THE SKELETON 



shortening of the thoracic wall, the shifting of the thoraco- 

 abdominal boundary, the changes in the axial skeleton, and the 

 numerical reduction of the thoracic metameres, must have a far- 

 reaching influence on the whole anatomy of the trunk, e.g. on the 

 position of the thoracic viscera (lungs, heart), and on the relation- 

 ships of the pleural cavities. Thus Ruge has shown, in a series 

 of excellent papers, that as the secondary type of thorax begins 

 to develop, the pleural boundary gradually recedes along the 

 anterior and inner wall of the thorax, so that the heart, which in 

 the primitive thorax almost always lies remote from the sternum, 

 approaches nearer the anterior thoracic wall. As a consequence 

 of this, the anterior edges of the pleural sacs, which are primarily 

 apposed behind the sternum, are forced apart, so that in Man, for 

 example, they are often separate as high as the fourth rib. 



A B 



FIG. 23, A AND B. DIAGRAMS OF THE VERTEBRAL AND COSTAL SKELETON. 



A, IN THE QUADRUPED ; B, IN MAN ; the arrows indicate the line of direct pressure 



of the thoracic viscera upon the wall of the thorax. 



Among the various factors recognisable as having played a 

 continuous role in the evolution of the Primates, not the 

 least weighty is the assumption of the upright position. The 

 alteration in the shape of the thorax above described, by shifting 

 back the centre of gravity of the body, favours the upright 

 position ; and the inter -dependence of these two modifications 

 is evident. 



To the same category, it appears to me, belongs the gradual 

 diminution in number and size of the sternal ribs. It is easy 

 to see how, with the shifting of the centre of gravity towards 

 the dorsal side of the body, and a consequent diminution of 



