COMPOSITION OF MILK. 787 



greatly under certain conditions, especially those of preg- 

 nancy and lactation. 



The secretion of milk, which under ordinary healthy 

 circumstances only occurs after parturition, if we except 

 the slight secretion which takes place in the latter months 

 of pregnancy, is effected by the epithelial cells lining the 

 ultimate follicles of the mammary gland. The process does 

 not differ from secretion in glands generally (see p. 409), 

 and need not here be particularly described. 



Under the microscope, milk is found to contain a num- 

 ber of globules of various size (fig. 24.1), the majority about 

 yoiroT of an inch in diameter. They are composed of oily 

 matter, probably coated by a fine layer of albuminous 

 material, and are called milk-globules ; while, accompanying 

 these, are numerous minute particles, both oily and albu- 

 minous, which exhibit ordinary molecular movements. The 

 milk, which is secreted in the first few days after parturi- 

 tion, and which is called the colostrum, differs from ordinary 

 milk in containing a larger quantity of solid matter ; and 

 under the microscope are to be seen certain granular masses 

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

 masses of albuminous and oily material, are probably 

 secreting cells of the gland, either in a state of fatty de- 

 generation, or, as Dr. Gedge remarks, old cells which in 

 their attempts 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. 



The specific gravity of human milk is about 1030. Its 

 chemical composition has been already mentioned (p. 257). 



