i 9 4 ELEMENTARY BIOLOGY. [CHAP. 



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- 

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

 beneath the superior oblique muscle. 



The sclerotic is cartilaginous but contains no ossifications, 

 and the lens is nearly spherical. There is 



The Ear consists of an essential part the membranous 

 labyrinth lodged in the periotic capsule, and accessory 

 parts, the columella auris, the tympanic membrane and the 

 tympanum. 



The former consists of the three ordinary semicircular 

 canals, with their vestibular dilatations, which open into a 

 vestibule divided into utriculus and sacculus. The latter, 

 especially, contains a great quantity of white crystalline 

 calcareous otoliths. 



On the outer side of the vestibule is a small dilatation 

 which is possibly a rudimentary cochlea. 



The membranous labyrinth is contained in the partly 

 cartilaginous, partly osseous, periotic capsule into which it 

 fits but loosely; the interval is filled with a fluid, the peri- 

 lymph. In the outer face of the periotic capsule is an oval 



