THE SKELETON OF THE CROCODILE.
The species chosen for description is _C. palustris_, a form occurring
throughout the Oriental region, but the description would apply almost
equally well to any of the other species of the genus _Crocodilus_,
and with comparatively unimportant modifications to any of the living
Crocodilia.
I. EXOSKELETON.
The exoskeleton of the Crocodile is strongly developed and includes
elements of both epidermal and dermal origin.
_a._ The =epidermal exoskeleton= is formed of a number of horny
=scales= or plates of variable size covering the whole surface of the
body. Those covering the dorsal and ventral surfaces are oblong in
shape, and are arranged in regular rows running transversely across
the body. The scales covering the limbs and head are mostly smaller
and less regularly arranged, and are frequently raised into a more or
less obvious keel. Those covering the dorsal surface of the tail are
very prominently keeled.
The epidermal exoskeleton also includes the horny =claws= borne by the
first three digits of both manus and pes.
_b._ The =dermal exoskeleton=. This has the form of bony =scutes=
which underlie the epidermal scales along the dorsal surface of the
trunk and anterior part of the tail. Except in very young individuals
the epidermal scales are rubbed off from these scutes, which
consequently come to project freely on the surface of the body. Each
scute is a nearly square bony plate, deeply pitted or sculptured, and
marked by a strong ridge on its dorsal surface, while its ventral
surface is smooth. Contiguous scutes are united to one another by
interlocking sutures.
The scutes are arranged in two distinct areas, viz. (1) a small
anterior =nuchal shield= which lies just behind the head and is formed
of six large scutes more or less firmly united together, and (2) a
larger posterior =dorsal shield= covering the whole of the back and
anterior part of the tail, and formed of smaller scutes, which are
arranged in regular transverse rows, and progressively diminish in
size when followed back.
The =teeth= are exoskeletal structures, partly of dermal, partly of
epidermal origin. They lie along the margins of the jaws and are
confined to the premaxillae, maxillae and dentaries. They are simple
conical structures, without roots; each is in the adult placed in a
separate socket, and is replaced by another which as it grows comes to
occupy the pulp cavity of its predecessor. In the young animal the
teeth are not placed in separate sockets but in a continuous groove.
This feature is met with also in the Ichthyosauria. The groove
gradually becomes converted into a series of sockets by the ingrowth
of transverse bars of bone. The anterior teeth are sharply pointed and
slightly recurved, the posterior ones are more blunt.
The upper jaw bears about nineteen pairs of teeth, the lower jaw about
fifteen pairs. The largest tooth in the upper jaw is the tenth, and in
the lower jaw the fourth.
The three living families of Crocodilia, the Crocodiles, Alligators
and Garials, can be readily distinguished by the characters of the
first and fourth lower teeth. In Alligators both first and fourth
lower teeth bite into pits in the upper jaw; in Garials they both bite
into notches or grooves in the upper jaw. In Crocodiles the first
tooth bites into a pit, the fourth into a notch in the upper jaw.
II. ENDOSKELETON.
1. THE AXIAL SKELETON.
This includes the vertebral column, the skull, and the ribs and
sternum.
A. THE VERTEBRAL COLUMN.
The vertebral column is very long, consisting of some sixty vertebrae.
It can be divided into the usual five regions, the cervical, thoracic,
lumbar, sacral, and caudal regions.
[Illustration FIG. 41. FIRST FOUR CERVICAL VERTEBRAE OF A CROCODILE
(_C. vulgaris_). (Partly after VON ZITTEL.)
1. pro-atlas.
2. lateral portion of atlas.
3. odontoid process.
4. ventral portion of atlas.
5. neural spine of axis.
6. postzygapophysis of fourth vertebra.
7. tubercular portion of fourth cervical rib.
8. first cervical rib.
9. second cervical rib.
10. convex posterior surface of centrum of fourth vertebra.]
THE CERVICAL VERTEBRAE.
Counting as cervical all those vertebrae which are anterior to the
first one whose ribs meet the sternum, there are nine cervical
vertebrae, all of which bear ribs.
As a type of the cervical vertebrae the fifth may be taken. It has a
short cylindrical =centrum= deeply concave in front and convex behind.
From the anterior part of the ventral surface of the centrum arises a
short =hypapophysis=, and on each side is a facet with which the lower
limb (=capitulum=) of the cervical rib articulates. The =neural arch=
is strongly developed and drawn out dorsally into a long =neural
spine=, in front of which are a pair of upstanding processes bearing
the prominent upwardly and inwardly directed =prezygapophyses=. At the
sides and slightly behind the neural spine are a corresponding pair of
processes bearing the =postzygapophyses=, which look downwards and
outwards. At the point where it joins the centrum the neural arch is
drawn out into a short blunt =transverse process= with which the upper
limb (=tuberculum=) of the cervical rib articulates. The sides of the
neural arch are slightly notched behind for the exit of the spinal
nerves.
The first or =atlas= vertebra differs much from any of the others, and
consists of four quite detached portions, a ventral arch, with two
lateral portions and one dorsal. The =ventral arch= (fig. 41, 4) is
flat below and slightly concave in front, forming together with two
flattened surfaces on the lateral portions a large articulating
surface for the occipital condyle of the skull. Its posterior face is
bevelled off and forms with a second pair of facets on the lateral
portions a surface with which the odontoid process of the second
vertebra articulates. The postero-lateral surfaces of the ventral arch
also bear a pair of little facets with which the cervical ribs
articulate. The lateral portions are somewhat flattened and expanded,
and bear in addition to those previously mentioned a pair of small
downwardly directed facets, the postzygapophyses, which articulate
with the prezygapophyses of the second vertebra. The dorsal portion
(fig. 41, 1) is somewhat triangular in shape, and overhangs the
occipital condyle. It is often regarded as the neural arch of a
vertebra in front of the atlas and is called the _pro-atlas_; but as
it is a membrane bone it is not properly a vertebral element.
The second or =axis vertebra= also differs a good deal from the other
cervicals. The centrum is massive, and is terminated in front by a
very large slightly concave articulating surface formed by the
=odontoid process= (fig. 41, 3) which is united with the centrum by
suture only, and is really the detached centrum of the first vertebra.
The cervical rib (fig. 41, 9) articulates with two little
irregularities on the odontoid process. The posterior surface of the
centrum is convex. The neural arch is strongly developed and
terminated dorsally by a long neural spine (fig. 41, 5), its sides are
notched, slightly in front and more prominently behind for the exit of
the spinal nerves. It is drawn out in front into two little processes
bearing a pair of upwardly and outwardly directed prezygapophyses,
while the postzygapophyses are similar to those of the other cervical
vertebrae.
The last two cervical vertebrae resemble the succeeding thoracic
vertebrae, in the increased length of the transverse processes, and
the shifting dorsalwards of the facet with which the capitulum of the
rib articulates.
THE THORACIC VERTEBRAE.
The thoracic vertebrae commence with the first of those that bears
ribs reaching the sternum. They are ten in number, and the first eight
are directly connected with the sternum by ribs.
The =third= of them may be taken as a type. It has a thick cylindrical
centrum, concave in front and convex behind, there is a slight
hypapophysis, and the centrum is suturally united with a strong neural
arch enclosing a narrow neural canal. The neural arch is drawn out
dorsally into a wide truncated neural spine, and laterally into two
prominent transverse processes, with the ends of which the tubercula
of the ribs articulate, while the capitulum articulates in each case
with a step-like facet (fig. 42, A, 3) on the anterior face of the
transverse process. The prezygapophyses (fig. 42, A, 2) are borne on
outgrowths from the bases of the transverse processes, and the
postzygapophyses on outgrowths at the base of the neural spine.
The thoracic vertebrae behind the third have no hypapophyses, and the
capitular facets gradually come to be placed nearer and nearer the
ends of the transverse processes, at the same time becoming less
prominent; otherwise these vertebrae are just like the third.
[Illustration FIG. 42. ANTERIOR VIEW OF A, A LATE THORACIC AND B, THE
FIRST SACRAL VERTEBRA OF A YOUNG CROCODILE (_C. palustris_). × 1/3.
1. neural spine.
2. process bearing prezygapophysis.
3. facet for articulation with the capitulum of the rib.
4. sacral rib.
5. surface which is united with the ilium.
6. concave anterior face of centrum.]
In the first and second thoracic vertebrae the capitulum of the rib
articulates, not with a facet on the transverse process, but with a
little elevation borne at the line of junction of the centrum and
neural arch.
THE LUMBAR VERTEBRAE.
These are five in number, and are precisely like the posterior
thoracic vertebrae, except in the fact that the transverse processes
have no facets for the articulation of ribs.
THE SACRAL VERTEBRAE.
These are two in number, and while the centrum of the first is concave
in front (fig. 42, B, 6) and nearly flat behind, that of the second is
flat in front and concave behind. Each has a pair of strong =ribs=
(fig. 42, B, 4) firmly ankylosed in the adult with a wide surface
furnished partly by the centrum, partly by the neural arch. The distal
ends of these ribs are united with the ilia. The character of the
neural spines and zygapophyses is the same as in the thoracic
vertebrae.
THE CAUDAL VERTEBRAE.
These are very numerous, about thirty-four in number. The first
differs from all the other vertebrae of the body in having a biconvex
centrum. The succeeding ones are procoelous and are very much like the
posterior thoracic and lumbar vertebrae, having high neural spines and
prominent straight transverse processes. They differ however in having
the neural spines less strongly truncated above, and the transverse
processes arise from the centra and not from the neural arches. When
followed further back the centra and neural spines gradually lengthen
while the transverse processes become reduced, and after the twelfth
vertebra disappear. Further back still the neural spines and
zygapophyses gradually become reduced and disappear, as finally the
neural arch does also, so that the last few vertebrae consist simply
of cylindrical centra.
Each caudal vertebra, except the first and the last eleven or so, has
a =V=-shaped =chevron bone= attached to the postero-ventral edge of
its centrum. The anterior ones are the largest and they gradually
decrease in size till they disappear.
B. THE SKULL[90].
The skull of the Crocodile is a massive depressed structure
presenting a number of striking characteristics, some of the more
important of which are:--
1. All the bones except the mandible, hyoid, and columella are firmly
united by interlocking sutures. In spite of this, however, growth of
the whole skull and of the component bones goes on continuously
throughout life, this growth being especially marked in the case of
the facial as opposed to the cranial part of the skull.
2. All the bones appearing on the dorsal surface are remarkable for
their curious roughened and pitted character; this feature is
prominent also in many Labyrinthodonts.
3. The size of the jaws and teeth is very great.
4. The mandibular condyle is carried back to some distance behind the
occipital condyle.
5. The occipital plane (see p. 386) of the skull is vertical.
6. The length of the secondary palate is remarkably great, and the
vomer takes no part in its formation.
7. The posterior nares are placed very far back, the nasal passages
being as in mammals separated from the mouth by the long secondary
palate.
8. There is a complicated system of Eustachian passages communicating
at one end with the tympanic cavity and at the other end with the
mouth cavity.
9. The interorbital septum is mainly cartilaginous, the presphenoidal
and orbitosphenoidal regions remaining unossified.
The =skull= is divisible into three parts:--
(1) the cranium, (2) the lower jaw, (3) the hyoid.
The =cranium= may again for purposes of description be divided into:--
1. the cranium proper or brain case;
2. the bones connected with the several special sense organs;
3. the bones of the upper jaw, and suspensorial apparatus.
1. THE CRANIUM PROPER OR BRAIN CASE.
[Illustration FIG. 43. PALATAL ASPECT A, OF THE CRANIUM, B, OF THE
MANDIBLE OF AN ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)
1. premaxillae.
2. maxillae.
3. palatine.
4. pterygoid.
5. posterior nares.
6. transpalatine.
7. posterior palatine vacuity.
8. anterior palatine vacuity.
9. basi-occipital.
10. opening of median Eustachian canal.
11. jugal.
12. quadratojugal.
13. quadrate.
14. dentary.
15. splenial.
16. coronoid.
17. supra-angular.
18. angular.
19. articular.
20. lateral temporal fossa.
21. openings of vascular canals leading into alveolar sinus.]
The cartilage and membrane bones of the cranium proper when taken
together can in most vertebrates be seen to be more or less arranged
in three rings or segments called respectively the =occipital=,
=parietal= and =frontal= segments; in the Crocodile however only the
occipital and parietal segments are clearly seen.
The =occipital segment= consists of four cartilage bones, three of
which together surround the =foramen magnum=.
The most ventral of these, the =basi-occipital= (figs. 43 and 45, 9),
forms the single convex =occipital condyle= for articulation with the
atlas, bounds the base of the foramen magnum, and is continuous
laterally with two larger bones, the =exoccipitals= (fig. 45, 24),
which meet one another dorsally and form the remainder of the boundary
of the foramen magnum. Each is drawn out externally into a strong
process, which is united below with the quadrate, and above with the
squamosal by a surface seen in a disarticulated skull to be very rough
and splintered. In a longitudinal section the anterior face of the
exoccipital is seen to be closely united with the opisthotic.
The exoccipital is pierced by a number of foramina, four lying on the
posterior surface. Just external to the foramen magnum is a small
foramen for the exit of the hypoglossal nerve (figs. 44 and 45, XII).
External to this is the foramen for the pneumogastric (fig. 44, X),
while more ventrally still is the foramen (fig. 44, 15) through which
the internal carotid artery enters the skull. Some distance further to
the side, and more dorsally, is a larger foramen which gives passage
to the facial nerve and certain blood-vessels.
In a median longitudinal section of the skull the hypoglossal foramen
is seen, and just in front of it a small foramen for a vein. Further
forwards the long slit-like opening between the exoccipital and
opisthotic is the =internal auditory meatus= (fig. 45, VIII) through
which the auditory nerve leaves the cranial cavity and enters the
internal ear.
The =supra-occipital= (fig. 45, 5) is a small bone which takes no part
in the formation of the foramen magnum, and is closely united in front
with the epi-otic. It is characteristic of Crocodiles that all the
bones of the occipital segment have their longer axes placed
vertically, and that they scarcely if at all appear on the dorsal
surface.
In front of the occipital segment is the =parietal segment=. The
dorsal and ventral portions of the two segments are in contact with
one another, but the lateral portions are widely separated by the
interposition of the =auditory= and =suspensorial bones=.
The =basisphenoid= (fig. 45, 12) is an unpaired wedge-shaped bone,
united along a deep vertical suture with the basi-occipital. The two
bones are, however, partially separated in the mid-ventral line by a
foramen, the opening of the =median Eustachian canal=, which leads
into a complicated system of Eustachian passages ultimately
communicating with the tympanic cavity.
The dorsal surface of the basisphenoid is well seen in a section of
the skull, but owing to the way it tapers ventrally, it appears on the
ventral surface only as a very narrow strip of bone wedged in between
the basi-occipital and pterygoids. In a lateral view it is seen to be
drawn out in front into an abruptly truncated process, the =rostrum=,
which forms part of the =interorbital septum.= On the anterior part of
the dorsal surface is a deep pit, the =pituitary fossa= or =sella
turcica=, at the base of which are a pair of foramina, through which
the carotid arteries pass. Dorsolaterally the basisphenoid articulates
with the =alisphenoids=.
[Illustration FIG. 44. LATERAL VIEW OF THE SKULL OF AN ALLIGATOR
(_Caiman latirostris_). × 1/3. (Brit. Mus.)
1. premaxillae.
2. maxillae.
3. lachrymal.
4. prefrontal.
5. jugal.
6. postfrontal.
7. squamosal.
8. quadrate.
9. palatine.
10. pterygoid.
11. transpalatine.
12. quadratojugal.
13. exoccipital.
14. basi-occipital.
15. foramen by which carotid artery enters skull.
16. external auditory meatus.
17. frontal.
18. supra-angular.
19. articular.
20. dentary.
21. coronoid.
22. angular.
III, VI, opening for exit of oculomotor and abducens nerves.
V, foramen ovale.
X, pneumogastric foramen.
XII, hypoglossal foramen.]
The =alisphenoids= (fig. 45, 13) are a pair of irregular bones which
arise from the basisphenoid antero-laterally, and are united dorsally
with the parietal, frontal, and postfrontals. They bound most of the
anterior part of the brain case, and each presents on its inner face a
deep concavity which lodges the cerebral hemisphere of its side.
Viewed from the ventral side the two alisphenoids are seen to almost
or quite meet one another immediately below the frontal, and then to
diverge, forming an irregular opening--partially closed by cartilage
in the fresh specimen,--through which the optic nerves leave the
cranial cavity. Further back the alisphenoids meet one another for a
narrow area, and then diverge again, so that between each and the
rostrum of the basisphenoid there appears an opening (fig. 44, III,
VI) through which the oculomotor and abducens nerves leave the
cranium. Further back still each is united for a short space with the
basisphenoid, pterygoid and quadrate, and then becomes separated from
the quadrate by a large foramen, the =foramen ovale= (fig. 44, V),
through which the whole of the trigeminal nerve passes out.
The dorsal portion of the parietal segment is formed by the _parietal_
(fig. 45, 4), which though double in the embryo, early comes to form a
single bone. It extends over the posterior part of the cranial cavity,
and is continuous in front with the frontal, behind with the
supra-occipital, and laterally with the postfrontals, squamosals,
alisphenoids, pro-otics and epi-otics. It forms the inner boundary of
a large rounded vacuity on the roof of the skull, the =supratemporal
fossa=.
The =frontal segment= is very imperfectly ossified, there being no
certain representatives of either the ventral member, the presphenoid,
or the lateral members, the orbitosphenoids. On the dorsal side there
is, however, a large development of membrane bones. There is a large
_frontal_ (fig. 45, 3), unpaired, except in the embryo, united behind
with the parietal and postfrontal, and drawn out in front into a long
process which is overlapped by the prefrontals and posterior part of
the nasals. The frontal ends off freely below, owing to the
orbitosphenoidal region being unossified, it forms a considerable part
of the roof of the cranial cavity, but takes no part in the formation
of the wall.
Each _prefrontal_ (fig. 45, 14) forms part of the inner wall of the
orbit and sends ventralwards a process which meets the palatine.
The _postfrontals_ (fig. 44, 6) are small bones lying at the sides of
the posterior part of the frontal. Each is united with a number of
bones, on its inner side with the frontal and parietal, behind with
the squamosal, and ventrally with the alisphenoid. It also unites by
means of a strong descending process with an upgrowth from the jugal,
and thus forms a =postorbital bar= separating the orbit from the
lateral temporal fossa. The postfrontal forms also part of the outer
boundary of the supratemporal fossa.
2. THE SENSE CAPSULES.
Skeletal capsules occur in connection with each of the three special
sense organs of sight, of hearing and of smell.
The =Auditory capsules= and associated bones.
Three bones, the =epi-otic=, =opisthotic= and =pro-otic=, together
form the auditory or =periotic= capsule of each side. They are wedged
in between the lateral portions of the occipital and parietal segments
and complete the cranial wall in this region. Their relations to the
surrounding structures are very complicated, and many points can be
made out only in sections of the skull passing right through the
periotic capsule. The relative position of the three bones is,
however, well seen in a median longitudinal section. The =opisthotic=
early becomes united with the exoccipital, while the =epi-otic=
similarly becomes united with the supra-occipital, the =pro-otic=
(fig. 45, 7),--seen in longitudinal section to be pierced by the
prominent =trigeminal foramen=--alone remaining distinct throughout
life. The three bones together surround the essential organ of hearing
which communicates laterally with the deep tympanic cavity by the
=fenestra ovalis=.
The =tympanic cavity=, leading to the exterior by the =external
auditory meatus= (fig. 44, 16), is well seen in a side-view of the
skull; it is bounded on its inner side by the periotic bones,
posteriorly in part by the exoccipital, and elsewhere mainly by the
quadrate. A large number of canals and passages open into it. On its
inner side opening ventro-anteriorly is the =fenestra ovalis=, opening
ventro-posteriorly the =internal auditory meatus= (fig. 45, VIII),
while dorsally there is a wide opening which forms a communication
through the roof of the brain-case with the tympanic cavity of the
other side. On its posterior wall is the prominent foramen through
which the facial nerve passes on its way to its final exit from the
skull through the exoccipital, this foramen is bounded by the
quadrate, squamosal, and exoccipital.
The opening of the fenestra ovalis is in the fresh skull occupied by
the expanded end of the auditory ossicle, the =columella=, whose outer
end articulates by a concave facet with a trifid =extra-columellar=
cartilage which reaches the tympanic membrane. The lower process of
this extra-columella passes into a cartilaginous rod which lies in a
canal in the quadrate and is during life continuous with Meckel's
cartilage within the articular bone of the mandible.
The columella and extra-columella are together homologous with the
chain of mammalian auditory ossicles.
* * * * *
The =Optic capsules= and associated bones.
Two pairs of bones are associated with the optic capsules, viz. the
_lachrymals_ and the _supra-orbitals_. The _lachrymal_ (fig. 44, 3) is
a fairly large flattened bone lying wedged in between the maxillae,
nasal, jugal, and prefrontal. It forms a considerable part of the
anterior boundary of the orbit, and is pierced by two foramina. On the
orbital edge is a large hole leading into a cavity within the bone
which lodges the naso-lachrymal sac, and communicates with the narial
passage by a wide second foramen near the anterior end of the bone.
The _supra-orbital_ is a very small loose bone lying in the eyelid
close to the junction of the frontal and prefrontal.
* * * * *
The =Olfactory capsules= and associated bones.
Two pairs of membrane bones, the _vomers_ and _nasals_, are developed
in association with the olfactory organ, but the =mesethmoid= is not
ossified.
The _vomers_ form a pair of delicate bones, each consisting of a
vertical plate (fig. 45, 15), which with its fellow separates the two
narial passages, and of a horizontal plate which forms much of their
roof. The vomers articulate with one another and with the pterygoids,
palatines, and maxillae.
The _nasals_ (fig. 45, 2) are very long narrow bones extending along
the middle line from the frontal almost to the anterior nares. They
are continuous laterally with the premaxillae, maxillae, lachrymals and
prefrontals. They form the roof of the narial passages.
3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.
These are enormously developed in the Crocodile and are firmly united
to the cranium. It will be most convenient to begin by describing the
bones at the anterior end of the jaw and to work back thence towards
the brain-case. The most anterior bones are the _premaxillae_. The
_premaxillae_ (figs. 44 and 45, 1) are small bones, each bearing five
pairs of teeth, set in separate sockets in their alveolar borders.
They constitute almost the whole of the boundary of the =anterior
nares=, which are confluent with one another and form a large
semicircular opening in the roof of the skull, leading into the wide
narial passage. They are also partially separated from one another in
the ventral middle line, by the small =anterior palatine vacuity=
(fig. 43, A, 8). They form the anterior part of the broad =palate=.
The alveolar border on each side between certain of the teeth is
marked by pits which receive the points of the teeth of the other jaw.
The first pair of these pits in the premaxillae are often so deep as to
be converted into perforations. Pits of the same character occur
between the maxillary and mandibular teeth.
[Illustration FIG. 45. LONGITUDINAL SECTION THROUGH THE SKULL OF AN
ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)
1. premaxilla.
2. nasal.
3. frontal.
4. parietal.
5. supra-occipital.
6. epi-otic.
7. pro-otic.
immediately in front of the figure 7 is the prominent foramen
for the trigeminal nerve.
8. opisthotic.
9. basi-occipital.
10. quadrate.
11. pterygoid.
12. basisphenoid.
13. alisphenoid.
14. prefrontal.
15. vomer.
16. maxilla.
17. palatine.
18. dentary.
19. splenial.
20. angular.
21. supra-angular.
22. articular.
23. coronoid.
24. exoccipital.
25. squamosal.
26. jugal.
27. external mandibular foramen.
28. internal mandibular foramen.
VIII. internal auditory meatus.
XII. hypoglossal foramen.]
The _maxillae_ (figs. 43, A, 2 and 44, 2) are a pair of very large
bones and bear the remaining teeth of the upper jaw, set in sockets
along their alveolar borders. On the dorsal side each maxillae is
continuous with the premaxillae, nasal, lachrymal, and jugal, while
ventrally it meets its fellow in a long straight suture and forms the
greater part of the long bony palate. The maxillae are separated in the
middle line posteriorly by processes from the palatines, while further
back they meet the transpalatines. The internal or nasal surface, like
that of the premaxillae, is excavated by a deep longitudinal groove,
the =narial passage=. In a ventral view of the skull a number of small
openings (fig. 43, A, 21) are seen close to the alveolar border, these
are the openings of small vascular canals which lead into the
=alveolar sinus=, a passage traversing the maxillae, and transmitting
the superior maxillary branch of the trigeminal nerve and certain
blood-vessels. This alveolar sinus opens posteriorly by the more
external of the two large holes in the maxillae, which lie close to the
anterior edge of the posterior palatine vacuity, to be described
immediately. The more internal of these holes, on the other hand,
leads into a cavity lodging the nasal sac. Behind the maxillae the
completeness of the palate is broken up by the large oval =posterior
palatine vacuities= (fig. 43, A, 7); these are separated from one
another in the middle line by the palatines, and are bounded elsewhere
by the maxillae, transpalatines, and pterygoids.
The _palatines_ (fig. 43, A, 3) are long and rather narrow bones
interposed between the maxillae in front and pterygoids behind. They
meet one another in a long suture and form much of the posterior part
of the palate, while the whole length of their dorsal surface
contributes to the floor of the narial passage. The dorsal surface of
each bone is also drawn out on its outer side into a prominent ridge
which forms much of the side and roof of the narial passage, being in
contact with the vomer and pterygoid, and at one point by means of a
short ascending process with the descending process of the
prefrontal.
The _pterygoids_ (figs. 43, A, 4, and 45, 11) are a pair of large
bones, each consisting of a median more or less vertical part, which
becomes ankylosed to its fellow in the middle line early in life, and
of a wide horizontal part which meets the transpalatine. They
completely surround the posterior nares (fig. 43, A, 5) and their
median portions form the whole boundary of the posterior part of the
narial passage, and assist the palatines and vomers in bounding the
middle part. The horizontal parts form the posterior part of the
secondary palate, while the dorsal surface of each looks into the
=pterygoid fossa=, a large cavity lying below the quadrate and
quadratojugal at the side of the skull. The lateral margin adjoining
the transpalatine is in the fresh skull terminated by a plate of
cartilage against which the mandible plays. Dorsally the pterygoid
articulates with the basisphenoid, quadrate, and alisphenoid.
The _transpalatines_ (fig. 44, 11) connect the pterygoids with the
jugals and maxillae, articulating with each of the three bones by a
long pointed process. The jugal process meets also a down-growth from
the postfrontal.
The _jugals_ or _malars_ (fig. 44, 5) are long somewhat flattened
bones which are united to the lachrymals and maxillae in front, while
passing backwards each is united behind to the _quadratojugal_ (fig.
44, 12), the two forming the =infratemporal arcade= which constitutes
the external boundary of the orbit and lateral temporal fossa. The
jugal is united below to the transpalatine, and the two bones together
form an outgrowth, which meeting that from the postfrontal forms the
=postorbital bar=, and separates the orbit from the lateral temporal
fossa. The quadratojugals are small bones and are united behind with
the quadrates.
The =quadrate= (figs. 43, A, 13 and 44, 8) of each side is a large
somewhat flattened bone firmly fixed in among the other bones of the
skull. It is terminated posteriorly by an elongated slightly convex
surface, coated with cartilage in the fresh skull, by which the
mandible articulates with the cranium. The dorsal surface of the
quadrate is flat behind, further forwards it becomes much roughened
and articulates with the exoccipital and squamosal; further forwards
still it becomes marked by a deep groove which forms the floor of the
external auditory meatus and part of the tympanic cavity. The anterior
boundary of the quadrate is extremely irregular, it is united dorsally
with the postfrontal, pro-otic, and squamosal, and more ventrally with
the alisphenoid. The smooth ventral surface looks into the pterygoid
fossa. In front the quadrate forms the posterior boundary of the
supratemporal fossa and foramen ovale, and is continuous with the
alisphenoid, while it sends down a thin plate meeting the pterygoid
and basisphenoid. On the inner side of the dorsal surface of the
quadrate near the condyle, is a small foramen which leads into a tube
communicating with the tympanic cavity, by a foramen lying in front of
and ventral to that for the exit of the facial nerve. By this tube air
can pass from the tympanic cavity into the articular bone of the
mandible.
The _squamosal_ (fig. 44, 7) meets the quadrate and exoccipital below,
and forms part of the roof of the external auditory meatus, while
above it forms part of the roof of the skull and has a pitted
structure like that of the other bones of the roof. It is continuous
with the postfrontal in front, forming with it the =supratemporal
arcade= which constitutes the outer boundary of the supratemporal
fossa. It meets also the parietal on its inner side, forming the
=post-temporal bar=, the posterior boundary of the supratemporal
fossa.
It may be useful to recapitulate the large vacuities in the surface of
the Crocodile's cranium.
_Dorsal surface._
1. =The Supratemporal fossae=. Each is bounded internally by the
parietal, behind by the =post-temporal bar= formed by the parietal and
squamosal, and externally by the =supratemporal arcade= formed by the
squamosal and postfrontal. The postfrontal meets the parietal in front
and forms the anterior boundary of the supratemporal fossa.
2. The =Lateral temporal= or =infratemporal fossae=. These lie below
and to the outer side of the supratemporal fossae. Each is bounded
dorso-internally by the supratemporal arcade; and behind by a
continuation of the post-temporal bar formed by the quadrate and
quadratojugal. The external boundary is the =infratemporal arcade=
formed of the quadratojugal and jugal, while in front the fossa is
separated from the orbit by the =postorbital bar= formed by the
junction of outgrowths from the postfrontal and jugal.
3. The =Orbits=. Each is bounded behind by the postorbital bar,
externally by the jugal forming a continuation of the infratemporal
arcade, in front by the lachrymal, and internally by the frontal and
prefrontal.
4. The =Anterior nares=. These form an unpaired opening bounded by the
premaxillae.
_Posterior surface._
5. The =Foramen magnum=. The exoccipitals form the chief part of its
boundary, but part of the ventral boundary is formed by the
basi-occipital.
6. The =Pterygoid fossae=. These form a pair of large cavities at the
sides of the occipital region of the skull. The dorsal boundary is
formed by the quadrate and quadratojugal, the ventral by the
pterygoid, the internal chiefly by the quadrate, pterygoid,
alisphenoid, and basisphenoid. The transpalatine forms a small part of
the external boundary which is incomplete.
_Ventral surface._
7. The =Posterior nares=. These form a median unpaired opening (fig.
43, A, 5) bounded by the pterygoids.
8. The =Posterior palatine vacuities=. Each is bounded by the maxillae
in front, the maxillae and transpalatine externally, the transpalatine
and pterygoid behind, and the palatine on the inner side (fig. 43, A,
7).
9. The =Anterior palatine vacuity=. This is unpaired and is bounded by
the premaxillae (fig. 43, A, 8).
(_b_) THE LOWER JAW OR MANDIBLE.
The mandible is a strong compact bony structure formed of two halves
or =rami=, which are suturally united at the symphysis in the middle
line in front. Each ramus is formed of six separate bones.
The most anterior and largest of these is the _dentary_ (figs. 44, 20,
and 45, 18), which forms the symphysis, and greater part of the
anterior half of the jaw, and bears along the outer part of its dorsal
border a number of sockets or =alveoli= in which the teeth are placed.
Lying along the inner side of the dentary is a large splint-like bone,
the _splenial_ (fig. 45, 19), which does not extend so far forwards as
the symphysis, and is separated from the dentary posteriorly by a
large cavity. Forming the lower part of all the posterior half of the
jaw is the large _angular_ (figs. 44, 22, and 45, 20), which underlies
the posterior part of the dentary in front and sends a long process
below that bone to the splenial. On the inner side of the jaw there is
an oval vacuity, the =internal mandibular foramen= (fig. 45, 28),
between the angular and the splenial; through this pass blood-vessels
and branches of the inferior dental nerve. Lying dorsal to the angular
is another large bone, the _supra-angular_ (figs. 44, 18, and 45, 21).
It extends back as far as the posterior end of the jaw and forwards
for some distance dorsal to the dentary and splenial. It forms part of
the posterior margin of a large vacuity, the =external mandibular
foramen=, which is bordered above and in front by the dentary and
below by the angular; it gives passage to the cutaneous branch of the
inferior dental nerve. The concave surface for articulation with the
mandible and much of the posterior end of the jaw is formed by a short
but solid bone, the =articular= (fig. 45, 22), which in young skulls
rather readily becomes detached. The remaining mandibular bone is the
_coronoid_ (fig. 45, 23), a very small bone of irregular shape
attached to the angular below, and to the supra-angular and splenial
above.
(_c_) THE HYOID.
The hyoid of the Crocodile consists of a wide flattened plate of
cartilage, the =basilingual plate= or =body of the hyoid=, and a pair
of =cornua=.
The =basilingual plate= (fig. 53, 1) is rounded anteriorly and marked
by a deep notch posteriorly. The =cornua= (fig. 53, 3), which are
attached at a pair of notches near the middle of the outer border of
the basilingual plate, are partly ossified, but their expanded ends
are formed of cartilage. They pass at first backwards and then upwards
and inwards. They are homologous with part of the first branchial
arches of Selachians.
The columella and extra-columella have been already described (p.
251).
C. THE RIBS AND STERNUM.
=Thoracic ribs.=
The Crocodile has ten pairs of =thoracic ribs=, all except the last
one or two of which consist of three parts,--a vertebral rib, an
intermediate rib and a sternal rib.
Of the =vertebral ribs= the third may be taken as a type, it consists
of a curved bony rod which articulates proximally with the transverse
process of the vertebra by two facets. The terminal one of these, the
=capitulum= or =head=, articulates with a notch on the side of the
transverse process; the other, the =tuberculum=, which lies on the
dorsal surface a short distance behind the head, articulates with the
end of the transverse process. From near the distal end an imperfectly
ossified uncinate =process= (see p. 190) projects backwards.
The =intermediate ribs= are short and imperfectly ossified; they are
united with the =sternal ribs= (fig. 46, 3), which are large,
flattened, likewise imperfectly ossified structures, and articulate at
their distal ends with a pair of long divergent =xiphisternal horns=
(fig. 46, 5), which arise from the posterior end of the sternum
proper. The last pair of sternal ribs are attached to the preceding
pair, not to the xiphisternal horns.
The first and second vertebral ribs differ from the others in the fact
that the tuberculum forms a fairly long outstanding process.
=Cervical ribs.=
Movable ribs are attached to all the cervical as well as to the
thoracic vertebrae. Those borne by the atlas and axis are long, narrow
structures attached by a fairly broad base, and tapering gradually.
The ribs borne by the third to seventh cervical vertebrae are shaped
like a =T= with a double base, one limb of which, corresponding to the
tuberculum (fig. 41, 7), articulates with a short transverse process
arising from the neural arch, while the other, corresponding to the
capitulum, articulates with a surface on the centrum. The ribs
attached to the eighth and ninth cervical vertebrae are intermediate
in character between the =T=-shaped ribs and the ordinary thoracic ribs.
The anterior limb of the =T= is shortened, the posterior one is drawn
out, forming the shaft of the rib. The distal portion of the rib of
the ninth cervical vertebra is unossified.
The =Sacral ribs= have been described in connection with the sacral
vertebrae.
THE STERNUM.
The =sternum= of Crocodiles is a very simple structure, consisting of
a plate of cartilage (fig. 46, 2) lying immediately dorsal to the
interclavicle, and drawn out posteriorly into a pair of long
=xiphisternal horns= (fig. 46, 5).
THE ABDOMINAL SPLINT RIBS.
Lying superficially to the recti muscles of the ventral body-wall,
behind the sternal ribs, are seven or eight series of slender curved
bones, the _abdominal ribs_ (fig. 46, 4). Each series consists of four
or more bones, arranged in a =V=-like form with the angle of the =V=
directed forwards. They show a considerable amount of variability in
number and character. They are really membrane bones, and are in no
way homologous with true ribs, but correspond rather with the more
posterior of the bones constituting the plastron of Chelonia.
[Illustration FIG. 46. STERNUM AND ASSOCIATED MEMBRANE BONES OF A
CROCODILE (_C. palustris_) × 1/3. (Brit. Mus.)
The last pair of abdominal ribs which are united with the epipubes by
a plate of cartilage have been omitted.
1. interclavicle.
2. sternum.
3. sternal rib.
4. abdominal splint rib.
5. xiphisternal horn.]
2. THE APPENDICULAR SKELETON.
This includes the skeleton of the two pairs of limbs and their
respective girdles.
[Illustration: FIG. 47. LEFT HALF OF THE PECTORAL GIRDLE OF AN
ALLIGATOR (_Caiman latirostris_) × 2/3. (Brit. Mus.)
1. scapula.
2. coracoid.
3. interclavicle.
4. glenoid cavity.]
THE PECTORAL GIRDLE.
The pectoral girdle of the Crocodile is less complete than is that of
most reptiles. It consists of a dorsal bone, the =scapula=, and a
ventral bone, the =coracoid=, with a median unpaired element, the
_interclavicle_; but there is no separate representative either of the
clavicle or precoracoid.
The =scapula= (fig. 47, 1) is a large bone, flattened and expanded
above where it is terminated by an unossified margin, the
=suprascapula=, and thickened below where it meets the coracoid. The
scapula forms about half the =glenoid cavity= (fig. 47, 4) for
articulation with the humerus, and has the lower part of its anterior
border drawn out into a roughened ridge.
The =coracoid= (fig. 47, 2) is a flattened bone, much expanded at
either end; it bears on its upper posterior border a flattened surface
which forms half the glenoid cavity, and is firmly united to the
scapula at its dorsal end. Its ventral end meets the sternum.
The _interclavicle_ (figs. 46, 1, and 47, 3) is a long narrow
blade-shaped bone lying along the ventral side of the sternum; about a
third of its length projects beyond the sternum in front.
THE ANTERIOR LIMB.
This is as usual divisible into three portions, the upper arm,
fore-arm and manus.
The =upper arm= or =brachium= contains one bone, the =humerus.=
The =humerus= (fig. 48, A, 1) is a fairly long stout bone,
considerably expanded at either end. The proximal end or head is
evenly rounded and is formed by an epiphysis ossifying from a centre
different from that forming the shaft. It articulates with the glenoid
cavity. The shaft bears on the flexor surface, at some little distance
behind the head, a prominent rounded protuberance, the =deltoid
ridge.= The distal end or trochlea is also formed by an epiphysis and
is partially divided by a groove into two convex surfaces; it
articulates with the two bones of the fore-arm, the radius and ulna.
[Illustration FIG. 48. A, RIGHT ANTERIOR, AND B, RIGHT POSTERIOR LIMB
OF A YOUNG ALLIGATOR (_Caiman latirostris_). (Brit. Mus.)
A × 1/2. B × about 1/3.]
1. humerus.
2. radius.
3. ulna.
4. radiale.
5. ulnare.
6. pisiform.
7. patch of cartilage representing carpalia 1 and 2; between it
and the radiale should be another flattened patch, the centrale.
8. carpalia 3, 4, and 5 (fused).
9. first metacarpal.
10. proximal phalanx of second digit.
11. second phalanx of fifth digit.
12. femur.
13. tibia.
14. fibula.
15. tibiale, intermedium and centrale (fused).
16. fibulare.
17. tarsalia 1, 2, and 3 (fused).
18. tarsalia 4 and 5 (fused).
19. first metatarsal.
20. ungual phalanx of second digit.
21. fifth metatarsal.
The =radius= and =ulna= are nearly equal in size and each consists of
a long shaft terminated at either end by an epiphysis.
The =radius= (fig. 48, A, 2) or pre-axial bone is slightly the smaller
of the two. It has a straight cylindrical shaft and is slightly and
nearly evenly expanded at either end. The proximal end which
articulates with the humerus is flat or slightly concave, the distal
end which articulates with the carpus is slightly convex.
The =ulna= (fig. 48, A, 3) or postaxial bone is a curved bone rather
larger than the radius. Its proximal end is large and convex, but is
not drawn out into an olecranon process.
The =Manus= consists of the =carpus= or =wrist=, and the =hand=.
The =Carpus=. This differs considerably from the more primitive type
met with in the Turtle. It consists of six elements arranged in a
proximal row of three and a distal row of two, with one intervening.
The bones of the proximal row are the radiale, the ulnare, and the
pisiform. The =radiale= (fig. 48, A, 4) is the largest bone of the
carpus: it is a somewhat hour-glass shaped bone, with its ends formed
by flattened epiphyses. It articulates by its proximal end with the
whole of the radius, and partly also with the ulna, and by its distal
end with the centrale.
The =ulnare= (fig. 48, A, 5) is a smaller bone, also somewhat
hour-glass shaped; it articulates proximally with the pisiform and
radiale, not quite reaching the ulna. The third bone of the proximal
row is the =pisiform= (fig. 48, A, 6), an irregular bone, articulating
with the ulna, radiale, and fifth metacarpal. The =centrale= is a
flattened cartilaginous element applied to the distal surface of the
radiale.
The distal row of carpals consists of two small structures. The first
of these forms a small cartilaginous patch, which is wedged in between
the first and second metacarpals, the centrale and the bone
representing carpalia 3, 4 and 5; this cartilaginous patch represents
=carpalia 1 and 2= (fig. 48, A, 7). The bone representing =carpalia 3,
4 and 5= is a good deal larger, rounded, and well-ossified; it
articulates with the ulnare, the pisiform, and the third, fourth, and
fifth metacarpals.
The =hand=. Each of the five digits consists of an elongated
metacarpal, terminated at each end by an epiphysis, and of a varying
number of phalanges. The terminal phalanx of each digit has an
epiphysis only at its proximal end, the others have them at both ends.
The first digit, or =pollex=, is the stoutest, and has two phalanges,
the second has three, the third four, the fourth three, and the fifth
two. The terminal phalanx of each of the first three digits is pointed
and sheathed in a horny claw; and is also marked by a pair of
prominent lateral grooves.
THE PELVIC GIRDLE.
The pelvic girdle of the Crocodile consists of four parts, a dorsal
element, the =ilium=, an anterior ventral element, the =pubis=, a
posterior ventral element, the =ischium=, and an accessory anterior
ventral element, the =epipubis=. All except the epipubis take part in
the formation of the =acetabulum=, which is perforated by a prominent
hole.
The =ilium= (fig. 49, 1) is a thick strong bone, firmly united on its
inner side with the two sacral ribs. Its dorsal border is rounded, its
ventral border bears posteriorly two irregular surfaces, completed by
epiphyses, which are united respectively with the ischium and pubis.
The =ischium= (fig. 49, 2)--the largest bone of the pelvis, is
somewhat contracted in the middle and expanded at either end. Its
proximal end, which is formed by an epiphysis, bears two surfaces, one
of which is united to the ilium, while the other forms part of the
acetabulum. The anterior border is also drawn out dorsally into a
strong process, which is terminated by a convex epiphysis, and is
united to the pubis. The ventral end of the ischium forms a flattened
blade, meeting its fellow in a median symphysis.
The =pubis= (fig. 49, 3) is much smaller than either the ilium or
ischium; it forms a small patch of unossified cartilage, interposed
between the anterior parts of the ilium and ischium.
[Illustration FIG. 49. PELVIS AND SACRUM OF AN ALLIGATOR (_Caiman
latirostris_) × 1/2. (Brit. Mus.)
1. ilium.
2. ischium.
3. true pubis.
4. epipubis (so-called pubis).
5. acetabular foramen.
6. neural spines of sacral vertebrae.
7. symphysis ischii.
8. process bearing prezygapophysis.]
The =epipubis= (fig. 49, 4) is a large bone with a thickened proximal
end, which is loosely articulated to the ischium, and a flattened
expanded distal end, which is united with its fellow, and with the
last pair of abdominal ribs by a large plate of cartilage. This bone
is generally described as the pubis.
THE POSTERIOR LIMB.
This is as usual divisible into three portions, the =thigh=, the
=crus= or =shin=, and the =pes=.
The =thigh= is formed by the =femur= (fig. 48, B, 12), a moderately
long stout bone, not unlike the humerus; it articulates with the
acetabulum by a fairly prominent rounded =head=. The distal end
articulating with the tibia and fibula is also expanded, and is
partially divided into equal parts by anterior and posterior grooves.
The flexor surface bears a fairly prominent trochanteric ridge. Each
end of the femur is formed by an epiphysis.
The =crus= or =shin= includes two bones, the =tibia= and =fibula=.
Both are well developed, but the tibia is considerably the larger of
the two.
The =tibia= (fig. 48, B, 13) is a strong bone with a flattened
expanded proximal end articulating with almost the whole of the end of
the femur, and a similarly expanded distal end articulating with a
bone representing the fused astragalus and centrale.
The =fibula= (fig. 48, B, 14) is flattened proximally, and articulates
with only quite a small part of the femur, while distally it is more
expanded, and articulates with the fibulare (calcaneum) and with a
facet on the side of the fused astragalus and centrale.
The =Pes= consists of the =tarsus= or =ankle=, and the =foot=.
The =Tarsus=. This, like the carpus, is much reduced and modified from
the primitive condition. It consists of only four bones, arranged in
two rows of two each. The two bones of the proximal row are much
larger than are those of the distal row. The pre-axial of them (fig.
48, B, 15) representing the fused =astragalus= (tibiale and
intermedium) and =centrale=, articulates proximally with the tibia and
fibula, and distally with the first metatarsal, and a small bone
representing the first three tarsalia. The postaxial bone, the
=calcaneum= (fibulare) (fig. 48, B, 16), is drawn out into a
prominent posterior process forming a heel such as is almost unknown
elsewhere except in mammals. It articulates with the fibula, the
tibiale-centrale, and distally with a bone representing the fourth and
fifth tarsalia, and with the fifth metatarsal.
The two bones forming the distal row of tarsals are both small and
rounded; one represents the first three tarsalia fused together, the
other tarsalia 4 and 5.
The =Foot=. The =foot= has five digits, but the fifth is much reduced,
consisting only of a short metatarsal. The first four =metatarsals=
are all long bones, slightly expanded at each end, and terminated by
small epiphyses. The first digit has two phalanges, the second three,
the third four, and the fourth five. The terminal or =ungual phalanx=
in each instance is grooved and pointed, and in the case of the first
three digits bears a horny claw. The ungual phalanx progressively
decreases in size from the first to the fourth. The fifth digit
consists only of a small, somewhat square metatarsal (fig. 48, B, 21),
attached to the bone representing the fused fourth and fifth
tarsalia.
FOOTNOTES:
[90] Free use has been made of L.C. Miall's _Studies in Comparative
Anatomy_, I., _The Skull in Crocodilia_, London, 1878. See also W.K.
Parker, _Tr. Z.S._, vol. XI. 1885, p. 263.