PHYLUM CHORDATA 



309 



FIG. 017. Transverse section of the nasal 

 region of the head of Lacerta to 

 show the relations of Jacobson's or- 

 gans. D, nasal glands ; /. /. Jacob- 

 son's organs ; N. N. nasal cavities. 

 (From Wiedersheim's Comparati'-' 

 Anatomy.) 



The external aperture opens into 

 a sort of vestibule, beyond which 



V 



is the nasal or olfactory cavity 

 proper, containing a convoluted 

 turbinal bone over which the 

 mucous membrane extends. Open- 

 ing into each nasal cavity, near 

 the internal opening, is Jacob- 

 sons organ (J. J.), an oval sac 

 with strongly pigmented walls 

 supported by cartilage. 



The eye has a cartilaginous 

 sclerotic having a ring of small 

 bones (Fig. 918) supporting it ex- 

 ternally. There is a pecten or vascular pigmented process similar 



to the falciform process in the eye of Teleo- 

 stomes (p. 199), 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 

 middle ear or tympanum, and the internal 

 ear or membranous labyrinth. The former 

 is closed externally by the tympanic membrane, the position of 

 which has been already mentioned. It communicates with the 

 cavity of the mouth by the 

 Eustachian 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 fenestrae - - the 

 fenestra oxalis and the 

 fenestra rotunda. The colu- 

 mella stretches across the 

 cavity from the tympanic 

 membrane, and is fixed in- 

 ternally into the membrane 

 covering over the fenestra 

 ovalis. 



The parts of the mem- 

 branous labyrinth (Fig. 919) 

 are enclosed by the bones 



FIG. 918. Ring of ossicles 

 in sclerotic of eye 

 of Lacerta. (After 

 Wiedersheim.) 



mn 



ca 



op 



FIG. 919. Membranous labyrinth of Lacerta 

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

 terior ampulla ; ac, auditory nerve ; ode, opening 

 of the ductus endolyniphaticus ; ae, external 

 ampulla ; ap. posterior ampulla ; br. basilar 

 branch of nerve ; ca. anterior semicircular canal : 

 ce. external semicircular canal ; cp. posterior 

 semicircular canal ; cus. canal connecting utriculus 

 and sacculus ; i!e. ductus endolyniphaticus ; (. 

 lagena ; 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 Wiedersheim, after Retzius.) 



