LACTATION 



557 



tains cells which are supposed to have been detached from the 

 epithelium, but, as will be seen presently, there is some doubt 

 regarding this point. 



The alveoli secrete milk during lactation, not merely while 

 suckling is going on, but also at other times, so that milk tends 

 to collect in the ducts and especially in the reservoirs. It has 



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FIG. 136. Section of mammary gland (human) during lactation 

 (highly magnified), a, acini ; b, duct. 



been calculated that the udders of a cow could not contain the 

 quantity of milk which can be obtained from them at one milking, 

 so that in such cases at least it seems certain that the process 

 of secretion must be carried on during the time that the milk is 

 being drawn. Furthermore, the milk which is drawn latest 

 has been shown to have a different composition from that which 

 is first obtained, the proportion of solids to liquids undergoing 

 an increase as the process of milking is continued. This, how- 



