28o 



COMPARATIVE ANATOMY 



Thoracic Muscles. The intrinsic muscles of the thorax are covered 

 by the extensive muscles of the shoulder and chest. When these are 

 removed, the same three muscles found in the abdomen are exposed. 

 The outer of these, corresponding with the external oblique of the abdo- 

 men, is the external intercostal ; the internal intercostal is the homologue 

 of the internal oblique of the abdomen; the m. transversus thoracis corre- 

 sponds to the transversus abdominis. In addition to these, an external 

 sheet of muscle which is continuous in many marsupials is represented in 



1ST LUMBAR VERTEBRA- 



..^Sasan^^®" PSOAS 



POSTCAVA 



TENDINOUS 

 DIAPHRAGM 



MUSCULAR 

 DIAPHRAGM 



STERNUM. _T^:^_ 



Fig. 233. — The human diaphragm viewed from above. Only the peripheral region 

 of the diaphragm is muscular. The central portion is tendinous. The diaphragm is 

 dome-shaped with the convex side uppermost. (Redrawn from Braus.) 



man by the serratus posterior, which is divided into superior and inferior 

 moieties. When certain parts of the intercostal muscles contract, the 

 rib-basket is raised, the thoracic cavity enlarged, and air is inspired to 

 fill the space thus created. The levators of the ribs are twelve pairs of 

 triangular muscles which connect the ribs with the transverse processes 

 of the vertebrae and are used to raise the ribs. (Fig. 232) 



Diaphragm. The diaphragm is a dome-shaped muscle separating 

 thoracic and abdominal cavities. The central portion is tendinous and 

 only the outer portion which is attached to the ribs is muscular. When 

 the muscles of the diaphragm contract, the dome is flattened and the 



