32 SECRETION 



by the cells of the mammary gland. During the period of 

 gestation the cells lining the alveoli are flat and have only a 

 single nucleus. When they begin -to secrete they increase in 

 height, the nuclei divide and that portion of the cell toward 

 the lumen undergoes fatty degeneration. This fatty ma- 

 terial is extruded into the lumen and apparently constitutes 

 a part of the secretion. The liquid constituents taken out 

 of the blood probably hold the proteid and carbohydrate 

 portions in solution, while the fatty particles constitute the 

 fat of the milk. Thus secreted, the liquid accumulates in the 

 ducts and sinuses until removed by the infant or otherwise. 

 The fact that the secretion of milk in woman is influenced 

 by emotions of fear, grief, etc., is strong evidence of a ner- 

 vous control of the procedure, but proof of secretory fibers 

 to the cells has not been established. 



The quantity of food required by the mother during the 

 time the child is nursed is increased, but no particular kind 

 of food seems to be especially required. The larger demand 

 for liquids is marked, however, and when the quantity of 

 milk is increased by a large ingestion of liquids, the solids in 

 the secretion are not relatively diminished. 



Composition and Properties of Milk. Human milk has 

 specific gravity of about 1030, and is not so white or so 

 opaque as cow's milk. Besides water, its chief constituents 

 are fats, lecithin, cholesterin, casein and lactose, of which 

 the two last named are the most important. Casein is the 

 main proteid constituent. Lactose is very abundant, and is 

 responsible for the sweet taste and for a large part of the 

 nutritive value of the fluid. 



Thyroid Gland. 



The thyroid gland consists of two glandular masses united 

 by an isthmus .of the same structure. It lies in front of the 

 trachea at the lower end of the larynx. It consists of a large 

 number of vesicles bound together by connective tissue. 



