CHAP. rv. THE FAT OF MILK. 281 



network of which surrounds every alveolus. Out of the blood thus 

 placed at their disposal the secreting cells manufacture milk, which 

 Hows along the ducts, and accumulates in the milk cistern at the top of 

 each teat. The general plan, here described, upon which the 

 mammary glands are constructed, is similar to that of the salivary 

 glands of the mouth. 



In the active gland each alveolus encloses a relatively large cavity, 

 varying in size in different alveoli ; it is lined internally by a single 

 layer of columnar epithelium cells, each containing protoplasm and a 

 nucleus. The transparent homogeneous granular-looking substance 

 called protoplasm is living matter, by and from which all the tissues of 

 animals, and of plants, are built up ; its composition is ever changing, 

 it is throughout active life continually receiving new matter, and is con- 

 stantly parting with material which has been elaborated within itself, 

 hence it is impossible to assign to it any definite chemical constitution. 

 " For the living and life-giving protoplasm is endowed with internal 

 forces, and as the result of this, with an internal and external varia- 

 bility which is wanting in every Wher known structure ; its active 

 molecular forces cannot, in short, be compared with those of any other 

 substance. The capacity which protoplasm has, in consequence of the 

 forces which become manifested in it, of assuming definite external 

 forms, and of varying these, as well as its capacity of secreting sub- 

 stances of different chemical and physical properties according to 

 definite laws, is the immediate cause of cell-formation and of every 

 process of organic life." l 



Each epithelial cell of the alveoli of the mammary gland is capable 

 of forming in its interior one or more oil globules of varying size. 

 These may and generally do run together, and, pressing the nucleus 

 which is a somewhat denser aggregation of the internal protoplasm 

 on one side of the cell, give to the latter the appearance of a fat cell. 2 

 And a fat cell it would probably become, and the whole mammary 

 gland would in all likelihood undergo a conversion into a mass of 

 adipose tissue, if the fat were not ejected from the epithelial cells, and 

 particularly if, in these circumstances, the production of fat were 

 exalted at the expense of the production of casein or of milk-sugar. 3 

 The fat can be seen to be gathered in the epithelium cell, and to be 

 ejected by the cell protoplasm through the wall of the cell into the 

 cavity of the alveolus. The cell resumes its former solid character, 

 and begins again to form oil globules in its protoplasm, and the epi- 

 thelial cells, so long as the secretion of milk is continuous, go on again 

 and again forming globules of fat without being themselves destroyed. 4 

 Thus, the fat of milk, whose myriad globules have become confluent in 

 every pat of butter, is formed in the microscopic epithelial cell through 

 the metabolism 5 of its protoplasm. The fat-globules as brought into 



1 Sachs, " Text-book of Botany, Morphological and Physiological." 



2 Klein, " Elements of Histology. " 



3 Foster, "Physiology." 



4 Langer, quoted by Klein, "Histology." 



5 Gr. metabole, change. 



