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 (pleura- 

 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. 



(diapophysis). 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 



