118 



STRUCTURE OF THE VERTEBRATES 



hairs with an under layer of hairs which are used for making 

 felt. 



5. Glands. The glands of mammals are apparently new de- 

 velopments within the class. Anal or other scent glands may be 

 present, but the principal integumentary glands are (1) sweat 

 and (2) sebaceous, or oil. The sweat glands are simple tubes 

 with the inner end highly coiled. The secretion assists in heat 

 regulation by the evaporation of water on the skin, and they are 

 to a certain extent excretory. Salts and a slight amount of nitro- 

 gen wastes are thrown out with the sweat. Dogs and some other 

 animals lack the sweat glands, the heat regulatory function 



Stratum comeum 

 Germiruitivum 



Dermis 



y^Ectodermal inpocketing^^^ 



Duct, 



Development at 

 4 to 5 months 

 of foetal life 



Development^^ 

 at 7 months 

 ,of foetal life 



Definitive 

 Structure 



Fig. 54. Development of the Sweat Glands. (Adapted from Kollman). 



being taken over by the vascular tongue and mouth, and the 

 correlated loss of heat from the lungs. 



The sebaceous glands empty into the follicles in which the 

 hairs are located and keep the hair and skin oiled. These glands 

 differ from most secretory epithelium cells in that the cell is 

 destroyed when the secretion is thrown out. Consequently the 

 gland repairs itself by cellular divisions in the layer next the 

 basal membrane which bounds the epithelial covering. 



Mavimary, or milk, glands are diagnostic of the mammals. 

 They are probably modified sweat glands. Although present in 

 both male and female they are rarely functional in the male. In 

 the female they are highly functional at the end of pregnancy. 

 Both periodic and sex differences in function are controlled by 



