BOXES OF THE THOEAX. 



135 



[ 261. The THOEAX is formed by the twelve dorsal ver- 

 tebrae, the ribs, and sternum ; the vertebrae have their elements 

 well developed in this region, to form an osseous cage for pro- 

 tecting the heart, lungs, and great bloodvessels (fig. 1 24). The 



Fig. 124. 



ribs, 



pophyses, are 

 attached by a 

 head to the cen- 

 trum, and by a 

 tubercle to the 

 parapophyses; 

 the hcemapo- 

 pJiyses, or car- 

 tilages, are un- 

 ossified, and 

 removed to the 

 distal end of the 

 ribs; they unite 

 before with the 

 hcema /bones, or 

 sternum, which 

 is here placed 

 in the median 

 line, 



The haemal 

 elements play 

 an important 

 part in the eco- 

 nomy of many 

 animals. In 



birds and tortoises, the sternum is widely expanded, its deep 

 keel affording a large surface for the attachment of the 

 pectoral muscles in birds (fig. 77), and for the same muscles 

 in the mole and the bat among mammals. In man, only seven 

 of the twelve ribs form a complete hoop, as the hcemapophyses 

 of the five inferior ribs are united together, and the haemal ele- 

 ments of these are wanting. In crocodiles, the hcemapophyses, 

 or sternal ribs, are ossified ; and similar ossified apophyses are 

 continued along the fore part of the abdomen to the pubis. 

 Rudiments of these abdominal ribs are seen in the transverse 

 tendinous intersections of the rectus abdominis muscles in 



