THE FIFTH NERVE, OR TRIGEMINAL 



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external strabismus, by the unbalanced action of the rectus externus and supe- 

 rior oblique, to which the sixth nerve is appropriated; and hence, from the 

 irregularity of the axes of the eyes, double sight, diplopia, is often experienced 

 when a single object is within view of both the eyes. 3. The eye cannot be 

 moved upward, downward, or inward. 4. The pupil becomes dilated, 

 mydriasis. 5. The eye cannot accommodate for short distances. 



The Fourth Nerve, or Trochlearis. Origin. The fourth nerve 

 arises from a nucleus consisting of large multipolar ganglion cells situated 

 ventral to the aqueduct of Sylvius, and the inferior corpus quadrigeminum. 



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FIG. 410. Section Across the Pons, About the Middle of the Fourth Ventricle, py, 

 Pyramidal bundles; po, transverse fibers passing po lt behind, and po 2 , in front of py; r t 

 raphe; o.s, superior olive; a.V, bundles of ascending root of V. nerve enclosed in a pro- 

 longation of the substance of Rolando; VI, the sixth nerve; nVI, its nucleus; VII, facial 

 nerve; Vll.a, intermediate portion; nVII, its nucleus; VIII, auditory nerve; nVIII, 

 lateral nucleus of the auditory. (After Quain.) 



The fibers from both sides sweep dorsally around the central gray matter, and 

 reach the valve of Vieussens, where they decussate in the mid-line of the roof, 

 then pass forward along the lateral aspect of the crus. The nucleus of the 

 fourth nerve on either side is connected with those of the third and sixth 

 nerves and with the optic reflex center previously described. 



Functions. The fourth nerve is exclusively motor, and supplies only the 

 trochlearis or superior oblique muscle of the eyeball. 



The Fifth Nerve, or Trigeminal. Origin. The fifth or trigeminal 

 nerve resembles the spinal nerves in that it has two roots; namely, the larger 

 or sensory, in connection with which is the Gasserian ganglion, and the small 

 or motor root, which has no ganglion, and which passes under the ganglion 

 of the sensory root. The fibers of origin of the fifth nerve come from the 

 floor of the fourth ventricle. The motor root arises to the inside of the sen- 

 sory about the middle of each lateral half of the fourth ventricle. The 



