THE OPTIC NERVES. 



519 



only with the brain, readily causes a movement of contraction. On the 

 other hand, division of the oculomotorius nerve, which paralyzes the 

 iris, puts an end to the movements of the pupil, although the eye may be 

 otherwise uninjured and the perception of light unimpaired. 



Decussation of the Optic Nerves. The decussation of these nerves 

 forms one of their most prominent anatomical features, being in all cases 

 readily visible on superficial examination, while in many classes of the 

 lower animals and in man it presents, on closer inspection, certain marked 

 varieties of detail. In fish, as a general rule (Fig. 1TO), the two optic 

 nerves cross each other's path from side to side at different levels, with- 

 out any admixture or even contact of their fibres ; that from the right side 

 of the brain passing independently to the left eye, and that from the left 

 side of the brain to the right eye. In the herring, according to Wagner, 

 the optic nerve of the right eye perforates that of the left, passing bodily 

 through it by a distinct slit, without forming a chiasma. In the sharks 

 and rays, which have a higher general grade of organization than other 

 fish, the fibres of the two nerves cross each other in separate fasciculi. 



Fig. 170. 



Fig. 171. 



INFERIOR SURFACE OF THE BRAIN 

 OF THE COB. 1. Optic nerve of right eye. 

 2. Optic nerve of left eye. 3. Right optic 

 tubercle. 4. Left optic tubercle. 5, 6. Cere- 

 bral hemispheres. 7. Medulla oblongata. 



INFERIOR SURFACE OF THE BRAIN 

 OF FOWL. 1. Optic nerve of right eye. 2. 

 Optic nerve of left eye. 3. Right optic 

 tubercle. 4. Left optic tubercle. 5, 6. Cere- 

 bral hemispheres. 7. Medulla oblongata. 



In birds the two optic nerves appear externally to be united at their 

 point of crossing (Fig. 171), but dissection shows that the decussation 

 of their fibres is complete. Those coming from the left side pass, in 

 the form of slender bundles, altogether over to the right, and those 

 from the right side pass in the same manner over to the left ; so that in 

 this class each optic tubercle is connected exclusively with the eye of 

 the opposite side. 



In man the decussation is more complicated, and is arranged in such 



