38 PHYSIOLOGY FOR DENTAL STUDENTS. 



bring about the right secretion at the right time. These mech- 

 anisms are of two essentially different types, a nervous reflex 

 control, and a chemical or "hormone" control. The nervous con- 

 trol is exercised through a nerve center which is called into activ- 

 ity by afferent stimuli which proceed from sensory nerve endings 

 or receptors (see p. 244) that are especially sensitized so as to 

 be stimulated by some property of food (its taste or smell, or 

 some local action on the nerve endings). This type of control 

 exists where prompt response of the glandular secretion is impor- 

 tant, as in the mouth and in the early stages of digestion in the 

 stomach. The hormone control consists in the action directly on 

 the gland cells of substances which have been absorbed into the 

 blood from the mucous membrane of the gastro-intestinal tract. 

 The production of these substances depends upon the nature of 

 the contents of the digestive tube. This is a more sluggish proc- 

 ess of control than the nervous, but it is all sufficient for the cor- 

 relation of most' of the disgestive functions. 



These considerations point the way to the scheme which we 

 must adopt in studying the process of digestion ; we must explain 

 how each digestive juice comes to be secreted, what action it has 

 on the foodstuffs, and what it is, after each stage in digestion is 

 completed, that controls the movement onwkrd of the food to 

 the next stage. And when we have followed each foodstuff to 

 its last stage in digestion, we may then proceed to study the 

 means by which the digested foodstuffs are absorbed into the cir- 

 culating fluids, and in what form they are carried to the tissues. 



On account of the varying nature of their food we find that 

 the digestive system differs considerably in different groups of 

 animals. In the omnivora, such as man, the digestive canal be- 

 gins with the mouth cavity, in which the food is broken up me- 

 chanically and is mixed with the saliva in sufficient amount to 

 render it capable of being swallowed. The saliva, by containing 

 starch-splitting ferment, also initiates the digestive process. The 

 food is then carried by way of the oesophagus to the stomach, in 

 the near or cardiac end of which it collects and becomes 

 gradually permeated by the acid gastric juice. It is then caught 

 up, portion by portion, by the peristaltic waves of the 



