316 INFLUENCE OF NERVES ON THE INTESTINE. 



parts of the floor of the pelvis during a strong effort at stool, are driven 

 downwards in the form of a cone, causing the mucous membrane of the 

 anus, which contains much venous blood, to be everted. The function of 

 the levator ani (Figs. 1 29, 1 30) is, to raise voluntarily the soft parts of the 

 floor of the pelvis, and to pull the anus to a certain extent upwards over 

 the descending fecal mass. At the same time, it prevents the distension 

 of the pelvic fascia. As the fibres of both levatores converge below and 

 become united with the fibres of the external sphincter, they aid the 

 latter, during energetic contraction of the sphincter; or, as Hyrtl puts it, 

 the levatores are related to the anus, like the two cords of a tobacco 

 pouch. During the periods between the evacuation of the gut, the 

 faeces appear only to reach the lower end of the sigmoid flexure. As a 

 rule, from thence downwards, the rectum is normally devoid of fceces. It 

 seems that the strong circular fibres of the muscular coat, which Nelaton 

 has called sphincter ani tertius, when they are well developed, contract 

 and prevent the entrance of the fa3ces. When the tendency to the 

 evacuation of the rectum is very pressing, the anus may be closed more 

 firmly from without, by energetically rotating the thigh outwards, and 

 contracting the muscles of the gluteal region. 



161. Influence of Nerves on the Intestinal 



Movements. 



Auerbach's Plexus. The intestinal canal contains an automatic motor 

 centre within its walls the plexus myenterieus of Auerlach which lies 

 between the longitudinal and circular muscular fibres of the gut. It is 

 this plexus which enables the intestine when cut out of the body to 

 execute, apparently spontaneously, movements for some time. 



[Structure. The plexus of Auerbach consists of non-medullated nerve-fibres 

 which form a dense plexus, groups of ganglion cells occurring at the nodes (Fig. 

 131). A similar plexus extends throughout the whole intestine between the 

 longitudinal and circular muscular coats from the oesophagus to the rectum. 

 Branches are given off to the muscular bundles. A similar, but not so rich a 

 plexus lies in the sub-mucous coat, Meissner's plexus, which gives branches to 

 supply the muscularis nmcosre, the smooth muscular fibres of the villi, and the 

 glands of the intestine (Fig. 132).] 



1. If this centre is not affected by any stimulus, the movements of 

 the intestine cease comparable to the condition of the medulla 

 oblongata in apnoea (Sig. Mayer and v. Basch). The same is true 

 just as in the case of the respiration during intra-uterine life, in con- 

 sequence of the foetal blood being well supplied with 0. This condition 

 may be termed aperistalsis. It also occurs during sleep, perhaps on 

 account of the greater amount of in the blood during that state. 



