XIII 



PHYLUM CHORDATA 



277 



missure (com, below, upper line) partly representing the hippo- 

 campal commissure of the brain of Reptiles and Mammals. The 

 metacoele is covered by a thick choroid plexus ; the mesocoele is 

 divisible into a median passage or iter (i.), and paired optocoeles 

 (opt. v.) in the optic lobes : the paracoeles are large cavities each com- 

 municating with a rhinoccele in the corresponding olfactory bulb. 

 The pineal body is vestigial in the adult, a lobe of the anterior choroid 

 plexus, with a vestige of the stalk (pin), taking the position which 

 it usually occupies : 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. 946, ISp.), 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 corre- 



mernb.Lab 



ch.plsc. 



o.st 



fen-ov 



FIG. 94-7. 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. 

 an. tump, annulus tympanicus ; b. hy. body of hyoid ; buc. cav. cavity of pharynx ; ch. plx. 

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

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

 nerve ; o. st. omosternum ; pig. pterygoid ; qu. ju. quadrato-jugal ; stp. stapes ; tymp. cav. 

 tympanic cavity ; tymp. m. tympanic membrane. 



spondence with the number of vertebrae there are only ten pairs 

 of spinal nerves, of which the second and third units to form a 

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

 seventh to the tenth join to form a lumbo-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 naris (Fig. 940, 

 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- 



VOL. IT s 



