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 Malpighian 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 an 

 important fact that in many of the lower mammals of the Monotreme 

 group the mammary glands consist of a large number of small 

 cutaneous glands without a nipple, which resemble enlarged 

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

 ing the region of the maternal abdomen in which these glands are 

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

 ordinary sebaceous glands contains a small amount of caseinogen, 

 which confirms the phylogenetic homology between sebaceous and 

 mammary glands (Neumeister). 



The development of the mammary glands commences in both 

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

 glandular tissue consists of tubes which branch two or three times 

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

 subdivide into more branches, but the glandular alveoli at the 

 extremity are not developed till the approach of puberty in the 

 female. 



In males the mammary gland is only a rudimentary, vestigial 

 organ, witnessing to an original hermaphroditism. In the adult 

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

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

 immediately undergo a process of degenerative involution. New- 

 born animals of both sexes constantly secrete a small quantity of 



