MOTOR NERVES OF THE FACE. 615 



producing contraction of the external rectus muscle and no pain. The same experiments 

 illustrate the function of the nerve, inasmuch as its irritation is followed by powerful 

 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 con- V 

 verging strabismus, due to the unopposed action of the internal rectus muscle. 



With regard to the associated movements of the eyeball, it is a curious fact that all 

 of the muscles of the eye that have a tendency to direct the pupil inward or to produce 

 the simple movements upward and downward, viz., 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 muscle that has a tendency to move the globe so as 

 to direct the pupil outward, except the inferior oblique, viz., the superior oblique and 

 the external rectus, is supplied by a special nerve. The various movements of the eyeball 

 will be studied more minutely in connection with the physiology of vision. 



Motor Nerves of the Face. 



The motor nerves of the face are, the small, or motor root of the fifth, and the portio 

 dura of the seventh, or the facial. The first of these nerves is distributed to the deep 

 muscles, those concerned in the act of mastication ; and the second, the facial, supplies 

 the superficial muscles of the face and is sometimes called the nerve of expression. 

 These nerves are not so simple in their anatomy and physiology as the motor nerves ot 

 the eyeball. The nerve of mastication, at its origin, is deeply situated at the base of the 

 brain and is exposed and operated upon with difficulty. It passes out of the cranium, 

 closely united with one of the great sensitive branches of the fifth, and its distribution 

 has been most successfully studied by experiments in which it is divided in the cranial 

 cavity. The origin of the facial is also reached with great difficulty. It communicates 

 with other nerves, and its physiology has been most satisfactorily studied by dividing it 

 at its origin or in different portions of its course. In treating of these nerves, we shall 

 first, as in the case of the motor nerves of the eye, study their properties at their roots, 

 noting the phenomena following their galvanization and section. It will be neces- 

 sary, also, to describe their origin and distribution, as far as has been ascertained by 

 dissection. 



Nerve of Mastication (the Small, or Motor Hoot of the Fifth Nerve). 



The motor root of the fifth nerve is entirely distinct from its sensitive 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 cavity has demonstrated its distribution to a par- 

 ticular set of muscles. 



Physiological Anatomy of the Nerve of Mastication. 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 from six to eight rounded filaments. If a thin 

 layer of the pons covering these filaments be removed, the roots will be found pene- 

 trating its substance, becoming flattened, passing under the superior peduncles of the 

 cerebellum, and going to the anterior wall of the fourth ventricle. At this point, they 

 change their direction, passing now from without inward and from behind forward toward 

 the median line, the fibres diverging rapidly. The posterior fibres pass to the median line, 

 and certain of them decussate with the 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 



