162 MANUAL OF PHYSIOLOGY. 



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 reach- 

 ing 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 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 

 applied. Being kept in motion in a large quantity of liquid 

 which dissolves the cases in which the food particles are con- 

 tained, the bolus of food soon falls asunder, and each. of its in- 

 gredients is fully exposed to the action of the gastric juice. The 

 acid reaction of the gastric fluid neutralizes the alkalinity of the 



