410 DIGESTION. 



have ceased to pass out of the sigmoid flexure, and the rec- 

 tum has been emptied. The mucous membrane of the rec- 

 tum, which is rather loosely held to the subjacent tissue, is 

 slightly prolapsed during an evacuation, but returns shortly 

 after the act has been completed. 



Yery little need be said concerning the influence of the 

 nervous system on the movements concerned in defecation. 

 The non-striated muscular fibres which form the muscular 

 coat of the rectum are supplied with nerves from the sympa- 

 thetic system ; and to the external sphincter are distributed 

 filaments from the last sacral pair of the spinal nerves. These 

 nerves bring the sphincter to a certain degree under the con- 

 trol of the will, and impart likewise the property of tonic 

 contraction, by which the anus is kept constantly closed. 



Gases found in the Alimentary Canal. 



In the human subject, a certain quantity of gas is gener- 

 ally found in the stomach and in the small and the large 

 intestine. The most accurate analyses of these gases, as 

 they may be supposed to exist in the human subject in 

 health, are those of Magendie and Chevreul, who had the 

 opportunity of examining the bodies of several criminals 

 immediately after execution. The previous analyses by Ju- 

 rine (who was the first to examine the gases of the alimentary 

 canal) and others, were made before the processes for the 

 analysis of gases had been brought to a sufficient degree of 

 perfection to insure accurate results. 



The gases in the stomach appear to have no definite func- 

 tion. They generally exist in very minute quantity, and are 

 sometimes absent. The oxygen and nitrogen are derived 

 from the little bubbles of air which are incorporated with 

 the alimentary bolus during mastication and insalivation. 

 The other gases are probably evolved from the food during 

 digestion ; at least, there is no satisfactory evidence that they 

 are produced in any other way. 



Yery little gas is ordinarily found in the stomach. Ma- 



