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PHYSIOLOGY 



CHAP. 



they projected irregularly into the lumen of the alveolus, had 

 usually two or three nuclei, and contained fatty globules 

 similar to those of the milk, at the end which projected into 

 the lumen. 



Later work on the mammary gland in full secretion did not 

 confirm these observations. Generally speaking, the epithelial 

 cells are uniformly flat or at most cubical, as shown by Fig. 

 141. Schafer regards it as probable that the columnar appearance 

 described by Partsch and Heidenhain is found only in collapsed 

 alveoli, due to the deformation of the basement membrane. On 

 the other hand, the same argument which Heidenhain urged 

 against the first view can with slight modifications be adduced 



FIG. 140. Alveoli of mammary gland of bitch under different conditions of activity. (Heidenhain.) 

 a, b, section through the middle of two alveoli at the commencement of lactation, the epi- 

 thelium cells being seen in profile ; c, part of the wall of an alveolus in a similar condition, 

 with epithelium cells seen flat; <7, alveolus in full secretory activity. 



against his own, which assumes the formation of the milk to 

 depend upon disintegration of the projecting, non-nucleated part 

 of the cell protoplasm. The regeneration of the protoplasm lost 

 by secretion would have to be so rapid that we know of no 

 analogous phenomena to give countenance to it. 



Since in all the other glands we have been studying the 

 secretory process takes place without any cellular destruction or 

 regeneration (except in the case of the sebaceous glands, the 

 function of which is no true secretion), it is difficult to see why 

 the simplest hypothesis should not also be accepted for the 

 lacteal secretion. According to this the epithelial cells of the 

 mammary gland have the power of forming the specific organic 

 constituents of milk from conversion of the crude materials drawn 

 from the lymph, not by any cytological process that involves the 

 disintegration of the protoplasm, but by an essentially chemical 

 method, the exact nature of which is unknown to us, as in the 



