336 MECHANISMS OF THE DIGESTIVE TRACT. 



When the faeces have reached the rectum, the passing is still kept 

 in check by the tonic contraction of the two sphincters of the anus, the 

 internal sphincter composed of involuntary and the external sphincter 

 of voluntary muscle. When the contents of the rectum are fluid, or 

 when they are forced against the sphincters by strong peristaltic con- 

 tractions of the lower bowel, the action of the sphincters may be 

 voluntarily reinforced by active contraction of the levator ani muscles, 

 which raise and compress the lower segment of the rectum or anal canal. 



Defalcation is normally started by a voluntary act, although it may 

 take place involuntarily, as is shown by the fact that it may occur in a 

 dog whose spinal cord has been divided in the mid-dorsal region (Goltz). 



The steps of normal defalcation are as follows : — The glottis being 

 closed, a forcible expiratory contraction of the abdominal muscles takes 

 place. The perineal muscles being relaxed at the same time, the lower 

 part of the rectum is straightened, and a portion of the contents of the 

 sigmoid flexure is forced down into the lower part of the rectum. The 

 presence of a foreign body in this part of the rectum irritates the 

 mucous membrane and excites reflexly the rest of the act. Strong 

 peristaltic contractions take place along the whole of the descending 

 colon, sigmoid flexure, and rectum, while both sphincters are relaxed, 

 thus forcing out the contents of the bowel. The last section of the 

 rectum at the close of the act is emptied by a forcible contraction of 

 the levator ani and the other perineal muscles. 



The carrying out of this reflex act is dependent on the integrity of 

 a certain part of the lumbar spinal cord. If this centre be destroyed, 

 the tonic contraction of the sphincter muscles disappears. Budge 1 

 found that stimulation of the spinal cord in the rabbit, at the level of 

 the fourth lumbar vertebra, caused contraction of the rectum. He con- 

 sidered this portion of the cord to be a centre for the rectum, bladder, 

 and vasa deferentia, and named it therefore the genito-spinal centre. 

 His view that motor fibres left this centre to run to the rectum by the 

 sacral nerves was confirmed later by Masius. 2 Nasse 3 showed that, in 

 addition to this supply from the sacral nerves, the rectum receives 

 fibres from the lumbar nerves through the sympathetic chain and 

 mesenteric plexus. 



The nerve supply to the pelvic viscera has been recently the subject 

 of careful investigation by Langley and Anderson. 4 These authors 

 come to the following conclusions with regard to the descending colon, 

 rectum, and anus. 



These parts of the alimentary canal receive fibres from two sources : 

 an upper supply in the rabbit from the second, third, fourth, and fifth 

 lumbar nerves ; in the cat from the second, third, and fourth lumbar ; and 

 a lower supply from third and fourth sacral nerves in the rabbit, with 

 some fibres either from the second sacral or first coccygeal nerve ; and 

 in the cat from the second and third sacral nerves, and sometimes also 

 from the first sacral. 



1 Virchow's Archiv, 1858, Bd. xv. S. 115 ; Ztschr.f. rat. Med.. Leipzig u. Heidelberg, 

 1864, Bd. xxi. 



2 Bull. Acad. roy. d. sc. de Belg., Brux., 1867, tome xxiv. p. 312 ; and 1868, tome xxv. 

 p. 491. 



3 " Beitr. z. Physiol, der Darmbewegung, " Leipzig, 1866 (quoted by Langley and 

 Anderson). 



4 Journ. Physiol., Cambridge and London, 1895, vol. xviii. p. 67 ; 1895, vol. xix. 

 p. 71 ; and 1896, vol. xx. p. 371. 



