332 DIGESTION 



splanchnic nerves contain fibres by which the intestinal movements 

 can be inhibited, and they appear to be always in action, for after 

 section of these nerves the movements are strengthened. On the 

 other hand, stimulation of the peripheral end of the cut splanchnic 

 causes arrest of the movements. Occasionally, however, it has 

 the opposite effect. Contractions of the small intestine are more 

 easily caused by excitation of the vagus after the inhibitory splanch- 

 nic nerves have been cut. The splanchnics also contain inhibitory 

 fibres for the stomach, and it is only when these are intact that 

 complete reflex inhibition of the organ can be obtained in the rabbit 

 (Auer). The gastric movements are not permanently affected by 

 section of these nerves alone, or even by simultaneous section of 

 the splanchnics and the gastric branches of the vagi. But if the 

 vagi are cut while the splanchnics remain intact, the peristalsis of 

 the stomach is weakened, its onset delayed, and the proper emptying 

 of the viscus through the pylorus interfered with. In all probability 

 these results are due to the uncontrolled action of the inhibitory 

 fibres. The splanchnics have a special relation to the ileo-colic 

 sphincter, which closes when they are stimulated, and becomes in- 

 sufficient when they are cut. The vagus does not affect it. 



The lower part of the large intestine is influenced by the sacral nerves 

 (second, third, and fourth sacral in tlie rabbit), and by certain lumbar 

 nerves, in the same way as the higher parts of the alimentary canal, and 

 particularly the small intestine, are influenced by the vagus and the 

 splanchnics. Stimulation of these sacral nerves within the spinal 

 canal, or of the pelvic nerves (nervi erigentes) into which they pass, 

 causes contraction of the parts of the large intestine concerned in 

 defaecation that is, in the dog, of the whole colon, with the exception 

 of the caecum; in the cat, of the distal two-thirds of the colon. The 

 colon first undergoes rapid shortening due to the contraction of the 

 longitudinal fibres and the recto-coccygeus muscle. After a few 

 seconds this is followed by contraction of the circular fibres, beginning 

 at the lower limit of the region in which antiperistalsis can occur, and 

 spreading downwards, so as to empty the portion of the bowel involved 

 in the contraction. This is a very close imitation of what occurs in 

 natural defaecation. In man the parts involved in these movements 

 are probably the sigmoid flexure and rectum. In addition to these 

 characteristic motor effects on the lower part of the large intestine, 

 stimulation of the pelvic nerves causes an increase in the antiperistalsis 

 of its upper portions. Stimulation of the lumbar nerves or of the por- 

 tions of the sympathetic into which their visceral fibres pass (lumbar 

 sympathetic chain from second to sixth ganglia, or the rami from it to 

 the inferior mesenteric ganglia) causes inhibition of the movements of 

 the caecum and the whole colon, including the antiperistaltic move- 

 ments. Excitation of the sacral nerves initiates or increases the con- 

 traction of both coats of the portions of the large intestine on which 

 they act, excitation of the lumbar nerves inhibits both. And in the small 

 intestine the same law holds good ; the two coats are contracted together 

 by the action of the vagus or inhibited together by that of the splanchnics. 



Defaecation is partly a voluntary and partly a reflex act. But 

 in the infant the voluntary control has not yet been developed; 



