30 MANUAL OF PHYSIOLOGY. 



only present in very small quantity and with no great constancy. 

 On the other hand, there are four elements, namely, carbon, oxy- 

 gen, hydrogen and nitrogen, which are found with such great regu- 

 larity, 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 a most important character of organic 

 tissues. 



Secondly, in organic textures the chemical elements are asso- 

 ciated in much more complex and irregular proportions. 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 of the complexity of bodies found in organic analy- 

 sis, a remarkable one, called lecithin, which appears in the analysis 

 of protoplasm and many tissues, may be mentioned. It may be 



expressed thus : 



C 44 H 90 NP0 9 . 



It is peculiar in containing nitrogen and phosphorus, and in con- 

 struction is said to be like a fat. 



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 present in living textures, in which it is combined 



