DIGESTION IN THE STOMACH 737 



vagi. On this account division of both vagi may give rise to entire absence of 

 gastric digestion, and death of the animal may ensue from inanition, or from 

 poisoning by the products of decomposition of food in the stomach, even 

 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 ansemia of the mucous membrane of the stomach. More- 

 over the stomach seems to be much more susceptible than the salivary glands to the 

 action of poisons, such as anaesthetics. 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 peripheral 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 induc- 

 tion 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 submaxillary gland, namely, the great length of the 

 latent period before gastric secretion begins. The length of this latent period has 

 not yet been satisfactorily 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 ch'anges 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 immediately 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 

 ost 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 

 in response to a normal meal consists of two parts, viz. (1) a large amount, 

 the secretion of which begins within five minutes of the taking of the food 

 and is determined by the reflex nervous mechanism described above ; and 

 2) a smaller portion, the secretion of which is excited by the presence of the 



(2 



