286 



THE COMPLETE GRAZIER. 



BOOK II. 



receive, while the latter apparently take up ready-made fat and pass 

 it on. Moreover, the mammary secreting cells are further capable 

 of elaborating milk-sugar and casein out of the constituents of the 

 blood. 



At the beginning and at the end of lactation, and equally during the 

 suspension of that function, the mammary gland presents features 

 which well deserve study. The re sting- gland, that is, the gland of a 

 non-pregnant or non-suckling animal, contains fewer alveoli than the 

 active gland, but a great deal of fibrous connective tissue. The alveoli, 

 too, are at this period solid cylinders with no internal lumen, but 

 during pregnancy these solid alveoli rapidly multiply, lengthen, and 

 thicken, owing to the division of the epithelial cells. When milk secre- 

 tion commences, the cells occupying the central part of the alveokis 



undergo the fatty degeneration, and 

 are at once excreted. It is the 

 presence of these cells which imparts 

 to the milk, for several days after 

 calving, the peculiar properties in 

 virtue of which it is called colostrum, 

 or colustrum (figs. 69 and 70). 

 The central cells of the alveoli are 

 appropriately termed colostrum cor- 

 puscles, and their elimination pro- 

 vides the cavity, or lumen, inside 

 each alveolus, into which the fat 

 globules formed in the peripheral 

 epithelial cells are ejected. The 

 peripheral cells, it is to be noticed, 

 do not sacrifice their position like 

 the central ones, or there would be 

 "<>ne left f ?5 the wol \ of secretion; 

 but bchmid asserts that even these 

 finally break up," one by one, and 





The 



lower 

 milk ; 



half of the figure represents 



the upper half colostrum. 



up," one y one, 



are replaced by new epithelial cells derived from the multiplication 

 of the other still active ones. The small bits of granular substance 

 met with here and there in milk are the remains of the worn out and 

 broken down protoplasm of such epithelial cells. 1 



The deep yellow colour, unctuous character, and higher specific gravity 

 of colostrum are thus readily explicable. Furstenberg thought that 

 the pieces of membrane and clusters of cells which appear in this early 

 mammary fluid were milk globules in a state of transition, that is, not 

 yet perfectly formed. 2 But now they are known to be the degraded 

 fat-laden cells of the axial region of the alveolus, which must, as it 

 were, be cleared for action before the mammary gland can attain its 

 full secretory power. The reason, too, that milk drawn within the 

 first week after calving should never be used for making cheese is that 

 such milk contains in relative abundance actual animal matter derived 



1 Klein, " Elements of Histology." 



2 Sheldon's " Dairy Farming." 



