LACTATION 601 



stiffen, and it is suggested that this action has the effect of keeping 

 open the orifices of the ducts, and thus permitting the free outflow 

 of milk. 1 



It is probable also that the discharge of the secretion is assisted 

 in some degree by the swelling of the entire mammary gland 

 resulting from a reflex dilatation of the vessels ; but if the secretory 

 process is very active, and the ducts are heavily charged, the ti<>\\ 

 of milk may take place almost automatically, and with hardly any 

 external stimulus. 



THE FORMATION OF THE ORGANIC CONSTITUENTS OF MILK 



The principal organic constituents of milk are peculiar to the 

 secretion, a fact which shows that they are elaborated in the 

 mammary glands themselves, and not elsewhere in the body. It is 

 stated, however, that a relatively small amount of caseinogen is 

 present in the secretion of the sebaceous glands, from which, as 

 already remarked, it is commonly supposed that the mammary 

 glands 2 have been derived in the course of evolution. Nothing 

 appears to be known definitely regarding the method of formation of 

 the caseinogen of niilk, 3 but it has been suggested that it is derived 

 from the degenerate nuclei of the gland-cells. 



The precise method by which the milk fat is formed is likewise 

 unknown. It may be derived from protein material, the change 

 being effected in the cells of the gland, or some of it may possibly 

 have its source in fat which has already been formed elsewhere, and 

 carried to the mammary glands in the blood or lymph. There is n 

 reason for supposing that the cells of the glands do not possess, in 

 common with most other tissues, the power to elaborate fat. On the 

 other hand, there is definite historical evidence that they have this 

 capacity (see above, p. 593). Moreover, the special composition of 

 the milk fat seems to be by itself conclusive evidence that it is 

 constructed within the mammary glands. 



The suggestion has been made that the leucocytes which migrate 

 through the epithelium and make their way into the secretory fluid 

 may help to bring fatty globules into it, 4 but there seems m 

 necessity for assuming that this is the case. 



The fat formation which takes place in the cells of the lacteal 

 glands in the process of milk manufacture has l>een compared with 

 the fatty degeneration which occurs in other tissues, milk bein- 



1 Sharper Sclmfer, lo<: <-it. 



- Neunieister, I^ehrbuch dtr phgtMomtcken <'/>enw', vol. ii., Jena, 1 w 



3 Thierfelder, "Zur Physiologic der Milchbildung," 1'tfm/,-,-'* .!/</,, vol. xxxii., 

 1883. 



4 Michaelis, "Beitrage zur Kenntniss der Milchsecretion," .!/</,. f. Mil,-,-. 

 Anat., vol. xxi., 1898. 



