xi AUDITORY ORGAN 535 



Between the wall of the orbit and the eyeball are two 

 glands, the secretion of which, passing through ducts 

 perforating 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. 186 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 is present on the inner side of 

 the edges of the eyelids, and produces 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, cc), is represented by a relatively larger struc- 

 ture, coiled on itself in a spiral manner, and specially 

 important as regards the sense of hearing. 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, con- 

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

 tympanic cavity at the fenestra ovalis and fenestra 

 rotunda (p. 193) by the membrane which closes each of 

 them. 



The membranous cochlea does not run up the middle of 

 the spiral of the bony cochlea, but is attached between its 



