138 FUNCTIONS OF NUTRITION. 



of the gastric cavity is soft and glandular, covered with transparent, 

 columnar epithelium, and thrown into minute folds, often reticulated 

 with each other. In the small intestine it presents larger transverse 

 folds, known as " valvulse conniventes," is covered with villosities 

 of various forms, and contains throughout an abundance of tubular 

 follicles. Finally, in the large intestine the mucous membrane is 

 smooth and shining, free from villosities, and provided with a gland- 

 ular apparatus different from that of the preceding parts. 



The accessory glandular organs of the digestive apparatus are the 

 salivary glands communicating with the cavity of the mouth, and the 

 liver and the pancreas connected with the duodenum. 



The digestive fluids, derived from these sources, are five in number ; 

 namely, 1st, the saliva secreted by the salivary glands, and discharged 

 into the mouth ; 2d, the gastric juice, supplied by the mucous mem- 

 brane of the stomach ; 3d, the pancreatic juice, -produced by the pan- 

 creas, and conveyed through its duct into the duodenum ; 4th, the bile 

 supplied by the liver, and also discharged into the duodenum ; and, 

 5th, the intestinal juice secreted by the glandules of the small intestine. 

 These fluids have, in general, certain well marked characters, by which 

 they are readily distinguished from each other, and which indicate cor- 

 responding differences in their physiological properties. At the same 

 time each one is a compound secretion, containing various organic in- 

 gredients, the product of different physiological acts. Thus the three 

 pairs of salivary glands and the buccal follicles unite their secretions 

 to form the saliva of the mouth ; the gastric juice contains an organic 

 ferment and a free acid, both essential to its physiological activity, 

 and produced in the stomach by dissimilar secretory operations ; the 

 pancreatic juice contains no less than three different albumenoid mat- 

 ters ; and the bile is equally complex in the number and quality of its 

 ingredients. This is a general feature of the secretions belonging to 

 the digestive apparatus. 



It is the aim of the physiologist to ascertain the constitution and 

 properties of each digestive fluid, and to learn, if possible, its action 

 on the ingredients of the food. For this purpose, the method of ex- 

 periment by artificial fistulas has been largely used, and with very 

 valuable results. By inserting a silver canula into the parotid or sub- 

 maxillary duct, in various animals, the secretion of either gland may 

 be obtained without admixture from other sources. A fistula of the 

 stomach, established through the abdominal walls, supplies us with 

 gastric juice, and similar methods have been adopted with the gall- 

 bladder and the pancreatic duct. By this means the time, rapidity, and 

 quantity of each secretion is ascertained, as well as its variations under 

 external or internal influences. The digestive fluids of different animals 

 are compared with each other, and with those obtained by accidental 

 fistulas in man. Lastly, the secretions are placed in contact with 

 different alimentary substances, in flasks or test tubes, at the tempera- 

 ture of the body, and their action investigated by the mode of artificial 



