as the insect remains in its larva stage, its condition
is analogous to that of the myriapode: but in the more ele-
vated state of its existence, its structure is subject to new
conditions and regulated by new laws.

While the number of members is thus reduced, ample
compensation is given by their increased activity and power,
derived from their augmented length, and the more distinct
lever-like forms of the pieces which compose them.

These pieces (see Fig. 150) are named, from their sup-
posed analogy to the divisions of the limbs of the higher or-
ders of vertebrated animals, the haunch (h,) the trochanter
(t,) the femur (f,) the tibia (s,) and the tarsus (r.) In ge-
neral the femur (or thigh) has nearly a horizontal, and the
tibia (or leg) a vertical position, while the whole tarsus (or
foot) is applied to the ground.

The haunch (h,) which is supposed to correspond to the

* For the details of this structure I must refer to writers on entomology,
and in particular to Kirby and Spence's « * Inti-oduction to Entomology,"
vol. iii. p. 701.

STRUCTURE OP INSECTS. 033

hip bone of quadrupeds, is a broad, but very short truncated
cone. The mode of its articulation with the trunk admits
of great variety; sometimes it is united by a ball and socket
joint, as in the CurciiUo and Ccramhyx; and it then has, of
course, great freedom of motion: at other times the joint is
of the hinge kind, as in the Mdolontha. The trochanter
(t,) and the femur (f,) though in reality distinct pieces, are
usually so firmly united as to compose only one division of
the limb. The articulation of this portion with the haunch
is always effected by a hinge-joint. Joints of this descrip-
tion, when formed, as they are in insects, by the apposition
of two tubular pieces, are constructed in the following man-
ner. One of the tubes has, at the end to be articulated, two
tubercles, which project from the margin, and are applied to
the adjacent end of the other tube at two opposite points of
its circumference; the line which passes through those two
points being the axis of motion. On the side where the
flexion is intended to be made both tubes are deeply notched,
in order to admit of their being bent upon one another at a
very acute angle: and the space left by these notches is filled
up by a pliant membrane, which performs the office of a li-
gament. These articular tubercles and dejnessions 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 ioint and of the next
are placed at right angles to each other; so that there results,
from the combination of both, a caj)ability in the thigh of
executing a circular motion in a manner almost as perfect
as if it had revolved in a spherical socket. The principle
of this compound 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 arc called gimhah arc used,
the parts of which have two axes of rotation, at right angles
Vol. I. 30

234 THE MECHANICAL FUNCTIONS.

to each other, so as to enable the compass to talie its proper
horizontal position, independently of any inclination of the
ship.

The tibia, or shank (s,) is joined at an acute angle with
the femur; and is frequently either beset with spines, or else
notched or serrated.

The tarsus, or foot (r,) is the last division of the limb: it
is divided into several joints, which have been supposed to
represent those of the toes of quadrupeds. The joints are
generally of the hinge kind, but some are met with of a
more rounded form, and approaching to that of the ball and
socket. The whole structure is most admirably adapted to
its exact application over all the inequalities of the surfaces
on which the insect treads. But as the habits and modes of
life of this numerous class are exceedingly diversified, so
the form of the feet admits of greater variety than that of
any other part of the limb.

The feet of insects diverge, and spread over a wide sur-
face; 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; con-
trary to what obtains in quadrupeds, where the feet are
more immediately underneath the points at which they are
connected with the trunk.

The last joint of the tarsus is generally terminated by a
claw, which is sometimes single and sometim.es double, and
which contributes to fasten the foot, under a variety of cir-
cumstances, both of action and of repose. With feet thus
armed, the insect can ascend or descend the perpendicular
sides of a rough body with the greatest ease; but it is scarce-
ly 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
afibrding the greatest possible securit}^ against displacement.

• It has been conjectured that the object In furnishing- this insect with leg's
of so great a length is that of enabling it to walk among- blades of g-rass.

STRUCTURE OF INSECTS.

235

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 other-
wise have to sustain. These cushions arc formed of dense
velvety tufts of hair, lining the underside of the tarsi, but
leaving the claw uncovered; and the filaments, by insinuat-
ing themselves among the irregularities of the surfaces to
which they are applied, produce a considerable degree of ad-
hesion. 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 ap-
pear to be unnecessary, because the lightness of their bodies
sufficiently secures them from any danger arising from falls.
Some insects are furnished with a still more refined and
effectual apparatus for adhesion, and one which even enables
them to suspend themselves in an inverted position from the
under surfaces of the bodies. It consists of suckers, the ar-
rangement and construction of which are exceedingly beau-
tiful; and of which the common house-fly presents us with
an example. In this insect that part of the last joint of the
tarsus which is immediately under the root of the claw, has
two suckers appended to it by a narrow funnel-shaped neck,
moveable by muscles in all directions. These suckers are
shown in Fig. 152, which represents the under side of tbe
foot oiMusca 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 Tahcmiis, or

horse-fly, each foot is furnished with three suckers. In
the Cimbex hUea, or yellow saw-fly, there are four, of which

236 THE MECHANICAL FUNCTIONS.

one is placed upon the under surface of each of the four first
joints of the toes, (Fig. 153;) and all the six feet are pro-
vided with these suckers. In the Dyiisciis niarginaliSy
suckers arc furnished to the feet of the male insect only.
The three first joints of the feet of the fore-legs of that in-
sect 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 pro-
portionally much narrower, and are covered on their under
surface with a multitude of very minute suckers. The Jicri-
diinn biguttiihivi, wdiich is a species of grasshopper, has
one large oval sucker, under the last joint of the foot, im-
mediately between the claws. On the under surface of the
first joint are three pairs of globular cushions, and another
pair under the second joint. Fig. 155 shows these parts.
The cushions are filled with an elastic fibrous substance;
which, in order to increase the elasticity of the whole struc-
ture, is looser in its texture towards the circumference.^

The mode in which these suckers operate may be dis-
tinctly seen, by observing with a magnifying glass the ac-
tions of a large blue-bottle fly in the inside of a glass tum-
bler. A fly will, by the application of this apparatus, remain
suspended from the ceiling for any length of time without the
least exertion; for the weight of the body pulling against
the suckers serves but to strengthen their adhesion: hence,
we find flies preferring the ceiling to the floor, as a place of
rest.

Insects which, like the gnat, walk much upon the surface
of water, have at the ends of their feet a brush of fine hair,
the dry points of which appear to repel the fluid, and pre-
vent the leg from being wetted. If these brushes be moist-
ened with spirit of wine, this apparent repulsion no longer
takes place; and the insect immediately sinks and is drowned.

* Philosophical Transactions for 1S26, p. 324.

AQUATIC INSECTS.

237

§ 7. */2quatic Insects.

Although many insects arc 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 alto-
gether a large portion of their bulk, and gives to the insect,
when it is immersed in water, a strong buoyant force. As
the largest volume of air is contained in the abdomen, this
part is comparatively lighter than either the trunk or head;
and the natural position of the insect in the fluid is oblique'
to the horizon, the head being depressed, and the abdomen
elevated. Any force impelling the body forwards in the
direction of its axis tends, therefore, to make italso descend.
The effect of this downward force is counteracted by the
sustaining pressure of the water, which is directed vertically
upwards: so that the real operation of the force in question
is to carry the body forwards nearly in a horizontal di-
rection.

In insects destined to move in water, sometimes all the
legs, but occasionally only one pair, are lengthened and ex-
panded into broad triangular surilices, capable of acting as

oars: and these surfaces arc farther extended by the addi-
tion of marginal fringes of hair, so disposed 'as to project
and act upon the water every time the im])ulsc is given, but
to bend down when the leg is again drawn up, prci)aratory

23S THE MECHANICAL FUNCTIONS.

to the succeeding stroke; thus imitating the action which is
called feathering an oar. The impulses are given with great
regularity, all the feet striking the water at the same mo-
ment.

Of all the coleopterous insects, the Dytiscus, or water-
beetle (of which Fig. 156 represents the upper, and Fig. 157
the under side,) is the one best constructed for swimming:
its body having a flattened form, very much resembling a
boat, narrower before than behind, and its surface present-
ing no projecting parts. The upper surface in particular is
extremely smooth, to enable it to glide under the water with
the least possible friction. Its centre of gravity is placed
very near the under surface. The posterior legs, which act
as powerful oars, are attached to very large haunches, for
the purpose of containing 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 required to be moveable:
and accordingly they are firmly united to the thorax; a
structure which renders the motion of the other joints more
regular and uniform. When the Dytiscus wishes to rise, it
need only desist from all action, and abandon itself to the
buoyant force of the fluid, which quickly carries it to the
surface.

The Notonecta^ or water-boatman (Fig. 15S,) is remarka-
ble for always swimming on its back, a peculiarity depend-
jrg ing on the form of its body, which is-

semi-cylindrical, with the legs aflixed to
the flat surface; so that, when lying on
its back in the fluid, the centre of gravity
is below the centre of the whole figure,
or the metacentre, as it is termed, and
the equilibrium is maintained. It is evident that, under these
circumstances, if it were placed in the water with its legs
undermost, it *would unavoidably tilt over, and resume its
usual position. Its long legs extending at right angles to
the body, present a striking resemblance to the oars of a boat;

PROGRESSIVE MOTION OF INSECTS. 039

and they act, indeed, in the same manner, and on the same
prineii:)les.

§ 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 anterior and posterior legs on one
side are combined with that of the middle one on the other
side; and the two sets of legs arc moved alternately. In
consequence of their relative positions with the trunk, the
anterior legs are advanced by the extension, and the poste-
rior legs by the flexion of the corresponding joints. When
the feet have fixed themselves on the ground, the contra-
ry actions take place, and the body is brought forwards.
During this period the legs which compose the other set arc^
called into play, and are advanced; and the same succession
of actions takes place with these as with the former. This
can easily be seen when the insect walks very leisurely; but
in a more quickened pace, the succession of actions is too
rapid to be followed by the eye.

The action of leaping is performed by the sudden exten-
sion of all the joints of the limb, which are prcviouslv
folded as close as possible. The joints principally concerned
in this action, are those of the tliigli and tibia, as they fur-
nish 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 Coleoptcra, 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 vio-
lently exerted, and by producing a sudden unbending of this
apparatus of folded springs, they project tlie whole body,

^40 THE MECHANICAL FUNCTIONS.

by the accumulated 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
contribute much less to it than the hind legs, and are more
useful in modifying the direction of the leap, than in adding
to its force. The power of leaping is derived principally
from the great size and strength of the extensor muscles of
the legs, which, being contained within the femur, necessa-
rily 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 in general, in-
deed, infer the particular kind of progressive motion for
which the insect is intended by observing the comparative
length of the different pairs of legs. When they are of equal
size, the pace is uniform:— swiftest in those that have the
# longest legs, — slowest when they are short. When the an-
terior legs are much longer than the posterior, the power of
prehension may be increased, but that of progression is im-
peded. The great prolongation of the posterior legs is ge-
nerally accompanied by the power of jumping, unless, in-
deed, 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 w^hich they intend to spring, and which give
to the limb a steady fulcrum. The Cicada spitmaria has
been known to leap to a distance of five or six feet; which
is two hundred and fifty times its own length: this, if the
same proportions were observed, is equivalent to a man of
ordinary stature vaulting through the air the length of a
quarter of a mile. When the same insect is laid on glass,
on which the spines cannot fasten, it is unable to leap far-
ther than six inches.*

The insects belonging to the genus Elciier are provided

• De Geer, III. 178, quoted by Kii-by and Spence.

PROGRESSIVE MOTION OP INSECTS. 241

with a peculiar mechanism for the special purpose of accom-
plishing a singular mode of leaping, 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, beino- unable
to reach with its feet the plane on which it is lying, and
procure a fulcrum for the action of its muscles. It is appa-
rently 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 upon the feet; an effect which is accomplished by
an exceedingly curious mechanism. The prothorax is pro-
longed beyond the length it usually lias in other coleop-
tera, and it is articulated with the mesothofax on the
dorsal side by two lateral tubercles, which form a hino-e
joint, limiting its motions to a vertical plane. The sternum,
or pectoral portion of the prothorax is also extended back-
wards, and terminates in an elastic spine, which is received
into a cavity in the mesothorax, and which, while the insect
is lying on its back, with the prothorax bent upon the meso-
thorax, recoils with the force of a spring, and communicates
to the body an impulse which carries it upwards to a consi-
derable height. If the elater should fail in its first attempts
to recover its feet, it repeats its leaps till it succeeds. We
find no example of a similar structure in any other part of
the animal kingdom.

The express adaptation of structure to the mode of life
designed for each species of insect is nowhere more strongly
marked than in those which are intended to burrow in the
earth: and of these the Gryllo-ia/pa, or mole cricket, pre-
sents a remarkable example. A minute account of the ana-
tomy of this insect has been given by Dr. Kidd," 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 peculiar-
ly calculated for burrowing, with a skin which, being co-
vered with a fine down, effectually prevents the adhesion of

• Phil. Trans, for 1825, p. 203.
Vol. I. 31

242 THE MECHANICAL FUNCTIONS.

the moist earth through which it moves, and with a form of
body enabling it to penetrate with least resistance the oppo-
sing medium. By being endowed with the power of moving
as easily in a backward as in a forward direction^ it is ena-
bled quickly to retreat in the narrow channel it has exca-
vated: and as a safeguard in these retrograde movements, it is
provided with a pair of posterior appendages, which are sup-
plied with large nerves, and may be regarded as serving the
purpose of caudal antennse.

The fore-legs, (one of which is represented in Fig. 158"^)
are the burrowing implements, and they are admirably cal-
158* .^^-^^^^^^ culated for their peculiar ofiice,

% ^ both in the shape and in the
'-.^^ mode of articulation of their
several divisions, which bear a
considerable analogy to the
corresponding member of the mole. Dr. Kidd observes,
that, compared with the other legs, and with the general size
of the animal, they are as if the brawny hand and arm of a
robust dwarf were set on the body of a delicate infant; and
the indications of strength which their structure manifests,
fully answer to their extraordinary size. For a more par-
ticular description of the mechanism of this instrument I
must refer the reader to the paper above quoted.

§ 9. Flight of Insects.

If the excellence of a mechanic art be measured by the
difficulties to be surmounted in the attainment of its object,
none surely would rank higher than that which has accom-
plished the flight of a living animal. No human skill has
yet contrived the construction of an automaton, capable, by
the operation of an internal force, of sustaining itself in the
air in opposition to gravity, for even a few minutes; and
far less of performing in that element the evolutions which
we daily witness even in the lowest of the insect tribes.
To the ultimate attainment of this faculty it would appear
that all the transformations they undergo in external appear-

PLIGHT OF INSECTS. 243

ance, and all the developments of their internal mechanism,
are expressly directed. Wings are added to the frame only
in the last stage of its completion; after it has disencum-
bered 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. Curiously folded up
in the pupa, the wings there attain their full dimensions,
ready to expand whenever the bandages that surround them
are removed. No sooner is the insect emancipated form its
confinement, than- these organs, which are composed of du-
plicatures of a dense but exceedingly fine membrane, iden-
tical in its composition with the general integuments, begin
to separate from the sides of the body, and to unfold all
their parts. Their moisture rapidly evgiporates, leaving the
delicate film dry and firm, so as to be ready for immediate
action. The fibres, or nervures, as they are called, form a
delicate net-work, for the support of this fine membrane,
like the frame of the arms of a windmill, which supports
the canvass spread over them. The microscope shows that
these fibres are tubular, and contain air; a structure the most
effectual for conjoining lightness with strength; and many
entomologists are of opinion that the insect has the power,
during the act of flying, of directing air into the nervures,
so as to dilate them to the utmost, and render them quite
tense and rigid.

In the great majority of insects, the wings are four in
number; of which the first pair are, as we have seen, affixed
to the mesothorax, and the second to the mctalhorax.
These two segments of the thorax, composing what has been
termed the alitrunk, constitute the most solid portion of
the skeleton, and are frequently strengthened hy ridges,
and other mechanical contrivances for support. The su-
perior extremities of these supports, which have been com-
pared to the clavicles, or furcular bones of birds, are always
curved inwards. This part of the trunk requires to be al-
ternately dilated and contracted during flight; and, hence,

244 THE MECHANICAL FUNCTIONS.

the several pieces of which its dorsal portion is composed,
are loosely connected together by ligaments.*

The shape of the wings is more or less triangular. They
are moved by numerous muscles, which occupy a large
space in the interior of the trunk, and consist of various
kinds of flexors, extensors, retractors, levators, and depress-
ors; the whole forming a very complicated assemblage of
moving powers. The largest, and consequently most pow-
erful 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 con-
tractions, which are capable of being renewed in very rapid
succession: for, indeed, unless they had this power, even so
lio-ht a body as that of an insect could not have been sus-
tained for a moment in so rare a medium as the atmosphere,
far less raised to any height by its resistance.

The simple ascent and descent of the wings would be suf-
ficient, 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 the result of the reaction of
the air. The axis of motion of the wings is a line inclined
at a small angle to the axis of the body, and directed, from
before, backwards, outwards, and downwards; and they move
in a plane which is not vertical, but inclined forwards. The
angle which the plane of the wing forms with the horizon
varies continually in the different positions of the wing; but
the general resultant of all these successive impulses is a
force directed forwards and upwards; the first part of this
force produces the horizontal progression of the insect, while
the second operates in counteracting the force of gravity;
and during the advance of the insect, either maintains it at
the same height, or enables it to ascend.

When the insect wishes to turn, or to pursue an oblique

• See Chabrier's "Essai sur le Vol des Insectes," Memoires du Museum
d'Histoire Naturelle? vi. 410, vii. 297, and viii. 47 and 349. See, also, Zoolo-
gical Journal, i. 391.

FLIGHT OF INSECTS. 245

course, it effects its purpose very easily by strikinir the air
with more force on one side than on the other; or, by em-
ploying certain muscles which bend the body to one side,
it shifts the situation of the centre of gravity, so that the re-
action 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 a force with the wings just sufficient to ba-
lance that of gravity, insects can poise themselves in the air,
and hover for a length of time over the same spot, without
rising or falling, advancing or retreating; and the body may,
all the while, be kept either in the horizontal, or in the erect
position. In the latter case, the motions are similar to those
which take place in ordinary flying, only they are more
feebly exerted, since all that is required is to sustain the
weight of the body without urging it to a greater speed.
LibeUulde, Sphinxes, and a great number of Diptera, exhibit
this kind of action: among the latter, the St7^aiiomys is most
remarkable for its power of remaining long in the same fixed
position.

The number, form, and structure of the wings have fur-
nished entomologists with very convenient characters for
their classification: on these are founded the orders of the
Coleopiera, Orthoptei^a, Rhipiptera, Hemiptera, Neurop-
tera, Hymenoptera, Diptera, 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 crea-
tion, would, it is obvious, far exceed the proper limits of
this treatise. I must, therefore, confine myself to a few lead-
ing points in their structure and modes of progression.

In the Coleopiera, an order which comprehends by far the
largest number of genera of insects, the lower pair of wino-s
(w, Fig. 150, p. 228) are light and membranous, and of a
texture exceedingly fine and delicate. They are of great
extent, compared with the size of the body, when fully ex-
panded: and are curiously folded when not in use. For the
protection of these delicate organs, the parts which cor-
respond to the upper pair of wings of other insects, are here

246

THE MECHANICAL FUNCTIONS.

converted into thick opaque, and hard plates, (e,) adapted to
cover the folded membranous wings when the insect is not
flying, and thus securing them from injurious impressions,
to which they might otherwise be exposed from heat, miois-
ture, or the contact of external bodies. These wing cases,
or elytra^ as they are termed, are never themselves em-
ployed as wings, but remain raised and m.otionless during
the flight of the insect. They probably, however, contri-
bute to direct the course of flight, by variously modifying
the resistance of the air.*

In the Orthoptera, (Fig. 159,) the coverings of the wings,
or tegmina, instead of being of a horny texture, are soft and
flexible, or semi-membranous. The wings themselves, being
broader than their coverings, are, when not in use, folded
longitudinally, like a fan.

In the new Order of Rhipiptera of Latreille,t which in-
cludes only two genera, the tegmina are anomalous both in

160

159

162

their situation and shape; being fixed at the base of the an-
terior legs, very long and narrow, and apparently incapable
of protecting the wings. The wings themselves are of am-

* The Elytra of insects have been regarded, by Oken, as corresponding to
the bivalve shells of the Mollusca, a notion which seems to be founded upon
a fanciful and strained analogy.

f The Strepsipiera of Kirby. See Transactions of the Linnjean Society,
XI. 86.

FLIGHT OF INSECTS. 247

pie extent, forming, when expanded, a quadrant of a circle,
with five or six nervurcs radiating from their base, and
folded longitudinally.

In the Hetniptera, the tegmina, or as they are here
called, the hemi-elytra^ are coriaceous towards their base,
but membraneous towards their extremity, and the true
wings are folded transversely, so as to cross one another.
These hemi-elytra are employed to strike the air in flight,
and their movements accompany those of the wings.

Insects having four thin membranous and transparent
wings are arranged under two orders; namely, the Neurop-
tera (Fig. 160,) in which the lesser nervures form an inter-
lacement of fibres, crossing one another nearly at right
angles, like net-work, or lace: and the Hymenoptcra (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. Libellulse, and JEschnse^ 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
greatest ease in all directions, sideways, or backwards, as
well as forwards, and can instantly change their course with-
out being obliged to turn their bodies. Hence they possess
great advantages both in chasing other insects, and in evading
the pursuit of birds. Bees^ which are hymenopterous in-
sects, have often been observed to fly to great distances
from their hive in search of food. The humble bee adopts
a very peculiar mode of flight, describing, in its aerial course,
segments of circles, alternately to the right and to the left.
The velocity with which these insects move through the
air, in general, much exceeds that of a bird, if estimated with
reference to the comparative size of these animals.*

* I have been favoured by Mr. George Newport with the following ac-
count of the structure of the sting of the Wild Bee, {Anthophora rctusOy
Kirby) which he has lately carefully examined, and from whose drawings
of the dissected parts the annexed figures (163) have been engraved, " The
sting of the bee, a, is formed of two portions placed laterally together, but

248

THE MECHANICAL FUNCTIONS.

Although the greater number of insects have four wings,
there are many, such as the common house fly, and the gnat,
which have only two. These compose the order Diptera
(Fig. 162.) In these insects we meet with two organs, con-
sistino" of .cylindrical filaments, terminated in 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 that have four wings.

capable of being- separated. The point, v, is directed a little upwards, and
is a little curved: the barbs, seen still more highly magnified at a, are about
six in number, and are placed on the under surface, and their points directed
backwards. At the base of the sting-, e, there is a semicircular dilatation

apparently intended to prevent the in-
strument from being thrust too far out
of the sheath (seen separately at y,) in
which it moves: it has also a long ten-
don to v/liich the muscles are attached.
It is between these plates, when ap-
proximated, that the poison flows from
the orifice of the somewhat dilated ex-
tremity 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 se-
cretes the poison, but merely a recep-
tacle for containing it: for it is conveyed
into this bladder by means of a long con-
voluted vessel, c, which receives it from
the secreting organs, s. These organs
consist of two somewhat dilated vessels
resembling caeca, 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 mus-
cles which move the sheath are distinct from those of the sting, and are at-
tached 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. Swam-
merdam has delineated these parts as caeca in his dissection of the common
hive bee, but has not noticed the secretory vessels. The sting of the hive
bee resembles that of the Anthophora retusa.''

PLIGHT or INSECTS. 249

They are named the hdUeres, or poisers, from their sup-
^Dosed 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 rudi-
ments 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 king-
dom, and modifies the structure of each individual part so as
to preserve its conformity to one general type.

The innumerable tribes of butterflies, sphinxes, and moths,
are all comprehended in the order Lcjndoptcra, and are dis-
tinguished by having wings covered with minute plumes or
scales. These scales are attached so slightly to the membrane
of the wing as to come off when touched with the fingers, to
which they adhere like fine dust. When examined with
the microscope, their construction and arrangement a[)pear
to be exceedingly beautiful, being marked with parallel and
equidistant striae, often crossed by still finer lines, the dis-
tinct visibility of which in many kinds of scales, as those of
Pontia brassica, or cabbage butterfly, and the Morp/io
Menelaus of America, constitutes a good criterion of the ex-
cellence of the instrument. The beautiful colours which
these scales possess may perhaps generally be owing to the
presence of some colouring material: but the more delicate
hues are probably the result of the optical effect of the striai
on the surface; and in some cases they result from the thin-
ness 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 liglit-

The forms of these scales arc exceedingly diversified, not
only in different species, but also in different parts of the
wings and body of the same insect; for the surface of the
body, generally, as well as the limbs, and even in some spe-
cies the antennae are more or less covered with these scales.*

• In the posthumous work of Lyonet, which lias lately appeared, nearly the
whole of six quarto plates are crowded with the delineations of tlic dillcrent
forms of the scales found in the Bomhyx Cossus.

Vol. I. 32

250

THE MECHANICAL FUNCTIONS.

Fig. 164 exhibits some of the more usual shapes as they ap-
pear when viewed with high magnifying powers.

Each scale is inserted into the membrane of the wing by
a short pedicle, or root, and overlaps the adjoining scales:
and the wliole are disposed in rows with more or less regu-

"^ V ^

larlty; one row covering the next, like tiles on the roof of a
house.* This imbricated arrangement, together with the
marks that are left on the membrane of the wing where the
scales have been rubbed off, are shown in Fig. 165, which
is a faithful delineation of the appearance of the wing of the
Hesperia Sloanus, seen through a powerful microscope.
The membrane of the wing itself, when stripped of its
scales, is as perfectly transparent as that of the bee, and is,
in like manner, supported by diverging nervures. Many
butterflies exhibit in some parts of the wing smooth pearly
spots, called by entomologists, ocelli, or eyes, which arise
from those parts being naturally destitute of scales. The
number of these scales necessary to cover the surface of the
wings must, from their minuteness, be exceedingly great.
The moth of the silk worm {Bornbyx mori, Fig. 148,)

* The scales on the wing of the Leplsma are of two kinds; one set being
arranged in rows, as usual, and the otliers, which are of a different shape,
being inserted between and over the former, so as to fasten each firmly in its
place.

FLIGHT OF INSECTS. 05^

which has hut a small wine;, contains, according to Lewen-
hoeck, more than two hundred thousand of these scales in
each wing.

These scales douhtless contrihute to the protection of the
wing; but they at the same time add considerably to their
weight, and impede the velocity of their action. This in-
convenience appears to have been in a great measure com-
pensated by the greater size of the wings, and by the extent
of the surface with which they strike the air. Still, how-
ever, 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 but-
terfly, by its irregular and apparently capricious movements,
alternately dipping and rising in the air, so as to describe a
series of zigzag lines, more easily eludes capture when pur-
sued, not only by naturalists, but also by birds that are ea-
gerly seeking to secure them. It is astonishing to what a
distance the silk worm motiis will fly: some have been
known to travel more than a hundred miles in a short time.
The PapUio Iris often rises to so great a heiglit in the air
as to be quite invisible.

A mechanical contrivance is adopted in many of the Le-
pidoptera for keeping their wings steady during flight, con-
sisting of a hook covered with hair and scales, attached to
the under side of the upper wings near their ijase, and con-
nected also by means of bristles to the base of the lower
wing: by this attachment. all the wings are locked together
and brought into action at the same time. Insects of the
Sphinx tribe are also provided with a kind of rudder formed
by the expansion of the tail, enabling them to steer their
course with more certainty. The Lcpidojitera 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 asaflbrding incontrovert-
ible proofs of the enormous power with which their muscles

252 THE MECHANICAL FUNCTIONS.

must be endowed. We have already had occasion to notice
a remarkable instance of the force and permanence of mus-
cular contraction in tliose caterpillars which frequently re-
main for hours together in a fixed attitude, with their bodies
extended from a twig, to which they cling with their hind
feet alone.* Ants will carry loads which are forty or fifty
times heavier than their own bodies: and the distance to
which many species, such as the Elater, the Locust, the
Lepisma, and above all the Pidex, are capable of leaping,
compared with the size of the insects themselves, appear
still more astonishing. Linnaeus has computed that the
Melolontha, or chaffer, is, in proportion to its bulk, more
than six times stronger than the horse: and has asserted that
if the same proportional strength as is possessed by the Lu-
canus, 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 as-
sailants, like the giants of ancient mythology.

But while we must admit that all these facts indicate a re-
markable degree of energy in the contractile power of the
miuscular fibres of insects, we should at the same time re-
collect that the diminutive size of the beings w^hich display
those powers is itself the source of a mechanical advantage
not possessed by larger animals. The efficacy of all mechani-
cal arrangements must ultimately depend on a due propor-
tion 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 dimensions
of the structures themselves. This will be evident when
we consider that the forces which are called into action are
resisted by the cohesion of the particles composing the solid
parts of the machine: and this cohesion being not a variable,
but a constant and definite force, must necessarily limit the
dimensions of every mechanical structure, whether intend-
ed for stability or for action. An edifice raised beyond a
certain magnitude, will not support itself, because the weight

* See Fig- 148*, p. 224.

FLIGHT OF INSECTS. 253

of the materials 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 perform-
ance when constructed on a larger scale? x\ny lever, of
whatever form, may be increased in its dimensions until the
force of gravity becomes superior to the cohesion of its own
particles: and consequently any structure, like a vegetable
or animal body, composed of a combination of levers, would,
if its size were to exceed a certain limit, fall to pieces mere-
ly by its own weight. This can be prevented either by em-
ploying materials of greater cohesive strength, or by in-
creasing, at the points where the strains are greatest, the thick-
ness of the parts compared with their length: but the choice
of materials is necessarily restricted within narrow limits,
and the latter expedient would entirely alter the relative
proportions of the parts, and would require a complete
change in the plan of their construction. In passing from
the smaller to the larger animals, we find, accordingly, that
new models are adopted, a new order of architecture intro-
duced, and new laws of development observed. "We have
next, then, to direct our attention to the procedure of na-
ture in the execution of this more enlarged and comprehen-
sive scheme of animal organization.

( 254 )

CHAPTER VL

vertebrata,
?» 1. Vertebrated Animals in general

§

If it be pleasing to trace the footsteps of nature in con-
structions so infinitely varied as those of the lower animals,
and to follow the gradations of ascent from the zoophyte to
the winged insect, which exhibits the greatest perfection com-
patible with the restricted dimensions of that class of beings,
still more interesting must be the study of those more ela-
borate eflbrts 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 deve-
lopments in her system of organization, resorting to new
models of structure on a scale of greater magnitude than be-
fore, devisino; new plans of economy, calculated for more ex-
tended periods of duration, and adopting new arrangements
of organs, fitted for the exercise of a higher order of facul-
ties. The result of these more elaborate constructions is
seen in the vast series of Vertebrated Animals, wdiich com-
prises 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 tlie size or external form of these animals, whatever
the activity or sluggishness of their movements, whether
they be inhabitants of the land, the waters, or the air, a
striking similitude may be traced, both in the disposition of
their vital organs, and in the construction of the solid frame-

VERTEBRATED ANIMALS. 255

work, or skeleton, which sustains and protects their fabric.
The quadruped, the bird, tlie tortoise, the serpent, and the
fish, however they may differ in subordinate details of or-
ganization, are yet constructed upon one uniform principle,
and appear like varied copies from the same original model.
In no instance do they present structures which are altoge-
ther 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 ano-
ther, separated by an immense gulf, containing no interme-
diate islands from which we might gather indications of these
tracts of land having been originally connected. At the
very first sight, indeed, the general fabrics of these two de-
scriptions of animals appear to have been constructed upon
opposite principles; for, in the one, as we have already seen,
the softer parts are internal, and are enclosed in a solid crust,
or shell, or horny covering, answering, at once, the purposes
of protection and mechanical support, and furnishing exten-
sive surfaces for the attachment of the orjrans of motion.
But, in the Vertebrata, the solid frame-work which serves
these purposes, occupies, for the most part, an internal situa-
tion, constituting a true-jointed skeleton, which is surround-
ed by the softer organs, and to which the muscles, destined
to move their several parts, are attached. The office of ex-
ternal defence is intrusted solely to the integuments, and
their different appendages. Such is the general character of
the arrangements which nature has here ado])ted; from
which, however, she has occasionally deviated with respect
to some important organs of extremely delicate texture, and
which require to be shielded from the slightest pressure.
This occurs with regard to the brain, and the spinal marrow,
which, we shall presently find, are specially guarded by a
bony structure, enclosing them on every side, and forming
an impenetrable case for their protection. The solid mass
of bone, thus provided to defend the brain, gives also the
opportunity of lodging safely the delicate apparatus subscr-

256 THE MECHANICAL FUNCTIONS.

vient to the finer senses, namely, those of siglit, of hearing,
and of smell. The security which these organs derive from
this protection allows of their being carried to a higher de-
gree of improvement than could be attained in the lower or-
ders.

There is also another advantage, of considerable moment,
which results from the internal situation of the skeleton,
namely, that it admits of an indefinite extension by growth,
without interfering with the corresponding enlargement of
the softer organs; for we have seen that in all the instances
in which this arrangement is reversed, that is, whenever the
enclosing surfaces become solid, and can no longer yield to
the dilatation of the contained organs, no alternative remains
but that of breaking up the exterior case, and vv-holly cast-
ing it off*, to make room for the farther growth of the ani-
mal; after which operation, it has to be replaced by another
coverincr of larger dimensions. This operation 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 perpetual moultings of the caterpillar; hence the
repeated castings of the shells of the Crustacea; and hence
also the successive metamorphoses of the insect. Nothing
of this kind takes place among the Vertebrata; where all the
organs are developed in regular and harmonious succession,
without the slighest mutual interference, and without those
vicissitudes of action, and of torpidity, which we witness in
the chequered existence of the insect.

§ 2. Structure and Comjiositioii of the Osseous Fabric.

The process employed for the formation and extension of
the solid frame-work of the Vertebrata diff'ers totally from
that which we have seen exemplified in the growth of shells,
or of the hard coverings of insects and of crustaceous ani-
mals. 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 w^hich
they are carried in the higher classes. In the more elabo-

CHEMICAL COMPOSITION OF BONE. 257

rate system of the vcrtcbrata, the skeleton is composed of
true bones; that is, of solid pieces, which, althouirh they arc
dense calcareous structures, yet continue organized during
the whole period of development, and form as much a part
of the living system as any other organ of the body. We
have formerly seen that the membrane in which the calca-
reous matter of the shell is deposited, should properly be
classed among the integuments; being analogous to them not
only in being situated externally, but also in their structure
and in their function. It is not so with bone, which is
essentially an internal structure.*

In their chemical composition, likewise,bones are striking-
ly contrasted with the calcareous products of the INIolIusca:
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. Where-
ver great strength and rigidity are required, this is the ma-
terial depended on for imparting these qualities; and it has,
accordingly, been employed for the osseous structures, which
are among the most elaborate results of organization. The
densest and hardest of these structures are those in which

* De Blainville rcg-ards the hard covering's of insects, tog-ether with the
shells of the Crustacea, as structures derived aUog-cther from the intcg-uments,
and as perfectly anaiog-ous, in this respect, to the scales, hoofs or other horny
productions of the skin in vertebrated animals. Geoffrey St. Hilaire con-
tends, on the contrary, that the former constitute the true skeleton of the
lower classes, and that a perfect analogy may be traced between the ring-s,
which arc the essential constituents of the frame-work of annulose animals,
and the vertebrae, which enclose the spinal cord of the higher classes. Pro-
fessor Carus appears in his system of org-anic formations, to have kept in view
both these analogies; g-iving- to the former class of structures the denomina-
tion of Uermo-skeleioJi, and to the latter that of Netiro-skeklon (See his Ta-
bulx Anatomiam Comparativam illustrantes, edited by Thienemann.) Ana-
log'ies have also been imag-ined to exist between the external and internal
situations of the woody fibres of plants belong-ing respectively to the endoge-
nous and exogenous classes, and that of the corres[)onding relative situations
of the skeletons of invertebrated and vertebrated animals. Sec a Memoir by
Dumortier, in the Nova Acta Physico-Medica Acad. Ciesar. Leopold. Caro-
lina Natur. Curios. XVL, 219.)

Vol. I. 33

258 THE MECHANICAL FUNCTIONS.

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 ce-
menting ingredient. The internal bony portions of the ear,
where, in order perfectly to transmit the sonorous vibra-
tions, the greatest solidity is required, are the densest parts
of the skeleton; and phosphate of lime enters most largely
into the composition of these bones. The tympanic portions
of the temporal bone of the whale and the cachalot, where
the great size of the organ gives us advantages in examining
them, are as dense and as hard as marble. The bony por-
tions of the teeth, likewise, afford instances of very hard cal-
careous 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
wdth steel. It is scarcely necessary to point out the obvious
intentions which are fulfilled by this peculiarity of structure,
conferring extraordinary hardness on a part, of which the
appropriate office is that of breaking down hard bodies sub-
jected to their mechanical action. But this extreme degree
of crystalline hardness v/ould 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 employed as
the basis of the structure, acting as a strong cement inter-
posed between the calcareous particles.

This composition of bone is rendered evident by subject-
ing it to certain chemical processes. On exposure to heat,
we find it first becoming black, from the development of
the charcoal attendant upon the destruction of the animal
membrane. The oil contained in the cavities exudes, and,
taking fire, is soon totally consumed. The bone then reco-
vers its whiteness, and undergoes no farther change by the

CHEMICAL COMPOSITION OF BONE. 259

action of llic fire. If it be now examined, it will be found
to have lost nearly half its original weight, and to have be-
come exceedingly brittle; this, as already mentioned, being
the natural ])roperty 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, showinc: 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 sufficiently diluted to prevent its injuring the animal
membrane, but yet sufficiently powerful to dissolve the
phosphate and carbonate of lime. Diluted nitric or muria-
tic acids may be used for this purpose, and will, in this way,
gradually separate the earthy particles from the membranous
portion of the bone. During the action of the acid a few
bubbles of carbonic acid gas make their appearance, indi-
cating 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 so-
lution of ammonia. This precipitate is readil}' dissolved,
without effiirvescence, by nitric, muriatic, or acetic acids.
A small quantity of sulphuric acid may also be detected in
the 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 pro-
perties. It is soft, flexible, and elastic; resembling in every
respect the muscular or fibrous structures, and being, like
them, resolvable into gelatin and albumen by long boiling
in water. This substance has sometimes, but erroneously,
been considered as identical with cartilage; for it has nei-
ther the whiteness, nor the elasticity, nor the texture of carti-
lage, nor is it at all similar to that substance in its chemical

260 THE MECHANICAL FUNCTIONS.

composition: for while cartilage isjormed almost wholly of
albumen, the animal basis of bone is almost entirely resol-
vable into gelatin.

Thus may a bone be analyzed into its two constituent
parts: by the process first described we obtain its earth de-
prived 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 in-
timate combination of these elements, two qualities, which,
in masses of homogeneous and unorganized matter, are
scarcely compatible with one another, are skilfully united.

The mechanical structure of bone is no less worthy of ad-
miration, 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 phos-
phate is deposited, partakes of the reticular structure belong-
ino- to the ordinary cellular texture; and a bone, when mi-
nutely examined, exhibits also the same appearance of plates
intermixed vvith fibres. In the outer compact portion, in-^
deed, the fibrous arrangement of the particles is not so easi-
ly distinguished: but it may be detected in young bones
while they are becoming ossified: and also in bones that
have been long exposed to the weather, or long macerated
in water. The interior of most bones, in the higher classes
of animals, presents distinctly the appearance of irregular
cavities, resulting from the partial 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 dif-
ferent forms. Where a part is intended to have compact-
ness and strength, with a very limited degree of motion, it
is divided into a great number of small pieces, united toge-
ther by ligaments, and the separate bones are short and com-
pressed, 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 protection, or the provision of broad surfaces for
the attachment of muscles, we find the osseous structure ex-

STRUCTURE OF BONE. 2G1

pjtndcd into flat plates; as is exemplified in the hones of the
skull, in the shoulder hlade, and still more remarkahly in the
bony shield which surrounds the body of the tortoise. On
the other hand, where a system of levers is wanted, as in
the limbs, which have to sustain the weight of the trunk,
and to confer extensive powers of locomotion, the honcS arc
modelled into lengthened cylinders, generally somewhat ex-
panded at the extremities, 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 dynamics are more or less
exemplified in the construction of this part of the animal
fabric. Levers of various kinds are most artificially com-
bined in the formation of the fins of fishes, the wines 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 at-
tention has been bestowed on every condition by whicli me-
chanical advantage could be gained. In the more perfect de-
velopments of structures, such as those which oi)tain in the
higher orders of mammalia, and also in the class of birds, all
the long bones are hollow cylinders, and their cavity is
largest in the middle of their length. This is shown in Fig.
172, which represents a longitudinal section of a human
thigh bone, and in Fig. 173, which is a similar section of
the humerus, or bone of the arm. The walls of these bones
consist of a dense and compact substance, formed by the
close coliesion of the osseous plates. These walls arc of

262

THE MECHANICAL FUNCTIONS.

greater thickness in the middle of the shank or shaft of the
column, and hecome thinner as we follow them towards

either of the ends. This gradual diminu-
tion in the thickness of the walls arises
from the continual separation of the plates,
which bend inwards, and crossing each
other, leave a multitude of irregular
spaces or cells, which are called cancelli.
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 extremi-
ties of the bone, where this spongy or can-
cellated 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 cal-
culated for such resistance than v/hen in the form of a tube,
or hollow cylinder.* To this mechanical principle I have
already had occasion to advert, when speaking of the hollow
stems of vegetables, which derive their chief strength from
their possessing this form;! and we now find it again ap-
plied 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. Porterfiekl, in a paper contained in the first volume of
Medical Essays and Observations, publislied by a Society in Edinburgh,
p. 95.

t P. 70.

OSSIFICATION. 263

§ 3. Formation and Development of Bone.

But it is not enough to contemplate the purposes so admi-
rably answered by tliesc arrangements. Our curiosity can-
"iiot but be powerfully excited to learn what processes and re-
fined 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 permitted to
penetrate a little farther than usual into the secrets of orsjanic
evolution: for the succession of changes can be better followed
by the eye in the slow development of the harder parts, than
in the quicker growth of mere yielding and expansible tex-
tures. The peculiar material, also, of which bone is formed,
is easily distinguished by its hardness, its whiteness, and
its opacity from the softer and more transparent animal sub-
stance with which it is intermixed. Hence we are allowed
an opportunity of observing the earliest stages of its deposi-
tion, and of accurately following the subsequent chano-cs it
undergoes. •

The parts of the embryo animal, which are destined to
become bone, partake of the soft and gelatinous consistence,
which, at that early period, characterizes all the textures of
the body; and they can hardly, indeed, be distinguished
from the semi-fluid portions which surround them. In pro-
cess of time, when tlic vascular circulation of the blood has
been established, and the newly formed arteries have extend-
ed their branches over every part of the nascent organization,
those vessels which are appropriated to the task of forming
the bones, arrive at the pulpy masses where their work is
to commence. As sculptors, before working upon the marble,
first execute a model of a coarser and more plastic material,
so the first business of these arteries is to prepare a model
of the future bone, constructed, not with the same material
of which it is afterwards to consist, but with 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 conii)arcd with tiic adjacent jelly. It

264 THE MECHANICAL FUNCTIONS.

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
development as to have acquired a certain degree of soli-
dity and firmness, and to bear, as well as to require, the
support of more massive and rigid structures, this flexible
and elastic cartilage may be employed with great advantage
as its substitute. A hard and unyielding structure would,
in the early stages of its formation, have even been injuri-
ous. But in proportion as the fabric is enlarged, the ne-
cessity for mechanical support increases, and farther provi-
sion must be made for resistance to external violence.

When, at length, all is prepared for the construction 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 car-
ried away, piece by piece, while the operation of fixing in
their position the beams and pillars of the edifice proceeds.
The way is cleared at first b}^ the absorption of the central
part of the cartilage, and a few particles of ossific matter
are deposited in its room. While this process is going on,
greater activity is 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 can-
not pursue them to their minuter ramifications. They now
assume more active functions, and hasten to execute their
task by depositing granules of calcareous phosphate: these
are laid down, particle by particle, in a certain determinate
order, and in regular lines, so as to foWn continuous fibres.
When a great number of these delicate fibres are gathered
together, and connected by other fibres, which shoot in va-
rious directions across them, a texture composed of an as-
semblage of long spicula, or thin plates, is constituted.
In the cylindrical bones, the spicula prevail, and they are

OSSIFICATION.

265

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 principally 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 depo-
sited, but by the addition of new particles at both their ex-
tremities. In like manner, the ring increases in thickness,
not by the deposition of phosphate of lime between the ori-
ginal 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 simi-
lar to what I have just described; only the fibres have a ra-
diated arrangement, shooting out from the spot where the
first deposite took place, as from a common centre. This is
seen in Fig. 174, which represents the parietal bone of the

175 iT/i 175

human skull, in an early stage of its ossification, and shows
the radiating fibres very distinctly. In the cubical, and
more irregularly shaped bones, the process is, doubtless,
conducted with the same order and regularity, although it
cannot so readily be followed by the eye.

The same process is repeated in different parts of the bone,
wherever nature has, in conformity with determinate laws
of development, appointed particular centres of ossification.
The bone continues to extend from each of these centres,
proceeding gradually towards the circumference, or the re-
moter parts of the cartilage, on which the ossific materials
are moulded, and by the form of which that of the future

Vol. I. 34

266 THE MECHANICAL FUNCTIONS.

bone is regulated. The process of ossification has, however,
this peculiarity, that the cartilage is progressively absorbed
to make room for the deposites of bony substance. When,
the bone is long, separate points of ossification appear in the
extremities, before the central portions 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 pro-
tect, requires their protection at a very early period of life.
The growth of so large a surface of bone, as would be re-
quired for covering the brain, could not have proceeded
with sufficient quickness for the exigencies of the occasion,
if it had originated from a single point. Therefore it is
that, besides being commenced at a very early age, the pro-
cess goes on from a great number of separate points at the
same time. The ossification is evidently hurried on in order
to complete the roofing in of the edifice by the time at which
the animal is to be ushered into the world, and exposed to
dangers from the contact of external bodies. The divergent
fibres shoot out rapidly, coalescing with those in their im-
mediate neighbourhood, which co-operate to form an exten-
sive 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 origi-
nated from other centres, and are proceeding in a contrary
direction. Yet the arteries still continuing to deposite ossific
matter, each set of fibres insinuate themselves between those
of the opposite set, for some little distance, and until their
farther 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

OSSIFICATION. 267

of a saw, presenting externally the zig-zag line of junction
which is called a sulicre. 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 com-
pletely ossified.

The union of bony fibres proceeding from different cen-
tres of ossification is not indiscriminate, but is found to be
regulated by definite laws, and to have certain relations to
the general plan of conformation originally established.
Each distinct bone is formed from a certain number of ossi-
fic centres, which altogether constitute a system appertain-
ing to that bone only, and not extending to the adjacent
bones. These pieces unite together, as if by a natural affi-
nity; and they refuse to unite with the bony fibres proceed--
ing from neighbouring centres, and belonging to other
groups. The groups themselves are not arbitrary, but are
pre-established parts of the original design. Circumstances
occasionally, indeed, arise, which may overrule this inhe-
rent tendency to preserve the line of separation between
two bones; and we then fiind them coalescing to form a sin-
gle piece. Such unions are technically called anchyloses.

Were this the whole of what takes place in the formation
of a bone, the process would not, perha])s, differ very mate-
rially from that by which a shell is produced; for a shell,
as we have seen, is the result of successive depositions of
calcareous matter, forming one layer after another, in union
with a corresponding deposite of animal membrane. But the
subsequent changes which occur, show that the constitution
of bone is totally dissimilar to that of shell: for no portion
of the shell that is once formed, and has not been removed,
is subject to any farther alteration. It is a dead, thougli per-
haps not wholly inorganic mass; appended, indeed, to the
living system, but placed beyond the sphere of its influence.
But a bone continues, during the whole of life, to be an in-
tegrant part of the system, partaking of its changes, modi-
fied by its powers, and undergoing continual alterations of
shape, and even renewals of its substance, by the actions of
the living vessels.

268 THE MECHANICAL FUNCTIONS.

The form which had at first been rudely sketched, slow-
ly advances towards perfection in the course of its 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 si-
milar internal cavities, but being frequently excavated in parts
which had before been solid. During all these gradual altera-
tions of shape, however, there is no stretching of elastic parts;
for all the osseous fibres and laminse are rigid and unyield-
ing, and in this respect retain an analogy with shell. The
changes thus observed can have been effected in no other
way than by the actual removal of such parts of the young
bone as had occupied the situations where vacuities are found
to exist in the old bone. We find, for instance, that in the
early state of a bone there are no internal cavities, but the
whole is a uniform solid mass. At a certain stage of ossifi-
cation cells are excavated by the action of the absorbent ves-
sels, which carry away portions of bony matter lying in the
axis of the cylindrical or in the middle layer of the flat bones.*
Their place is supplied by an oily matter, which is the mar-
row, 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 ab-
sorbent vessels, so that the cavity is farther enlarged. In
this manner the outermost layer of the young bone gradual-
ly changes its relative situation, becoming more and more
deeply buried by the new layers which are successfully de-
posited, and which cover and surround it; until by the re-
moval of all the layers situated nearer to the centre, it be-
comes the innermost layer; and is itself destined in its turn
to disappear, leaving the new bone without a single particle
which had entered into the composition of the original struc-
ture.

It has been found that by mixing certain colouring sub-

• The bones of the lower classes of vertebrated animals, as of Fishes and
Reptiles, seldom reach this stage of ossification, but remain solid throughout;
corresponding to the bones of the higher classes at the early periods of their
development.

SKELETON OF THE VERTEBRATA. 209

stances wllh the food of animals the bones will soon become
deeply tinged by them. This fact was discovered acciden-
tally by Mr. Belchier, who gives the following account of
the circumstances that led him to notice it.* Happening
to be dining with a calico printer on a leg of fresh pork, he
was surprised to observe that the bones, instead of being
white as usual, were of a deep red colour; and on inquiring
into the circumstances, he learned that the pig had been fed
upon the refuse of the dying-vats, which contained a large
quantity of the colouring substance of madder. So curious
a fact naturally attracted a good deal of attention among
physiologists, and many experiments were undertaken to
ascertain the time required to produce this change, and to de-
termine 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 the 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 solidity, but more remote from the
heart, the tinge was fainter. The bone was of a deeper dye
in proportion to the length of time the animal had been fed
upon the madder. When this diet was discontinued, the
colour became gradually more faint, till it entirely disap-
peared. I shall have occasion in the sequel, to discuss the
inferences which have been drawn from these curious facts.

§4. Skeleton of the Vertchrata.

The purposes to be answered by the Skeleton, in vertc-
brated animals, resolve themselves into the three following;
first, the affording mechanical support to the body generally,
and also to different portions of the body; secondly, the pro-

* Philosophical TransucUons for 1736, vol. xxxix. 287 and 289.

279 THE MECHANICAL FUNCTIONS.

viding a solid basis for the attachments of the muscles which
are to effect their movements; and, thirdly, the giving pro-
tection to the vital organs, but more particularly to the cen-
tral parts of the nervous system. Of these, the last is the
circumstance that has the greatest influence in determining
the principles on which the osseous frame-work has been
constructed. In the 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 sub-
stance, are the most important parts of that sj^stem, 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 deno-
minated the spi)io -cerebral axis, in contradistinction 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 ner-
vous system, characterizing these two great divisions of the
animal kingdom. In the embryo state of the vertebrata, the
central parts of that system consist of two separate filaments,
running parallel to each other the whole length of the body:
but, in process of time, these two filaments unite, and con-
stitute a single spinal cord: and the primary type of the ske-
leton is determined by the peculiar form of this, the central
organ of the nervous system.

In laying the foundations of the skeleton, then, the first
object is to provide for the security of the spinal cord: and
this is accomplished by enclosing it within a series of carti-
laginous 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. These rings form a co-
lumn, extending, in a longitudinal direction, along the trunk;
retracing to us the series of horny rings, in which the bodies
of worms, of insects, and, indeed, of all the %/irticulata^ are

VERTEBRAL COLUMN.

271

incased. When ossified, these several rings arc termed vo'-
Uhrx; and the entire column which they compose is the
Spine. Fig. 177 shows the form of one of the verteb^ of
the back in the human skeleton. Fig. 178 is a side view of
four vertebrae joined together, and Fig. 179 is a vertical sec-

tion of the same part of the spine, showing the canal formed
by the rings. From the constancy with which the spinal
column is found in all animals of this type, and from the
uniformity of the plan on which, amidst endless variations,
it is modelled, it has been chosen as the distinctive charac-
ter of this great assemblage of animals, which have, accord-
ingly, been denominated the Verlebrata, or Vcrtcbratcd
Jinimals,

Nor is the spine of less importance when viewed in its
mechanical relations to the rest of the skeleton. It is the
great central beam of the fabric, establishing points of union
between all its parts, and combining them into one conti-
nuous frame-work: it is the o;eneral axis of all their motions,
the common fulcrum on which the principal bones of the
extremities are made to turn: it furnishes lixed points of at-
tachment to all the large muscles which act upon these bones
as levers, and, also, to those which move the trunk itself.

If this column had been perfectly rigid, the whole frame-
work would Ti^'C been exposed to inconvenience and even
danger, amidst the shocks it must encounter during all tlic
quick and sudden movements of the body. Not only must
its mechanism be framed to sustain these shocks, but also to

272 THE MECHANICAL FUNCTIONS.

accommodate itself to various kinds of flexions, and twist-
ings of the trunk. While these objects are provided for,
car^ust at the same time be taken that the spinal marrow
it encloses shall, amidst all these motions, remain secure from
pressure; for so delicate is its structure that the least degree
of compression would at once interrupt its functions, and
lead to the most fatal consequences. A safe passage is like-
wise to be afforded to the nerves, which issue from the spi-
nal marrow, at certain intervals, on each side throughout
its whole length.

No where has mechanical art been more conspicuously
displayed than in the construction of a fabric capable of ful-
filling these opposite, and apparently incompatible functions.
The principal difficulty was to combine great strength with
sufficient 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 connecting ligaments, is al-
lowed but a very small degree of flexion at the point of junc-
tion. This slight flexion at each single joint, however, by
becomino- multiplied along the series, amounts to a consider-
able 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
178 and 179,) composed of a peculiar substance, intermediate
in its texture to ligament and cartilage, and possessing in a
remarkable 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, how-
ever the spine may be bent, no chasm is lefLby the flexions
of the vertebrae upon one another, nor is tlie continuity of
the column in the smallest degree interrupted.

The motions of the vertebrae upon each other are farther
regulated by the mode in which their articular processes,

VERTEBRAL COLUMN. 273

which are the pieces that project ohllquely on each side,
play upon each other. These processes, which are seen at
A, A, in the preceding figures (177 and 178) arc of great use
in preventing the sudden displacement of the vertebrae; 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 together, and the skill displayed in combining great
flexibility with such powerful resistance to every effort that
can be made to separate them.

For the purpose of allow^ing a passage to the spinal mar-
row, tlie 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 passage of the nerves
as they issue from the spinal marrow. All these circum-
stances are visible in the figures, particularly in the section.
Fig. 179, where c, c, is the canal for the spinal marrow, and
in which the apertures just mentioned are distinctly seen, at

o, o.

In order to give an advantageous purchase to the muscles
which are attached to the spine, each vertebra lias, besides
the parts above described, a projecting piece of bone, ex-
tending upwards from the crown of the arch, and denomi-
nated 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 vertebraj, tw^o other projecting pieces, which
are denominated the transverse processes (t,) and which
serve as levers for bending the column laterally, that is, ei-
ther to the right or to the left. All these com])onent parts
of the spine are subject to considerable modifications, in dif-
ferent tribes of animals, according to the particular mccha-

VoL. I. 35

274

THE MECHANICAL FUNCTIONS.

nical circumstances of the system, and to the particular in-
tentions of their formation.

There is scarcely any part of the osseous fabric of which
the variations better illustrate the strict unity of plan and
the beautiful law of gradation observed by nature in all her
operations, than the spine. In studying the various modifi-
cations which this part of the skeleton undergoes, it will be
useful to bear in mind the principles which appear to regu-
late its formation, and which Geoffroy St. Hilaire has de-
duced by following the history of its early growth, and no-
ticing the order in which its several parts are developed.*
In common wath all bones, the vertebrae take their rise from
certain determinate points, or centres of ossification, where,
at first, detached pieces of bone are formed, destined to
unite together so as to compose the entire bone. An accu-
rate knowledge of the general forms and relative situations
of these elementary pieces is of much importance, because
we find that particular circumstances determine the deve-
lopment of some of these parts much earlier, and to a greater
extent than other parts, and thus lead to great differences in
the shapes and proportions of various bones, at different pe-
riods of their growth, although their origin and composition
are essentially the same.

The number of elements which enter into the composi-
tion of a vertebra has been differently estimated by different

physiologists: but the following are
certainly entitled to that character.
They are represented in their relative
situations in Fig. 180. The first is
the part which forms the nucleus, or
body (b) of the vertebra; and its ossi-
fication begins at the centre. Next
in importance are the two bony plates,
or leaves, as they may be called (l,
L,) which proceed from the sides of
the body, and embrace the spinal
marrow which is situated between
them. The fourth essential element
* Memoii'es du Museum, ix. 79 and 89.

STRUCTURE OF VERTEBRA. 275

is the spinous process, (s,) which unites the two leaves, and
thus completes the superior arch, of which it may he re-
garded as the key stone, for the protection of the spinal
marrow. Then come the two transverse jyrocesses (t, t)
which extend outwards from the sides, and with which the
arches of bone, that constitute the ribs (r, r,) are «renerally
connected. These are the six parts which may be consi-
dered as the elements that are most essential, and most con-
stantly present in the composition of the vertebrae. But
some other parts may also be noticed as of very frequent
occurrence: such are the bony plates which cover the two
flat portions of the bodies of the vertebrae, forming the sur-
faces immediately contiguous to the intervertebral ligament;
which surfaces, in some of the lower orders of the verte-
brata become articular. There is frequently, also, a deve-
lopment of processes, (f,) forming arches and spines at the
lower surface of the vertebrae, or the one opposite to that
which gives rise to the superior arches already mentioned.
This structure is very generally met with in fishes, and it
is observed also in the cetacea. The arches thus formed
enclose a large artery, which is the continuation of the aor-
ta, 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 surfiices
for the purpose of being connected with the surfaces of cor-
responding processes 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, should not be
included among the primary elements of the vertebrae, be-
cause we find them, in different instances, occupying differ-
ent positions, and formed sometimes by extensions of the
bodies, and at other times of the leaves. In following tliem
through the several tribes of animals, we observe them shift-
ing 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

276 THE MECHANICAL FUNCTIONS.

one another, and a third posterior to these. In the Orni-
thorhyncus, while the latter retains its situation in the mid-
dle, 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 Avholly
disappeared, and the outer surfaces have risen into what are
termed the oblique processes.

In addition to these, accessory bones are often 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 distance,
as if they had slipped from their proper situations; they are
then found between the spinous processes, and receive the
name of interspinous hoiies.

The spinous processes have a tendency, when their de-
velopment proceeds, to divide into two branches, and this bi-
furcation frequently takes place also in the interspinous bones.
The transverse 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 po-
sition, as we follow them backwards along the spinal column,
where they descend tow^ards the abdominal region.

The flexibility of particular portions of the spinal column
is regulated by the size and form of its processes. When
these are much developed, they necessarily obstruct the flex-
ion of the vertebrae in the directions in which they are situ-
ated : 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 principally
wanted. In dolphins, and other cetacea, on the contrary,
where the actions are required to be vertically upwards and
downwards, the spinous processes are small, and the trans-
verse processes very long and broad.

STRUCTURE OF THE SPINE. 277

Every instance of variation in the forms of these impor-
tant parts of the osseous system, will, in like manner, be
found to have a relation to some particular circumstance in
the living habits of the animal, and to be subordinate to the
general plan of its economy. But, in order to understand
the mode in which nature has effected these changes, it is
necessary to study the elements of each part of the osseous
system: for these constitute the alphabet by which the com-
binations she presents to us become legible, and their ori-
gin and jDrogress are unfolded to our comprehension. Ac-
cording as each of these elements of ossification receives dif-
ferent degrees of development, so do the different bones
they compose acquire their particular shapes and relative di-
mensions. Sometimes, indeed, we find that one or other of
these elements has disappeared; or, at least, we can discover
no trace of its development; in other cases, we see it ex-
ceedingly 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 spinous bones of fishes,
its accessory structures are multiplied, as if continued eflbrts
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 im-
portance, and wdiich are constituent parts of the primordial
type of the class to which the individual animal belongs.

The spinal column is generally prolonged at its posterior
extremity into a scries of vertebra}, which are sometimes
exceedingly numerous; decreasing in their size as they ex-
tend 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 be-
come stinted in their growth and ossification, until w^e find
the terminal pieces generally remaining in the state of car-
tilage. Such is the structure of the osseous support of the tail,
as seen in many quadrupeds in its most developed forms. It
illustrates the law, that when in any system there occurs a
frequent repetition of the same structure, the evolution, in

278 THE MECHANICAL FUNCTIONS.

the latest of those repetitions, becomes less perfect, and ends
by being abortive. In the present instance, the consequences
of this law are highly advantageous, since it provides for
the flexibility of the tail, and qualifies it for being applied
to a great variety of useful purposes, as we find more espe-
cially exemplified in the Ateles, or spider monkey, and in
the Kangiiroo,

Next in importance to the spine is the craniuvi, or osse-
ous covering of the brain; together with the bones of the
face, which protect the organs of the finer senses. An ac-
curate 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 cokimn, and
that they are, in fact, developments of vertebrae, much al-
tered, indeed, in shape, in consequence of the new condi-
tions to which they have been subjected; but still possessing
all the essential elements of vertebrae. In the embryo con-
dition of these organs, and while the brain is yet undeve-
loped, the resemblance of the bony circles which enclose it
to vertebrae is certainly very striking; but in proportion as
the brain becomes expanded, the similarity diminishes; for
the rapid growth of the brain in the higher orders of animals
is necessarily attended with an equally sudden expansion of
the bones of the skull. Hence, their several elements are
thrown into unusual positions, and being variously distorted
and disfigured, can hardly be recognised 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 tend to support the hypothesis that the bony
coverings of the brain are the result of the development of
three vertebrae. According to this theory, the first of these
supposed cranial vertebrse, beginning our enumeration from
the neck, is the origin of the occipital bone, of which the
lower part, or that which immediately supports the cerebel-
lum, 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,

SKELETON OF VERTEBRATA.

279

in process of time, the posterior half of the sphenoid bone,
which lies in the middle of the basis of the skull; the tem-
poral bones being formed by its leaves, and the parietal bones
by the lateral halves of its spinous process. The third cra-
nial vertebra is constituted by the anterior half of the sphe-
noid bone, which is its body, and the frontal bones, which
are its leaves. This theory, which originated with Oken,
has been farther extended to the bones of the face, by Geof-
froy St. Hilaire, who conceives them to be likewise deve-
lopments 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.

All the other parts of the skeleton may be regarded as ac-
cessory to the spine: and they are far from exhibiting the
same constancy either in form or number, as the vertebral
column. In some instances, as in serpents, these accessory
parts are altogether wanting; in others, they exist only in
rudimental states ; and it is but in a few that they can be
considered as having reached their full development. In or-
der to obtain a standard of comparison by which to estimate
all their gradations of evolution, it will be best to consider

* In this theory of G. St. lliluire, the number of cranial vci-tebra; is seven,
each composed of nine elementary pieces.

280 THE MECHANICAL FUNCTIONS.

them first in their more perfectly developed forms, as they
are presented in the higher classes of quadrupeds. In the
following descriptions, the skeleton of the Hog (Fig. 181) will
be taken for the purpose of reference.

The ribs consist of arches of bone affixed at their upper
ends to the bodies of the vertebrae, and also, by a separate
articulation, to their transverse processes ; where, in general,
they are allowed a slight degree of motion. Their primary
use is to defend the vital organs situated in the region of the
chest, or thorax, (namely, the heart and the lungs ;) but they
are subservient also to the function of respiration, by the al-
ternate movements that are given to them by their mus-
cles. The two parts, of which they are composed, often form
an angle by their junction, and at this angle a process occa-
sionally 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 continua-
tions of them, and are provided apparently to eke out the re-
mainder of the semicircle. These cartilages, which have
been termed the sterno-costal appendices, often become ossified
either wholly or in part.

The sternum is formed of nine elementary pieces, each pro-
ceeding from a separate centre of ossification. Two of these
occupy the end which is nearest to the head, four are lateral,
and two are situated at the opposite extremity : one only be-
ing central and surrounded by the rest. Few subjects in
comparative osteology are more curious and instructive than
to trace the development of these several elementary parts
in the different classes of animals, from the rudimental states
of this bone as it occurs in fishes, to its greatly expanded con-
ditions in the tortoise and the bird, wdiich severally exhibit
the most opposite proportions of these elements.

Last in the order of constancy come the bones of the ex-
tremities. As we ascend in the scale of animals we may
observe the prevalence of a tendency to the concentration
of organs, and consequently to the diminution of their num-

SKELETO.V OF VERTERRATA. 281

ber. While in animals of llic inferior orders, which arc
possessed of extremities, we find a considcrahle 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 interme-
dium of large and broad bones, which are intended to serve
as a basis for their more secure attachment, and for giving,
at the same time, extensive and advantageous purchase to
the muscles, which are to move the limbs. The two bones
by which the anterior extremity is connected with the
trunk are the hlade-hone, or Scapula, (u,) which sends out
a process called the coracoid bone; and the collar-hone, or
the Clavicle,'^' which extends from the scapula to the ster-
num. The corresponding connecting bones of the posterior
extremity are three in number, and constitute, together with
the part of the spine to which they are attached, what is
called the Pelvis (p.) The part of the spine which is thus
included in the pelvis, is termed the Sacrum. In its com-
plete state of ossification it is a single bone; but it was ori-
ginally composed of a number of separate vertebras, which
have afterwards become consolidated into a single bone, and
which bear the marks of having been compressed from be-
hind forwards during their growth, so^that they could only
expand laterally. The vertebrae which succeed to these,
and which are not consolidated with the sacrum, compose
what is called the os coccygis, (q,) or more properly the
coccygeal verlehrx: when they are sufliciently numerous to
compose a tail, they come under the denomination of caudal
vertebrx. The three bones of the pelvis, are the iliu7?i, the

• This bone does not exist in the skeleton of the hog-: but its form and
connexions with the sternum and scapula, in the liuman skeleton, arc slio\sn
in Fig-. 182, where s is the sternum; c, the clavicle; h, the scapula; a, the
acromion; k, the coracoid process; and g-, the glenoid cavity for the articula-
tion of the humerus.

Vol. I. 36

282 THE MECHANICAL FUNCTIONS.

ischhim, and the pubis. They all concur in the formation
of a large cup-like cavity, called the aceiabidum, which
receives the head of the thigh bone (r,) constituting, gene-
rally, the largest joint in the body.

A single bone composes the first division of each limb,
both in the fore and hind extremities. In the fore leg it is
termed the humerus (n,) in the hind leg, the femur (f.)
The next division contains two bones, placed parallel to each
other; they are, in the former, the radius (r,) and the ulna
(u;) in the latter, the tibia (t,) and fibula (f.) These are
followed by a number of small, rounded, or cubical bones,
collected together in a group, which constitutes the Carpus
(w,) in the fore leg, and the Tarsus (t,) in the hind leg.
Next come a set of long cylindrical bones, composing the
metacarpus (m,) in the former, and the metatarsus (m,) in
the latter case. In the most complete forms of development,
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 ex-
terior part of the angles which they form by their junction,
serving the purpose of giving a more advantageous position
to the tendons of tl\e muscles which extend those joints.
i:he patella, or knee pan (k,) is the largest of these, and is
pretty constantly present. Smaller bones of this description
are met with on the joints of the fingers, and are termed ^e-
samoid bones.

On comparing these divisions of the limbs of quadrupeds
with those of insects, we cannot fail to perceive that there
exists between them a marked analogy; and that naturalists
were not led away by mere fancy when they applied to the
latter the same names as those borne by the former. This,
however, is not the only instance of analogy that may be
discovered between the structures of articulated and of ver-

SKELETON OF VERTEIJRATA. 283

tebratcd animals, however strong may be the contrast wliich
they offer in all the essential features of their conformation.
The rings whicli compose the skeleton of the insect, and
which enclose its principal nervous chords, have been sup-
posed to have an analogy with the circles of bone which con-
stitute the primary forms of the vertebrae, and which con-
lain the spinal chord; although, in the first case, it is true,
other viscera are included within the arches, whereas, none
are contained in the last case. They agree, also, in having
the head placed at one extremity, distinct from the trunk,
and containing the principal organs of the senses. Farther
correspondences have been likewise traced in the minuter
anatomy of these parts, which it would here occupy too
much space to examine in detail.

An approximation is evidently made towards an internal
skeleton in the cephalopodous mollusca; wlicre 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 cartila-
ginous spine of the fish called the Myxine, or Gasirobran-
chus, which does not enclose the spinal marrow, but only
admits it to pass along a groove in its upper edge.

All these multiplied instances, when weighed together,
and united in a comprehesive view, are sufiicient to prove,
that there exist very perceptible links of connexion among
all the classes of created beings, even in those apparently
the most remote from one anotlier. They render it clear to
the discerning eye of the philosophic naturalist, that all tiie
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 dis-
penses all his gifts with the most salutary regard to the ge-
neral welfare of his creatures.

( 284 )

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 b}^ Fishes, a class com-
prehending an immense number of species, which are all
inhabitants of the water, which exhibit an endless variety
of forms, and open to the physiologist a wide field of in-
teresting research. We cannot fail to perceive, on the most
cursory glance, the beautiful adaptation of the form and struc-
ture of all these animals to the properties of the element in
which they are destined to reside. In order that the fish
might glide through the fluid with the least resistance, all
its vital organs have been collected into a small compass,
and the body has been reduced into the shape of a compact
oval, compressed laterally: and tapering to a thin edge, both
before and behind; for the purpose of readily cleaving the
water as the fish darts forward, and also of obviating the re-
tardation that might arise from the reflux of the water col-
lected behind. With a view^ to diminish friction as much
as possible, the surface of the body has been rendered smooth,
and the skin impregnated with oil, which defends it from
injurious impressions, and at the same time prevents the
water from penetrating into its substance.

The body of a fish is nearly of the same specific gravity
as the water it inhabits; and the efiect of gravity is therefore
almost wholly counterbalanced 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 w^ere precisely the same as that of the fish, the
animal would be able to remain suspended in any pari of the

FISHES. 285

.fluid without the necessity of employing any voluntary mo-
tion or exertion for that purpose: but as the body of a fish
is generally a little heavier than the fluid medium, csj)ecial-
ly if it be fresh water, it is necessary for the animal to give
its body some degree of motion, in order to prevent its
sinking.

In land quadrupeds, the limbs have to perform the double
office of supporting the body, and of eficcting at the same
time its locomotion: but as nearly the whole of the weight
of a fish is already sustained by the clement in which it is
immersed, its instruments of motion may be employed ex-
clusively for progression, and the powerful hydrostatic pres-
sure, which supports the body on all sides, supersedes the
necessity of that cohesive rigidity of frame, which is essen-
tial to the safety of terrestrial animals. Hence we find that
in one whole tribe of fishes, the skeleton is composed mere-
ly of cartilage; and, in all, it exhibits much less of the osse-
ous character than in the higher classes. The frame-work
of the skeleton, even of osseous fishes, has not the compact-
ness possessed by that of quadrupeds or reptiles: the pieces
which compose it are joined together less firmly; many of
them, indeed, remain in an imperfectly ossified condition,
their elementary pieces being detached from one andther, 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 which enters into their composition, the
bones of fishes contain but a small proportion of earthy in-
gredient, a circumstance which explains the pellucidity of
the mass, and the readiness with which the osseous fibres it
contains can be distinguished. Another consequence of the
want of density in the bones of fishes is, that their articula-
tions are less regular and perfect than the corresponding

286

THE MECHANICAL FUNCTIONS.

joints of terrestrial animals; for it is evident that where the,
parts are soft and flexible, joints are not required.

In the osseous fishes, the bony structures are more finished;
and they even arrive at a degree of hardness, equal to that
of the higher classes. But this development is not uniform in
all the bones; in the head of the pike, for instance, while
some of the bones have acquired a great hardness, others re-
main wholly and permanently in a cartilaginous condition.
The bones of fishes, however advanced in their ossification,
never reach that stage of the process in wdiich cavities are
formed ; thus there is no space for marrow, nor even for the
cellular or cancellated structure which we have noticed in
the more perfect bones.* The general disposition of the

bones which compose the entire skeleton will be understood
from Fig. 184, w'hich represents that of the Cijpriniis 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 m.ore pulpy texture.

Progressive motion in fishes is effected by the simplest
means, the principal instrument employed for this purpose
beina; the tail ; for the fins, as we shall presently find, are
merely auxiliary organs, serving chiefly to balance the body
while it receives its propulsion from the tail. A fish moves
in the water upon the same principle as a boat is impelled

Cuvier, sur les Polssons. Tom. i. p. 218.

FISHES. 287

in sculling ; for the action of the tail upon the \vatcr is late-
ral, like that of an oar, which it resembles in the vertical po-
sition of its plane; and the effect is transferred by the resist-
ance of the water to the body where the
impulse originates. Let us suppose, for
example, that the tail is slightly inclined
to the right, as shown in Fig. 185. If, in
this situation, the muscles on the left side,
tending to bring the tail in a right line
with the body, are suddenly thrown into
action, the resistance of the water, by re-
acting against the broad surface of the
tail in the direction p r, perpendicular to
that surface, will cause the muscular ac-
tion to give the whole body an impulse in that direction; and
the centre of gravity, c, will move onwards in the direction
c B, parallel to p r. This impulse is not destroyed by the far-
ther flexion of the tail towards the left side, because the
principal force exerted by the muscles has already been ex-
pended in the motion from r to m, in hringing it to a straight
line with the body ; and the force which carries it on to l is
much weaker, and, therefore, occasions a more feeble reac-
tion. When the tail has arrived at the position l, indicated
by the dotted outline, a similar action of the muscles on the
right side will create a resistance and an impulse in the di-
rection of K L, and a motion of the whole body in the same
direction, c a. These impulses being repeated in quick suc-
cession, the fish moves forwards in the diagonal c d, interme-
diate between the directions of the two forces. By bending
the whole body almost in a circle, and then suddenly straight-
ening it, fishes are often able to leap to the top of a high ca-
taract, in ascending against the stream of a river.

Such being the plan upon which progression is to be ef-
fected, we find that every part of the mechanism of the fish
is calculated to promote its execution. The principal mus-
cular strength is bestowed upon the movements of the tail;
and the largest assemblage of muscles consists of those which
give it the lateral flexions that have been just described.

288

THE MECHANICAL FUNCTIONS.

For this purpose, all the important viscera are placed for-
wards, 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 inter-
vening 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 com-
pose more than half, the bulk of the animal.

Having seen how all impediments to the free motion of
the tail have been carefully removed, let us next inquire into
the mechanism by which mobility has been given to that
organ. The first peculiarity we meet with in the structure
of the spine of fishes is the mode in which the vertebras are
connected together. The bodies of each vertebra, as may
be seen in Figures 1S6 and 187, are hollowed out, both be-

fore 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 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 vertebrae of a cod. The edges that are in
contact, are united all round by an elastic ligament, which
readily yields to the bending of the vertebrae upon one ano-

SKELETON OF FISHES. 289

ther 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 multiplied in the whole series, the resulting effect is
considerable. The cavity itself is filled with a gelatinous,
but incompressible fluid substance, which constitutes a sphe-
rical pivot for all the motions of the joint.

This singular kind of articulation would appear framed
with a view to allow of motion in all directions. 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
*. that are wanted in the fish, for striking the water laterally,
with the broad vertical surface of the tail. Processes of a si-
milar form and appearance, 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 por-
tion of the column. These are the i7ifenor spinous pro-
cesses, 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-
ber of vertebrae is very various in different fishes: in some
they are multiplied exceedingly, as in the shark, where there
are more than two hundred.

^ There are few parts of the structure of animals that ex-
hibit more remarkable instances of the law of gradation than
the spine of fishes, in which we may trace a regular progress
of development from the simplest and almost rudimental
condition in which it exists in the M^xi?ie and the Lam-
prey, 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 resem-
blance of the spinal column of the myxine, more especially,
to the annular condition of the frame-work of the vermes,
Vol. I. 37

290 THE MECHANICAL FUNCTIONS.

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 among ihe
vertebrata are, indeed, but slender and equivocal; for, in
place of a scries of bones composing the vertebral column,
it has merely a soft and flexible tube of a homogeneous and
cartilaginous substance, exhibiting scarcely any trace of divi-
sion into separate rings, but appearing as if it were formed
of a continuous hollow cylinder of intervertebral substance,
usurping the place of the vertebras, which it is the usual oflSce
of that substance to connect together, and having in its axis
a continuous canal filled with gelatinous fluid. This, how-
ever, 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 protection. The nervous matter here consists
merely of two slender cords, which run parallel to one ano-
ther in a groove on the upper part of the spinal column; and
these cords are covered only by a thin membrane, the pre-
sence of which it requires very 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-fish, which in form, texture, and situa-
tion, is very 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 vertebras 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
continuous canal throughout the whole column.

Proceeding to more advanced developments, we find, in

the sturgeon and other cartilaginous fishes, a greater conden-

# sation of substance produced by the deposition of granules

of osseous matter; the central canal becomes divided into

Jozenge-shaped compartments by the closing in of the sides

STRUCTURE OF FISHES. 291

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 these fishes remains in the incipient
stage of ossification, 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 with-
out uniting, they form no sutures', but overlap one another.
Thus the bony structures are detached, and often complete-
ly isolated; affording to the 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. This
knowledge is more especially of importance towards under-
standing the formation and connexions of the bones of the
head, which are very numerous and complicated; and the
investigation of which has been prosecuted with extraordi-
nary diligence by Geoflfroy St. Hilaire and other continental
zootomists.

It is here, more especially, that we obtain the clearest evi-
dence of the derivation of the cranial bones from vertebrae
analogous to those of the spine. The occipital bone, in par-
ticular, corresponds to a spinal vertebra in all its essential
elements. In many fishes, the body of this bone, being
lengthened out to form the posterior part of the basis of the
skull, becomes the 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

• A small aperture still remains, establisliing- a communication between
the cavities the whole length of the spine. This is supposed to be dcsig-ned
to obviate the compression of the fluid in the different cells or cavities during-
the motions of the spine. The vertical sections, Fig. 189 and 190, of two
contiguous vertebra in different fishes, will convey an idea of this gradation
of development.

292 THE MECHANICAL FUNCTIONS.

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 processes of the vertebrse, form together
a long cranial cavity. This cavity is placed in a direct line
with the spinal canal, and contains the nervous tubercles
which constitute the brain. Yet the brain does not com-
pletely fill this cavity; for a space is still left, which is occu-
pied 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 cen-
tral parts of the spine, this correspondence is less distinct, in
consequence of the various degrees of development which
these several elements have received, in order to adapt them
to particular purposes relating to sensation, to the prehen-
sion and deglutition of the food, and also to aquatic respira-
tion. It is impossible, however, without exceeding the li-
mits within which I must here confine myself, to enter into
the details of structure which would be requisite in order to
render this subject sufficiently intelligible.

The rest'of the skeleton of fishes is extremely simple. In
many, as in the Ray and Tetrodon, there are no ribs.
Where these bones exist, they are articulated with the ex-
tremities of the* transverse processes of the vertebrse, of
which they appear to be merely continuations, or appendi-
ces. There is generally no sternum to which they can be
attached below: in a few fishes only, such as i\iQ 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 correspond to
the arms and legs of quadrupeds, are the pectoral and ven-
tral fins (marked respectively by the letters p and v in Fig.
184.) The former are met with, with but few exceptions,

• The bony arches arising from the skull, which support the bronchiae, or
gills, have been considered as the bones corresponding to the ribs of terres-
trial quadrupeds; and if this view were taken of them, it would tend to con-
firm the analogy of the cranial bones to the spinal vertebra.

STRUCTURE OF FISHES.

293

in all fishes; and they consist of a scries of osseous pieces,
in which we may often recognise with tolerable precision
the analogous bones composing the anterior extremities of
a quadruped; such as the scapula, clavicle, humerus, ulna,
and radius.*' These two latter bones are very distinctly
marked in the Lophhts jmcatorius, or Ajigler, as may be
seen in Fig. 191, where b is the scapula; c, the clavicle; u,
the ulna; and r, the radius. The carpus may also be recog-

nised in a chain of small bones, w^, interposed between the
radius and the Phalanges, z. In the Ray 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 de-
tached from the rest of the skeleton, and at other times con-
nected with the spine: in most cases, however, it is sus-
pended from the cranium; a fact which may be cited in

• Those anatomists who are fond of pursuing" the theory of analogies,
maintain that all these bones are merely developments of certain ribs, pro-
ceeding- from the spine in its anterior parts. A similar orig-in has been as-
signed to the pieces of bone to which the ventral fins are attached: but it is
difficult to reconcile this tlieory with the fact that these bones do not pro-
ceed 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 analog-ous to the
bones of the hinder extremities in the mammalia, they arc in a condition of
very imperfect development.

294

THE MECHANICAL FUNCTIONS.

farther corroboration of the analogy which the cranial bones
have to vertebrae.

In the ray and the shark tribes, both the anterior and pos-
terior extremities are sup-
ported by arches of bones,
formino; a sort of belt. This
structure is an approach to
that which obtains in many
reptiles, and indicates a farther step in the regular progress
of development. This belt in the ray is shown in Fig. 193.
In examining that part of the skeleton of fishes which
corresponds to the posterior extremity, we observe the total
absence of both femur and tibia; but the bones of the toes
are attached to a set of small bones, which appear to act the
part of a pelvis, but which, in consequence of their not being
connected with the spine, have no determinate situation, and
are found at various distances from the head in different
fishes. They appear emancipated 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: and we
find them, accordingly, often placed considerably forwards;
and in some instances, as in the Subhrachieni, even anteri-
orly to the pectoral fins, which are the true arms of the ani-
mal. But in one whole order of fishes, the ^djjodes, there
is not even a vestige of ventral fins, nor are any pelvic bones
provided for their support. This is the case with the Eel,
the Gymnotus, &c. In a few species there is also a total
absence of pectoral as well as ventral fins.

The dorsal fins are supported by a series of slender bones
(d, Fig. 1S4,) which are joined to the spinous processes of
the vertebras, and are formed from distinct centres of ossi-
fication. These rays, as they are called, are sometimes
destined to grow to so considerable a length, as to require
being subdivided into many pieces, in order to lessen the
danger of fracture, to which a very long filajiient of bone
would have been exposed, and also to allow of a greater de-
gree of flexibility. These rays assume branched forms from

MUSCULAR SYSTEM OP FISHES.

295

the farther subdivision of their parts, and when, for the pur-
pose of adding strength to the fin, it becomes necessary to
multiply the points of support, intermediate bones are de-
veloped, serving as the basis of the rays. Convenience re-
quires that they should be detached from the ends of the
spinous processes, which is their usual position, and placed
between them: wdieh in this situation, they bear the name
of interspinoiis bones; and when a still greater length of
osseous support is wanted, new centres of ossification are
developed at their extremities, giving rise to a series of ad-
ditional pieces, joined end to end, and carrying out the in-
terspinous bone, and the ray which terminates it, to a con-
siderable distance. This structure is distinctly seen in the
small dorsal fins of the Mackeral. The anal fins, which
are situated on the lower side of the body, in the vertical
plane, and next to the tail, are, in like manner, supported
by rays, having the same parallel, or fan-like 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 man-
ner than those of the animals of the higher classes. Those
which appear immediately underneath the integuments are
shown in Fig. 194, where m, m are the great lateral muscles,

producing the flexion of the body and tail: d is the dorsal fin,
which is raised by the muscle d; the pectoral fin, expanded
by the muscle p: v, the ventral fin, moved by the mu.scles
situated at v: a, the anal fin, in like manner moved by nms-
cles at its base a: and c, the caudal fin, the muscles for
moving which arc seen at c: o is the operculum, or flap.

296 THE MECHANICAL FUNCTIONS.

which covers the gills : and n, the nasal cavities, or organs
of smell. The form of the hody, and disposition of the skele-
ton, allow of their being inserted immediately on the parts
which they are intended to approximate. Hence the use of
long tendinous chords is dispensed with.^'

The actions of the muscles are easily understood from the
nature of their insertions. In general, the direction of the
fibres is, in some degree, oblique, with reference to the mo-
tion performed. Two series of muscles are provided for the
movements of the tail, which consist almost exclusively of
lateral flexion, the whole spine in some degree participating
in this motion. These muscles occupy the upper and lower
portions of the trunk : their limits being strongly marked by
a line running longitudinally the whole length of the body on
each side. The inchnation of their fibres is somewhat diffe-
rent in each. The advantage in point of velocity of action
which results from this obhquity 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 ex-
panding and closing the rays ; for each of which motions ap-
propriate muscles are provided : and, indeed, each ray is fur-
nished with a distinct muscular apparatus for its separate mo-
tion ; and these smaller muscles regulate with great nicety all
the movements of the fins, expanding or closing them like a
fan, according as their action is to be strengthened or relaxed.
This feathering of the fin, as it may be called, takes place in
most fishes, and is particularly observable in the tail of the
Esox, or pike tribe. Each ray of these fins, indeed, is fur-
nished with a distinct muscular apparatus, for its separate
motion.

Whatever analogy may exist in the structure of the fins

* Between the layers of flesh, however, there occur slender semi-transpa-
rent tendons, which give attachment to a series of short muscular fibres pass-
ing nearly at right angles between the surfaces of the adjoining plates.
See Sir A. Carlisle's account of this structure in the Philosophical Transac-
tions for 1806.

SWIMMING BLADDER OF FISHES. 297

of fishes and the feet of quadrupeds, there is none in the
manner in which they are instrumental in clFccting pro-
gressive motion. Tlie great agent h}^ which the fish is im-
pelled forwards is the tail: the fins, wliich correspond to the
extremities of land animals, are useful chiefly for the pur-
poses of turning, stopping, or inclining the hody, 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 hody, (the mesial plane,) prevent the rolling of
the body, wdiile the fish darts forwards in its course. The
fins that are in pairs (that is the pectoral and the ventral
fins,) by their alternate flexions and extensions, act like
oars; while they are capable, at the same time, of expanding
and of closing the rays, like the opening and shutting of a
fan, according as their action is required to be eflective, or
the contrary. All these auxiliary instruments arc chiefly
serviceable in modifying the direction, and adjusting the
variations of force derived from the impulse of the tail.
:^They are employed, also, in suddenly checking or stopping
the motion, and giving it a more rapid acceleration. But
still the tail is the most powerful of the instruments for pro-
gression, being at once a vigorous oar, an accurate rudder,
and a formidable weapon of ofience.

Independently of these external instruments of progres-
sion, most fishes are provided with internal means of changing
their situation in the water. The structure by which this
efiect is accomplished is one of the most remarkable in-
stances that is met with of an express contrivance for a spe-
cific purpose, 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 muscular 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. Were
the animal to acquire the power of altering, at pleasure, its
specific gravity, it would then possess the means of rising
Vol. I. 38

298

THE MECHANICAL FUNCTIONS.

or sinking, without calling into action either the fins or the
tail. Such is precisely the object of a peculiar mechanism,
which nature has provided in the interior of the body of the
fish. A large bladder, filled with air, has been placed im-
mediately under the spine, in the middle of the back, and
above the centre of gravity. This is known by the name
of the air-bladder, or the swhnming bladder, and in the
cod-fish it is called the sound. It frequently, as in the
Carp, consists of two bladders (a, b. Fig. 195) joined end-

wise, and communicating with each other by a narrow neck.*
When distended with air, it renders the whole fish specifi-
cally lighter than the surrounding water; and the fish is
thus buoyed up, and remains at the surface without any ef-
fort 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 de-
sign than the placing of this hydrostatic apparatus, acting
upon philosophical principles, in the interior of the organi-
zation, for a purpose so definite and unequivocal?

In several tribes of fishes there is a canal (c d) establish-
ing a communication between this bladder and the stomach,
or the gullet (o;) so that by compressing the bladder, a quan-
tity of air may be forced out, and a very sudden increase of

* There is great variety in the form and structure of tlie air-bladder in
different fishes. Sometimes it contains a large glandular body of a peculiar
structure, which has been conjectured to be an apparatus for secreting air
from the blood: but it is by no means very generally met with.

SWIMMING BLADDER OP FISHES. 29.Q

specific gravity produced; followed, of course, by a quick
descent. When, by any accident, the air bladder has been
opened, or has burst, so that all the air has escaped, the fish
is seen to grovel at the bottom, lying on its back, and can
never afterwards rise to the 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 beconie 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 blad-
der to its former bulk, and restored the power of descending.
Some tribes of fish are totally unprovided with an air-
bladder. This is the case with the flounder, the sole, and
other genera of a flat shape, forming the family of Pleuron-
cetes. They are chiefly inhabitants 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 eflfort, for their ascent
must be accomplished entirely by the continued beating and
flapping of the water with their expanded pectoral fins. It
is only the larger fish of this form, such as rays, which have
very voluminous and powerful pectoral fins for striking the
water downwards with considerable force, that can rise with
facility without the assistance of an air-bladder. In these,
the lateral fins, which are enormous expansions of the pec-
toral fins, may be compared to wings, their vertical action
on the water being similar in eflfect to the corresponding
movements of a bird, when it rises vertically in the air.
Those fishes which swim rapidly, and frequently ascend and
descend in the water, are, in general, provided with the
largest air-bladders.

In studying the varieties presented by the forms of the
fins in different tribes of fishes, we find the same constant
relation preserved with the particular situations and circum-
stances in which they are placed. The dorsal fins, which

^'V

300 THE MECHANICAL TUNCTIONS.

are more especially useful for steadying the body, arc long-
est in those fishes which inhabit the most stormy seas. The
most voracious tribes, which incessantly pursue their prey,
are furnished with most pow^erful muscles, and possess the
greatest means of rapid progression. On the other hand,
many of the more pacific, and weaker species are studiously
guarded by a dense and hard integument, serving as a shield
against the attacks of enemies, and often armed v^ith sharp
points, which are sufficient to repel the most daring assail-
ant. The Balisies is covered with scales of singular hard-
ness, closely set together, and frequently having rough
edo-es. The Ostrucioii, or trunk fish, instead of these scales,
is provided wnth a kind of coat of mail, composed of osseous
plates, curiously joined together, like a tesselated pavement,
and reminding us of the arrangements we have seen adopted
in the calcareous coverings of the echinida.

Some of the cartilaginous fishes are, in like manner, pro-
tected by calcareous plates, appended to the integuments.
There is a row of plates of this kind, of a quadrangular
shape, which passes along the middle of the back in the stur-
geon: and the whole body of the Ostracion, or Trunk-fish,
is covered with osseous scales. All these have no imme-
diate relation to the skeleton, but are apparently remnants
of inferior types, of which one of the prevailing characters
is the external situation of the protecting organs.

Diodons and Tetrodons are remarkable for being provided
with the means of suddenly assuming a globular form, by
swallowing air, which, passing into the crop or first stomach,
blows up the whole animal like a balloon. The abdominal
renion beino; thus rendered the lightest, the body turns over,
the stomach becoming the uppermost part; and the fish floats
upon its back, without having the power of directing itself
durino: this state of forced distention. 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 present-

MOVEMENT OF FISHES. 301

mg 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 tlie seas of
India, so constructed as to be able to crawl on land to some
distance from the shore. One of these, the Ferca 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 ap-
parently as much ease and rapidity as a bird flics through
the air. Although this may partly be accounted for by the
size of their muscles, and the advantageous mode of their in-
sertion, 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 construction.!
Most fishes, however, move with the utmost rapidity, and
with scarcely any visible effort; and perform long journeys
without apparent fatigue. The Salmon has been known to
travel at the rate of sixteen miles an hour for many days to-
gether. Sharks often follow ships across the Atlantic, not
only outstripping them in their swiftest sailing, but playing
round them on every side, just as if the vessel were at rest.

* See the account given by Lieutenant Daldorff; Linnean Transactions, III.
62. I shall have occasion to notice, in the sequel, the remarkable conforma-
tion of the respiratory organs of these and other fishes, which enable them to
live, for a time, out of their natural element.

■J- Carlisle, Phil. Trans, for 1806, p. 9.

( 302 )

CHAPTER VIII.

REPTILIA.

§ 1. Terrestrial Vertehrata in general.

The numerous tribes of vertebrated animals wbich are
Strictly terrestrial, or destined to move on land, differ widely
in their modes of progression, and in the mechanical advan-
tages of their formation. The greater number are quadru-
peds; 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-
vancing; others outstrip the winds in fleetness. Some fami-
lies of reptiles are entirely destitute of any external organs
of motion, the whole trunk of the body resting on the ground :
while man occupies a place where he stands, alone, being
distinguished by the exclusive faculty of permanently sus-
taining himself on the lower extremities.

In reviewing the developments and the mechanical func-
tions 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 con-
necting the strictly aquatic, with the terrestrial vertebrated
animals: then, taking up this latter series, I shall consider
the more sim.ple 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 w^ith the human structure, which terminates this
extensive series.

BATRACHIA.

303

§ 2. Balrachia.

The order of Balrachia, or Amphibious Reptiles, con-
stitutes the first step in the transition from aquatic to terres-
trial vertebrata. It is more particularly the function of res-
piration that requires to be modified, in consequence of the
change of element in which the animal is to reside; and as
if it had been necessary, conformably to the laws of animal
creation, that this change should not be abruptly made, we
find that Batrachian reptiles, with which this series com-
mences, are constructed, at first, on the model of fishes;
breathing the atmospheric air contained in the water by
means of gills, and moving through the fluid by the same
instruments of progression as fishes, which, indeed, they ex-
actly resemble in every part of their mechanical conforma-
tion. The tadpole, which is the young of the frog, is, at
first, not distinguishable in any circumstance of its internal
skeleton, or in the disposition of its vital organs from the
class of fishes. The head, indeed, is enlarged, but the body

19;

immediately tapers to form a lengthened tail, by the pro-
longation of the spinal column, which presents a numerous
series of coccygeal vertebrae, furnished with a vertical ex-
pansion of membrane to serve as a caudal fin, and with ap-
propriate muscles for executing all the motions required in #
swimming. The appearance of the tadpole, in its early
stage of development, is seen in Fig. 197 and 198, the

304 THE MECHANICAL FUNCTIONS.

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 higher development. Prepara-
tions are silently making for a change of habitation, for the
animal's emerging from the waters, for the reception of at-
mospheric air into new cavities, for the acquisition of limbs
suited to new modes of progression; in a word^ for a terres-
trial life, and for all the attributes and powers which belong
to quadrupeds. The succession of forms, which these meta-
morphoses present, are in themselves exceedingly curious,
and bear a remarkable analogy with the progress of the trans-
formations of those insects, which in the first stages of their
existence are aquatic. To the philosophic inquirer into the
marvellous plans of creation, the series of changes which
mark these singular transitions cannot fail to be deeply in-
teresting; and occurring as we here find them, among a tribe
of animals allied to the more perfect forms of organization,
they afibrd us a better opportunity of exploring the secrets
of their development by tracing them from the earlier stages
of this complicated process so full of mystery and of won-
der.

The egg of the frog (Fig. 196) is a round mass of trans-
parent nutritive jelly, 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, however, are mere tempo-
rar}^ organs, for they serve the purposes of respiration only
until the proper gills are formed, and they then shrink and
become obliterated. The true gills, or branchise, are con-
tained within the body, and are four in number on each
side, constructed on a plan very similar to those of fishes.
Retaining this aquatic constitution, the tadpole rapidly in-

DEVELOPMENT OF THE BATRACHIA. 305

creases in size and in activity for several weeks. In the mean
time the legs, of which no trace was at first apparent, have
commenced tlicir o;rowlh. The hind ]e2;s are the first to
make their appearance, showing their emhryo forms with-
in the transparent coverings of the hinder part of the
trunk, just at the origin of the tail. These are soon suc-
ceeded by the four legs, which exactly follow the hind
legs, in all the stages of their development, 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 inter-
val 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 mouthful of air, which is re-
ceived into its newly developed lungs, and afterwards dis-
charging it in the form of a small bubble. AVhen the ne-
cessary internal 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, dimi-
nishes by degrees leaving only a short stump, which is soon
removed. The gills are by this time shrunk, and rapidly
disappear, their function being superseded hy 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 it 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 integuments.

No less curious are the changes which take place in all the
other organs for the purpose of effecting the transformations
rendered necessary by this entire alteration in all the ex-
ternal circumstances of that animal, — this total reversal of

Vol. I. ,39

306

THE MECHANICAL FUNCTIONS.

its wants, of its habits, of its functions, and of its very con-
stitution. I shall have occasion to notice several 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 progres-
sive motion. In order to form a correct idea of these re-
lations, it will be necessary to notice the leading peculiari-
ties of the skeletons of this tribe of animals.

The skeleton of the adult frog is shown in Fig. 200; from
which it will be seen that the spinal column is comparatively

much shorter than that of fishes, or, indeed, of any other
class of animals; for it consists of only eight vertebrae, ex-
clusive of those which have united to form the os coccygis.
It was evidently the intention of nature to consolidate the
frame-work of the trunk, in which flexibility was not re-
quired for progressive motion: the performance of that func-
tion being transferred to the hind extremities, which are ex-
ceedingly large in proportion to the rest of the body. There
is a tendency in every part of the skeleton to develope itself
in a transverse direction, while the trunk is shortened as
much as possible.

The mode in which the vertebras are articulated together,

SKELETON OF THE BATRACHIA. 307

differs widely from what we have seen in fishes, and ap-
proaches to the structure of tlie higher chisses of vertel)rata.
The body of each vertebra, instead of having at its posterior
surface a cup-like cavity, terminates by a projecting ball,
which is received into the cavity in the anterior surface of
the next vertebra, so as to compose a true ball and socket
joint, capable, when other circumstances permit, of a rotato-
ry 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 accom-
plished. By carefully watching 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 vertebra which is in front of it;
and the whole then becomes ossified, so as to compose only
one bone, its posterior surface remaining distinct, and 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, whicii, being joined
to the sacrum at an angle, gives rise to the strange deformi-
ty 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 j)ortion of the spinal marrow
which had passed through it, in the aquatic life of the ani-
mal, being now withdrawn. *

The theory of the spinal origin of the cranial bones re-
ceives 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 ca-

308 THE MECHANICAL FUNCTIONS.

nal, 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 sur-
faces, formed upon the root of each leaf of the occipital bone,
while its body, or basilar portion, is scarcely connected with
the first cervical vertebra, and has no articular surface.

In place of ribs, we find only small, slender, detached
bones, or rather cartilages, affixed to the extremities of the
transverse processes of some of the vertebras. They may
be regarded as rudi mental 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 pro-
cesses of the last vertebra of the back. This vertebra is much
broader than the rest, and, although it consists but of a single
vertebra, must be considered as a sacrum. The two pubic and
ischiatic bones are exceedingly small, but still contribute to
form the acetabulum, or cavity for the reception of the thigh
bone, at the hinder extremity of the slender bones above
mentioned. This is the simplest possible form to which the
pelvis can be reduced, while it preserves its attachments to
the spine. It presents, in this respect, a more advanced
stase of development than that of fishes.

The connexion of the bones of the anterior extremities
with the spine is analogous to that which takes place in rays
and sharks: there being an osseous belt formed by the sca-
pula, clavicle, and coracoid bone, with the latter of which
the humerus is connected. The sternum is large and con-
siderably developed; making some slight approach to the
expansion it receives in the Chelonia. The radius and ulna
are united into oneiDone: the bones of the arm and leg, in

* The plan of reproduction in these animals requires that the ovary, or or-
ffan which contains the eggs, should be capable of enormous dilatation, in
order to contain the immense bulk to which these eggs are expanded, pre-
viously to their being brought fortli. It was probably in order to make room
for this dilated ovary that the ribs have not been developed.

PROGRESSIVE MOTION IN BATRACIIIA. 309

general, resemble, in their figure and connexions, those of
the higher orders oi Mammalia, to the type of which this
order of reptiles is evidently making an approximation.
There are five toes in the foot, with sometimes the rudiment
of a sixth: the anterior extremity has only four toes, which
are without claws.

The necessity of employing the same instruments for pro-
gression in the water and on land, is probably the cause
which prevents their having the form best adapted for ei-
ther function. The hind feet of the frog, being well con-
structed for striking the water backwards in swimming, are,
in consequence, less capable of exerting a force sufficient to
raise and support the weight of the body in walking: and
this animal accordingly is exceedingly awkward in its at-
tempt to walk. On a short level plane it can proceed only
by leaps; an action which the length and great muscularity
of the hind legs particularly fit them for performing. The
toad, on the other hand, whose hind legs are^hort and fee-
ble, 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 develop-
ment of particular parts of the skeleton. The anterior ex-

• It is singular that the frog", though so low in the scale of vertebratcd ani-
mals, should bear a striking resemblance to the human conformation in its or-
gans of progressive motion. This ai-ises 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 sariorius, the use
of which is to turn the foot outwards, both in stepping and in swimming.

310 THE MECHANICAL FUNCTIONS.

tremitles 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 exceedingly small,
and the sternum continues cartilaginous. The pelvis has no
osseous connexion with the spine, but is merely suspended
to it by ligaments. The land salamanders have a rounded
tail, but the aquatic species, or Tritons, have it compressed
vertically; thus retaining the fish-like form of the tadpole,
and the same radiated disposition of the muscles.

§ 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 length-
ened 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.^ In

* Professor Mayer has, however, traced obscure rudiments of pelvic bones
in the Unguis fragilis, the Anguis ventralis, and the Typhlops avcotatus,
and is of ophiion that they exist much more generally in this order of rep-
tiles than lias been commonly imag-ined. Some serpents, as the Boa, Python,
Tortryx and Eryx, have claws, which may be considered as rudiments of feet,
visible externally. Ln others, as the Jnguis, Typhlops, and Amphisbocna,

SERPENTS.

311

the conformation of the skull and bones of the fiice, tlicy pre-
sent strong analogies with batrachian reptiles, and also with
fishes, one tribe of which, namely, theapodous oranguilliform
fishes, they greatly resemble by the length and flexibility of
the spine. These peculiarities of conformation may be in a
great measure traced to the mode of life for w^hich they are
destined. The food assigned to them is living prey, which
they must attack and vanquish before they can convert it into
nourishment. The usual mode in which the boa seizes and de-
stroys its victims is bycoilingthe hinder part ofits 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,
fasten upon the defenceless object of its attack, and twining
round its body so as to compress its chest, and put a stop to

they exist concealed under the skhi. In others, he has discovered cartilagi-
nous filaments, which he conceives to correspond to these parts. (Annales
des Sciences Naturelles, VII. 170.) Some of these are represented in the
following figures. Fig. 203 exhibits the claw of the Boa constrictor y placed

203

205 206

2or

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, repre-
sent the claws and rudi mental bones of the Tortrix scijtak, Tortrix coral-
linus, ^Wil Anguis fragilis, respectively. Tliose of the Jmphisbaena alba, Fig.
208, and the Coluber pullatus. Fig. 209, are still less developed. The Clial-
cides, or snake lizard, which has four minute feet, is represented in Fig. 210.

312

THE MECHANICAL FUNCTIONS.

its respiration. Venomous serpents, on the other hand, coil
themselves into the smallest possible space, and suddenly
darting upon the unsuspecting or fascinated straggler, inflict
the quickly fatal wound.*

It is evident, from these considerations, that, in the ab-
sence of all external instruments of prehension and of pro-
gressive 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 number of small pieces; secondly, by the great free-
dom of their articulations; and thirdly, by the peculiar mo-
bility and connexions of the ribs.

Numerous as are the vertebrae of the eel, the spine of
which consists of above a hundred, that of serpents is in
general formed of a still greater number. In the rattle-
snake [Crotahcs horridus) there are about two hundred;

and above three hundred have
been counted in the spine of
the Coluber natrix. These
vertebrae are all united by ball
and socket joints, as in the
adult batrachia; the posterior
rounded eminence of each ver-
tebra being received into the
anterior surface of the next.
Fig. 202 is a view of this por-
tion of the skeleton in the Boa
constrictor, showing the arti-
culation of the ribs with the
vertebrae.

While provision has thus been made for extent of mo-
tion, extraordinary care has at the same time been bestou'ed
upon the security of the joints. Thus, we find them efiectu-

* Their prey is swallowed entire; and therefore, as we shall afterwards find,
the bones of the jaws and face are formed to admit of great expansion, and
of great freedom of motion upon one another.

SERPENTS. 313

ally protected from dislocation by the locking; in, above
and below, of the articular processes, and by the close in-
vestment 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 flex-
ion, but to limit the vertical and longitudinal motions: and
whatever degree of freedom of motion may exist between
the adjoining vertebra?, 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 mo-
tions of the ribs, would have impeded all these movements,
and would have also been an insurmountable bar to the di-
latation of the stomach, which is rendered necessary by the
habit of the serpent of gorging its prey entire.

The mode in which the boa exerts a powerful pressure on
the bodies of the animals it has seized, and which it has en-
circled within its folds, required the ribs to be moveable la-
terally, as well as backwards, in order to elude the force
thus exerted. The broad convex surfaces on which they
play give them, in this respect, an advantage which the or-
dinary mode of articulation would not have aflforded. The
spinous processes in this tribe of serpents are short and wide-
ly separated, so as to allow of flexion in every direction. In
the rattle-snake, on the other hand, their length and oblique
position are such as to limit the upward bending of the spinal
column, although, in other respects, its motion is not restricted.
The vertebrae at the end of the tail are furnished with broad
transverse processes for the attachment of the first joints of
the rattle.

But of whatever variety of flexions we may suppose the
lengthened body of a serpent to be capable, it will, at first
view, be diflacult to conceive how these simple actions can
be rendered subservient 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. Tlifiy

Vol. I. 40

314 THE MECHANICAL FUNCTIONS.

raise themselves without difficulty to the tops of the highest
trees, and escape to their hiding places with a quickness
which eludes observation and baffles the efforts of their pur-
suers.

The solution of this enigma is to be sought for partly in
the structure of the skin, which, in almost every species, is
covered with numerous scales: and partly in the peculiar
conformation of the ribs. The edges of the scales form rough
projections, which are directed backwards, so as to catch the
surfaces of the bodies to which they are appUed, and to pre-
vent any retrograde motion. In some species, the integu-
ment is formed into annular plates, reminding us of the struc-
tures 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
upon the ground by numerous fixed points of support.

This support is farther strengthened by the connexion of
the ribs with the abdominal scuta, or the scales on the under
side of the body. The mode in which the ribs become aux-
iliary 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 succession, like the feet of a caterpillar. Sir
Everard Home, to wham 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 pe-
culiar, 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, possessed by most
vertebrated animals, and characterizing the type of their os-
seous fabric. In the ordinary structure, the head of each
rib has a convex surface, that plays either on the body of a

♦ Philos, Trans, for 1812, p. 163.

PROGRESSIVE MOTIOlff IN SERPENTS. 315

single vertebra with which it is connected, or upon the two
bodies of adjacent vertebrae : but in serpents the extremity
of the head of the rib has two slightly concave articular sur-
faces, which play on a convex protuberance of the vertebra.
This structure is attended with the advantage of preventing
the ribs from interfering with the motions of the vertcbrse
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
Ophiosaiirus and the Blind-worm {Anguis fragilis,) there is no
vestige either of a sternum or scapula, in any animal of this
class. Each rib terminates in a slender cartilage, tapering
to a point, which rests, for its whole length, upon the upper
surface of one of the scuta, or broad scales on the lower side
of the body. These scuta, which are thus 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 performing the office of legs. The
ribs move in pairs ; and the scutum under each pair, being
carried along with it in all its motions, and laying hold of the
ground by its projecting edge, becomes a fixed point for the
advance of the body. This motion. Sir E. Home observes,
is beautifully seen when a snake is climbing over an angle to
get upon a flat surface. When the animal is moving on a
plane, it alters its shape from a circular or oval form, to one
that approaches to a triangle, of which the surface applied
to the ground forms the base. Five sets of muscles are pro-
vided for the purpose of giving to the ribs the motions back-
wards and forwards, by which, as levers, they effect this
species of progression. These muscles are disposed in regular
layers ; some passing over one or two ribs to be attached to
the succeeding rib. In all snakes the ribs are continued
backwards much beyond the region occupied by the lungs;
and although the anterior set are subservient to respiration,
as well as to progressive motion, it is evident, that all those
posterior to the lungs must be employed solely for the latter
of these purposes.

316 THE MECHANICAL FUNCTIONS.

It is easy to understand how the serpent can slowly ad-
vance, 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 is fixed,
and the head projected forwards. By an alteration of these
movements, assisted by the actions of the ribs, the serpent
is enabled to glide onwards with considerable rapidity, and
without attracting observation. But where greater expedi-
tion 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 sepa-
rate from each other, the body being propelled by bringing
it from a curved to a straight line.

There is yet a third kind of motion, which serpents oc-
casionally 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 throw-
ing into violent action all the muscles on the opposite side,
the whole body is propelled, as if by the release and un-
winding of a powerful spring, with an impulse which raises
it to some height from the ground, and projects it to a con-
siderable distance.

Thus these animals, to which nature has denied all exter-
nal members, are yet capable, by the substitution of a differ-
ent 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 exe-
cuting leaps w^ith a vigour and agility which astonish the
beholder, and which, in ages of ignorance and superstition,
were easily ascribed to supernatural agency.

SAURIAN REPTILES. 317

§ 4. Sauria.

The conformation of those parts of the frame which are
subservient to progressive motion becomes more perfect in
the class of Saurian reptiles, which includes all the Lizard
tribes. Several links of connexion with the preceding class
may still be noticed, marking the progress of development,
as we follow the ascending series of animals. Rudiments
of the bones of the extremities, and, also, of the sternum,
make their appearance very visibly in the Ophiosauriis,
and in the blind worm, {*Bnguis fragilis.) The Siren la-
certina has two diminutive fore feet, placed close to the
head. The Lacerta lumbricoides of Linnaeus, or the Bipes
canaliculatus of Lacepede, which is found in Mexico, and
of which a specimen is preserved in the collection at Paris,
has a pair of very short feet, also placed near the head, and
divided into four toes, with the rudiment of a fifth. The
Lacerta bipes (Linn.) or Sheliopiisic of Pallas, has, on the
other hand, a pair of hind feet only, but extremely small, to-
gether with rudiments of a scapula and clavicle concealed
under the skin. Next in order must be placed the Chal-
cides, or Snake-lizard, (Fig. 210,) and the Lacerta seps, ani-
mals 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 pro-
gressive undulations.

Ascending from these, we may form a series of reptiles,
in which the development 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 development of the skeleton, we find the
trunk divisible into separate regions. We now, for the first
time, meet with a distinct neck, separating the head from the
thorax, which is itself distinguishable from the abdomen;
and a distinct sacrum is interposed between the lumbar and
the caudal vertebrae.

318 THE MECHANICAL TUNCTIONS.

A farther approach to the higher classes, is observable in
the number of cervical vertebrae, which is almost constantly
seven; as we shall find it to be in the mammalia. The arti-
culations of the vertebrae are similar to those of serpents, in-
asmuch 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 con-
dyle projecting from the body of the occipital bone, instead
of lateral condyles proceeding from its leaves, as we noticed
was the structure in the batrachia. The caudal vertebrae are
always numerous, and the tail is compressed vertically,
which is the form most favourable for progression in water.
They are remarkable, also, for having inferior spinous pro-
cesses attached to the bodies by cartilages; a structure ana-
logous to that which we have seen in fishes.

The number of ribs differs in different species of Sauria:
they are always articulated to the extremities of the trans-
verse processes of the vertebrae, of which they appear to be
continuations. Processes of this description also occur in
the neck, attached to the transverse processes of the cervical
vertebrae; and these have been regarded as cervical ribs.
Their presence are impediments to the flexions of the neck;
whence arises the difficulty which the crocodile appears to
have in bending the neck, while turning round upon the ani-
mal he is pursuing. In the thorax, the ribs are connected
with a broad sternum; but there are other ribs, both before
and behind, which have no such termination, and therefore
bear the name oi false ribs.

The pelvis consists chiefly of the iliac bones, which, as
in the batrachia, pass backwards to form the articular cavity
for the thigh bone. Two small and slender bones extend
forwards from the pubic bones, on the under side of the
body, apparently for the purpose of supporting the abdomi-
nal viscera.* The bones of the extremities are very perfectly
formed, approaching in their shape and arrangement very

* They appear to be analogous to the marsupial bones peculiar to a family
of mammalia.

FEET or THE GECKO.

31S

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 swim-
ming. The form of the tail, which is generally compressed
vertically, like that of fishes, though perhaps not to an equal
degree, is another indication of their being formed for an
aquatic life: for where the tail has this shape, we always find
that the chief muscular power is bestowed upon it as an in-
strument of aquatic progression, producing, by its lateral
flexions, a horizontal movement of the body. Crocodiles
and alligators, for instance, which have this conformation,
are comparatively weak when on land, and as soon as they
have seized their prey, their efforts are always directed to
drag it with them into the water; knowing that when in
their own element they can readily master its struggles, and
dispose of it as they please.

In the Gecko tribe, we find a particular mechanism pro-
vided 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 toe
(represented in Fig. 211) there are as many as sixteen trans-
212 211 verse slits, leading to the same

number of cavities, or sacsj
these open forwards, and their
external edge is serrated, ap-
pearing like the teeth of a small-
toothed comb. A section of
the foot, showing these cavi-
ties, is seen in Fig. 212. All
these parts, together with the
cavities are covered or lined
with cuticle. Below them are
large muscles wh ich draw do wn
the claw; and from the tendons
of these muscles arise two sets of smaller muscles, situated so

320 THE MECHANICAL FUNCTIONS.

as to be put upon the stretch, when the former are in action.
By the contractions of these muscles, the orifices of the cavi-
ties, 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 upon
the smoothest surfaces, even in opposition to the tendency
of gravity. It can run very quickly along the walls or ceil-
ing of a building, in situations where it cannot be supported
by the feet, but must depend altogether upon the suspension
derived from a succession of rapid and momentary adhe-
sions.

Although the Sauria are better formed for progressive
motion than any of the other orders of reptiles, yet the
greater shortness and oblique position of their limbs, com-
pared with those of mamrniferous quadrupeds, obliges them
in o-eneral 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 walk-
ing, 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 exist-
ence, which we find in the higher orders of the Mammalia.
But before proceeding to consider these, we have to notice
a sino-ular group of animals, whose conformation appears to
be exceedingly anomalous, and as if it interrupted the regu-
larity of the ascending series, of which it seems to be a col-
lateral ramification.

* Philosophical Transactions for 1816, p. 151, and 323.

CHELONIAN REPTILES. 321

§ 5. Chelonia,

The order of Chelonian Peptiles, which comprises all
the tribes of Tortoises and Turtles, appears to constitute an
exception to the general laws of conformation, which pre-
vail among Vertebrated Animals: for instead of presenting
a skeleton wholly internal, the trunk of the body is found
to be enclosed on every side in a bony case, which leaves
openings only for the head, the tail, and the fore and hind
extremities. That portion of this osseous expansion which
covers the back is termed the Carapace; and the flat plate
which defends the lower part of the body is termed the
plastron. It is a form of structure that reminds us of the
defence provided for animals ver}'' low in the scale of or-
ganization, such as the echinus, the Crustacea, and the bi-
valve 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 struc-
tures. The great purpose which nature seems to have had
in view in the formation of the Chelonia is security; and for
the attainment of this object she has constructed a vaulted
and impenetrable roof, capable of resisting enormous pres-
sures 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 remarkable inversion they appear to have un-
dergone, 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 en-
velops all the other parts is really formed by an extension

Vol. I. 41

322

THE MECHANICAL FUNCTIONS.

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 sternocostal ap-
pendices, which become ossified. Thys, no new element
has been created; but advantage has been taken of those al-
ready existing in the general type of the vertebrata, to mo-

dify their forms, by giving them different degrees of rela-'
tive development, and converting them, by these trans-
formations, into a mechanism of a very different kind, and
subservient to other objects than those to which they are
usually applied. It is scarcely possible to have stronger"
proofs, if such were wanting, of the unity of plan which has
regulated the formation of all animal structures, than those
afforded by the skeleton of the tortoise.

The first step taken to secure the relative immobility of
the trunk, is to unite in one rigid bony column all its verte-

CIIELONIAN REPTILES. 323

brae, and to allow of motion only in those of the neck, and
of the tail. The former, accord ini»;ly, are all anchylosed to-
gether, leaving, indeed, traces of their original forms as se-
parate vertebrae, but exhibiting no sutures at the place of
junction. The canal for the spinal marrow is preserved, as
usual, above tlic bodies of these coalesced vertcbric, and is
formed by their united leaves; the arches being completed
by the spinous processes. But these processes do not ter-
minate in a crest as usual; they are farther expanded in a
lateral direction, forming flat pieces along the back, which
are united to one another by sutures, and which are also
joined to the expanded ribs, so as to form the continuous
plane surface of the carapace. The transverse processes of
the vertebrae are well marked, but, though firmly united to
the ribs, do not give rise to them; for the ribs, which are
flattened and expanded, so as to touch one another along
their whole length, are inserted below, between the bodies
of every tvvo adjoining vertebras; while above, they are
united by suture with the plates of the spinous processes.
This change in the situation of the ribs is the consequence
of the change in their oflice. When designed to be very
moveable, we find them attached either to the extremities of
the transverse processes, or to the articular surfaces of a sin-
gle vertebra; but where solidity and security are aimed at,
they are always inserted between the bodies of two verte-
brae. This we shall find to be the case also in birds, where
the bones of the thorax are required to be immoveable. It
is remarkable, indeed, that a great number of the peculiari-
ties 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 ani-
mals to be an intermediate link of gradation to that new and
important type of animals destined for a very diflerent mode
of existence.

The sterno-costal appendages, which connect the ribs to
the sternum, are, in most animals, cartilaginous: thougli oc-
casionally we find them partially ossified. In the tortoise,

1^-

324 THE MECHANICAL FUNCTIONS.

however, their ossification is not only complete, but has
been expanded laterally, so as to form a continuous surface
with the extremities of the ribs and with the edges of the
plastron, and completely to fill up the vacancy between
diem; constituting a dense and solid wall, which entirely
closes the sides of the general bony case. So strong is the
tendency to ossification in all these pieces, that the sutures
at first formed between them are often, in process of time,
obliterated; and the bony fibres are continuous throughout a
ffreat extent of surface.

The most remarkable metamorphosis in the osseous sys-
tem of this new type is that which occurs in the sternum.
So expanded are all its parts, that it is difficult to recognise
this bone under the disguised form in which it constitutes
the plastron, or broad plate, which, as we have seen, covers
the whole of the under side of the body. Yet, by a careful
examination of its structure, both in the young animal, and
also in the adult, wiien the sutures are not obliterated, we
may easily recognise the nine elements of the sternum;
namely, the one in the middle and fore part, and the four
pairs of lateral pieces; each having been formed from its re-
spective centre of ossification. In form and relative propor-
tion, indeed, they are widely different from the same parts
as they are presented in the skeletons of other animals: yet
in number and in relative situations they preserve that con-
stancy and uniformity so characteristic of the beautiful har-
mony 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 atmo-
sphere. Hence we find them covered throughout with a thin
horny plate, originally a production of the integument. It
is this substance which is commonly known by the name of
tortoise shell.*

• It should be observed, that the divisions of these plates, which appear
externally, bear no relation to the sutures which scpai'atc the subjacent bones,

* CHELONIAN REPTILES. ^25

The immobilil}- of the trunk is compensated, as far as re-
gards 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 ca-
rapace. These vertebrae are joined by the ball and socket
articulation common to all the existing species of reptiles.*
The articulation of the head with the neck is effected in the
same manner; but it is interesting to remark that the occipi-
tal condyle, which is situated at the lower margin of the
great aperture, though presenting a single convex surface,
225 p^.^__,^__p yet has that surface evidently di-

vided into three parts; the two up-
per portions being lateral, and the
lower portion in the middle. These
three articular surfaces are seen im-
mediately below the central aper-
ture, F, in Fig. 215, which exhi-
bits the skull of the Testudo my das, viewed from behind.
Although closely approximated, a faint line of demarcation,
which divides their surface, indicates an incipient tendency
to separate; we shall find that, in the farther steps of deve-
lopment which occur in the higher classes, this separation
actually takes place by the obliteration of the lower articu-
lar surface, and the transfer of the two lateral surfaces to the
condyloid processes arising from the development of the
leaves of the occipital bone.

The singular conformation of the bones of the head, in the
turtle, affords fresh evidence in support of the theory that
these bones were originally vertebrae. The brain of the tor-
toise is exceedingly small; and, yet, the skull, when viewed
from above, presents an appearance of great brcadth^as if it
enclosed a cavity of large dimensions. But if we look upon

so that it is not possible to draw inferences respecting the form of the latter
from the mere inspection of the external shell.

* The expression of this fi\ct is thus qualified, because it does not apply
to many fossil or extinct species, such as the Iddhyosaurus.

326 THE MECHANICAL FUNCTIONS.

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 admit-
ting the end of the finger, and that the broad plates of bone,
p, p, which form the upper surface of the skull, have no re-
lation to this cavity, and are merely extended over the tem-
poral muscles, which are of very large size, occupying the
whole of the spaces s, s; which spaces are completely sur-
rounded by these bones. It would appear that the same ten-
dency to lateral expansion, which exists in the spinous pro-
cesses of the dorsal vertebrae, prevails, also, among those
which contribute to form the skull. The parietal bones,
which represent the spinous processes of the second cranial
vertebra, after having performed their primary office of pro-
tecting the hemispheres of the brain by closing over them,
still proceed in their development, 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 tem-
poral muscles. This great breadth of the head in the turtle
gives the animal an aspect of superior intelligence, to which
character, from the really diminutive size of its brain, it is,
in no respect, entitled. As the turtle is unable to withdraw
its head \vithin the carapace, such extraordinary protection
appears to have been necessary: for it is not met with in the
tortoise, which has a carapace sufficiently capacious to give
shelter to the head whenever occasion may require.*

This arrangement of the expanded spinous processes and
ribs gives rise to a singular inversion in the position of the
scapula; for it is here placed on the inside of the ribs and
sternum, that is, between the carapace and plastron.t The

* The analogy of the spine of the occipital bone with that of a vertebra is
farther shown by this bone extending- backwards to a considerable length,
exactly in the manner of the spinous processes of the cervical vertebrae in
other animals.

I The anomalous situation of these bones, and the strangely disguised
f<jrms which their several parts assume, render it veiy difficult to recognise.

i CHELONIAN REPTILES. 327

humerus is remarkably curved, especially in the tortoise,
where it has the form nearly of a semi-circle. The radius
and ulna are distinct from each other; the carpus and pha-
langes are short and stunted, forming a compressed kind of
hand.

The pelvis, like the scapula and clavicle, is enclosed with-
in the bony shell which protects the trunk. The sacrum is
moveable upon the last dorsal vertebra; and the coccygeal
vertebrae 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 ex-
tremity are similar to those of the fore leg.* 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 efficacious
for progression in the water than on land: this circumstance,
in conjunction with the constitutional torpor of the animal,
sufficiently accounts for the excessive, and, indeed, proverbial
tardiness of its movements.

Security appears still to be the object aimed at in the me-
chanism of all the other parts of the skeleton. The articu-
lations 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 serpen-
tine 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 meta-
carpus are exceedingly flattened, and approximate to the lin-

in the skeleton, the several pieces whicli correspond to the normal type of
the scapula, acromion, coracoid bone, and clavicle; and anatomists arc not
yet ag-recd as to the proper designations which are applicable to these bones,
in the Chelonia.

* The cylindrical bones of the tortoise are solid throug-hout, and have no
cavity for containing- marrow, as in the more hig-hly developed bones of the
mammalia. This is seen iu the section of the femur, Fig'. 214.

328 THE MECHANICAL FUNCTIONS.

like form which we shall presently see exemplified in the
cetaceous tribes. The phalanges are also large and length-
ened, forming a kind of oval hand, or rather paddle, the
functions of which it is w^ell calculated to perform. The
curvature of the humerus is of great advantage to the .tor-
toise in assisting it to turn itself, when, by any accident, it
has been laid on its back.

Considerable differences may be noticed in the structure
of the several species of Chelonia, according to the diversity
of their habits. Tortoises which live on land, require more
complete protection by means of their shell than turtles, or
Emydes, which dwell only in the water: hence the convex-
ity of their carapace, the solidity of its ossification, its im-
moveable connexion with the plastron, and the complete
shelter it affords to the head and limbs. Turtles, on the
other hand, receiving support from the element in which
they reside, require less provision to be made for these ob-
jects. Their carapace is smaller, has a more flattened form,
and cannot afford protection to the head and limbs. These
latter organs are proportionally larger, present a greater de-
velopment of the radius and ulna, and are compressed into
a flat expanded surface. Previously to the retraction of the
head and limbs within the shell, the air is expelled from the
large cavities of the lungs, by the vigorous actions of the ab-
dominal 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 first assume the form of broad
plates immoveably united to the spine, when they have pro-
ceeded a certain distance, separate from each other, and re-
sume 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

CHELONIAN REPTILES. 329

the intervals between its imperfectly developed elements
also membranous. All this renders the whole shell less
compact, more flexible, and more feeble: but the movements
of the animal are quicker and more energetic.

These characteristic differences between Ihe aquatic Che-
Ionia and those that live on land are still more strongly-
marked in the genus Trionyx, or soft tortoise; which is
destitute of scales, and in which many of the pieces that are
bony in the tortoise are replaced by simple cartilage or
membrane.

The enormous weight of the shell of the turtle would be
a serious impediment to the motion of this animal in the
water, were there not some provision made for diminishing
the specific gravity in the body. This purpose is answered
by the great capacity of the lungs, which, when inflated
with air, nearly fill the thorax, and give great buoyancy to
the whole mass. Thus, wherever there exists a supposed
inconvenience, dependent on the fulfilment of one condi-
tion, we are certain to meet with a compensation in the
structure of some other part, and in the mode of executing
some o^her function. An express provision for giving buoy-
ancy has been made in the construction of the shell of a spe-
cies of tortoise inhabiting the coasts of the Scychelle Islands.
The under surface of the shell, instead of being gently con-
cave, 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 ele-
ment* The greater size of turtles, when compared with
tortoises, is a farther instance of the superior facility with
which organic growth proceeds in aquatic than in land ani-
mals formed on the same model of construction.

* Home's Lectures, vi. 37.

Vol. I. 42

( 330 )

CHAPTER IX.

MAMMALIA.

§ 1. Mammalia in general.

The singular animals, so remarkable for their anomalous
shapes, their torpid vitality, and their amphibious constitu-
tion, which have lately occupied our attention, appear placed
by nature as forms of transition, in the passage from those
vertebrated animals which dwell in the water, to those
which inhabit the land. The class of Mammifera^ or Mam-
malia^ comprehends all the animals which possess a spinal
column, breathe air by means of lungs, and are also warm-
blooded, and viviparous, conditions which render it neces-
sary that they should possess organs, called m^ammse, en-
dowed with the power of preparing milk for the nourish-
ment of their young; a peculiarity from which the lame of
the class is derived. But they are not exclusively 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 narwal, the cachalot, and the
whale; animals which, however widely they may differ in
their habits and external conformation from terrestrial quad-
rupeds, possess, in common with the latter, all the essential
characters of internal structure and of Junctions above enu-
merated. These characters belong also to the human spe-
cies, which must consequently, in its zoological relations, be
ranked as a genus of the class mammalia. So numerous,
indeed, are the analogies wjiich connect the natural families
of this class with our own race, that we must ever feel a
deep interest in the accurate investigation of their compara-
tive anatomy and physiology; and it has been found, accord-
ingly, that the progress which has, of late years, been made

MAMMALIA. 331

in this branch of science has materially enlarged our know-
ledge of the structure, the functions, and the physical his-
tory of man: subjects with which our welfare has obviously
the closest and most intimate relation.

The principle of analogy which prevails so generally in
the inferior departments of the animal creation, may be also
traced in the class mammalia; for we always fmd its influ-
ence more conspicuous in proportion as the objects compre-
hended in the natural series of beings are more numerous
and more diversified. Scarcely any of the great natural as-
semblages 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 wase
war with the larger animals, whom they either spring upon
unawares, or openly pursue and overpower, displaying the
savage energies of their nature, in practising all the arts of
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 like manner,
a great diversity of constructions, adapted to the particular
nature of that subsistence, whether it be herbage, or the
leaves of trees, or fruits, or seeds, or the coarse fibres of the
wood and bark. While all are gifted with powers to ob-
tain the nourishment they require, those that have not been
armed with weapons of attack, are still provided with in-
struments of defence, or with means of flight. Each has its
respective sphere of operation; and to each has its appropri-
ate soil, habitation, climate, and element been assigned.

It is easy to conceive that all these various circumstances
must lead to great diversities in the apparatus for mastica-
tion and for digestion, in the organization of the senses, in
the construction of the instruments of locomotion and of pre-
hension, and in the general form of the body to which these

332 THE MECHANICAL FUNCTIONS.

various parts are to be adapted. Yet, amidst all these varia-
tions, we may perceive the same laws of analogy connecting
the whole into one series, and assimilating all these multi-
form structures to one common standard. The same organ,
however modified in its shape and size, however stinted in
one, or developed in another, is ever found in its appropriate
place, and retains the same connexions with adjacent organs,
whether we seek, it in the carnivorous or the herbivorous
quadruped, in the inhabitant of the land or of the water, in
the denizen of the frigid or of the torrid zone; or in animals
of the most diminutive or most colossal statures.

As an example, we may take the vertebrae of the neck.
It is a universal law, that this part of the spinal column
shall, in every animal of the class mammalia, consist of
neither more nor less than seven vertebrae. Whatever be
the length or shortness of the neck, whether it be compressed
into a small space, as in the elephant and the mole, whether
it be lengthened to allow the head to reach the ground, as
in the horse and the ox, or whether it be excessively pro-
longed to allow the animal to reach the tops of the trees, as
in the cameleopard, still this same constant number is pre-
served in the vertebrae which it contains. When the neck
is long, each individual vertebra must necessarily be length-
ened in the same proportion. Thus, in the Cameleopard,
the vertebrae of the neck consist of seven very long tubes,
joined together endwise, w^ith scarcely any development of
spinous processes, lest they should impede the bending of
the neck. The greatest contrast to this structure is met
with in the Dolphin, and other Cetacea, which present ex-
ternally no appearance whatever of a neck, but whose skeleton
exhibits cervical vertebrae, closely compressed together, and
exceedingly thin, and most of them united together;* every
bone, thus formed, however, retains the marks of having
originally consisted of separate vertebrae; and still, in this

* In the cachalot, the whole of these seven vertebrae are usually anchy-
losed into one bone.

CETACEA. 333

extreme case, the number of primary pieces is constantly
seven.*

§ 2. Cetacea.

Remarkable exemplifications of the law of uniformity
of organic structure are furnished by the family of the Ce-
tacea, 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)^,
or rudimental stage of development. Here, as before, we
have to seek these first elements among the inhabitants of
the vvater: for whenever, in our progress through the ani-
mal kingdom, we enter upon a new division, aquatic tribes
are always found to compose the lowest links of the ascend-
ing chain. Here, also, we observe organic development
proceeding with more rapidity, and raising structures of
greater dimensions in aquatic than in 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 Bradypus tridadylus, or three-toed sloth, was, till very lately,
thoug-ht 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 supernumerarv,
ought properly to be classed among- the dorsal vertebrae, of wiiich they possess
the distinctive characters, not only from the form and size of tiieir transverse
processes, but also from their having- small bony appendices, articulated with
them by a reg-ular 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 confirmed the observations of these anatomists, but has farther discovered,
that similar rudimental ribs are attached also to the eighth vertebra. (See
Philosophical Magazine, third series, iii. 376.) The Brudypiis forquaius,
which has been said to possess eight cervical vertebrae, will, perhaps, on closer
examination, be hereafter found not to deviate, any more than the three-toed
sloth, from the normal type, as reg-ards the number of these vertebrx. In-
stances have occiu'red of supernumerary cervical processes, or ribs in the hu-
man skeleton. (See Edinburg-li Medical and Surg-icid Journal, xl. 304.)

#

334 THE MECHANICAL FUNCTIONS.

the bosom of the primeval ocean, or stalked with gigantic
strides across antediluvian plains, and whose scattered re-
mains bear fearful testimony of the convulsions 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 belong-
ing to this semi-amphibious family, will impress us with the
resemblance they bear to fishes; for they present the same
oval outline of the body, the same compact form of the
trunk, which is united with the head without an intervening
neck; the same fin-like shape of the external instruments of
motion, and the same enormous expansion and prolongation
of the tail, which is here also, as in fishes, the chief agent in
progression. With all this agreement in external charac-
ters, their internal economy is conducted upon a totally dif-
ferent 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 conse-
quences which flow from this difference are of great import-
ance. The necessity of aerial respiration compels them to
rise, at short intervals, to the surface of the water; and this
air, with which they fill their lungs in respiration, gives
their bodies the buoyant force that is required to facilitate
their ascent, and supersedes the necessity of a swimming
bladder, an organ which is so useful to the fish.

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 an-
swers, also, the purpose of preserving the vital warmth of
the body. A great accumulation of this lighter substance is
formed on the upper part of the head, apparently with a view
to facilitate the elevation to the surface of the blowing hole,
or orifice of the nostrils, which is placed there.*

Another peculiarity of conformation, in which the cetacea

* The substance called Spermaceti is lodg-ed in cells, formed of a cartilagi-
nous substance, situated on the upper part of the head of the Cachalot.

CETACEA. 335

differ from fishes, and which has also an obvious relation to
their peculiar mode of breathing, is in the form of the tail,
which, instead of being compressed laterally, and inflected
from side to side, as in fishes, is flattened horizontally, and
strikes the water in a vertical direction, thereby giving the
body a powerful impulsion, either towards the surface, when
the animal is constrained to rise, or downwards, when, by
diving, it hastens to escape from danger.

All the essential and permanent parts of the skeleton of
vertebrated animals, that is, the spinal column, and its im-
mediate 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
object of the conformation is here, as in that of the fish, 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 transverse processes of the dorsal and lumbar
vertebrae are expanded both in length and breadth, and, be-
ing situated horizontally, offer no impediment to the vertical
flexure of the spine. For the same reason, the ribs are con-
tinued in a line with the transverse processes, and articu-
lated with their extremities, thus giving still farther 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 vertebrae are uninterrupted continua-
tions of those of the trunk. They develope, however, parts
which are met with only among fishes and reptiles, namely,
arches composed of inferior leaves* and spinous processes,
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, appa-

* These leaves being- formed of cartilag-e, are generally lost when tlie
bones are macerated for the purpose of preparing- the skeleton.

336

THE MECHANICAL FUNCTIONS.

rently corresponding to pelvic bones, for the presence of
which no more probable reason can be assigned than the
tendency to preserve an analogy with the more developed
structures of the same type.

A similar adherence to the law
of uniformity in the plan of con-
struction of all the animals belong-
ing to the same class, is strikingly
shown in the conformation of the
bones of the anterior extremities of
the cetacea; for, although they pre-
sent, externally, no resemblance to
the leg and foot of a quadruped, being
fashioned into fin-like members, wdth
a flat, oval surface, for striking the wa-
ter, 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.
216) exhibiting the same divisions
into carpal and metacarpal bones, and
phalanges of fingers, as exist in the
most highly developed organization,
not merely of a quadruped, but also
of a monkey, and even of man.

§ 3. */lmphibia.

In the small tribe denominated by Cuvier Amphibia, and
consisting of the Phoca, or Seal, and the Tricheciis, or
Walrus, we perceive that an advance is made towards a
fuller development of the limbs: these animals having a dis-
tinct neck and pelvis, and both hind and fore extremities.
In the seal the hind legs are drawn out posteriorly to a con-
siderable length, and placed parallel to each other: when
united and alternately raised and depressed, they perform

AMPHIBIA. 337

the same office as the tail of the cetacea, and propel the ani-
mal forwards: but when employed scparatcl}', they are more
qualified to act as oars. The walrus has feet still more de-
veloped, and distinctly divided into toes, which arc disposed
so as to strike backwards against the water.

§ 4. Ma 7717711 fcr oil s Quachnipeds in general.

From the imperfectly developed a'quatic and amphibious
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 development,
not being expended in the mere effort of giving expansion
to the several textures, and of swelling the bulk of the frame,
sometimes to inordinate dimensions, are employed rather in
reducing the elements of the organization into compact forms,
and in concentrating their energies, so as ultimately to at-
tain the extent of power and harmony of action, which are
displayed in the higher orders of warm-blooded quadrupeds.

It is to these favoured tribes that we must look for exam-
ples of the most complete development of the skeleton, and
the most advantageous disposition of mechanic force. We
have seen that reptiles, from the comparative shortness of
their limbs, and the torpidity of their muscular powers, are
but ill adapted for rapid progression. In all the more per-
fectly formed quadrupeds of the class mammalia, the trunk
of the body, being raised high upon the limbs, possesses great
range of motion, and can traverse with fewer steps a given
space.

The office of the limbs, as far as they are concerned in
progressive motion, is two-fold. They have, first, to sus-
tain the weight of the body, which they must do by acting
in opposition to the force of gravity; and they must, second-
ly, give the body an impulse forwards. Let us consider
more particularly the relations which the structures bear to
each of these two functions.

Vol. I. 43

338 THE MECHANICAL FUNCTIONS,

The limbs of quadrupeds constitute four columns of sup-
port to the trunk, which is placed horizontally above them;
but the whole weight of the body, together with that of the
head and neck, does not bear equally upon them; the fore
extremities almost always sustain the greater- part of that
weight, both because the fore part of the trunk is itself hea-
vier than the hind part, and because it is loaded with the
additional weight of the head and neck. Hence, in the usual
attitude of standing, the pieces of which the fore limbs are
composed are required to be placed more in a straight line
than those of the hinder limb: for the power of a column to
support a weight is the greater in proportion as it approaches
to the perpendicular position. The hind limbs are composed
of exactly the same number of divisions; but the separate
portions are usually longer than those of the fore 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 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 extremities 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 sca-
pula proceeds forwards, the humerus backwards; the radius
and ulna again forwards, and the fore foot backwards, posi-
tions which are exactly the reverse of the corresponding
bones of the hind limb. (See Fig. 218, page 350.)

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;

MAMMIFEROUS QUADRUPEDS. 339

a tendency which is required to be counteracted by the ac-
tions of the muscles whicli are situated on the external side
of each of those angles. These muscles are tlie extensors of
the joints; that is, the muscles whicli tend to bring their
parts into a straight line. It is, in fict, by this muscular ac-
tion, much more than by simple rigidity, that the limb sup-
ports the superincumbent weight of the body. It is evi-
dent that greater muscular force is necessary for this purpose
when the joints are bent, than when they are already ex-
tended; and the portions of the fore legs being naturally in
this condition, require less power than those of the hinder
legs to retain them in their proper relative positions.

The most complete instance of a vertical arrangement of
the bones of the extremities is seen in the Elephant; where,
in order to sustain the enormous weight of the body, the
limbs are shaped into four massive columns, of which the
several bones are disposed nearly in perpendicular lines. By
this means, the body is supported with scarcely any muscu-
lar effort, and the attitude of standing is, in this anim.al, a
state of such complete repose, that it often sleeps in that po-
sition. ' 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 transverse-
ly in that organ, and thus formed a vertical prop for the
head. But in almost all other quadrupeds, the mere act qf
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 re-
cline upon the ground.

The conformation of the hind extremities, which, as we
have seen, is not so well calculated for the simple support ol

340 THE MECHANICAL FUNCTIONS.

the trunk, is, on the other hand, better adapted to give it
those impulses which are to effect its progressive movements.
The nature of those movements, and the order in Which they
succeed each other, are different according to the peculiar
mode of progression which the animal practises, the degree
of speed it is desirous of exerting, and the particular end
it has in view. The paces of a quadruped usually distin-
guished, 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 mid-
dle of the quadrangular base formed by the feet, while they
rest upon the ground, the first effort to advance which the
quadruped makes, propels the centre of gravity forwards.
This it 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 move-
ments. 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 ac-
tion are relaxed, and the leg is raised from the ground,
brought forwards, and laid down close to the fore foot of the
same side. This fore 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 suc-
cessive positions will show that, amidst all the changes which
take place in the points of support, the stabilit}^ of the body
is constantly preserved. It is an elementary proposition in
mechanics that all that is necessary for ensuring the sup-
port of a body on any given base, is that the vertical line
drawn from the centre of gravity shall fall within that base.
When the animal is standing, the feet form a quadrilateral
base, and the centre of gravity is in a vertical line passing
either through the centre of the base, or, as, for the reasons

PROGRESSIVE MOTION IN QUADRUPEDS. 341

already mentioned, more frequently happens, through a point
a little in front of the exact centre. At the time when the
hind foot which hegan the action is raised from the ground,
the centre of gravity, having heen by that action impelled
forwards, still remains above the base formed by the other
three feet, and wdiich is now reduced to a trian^i-lc. That
hind foot being set down, while the corre?pondino- fore foot
is raised, a new triangular base is formed by the same hind
foot, together witli the tw^o of the other side, which have
not yet been raised. The centre of gravity is still situated
above this triangle, and the body is consequently still sup-
ported 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 basis with the other feet, the centre of o-ra-
vity has arrived above the centre of this new base. But at
this moment the centre of gravity is ao;ain urn-ed 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 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 quadrupeds 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 on 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 occa-
sioned; but these undulations are only lateral. A trot may
be considered as a succession of short leaps made by each
set of feet taken diagonally; that is, by the right fore foot,
and the left hind foot; or, vice versa, the one set being raised

342 THE MECHANICAL FUNCTIONS.

too-ether 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 re-
maining portion 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 cen-
tre of gravity is lifted higher from the ground, and is pro-
jected in a wide arch, and with great velojcity.

In the amble, both the legs on one side are raised toge-
ther; 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 prac-
tised by deer, and is performed by striking the ground with
all the legs at the same moment. It consists, therefore, like
the gallop, of a series of leaps; but their direction is more
uniformly upwards, from the concurrence of all the legs in
the same action.

Nature has purposely endowed different tribes 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
mobility of the extremities. Of all the large animals, the
Lion has been constructed with the finest proportions for
conferring both strength and activity. The mass of his
body is supported more by the fore than by the hind ex-
tremities. In walking, the lion takes long strides, and ex-
hibits strongly the lateral undulations of the trunk.

Quadrupeds having a very long, or a very massive body,
or whose limbs are short, and nearly of equal height, are in-
capable 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 accelera-
tion of its walking pace. It excels principally in the vigour

PROGRESSIVE MOTION IN QUADRUPEDS. 343

and extent of its bounds; for which it is admirably qualified
by prodigious power of its muscles, enabling it to spring
forwards upon its victim witii 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 ex-
tent 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 leaping.
This is exemplified in the Jerboa and the Kanf;nroo^ ani-
mals, which, from the disproportionate shortness of their
fore legs, are totally incapacitated from walking; and for
the same reason, they cannot run with any degree of swift-
ness. It is only in climbing up a steep acclivity that the
jerboa is enabled to employ all its limbs: in a descent, on
the contrary, it uses only its fore legs, the hinder being
dragged after them. But, when pursued, these animals are
capable, for a long continuance, of taking leaps of nine feet
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 furnished with very strong
muscles, and maybe considered as constituting a fifth limb.
It is of great assistance to the animal in taking leaps, and
during its repose, contributes, together with the hind feet,
to support the weight of the body, as on a tripod, and to
leave at liberty the fore legs, which may then be employed
as arms.

The Hare and the Babbit furnish other instances of an
extraordinary length of the hinder legs depriving the ani-
mal of the power of walking, and obliging it to move for-
wards only by a succession of leaps. The hare may be said,
indeed, to walk with its fore legs only, while it gallops with
the hinder: but this disadvantage is amply compensated by
its amazing swiftness when runi;^ing at full speed.

344 THE MECHANICAL TUNCTIONS.

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 a declivity 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 occa-
sionally to the Agouti and the Guinea pig^ when these ani-
mals attempt to run down hill.

The Sloth, which is formed for clinging with great tena-
city 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 proverbi-
ally 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 Camcleopard^ likewise, has the fore legs much longer
than the hinder. The object of this conformation was pro-
bably to elevate the anterior part of the spine, so as to raise
the head as much as possible, and also to give a considera-
ble inclination to the whole column, for the purpose of dis-
tributing 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 usual horizontal position, the
anterior extremities would have had to support the whole
of the enormous weight of this neck and head. This pecu-
liarity of structure, however, introduces considerable modi-
fications in the mode of progression of the animal. The
ordinary pace of the cameleopard is the amble; but it has
also a slower walking pace, and occasionally a gallop. In
the amble, its undulation is so considerable as to give it the
appearance of being lame. A similar kind of limping gait,
arising from the same cause, namely, the disproportionate
elevation of the fore part of the spine, has been observed in
the Hyena,

RUMINANT QUADRUPEDS. 34/5

§ 5. Buminantia.

In following the series of Mammalia in the order which
best exhibits their successive stages of development, I shall
commence with those whose digestive apparatus is formed
to extract nourishment exclusively from the vegetable kino-,
dom. The first assemblage that presents itself to our notic'^e
is the remarkable hmWy oi Ru77iinants, which feed princi-
pally on herbage. Wherever the earth is clothed with ve-
getation, 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
employments of their lives, and satisfy the chief conditions
of their existence. To these purposes the whole conforma-
tion 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 com-
plicated arrangement of joints, and exhibit many of those
consolidations of the bones, which tend to simplify the
structure, and to contribute to its strength.

But though never incited by the calls of appetite to en-
gage in sanguinary warfare, they are yet liable to the as-
saults of many ferocious and well armed adversaries, and
often unprovided with any adequate means of defence; their
only resource, therefore, is to avoid the dangers of the en-
counter 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 struc-
ture belonging to this type. We may observe that the
parts composing the hind legs are longer, and inclined to
Vol. I. 44

346

THE MECHANICAL FUNCTIONS.

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 350.)
As it was necessary, from the situation of their food, that
their heads should reach the ground in grazing, w^e find that
the neck has been much elongated, that the muscles which
raise the head have been enlarged and strengthened, and that
the spinous processes of the back and neck have been much
expanded, in order to allow of suiFicient surface for the at-
tachments of these muscles. The effort requisite to raise,
and even support the head, is very considerable; as will ap-
pear when we reflect that its weight acts by means of an ex-
tremely 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 Ugamentum niichse,
and is represented at n, in Fig. 217, is formed of a great
number of bands wiiich 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 successively joined together, and com-
posing 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

RUMINANT QUADRUPEDS. 317

lowers its head, and gives considerable assistance to the mus-
cles in raising it. In the deer and the ox, which toss tiieir
heads with force, and especially in the males, which are
armed with antlers or horns, the muscles performing those
motions are remarkably strong, and the spinous processes of
the back particularly prominent. In the loins, on the con-
trary, 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 resem-
ble 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 corresponding to a clavicle; but they are connected
with the spine and ribs only by ligaments and muscles; so
that the fore part of the trunk is, in fact, suspended between
the limbs by its muscular attachments alone. This is not
the case with the hind extremities; for their bones commence
with the pelvis, which proceeds backwards from the sacrum,
but with a considerable inclination downwards, and has a
deep hemispherical cavity for the lodgement of the round
head of the thigh bone. 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
veloc^y than could 4iave been effected by muscles of equal
strength, but witl:^horter fibres.

Both the humerus in front, and the femur behind, are so
short as to appear, on a superficial view, to form part of the
trunk, being entirely enveloped and concealed by the large
muscles connecting them with the body. The heads of the
two humeri, in consequence of the absence of the clavicle,
are brought very near 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

,

348 THE MECHANICAL FUNCTIONS.

arm, although completely separate at an early period of
growth, soon unite to form but one bone. This union be-
gins 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 ap-
pears 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 afibrd 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 oc-
cur in these joints. The consolidation of parts is most con-
spicuous in the succeeding division of the limb, namely,
that constituting the metacarpus in the anterior, and the
metatarsus in the hind extremity. In either case we find it
consisting not of five bones, as in the more highly organized
carnivorous mammalia, but of a single bone only, termed
the cannon bone. In the early periods of ossification, how-
ever, they each consisted of two slender bones, lying close
and parallel to each other; but afterwards united by an ossi-
fic deposition, which fills up the interval between them,
and leaves behind no trace of suture.* In proportion as the
young animal acquires strength, the uni^n of these twcfbones
becomes still more intimate by the absodtttion of the parti-
tion 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

* The observations which establish this fact are detailed by G. St. Hilaire,
in a paper in the " Memoires du Museum," x. 173.

RUMINANT QUADRUPEDS. 349

above the ground. It is a common mistake, arising from
the height of these joints, and the names they bear in ordi-
nary language, to consider them as the knees of the animal.
The slightest inspection of the skeleton will be suflicient 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 re-
ally the heel. Thus, the foot, especially in the posterior ex-
tremity, is of great length; a structure which is evidently
intended to give greater velocity to the actions of the mus-
cles, while it at the same time ensures the utmost steadiness
and security of motion.

At the lower extremity of the cannon bone there are two
articular surfaces, indicating the originally separate ends of
its two component bones. They are for the articulation of
the two following bones, which are also very long, and which
correspond in situation to the first phalanges of the fino-ers
and toes. These are followed by a second and third set of
phalanges ; the last of which terminate in hoofs. All rumi-
nant quadrupeds have thus a double hoof; a character which
is peculiar to this family.

Thus, then, has Nature moulded the organs of progressive
motion in this^-emarkable tribe of animals to accommodate
them to the peculiar conditions of their existence, while she
has still preserved their relations to the primitive type of
the class to which they belong. Thus has she bestowed
upon them the slender and elegant forms, so pleasing to the
eye, which characterize the fleetest racer, and has provided
for the agile, yet firm and secure movements which they are
to exercise in various ways in eluding the observation, and
escaping from the pursuit of their stronger and more saga-
cious foes. This purpose they efiect, at one time by rapid
flight across extensive tracts of country; at another, by re-
tirement into unfrequented forests, or mountains of difficult
access, crossing their rugged surfaces in all directions, clam-
bering their precipitous acclivities, and fearlessly bounding
over intervening abysses, from point to point, till the place
of safety is attained on some rocky eminence. From this

350

THE MECHANICAL FUNCTIONS.

secure station the Alpine chamois looks down upon its pur-
suers, and defies their farther efforts at capture or molestation.

The astonishing feats of agility practised by this animal, and
by which the most experienced hunters are perpetually baf-

RUMINANT QUADRUPEDS. 351

fled in their attempts to approach it, sufliciently attest the
perfection of its orj^anization in reference to all these ob-
jects. The chamois has often been seen to leap down a per-
pendicular precipice of twenty or thirty feet in hei<rht,
without sustaining the slightest injury. How the ligaments
that bind the joints can resist the violent strains and concus-
sions 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 dead-
ly effect in their mutual combats. Even when not furnished
with horns, the animal instinctively strikes with its fore-
head where the frontal bone has been expanded and forti-
fied 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 interest-
ing to inquire**into the nature of these- organs, and the phe-
nomena of their production.

The antlers of the male stag are osseous structures, sup-
ported on short and solid tubercles of the frontal bone: af-
ter remaining nearly a year they are cast off, and soon re-
placed by a newly formed antler, which is of larger size than
the one which was lost. Previously to the formation of
this structure, those branches of the artery, termed the ca-
rotid, which supply blood to the frontal bone, are observed
very rapidly to dilate, and to throb with unusual force; and
all the blood vessels of the skin of the part where the antler
is to arise, soon become distended with blood; an effect
which is accompanied by general heat and redness, like a
part in a state of high inflammation.* Presently the skin is

* These phenomena are connected with periodical changes in the consti-
tution relating to the productive functions.

352 THE MECHANICAL FUNCTIONS.

elevated by the growth of a tubercle from the subjacent
bone: this tubercle is at first a cartilage, and after it has at-
tained a certain size, becomes ossified, and grows like other
osseous structures, first shooting into the form of a length-
ened cylinder, and then dividing into branches. It is fol-
lowed in its elongation by the skin, which during the whole
time that the antler is growing is extended over it in every
part, forming what is called, from the delicate investment of
hair, its velvet coat. The blood vessels of the proper mem-
brane of the antler, or periosteum, still continuing to sup-
ply it with the materials required for its growth and conso-
lidation, deposite 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 remarka-
able from its occurring in a small part of the system, and
in a bony structure.

After the antler has attained its full size, a deposition of
osseous substance still continuous at its base, around the
trunks of the arteries which are proceeding along the invest-
ing membrane of the bone for the purpose of conveying
nourishment. The accumulation of this substance raises a
ring called the burr, round that part of the antler: and by
encroaching on the arteries themselves, it gradually dimi-
nishes their capacity of conveying blood, and they at length
become entirely obliterated. The bone, no longer receiving
a superabundant nourishment, ceases to grow; the integu-
ments which covered it, decay, and becoming dry and shri-
velled, are torn by rubbing against trees, and peel o^ 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 sufficiently nourished
by its own interior vessels, continues in a living state, and
preserves its connexion with the system. But, at length, the

RUMINANT QUADRUPEDS. 353

arteries, whether 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 ves-
sels 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 occa-
sions the fall of the whole structure. After the separation of
the antler, the eminence of the frontal bone, on which it stood,
is left rough and uneven, like that of a fractured part : but
the surrounding integuments soon close over, and cover it
completely ; until the period arrives when it is to be replaced
by a new antler, which exhibits the same succession of phe-
nomena, in its growth and decay, as its predecessor, only that
its development is usually carried farther, the new stem being
both thicker and longer, and the branches wider and more
numerous. The antler of each successive year has, conse-
quently, a different form from that of the preceding; and,
when the animal has attained a certain age, the extremities
of the branches present broad expansions of bone, which the
antlers of an earlier growth had neveV exhibited.

The short bony processes which extend in a perpendicular
direction on the head of the cameleopard, 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 re-
tain, at the extremities, a tuft of hair. The development of
these processes, in the young animal, takes place in the same
manner as that of an antler, but it reaches only to a certain
point, upon attaining which, the growth is arrested, and never
proceeds farther. The arteries cease to deposite superabun-

VoL. I. 45

354 THE MECHANICAL FUNCTIONS.

dant nourishment, but continue to maintain an exact equili-
brium between the expenditure and the supply ; so that the
horns of the cameleopard are never shed, and remain perma-
nent bony structures.

A farther modification of this process occurs in the con-
struction 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 the
forehead: 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 integu-
ment 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 eflfect-
ual protection to the subjacent bony layers which are form-
ing underneath it. The highly vascular membrane, from
which these new structures chiefly arise, appears to have
different powers of production at its two surfaces: for while
the inner surface is forming the osseous portion of the horn,
and supplying the phosphate of lime required for the con-
struction of its plates and fibres, the exterior surface is add-
ing successive layers of horny substance to the inner side
of those portions which had been before deposited. These
two operations, w^hich oflfer 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 vessels 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 succession; each new^ layer is agglutinated
to the inner surface of the preceding; and each has the shape

RUMINANT QUADRUPEDS.

355

of a hollow cone, occupying the part towards the apex of
the former cone, and extending farther towards the base.
Hence a longitudinal section of the whole presents the ap-
pearance represented in the annexed figures (218*,) where
A is the section of the horn of an Ox, and b, a similar sec-
tion of the horn of an Antelope. C is a magnified view of
the extremity of the latter, together with a portion of the
bone D, which occupies the axis of the horn.

In this process of the formation of horn, as happens in
that of shells, there sometimes occur irregularities, or peri-
odical intermissions and increase of action in the secreting
organs, giving rise to transverse grooves, or ridges. Tliese
may be seen in the horns of the goat, in which the fibres
are short, and laid one over another with the same regula-
rity as the tiles of a house. The tendency in these horns

to assume a spiral form is explicable on the same principles
as those which regulate the growth of turbinated shells.

356 THE MECHANICAL FUNCTIONS.

The horns of the ox and of the antelope tribes are formed
of longer and more continuous fibres, which are closely
compacted 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 two
horned species, grow from the integument 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, agglu-
tinated together into a solid mass by a material which acts
as a cement. This fibrous structure is most distinctly seen
at the base of the horn, where the ends of the fibres project,
like those of a brush, from the surface. When these horns
are sawn transversely, and examined with a magnifying
glassy a great number of orifices are seen, marking the
empty spaces that intervene between the hairs; and if the
section be made in a longitudinal direction, the same spaces
give rise to the appearance of parallel grooves. These horns
are not deciduous, like those of the stag: but continue to
adhere to the skin, and to grow from the root, in proportion
as they are worn at the extremity.

§ 6. Solipeda,

The Solipeda form a natural family of quadrupeds, in-
cluding the Horse, the *dss, the Quagga, the Zebra, &c.
which are very nearly allied in their conformation to the
ruminant tribe. To combine fleetness w^ith strength has
been the obvious design of nature in the construction of
these animals. We find, accordingly, that the consolidation
of the bones of the foot is carried still farther than in the
ruminant tribe; for, in place of the two parallel phalanges,
which are, in the latter, articulated with the cannon bone,
there is here only a single metatarsal bone. The three pha-
langes, of which that single finger consists, bear the names of
ihQ pastern, the coronet, and the coffin bo?ie; and the hoof; of

MAMMALIA SOLIPEDA. , 357

course, is single, likewise; there is, also, a small bone, con-
nected with the last, and called the shuttle hone. To the
cannon bone are joined, behind, and on the side, two much
shorter and very slender bones, which are rudiments of the
other metacarpal bones. They have been termed the sty-
loid, or splint hones; and are generally united by ossifica-
tion with the cannon bone. The scapula of the horse is
very narrow, and placed very nearly in a straight line with
the humerus; which latter bone is very short, and scarcely
descends below the line of the chest. The thigh-bone is also
unusually short. The muscles, which extend the joint, and 'UJk
throw the thigh backwards, in kicking, are particularly pow-
erful. This is the natural defensive action of the horse: and
its force is increased 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 tro-
chanter,* giving to the muscles the advantage of a long le-
ver. The cervical vertebrae have only short spinous pro-
cesses, that they might not interfere with the motions of the
neck. In the vertebrae of the back, on the other hand, these
processes are remarkably long, especially at the part where
the shoulder rests; their projection constituting what is
called the Withers.

§ 7. Pachydermata,

From the horse we pass, by a natural transition, to the
Pachydermata^ a small group of animals interesting by their
peculiarities, and by their being remnants of a very extensive
tribe, which formerly inhabited the earth, but have now al-
most entirely disappeared. Although they feed upon grass,
they do not ruminate, nor are they cloven-footed. They are,
for the most part, huge and unwieldy animals, with thick in-
teguments, rendered tough by a large mass of condensed cel-
lular substance, which forms the chief defensive armour of

* This process has been termed ^\q, jiroccssus recurvatusfemoris.

358 THE MECHANICAL FUNCTIONS.

those that are destitute of either tusk, proboscis, or nasal
horn.

The most remarkable genus of this family is the Elephant,
the colossal giant of quadrupeds. The many peculiarities that
are observable in the conformation of this animal have all an
obvious 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 mas-
tication are powerful, and its teeth of great size. The whole
of this apparatus requires an immense development of bone
to render it efficient ; so that the head, with its huge tusks
and grinders, is of enormous weight. Had this ponderous head
been suspended at the end of a neck of such length as to ad-
mit 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 was competent to any degree of
muscular power. Nature has accordingly abandoned this
form of structure, and has at once curtailed the neck, bring-
ing 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 inte-
rior cellular structure, and, thus, the muscles are attached to
a considerable extent of bone, instead of being affixed to a
single process, which would have incurred great risk of being
broken off by their action. These large cells are constructed
with a view to combine strength with lightness ; the plates
which form their sides being disposed, in a radiated manner,
towards the circumference, and arranged with great regulari-
ty; and the cells themselves, instead of containing marrow, are
filled with air, by means of communications with the Eusta-
chian tubes, which open into the nostrils : thus, a great extent
of surface is given to the skull, without any addition to its

MAMMALIA PACHYDERM AT A. 359

weight. The hgamentum niichae also comes in aid of the
muscular power, being here of vast size and strength.

The head being limited in its range of motion by its approxi-
mation to the trunk, the mouth cannot be applied directly to
seize the food : and some means were, therefore, to be pro-
vided for bringing the food to the mouth. For this purpose,
a new organ, the proboscis, has been constructed : it consists
of a cylinder, perfectly flexible, and of a length sufficient to
reach the ground, when the elephant is standing. The ani-
mal 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 ra-
diating from the interior towards the circumference, others si-
tuated more obliquely, and a third set running longitudinally,
and forming an exterior layer : but they are all variously in-
terlaced together so as to compose a very complicated ar-
rangement. The extremity of the proboscis, w-hich is endowed
with great sensibility, is furnished with an appendix, resem-
bhng a finger, most of the functions of which, indeed, it is
capable of performing.

For the formation of this admirable member it has not
been necessary to deviate from the ordinary laws of deve-
lopment by the creation of a new organ; the same end being
accomplished by the extension of a structure already be-
longing to the type of mammiferous animals. In several of
the pachydermata the nostrils are already considerabl}^ ad-
vanced, 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 far smaller
scale, the proboscis of the elephant. This latter organ, then,
may be considered as merely an elongation of the nostrils,
which have been drawn out to suit a special purpose, very
different from the function to which that part is usually sub-
servient.^

♦ A defective development of the bones of the nasal cavity, while the na-
tural growth of the soft parts has continued, has often, in the case of thehu-

360 THE MECHANICAL FUNCTIONS.

While fleetness and elasticity are the results of the me-
chanical conformation of the horse, solidity and strength are
the objects chiefly aimed at in the construction of the pachy-
dermata. The limbs have a great weight to sustain, in con-
sequence of the huge size of the body; and hence the seve-
ral 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 forelegs,
and the fibula in the hinder, are fully developed, and are
distinct from the radius and the tibia. The number of the
toes, instead of being reduced to one, as in the horse, or to
two, as in ruminants, is here increased to five: though, in
consequence of their being very short, and of the skin which
covers and surrounds them being very thick, they hardly
appear 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 examination of all the
varieties of forms and structures that occur in the mecha-
nism 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 must ne-
cessarily pass over a multitude of instances of express adap-
tation, which are suited only to particular cases, and are,
consequently, of minor importance as regards the general
plans of organization. In the sort of bird's-eye view which
I am taking of the endless modifications of structure that
have been executed in conformity with those plans, I am
able particularly to notice only such as are most remarkable.

§ 8. Rodentia,

As the tribes of mammalia we have hitherto examined,
employ the anterior extremities for the purposes of progres-

man foetus, given rise to a monstrosity veiy much resembling' the trunk of the
tapir or of the" elephant. (See Geoffroy St. Hilaire.)

MAMMALIA RODENTIA. 361

slon only, they are destitute of a clavicle. In most of those
which follow, and where a greater development of the limb
confers more extensive and more varied powers of motion,
applicable to a greater range of objects, this bone is found.
In the greater number, however, it is merely in a rudimental
state ; that is, developed only to a certain extent, one portion
being bony, and the rest cartilaginous ; as if the ossification
had been arrested at an early stage. These imperfect clavi-
cles are too short to connect the scapula with the sternum ;
the rest of the space being eked out by cartilage, and by li-
gaments: but, still, they are of great use in affording points
of attachment to the muscles of the limb, and giving them the
advantage of acting by a rigid lever. The carnivorous tribes,
which make considerable use of their fore paws in striking
and seizing their prey, have clavicles of this description.
Those quadrupeds which have to execute still more complex
actions with their fore feet, have perfect clavicles, extending
from the shoulder to the chest, and connecting the bones of
the anterior extremity with the general frame-work of the
skeleton. This is the case in a large proportion of the family
of Rodentia, such as the Squirrel^ which employs its paws
for holding objects ; and the Beaver,^ which, likewise, makes
great use of its feet for moving and arranging the materials
of 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 keep-
ing the shoulders at the same constant distance from the
trunk, and affording a firm axis for the rotatory motions of
the limb, materially assist them in the performance of these
actions.

* The beaver presents a singular modification in the structure of the tail,
which is expanded into a flattened oval disk, covered by a skin beset with
scales: the whole forming- a mechanical instrument, which maybe compared
to a trowel, exceedingly well adapted for the purposes to which it is applied
by the animal in constructing its mud habitation.

Vol. I. 46

362 THE MECHANICAL FUNCTIONS.

§ 9. Lisectivora.

In the tribe of Insectivorous quadrupeds we meet with
several races which present singular conformations. In none
are these anomalies more remarkable than in the mole, an
animal which nature has formed for subterranean residence,
and whose limbs are constructed with a view to the rapid
excavation of passages under ground. The hands of the
mole, for its fore paws almost deserve that appellation, are
turned upwards and backwards for scooping the soil, while
the feet are employed to throw it out with great 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 overcomie the resistances that may be op-
posed to the progress of the animal. That no impediment
might be offered to these motions of the head, the spinous
processes of the cervical vertebrae have not been suffered to
extend upwards. Large muscles are provided for bending the
head backwards upon the neck; and they are assisted by a
cervical ligament of great strength, which is generally in
part ossified. The muscles of the fore extremities are also
of extraordinary power. The scapula is a long and slender
bone, more resembling a humerus in its shape than an ordi-
nary scapula: the humerus, on the contrary, 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 expanded; the palms being turned outwards and back-
wards, and its lower margin being fashioned into a sharp
cutting edge. The carpal bones and the phalanges of the
fingers are very much compressed; but they are 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 af-

INSECTIVOROUS MAMMALIA. 363

fording an extensive surface of attachment for the large pec-
toral muscles, from which the limb derives its principal
force. The head terminates in front by a pointed nOvSe,
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 development
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, w^alks very awkwardlj^, and is unable to ad-
vance but by an irregular and vacillating pace.*

We have seen that there is a tribe of fishes armed ex-
ternally with sharp spines, which they are capable of erect-
ing when in danger of attack. A similar kind of defensive
armour is furnished to the Porcupine and the Hedgehog,
which belong to the family of insectivorous quadrupeds.
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 pannicidus carnosus. In the hedgehog
these muscles are very complicated, and give the animal
the power of rolling itself into a ball. A minute descrip-
tion 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

* The only quadrupeds which resemble the mole in the perfect adaptation
of their structure to the pui-poses of burrowing", are the Wombat and the
Koala, which are among" the many extraordinary animals inhabiting- the con-
tinent of Australia. Their hind leg's are constructed in a manner vei'y mucli
resembling- the human fore-arm. (See Home, Lectures, See. i. 134.)

364 THE MECHANICAL FUNCTIONS.

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 nyiscles tends to draw the skin down-
wards, and to coil it over the head and paws, in the manner
shown in Fig. 220, like the closing of the mouth of a great
bag.

§ 10. Carnivora,

The type of the Mammalia may be considered as having
attained its full development in the carnivorous tribes, which
comprehend the larger beasts of prey. As their food is ani-
mal, they acquire a less complicated apparatus for digestion
than herbivorous quadrupeds, possess greater activity and
strength, and enjoy a greater range of sensitive and intel-
lectual faculties. In accordance with these conditions, we
may notice the greater expansion of their brain, the su-
perior acuteness of their senses, and their enormous mus-
cular power. The trunk of the body is lighter than that
of vegetable feeders, especially in the abdominal region,
and is compressed laterally: the spine is more pliant and
elastic,* the limbs have greater freedom of motion, the ex-
tremities are more subdivided, and they are armed with

* The suppleness of the spine might at once be inferred, on the simple in-
spection of the skeleton, from the circumstance that the vertebrae of the
neck and loins have a comparatively small development of their spinous pro-
cesses.

CARNIVOROUS MAMMALIA. 365

formidable weapons of offence and destruction. Great me-
chanical 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 out-
line of the body, in Fig. 221, the first vertebra of the neck,
or atlas, has very widely expanded transverse processes,
while the second vertebra has a largely developed spinous
process, for supplying levers for the muscles which have to
perform these and other actions in which the head is con-
cerned.

The whole of the remaining part of the skeleton of these
animals is constructed with reference to their predatory na-
ture. The sudden springs with which they pounce upon
their prey must impart to the whole osseous frame the most
violent concussion. The first stroke with which they attempt
the destruction of their victims is given with the fore leg:
so that, had the limb been rigidly connected with the ster-
num, by means of an entire clavicle, its motions would have

been too limited, and danger of fracture would have been in-
curred. The scapula is broad, and the humerus of great

366 THE MECHANICAL FUNCTIONS.

length, compared with the same bones in ruminants; and the
latter has, besides, a large surface for its articulation with
the former of these bones, thus allowing of a great range of
motion: the radius and ulna are perfectly distinct, and play
extensively on each other.

The fore feet rest on the ground by means of the second
of the three joints of which each toe is composed. The last
phalanges are raised at right angles to the former, for the
purpose of supporting the claws in an erect position. It has
been considered of such importance to preserve these formi-
dable instruments constantly sharp, and in a condition fitted
for immediate use, that an express contrivance has been re-
sorted to for this purpose. It consists in a sheath, within
which the claws, when not 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 throw^n 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 extre-
mities, we find the pelvis extending farther backwards, and
more in a perpendicular line with the femur. This latter
bone is longer and more slender than in the horse, but it is
more compact in its form, and its processes are more strong-
ly 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 diversity of action, at the same
time that they have greater power than those of herbivorous
quadrupeds. The articular surfaces are of greater extent, arid
are lubricated with a more copious supply of synovia; their
ligaments are more delicate and more numerous; and the

* There exists, concealed in the tuft of hair, at the extremity of the lion's
tail, a small conical and slightly cui-ved claw, which is attached to the skin
only, and not to the last caudal vertebra: it is difficult to conjecture what can
be its use.

MAMMALIA QUADRTJMANA. 367

joints, in general, adapted to a greater variety of movements.
All these provisions are evidently directed to confer great
freedom and facility of motion, and to enlarge the sphere of
action of the body generally, as well as of the limbs.

§ 11. Quadrumana.

We may trace in the series of quadrupeds which have
come under our review, a gradual increase in the develop-
ment 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 secure-
ly on sand; then come the Rhinoceros, which has three
hoofed toes; the Hippopotamus, which has four; and the
Elephant, which has five. To these succeed another series,
where nails, or claws, are substituted for hoofs, as is the case
with all the Carnivora, which, standing on the extremities
of their toes, have been termed Digitigrades. Then follow
the Plantigrade quadrupeds, such as the bear, the badger,
the hedgehog and the mole, which rest with the whole foot
on the ground, and are, in consequence, able to make great
use of their fore paws. These conduct us to the family of
the Quadrumana^ comprehending the Monkey and the Le-
mur 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 approximation 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 en-
gage in successful combats with serpents and other enemies;
retaining their positions in perfect security on the moving

368 THE MECHANICAL FUNCTIONS.

branches, or sportively swinging by their extremities in the
air. Both the feet and the hands are formed for this species
of prehension; and many are farther provided with a strong-
ly prehensile tail, which is an instrument admirably adapted
for 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 quadrumana will af-
ford the key to most of the peculiarities of structure they
present to our observation. The head, being no longer sus-
pended at the end of a horizontal, or recurved neck, is, in
the usual attitude of the animalj supported chiefly by the
cervical vertebrae. The greater development of the brain,
and, miore especially, of its posterior lobes, creates a neces-
sity for an extension of the occipital bone in that direction,
a portion of the weight to be sustained by the atlas, is, ac-
cordingly, thrown behind the centre of motion, which is at
its articulation with the latter bone; and this weight tends,
therefore, to balance that of the anterior part of the head;
hence, there is no need of the strong cervical ligament, which
is so universally met with in quadrupeds, and, 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 gene-
rally greatly multiplied to form a tail of considerable length,
which, in the Jiteles, or spider monkey of America, is
moved by powerful muscles, and is an organ of great flexi-
bility and strength. Monkeys possess a distinct clavicle, a
lengthened humerus and a femur, a radius and ulna movea-
ble 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 mus-
cles which are to move their several parts; by this means,

THE HUMAN FRA3IE. 3G9

although the force with which they act may be somewhat
lessened, yet the velocity of the motion they produce is in-
creased in the same proportion. The fibula is here a bone
of more importance than in quadrupeds; for it performs a
motion of rotation round the tibia, analogous to that of the
radius upon the ulna, giving a great extent of action to the
foot, and converting the leg into an arm, as we have already
seen that the foot itself is 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 inw^ards, and causing the monkey
while walking to rest chieflj^on its outer edge. This seem-
ing defect gives a slight appearance of awkwardness to the
gait; it is not, however, to be view^ed as an imperfection^ for
it is evidently designed to assist the animal in climbing trees,
which is its most usual action, the oblique position of the
foot enabling it most effectually to lay hold of the branches.
Monkeys are evidently not formed to excel in swiftness; for
the heel, in these animals, presents no large projection as in
other orders of mammalia; nor are the muscles that are in-
serted into the heel particularly powerful; they hardly, in^
deed, can be said to compose a calf as in the human leg.

§ 12. Man.

The series of structures modelled on the characteristic
type of the Mammalia, after having exhibited the successive
development of all its elements, attains the highest perfec-
tion 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 ph3^sical structure and his physiological constitution place
him incontestably at the summit of tlie scale of terrestrial
beings. Considered zoologically, indeed, the human species
must rank among the Mammalia, and it even makes a near ap-
proach to the Quadrumana; yet there exists many peculiari-
ties of structure, which entitle Man to be placed in a sepa-

Vol. I. 47

370 THE MECHANICAL FUNCTIONS.

rate order, where disclaiming any close alliance with inferior
creatures, he proudly stands alone, towering far above them
all.

It is not, however, on a pre-eminence in any single phy-
sical 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 adapta-
tion to an incomparably greater variety of objects, and an infi-
nitely more expanded sphere of action. As the beauty of an
edifice depends not on the elaborate finishing of any one por-
tion, 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 pro-
portion and harmony subsisting among all its parts, and per-
vading the whole system of its functions. The design of
its structure and economy embraces widely different, and far
higher aims than those contemplated in the organization of
any of the inferior animals. Destined to an intellectual, a
social, and a moral existence, Man has had every part of his
organization modified with an express relation to these great
objects of his formation. This will best appear when we
come to examine the organs which are subservient to the
sensitive and active faculties; but even here, where our views
must, for the present, be limited to the mechanical circum-
stances of his structure, the proofs are sufficiently numerous
to warrant this general conclusion.

Man presents the only instance among the mammalia of
a conformation by which the erect posture can be perma-
nently maintained, and in which the office of supporting the
trunk of the body is consigned exclusively to the lower ex-
tremities. To this intention the form and arrangement of
all the parts of the osseous fabric, and the position and ad-
justments of the organs of sense have a well marked refer-
ence.* The lower limbs are qualified to be the efficient

* In most quadrupeds, as we have seen, the thorax is deep in the direc-
tion fi-om the sternum to the spine, but is compressed latemlly, for the evi-

THE HUMAN FRAME. 371

instruments of progression by their greater length and mus-
cularity, 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 Quadru-
mana, which come nearer to the human form than any of
the other tribes, have the lower limbs comparatively weak.
In almost all other quadrupeds the disproportion is still
greater, the thigh being short, and almost concealed by the
muscles of the trunk, and the remainder of the limb being
slender, and not surrounded by any considerable mass of
muscles.

The articular surfaces of the knee joint are broader, and
admit of greater extent of motion in man than in quadru-
peds: hence the leg can be brought into the same line with
the thigh, and form with it a straight and firm column of
support to the trunk; and the long neck of the thigh bone
allows of more complete rotation. The widely spread basin
of the pelvis effectually sustains the weight of the digestive
organs, and they rest more particularly upon the broad ex-
pansion of the iliac bones: in quadrupeds, these bones, havino-
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 conca-
vity in two directions, the one longitudinal, the other trans-
verse, constituting a double arch. This construction, be-
sides "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

dent purpose of brlng-Ing- the fore limbs nearer to each other, that they
might more effectually support the anterior part of the trunk. In Man, on
the contrary, the thorax is flattened anteriorly, and extends more in widtli
than in depth} thus throwing- out the shoulders, and allowing an extensive
range of motion to the arms.

■>ts

372 THE MECHANICAL FtrNCTIONS.

also for the lodgement of smaller muscles affixed to each in-
dividual 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 which compose the calf of the
leu, are of creat size and streno;th, 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 decided advan-
tages over that of the monkey, with reference to the specific
objects of its formation.

It is impossible to doubt that nature intended man to as-
sume the erect attitude, when we advert to the mode in
which the head is placed on the spinal column. The enor-
mous development of the brain, and of the bones which in-
vest it, increases so considerably the weight of that part of
the head, which is situated behind its articulation with the
vertebrse 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 very greatly preponderates. The muscles
which bend the head back upon the neck, and retain it in
its natural position, are therefore not required to be so pow-
erful as they must be in quadrupeds, especially in those
which graze, and in which the mouth and eyes must fre-
quently be directed downwards, for the purpose of procuring
food. In man this attitude would, if continued, be extreme-
ly fatiguing, from the weakness of those muscles, and the
absence of that strong ligament which sustains the w^elght
of the head in the ordinary horizontal attitude of quadru-
peds.

"Pronaque cum spcclant anlmalia cxtera terram,
Os hominl sublime dedit, cxlumque tueri
Jusslt, et erectos ad sidera toilere vultus." — Otid.

The space comprehended by the two feet is extremely
narrow, when compared with the extended base on which
the quadruped is supported. Hence, the stability of the body

THE HUMAN FRAME. S*!?

must be considerably less. The statue of an elephant, placed
upon a level surface, would stand without danger of overset-
ting: but the statue of a man, resting on the feet, in the usual
attitude of standing, Avould he 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 body from falling. But the actions of
the muscles are continually exerted to prevent the yielding
of the joints under the weiglit of the body, which tends to
bend them. In quadrupeds, less exertion is requisite for
that purpose; and standing is in them, as we have seen, a
posture of comparative repose: in man it requires nearly as
great an expenditure of muscular power as the act of walk-
ing. Soldiers, on parade, experience more fatigue by re-
maining 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 altcrnatel}' 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 direc-
tion. Long habit has rendered us unconscious of these ex-
ertions, which we are, nevertheless, continually making;
but a child learning to walk finds it difficult to accomplish
them successfully. It is one among those arts which he has
to acquire, and which costs him, in the apprenticeship, many
painful efforts, and many discouraging falls. But whenever
nature is the teacher, the scholar makes rapid progress in
learning; and no sooner have the muscles acquired tJie ne-
cessary strength, than the child becomes an adept in ba-

374 THE MECHANICAL FUNCTIONS.

lancing its body in various attitudes, and, in a very short
time, is unconscious that these actions require exertion.

In walkins:, the first effort that is made consists in trans-
ferring 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 unsupported,
falls through a certain space, and would continue its descent,
were it not that it is received on the other foot, wjiich, by
this time, has been set upon the ground. This falling of the
body would, if not immediately checked, become very sen-
sible; 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 disagreea-
ble jar is given to the whole frame.

While the weight of the body is thus transferred, alter-
nately, from one foot to the other, the centre of gravity not
only rises and falls, so as to describe, at every step, a small
arch, but also vibrates from side to side, so that the series of
curves it describes, are somewhat complicated in their form.
This undulation of the body, from one foot to the other,
would scarcely ever be performed w^ith perfect equality on
both sides, if we trusted wholly to the sensations communi-
cated by the muscles, and if we were not guided by the sense
of sight, or some other substitute. Thus, a person blind-
folded cannot walk far in a straight line; for, even on a le-
vel plane, he will incline unconsciously either to the right
or to the left.

In all quadrupeds, and even also in the quadrumana, 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 ofhces, and are at liberty to be
applied to other purposes, and employed as instruments of
prehension and of touch. In the power of executing an in-

THE HUMAN FRAME. 375

finite variety of movements and of actions, requiring either
strength, delicacy, or precision, the human arm and hand,
considered in their mechanism alone, are structures of unri-
valled excellence; and, when viewed in relation to the in-
tellectual energies to which they are subservient, plainly re-
veal to us the divine source, from which have emanated this
exquisite workmanship, and these admirable adjustments, so
fitted to excite in our breasts tlie 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 se-
parate treatise: but I must refrain from pursuing this inte-
resting subject, as, fortunately, the task ]|as devolved upon
one far more able than myself to do it justice.

( 376 )

CHAPTER X.

VERTEBRATA CAPABLE OP FLYING.

§ 1. Verlebrata ivilhout Feathers, for^med for flying.

Few problems in mechanic art present greater practical
difficulties than thft of raising from the ground, and of sus-
taining and moving rapidly through the air an animal body^
composed as it must be of many ponderous organs, that are
requisite for the performance of the higher functions of life:
yet Nature has achieved all this, not only in endless tribes
of the more diminutive invertebrate animals, but also in the
more solid and massive organizations which are modelled on
the vertebrate type. These objects have been accomplished,
in all cases, without the employment of any other than the
ordinary elements of those organizations; modified, indeed,
to suit the particular purpose in view, but yet essentially the
same, and regulated by the same laws of development which
prevail throughout the whole animal system. The adapta-
tion of these elements to the construction of an apparatus of
so refined a nature as that which is required for flying, im-
plies 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 must take place before the completion
of the finished structures. Of this we have already had a
remarkable example in the metamorphoses of insects, which
exhibit, in their last stage of development, the highest de-
gree of perfection compatible with the articulate type.
Birds, in like manner, present us with the highest refine-

POWER OF FLYING. 377

ment of mechanical conformation which can be attained by
the development of a vertcbratcd structure.

The power of flying is derived altogether from the resist-
ance which the air opposes to bodies moving throuo-h it or
acting upon it by mechanical impulse. In the ordinary
movements of our own bodies, this resistance is scarcely
sensible, and hardly ever attracts notice: but it increases in
proportion to the surface which acts upon the air, and still
more according to the velocity of the moving body; for the
increase is not merely in the simple ratio of the velocity,
but as its square, or perhaps, even a higher power. In order
that an animal may be able to fly, therefore, two principal
conditions are required: there must, first, be a considerable
extent of surface in the wings, or instruments which act
upon the air; and there must, secondly, be sufficient mus-
cular 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 possess them in the same perfection. No quadruped,
except the bat, has sufficient muscular power in its .limbs,
however aided by an expansion of surface, to strike the air
with the force requisite for flight. No refinement of me-
chanic ingenuity has ever placed the Daedalian art of flying
within the reach of human power; for even if the lightest
possible wings could be so artificially adapted to the body as
to receive the full force of the actions of the limbs, however
these actions might be combined, they would fall very far
short of the exertion necessary for raising the body from
the ground.

Examples, however, occur in every one of the classes of
vertebrated animals, where an approach is made to this fa-
culty. In the Exocastus, or flying-fish, the pectoral fins have
been enormously expanded, evidently for the purpose of en-
abling the animal to leap out of the water, and support itself
for a short interval in the air: but its utmost efforts are inade-
quate to sustain it beyond a few moments in that element,
Vol. I. 48

37S THE MECHANICAL FUNCTIONS.

and it can never rise to more than five or six feet above the
surface of the water.

A species of lizard, called the Draco Fb/a7Z5, has a singu-
larly constructed apparatus, which appears like two wings,
affixed to the sides of the back, and quite independent of
either the fore or the hind extremities. By the aid of these
moveable flaps, the animal is able to 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 imper-
fect organs. The construction of these anomalous mem-
bers is highly curious in a physiological point of view; as
showing how Nature, in effecting a new purpose, is in-
clined to resort to the modification of structures already
established as constituent parts of the frame, in preference
to creating new organs, or such as have no prototype in the
model of its formation. Frequent proofs of this law, indeed,
are afforded by the comparative examination of the anatomy
of the organs of progressive motion. The ribs, in particular,
are often the subject of these conversions to uses very dif-
ferent from their ordinary function, which is that of assist-
ing in respiration. Thus, we have seen that in the Tortoise
they are expanded to form the carapace, uniting with corre-
sponding dilatations of the sternum, and sterno-costal appen-
dages, in composing a general osseous ineasement 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 Cobra de Capello, or
hooded snake of the East Indies, are employed for the me-
chanical purpose of supporting an expansion of the skin of
the neck, which forms a kind of hood, capable of being
raised or depressed at the pleasure of the animal."^ These
ribs are entirely unconnected with the respiration of the ser-
pent.

In the Draco volans, which was to be furnished with rii-

* Phil. Trans, for 1804, p. 346.

FLYING LIZARD.

379

struments for assisting it in its distant leaps through the air,
it is again the ribs which are resorted to for furnisliing the
basis of such an apparatus. On each side of the dorsal ver-
tebrae, 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 elon-
gated, and are covered with a thin cuticle, derived from the
common integuments. The ordinary muscles which move
the ribs still remain, but with greatly increased power, and
serve to flap these strangely formed wings at the pleasure of
the animal, during its short aerial excursions.

222

Among the mammalia, we meet with a few species which
have a broad membrane, formed of a 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. Struc-
tures of this kind are possessed by the Sciurus volans^ or

380

THE MECHANICAL FUNCTIONS.

flying squirrel, and also by some other species of the same
genus. They are seen on a still larger scale in the Lemur
volans, or Galeopitheciis. The resistance which these broad
expansions of skin oppose to the air, when the limbs are
spread out, enables the animal to descend in perfect safety
through that medium from very considerable heights: but
these appendages to the body are mere 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 en-
dowed 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
extremities, 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 between these lengthened
fingers, has its origin in the sides of the neck, and reaches
all along the body to the extremities of the hinder legs,
which it encloses in its folds. Thus, not only is the sur-
face, by which it acts upon the air, sufiiciently extensive,
but the muscular powder, 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

BAT. 381

office of a real wing. The bat flies with perfect ease, even
while carrying along with it one or two of its young: it is
not, however, fitted for very long flights.

The conformation of the skeleton is adapted to this new
and important function. The chest is broad and capacious
to admit of free respiration while the animal is flying, and
to afi'ord ample space for the attachment of the large mus-
cles which have become necessary. The scapulae (s) are
large, and of a singular form, and they are kept at a consi-
derable distance asunder by the expanded chest: their cora-
coid 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 remarkably curved in their shape. The sternum is
much developed, extending laterally, and having a project-
ing crest along the middle of its lower surface. The hu-
merus (h) is strong, but short; apparently in order to avoid
the danger of its being snapped asunder by the violent ac-
tions 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, ex-
cept the process of the olecranon, or elbow, which has be-
come soldered to the radius.

These advantages in the construction of the fore extremi-
ties are obtained at the expense of the hinder, which are too
feeble to support the weight of the body in the upright posi-
tion required for walking, in consequence of the centre of gra-
vity being between the wings. On a level plane, indeed, the
bat can advance only by a kind of crawling or hoj^ping motion.
The whole anterior half of the trunk is much more fully de-
veloped than the posterior half, which appears as if it had
been checked in its growth. The pelvis (p) is of diminutive
size, compared with the rest of the skeleton : the pubic bones
are lengthened backwards, and are joined merely at a small
point. The whole posterior limb is short, the femur (f ) com-

382 THE MECHANICAL FUNCTIONS.

paratively long, and the fibula is a very slender bone, yet
quite distinct from the tibia (t.) The ^slight degree of mo-
tion which is thus allowed between them is useful to the ani-
mal, in enabling the feet to lay hold of cornices or other pro-
jecting parts of the roofs of buildings, on which the animal
fastens itself, and hangs with the head downwards. It is
probably with the intention 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 mem-
brane, which often extends between the hind feet, and is far-
ther sustained by the tail, in those species which have the
spine prolonged to form one.

Bats are also provided with another instrument for sus-
pending 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 ter-,
minated by a strong claw, which projects, even when the
wings are folded, and is useful in progression, serving as a
point of support.

§ 2. Birds.

It is in birds alone that we find the most perfect adapta-
tion of structure to the purposes of rapid and extensive
flight: in them the frame of the skeleton, the figure, position,
and structure of the wings, the size of the muscles, the pecu-
liar nature of their irritability, and even the outward form
of the body, have all a direct and beautiful relation to the
properties of the element in which nature has intended them
to move. In their formation, a new, and in as far as relates
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 development.

The skeleton of birds has the same constituent parts as
that of other vertebrated classes: the bones of the anterior

BIRDS. 38

rt

extremity, though destined exclusively to support the wing,
retain the same divisions, and are composed of the usual
elements: and the general form of the body is that best cal-
culated to glide through the air with the least resistance.
As birds swallow their food entire, there is no necessity for
any part of the bulky apparatus of hard and solid teeth, large
muscles and heavy jaws which are required by most quad-
rupeds: hence the head admits of being greatly reduced in
its, dimensions; and the form of the beak, which is drawn to
a point, and cuts the opposing air, tends to facilitate the pro-
gress of the bird in its flight.

In the conformation of the body, also, every circumstance
that could contribute to give it lightness has been sedulously
studied. The general size of birds is considerably smaller
than quadrupeds of corresponding habits. No where has
Nature attempted to endow a huge ponderous animal, like
the fabled Pegasus, with the power of flight. Great con-
densation has been given to the osseous substance,'^' in order
that the greatest degree of strength might be procured with
the same weight of solid materials; and the mechanical ad-
vantage derived from their being disposed in the circum-
ference rather than in central masses, has been obtained to
the utmost extent. The horny material, of which the stems
of the feathers are constructed, arc, 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 likewise
been made hollow, and instead of their cavities being filled
with marrow, they contain only air.t Thus, the whole ske-
leton is rendered remarkably light: that of the Pelicanus

* Ossification not only proceeds more rapidly, bat 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 ossi-
fied.

•j- In the bat there is no provision of this kind for lightening the boneSy
and we find them containing marrow, as in other mammalia, and not air.

384 THE MECHANICAL FUNCTIONS.

07iocrotalus, for instance, or white Pelican, which is five
feet in length, was found by the Parisian Academicians to
weigh only twenty-three ounces, while the entire bird
weighed nearly twenty-five pounds. The cavities in the
bones communicate with large air cells, which are distri-
buted 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 as-
cent of the bird into the higher regions of the atmosphere.*
The conditions in which a bird is placed with regard to
the density of the surrounding medium, as well as their mode
of progression, are so opposite to those of fishes, that we
should expect to find great corresponding differences in
their conformation. These two classes of vertebrata, accord-
ingly, 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 con-

* This air, being" contained in the interior of the body, which preserves
a very elevated temperature, must be constantly in a state of greater rarefac-
tion than the cooler external air; a condition which must contribute in some
slight degree to render the whole body lighter than it would otherwise have
been. It appears to me, however, that considerably greater importance has
been attached to this circumstance than it really possesses. Many have gone
so far as to represent the condition of a bird as approaching to that of a bal-
loon filled with a lighter gas than atmospheric air: and have been lavish in
their expressions of admiration at the beauty of the contrivance which thus
converted a living structure into an aerostatic machine. A little sober con-
sideration will suffice to show that the amount of the supposed advantages
resulting to the bird from the diminution of weight, occasioned by the dif-
ference of temperature between the air included in its body and the exter-
nal atmosphere, is perfectly insignificant. Any one who will take the trou-
ble to calculate the real diminution of weight arising from this cause, under
the most favourable circumstances, will find that, even in the case of the
largest bird, it can never amount to more than a few grains.

BIRDS.

'3S5

trary, 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, re-
presented in Fig. 224. In the fish, progressive motion is

eff'ected principally by the movements of the tail, which im-
pels the body alternately from side to side: in the bird, the
only instruments of motion are the wings, which are affixed
to the fore part of the trunk, and are moved by muscles situ-
ated in that region. In the fish, the spine is flexible ncar-
VoL. I. 49

3S6 THE MECHANICAL FUNCTIONS.

ly 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 wdth
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 resting upon its legs, the centre of gravity must, in like
manner, be brought immediately over the base of support
formed by the toes: it becomes necessary, therefore, to
provide means for shifting the centre of gravity from one
place to another, according 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 adjust-
ments consist in the motions of the head and neck, which last
is, for that purpose, rendered exceedingly long and flexible.
The number of cervical vertebrae is generally very consi-
derable: in the mammialia,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 together by articulations, generally al-
lowing free motion in all directions; that is, laterally, as well
as forw^ards and backwards. This unusual degree of mobi-
lity is conferred by a peculiar mechanism, w^hich is not met
with in the other classes of vertebrated 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, how^ever, that in consequence of the
positions of the oblique processes, the upper vertebr aeof

* See Mr. H. Earle's paper on this subject in the Philosophical transac-
tions for 1833, p. 277.

BIRDS. 387

the neck bend with more facility forwards than backwards;
while those in the lower half of the neck bend more rcacfily
backwards: hence, in a state of repose, the neck naturally as-
sumes a double curvature, like that of the letter S, as is well
seen in the graceful form of the swan's neck. By extend-
ing the neck in a straight line, the bird can, while flying,
carry forwards the centre of gravity, so as to bring it under
the wings; and when resting on its feet, or floating on the
water, it can transfer that centre backwards, so as to bring it
towards the middle of the body, by merely bending back the
neck into the curved form which has just been described; and
thus the equilibrium is, under all circumstances preserved, by
movements remarkable for their elegance and grace.*

Another advantage arising from the length and mobility
of the neck is, that it facilitates 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 wetted, and which is copiously provided for
that purpose by glands situated near the tail. The flexibili-
ty 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 farther provision made for the accomplish-
ment of this object in the mode of articulation of the head
with the neck. We have seen that, in fishes, and in most
reptiles, this articulation consists of a ball and socket joint;
a rounded tubercle of the occipital bone being received into
a hemispherical depression in the first vertebra of the neck.
In the mammalia the plan is changed, and there are two ar-

* 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
necessary in consequence of the conversion of the fore extremities into wing^
of which the structure is incompatible with any prehensile power, such as is
often possessed by the anterior extremity of a quadruped.

383

THE MECHANICAL EUNCTIONS.

ticular surfaces, one on each side of the spinal canal, formed
on' processes corresponding to the leaves of the first cranial
vertebra, and assimilating it more to a hinge joint. In birds,
however, where, as we have just seen, the most extensive
lateral motions are required, the plan of the ball and socket
joint is again resorted to; and the occipital bone is made to
turn upon the atlas by a single pivot. So great is the free-
dom 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 scarce-
ly a trace of these processes in the cervical vertebrae of birds.
But another, and a still more important consideration was to
be attended to in the construction of this part of the spine.
It must be recollected that the spinal marrow passes down
along the canal formed by the arches of the vertebrae, and
that any pressure applied to its tender substance would in-
stantly paralyze the w^hole 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 accom-
plished in the simplest and most effectual manner, by en-
larging the diameter of the canal at the upper and lower part
225 ^ of each vertebra, while, at the

middle, it remains of the usual
size, so that the shape of the ca-
vity, as is well seen in Fig. 225,
which shows a vertical 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, allow-
ing of very considerable flexion,

• For the specimen from which this engraving wa,s made, I am indebted
to the kindness of Mr. Owen.

BIRDS. 3S9

without reducing the diameter of the canal beyond that of
the narrow portion, and, tlierefore, without producing com-
pression of the spinal marrow. Mr. Earle found' that ver-
tebrae united in this manner may be bent backwards to a
right angle, and laterally to half a right angle, without inju-
ry to the enclosed nervous substance. The design of this
structure is farther evident from its not existing in the dor-
sal and lumbar portions of the spine, which admit of no mo-
tion whatever, and where there is no variation in the diame-
ter of the spinal canal.

A plan entirely different is followed in the vertebrse of
the back and loins. For the purpose of ensuring the proper
actions of the wings, the great object here is to prevent mo-
tion, and to give all possible strength and security; and ac-
cordingly the whole of this portion of the spine, together
with the sacrum, is consolidated into one piece. All the
processes are largely 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 tl:;^^
bodies of the dorsal vertebrae are immoveably soldered to-
gether 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 being anchylosed
together for this purpose; so that they form a bone of great
length. The coccygeal vertebrae (q) are also numerous, but
are compressed into a small space, and enjoy great latitude
of motion, being subservient to the movements of the tail.

The ribs are numerous, and of considerable strength: they
send out processes, which are directed backwards, passing
over the next rib before they terminate, and giving very ef-
fectual support to the walls of the chest. The ribs are con-
tinued along the abdomen, and afford protection to the vis-
cera in that cavity; and some arise even from the sacrum,
and from the iliac bones. Those which are in front are

* In the paper already quoted, p. 278.

390 THE MECHANICAL FUNCTIONS.

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 con-
siderable part of the abdomen, and having a large perpendi-
cular crest descending, like the keel of a ship, from its lower
surface. The object of this great development is to furnish
extensive attachment to the large pectoral muscles employed
to move the wings, and which, taken together, are generally
heavier than the rest of the body. Considered with refe-
rence 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
fatal. The bat is the only instance, among the mammalia,
where the sternum presents this peculiar carinated^ or keel-
like shape: and the purpose is evidently the same as in the
bird.*

The scapula is generally a small and slender bone. The
Jfcpracoid bone (k) is largely developed, and assumes much of
the appearance of a clavicle.t But the real clavicles (c) are
united below, where they join the fore part of the sternum,
appearing as one bone, which, from its forked shape, has
been denominated the furcidar 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
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

* Notwithstanding the great modification 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
developed in very different proportions.

\ 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 ana-
logies of position and of development are in favour of the views stated in the
text.

BIRDS. 391

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 quadruped: but on a
closer examination we find it to contain all the principal
bones of the latter, though somewhat altered in shape, and
still more changed in their functions. Yet still the same
unity of plan, and perfect harmony of execution may be
discerned in the mechanism of this refined instrument of a
higher mode of progression.

The head of the humerus (h) has a compressed form; and
in order to obtain great extent of motion, it is made to play
by a very small cylindrical surface upon the scapula; thus
admitting of the complete descent of the wing, unobstructed
by any opposing process, but at the same time limiting its
motion to one plane. It is connected below, by broad at-
tachm.ents, to the radius and ulna, forming with them a hinge
joint. These latter bones are separate, and of great length,
but so firmly united together by ligament as scarcely to have
any motion on one another. The carpus (w,) consists of
two bones only, the one articulated with the radius, the
other with the ulna. They move together as one piece;
but, contrary to what takes place in quadrupeds, the move-
ments are made from side to side, instead of their consistino-
of flexion and extension; this variation from the usual struc-
ture being for the purpose of folding down the joints of the
wing, and bringing them close to the bod}^ .The metacar-
pus (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 process a small pointed bone is connected, cor-
responding 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

392 THE MECHANICAL FUNCTIONS.

a little finger. The degree of development 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 occa-
sionally 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 else-
where rarely find united. 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 ad-
vantageous manner for resisting flexion by being formed
into a hollow cylinder. But the vane of the feather is still
more artificially constructed; being composed of a number
of flat threads, or filaments, so arranged as to oppose a much
greater resistance to a force striking perpendicularly against
their surface, than to one tvhich is directed laterally; that
is, in the plane of the stem. They derive this power of re-
sistance 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 fila-
ments of the feather have to encounter the resistance and
impulse of the air. It is here that strength is wanted, and
it is here that strength has been bestowed.

On examining the assemblage of these 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 effected 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 laminas, and
fitted to catch upon and clasp one another, whenever the la-
minae are brought w^ithin a certain distance. The fibrils of
a feather from the wing of a goose are represented magnified

FEATHERS OF BIRDS.

393

at a, a, b, b, Fig. 226, as they arise from the two sides of
the edges of each lamina: they are exceedingly numerous,
above a thousand being contained in the space of an inch;

and they are of two kinds, each kind having a different form
and curvature. Those marked a, a, which arise from the
side next to the extremity of the feather are branched or
tufted, and bend downwards, while those marked b, b, pro-
ceeding from the other side of the lamina, or that nearest
the root of the feather, are shorter and firmer, and do not
divide into branches, but are hooked at the extremities, and
Ifire directed upwards. When the two laminae are brought
close to one another, the long, curved fibrils of the one be-
ing carried over the short and straight fibrils of the other,
both sets becom.e entangled together; their crooked ends
fastening into one another, just as the latch of a door falls
into the cavity of the catch which is fixed in the door-post
to receive it. The way in which this takes place will be
readily perceived by making a section of the vane of a fea-
ther across the lam.ina^, and examining, with a good micro-
scope, 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 re-
peated over every part of the feather, and constitutes a close-
VoL. I. 50

394 THE MECHANICAL FUNCTIONS.

!y 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 intended for flight,
as in those of the ostrich, the fibrils are altogether wanting:
in those of the peacock's tail, the fibrils, though large, have
not the construction which fits them for clasping those of
the contiguous lamina; and in other instances they do so
very imperfectly.

A construction so refined and artificial as the one I have
been describing, and so perfectly adapted to the mechanical
object which it is toansw^er, cannot be contemplated without
the deepest feeling of admiration, and without the most eager
curiosity to gain an insight into the elaborate processes,
v^^hich, we cannot doubt, are employed by nature in the for-
mation of a fabric so highly finished, and displaying such
minute and curious workmanship. It is only very recently
that we have been, admitted to a close inspection of the com-
plicated 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 o%
the minutest and apparently most insignificant portions of
the organized frame, whether it be the down of a thistle, the
scales of a moth, or the fibrils of a feather, as well as to
the completion of the larger and more important organs of
vitality.

Every bird, on quitting the egg, is found to be covered
on all parts except the under side, with a kind of down con-
sisting of minute filaments, collected in tufts, and resem-

* 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 mi-
nuter details 1 have supplied from my own observations with the microscope.
The branched forni of the upper fibrils, and the reticulated structure of the
laminx themselves, when viewed witli a high magnifying power, are parti-
cularly beautiful microscopic objects.

FEATHERS OF BIRDS. 395

bllng in their arrangement the fibres of a camcl-liair 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 that afterwards
form the feathers, of which this down, serving the purpose
of a first garment, hastily spread over the young bird, is but
the precursor; for the tufts generally soon fall off and disap-
pear, except in the rapacious tribes, as the eagle and the vul-
ture, where they remain attached to the feathers for a consi-
derable time.

While this temporary protection is given to the integu-
ment, extensive preparations are making underneath for fur-
nishing 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 re-
ceiving the necessary supply of nuti-icnt juices, and of ves-
sels for their circulation, together with their usual comple-
ment of nerves and absorbents. When first visible, this
organ has the form of a very minute cone, attached by a fila-
ment proceeding from its base to one of the papillae of the
skin, and establishing its connexion with the living system.
In the course of a few days, this cone has become 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 rudi-
ments 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 j)roper shapes
in different places: for while the first lamina? are construct-
ing in one portion of the cylinder, the next are only just be-
^innins; to be formed in another; and while the outer covcr-
ing of the stem is growing from one membrane, the interior
spongy tissue is dejiosited in other places, in various stages
of softness or consolidation: so that the whole comj)oses a
system of operations, which may be said to resemble in its

396

THE MECHANICAL FUNCTIONS.

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 it-
self only developed as the work proceeds, its different parts
being produced successively in proportion as they are want-
ed, and their form and structure undergoing frequent varia-
tion in the course of their development.

230 231

228 .- 229

i-!'!"il)llllK H A

S-^-

nriiiii!.

The most elaborate and apparently accurate researches on
this intricate subject, are those lately undertaken by M.
Frederick Cuvier, from whose memoir* I have selected
the following abridged statement of the principal results of
his observations. It will be necessary, in order to obtain a

* M^moires du Museum, xiii. 327; and Annales des Sciences Nuturelles,
ix. 113.

FEATHERS OF BIRDS, 397

clear idea of the several steps of the process to be described,
to advert to the structure of a feather in its fiiiislicd state.
For this purpose we need only examine a common feather,
such as that represented 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 scries of laminae, composing,
with their fibrils, what is termed the vane of the feather
(v.) The lines from which these laminsc arise, approach
one another at 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 transparent tubular portion of the
quill (t) commences; and at its lower extremity (l) there
exists a second, or lower orifice.

The entire organ which forms the feather, and which may
be termed its matrix, is represented in Fig. 229, when it
has attained the cylindric form already described; of which
A is the apex, or conical part, that rises above the cuticle,
and B the base, by which it is attached to the cerium, or true
skin. A white line is seen running longitudinally the whole
length of the cylinder, and anotlier, exactly similar to it, is ^
met with on the opposite side: the one corresponds in situ-
ation to the front, and the otlicr to the back o^the stem of
the future feather. On laying open the matrix longitudi-
nally, as is shown in Fig. 230, it is found to be composed
of a sheath or capsule, and of a central pulpy mass, termed
the hulh. The capsule consists of several membranous lay-
ers (c, E, s, I,) which are more consolidated near the apex,
and become gradually softer and more delicate, as we trace
them towards the base of the matrix, where their formation
is only beginning to take place.

The laminre and theiV fibrils, the assemblage of whicli
constitutes the vane of the feather, arc tlic parts which arc
first formed; and their construction is cflected in the sjiace
between the outer capsule (c,) and the centKil bulb (n,) in a
mode which is exceedingly remarkable, and dillcrcnt from

39S THE MECHANICAL FUNCTIONS.

that of the formation of any other organic product with
which we are acquainted. Instead of growing from a base,
like hairs, and other productions of the integuments, by suc-
cessive depositions of layers, the materials which are to
compose the laminae are cast in moulds, where they harden
and acquire the exact shape of the recipient ca^aties. The
next object of our curiosity, then, is to learn the way in
which these moulds are constructed; and on careful exami-
nation they appear to be formed by two striated membranes,
the exterior one (e) enveloping the other (i,) or interior
membrane. These membranes are separated by a series of
partitions, which commence at the edges of the longitudinal
white band, seen in Fig. 229, and wind obliquely upwards
till they reach the opposite longitudinal band already de-
scribed, where they join a longitudinal partition which oc-
cupies a line answering to that posterior band. Thus they
leave between them narrow spaces, which constitute so
many compartments for the deposition, as in a mould, of the
material of each lamina. The course of these channels, and
their junction at the back of the matrix is seen at s, Fig.
230. It is exceedingly probable, though from the minute-
ness of the parts it is scarcely possible to obtain ocular de-
monstration of the fact, that the fibrils of the laminae are
formed 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 supplied the
materials for the formation of the laminae, is to construct
the stem of the feather, and unite the laminae to its sides.
For this purpose the anterior portion of the bulb deposites
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 ante-
rior and the other posterior. The former of these, after
having finished the external plate, proceeds to form the
spongy substance, which is to connect the two plates, and
the posterior portion of the bulb embraces the inner plate,

i,
FEATHERS OP BIRDS. 399

and gradually folds it inwards till its sides meet at the mid-
dle groove along the back of the stem. The anterior part
of the bulb, during the process of fdling up the stem, exhi-
bibits a series of conical shaped membranes, as is seen in
the section, Fig. 231; the points of the cones being directed
upwards, and their intervals being occupied hy the spongy
substance in different stages of consolidation, and more per-
fected in proportion as they are situated nearer the apex of
the stem.

While the construction of tlie feather, in Its different
stages, is thus advancing from below, those parts which
are completely formed, are rising above the surAice of the
skin, still enveloped in the capsule which originally protected
them, but the upper portions of which, from the action of
the air, and the obliteration of the vessels that nourished
them, now decaying, shrivel and fall off in shreds, allowing
the successive portions of the feather to come forth, and the
lamincE to unfold themselves as they rise and assume their
proper- shapes. This successive 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 poste-
rior part of the bulb now contracts itself, and brino-ino; the
edges of that surf^ice of the stem closer toiiether, at lenirth
unites them at the superior orifice (o,) Fig. 22S; 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 depositing a horny
covering to the feather: all that remains of its substance is a
thin membrane which adheres to the outside of the tubular
part or barrel of the quill, and which must be scraped off
before the latter can be used as a pen. The tubular part is
the product of the anterior part of the bulb, which now
ceases to deposite the spongy substance, but forms a transpa-
rent horny material 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, form what is termed the pith of the quill. The

400 THE MECHANICAL FUNCTIONS.

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, preparato-
ry to the formation of another, which, in due season, is to
replace it. All the feathers are, in general, moulted annual-
ly, or even at shorter periods; and the same complicated
process is again begun and completed by a new matrix pro-
duced for the occasion, every time a new feather is to be
formed.

It is impossible, on reviewing these curious facts, not to
be struck with the admirable art and foresight which are
implied in all this long and complicated series of operations.
While the bird was yet nourished by the fluids of the egg,
the ground had already been prepared for its future plumage,
and for the formation of instruments of flight. A tempora-
ry investment of down is in readiness to shelter the tender
chicken from the rude impressions of the air, and an
apparatus is preparing for the construction of the most re-
fined instruments for clothing and for motion: first, the scaf-
folding, 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
quill, which is prolonged for the purpq^e of converting it
into a lever of sufficient length for the mechanical office it
has to perform. In proportion as each structure is finished,
she neglects not to remove the scaffolding which had been
setup as a temporary structure; the membranes, with all
their partitions, are carried away, the vascular pulp of the
bulb is absorbed, and its place supplied by air, tiius 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?

WING OF BIRDS. 401

Several circumstances remain to be noticed respecting the
structure and actions of the wings of birds. If we attend to the
mode of their articulation with the scapula, we find it ])roducing
a motion oblique with regard to the axis of the body, so that
the stroke which they give to the air is directed both down-
wards and backwards; and the bird, while moving forwards, is
at the same time supported in opposition to the force of gra-
vity. The different portions of the wing are likewise so dis-
posed as to be contracted and folded together when the wing
is drawn up, but fully expanded when it descends in order
to strike the air. It is obvious that, without this provision,
a great part of the motion acquired by the resistance of the
air against the wing in its descent would have been lost by
a counteracting resistance during its ascent. The disposi-
tion of the great feathers is such that they strike the air with
their flat sides, but present only their edges in rising; what
is c2L\\edi feathering the oar in rowing is a similar operation,
performed with the same intention, and deriving its name
from this resemblance.

As the inclination of the wing is chiefly backw^ards,
the greatest part of the effect produced by its action is
to move the body forwards. Birds of prey have a great
obliquity of wing, and are consequently better formed for
horizontal progressive motion, which is what they chiefly
practise in pursuing their prey, than for a rapid perpendi-
cular 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 disposed as to strike di-
rectly 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 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 expand-
ed, present a very large extent of surface.

Vol. I. 51

402 THE MECHANICAL FUNCTIONS.

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
backwards in a straight line, are of considerable assistance
in the steerage of the animal. In many birds, as in the
wood-pecker, the tail is much employed as a support to the
body in climbing trees. The caudal vertebrae are often nu-
merous, but are short and compressed together; they are re-
markable for the great development of their transverse pro-
cesses, and for having spinous processes both on their lower
and upper sides. The last vertebra, instead of being cylin-
drical, has a broad carinated spine for the insertion of large
feathers.

Birds could not, of course, be always on the wing; for a
great expenditure of muscular power is constantly going on
while they support themselves in the air. Occasional rest
is necessary to them as well as to other animals, and means
are accordingly provided by nature for their mechanical
support and progressive motion while on land.

The anterior extremities having been exclusively appro-
priated to flight, and constructed with reference to the pro-
perties of the atmosphere, the offices of sustaining and of
moving the body along the ground must be intrusted wholly
to the hind limbs. The centre of gravity, before sustained
by the wings, must now be brought over the new basis of
support formed by the feet; or rather, as it is placed far for-
wards, 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 the remote part
of the pelvis, which is elongated backwards, at a considera-
ble 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 for-
mation of the limb, has preserved an accordance with the
vertebrated type, both as to the number of pieces which
compose it, and as to their relative situations, she has devi-

FEET OF BIRDS. 403

ated 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 ex-
tent 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 ge-
nerally large, especially in the order of Grallx^ or wading
birds: the fibula is exceedingly slender, and always united,
at its lower part, with the tibia; and there is a total deficien-
cy of tarsal bones, except in the Ostrich, where rudiments
of them may be traced. Already we have seen, in ruminant
quadrupeds, that these bones have dwindled to a very small
size, but here they have wholly disappeared. The long
bone which succeeds to the tibia, though considered by some
anatomists as the tarsus, is, properly, the metatarsal bone, and
in the Grallcc is of great length. At its lower end it has
three articulations, shaped like pulleys, 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 uniforjn law.
The innermost toe, which may be compared to a thumb, con-
sists 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 eve-
ry case being affixed to the last joints, which have, therefore,
been termed the ungual bones. This remarkable numerical
relation, among the several bones of the toes, exists quite in-
dependently of their length.

There is one whole order of ])irds which are particularly
fitted for climbing and perching upon trees, having the two
middle toes parallel to each other, and the inner and outer
toes turned back, so as to be opposed to them in their action.
They are thus enabled to grasp objects with the greatest fa-

404 THE MECHANICAL FUNCTIONS.

cility; having, in fact, two thumbs, which are opposable to
the two fingers. They have been termed Scansores, or Zi/- •
godactyli. Almost all other birds have three toes before,
and one behind.

From this enumeration, it would appear as if Nature, in
modifying the type of vertebrated animals to suit the pur-
poses required in the bird, had purposely omitted one of the
toes, which are usually five in number. But instances occur
of birds, in which we may trace the rudiment of a fifth toe
high upon the metatarsus, and upon its inner side. The
spur of the cock may be regarded as having this origin.
What confirms this view of the subject, is, that in those
birds which have only three toes, namely, in the Emit, 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 phalanges.
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 suppressed.*

A bird is capable of shifting the position of the centre of
gravity of its body according as circumstances require it,
simply by advancing or drawing back its head. While fly-
ing, 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 sup-
port. When preparing to sleep, they bring the centre of
gravity still lower, by turning the head round and placing
it under the w^ing. These motions of the head are again re-
sorted to when the bird walks; and the centre of gravity is
thus transferred alternately from one foot to the other: hence,

* The last bone of the outer toe of the ostrich is very small, and being
usually lost in preparing the skeleton, has been overlooked by naturalists;
but Dr. Grant has ascertained, by the careful dissection of a recent specimen,
the existence of tiiis fifth phalanx.

FEET OP BIRDS.

405

in walking, the head of a bird is in constant motion: whilst
the duck and other birds, whose legs are very short, have a
waddling gait. It may be observed that the more perfect-
ly predaceous birds are not the best formed for walking; be-
cause where they 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 win^s.

In roosting, birds support themselves upon their perch
by means of one leg only, the other being folded close to
the body. They even maintain this attitude with greater
ease and security than if they rested upon both feet. ' The
true explanation of this curious fact was long ago given by
Borelli. On tracing the tendons (t, t Fig. 233) of the mus-
cles (m, m) which bend the claws, and enable them to grasp
an object, we find them passing over the outer angles of each
of the intervening joints, so that whenever these joints are
bent, as shown in Fig. 234, those tendons are put upon the

stretch, and mechanically, or without any action of the inus-
cles, tend to close the foot. When the bird is on its perch,
this effect is produced by the mere weight of the body,
which of course, tends to bend all the joints of the limb on
which it rests; so that the greater that weight, the greater is

406 THE MECHANICAL FUNCTIONS.

the force with which the toes grasp the perch. All this
takes place without muscular effort or volition on the part
of the bird. It remains in this position with more security
on one foot than it would have done by resting upon both;
because, in the latter case, the weight of the body being di-
vided between them, does not stretch the tendons sufficient-
ly. In this position, the bird not only sleeps in perfect se-
curity, but resists the impulse of the wind and the shaking
of the bough.

The great length of the toes of birds enables them to stand
steadily on one leg: and in this attitude many employ the
other foot as a hand; especially parrots, whose head is too
heavy to be readily brought to the ground. Some birds,
which frequent the banks of rivers, are in the practice of
holding a stone in one foot, while they rest upon the other:
this contributes to increase their stability in two ways; first,
it adds to the weight of the body, which is the force that
stretches the tendons, and causes them to grasp the bough;
and, secondly, it also lowers the centre of gravity.

The stork, and some other birds belonging to the same
order, which sleep standing on one foot, have a curious me-
chanical contrivance for locking the joint of the tarsus, and
preserving the leg in a state of extension without any mus-
cular effort. The mechanism is such as to withstand the ef-
fect of the ordinary oscillations of the body, when the bird
is reposing; but it is easily unlocked by a voluntary muscu-
lar 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 me-
chanism of birds; as it would exceed the limits within which
I must confine myself, to enter more fully into the peculi-
arities which distinguish the different orders and families.

* This mechanism is noticed by Dr. Macartney, in the Transactions of the
Royal Irish Academy, vol. xiii. p. 20, and is more fully described in Rees's
Cyclopjedia, Art. Bird. He observes that both Cavier and Dumeril have
committed an en-or in referring this peculiarity of structure to the knee in-
stead of the tarsal joint.

FEET OF BIRDS. 407

Some of the more remarka!)lc deviations from what may be
considered as the standard conformation, may, however, for
a moment arrest our attention.

The Ostrich is of all birds the one that 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 wnngs,. 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. The clavicles do not reach the sternum, nor
even meet at the anterior part of the chest to form the fur-
cular bone: for as the wings are not employed in flying, the
usual office of that bone is not wanted. The form of the
pelvis is diflerent 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 universally met with in
birds that fly. The filaments of the ostrich's feathers, in
consequence of having none of these fibrils, hang loose and
detached from one another, forming the fine hair or down^
which, however ornamental as an article of dress, must be
viewed, when considered physiologically, as a species of de-
generacy 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 diflferent destination, being
formed for swimming. In external form, it resembles the
anterior extremity of the turtle; but, still, we find it con-
structed on the model of the wings of birds; as if nature had
bound herself, by a law, not to depart from the standard of
organization, although the purpose of the structure is alto-
gether changed. As penguins are intended for a maritime

408 THE MECHANICAL FUNCTIONS.

life, all their extremities are formed for swimming. Their
legs are exceedingly short, and placed far backwards; so that
these birds are compelled, when resting on their feet on the
shore, to raise their bodies in a perpendicular attitude, in
order to place the centre of gravity immediately above the
base of support: a posture which gives them a strange and
grotesque appearance.

I have already alluded to the lengthened legs and feet of
the waders, the utility of which to birds frequenting marshy
places, and shallow waters, is very obvious. Their legs are
not covered with feathers, which would have been injured
by continual exposure to wet. But birds of a truly aquatic
nature have their toes webbed, that is, united by a mem-
brane, a mechanism which qualifies 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 prevent the water from penetrating
through it; and for the purpose of preserving it in this con-
dition, 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 consi-
derable velocity, by the impulse of the wind.

Birds excel all other vertebrated animals in the energy of
their muscular powers. The promptitude, the force, and
the activity they display in all their movements, and the un-
wearied vigour with which they persevere, for hours and
days, in the violent exertions required for flight, far exceed
those of any quadruped, and implies a higher degree of irri-
tability, dependent, probably, on the great extent of their
respiratory functions, than is possessed by any other class of
animals.

ST^^

END OF VOL. I.

.1

J

i

1^

)

^

/^.i:^*

/