628 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



According to the results of Heidenhain, the histological appearance 

 of the cells of the mammary glands differs according as they are examined 

 at the commencement or termination of secretion or while the secretion 

 is at its height. 



In the first stage of secretion the cells are flattened and lie against 

 the walls of the alveoli, of which they may be regarded as forming a 

 protoplasmic boundary. Their nuclei are at this period spindle-formed, 

 lying close to the contours of the cells, scarcely detectable on examina- 



FIG. 262. MAMMARY GLAND OP THE DOG IN FIRST STAGE OF SECRETION. 



(Heidenhain.) 



a, b, section through the centre of two alveoli of the mammary gland of the dog, the epithelial cells seen 

 in profile ; c, surface view of the epithelial cells. 



tion in transverse section. Seen from above, however, the epithelial 

 cells are found to be polygonal, and each containing a round nucleus. 

 In the terminal period of secretion the cells may be found to have greatly 

 increased in size, possess one to three nuclei, and contain in the portions 

 directed toward the alveoli large numbers of fat-globules. Often the 

 cells may be seen to undergo subdivision, a part falling free into the 

 alveolus (Figs. 262, 263, and 264). Between these two extreme periods 



FIG. 203. MAMMARY GLAND OF THE DOG IN SECOND 

 STAGE OF SECRETION. (Heidenhain.) 



FIG. 264. MAMMARY GLAND OF 

 THE DOG IN MIDDLE STAGE 

 OF SECRETION. (Heidenhain.) 



various intermediary stages may be recognized. From these histological 

 changes Heidenhain concludes that in the formation of colostrum the 

 epithelial cells are not dissolved, and that, therefore, the colostrum 

 corpuscles are not fatty, degenerated epithelial cells, but that only the 

 free end of the epithelial cells with their contained oil-globules is liber- 

 ated ; that the broken-down protoplasm becomes dissolved in the milk, 

 and the fat-globules are thus set free. 



