300 PHYSIOLOGICAL CHEMISTRY. 



CHAPTER XII. 

 MILK. 



THE chemical constituents of the mammary glands have beei* 

 little studied. The protoplasm of the cells is rich in proteids and, 

 as is ^generally admitted, consists in great part of casein or a sub- 

 stance nearly related, l^all the milk is removed from the mam- 

 mary gland by thorough washing, the cells still contain a large 

 quantity of proteids which swell up to a slimy, ropy, or fibrous 

 mass when very dilute alkali (1-2 p. m. KOH) is added. These 

 proteids consists mainly of nucleoalbumin, which is gradually 

 changed by the action of the alkali. If the mammary gland is 

 boiled with water, the protoplasm of the cell is decomposed and a 

 nucleoalbumin passes into solution, which may be precipitated by 

 the addition of acetic acid, and which is characterized by its greater 

 insolubility in acetic acid, compared with casein. This nucleo- 

 albumiu, which may well be considered as a protoplasm-nucleo- 

 albumin changed by heat, also gives on boiling with dilute mineral 

 acids a reducible substance whose nature is not known. The 

 relation the above-mentioned nucleoalbumin, which is more cor- 

 rectly designated as a proteid when the mother-substance of the 

 reducible body does not occur as an impurity, bears to the sugar of 

 milk or the mother-substance of the same, has not been determined. 

 According to BERT, the secreting glands contain a body which on 

 boiling with dilute mineral acids yields a reducible substance. 

 Such a substance, which acts as a step towards the formation of 

 milk-sugar, has also been observed by THIERFELDER. Fat seems 

 to be a never-failing constituent of the cell, at least in the secreting 

 gland, and this fat may be observed in the protoplasm as large or 

 small globules similar to milk-globules. The extractive bodies of 



