XIII 



PHYLUM CHORDATA 



321 



FI3. 084. Ring of ossicles 

 in sdi-rot ie of eye 

 of Lacerta. (After 

 Wiedersheim.) 



of the two pairs of limbs and tapering greatly towards the posterior 

 end of the tail. 



The cerebral nerves resemble those of the Frog as regards their 

 origin and distribution in- most respects, the principal difference 

 being that a spinal accessory is intercalated in front of the hypo- 

 glossal, and that the hypoglossal arises from the medulla oblongata, 

 not from the spinal cord, and is therefore a cerebral nerve. 



The nasal cavities (Fig. 983) open at the extremity of the snout by 

 the external nares, and into the cavity of the mouth by a pair of 

 slit-like internal nares situated near the middle 

 line of the palate. The external aperture opens 

 into a sort of vestibule, beyond which is the 

 nasal or olfactory cavity proper, containing 

 a convoluted turbinal bone over which the 

 mucous membrane extends. Opening into each 

 nasal cavity, near the internal opening, is 

 Jacobson's organ (J, J.), an oval sac with 

 strongly pigmented walls supported by carti- 

 lage. 



The eye has a cartilaginous sclerotic having a ring of small bones 

 (Fig. 984) supporting it externally. There is a cushion-like pecten 

 or vascular pigmented process similar to the structure of the same 

 name occurring in Birds (see below, Class Aves), projecting into the 

 inner chamber of the eye. In essential structure the rest of the 



eye agrees with that of 

 the Craniata generally as 

 already described. Two 

 glands lie in the orbit, the 

 lacrymal and the Harderian. 

 The ear consists of two 

 principal parts, the internal 

 ear or membranous laby- 

 rinth, and the middle ear or 

 tympanum. The latter is 

 closed externally by the 

 tympanic membrane, the 

 position of which has been 

 already mentioned. It com- 

 municates with the cavity 

 of the mouth by the Eus- 

 tachian passage, which is 

 narrower and longer than 

 in the Frog. The inner 

 wall of the tympanic cavity 

 is formed by the bony wall 

 of the auditory region of the skull, in which there are two fenestrse 

 the fenestra ovalis and the fenestra rotunda. The columella stretches 



cut 



ca 



sc 



1 IG. _98_5. Membranous labyrinth of Lacerta 

 viridis, viewed from the outer side. aa. an- 

 terior ampulla ; ac. auditory nerve ; ade. opening 

 of the ductus endolymphaticus ; ae. external 

 ampulla ; aj>. posterior ampulla ; br. basilar 

 branch of nerve ; ca. anterior semicircular canal ; 

 ce. external semicircular canal ; cp. posterior 

 semicircular canal ; CMS. canal connecting utri- 

 rulus and sacculus ; de. ductus endolymphaticus ; 

 I. cochlea ; mb. basilar membrane ; raa, rae, rap. rl. 

 branches of auditory nerve ; s. sacculus ; ss. com- 

 mon canal of communication between anterior 

 and posterior semicircular canals and utricle ; 

 u. utriculus. (From Wieclersheim's Comparative 

 Anatomy, after Retzius.) 



