The alligator and its allies

by A. M. Reese

The Abdominal Ribs

([Fig. 26], 4). While these ribs are membrane bones and are not homologous with the other ribs, they may as well be mentioned at this time. They consist of about seven V-shaped sets of slender bones, the point of each V being directed cephalad. Each V is made up of from two or five slender bones, the number and arrangement being subject to considerable variation. The last V of the series (not well shown in the figure) is considerably larger than the rest and is made up of four curved bones that extend around the anterior ends of the pubic bones and are united to them by a broad tough membrane. The first or most anterior V is united by a narrow membrane (not shown in the figure) with the membrane that extends between the xiphisternal horns. All of the V’s are more or less connected with each other by fibrous membranes. Since these ribs lie superficial to the recti muscles of the ventral body wall they are sometimes missing in carelessly prepared skeletons.

The Sternum

([Fig. 26]). The sternum consists of the cartilaginous sternum proper (2), the xiphisternal horns (5), and the bony episternum or interclavicle (1). The latter is an elongated, flattened bone of somewhat spatulate outline, lying in the midventral line; it projects forwards to about the sixth cervical vertebra, while the anterior edge of the sternum is below the eighth cervical. Lying dorsal and lateral to the episternum is the flat, almost membranous sternum, to the posterior border of which the first two thoracic ribs are attached. The xiphisternum consists of two long, slender rods of cartilage; the anterior ends of these rods are in contact with each other and with the posterior border of the sternum; from this point they gradually diverge from each other as they extend caudad. A membrane extends between the horns as far back as the attachment of the last thoracic ribs.

IV. The Appendicular Skeleton.

The Pectoral Girdle and Anterior Limb.

The pectoral girdle ([Fig. 27]) is of a very simple type, consisting, unless the episternum (interclavicle) be counted, of but two bones, the scapula (s) and coracoid (c). The former consists of an upper, flat, paddle-shaped portion and a thicker lower portion which articulates anteriorly with the coracoid, and posteriorly forms about half of the notch-like glenoid cavity. The dorsal edge of the flattened portion is continued as a small, cartilaginous suprascapula. The coracoid is a flattened bone, wide at either end and narrow in the middle, so that in a dorsal view it is shaped like an hourglass. It is decidedly curved, with the convex side down. Its outer edge articulates with the scapula and is thickened to form the anterior border of the glenoid cavity. Its median end is attached to the sternum. Near its scapular articulation there is a well-marked foramen that passes entirely through the bone. The episternum (e) or interclavicle was described in connection with the sternum and ribs. There is no clavicle nor other coracoid elements.

Fig. 27. Pectoral Girdle and Anterior Limb.

c, coracoid; ce, centrale; cl, claw; e, episternum; h, humerus; m, metacarpals; p, pisiform; r, radius; r′, radiale; s, scapula; u, ulna; u′, ulnare.