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; 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.); 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 Babbit 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 
o 2 
