SECRETION. 331 



THE MAMMARY SECRETION: MILK. 



Under tKe microscope, milk is found to contain a number of globules 

 of various sizes (Fig. 224), the majority about TTjFO - of an inch in diam- 

 eter. They are composed of oily matter, probably coated by a fine layer 

 of albuminous material, and are called milk-globules; while, accompany- 

 ing these, are numerous minute particles, both oily and albuminous, 

 which exhibit ordinary molecular movements. The milk which is 

 secreted in the first few days after parturition, and which is called the 

 rnlnsfrum, differs from ordinary milk in containing a larger quantity of 

 solid matter; and under the microscope are to be seen certain granular 



FIG. 224. Globules and molecules of Cow's milk. X 400. 



masses called colostrum-corpuscles. These, which appear to be small 

 masses of albuminous and oily matter, are probably secreting cells of the 

 gland, either in a state of fatty degeneration, or old cells which in their 

 attempt at secretion under the new circumstances of active need of milk, 

 are filled with oily matter; which, however, being unable to discharge, 

 they are themselves shed bodily to make room for their successors. Colos- 

 trum-corpuscles have been seen to exhibit contractile movements and 

 to squeeze out drops of oil from their interior (Strieker). 



Chemical Composition. Milk is in reality an emulsion consisting 

 of numberless little globules of fat, coated with a thin layer of albumi- 

 nous matter, floating in a large quantity of water which contains in solu- 

 tion casein, serum-albumin, milk-sugar (lactose), and several salts. Its 

 percentage composition has been already mentioned, but may be here 

 repeated. Its reaction is alkaline: its specific gravity about 1030. 



