284 DIGESTION. 



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 

 quantities in the faeces; their contents, probaby, 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, some, as the casein of milk, 

 are coagulated by the acid of the gastric fluid ; and thus, 

 before they are digested, come into the condition of the 

 other solid principles 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 



