298 DIGESTION. 



sphincter, which is a striated muscle of considerable power. There is always, however, 

 a voluntary relaxation of this muscle, or rather a cessation of its semi-voluntary con- 

 traction, which immediately precedes the expulsive act. The dilatation of the anus is 

 also facilitated by the action of the levator ani, which arises from the posterior surface 

 of the body and ramus of the pubis, the inner surface of the spine of the ischium, and a 

 line of fascia between these two points, passes downward, and is inserted into the me- 

 dian raphe of the perineum and the sides of the rectum, the fibres uniting with those of 

 the sphincter. While this muscle forms a support for the pelvic organs during the act 

 of straining, it steadies the end of the rectum, and, by its contractions, favors the relaxa- 

 tion of the sphincter and draws the anus forward. 



The action of the diaphragm and the abdominal muscles is very simple. They merely 

 compress the abdominal organs, and consequently those contained in the pelvis, and as- 

 sist in the expulsion of the contents of the rectum. The diaphragm is the most impor- 

 tant of the voluntary muscles concerned in this process; and, during the act of straining, 

 the lungs are moderately filled and respiration is interrupted. The vigor of these efforts 

 depends greatly upon the consistence of the fascal mass, very violent contractions being 

 frequently required for the expulsion of hardened faeces after long constipation. Al- 

 though more or less straining generally takes place, the contractions of the muscular 

 coats of the rectum are frequently competent of themselves to expel the faeces, espe- 

 cially when they are soft. This can be shown by arresting all voluntary muscular action 

 during an easy act of defecation, when the faeces may be passed by contractions of the 

 rectum alone. 



By a combination of the movements above described, the floor of the perineum is 

 pressed outward, the anus is dilated, the sharp bend in the lower part of the rectum is 

 brought more into line with the rest of the canal, and a portion of the contents of the 

 rectum is expelled. Very soon, however, the passage of faeces is interrupted by a con- 

 traction of the levator ani and the sphincter, by which the anus is suddenly and rather 

 forcibly retracted. This muscular action may be effected voluntarily; but, after the 

 sphincter has been dilated for a time, the evacuation is interrupted in this way, notwith- 

 standing all efforts to oppose it. After a time, another portion of fasces is discharged, 

 until the matters have ceased to pass out of the sigmoid flexure and the rectum has 

 been emptied. The mucous membrane of the rectum, which is rather loosely held to 

 the subjacent tissue, is slightly prolapsed during an evacuation, but it returns shortly after 

 the act has been completed. 



Very little need be said concerning the influence of the nervous system on the move- 

 ments concerned in defsecation. The non-striated muscular fibres which form the mus- 

 cular coat of the rectum are supplied with nerves from the sympathetic 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 control 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 generally found in the stomach and 

 in the small and 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 Chev- 

 reul, who had the opportunity of examining the bodies of several criminals immediately 

 after execution. 



The gases in the stomach appear to have no definite function. They generally exist 

 in very small quantity, and they are sometimes absent. The oxygen and nitrogen are de- 

 rived from the little bubbles of air which are incorporated with the alimentary bolus dur- 

 ing 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 



