266 THE TISSUES AND MECHANISMS OF DIGESTION. 



are brought about in the gland itself. In both animals the cerebro-spi- 

 ual fibres are vaso-dilator, and the sympathetic fibres vaso-constrictor in 

 action. 



200. The secretion of gastric juice. Though a certain amount of gastric 

 juice may sometimes be found in the stomachs of fasting animals, it may be 

 stated generally that the stomach, like the salivary glands, remains inactive, 

 yielding no secretion, so long as it is not stimulated by food or otherwise. 

 The advent of food into the stomach, however, at once causes a copious flow 

 of gastric juice; and the quantity secreted in the twenty-four hours is pro- 

 bably very considerable, but we have no trustworthy data for calculating the 

 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 repeat- 

 edly stimulated. The most efficient stimulus is the natural stimulus, viz., 

 food ; though dilute alkalies 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 gray color, 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 dilatation in the same way as is the secretion of saliva. 



201. Seeing that, unlike the case of the salivary secretion, food is 

 brought into the immediate neighborhood of the secreting cells, it is exceed- 

 ingly probable that a great deal of the secretion is the result of the working 

 of a local mechanism ; and this view is supported by the fact that when a 

 mechanical stimulus is applied to one spot of the gastric membrane the 

 secretion is limited to the neighborhood of that spot and is not excited in 

 distant parts. This local mechanism may be nervous in nature or the effect of 

 the stimulus may perhaps be conveyed directly from cell to cell, from the 

 mouth of the gland to its extreme base, without the intervention of any 

 nervous elements ; but the vascular changes at least would seem to imply 

 the presence of a nervous mechanism. 



The stomach is supplied with nerve fibres from the two vagi nerves and 

 from the solar plexus of the splanchnic system. The two vagi after forming 

 the cesophageal plexus on the oesophagus are gathered together again as two 

 main trunks which run along the oesophagus the left in the front, the right 

 at the back to the stomach. The left or anterior nerve is distributed to the 

 smaller curvature and the front surface of the stomach, forming a plexus in 

 which nerve-cells are present ; and branches pass on to the liver and proba- 

 bly to the duodenum. The right or posterior nerve is distributed to the 

 hinder surface of the stomach, but only to the extent of about one-third of 

 its fibres ; about two-thirds of the fibres pass on to the solar plexus. The 

 fibres of the vagus nerves thus distributed to the stomach are for the most 

 part non-medullated fibres ; by the time the vagus reaches the abdomen it 

 consists almost exclusively of non-medullated fibres, medu Hated fibres being 

 very few ; the large number of medullated fibres which the nerve contains 

 in the upper part of the neck pass off into the laryngeal, cardiac, and other 

 branches. 



From the solar plexus nerves, arranged largely in plexuses, pass in com- 

 pany with the divisions of the coeliac artery, coronary artery of the stomach 

 and branches of the hepatic artery, to the stomach. Though the two abdo- 



