678 



COLLEGE ZOOLOGY 



and secrete a greasy substance which keeps the surface soft and 

 the hair glossy. The sweat-glands (Fig. 347, SD) secrete a fluid 

 composed chiefly of water containing a small amount of solid 

 matter in solution; this fluid evaporates, thereby cooling the 

 skin and regulating the bodily temperature. The lachrymal 

 glands, whose secretions keep the eyeballs moist, the scent glands 



of many mammals, and the 

 mammary glands, are all modi- 

 fications of cutaneous glands. 



b. The Teeth of Mammals 



The teeth of mammals are of 

 considerable value in classifica- 

 tion, and indicate also the food 

 habits of their possessors. Most 

 mammals are provided with 

 teeth, but the whalebone whales, 

 the monotremes, and many eden- 

 tates are without them in the 

 adult stage, and in some forms 

 (e.g. the spiny anteater, Echidna] 

 they have never been found even 

 in the embryo. 



The teeth are embedded in 

 sockets in the bone, but arise in- 

 dependently of the endoskeleton, 

 taking their origin from calci- 

 fications of the mucous mem- 

 brane of the mouth. The prin- 

 cipal forms of teeth and the 

 relations of the three constituents 

 are shown in. Figure 546. The 

 enamel (in black) is the outer 

 hard substance ; the dentine 

 (horizontal lines) constitutes the 



FIG. 546. Diagrammatic section 

 of various forms of teeth. I, incisor 

 or tusk of elephant with pulp cavity 

 open at base. II, human incisor, 

 during development, with pulp cav- 

 ity open at base. Ill, completely 

 formed human incisor, opening of 

 pulp cavity small. IV, human 

 molar with broad crown and two 

 roots. V, molar of ox, enamel 

 deeply folded and depressions filled 

 with cement. Enamel, black; pulp, 

 white ; dentine, horizontal lines ; 

 cement, dots. (From Flower and 

 Lydekker.) 



