THE CRANIAL NERVES. 



451 



That this is a reflex action is shown by dividing and irritating the 

 optic nerves. After section of the nerve, according to the experiments 

 of Mayo and Longet, upon pigeons, dogs, and rabbits, irritation of its 

 peripheral end, that is, the portion still connected with the eyeball, 

 produces no effect on the pupil ; but irritation of its central portion, 

 which is connected with the brain, at once causes contraction. On the 

 other hand, division of the oculomotorius nerve paralyzes the iris and 

 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 the optic 

 nerves, which in man and all vertebrate animals is visible on super- 

 ficial examination, varies considerably in its details in different classes. 

 These variations may be mainly referred to three distinct types, gen- 

 erally known to comparative anatomists, and more distinctly recog- 

 nized in the recent investigations of Nicati.* 



FIG. 119. 



FIG. 120. 



INFERIOR SURFACE OF THE BRAIX OF THE 

 COD. 1. Optic nerve of right eye. 2. Optic 

 nerve of left eye. 3. Right optic tubercle. 

 4. Left optic tubercle, o, 6. Cerebral hemi- 

 spheres. 7. Medulla oblongata. 



INFERIOR SURFACE OF THE BRAET OF FOWL. 

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

 of left eye. 3. Right optic tubercle. 4. Left 

 optic tubercle. 5, 6. Cerebral hemispheres. 

 7. Medulla oblongata. 



I. In fishes and reptiles the optic nerves cross each other without com- 

 mingling, so that their complete decussation is visible to the unaided 

 eye. In many instances, as in the cod (Fig. 119), each nerve, preserv- 

 ing its cylindrical form, passes either above or below its fellow to the 

 eye of the opposite side. In others, as in the herring, the nerve of the 

 right side passes through a slit or fenestra in that of the left ; and in 

 others still, the decussation takes place by several distinct bundles of 

 fibres crossing at various levels above or below each other. Through- 

 out these two classes it is plainly evident that all the optic fibres com- 



* Archives de Physiologic. Paris, 1878, 2^6 serie, tome v., p. 658. 



