570 THE PHYSIOLOGY OF REPRODUCTION 



The precise method by which the milk fat is formed isUke- 

 wise 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 no 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 his- 

 tological evidence that they have this capacity (see above, p. 560). 

 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,^ but 

 there seems no 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 been compared 

 with the fatty degeneration which occurs in other tissues, milk 

 being nothing more than an emulsion of the fat of butter in 

 a solution of salts, proteins, and sugar. " What occurs as a 

 normal process in the cells of the lacteal glands occurs under 

 pathological conditions in much greater extent in very various 

 tissues, and leads almost always to incurable and fatal losses, 

 since as a rule no reparation is made by the younger cells." ^ 

 " The production of milk," says Virchow,^ " in the brain instead 

 of in the lacteal glands, constitutes a form of brain softening. 

 The same process that in the one place affords the happiest 

 and sweetest results, in another induces a painful and bitter 

 wound." It has already been mentioned, however, that the 

 fat of miUc has a special composition of its own, so that too 

 much stress must not be laid upon a resemblance between the 

 secretion of milk and the pathological formation of other fluid 

 substances in different parts of the body. 



The mode of formation of the sugar of milk has been the 



^ Michaelis, " Beitrage zur Kenntniss der Milohsecretion," Arch. f. 

 Mikr. Anat., vol. xxi., 1898. 



' Verworn, General Physiology, Lee's Translation from the second German 

 edition, London, 1899. ' Virchow, he. cit. 



