HUMAN PHYSIOLOGY. 



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HUMAN PHYSIOLOGY. V. 



DESTRUCTION of tiasue and change of structure are the neoes- 

 aary consequences of ovon thoiio functions that are indispensable 

 uuanoe of lii'.-. Kvry breath we draw, every beat 

 df our heart*, causes wan to of the material of the body ; and 

 thin wiiate ia largely increased when, in addition to those passive 

 and involuntary actions, wo add active exertion. We cannot 

 lift u finger or walk a yard without helping forward this change, 

 whit-li in for over proceeding. 



as waste and decay take place thus continually, it 

 follows that if the body ia to be sustained if provision is to bo 

 r its growth and increase new material must be as 

 tly furnished as the old tissues waste. It is to provide 

 .v iimtfriul to supply the place of the destroyed struc- 

 iiat we take food. In considering the subject of food, 

 ill have to look at it in two lights first, as to its 

 ; secondly, as to the sources whence it is derived. It 

 has boon already said that tho object of food is to sustain and 

 nourish tho body, to enable it to perform the work it has to do, 

 and in youth to increase its size and grow to its full pro- 

 portioii. For tho proper performance of those duties two groat 

 differing classes of nourishment are necessary one called tho 

 plastic, or nitrogenous, the elements included in which are sup- 

 posed to go directly to tho formation of the tissues ; all of tho 

 members of this class contain, as its narao implies, a certain 

 proportion of nitrogen, and are derived mainly, though not 

 entirely, from tho animal kingdom ; the chief of these are 

 fibrine, albumen, caseino, gelatine, and gluten. The first four 

 are almost exclusively of animal origin, whilst tho gluton is a 

 vegetable product ; fibrine, albumen, and gelatine enter into 

 the composition of butchers' meat, poultry, and fish ; caseino is 

 found in milk ; gluten in most vegetable foods, in tho largest 

 proportion in wheat. Tho other class of food is called car- 

 bonaceous, or heat-producers ; this supplies the fuel for tho 

 respiratory process, and furnishes tho heat of tho body. Tho ele- 

 ments included in this class come from both the animal and vege- 

 table, and include the fats, starches, and saccharine principles. 

 The office of all these, as has been said, is to produce heat, and 

 for this purpose the fats are the most powerful agents ; but, in 

 addition to this their main office, they serve other important 

 purposes in aiding in the digestion of the nitrogenous elements 

 of the food. The digestive power of the pancreatic fluid de- 

 pends in a great measure upon tho presence of fat, and it is 

 believed that it is largely concerned in the production of bile. 

 Its presence in largo quantities in the nerve-centres shows 

 that it serves some important office in nervous action, and by 

 its mechanical presence it gives roundness and suppleness to 

 the form, and, owing to its bad conducting power, helps to 

 preserve the heat of tho body. The starches and sugars are 

 also heat-producers, but by no means such energetic ones as 

 tho fata ; these, by their digestion, become changed into various 



ids. They are largely concerned in the production of fat, 

 as is shown by tho rapidity with which animals fatten that are 

 fed on diets rich in farina and sugar. 



As to the sources of food, they are confined entirely to tho 

 animal and vegetable kingdoms. No instance has yet been 

 met with where an animal had tho power of deriving its nou- 

 rishment directly from tho mineral world ; although it is a curious 

 fact, that in some of the deep sea-dredgings recently made, 

 when the inud from the bottom of the ocean was examined, it 

 was found to teem with abundance of tho lowest forms of animal 

 life, but did not present a trace of any vegetable organism. 

 Man almost universally derives his food from both tho animal 

 and vegetable kingdoms ; and there can bo little doubt, from 

 the formation of his teeth and the structure and shortness, 

 as compared with vegetable feeders, of his alimentary canal, 

 bhat he was intended so to do ; for, although it cannot be denied 

 that it is possible for him to derive all the elements necessary 

 for his support from the vegetable world alone, it is quite certain 

 that, to enable him to do so, he must expend a much larger 

 amount of vital force than is called for when he lives on a 

 mixed diet ; he does, in fact, in the latter case, utilise the work 

 already done by the lower animal on which he feeds, as that in 

 its turn has appropriated and made its own the work the vege- 

 table has accomplished, in absorbing and preparing the mineral 

 elements which, without this preparation, would have been un- 

 Available for the support of n.nimft.1 life. 



Bach, then, being the object* and the xmroe* of the food of 

 man, we moat now consider in detail tho different varietien 

 of which tliia food U composed. But, before doing ao, it 

 may be well to atop and consider for a minute the quantity 

 of tho primary element* that are absolutely neoeaiary for 

 the anpport of life. By a careful calculation it haa bean found 

 that a man requires, to keep himaelf alive, a daily supply of 

 4,100 grains of carbon and 100 grain* of nitrogen. These 

 quantities are contained in two pound* four oancea of bread ; 

 but though thin or even a larger quantity of bread or of any 

 other article containing an equal amount of carbon and nitro- 

 gen would sustain life for a short time, it haa been found that 

 health cannot be maintained on any one article of diet save 

 milk, though it contain within itoelf all the necessary element* 

 for tho support of life. Doga fed on white bread and water died 

 of starvation in fifty daya ; rabbits and guinea-pigs fed exclu- 

 sively on any of the following substance* wheat, oata, barley, 

 cabbage, or carrots died in fifteen days, whilst if they wero 

 given in succession or alternately, no evil results followed. 



The first great necessity for the support of animal life ia 

 water, for without it the others would be useless. The impor- 

 tance of water will bo at once perceived when we remember 

 that 75 per cent, of the blood and of the fleshy covering of 

 tho body is composed of it, and that from the body in the 

 course of the day about five pounds and a half of water is ex- 

 creted by tho lungs, the skin, and the kidneys. It is taken 

 into the body, both separately and in combination with other 

 foods, there being, in fact, no food, however solid, which does not 

 contain a certain proportion of it, and of many it forms the 

 largest part. Many succulent vegetables contain 80 to 90 per 

 cent. potatoes, 75 per cent. ; new milk, 88 per cent. ; and 

 beer, ale, skim milk, etc., 90 per cent. The duties water has to 

 perform in the body are mainly two : it serves to dissolve the 

 food, and enable the nutritive parts of it to be carried into the 

 circulation ; and on tho other hand, it dissolves out the worn 

 materials and conveys them out of the body. In addition to 

 these functions, by its evaporation from the surface of the body 

 and the air-passages, it helps to regulate its temperature,. 



Of tho foods derived from the animal kingdom, we must first 

 consider milk. This may be taken as the type of what every 

 food should be, for it contains within itself all the elements 

 necessary for tho support of the animal organism. In some 

 countries it forms tho chief diet of the people, and wherever it 

 can be easily procured it is largely consumed. Its component 

 parts are water ; caseine, which represents tho albuminous 

 group ; cream or butter, representing tho fats and sugar of milk, 

 the saccharine, the latter two being the carbonaceous elements. 

 The proportion of those constituents varies according to the 

 animal from which it is derived. Asses' milk is that which 

 most nearly approaches in composition the human milk, but 

 this has less than a third of the quantity of the fatty element ; 

 whilst cows' milk, containing nearly the same proportion of 

 cream, has three times as much caseine, and only two-thirds aa 

 much sugar of milk. To make cow's milk, therefore, a fit 

 food for young infants, it is necessary that it should bo diluted 

 with water, and have sugar of milk added to it. Cheese, which 

 is tho caseine of milk separated from it by the addition of an 

 acid, contains a large proportion of nitrogen, and is a very 

 nutritious, but not very digestible substance. 



Our supplies of meat food are derived from several sources, 

 but mainly from the ox, sheep, and pig; its nutritive value 

 depends principally upon the proportion of lean which it con- 

 tains, but there is little difference in the relative nutrition of 

 the meats derived from these three sources. Horseflesh is 

 but little used in this country, but has within the last few 

 years come into rather extensive demand in France. Venison, 

 and the flesh of other wild animals, is equally nutritious with 

 beef and mutton, and when properly cooked is more digestible. 

 Poultry and rabbits are not in themselves very nourishing, as 

 they contain too little fat. Fish also, with the exception of 

 mackerel, eels, and salmon, are deficient in fat, and in nutritive 

 value very inferior to flesh. Eggs contain a certain proportion 

 of fatty matter, but a much larger proportion of nitrogen ; the 

 fatty matter is found in the yolk, but it is preferable to take 

 the whole egg than an equal weight of the yolk alone. Fats in 

 some form are an absolute necessity as an article of food. Those 

 derived from the animal kingdom are mainly butter, lard, suet, 

 and dripping; in most cases it exists in sufficient quantity in 



