I.J THE FROG. 33 



The Eyeball is lodged in the orbit and protected by the 

 eyelids. It has four recti and two oblique muscles (which 

 proceed from the orbit to the circumference of the globe) 

 and one retractor muscle (cf. p. 113). In addition, a fine 

 tendon passes between the outer end of the lower eyelid, or 

 nictitating membrane, and the fibres of the retractor, so 

 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 Harderian 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 

 and the tympanic membrane. 



The labyrinth consists of three semicircular canals which 

 open into a vestibule divided into utriculus and sacculus. 

 The latter, especially, contains a great quantity of white 

 crystalline calcareous otoliths. There is developed inter- 

 cranially a sac-like derivative of the labyrinth, the saccus 

 endolymphaticus, lying about the brain and filled with otolith- 

 like crystals. It is from this that the periganglionic glands 

 referred to on p. 32 are derived. 



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. 3 



