278 DIGESTIVE SYSTEM. 



walls. The splanchnic nerves are, therefore, to the intes- 

 tines what the pneumo-gastric nerve is to the heart, that is 

 an arresting nerve (Experiments by Pfliiger). 



Onimus and Legros studied the movements of the differ- 

 ent parts of the digestive canal by means of a registering 

 apparatus, upon which a lever (set in motion by an india- 

 rubber bag introduced into the intestinal tube, and which 

 set in action its contractions) recorded these contractions as 

 they occurred. While engaged in this study, they observed 

 that, by galvanizing the pneumo-gastric nerve by means of 

 interrupted currents, the movements of the intestine may be 

 checked, and checked, not when in a state of contraction, bat 

 when entirely relaxed. " In this case a very remarkable 

 depression is obtained in the tracing, and it is important to 

 associate the fact of the checking of the heart in diastole, 

 and the checking of the respiratory movements in inspira- 

 tion, during the excitation of the pneumo-gastric nerve by 

 interrupted currents " (see p. 40). 



It is easier to explain what goes on at the lower extremity 

 of the digestive canal, this part being more accessible, and 

 the phenomenon of defecation thus becoming perfectly plain. 

 First, it must be recollected that the longitudinal muscular 

 fibres form in the rectum an extremely thick and powerful 

 stratum, and that, on the other hand, the circular iibres are 

 grouped together and multiplied in such a manner as to form 

 a sphincter or ring, called an internal sphincter, formed of 

 smooth muscular fibres, and enclosed in another and more 

 powerful sphincter, called the external sphincter, which is 

 formed of striated fibres. These sphincters do not exactly 

 form a ring, but rather an antero-posterior button-hole, con- 

 fined by two muscular bands, which, during the state of 

 repose, are quite close together. When in repose, this 

 sphincter, by virtue of its elasticity alone, completely closes 

 the opening which it surrounds, as is the case indeed with all 

 the sphincters (see Physiology of the muscle, natural form 

 of the muscle and sphincters when in the state of repose, p. 

 72). These contractions, therefore, are no more permanent 

 here than elsewhere : the ring-like aperture is normally oblit- 

 erated by the natural form of the sphincter, and the sphincter 

 contracts only when some body seeks to modify its form, in 

 order to dilate the orifice which it surrounds. Under these 

 circumstances either the sphincter does not react, but dilates 

 readily, on account of its great elasticity, and the passage 

 takes place ; or else the sphincter reacts, and by its contrac- 



