STOMACH AND INTESTINE 



397 



undigested or indigestible from the alimentary 

 canal. And the same caution may be applied, 

 with still more force, to that substitution of 

 hydro-carbon, or fat, for hydrate of carbon, or 

 starch and sugar, which some authors have 

 regarded as so easy and natural an exchange. 

 In all probability these substances are not 

 by any means convertible or inter-changeable 

 in any scale of diet. The cell- wall of the 

 adipose tissue is dissolved with great diffi- 

 culty; its liberated contents are next ab- 

 sorbed in but small quantities ; and they then 

 pass through glands which apparently have a 

 slow but definite office to execute upon them, 

 before they are admitted into the general 

 circulating current of the blood. And, lastly, 

 the rudest numerical contrast of their final 

 combustive metamorphosis with that of the 

 hydrates of carbon, shows that they require 

 the combination of a much larger quantity of 

 oxygen * before they can leave the body in 

 the form of carbonic acid and water. 



The total amount of food required by the 

 body is also exposed to circumstances which 

 are just as certain to baffle all such calcu- 

 lations. For this important quantity will evi- 

 dently vary with the rate of waste sustained 

 by each individual : and hence with the 

 activity of his life ; the nature of his habitual 

 exertion; and the state of his mind ; as well 

 as with the climate, race, temperament, and 

 education, which help to form the microcosm 

 of every man's personality. The degree of 

 variation which may be brought into play 

 by each of these circumstances it is impossible 

 to specify ; though it would often receive no 

 inapt illustration from a comparison of the 

 habits of the various members of a family or 

 other smallest social aggregate. 



Hence the true value of physiological che- 

 mistry, in respect to the principles of dietetics, 

 is that of being an admirable guide to the 

 general composition of a proper food. In this 

 capacity, it is not too much to say that its 

 veto ought to be absolute. But with this 

 negative function terminates its practical use- 

 fulness. Our choice of the exact quantities 

 and qualities of alimentary substances neces- 

 sary to construct a perfect scale of diet, may 

 indeed be sometimes explained by chemistry. 

 But it must always be dictated by experience. 

 And the dietaries of gaols, workhouses, and 

 hospitals, corrected, as they have too often 

 been, by the ghastly hand of Death himself, 

 have fixed the limits of the food necessary for 

 health, with an accuracy which, considering 

 the price of human life that has been paid 

 for it, ought surely to satisfy the most rigid 

 economist. 



From such sources of information we may 

 deduce, that a healthy aclnlt male, of active 

 habits, requires daily about two pounds of 

 solid food. Of this food, six or eight ounces 

 arc, preferably, meat. While, if the quality of 

 such a diet be lowered (as, for example, by the 



* It would not be difficult to point out, how these 

 views concur to explain the preference of fat as a 

 calorific food, by the inhabitants of cold climates. 



introduction of much potatoes or rice), its 

 quantity ought to be proportionally raised, 

 so as to compensate this diminution of its 

 nutritious characters. 



Relations of digestion to nutrition generally. 

 We have thus specified the various ali- 

 mentary substances which are normally sub- 

 mitted to the action of the digestive canal. 

 And the functions of the different segments 

 and structures of this tube have already, so 

 far as possible, been assigned to each. 



All these functions, however, together 

 make up but a small part of the complex act 

 of digestion. Nay, more, digestion itself is 

 only a part of a still wider and more complex 

 process of nutrition. And, further, the rela- 

 tion borne by digestion to nutrition is by no 

 means limited to an absorption of new matter 

 into the body ; but also involves a revolution 

 or cycle of much of the existing substance of 

 the organism, between those acts of ingestion 

 and egestion, which mark the respective ex- 

 tremes of its nutritional life. Hence it seems 

 necessary to end this description of the ali- 

 mentary canal by a succinct enumeration of 

 (1st) the series of phenomena which consti- 

 tute the digestive act, and (2nd) the share 

 which digestion itself takes in nutrition 

 generally. 



We may best review the various stages of 

 digestion generally, by supposing that we could 

 track a mass of mixed food through the whole 

 extent of the alimentary canal, and could 

 observe the changes which it gradually under- 

 went in this course. Such a food must of 

 course be assumed to consist of proper pro- 

 portions of all the alimentary principles, in 

 the states in which they are ordinarily found 

 in any diet suitable for the maintenance of 

 health in the human subject. 



The entry of such a food into the mouth 

 would mark the end of what is generally de- 

 scribed by systematic writers as the first stage 

 of digestion: namely, the act of prehension. 



The food having arrived in the cavity of 

 the mouth, is next subjected to the operations 

 of mastication and insalivation. 



Of these two processes, the first effects the 

 mechanical division of the food : reducing 

 it to small particles ; increasing, therefore, 

 its relative surface; and hence preparing 

 it for the action of all those secretions to 

 which it is exposed in its further course 

 through the alimentary canal. The mechanism 

 of this act is greatly aided by the simultaneous 

 admixture of the saliva. This liquid is added 

 to the food in quantities that vary, according 

 to its dryness and the consequent need of such 

 an addition, from 4 (apples) to 80 (bread) 

 per cent, of the alimentary substance that is 

 undergoing mastication. 



But insalivation also adds an important 

 chemical influence to the preceding mecha- 

 nical advantage. The united secretions of 

 the parotid, sublingual, and submaxillary 

 glands, and the mucous membrane of the 

 mouth, together furnish a liquid mixture, 

 which converts starch into grape sugar with 

 the greatest rapidity and energy. The perfect 



