208

SKULL OF THE CROCODILE.

is the normal or the most constant point of its suspension, the bone, 20, being the pleurapophysial element of the maxillary arch: it is called the "palatine," because the under surface forms a portion of the bony roof of the mouth, called the "palate." It is articulated at its fore part with the bone, 21, in the same plates, which bone is the hæmapophysial element of the maxillary arch: it is called the "maxillary," and is greatly developed both in length and breadth; it is connected not only with 20 behind, and 22 in front, which are parts of the same arch, and with the diverging appendages of the arch, viz., 26, the malar bone, and 24, the pterygoid, but also with the nasals, 15, and the lacrymal, 16, as well as with its fellow of the opposite side of the arch. The smooth, expanded horizontal plate, which effects the latter junction, is called the palatal plate of the maxillary; the thickened external border, where this plate meets the external rough surface of the bone, and which is perforated for the lodgment of the teeth, is the "alveolar border" or "process" of the maxillary. The hæmal spine or key-bone of the arch, 22, is bifid, and the arch is completed by the symphysial junction of the two symmetrical halves; these halves are called "premaxillary bones:" these bones, like the maxillaries, have a rough facial plate, and a smooth palatal plate, with the connecting alveolar border. The median symphysis is perforated vertically through both plates; the outer or upper hole being the external nostril, the under or palatal one being the prepalatal or naso-palatal aperture.

Both the palatine and the maxillary bones send outwards and backwards parts or processes which diverge from the line of the hæmal arch, of which they are the chief elements; and these parts give attachment to distinct bones which form the “diverging appendages" of the arch, and serve to attach it, as do the diverging appendages of the thoracic hæmal arches in the bird, to the succeeding arch.

The appendage, 24, called "pterygoid," effects a more extensive attachment, and is peculiarly developed in the crocodilia. As it extends backwards it expands, unites with its fellow below the nasal canal, and encompassing that canal, coalesces above it with the vomer, and is firmly attached by suture to the presphenoid and basisphenoid : it surrounds the hinder or palatal nostril, and, extending outwards, it gives attachment to a second bone, 25, called " ectopterygoid," which is firmly connected with the maxillary, 25, the malar, 26, and the post-frontal, 12. The second diverging ray is of great strength; it extends from the maxillary, 21 ("hæmapophysis" of the maxillary arch), to the tympanic, 28 (" pleurapophyses" of the mandibular arch), and is divided into two pieces, the malar, 26, and the squamosal, 27. Such are the chief crocodilian modifiations of the hamal arch, and appendages of the anterior or nasal vertebra of the skull.

The hæmal arch of the frontal vertebra is somewhat less metamorphosed, and has no diverging appendage. It is slightly displaced backwards, and is articulated by only a small proportion of its pleurapophysis, 28, to the parapophysis, 12, of its own segment; the major part of that short and strong rib articulating with the parapophysis, 8, of the succeeding segment. The bone, 28, called "tympanic," because it serves to support the "drum of the ear" in air-breathing vertebrates, is short, strong, and immoveably wedged, in the crocodilia, between the paroccipital, 4, mastoid, 8, post-frontal, 12, and squamosal, 27; and the conditions of this fixation of the pleurapophysis are exemplified in the great development of the hæmapophysis (mandible), which is here unusually long, supports numerous teeth, and requires, therefore, a firm point of suspension, in the violent actions to which the jaws are put in retaining and overcoming the struggles of a powerful living prey. The moveable articulation between the pleurapophysis, 28,

SKELETON OF THE CROCODILE.

209

and the rest of the hæmal arch is analogous to that which we find between the thoracic pleurapophysis and hæmapophysis in the ostrich and many other birds. But the hæmopophysis of the mandibular arch in the crocodiles is subdivided into several pieces, in order to combine the greatest elasticity and strength with a not excessive weight of bone. The different pieces of this purposely subdivided element have received definite names. That numbered 29, which offers the articular concavity to the convex condyle of the tympanic, 28, is called the "articular" piece; that beneath it, 30, which developes the angle of the jaw, when this projects, is the "angular" piece; the piece above, 29′, is the "surangular;" the thin, broad, flat piece, 31, applied, like a splint, to the inner side of the other parts of the mandible, is the "splenial;" the small accessory ossicle, 31', is the "coronoid," because it developes the process, so called, in lizards; the anterior piece, 32, which supports the teeth, is called the "dentary." This latter is the homotype of the premaxillary, or it represents that bone in the mandibular arch, of which it may be regarded as the hæmal spine; the other pieces are subdivisions of the hæmapophysial element. The purport of this subdivision of the lower jaw-bone has been well explained by Conybeare * and Buckland,† by the analogy of its structure to that adopted in binding together several parallel plates of elastic wood or steel to make a crossbow, and also in setting together thin plates of steel in the springs of carriages. Dr. Buckland adds-"Those who have witnessed the shock given to the head of a crocodile by the act of snapping together its thin long jaws, must have seen how liable to fracture the lower jaw would be, were it composed of one bone only on each side." The same reasoning applies to the composite structure of the long tympanic pedicle in fishes. In each case the splicing and bracing together of thin flat bones of unequal length and of varying thickness, affords compensation for the weakness and risk of fracture that would otherwise have attended the elongation of the parts. In the abdomen of the crocodile the analogous subdivision of the hamapophyses, there called abdominal ribs, allows of a slight change of their length, in the expansion and contraction of the walls of that cavity; and since amphibious reptiles, when on land, rest the whole weight of the abdomen directly upon the ground, the necessity of the modification for diminished liability to fracture further appears. These analogies are important, as demonstrating that the general homology of the elements of a natural segment of the skeleton is not affected or obscured by their subdivision for a special end. Now this purposive modification of the hæmapophyses of the frontal vertebra is but a repetition of that which affects the same elements in the abdominal vertebræ. Passing next to the hæmal arch of the parietal vertebra, we are first struck by its small relative size. Its restricted functions have not required it to grow in proportion with the other arches, and it consequently retains much of its embryonic dimensions. It consists of a ligamentous "stylohyal," its pleurapophysis retaining the same primitive histological condition which obstructs the ordinary recognition of the same elements of the lumbar hæmal arches. A cartilaginous "epihyal," 39, intervenes between this and the ossified "hæmapophysis," 40, which bears the special name of ceratohyal. The hæmal spine, 41, retains its cartilaginous state, like its homotypes, in the abdomen; there they get the special name of "abdominal sternum," here of "basihyal." The basihyal has, however, coalesced with the thyrohyals to form a broad cartilaginous plate, the anterior border rising like a valve to close the fauces, and the

"Geol. Trans.," 1821, p. 565.

+ "Bridgewater Treatise," 1836, vol. i., p. 176.

ORGANIC NATURE.-No. VII.

210

LIMBS OF THE CROCODILE.

posterior angles extending beyond and sustaining the thyroid and other parts of the larynx. The long bony "ceratchyal" and the commonly cartilaginous "epihyal" are suspended by the ligamentous "stylohyal" to the paroccipital process; the whole arch having, like the mandibular one, retrograded from the connection it presents in fishes.

This retrogradation is still more considerable in the succeeding hæmal arch. In comparing the occipital segment of the crocodile's skeleton with that of the fish, the chief modification that distinguishes that segment in the crocodile is the apparent absence of its hæmal arch. We recognise, however, the special homologues of the constituents of that arch of the fish's skeleton in the bones 51 and 52 of the crocodile's skeleton (Fig. 18); but the upper or suprascapular piece, 50, retains, in connection with the loss of its proximal or cranial articulations, its cartilaginous state; the scapula, 51, is ossified, as is likewise the coracoid, 52, the lower end of which is separated from its fellow by the interposition of a median, symmetrical, partially-ossified piece called "episternum." The power of recognising the special homologies of 50, 51, and 52 in the crocodile, with the similarly-numbered constituents of the same arch in fishes— though masked, not only by modifications of form and proportion, but even of very substance, as in the case of 50-depends upon the circumstance of these bones constituting the same essential element of the archetypal skeleton, viz., the fourth hæmal arch, numbered pl, 52, in Fig. 7 : for although in the present instance there is superadded, to the adaptive modifications above cited, the rarer one of altered connections, Cuvier does not hesitate to give the same names, "suprascapulaire" to 50, and "scapulaire" to 51, in both fish and crocodile; but he did not perceive or admit that the narrower relations of special homology were a result of, and necessarily included in, the wider law of general homology. According to the latter law, we discern in 50 and 51 a compound " pleurapophysis," in 52 a "hæmapophysis," and in hs, the "hæmal spine," completing the hæmal arch.

The scapulo-coracoid arch, both elements, 51, 52, of which retain the form of strong and thick vertebral and sternal ribs in the crocodile, is applied in the skeleton of that animal over the anterior thoracic hæmal arches. Viewed as a more robust hæmal arch, it is obviously out of place in reference to the rest of its vertebral segment. If we seek to determine that segment by the mode in which we restore to their centrums the less displaced neural arches of the antecedent vertebræ of the cranium or in the sacrum of the bird, we proceed to examine the vertebræ before and behind the displaced arch, with the view to discover the one which needs it, in order to be made typically complete. Finding no centrum and neural arch without its pleurapophyses from the scapula to the pelvis, we give up our search in that direction; and in the opposite direction we find no vertebra without its ribs, until we reach the occiput; there we have centrum and neural arch, with coalesced parapophyses, but without the hæmal arch, which arch can only be supplied by a restoration of the bones 50-52 to the place which they naturally occupy in the skeleton of the fish. And since anatomists are generally agreed to regard the bones 50-52 in the crocodile (Fig. 18) as specially homologous with those so numbered in the fish (Fig. 9), we must conclude that they are likewise homologous in a higher sense; that in the fish the scapula-coracoid arch is in its natural or typical position, whereas in the crocodile it has been displaced for a special purpose. Thus, agrecably with a general principle, we perceive that, as the lower

* See "On the Archetype and Homologics of the Vertebrate Skeleton," pp. 117 and 159.

FORE-LIMB OF THE CROCODILE.

211

vertebrate animal illustrates the closer adhesion to the archetype by the natural articulation of the scapulo-coracoid arch to the occiput, so the higher vertebrate manifests the superior influence of the antagonizing power of adaptive modification by the removal of that arch from its proper segment.

The anthropotomist, by his mode of counting and defining the dorsal vertebræ and ribs, admits, unconsciously perhaps, the important principle in general homology which is here exemplified; and which, pursued to its legitimate consequences, and further applied, demonstrates that the scapula is the modified rib of that centrum and neural arch, which he calls the "occipital bone;" and that the change of place which chiefly masks that relation (for a very elementary acquaintance with comparative anatomy shows how little mere form and proportion affect the homological characters of bones), differs only in extent, and not in kind, from the modification which makes a minor amount of comparative observation requisite, in order to determine the relation of the shifted dorsal rib to its proper centrum in the human skeleton.

With reference, therefore, to the occipital vertebra of the crocodile, if the comparatively well-developed and permanently-distinct ribs of all the cervical vertebræ prove the scapular arch to belong to none of those segments, and if that hæmal arch be required to complete the occipital segment, which it actually does complete in fishes, then the same conclusion must apply to the same arch in other animals, up to man himself.

The anterior locomotive extremity is the diverging appendage of the arch, under one of its numerous modes and grades of development. The proximal element of this appendage, or that nearest the arch, is called the "humerus," 53 (Fig. 18). The second segment of the limb consists of two bones; the larger one, 54, is called the "ulna" it articulates with the outer condyle of the humerus by an oval facet, the thick convex border of which swells a little out behind, and forms a kind of rudimental olecranon;" the distal end is much less than the proximal one, and is most produced at the radial side.

66

The radius, 55, has an oval head; its shaft is cylindrical; its distal end oblong and subcompressed.

The small bones, 56, which intervene between these and the row of five longer bones, are called "carpals;" they are four in number in the crocodilia. One seems to be a continuation of the radius, another of the ulna; these two are the principal carpals; they are compressed in the middle, and expanded at their two extremities: that on the radial side of the wrist is the largest. A third small ossicle projects slightly backwards from the proximal end of the ulnar metacarpal; it answers to the bone called "pisiforme" in the human wrist. The fourth ossicle interposed between the ulnar carpal and the metacarpals of the three ulnar digits.

These five terminal-jointed rays of the appendage are counted from the radial to the ulnar side, and have received special names; the first is called "pollex," the second "index," the third "medius," the fourth "annularis," and the fifth "minimus." The first joint of each digit is called "metacarpal;" the others are termed "phalanx." In the crocodilia the pollex has two phalanges, the index three, the medius four, the annularis four, and the minimus three. The terminal phalanges, which are modified to support claws, are called "ungual" phalanges.

As the above-described bones of the scapular extremity are developments of the appendage of the scapular arch, which is the hæmal arch of the occipital vertebra, it follows, that, like the branchiostegal rays and opercular bones in fishes, they are essen

212

PELVIS AND HIND-LIMB OF THE CROCODILE.

tially bones of the head. But the enumeration of the bones of the crocodile's skull is not completed by these; there is a bone anterior to the orbit, which is perforated at its orbital border by the duct of the lacrymal gland, whence it is termed the "lacrymal bone," and its facial part extends forwards between the bones marked 14, 15, 21, and 26. In many crocodilia there is a bone at the upper border of the orbit, which extends into the substance of the upper eyelid; it is called "superorbital." In the crocodilus palpebrosus there are two of these ossicles.

Both the lacrymal and superorbital bones answer to a series of bones found commonly in fishes, and called "suborbitals" and "superorbitals." The lacrymal is the most anterior of the suborbital series, and is the largest in fishes; it is also the most constant in the vertebrate series, and is grooved or perforated by a mucous duct. These ossicles appertain to the dermal or muco-dermal system or "exoskeleton," not to the vertebral system or "endoskeleton."

"tro

There remains, to complete this sketch of the osteology of the crocodile, a brief notice of the bones composing the diverging appendage of the pelvic arch: these being a repetition of the same element as the appendage of the scapular arch, modified and developed for a similar office, manifest a very close resemblance to it. The first bone, called the "femur," is longer than the humerus, and, like it, presents an enlargement of both extremities, with a double curvature of the intervening shaft, but the directions are the reverse of those of the humerus, as may be seen in Fig. 18, where the upper or proximal half of the femur is concave, and the distal half convex, anteriorly. The head of the femur is compressed from side to side, not from before backwards as in the humerus; a pyramidal protuberance from the inner surface of its upper fourth represents a chanter;" the distal end is expanded transversely, and divided at its back part into two condyles. The next segment of the hind-limb or "leg," includes, like the corresponding segment of the fore-limb called "fore-arm," two bones. The largest of these is the "tibia," 66, and answers to the radius. It presents a large, triangular head to femur; it terminates below by an oblique crescent with a convex surface. The "fibula" is much compressed above; its shaft is slender and cylindrical, its lower end is enlarged and triangular. The group of small bones which succeed those of the leg are the tarsals; they are four in number, and have each a special name. The "astragalus" articulates with the tibia, and supports the first and part of the second toe. The calcaneum intervenes between the fibula and the ossicle supporting the two outer toes; it has a short but strong posterior tuberosity. The ossicle referred to represents the bone called "cuboid" in the human tarsus. A smaller ossicle, wedged between the astragalus and the metatarsals of the second and third toes is the "ectocuneiform."

Four toes only are normally developed in the hind-foot of the crocodilia; the fifth is represented by a stunted rudiment of its metatarsal, which is articulated to the cuboid and to the base of the fourth metatarsal. The four normal metatarsals are much longer than the corresponding metacarpals. That of the first or innermost toe is the shortest and strongest; it supports two phalanges. The other three metatarsals are of nearly equal length, but progressively diminish in thickness from the second to the fourth. The second metatarsal supports three phalanges; the third four; and the fourth also has four phalanges, but does not support a claw. The fifth digit is represented by a rudiment of its metatarsal in the form of a flattened triangular plate of bone, attached to the outer side of the cuboid, and slightly curved at its pointed and prominent end.

The forms and proportions of the entire skeleton of the crocodile are adapted to the necessities of an amphibious animal, but minister to much more rapid and energetic