GENERAL STRUCTURE AND COMPOSITION 17 



ing out of complex chemical processes such as go on in the 

 Body. Although the water of the Body does not itself take 

 active part in the life processes, these processes could not go on in 

 its absence. 



Crystalloids. Substances which form crystals when solutions 

 containing them are evaporated are classed as crystalloids. Ex- 

 amples are common salt and cane sugar. From the standpoint 

 of physical chemistry as applied to the Body the important fact 

 concerning crystalloids is not their crystal-forming ability, but the 

 relatively small size of their molecules, which gives them a cor- 

 respondingly high degree of freedom of motion in solution and 

 facility in entering and leaving chemical combinations. All living 

 tissues contain crystalloids as part of their substance. The 

 amounts are relatively small, although the materials are as neces- 

 sary to living protoplasm as are those that make up the greater 

 part of its mass. 



Colloids. These are substances which do not form crystals 

 when solutions of them are evaporated, but appear as gelatinous 

 or gummy masses. Examples are white of egg, and ordinary 

 table gelatin. Colloids are composed, in general, of much larger 

 molecules than are crystalloids. They have correspondingly less 

 facility of chemical action. Proteins are colloidal in structure; 

 hence living protoplasm, which is chiefly protein in its constitution, 

 is colloidal. 



Cell Membranes. To preserve definite structure in the watery, 

 gelatinous protoplasm individual cells are enclosed in cell mem- 

 branes. These should not be confused with the woody envelopes 

 in which many plant cells are enclosed. Reference here is to the 

 delicate sheaths which surround all cells, both plant and animal, 

 and which serve to keep the semi-liquid contents of the cells from 

 running together into a formless mass. The chemical nature of 

 the membranes is not certainly known, although there is reason 

 to believe that they consist of protoplasm which differs from that 

 of the cells at large chiefly in its greater density. The significance 

 of cell membranes in Physiology lies in the fact that all interchanges 

 between living protoplasm and its surroundings must take place 

 through them. Any nourishment any cell receives must pass 

 through the cell membrane before it reaches active protoplasm. 

 Similarly, all materials which cells discharge have to be expelled 



