CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 339 



exact amount. So also when the gastric mucous membrane is 

 stimulated mechanically, as with a feather, secretion is excited ; 

 but to a very small amount even when the whole interior surface 

 of the stomach is thus repeatedly stimulated. The most efficient 

 stimulus is the natural stimulus, viz. food ; though dilute alkalis 

 seem to have unusually powerful stimulating effects ; thus the 

 swallowing of saliva at once provokes a flow of gastric juice. 

 During fasting the gastric membrane is of a pale grey colour, 

 somewhat dry, covered with a thin layer of mucus, and thrown 

 into folds ; during digestion it becomes red, flushed, and tumid, 

 the folds disappear, and minute drops of fluid appearing at the 

 mouths of the glands, speedily run together into small streams. 

 When the secretion is very active, the blood flows from the 

 capillaries into the veins in a rapid stream without losing its 

 bright arterial hue. The secretion of gastric juice is in fact 

 accompanied by vascular dilation in the same way as is the secre- 

 tion of saliva. 



193. Seeing that, unlike the case of the salivary secretion, 

 food is brought into the immediate neighbourhood of the secreting 

 cells, it is exceedingly probable that a great deal of the secretion 

 is the result of some direct local action ; and this view is sup- 

 ported by the fact that when a mechanical stimulus is applied 

 to one spot of the gastric membrane the secretion is limited 

 to the neighbourhood of that spot and is not excited in distant 

 parts. 



The stomach is supplied with nerve-fibres from the two vagi 

 nerves and from the solar plexas of the splanchnic system. Our 

 knowledge however of the action of the nervous system upon the 

 stomach by means of these two sets of fibres is very imperfect. 

 There are many facts which shew that the central nervous 

 system may affect the secretion of gastric juice. On the other 

 hand a secretion of quite normal gastric juice will go on after 

 both vagi, or the nerves from the solar plexus going to the 

 stomach have been divided, and indeed when all the nervous 

 connections of the stomach are so far as possible severed. 



194. The contrast presented between the scanty secretion 

 resulting from mechanical stimulation and the copious flow which 

 actual food induces is interesting because it seems to shew that 

 the secretory activity of the cells is heightened by the absorption 

 of certain products derived from the portions of food first digested. 

 This is well illustrated by the following experiment of Heidenhain. 

 This observer, adopting the method employed for the intestine, 

 of which we shall speak later on, succeeded in isolating a portion 

 of the fundus from the rest of the stomach ; that is to say, he cut 

 out a portion of the fundus, sewed together the cut edges of the 

 main stomach, so as to form a smaller but otherwise complete organ, 

 while by sutures he converted the excised piece of fundus into a 

 small independent stomach opening on to the exterior by a fistulous 



