2 PHYSIOLOGY AT THE FAKM. 



fifteen simple substances that is, chemical substances that 

 have hitherto resisted decomposition in the bodies of ordi- 

 nary quadrupeds ; and to insure the thriving condition of 

 health, all these must be supplied more or less constantly in 

 their food. The most important of these simple substances 

 must be contained in the daily food ; while it may be sufficient 

 that others of them, which exist in minute quantity in the 

 living system, should be afforded in occasional food. But this 

 question belongs to the consideration of the minute chemical 

 composition of the several articles of food proper for each 

 animal, to be spoken of elsewhere in this treatise. In the 

 mean time, it will be enough to observe that the necessity for 

 the food affording a more or less constant supply of all the 

 elementary constituents of the animal body, arises from the 

 unceasing disintegration of the living substance in almost 

 every part of the frame. The refuse, so uninterruptedly thrown 

 off by a living animal, includes every kind of elementary 

 particles which enter into the composition of the frame, and 

 all those elementary particles contained in the food which are 

 either superfluous at that particular moment, or which do not 

 enter at any time into its chemical constitution. Thus, through- 

 out any given period, not too brief, the particles contained in 

 the food are identical in kind with the particles contained in 

 the whole refuse ; and the particles contained either in the food 

 or in the whole refuse, after the deduction of a variable super- 

 fluity, are the same in kind as the particles that enter into the 

 constitution of a living body. 



By the mouth alone food is supplied to the animal frame. 

 Under some circumstances the skin absorbs, but what it takes 

 up in general is merely water, so that it can hardly give passage 

 to food, but only to drink or medicine. By the fundament an 

 attempt is sometimes made to administer nourishment, but 

 barely with success. It is at times maintained that increase is 



