Now, in order to carry on this great process of nutrition, we have said that food and stimulus are equally indispensable. We must have food or we can have nothing to assimilate; but we must also have stimulus, or no assimilation will take place. _The unstimulated tissue will not assimilate food._ The nutritive material rushes by it, unsought for and unappropriated, and no repair takes place. There are some people whom no amount of eating will build up: what they need is not more food, but more nerve stimulus; they doubtless eat already more than their tissues are able to assimilate. In pulmonary consumption, the chief monster which we have to fight against is impaired nutrition, the tubercles being only a secondary and derivative symptom.[12] The problem before us, in dealing with consumption, is to improve nutrition, to make the tissues assimilate food. And to this end we prescribe, for example, whisky and milk--a food which easily reaches the tissues, and a stimulant which urges them to take up the food sent to them. We define, therefore, a stimulant as _any substance which, brought to bear in proper quantities upon the nervous system, facilitates nutrition_. [12] Indeed, there are many fatal cases in which tubercles never appear. See Niemeyer on _Pulmonary Phthïsis_. At the head of all stimulants stands oxygen, concerning which, for further illustration, we shall quote the following passage from Dr. Anstie: "It needs but a glance at the vital condition of different populations in any country to arrive at a tolerably correct idea of the virtues of oxygen as a promoter of health and a curer of disease. If we compare the physical condition of the inhabitants of a London alley, an agricultural village, and a breezy sea-side hamlet, we shall recognize the truth of the description which assigns to it the same therapeutic action as is exercised by drugs, to which the name of stimulant seems more naturally applicable than to such a familiar agent as one which we are constantly breathing in the common air. A child that has been bred in a London cellar may be taken to possess a constitution which is a type of all the evil tendencies which our stimulants are intended to obviate.... It is highly suggestive to find that that very same quiet and perfect action of the vital functions, without undue waste, without pain, and without excessive material growth, is precisely what we produce, when we produce any useful effect, by the administration of stimulants, though, as might be expected, our artificial means are weak and uncertain in their operation, compared with the great natural stimulus of life."[13] [13] _Stimulants and Narcotics_, p. 144.