THE MECHANICS OF DIGESTION 1019 



give rise to a harmonious back and forth motion of the feces. Further- 

 more, since the proximal colon is subdivided into successive recesses 

 by incomplete transverse partitions, a whirlpool effect is produced 

 which carries the contents from haustrum to haustrum. It is for this 

 reason that the movements in the proximal colon and cecum are often 

 designated as haustral churning. 



Gradually as the water is absorbed, the fecal material assumes 

 a more solid consistency and escapes into the transverse colon, whereas 

 its more fluid portion is forced back into the cecum. Eventually, 

 however, all of it is lodged in the transverse colon and is held here 

 until forced into the rectum by long and forceful peristaltic waves. 

 This segment, therefore, plays the part of a storehouse and hence, it 

 cannot surprise us to find that any retardation of the feces must 

 result in a loss of an excessive quantity of water and a firm lodgment 

 of these masses in the haustrse. Its gradually increasing weight then 

 gives rise to a sagging which causes the hepatic flexure to assume a 

 much lower level than the splenic flexure. This condition is not at 

 all uncommon and is responsible for the peculiar outline of this part 

 of the intestine when observed with the Rontgen rays. It then dis- 

 plays the contours of a snake when assuming the position of striking. 

 The descending colon and rectum are usually empty and are filled only 

 a short time before the beginning of the act of defecation.^ This 

 filling of the rectum is accomplished by two or three powerful long 

 peristaltic waves which begin in the transverse colon and slowly trav- 

 erse the descending colon, forcing the feces into the rectal receptacle. 

 They are usually accompanied by noises which have been designated by 

 Kussmaul as the ''tormina intestinorum.'' In accordance with Scharz, ^ 

 it may be concluded that these waves are the direct cause of defecation, 

 because they force a certain amount of fecal material into the rectum 

 which then serves as the initial stimulus to the receptors initiating this 

 process. 



Cannon^ divides the large intestine into two parts, the first of 

 which includes the very active cecum and ascending colon and the 

 second, the relatively inactive transverse and descending colons. 

 This line of demarcation transects it distally to the hepatic flexure. 

 Furthermore, Elliott and Barclay-Smith^ have shown that the intense 

 antiperistaltic movements of the upper large intestine are present in 

 a great variety of animals, and are especially prominent in the her- 

 bivora in which the cecum plays the part of a large thin-waUed 

 reservoir for the food while undergoing bacterial decomposition. 



Defecation. — The distal portion of the colon leads into the sigmoid 

 flexure and rectum. Under normal conditions, the latter receives 



1 Roith, Anat. Hefte, 1902, and Reider, Fortschr. auf dem Geb. der Rontgen- 

 strahlen, 1912. 



2 Munchener med. Wochenschr., 1911. 



3 Am. Jour, of Physiol., xxix,.1911, 238. 

 * Jour, of Physiol., xxxi, 1904, 272. 



