CHAP. XIX.] FOURTH PAIR OF NERVES. 103 



iris. The same effect is produced by mechanical irritation of the 

 trunk of the nerve. 



Section of the trunk of the third nerve in rabbits or dogs, gives 

 rise to external strabismus, [with paralysis of the upper eyelid 

 (ptosis) and dilatation and immobility of the pupil. The eyeball is 

 given up to the influence of the external rectus and of the superior 

 oblique muscles, the former of which being the more powerful de- 

 termines its permanent position. 



The paralysis of the iris, after section of this nerve, is so com- 

 plete that the most powerful light directed into the eye/is incapable 

 of exciting the least contraction of the pupil. And Mayors expe- 

 riments demonstrate that the fifth nerve, which is the only other, 

 except, perhaps, the sympathetic, connected with the iris, does not 

 exert any motor influence upon that membrane. 



When the optic nerve is irritated, the third remaining intact and 

 retaining its connexion with the brain, the pupil contracts. This 

 action does not take place if the third nerve have been previously 

 cut. The motor action of the third nerve may, therefore, be ex- 

 cited through the optic nerve. There can be no doubt, indeed, that 

 this is the ordinary method by which contraction of the pupil is pro- 

 duced during life ; the stimulus of light falling upon the retina ex- 

 cites the optic nerve and, through it, that portion of the brain in 

 which the third nerve is implanted. 



The effects produced by pathological changes affecting the third 

 nerve, or that part of the brain with which it is connected, are in 

 exact accordance with the results derived from experiment. 



Paralysis of the levator palpebra3 superioris muscle, permanent 

 squinting of the eyeball in the outward direction, and a dilated 

 motionless pupil are the unerring signs of a paralytic lesion affect- 

 ing the third nerve either at its central extremity or in some part 

 of its course. 



The third nerve may, therefore, be stated to be the nerve of 

 motion to the elevator muscle of the upper lid, and a principal nerve 

 of motion to the eyeball, and to the muscular apparatus within the 

 eye. It is a nerve of great importance to vision, not only from its 

 influence over the eyeball itself, but also from its connexion with 

 the muscular structures in the interior, on which the power of ad- 

 justment probably depends. 



Of the Fourth Pair of Nerves. These, which were called by 

 Willis " nervi pathetici" are the smallest of the encephalic nerves. 

 They are also remarkable for the very long course which they take 

 from their origin to their point of exit from the cranium. 



