32 SECRETION 



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 material is extruded into the 

 lumen and apparently constitutes a part of the secretion. The 

 liquid constituents taken out of the blood probably hold the pro- 

 teid and carbohydrate portions in solution, while the fatty par- 

 ticles 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 in- 

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

 nervous 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 a 

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

 as cow's milk. Besides water, its chief constituents are fats, lec- 

 ithin, 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. Each vesicle 

 is lined by cuboidal epithelial cells, which secrete a semi-gelatin- 

 ous substance, colloid. 



