288 



PHYSIOLOGICAL CHEMISTRY. 



essentially protein diet, milk fat is formed, and in the herbi- 

 vora on a diet containing largely pentoses and other carbo- 

 hydrates milk fat is likewise produced normally and in 

 quantity. 



These facts then seem to be clear, that while under some 

 circumstances fats as such pass from the blood into the milk, 

 and this is further evident by the experience of feeding cows 

 with certain foods rich in fats, the milk glands have the 

 power of producing the several individual fats as occurring in 

 milk from compounds which are not fat to begin with. In 

 discussing the chemistry of proteins in an earlier chapter it 

 was shown that in the breaking down of these bodies under 

 the influence of various agents fatty acids are found among 

 the decomposition products. The complex protein molecule 

 may be all that is necessary to give rise to the milk fats if 

 other things are not available. 



The origin of milk sugar is not at all clear. Lactose is not 

 a constituent of our ordinary foods and at best the blood con- 

 tains probably only inverted sugars or monosaccharides. In 

 some cases the formation of milk sugar may be traced indi- 

 rectly to the carbohydrates of the food ; but this will not ex- 

 plain the production of sugar in the carnivora. Here as be- 

 fore we are probably obliged to fall back on the behavior of 

 the complex proteins. Among the groups they contain, or at 

 any rate yield in decomposition, the presence of sugar groups 

 has been certainly shown. This was explained in a former 

 chapter. The milk lactose probably results from a synthesis 

 of these simpler sugars. 



From what is in general known of the nature of complex 

 protein matter such as exists in the milk glands it seems there- 

 fore possible to trace the origin of the milk proteins, sugar and 

 fats to the disintegration of this original protein substance. 

 But of the agents of disintegration, and following necessary 

 syntheses, we know absolutely nothing. The presence of cer- 

 tain enzymes has been assumed, but as they have not been 

 isolated or identified, their part in the reactions remains 

 speculative. 



