v.] SKELETON OF LOWER LIMB. 189 



from its fellow of the opposite side ventrally, is a condition 

 which may occur, as e.g. in the very imperfectly developed os 

 innominatum of Cetaceans and Sirenia, and certain Reptiles, 

 e.g. the Boa Constrictor. 



In that the os innominatum joins the backbone by one 

 attachment only (namely, that of the ilium), man agrees with 

 the great majority of Vertebrates. But even in some of his 

 own class (as in the Armadillos, some other Edentates, and 

 some Bats) a 'second, ischiatic, bony attachment may be 

 formed, answering to what would result from an ossification 

 of the great sacro-sciatic ligaTnent in us. 



As has been said, the form of man's os innominatum is 

 absolutely peculiar. Still he shares the normal characters 

 of his class and of Vertebrates generally, in that the inferior 

 bony elements on each side are rather short and mainly 



FIG. 161. RIGHT SIDE OF PELVIS OF A BIRD. 



a, acetabulum ; c, caudal vertebrae ; d l d 5 , three dorsal vertebrae ; z7, ilium : 

 to, great foramen caused by the post-axial ujiion of the ilium and ischu. m ; 

 is, ischium ; o, very elongated obturator foramen ; /, pubis. 



directed ventrally. These elements may, however, be very 

 much elongated and mainly directed post-axially (i.e. back- 

 wards, if the body is placed horizontally), as in Birds. The 

 ventral elements may be single, as in Batrachians, and if 

 double, as in man, they may apparently consist of parts which 

 do not correspond with the two elements existing in him and 

 other Mammals : such a diversity of structure seems to exist 

 in Reptiles. 



In so far as the ilium of man is a broad flat bone, it differs 

 from that of Mammals generally, and only agrees with certain 

 exceptional forms, such as the Elephant, Sloth, and Gorilla, 



The extent of the ilium in the line of the backbone may be 

 very much greater than in man, as is the case in Birds, where 

 it is prolonged both pre- and post-axially to a very considerable 

 extent, thus connecting in one whole many vertebra?. It is 

 probably the' pre-axial part of the bird's ilium which corre- 

 sponds with that of man and of Mammals, while its post- 

 axial part seems to correspond with the ilium of Reptiles. 



