GASTRIC DIGESTION. 153 



into true peptone. The delicate sheets of elastic tissue, such as 

 basement membranes and those of small vessels, are dissolved, but 

 larger masses of yellow elastic tissue are not affected by the gas- 

 tric digestion. The horny part of the epidermis, hairs, etc., are 

 quite unaltered, and also the mucus, which passes along the ali- 

 mentary tract without change. Bone dissolves slowly, the animal 

 part being attacked at the surface by the gastric juice and the 

 acid slowly removing the salts. 



The action of the gastric juice on milk is peculiar. On reaching 

 the stomach milk is curdled by a special ferment formed in the 

 gastric mucous membrane. This is known as "Rennet," which is 

 made from the stomach of the calf, and used in the manufacture 

 of cheese. The precipitation of the Casein (alkali albumin), 

 which gives rise to the curdling of the milk, is not brought about 

 by the hydrochloric acid (although the acidity would be sufficient 

 cause), because neutralized gastric juice has the same effect. It 

 appears that a special ferment (not pepsin) which directly affects 

 the casein and causes its coagulation, must exist. It is not due 

 to common lactic ferment, for though lactic acid is produced, it is 

 formed too slowly to account for the very rapid coagulation of 

 milk which occurs in the stomach. 



The gastric juice has little effect on vegetable food in general, 

 though well-masticated bread may be very materially altered, 

 owing to the action of the saliva on the starch continuing until 

 the mass is broken 1 up, and the gastric juice then dissolving the 

 proteids (gluten). The greater part of the substance of bread, 

 however, leaves the stomach in an imperfectly digested state. 



In short, the amount of change which any given form of food 

 will undergo in the stomach will depend on the amount and ex- 

 posed condition of the proteid it contains. 



In recapitulating the chief events of gastric digestion, it must 

 be remembered that while the food is yet in the mouth the secre- 

 tion of the gastric juice commences, and is greatly increased by 

 the arrival of a bolus of food and a quantity of frothy alkaline 

 saliva. As the stomach is filled, more and more secretion is pro- 

 duced, and as some food is absorbed an additional stimulus is 



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