GROWTH OF THE HUMAN BODY 705 



K. Basch conceives that casein is formed by combination of a pseudonuclein, 

 which is set free from the nuclei of the gland cell, with proteid. He bases 

 this view on the fact, among other things, that by the action of a xanthin-free 

 and sugar-free nucleic acid, prepared from the mammary gland, on the serum 

 of ox blood, he was able to get a substance which had all the physical and 

 chemical properties of cow's milk casein. 



The percentage of fat in the milk can be raised by feeding fat. This increase 

 is explained in different ways by different authors ; but it is fairly probable that 

 some of the fat of the food at least is carried over into the secretion. The fact 

 that w r hen iodized swine fat has been fed, it can be demonstrated in the milk 

 in fairly large quantities (Winternitz) would favor this view. When iodine 

 alone or potassium iodide is fed only the merest traces of iodine are found in 

 the milk (Caspari). 



On the other hand we have observations by Henriques and Hansen which 

 indicate that the fat of the food in its passage through the cells of the milk 

 glands suffers changes in its composition. 



Even when an animal in lactation receives a. diet which is poor in fat, the 

 milk contains a considerable quantity of fat. In this case the milk fat must 

 have been formed either from the carbohydrates fed or from the large deposits 

 of fat in the body. This is demonstrated especially by the appearance of iodized 

 fat in the milk at a time when there was no iodized fat in the food, but when 

 iodized fat had previously been stored in the body (Caspari). 



A cow on a restricted diet naturally secretes less milk than on a full diet; 

 but the fat in her milk still has a lower melting point than the body's fat. This 

 means that when the fat deposited in the body is called into requisition, rela- 

 tively more olein is mobilized than palmatin and stearin (Henriques and 

 Hansen). 



It has ofttimes been confirmed that both the quantity of milk and the per- 

 centage of fat secreted increases on a diet which is rich in proteid, and this 

 can only mean that proteid either directly or indirectly is a source of fat. 



According to Thierfelder the sugar of milk arises from some mother sub- 

 stance not yet definitely identified, by the action of an enzyme associated with 

 the gland cells. 



The following may be mentioned here among the many different external 

 agencies which influence the quantity and quality of the milk: (1) Frequent 

 milking favors the activity of the glands. When the milking is done at stated 

 intervals, the last milk stripped from the glands is richest in fat. This is 

 probably due to the adherence of many fat particles to the walls between the 

 folds of the mucous membrane as the earlier milk flowed through, the stripping 

 process being necessary to dislodge them. (2) Too vigorous exercise diminishes 

 the quantity of the milk, probably because the blood stream is diverted from 

 the glands to the muscles. On the other hand, moderate exercise increases the 

 secretion of milk in women, probably owing to its favorable effects on respira- 

 tion, circulation, digestion, etc. (II. Munk). 



SECOND SECTION 



GROWTH OF THE HUMAN BODY 



Several different periods may be recognized in the life of an individual 

 human being. They are not marked off by sharply defined boundaries, so 

 that one can say for example that on a certain day the individual is a youth 



