474 



COLLEGE ZOOLOGY 



Nail plate 



Sole 



Phalanges y'i^A»wi 



Digital pad 



Digital pad 



Clow of cat 



Nail of man 



Hoof of horse 



Figure 346, Diagrammatic longitudinal sections through the distal ends of the digits of mam- 

 mals. Claws, nails, and hoofs are all modified scales. The hoof of the horse is a modified nail 

 and essentially similar to a human fingernail. (Partly after Wiedersheim, after Gegenbaur and 

 Boas.) 



the "whalebone" (baleen) of certain whales. 

 Dermal plates of bone form the exoskeleton 

 of the armadillos (Fig. 354E). 



Skin glands 



Mammals possess a greater number of 

 glands than reptiles or birds; these are for 

 the most part sebaceous and sweat glands, 

 or modifications of them (Fig. 211). The 

 sebaceous glands usually open into the hair 

 follicles and secrete a greasy substance 

 which keeps the surface of the skin soft and 

 the hair glossy. The sweat glands secrete a 

 fluid composed chiefly of water, containing 

 a small amount of solutes, mainly sodium 

 chloride; this fluid evaporates, thereby cool- 

 ing the skin and regulating the body tem- 

 perature. In carnivores generally, sweat 

 glands are much reduced in numbers; the 

 panting of a dog utilizes evaporation on the 

 tongue for the same purpose. The lacrimal 

 glands, the secretions of which keep the eye- 

 balls moist, the scent glands of many spe- 

 cies, and the mammary glands are all modi- 

 fications of skin glands. 



Teeth of mammals 



The teeth of mammals are of consider- 

 able value in classification and indicate also 

 food habits. Whalebone whales, mono- 

 tremes, and many edentates are without 

 teeth in the adult stage; and in some forms, 

 for example, the spiny anteater, they have 

 never been found even in the embryo. 



The embryologic development of mam- 



malian teeth is like that of other vertebrates. 

 The principal forms and the relations of 

 their three constituents are shown in Fig. 

 347. The enamel (in black) is the outer 

 hard substance; the dentine (stippled) con- 

 stitutes the largest portion of the tooth; and 

 the cement (dotted line) usually covers the 

 part of the tooth embedded in the tissues of 

 the jaw. The central pulp cavity of the tooth 

 contains nerves, blood vessels, and connec- 

 tive tissue. A tooth has an open pulp cavity 

 during growth, which in some cases con- 

 tinues throughout life. 



The teeth of fishes, reptiles, and amphi- 

 bians are, with some exceptions, similar, and 

 the dentition of these animals is therefore 

 said to be homodont (same). The dentition 

 of mammals, on the other hand, is almost 

 always heterodont (different); there being 

 usually 4 kinds of teeth in each jaw: (1) 

 the chisel-shaped cutting teeth or incisors 

 (Fig. 347), (2) the conical tearing teeth or 

 canines, (3) the anterior grinding teeth or 

 premolars, and (4) the posterior grinding 

 teeth or molars. 



In most mammals the first set of teeth, 

 known as the millc (deciduous) dentition, 

 is pushed out by the permanent teeth which 

 last throughout the life of the animal. The 

 milk molars are followed by the premolars, 

 but the permanent molars have no predeces- 

 sors. 



The relation of the form of the teeth to 

 the food habits of the animal is quite ob- 

 vious; for example, sharp conical teeth are 

 adapted for capturing fish, large canine teeth 

 are suitable for capturing and killing prey, 



