THE GASTRIC JUICE AND STOMACH DIGESTION. 159 



is not affected in the same way, however, by all the agents which cause 

 its coagulation. After precipitation by alcohol or by lime phosphate, it 

 is unchanged in its chemical qualities, and may be again dissolved in 

 water without losing any of its original digestive properties ; but when 

 coagulated by boiling, it is permanently altered and cannot again be 

 brought into an active condition. 



Pepsine may also be k precipitated from the gastric juice by contact 

 with the bile. If ten to fifteen drops of dog's bile be added to 10 cubic 

 centimetres of fresh gastric juice from the same animal, a complete pre- 

 cipitation takes place ; so that the whole of the biliary coloring matter 

 is thrown down as a deposit and the filtered fluid is found to have lost 

 its digestive power although it still retains an acid reaction. This ex- 

 plains the disturbing effect upon digestion produced by a regurgitation 

 of bile through the pylorus into the stomach. 



Pepsine may be extracted from the fresh mucous membrane of the 

 stomach by cutting it into small pieces and macerating it for some hours 

 with distilled water. The filtered fluid, acidulated with dilute hydro- 

 chloric acid until it presents a similar grade of acid reaction to that of 

 the fresh gastric juice, is found to possess the peculiar digestive proper- 

 ties of the natural secretion. 



These digestive properties depend accordingly upon the presence of 

 both the pepsine and the free acid. They are not exhibited by a dilute 

 acid alone, nor by a solution of pepsine which is neutral or alkaline in 

 reaction. The pepsine, which acts in some unexplained manner, like 

 other so called " catalytic" bodies, requires, as a special condition of its 

 activity, the presence of a free acid. Accordingly, the fluids of the sto- 

 mach, even though they contain pepsine, will not act upon the food unless 

 they have also an acid reaction. If the fresh gastric juice be neutralized 

 by the addition of an alkali, it loses its digestive properties as soon as 

 the point of neutralization is reached; but these properties may be 

 restored by again acidulating the fluid. For this purpose, either lactic 

 or hydrochloric acids may be used, both of which yield very active 

 digestive fluids with pepsine. Dilute sulphuric, nitric, and acetic acids, 

 on the other hand, according to Lehmann, 1 produce a mixture of only 

 slight digestive power ; while phosphoric, oxalic, and tartaric acids are 

 nearly inert in this respect. If the gastric juice, again, be subjected to 

 a boiling temperature, it is found to have lost its digestive properties 

 owing to the chemical alteration of its pepsine, notwithstanding its acid 

 reaction may remain the same as before. 



The characteristic property of the fresh gastric juice, as well as of 

 acidulated solutions of pepsine, is that it has the power of digesting and 

 dissolving substances of an albuminous nature. This is best shown by 

 suspending in gastric juice pieces of coagulated fibrine and keeping 

 the fluid at the temperature of 38 (100 F.). The fibrine rapidly swells 



1 Physiological Chemistry. Cavendish Society edition. London, 1853, vol. ii. 

 p. 58. 



