CHAPTER XV.THE SKELETON OF THE CROCODILE.

Fig.40. A.Anterior limb of a young Hawksbill Turtle(Chelone imbricata) × ¼ (Brit. Mus.). B.Posterior limb of a large Green Turtle(Chelone midas) × 1/8 (Camb. Mus.).1. humerus.12. femur.2. radius (almost hidden by the13. tibia.ulna).14. fibula.3. ulna.15. tibiale intermedium and4. radiale.centrale fused.5. intermedium.16. fibulare.6. ulnare.17. tarsale 1.7. centrale.18. tarsale 2.8. carpale I.19. tarsalia 4 and 5 fused.9. carpale IV.20. first metatarsal.10. pisiform.21. fifth metatarsal.11. first metacarpal.I, II, III, IV, V, digits.

The Pelvic Girdle.

The pelvic girdle consists of three bones; a dorsal bone, theilium, an anterior ventral bone, thepubis, and a posterior ventral bone, theischium. All three bones contribute largely to the formation of theacetabulum, with which the head of the femur articulates.

Theiliumis 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.

Thepubisis 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 cartilaginousepipubis, while behind, the two pubes are terminated by a wide rounded cartilaginous area.

Theischium, 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 twoobturator foraminafrom one another.

The Posterior Limb.

This is divisible into three portions, thethigh, thecrusorshin, and thepes.

Thethighincludes a single bone, thefemur.

Thefemur(fig. 40, B, 12) is a short thick bone, with a prominent roundedheadarticulating 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 thecrusorshinare thetibiaandfibula. These are both straight rod-like bones with expandedterminations which closely approach one another, while elsewhere the bones diverge considerably.

The terminations of all three of the leg bones are formed by epiphyses.

ThePesconsists of thetarsusorankle, and thefoot, which is made up of five digits.

Thetarsus. 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 thefibulare; the larger pre-axial one (fig. 40, B, 15) represents thetibiale,intermedium, andcentralefused, 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.

Eachdigitconsists of a metatarsal and of a varying number of phalanges.

Themetatarsals. 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.

Thephalanges. 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.

The species chosen for description isC. palustris, a form occurring throughout the Oriental region, but the description would apply almost equally well to any of the other species of the genusCrocodilus, 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.Theepidermal exoskeletonis formed of a number of hornyscalesor 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 hornyclawsborne by the first three digits of both manus and pes.

b.Thedermal exoskeleton. This has the form of bonyscuteswhich 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 anteriornuchal shieldwhich lies just behind the head and is formed of six large scutes more or less firmly united together, and (2) a larger posteriordorsal shieldcovering 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.

Theteethare 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 inthe 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.

Fig. 41. First four cervical vertebrae of a Crocodile(C. vulgaris). (Partly aftervon Zittel.)1. pro-atlas.7. tubercular portion of fourth2. lateral portion of atlas.cervical rib.3. odontoid process.8. first cervical rib.4. ventral portion of atlas.9. second cervical rib.5. neural spine of axis.10. convex posterior surface of6. postzygapophysis ofcentrum of fourth vertebra.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 cylindricalcentrumdeeply concave in front and convex behind. From the anterior part of the ventral surface of the centrum arises a shorthypapophysis, and on each side is a facet with which the lower limb (capitulum) of the cervical rib articulates. Theneural archis strongly developed and drawn out dorsally into a longneural spine, in front of which are a pair of upstanding processes bearing the prominent upwardly and inwardly directedprezygapophyses. At the sides and slightly behind the neural spine are a corresponding pair of processes bearing thepostzygapophyses, which look downwards and outwards. At the point where it joins the centrum the neural arch is drawn out into a short blunttransverse processwith 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 oratlasvertebra differs much from any of the others, and consists of four quite detached portions, a ventral arch, with two lateral portions and one dorsal. Theventral 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 thepro-atlas; but as it is a membrane bone it is not properly a vertebral element.

The second oraxis vertebraalso 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 theodontoid 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.

Thethirdof 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) areborne 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.

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.4. sacral rib.2. process bearing5. surface which is united withprezygapophysis.the ilium.3. facet for articulation with the6. concave anterior face ofcapitulum of the rib.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 strongribs(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 aV-shapedchevron boneattached 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 structurepresenting 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.

Theskullis divisible into three parts:—

(1) the cranium, (2) the lower jaw, (3) the hyoid.

Thecraniummay 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.

Fig. 43. Palatal aspect A, of the cranium, B, of the mandible of an Alligator(Caiman latirostris). × 1/3. (Brit. Mus.)1. premaxillae.12. quadratojugal.2. maxillae.13. quadrate.3. palatine.14. dentary.4. pterygoid.15. splenial.5. posterior nares.16. coronoid.6. transpalatine.17. supra-angular.7. posterior palatine vacuity.18. angular.8. anterior palatine vacuity.19. articular.9. basi-occipital.20. lateral temporal fossa.10. opening of median21. openings of vascular canalsEustachian canal.leading into alveolar sinus.11. jugal.

The cartilage and membrane bones of the cranium properwhen taken together can in most vertebrates be seen to be more or less arranged in three rings or segments called respectively theoccipital,parietalandfrontalsegments; in the Crocodile however only the occipital and parietal segments are clearly seen.

Theoccipital segmentconsists of four cartilage bones, three of which together surround theforamen magnum.

The most ventral of these, thebasi-occipital(figs. 43 and 45, 9), forms the single convexoccipital condylefor articulation with the atlas, bounds the base of the foramen magnum, and is continuous laterally with two larger bones, theexoccipitals(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 theinternal auditory meatus(fig. 45, VIII) through which the auditory nerve leaves the cranial cavity and enters the internal ear.

Thesupra-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 theparietal 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 theauditoryandsuspensorial bones.

Thebasisphenoid(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 themedian 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, therostrum, which forms part of theinterorbital septum.On the anterior part of the dorsal surface is a deep pit, thepituitary fossaorsella turcica, at the base of which are a pair of foramina, through which the carotid arteries pass. Dorsolaterally the basisphenoid articulates with thealisphenoids.

Fig. 44. Lateral view of the skull of an Alligator(Caiman latirostris). × 1/3. (Brit. Mus.)1. premaxillae.9. palatine.16. external auditory22. angular.2. maxillae.10. pterygoid.meatus.III, VI, opening for exit3. lachrymal.11. transpalatine.17. frontal.of oculomotor and4. prefrontal.12. quadratojugal.18. supra-angular.abducens nerves.5. jugal.13. exoccipital.19. articular.V, foramen ovale.6. postfrontal.14. basi-occipital.20. dentary.X, pneumogastric foramen.7. squamosal.15. foramen by which carotidXII, hypoglossal foramen.8. quadrate.artery enters skull.21. coronoid.

Thealisphenoids(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 almostor 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, theforamen ovale(fig. 44, V), through which the whole of the trigeminal nerve passes out.

The dorsal portion of the parietal segment is formed by theparietal(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, thesupratemporal fossa.

Thefrontal segmentis 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 largefrontal(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.

Eachprefrontal(fig. 45, 14) forms part of the inner wallof the orbit and sends ventralwards a process which meets the palatine.

Thepostfrontals(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 apostorbital barseparating 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.

TheAuditory capsulesand associated bones.

Three bones, theepi-otic,opisthoticandpro-otic, together form the auditory orperioticcapsule 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. Theopisthoticearly becomes united with the exoccipital, while theepi-oticsimilarly becomes united with the supra-occipital, thepro-otic(fig. 45, 7),—seen in longitudinal section to be pierced by the prominenttrigeminal 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 thefenestra ovalis.

Thetympanic cavity, leading to the exterior by theexternal auditory meatus(fig. 44, 16), is well seen in aside-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 thefenestra ovalis, opening ventro-posteriorly theinternal 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, thecolumella, whose outer end articulates by a concave facet with a trifidextra-columellarcartilage 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.

TheOptic capsulesand associated bones.

Two pairs of bones are associated with the optic capsules, viz. thelachrymalsand thesupra-orbitals. Thelachrymal(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. Thesupra-orbitalis a very small loose bone lying in the eyelid close to the junction of the frontal and prefrontal.

TheOlfactory capsulesand associated bones.

Two pairs of membrane bones, thevomersandnasals, are developed in association with the olfactory organ, but themesethmoidis not ossified.

Thevomersform 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.

Thenasals(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 thepremaxillae. Thepremaxillae(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 theanterior 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 smallanterior palatine vacuity(fig. 43, A, 8). They form the anterior part of the broadpalate. 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.

Fig. 45. Longitudinal section through the skull of an Alligator(Caiman latirostris). × 1/3. (Brit. Mus.)1. premaxillae.for the trigeminal nerve.16. maxillae.25. squamosal.2. nasal.8. opisthotic.17. palatine.26. jugal.3. frontal.9. basi-occipital.18. dentary.27. external mandibular4. parietal.10. quadrate.19. splenial.foramen.5. supra-occipital.11. pterygoid.20. angular.28. internal mandibular6. epi-otic.12. basisphenoid.21. supra-angular.foramen.7. pro-otic.13. alisphenoid.22. articular.VIII. internal auditoryimmediately in front of the14. prefrontal.23. coronoid.meatus.figure 7 is the prominent foramen15. vomer.24. exoccipital.XII. hypoglossal foramen.

Themaxillae(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, thenarial 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 thealveolar 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 ovalposterior 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.

Thepalatines(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 shortascending process with the descending process of the prefrontal.

Thepterygoids(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 thepterygoid 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.

Thetranspalatines(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.

Thejugalsormalars(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 thequadratojugal(fig. 44, 12), the two forming theinfratemporal arcadewhich 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 thepostorbital bar, and separates the orbit from the lateral temporal fossa. The quadratojugals are small bones and are united behind with the quadrates.

Thequadrate(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 anelongated 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.

Thesquamosal(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 thesupratemporal arcadewhich constitutes the outer boundary of the supratemporal fossa. It meets also the parietal on its inner side, forming thepost-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 thepost-temporal barformed by the parietal and squamosal, and externally by thesupratemporal arcadeformed by the squamosal and postfrontal. The postfrontal meets the parietal in front and forms the anterior boundary of the supratemporal fossa.

2. TheLateral temporalorinfratemporal 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 theinfratemporal arcadeformed of the quadratojugal and jugal, while in front the fossa is separated from the orbit by thepostorbital barformed by the junction of outgrowths from the postfrontal and jugal.

3. TheOrbits. 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. TheAnterior nares. These form an unpaired opening bounded by the premaxillae.

Posterior surface.

5. TheForamen magnum. The exoccipitals form the chief part of its boundary, but part of the ventral boundary is formed by the basi-occipital.

6. ThePterygoid 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. ThePosterior nares. These form a median unpaired opening (fig. 43, A, 5) bounded by the pterygoids.

8. ThePosterior palatine vacuities. Each is bounded by the maxillae in front, the maxillae and transpalatineexternally, the transpalatine and pterygoid behind, and the palatine on the inner side (fig. 43, A, 7).

9. TheAnterior 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 orrami, 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 thedentary(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 oralveoliin which the teeth are placed. Lying along the inner side of the dentary is a large splint-like bone, thesplenial(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 largeangular(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, theinternal 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, thesupra-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, theexternal 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, thearticular(fig. 45, 22),which in young skulls rather readily becomes detached. The remaining mandibular bone is thecoronoid(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, thebasilingual plateorbody of the hyoid, and a pair ofcornua.

Thebasilingual plate(fig. 53, 1) is rounded anteriorly and marked by a deep notch posteriorly. Thecornua(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 ofthoracic 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 thevertebral ribsthe 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, thecapitulumorhead, articulates with a notch on the side of the transverse process; the other, thetuberculum, 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 uncinateprocess(see p. 190) projects backwards.

Theintermediate ribsare short and imperfectly ossified; they are united with thesternal ribs(fig. 46, 3), which are large,flattened, likewise imperfectly ossified structures, and articulate at their distal ends with a pair of long divergentxiphisternal 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 aTwith 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 theT-shaped ribs and the ordinary thoracic ribs. The anterior limb of theTis 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.

TheSacral ribshave been described in connection with the sacral vertebrae.

The Sternum.

Thesternumof 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 longxiphisternal horns(fig. 46, 5).

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