.SECT. XIII 



PHYLUM CHORDATA 



. 265 



cavities each communicating with a rhinocoele in the corresponding 

 olfactory lobe. The pineal body is absent in the adult, its place 

 being taken by a lobe of the anterior -choroid plexus : in the larva 

 it is found outside the skull and immediately beneath the skin. 



The first spinal nerve performs the function of the hypoglossal 

 (Fig. 734, p. 98), supplying the muscles of the tongue : it passes 

 out between the first and second vertebrae. The spinal cord is 

 short and ends in a delicate filament, the filum terminate. In 

 correspondence with the number of vertebrae there are only ten 

 pairs of spinal nerves, of which the second and third unite to form 

 a brachial plexus giving off the nerves to the fore-limb, while the 

 seventh to the tenth join to form a luiiibo-sacral plexus giving off 

 the nerves to the hind-limb. 



Sensory Organs. The olfactory sacs have each two openings : 

 the anterior naris or external nostril and the posterior narjs 



7n.e.rnb.Lab 



o.sl 



Fie. S8L'. Transverse section of head of Frog to show the relations of the accessory auditory 

 apparatus (diagrammatic). Skeletal structures black, with the exception of the columella ; 

 a, t. linnp. annulus tympanicus ; b. k//. body of hyoid ; buc. cav. buccal cavity; ch. pU: 

 choroid plexus ; col. columella ; eus. t. Eustachian tube ; fen. or. fenestra ovalis ; med. obi. 

 medulla oblongata ; mentb. lab. membranous labyrinth ; mnd. mandible ; Nv. VIII. auditory 

 nerve; o. st. epi-sternuin ; ptg. pterygoid ; qu. ju. quadra to-j ugal ; stp. stapes; tymp. cat: 

 tympanic cavity ; tymp. m. tympanic membrane. 



v 



(Fig. 876, p. na.) or internal nostril, which opens into the mouth 

 immediately external to the vomer. 



The eye and the auditory organ have the usual structure, but in 

 connection with the latter there is an important accessory organ 

 of hearing not hitherto met with. Bounded externally by the tym- 

 panic membrane and internally by the outer wall of the auditory 

 capsule is a considerable space, the tympanic cavity (Fig. 882, tymp. 

 cav.), which communicates with the mouth by the short Eustachian 

 tube (eus. t.) already noticed (Fig. 876, eus. t.), so that a probe thrust 

 through the tympanic membrane from outside passes directly 

 into the mouth. In the roof of the tympanic cavity lies the 

 columella (col.\ its head, or extra-columella, attached to the inner 



