CAMBRIDGE BIOLOGICAL SERIES.
GENERAL EDITOR:--ARTHUR E. SHIPLEY, M.A.
FELLOW AND TUTOR OF CHRIST'S COLLEGE, CAMBRIDGE.
THE
VERTEBRATE SKELETON.
London: C.J. CLAY AND SONS,
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE,
AVE MARIA LANE,
AND
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Glasgow: 50, WELLINGTON STREET.
Leipzig: F.A. BROCKHAUS.
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Bombay and Calcutta: MACMILLAN AND CO., LTD.
[_All Rights reserved._]
THE
VERTEBRATE SKELETON
BY
SIDNEY H. REYNOLDS, M.A.,
TRINITY COLLEGE, CAMBRIDGE;
LECTURER AND DEMONSTRATOR IN GEOLOGY AND ZOOLOGY AT UNIVERSITY
COLLEGE, BRISTOL.
Cambridge:
AT THE UNIVERSITY PRESS.
1897
[_All Rights reserved._]
=Cambridge=:
PRINTED BY J. & C.F. CLAY,
AT THE UNIVERSITY PRESS.
PREFACE.
IN the following pages the term skeleton is used in its widest sense,
so as to include exoskeletal or tegumentary structures, as well as
endoskeletal structures. It was thought advisable to include some
account of the skeleton of the lowest Chordata--animals which are not
strictly vertebrates, but it seemed undesirable to alter the title of
the book in consequence.
The plan adopted in the treatment of each group has been to give first
an account of the general skeletal characters of the group in question
and of its several subdivisions; secondly to describe in detail the
skeleton of one or more selected types; and thirdly to treat the
skeleton as developed in the group organ by organ.
A beginner is advised to commence, not with the introductory chapter,
but with the skeleton of the Dogfish, then to pass to the skeletons of
the Newt and Frog, and then to that of the Dog. After that he might
pass to the introductory chapter and work straight through the book. I
have endeavoured to make the account of each type skeleton complete in
itself; this has necessitated a certain amount of repetition,--a
fault that I have found it equally difficult to avoid in other parts
of the book.
Throughout the book generic names are printed in italics; and italics
are used in the accounts of the type skeletons for the names of
membrane bones. Clarendon type is used to emphasise certain words. In
the classificatory table the names of extinct genera only, are printed
in italics.
In a book in which an attempt is made to cover to some extent such a
vast field, it would be vain to hope to have avoided many errors both
of omission and commission, and I owe it to the kindness of several
friends that the errors are not much more numerous. I cannot however
too emphatically say that for those which remain I alone am
responsible. Messrs C.W. Andrews, E. Fawcett, S.F. Harmer, J. Graham
Kerr, and B. Rogers have all been kind enough to help me by reading
proofs or manuscript, while the assistance that I have received from
Dr Gadow during the earlier stages and from Prof. Lloyd Morgan and Mr
Shipley throughout the whole progress of the work has been very great.
To all these gentlemen my best thanks are tendered.
All the figures except 1, 35, 55, and 84 were drawn by Mr Edwin
Wilson, to whose care and skill I am much indebted. The majority are
from photographs taken by my sister Miss K.M. Reynolds or by myself in
the British Museum and in the Cambridge University Museum of Zoology,
and I take this opportunity of thanking Sir W.H. Flower and Mr S.F.
Harmer for the facilities they have afforded and for permission to
figure many objects in the museums respectively under their charge. I
have also to thank (1) Prof. von Zittel for permission to reproduce
figs. 27, 41, 52, 69, 70, 80, 106 A, and 107 C; (2) Sir W.H. Flower
and Messrs A. and C. Black for figs. 1 and 84; (3) Prof. O.C. Marsh
and Dr H. Woodward for fig. 35; (4) Dr C.H. Hurst and Messrs Smith,
Elder, and Co. for fig. 55.
A few references are given, but no attempt has been made to give
anything like a complete list. The abbreviations of the titles of
periodicals are those used in the _Zoological Record_.
I have always referred freely to the textbooks treating of the
subjects dealt with, and in particular I should like to mention that
the section devoted to the skeleton of mammals is, as it could hardly
fail to be, to a considerable extent based on Sir W.H. Flower's
_Osteology of the Mammalia_.
SIDNEY H. REYNOLDS.
_March 10, 1897._
CONTENTS.
PAGE
CHAPTER I.
Introductory account of the skeleton in general 1
CHAPTER II.
Classification 30
CHAPTER III.
Skeleton of Hemichordata, Urochordata and Cephalochordata 50
CHAPTER IV.
Skeletal characters of the Vertebrata. The skeleton in the
Cyclostomata 53
CHAPTER V.
Skeletal characters of the Ichthyopsida. Characters of the
several groups of Pisces 59
CHAPTER VI.
The skeleton of the Dogfish (Scyllium canicula) 71
CHAPTER VII.
The skeleton of the Codfish (_Gadus morrhua_) and the skull of
the Salmon (_Salmo salar_) 83
CHAPTER VIII.
General account of the skeleton in Fishes 104
CHAPTER IX.
Characters of the several groups of Amphibia 133
CHAPTER X.
The skeleton of the Newt (_Molge cristata_) 138
CHAPTER XI.
The skeleton of the Frog (_Rana temporaria_) 151
CHAPTER XII.
General account of the skeleton in Amphibia 168
CHAPTER XIII.
Skeletal characters of the Sauropsida. Characters of the
several groups of Reptiles 189
CHAPTER XIV.
The skeleton of the Green Turtle (_Chelone midas_) 214
CHAPTER XV.
The skeleton of the Crocodile (_Crocodilus palustris_) 237
CHAPTER XVI.
General account of the skeleton in Reptiles 270
CHAPTER XVII.
Characters of the several groups of Birds 295
CHAPTER XVIII.
The skeleton of the Wild Duck (_Anas boschas_) 302
CHAPTER XIX.
General account of the skeleton in Birds 328
CHAPTER XX.
Characters of the several groups of Mammalia 343
CHAPTER XXI.
The skeleton of the Dog (_Canis familiaris_) 374
CHAPTER XXII.
General account of the skeleton in Mammalia. The exoskeleton
and vertebral column 416
CHAPTER XXIII.
General account of the skeleton in Mammalia (_continued_). The
skull and appendicular skeleton 455
LIST OF ILLUSTRATIONS.
FIG. PAGE
1 Diagrammatic sections of various forms of teeth 6
2 Cervical vertebrae of an Ox (_Bos taurus_) 15
3 Diagram of the skeleton of _Amphioxus lanceolatus_ 51
4 Dorsal, lateral, and ventral views of the skull of
_Petromyzon marinus_ 56
5 Skull of a male _Chimaera monstrosa_ 65
6 Lateral view of the skull of a Dogfish (_Scyllium canicula_) 75
7 Semidorsal view of the pectoral girdle and fins of a Dogfish
(_Scyllium canicula_) 80
8 Dorsal view of the pelvic girdle and fins of a male Dogfish
(_Scyllium canicula_) 81
9 Dorsal and ventral views of the cranium of a Salmon (_Salmo
salar_) from which most of the membrane bones have been
removed 88
10 Lateral view of the chondrocranium of a Salmon (_Salmo
salar_) 90
11 Lateral view of the skull of a Salmon (_Salmo salar_) 92
12 Mandibular and hyoid arches of a Cod (_Gadus morrhua_) 99
13 Right half of the pectoral girdle and right pectoral fin of
a Cod (_Gadus morrhua_) 102
14 Diagram of a section through the jaw of a Shark
(_Odontaspis americanus_) showing the succession of
teeth 107
15 Part of the lower jaw of a Shark (_Galeus_) 108
16 Skulls of _Notidanus_ and _Cestracion_ 118
17 Dorsal view of the branchial arches of _Heptanchus_ 120
18 Lateral view of the skull of a Sturgeon
(_Acipenser sturio_) 122
19 Dorsal and ventral views of the cranium of
_Ceratodus miolepis_ 125
20 Lateral view of the skeleton of _Ceratodus miolepis_ 128
21 Dorsal, ventral and lateral views of the skull of a Newt
(_Molge cristata_) 142
22 Ventral and lateral views of the shoulder-girdle and
sternum of an old male Crested Newt (_Molge cristata_) 146
23 Right posterior and anterior limbs of a Newt (_Molge
cristata_) 148
24 Dorsal and ventral views of the cranium of a Common Frog
(_Rana temporaria_) 155
25 Dorsal and ventral views of the cranium of a Common Frog
(_Rana temporaria_) from which the membrane bones have
mostly been removed 157
26 Lateral view of the skull and posterior view of the
cranium of a Common Frog (_Rana temporaria_) 159
27 Dorsal view of the skull of a Labyrinthodont (_Capitosaurus
nasutus_) 176
28 Ventral view of the cranium, and lateral view of the
cranium and mandible of _Siphonops annulatus_ 178
29 Visceral arches of Amphibia: A, _Molge cristata_; B, _Rana
temporaria_, adult; C, Tadpole of _Rana_; D, _Siredon
pisciformis_ 181
30 Shoulder-girdle and sternum of an adult male Common Frog
(_Rana temporaria_), and of an adult female
_Docidophryne gigantea_ 183
31 A, Right antibrachium and manus of a larval Salamander
(_Salamandra maculosa_); B, Right tarsus and adjoining
bones of _Molge sp._ 186
32 Lateral and dorsal views of the skull of an _Ichthyosaurus_ 196
33 Lateral view and longitudinal section of the skull of a
Lizard (_Varanus varius_) 201
34 Lateral view of the shoulder-girdle of a Lizard (_Varanus_) 202
35 Restored skeleton of _Ceratosaurus nasicornis_ 206
36 Dorsal and ventral views of the carapace of a Loggerhead
Turtle (_Thalassochelys caretta_) 216
37 Plastron of a Green Turtle (_Chelone midas_) 218
38 The skull of a Green Turtle (_Chelone midas_) 223
39 Longitudinal vertical section through the cranium of a
Green Turtle (_Chelone midas_) 226
40 Anterior limb of a young Hawksbill Turtle (_Chelone
imbricata_), and posterior limb of a large Green Turtle
(_Chelone midas_) 234
41 The first four cervical vertebrae of a Crocodile
(_Crocodilus vulgaris_) 239
42 Anterior view of a late thoracic and the first sacral
vertebrae of a Crocodile (_Crocodilus palustris_) 242
43 Palatal aspect of the cranium and mandible of an Alligator
(_Caiman latirostris_) 245
44 Lateral view of the skull of an Alligator (_Caiman
latirostris_) 248
45 Longitudinal section through the skull of an Alligator
(_Caiman latirostris_) 253
46 Sternum and associated membrane bones of a Crocodile
(_Crocodilus palustris_) 261
47 Left half of the pectoral girdle of an Alligator
(_Caiman latirostris_) 262
48 Right anterior and posterior limbs of an Alligator
(_Caiman latirostris_) 264
49 Pelvis and sacrum of an Alligator (_Caiman latirostris_) 267
50 Preparation of part of the right mandibular ramus of
_Crocodilus palustris_ 274
51 Dorsal and ventral views of the skull of a Common Snake
(_Tropidonotus natrix_) 279
52 Skull of Hatteria (_Sphenodon punctatus_) 282
53 Hyoids of an Alligator (_Caiman latirostris_), and of a
Green Turtle (_Chelone midas_) 285
54 Ventral view of the shoulder-girdle and sternum of
_Loemanctus longipes_ 287
55 Left half of the skeleton of a Common Fowl (_Gallus
bankiva_) 301
56 The wing of a Wild Duck (_Anas boschas_) 304
57 Wings of a Wild Duck with the coverts removed (_Anas
boschas_) 305
58 Dorsal and ventral views of the pelvis and sacrum of a Duck
(_Anas boschas_) 311
59 Skull of a Duck (_Anas boschas_) 312
60 A, Ventral view of the cranium of a Duck (_Anas boschas_);
B, Cranium and mandible seen from the left side 313
61 Lateral view of the pelvis and sacrum of a Duck (_Anas
boschas_) 325
62 Third cervical vertebra of an Ostrich (_Struthio camelus_) 331
63 Shoulder-girdle and sternum of A, Black Vulture (_Vultur
cinereus_); B, Peacock (_Pavo cristatus_); C, Pelican
(_Pelicanus conspicillatus_) 337
64 Bones of the right wing of A, a Penguin; B, an Ostrich
(_Struthio camelus_) and C, a Gannet (_Sula alba_) 339
65 Pelvic girdle and sacrum of A, Cassowary (_Casuarius
galeatus_); B, Owen's Apteryx (_A. oweni_); C,
Broad-billed Rhea (_R. macrorhyncha_); D, Ostrich
(_Struthio camelus_) 340
66 Ventral view of the shoulder-girdle and sternum of a
Duckbill (_Ornithorhynchus paradoxus_) 347
67 Cervical vertebrae of a Ca'ing Whale (_Globicephalus
melas_) 354
68 Dentition of a Dog (_Canis familiaris_) 375
69 Atlas and axis vertebrae of a Dog (_Canis familiaris_) 379
70 Second thoracic and second lumbar vertebrae of a Dog
(_Canis familiaris_) 382
71 Diagram of the relations of the principal bones in the
Mammalian skull 385
72 Vertical longitudinal section through skull of a Dog
(_Canis familiaris_) 387
73 Dorsal view of the cranium of a Dog (_Canis familiaris_) 389
74 Diagram of the mammalian tympanic cavity and associated
bones 391
75 Ventral view of the cranium of a Dog (_Canis familiaris_) 396
76 Sternum and sternal ribs of a Dog (_Canis familiaris_) 403
77 Bones of the left upper arm and fore-arm of a Dog (_Canis
familiaris_) 407
78 Right innominate bone, A, of a full-grown Terrier; B, of a
Collie Puppy 410
79 Left leg bones of a Dog (_Canis familiaris_) 411
80 A, Right manus; B, Right pes of a Dog (_Canis familiaris_) 413
81 Skull of a young Indian Rhinoceros (_R. unicornis_) showing
the change of the dentition 421
82 Palatal aspect of the cranium and mandible of a Donkey
(_Equus asinus_) 431
83 Skull of _Procavia (Dendrohyrax) dorsalis_ 433
84 Carnassial or sectorial teeth of Carnivora 436
85 Mandible of Isabelline Bear (_Ursus isabellinus_) 438
86 Left mandibular ramus of the Sea Leopard (_Ogmorhinus
leptonyx_) 439
87 Cervical vertebrae of a young Fin Whale (_Balaenoptera
musculus_) 444
88 Atlas and axis vertebrae of an Ox (_Bos taurus_) 445
89 First and second thoracic vertebrae of an Ox (_Bos taurus_) 449
90 Skulls of Tasmanian Wolf (_Thylacinus cynocephalus_) and
Hairy-nosed Wombat (_Phascolomys latifrons_) 456
91 Skull of Two-fingered Sloth (_Choloepus didactylus_) 458
92 Skull of _Rhytina stelleri_ 460
93 Lateral view and longitudinal section of the skull of a
young Ca'ing Whale (_Globicephalus melas_) 463
94 Cranium and mandible of a Pig (_Sus scrofa_) 466
95 Mandible of a Hippopotamus (_Hippopotamus amphibius_) 467
96 Skull of a young Indian Elephant (_Elephas indicus_) 474
97 Longitudinal section of the skull of a young Indian
Elephant (_Elephas indicus_) 475
98 Half-front view of the skull of a Porcupine (_Hystrix
cristata_) 477
99 Skulls of an old and of a young Gorilla (_Gorilla savagei_) 483
100 Malleus, stapes, and incus of Man, Dog, and Rabbit 485
101 Skeleton of a Cape Buffalo (_Bubalus caffer_) 492
102 Lateral and dorsal views of the shoulder-girdle and part
of the sternum of the Spiny Anteater (_Echidna
aculeata_) 494
103 Skeleton of a Llama (_Auchenia glama_) 496
104 Dorsal view of the sternum and right half of the shoulder
girdle of _Mus sylvaticus_ 498
105 Anterior surface of the right humerus of a Wombat
(_Phascolomys latifrons_) 500
106 Manus of Perissodactyles: A, Left manus of _Tapirus_; B,
Right manus of _Titanotherium_; C, Left manus of
_Chalicotherium giganteum_ 508
107 Left manus of A, _Coryphodon hamatus_; B, _Phenacodus
primaevus_; C, _Procavia (Dendrohyrax) arboreus_ 510
108 Left anterior and posterior limbs and limb girdles of
_Uintatherium mirabile_ 516
109 Left femur of an Ox (_Bos taurus_) and of a Sumatran
Rhinoceros (_Rhinoceros sumatrensis_) 518
110 Pes of A, a Tapir (_Tapirus americanus_); B, a Rhinoceros
(_Rhinoceros sumatrensis_); C, _Hipparion gracile_; D,
a Horse (_Equus caballus_) 524
ERRATA.
p. 172, note, _for_ 14 _read_ 15.
p. 279, description of figure, _for_ Tropidinotus _read_
Tropidonotus.
p. 287, description of figure, _for_ shoulder-girdle of sternum
_read_ shoulder-girdle and sternum.
p. 393, middle of page, _for_ VIII _read_ VII.
p. 427, line 2, _for_ Grampus _read_ Killer.
CHAPTER I.
INTRODUCTORY ACCOUNT OF THE SKELETON IN GENERAL.
BY the term =skeleton= is meant the hard structures whose function is
to support or to protect the softer tissues of the animal body.
The skeleton is divisible into
A. The EXOSKELETON, which is external;
B. The ENDOSKELETON, which is as a rule internal; though in some
cases, e.g. the antlers of deer, endoskeletal structures become, as
development proceeds, external.
In Invertebrates the hard, supporting structures of the body are
mainly =exoskeletal=, in Vertebrates they are mainly =endoskeletal=;
but the endoskeleton includes, especially in the skull, a number of
elements, the =dermal= or =membrane= bones, which are shown by
development to have been originally of external origin. These membrane
bones are so intimately related to the true endoskeleton that they
will be described with it. The simplest and lowest types of both
vertebrate and invertebrate animals have unsegmented skeletons; with
the need for flexibility however segmentation arose both in the case
of the invertebrate exoskeleton and the vertebrate endoskeleton. The
exoskeleton in vertebrates is phylogenetically older than the
endoskeleton, as is indicated by both palaeontology and embryology.
Palaeontological evidence is afforded by the fact that all the lower
groups of vertebrates--Fish, Amphibia, and Reptiles--had in former
geological periods a greater proportion of species protected by
well-developed dermal armour than is the case at present.
Embryological evidence tends the same way, inasmuch as dermal
ossifications appear much earlier in the developing animal than do the
ossifications in the endoskeleton.
Skeletal structures may be derived from each of the three germinal
layers. Thus =hairs= and =feathers= are =epiblastic= in origin,
=bones= are =mesoblastic=, and the =notochord= is =hypoblastic=.
The different types of skeletal structures may now be considered and
classified more fully.
A. EXOSKELETAL STRUCTURES.
I. EPIBLASTIC (epidermal).
Exoskeletal structures of epiblastic origin may be developed on both
the inner and outer surfaces of the Malpighian layer of the
epidermis[1]. Those developed on the outer surface include =hairs=,
=feathers=, =scales=, =nails=, =beaks= and =tortoiseshell=; and are
specially found in vertebrates higher than fishes. Those developed on
the inner surface of the Malpighian layer include only the =enamel= of
teeth and some kinds of scales. With the exception of feathers, which
are partly formed from the horny layer, all these parts are mainly
derived from the Malpighian layer of the epidermis.
=Hairs= are slender, elongated structures which arise by the
proliferation of cells from the Malpighian layer of the epidermis.
These cells in the case of each hair form a short papilla, which sinks
inwards and becomes imbedded at the bottom of a follicle in the
dermis. Each hair is normally composed of an inner cellular pithy
portion containing much air, and an outer denser cortical portion of a
horny nature. Sometimes, as in Deer, the hair is mainly formed of the
pithy portion, and is then easily broken. Sometimes the horny part
predominates, as in the bristles of Pigs. A highly vascular dermal
papilla projects into the base of the hair.
=Feathers=, like hairs, arise from epidermal papillae which become
imbedded in pits in the dermis. But the feather germ differs from the
hair germ, in the fact that it first grows out like a cone on the
surface of the epidermis, and that the horny as well as the Malpighian
layer takes part in its formation.
=Nails=, =claws=, =hoofs=, and the =horns of Oxen= are also epidermal,
as are such structures as the =scales= of reptiles, of birds' feet,
and of _Manis_ among mammals, the =rattle= of the rattlesnake, the
=nasal horns= of _Rhinoceros_, and the =baleen= of whales. All these
structures will be described later.
=Nails= arise in the interior of the epidermis by the thickening and
cornification of the stratum lucidum. The outer border of the nail
soon becomes free, and growth takes place by additions to the inner
surface and attached end.
When a nail tapers to a sharp point it is called a =claw=. In many
cases the nails more or less surround the ends of the digits by which
they are borne.
Horny =beaks= of epidermal origin occur casing the jaw-bones in
several widely distinct groups of animals. Thus among reptiles they
are found in Chelonia (tortoises and turtles) as well as in some
extinct forms; they occur in all living birds, in _Ornithorhynchus_
among mammals, and in the larvae of many Amphibia.
In a few animals, such as Lampreys and _Ornithorhynchus_, the jaws
bear horny tooth-like structures of epidermal origin.
The =enamel= of teeth and of placoid scales is also epiblastic in
origin[2], and it may be well at this point to give some account of
the structure of teeth, though they are partly mesoblastic in origin.
The simplest teeth are those met with in sharks and dogfish, where
they are merely the slightly modified scales developed in the
integument of the mouth. They pass by quite insensible gradations into
normal placoid scales, such as cover the general surface of the body.
A =placoid scale=[3] is developed on a papilla of the dermis which
projects outwards and backwards, and is covered by the columnar
Malpighian layer of the epidermis. The outer layer of the dermal
papilla then gradually becomes converted into dentine and bone, while
enamel is developed on the inner side of the Malpighian layer, forming
a cap to the scale. The Malpighian and horny layers of the epidermis
get rubbed off the enamel cap, so that it comes to project freely on
the surface of the body.
As regards their attachment teeth may be (1) attached to the fibrous
integument of the mouth, or (2) fixed to the jaws or other bones of
the mouth, or (3) planted in grooves, or (4) in definite sockets in
the jaw-bones (see p. 107).
Teeth in general consist of three tissues, =enamel=, =dentine= and
=cement=, enclosing a central pulp-cavity containing blood-vessels and
nerves. Enamel is, however, often absent, as in all living Edentates.
=Enamel= generally forms the outermost layer of the crown or visible
part of the tooth; it is the hardest tissue occurring in the animal
body and consists of prismatic fibres arranged at right angles to the
surface of the tooth. It is characterised by its bluish-white
translucent appearance.
II. MESOBLASTIC (mesodermal).
=Dentine= or =ivory= generally forms the main mass of a tooth. It is a
hard, white substance allied to bone. When examined microscopically
dentine is seen to be traversed by great numbers of nearly parallel
branching tubules which radiate outwards from the pulp-cavity. In
fishes as a rule, and sometimes in other animals, a variety of dentine
containing blood-vessels occurs, this is called =vasodentine=.
=Cement= or =crusta petrosa= forms the outermost layer of the root of
the tooth. In composition and structure it is practically identical
with bone. In the more complicated mammalian teeth, besides enveloping
the root, it fills up the spaces between the folds of the enamel.
The hard parts of a tooth commonly enclose a central pulp-cavity into
which projects the pulp, a papilla of the dermis including
blood-vessels and nerves. As long as growth continues the outer layers
of this pulp become successively calcified and added to the substance
of the dentine. In young growing teeth the pulp-cavity remains widely
open, but in mammals the general rule is that as a tooth gets older
and the crown becomes fully formed, the remainder of the pulp becomes
converted into one or more tapering roots which are imbedded in the
alveolar cavities of the jaws. The opening of the pulp-cavity is then
reduced to a minute perforation at the base of each root. A tooth of
this kind is called a =rooted= tooth.
But it is not only in young teeth that the pulp-cavity sometimes
remains widely open; for some teeth, such as the tusks of Elephants
and the incisor teeth of Rodents, form no roots and continue to grow
throughout the animal's life. Such teeth are said to be rootless or to
have persistent pulps.
An intermediate condition is seen in some teeth, such as the grinding
teeth of Horses. These teeth grow for a very long time, their crowns
wearing away as fast as their bases are produced; finally however
definite roots are formed and growth ceases.
[Illustration FIG. 1. DIAGRAMMATIC SECTIONS OF VARIOUS FORMS OF TEETH
(from FLOWER).
I. Incisor or tusk of elephant, with pulp-cavity persistently open at
base. II. Human incisor during development with root imperfectly
formed, and pulp-cavity widely open at base. III. Completely formed
human incisor, with pulp-cavity contracted to a small aperture at the
end of the root. IV. Human molar with broad crown and two roots. V.
Molar of Ox, with the enamel covering the crown, deeply folded and the
depressions filled with cement. The surface is worn by use, otherwise
the enamel coating would be continuous at the top of the ridges. In
all the figures the enamel is black, the pulp white, the dentine
represented by horizontal lines, and the cement by dots.]
The teeth of any animal may be =homodont=, that is, all having the
same general character, or =heterodont=, that is, having different
forms adapted to different functions. The dentition is heterodont in a
few reptiles and the majority of mammals.
SUCCESSION OF TEETH. In most fishes, and many amphibians and reptiles
the teeth can be renewed indefinitely. In sharks, for example,
numerous rows of reserve teeth are to be seen folded back behind those
in use (see fig. 15). The majority of mammals have only two sets of
teeth, and are said to be =diphyodont=; some have only a single series
(=monophyodont=).
DEVELOPMENT OF TEETH. A brief sketch of the method in which
development of teeth takes place in the higher vertebrates may here be
given. Along the surface of the jaws a thickening of the epiblastic
epithelium takes place, giving rise to a ridge, which sinks inwards
into the tissue of the jaw, and it is known as the primary enamel
organ. At the points where teeth are to be developed special ingrowths
of this primary enamel organ take place, and into each there projects
a vascular dental papilla from the surrounding mesoblast of the jaw.
Each ingrowth of the enamel organ forms an =enamel cap=, which
gradually embraces the dental papilla, and at the same time appears to
be pushed on one side, owing to the growth not being uniform. The
external layer of the dental papilla is composed of long nucleated
cells, the =odontoblasts=, and it is by these that the dentine is
formed. Similarly the internal layer of the enamel organ is formed of
columnar enamel cells, which give rise to the enamel. The mesoblastic
cells surrounding the base of the tooth give rise to the cement.
=Bone= is in many cases exoskeletal, but it will be most conveniently
described with the endoskeleton.
The =scales of fish= are wholly or in part mesoblastic in origin,
being totally different from those of reptiles. The =cycloid= and
=ctenoid= scales of Teleosteans (see p. 105) are thin plates coated
with epidermis. They are sometimes bony, but as a rule are simply
calcified. =Ganoid= scales are flat plates of bone coated with an
enamel-like substance, and articulating together with a peg and socket
arrangement; they are probably identical with enlarged and flattened
placoid scales.
The =armour plates= of fossil Ganoids, Labyrinthodonts, and Dinosaurs,
and of living Crocodiles, some Lizards and Armadillos, are composed of
bone. They are always covered by a layer of epidermis.
The =antlers of deer= are also composed of bone; they will be more
fully described in the chapter on mammals. It may perhaps be well to
mention them here, though they really belong to the endoskeleton,
being outgrowths from the frontal bones.
B. ENDOSKELETAL STRUCTURES.
I. HYPOBLASTIC.
(_a_) The =notochord= is an elastic rod formed of large vacuolated
cells, and is surrounded by a membranous sheath of mesoblastic origin.
It is the primitive endoskeleton in the Chordata, all of which possess
it at some period of their existence; while in many of the lower forms
it persists throughout life. Even in the highest Chordata it is the
sole representative of the axial skeleton for a considerable part of
the early embryonic life. A simple unsegmented notochord persists
throughout life in the Cephalochordata, Cyclostomata, and some Pisces,
such as Sturgeons and Chimaeroids.
(_b_) The enamel of the pharyngeal teeth of the Salmon and many other
Teleosteans is hypoblastic in origin. The epiblast of the stomodaeum,
in which the other teeth are developed, passes into the hypoblast of
the mesenteron in which these pharyngeal teeth are formed.
II. MESOBLASTIC.
The most primitive type of a mesoblastic endoskeleton consists of a
membranous sheath surrounding the notochord, as in _Myxine_ and its
allies. The first stage of complication is by the development of
cartilage in the notochordal sheath, as in _Petromyzon_. Often the
cartilage becomes calcified in places, as in the vertebral centra of
_Scyllium_ and other Elasmobranchs. Lastly, the formation of bone
takes place; it generally constitutes the most important of the
endoskeletal structures.
=Bone= may be formed in two ways:--
(1) by the direct ossification of pre-existing cartilage, when it is
known as =cartilage bone= or =endochondral bone=;
(2) by independent ossification in connective tissue; it is then known
as =membrane= or =dermal= or =periosteal bone=.
With the exception of the _clavicle_[4] all the bones of the trunk and
limbs, together with a large proportion of those of the skull, are
preformed in the embryo in cartilage, and are grouped as cartilage
bones; while the clavicle and most of the roofing and jaw-bones of the
skull are not preformed in cartilage, being developed simply in
connection with a membrane. Hence it is customary to draw a very
strong line of distinction between these two kinds of bone; in reality
however this distinction is often exaggerated, and the two kinds pass
into one another, and as will be shown immediately, the permanent
osseous tissue of many of those which are generally regarded as
typical cartilage bones, is really to a great extent of periosteal
origin. The palatine bone, for instance, of the higher vertebrates in
general is preceded by a cartilaginous bar, but is itself almost
entirely a membrane bone.
Before describing the development of bone it will be well to briefly
describe the structure of adult bone and cartilage.
The commonest kind of =cartilage=, and that which preforms so many of
the bones of the embryo, is =hyaline= cartilage. It consists of oval
nucleated cells occupying cavities (=lacunae=) in a clear
intercellular semitransparent matrix, which is probably secreted by
the cells. Sometimes one cell is seen in each lacuna, sometimes
shortly after cell-division a lacuna may contain two or more cells.
The free surface of the cartilage is invested by a fibrous membrane,
the =perichondrium=.
=Bone= consists of a series of lamellae of ossified substance between
which are oval spaces, the =lacunae=, giving rise to numerous fine
channels, the =canaliculi=, which radiate off in all directions. The
lacunae are occupied by the =bone cells= which correspond to cartilage
cells, from which if the bone is young, processes pass off into the
canaliculi. It is obvious that the ossified substance of bone is
intercellular in character, and corresponds to the matrix of
cartilage.
Bone may be compact, or loose and spongy in character, when it is
known as =cancellous bone=. In compact bone many of the lamellae are
arranged concentrically round cavities, the =Haversian canals=, which
in life are occupied by blood-vessels. Each Haversian canal with its
lamellae forms a =Haversian system=. In spongy bone instead of
Haversian canals there occur large irregular spaces filled with
marrow, which consists chiefly of blood-vessels and fatty tissue. The
centre of a long bone is generally occupied by one large continuous
marrow cavity. The whole bone is surrounded by a fibrous connective
tissue membrane, =the periosteum=.
THE DEVELOPMENT OF BONE.
=Periosteal ossification.= An example of a bone entirely formed in
this way is afforded by the parietal. The first trace of ossification
is shown by the appearance, below the membrane which occupies the
place of the bone in the early embryo, of calcareous spicules of bony
matter, which are laid down round themselves by certain large cells,
the =osteoblasts=. These osteoblasts gradually get surrounded by the
matter which they secrete and become converted into bone cells, and in
this way a mass of spongy bone is gradually produced. Meanwhile a
definite periosteum has been formed round the developing bone, and on
its inner side fresh osteoblasts are produced, and these with the
others gradually render the bone larger and more and more compact.
Finally, the middle layer of the bone becomes again hollowed out and
rendered spongy by the absorption of part of the bony matter.
=Endochondral ossification=[5]. This is best studied in the case of a
long bone like the femur or humerus. Such a long bone consists of a
shaft, which forms the main part, and two terminal portions, which
form the =epiphyses=, or portions ossifying from centres distinct from
that forming the shaft or main part of the bone.
In the earliest stage the future bone consists of hyaline cartilage
surrounded by a vascular sheath, the perichondrium.
Then, starting from the centre, the cartilage becomes permeated by a
number of channels into which pass vessels from the perichondrium and
osteoblasts. In this way the centre of the developing shaft becomes
converted into a mass of cavities separated by bands or trabeculae of
cartilage. This cartilage next becomes calcified, but as yet is not
converted into true bone. The osteoblasts in connection with the
cavities now begin to deposit true endochondral spongy bone, and then
after a time this becomes absorbed by certain large cells, the
osteoclasts, and resolved into marrow or vascular tissue loaded with
fat. So that the centre of the shaft passes from the condition of
hyaline cartilage to that of calcified cartilage, thence to the
condition of spongy bone, and finally to that of marrow. At the same
time beneath the perichondrium osteoblasts are developed which also
begin to give rise to spongy bone. The perichondrium thus becomes the
periosteum, and the bone produced by it, is periosteal or membrane
bone. So that while a continuous marrow cavity is gradually being
formed in the centre of the shaft, the layer of periosteal bone round
the margin is gradually thickening, and becoming more and more compact
by the narrowing down of its cavities to the size of Haversian canals.
The absorption of endochondral and formation of periosteal bone goes
on, till in time it comes about that the whole of the shaft, except
its terminations, is of periosteal origin. At the extremities of the
shaft, however, and at the epiphyses, each of which is for a long time
separated from the shaft by a pad of cartilage, the ossification is
mainly endochondral, the periosteal bone being represented only by a
thin layer.
Until the adult condition is reached and growth ceases, the pad of
cartilage between the epiphysis and the shaft continues to grow, its
outer (epiphysial) half growing by the formation of fresh cartilage as
fast as its inner half is encroached on by the growth of bone from the
shaft. The terminal or articular surfaces of the bone remain
throughout life covered by layers of articular cartilage.
Even after the adult condition is reached the bone is subject to
continual change, processes of absorption and fresh formation going on
for a time and tending to render the bone more compact.
METHODS IN WHICH BONES ARE UNITED TO ONE ANOTHER.
The various bones composing the endoskeleton are united to one another
either by =sutures= or by movable =joints=.
When two bones are suturally united, their edges fit closely together
and often interlock, being also bound together by the periosteum.
In many cases this sutural union passes into fusion or =ankylosis=,
ossification extending completely from one bone to the other with the
obliteration of the intervening suture. This feature is especially
well marked in the cranium of most birds.
The various kinds of joints or articulations[6] may be subdivided into
imperfect joints and perfect joints.
In =imperfect joints=, such as the intervertebral joints of mammals,
the two contiguous surfaces are united by a mass of fibrous tissue
which allows only a limited amount of motion.
In =perfect joints= the contiguous articular surfaces are covered with
cartilage, and between them lies a synovial membrane which secretes a
viscid lubricating fluid.
The amount of motion possible varies according to the nature of the
articular surfaces; these include--
_a._ =ball and socket joints=, like the hip and shoulder, in which the
end of one bone works in a cup provided by another, and movements can
take place in a variety of planes.
_b._ =hinge joints=, like the elbow and knee, in which as in
ball-and-socket joints one bone works in a cup provided by another,
but movements can take place in one plane only.
THE ENDOSKELETON.
The endoskeleton is divisible into =axial= and =appendicular= parts;
and the =axial= skeleton into--
1. the spinal column,
2. the skull {_a._ the cranium,
{_b._ the jaws and visceral skeleton,
3. the ribs and sternum[7].
I. THE AXIAL SKELETON.
1. THE SPINAL COLUMN.
The spinal column in the simplest cases consists of an unsegmented
rod, the notochord, surrounded by the =skeletogenous layer=, a sheath
of mesoblastic origin, which also envelops the nerve cord. Several
intermediate stages connect this simple spinal column with the
vertebral column characteristic of higher vertebrates. A typical
vertebral column may be said to consist of (1) a series of
cartilaginous or bony blocks, the vertebral =centra=, which arise in
the sheath surrounding the notochord. They cause the notochord to
become constricted and to atrophy to a varying extent, though a
remnant of it persists, either permanently or for a long period,
within each centrum or between successive centra. (2) From the dorsal
surface of each centrum arise a pair of processes which grow round the
spinal cord and unite above it, forming a =dorsal= or =neural arch=.
(3) A similar pair of processes arising from the ventral surface of
the centrum form the =ventral= or =haemal arch=. To the ventral arch
the ribs strictly belong, and it tends to surround the ventral
blood-vessels and the body cavity with the alimentary canal and other
viscera.
A =neural spine= or spinous process commonly projects upwards from the
dorsal surface of the neural arch, and a pair of =transverse
processes= project outwards from its sides. When, as is commonly the
case, the two halves of the haemal arch do not meet, the ventral
surface of the centrum often bears a downwardly-projecting
=hypapophysis=.
The character of the surfaces by which vertebral centra articulate
with one another varies much. Sometimes both surfaces are concave, and
the vertebra is then said to be =amphicoelous=; sometimes a centrum is
convex in front and concave behind, the vertebra is then
=opisthocoelous=, sometimes concave in front and convex behind, when
the vertebra is =procoelous=. Again, in many vertebrae both faces of
the centra are flat, while in others they are saddle-shaped, as in the
neck vertebrae of living birds, or biconvex, as in the case of the
first caudal vertebra of crocodiles.
In the higher vertebrates pads of fibrocartilage--the =intervertebral
discs=--are commonly interposed between successive centra, these or
parts of them often ossify, especially in the trunk and tail, and are
then known as =inter centra=.
[Illustration FIG. 2. CERVICAL VERTEBRAE OF AN OX (_Bos taurus_).
A, is the fifth; B, the fourth; C, the third. x 1/4 (Camb. Mus.)
1. neural spine.
2. transverse process.
3. hypapophysis.
4. convex anterior face of the centrum.
5. concave posterior face of the centrum.
6. prezygapophysis.
7. postzygapophysis.
8. vertebrarterial canal.
9. neural canal.
10. inferior lamella of transverse process.]
The vertebrae of the higher forms can generally be arranged in the
following five groups, each marked by certain special characteristics:
1. The =cervical= or =neck vertebrae=. These connect the skull with
the thorax, and are characterised by relatively great freedom of
movement. They often bear small ribs, but are distinguished from the
succeeding thoracic vertebrae by the fact that their ribs do not reach
the sternum. The first cervical vertebra which articulates with the
skull is called the =atlas=, but a study of the nerve exits shows that
the first vertebra is not serially homologous throughout the
Ichthyopsida, so that it is best to reserve the term atlas for the
first vertebra in Sauropsida and Mammalia.
2. The =thoracic vertebrae= (often called dorsal) bear movably
articulated ribs which unite ventrally with the sternum.
3. The =lumbar vertebrae= are generally large, and are often more
movable on one another than are the thoracic vertebrae. They bear no
ribs.
4. The =sacral vertebrae= are characterised by the fact that they are
firmly fused together, and are united with the pelvic girdle by means
of their transverse processes and rudimentary ribs.
5. The =caudal= or =tail vertebrae= succeed the sacral. The anterior
ones are often fused with one another and with the sacrals, but they
differ from true sacral vertebrae in that there are no rudimentary
ribs between their transverse processes and the pelvic girdle. They
often bear V-shaped =chevron bones=.
In fish and snakes the vertebral column is divisible into only two
regions, an anterior trunk region, whose vertebrae bear ribs, and a
posterior tail region, whose vertebrae are ribless.
2. THE SKULL.
Before giving a general account of the adult skull it will be well to
briefly describe its development.
GENERAL DEVELOPMENT OF THE CRANIUM[8].
Shortly after its appearance, the central nervous system becomes
surrounded by a membranous mesodermal investment which in the region
of the spinal cord is called the =skeletogenous layer= or =perichordal
sheath=, while in the region of the brain it is called the
=membranous cranium=. Ventral to the central nervous system is the
notochord, which extends far into the region of the future cranium,
and like the nervous system, is enclosed by the skeletogenous layer.
The primitive cartilaginous cranium is formed by histological
differentiation within the substance of the membranous cranium and
always consists of the following parts:
(_a_) the =parachordals=. These are a pair of flat curved plates of
cartilage, each of which has its inner edge grooved where it comes in
contact with the notochord. The parachordals, together with the
notochord, form a continuous plate, which is known as the =basilar
plate=. The basilar plate is the primitive floor below the hind- and
mid-brain. In front the parachordals abut upon another pair of
cartilaginous bars, the trabeculae, the two pairs of structures being
sometimes continuous with one another from the first;
(_b_) the =trabeculae= which meet behind and embrace the front end of
the notochord. Further forwards they at first diverge from one
another, and then converge again, enclosing a space, the =pituitary
space=. After a time they generally fuse with one another in the
middle line, and, with the parachordals behind, form an almost
continuous basal plate. The trabeculae generally appear before the
parachordals. They form the primitive floor below the fore-brain;
(_c_) the cartilaginous =capsules= of the three pairs of =sense
organs=. At a very early stage of development involutions of the
surface epiblast give rise to the three pairs of special sense
organs--the olfactory or nasal organs in front, the optic in the
middle, and the auditory behind. The olfactory and auditory organs
always become enclosed in definite cartilaginous capsules, the eyes
often as in the Salmon, become enclosed in cartilaginous sclerotic
capsules, while sometimes, as in mammals, their protecting capsules
are fibrous.
Each pair of sense capsules comes into relation with part of the
primitive cranium, and greatly modifies it. Thus the auditory or
periotic capsules press on the parachordals till they come to be more
or less imbedded in them. Perhaps owing to the pressure of the nasal
capsules the trabeculae fuse in front, and then grow out into an
anterior pair of processes, the =cornua trabeculae=, and a posterior
pair, the =antorbital processes=, which together almost completely
surround the nasal capsules. The sclerotic capsules of the eyes
greatly modify the cranium, although they never become completely
united with it.
The cartilaginous cranium formed of the basal plate, together with the
sense capsules, does not long remain merely as a floor. Its sides grow
vertically upwards, forming the =exoccipital= region of the cranium
behind, and the =alisphenoidal= and =orbitosphenoidal= regions further
forwards. In many forms, such as Elasmobranchs, all these upgrowths
meet round the brain, roofing it in and forming an almost complete
cartilaginous cranium. But in most vertebrata, while in the occipital
region, the cartilaginous cranium is completed dorsally, in the
alisphenoidal and orbitosphenoidal regions the cartilage merely forms
the lateral walls of the cranium, the greater part of the brain having
dorsal to it a wide space, closed by merely membranous tissue in
connection with which the large frontal and parietal bones are
subsequently formed.
The SKULL includes
_a._ the cranium,
_b._ the jaws and visceral skeleton.
The =cranium= can be further subdivided into
(1) an axial portion, the =cranium proper= or =brain case=;
(2) =the sense capsules.= The capsules of the auditory and olfactory
sense organs are always present, and as has been already mentioned,
in many animals the eye likewise is included in a cartilaginous
capsule.
(1) THE CRANIUM PROPER OR BRAIN CASE.
The cranium varies much in form and structure. In lower vertebrates,
such as Sharks and Lampreys, it remains entirely cartilaginous and
membranous, retaining throughout life much of the character of the
embryonic rudiment of the cranium of higher forms. The dogfish's
cranium, described on pp. 73 to 76, is a good instance of a cranium of
this type. But in the majority of vertebrates the cartilage becomes
more or less replaced by cartilage bone, while membrane bones are also
largely developed and supplant the cartilage.
The cranium of most vertebrates includes a very large number of bones
whose arrangement varies much, but one can distinguish a definite
=basicranial axis= formed of the basi-occipital, basisphenoid, and
presphenoid bones, which is a continuation forwards of the axis of the
vertebral column. From the basicranial axis a wide arch arises,
composed of a number of bones, which form the sides and roof of the
brain-case These bones are arranged in such a manner that if both
cartilage and membrane bones are included they can be divided into
three rings or segments. The hinder one of these segments is the
occipital, the middle the parietal, and the anterior one the frontal.
The occipital segment is formed of four cartilage bones, the
=basi-occipital= below, two =exoccipitals= at the sides, and the
=supra-occipital= above. The parietal segment is formed of the
=basisphenoid= below, two =alisphenoids= at the sides and two membrane
bones, the _parietals_ above, and the frontal segment in like manner
consists of the =presphenoid= below, the two =orbitosphenoids= at the
sides, and two membrane bones, the _frontals_, above. The parietals
and frontals, being membrane bones, are not comparable to the
supra-occipital, in the way that the presphenoid and basisphenoid are
to the basi-occipital.
The cartilage bones of the occipital segments are derived from the
parachordals of the embryonic skull, those of the parietal and frontal
segments from the trabeculae.
In front of the presphenoid the basicranial axis is continued by the
=mesethmoid=.
(2) THE SENSE CAPSULES.
These enclose and protect the special sense organs.
(_a_) =Auditory capsule.=
The basisphenoid is always continuous with the basi-occipital, but the
alisphenoid is not continuous with the exoccipital as the =periotic=
or =auditory capsule= is interposed between them. Each periotic
capsule has three principal ossifications; an anterior bone, the
=pro-otic=, a posterior bone, the =opisthotic=, and a superior bone,
the =epi-otic=.
These bones may severally unite, or instead of uniting with one
another they may unite with the neighbouring bones. Thus the epi-otic
often unites with the supra-occipital, and the opisthotic with the
exoccipital.
Two other bones developed in the walls of the auditory capsule are
sometimes added, as in Teleosteans; these are the =pterotic= and
=sphenotic=.
(_b_) =Optic capsule.=
The eye is frequently enclosed in a cartilaginous sclerotic capsule,
and in this a number of scale-like bones are often developed.
Several membrane bones are commonly formed around the orbit or cavity
for the eye. The most constant of these is the _lachrymal_ which lies
in the anterior corner; frequently too, as in Teleosteans, there is a
_supra-orbital_ lying in the upper part of the orbit, or as in many
Reptiles, a _postorbital_ lying in the posterior part of the orbit.
(_c_) =Nasal capsule.=
In relation to the nasal capsules various bones occur.
The basicranial axis in front of the presphenoid is ossified, as the
=mesethmoid=, dorsal to which there sometimes, as in Teleosteans,
occur a _median ethmoid_ and a pair of =lateral ethmoids=[9]. Two
pairs of membrane bones very commonly occur in this region, viz. the
_nasals_ which lie dorsal to the mesethmoid, and the _vomers_
(sometimes there is only one) which lie ventral to it.
The part of the skull lying immediately in front of the cranial cavity
and in relation to the nasal capsules constitutes the =ethmoidal
region=.
There remain certain other membrane bones which are often found
connected with the cranium. Of these, one of the largest is the
_parasphenoid_ which, in Ichthyopsids, is found underlying the
basicranial axis. _Prefrontals_ often, as in most reptiles, occur
lying partly at the sides and partly in front of the frontal, and
_postfrontals_ similarly occur behind the orbit lying partly behind
the frontals and partly at their sides. Lastly a _squamosal bone_ is,
as in Mammals, very commonly developed, and lies external and partly
dorsal to the auditory capsules.
THE JAWS AND VISCERAL SKELETON.
In the most primitive fish these consist of a series of cartilaginous
rings or arches placed one behind another and encircling the anterior
end of the alimentary canal. Originally they are mainly concerned with
branchial respiration.
The first or =maxillo-mandibular= arch forms the upper jaw and the
lower jaw or mandible.
The second or =hyoid= arch bears gills and often assists in attaching
the jaws to the cranium. The remaining arches may bear gills, though
the last is commonly without them.
The above condition is only found in fishes, in higher animals the
visceral skeleton is greatly reduced and modified.
The first or maxillo-mandibular arch is divisible into a dorsal
portion, the =palato-pterygo-quadrate bar=, which forms the primitive
upper jaw and enters into very close relations with the cranium, and a
ventral portion, =Meckel's cartilage=, which forms the primitive lower
jaw. The cartilaginous rudiments of both these portions disappear to a
greater or less extent and become partly ossified, partly replaced by
or enveloped in membrane bone.
The posterior part of the palato-pterygo-quadrate bar becomes ossified
to form the =quadrate=, the anterior part to form the palatine and
pterygoid, or the two latter may be formed partially or entirely of
periosteal bone, developed round the cartilaginous bar. Two pairs of
important membrane bones, the _premaxillae_ and _maxillae_ form the
anterior part of the upper jaw, and behind the maxilla lies another
membrane bone, the _jugal_ or _malar_, which is connected with the
quadrate by a _quadratojugal_. The premaxillae have a large share in
bounding the external nasal openings or anterior nares.
In lower vertebrates the nasal passage leads directly into the front
part of the mouth cavity and opens by the posterior nares. In some
higher vertebrates, such as mammals and crocodiles, processes arise
from the premaxillae and palatines, and sometimes from the pterygoids,
which meet their fellows in the middle line and form the palate,
shutting off the nasal passage from the mouth cavity and causing the
posterior nares to open far back.
The cartilage of the lower jaw is in all animals with ossified
skeletons, except the Mammalia, partly replaced by cartilage bone
forming the =articular=, partly overlain by a series of membrane bones
the _dentary_, _splenial_, _angular_, _supra-angular_ and _coronoid_.
In many sharks large paired accessory cartilages occur at the sides of
the jaws; and in a few reptiles and some Amphibia, such as the Frog,
the ossified representative of the anterior of these structures occurs
forming the =mento-meckelian= bone. In mammals the lower jaw includes
but a single bone.
The quadrate in all animals with ossified skeletons, except the
Mammalia, forms the suspensorium of the mandible or the skeletal link
between the jaw and the cranium; in the Mammalia, however, the
mandible articulates with the squamosal, while the quadrate is greatly
reduced, and is now generally considered to be represented by the
tympanic ring of the ear.
The second visceral or hyoid arch in fishes consists of two pieces of
cartilage, a proximal[1] piece the =hyomandibular=, and a distal[10]
piece the =cerato-hyal=. The cerato-hyals of the two sides are commonly
united by a median ventral plate, the =basi-hyal=. The hyoid arch
bears gills on its posterior border, but its most important function
in most fishes is to act as the suspensorium. In higher vertebrates
the representative of the hyomandibular is much reduced in size, and
comes into relation with the ear forming the =auditory ossicles=; the
cerato-hyal looses its attachment to the hyomandibular and becomes
directly attached to the cranium, forming a large part of the hyoid
apparatus of most higher vertebrates.
Behind the hyoid arch come the branchial arches. They are best
developed in fishes, in which they are commonly five in number and
bear gills. Their ventral ends are united in pairs by median pieces,
the =copulae=.
In higher vertebrates they become greatly reduced, and all except the
first and second completely disappear. In the highest vertebrates, the
mammals, the second has disappeared, but in birds and many reptiles it
is comparatively well developed.
3. THE RIBS AND STERNUM.
The =ribs= are a series of segmentally arranged cartilaginous or bony
rods, attached to the vertebrae; they tend to surround the body
cavity, and to protect the organs contained within it. Ribs are very
frequently found attached to the transverse processes of the
vertebrae, but a study of their origin in fish shows that they are
really the cut off terminations of the ventral arch, not of the
transverse processes which are outgrowths from the dorsal arch. In the
tail their function is to surround and protect structures like the
ventral blood-vessels which do not vary much in size, consequently
they meet one another, and form a series of complete ventral or haemal
arches. But the trunk contains organs like the lungs and stomach which
are liable to vary much in size at different times, consequently the
halves of the haemal arch do not meet ventrally, and then the ribs
become detached from the rest of the haemal arch. Having once become
detached, they are able to shift about and unite themselves to various
points of the vertebra. They frequently, as has been already
mentioned, become entirely attached to the transverse process, or they
may be attached to the transverse process by a dorsal or =tubercular=
portion and to the centrum or to the ventral arch by a ventral or
=capitular= portion.
In all animals above fishes the distal ends of the thoracic ribs unite
with a median breast bone or sternum which generally has the form of a
segmented rod. The =sternum= is really formed by the fusion of the
distal ends of a series of ribs. In many animals elements of the
shoulder girdle enter into close relation with the rib elements of the
sternum.
II. THE APPENDICULAR SKELETON.
This consists of the skeleton of the anterior or =pectoral=, and the
posterior or =pelvic= limbs, and their girdles. In every case (except
in Chelonia) the parts of the appendicular skeleton lie external to
the ribs.
1. THE LIMB GIRDLES.
=The Pectoral girdle=[11]. In the simplest case the pectoral or
shoulder girdle consists of a hoop of cartilage incomplete dorsally.
It is attached by muscle to the vertebral column, and is divided on
either side into dorsal and ventral portions by a cavity, the =glenoid
cavity=, at the point where the anterior limb articulates. In higher
fishes this hoop is distinctly divided into right and left halves; it
becomes more or less ossified, and a pair of important bones, the
clavicles, are developed in connection with its ventral portion.
In higher vertebrates ossification sets up in the cartilage and gives
rise on each side to a dorsal bone, the =scapula=, and frequently to
an anterior ventral bone, the =precoracoid=, and a posterior ventral
bone, the =coracoid=. The precoracoid is often not ossified, and upon
it is developed the clavicle which more or less replaces it. In some
forms a =T= shaped _interclavicle_ occurs, in others =epicoracoids= are
found in front of the coracoids. In all vertebrata above fish, except
the great majority of mammals, the coracoids are large and articulate
with the sternum. But in mammals the coracoids are nearly always quite
vestigial, and the pectoral girdle is attached to the axial skeleton
by the clavicle or sometimes by muscles and ligaments only.
The =Pelvic girdle=[12] like the pectoral consists primitively of a
simple rod or hoop of cartilage, which in vertebrata above fishes is
divided into dorsal and ventral portions, by a cavity, the
=acetabulum=, with which the posterior limb articulates. In the pelvic
girdle as in the pectoral one dorsal, and (commonly) two ventral
ossifications take place. The dorsal bone is the =ilium= and
corresponds to the scapula. The posterior ventral bone is the
=ischium= corresponding to the coracoid. The anterior ventral bone is
the =pubis= and is generally compared to the precoracoid, but in some
cases a fourth pelvic element, the =acetabular= or =cotyloid= bone is
found, and this may correspond to the precoracoid.
The pelvic girdle differs from the pectoral in the fact that the
dorsal bones--the ilia--are nearly always firmly united to transverse
processes of the sacral vertebrae, by means of rudimentary ribs. The
pubes and ischia generally meet in ventral symphyses.
2. THE LIMBS.
It will be most convenient to defer a discussion of the limbs of
fishes to chap. VIII.
All vertebrates above fishes have the limbs divisible into three main
segments:--
=Anterior or Fore limb.= =Posterior or Hind limb.=
Proximal segment. upper arm or _brachium_. thigh.
Middle segment. fore-arm or _antibrachium_. shin or _crus_.
Distal segment. _manus_. _pes_.
The proximal segments each contain one bone, the =humerus= in the case
of the upper arm, and the =femur= in the case of the thigh. The middle
segments each contain two bones, the =radius= and =ulna= in the case
of the fore-arm, and the =tibia= and =fibula= in the case of the shin.
* * * * *
The manus and pes are further subdivided into
(_a_) two or three proximal rows of bones forming the wrist or
=carpus= in the case of the manus, and the ankle or =tarsus= in the
case of the pes.
(_b_) a middle row called respectively the =metacarpus= and
=metatarsus=.
(_c_) a number of distal bones called the =phalanges= which form the
skeleton of the fingers and toes, or =digits=.
Typically the manus and pes both have five digits (pentedactylate).
The first digit of the manus is commonly called the =pollex=, and the
first digit of the pes the =hallux=.
In a very simple =carpus= such as that of _Chelydra_, there are nine
bones. They are arranged in a proximal row of three, the radiale,
intermedium, and ulnare,--the first being on the radial side of the
limb, and a distal row of five called respectively carpale 1, 2, 3, 4,
5, beginning on the radial side. Between these two rows is a single
bone the centrale, or there may be two.
Similarly there are nine bones in a simple =tarsus= such as that of
_Salamandra_. They form a proximal row of three, the tibiale,
intermedium and fibulare, and a distal row of five, called
respectively tarsale 1, 2, 3, 4, 5, beginning on the tibial side.
Between the two rows there is a centrale as in the carpus, or there
may be two.
* * * * *
The following names derived from human anatomy are commonly applied to
the various carpal and tarsal bones:
=Carpus.=
radiale = scaphoid
intermedium = lunar
ulnare = cuneiform
centrale = central
carpale 1 = trapezium
" 2 = trapezoid
" 3 = magnum
" 4 } = unciform
" 5 }
=Tarsus.=
tibiale }
intermedium } astragalus
fibulare = calcaneum
centrale = navicular
tarsale 1 = internal cuneiform
" 2 = middle "
" 3 = external "
" 4 } = cuboid
" 5 }
NOTE. The above is the view commonly accepted concerning the
homology of the carpal and tarsal bones. But with regard to the
proximal row of tarsal bones there is difference of opinion.
All anatomists are agreed that the calcaneum is the fibulare
and that the intermedium is contained in the astragalus, but
while the majority regard the astragalus as the fused tibiale
and intermedium, Baur considers that a small bone found on the
tibial side of the tarsus in _Procavia_, many Rodents,
Insectivores, and the male _Ornithorhynchus_, is the vestigial
tibiale, and regards the astragalus as the intermedium
alone[13]. He also considers that the mammalian scaphoid
represents a centrale.
MODIFICATIONS IN THE POSITIONS OF THE LIMBS[14].
In their primitive position the limbs are straight and are extended
parallel to one another at right angles to the axis of the trunk. Each
limb then has a dorsal surface, a ventral surface, an anterior or
=pre-axial= edge, and a posterior or =postaxial= edge.
In the anterior limb the radius and the pollex are pre-axial, the ulna
and the fifth finger are postaxial. In the posterior limb the tibia
and the hallux are pre-axial, the fibula and the fifth toe are
postaxial. The Cetacea and various extinct reptiles, such as
_Ichthyosaurus_ and _Plesiosaurus_, have their limbs in practically
this primitive position.
The first modification from it is produced by the bending ventrally of
the middle segments of both limbs upon the proximal segments, while
the distal segment is bent in the opposite direction on the middle
segment. Then the ventral surfaces of the antibrachium and crus come
to look inwards, and their dorsal surfaces to look outwards. The
brachium and manus, thigh and pes still have their dorsal surfaces
facing upwards and their ventral surfaces facing downwards as before,
and the relations of their pre-and postaxial borders remain as they
were. Many Amphibians and Reptiles, such as tortoises, carry their
limbs in this position.
In all higher vertebrates, however, a further change takes place, each
limb is rotated as a whole from its proximal end, the rotation taking
place in opposite directions in the fore and hind limbs respectively.
The anterior limb is rotated backwards from the shoulder, so that the
brachium lies nearly parallel to the body, and the elbow points
backwards, the antibrachium downwards, and the manus backwards; the
pre-axial surface of the whole limb with the radius and pollex now
faces outwards, and the postaxial surface with the ulna and fifth
finger now faces inwards. In the Walrus and, to a certain extent, in
the Sea lions the anterior limb remains throughout life in this
position. The posterior limb is also rotated, but the rotation in this
case takes place forwards, so that the thigh lies nearly parallel to
the body, the knee-joint pointing forwards; the crus downwards and the
pes forwards. The pre-axial surface of the whole limb with the tibia
and hallux looks towards the middle of the body, the postaxial surface
with the fibula and fifth toe looks outwards. This is the position in
which the hind limb is carried in nearly all mammals.
In nearly all mammals a further change takes place in the position of
the anterior limb. The radius and ulna have hitherto been parallel to
one another, but now the lower end of the radius, carrying with it the
manus, comes to be rotated forwards round the ulna, so that the manus,
as well as the pes, comes to be forwardly-directed, and its pre-axial
surface faces inwards.
In the majority of mammals the radius and ulna are permanently fixed
in this, which is known as the =prone= position, but in man and some
other mammals the manus can be pronated or turned into this position
at will. When the radius and ulna are parallel throughout their whole
length the manus is said to be in the =supine= position.
The =extensor= side of a limb is that to which the muscles which
straighten it are attached, the =flexor= side is that to which the
muscles which bend it are attached.
FOOTNOTES:
[1] The skin consists of an outer layer of epiblastic origin, the
epidermis, and an inner layer of mesoblastic origin, the dermis. The
epidermis is divided into two principal layers, an outer one, the
horny layer or _stratum corneum_, and an inner one, the _stratum
Malpighii_. The innermost part of the stratum corneum is distinguished
as the _stratum lucidum_, and the outermost part of the stratum
Malpighii as the _stratum granulosum_.
[2] The enamel of the pharyngeal teeth of some Teleosteans is
hypoblastic in origin.
[3] See also p. 71.
[4] It is usual to regard the clavicle as a membrane bone, but
Kölliker has shown that in rabbit embryos of about the 17th day it is
cartilaginous.
[5] In compiling these paragraphs on Histology, free use has been made
of Klein and Noble Smith's _Atlas of Histology_, the small Histologies
of Klein and Schäfer, Huxley's _Elementary Physiology_, and Lloyd
Morgan's _Animal Biology_.
[6] See Huxley's _Elementary Physiology_, Revised edition, London,
1886, p. 180.
[7] Strictly speaking the jaws, visceral skeleton, ribs and sternum do
not form part of the axis, but it is convenient to group them as parts
of the axial skeleton.
[8] F.M. Balfour, _Comparative Embryology_, vol. II., London, 1881, p.
465. W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
London, 1877.
[9] Sometimes also called ectethmoids or parethmoids.
[10] The _proximal_ end of anything is the one nearest the point of
origin or attachment, the _distal_ end is the one furthest from the
point of origin or attachment.
[11] W.K. Parker, _A Monograph of the Shoulder Girdle and Sternum_,
Ray Soc. London, 1868.
[12] See R. Wiedersheim, _Zeitschr. wiss. Zool._ vol. LIII. suppl. p.
43, 1892.
[13] G. Baur, _Beiträge zur Morphogenie des Carpus und Tarsus der
Vertebraten_, Theil 1. Batrachia. Jena, 1888, and _Amer. Natural._,
vol. XIX. 1885 (several papers).
[14] This account is based on Chapter XX. of Flower's _Osteology of
the Mammalia_. London 1876.