4 MANUAL OF MILK PRODUCTS 



the alveoli are smaller glands or ducts within the lobule. The 

 alveoli are exceedingly small and can be seen only under a 

 microscope of high power. 



The actual secretion of the milk goes on in the alveoli. Ex- 

 actly how the milk is secreted is not known. 



THE ELABORATION OF MILK IN THE ANIMAL ORGANISM 



Early theories. 



"There has been a large number of theories advanced as to 

 the methods by which milk is elaborated, most of them based 

 upon the assumption that it is a comparatively simple chemical 

 and physical problem. All the earlier theories were based upon 

 such assumption, the physiologists regarding the mammary 

 gland as an organ to separate certain elements from the blood 

 in definite proportions as milk. It was regarded that the pro- 

 cess was largely one of transudation through a special mem- 

 brane, on the same principle that exchange of gases by osmosis 

 occurs rapidly in the tissues of the lungs. It was assumed 

 that the fat of the food and the water and the salts taken into 

 the alimentary canal were absorbed and taken into the blood 

 and then eliminated by the mammary gland. The milk-serum 

 was regarded as escaped blood serum, and that the other particles 

 were derived from the blood or epithelial cells. The gland was 

 assumed to be a semipassive organ, receiving the milk already 

 prepared, and only requiring elimination in the proper propor- 

 tions." Bitting. 



In order to test the accuracy of this theory, Jordan and 

 Jenter 1 conducted a very careful experiment for the purpose 

 of determining whether or not the fat in milk is derived from 

 the fat in the food which the cow consumes. Analyses were 

 made of the feeds and milk and the urine and foeces collected, 



1 New York Expt. Sta. Bui. 132. 



