CHEMICAL CONSTITUENTS OF CELLS. 87 



In addition to the elements which have been already mentioned as 

 forming part of the organic constituents of cells, and which, of course, 

 may exist in other forms, we find also, when organic matter is subjected 

 to combustion, chlorine, fluorine, silicon, potassium, sodium, calcium, 

 magnesium, manganese, iron, and occasionally copper and lead, in the 

 ash. Of the other elements of organic bodies in incineration the carbon 

 is converted into carbon-dioxide, part of which remains in the form of 

 carbonates in the ash, part of the hydrogen uniting with oxj-gen to form 

 water. Another portion unites with nitrogen to form ammonia, while the 

 phosphorus and sulphur remain as oxygen compounds, sulphuric and 

 phosphoric acids, united with different bases also in the ash. 



Of the various chemical compounds which are found in the interior 

 of cells, and which have entered it, either from accidental contact or as 

 foods, or as resulting from the chemical processes in cells, we may make 

 three different groups : 



1. Those which, already formed, exist in inanimate nature, are 

 absorbed, and again leave cells without undergoing any change while 

 forming constituents of organized bodies. Such substances are repre- 

 sented by the inorganic constituents of animal cells. 



2. This group comprises those which are already formed exterior to 

 the cells, and which, in the process of assimilation by the cells, undergo 

 a change simply in their mode of molecular arrangement, without under- 

 going any profound chemical metamorphosis. Such constituents are 

 seldom, if ever, removed from the cells in the form in which they entered 

 it, and are, in the chemical process occurring in the interior of the cells, 

 always reduced to simpler forms. The organic constituents of cells form 

 this group. They may be either nitrogenous or non-nitrogenous in 

 composition. 



3. We meet also with a class of compounds which are themselves 

 developed in the vital processes in cells, as the result of the metamorphosis 

 of either the organic constituents of cells or of the food-products which 

 have been assimilated by the cells. Such bodies may be removed from 

 cells either as complex, organic, excretory products (as types of which 

 urea and kreatin may be mentioned), or they themselves may undergo 

 more profound decomposition before being removed from the interior of 

 the cells. The examination of protoplasm, wherever found in the animal 

 or vegetable kingdom, will show that it contains examples of each of 

 these three classes of compounds. 



The chemical constituents of organic bodies may, then, be divided 

 into two different groups, the organic and the inorganic. The organic 

 may again be subdivided into the nitrogenous and the non-nitrogenous. 

 Proteids, with their derivatives, represent the nitrogenous group ; the 

 hydro-carbons and carbo-hydrates, with their derivatives, the non- 



