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TEXT-BOOK OF PHYSIOLOGY. 



thrown on the sensitive side of the retina the usual response, a con- 

 traction of the sphincter and a narrowing of the pupil, is at once 

 observed. 



FOURTH PAIR. THE PATHETICUS. 



The fourth cranial nerve, the patheticus, 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 nu- 

 cleus the nerve-fibers pass down- 

 ward for a short distance, then 

 curve dorsally around the aque- 

 duct of Sylvius, and enter the 

 valveof_yieussens, where they 

 completely decussate with the 

 nerve-fibers of the opposite side. 

 Cortical Connections. 

 The nucleus of the pathetic nerve 

 is in histologic and physiologic 

 connection with the motor area 

 of the cerebral cortex. Nerve 

 cells in this region give off axons 

 which enter the pyramidal tract 

 and descend through the inter- 

 nal 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 below the posterior 

 quadrigeminal body, crosses the 

 superior cerebellar peduncle, 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 tEe 

 cavernous plexus of the sympathetic and the ophthalmic division of 

 the trigeminal. 



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

 modic 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. 



FIG. 246. DISTRIBUTION OF THE PATHETI- 

 CUS. I. Olfactory nerve. II. Optic 

 nerves. III. Motor oculi communis. 

 IV. Patheticus, by the side of the 

 ophthalmic branch of the fifth, and 

 passing to the superior oblique muscle. 

 VI. Motor oculi externus. i. Gang- 

 lion of Gasser. 2, 3, 4, 5, 6, 7, 8, 9, 10. 

 Ophthalmic division of the fifth 

 nerve, with its branches. (Hirsch- 

 feld.} 



