THE ENCEPHALIC OR CRANIAL NERVES 635 



The eye of the left side is first carefully shielded from the light. A fine ray 

 of light is then projected into the right eye in such a manner that it falls en- 

 tirely on the non-sensitive (the temporal) side of the retina. There will be an 

 absence of the usual pupillary response, or rather the pupil remains inactive; 

 but if the light is gradually directed toward the sensitive (the nasal) side of 

 the retina, there will come a moment, as the central line is crossed and the 

 light falls on the sensitive side, when the usual pupillary response manifests 

 itself, viz.: a contraction of the sphincter pupillae and a diminution in the 

 size of the pupil. The explanation of these facts will become apparent 

 from an examination of Fig. 264 in which the course of the pupillary fibers 

 is shown and especially if it be accepted that these fibers at their central 

 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. 264. 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. 



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 (see Fig. 266). 



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 crus 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 crus cerebri to the anterior border of 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. 



