FOOD. [CH. xxvin. 



be confounded with the formation of casein from caseinogen 

 which is produced by rennet. There are, however, some bacterial 

 growths which, like rennet, produce true coagulation. 



Alcoholic Fermentation in Milk. When yeast is added to 

 milk, the sugar does not readily undergo the alcoholic fermenta- 

 tion. Other somewhat similar fungoid growths are, however, able 

 to produce the changes, as in the preparation of koumiss; the milk 

 sugar is first inverted, that is dextrose and galactose are formed 

 from it (see p. 388), and it is these sugars from which alcohol 

 and carbonic acid originate. 



The Salts of Milk. The chief salt present is calcium phos- 

 phate ; a small quantity of magnesium phosphate is also present. 

 The other salts are chiefly chlorides of sodium and potassium. 



The Mammary Glands. 



The mammary glands are composed of large divisions or lobes, and these 

 are again divisible into lobules ; the lobules are composed of the con- 

 voluted and dilated subdivisions of the main ducts held together by con- 

 nective-tissue. Covering the general surface of the gland, with the exception 

 of the nipple, is a considerable quantity of fat, itself lobulated by sheaths 

 and processes of areolar tissue (fig. 389) connected both with the skin in 

 front and the gland behind ; the same bond of connection extends also 

 from the under surface of the gland to the sheathing connective-tissue of the 

 great pectoral muscle on which it lies. The main ducts of the gland, fifteen 

 to twenty in number, called the lactiferous ducts, are formed by the union 

 of the smaller (lobular) ducts, and open by small separate orifices through 

 the nipple. At the points of junction of lobular ducts to form lactiferous 

 ducts, and just before these enter the base of the nipple, the ducts are 

 dilated ; .and during the period of active secretion by the gland, the dilatations 

 form reservoirs for the milk, which collects in and distends them. The walls 

 of the gland-ducts are formed of areolar with some unstriped muscular tissue, 

 and are lined internally by short columnar and near the nipple by flattened 

 epithelium. The alveoli consist of a basement membrane of flattened cells 

 lined by low columnar epithelium. 



The nipple is composed of areolar tissue, and contains unstriped muscular 

 fibres. Blood-vessels are also freely supplied to it, so as to give it an erectile 

 structure. On its surface are very sensitive papillas ; and around it is a 

 small area or areola of pink or dark-tinted skin, on which are to be seen 

 small projections formed by minute secreting glands. 



Blood-vessels, nerves, and lymphatics are plentifully supplied to the 

 mammary glands ; the calibre of the blood-vessels, as well as the size of the 

 glands, varies very greatly under certain conditions, especially those of 

 pregnancy and lactation. The secretory nerves of the mammary glands have 

 not yet been discovered. 



The alveoli of the glands during the secreting periods are found to be 

 lined with very short columnar cells, with nuclei situated towards the centre. 

 The edges of the cells towards the lumen may be irregular and jagged, and 

 the remainder of the alveolus is filled up with the materials of the milk. 

 During the intervals between the acts of discharge, the cells of the alveoli 

 elongate towards the lumen, their nuclei divide, and in the part of the cells 

 towards the lumen a collection of oil globules and of other materials takes 

 place. 



The next stage is that the cells divide and the part of each towards the 



