404 THE VERTEBRATE SKELETON. 



increase in length from the first which is very stout, and has 

 the capitulum and tuberculum very distinct, to about the 

 eighth or ninth ; afterwards they gradually diminish in size. 

 The first nine to eleven have the capitula and tubercula sepa- 

 rate, afterwards they gradually merge together. 



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 ex- 

 panded at the ends. The last segment or xiphisternum 

 (tig. 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. De- 

 velopment 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 



