652 



THE NERVOUS SYSTEM 



ganglion, of which it forms what is called the short root, it also supplies 

 motor filaments to the iris and ciliary muscle. The fibers which subserve 

 the three functions, accommodation, contraction of the pupil, and nerve- 

 supply to the external ocular muscles, arise from three distinct groups of 

 cells. Optic reflexes involving movements of the eyeballs are mediated 

 through fibers from cells of the superior corpora quadrigemina (which re- 

 ceive fibers from the optic nerve). These fibers from the corpora quadri- 

 gemina descend, chiefly through the posterior longitudinal bundle, figure 



FIG. 409. Fourth Ventricle with the Medulla Oblongata and the Corpora Quad- 

 rigemina. The Roman numbers indicate superficial origins of the cranial nerves, while the 

 other numbers indicate 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 genic- 

 ulatum; p.c. pedunculus cerebri; m.c.p, middle cerebellar peduncle; s.c.p, superior cere- 

 bellar peduncle; i.c.p, inferior cerebellar peduncle; l.c, locus ceruleus; e.t, eminentia teres; 

 a.c, ala cinerea; a.n, accessory nucleus; o, obex; c, clava; f.c, funiculus cuneatus; f.g, funicu- 

 lus gracilis. 



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 within 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. 



