NERVE OF MASTICATION. 547 



have shown that the motor oculi externus is entirely insensible at its origin, its 

 stimulation producing contraction of the external rectus muscle and no pain. 

 The same experiments illustrate the action of the nerve, inasmuch as its 

 stimulation is followed by contraction of the muscle and deviation of the eye 

 outward. Division of the nerve in the lower animals or its paralysis in the 

 human subject is attended with internal, or converging strabismus, due to the 

 unopposed action of the internal rectus muscle. 



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

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

 pupil inward or to 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 the inferior oblique ; and 

 that each of the two muscles that move the globe so as to direct the pupil 

 outward, except the inferior oblique (the superior oblique and the external 

 rectus), is supplied by a special nerve. The movements of the eyeball will 

 be described more minutely 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 entirely 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 branch of the large root ; but 

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

 ity 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 substance of the pons. At 

 the point of apparent origin, the small root presents six to eight rounded fila- 

 ments. If a thin layer of the pons covering these filaments be 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 rapidly. The posterior fibres pass to the median line, and cer- 

 tain 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 ante- 

 rior wall of the fourth ventricle. Here they lose their white color, become 

 gray and present a number of globules of gray substance between their fila- 

 ments. 



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

 glion of Gasser from which it sometimes, though not constantly, receives a 

 filament of communication lies behind the inferior maxillary branch of the 

 large root, and passes out of the cranial cavity, by the foramen ovale. With- 



