i8 



,. ,y-, n LECTURE III. 



Delivered on June 27th. 

 THE CHEMICAL REFLEXES OF THE ALIMENTARY TRACT. 



MR. PRESIDENT AND GENTLEMEN, It is not until the food arrives at 

 the stomach that any use is made by the organism of the more primitive 

 chemical method of adjusting the secretion of the digestive juices to the 

 presence of food at various sections of the canal. We have searched in 

 vain for evidence of the production of a secretin in the mouth or its 

 mucous membrane, as the result of the taking of food into this cavity, 

 which might serve as a stimulus to the salivary glands, and the processes 

 in the first part of the alimentary canal are so closely associated with 

 consciousness and volition that one would expect their complete subordi- 

 nation to the central nervous system. In the stomach, however, the 

 researches of Pawlow have already pointed to the possibility of some 

 chemical mechanism being involved in the secretion of gastric juice. This 

 observer has shown that gastric secretion begins even before the entry of 

 food into the stomach, as a result either of stimulation of the nerves of 

 taste or of the mental reproduction of sensations of smell and taste. 

 This immediate secretion is therefore largely psychical and is so desig- 

 nated by Pawlow. The efferent channel of the nervous process is repre- 

 sented by the vagus nerves, and Pawlow has shown that division of both 

 vagus nerves absolutely prevents this immediate secretion of gastric juice. 

 There can be no doubt that the psychical secretion is the more important 

 factor in the production of gastric juice in normal circumstances. The 

 dependence of normal digestion in the stomach upon the free play of 

 these nervous influences is shown by the indigestion which is a familiar 

 result of mental disturbance. Gastric secretion has, however, two stages. 

 In the first stage there is the rapid secretion due to the psychical stimu- 

 lation. Two or three hours after the taking of food into the stomach a 

 second rise in the secretion of juice may be observed, and this second rise 

 is apparently dependent, not on the taste of the food or mental condition 

 of the animal, but on the nature of the stomach contents. Pawlow 

 states that this second stage occurs even when both vagus nerves are 

 divided, and the possibility is at once suggested that this stage is not due 

 to nervous processes at all, but is determined by some chemical mechan- 

 ism similar to that which we have studied in the case of the pancreas. 

 Pawlow is apparently of opinion that the secondary secretion is due to 

 local reflexes in the wall of the viscus, but recent experiments by Edkins 

 have shown that it is unnecessary to invoke the aid of any such obscure 

 mechanism. 



