826 



PHYSIOLOGY. 



and outward. That is, he cannot make a movement directed by the 

 superior oblique and still keep the head perfectly vertical. It 

 becomes necessary that the head be inclined to one side, and at the 

 time this inclination is produced the rotation of the eyeball occurs 

 without the will having the power to prevent it. By the very act 

 of inclination of the head the necessary parallelism of the two eyes 

 is positively destroyed; hence this involuntary action of the supe- 

 rior oblique to place the visual axes upon the same plane. 



Nucleus of oculo-motor. 



Edinger-Westphal nucleus. 

 Principal nucleus. 

 Median nucleus. 



Nucleus of fourth nerve. 



Fig. 374. Nuclei of Origin of the Third and Fourth Nerves. 

 (PoiRiEB and CHARPY.) 



The fourth pair of cranial nerves arise from a collection of cells 

 beneath the anterior part of the posterior corpus quadrigeminum. 

 It completely decussates in the superior medullary velum. It starts 

 behind the quadrigeminal body and then appears like a white thread 

 winding around the outer side of the eras of the cerebrum. It then 

 pierces the dura mater, runs along the outer wall of the cavernous 

 sinus, and enters the sphenoidal foramen with the oculomotor and 

 abducent. It supplies the superior oblique muscle of the eye. 



Pathology. Usually the first sign of any disorder of the pathetic 

 is a giddiness when ascending or descending a stairs, owing to the 



