VERTEBRATA. 139 



catch and reflect the sound-waves into the tube. It should 

 be mentioned that the ear is more than an organ of hearing ; 

 it is also an organ for maintaining the balance, for if the 

 ear or the auditory nerve be injured the animal can no 

 longer maintain its equilibrium. 



The eye is built on the plan of a photographic camera. 

 The essential parts are a lens which brings the rays of light 

 to a focus on the retina, and means for causing the image 

 on the retina to stimulate the optic nerve. To these are 

 added various accessory structures for protection, for regu- 

 lating the amount of the light, etc. In the lower forms 

 eyelids are absent, but higher in the scale folds of flesh are 

 developed which can close over the organ. Many animals 

 have three of these eyelids, two working vertically, the 

 third, the nictitating membrane, extending from the inner 

 angle eye' over the transparent cornea. This nictitating 

 membrane occurs in the eye of man as a small fold (semi- 

 lunar fold), which has entirely lost its primitive protective 

 function. 



Over the whole globe of the eye is a tough layer, the 

 sclerotic coat, which is usually white, and which may be 

 cartilaginous or may even have bone deposited in it, as in 

 many reptiles and birds. In front this layer becomes per- 

 fectly transparent, and is there known as the cornea. Inside 

 of the sclerotic is found a densely black layer (choroid), and 

 still within this the transparent retina, the outer portion of 

 which is imbedded in the choroid. In front the choroid is 

 continued into the iris, a circular muscle with an aperture 

 the pupil, in its centre. This iris, which is colored, regu- 

 lates by its enlargement and contraction the amount of light 

 which is admitted to the visual parts of the eye. Back of 

 the iris and held in position by a circular muscle and liga- 

 ment is the transparent lens. In front of this lens is a 



