i9o6.] 



The Secretion of Milk. 



219 



the blood capillaries, so that in this state the secretory activity 

 of the cells is suspended. 



As soon as suckling commences the acinus is emptied, and 

 the pressure upon the capillaries is released. The blood begins 

 to flow freely around the acinus, the cells gradually regain the 

 cylindrical shape which characterizes their active phase, and the 

 elaboration of milk commences. The growth of the cells in- 

 wards is irregular. While remaining closely attached at their 

 bases they become free internally, and project into the cavity of 

 the acinus-like papillae. At the same time fat globules appear 

 in the cells in the form of minute drops, accumulating especially, 

 however, in the projecting free inner parts of the cells. The 

 nucleus may also be divided. The free portion of the cell with 

 its contents is now detached,, and the cytoplasm dissolves in the 

 acinus, liberating the fatty contents, and a nucleus if such be 

 present. 



The secreting cells, after undergoing this change, continue to 

 secrete fat globules of various sizes, which gradually approach 

 the free surface, and are projected into the cavity of the acinus 

 by cytoplasmic contraction. In this case, however, there is no 

 sacrifice of the protoplasmic structure. The liquid portions of 

 the milk are also added to the alveolar contents. This goes on 

 until the ducts, and finally the acini, become onces more gorged 

 with milk, and the secretion in consequence gradually ceases 

 with the increase of alveolar tension. The cells become 

 stretched and flattened, the flow of blood is lessened, and the 

 acinus ceases to function until it is emptied again. 



In such a secretion period there is therefore (i) a short phase, 

 when the cells are preparing for their function, which is com- 

 summated with a sacrifice of their free borders ; and (2) a long 

 phase, which lasts until the gland is filled with milk, and during 

 which no further loss of protoplasnl takes place. The Activity 

 of each cell during the period of secretion may be , expressed by 

 a curve of a wave-like shape, the anterior side of which, after 

 gradually leaving the horizontal, rises rapidly to the crest, the 

 posterior descending more leisurely to the base line. 



If these m,ay be taken to be the conditions which, accompany 

 the successive secretions in the cow, it may be said that a 

 certain part at all events of the albumenoid constituents of milk 



