394 THE DOGFISH LESS. 



and opening externally by the nostrils. They are lined with 

 mucous membrane, which is raised up into ridges so as to 

 increase the surface. The actual organ of smell is the 

 epithelium forming the superficial layer of the mucous 

 membrane ; it is developed as an in-pushing of the ectoderm, 

 and is supplied by the olfactory nerve. 



The eyes are a pair of nearly globular organs, lying in the 

 orbits and moved each by six muscles. Their structure is, 

 in all essential respects, the same as in man. There is an 

 outer capsule, the sclerotic, formed of cartilage, lined by a 

 vascular membrane, the choroid, within which is a delicate 

 membrane, pigmented externally, the retina or actual organ 

 of sight. In the front or exposed part of the eye the 

 sclerotic passes into a transparent, watch-glass-like cornea, 

 and the choroid into a curtain or diaphragm, the iris, having 

 a central aperture, the pupil^ to admit the rays of light to 

 the interior of the eye. Behind the pupil is a gelatinous, 

 biconvex crystalline lens of glassy transparency. The 

 space between the cornea and the iris is called the aqueous 

 chamber of the eye, and is filled by a watery fluid, the 

 aqueous humour. The main part of the cavity of the eye, 

 bounded in front by the lens, and for the rest of its extent 

 by the retina, is the vitreous chamber, and is filled with a 

 gelatinous substance, the vitreous humour. The cornea, 

 aqueous humour, lens, and vitreous humour together form a 

 series of adjustable lenses serving to focus objects on the 

 retina, and the stimulus thus applied to that membrane is 

 conveyed by the fibres of the optic nerve to the brain. 



The auditory organ is a sac of complex form, the mem- 

 branous labyrinth, enclosed in the auditory capsule of the 

 skull, where it floats in a watery fluid, the perilymph. It 

 consists of a sac called the vestibule, with which are con- 

 nected three tubes, called from their form the semicircular 



