632 TEXT-BOOK OF PHYSIOLOGY 



forward between the sclera and chorioid coat, and terminate in the ciliary 

 muscle and the sphincter of the pupil. The ciliary nerves are not portions 

 of the third nerve proper, but peripheral sympathetic neurons. As the 

 ciliary ganglion receives filaments from the cavernous plexus of the 

 sympathetic and filaments which come from the trigeminal nerve, it 

 is* probable that the ciliary nerves contain not only motor, but vaso- 

 motor and sensor fibers as well. 



Properties. Stimulation of the nerve near its exit from the encepha- 

 lon is followed by contraction of the muscles to which it is distributed 

 with the following results, viz.: 



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 pote 



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 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 paralyzed 



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 pupils widely dilate, due 

 largely to the relaxation of the sphincter pupilla muscles and partly to a 



