THE SKELETON OF THE GREEN TURTLE.
(_Chelone midas._)
The most striking feature as regards the skeleton of the Turtle is
that the trunk is enveloped in a bony box, the dorsal portion of which
is called the =carapace=, while the ventral portion is the =plastron=.
I. EXOSKELETON.
_a._ The =epidermal exoskeleton= in the Green Turtle as in all other
Chelonia except _Dermochelys_, _Trionyx_ and their allies is strongly
developed, its most important part consisting of a series of horny
=shields= which cover over the bony plates of the carapace and
plastron but do not at all correspond to them in size and arrangement.
The shields covering over the =carapace= consist of three rows of
larger central shields,--five (=vertebral=) shields being included in
the middle row and four (=costal=) in each lateral row,--and of a
number of smaller =marginal= shields.
Of the marginal shields, that lying immediately in front of the first
vertebral is termed the =nuchal=, while the two succeeding the last
vertebral are called sometimes =pygal=, sometimes =supracaudal=; the
remainder are the marginal shields proper.
The epidermal covering of the =plastron= consists principally of six
pairs of symmetrically arranged shields, called respectively the
=gular=, =humeral=, =pectoral=, =abdominal=, =femoral=, and =anal=,
the gular being the most anterior. In front of the gular shields is an
unpaired =intergular=, and the shields of the plastron are connected
laterally with those of the carapace, by five or six pairs of rather
irregular =infra-marginal= shields. Smaller horny plates occur on
other parts of the body, especially on the limbs and head.
Two other sets of structures belong also to the epidermal exoskeleton,
viz. (_a_) horny =beaks= with denticulated edges which ensheath both
upper and lower jaws, (_b_) =claws=, which as a rule are borne only by
the first digit of each limb. Sometimes in young individuals the
second digit is also clawed.
_b._ The =dermal exoskeleton= is strongly developed, and is combined
with endoskeletal structures derived from the ribs and vertebrae to
form the carapace.
The =Carapace= (fig. 36) consists of a number of plates firmly united
to one another by sutures. They have a very definite arrangement and
include:
(_a_) the =nuchal= plate (fig. 36, 1), a wide plate forming the whole
of the anterior margin of the carapace. It is succeeded by three
series of plates, eight in each series, which together make up the
main part of the carapace. Of these the small
(_b_) =neural plates=[1] (fig. 36, A, 2) form the middle series. They
are closely united with the neural arches of the underlying vertebrae;
(_c_) the =costal plates=[89] (fig. 36, A, 3) are broad arched plates
united to one another by long straight sutures. They are united at
their inner extremities with the neural plates, but the boundaries of
the two sets of plates do not regularly correspond. Each is united
ventrally with a rib which projects beyond it laterally for some
distance; (_d_) the =marginal plates= (fig. 36, 4) are twenty-three
in number, eleven lying on each side, while an unpaired one lies in
the middle line posteriorly. Many of them are marked by slight
depressions into which the ends of the ribs fit;
(e) the =pygal= plates (fig. 36, 5) are two unpaired plates lying
immediately posterior to the last neural.
[Illustration Fig. 36. A, DORSAL AND B, VENTRAL VIEW OF THE CARAPACE
OF A LOGGERHEAD TURTLE (_Thalassochelys caretta_), (after Owen).
1. nuchal plate.
2. first neural plate.
3. second costal plate.
4. marginal plate.
5. pygal plate.
6. rib.
7. thoracic vertebra.
8. first vertebral shield.
9. costal shield.]
The sculpturing due to the epidermal shields is very obvious on the
carapace.
The =plastron= (fig. 37) consists of one unpaired ossification, the
=entoplastron=, and four pairs of ossifications called respectively
the =epiplastra=, =hyoplastra=, =hypoplastra=, and =xiphiplastra=.
The =epiplastra= (fig. 37, 1) are the most anterior, they are expanded
and united to one another in the middle line in front, while behind
each tapers to a point which lies external to a process projecting
forwards from the hyoplastron. They are homologous with the
_clavicles_ of other vertebrates.
The =entoplastron= or =episternum= (fig. 37, 2) which is homologous
with the _interclavicle_ of other reptiles, is expanded at its
anterior end and attached to the symphysis of the epiplastra, while
behind it tapers to a point and ends freely.
The =hyoplastra= are large irregular bones each closely united
posteriorly with the corresponding hypoplastron, and drawn out
anteriorly into a process which lies internal to that projecting
backwards from the epiplastron. Each gives off on its inner surface a
slender process which nearly meets its fellow, while the anterior half
of the outer surface is drawn out into several diverging processes.
The =hypoplastra= (fig. 37, 4) are flattened bones resembling the
hyoplastra, with which they are united by long sutures; the posterior
half of both outer and inner surfaces is drawn out into a number of
pointed processes.
The =xiphiplastra= are small flattened elongated bones meeting one
another in the middle line posteriorly. In front they are notched and
each interlocks with a process from the hypoplastron of its side. The
hyoplastra, hypoplastra and xiphiplastra are homologous with the
abdominal ribs of Crocodiles.
[Illustration FIG. 37. THE PLASTRON OF A GREEN TURTLE (_Chelone
midas_). × 1/7. (Camb. Mus.)
1. epiplastron (clavicle).
2. entoplastron (interclavicle).
3. hyoplastron.
4. hypoplastron.
5. xiphiplastron.]
II. ENDOSKELETON.
1. THE AXIAL SKELETON.
The axial skeleton includes the vertebral column, the ribs, and the
skull.
A. THE VERTEBRAL COLUMN AND RIBS.
The number of vertebrae in the Green Turtle is thirty-eight, not a
great number as compared with that in many reptiles, and of these
eighteen are caudal.
The vertebral column is divisible into four regions only--=cervical=,
=thoracic=, =sacral=, and =caudal=.
THE CERVICAL VERTEBRAE.
These are eight in number, and are chiefly remarkable for the great
variety of articulating surfaces which their centra present, and for
their mobility upon one another.
The first or =atlas= vertebra differs much from all the others and
consists of the following parts:--
_a._ the =neural arch=, formed of two separate ossifications united in
the mid-dorsal line;
_b._ the =inferior arch=;
_c._ the =centrum=, which is detached from the rest and forms the
odontoid process of the second vertebra.
Each half of the =neural arch= consists of a ventral portion, the
=pedicel=, which lies more or less vertically and is united ventrally
to the inferior arch, and of a dorsal portion, the =lamina=, which
lies more or less horizontally and meets its fellow in the middle line
in front, partially roofing over the neural canal. Each pedicel bears
a facet on its anterior surface, which, with a corresponding one on
the inferior arch, articulates with the occipital condyle of the
skull. Three similar facets occur also on the posterior surface of the
pedicel and inferior arch, and articulate with the odontoid process.
The laminae meet one another in front, but do not fuse, while behind
they are separated by a wide triangular space. They bear a pair of
small downwardly-directed facets, the =postzygapophyses=, for
articulation with the prezygapophyses of the second vertebra.
The =inferior arch= is a short irregular bone bearing two converging
facets for articulation with the occipital condyle and odontoid
process respectively.
The =centrum= or =odontoid process= has a convex anterior surface for
articulation with the neural and inferior arches, and a concave
posterior surface by which it is united with the centrum of the second
or axis vertebra. It bears posteriorly a small epiphysis which is
really a detached portion of the inferior arch.
The second or =axis= and following five cervical vertebrae, though
showing distinct differences, resemble one another considerably, each
having a fairly elongated centrum with a keel-like =hypapophysis=,
each having also a neural arch with prominent articulating surfaces,
the anterior of which, or =prezygapophyses=, look upwards and inwards,
while the posterior ones, the =postzygapophyses=, look downwards and
outwards. They however, as was previously mentioned, differ very
remarkably in the character of the articulating surfaces of the
centra. Thus the second and third vertebrae are convex in front and
concave behind, the fourth is biconvex, the fifth is concave in front
and convex behind. The sixth is concave in front and attached to the
seventh by a flat surface behind, the seventh has a flat anterior face
and two slightly convex facets behind. The vertebrae all have short
blunt transverse processes and the second has a prominent =neural
spine=.
The =eighth cervical vertebra= is curiously modified, the centrum is
very short, has a rather prominent hypapophysis, and is convex behind,
while in front it articulates with the preceding centrum by two
concave surfaces. The neural arch is deeply notched in front and bears
two upwardly-directed prezygapophyses, while behind it is very massive
and is drawn out far beyond the centrum, bearing a pair of flat
postzygapophyses. The top of the neural arch almost or quite meets a
blunt outgrowth from the nuchal plate.
THE THORACIC VERTEBRAE.
These are ten in number and are all firmly united with the ribs and
elements forming the carapace.
The first thoracic vertebra differs from the others, the centrum is
short and has a concave anterior surface articulating with the centrum
of the last cervical vertebra, and a pair of prezygapophyses borne on
long outgrowths. The neural spine arises only from the anterior half
of the centrum, and is not fused to the carapace. Arising laterally
from the anterior part of the centrum are a small pair of ribs each of
which is connected with a process arising from the rib of the
succeeding vertebra.
The next seven thoracic vertebrae are all very similar, each has a
long cylindrical centrum, expanded at the ends, and firmly united to
the preceding and succeeding vertebrae. The neural arches are
flattened and expanded dorsally, and are united to one another and to
the overlying neural plates; each arises only from the anterior half
of its respective centrum, and overlaps the centrum of the vertebra in
front of it. Between the base of the neural arch and its successor is
a small foramen for the exit of the spinal nerve. There are no
transverse processes or zygapophyses.
To each thoracic vertebra from the second to ninth inclusive, there
corresponds a pair of =ribs= (fig. 36, 6) of a rather special
character. Each is suturally united with the anterior half of the edge
of its own vertebra, and overlaps on to the posterior half of the edge
of the next preceding vertebra. The ribs are much flattened, and each
is fused with the corresponding costal plate, beyond which it projects
to fit into a pit in one of the marginal plates.
The tenth thoracic vertebra is smaller than the others, and its neural
arch does not overlap the preceding vertebra, it bears a pair of small
ribs which are without costal plates, but meet those of the ninth
vertebra.
There are no =lumbar= vertebrae.
THE SACRAL VERTEBRAE.
The =sacral vertebrae= are two in number, they are short and wide,
their centra are ankylosed together, and their neural arches are not
united to the carapace.
The first has the anterior face of the centrum concave and the
posterior flat, while both faces of the second are flat. Each bears a
pair of short ribs which meet the ilia, but are not completely
ankylosed either with them or the centra.
THE CAUDAL VERTEBRAE.
The =caudal vertebrae= are eighteen in number. The centrum of the
first is flat in front and is ankylosed to the second sacral; behind
it is convex. The others are all very similar to one another, and
decrease gradually in size when followed back. Each has a moderately
long centrum, concave in front and convex behind, both terminations
being formed by epiphyses. The neural arch arises only from the
anterior half of the vertebra; it bears a blunt truncated neural spine
and prominent pre- and post-zygapophyses. The first seven caudal
vertebrae bear short ribs attached to their lateral margins, the
similar outgrowths on the succeeding vertebrae do not ossify from
distinct centres, and are transverse processes rather than ribs.
B. THE SKULL.
The skull of the Turtle is divisible into the following three parts:--
(1) the cranium;
(2) the lower jaw or mandible;
(3) the hyoid.
(1) THE CRANIUM.
The =cranium= is a very compact bony box, containing a cavity in which
the brain lies, and which is a direct continuation of the neural canal
of the vertebrae.
[Illustration FIG. 38. THE SKULL OF THE GREEN TURTLE (_Chelone
midas_). × 1/2. A, POSTERIOR HALF, B, ANTERIOR HALF. (Brit. Mus.)
1. parietal.
2. squamosal.
3. quadrate.
4. basisphenoid.
5. basi-occipital.
6. quadratojugal.
7. opisthotic.
8. exoccipital.
9. foramen magnum.
10. splenial.
11. articular.
12. dentary.
13. angular.
14. supra-angular.
15. premaxilla.
16. maxilla.
17. jugal.
18. postfrontal.
19. vomer.
20. prefrontal.
21. frontal.
22. external auditory meatus leading into tympanic cavity.]
Like those of the skull as a whole its component bones may be
subdivided into three sets:--
1. those forming the brain-case or =cranium proper=;
2. those developed in connection with the special sense organs;
3. those forming the upper jaw and suspensorial apparatus.
Both cartilage and membrane bones take part in the formation of the
skull, and a considerable amount of cartilage remains unossified,
especially in the ethmoidal and sphenoidal regions.
1. THE CRANIUM PROPER OR BRAIN-CASE.
The cartilage and membrane bones of the brain-case when taken together
can be seen to be more or less arranged in three rings or segments,
called respectively the occipital, parietal, and frontal segments.
The =occipital segment= is the most posterior of these, and consists
of four cartilage bones, the =basi-occipital=, the two =exoccipitals=
and the =supra-occipital=; these bound the =foramen magnum=.
The =basi-occipital= (figs. 38 and 39, 5) lies ventral to the foramen
magnum and only bounds a very small part of it; it forms one-third of
the =occipital condyle= by which the skull articulates with the atlas
vertebra. It unites dorsally with the exoccipitals and anteriorly with
the basisphenoid.
The =exoccipitals= are rather small bones, which form the sides and
the greater part of the floor of the foramen magnum, and two-thirds of
the occipital condyle. Laterally each is united with the pterygoid and
opisthotic of its side. At the sides of the occipital condyle each
exoccipital is pierced by a pair of foramina, the more dorsal and
posterior of which transmits the hypoglossal nerve.
The =supra-occipital= (fig. 39, 14) is a larger bone than the others
of the occipital segment. It forms the upper border of the foramen
magnum and is drawn out dorsally into a large crest which extends back
far beyond the occipital condyle. In the adult the supra-occipital is
completely ankylosed with the epi-otics.
The =Parietal segment=.
The ventral portion of the parietal segment is formed by the
=basisphenoid= (figs. 38 and 39, 4) which lies immediately in front of
the basi-occipital. A triangular portion of it is seen in a ventral
view of the skull, but it is quickly overlapped by the pterygoids. It
gives off dorsally a pair of short processes which meet the pro-otics.
The alisphenoidal region is unossified and the only other constituents
of the parietal segment are the _parietals_ (fig. 39, 1). These are
large bones which, after roofing over the cranial cavity, extend
upwards and become expanded into a pair of broad plates which unite
with the squamosal and bones of the frontal segment to form a wide,
solid, false roof to the skull. Each also sends ventralwards a plate
which meets an upgrowth from the pterygoid and acts as an alisphenoid.
The =Frontal segment=.
Of the frontal segment the basal or presphenoidal and lateral or
orbitosphenoidal portions do not become ossified, the dorsal portion
however includes three pairs of membrane bones, the _frontals_,
_prefrontals_ and _postfrontals_.
The _frontals_ are a pair of small bones lying immediately in front of
the parietals, and in front of them are the _prefrontals_ (figs. 38
and 39, 20), a pair of similar but still smaller bones, which are
produced ventrally to meet the vomer and palatines. They form also the
dorsal boundary of the anterior nares. The _postfrontals_ (figs. 38
and 39, 18) are larger bones, united dorsally to the frontals and
parietals, posteriorly to the squamosals, and ventrally to the jugals
and quadratojugals. All three pairs of frontal bones, especially the
postfrontals, take part in the bounding of the orbits.
[Illustration FIG. 39. LONGITUDINAL VERTICAL SECTION THROUGH THE
CRANIUM OF A GREEN TURTLE (_Chelone midas_). × 2/3. (Camb. Mus.)
1. parietal.
2. squamosal.
3. quadrate.
4. basisphenoid.
5. basi-occipital.
6. quadratojugal.
7. pro-otic.
8. opisthotic.
9. pterygoid.
10. palatine.
11. rod passed into narial passage.
12. exoccipital.
13. epi-otic fused to supra-occipital.
14. supra-occipital.
15. premaxillae.
16. maxillae.
17. jugal.
18. postfrontal.
19. vomer.
20. prefrontal.
21. frontal.
V, VII, VIII, IX, X, XI, XII,
foramina for the exit of
cranial nerves.]
2. THE SENSE CAPSULES.
Skeletal structures occur in connection with each of the three special
sense organs of hearing, sight, and smell.
The =Auditory capsules=.
The auditory or periotic capsule of the turtle is rather large and its
walls are well ossified, epi-otic, pro-otic and opisthotic bones being
present.
The =epi-otic= (fig. 39, 13) is the more dorsal of the three bones,
and in the adult is completely ankylosed with the supra-occipital.
The =opisthotic= (fig. 39, 8) is the ventral posterior element. On its
inner side it is united to the supra-occipital above, and to the
exoccipital below; it sometimes becomes completely fused with the
exoccipital. In front it meets the pro-otic, and on its outer side the
squamosal and quadrate. Its anterior portion is hollowed out by the
cavity in which the auditory organ lies, it gives off also a process
which is separated from the exoccipital by an oval foramen through
which the glossopharyngeal, pneumogastric, and spinal accessory nerves
leave the cranial cavity.
The =pro-otic= is the anterior element; it meets the supra-occipital
and opisthotic posteriorly, while anteriorly it is separated from the
alisphenoidal plate of the parietal and pterygoid by a large oval
foramen through which the maxillary and mandibular branches of the
trigeminal nerve pass out (fig. 39, V 1 & 2). It is hollowed out
posteriorly by the cavity in which the auditory organ lies, and its
inner wall as seen in longitudinal section is pierced by a foramen
through which the external carotid artery and facial nerve leave the
cranial cavity,--the nerve finally leaving the skull through a small
oval foramen on the anterior face of the pro-otic near its junction
with the quadrate.
Between the pro-otic and opisthotic as seen in a longitudinal section
of the skull is a large opening constricted in the middle. This is
the =internal auditory meatus= (fig. 39, VIII.). Through it the
auditory nerve leaves the cranial cavity and enters the ear. The ramus
vestibularis leaves through the dorsal part of the hole, the ramus
cochlearis through the ventral.
The cavity of the auditory or periotic capsule communicates with the
exterior by a fairly large hole, the =fenestra ovalis=, which lies
between the opisthotic and pro-otic, and opens into a deep depression,
the =tympanic cavity=, which is seen in a posterior view of the skull
lying just external to the exoccipital. The cavity communicates with
the exterior by a large opening, the =external auditory meatus= (fig.
38, 22).
Several other openings are seen in the tympanic cavity; through one at
the extreme posterior end the pneumogastric and spinal accessory
nerves leave the skull, and through another, a little further
forwards, the glossopharyngeal.
The auditory ossicles consist of a long bony =columella=, whose inner
end fits into the fenestra ovalis, while the outer end is attached to
a small cartilaginous plate, the =extra-columella=, which is united to
the tympanum.
The =Optic capsules=.
The skeletal structures developed in connection with the optic capsule
do not become united to the skull. They consist of:--
(_a_) the =sclerotic=, a cartilaginous sheath investing the eye and
bearing
(_b_) a ring of ten small bony scales.
There is no _lachrymal_ bone.
The =Olfactory= or =Nasal capsules=.
The basicranial axis in front of the basisphenoid remains
cartilaginous, neither presphenoid nor mesethmoid bones are developed,
and the orbits in a dry skull communicate by a wide space through
which the second, third, fourth, and sixth cranial nerves pass out.
Separate _nasal_ bones also do not occur, the large prefrontals
extending over the area usually occupied by both nasals and
lachrymals.
The only bone developed in connection with the nasal capsules is the
_vomer_ (fig. 39, 19), an unpaired bone lying ventral to the
mesethmoid cartilage, and in contact laterally with the maxillae,
premaxillae and palatines.
3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.
A number of pairs of bones are developed in connection with the upper
jaw and suspensorial apparatus, one pair, the =quadrates=, being
cartilage bones, while the rest are all membrane bones.
The _squamosals_ (fig. 38, 2) are large bones which, lying external to
the auditory bones, extend dorsalwards to meet the parietals and
postfrontals, and form a large part of the false roof of the skull.
They are united ventrally with the quadrates and quadratojugals.
Each =quadrate= (fig. 38, 3) forms the outer boundary of the tympanic
cavity, and is firmly united on its inner side with the opisthotic,
exoccipital, and pterygoid. Dorsally it is fixed to the squamosal and
anteriorly to the quadratojugal. Its outer surface is marked by a deep
recess, and it ends below in a strong condyle with which the mandible
articulates. In front of the quadrates are a pair of thin plate-like
bones, the _quadratojugals_ which are united in front to the jugals or
malars.
The _jugals_ (fig. 38, 17) are also thin plate-like bones, and form
part of the posterior boundary of the orbit. They are attached
dorsally to the postfrontals, and anteriorly to the maxillae, while
each also sends inwards a horizontal process which meets the pterygoid
and palatine.
The _maxillae_ (figs. 38 and 39, 16) are a pair of large
vertically-placed bones, each drawn out ventrally into a straight,
sharp, cutting edge. They form the lateral boundaries of the anterior
nares, and each sends dorsalwards a process which meets the
postfrontal. Each also sends inwards a horizontal =palatine process=,
which meets the palatine and vomer, and also forms much of the floor
of the narial passage.
The _premaxillae_ (figs. 38 and 39, 15) are a pair of very small bones
forming the floor of the anterior narial opening, they are wedged in
between the two maxillae, and send back processes which meet the vomer
and palatines.
The _palatines_ (fig. 39, 10) are a pair of small bones firmly united
with the pterygoids behind, with the maxillae and jugals externally,
and with the vomer in the middle line. Each also gives off a palatine
plate which unites with the expanded lower edge of the vomer, and
forms the ventral boundary of the posterior nares. Anteriorly the
palatines form the posterior boundary of a large foramen through which
the ophthalmic branches of the fifth and seventh nerves pass to the
olfactory organs.
The _pterygoids_ (fig. 39, 9) are a pair of large bones which unite
with one another by a long median suture. They are united also with
the palatines in front, and with the quadrate, basisphenoid,
basi-occipital, and exoccipitals behind. Each also sends dorsalwards a
short =alisphenoid plate= which meets that from the parietal.
Piercing the posterior end of the _pterygoid_ is the prominent opening
of the carotid canal; a bristle passed into this hole emerges through
a foramen lying between the pro-otic and the alisphenoid process of
the pterygoid.
(2) THE LOWER JAW OR MANDIBLE.
The =mandible= consists of one unpaired bone, formed by the fusion of
the two _dentaries_, and five pairs of bones, called respectively the
=articular=, _angular_, _supra-angular_, _splenial_ and _coronoid_.
The fused _dentaries_ (fig. 38, 12) form by far the largest of the
bones; they constitute the flattened anterior part of the mandible,
and extend back below the other bones almost to the end of the jaw.
The _coronoid_ is the most anterior of the paired bones, it forms a
prominent process to which the muscles for closing the jaw are
attached.
The =articular= (fig. 38, 11) is expanded, and with the
_supra-angular_ forms the concave articulating surface for the
quadrate.
The _splenial_ (fig. 38, 10) is a thin plate applied to the inner
surface of the posterior part of the mandible.
The _angular_ (fig. 38, 13) is a slender plate of bone lying below the
supra-angular and splenial.
(3) THE HYOID.
The hyoid apparatus is well developed, parts of the first two
branchial arches being found, as well as of the hyoid proper. It
consists of a more or less oblong flattened =basilingual plate= or
=body of the hyoid= which represents the fused ventral ends of the
hyoid and branchial arches of the embryo, and is drawn out into a
point anteriorly. The greater part is formed of unossified cartilage,
but at the posterior end it is bilobed, and a pair of ossified tracts
occur. To its sides are attached three pairs of structures, which are
portions of the hyoid and first and second branchial arches
respectively.
The free part of the =hyoid= consists of a small piece of cartilage
attached to the anterior part of the basilingual plate at its widest
portion (fig. 53, 2).
The =anterior cornu= or free part of the =first branchial arch= is
much the largest of the three structures. Its proximal portion
adjoining the basilingual plate is cartilaginous, as is its distal
end; the main part is however ossified.
The =posterior cornu= or free part of the =second branchial arch=
(fig. 53, 4) consists of a short flattened cartilaginous bar arising
from the bilobed posterior end of the basilingual plate.
The hyoid apparatus has no skeletal connection with the rest of the
skull.
2. THE APPENDICULAR SKELETON.
This includes the skeleton of the two pairs of limbs and their
girdles.
THE PECTORAL GIRDLE.
The pectoral girdle has an anomalous position, being situated
internal or ventral to the ribs. It consists of three bones, a dorsal
bone, the =scapula=, an anterior ventral bone, the =precoracoid=, and
a posterior ventral bone, the =coracoid=.
The =scapula= is a small somewhat rod-shaped bone forming about
two-thirds of the glenoid cavity. At its proximal end it is closely
united with the precoracoid, the two bones ossifying continuously. It
tapers away distally, and is directed dorsalwards towards the
carapace.
The =precoracoid= forms an angle of about 130° with the scapula, with
which it is completely fused at its proximal end. Its distal end is
somewhat expanded and flattened, and is terminated by a
fibrocartilaginous =epiprecoracoid= which meets its fellow. It takes
no part in the formation of the glenoid cavity.
The =coracoid= is a large flattened blade-shaped bone forming about
one-third of the glenoid cavity. It does not meet its fellow in a
ventral symphysis, and is terminated by a cartilaginous =epicoracoid=.
The glenoid articulating surfaces of both scapula and coracoid are
lined by a thick pad of cartilage.
THE ANTERIOR LIMB.
This is divisible into three portions, the =upper arm=, =fore-arm= and
=manus=.
The =upper arm= contains a single bone, the =humerus=.
The =humerus= (fig. 40, A, 1) is a stout, nearly straight, somewhat
flattened bone widely expanded at both ends. At the proximal end is
the large hemispherical =head=, which articulates with the glenoid
cavity. Behind the head the bone is drawn out into another large
rounded process. Below the head the shaft bears a small outgrowth
which is continuous with a larger one on the flexor surface (see p.
29). The bone is terminated distally by the =trochlea=, consisting of
three partially distinct convex surfaces which articulate with the
bones of the fore-arm.
The =fore-arm= includes two bones, the =radius= and =ulna=; both these
are small bones, and are immovably fixed to one another proximally and
distally.
The =radius= or pre-axial bone is the larger of the two, and is a
rod-like bone terminated at either end by an epiphysis. It articulates
at its proximal end with the humerus, and at its distal end with the
radiale or scaphoid bone of the carpus.
The =ulna= (fig. 40, A, 3) or postaxial bone is shorter than the
radius, and more expanded at its proximal end, where it articulates
with the humerus. It articulates distally with the intermedium (lunar)
and the ulnare (cuneiform) bones of the carpus. All three bones of the
arm have their terminations formed by epiphyses which ossify from
centres distinct from those forming the shafts.
The =Manus= consists of the =carpus= or =wrist= and the =hand= which
includes the metacarpals and phalanges.
The =carpus= consists of a series of ten small bones, one of which,
the =pisiform= (fig. 40, A, 10), differs from the others in being
merely an ossification in the tendon of a muscle. The remaining nine
bones are arranged in a proximal row of three, the =ulnare= (fig. 40,
A, 6), =intermedium=, and =radiale=, and a distal row of five
(carpalia 1-5), each of which supports one of the metacarpals. A ninth
bone, the =centrale= (fig. 40, A, 7), is wedged in between the two
rows. The ulnare, intermedium and pisiform are comparatively large
flattened bones, the others are small and cubical.
The =hand=. This is composed of five digits, each of which consists of
a metacarpal and of a varying number of phalanges.
The =metacarpals=. The first metacarpal (fig. 40, A, 11) is a short
flattened bone, the others are all elongated and cylindrical, and are
terminated proximally by slightly concave surfaces, and distally by
slightly convex ones.
The =phalanges=. The first and fifth digits both have two phalanges,
the second, third, and fourth have each three. The distal phalanx of
the first digit is stout and curved, and bears a horny claw; those of
the other digits are flattened and more or less pointed.
[Illustration FIG. 40. A. ANTERIOR LIMB OF A YOUNG HAWKSBILL TURTLE
(_Chelone imbricata_) × 1/4 (Brit. Mus.). B. POSTERIOR LIMB OF A LARGE
GREEN TURTLE (_Chelone midas_) × 1/8 (Camb. Mus.).
1. humerus.
2. radius (almost hidden by the ulna).
3. ulna.
4. radiale.
5. intermedium.
6. ulnare.
7. centrale.
8. carpale I.
9. carpale IV.
10. pisiform.
11. first metacarpal.
12. femur.
13. tibia.
14. fibula.
15. tibiale intermedium and centrale fused.
16. fibulare.
17. tarsale 1.
18. tarsale 2.
19. tarsalia 4 and 5 fused.
20. first metatarsal.
21. fifth metatarsal.
I, II, III, IV, V, digits.]
THE PELVIC GIRDLE.
The pelvic girdle consists of three bones; a dorsal bone, the =ilium=,
an anterior ventral bone, the =pubis=, and a posterior ventral bone,
the =ischium=. All three bones contribute largely to the formation of
the =acetabulum=, with which the head of the femur articulates.
The =ilium= is a small slightly curved bone, which unites ventrally
with the pubis and ischium, and extends dorsalwards and backwards to
meet the distal ends of the sacral ribs.
The =pubis= is the largest bone of the three; its distal end forms a
wide bilobed plate, the inner lobe meeting its fellow in a median
symphysis, while the other lobe or lateral process extends outwards.
Attached to the symphysis in front is a cartilaginous =epipubis=,
while behind, the two pubes are terminated by a wide rounded
cartilaginous area.
The =ischium=, the smallest bone of the three, is flattened and like
the pubis meets its fellow in a median symphysis. A narrow band of
cartilage connects the symphysis pubis with the symphysis ischii, and
separates the two =obturator foramina= from one another.
THE POSTERIOR LIMB.
This is divisible into three portions, the =thigh=, the =crus= or
=shin=, and the =pes=.
The =thigh= includes a single bone, the =femur=.
The =femur= (fig. 40, B, 12) is a short thick bone, with a prominent
rounded =head= articulating with the acetabulum. Behind this head is a
deep pit, beyond which is a roughened area corresponding with the
great trochanter of mammals. The distal end is expanded and somewhat
convex.
The bones of the =crus= or =shin= are the =tibia= and =fibula=. These
are both straight rod-like bones with expanded terminations which
closely approach one another, while elsewhere the bones diverge
considerably.
The terminations of all three of the leg bones are formed by
epiphyses.
The =Pes= consists of the =tarsus= or =ankle=, and the =foot=, which
is made up of five digits.
The =tarsus=. The tarsal bones of the Turtle do not retain their
primitive arrangement to such an extent as do the carpals. They are
arranged in a proximal row of two and a distal row of four. Of the
bones in the proximal row the postaxial one is much the smaller and is
the =fibulare=; the larger pre-axial one (fig. 40, B, 15) represents
the =tibiale=, =intermedium=, and =centrale= fused, and articulates
with both tibia and fibula. The first three distal tarsalia are all
small bones and are very similar in size, and each articulates
regularly with the corresponding metatarsal. The fourth bone (fig. 40,
B, 19) is much larger, and represents tarsalia 4 and 5 fused. The
first two distal tarsalia articulate with the pre-axial tarsal of the
proximal row, the third only with its neighbours the second, and the
fused fourth and fifth. The latter articulates with both bones of the
proximal row.
Each =digit= consists of a metatarsal and of a varying number of
phalanges.
The =metatarsals=. The first metatarsal (fig. 40, B, 20) is broad and
flattened, the second, third and fourth, are all elongated bones with
nearly flat terminations formed by small epiphyses. The fifth is large
and flattened, and the articular surface for the phalanx is situated
somewhat laterally.
The =phalanges=. The first digit has two phalanges and is the stoutest
of them all; its distal phalanx is sheathed in a large horny claw. The
other digits, of which the third is the longest, have each three
phalanges. The distal phalanges of the second and third digits are
flattened and pointed and bear small horny claws.
FOOTNOTES:
[89] Another view commonly held is that the neural and costal plates
are respectively formed by the expanded neural arches and ribs.