554 



THE NERVOUS SYSTEM 



The Sixth Nerve, the Abducens. Origin. The sixth nerve arises 

 from a compact oval nucleus, situated somewhat deeply at the back part of 

 the pons near the middle of the floor of the fourth ventricle. The eminentia 

 teres marks its position. It contains moderately large nerve cells with large 

 axis-cylinder processes. It is connected, figure 3 7 3 , with the nuclei of the third, 

 fourth, and seventh nerves, and with reflex centers of the optic tracts, as pre- 

 viously mentioned. The root is thin, and passes ventrally and laterally through 



i.e. 



OG. 



Sty. hy. 



FIG. 387. The Seventh Nerve and Its Branches. 



S.M. 



" LM. 



Facial nerve; P.I, pars in- 



termedia; " VIII, auditory nerve; Aq.Fal, aqueduct of Fallopius; G.G, gemculate ganglion; 

 E.S.P, external superficial petrosal nerve; MM, middle meningeal artery; G.5.P., great super- 

 ficial petrosal nerve : G.P.D, great deep petrosal nerve; I.C, internal carotid artery; Via, Vidian 

 nerve; M.G., Meckel s ganglion: Ty.Pl, tympanic plexus; S.D.P, small deep petrosal nerve; G.Ph, 

 Glosso-pharyngeal nerve Ty, tympanic branch: S.S.P., small superficial petrosal nerve; U.G, 

 optic ganglion: Stap, nerve to stapedius 1 C.T, chorda tympani nerve; L, lingual nerve; A.Va, 

 communication with auricular branch of vagus; P. A, posterior auricular nerve- Sty.hy, nerve to 

 stylo-hyoid; Di, nerve to digastric (posterior belly); T.F, temporal-facial division; C.F, cervico- 

 facial division; T, temporal M, malar; I.O, infra-orbital; B, buccal. S.M, supra-mandibular ; 

 LM, infra-mandibular branches. (Cunningham.) 



the r^ticular formation, to the surface, which it reaches at the lower edge of 

 the pons, opposite the front end of the pyramid. 



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

 rectus externus muscle of the eye. The muscle is paralyzed when the nerve 

 is divided. In all such cases of paralysis the eye squints inward and cannot 

 be moved outward. 



The Seventh Nerve, or Facial. Origin. The facial or seventh pair of 

 nerves arises from the floor of the central part of the fourth ventricle, behind 

 and in line with the motor nucleus of the fifth, to the outside of and deeper 

 down than the nucleus of the sixth. The nucleus is narrower in front than 

 behind, and consists of large motor cells with well-marked axis-cylinder proc- 



