Chap. 9 



PROTECTION, SUPPORT, AND MOVEMENT SKELETONS 



149 



Fig. 9.14. Skeleton of the running horse, Sysonby, mounted after photographic 

 studies from life. The versatility of the skeleton: the pillars of the bridge working 

 as springs. (Skeleton mounted by S. H. Chubb. Photograph, courtesy, American 

 Museum of Natural History.) 



peculiarly human shape of three slight curves, two outward and one inward. 

 In the thoracic and pelvic regions the outward curves create shallow bays 

 filled respectively by the lungs and abdominal organs. The latter are suspended 

 by mesenteries attached to the wall of the inward curve or small of the back. 

 Thus there is a strain upon this part of the back even though the abdominal 

 organs rest mainly upon the pelvic girdle. In the upright human body the 

 weight of the organs comes only indirectly upon the front wall of the ab- 

 domen. In quadrupeds, the abdominal organs are strung more evenly along 

 the back, rest directly on the ventral body wall and scarcely at all upon the 

 pelvic girdle except in those that sit — cats, kangaroos, and others. 



Ribs. Articulated to the thoracic vertebrae are the 12 pairs of ribs, 13 pairs 

 in about 6 per cent of persons. These with the sternum or breastbone form a 

 protecting basket for the heart and lungs. In the evolution of vertebrates the 

 number of ribs has gradually decreased. There are many more in reptiles than 

 in birds and more in lower than in higher mammals. 



Pectoral and Pelvic Girdles, Arms and Legs. The human pectoral or shoul- 

 der girdle and the arms are carried about as passengers, important and active 

 to be sure, but not burden-bearers like the hip girdle and legs. The human 

 arms are legs freed from the former activities of legs and now engaged in every 

 kind of business. The size, structure, and attachments of their bones allow for 

 freedom of movement but not support. 



