THE DIGESTION OF FOOD 111 



tunity to act on the food residues, and increase their 

 effect. Recent results appear to indicate that the syn- 

 thetic activities of intestinal bacteria may be a matter of 

 some importance in the utilization of amides (Par. 273). 



160. Stimuli to digestion. The gastric juice is not 

 constantly poured into the stomach to accumulate there, 

 but is secreted as it is needed under the influence of cer- 

 tain stimuli. These stimuli may be classed as psychic and 

 chemical. Appetizing odors when there is a strong desire 

 to eat, and the agreeable taste of food in the mouth of a 

 hungry person are important psychic or "nervous" 

 influences that promote gastric digestion through stimu- 

 lating the secretion of an adequate supply of the digest- 

 ing fluid. Other stimuli that may be called chemical 

 result from the indirect reaction of certain substances 

 upon the secretory glands. With man, meat extracts, 

 proteoses, acids, sugars, alcohol, and condiments seem to 

 be effective in this way. This stimulus comes later than 

 the psychic, but is more prolonged. 



161. Secretins. The more recent researches indicate 

 that the first products of digestion, reacting on the inner 

 membranes of the stomach and duodenum, cause the 

 formation of substances called secretins that belong to 

 the general class of excitants known as hormones, which, 

 carried by the blood stream to the cells of secretory glands, 

 excite the secretion of the digestive juices. It now seems 

 possible that sometime we shall have a definite dietetic 

 method of influencing human digestion, at least, other than 

 a medicinal, for it appears that certain food compounds 

 may stimulate and others retard the activity of digestion. 



162. The psychic factor. The psychic factor is no 

 less important. This being so, it is seen how necessary it 

 is that eating shall be pleasurable. Satisfaction with the 



