776 PHYSIOLOGY 



absence of gastric digestion, and death of the animal may ensue 

 from inanition, or from poisoning by the products of decomposi- 

 tion 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 stimula- 

 tion 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 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 fistulas 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 stimu- 

 lated 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 submaxillary gland, 

 namely, the great length of the latent period before gastric secretion 

 occurs. 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 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 immediately followed by the 

 production of saliva, for the tasting and testing of the fpod 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, 



