THE BRAIN. 



needle, the result is a contraction of the pupil. This influence is 

 not communicated directly from the nerve to the iris, but is first 

 sent inward to the tubercles, to be afterward reflected outward by 

 the motor nerves. So long as the eyeball remains in connection 

 with the brain, mechanical irritation of the optic nerve, as we have 

 shown above, causes contraction of the pupil ; but if the nerve be 

 divided, and the extremity which remains in connection with the 

 eyeball be subjected to irritation, no effect upon the pupil is pro- 

 duced. 



The anatomical arrangement of the optic nerves, and the connec- 

 tions of the optic tubercles, are modified in a remarkable degree in 

 different animals, to correspond with the position of the two eyes. 

 In fish, for example, the eyes are so placed, on opposite sides of the 

 head, that their axes cannot be brought into parallelism with each 

 other, and the two eyes can never be directed together at the same 

 object. In these animals, the optic nerves cross each other at the 

 base of the brain without any intermixture of their fibres ; that 

 from the right optic tubercle passing to the left eye, and that from 

 the left optic tubercle passing to the right eye. (Fig. 147.) The two 



Fig. 147. 



Fig. 148. 



INFERIOR SURFACE OF BRAIN 

 OF COD. 1. Right optic nerve. 2. Left 

 optic nerve. 3. Right optic tubercle. 4. 

 Left optic tubercle. 5, 6. Hemispheres. 

 7. Medulla oblongata. 



INFERIOR SURFACE OF BRAIN OF 

 FOWL. 1. Right optic nerve. 2 Left optic 

 nerve. 3. Right optic tubercle. 4. Left 

 optic tubercle. 5, 6. Hemispheres. 7. Me- 

 dulla oblougata. 



nervous cords are here totally distinct from each other throughout 

 their entire length ; and are only connected, at the point of cross- 



