GASTEIC DIGESTION. 355 



is dissolved and converted into peptone, and the oil is again liberated. 

 When, therefore, artificial gastric juice is added to milk warmed to the 

 temperature of the body, the casein is rapidly coagulated and a toler- 

 ably firm, white curd is formed, floating in the clear fluid, the whey, which 

 contains the salts, milk-sugar, water, and albumen of the milk. As 

 digestion progresses, the casein being turned into peptone, the oil is set 

 free and the whey again becomes milky, from the oil again passing into 

 the state of partial emulsion. 



Gelatin and connective tissues are dissolved and peptonized by the 

 gastric juice. When gelatin has been subjected to the action of gastric 

 secretion, its solutions no longer solidify when cold, but a gelatin-pep- 

 tone is formed which is soluble and diffusible, although it differs from a 

 true peptone. When muscular tissue is subjected to the action of gastric 

 juice, the sarcolemma becomes dissolved, the muscle-fibre breaks up 

 traiisversel}' into disks, which become dissolved and converted ultimately 

 into a true peptone. Horny tissues are unchanged by gastric juice, as 

 is also anryloid substance. Red blood-corpuscles are dissolved in the 

 stomach, the haemoglobin being decomposed into haematin, and the glob- 

 ulin is ultimately transformed into peptone, while the hsematin is partly 

 unchanged and partly converted into bile-pigment. 



That the stomach is able to digest albuminous bodies of the most 

 varying nature, and yet escape digestion itself by its own secretion, is a 

 fact of which explanation has as yet never been clearty determined. It has 

 been attributed to the alkalinity of the blood in the tissues neutralizing 

 the gastric juice and so protecting the tissues of the stomach ; if that 

 were so, we would expect that the pancreatic secretion, being most active 

 in an alkaline medium, would be aided by the alkalinit} r of the blood in 

 digesting the walls of the intestine. The protection of the walls of the 

 stomach during digestion has been attributed to the mucus or the 

 epithelium lining the stomach ; both may, however, be mechanically 

 removed through a gastric fistula and both are, undoubtedly, at least 

 partially removed in the use of the stomach-sound and yet without the 

 walls of the stomach being digested. 



The obscurity is rendered still more intense by the fact that under 

 certain circumstances the stomach does digest itself. If an animal be 

 killed during active digestion, and the body kept at an elevated temper- 

 ature, the walls of the stomach will be digested ; or if the stomach be 

 excised from an animal and covered with dilute hydrochloric acid, it will 

 be almost -completely dissolved. These facts have been attributed to the 

 absence of vitality ; but if the leg of a living frog be inserted through 

 a fistula into the stomach of a dog it will be completely digested, and 

 in the maintenance of a gastric fistula in dogs it will often be noticed 

 that the edges of the wound become corroded from the escape of gastric 



