PERISTALTIC MOVEMENT DEFECATION. 195 



function ; but in most herbivorous animals it is larger (as 

 in the Monkey, fig. 30) ; and it is found to secrete an acid 

 fluid, which resembles the gastric juice, and which may have 

 for its office to perform a second digestion upon the sub- 

 stances which have escaped the first. These coeca are some- 

 times very large in the intestinal canal of Birds (fig. 111). 

 From the coecum, the Large Intestine ascends as high as the 

 liver, crosses the upper part of the abdomen, and then 

 descends again, as shown in fig. 108 ; this portion is termed 

 the colon ; and it terminates in the rectum, which forms the 

 extremity of the intestinal tube. 



215. The alimentary mass is propelled along the first part 

 of the intestinal canal, and the residue left after the absorp- 

 tion of the nutritive materials is carried along the continua- 

 tion of it, by the contraction of its muscular coat, producing 

 what is termed the peristaltic motion of the bowels. The fibres 

 of this muscular coat are chiefly arranged in a ring-like 

 manner around the tube j so that, when they contract, they 

 narrow the diameter of the tube. They are stimulated to 

 contract by the contact of the solid or liquid matter passing 

 through it (Chap, xn.) j and their contraction forces this matter 



.onwards, into the succeeding portion of the tube. This con- 

 tracts in its turn, so as to propel its contents further ; and thus 

 the mass is gradually driven from one extremity of the canal 

 to the other. The peristaltic movement does not seem to 

 depend (as do the contractions of the muscles concerned in 

 swallowing, 195) upon the nervous system; for it will take 

 place after the intestinal tube has been completely separated 

 from the principal nervous centres ; and also after the death 

 of the animal, if this have been produced by a sudden cause. 

 Thus, if a Rabbit be killed by a smart blow at the top of the 

 neck, and the abdomen be immediately opened, the peristaltic 

 movement will be seen in vigorous action, especially if the 

 animal have eaten a full meal an hour or two previously. 



Defecation. 



216. In passing through the large intestine, the undigested 

 residue is still more completely deprived of the nutritive 

 matter it may contain ; and its fluid portion is absorbed, so 

 that it becomes more solid. It is allowed to accumulate in the 

 rectum, until its bulk occasions inconvenient pressure upon 



o2 



