382 



STOMACH AND INTESTINE. 



they from time to time convey. While on 

 very simple hydraulic principles, these re- 

 peated cross branches would so diminish the 

 various resistances (of impact and adhesion) 

 offered to the blood within the vessels, as to 

 permit, either a greater rapidity of the cur- 

 rent, and hence a more rapid renewal of the 

 mass of blood contained in the capillaries ; or 

 a more forcible pressure upon the latter fluid ; 

 or even both of these effects simultaneously. 



It is perhaps a corroboration of the above 

 conjectures, to trace their close relation to 

 those which might be gathered from an inde- 

 pendent consideration of the circumstances of 

 the portal system. The trunk vein of this set 

 of vessels leads to a second set of capillaries 

 in the liver ; through which there is nothing 

 to propel the portal blood, save the force of 

 the heart, aided by a small amount of suc- 

 tion, which the thorax exerts as it ex- 

 pands during inspiration. And hence, how- 

 ever large a quantity of the original cardiac 

 pressure may be again amassed by the con- 

 vergence of the various intestinal veins, still 

 there can be no doubt that at least so 

 much of it will have been lost, as to require 

 all the aid which the above disposition of the 

 arteries can afford it. But in spite of all such 

 assistance, it seems probable, that the current 

 of the portal blood is both far slower, and 

 much more feeble, than that which occupies 

 any of the arteries. Still it is no doubt quite 

 sufficient for the exigencies of the circulation 

 in the liver; and especially for that secretion 

 of bile, to which the various details of the 

 organization of this gland chiefly refer. 



THE FOOD. The function of digestion has 

 for its chief object the replacement of that 

 loss of substance which the body is con- 

 stantly undergoing. 



Even the hardest materials of the globe we 

 inhabit experience a gradual disintegration ; 

 as the result of the various physical processes 

 to which they are exposed. Such processes 

 may be instanced in the attrition and solution 

 of solids, the evaporation of liquids, and the 

 diffusion of gases. And hence, when we turn 

 from these inorganic substances to the animal 

 fabric ; and consider its slight cohesion, the 

 friction which its locomotion implies, its 

 large watery constituent, and the feeble 

 chemical affinities which enchain its elemen- 

 tary atoms we shall scarcely be surprised 

 to find, that the rapidity of its waste far ex- 

 ceeds that of the inanimate solids around us. 



But the rate of waste, and the consequent 

 need of replacement, both depend, far less on 

 simple physical causes of this kind, than on 

 certain actions which are specific to the or- 

 ganized body. These actions, which, in the 

 aggregate, make up what we term LIFE, do not 

 so much imply, as actually consist in, a per- 

 petual process of flux and metamorphosis. 

 This multiform change engages the whole of 

 the corporeal tissues; and conducts their 

 various ingredients, through a number of suc- 

 cessive phases of composition, to an effete and 

 useless state, in which they are finally ejected 

 from the organism. 



And hence, whatever the share taken by the 

 physical actions of diffusion, solution, fric- 

 tion, and evaporation, in the removal of the 

 substance of the body, they are not in any 

 sense the true causes of its process of waste ; 

 or the real sources of its egcsta or losses. 

 They are but, as it were, the janitors of the 

 animal fortress ; the nature and amount of 

 the matters which pass out by them being 

 controlled and regulated by the higher life that 

 rules within. 



The ingesta, which replace these egesta, 

 and thus form the opposite extreme of nu- 

 tritional life, are equally influenced by the 

 general requirements of the animal. Ex- 

 cluding, for the present, all consideration of 

 that preponderance of absorption which de- 

 termines the growth of a young animal, or 

 the converse excess of excretion which results 

 in the decrease and decay of an old one ; and 

 limiting our attention to the mere maintenance 

 of the adult body : we shall find that it is the 

 composition of its structures, and the rate of 

 their wear and tear, that chiefly determine the 

 kind of food it makes use of, and the quantity 

 it consumes within a given space of time. 

 While as regards the exact degree of this 

 dependence, we shall further find that, here 

 as elsewhere, the operations of organized 

 nature are only limited by wide general prin- 

 ciples ; within which are apparently conceded 

 great variety and fluctuation. The laws of nu- 

 trition are, so to speak, universal in their 

 range, but elastic in their application. 



In respect to the nature of the food, we 

 may first notice, that by far the larger part of 

 it is always derived from the organic, and 

 never from the inorganic, world. In other 

 words, the chemistry of the organism has 

 little power of construction or synthesis. 

 So that, although a proximate analysis of the 

 tissues of the animal body presents us with 

 compounds, which may be shown to consist 

 chiefly of a few elementary substances united 

 to each other in varying proportions, still the 

 carbon, oxygen, hydrogen, and nitrogen, which 

 surround or penetrate the living animal, are 

 never directly built up into these tissues. On 

 the contrary, the various substances which 

 form the proximate principles of the several 

 structures of the organism are themselves 

 produced by the metamorphosis of kindred 

 compounds introduced in the food ; com- 

 pounds which have been in their turn derived 

 from the vegetable kingdom ; either directly, 

 in the shape of plants, or indirectly, from sub- 

 stances constructed out of" vegetable tissues 

 by the organism of another animal. And the 

 inorganic substances introduced into the body 

 seem to be almost restricted to the subordinate 

 (though equally indispensable) office, of com- 

 bining with these products of vegetable life, 

 and modifying their actions in obedience to 

 the necessities of the existing individual. 



The above statement as to the organic 

 nature of the food suggests some interesting 

 considerations. 



In the first place, it seems to shew that 

 the living animal of to-day pre-supposes 



