Pflll.ONOPHY OF ZOOJXJGY. 



this subject, however, there are no satisfactory observations, 

 neither is it determined, whether the saliva acts mechanical- 

 Iv, 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 protected 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 oesophagus. 



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 intermediate 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 is a layer 

 of cellular substance, lined, 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 



