654 FUNCTIONS OF ANIMALS. 



Duhamel.* In the same manner, musk, indigo, &c. when taken 

 into the stomach, make their way into many of the secretions. 



These facts show us that assimilation is a chemical process 

 from beginning to end ; that all the changes are produced ac- 

 cording to the laws of chemistry ; and that we can even derange 

 the regularity of the process by introducing substances whose 

 mutual affinities are too strong for the organs to overcome. 



It cannot be denied, then, that the assimilation of food con- 

 sists merely in a certain number of chemical decompositions 

 which that food undergoes, and the consequent formation of 

 certain new compounds. But are the agents employed in assi- 

 milation merely chemical agents? We cannot produce any 

 thing like these changes on the food out of the body, and there- 

 fore we must allow that they are the consequence of the action 

 of the animal organs. But this action, it may be said, is merely 

 the secretion of particular juices, which have the property of in- 

 ducing the wished-for change upon the food; and this very 

 change would be produced out of the body, provided we could 

 procure these substances, and apply them in proper quantity to 

 the food. If this supposition be true, the specific action of the 

 vessels consists in the secretion of certain substances ; conse- 

 quently the cause of this secretion is the real agent in assimila- 

 tion. Now, can the cause of this secretion be shown to be merely 

 a chemical agent ? * Certainly not. For in the stomach, where 

 only this secretion can be shown to exist, it is not always the 

 same, but varies according to circumstances. Thus eagles at 

 first cannot digest grain, but they may be brought to do it by 

 persisting in making them use it as food. On the contrary, a 

 lamb cannot at first digest animal food, but habit will also give 

 it this power. In this case, it is evident that the gastric juice 

 changes according to circumstances. 



The presence of some agent, different from a mere chemical 

 power, will be still more evident, if we consider the immunity of 

 the stomach of the living animal during the process of digestion. 

 The stomach of animals is as fit for food as any other substance. 

 The gastric juice, therefore, must have the same power of acting 

 on it, and of decomposing it, that it has of acting on other sub- 



* Phil. Trans. 1740,'p. 390. The fact'was mentioned by Mizaldus in a book 

 published in 1566, entitled, Memoiabilium, utilium ac jucundorum Centuries 

 novem. 



