FORMATION OF MILK. 



the whole lifetime of the animal. Their action is broken by alter- 

 nate periods of rest. Even during the same lactation period the 

 work of the glands does not continue at the same rate, but varies, 



Fig. 14. Section of Sebaceous Gland. a,Sebaceous gland ; b, superficial layer of epidermis ; 

 c, epidermis ; d, horny layer, (x 100.) (Fiirstenberg.) 



on the one hand, with the period of lactation, and, on the other 

 hand, according to the surrounding physical conditions. 



According to the theory regarding the origin of milk which obtained 

 prior to the year 1840, it was believed that the milk-glands acted as a 

 sort of filter with a wide surface, for certain constituents of the blood, and 

 that in milk we were dealing with a filtrant from the blood, the amount 



Fig. 15. Tallow Follicle of the Nipple, (x 100.) 

 a, Outlet ducts. (Furstenberg.) 



Fig. 16, -Tallow Follicle of 

 Nipple, (x 180.) (Furstenberg.) 



and quality of which was determined solely by the amount and quality of 

 the food. When, however, it had been proved, by chemical investigation, 

 that not one of the organic constituents of milk occurred ready formed in 

 the blood, but that they were all formed in the milk-gland, this theory 

 had to be abandoned. 



The labours, during the last forty years, of different physiologists, such 



