178 PHYSIOLOGY CHAP. 



never accumulates in the chyme or pulp of the gastric contents 

 during digestion. If this accumulation can take place under 

 pathological conditions, the phenomenon most likely depends on 

 the reduced absorbing capacity of the epithelium, consequent on 

 its catarrhal alterations. 



Still it is undeniable that the peptonisation of proteins is very 

 imperfectly accomplished in the stomach, the dissolved protein 

 passing from the pylorus to the intestine before it has time to 

 become peptonised. This is evident from the observations made 

 by Busch (1858) on the famous case of fistula established in the 

 upper part of the jejunum in a woman of thirty-one. Four hours 

 after ingestion of raw egg-white, a ropy fluid which was faintly 

 alkaline, mixed with bile, and free of coagulum, began to flow 

 from the upper end of the intestine ; on dilution with water and 

 heating, or treatment with nitric acid, this coagulated in large 

 flocculi. It follows that a considerable part of the egg-albumin 

 ingested passes not only the stomach but also the duodenum, 

 without being attacked by the digestive juices. We shall return 

 elsewhere to the significance of this fact. 



Our knowledge of the changes which natural food-stuffs and 

 viands, i.e. foods modified by cooking and other manipulations, 

 undergo in the living stomach, rests more particularly on the 

 classical work of Frerichs and Schroder, as confirmed by later 

 observers. 



Milk is curdled previous to peptonisation. The caseinogen of 

 cows' and goats' milk forms a firm clot which is more resistant to 

 the action of gastric juice than the caseinogen of human milk, 

 the latter accordingly being the most suited for the alimentation 

 of infants. 



Of muscular flesh, viscera, and membranes of animals the 

 collagenous substances of the connective tissues are first digested ; 

 they soften and become transparent and eventually dissolve ; next 

 follows the digestion of the muscular fibrils, and parenchymatous 

 cells. The fat which infiltrates the connective tissues, and that 

 with which many viands are impregnated, resists the digestive 

 action of the gastric juice, and greatly delays the digestion of the 

 proteins of which tissue protoplasm is built up. For this reason 

 pork is more difficult to digest than the lean meat of beef or veal. 



Bone, too, is digested by the combined action of the hydro- 

 chloric acid which attacks the phosphates and carbonates of 

 calcium, converting them into soluble carbonates and phosphates, 

 and the pepsin which digests the ossein. 



Vegetables and plant tissues in general are more slowly 

 digested than animal tissues, owing especially to the comparative 

 resistance to the action of gastric juice of the cellulose and starch 

 which surround the proteins. Bread, which is the commonest 

 form of food, is reduced to a soft and partially digested pulp in 



