DIGESTION IN THE STOMACH. 291 



pass unchanged through the stomach and intestines, and 

 may be found in the faeces. The interstitial tissues of these 

 structures are converted into pulpy textureless substances 

 in the artificial digestive fluid, and are not discoverable in 

 the faeces. Elastic fibres are unchanged in the digestive 

 fluid. Fat- cells are sometimes found quite unaltered in the 

 faeces : and crystals of cholesterin may usually be obtained 

 from faeces, especially after the use of pork fat. 



As regards vegetable substances, Dr. Rawitz states, that 

 he frequently found large quantities of cell-membranes un- 

 changed in the faeces ; also starch- cells, commonly deprived 

 of only part of their contents. The green colouring prin- 

 ciple, chlorophyll, was usually unchanged. The walls of 

 the sap-vessels and spiral vessels were quite unaltered by 

 the digestive fluid, and were usually found in large quan- 

 1 tities in the faeces ; their contents, probably, were removed. 



From these experiments, we may understand the structural 

 changes which the chief alimentary substances undergo in 

 their conversion into chyme ; and the proportions of each 

 which are not reducible to chyme, nor capable of any 

 further act of digestion. The chemical changes undergone 

 in and by the proximate principles are less easily traced. 



Of the albuminous principles, the casein of milk, and, 

 according to Dr. Beaumont, fluid albumen, are coagulated 

 by the acid of the gastric fluid ; and thus, before they are 

 digested, come into the condition of the other solid prin- 

 ciples of the food. These, including solid albumen and 

 fibrin, in the same proportion that they are broken up and 

 anatomically disorganized by the gastric fluid, appear to 

 be reduced or lowered in their chemical composition. This 

 chemical change is probably produced, as suggested by 

 Dr. Prout, by the principles entering into combination 

 with water. It is sufficient to conceal nearly all their 

 characteristic properties ; the albumen is rendered scarcely 

 coagulable by heat; the gelatin, even when its solution 

 is evaporated, does not congeal in cooling ; the fibrin and 



