224 



ASSIMILATION. 



it is received by the mouth approaches to the nature 

 of the animal receiving it, the apparatus of assimila- 

 tion is the more simple, and the operation the more 

 easily performed. But still, so much is each animal 

 an independent being in its substance, that no kind 

 of food goes into the substance or constitution of an 

 animal without undergoing a total change in the pro- 

 cess of assimilation. Even if the animal preys upon 

 its own species, as is by no me;ins uncommon among 

 some of the fishes, in which case the food and the 

 feeder may be supposed to make the nearest approxi- 

 mation to each other, the food undergoes as complete 

 a change as if it were the substance most foreign and 

 opposite to the nature of the animal that can be ima- 

 gined. It is worthy of being borne in mind, too, that 

 in the same animal, or the same species, the product 

 of assimilation is nearly the same, however dif- 

 ferent the food may be ; or that different kinds of 

 food produce differences in the quantity of assimilated 

 product rather than in the quality. Consequently, 

 when we say that the food of an animal is of bad 

 quality, we simply mean that it is deficient in nutri- 

 ment ; or if we have any further meaning, it must be, 

 that the food contains some deleterious ingredient 

 which in so far acts as a poison. 



The second part of the process is digestion, which 

 usually takes place in the true stomach of the ani- 

 mal, although some animals appear to have a diges- 

 tive power in the gullet, or passage leading from the 

 mouth to the stomach. It is here that the mystery 

 begins ; for though the stomach may be said to ex- 

 ert both mechanical and chemical powers in the pro- 

 cess of digestion, yet there are other results produced 

 in it which cannot well be attributed to either. While 

 the process of digestion is going on, the stomach is 

 in continual motion, contracting in one place and ex- 

 panding in another, as if it were wriggling as a worm 

 does ; and hence this is called its vermicular, or 

 worm-like motion. 



The most powerful, as well as the most singular 

 agent in the stomach, appears, however, to be the 

 gastric juice, a peculiar fluid which is secreted or 

 given out by the inner coat of that organ, and which not 

 only exerts a very powerful action in chemically dis- 

 solving the food, but also produces some changes in 

 it which cannot be explained upon any known prin- 

 ciples of chemistry. The energy of this fluid has 

 been proved by direct experiment : it has been ob- 

 tained from the stomach of the living subject (which 

 is neither difficult, nor attended with the slightest 

 danger) ; and by being kept at nearly the natural 

 temperature, it has, to a considerable extent, effected 

 the process of digestion in a separate vessel, in which 

 there could be no vermicular motion to assist its 

 action. 



This gastric juice is somewhat singular in its ope- 

 ration. It will dissolve cartilage, bone, and even, in 

 some cases, iron ; but it will not dissolve the skin of 

 a berry, the least bit of cork, or the smallest fibre of 

 cotton wool. It is one of those animal fluids which 

 are so perplexing to the chemist in his investigations, 

 and might alone demonstrate the fact, that life is 

 something which neither mechanics nor chemistry can 

 reach, and in the production of which they can con- 

 sequently have no concern. Not only while in the 

 stomach of the living subject, but after it is removed, 

 this fluid produces what appear to be very powerful 

 chemical effects ; but when we subject it to chemical 

 analysis, we arc unable to detect in it any ingredient 



to which such effects could, chemically speaking, be 

 attributed : in other words, we discover nothing. 



There are two properties of the gastric juice, which 

 seem worthy of separate notice. These are, the pre- 

 vention of putrefaction, and the coagulation of albu- 

 men. The first of these is in opposition to one of 

 the theories of digestion, by which it was maintained 

 that that process is a species of putrefaction. But 

 so far is that theory from being true, that it is well 

 known that if putrid matter of any kind is admitted 

 into the stomach or any part of the digestive organs, 

 in many animals, it is attended with serious and even 

 fatal effects. The gastric juice while it performs that 

 sort of decomposition which is necessary for pre- 

 paring the food for assimilation, prevents the putre- 

 faction to which the food might otherwise have it 

 tendency, and it seems also to prevent chemical solu- 

 tion by the action of one part of the food upon 

 another. Chemical actions, such as the production 

 of an acid or a gas, do indeed sometimes take place 

 in the stomach ; but these are always disagreeable, 

 and consequently the results of something wrong in 

 the functions of that organ. The coagulative power 

 is, perhaps, more singular ; but coagulation is a sub- 

 ject upon which our knowledge is very obscure and 

 vague. The fact is well known, however, even to 

 those who are equally ignorant of chemistry and 

 physiology. There is not a nurse in the country, 

 but knows that the stomach of an infant is disordered, 

 when it returns the milk of its nurse uncurdled ; and 

 dairymaids have from time immemorial employed 

 the rennet, the red or gastric juice from the stomach 

 of animals, for curdling milk in the manufacture of 

 cheese. 



The action of the gastric juice, assisted by the ver- 

 micular motion of the stomach, converts the food 

 into an uniform pulpy mass, changed in appearance 

 and odour, and which is known by the name of 

 cliymc. This name means that which is softened and 

 mixed together ; and, therefore, it is expressive of the 

 substance, though not of the process by which it is 

 formed. The solution is, indeed, a peculiar one ; the 

 food is reduced to a pulpy mass, and though there 

 may be a difference in the food, that mass is in the 

 same species of animal in the same state of health 

 always nearly the same. The solution which it has 

 undergone is a peculiar one ; for though it is soft- 

 ened and moistened, it is not dissolved, neither is it 

 soluble in water. 



When the food has been properly reduced to 

 chyme, the action of the stomach forces it through 

 the pyloric opening into the duodenum, where it mixes 

 with new secretions from the animal, and undergoes 

 another change. Not far from the commencement 

 of the duodenum, at the pyloric orifice of the sto- 

 mach, the gall and pancreatic ducts pour their con- 

 tents into it ; the first the bile, and the second the 

 pancreatic juice. What specific part these perform 

 in the general process of assimilation, is not known ; 

 but it is presumed that their action, especially that 

 of the bile, is very important, both because of the size 

 of the liver by which it is produced, and because of 

 the great derangement which takes place in the 

 whole process of assimilation when that organ is 

 diseased. 



At or near that part of the duodenum into which 

 the biliary and pancreatic secretions are discharged, 

 the chyme begins to be separated into two parts, the 

 relative proportions of which vary with the quality of 



