226 DIGESTION. 



fistulse, have illustrated many of the changes through which 

 the chief alimentary principles pass, and the times and modes 

 in which they are severally disposed of. These must now be 

 traced. 



The readiness with which the gastric fluid acts on the several 

 articles of food is, in some measure, determined by the state of 

 division, and the tenderness and moisture of the substance pre- 

 sented to it. By minute division of the food, the extent of 

 surface with which the digestive fluid can come in contact is 

 increased, and its action proportionably accelerated. Tender 

 and moist substances offer less resistance to the action of the 

 gastric juice than tough, hard, and dry ones do, because they 

 may be thoroughly penetrated by it, and thus be attacked not 

 only at the surface, but at every part at once. The readiness 

 with which a substance is acted upon by the gastric fluid does 

 not, however, necessarily imply the degree of its nutritive 

 property ; for a substance may be nutritious, yet, on account 

 of its toughness and other qualities, hard to digest ; and many 

 soft, easily digested substances contain comparatively a small 

 amount of nutriment. But for a substance to be nutritive, it 

 must be capable of being assimilated to the blood ; and to h'nd 

 its way into the blood, it must, if insoluble, be digestible by 

 the gastric fluid or some other secretion in the intestinal canal. 

 There is, therefore, thus far, a necessary connection between 

 the digestibility of a substance and its power of affording nutri- 

 ment. 



Those portions of food which are liquid when taken into the 

 stomach, or which are easily soluble in the fluids therein, are 

 probably at once absorbed by the bloodvessels in the mucous 

 membrane of the stomach. Magendie's experiments, and 

 better still, those of Dr. Beaumont, have proved this quick 

 absorption of water, wine, weak saline solutions, and the like ; 

 that they are absorbed without manifest change by the diges- 

 tive fluid, and that, generally, the water of such liquid food 

 as soups is absorbed at once, so that the substances suspended 

 in it are concentrated into a thicker material, like the chyme 

 from solid food, before the digestive fluid acts upon them. 



The action of the gastric fluid on the several kinds of solid 

 food has been studied in various ways. In the earliest experi- 

 ments, perforated metallic and glass tubes, filled with the ali- 

 mentary substances, were introduced into the stomachs of ani- 

 mals, and after the lapse of a certain time withdrawn, to ob- 

 serve the condition of the contained substances ; but such ex- 

 periments are fallacious, because gastric fluid has not ready 

 access to the food. A better method was practiced in a series 

 of experiments by Tiedemann and Gmelin, who fed dogs with 



