I.] THE FROG. 33 



The Eyeball is lodged in the orbit and protected by the 

 eyelids, described above. It has four recti muscles which 

 proceed from the inner wall of the orbit, and are attached 

 to the circumference of the globe; within these is a retractor 

 muscle with similar attachments, ensheathing the optic 

 nerve, while two oblique muscles proceed from the anterior 

 and inner wall of the orbit and are attached to the dorsal 

 and ventral faces of the bulb. In addition, a fine tendon 

 passes from the outer end of the lower eyelid, or nictitating 

 membrane, and is attached to the fibres of the retractor 

 bulbi — the effect of which is that when the bulb is retracted 

 the nictitating membrane is raised over the eye. The upper 

 lid has no muscles. A secretory organ, termed the Harde- 

 riaii gland, is situated in the anterior part of the orbit 

 beneath the superior oblique muscle. 



The essential part of the eye is the inner lining or retina, 

 which receives the fibres of the optic nerve ; to this there 

 are superadded a vascular pigmented choroid and a car- 

 tilaginous sclerotic, which together constitute an accessory 

 capsule. The lens is nearly spherical. 



The Ear consists of an essential part — the membranous 

 labyrinth — receiving the fibres of the auditory nerve, lodged 

 in an accessory partly osseous, partly cartilaginous, periotic- 

 capsule; to the latter are superadded the columella auris, 

 the tympanic membrane and the tympa?tu?n. 



The labyrinth consists of three semicircular canals which 

 open into a vestibule divided into utricidus and sacculus. 

 The latter, especially, contains a great quantity of white 

 crystaUine calcareous otoliths. 



On the outer side of the vestibule is a small dilatation 

 which is possibly a rudimentary cochlea. 



The membranous labyrinth is filled with a fluid {endolymph), 



M. X 



