THE PELVIS AND THE VISCERA 125 



than in a dorsi- ventral direction. This change produces 

 an effect upon the sacro-iliac joint that we may sum up 

 by saying that an elongated dorsi-ventral contact area 

 becomes unnecessary, and is gradually replaced by an 

 elongated cranio-caudal contact area. 



Although there are some exceptions and irregularities 

 in the application of this principle to the pelves of existing 

 Mammals, still the exceptions are capable of explanation 

 and do not detract from the general rule that with a 

 change from quadrupedal to bipedal progression more 

 sacral vertebrae enter into the formation of the joint, but 

 less of the dorsi-ventral area of each vertebra is engaged 

 by the ilia. 



A common type of sacro-iliac joint, in purely jprono 

 grade quadrupedal Mammals, is that in which but one 

 sacral element is articulated with the ilia; and this one 

 element is sunk deep between the two ilia, so that the 

 joint surface involves the whole of its dorsi-ventral area, 

 engaging both pleurapophysis and diapophysis elements 

 which are represented in this area. In most of the 

 Lemurs one whole sacral element, and from a quarter to 

 a half of the next caudal element, are engaged in the 

 sacro-iliac joint. In many New-World Monkeys the 

 condition is the same, and in both cases these elements 

 are deeply sunk between the ilia, so that the diapophyses 

 are articular. In the Baboons one whole element, and 

 three-quarters of another, are engaged. In most Old- 

 World Monkeys nearly the whole of two sacral elements 

 enter into the articulation, and the same condition is 

 present in the pelvis of the upright Indris among the 

 Lemurs. In the Anthropoids from two and a half to 

 three sacral elements~are involved, but the condition is 

 subject to a considerable degree of variation in different 

 individuals. In the Gibbons (Hylobates lar) nearly the 

 whole of three elemenfsTs articular, and the articulation 

 involves both pleurapophysis and diapophysis of the 

 sacral vertebrae. In the Orang-utan the articulation 



