The exact relations of fatty tissue to the states of health are not as yet well understood; but, since on great exertion or prolonged mental or moral strain or in low fevers we lose fat rapidly, it may be taken for granted that each individual should possess a certain surplus of this readily-lost material.  It is the one portion of our body which comes and goes in large amount.  Even thin people have it in some quantity always ready, and, despite the fluctuations, every one has a standard share, which varies at different times of life.  The mechanism which limits the storing away of an excess is almost unknown, and we are only aware that some foods and lack of exertion favor growth in fat, while action and lessened diet diminish it; but also we know that while any one can be made to lose weight, there are some persons who cannot be made to gain a pound by any possible device, so that in this, as in other things, to spend is easier than to get; although it is clear that the very thin must certainly live, so to speak, from hand to mouth, and have little for emergencies.  Whether fat people possess greater power of resistance as against the fatal wasting of certain maladies or not, does not seem to be known, and I fancy that the popular medical belief is rather opposed to a belief in the vital endurance of those who are unusually fat.

That I am not pushing too far this idea of the indicative value of gain of weight may be further seen in persons who suffer from some incurable chronic malady, but who are in other respects well.  The relief from their disease, even if temporary, is apt to be signalled by abrupt gain in weight.  A remarkable illustration is to be found in those who suffer periodically from severe pain.  Cessation of these attacks for a time is sure to result in the putting on of flesh.  The case of Captain Catlin[10] is a good example.  Owing to an accident of war, he lost a leg, and ever since has had severe neuralgic pain referred to the lost leg.  These attacks depend almost altogether on storms.  In years of fewest storms they are least numerous, and the bodily weight, which is never insufficient, rises.  With their increase it lowers to a certain amount, beneath which it does not fall.  His weight is, therefore, indirectly dependent upon the number of storms to the influence of which he is exposed.

At present, however, we have to do most largely with the means of attaining that moderate share of stored-away fat which seems to indicate a state of nutritive prosperity and to be essential to those physical needs, such as protection and padding, which fat subserves, no less than to its aesthetic value, as rounding the curves of the human form.

The study of the amount of the different forms of diet which is needed by people at rest, and by those who are active, is valuable only to enable us to construct dietaries with care for masses of men and where economy is an object.  In dealing with cases such as I shall describe, it is needful usually to give and to have digested a surplus of food, so that we are more concerned now to know the forms of food which thin or fatten, and the means which aid us to digest temporarily an excess.