DEFECATION. 335 



emotions. They are certainly checked retiexly by painful stimulation 

 of sensory nerves, especially if the stimulus affect the abdominal 

 viscera. If the splanchnic nerves be intact, the application of an 

 interrupted current to any point in the intestines causes a ring of 

 constriction at the excited spot, with a simultaneous reflex inhibition 

 of the spontaneous contractions, and tonus of all other parts of the 

 small intestine. This inhibitory effect is almost abolished by section of 

 both larger splanchnic nerves. 



The innervation of the alimentary canal in the frog.— It has been 

 stated by Steinach and Wiener, 1 that the alimentary canal of the frog 

 receives its motor innervation chiefly from the -posterior roots of the spinal 

 nerves. The following table shows the distribution of these nerves to the 

 various parts of the canal, according to these observers : — 



(Esophagus . . Vagus, third and fourth posterior roots. 



Stomach ... ,, fourth and fifth (aud sixth) posterior roots. 



Anterior half of small in- 

 testine ... ,, fifth and sixth ,, 



Posterior half of small in- 

 testine ... ,, sixth and seventh ,, 



-n ("seventh and eighth ,, 



Rectura • • • "J „ „ anterior roots. 



Stimulation of the anterior roots of all these nerves (with the exception of 

 the seventli and eighth) had no effect on the intestines. Somewhat similar 

 results had already been obtained by Waters,- without, however, distinguish- 

 ing between the anterior and posterior roots. The following represents the 

 results obtained by this observer : — 



Fourth spinal nerve . Contraction and vaso-constriction in oesophagus. 



Fifth ,, . ,, ,, ,, stomach. 



Sixth ,, . Feeble contractions of intestines ; vaso-constric- 



tion in the upper part of intestines. 



Seventh ,, . Vaso-constriction in lower tyro-thirds of in- 



testine ; occasional peristalsis. 



Eighth ,, . Vasoconstriction in rectum. 



'B' 



A repetition of these experiments by Horton Smith 3 under Langley's 

 directions failed to confirm them. Horton Smith regards the vagus as the 

 sole motor nerve for the oesophagus, stomach, small intestine, and upper part of 

 the large intestine, while the lower part of the large intestine and the rectum 

 are supplied by the anterior roots of the ninth and tenth spinal nerves. The 

 matter cannot be regarded as definitely settled. 



Defecation. 



As the products of digestion pass down the colon they lose their 

 fluid consistence, in consequence of the absorption of water. The mass 

 thus formed is carried down by the peristaltic contractions of the colon 

 towards, and tends to accumulate in, the sigmoid flexure. That it is not 

 driven down at once into the rectum is due largely to the fact that the 

 contractions which start at the ileo-csecal valve become less powerful on 

 their way down the intestine, and do not as a rule affect the rectum. 

 In healthy subjects, except just before defalcation, the rectum is empty, 

 and the presence of faeces in this part of the bowel at once gives 

 rise to discomfort, and to a desire to defecate. 



1 Arch./, d. gas. Physiol., Bonn, 1895, Bd. lx. 



2 Joum. Physiol., Cambridge and London, 1885, vol. vi. 

 s Ibid., 1897, vol. xxi. p. 101. 



