pjiYsioLoay OF organs . 147 



are passed on to the other cells. In animals with a circulatory system, 

 the simpler substances pass through the absorbing cells directly into the / ^^ 

 blood stream, or into the lymphatic system and then into the blood 

 stream. The principles of osmosis, diffusion and imbibition are usually 

 invoked to explain absorption through the walls of the ahmentary tract. 

 Numerous experiments, however, conclusively show that absorption is 

 often greatl}" modified ])y the activity of the absorbing cells. 



Sjmthesis of Absorbed Substances. — ^During passage through the ab- 

 sorbing cells some of the simpler substances may be synthesized into more 

 complex ones. This is certainly true of fats which are absorbed as glyc- 

 erol and fatty acids, for these latter substances are not found as such in 

 the blood stream nor in the lymphatics. Globules of fat can be demon- 

 strated by proper staining methods in the epithehal cells of the intestine 

 during the process of digestion, and after a meal the branches of the 

 lymphatics which arise in the intestinal walls are white from the emulsified 

 fat contained in the lymph. 



There is still considerable discussion regarding the history of the 

 amino-acids after absorption. Recent investigations show that they 

 are absorbed as such and that they occur in small quantities in the blood. 

 It seems probable that they circulate in the blood stream and are selected 

 as needed by the various organs for the synthesis of the particular 

 proteins found in these organs. It is believed that some of the amino- 

 acids not used in the tissues are deaminized, that is, their NH2 groups are 

 removed, and the nitrogen is ehminated as urea, while the organic 

 compound remaining maj^ be oxidized yielding energy. 



The carbohydrates absorbed as dextrose or levulose are collected 

 in the liver where they are transformed into glycogen, (CeHioOs)!, a 

 substance having the same empirical formula as starch. This trans- 

 formation is accomplished by means of enzymes. From time to time 

 glycogen is released into the circulation as dextrose which is carried to 

 the muscles and other tissues of the body where it is reconverted to glyco- 

 gen and is finally used as a source of energy. In animals gh^cogen maj' be 

 synthesized from the products of protein hydrolysis and also from fats, 

 especially from the glycerol portion of the fat molecule. 



Whether, in animals, fats and carbohydrates can be utilized in the 

 synthesis of protein is not certain, but plants are capable of synthesis 

 of this type. In animals the use of fats and carbohydrates effects a 

 saving of proteins. Reserve fats are stored in special fat-storing organs 

 ■as the fat-bodies {corpora adiposa) of frogs and toads; in connective 

 tissues between the skin and muscles, or between muscles ; or about the 

 abdominal viscera along the mesenteries. A small quantity of fat can 

 be demonstrated in all the tissue cells where it occurs in minute globules. 



The utilization of reserve foods requires the use of enzymes which 

 occur in all the tissues of the body. These are proteolytic, amylolytic. 



