H2 HUMAN PHYSIOLOGY 



lactis). The lactic acid thus formed precipitates the case- 

 inogen (souring of milk). 



(c) Fats (4$), in fine emulsions, not dissolved. Besides 

 the glycerides of palmitic, stearic, and oleic acid, milk 

 contains the glycerides of the lower fatty acids (butyric, 

 caproic, caprylic acid). On standing, the specifically lighter 

 fat globules rise upward, forming the cream. 



(d) Cholesterin, lecithin, and a yellow pigment in small 

 quantities. Besides these the milk is said to contain citric 

 acid, a product of the activity of the gland. 



(e) Salts (o. 5$), especially calcium phosphate, also potas- 

 sium chloride, a little sodium chloride, and a very little 

 magnesium sulphate and traces of iron. The calcium phos- 

 phate is present partly in solution as the acid salt and partly 

 in suspension as the neutral salt. 



Furthermore the milk contains gases, chiefly carbon 

 dioxide, and less nitrogen and oxygen. 



2. Conditions of the secretion of milk. The secretion of 

 milk takes place only during the lactation period which lasts 

 about ten months. 



The milk gland consists of 15-20 single tubular glands, 

 each one of which opens by a duct in the nipple. Just before 

 opening at the exterior, the tubules have a sac-like dilation. 



The lactiferous ducts have a wall of cylindrical epithelium. 

 The cells of the gland proper form a single layer of epithelial 

 cells, whose height varies greatly. When the ducts are 

 filled with secretion these cells are low, but when the ducts 

 are empty they are cylindrical and filled with numerous fat 

 droplets. The cells of the gland do not perish during secre- 

 tion, hence only form the secretion and excrete it. 



During the first days after delivery there are present in 

 the milk the so-called colostrum corpuscles which are 

 nucleated rudiments of cells containing many fat droplets. 



The nervous system has an influence upon milk secretion, 

 for the emotions can alter the quantity and character of the 

 milk. As to the secretory nerves authors differ. 



Nourishment has an effect upon the quantity and com- 



