The Senses. 295 



lens, the light passes again through a fluid medium, and 

 comes to a focus on a delicate expansion of nerve tissue, 

 which takes a reduced and inverted impression of the 

 picture presented. The physical arrangement of the 

 camera and the eye are, therefore, practically identical. 



The shape of the eyeball is spherical, its vertical being 

 somewhat longer than its transverse axis ; the optic nerve 

 penetrates it close to the floor, and inclined to the 

 temporal side of the eyeball. 



The optic nerve has a deep-seated origin in the optic 

 thalamus, corpora geniculata externum and internum, and 

 corpora quadrigemina ; these unite on the base of the brain 

 to form the optic nerves, which decussate in the well- 

 known manner. This decussation, so productive of sym- 

 pathetic ophthalmia in man, never, in my experience, gives 

 rise to sympathetic ophthalmia in the horse. The nerve 

 runs in the substance of the retractor muscle, and makes a 

 distinct dip downwards before penetrating the globe. 



By means of the optic nerve the impulses due to the 

 action of light on the retina are conveyed to the brain ; 

 the optic nerve conveys nothing else but luminous im- 

 pulses ; stimulation of the nerve causes flashes of light, but 

 no pain ; division of the nerve is therefore painless, but the 

 sheath of the nerve causes pain when cut. The optic nerve, 

 by its expansion the retina, collects the impulses due to the 

 action of light, which are then referred to the brain by 

 means of the nerve itself. The natural stimulus of the 

 retina is light ; we must therefore briefly study the changes 

 occurring in the rays of light while passing through the 

 various refracting media of the eye, which result in the 

 formation of images on the retina. 



A beam of light is said to consist of a number of rays, 

 each ray running parallel to its fellow, and continuing in 

 this position unless it meets with a body which turns it 

 back in nearly the opposite direction to which it has been 

 travelling, viz., reflects it, or with a body which will allow 

 it to pass, but only on condition that the parallel rays 

 become bent. This bending is termed refraction. 



