544 SPECIAL PHYSIOLOGY. 



longer, wider, more numerous, and more closely arranged than those 

 of the small intestine. The entire surface presents, when examined 

 with a lens, a cribriform aspect, due to the numerous orifices, which, 

 in the lower part of the intestine, are almost visible to the naked eye; 

 they are lined with columnar epithelium. Besides these crypts, there 

 are found, scattered over the mucous membrane of the large intestine, 

 small depressions, resembling saccular glands ; they were formerly de- 

 scribed as solitary glands, but they are lined with a columnar epithe- 

 lium only, and are placed over certain closed sacs, exactly similar to 

 those of the so-called solitary glands of the stomach and small intes- 

 tine, and of the agminated glands of the latter. 



The intestinal juice of the large intestine resembles, so far as is 

 known, that of the small intestine, being composed partly of mucus, 

 but chiefly of a special secretion, which is said to be alkaline, though, 

 in the caecum, the intestinal contents are acid. 



CHEMICAL PROCESSES OF DIGESTION ACTION OF THE DIGESTIVE 

 FLUIDS, WITH HEAT. 



As already stated, the purpose of the digestive process in the ani- 

 mal economy, is the reduction of alimentary substances into a soluble 

 arid absorbable condition, a state of solution, or of exceedingly minute 

 subdivision and suspension in a fluid, being an essential condition, 

 antecedent to the absorption of any nutrient substance into the living 

 tissues. Food, as we have seen, considered chemically, consists, of 

 water, alkaline and earthy salts, and certain important organic proxi- 

 mate constituents, which are classified into non-nitrogenous and nitro- 

 genous substances. 



Of these, the water, the natural medium of solution or suspension of 

 the solid alimentary substances, and likewise the saline substances, 

 both alkaline and earthy, which are mostly dissolved in it, correspond 

 with the water which forms three-fourths of the soft tissues of the 

 body, and with the water and salts of the blood: they are directly 

 absorbed without any digestive change. The organic constituents, 

 whether non-nitrogenous or nitrogenous, are some of them soluble, and 

 some insoluble in water at the temperature of the interior of the body, 

 viz., about 102. The soluble non-nitrogenous bodies are pectin, gum, 

 dextrin, sugars, alcohol, organic acids, and ethers. The soluble ni- 

 trogenous substances are certain forms of albumen, fibrin, casein, gela- 

 tin, and chondrin ; the albuminoid principles of the digestive fluids, 

 viz., salivin, pepsin, and pancreatin, which are probably in a state of 

 solution in the living body; creatin and creatiniu ; cerebric acid; and 

 thein, caffein, and theobromin. Many of these also are possibly di- 

 rectly absorbed. The insoluble organic constituents are the non-ni- 

 trogenous cellulose, starch, and fatty matters ; and the nitrogenous 

 solid forms of albumen, syntonin, casein, fibrin, gluten, and legumin, 

 and the gelatin- and chondrin-yielding tissues. All these, however 

 soft or minutely divided, must be dissolved, before they can be ab- 

 sorbed. They are the most abundant constituents of our food : in all 

 kinds of bread and biscuit, in cooked potatoes, rice, sago or tapioca, 



