ALIMENTARY SYSTEM 



363 



both are pierced by a core of soft tissue called the pulp 

 (Fig. 278). The teeth are all alike, and are fused to the surface of 

 the bones that carry them. As they are destroyed by use they 



o.l. 



pn.s. a.c'i.p. 



IV. v. p.ch.f. ch. ag. 



tn.ob. 



Fig. 279. — A longitudinal median section through the head of a 



aq., Aquaeductus cerebri ; a.ch.p., anterior choroid plexus ; br., bronchus ; ex., central canal of spinal cord ; 

 cb., cerebellum ; cer.h., left cerebral hemisphere ; e.n., nostril ; Eu., Eustachian tube i/.M., foramen of 

 Monro ; gls., glottis ; i.n., internal narial opening ; inf., infundibulum ; lar., larynx ; Ig., left lung ; 

 m.ob., medulla oblongata ; o.l., optic lobe ; ces., oesophagus ; olf.l.. olfactory lobe ; p.ch.p., posterior 

 choroid plexus ; pit., pituitary body ; pn.s., pineal stalk ; III.v., third ventricle ; IV.v., fourth ventricle ; 

 t., tongue. 



rrut. 



~>rC 



.,-&u^ 



~oes. 



fall out and are replaced one by one. On the front part of 

 the roof of the mouth, beside the vomerine teeth, open the 

 internal nares. The tongue is a 

 muscular structure arising, unlike 

 that of man, from the front part 

 of the floor of the mouth and 

 forked at its free end, which is 

 directed backwards when it is at 

 rest. In taking food the tongue is 

 turned over and its free end thrown 

 out of the mouth, wiping up, as it 

 goes, a sticky substance secreted 

 by glands in the roof of the mouth 

 so that the prey adhere to it. The 

 extrusion of the tongue is produced 

 by the sudden passage into it of 

 lymph, squeezed from one lymph 

 sac to another by muscular con- 

 traction. Behind the angle of the 



-9U. 



- t 



Fig. 280. — The mouth of a frog 



Protuberance caused by eyeball ; eu.. 

 Eustachian tube ; gh., glottis ; in., internal 

 narial opening ; m.t., maxillary teeth ; 

 md., lower jaw; as., oesophagus; t., 

 tongue; v.t.. vomerine teeth. 



