xi AUDITORY ORGAN 535 



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-lacrymal 

 duct (pp. 1 86 and 494). 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 lacrymal gland 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. 



The essential part of the auditory organ consists, as in 

 other Vertebrates, of the membranous labyrinth with its 

 three semicircular canals (pp. 186 and 465) enclosed in the 

 auditory capsule (periotic bone, p. 492), 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. 143). 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. 493) by a membrane which closes them. 



The membranous cochlea does not run up the middle of the spiral of 

 the bony cochlea, but is attached between its outer wall and a spiral 

 shelf arising from its inner wall. Thus the entire cochlea shows 



