THE ENCEPHALIC OR CRANIAL NERVES 



619 



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.: 



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 

 line due to the unopposed 

 action of external- rectus and 

 the superior oblique muscles. 



5. Double vision or diplopia. The 



image of the eye of the para- Sensory 

 i . i . . , , ' *** 



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- 

 traction of the radiating fibers of the iris, which collectively constitute the 

 dilatator pupillce muscle. With the entrance of light into the eye, the pupil 



10 



FIG. 267. INTRA-ORBITAL PORTION OF THE 

 THIRD NERVE, i. Optic nerve. 2. Third 

 nerve. 3. Superior branch. 4. Injerior branch. 

 5. Abducens. 6. Trifacial. 7. Ophthalmic 

 branch divided.' 8. Nasal branch. 9. Ciliary 

 ganglion. 10. Motor branch to this ganglion 

 from the inferior branch of the third nerve. 1 1 . 

 fibers. 12. Sympathetic fibers. 13. 

 Ciliary nerves. (Sappey.) 



