DIGESTION. 587 



cretions, useful or indispensable for the animal economy. In this 

 way are formed the seminal fluid of the male, the milk of the fe- 

 male destined for the nourishment of the offspring ; and in this 

 way are formed the saliva, the bile, the pancreatic juice, the mu- 

 cous matter which lines the cavities of the body, and all the dif- 

 ferent secretions so indispensable for the use of the living animal. 

 How these changes are induced has hitherto eluded the utmost 

 sagacity of physiologists. But, in man and the greater number 

 of animals, the agency of the nervous filaments which are spread 

 through all the essential organs of the body, is indispensable. 

 Accordingly, when these nerves are cut or diseased, the organ 

 which they supply ceases to perform any of its functions. Hence 

 in man and in most animals, we may say that the nervous ener- 

 gy, whatever it may be, constitutes the indispensable part of the 

 living structure. Yet it cannot be maintained that life cannot 

 exist without nerves ; for plants are undoubtedly living beings. 

 They require food and digest it, just as animals do ; and the di- 

 gested food is afterwards applied to all the purposes of secretion 

 and assimilation, just as in animals. Yet nothing like nervous 

 structure has ever been observed in vegetables ; nor is there the 

 least reason for supposing them supplied with nerves. 



The digestion of food, or the conversion of it into blood, though 

 we are utterly incapable of imitating it by artificial processes,* [is 

 purely a chemical process. We can only expect to learn the 

 contrivances which nature follows in it by investigating the dif- 

 ferent changes which the food undergoes as it passes in succes- 

 sion through the different organs employed in digestion, and by 

 ascertaining the chemical nature of the different substances which 

 are employed in the successive steps by which the food is con- 

 verted into blood. 



Let us then examine in succession the changes which the food 

 undergoes, and the liquids employed in producing these changes. 

 We must confine ourselves chiefly to the human species, though 

 a very great proportion of the facts which have been acquired 

 were obtained by experimenting upon the inferior animals, par- 

 ticularly dogs, whose food and whose organs of digestion bear a 

 close resemblance to those of the human body. 



The food of man is of two kinds, partly animal and partly ve- 

 getable ; and the structure of his teeth shows that nature intend- 

 ed him to make use of both. The vegetable substances which 

 answer best for food are, sugar, gum, and starch. And, as has 

 been well observed by Dr Front, those vegetable substances are 



