Chemical Constituents of the Animal Body. 265 



is evidence tJiat these compounds form the chief con- 

 stituents of the nuclei of cells. Nucleic acid itself is a 

 polynucleotide, a substance formed by the conjugation of 

 several nucleotide groups. The latter are formed by the 

 conjugation of phosphoric acid, a sugar, and a base, 

 which is either an alloxuric or a pyrimidine substance. 

 Both classes are chemically related to uric acid, and for 

 this reason the nucleic acids have been extensively investi- 

 gated on account of their pathological interest. It is 

 supposed that the plant nucleic acids differ from the 

 nucleic acids of animal origin, in that the former contain 

 a pentose sugar group whereas the latter contain a hexose. 



Another interesting group of conjugated proteins is 

 that of the hcemoglobins, in which the protein is combined 

 with a pigment. The latter contains iron, and is a 

 derivative of aetioporphyrin, from which the green chloro- 

 phyll pigments of plants are also derived. 



By means of gentle treatment with acids or with 

 certain ferments, intermediary hydrolysis products of pro- 

 teins can be obtained, which are polypeptides, less complex 

 than the original proteins. These on complete hydrolysis 

 undergo scission into mixtures of amino-acids. The 

 more complex of these hydrolysis products can be precipi- 

 tated from solutions by salts (that is, can be " salted out "), 

 such as ammonium sulphate or zinc sulphate, and are 

 known as proteoses or albumoses. The less complex products 

 which cannot be precipitated in this way, but which are 

 precipitable by phosphotungstic acid, are known as peptones. 



As a protein undergoes progressive hydrolysis, the 

 number of free amino and carboxyl groups increases. 

 The former may be estimated by treatment with nitrous 

 acid and measurement of the nitrogen evolved, and the 

 latter by titration with standard alkali in the presence of 

 formaldehyde, 



