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 cceca are some- 
_ times very large in the intestinal canal of Birds (fig. 111).— 
_ From the cecum, 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. x1.) ; 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 intestimal 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 
tectum, until its bulk occasions inconvenient pressure upon 
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