THE HEAD. 381 



4. The pharynx is a continuation backwards of the buccal 



cavity, beyond the margin of the soft palate. The 

 mouth and the posterior narial chamber open 

 into it in front ; behind it is continued into the 

 oesophagus ; and ventrally it communicates through 

 the glottis and trachea with the lungs. 



a. The glottis is a large opening in the floor of the 



pharynx, a short way behind the tongue, leading 

 into the trachea or windpipe. 



b. The epiglottis is a thin bilobed cartilaginous flap, 



forming the anterior boundary of the glottis, 

 and projecting into the pharynx behind the 

 tongue, opposite the free edge of the soft palate. 

 During the act of swallowing it is folded back 

 over the glottis, so as to close its entrance. 



5. The posterior nasal chamber lies above the soft palate, 



which separates it from the buccal cavity. 



a. The orifices of the Eustachian tubes are a pair of 



oval apertures at the sides of the roof of the 

 chamber, about the middle of its length. 



The Eustachian tubes connect the tympanic 

 cavities with the posterior nasal chamber. The 

 tympano-Eustachian passage is the modified 

 hyomandibular pouch of the embryo, corre- 

 sponding to the spiracle of the dog-fish. Its 

 connection with the ear is purely secondary, the 

 passage serving to keep the pressures of air on 

 the two sides of the tympanic membrane equal. 



Pass a seeker backwards and outwards, along one of the 

 Eustachian tubes, into the tympanic cavity (cf. fig. 66). 



b. The organs of Jacobson are a pair of small tubular 



bodies enclosed in the palatal processes of the 

 premaxillse, and lying in the floor of the anterior 

 part of the nasal cavities. Each communicates 



DD 



