14 ESSENTIALS OF PHYSIOLOGY. 



grouped to form the protein molecule. Moreover, when a protein is 

 hydrolysed by boiling with a mineral acid, the proportion of nitrogen 

 which is recovered in the form of ammonia, of monoamino-, and of 

 diamino-nitrogen varies considerably according to the protein examined. 

 It is especially noticeable that, whereas monoamino-acids are most 

 abundant in albumin and globulin, protamines, for example salmine, 

 derived from the spermatozoa of the salmon, are especially rich in basic 

 (diamino) nitrogen. 



PHYSICAL PROCESSES WHICH OCCUR IN 

 THE BODY. 



Ions. When sodium chloride is dissolved in water its molecules 

 undergo dissociation into sodium ions, which are charged with positive 

 electricity, and chlorine ions, which are charged with negative electricity. 

 Ions are not necessarily the same as atoms, since a solution of sulphuric 

 acid in water contains hydrogen ions and S0 4 ions. Substances which 

 undergo dissociation when in solution are called electrolytes, since an 

 electrical current passed through such a solution is conducted by the 

 movement of the ions ; and owing to the presence of electrolytes in 

 the tissues of the body, these are able to conduct electrical currents. 

 Many substances, however, such as sugar, when dissolved in water 

 do not undergo dissociation, and the dissolved molecules carry no 

 electrical charge. 



Diffusion and Osmosis. When a substance such as sodium chloride 

 is dissolved in water, the dissolved molecules behave like the molecules 

 of a gas ; they are in constant movement and exert pressure upon the 

 walls of the vessel containing them. If, for example, a vessel is divided 

 into two compartments by a vertical membrane, and if one compartment 

 is filled with water and the other with 1 per cent, salt solution, the 

 molecules of salt in their movements will beat upon the membrane ; 

 and if the latter is permeable to molecules of salt, they will pass througli 

 it into the distilled water until the amount of sodium chloride on the 

 two sides of the membrane becomes equal. This process is known as 

 diffusion, and the rate at which it occurs varies with the percentage of 

 sodium chloride originally present in the solution. 



If the membrane allows water but not sodium chloride to pass 

 through it, it is said to be semi-permeable, and the pressure exerted by 

 the molecules of salt upon the membrane is called osmotic pressure and 

 can be measured in the following manner. A semi-permeable mem- 

 brane is made by filling the pores of an earthenware cell with silicic 

 acid or copper ferrocyanide ; the cell is filled with 1 per cent, sodium 



