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TEXT-BOOK OF PHYSIOLOGY. 



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. 270 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. 270. 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 pupil- 

 lary fibers. 



FOURTH NERVE. THE TROCHLEAR. 



The fourth cranial nerve, the troch- 

 lear, 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 be- 

 neath the aqueduct of Sylvius just pos- 

 terior to the last nucleus of the third 

 nerve. After emerging from the nucleus 

 the nerve-fibers pass downward 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 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 



FIG. 271. DISTRIBUTION OF THE 

 PATHETICUS. I. Olfactory nerve. II. 

 Optic nerves. III. Motor oculi commu- 

 nis. IV. Trochlear, by the side of V the 

 ophthalmic branch of the fifth, and passing 

 to the superior oblique muscle. VI. Motor 

 oculi externus. i. Ganglion of Gasser. 

 2 > 3> 4> S 6, 7, 8, 9, 10. Ophthalmic divi- 

 sion of the fifth nerve, with its branches. 

 (Hirschfeld.) 



