22 MANUAL OF PHYSIOLOGY. 



such great regularity, and in so great quantity, that they may be 

 said to make up the great bulk (97 per cent.) of the animal frame. 

 The great constancy with which the first three of these elements 

 occurs must be regarded as the most important character of or- 

 ganic tissues. 



Secondly, in organic textures the chemical elements are asso- 

 ciated in a much more complex and irregular proportion. Gener- 

 ally a large number of atoms, of each element, are grouped 

 together to form the molecule, and often the compound is so com- 

 plex that its chemical formula remains a matter of doubt. As 

 an example, a remarkable body, called lecithin, which appears 

 in the analysis of protoplasm and many tissues, may be mentioned ; 

 it is a peculiar compound containing nitrogen and phosphorus, 

 and in construction said to be like a fat. It may be expressed 

 thus: 



In inorganic substances, on the other hand, the elements are 

 found to be combined, as a general rule, in simple and regular 

 proportions. The molecules are made up of but few elements 

 arranged in a definite manner and firmly bound together, so that 

 they are not prone to undergo spontaneous decomposition. As 

 an example, we may take water, which has the well-known 

 formula, 



H 2 0. 



Though these bodies may be taken as types of organic and in- 

 organic substances respectively, it must not be imagined that all 

 organic bodies are as complex, irregular, and unstable as lecithin, 

 or that inorganic compounds, as a rule, are invariably simple and 

 stable like water. 



It is further remarkable that Carbon an element which is 

 exceptional in forming but few associations in the mineral world, 

 where it chiefly combines with oxygen to form CO 2 is almost 

 invariably pn-srnt in living textures, in which it is combined 

 with hydrogen and nitrogen as well as oxygen in various propor- 

 tions. The constancy of carbon as an ingredient of organic bodies 

 is so great that what formerly was called organic chemistry is 

 now often called the chemistry of the carbon compounds. 



