254: HANDBOOK OF PHYSIOLOGY. 



whether they be taken into the body in meat, eggs, milk, bread, or other 

 foods, the resultant still is peptone. 



Milk is curdled, the casein being precipitated, and then dissolved. 

 The curdling is due to a special ferment of the gastric juice (curdling or 

 rennet ferment), and is not due to the action of the free acid only. The 

 effect of rennet, which is a decoction of the fourth stomach of a calf in 

 brine, has long been known, as it is used extensively to cause precipita- 

 tion of casein in cheese manufacture. The ferment which produces this 

 curdling action is distinct from pepsin. 



Gelatin is dissolved and changed into peptone, as are also chondrin 

 and elastin; but Mucin, and the Horny tissues, which contain keratin 

 generally are unaffected. 



On the Amylaceous articles of food, and upon pure Oleaginous prin- 

 ciples, the gastric juice has no action. In the case of adipose tissue, its 

 effect is to dissolve the areolar tissue, albuminous cell-walls, etc., which 

 enter into its composition, by which means the fat is able to mingle more 

 uniformly with the other constituents of the chyme. 



The gastric fluid acts as a general solvent for some of the saline con- 

 stituents of the food, as, for example, particles of common salt, which 

 may happen to have escaped solution in the saliva ; while its acid may 

 enable it to dissolve some other salts which are insoluble in the latter or 

 in water. It also dissolves cane sugar, and by the aid of its mucus 

 causes its conversion in part into grape sugar. 



The action of the gastric juice in preventing and checking putrefac- 

 tion has been often directly demonstrated. Indeed, that the secretions 

 which the food meets with in the alimentary canal are antiseptic in 

 their action, is what might be anticipated, not only from the proneness 

 to decomposition of organic matters, such as those used as food, espe- 

 cially under the influence of warmth and moisture, but also from the 

 well-known fact that decomposing flesh (e. g., high game) may be eaten 

 with impunity, while it would certainly cause disease were it allowed to 

 enter the blood by any other route than that formed by the organs of 

 digestion. 



Time occupied in Gastric Digestion. Under ordinary conditions, 

 from three to four hours may be taken as the average time occupied by 

 the digestion of a meal in the stomach. But many circumstances will 

 modify the rate of gastric digestion. The chief are : the nature of the 

 food taken and its quantity (the stomach should be fairly filled not 

 distended) ; the time that has elapsed since the last meal, which should 

 be at least enough for the stomach to be quite clear of food; the amount 

 of exercise previous and subsequent to a meal (gentle exercise being fa- 

 vorable, over-exertion injurious to digestion) ; the state of mind (tran- 

 quillity of temper being essential, in most cases, to a quick and due di- 

 gestion) ; the bodily health ; and some others. 



