DIGESTION AND THE DIGESTIVE AGENTS. 331 



mentary canal, but by its conversion into maltose this diffi- 

 culty is at once remedied, maltose being quite dialyzable. 

 On account of the short time during which the food remains 

 in the mouth but a very small part of the starch is con- 

 verted into maltose. But we shall see in the discussion of 

 the pancreas that this process is again picked up in the in- 

 testine and there completed. The ptyalin is unable to act 

 in an acid medium, and so as soon as the food reaches the 

 stomach its digestive action ceases, but when later the food 

 is passed into the small intestine and the acid of the stom- 

 ach gives way to the alkalinity of the intestine, the saliva 

 renews its digestive action, so that possibly the greatest ef- 

 fect of the saliva is produced, not in the mouth, but in the 

 intestine. The digestive action of the ptyalin may be easily 

 demonstrated by taking a dish of boiled starch and adding 

 a little saliva containing ptyalin to it. By keeping this dish 

 then at a temperature of about 98 Fahrenheit the starch is 

 rapidly converted into sugar and may be detected quite 

 easily by the taste and by chemical re-actions. 



2. The Stomach and Gastric Digestion. The process 

 of digestion in the stomach is almost wholly a chemical one 

 and is only incidentally mechanical. The food remains 

 here one or more hours, varying with its digestibility, dur- 

 ing which time it is subjected to a slight churning process. 

 Slight peristaltic movements creep across the stomach, the 

 effect of which is to more or less mix and re-mix the gas- 

 tric contents and so materially aid the juice in reaching all 

 the particles of the food. The stomach owes its power to 

 digest to the gastric juice. This juice is by no means a 

 simple substance, but is a mixture of several things. Its 

 composition is in 1,000 parts about as follows: 



993 parts water. 



2 " hydrochloric acid. 



3 " pepsin. 

 2 " salts. 



In addition to these substances, which can be more or less 

 quantitatively determined, there are present in the gastric 



