MAMMALIA 443 



the sweat glands in their secretion. They are much lobed, and 

 usually have teats or nipples ; but in the monotremes these are 

 wanting, and the young merely lick the secretion from a "milk 

 area." The glands may be distributed along the entire ab- 

 dominal surface (carnivora) or confined either to the anterior 

 (primates) or posterior portion (ruminants). The number of 

 the milk glands is correlated in a general way with the number 

 of young produced at a birth. 



454. Skeleton. — Some of the more elementary facts concerning the skeleton 

 may be summarized as follows. The vertebras usually unite by flat faces, and the 

 five regions of the vertebral column (see §349) have a fair degree of constancy as to 

 numbers. The neck, with a very few exceptions, has seven vertebrae, the length 

 of the neck depending on the length of the vertebrae and not on their number. The 

 trunk vertebrae, made up of the thoracic and lumbar, usually vary within the limits 

 19-23. The caudal vertebrae are most variable of all. The bones of the skull in 

 the adult have their edges closely united by means of sutures (a species of close 

 joint, which does not allow of motion). The lower jaw, the hyoid bone, and the 

 small bones of the ear are the only movable bones in the mammalian skull. The 

 lower jaw articulates directly with the cranium. The quadrate, which in reptiles 

 and birds serves to articulate the jaw with the cranium, has apparently changed 

 its position and given rise to one of the small bones of the middle ear. 



The pectoral girdle and arm bones are always present, but in the whales and 

 sea-cows the posterior are lacking. The digits are typically five in number. In 

 many carnivores these may be reduced to four, terminating in claws. In the hoofed 

 forms the toes are often reduced to four, three, two, or even one (the horse). In 

 such cases rudiments of the remaining digits may occur in the form of splints. 



455. Teeth. — The teeth are produced by the skin, and come 

 to be lodged in pits in the bones of the jaws. While differing 

 in shape, the teeth always possess the crown, the fang or root, 

 the neck and the pulp-cavity. The btdk of the tooth is dentine 

 deposited by the dermis. Over this is a layer of enamel formed 

 by the epiderrnis. The teeth differ from horny outgrowths in 

 that both layers of the skin are involved. ' In this respect they are 

 like the scales of fishes and the plates of the armadillo 



The cavity is more or less filled with "pulp tissue" which is 

 supplied with nerves and blood vessels. Most mammals have 

 only two sets of teeth, — a milk set which appears early and is 

 lost and a permanent set which replaces the former. In some 

 cases, however, there is only one set, and in a few {e.g., certain 

 whales) no teeth appear above the surface of the gums in the 

 adult. 



In the porpoises, dolphins and similar forms the teeth are 



