THE PATHETICUS. 525 



different degrees of obliquity to the two eyes, producing uncertainty 

 and double vision. 



The apparent origin of the patheticus nerve is directly behind the 

 tubercula quadrigemina, on the upper surface of the valve of Vieussens, 

 a thin lamina of white substance, extending from this situation back- 

 ward to the cerebellum, and thus covering in the anterior part of the 

 fourth ventricle. The fibres of the nerve, however, can be traced from 

 without inward in a transverse direction through the substance of the 

 valve. According to Henle and Meynert, a great part of these fibres 

 cross the median line, decussating with those coming in the opposite 

 direction from the corresponding nerve on the other side; then, turning 

 downward and forward, they reach a collection of gray matter seated 

 just behind the nucleus of the oculomotorius nerve, and continuous 

 with it anteriorly. According to Henle, a portion of the fibres also 

 remain upon the same side of the median line, and terminate, without 

 crossing, in this and another nucleus not far distant. The collection 

 of gray matter just described is, however, regarded as the main nucleus 

 or point of origin for the fibres of the patheticus nerve. This nucleus 

 is situated beneath the aqueduct of Sylvius, near the median line, and 

 at a situation corresponding with the anterior tubercula quadrigemina ; 

 while the point of exit of the nerve is above the aqueduct of Sylvius 

 and behind the posterior tubercula quadrigemina. Its fibres, accord- 

 ingly, after leaving the gray matter in which they originate, encircle 

 the lateral walls of the aqueduct, running obliquely upward and back- 

 ward, and then, curving inward, cross the median line to their point of 

 emergence on the opposite side. 



From this point the nerve passes forward, as a slender, rounded fila- 

 ment, not more than one millimetre in diameter, but containing, accord- 

 ing to the estimate of Rosenthal, about 1100 ultimate nerve fibres. It 

 passes along the upper wall of the cavernous sinus, where it lies in 

 immediate proximity to the oculomotorius; and thence, entering the 

 cavity of the orbit by the sphenoidal fissure, terminates in the sub- 

 stance of the superior oblique muscle of the eyeball. 



The course of the fibres of the oculomotorius and patheticus, when 

 compared with each other, shows a remarkable relation between two 

 nerves which are apparently distinct. The fibres of both originate from 

 adjacent portions of the same nucleus, situated in the thickness of the 

 crus cerebri. Those of the oculomotorius pass downward and forward, 

 to emerge from the inner free border of the crus, at the base of the 

 brain ; while those of the patheticus pass upward and backward, to 

 emerge from the upper and posterior part of the cerebro-spinal axis, 

 between the cerebrum and cerebellum. But the two nerves afterward 

 pass side by side, in their passage toward the orbit, and are finally dis- 

 tributed to muscles which are associated in the accomplishment of the 

 same movements. 



Physiological Properties of the Patheticus Nerve. The anatomical 

 distribution of this nerve to a muscle which receives filaments from no 



