PATHETICUS, OR TROCHLEARIS. 545 



or downward. Upon 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 uncomplicated paralysis of the third nerve. 



PATHETICUS, OR TROCHLEARIS (FOURTH NERVE). 



The physiology of the patheticus is very simple and 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 be easily followed to the 

 valve of Vieussens. The deep roots can be traced, passing from without in- 

 ward, to the following parts : One filament is lost in the substance of the 

 peduncles ; other filaments pass from before backward into the valve of Vieus- 

 sens and are lost, and a few pass into the f renulum ; 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 aque- 

 duct of Sylvius, beneath the nucleus of the third nerve. The decussation of 

 the fibres of origin of the fourth nerve has the same physiological signifi- 

 cance as the decussation of the roots of the third. From this origin, the 

 patheticus passes into the orbit, by the sphenoidal fissure, and is distributed to 

 the superior oblique muscle of the eyeball. In the cavernous sinus it receives 

 branches of communication from the ophthalmic branch of the fifth, but 

 these are not closely united with the 

 nerve. A small branch passes into the 

 tentorium, and one joins the lachrymal 

 nerve, these, however, being exclusively 

 sensory and coming from the ophthal- 

 mic branch of the fifth. It also re- 

 ceives a few filaments from the sympa- 

 thetic. 



Properties and Uses of the Pathet- 

 icus. Direct observations upon the pa- 

 theticus in living animals have shown 

 that it is motor, and its stimulation ex- 

 cites contraction of the superior oblique 

 muscle only. This muscle arises just 

 above the inner margin of the optic fo- 

 ramen, passes forward, along the upper FIG. lOS. ^Distribution of the patheticus (Hirsch- 



wall of the orbit at its inner angle, to a Ii0lfactory nerve . jrj^ nerves ; m , motor 

 little, cartilaginous ring which serves as ^^^^SSS^^^S, and 

 a pulley. From its origin to this point S^^S^^^i^onSf^l 

 it is muscular. Its tendon becomes ?i 3 'A?< 6 ' 7 < 8 ' 9, 10, ophthalmic division of 



the fifth nerve, with its branches. 



rounded just before it passes through 



the pulley, where it makes a sharp curve, passes outward and slightly back- 

 ward, and becomes spread out to be attached to the globe, at the superior and 



