502 PHYSIOLOGY CHAP. 



The formation of sebum is therefore not so much the result of 

 a true secretory process as of the perpetual renewal and fatty 

 degeneration of epithelial cells, perfectly analogous to the renewal 

 of the stratum corneum of the epidermis, in consequence of the 

 keratinous degeneration of the cells of the Malpighiah layer. 



Nothing definite is known as to the influence of the nervous 

 system on the sebaceous glands. It would not be surprising if, 

 like the glands of plants, they function independent of any regu- 

 latory nervous influence. Yet .certain observations of Arloing 

 (1899) suggest they are probably no exception to the general rule. 

 After dividing the cervico-sympathetic in the donkey, he saw that 

 a quantity of wax collected in the sebaceous glands of the skin 

 of the ear, which reached its maximum 15 hours after section, 

 and ceased about 64 hours after. Excitation of the peripheral 

 end of the nerve also seems to cause a perceptible increase in the 

 secretion of these glands. 



VI. The Mammary Glands, from which the highest class of 

 vertebrates has been named, belong to the skin no less than the 

 sudoriferous and sebaceous glands. 



They are compound acinar glands which may be regarded as 

 a collection of enlarged sebaceous glands with modified functions. 

 In man there are only two, in the region of the breast ; in the 

 mare and the goat two, and four in the cow, in the lower abdominal 

 region ; multiparous animals have ten, twelve, or even more along 

 the abdominal wall. From the phylogenetic point of view it is 

 important fact that in many of the lower mammals of the Monotre, 

 group the mammary glands consist of a large number of sma 

 cutaneous glands without a nipple, which resemble enlarge 

 sebaceous glands. The new-born offspring are nourished by licl 

 ing the region of the maternal abdomen in which these glands 

 situated. For the rest we have seen that the secretion of even tl 

 ordinary sebaceous glands contains a small amount of caseinogei 

 which confirms the phylogenetic homology between sebaceous am 

 mammary glands (Neumeister). 



The development of the mammary glands commences in botl 

 sexes in the third month of intra- uterine life. At birth the 

 glandular tissue consists of tubes which branch two or three time 

 and terminate in a blind sac. At the twelfth year these tul 

 subdivide into more branches, but the glandular alveoli at tl 

 extremity are not developed till the approach of puberty in 

 female. 



In males the mammary gland is only a rudimentary, vestigu 

 organ, witnessing to an original hermaphroditism. In the adi 

 they are no more developed than in the foetus. At the age 

 puberty they may develop to a certain extent and harden, b 

 immediately undergo a process of degenerative involution. New- 

 born animals of both sexes constantly secrete a small quantity of 



