214 



COMPARATIVE ANATOMY 



The Sternum. The sternum is a flat, dagger-shaped bone lying 

 mid- ventral ly of the chest. 



Three parts are distinguished, i. The manubrium or presternum, 

 triangular, the widest portion and the most anterior. It articulates with 

 the clavicle. 2. The gladiolus or mesosternum, the longest portion, 

 formed by the fusion of four sternal elements or sternebrae, 3. The 

 posterior metasternum, xiphoid or ensiform process. The xiphoid 

 process is sometimes perforated by a foramen and is sometimes forked 

 posteriorly. 



Development of the Sternum. The sternum arises from connective 

 tissue which is afterwards chondrified to become a pair of cartilaginous 



c p 



X 



1 



A 



Fig. 171. — Scheme of development of mammalian sternum. A, early stage; B, 

 cartilage, the halves uniting; c, Pcoracoid precartilage; d, clavicle; co, centers of ossifica- 

 tion; ni, mesosternal parts; ?««, manubrium; p, presternum; st, sternebrae; x, xiphister- 

 num. (From Kingsley's "Comparative Anatomy of Vertebrates.") 



bars, which secondarily unite in the mid-ventral line and only later 

 connect with the costal cartilages. Ossification begins in a series of paired 

 centers, but the manubrium usually has one center only. Ossification of 

 the metasternum or xiphoid process remains incomplete until very late in 

 life. 



Evolution of the Stemtmi. Opinion is divided as to the beginnings of 

 the sternum. Some morphologists take the median portion of the 

 elasmobranch pectoral girdle to be the homologue of the mammalian 

 presternum, notwithstanding the fact that in some urodeles the sternum 

 is a midventral plate of cartilage quite unconnected with the pectoral 

 girdle. Since, however, the median ventral portion of the elasmobranch 

 pectoral girdle is limited to a single intersegment, while the sternum of 

 higher vertebrates extends through several segments and in mammals is 

 clearly metameric, this hypothesis leaves the metamerism of the sternum 

 unexplained. To meet this difficulty, it would be necessary to assume an 



