MAMMALIA— PHYSIOLOGY 435 



Chemical Characteristics of Protoplasm. — When we attempt an 

 analysis of the chemical constituents of living protoplasm, we induce 

 important changes. By weighing the material before treating it, 

 and then comparing the weights of all substances determined, we 

 find that it is possible to learn most of the constituents, except that 

 all-important one — life. Protoplasm consists of proteins, carbo- 

 hydrates, fats, inorganic salts, enzymes, water and the " vitamins." 



Proteins are compounds with high molecular weights, which 

 contain carbon, oxygen, hydrogen and nitrogen. Usually the/ 

 contain sulphur, and sometimes phosphorus. They are apparently 

 a condensation of the molecules of numerous amino-acids and by 

 hydrolysis they yield amino-acids in different quantities and of 

 different characteristics. 



Proteins differ in their color reactions and are readily classified 

 by such reactions. Proteins also differ in precipitation reactions. 

 Some are precipitated by the mineral acids such as nitric, hydro- 

 chloric, and sulphuric, others by salts of the heavy metals, par- 

 ticularly mercuric salts. The alkaloidal reagents which precipitate 

 the vegetable alkaloids are precipitants for proteins. Eggs, rich in 

 proteins, are used as an antidote for copper, lead or mercury poison, 

 since they render the metallic compounds insoluble so that they 

 may be removed by the stomach-pump or laxatives. 



Colloids. — Proteins are colloids. The term " colloid " which 

 comes from the Greek word for " glue " referred at first to substances 

 like glues and gums, but now is used to indicate finely divided mat- 

 ter suspended in any medium. Colloids may include liquid dis- 

 persed in a gas or another liquid or even a solid; or may refer to 

 solids similarly dispersed. Blood, lymph, bile and the various 

 digestive secretions are common examples of colloidal solutions. 



Proteins, as colloids, do not readily diffuse through membranes 

 nor go into solution They readily absorb substances however and 

 in the cell they synthesize and oxidize them. Protein individuality 

 is such that the blood of one order cannot, as a rule, be transfused 

 successfully into the circulation of another order without fatal 

 results. Foreign proteins are apparently incompatible. It is 

 interesting to note, however, that human blood may be successfully 

 administered to certain of the anthropoid apes. Clumping together 

 (agglutination) of blood corpuscles occurs when incompatible blood 

 is transfused. 



