502 NERVOUS SYSTEM 



internal, or converging strabismus, due to the unopposed action of the 

 internal rectus muscle. 



In regard to the associated movements of the eyeball, it is important 

 to note that all the muscles of the eye that have a tendency to direct 

 the pupil inward or produce the simple movements upward and 

 downward (the internal, inferior, and superior recti) are animated by a 

 single nerve, the motor oculi communis, this nerve also supplying, how- 

 ever, the inferior oblique ; and that each of two muscles that move the 

 globe so as to direct the pupil outward (the superior oblique and the 

 external rectus) is supplied by a special nerve. The movements of 

 the eyeball will be described more in detail in connection with the 

 physiology of vision. 



NERVE OF MASTICATION (THE SMALL, OR MOTOR ROOT OF THE 



FIFTH NERVE) 



The motor root of the fifth nerve is distinct from its sensory portion 

 until it emerges from the cranial cavity by the foramen ovale. It is then 

 closely united with the inferior maxillary division of the large root ; but 

 at its origin it has been shown to be motor, and its section in the cranial 

 cavity has demonstrated its distribution to a particular set of muscles. 



Physiological Anatomy. The apparent origin of the fifth nerve is 

 from the lateral portion of the pons Varolii. The small, or motor root 

 arises from a point a little higher and nearer the median line than the 

 large root, from which it is separated by a few fibres of the white sub- 

 stance of the pons. At the point of apparent origin, the small root 

 presents six to eight rounded filaments. If a thin layer of the pons 

 covering these filaments is removed, the roots will be found penetrating 

 its substance, becoming flattened, passing under the superior peduncles 

 of the cerebellum and going to a gray nucleus, with large multipolar 

 cells, in the anterior wall of the fourth ventricle near the median line. 

 At this point the fibres change their direction, passing from without 

 inward and from behind forward toward the median line, the fibres 

 diverging widely. The posterior fibres pass to the median line and 

 certain of them decussate with fibres from the opposite side. The 

 anterior fibres pass toward the aqueduct of Sylvius and are lost. 

 The fibres become changed in their character when they are followed 

 inward beyond the anterior wall of the fourth ventricle. Here they lose 

 their white color, become gray and present a number of globules of 

 gray substance between their filaments. 



From the origin above described, the small root passes beneath the 

 ganglion of Gasser, from which it sometimes, though not constantly, 



