106 HUMAN PHYSIOLOGY 



the judgment will depend upon the accuracy of the picture, 

 and upon the previous experience and training of the nerve 

 structures involved. 



But, further, while the parts of the brain connected with 

 the visual sense are usually stimulated by changes in the 

 cells of the eye, they may be directly stimulated ; and when 

 this is the case, a sensation of light, apparently in front of 

 the eye, is experienced, because the centres are naturally 

 always stimulated by such illumination. Sensations thus 

 produced are called illusions, and they are well illustrated by 

 the flashes of light before the eyes which sometimes precede 

 tin epileptic attack and which are caused by direct irritation 

 of the surface of the brain. 



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 (diop- 

 tric mechanism). 



B. The method in which the retina is stimulated. 



(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. 



(1) MONOCULAR VISION. 

 A. The Dioptric Mechanism. 



Anatomy. Before attempting to study the physiology 

 of the eye, the student must dissect an ox's or a pig's eye, 

 and then make himself familiar with the microscopic struc- 

 ture of the various parts. 



