Skeleton of Mammalia. 75 



in pits or follicles, and into these follicles there open 

 sebaceous glands, which secrete an oily material for 

 the lubrication of the hairs. 



The neck-region of the vertebral column or back- 

 bone in all mammals consists of seven vertebrae, 

 except in three cases; 1 the back region consists of 

 about twenty, but the number is more variable; the 

 shoulder girdle is never connected directly to the 

 spine, but the pelvic girdle always is so, and hence 

 there are always certain vertebrae thickened and united 

 for the purpose of supporting the pelvis; these are 

 "known as the sacrum, and behind this in most 

 mammals is the tail, which varies extremely in length, 

 sometimes, as in the long-tailed manis of Western 

 Africa, having over forty vertebrae, in others, as in 

 some bats, having only three. In man there are four 

 very small rudimental tail vertebrae, and the same 

 number exists in the gorilla, chimpanzee, and orang- 

 outang. In many mammals, as the South American 

 monkeys, opossums, and kinkajous, the tail is prehen- 

 sile and is used as an additional hand in climbing. 



50. Skeleton. The skeleton consists of two classes 

 of bones, some with an interior of spongy cells, others 

 with an internal cavity. In both cases the hollow 

 spaces are filled with marrow. The skull in mammals 

 is a solid box to which the upper jaw is immovably 

 fixed, and it articulates with the first vertebra of the 

 neck by means of two articular knobs or condyles. 

 The lower jaw is composed of two pieces only, one 

 on each side, and it forms a joint directly with the 



1 These are two sloths, one having 9, the other 6 vertebrae, 

 and an aquatic American animal, the manatee, which has 6. 



