622 TEXT-BOOK OF PHYSIOLOGY 



terminations decussate or are in relation either directly or indirectly with the 

 sphincter centers. 



The eye of the right side is then in turn shielded from the light and the 

 same method of examination is carried out. In this case, however, the light 

 is projected first on the nasal, which is the non-sensitive side of the retina; 

 there will again be no response in the pupil. But if the light is gradually 

 directed toward the sensitive (the temporal) side, there will come a moment, 

 as the central line is crossed and the light falls on the sensitive portion of 

 the retina, when the usual pupillary response manifests itself. The course 

 of the pupillary fibers in this instance will also become apparent from an 

 examination of Fig. 268. It is evident, however, that in either case a bilateral 

 pupillary reaction will follow stimulation of the sensitive side of either eye 

 because of the central decussation of the pupillary fibers. 



THE FOURTH NERVE. THE TROCHLEAR 



The fourth cranial nerve, the trochlear, consists of peripherally coursing 

 axons which serve to bring the cells from which they arise into relation with 

 the superior oblique muscle. t 



Origin. The axons of this nerve arise from a group of cells located 

 beneath, the aqueduct of Sylvius just posterior to the last nucleus of the 

 third nerve. After emerging from the nucleus the nerve-fibers pass down- 

 ward for a short distance, then curve dorsally around the aqueduct of 

 Sylvius, and enter the valve of Vieussens, where they completely decussate 

 with the nerve-fibers of the opposite side. 



Cortical Connections. The nucleus of the trochlear nerve is in his- 

 tologic and physiologic connection with the motor area of the cerebral cor- 

 tex. Nerve-cells in this region give off axons which enter the pyramidal 

 tract and descend through the internal capsule and the cms cerebri, after 

 which they cross to the opposite side. Their end-tufts arborize around the 

 cells of the nuclei already described. 



Distribution. After its decussation the nerve-trunk emerges just be- 

 low the posterior quadrigeminal body, crosses the superior cerebellar pe- 

 duncle, and winds around the cms cerebri to the anterior border pf the pons 

 Varolii. It then enters the orbit cavity through the sphenoid fissure and 

 finally terminates in the superior oblique muscle. In its course the nerve 

 receives filaments from the cavernous plexus of the sympathetic and the 

 ophthalmic division of the trigeminal. 



Properties. Stimulation of. the nerve- trunk is followed by spasmodic 

 contraction of the superior oblique muscle, the anterior pole of the eyeball 

 being turned downward and outward, combined with slight torsion away 

 from the middle line. . 



Division of the nerve is followed by a relaxation or paralysis of the 

 muscle. In consequence of the now unopposed action of the inferioi 

 oblique muscle, the anterior pole of the eyeball is turned upward and in- 

 ward with slight torsion toward the middle line. The diplopia consequent 

 upon this paralysis is homonymous, the images appearing one above the 

 other. The image of the paralyzed eye is below that of the normal eye and 

 its upper end inclined toward that of the normal eye. 



