688 THE SKELETAL SYSTEM 



As indicated above, ribs may be considered as outward extensions or proc- 

 esses of the vertebrae. In the frog, the much-abbreviated ribs become firmly 

 ossified to the basidorsal elements of the vertebrae and extend outward as 

 the transverse processes. However, in most vertebrates, they are articulated 

 with the vertebrae by means of lateral extensions or processes from the 

 vertebrae. 



Chondrification of the rib occurs separately from the chondrification of 

 the vertebra, and articulations develop between the rib and the vertebrae 

 (fig. 3210). Similarly, when ossification develops, a separate center of ossi- 

 fication arises in the body of the rib (fig. 32 lO). However, epiphyseal cen- 

 ters arise in the tubercular and capitular heads, which later unite with the 

 shaft of the rib. The student is referred to Kingsley, '25, for a full discussion 

 of vertebrae and ribs. 



e) Sternum. A sternum connected with the ribs, and thus forming a 

 part of the protective thoracic basket, is found only in reptiles, birds, and 

 mammals. A sternum is absent in the gymnophionan Amphibia (Apoda), is 

 reduced to a midventral cartilaginous series of bars in Nectiirus, and forms 

 a part of the pectoral girdle in the frog (fig. 323C). 



In its formation in the mammal, the sternum begins as a bilateral series 

 of mesenchymal aggregations between the ventro-mesial ends of the clavicular 

 and costal concentrations of mesenchyme (fig. 322A). These mesenchymal 

 aggregations move toward the midline, form pre-cartilage, and then form 

 cartilage. The median cartilaginous mass at the anterior end forms the pre- 

 sternum or episternum; the portion between the rib elements forms the meso- 

 sternum, and the posterior free area is the metasternum or xiphisternum 

 (fig. 322B). In forms which have a clavicle, the latter articulates with the 

 episternum. The anterior portion of the mesosternum unites ultimately with 

 the presternum to form the rudiment of the manubrium. The mesosternum 

 segments into blocks or sternebrae, while the caudal free end of the sternum 

 forms the xiphisternum (fig. 322C). Centers of ossification arise in these 

 areas and convert them to bone. In the human, the sternebrae of the meso- 

 sternum unite to form the body or corpus sterni, but, in the cat, pig, and 

 many other mammals, they remain distinct. 



2) Axial Skeleton of the Tail. The axial skeleton of the tail is modified 

 greatly from that of the trunk region. In water-living vertebrates, the tail 

 forms a considerable portion of the body. As the tail is used for swimming 

 purposes, the contained vertebrae are developed to serve this end. In con- 

 sequence, rib processes are reduced or lost entirely, and hemal arches for 

 the protection of the caudal blood vessels are strongly developed features. 

 Another feature subserving the swimming function is the tendency toward 

 diplospondyly, i.e., the development of two vertebral centra per segment (fig. 

 32 IH'). In land forms, the tail tends to be reduced. However, in the 

 armadillo, kangaroo, etc., the tail is a formidable structure, and hemal-arch 



