DIGESTION 



169 



The nerve elements are partly intrinsic and partly extrinsic. The in- 

 trinsic nerve elements consist of two nerve plexuses, one which lies between 

 the muscle coats, another which lies between the muscle and the mucous 

 coats. They have been named after their discoverers, Auerbach's and 

 Meissner's plexuses respectively. Each plexus consists of microscopic 

 ganglia and nerve processes which interlace with one another to form a 

 very complicated network. The nerve processes are distributed mainly to 

 muscle-fibers. As these plexuses are associated with a large part of the 

 alimentary canal they together have been termed the myenteric plexus. 

 The extrinsic nerves, which associate the central nerve system with the 

 gastric structures are the vagus and the great splanchnic. The vagus nerve, 

 through its efferent nerve-fibers is in physiologic relation with muscle-fibers 

 and gland-cells, through the intermediation of gang- 

 lionic cells. The great splanchnic through its efferent 

 nerve-fibers is in physiologic relation with the muscle- 

 fibers and blood-vessels through the intermediation 

 of the post-ganglionic fibers derived from the cells of 

 the semilunar ganglion. 



Gastric Fistulae. The general process of diges- 

 tion, as it takes place in the stomach, has been 

 studied in human beings and animals with a fistula 



in the walls of the stomach and abdomen, the result 



either of accident, of necessary surgical or of experi- 

 mental procedures. 



The earliest observations on gastric digestion 



were made by Dr. Beaumont on Alexis St. Martin, 



who, as the result of a gunshot wound, was left with 



a permanent fistulous opening into the fundus of the 



stomach. This opening two years after the acci- 

 dent was about two and a half inches in circum- 

 ference and usually closed from within by a fold of 



mucous membrane which prevented the escape of 



the food. This valve could be readily displaced by 



the finger and the interior of the stomach exposed 



to view. After the complete recovery of St. Martin, 



Dr. Beaumont during the years between 1825 and 



1831 at intervals made numerous experiments on 



the nature of gastric digestion. As the result of an 



admirable series of investigations it was established 



,, ii> r -, ? , . i > . , 



that the digestion of the food is largely a chemic (Landois.) 

 act, due to the presence of an acid fluid secreted by 



the mucous membrane; that this fluid is secreted most abundantly after the 

 introduction of food into the stomach; that different articles of food possess 

 varying degrees of digestibility; that the duration of digestion varies ac- 

 cording to the nature of the food, exercise, mental states, etc., and that the 

 process is aided by continuous movements of the muscle walls. 



Since Dr. Beaumont's time the establishing of a gastric fistula in human 

 beings has been necessitated by pathologic conditions of the esophagus. 

 After recovery these cases offered fair facilities for the study of the process 

 when the food was introduced through the opening. Similar, fistulae have 



FIG. 72. PYLORIC 

 or THE STOMACH. 

 t, Mouth of duct; n, neck. 



