| 1. presternum. | 4. xiphisternum. The flattened |
| 2. first sternebra of | cartilaginous plate |
| mesosternum. | terminating the xiphisternum is |
| 3. last sternebra of | not shown. |
| mesosternum. | 5. first sternal rib. |
The Sternum.
This is an elongated cylindrical structure lying in the mid-ventral wall of the thorax, and is divided into eight segments or sternebrae. The anterior segment, the presternum (fig. 76, 1) or manubrium sterni is expanded in front; the next six segments, which, together form the mesosternum are elongated, somewhat contracted in the middle and expanded at the ends. The last segment or xiphisternum (fig. 76, 4) is long and narrow, and terminates in a flattened expanded plate of cartilage. The first pair of sternal ribs articulate with the sides of the presternum, and the remaining pairs between the successive sternebrae. Between the last sternebra and the xiphisternum two pairs articulate. Development shows that the sternum is formed by the union in the middle line of two lateral portions; this can be well seen in the presternum and xiphisternum of the puppy, but no traces of this median division remain in the adult dog.
2. The Appendicular Skeleton.
The appendicular skeleton consists of the bones of the anterior and posterior limbs, and of their respective supports, the pectoral and pelvic girdles.
The Pectoral Girdle.
The pectoral girdle lies external to the ribs, and has no bony attachment to the axial skeleton. In almost all Mammalia it is, as compared with that in Sauropsids, very incomplete; and in the dog it is even more reduced than in the majority of Mammalia. The dorsal portion or scapula is well developed, but the ventral portion is almost entirely absent.
The scapula is somewhat triangular in shape, the apex being directed downwards and forwards, and being expanded to form the shallow glenoid cavity with which the head of the humerus articulates. The inner surface of the scapula is nearly flat, while the outer is drawn out into a very prominent ridge, the spine, which, arising gradually near the dorsal end, runs downwards, dividing the surface into two nearly equal parts, the prescapular and postscapular fossae, and ends in a short blunt process, the acromion. The anterior border of the scapula is somewhat curved, and is called the coracoid border; it is terminated ventrally by a slight blunt swelling, the coracoid process, which ossifies from a different centre from the rest of the scapula, and is probably the sole representative of the coracoid. The dorsal or suprascapular border of the scapula is rounded, while the posterior or glenoid border is nearly straight. The clavicle[144] or collar bone, which in a large proportion of mammals is well seen, in the dog is very imperfectly developed; it is short and broad, and is suspended in the muscles, not reaching either the scapula or sternum.
The Anterior Limb.
The anterior limb of the dog is divisible into the usual three portions, the brachium or upper arm, the antibrachium or fore-arm, and the manus or wrist and hand.