110 GENERAL PHYSIOLOGY 



which stand near the fats, but contain phosphorus ; they are pro- 

 bably present in every living cell and, according to Hoppe-Seyler, 

 are to be regarded as cleavage-products of proteids, especially of 

 nucleins, with which they occur. 



Among the non-nitrogenous end-products of proteid-decompo- 

 sition carbonic acid, which is produced by every cell, comes first 

 in importance. Lactic acid, oxalic acid, and sulphuric acid are im- 

 portant. The cholesterins also are to be regarded at least as deriva- 

 tives of proteids ; they seem to occur in all living substance, but 

 appear in great quantity only under certain circumstances in the 

 form of iridescent scales, as upon the surface of the skin and the 

 beak of birds, and in pathological conditions as gall-stones in the 

 bile. Chemically the cholesterins are univalent alcohols, which with 

 fatty acids can form fat-like compounds. Finally, there appear as 

 decomposition-products of proteids certain carbohydrates, particu- 

 larly grape-sugar and glycogen, and fats, which must be considered 

 somewhat in detail in connection with allied substances. 



I. Carbohydrates 



In contrast to its presence in the proteids, nitrogen is wanting 

 in the carbohydrates. The latter contain only the three elements, 

 carbon, hydrogen and oxygen ; in the natural carbohydrates the 

 number of carbon atoms within the molecule is always six or a 

 multiple of six, while the number of hydrogen atoms is always 

 double that of the atoms of oxygen ; hence hydrogen and oxygen 

 are present in the same relative proportions as in water a foct 

 which led to the designation " carbohydrates." The carbohydrates 

 are very wide-spread and are of great importance, especially in the 

 manufacture of living substance in plant-cells; but there are 

 varieties of living substance in which they cannot be demonstrated ; 

 in other words, they are not general constituents of such substance. 

 They present far simpler chemical relations than the proteids, and 

 a brief glance will show their most essential features. 



The natural carbohydrates may be divided into monosaccharids, 

 disaccharids and polysaccharids, of which the two latter groups are 

 different anhydride forms of the first group. 



The monosaccharids all have the formula C 6 H 12 Og, and are, there- 

 fore, isomeric ; but they are not all stereo-isomeric, that is, their 

 individual atoms are not grouped alike in all. To the mono- 

 saccharids belong chiefly grape-sugar (dextrose or glucose) andfruit- 

 sugar (Isevulose), both of which are wide-spread in plant juices, the 

 former in great quantity also in animal tissues. One of the most 

 remarkable characteristics of the monosaccharids is that they 

 readily take up oxygen from their surroundings and thus reduce 

 bodies that are rich in oxygen, a peculiarity upon which depend 

 the most important tests for their recognition. The most re- 



