DIGESTION. 379 



pass slowly along it, normally in a direction towards the 

 rectum; these contractions push before them part of the con- 

 tents of the intestine. The simultaneous contractions of the 

 outer longitudinal layer of the muscular coat are not so 

 marked or so easily directly observable. If the bowels be 

 entirely removed from the body of the animal the movements 

 go on for some time, so they are obviously not directly de- 

 pendent on extrinsic nerves. They are probably primarily 

 due to a slight automaticity of the muscle itself, which as in 

 the case of the heart (Chap. XVII) is favored by distension, but 

 they may be due to nerve impulses arising in the cells of the 

 plexus of Auerbach. As in the case of the heart these move- 

 ments are under control of extrinsic nerve-fibres, originating 

 in the cerebro-spinal centre, and these fibres are excitor and 

 depressor. Exactly contrary to that which we find in the 

 case of the heart, the fibres reaching the intestines through 

 the pneumogastrics are excitor, causing more powerful con- 

 tractions, and the fibres coming from the sympathetic through 

 the splanchnics (where they are mixed with but quite dis- 

 tinct from the vaso-constrictor fibres) are inhibitory. Stimu- 

 lation of the splanchnic nerves will bring actively contract- 

 ing intestines to rest. The influence of the central nervous 

 system on the motions of the bowel is shown by the contrac- 

 tions caused by fright or other strong emotions, illustrated 

 by the Hebrew phrase "bowels moved with compassion." 

 Deficiency of arterial blood excites powerful intestinal con- 

 tractions. The various purgative medicines act in very differ- 

 ent ways ; some directly on the intestinal neuro-muscular 

 apparatus; some on the extrinsic nerve centres concerned; 

 some (as Epsom salts) mainly by causing a great secretion of 

 liquid into the bowel and so distending it. 



