122 VETERINARY PHYSIOLOGY 



The blood vessels of the retina enter in the middle of the 

 optic nerve, and run out and branch in the anterior layer of 

 the retina. 



The interior of the eye may be examined by the Ophthal- 

 moscope, which consists essentially of a small mirror from 

 which light can be reflected ' into the back of the eye, with a 

 small hole in the centre through which the observer can study 

 the illuminated part of the chamber. (Practical Physiology.} 



C. Physiology 



The study of vision may be taken up in the following 

 order : 



1. The mode of formation of pictures on the nerve struc- 

 tures (retina) of the eye. 



(1) One eye (monocular vision). 



A. The method in which rays of light are focussed (di- 



optric mechanism). 



B. Stimulation of the retina. 



(2) Two eyes (binocular vision). 



2. The conduction of the nerve impulses from the retina to 

 the brain. 



3. The position and mode of action of the parts of the brain 

 in which the changes are set up which accompany visual 

 sensations (the visual centre). 



1. THE MODE OF FORMATION OF PICTURES UPON THE 

 RETINA 



(I.) MONOCULAR VISION 

 A. The Dioptric Mechanism 



Distant Vision. The eye may be compared to a photographic 

 camera, having in front a lens, or lenses, to focus the light 

 upon the sensitive screen behind (fig. 54). The picture is formed 

 on the screen by the luminous rays from each point outside 

 being concentrated to a point upon the screen. This is brought 

 about by refraction of light as it passes through the various 

 media of the eye the cornea, aqueous, crystalline lens, and 



