34 PHYSIOLOGY FOR DENTAL STUDENTS. 



stances to become condensed or concentrated at the surface of 

 colloidal molecules. An example is the well known action of 

 charcoal when shaken with colored solutions. It removes the pig- 

 ment by adsorbing it. Adsorption is due to surface tension, 

 which is the tension created at the surface between a solid and a 

 liquid, or between a liquid and a gas. It is in virtue of surface 

 tension that a raindrop assumes more or less spherical shape. 

 Since colloids exist as particles, there must be an enormous num- 

 ber of surfaces throughout the solution, that is, an enormous sur- 

 face tension. Now many substances, when in solution, have the 

 power of decreasing the surface tension, and in doing so it has 

 been found that they accumulate at the surface, that is to say, 

 in a colloidal solution, at the surface of the colloidal molecules. 

 The practical application of this is that it helps to explain the 

 physical chemistry of the cell, the protoplasm of which is a col- 

 loidal solution containing among other things proteins and 

 lipoids. The latter depress the surface tension and therefore 

 collect on the surface of the cell and form its supposed mem- 

 brane, whilst the proteins exist in colloidal solution inside. It is 

 possibly by their solvent action on lipoids that ether and chloro- 

 form so disturb the condition of the nerve cells as to cause anes- 

 thesia. 



General Nature of Enzymes or Ferments. 



To decompose proteins, fats or carbohydrates into simple mole- 

 cules in the laboratory necessitates the use of powerful chemical 

 or physico-chemical agencies. Thus, to decompose the protein 

 molecule into amino bodies requires strong mineral acid and a 

 high temperature. In the animal body similar processes occur 

 readily at a comparatively low temperature and without the use 

 of strong chemicals in the ordinary sense. The agencies which 

 bring this about are the enzymes or ferments. These are all col- 

 loidal substances (see p. 32), so that they are readily destroyed 

 by heat and are precipitated by the same reqgents as proteins. 

 They are capable of acting in extremely small quantities. Thus, 

 a few drops of saliva can convert large quantities of starch solu- 

 tion into sugar. During their action, the enzymes do not them- 



