COMPLEX APPARATUS FOR NUTRITION. 83 



or Aorta^ is seen at A,) to every part of the system; thence 

 returning by the veins (v, v^ v,) to the heart. The various 

 modes in which these functions are conducted in tlie seve- 

 ral tribes of animals will be described hereafter. It will be 

 sufficient for our present purpose to state, by way of com- 

 pleting the outline of this class of functions, that, like the 

 returning sap of plants, the blood is made to undergo far- 

 ther modifications in the minute vessels through which it 

 circulates; new arrangements of its elements take place 

 during its passage through the subtle organization of the 

 glands, which no microscope has yet unravelled: new pro- 

 ducts are here formed, and new properties acquired, adapted 

 to the respec^e purposes which they are to serve in the 

 animal economy. The whole is one vast Laboratory, where 

 mechanism is subservient to Chemistry, where Chemistry 

 is the agent of the higher powers of Vitality, and where 

 these powers themselves minister to the more exalted fiicul- 

 ties of Sensation and of Intellect. 



The digestive functions of animals, however complex 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 efTect 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 chemi- 

 cal changes which the food must necessarily undergo during 

 its assimilation. 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 we are furnished with a kind of scale, whereby, 

 whenever we can ascertain the degrees of difference exist- 

 ing between the chemical condition of the substance taken 

 into the body, and that of the product derived from it, we 

 may estimate the length of the process required, and the 



