438 NUTRITION ANIMAL HEAT AND FORCE. 



name, in this subdivision, being restricted to certain albuminoids which 

 closely resemble proteids but possess some distinctive characters. Inasmuch 

 as proteine is an hypothetical compound and the so-called proteids do not 

 differ much from other nitrogenized substances, it seems better to designate 

 the entire class as albuminoids. 



The so-called proteids are the albuminoid constituents of the blood, 

 lymph and chyle, and the characteristic albuminoid constituents of the vari- 

 ous tissues. These are sometimes called colloids. They pass through mem- 

 branes with difficulty, or are very slightly osmotic. In this regard they pre- 

 sent a striking contrast to the peptones, which are very osmotic, passing 

 easily through animal membranes. This distinction is important, and it has 

 already been fully described in connection with the physiology of digestion 

 and absorption. 



Nitrogenized matters constitute an important class of alimentary sub- 

 stances, and the corresponding constituents of the body are all originally 

 derived from food. The condition of existence of these substances in the 

 body is always one of union with more or less of the class of inorganic mat- 

 ters. Nitrogenized matters are found in all of the tissues and liquids of the 

 body, except the bile and urine. They undergo changes in digestion before 

 they become a part of the blood, they are changed in the blood into the 

 nitrogenized constituents of this fluid and are again changed as they are de- 

 posited in the tissues in the process of nutrition. They are not discharged 

 from the body in health, but are destroyed or changed into excrementitious 

 matters, chiefly urea, and in this form are eliminated in the excretions. An 

 excess of these substances taken as food is not discharged in the faeces, nor 

 does it pass out, in the form in which it entered, in the urine ; but it under- 

 goes digestion, becomes absorbed by the blood, and increases the quantity of 

 nitrogenized excrementitious matters discharged, particularly the urea. This 

 fact is shown by the great increase in the elimination of urea produced by 

 an excess of nitrogenized food. Whether the nitrogenized matter that is /iot 

 actually needed in nutrition be changed into urea in the blood, in the so- 

 called luxus-consumption process, or whether it be appropriated by the tissues, 

 increasing the activity of their disassimilation, is a question difficult to deter- 

 mine experimentally. Certain it is, however, that an excess of nitrogen- 

 ized food is thrown off in nearly the same way as an excess of inorganic 

 matter ; the difference being that the latter passes out in the form in which 

 it has entered, and the former is discharged in the form of nitrogenized ex- 

 crementitious matters. 



The nutrition of the nitrogenized constituents of the tissues may be greatly 

 modified by the supply of new matter. For example, a diet composed of nitro- 

 genized matter in a readily assimilable form will undoubtedly affect favorably 

 the development of the corresponding tissues of the body ; and on the other 

 hand, a deficiency in the supply will produce a corresponding diminution in 

 power and development. The modifications in nutrition due to supply have, 

 however, certain well defined limits. As regards the muscular tissue, proper 

 exercise increases nutritive activity, the development and power of muscles 



