5i8 THE RAHBIT chai'. 



Between the wall of the orbit and the eyeball are two 

 glands, the secretion of which, passing through ducts per- 

 forating the conjunctiva lining the eyelids, serves to keep 

 the outer surface of the eye moist, and is then conducted 

 into the nasal chambers by means of the naso-lachrymal 

 duct (pp. i86 and 477). These two glands correspond to 

 special differentiations of a primarily continuous structure : 

 one, the Harderian gland — already met with in the frog— is 

 situated in the antero-ventral region of the orbit : the other, 

 or lachrymal gla/id proper, in its postero-dorsal region. 

 Besides these, a series of small Meibomian glands are present 

 on the inner side of the edges of the eyelids, and produce 

 a fatty secretion. 



'l"he essential part of the auditory organ consists, as in 

 other \^ertebrates, of the membranous labyrinth with its 

 three semicircular canals (pp. 186 and 449) enclosed in the 

 auditory capsule (periotic bone, p. 475), and constituting the 

 internal ear. The small outgrowth of the sacculus seen in 

 the dogfish and frog, and known as the cochlea (Fig. 59, /), 

 is represented by a relatively larger structure, coiled on itself 

 in a spiral manner. The part of the periotic bone which 

 directly surrounds the cavity in which the membranous 

 labyrinth lies is especially hard, and when the outer portion 

 of the bone is cut away, is seen to form a sort of cast of the 

 enclosed organ, the form of which it repeats ; this is known 

 as the bony labyrinth (Fig. 133). Internally it is separated 

 from the membranous labyrinth by a narrow space all round, 

 containing the perilymph (p. 189) and only shut off from 

 the tympanic cavity at the fenestra ovalis and fenestra 

 rotunda (p. 475) by a membrane which closes them. 



Tlie membranous coclilea iIols not nin up the middle of llie spiral ui 

 llie l>ony coclden, Ijiit is aUachcd Ijelween ils ouler wnll and a .spiral 

 shelf arising from ils inner uall. Thus the entire eochlea sho«s 



