Page:EB1911 - Volume 23.djvu/174

ANATOMY] absent, and clavicles and inter clavicles forming the epi- and endo-plastral elements of the plastron, the shoulder-girdle is nowhere in contact with the skeleton except at its dorsal end.

The Fore Limbs.—The humerus has near its upper end a median process, and at a variable distance a lateral process, near which is the biceps-fossa. Above the radial or outer condyle exists a foramen for the passage of the radial nerve in Sphenodon, in the Lacertilia, and in many Chelonians, e.g. Cholone and Sphargis; such an ectepicondylar foramen is absent in Crocodiles. Above the ulnar condyle exists, but only in Sphenodon, the entepicondylar foramen, for the passage of the nervus medianus and brachial vessels. Thus Sphenodon alone possesses both foramina, the crocodiles neither.

Ulna and radius always remain distinct; the former is generally the stouter although not always the larger bone. The carpus may contain as many as 12 separate elements: ulnare, intermedium, radiale, 2 centralia, a pisiform on the ulnar and a small nodule in a corresponding position on the medial side, and 5 distal carpals. In Sphenodon the centralia are sometimes fused into one, and the radial nodule is absent; the numbers of phalanges are, 2, 3, 4, 4 and 3 proceeding from the first to the fifth finger. The carpus of the Chelonia is likewise primitive, with various unimportant reductions; Chelydra possesses one or two centralia, whilst pisiform and extra radial are absent; both these bones are present in Emys, but the centrale fuses with the radial carpal, and the fourth and fifth distal carpal are fused together. In Testudo the pisiform is small; intermedium, centrale and radiale are represented by one bone only, and the first, second and third distal carpals are fused, whilst the two remaining are free. In the marine turtles the fore limbs are transformed into paddles; the ulna is considerably shorter than the radius; all the normal nine carpal elements remain distinct; the pisiform is much enlarged, helping to increase the paddling surface, and it has moved from the ulnar carpal to the side of the fifth distal carpal. The three middle fingers and toes have mostly 3 phalanges; the pollex and hallux have always 2; the number of phalanges of the fifth finger varies from 3 to 1, of the fifth toe from 2 to 0. The greatest reduction occurs in Testudo and its allied genera of typical land-tortoises, Homopus, Pyxis and Cinixys, the formula for the fingers being 2, 2, 2, 2, 2 or 1, and 2, 2, 2, 2, 0 for the toes. In Pelomedusa all the fingers possess 2 free phalanges only, owing to fusion of the first and second phalanges with each other.

Considerable advance is marked by the Crocodiles. The intermedium and centrale are lost, the pisiform is small, ulnar and radiale are considerably elongated and enlarged. Of the distal carpals the two last are fused into one bone, and the three first, together with the central, are transformed into a pad-like cartilaginous and ligamentous piece between the large radial and the first and second finger, to which the pad is firmly attached. The other fingers articulate with the “humatum.” The result of the whole arrangement is the formation of two main joints, one between fore arm and carpus, the other intercarpal. The number of phalanges is 2, 3, 4, 4, 3.

The conditions prevailing in Lacertilia are connected with those of Sphenodon. The intermedium is lost, the other normal carpalia are present, also the pisiform; the first distal carpal is much reduced and the correspondingly enlarged radial carpal comes into articulating contact with the first metacarpal. The numbers of phalanges are 2, 3, 4, 4, and 2 or 3 for the fifth finger. The hand of the chameleons is most modified; the first three fingers form an inner bundle opposed to the outer or fourth and fifth fingers; in correlation herewith the third and fourth distal carpals are fused into one rather large mass; the other elements remain free, and A. Stecker has found a small intermedium present in the young, in a position which indicates that its subsequent absence is due to loss, not fusion with neighbouring elements.

The Pelvic Girdle.—The ilium is attached to the vertebral column by means of the two sacral ribs. The ischia and the pubic bones join the ilium at the acetabulum, which is not perforated, except in crocodiles. The ischia and pubes invariably form symphyses at their ventral ends, except the so-called pubes of the crocodiles, and these two symphyses are further continuous with each other, dividing the pubo-ischiadic space into a right and left foramen obturatum of very variable size. They are small and round in Testudo, divided by a broad, bony bridge, larger in Chelone, separated by a chiefly ligamentous, partly cartilaginous string; largest they are in Sphenodon and in the Lacertilia. Frequently the symphysial portion at the anterior end of the pubic symphysis remains cartilaginous, unpaired, e.g. in most Chelonians and Lacertilians, Comparable with the epipubis of Urodela. A corresponding cartilage, the os cloacae or hypoischium, is Continued backwards, from the ischiadic symphysis towards the vent, serving for the attachment of sphincter muscles; it occurs in many lizards and tortoises. In the Chelonians the pubic bones are generally much stronger than the ischia, and they send out each a strong lateral pubic process, directed forwards and outwards; the obturator nerve passes through the wide obturator foramen. In the pleurodirous tortoises the ends of the ilia and those of the lateral processes of the pubes are much broadened and firmly anchylosed with the posterior costal plates and with the xiphiplastron respectively. The whole pelvis, like the shoulder-girdle, lies inside the body. The pelvis of Sphenodon is essentially like that of the Lacertilia. The pubes are slender; they send out a pair of lateral processes, near the base of which the obturator nerve pierces the shaft of its pubis. This lateral process is the homologue of the long, slender pubis of birds. The chameleons' pelvis is peculiar. The pubes are devoid of lateral processes, but from their anterior end arises a pair of small cartilages, in a transverse direction; their ends are connected by ligament with the median anterior portion of the ischiadic symphysis. The crocodilian pelvis is very aberrant. The ilium is broad and sends two processes to the acetabulum, which retains a foramen; the posterior process articulates movably with the ischium; the preacetabular process fuses in very young specimens with a separate, ossifying, cartilaginous piece, which then forms a rough joint with the anterior portion or process of the ischium, which closes the acetabulum on its ventral side. To this anterior ischiadic process is attached the freely-movable, club-shaped bone, generally called pubis. The homologies of these club-shaped bones and of the small bone mentioned above are not clear. The club-shaped bones remain asunder; the ischia form a long and firm symphysis. The obturator nerve passes out of the pelvis between the ischium and the club-shaped bone, close to the posterior margin of the latter.

The posterior limbs show essentially the same composition as the fore limbs, but the modifications in the various reptilian orders are much greater. The femur has generally a well marked neck. Fibula and tibia remain distinct; the former usually shows a reduction in thickness. In the tarsus we observe never more than two proximal tarsal elements, a reduction due either to the suppression of the intermedium or to its enlargement and concomitant loss of the tibial element. The least-modified foot-skeleton is that of the Chelydridae, the lowest Chelonians. The proximal row is composed of a fibulare, and a much larger piece articulates with both tibia and fibula, the “astragalus”; the centrale is present; the first threes distal tarsals remain separate, each carrying a toe. The fused fourth and fifth tarsals carry the fourth toe, and, laterally attached, the hook-shaped fifth metatarsal. Chelone shows the same arrangement, except that the centrale is fused with the astragalus; in Tesludo, Emys, the fibulare, astragalus and centrale are fused into one broad mass, with the result of forming a crurotarsal and an intertarsal joint. The same arrangement reached by the Testudinidae is universal in the Lacertae, with the further modification that the three first distal tarsals fuse on to the proximal ends of their respective metatarsals. Most aberrant is the tarsus of Chameleons, in which the first and second toe