38 ZOOLOGY. 



of its contents, as water, alcoholic liquids, &c., are taken 

 up or absorbed by the walls of the stomach, and thus enter 

 the blood without further change. Others pass the pylorus 

 unaltered, and escape with the fseces; but the greater part 

 undergoes the action of digestion, and is by this transformed 

 into a pulpy semi- liquid mass, called chyme. 



It would appear that the alimentary fragments placed on 

 the surface of the mass, and more immediately in contact 

 with the walls of the stomach, imbibe the gastric juice, be- 

 come acid, and soften from the circumference towards the 

 centre. With time the whole mass undergoes this change, 

 and becomes converted finally into a soft pultaceous greyish 

 mass, of a faint and peculiar odour : this is the chyme, mingled 

 with the debris of the food. 



65. Nature of the Digestive Process. Prior to the 

 experiments of Spallanzani, the true nature of the digestive 

 process was not understood. He it was who first showed, 

 by direct experiment, that the solution of the food in the 

 living stomach was due to the action of the gastric juice; 

 and this he proved, by enclosing the food in tubes of wood, 

 perforated so as to allow of the action of the secreted liquids, 

 but strong enough to resist the action of the walls of the 

 stomach : thus proving that trituration was not the cause of 

 digestion. He carried his experiments still further, for by 

 withdrawing, by means of little bits of sponge secured with 

 threads, a sufficient quantity of the gastric juice from the 

 stomachs of crows and other birds, he imitated successfully 

 the digestive process on food placed in close vessels, heated 

 to a proper temperature. 



It is evident, then, that the gastric juice is the true solvent 

 of the food in the stomach, and a question arises, namely, to 

 what property it owes its active power. 



66. Ascribed hitherto to the presence of the hydro- 

 chloric and lactic acids which always enter into its composi- 

 tion, the experiments of Eberle, Schwan, and Muller seem to 

 prove the existence of a peculiar principle (pepsin) analogous 

 to the action of diastase on tinder. But this substance or 

 principle requires to be combined with an acid, the hydro- 

 chloric or acetic for example. It can then dissolve fibrin, 

 coagulated albumen, and the greater number of the solid 

 alimentary bodies ; and it further produces important changes 

 in the chemical nature of some of these bodies, as for ex- 

 ample, in albumen. 



