LIVING SUBSTANCE 103 



stantly be borne in mind that the chemical relations of the latter 

 are to be distinguished sharply from those of the former. 



Although there is no fundamental difference between the ele- 

 ments composing living and those composing lifeless substance, in 

 other words, although no special vital element exists in the organic 

 world, some of the elements in living substance form unique com- 

 pounds which characterise it only, and are never found in lifeless 

 substance. Thus, there exist in the former, besides chemical 

 compounds that occur also in the latter, specific organic complexes 

 of atoms. 



Many of these organic compounds, especially those that are of 

 special importance to living substance, possess so complicated a 

 constitution that thus far chemistry has not succeeded in obtain- 

 ing an insight into the spatial relations of the atoms in their 

 molecules, although the percentage composition of the molecules 

 is known to a greater extent. 



There are especially three chief groups of chemical bodies and 

 their transformation-products, by the presence of which living sub- 

 stance is distinguished from lifeless substance ; these are proteids, 

 fats, and carbohydrates. Of these only the proteids and their deri- 

 vatives have been demonstrated with certainty as common to all 

 cells ; hence they must be set apart among the organic constitu- 

 ents of living matter as the essential or general substances, in 

 contrast to all special substances. 



a. Proteids 



The proteids play the most important rdle in the composition 

 of living substance, since they are absolutely indispensable to all 

 life that exists at present upon the surface of the earth, and quan- 

 titatively they constitute the chief constituent of all the organic 

 compounds of the cell. Without exception they consist of the 

 elements carbon, hydrogen, sulphur, nitrogen, and oxygen ; of 

 these, nitrogen especially distinguishes proteids from the two other 

 chief groups of organic bodies, carbohydrates and fats, so that the 

 former, as nitrogenous bodies, are to be contrasted with the latter 

 two as non-nitrogenous. The stereo-chemical composition of the 

 proteid molecule is not yet known, but from analyses, in which the 

 molecule is split up into a large number of still very complex 

 molecules, it is known that it must have an excessively complex 

 constitution ; although it contains only the five elements C, H, N, S, 

 and O, the number of its atoms often reaches far beyond a thou- 

 sand. In the year 1866 Preyer made the first analysis of 

 haemoglobin, the proteid that gives the characteristic colour to 

 the blood, more exactly to the red corpuscles, and, as a carrier of 

 oxygen from the lungs through the blood to the cells of the tissues, 



