550 



THE NERVOUS SYSTEM 



fibers from the corpora quadrigemina descend (chiefly through the posterior 

 longitudinal bundle) to the nuclei of the third, fourth, and sixth nerves, thus 

 rendering possible coordinated reflex movements of the eye muscles. 



When the third nerve is stimulated within the skull, all those muscles to 

 which it is distributed are convulsed. 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- 



FIG. 384. Fourth Ventricle with the Medulla Oblongata and the Corpora Quadrigemina. The 

 roman numbers indicate superficial origins of the cranial nerves, while the other numbers in- 

 dicate their deep origins, or the position of their central nuclei. 8, 8', 8", Auditory nuclei nerves; 

 t, funiculus teres; A, B, corpora quadrigemina; c.g, corpus geniculatum; p.c, pedunculus cerebri; 

 m.c.p, middle cerebellar peduncle; s.c.p, superior cerebellar peduncle; i.c.p, inferior cerebellar 

 peduncle; l.c, locus ceruleus; e.t, eminentia teres; a.c, ala cinerea; a.n, accessory nuleus; 0, 

 obex; c, clava; f.c, funiculus cuneatus; f.g, funiculus gracilis. 



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 

 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 



