LECT. V. DIGESTION. 99 



tion and renewal with greater or less rapidity. We are led 

 to this conclusion by a number of experiments presented to 

 us by experimental physiology, but which require to be 

 varied and extended. 



The division of alimentary substances, the rendering of 

 them soluble, and the consequent facilitation of their absorp- 

 tion, are, in short, the changes which take place during 

 digestion. Nothing can be more physical than a function 

 which is exercised merely to modify the physical condition 

 of matter. It is desirable, however, to see this character 

 of digestion verified in its details. 



Before we begin to speak of the physico-chemical phe- 

 nomena of this process, I must explain to you briefly some 

 generalities . 



Varieties of Aliments. All alimentary substances may, 

 with respect to their composition, be reduced to three well- 

 characterized classes : the first comprises substances which 

 are azotised and neutral, namely, albumen, fibrine and case- 

 ine ; the second includes fatty bodies ; and the third compre- 

 hends gum, starch, and sugar, whose composition may 

 perhaps be represented by water and carbon. ^ Experiment 

 has demonstrated, that the alimentary substances of the two 

 latter classes are by themselves insufficient for the nourish- 

 ment of an animal; and that they must always be conjoined 

 with those of the first. 



We shall hereafter find that the alimentary substances of 

 these classes serve distinct purposes in the animal economy. 



Jizotised Substances. With respect to the substances in- 

 cluded in the first class : I cannot pass over in silence the 

 important discoveries recently made by Mulder and 

 Leibig. 



Albumen, fibrine, and caseine, are identical in their com- 

 position ; the proportion of the carbon to azote, in all these 

 three, being 8 equivalents of the former to 1 equivalent of 



