322 PHILOSOPHY OF ZOOLOGY. 
this subject, however, there are no satisfactory observations, 
neither is it determined, whether the saliva acts mechanical- 
ly, merely, by assisting the~teeth to reduce the food to a 
pulp, or whether it acts chemically, by aiding the conver- 
sion of the food into chyle. 
After the food has thus been reduced to a pulpy 
mass, by the attrition of the teeth, and sufficiently mixed 
with saliva, it is fit to be conveyed to the stomach. Be- 
tween the mouth and the stomach, however, in the more 
perfect animals, there intervene two cavities, the pharynx 
and the gullet. ‘The former occupies the back part of 
the mouth, behind the base of the tongue and the soft pa- 
late, and the posterior openings of the nostrils. The wind- 
pipe and gullet open into it. By means of the tongue and 
cheeks, the food is thrust into this cavity, and at this time, 
its entrance into the nostrils is pretected by the soft palate, 
and its suspended uvula. The opening into the windpipe 
is also closed by a valvular arrangement, and concealed by 
the base of the tongue and a cartilaginous valve, termed the 
epiglottis. The muscular structure of the walls of the 
pharynx propels its contents into the gullet or cesophagus. 
In many animals, the mouth may be said to open im- 
mediately into the stomach. In the more perfect kinds, 
however, the gullet forms the imtermediate connection, 
and consists of a tube, varying greatly in its dimensions,, 
according to the species. Its external coat consists of 
longitudinal or transverse muscular fibres, which assist in 
propelling the food to the stomach; and within this isa layer 
of cellular substance, lmed, on the interior, with a mucous 
membrane. ‘The food is here still farther softened, by be- 
ing mixed with the secretions of the gullet, and becomes: 
prepared for entering the stomach, to experience still far- 
ther changes. 
The stomach exhibits such remarkable variations of form 
