There are no characters in the chemical composition of the textures of organic beings which can be said to be absolutely distinctive or to separate them from inorganic matter. No doubt their chemical construction frequently exhibits certain peculiarities, not seen in dead matter, which may be taken as characteristic, but living textures only differ in the general plan of arrangement and composition from that most commonly met with in the construction of inorganic materials.

In the first place, the great majority of the chemical elements which we know of, take no share in the formation of living creatures, and are never found to enter into their composition. Practically, only fifteen of some seventy elements known to chemists take part in making up the tissues of animals. The majority of these are only present in very small quantity and with no great constancy. On the other hand, there are four elements, namely, carbon, oxygen, hydrogen and nitrogen, which are found with 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 occur must be regarded as a most important character of organic tissues.

Secondly, in organic substances the chemical elements are associated in much more complex and irregular proportions. Generally, a large number of atoms, of each element, are grouped together to form the molecule, and often the compound is so complex that its chemical formula remains a matter of doubt. As an example of the complexity of bodies found in organic analysis, a remarkable substance, called lecithin, which appears in the analysis of protoplasm and many tissues, may be mentioned. Its formula may be expressed thus: -

C3 H5.

C18 H35 O2 C18 H35 O2.

0 - P0.

OHN(CH3)3 O - C2H1 - OH.

It is peculiar in containing nitrogen and phosphorus, and in construction 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 decomposition. As an example, we may take water, which has the well-known formula, H20.

Though these bodies may be taken as types of organic and inorganic 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 C02 - is almost invariably present in living textures, in which it is combined with hydrogen and nitrogen as well as oxygen in various proportions. 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.

These complex associations of many atoms of carbon with many atoms of other elements, are readily dissociated when exposed to the air under even slightly disturbing influences. When heated to a certain degree they burn, i. e., unite rapidly with the oxygen of the air: and in the presence of minute organisms they putrefy. Thus instability is a general feature commonly met with in most substances of organic origin.

Chemical instability reaches the highest pitch in tissues which are actually alive and engaged in vital processes. So long as any texture lives, i. e., is capable of performing its functions, it must constantly undergo certain chemical changes, a kind of decomposition, tending to produce disintegration, and a reintegration by means of new chemical associations with fresh materials. A tissue may then be said to deserve the term living, only as long as it undergoes these antagonistic chemical changes. The tendency to destructive oxidation or disintegration is intimately connected with the functional activity of the living texture and increases with this activity. The reintegration or constructive process requires the presence of suitable materials with which the texture may combine, in order to make up for the loss. Thus living tissues are ever on the point of destruction, which can only be warded off by the timely reconstruction of their chemical ingredients by suitable fresh materials. This reconstruction by means of fresh matter from without is called assimilation, and forms the most, if not the only, satisfactory criterion by which adequately to distinguish living beings from inorganic matters.

The object of assimilation is to supply suitable fresh materials to the various textures for the chemical processes required for their function while living. This will be found to form a great part of physiological study. Further, the energy manifested in the living activity of the textures depends upon the various oxidizing processes, and the exact laws which govern these combustions, and the results they produce in the various tissues, practically make up the other part of physiology.