20 PHYSIOLOGY FOR DENTAL STUDENTS. 



activity the protoplasm produces the secondary constituents of 

 the cell, which arc not the same in all cells, and which include the 

 granules of pigment or other material, the masses of glycogen, 

 the globules of fat or the vesicles of fluid which are found em- 

 bedded in the protoplasm. 



By whatever process we attempt to isolate its constituents, we 

 of course kill the cell, so that we can never learn by analysis what 

 may have been the real manner of union of these substances in 

 the living condition. All we can find out is the nature of the 

 building material after the structure (the cell) into which it is 

 built has been pulled to pieces. If the chemical process by which 

 we disintegrate the cell is a very energetic one, for example, com- 

 bustion, we always find the elements, carbon, hydrogen, nitrogen, 

 oxygen, sulphur, phosphorus, sodium, potassium, calcium, chlo- 

 rine, and usually traces of other elements, such as iodine, iron, 

 etc. If the decomposition be less complete, definite chemical 

 compounds are obtained, namely, water, proteins, lipoids, car- 

 bohydrates, and the phosphates and chlorides of sodium, potas- 

 sium and calcium. We shall proceed to consider briefly the main 

 characteristics of each of these substances and their place in the 

 animal economy. 



WATER. This is the principal constituent of active living 

 organisms, and is the vehicle in which the absorbed foodstuffs 

 and the excretory products are dissolved. It may be said indeed 

 that protoplasm is essentially an aqueous solution, in which other 

 substances of vast complexity are suspended. Water, on account 

 of its very unique physical and chemical properties, is of prime 

 importance in all physiological reactions. These properties are : 

 its chemical inactivity at body temperatures; its great solvent 

 power (it is the best known universal solvent) ; its specific heat, 

 or capacity of absorbing heat ; and, depending on this, the large 

 amount of heat which it takes to change water into a vapor 

 latent heat of steam. These last mentioned properties are made 

 use of in the higher animals for regulating the body temperature. 



Of great importance in the maintenance of the chemical bal- 

 ance of the body are the electric phenomena which attend the 

 solution of certain substances in water. This will be discussed 



