chap, vi.] CAUDAL VERTEBRA. 61 



Man, the Chimpanzee, Orang, Cat, Sheep, Elephant, Sloth, 

 and Wombat. 



The ankylosis of additional vertebrae in the Mammalia is 

 probably related to the greater fixity and more complete 

 attachment of the pelvis to the vertebral column in this 

 class; 1 for the innominate bone is not only articulated 

 by its iliac portion to the true sacral vertebrae, but it has 

 also a posterior connection with the vertebral column by 

 its ischial portion, by means either of very strong ligaments, 

 or in some cases by bony union. 2 



In Man there are usually five ankylosed vertebrae, con- 

 stituting the " os sacrum " of anthropotomy, but only two, or 

 sometimes three, have distinct costal elements. The re- 

 mainder may be called pseudo-sacral, and belong more pro- 

 perly to the caudal series. The sacrum as a whole is broad, 

 strongly curved in the longitudinal direction, with the con- 

 cavity downwards, and its anterior extremity forms with the 

 body of the last lumbar vertebra a more prominent " sacro- 

 vertebral angle" than in other Mammals. 



In the Gorilla, Chimpanzee, and Orang, there are 

 generally five ankylosed vertebrae, to which the last lumbar 

 not unfrequentiy becomes united in old animals. The 

 whole sacrum thus formed is long and narrow, gradually 

 tapering posteriorly, and much less curved than in Man. 

 In the other Monkeys, there are usually two or three, 

 rarely four, ankylosed vertebrae ; the first two, or true sacrals, 

 are broad, and behind these the sacrum suddenly contracts. 



1 This is carried to a still greater extent in birds. 



2 Hence the following definition of the sacrum : " The posterior 

 limit of the sacral region is characterized, not by the union of the 

 different osseous pieces, which varies according to age, but by the place 

 of insertion of the ischio-sacral ligaments." (A. Milne Edwards, 

 Famille ties Chevrotains, p. 52. )' 





