FORMATION OF FAECES AND DEFECATION 299 



tory products (cf. pages 133, 308). The quantity of faeces evacuated daily 

 varies somewhat according to the nature and quantity of the food eaten. With 

 an ordinary diet it is estimated for the adult man at 120-150 g. with 30-37 g. 

 dry substance. 



The hardened masses to be removed collect in the large intestine and in 

 the rectum, and are from time to time discharged from the body. The 

 herbivorous animals (whose food is itself very voluminous, and in which the 

 work of digestion goes on continuously) have frequent evacuations, notwith- 

 standing the great diameter of the intestine. With carnivorous animals, whose 

 food is very concentrated, the fasces are voided less frequently. Man ordina- 

 rily has one stool per day. 



The intestinal contents are retained in the rectum by the tonic contraction 

 of the two sphincters, the sphincter ani externus and internus. The sigmoid 

 flexure of the descending colon has the effect of lessening the load to be borne 

 by the sphincters. The action of the outer sphincter is strengthened by the 

 levator ani, this muscle being thrown round the rectum like a loop. 



The center for the external sphincter in the rabbit lies within the spinal cord 

 in the region of the sixth to the seventh lumbar vertebra, in the dog at about the 

 lower end of the fifth lumbar vertebra. Since this sphincter can be strengthened 

 voluntarily, this center is also under the influence of the higher nerve centers. 

 Contractions of the sphincters are obtained by stimulation of the motor zone 

 of the cerebral cortex (dog). On the other hand, the tonus of the sphincters may 

 be abolished by strong psychic excitation (involuntary defecation), and by stimu- 

 lation of the motor zone of the cortex with the nervi errigentes cut (Frankl- 

 Ilochwart and Frohlich). 



From the spinal cord the nerves to the sphincters run partly in the hypo- 

 gastric nerves and partly in the nervi errigentes, and in the dog the former are 

 said to be inhibitory, the latter motor. Besides, the latter mediate contractions 

 of the rectococcygealis and of the other longitudinal muscles of the rectum. 



The tonus of the anal sphincters is not obliterated even by destruction of 

 the spinal cord (Goltz and Ewald), a fact explicable in part at least by the pres- 

 ence of a center for the sphincter nerves in the inferior mesenteric ganglion 

 (Frankl-Hochwart and Frohlich). 



Defecation is mediated by a reflex process not yet thoroughly investigated, 

 which is induced from the rectum, and which is modified by influences of the 

 will upon the muscles concerned. The sphincters relax and the hardened 

 masses are discharged by contraction of the rectal musculature with the assist- 

 ance of the abdominal pressure. The levator ani muscle may contribute to 

 the general effect. By its contraction it presents a point of insertion for the 

 longitudinal muscles of the rectum, and the compression of the rectum pro- 

 duced by it, coincident with the relaxation of the sphincters and the power- 

 ful effect of the abdominal pressure, assists in discharging the contents 

 (Henle). 



By abdominal pressure we mean the pressure upon the abdominal viscera 

 produced by simultaneous contraction of the diaphragm and of the abdominal 

 muscles. The part it plays in defecation depends upon the consistency of the 

 contents of the rectum. If this is soft, defecation can take place without 



