THE SKELETON 37 
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, ¢.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. 
y 
y 
Fic. 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 ré/e 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 
