THE BUCCAL CAVITY 17 



b. Paired apertures, 



i. The Nostrils or anterior nares are two small open- 

 ings on the dorsal surface of the head, close to its 

 anterior end. 



B. The Buccal Cavity. 



Open the ttiouih to its full extent : note the wide buccal w mouth 

 cavity, of which the hinder pwH or pharynx is continued bach into 

 the oesophagus. Note also the following structures : 



1. On the Eoof of the Mouth. 



a. The Teeth. 



i. The maxillary teeth are a row of iine teeth, attached 



round the edge of the upper jaw. 

 ii. The vomerine teeth are two small patches of sharp 



teeth in the fore part of the roof of the mouth 



and near the middle line. 



b. The posterior nares are two small holes lying to the 



outer sides of and slightly in front of the two patches 

 of vomerine teeth. 



Pass bristles through the nostrils, cmd see that they 

 come out through the posterior nares into the buccal 

 cavity. 



c. The Eustachian tubes or recesses are a pair of much 



larger holes, at the sides of the posterior part of the 

 buccal cavity. Each hole opens into a slightly dilated 

 chamber — the tympanic cavity — which is closed ex- 

 ternally by the tympanic membrane already seen on 

 the surface of the head. 



Perforate the tympanic membrame on one side with a 

 needle, and pass a bristle or seeher tlvrough the hole and 

 down the Eustachian tube into the mouth. 



d. Two rounded prominences at the sides of the roof of 



the mouth are caused by the eyeballs. 



Press down one of the eyes with your finger, and note 

 that it can be made to project very considerably into the 

 buccal cavity. 



2. On the Floor of the Mouth. 



a. The lower jaw, which is devoid of teeth, fornas a bony 

 margin to the floor of the mouth : the rest of the floor 



B 



