124 



ARBOREAL MAN 



the new conditions, is evidenced in a wide series of 

 vertebrate forms, and, with an exchange of pronograde 

 quadrupedal progression for arboreal uprightness, the 

 pelvis becomes greatly modified. In a simple mechanical 

 way we may regard the sacrum of the pronograde as 

 slung between the two ilia, slung from two separate 

 points of suspension, the one on its costal portion {phiira- 

 pophysis), and the other on its transverse process element 



Fig. 44. — Purely Diagrammatic Representation of the 

 Pelvis of a thoroughly Quadrupedal Mammal. 



Note the way in which the sacrum is articulated with the ilia at 

 the sacro-iliac joint, and the meeting of the pubes and 

 ischia at the ischio-pubic symphysis. 



(diajyophysis). The visceral weight is supported upon an 

 elongated ventral symphysis, which constitutes only one 

 element in the supporting developments of the structures 

 in the mid-ventral line of the body. Such a pelvis tends 

 to be narrow from side to side, but elongated in its dorsi- 

 ventral axis. With the assumption (even to a partial 

 extent) of arboreal uprightness of the body axis, the 

 body weight tends to be disposed round the vertebral 

 column as round a vertical pillar, rather than to be slung 

 from it as from a horizontal pole; and now the sacrum 

 tends to be wedged between the two iliac bones, as the 

 keystone of an arch disposed in a cranio-caudal rather 



