DIGESTION IS MAINLY CHEMICAL. 223 



substance which must be got rid of by re-absorption 

 into the veins — as we see when a vessel is ruptured, 

 and the blood gets deposited in the parenchyma. 

 Blood is, in fact, as Bergmann and Leuckart well ex- 

 press it, " a depot of assimilable and secretory sub- 

 stances ; and its purpose in the economy is that of a 

 regulating apparatus, which is necessitated 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 liver, they are entirely assimilated. 



Thus we see that solubility and transformation are 

 the two digestive effects, to produce which, two agen- 

 cies are needful, the mechanical and chemical. From 

 these tw^o points all other questions expansively radiate, 

 to them all converge. A single fact strikingly im- 

 presses 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 alimentary canal, under 



* Bergmann ti.. Leuckart : Vergleichende A naiomie und Physiologie, 

 p. 164. 



