526 THE CRANIAL NERVES. 



other source indicate in great measure its motor character, which is 

 furthermore fully established by the results of observation. Both the 

 experiments of Chauveau on the horse and rabbit, and those of Longet 

 on the horse, ox, and dog, show that galvanization of this nerve in 

 the interior of the cranium produces always contraction of the supe- 

 rior oblique muscle of the eyeball, with rotation of the eyeball on its 

 longitudinal axis from without inward ; and in those of Longet there 

 was also a perceptible deviation of the pupil outward. In cases quoted 

 by Longet, in the human subject, attributed to paralysis of this nerve, 

 there was incapacity of rotation of the eyeball on the affected side, and 

 consequently double vision, the image perceived by the affected eye being 

 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 ob- 

 lique muscle, and acts in harmony with the oculomotorius to preserve 

 the horizontal plane of the eyeball. 



Fifth Pair. The Trigeminus. 



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 communicating branches sent to 

 the corresponding motor nerves, it also provides for the imperfect 

 degree of sensibility belonging to the facial muscles. While in its main 

 portion, however, it is thus pre-eminently a sensitive nerve, it also pos- 

 sesses motor fibres, derived from a distinct root, and distributed to 

 muscles of a distinct group. Before emerging from the cranial cavity 

 it separates into three main divisions, destined for the corresponding 

 regions of the face ; and its name, trigeminus, is derived from the fact 

 that these three primary divisions of the nerve 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, 

 being about two millimetres in diameter; the posterior or sensitive root 

 is the larger, and having a diameter of about five millimetres. Both 

 these roots may be traced, through the fibrous bundles of the pons 

 Varolii, backward, upward, and inward toward the gray substance be- 

 neath the floor of the anterior part of the fourth ventricle. The two 

 roots, which remain distinctty separated throughout most of their pas- 

 sage through the pons, join each other above and become closely entan- 

 gled by the interweaving of their bundles ; though their fibres may still 

 be distinguished, on microscopic examination, by the generally larger 

 size of those belonging to the motor root. They finally reach a collec- 

 tion of gray matter, the u nucleus of the fifth nerve," which is situated 



