120 TEXT-BOOK OF PHYSIOLOGY. 



According to Pfliiger and others, this view is not tenable. Pfliiger 

 asserts that, as material changes or metabolism can take place only within 

 living cells, all the protein must first be assimilated and organized by the 

 cells before it can undergo metabolic changes. Metabolism by contact 

 action is denied, and the division of protein into organ and circulating 

 protein is not justifiable. 



In the process of metabolism the protein suffers disintegration, giving 

 rise through oxidation to some carbon-holding compound, possibly fat, 

 possibly sugar and to some nitrogen-holding compounds, which eventually 

 give rise to urea. The intermediate stages, however, are not definitely 

 known; the immediate antecedents of urea are probably carbamate and 

 carbonate of ammonia. The disintegration of the proteins is attended by 

 the liberation of heat, thus contributing to the general store of the 

 energy of the body. 



The amino-acids that are not utilized in the synthesis of the necessary 

 blood proteins are absorbed by the intestinal epithelium and deprived of their 

 amidogenic nitrogen (NH 2 ). The latter is then converted into ammonium 

 carbonate, which is then carried to the liver and converted into urea. The 

 remainder of the amino-acid is carried into the circulation, and is eventually 

 oxidized, thus giving rise to heat. It is also possible that some of the amino- 

 acids are carried to the tissues and there directly used in tissue formation. 



The fat principles while in the alimentary canal also undergo a series of 

 changes whereby they are reduced to soap and glycerin, under which forms 

 they are absorbed. During the act of absorption the soap and glycerin 

 are synthesized to human fat. The fine particles thus formed in the 

 intestinal wall are carried by the lymph vessels to the thoracic duct, and 

 thence into the blood stream, from which they rapidly disappear. Though 

 it is possible that a portion of the fat enters directly into the formation of 

 the living material, it is generally believed that it is at once oxidized and 

 reduced to carbon dioxid and water with the liberation of energy. The 

 natural supposition that a portion of the ingested fat is directly stored 

 up in the cells of the areolar connective tissue, thus giving rise to adipose tissue, 

 has been a subject of much controversy, though modern experimentation 

 renders this very probable. The body-fat, under physiologic conditions, 

 is also a product of the metabolic activity of connective-tissue cells and is a 

 derivative of both proteins and carbohydrates. 



The carbohydrate principles are reduced during digestion to simple forms 

 of sugar, chiefly dextrose and levulose. Under these forms they are absorbed 

 into the blood. These compounds are then carried to the liver and to the 

 muscles where they are dehydrated and stored under the form of starch, 

 termed animal starch or glycogen. Subsequently glycogen is transformed 

 by hydration to sugar, after which it is oxidized to carbon dioxid and water. 

 The intermediate stages through which sugar passes before it is reduced 

 to carbon dioxid and water are only imperfectly known. Though a large 

 part of the carbohydrate material is at once oxidized, it is now well estab- 

 lished that another portion contributes to the formation of, if it is not directly 

 converted into, fat. As the carbohydrates form a large portion of the food, 

 they contribute materially to the liberation of energy. 



The inorganic principles, though apparently not playing as active 



