1314 



THE ORGANS OF DIGESTION 



mucosa without advancing it appreciatively along the canal." 1 In this process constrictions occur 

 in the circular fibres, with the result that a collection of stationary food is divided into a number 

 of segments. In the middle of each segment constrictions appear and the earlier constrictions 

 relax. Then the latter constrictions relax and the earlier reappear, and so on until the food is 

 thoroughly mixed with digestive secretions. Finally, the food is driven on by peristalsis, coining 

 again to rest, and being again subjected to "rhythmic segmentation." Cannon says that in the 

 duodenum "rhythmic segmentation" lasts for several minutes, but in qther parts of the intestine 

 it may continue for half an hour or more, the food which is being subjected to it scarcely moving 

 along the canal. It is probable that in man there are from seven to eight segmentations per 

 minute in a given area. It is also probable that there is a sphincter action at the ileocecal opening. 

 Cannon divides the large intestine into two parts a distal part, in which the material is hard 

 and lumpy and is "advanced by rings of tonic contraction," and a proximal part, in which the 

 material is soft. In this part " the common movements are waves of constriction running back- 

 ward toward the cecum." The resistance of 

 the valve or sphincter enables reversed peri- 

 stalsis or antiperistalsis to mix the food. 

 When more food enters from the small intes- 

 tine, antiperistalsis ceases, tonic contraction of 

 the cecum and proximal portion of the colon 

 occurs, some of the food is merged into the 

 transverse colon, and antiperistalsis again 

 begins to act on what remains. The above 

 facts have been observed in animals and are 

 probably true in man. 



Innervation. The vagus fibres of the 

 small intestine seem to excite contraction of 

 the circular fibres after a brief preliminary 

 period of inhibition. 2 Some observers main- 

 tain that the splanchnic fibres are inhibitory, 

 but others claim that they are also motor. 

 The local reflex of the small intestine is in 

 Auerbach's plexus. Cannon quotes Bayliss 

 and Starling to the effect that the pelvic visceral 

 nerves to the large intestine, "arising like 

 the vagus from the central nerve system, are 

 augmentary nerves, whereas the supply from 

 the sympathetic system is purely inhibitory 



Gland of Lieberkufm 



Muscularis mucosa:. 



Solitary gland 



Circular muscle 

 fibres 



Longitudinal muscle 



jlbres 



^y Subperitoneal 

 connective tissue 



FIG. 1057. Transverse section of wall of large intestine. 



in its action." It is further contended that 

 the pelvic visceral nerves are distributed to 

 the distal colon only. "The region of anti- 

 peristalsis does not, therefore, receive motor 

 impulses from the pelvic nerves." 



Surface Form. The coils of the small intestine occupy the front of the abdomen below the 

 transverse colon, and are covered more or less completely by the great omentum. For the most 

 part the coils of the jejunum occupy the left side of the abdominal cavity ?'. e., the left lumbar 

 and inguinal regions and the left half of the umbilical region while the coils of the ileum are 

 situated to the right, in the right lumbar and inguinal regions, in the right half of the umbilical 

 region, and also in the hypogastric region. The cecum is situated in the right inguinal region. 

 Its position varies slightly, but the mid-point of a line drawn from the anterior superior spinous 

 process of the ilium to the symphysis pubis will about mark the middle of its lower border. It is 

 comparatively superficial. From it the ascending colon passes upward through the right lumbar 

 and hypochondriac regions, and becomes more deeply situated as it ascends to the hepatic flexure, 

 which is deeply placed under cover of the liver. The transverse colon crosses the belly trans- 

 versely on the confines of the umbilical and epigastric regions, its lower border being on a level 

 slightly above the umbilicus, its upper border just below the greater curvature of the stomach. 

 The splenic flexure of the colon is situated behind the stomach in the left hypochondrium, and 

 is on a higher level than the hepatic flexure. The descending colon is deeply seated, passing 

 down through the left hypochondriac and lumbar regions to the sigmoid flexure, which is situ- 

 ated in the left inguinal region, and which can be felt in thin persons, with relaxed abdominal 

 walls, rolling under the fingers when empty, and when distended forming a distinct bulge. The 

 usual position of the base of the vermiform appendix is indicated by a point on the cutaneous 

 surface two inches (5 cm.) from the anterior superior spinous process of the ilium, on a line 

 drawn from this process to the umbilicus. This is known as McBurney's point. Another mode 

 of defining the position of the base of the appendix is to draw a line between the anterior superior 

 spines of the ilia and marking the point where this line intersects the right semilunar line. 



1 Medical News, May 20, 1905. 



2 Bayliss and Starling, Journal of Physiology, 1899. 



