98 THE VITAL FUNCTIONS. 



the respective purposes which they are to serve in 

 the animal economy. The whole is one vast Labo- 

 ratory, where Mechanism is subservient to Che- 

 mistry, where Chemistry is the agent of the higher 

 powers of Vitality, and where these powers them- 

 selves minister to the more exalted faculties of 

 Sensation and of Intellect. 



The digestive functions of animals, however com- 

 plex and varied, and however exquisitely contrived 

 to answer their particular objects, yet afford less 

 favourable opportunities of tracing distinctly the 

 adaptation of means to the respective ends, than 

 the mechanical functions. This arises from the 

 circumstance that the processes they effect imply 

 a refined chemistry, of which we have as yet but a 

 very imperfect knowledge ; and that we are also 

 ignorant of the nature of the vital agents concerned 

 in producing each of the chemical changes which 

 the food must necessarily undergo during its assimi- 

 lation. We only know that all these changes are 

 slowly and gradually effected, the materials having 

 to pass through a great number of intermediate 

 stages before they can attain their final state of 

 elaboration. 



Hence, whenever we can ascertain the degrees 

 of difference existing between the chemical condi- 

 tion of the substance taken into the body, and that 

 of the product derived from it, we are furnished 

 with a kind of scale whereby we may estimate the 

 length of the process required, and the amount of 

 power necessary for its conversion into that pro- 

 duct. It is obvious, for example, that the chemical 

 changes which vegetable food must be made to 

 undergo, in order to assimilate it to blood, must be 



