ALIMENTARY SYSTEM 



439 



by the horizontal flanges of the vomer, representing the true 

 roof of the mouth. Into this hinder part, the nasopharynx, open 

 the Eustachian tubes which communicate with the middle ear. 

 The tonsils are a pair of pits at the sides of the soft palate near its 

 hinder border. The tongue is an elongate, muscular mass attached 

 along most of its length to the floor of the mouth, but with a 



n.e 



pvix 



mn 



Fig. 350. — A vertical section through a rabbit's head. — From Thomson. 



b 0. basioccipital ; bs., basisphenoid with depression for pituitary body ; cb., cerebellum ; c.c, position of 

 corpus callosum ; epg., epiglottis ; e.t., ethmoidal turbinal ; g., gullet ; m.e., part of mesethmoid in front 

 region where narial passage is not separate from olfactory chamber ; m.e'., part of same m hmdcx 

 region, where it divides from one another only the two olfactory chambers, which are here separated 

 by a horizontal partition from the single narial passage (the intermediate part of the mesethmoid is 

 cut away) ; mn., anterior end of mandible ; n.p., narial passage ; olf.l., olfactory lobe of cerebrum ; 

 pL, transverse portion of palatine ; pmx., premaxilla with incisors ; ps., presphenoid ; s.c, spinal cord ; 

 S.I., sublingual saUvary gland ; smx., submaxillary saUvary gland ; T., tongue ; Lb., maxillary turbinals ; 

 tr. trachea. 



free tip in front. It bears papillae of several kinds which are 

 sensitive to dissolved chemical substances and are thus organs of 

 taste. 



The teeth have the same basic structure as those of the dogfish 

 and frog and all other vertebrates, but they differ in a number 

 of important respects which characterise the mammals. They 

 are, as we have seen (p. 431) thecodont, or carried in sockets in 

 the jaw, and they are heterodont, or of a number of different 

 shapes ; finally, there is not continual replacement throughout 

 life, but instead there are two definite sets, the milk teeth in 

 the young animal and the permanent teeth in the adult. Such an 



