THE FOURTH NERVE, OR TROCHLEARIS 



383, to the nuclei of the third, fourth, and sixth nerves, thus rendering 

 possible co-ordinated reflex movements of all the eye muscles. 



When the third nerve is stimulated w r ithin the skull, all those muscles to 

 which it is distributed are contracted. When it is paralyzed or divided, the 

 following effects ensue: i. The upper eyelid can be no longer raised by the 

 levator palpebrae, but droops, ptosis, and remains gently closed over the eye, 

 under the unbalanced influence of the orbicularis palpebrarum, which is sup- 

 plied by the facial nerve. 2. The eye is turned outward and downward, 

 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 



cq.s 



c s 



ILC 



FIG. 408. Section through Superior Corpus Quadrigeminum and Part of the Thal- 

 amus. s, Aqueduct of Sylvius; gr, gray matter of the aqueduct, c.q. s., superior corpus 

 quadrigeminum; /, stratum lemnisci; o, stratum opticum, c, stratum cinereum; Th, pulvinar 

 of optic thalamus; c.g.e, c.g.i, lateral and median corpora geniculata; br.s, br.i, superior and 

 inferior brachia-/, fillet; p.l, posterior longitudinal bundle; r, raphe; 777, third nerve, and 

 n.III, its nucleus; l.p.p, posterior perforated space; s.n, substantia nigra above this is the 

 tegmentum with the circular area of the red nucleus; cr, crusta; 77, optic tract; M , medullary 

 center of hemisphere; n.c, nucleus caudatus; st, stria terminalis. (After Quain, from 

 Meynert.) 



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. 

 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. 



