762 SPECIAL PHYSIOLOGY. 



The first secretion of milk is preceded by an enlargement of these 

 glands, which causes a certain hardness and tenderness of the part 

 and a febrile disturbance of the system, known as milk fever. The 

 first milk secreted, much thicker and darker than the subsequent 

 secretion, is named the colostrum. After lactation is established the 

 secretion is not uniform but remittent, proceeding slowly during the 

 intervals of sucking, so as not usually to accumulate and cause suffer- 

 ing, but suddenly increasing, in accordance with the great afflux of 

 blood to the glands during the act of nursing. The fulness and in- 

 creased secretion experienced at this time constitute the phenomenon 

 called the draught. From each distended gland the quantity obtain- 

 able by pressure is about two ounces, but the daily quantity secreted 

 by both fluctuates according to so many circumstances that no correct 

 average is attainable. The composition of the milk also varies exceed- 

 ingly. Its specific gravity ranges from 1030 or less, to 1035. The 

 color of human milk is bluish-white, owing to its greater transparency 

 as compared with cow's milk. It is opalescent, and perhaps fluores- 

 cent. It contains from 860 to 910 parts of water in 1000, the solid 

 matter varying accordingly from 14 to 9 per cent. ; its average com- 

 position is 89 parts of water to 11 of solid constituents. These latter 

 consist of 4.5 of lactin or sugar of milk, 3.5 of casein, 2.5 of fatty 

 matters or butter, .3 of extractives, and .2 of alkaline and earthy 

 salts, together with traces of iron. Milk likewise contains, like the 

 blood, carbonic acid gas, nitrogen, and oxygen, the total amount of 

 these gases being about 3 per cent, of its volume ; more than half of 

 this is carbonic acid gas, and only ^jth part oxygen, the remainder 

 being nitrogen. 



The milk is a true secretion, formed out of the materials of the exu- 

 ded plasma of the blood, by the agency of the epithelial cells of the ter- 

 minal vesicles of the gland. It is composed of a slightly turbid fluid, 

 containing suspended in it a vast number of minute, more or less spher- 

 ical particles, named the milk globules ; these are composed of an oily 

 matter, surrounded by a thin film or pellicle of albuminoid substance, 

 probably of casein, for neither ether nor an alkali, which would dis- 

 solve fatty matter, attacks them, unless they are first acted on by acetic 

 acid, or are strongly agitated, so as to dissolve or break the albu- 

 minoid film, which does not appear to be organized. These milk glob- 

 ules vary from ysj^^th to ^^^th of an inch in diameter; other and 

 much smaller spherical particles, manifesting the molecular movement, 

 exist in the fluid, and probably cause its turbidity ; some of these may 

 consist of casein, but they are chiefly fatty, and readily dissolve in 

 ether. The milk also contains a few epithelial cells from the ducts. 

 Owing to the thin pellicle around the milk globules, these do not at 

 once run together, but only coalesce, after a time, in the formation of 

 the cream. In the colostrum the milk globules are very minute, but 

 there also exist in it peculiar large, yellowish, closely and finely gran- 

 ular fatty corpuscles, which resemble the so-called exudation cells, or 

 compound inflammation cells; these appear to result from the fatty 

 degeneration or transformation of the glandular epithelial cells. The 

 colostrum contains albumen, or at least it coagulates on boiling; it also 



