SKELETON 



63 



ture, encircling the viscera like the hoops of a barrel. Those ribs 



which do not reach the sternum are called false ribs. In most 



reptiles and some birds most of the thoracic ribs bear an uncinate 



I process directed upward and backward (fig. 60), overlapping the 



rib behind and strengthening the thorax. In the chelonia the ribs 



are confined to the dorsal side of the body and are fused to the 



j costal plates (dermal skeleton) to form the carapace. Single- and 



1 double-headed ribs often occur in the same individual of various 



I groups, and in the mammals the capitular head, instead of articu- 



I lating with a distinct parapophysis, may rest in a socket formed by 



two successive vertebrae. 



Fig. 61. — Sacral vertebrae, ribs and pelvis of Trionyx, obliquely from below. /, 

 . head and trochanter of femur; il, ilium; is, ischium; p, pubis; sr, sacral ribs; sv, sacral 

 I vertebrae. 



The pelvis is never directly united to the sacrum, but sacral ribs 

 intervene. These are distinct in the reptiles (fig. 61), but are fused 

 to the transverse processes in other groups. 



The Sternum (Breastbone) 



The sternum includes the skeletal parts on the ventral side of 

 the body, which are closely connected with the shoulder girdle and, 

 except in the amphibia, with the ribs. The fact that it occurs only 

 in vertebrates with legs (it is lacking in snakes and caecilians) shows 

 that it has arisen in adaptation to terrestrial locomotion. In man it 

 consists of three parts, a manubrium above, a middle piece (gladiolus), 

 and a xiphoid fensiform) process below, and these terms have been 

 carried into other groups. 



