DIGESTION IS MAINLY CHEMICAL. 211 



able and secretory substances ; and its purpose in the eco- 

 nomy is that of a reguhiting apparatus, which is necessi- 

 tated by the fluctuations in the procuring of food."* 



Remember also, that, before Assimilation can take place, 

 the food must be rendered soluble. Solubility is a primary 

 condition, but not the only one. Many soluble substances 

 have to undergo chemical changes, both of decomposition 

 and allotropism, before they form parts of the living body. 

 If albumen or sugar be injected into the veins, they will not 

 be assimilated, but cast out unaltered in the excretions ; 

 whereas, if injected into the alimentary canal, or into the 

 portal vein, which would carry them through the laboratory 

 of the hver, they are entirely assimilated. 



Thus we see that solubility and transformation are the 

 two digestive effects, to produce which, two agencies are 

 needful, the mechanical and chemical. From these two 

 points all other questions expansively radiate, to them 

 all converge. A single fact strikingly impresses the mind 

 with a sense of the extent to which chemical agency reaches, 

 namely, that in the course of four-and-twenty hours a sixth 

 part of the whole weight of the body is poured into the ali- 

 mentary canal, under the form of various secretions. ]\Iuch 

 more fluid is secreted from the blood and poured into this 

 canal during a single day, than would make up the whole 

 mass of fluid circulating in the blood-vessels at any given 

 period. -|- 



The reader's attention has been so fully directed to this 



* Bergmann 0. Leuckart : Verc/leichende A natomie und rinjsiologie, p. 1 64. 

 + Lehmann : Lehrhuch der Physiol. Chcmic, iii. 226, 2d edit. 



