692 PHYSIOLOGY 



when care has been taken to avoid injury to the pulmonary and tracheal 

 branches of these nerves. 



The converse experiment of exciting secretion by direct stimulation of 

 the vagus presents greater difficulties. Stimulation of the vagus in the 

 neck causes stoppage of the heart, and consequent anaemia of the mucous 

 membrane of the stomach. Moreover, the stomach seems to be much more 

 susceptible than the salivary glands to the action of poisons, such as anaes- 

 thetics. Its activity is also easily affected by inhibitory impulses arising 

 in the central nervous system as the result of either painful impressions or 

 emotional states of the animal. In order to avoid these disturbing factors 

 Pawlow proceeded as follows : An animal with fistulae of oesophagus and 

 stomach had one vagus nerve divided. A thread was attached to the peri- 

 pheral end of the cut vagus and allowed to hang out through the wound. Four 

 days after the operation the vagus was drawn out of the wound by carefully 

 pulling on the thread, so as not to hurt or frighten the animal in any way, 

 and its peripheral end stimulated by means of induction shocks. No effect 

 was produced on the heart, owing to the degeneration of the cardio-inhibitory 

 fibres, which is well known to occur within this period after section. Five 

 minutes after the commencement of the stimulation the first drop of gastric 

 juice appeared from the gastric cannula, and a steady secretion of juice was 

 obtained with continuation of the stimulation. This experiment furnishes 

 the decisive and final evidence that the secretory nerves to the stomach run in 

 the two vagi. There is one marked difference, however, between the action 

 of these nerves and the action of the chorda tympani nerve on the submaxil- 

 lary gland, namely, the great length of the latent period before gastric 

 secretion occurs. The length of this latent period has not yet been satisfac- 

 torily explained. It cannot be due to delay occurring between the vagus 

 fibres and the local nervous mechanism in the stomach. It may be that 

 the chemical changes finally resulting in secretion require a longer period for 

 their accomplishment than is the case in the salivary gland. Physiologically 

 there is, indeed, no special need for a rapid secretion of gastric juice, whereas 

 in the mouth it is essential that the introduction of food should be immedi- 

 ately followed by the production of saliva, for the tasting and testing of the 

 food and for its subsequent mastication or rejection. 



These experiments show conclusively that an important probably the 

 most important part of the gastric secretion is determined by a nervous 

 mechanism. This nervous secretion does not, however, account for the whole 

 of the gastric juice obtained as the result of a meal. If an animal provided 

 with two gastric fistulae, one into a diverticulum and the other into the 

 main stomach, has both its vagi divided, it is found that the introduction of 

 meat into the large stomach is followed, after a period of twenty to forty-five 

 minutes, by the appearance of a secretion of gastric juice from the small 

 stomach. Moreover, when an animal is given a normal meal and is allowed 

 to swallow the food after mastication, the total amount of gastric juice 

 obtained is greater than that produced by the sham feeding alone and the 

 flow is of longer duration. In fact, we may say that the gastric juice secreted 



