384- 



mical affinity on the one hand, and homoge- 

 neous and heterogeneous adhesion on the 

 other; and that while they are sometimes * 

 akin to the formation of hydrates, they oc- 

 casionally resemble those still more recon- 

 dite phenomena which are concerned in the 

 production of isomeric or isomorphous com- 

 pounds : substances which, though identical 

 in their composition, offer striking differences 

 in their solubility, as well as in many of their 

 chemical properties and reactions. 



This very limited convertibility of the main 

 components of the food, renders their variety 

 almost as essential, as though each different 

 tissue of the body had required the entry of 

 its corresponding substance from without. In 

 other words, within the range of the chemical 

 parallelism just mentioned, the organism de- 

 mands alimentary compounds containing all 

 the different ingredients necessary to cover its 

 own waste. 



This fact receives a good illustration from 

 that selection which the instinct of most per- 

 sons would impel them to make. Left to 

 himself, Man always chuses a mixed diet, 

 composed of proper quantities of animal and 

 vegetable, liquid and solid, matter. Nay more, 

 that almost equally imperious instinct which 

 urges him to vary his diet, though often con- 

 fused with the morbid cravings of luxury, is 

 essentially nothing less than an expression of 

 the natural wants of a healthy organism. 



Obscured, however, as these really natural 

 instincts often are by the stereotyped tastes 

 and habits of a highly artificial state of 

 society, we gain a far better insight into the 

 proper composition of food, by examining 

 that store of nutriment which, in the shape 

 of the yolk of the Bird's egg, or the milk of 

 the Mammal, Nature herself provides for the 

 maintenance of the young of these classes. 

 Of these two substances, the milk is justly 

 regarded as forming the very best example of 

 a proper food : both as regards the nature of 

 its several ingredients, and the proportions in 

 which they are mingled with each other. 



Milk. The alimentary properties of the 

 milk are due to the presence of a number of 

 proximate constituents, the more impor- 

 tant of which may be enumerated as fol- 

 lows. (1) A protein- compound, casein; 

 (2) a hydro-carbon or fat ; (3) a hydrate of 

 carbon or sugar; (4) certain salts; and 

 (5) the water in which the whole of these 

 materials are suspended or dissolved. Of 

 these five groups of substances, at least four 

 are indispensable ingredients of every proper 

 food. The hydrate of carbon and the hydro- 

 carbon are, to some extent, capable of forming 

 substitutes for each other. But with this ex- 

 ception, (an explanation of which will be 

 attempted by and by), the absence of any one 

 of these constituents, or even its presence in 

 insufficient quantity, suffices to destroy the 

 capacity of any particular food for maintaining 

 life ; so that an animal limited to such a diet 



* Compare the remarks on the gastric juice at 

 p. 337. 



STOMACH AND INTESTINE. 



ultimately dies with appearances of inanition. 

 And a fortiori, the ingestion of but one of 

 these alimentary ingredients, such as albu- 

 men, fat, or sugar, is soon attended with 

 effects which still more closely resemble those 

 of starvation. Such a diet does indeed essen- 

 tially starve the entire organism, even while 

 it supplies some of the constituents of its 

 lost substance. For although the unchecked 

 waste of the remaining constituents of its 

 mass tells upon certain of its textures with 

 greater rapidity and energy than on others, 

 still it ultimately involves the whole in a 

 common destruction : a fact which need 

 little surprise us, when we recollect the 

 mixed composition of the simplest tissues, and 

 the intimate mutual dependence of the most 

 distant and isolated parts. 



Constituents of food. 1. The first group, 

 consisting of what are called the protein- 

 compounds, includes a number of proxi- 

 mate principles, which are derived from both 

 the animal and vegetable kingdoms of nature. 

 The chief of these principles are albumen, 

 fibrin, and casein. By digestion in solution 

 of potash, and precipitation with an acid, 

 either of these yields a substance called pro- 

 tein: a name that alludes to the relation 

 this principle is supposed to bear to all the 

 compounds from which it is thus obtained. 

 It is regarded as their common starting-point 

 (TTjOfam ua, primaspartes teneo), and most essen- 

 tial component. And the slight differences 

 of composition offered by each particular pro- 

 tein-compound, are explained as chiefly due 

 to variations in the nature and amount of 

 certain collateral ingredients, the addition of 

 which to protein imparts the specific charac- 

 ters of albumen, fibrin, or the like. Hence 

 the various protein-compounds are supposed 

 to differ, not so much in elementary com- 

 position, as in certain characters which might 

 almost be termed morphological : namely, 

 outward form, physical properties, degree of 

 solubility, and the like. 



The exact process by which one of these 

 protein-compounds undergoes conversion into 

 another is still a complete mystery. But that 

 such changes do constantly obtain, cannot be 

 doubted. And hence, while the quantity of 

 albumen in the animal body, and the constancy 

 with which it is present, assign to this proxi- 

 mate principle the leading position in the 

 above group of proteinous substances, it is on 

 its generic, and not its specific, properties 

 that our attention ought chiefly to be fixed. 



The protein of the food may be regarded 

 as its most essential constituent. The reason 

 why such an importance is ascribed to it 

 becomes sufficiently evident, when we com- 

 pare its composition with that of the body 

 which it is intended to nourish. The highly 

 azotized constitution it possesses (C 54'7 

 + H 6-8 + N 14-2 -f O 24-3 = 100) closely 

 approaches that of the solids of the organism 

 generally. And it shows an equally important 

 relation to most of the tissues in detail. It 

 forms a large constituent of the blood ; and 

 therefore of the plastic nutritional fluid that 



