126 ARBOREAL MAN 



varies ; it may involve no more than the best part of two 

 sacral vertebrae, or nearly the whole of three may enter 

 into the joint. But the diapophyses of only two of the 

 sacral vertebrae are, as a rule, involved. In both the 

 Gorilla and the Chimpanzee the joint surface occupies 

 from two and a half to three sacral vertebrae, but involves 

 the diapophysis of only two elements. In Man the 

 greatest variation is sexual, and the female articulation, 

 as a rule, comprises only two or two and a half sacral 

 vertebrae, while that of the male embraces from two and 

 a half to three. In any case, an articulation evolving 

 a diapophyseal element is a somewhat uncommon human 

 anomaly. 



It would therefore seem that the sacrum is being 

 received between the ilia in a greater proportion of its 

 length, but is freeing itself from articulation in some 

 portion of its depth by (if it may be expressed thus) 

 liberating its dorsal surface from joint contact with the 

 ilia. 



Hand in hand with this alteration of the sacro-iliac 

 joint a change is proceeding in the ventral symphysis. 

 This change may best be summed up by saying that the 

 symphysial area becomes shortened progressively in the 

 transition -from pronograde poise to arboreal orthograde 

 habit. The symphysis, which is first of all an ischio- 

 pubic union in the mid-line, becomes opened from its 

 caudal aspect, and more and more of the ischia are freed 

 from the ventral union. By gradual stages the whole 

 length of the ischia becomes liberated from the symphysis, 

 and the two bones are splayed aside from each other. 

 Next, the pubes share in the process, and from the condi- 

 tion of a true pubic symphysis, in which the whole of 

 the ventral ends of the pubic bones are involved, there is 

 developed the pubic symphysis typical of Man,, in which 

 only about a half of the ventral ends of the pubes enter 

 into the articulation. 



That this is an outcome of the modified method of 



