OH. xxxv.] INTESTINAL MOVEMENTS. 527 



to the greater strength required for the onward moving of the 

 fteces, which are gradually, owing to the absorption of water, 

 becoming firmer. The greatest strength is in the rectum, at the 

 termination of which the circular unstriped muscular fibres form 

 a strong band called the internal sphincter ; while an external 

 sphincter muscle with striped fibres is placed rather lower down, 

 and more externally, and holds the orifice close by a constant 

 slight tonic contraction. 



Nervous mechanism. Experimental irritation of the brain 

 or cord produces no evident or constant effect on the movements 

 of the intestines during life ; yet in consequence of certain mental 

 conditions the movements are accelerated or retarded ; and in 

 paraplegia the intestines appear after a time much weakened in 

 their power, and costiveness, with a tympanitic condition, ensues. 



As in the case of the oesophagus and stomach, the peristaltic 

 movements of the intestines may be directly set up in the 

 muscular fibres by the presence of food or chyme acting as the 

 stimulus. Few or no movements occur when the intestines are 

 empty. 



The small intestines are connected with the central nervous 

 system by the vagi and by the splanchnic nerves. The fibres 

 which leave the medulla in the vagal rootlets are fine medullated 

 . ones : they are connected with cells in the ganglion trunci, and 

 then continue as non-medullated fibres to the intestinal walls ; 

 they pass through the solar plexus, but are not connected with 

 iR'rve-cells in that plexus. In animals stimulation of the pneumo- 

 gastric nerves induces peristaltic movements of the intestines. If 

 the intestines are contracting peristaltically before the stimulus 

 is applied, the movements arc inhibited for a brief period, after 

 which they are greatly augmented. The sympathetic fibres leave 

 the cord as fine medullated fibres by the anterior roots from the 

 sixth thoracic to the first lumbar, pass through the lateral chain, 

 but do not reach their cell stations until they arrive at the superior 

 mesenteric ganglia : thence they pass as non-medullated fibres to 

 the muscular coats. Stimulation of these fibres causes inhibition 

 of any peristaltic movements that may be present. These nerves 

 also contain vaso-motor fibres, and section of these leads to vaso- 

 dilatation and a great increase of very watery succus entericus. 



Peristalsis in the small intestine can be excited artificially 

 i \t n when all nerves running to it from the central nervous 

 system have been cut through. After pinching any particular 

 spot a wave of inhibition travels downwards, and a wave of 

 contraction upwards. (Starling.) 



In the case of the large intestine there is no supply from the 



