ABDOMINAL CAVITY 357 



comparatively few of them pass superficially over the duodeno- 

 pyloric constriction to become continuous with the corre- 

 sponding fibres of the muscular coat of the duodenum. 

 As they approach the duodenum the deeper longitudinal 

 fibres of the pyloric canal leave the surface and penetrate the 

 substance of the pyloric sphincteric ring. There can be little 

 doubt that the arrangement forms an effective apparatus, 

 antagonistic to the pyloric sphincteric ring, by means of 

 which, when the sphincter relaxes, the pyloric orifice may 

 be dilated. There is thus a constrictor and a dilatator of 

 the pylorus. 



In suitable specimens this arrangement of the muscle 

 fibres may be seen by the naked eye when a longitudinal 



Orifice of pylorus 



i 

 .1 



Pyloric part of stomach 



Termination of pylorus 

 protruding into duodenum 



Duodenum 



KIG. 167. Small portion of the Pyloric part of the Stomach with part 

 of the Duodenum attached. 



section is made through the pyloric canal in the plane of the 

 two curvatures of the stomach (Fig. 168). 



Dissection. Open the duodenum from end to end by an 

 incision carried along its convex border ; then clean it with a 

 sponge. 



Structure of the Duodenum. The structure of the duo- 

 denum is generally similar to that of other parts of the small 

 intestine. That is, its wall is formed by five main coats or 

 layers: (i) serous; (2) subserous ; (3) muscular; (4) sub- 

 mucous; (5) mucous. The serous coat is complete only 

 over the greater portion on the superior part (see p. 342). 

 All the other parts are covered by peritoneum only on two 

 faces the descending part in front, except where it is 

 crossed by the transverse colon, and on the right ; the 

 horizontal section of the inferior part in front and below, 

 and the ascending section of the inferior part in front and 



