XIII 



PHYLUM CHORDATA 



309 



FIG. 917. 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 ; 2f. N. nasal cavities. 

 (From Wiedersheim's Comparative 

 Anatomy.) 



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. Open- 

 ing into each nasal cavity, near 



the internal opening, is Jacob- 



-sons organ (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 fenestra3 the 



fonestra ovalis and the 



fonestra 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 Iiacerta. (After 

 Wieclersheim.) 



mn 



ap 



FIG. 919. Membranous labyrinth of Lacerta 

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

 terior ampulla ; ac, auditory nerve ; ode, opening 

 of the ductus endolymphaticus ; 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 ; de. ductus endolymphaticus ; I. 

 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 ; 

 it. utriculus. (From Wiedersheim, after Retzius.) 



