PATHETIOUS, OR TROCHLEARIS (FOURTH NERVE). 613 



The following case, kindly sent for examination by Dr. Althof, of the New York Eye 

 Infirmary, illustrates, in the human subject, nearly all of the phenomena following paraly- 

 sis of the motor oculi communis in experiments upon the lower animals : 



The patient was a girl, nineteen years of age, with complete paralysis of the nerve 

 upon the left side. There was slight protrusion of the eyeball, complete ptosis, with the 

 pupil moderately dilated and insensible 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 any deviation 

 upward 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). 



Except as regards the influence of the motor oculi communis upon the iris, the pa- 

 theticus is to be classed with the other motor nerves of the eyeball. Its physiology 

 is extremely simple and resolves itself into the action of a single muscle, the superior 

 oblique. It will be necessary, therefore, only to describe its origin, distribution, and 

 connections. 



Physiological Anatomy. The apparent origin of the patheticus is from the superior 

 peduncles of the cerebellum ; but it may be easily traced to the valve of Vieussens. The 

 deep roots, which are covered by an extremely thin layer of nerve-substance, can be 

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

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

 of Vieussens and are lost, and a few pass into the frenulum ; a few filaments pass back- 

 ward and are lost in the corpora quadrigemina ; bnt the greatest number pass to the 

 median line and decussate with corresponding filaments from the opposite side. The 

 decussation of the fibres of origin of the fourth nerves 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 sensitive and coming from the ophthalmic branch of the fifth. It also 

 receives a few filaments from the sympathetic. 



Properties and Functions of the Patheticus. Direct observations upon the patheticus 

 in living animals have shown that it is motor, and its galvanization excites contraction 

 of the superior oblique muscle only. The question of the function of the nerve, there- 

 fore, resolves itself simply into the mode of action of the superior oblique muscle. This 

 muscle arises just above the inner margin of the optic foramen, passes forward, along the 

 upper wall of the orbit at its inner fingle, to a little cartilaginous ring which serves as a 

 pulley. From its origin to this point it is muscular. Its tendon becomes rounded just 

 before it passes through the pulley, where it makes a sharp curve, passes outward and 

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

 and external part of its posterior hemisphere. It acts upon the eyeball from the pulley 

 at the upper and inner portion of the orbit as the fixed point and rotates the eye upon 

 an oblique, horizontal axis, from below upward, from without inward, and from behind 

 forward. By its action, the pupil is directed downward and outward. It is the direct 

 antagonist of the inferior oblique, the action of which has been described in connection 



