330 PHILOSOPHY OF ZOOLOGY. 
The inner surface of the intestines is lubricated by a 
mucous secretion, and the progress of the contents 1s ac- 
complished by the contraction of the muscular coats, occa- 
sioning what is termed the peristaltic motion of the intes- 
tines. 
During the passage of the contents of the intestines, va- 
rious gases may be detected in different parts. Oxygen, 
azote, and hydrogen appear, together with carbonic acid, 
and even traces of sulphur. These may in part be deriv- 
ed from the air swallowed along with the food, and from the 
decomposition of the drink; and it is probable that the 
action of the bile on the chyme may, in certain cases, aid 
their production. But neither the experiments of Ma- 
GENDIE and CHEVREUL on the intestinal gases of man, nor 
those of Vavauetin on the elephant, lead to any satisfac- 
tory results. 
The general view of the digestive process, and of the 
organs which are employed, which has now been given, 1s 
applicable only to the higher orders of animals. In many 
species the stomach is merely a bag with one orifice, serv- 
ing for the entrance and ejection of the food. Here no gas- 
tric, pancreatic, or biliary juices appear necessary. In other 
species, the alimentary canal is open at both its extremities, 
but exceeding simple throughout, so that there is some 
difficulty in distinguishing between the stomach and intes- 
tines. In all animals, there must exist digestive organs, 
but nature here exhibits such an astonishing variety of 
forms and structures, as to baffle all attempts at generaliza- 
tion. Very remarkable differencat prevail in stomachs des- 
tined to receive the same kind of food, and a striking simi- 
larity sometimes prevails between stomachs destined to re- 
ceive different kinds of food. These irregularities will 
again occupy our attention. 
Before concluding the remarks on this part of the subject, 
it is necessary to take notice of some parts which are more 
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