38 . SPECIAL SENSES. 



such works, those specially devoted to the subject will refer ; 

 particularly as, in such treatises, the bearings of physiology 

 upon pathology and treatment are fully considered. Our sec- 

 tion on the subject of vision is not intended to meet the 

 wants of the ophthalmologist, nor does it seem desirable that 

 it should. Our aim is to treat of the anatomy and physiology 

 of the organs of vision so as to make the subject clear, merely 

 as far as its physiology is concerned. In doing this, we shall 

 omit certain anatomical details, as well as most of the elabo- 

 rate mathematical formulae of modern physicists, and all re- 

 searches of a purely historical interest, which, 'if fully dwelt 

 upon, would swell 'the chapters on vision to the dimensions 

 of a large volume. 



The chief important points to be considered in the physi- 

 ology of vision are the following : 



1. The physiological anatomy and the general properties 

 of the optic nerves. 



2. The physiological anatomy of the parts essential to 

 correct vision. 



3. The laws of refraction, diffusion, etc., bearing upon 

 the physiology of vision. 



4. The action of the different parts of .the eye in the 

 production and appreciation of correct images. 



5. Binocular vision. 



6. The physiological anatomy and the functions of acces- 

 sory parts, as the muscles which move the eyeball. 



7. The physiological anatomy and the functions of the 

 parts which protect the eye, as the lachrymal glands, eye- 

 lids, etc. 



Physiological Anatomy of the Optic Serves* The optic 

 nerves, or tracts, take their origin, each by two principal roots 

 of white matter and a few filaments from what is described as 

 the gray root, chiefly from the tubercula quadrigemina, but 

 in part from those portions of the encephalon over which the 

 nerves pass to go to the eyes. The internal white root arises 



