n.] THE SPINAL SKELETON. 57 



or five, though when that bony element which we shall find 

 to be called the ischium also effects a bony union, the num- 

 ber may be raised to ten as in some Armadillos. 



When the osteological sacrum embraces as many as 

 twenty (as in the Ostrich), then the lumbar vertebra? are 

 encroached on by the extent of the hip-bones. Coccygeal 

 vertebra? indeed may take an extensive share in it, as in some 

 Edentates and Bats, where the osteological sacrum not only 

 unites with the so-called ilium, but also anchyloses with the 

 bone which is commonly termed the ischium. 



The possession of a true osteological sacrum is a character 

 man shares only with Vertebrates above Fishes. Very rarely, 

 however, as in the Turbot, there is in Fishes even a kind of 

 false sacrum, formed by the anchylosis of the bodies and 

 ventral spines of the first two coccygeal vertebra?. 



Compared with his own class only, man exhibits a sacrum 

 which includes a number of vertebrae attained by few others, 

 and but rarely exceeded, though sometimes there are six in 

 the highest Apes. The number may reach eight, or even (as 

 before said) ten in certain Armadillos ; but this is the highest 

 number of man's class, though about the smallest number 

 found in the class of Birds, where, as we have seen, there 

 may be as many as twenty. 



Apart from union with the hip-bones, coccygeal vertebrae: 

 become anchylosed with the true sacrum in Mammals with 

 the advance of age, so as to form altogether four or five sacral 

 vertebra\ That the development of the sacrum is not always 

 in proportion to that of the pelvic limbs, is proved by the 

 little lizard Seps, in which, in spite of the rudimentary con- 

 dition of the limbs, there are three sacral vertebra?. The 

 sacro-vertebral angle is generally replaced by almost a 

 straight line, and it is not nearly so marked in any Mammals 

 as in man, except in some Baboons. 



The concavity of its ventral surface, which is so variable in 

 man himself, is generally much less marked in other Mam- 

 mals, but sometimes it is quite as great, as in certain Baboons. 



The tapering form of the sacrum of man is also exceptional, 

 but it exists in the highest Apes, in Bears, and some other 

 Mammals. Generally, however, in man's class the sacrum 

 contracts suddenly at its post-axial end. 



The wing-like processes (the sacral ribs) make it relatively 

 wider than long in most Mammals. 



The sacral spinous processes of man are but rudiments of 

 what sometimes exist, as they are very long in Carnivora and 



