THE CRANIAL NERVES. 581 



1. Diminution in the size of the pupil. 



2. Accommodation of the eye for near vision. 



3. Elevation of the upper eyelid. 



4. Internal deviation and rotation upward and inward of the anterior pole 



of the eye, combined with a small amount of torsion toward the mesial 

 line, due to preponderating action of the internal rectus and inferior 

 oblique muscles. 

 Division of the nerve either experimentally or as a result of compression 



from a pathologic cause is followed by a relaxation of the muscles, with the 



following effects, viz.: 



1. Dilatation of the pupil, the iris 



responding neither to light nor 

 to efforts of accommodation. 



2. Loss of the accommodative 



power. 



3. Falling of the upper eyelid 



(ptosis). 



4. External deviation and rotation 



downward and outward of the 

 anterior pole of the eyeball 

 combined with a small amount 

 of torsion toward the mesial ^ ^r^pT^ A 



line due tO the Unopposed nerve. 3. Superior branch. 4. Inferior branch. 



action Of external rectUS and 5- Abducens. 6. Trifacial. 7. Ophthalmic 



trip Qnnprirvr nhlimip mimrlpc branch divided. 8. Nasal branch. 9. Ciliary 



the Superior Oblique muscles. gangKonm I0 . Motor branch to this ganglion 



5. Double Vision Or diplopia. The from the inferior branch of the third nerve, n. 



image Of the eye Of the para- Sensory fibers. 12. Sympathetic fibers. 13. 

 *\ . T J . Ciliary nerves. (Sappey.) 



lyzed side is projected to the 



opposite side of the true image and to the upper part of the visual field. 

 Owing to the slight mesial torsion the false image is inclined away 

 from the true image. 



6. Immobility and slight protrusion of the eyeball. 



Function. The function of the third nerve is to transmit nerve im- 

 pulses from the nuclei of origin to all the muscles of the eye except the ex- 

 ternal rectus and superior oblique and excite them to activity. The majority 

 of the ocular movements, the power of accommodation, the variations in 

 the size of the pupil in accordance with variations in the intensity of the light, 

 the power of convergence of the visual axes, are all excited by the trans- 

 mission of nerve impulses by the constituent fibers of the nerve from their 

 related nuclei. This is made evident by the effects which follow stimula- 

 tion and division of the nerve or lesions of the nuclei themselves. 



The central nuclei can be excited to activity (i) by nerve impulses de- 

 scending the motor tract, from the cerebral cortex, (2) by nerve impulses 

 coming through various afferent nerves. This holds true more especially 

 for the sphincter pupillae nucleus. 



The Iris Reflex or the Pupillary Reflex. These are terms applied 

 to the variations in the size of the pupil that follow variations in the inten- 

 sity of the light. In the absence of light the pupil widely dilates, due 

 largely to the relaxation of the sphincter pupillce muscle and partly to a con- 



