CHAPTER XIX 

 THE PELVIS AND THE VISCERA 



THE arboreal alteration of body poise makes itself felt 

 in other skeletal and visceral features than those related 

 solely to the backbone and the skull; for, at the other 

 end of the vertebral column, the pelvis undergoes marked 

 changes in arboreal life. The primitive pelvis, such as 

 the earliest Mammals inherited, is a very definite structure 

 articulated in a very definite manner, and in all essentials 

 it is of the same type as that seen in the generalized 

 Reptiles both living and extinct. 



Such a primitive pelvis consists of two lateral halves, 

 each half being composed of three elements : one a dorsal 

 element, articulating with the vertebral column, and the 

 other two, which are ventral elements, articulating with 

 each other in the middle line of the ventral surface of the 

 body. The dorsal element (ilium) articulates with the 

 vertebra] column at the sacrum, over a sacro-iliac joint 

 surface which involves both the rib element (pleurapo- 

 physis] and the transverse process element (diapophysis), 

 which enter into the formation of the sacrum. The two 

 ventral elements articulate at an elongated symphysis, 

 which involves both bones (pubis and ischium), and is 

 therefore an ischio-pubic symphysis. These types of 

 sacro-iliac and ischio-pubic joints are characteristic of 

 quadrupedal animals that have four equally developed 

 supporting limbs, and they are obviously dependent upon 

 the mechanical demands for supporting the body upon 

 the limbs in pronograde animals. With a change of 

 body poise, an alteration in pelvic architecture, to meet 

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