HISTORY OF MATTER AND ENERGY, 163* 



Digestion and Absorption. Lumbricus takes daily into its ali- 

 mentary canal a certain amount of necessary food-stuffs, but these 

 are not really inside the body so long as they remain in the ali- 

 mentary canal; for this is shown by its development to be only a 

 part of the outer- surface folded in to afford a safe receptacle 

 within which the food may be worked over. Before the food 

 can be actually taken into the body, or absorbed, it must undergo 

 certain chemical changes collectively called digestion. The most 

 essential part of this process consists in rendering non-diffusible 

 substances diffusible, in order that they may pass through the 

 walls of the alimentary canal into the blood. Proteids, for ex- 

 ample, have been shown to be non-diffusible (Chap. III.). In 

 digestion they are changed by the fluids of the alimentary canal 

 into peptones substances closely like proteids, but readily 

 diffusible. In like manner the non-diffusible starch is changed 

 into diffusible sugar (glucose, C 6 H 12 O 6 ), and becomes capable of ab- 

 sorption. It is highly probable that all carbohydrates are thus 

 turned into glucose or grape-sugar. The fats are probably con- 

 verted in part into soluble and diffusible soaps which are readily 

 absorbed; and in part emulsified and directly passed into the cells 

 of the alimentary tract in a finely divided state, but nothing is 

 known of this save by analogy with higher animals. In every 

 case, however, digestion takes place outside the body, and is only 

 preliminary to the real entrance of food into the living matter. 



Metabolism. After absorption the incoming matters are dis- 

 tributed by the circulation to the ultimate living units or cells,. 

 and are finally taken up by them and built into their substance. 

 There is reason to believe that each cell takes from the common 

 carrier, the blood, only such materials as it needs, leading a some- 

 what independent life as to its own nutrition. It co-operates 

 with other cells under the direction of the nervous system (co- 

 ordinating mechanism), but to a great degree is independent in 

 its choice of food just as a soldier in a well-fed army obeys 

 orders for the common good, but takes what he pleases from the 

 daily ration supplied to all. 



What takes place within the cell upon the entrance of the 

 food is almost wholly unknown, but somehow the food-matters, 

 rich in potential energy, are built up into the living substance 

 probably by a series of constructive processes culminating in pro- 

 toplasm. Alongside these constructive processes a continual 



