Pranamayakosha

The feeling of health or vitality is not due merely to the correct working of the more or less static structures and more or less dynamic (though routine) functions of the various parts of the body, but is something additional, which does not come into the body from an external place but which wells up within the body through certain centers.

Prana is not the breath, but is the breath of life. And as breathing is the primary and fundamental function of the body, so is this 'electricity' required for the vitality of all the organs, and although the old teachers of yoga were not able to describe it in terms of something else already known, something other than itself (which is the basis of describing), they came as near as they could to describing it by calling it vital air or vitality. In our day we have the idea of elec. trinity to draw upon; we can say that the body, like a battery, is well or ill charged with vitality.

Linking this whole idea with the modern science of evolution, we may say that the presence of the mind (in the large sense) is one necessary ingredient of living or, in other words, the same particularized will to live which played its part in the gradual formation of the functioning structures must be still present for their continuation, and must not run contrary to the conditions which provided for their formation in the course of their evolution in Nature. Briefly, 'bad thoughts' are unnatural thoughts, but the good thoughts provided for in the meditational reverence in the centers and helped by various symbols and images are natural and also conducive to further growth and improvement. In the course of evolution the beneficial habits of structure and function were preserved, and the bad were eliminated.

All this may be condensed down to a statement that the yogis believe in  and tell us that they 'see' and anyhow know  that there is a 'subtle body' (sukshma sharira), one part of which is especially concerned with the welfare of the dense body. This part (named pranamayakosha  vessel or body composed of prana) is in a sense the brain of the body, in that it contains the body-memory which marshals all the newly incoming elements from food and air into their Proper places (something like the ring-master in a circus) Without which when introduced into the body they would Plot fall into line and do their parts, for the newly-incoming materials have to respond to the 'obedience’s' which have been stored in the long process of evolution in Nature in This 'brain of the body'. Think, for example, of the vast Number of 'habits' from the past which are coordinated in the course of the development of a child in its mother's womb, all taking place under the direction of this inner  mechanism. One is almost tempted to call it automation, or Mechanical brain, and would do so but for the fact that it was compounded from the life or mind element, which is still there and is still responsive to what is going on.