PATHETICUS, OR TROCHLEARIS 499 



facts show that the third is not the only nerve capable of acting on the 

 iris and that it is not the sole avenue for the transmission of reflex in- 

 fluences. 



When the eye is turned inward by a voluntary effort, the pupil is 

 contracted ; and when the axes of the two eyes are made to converge 

 strongly, as in looking at near objects, the contraction is very con- 

 siderable. 



The third nerve contains filaments that preside over voluntary 

 movements of the. ciliary muscle in the accommodation of the eye to 

 vision at different distances. 



The following case illustrates, in the human subject, nearly all the 

 phenomena following paralysis of the motor oculi communis in experi- 

 ments on the lower animals : 



The patient was a girl, nineteen years of age, with complete paraly- 

 sis of the nerve on the left side. There was slight protrusion of the 

 eyeball, complete ptosis, with the pupil moderately dilated and insen- 

 sible to ordinary impressions of light. The sight was not affected, but 

 there was double vision, except when objects were placed before the 

 eyes so that the axes were parallel, or when an object was seen with 

 but one eye. The axis of the left eye was turned outward, but it was 

 not possible to detect deviation upward or downward. On causing 

 the patient to incline the head alternately to one shoulder and the other, 

 it was evident that the affected eye did not rotate in the orbit but 

 moved with the head. This seemed to be a case of complete and un- 

 complicated paralysis of the third nerve. 



PATHETICUS, OR TROCHLEARIS (FOURTH NERVE) 



The physiology of the patheticus resolves itself into the action of 

 a single muscle, the superior oblique. 



Physiological Anatomy. The apparent origin of the patheticus is 

 from the superior peduncles of the cerebellum ; but it may easily be 

 followed to the valve of Vieussens. The deep roots can be traced, pass- 

 ing from without inward, to the following parts : One filament is lost in 

 the substance of the peduncles ; other filaments pass from before back- 

 ward into the valve of Vieussens and are lost, and a few pass into the 

 frenulum ; a few filaments pass backward and are lost in the corpora 

 quadrigemina ; but the greatest number pass to the median line and 

 decussate with corresponding filaments from the opposite side. The 

 fibres can be traced to a nucleus in the floor of the aqueduct of Sylvius, 

 beneath the nucleus of the third nerve. The decussation of the fibres 

 of origin of the fourth nerve has the same physiological significance as 



