THE CRANIAL NERVES. 459 



oblique and inferior in regard to the other ; but these disturbances of 

 vision disappeared when the head was inclined toward the opposite side. 

 The patheticus is, accordingly, the motor nerve of the superior 

 oblique muscle, and acts in harmony with the oculomotorius to 

 preserve the horizontal plane of the eyeball. 



Fifth Pair. The Trigemjims. 



The fifth pair occupies, in every respect, a prominent place among 

 the cranial nerves. It is the great sensitive nerve of the face, being 

 the only source of general sensibility for the integument and mucous 

 membranes of this region ; and, by branches of communication to the 

 corresponding motor nerves, it provides for the imperfect sensibility 

 of the facial muscles. But while in its main portion it is preeminently 

 sensitive, it also possesses motor fibres, derived from a distinct root, and 

 distributed to muscles of a special group. Before emerging from the 

 cranial cavity it separates into three main divisions, destined for the 

 corresponding regions of the face ; and its name, triireininus, is derived 

 from the fact that in man these three primary divisions are nearly 

 alike in size and importance. 



The apparent origin of the fifth nerve is from the lateral portion of 

 the pons Yarolii, where its two roots emerge in close approximation 

 to each other, but usually separated by a narrow band of the trans- 

 verse fibres of the pons. The anterior or motor root is the smaller 

 of the two, being about two millimetres in diameter ; the posterior or 

 sensitive root is the larger, having a diameter of about five millimetres. 

 Both roots may be traced, through the pons Yarolii, backward, upward, 

 and inward, to the gray substance beneath the anterior part of the 

 fourth ventricle. During the greater part of this passage they remain 

 distinct, but join each other above and become closely entangled by the 

 interweaving of their bundles ; though their fibres may still be distin- 

 guished, on microscopic examination, by the generally larger size of 

 those belonging to the motor root. They finally reach a collection of 

 gray substance, the " trigeniinal nucleus," situated next behind that 

 of the oculomotorius and patheticus, but farther outward from the 

 median line, occupying the extreme lateral part of the fourth ven- 

 tricle, where its floor forms an angle with the roof. The fibres of the 

 nerve terminate partly in or among the large, stellate, and dark-colored 

 cells of this nucleus. According to Henle, a portion also pass through 

 the nucleus, and across the median line to the opposite side ; the two 

 sets together forming, in this way, partly a direct and partly a crossed 

 connection between the peripheral organs and the nervous centres. 



After emerging from the pons Yarolii. the two roots of the fifth nerve 

 pass outward and forward in company with each other, the larger, pos- 

 terior, or sensitive root being placed above, the smaller, anterior, or 

 motor root underneath. At the apex of the petrous portion of the 

 temporal bone, a little outside and behind the posterior clinoid process 

 of the sella turcica, the fibres of the sensitive root spread out into a 



