270 INTESTINAL DIGESTION. 



straining generally takes place, the contractions of the muscular coats of the 

 rectum frequently are competent of themselves to expel the faeces, especially 

 when they are soft. 



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 contraction 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, notwithstanding 

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

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

 tum has been emptied. 



Very little need be said concerning the influence of the nervous system 

 on the movements concerned in defaecation. The non-striated muscular 

 fibres which form the muscular 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 spinal nerves. These nerves bring the 

 sphincter in a certain degree under the control of the will, and impart like- 

 wise the property of tonic contraction, by which the anus is kept constantly 

 closed. The nerve-centre for defaecation in the dog, or the ano-spinal 

 centre, is in the spinal cord, at the site of the fifth lumbar vertebra (Budge), 



GASES FOUND IN THE ALIMENTARY CANAL. 



The gases in the stomach appear to have no definite office. They gener- 

 ally exist in very small quantity and they are sometimes absent. The oxy- 

 gen and nitrogen are derived from the little bubbles of air which are incor- 

 porated 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. Magendie and Chevreul collected and analyzed a small quantity of gas 

 from the stomach of an executed criminal a short time after death and as- 

 certained that it had the following composition : 



GASES CONTAINED IN THE STOMACH. 

 Oxygen 

 Carbon dioxide 



Pure hydrogen 3'55 



Nitrogen 71-45 



100-00 



Magendie and Chevreul found three different gases in the small intes- 

 tine. Their examinations were made upon three criminals soon after execu- 

 tion. The first was twenty-four years of age, and two hours before execu- 

 tion, he had eaten bread and Gruyere cheese and had drunk red wine and 

 water. The second, who was executed at the same time, was twenty-three 



