581 



Comparative Morphology of Chordates 



and emptied by alternate expansion and contraction of the thoracic 

 wall, assisted by contraction and relaxation of the diaphragm. Mechan- 

 ical protection of the thoracic organs and this bellows-like breathing 

 movement of the wall both require that it should have a considerable 

 degree of stiffness. The chief contents of the abdominal cavity are 

 organs of digestion, excretion, and reproduction. Not only are the 

 functions of these organs less susceptible to external mechanical inter- 

 ference, but a rigid abdominal wall would be positively objectionable. 

 The stomach is sometimes full, sometimes empty. The uterus may be 

 collapsed and shrunken or it may be greatly distended by developing 

 young. The abdominal wall must be able to adapt itself to great 

 variations in the volume of its contents. 



Throughout the entire extent of the thoracic region, ribs are 

 present (Figs. 451, 452). Each usually consists of two parts: a dorsal 

 vertebral rib articulated to the vertebral column, and a ventral 

 sternal rib joined to the sternum. In the egg-laying duckbill and spiny 

 anteater (monotremes), the thoracic rib is in three parts, a short 

 intermediate segment being interposed between the vertebral and 

 sternal parts. (A similar condition exists in the trunk-ribs of croco- 

 dilians and some lizards.) The vertebral rib, in the adult mammal, is 

 bony, but the sternal rib usually remains cartilaginous, although in 



^^rr*?^-v 



Fig. 451. Lion. Owen's idea of showing the animal's external outline in relation 

 to the skeleton results in a picture which reflects the perfect correlation between 

 the distribution of the dynamic musculature and the arrangement of the static 

 skeletal parts which provide support and leverage. (Courtesy, Owen: "Compara- 

 tive Anatomy and Physiology of Vertebrates," London, Longmans, Green & Co. 

 Ltd.) 



