710 THE NEBVES. 



one side or the other, is necessary for the unity of perception of visual 

 objects ; if one of the two eyes remained fixed while the other turned on its 

 axis, we would perceive two images, a superior corresponding to the healthy 

 eye, and an inferior to the diseased one. These two images are visible 

 when the head is vertical, and particularly when it is inclined to the affected 

 side , they are confounded into a single one when the head is carried to 

 the healthy side." " 



The involuntary action of the oblique muscles of the eye in this rotatory 

 movement strongly attracts atttention to the nerves which these muscles 

 receive, and stimulates a desire to learn the particular conditions which 

 permit them to act as excito-motors independently of the will ; although they 

 as well as the muscles to which they are destined, belong to those of animal 

 life. In the present state of science, nothing positive can be affirmed on so 

 delicate a subject. There are, nevertheless, two interesting remarks to 

 make the pathetic nerve is exclusively destined to the superior oblique 

 muscle, and the long branch sent by the common ocnlo-motor nerve, to the 

 inferior oblique does not give any filament to the neighbouring parts. This 

 branch is, therefore, also the exclusive nerve of the inferior oblique, and may 

 bo considered as a second pathetic. 



^Sir Charles Bell designated the fourth nerve the " respiratory nerve of 

 the eye," and asserted that it was large in all animals capable of much 

 expression.) 



5. Fifth Pair, or Trigeminii. (Figs. 110, 322, 335, 336, 337, 338, 342.) 



The nerve we are about to describe has also been named by Chaussier 

 the trifacial nerve. It is distinguished among all the cranial nerves by its 

 enormous volume, the multiplicity of its branches, the variety of its uses, 

 and its connections with the great sympathetic system. It therefore 

 requires to be described as completely as possible ; and in this description 

 we will include the study of the cephalic ganglia of the great sympathetic 

 system, which ought to be regarded as annexes of the fifth pair. 



Origin. — The trigeminus belongs to the category of mixed nerves, as it 

 possesses two roots — one sensitive, the other motor. 



Sensitiiie Boot (Figs. 337, 338, i). — This is the largest root. It emanates 

 from the outside of the pons Varolii, near the middle cerebellar peduncle, 

 and is directed forward and downward to gain the anterior portion of the 

 foramen lacerum, where it terminates in a very great semilunar enlargement 

 constituting the Gasserian ganjlion. Flattened from above to below, and 

 wider in front than behind, this root on the outer side is about 6-lOths of 

 an inch in length, but the inner side is double that measurement because of 

 the oblique position of the ganglion which continues it. 



If it be traced into the substance of the pons, it will be found that the 

 fibres of the latter separate for its passage from the deep plane it at first 

 occupies. The following is the manner in which it comports itself in this 

 plane : — This root is separated into two orders of fibres, posterior and 

 anterior. The first pass beneath the arciform fasciculi of the pons Varolii, 

 to be continued with the restiform body, and consequently with the pos- 

 terior columns of the spinal cord ; the second separate from each other, and 

 soon Lbeeome confounded with the cells amassed in the interior of the 

 isthmus, at the anterior cerebellar peduncles, and above the intermediate 

 fasciculus .of the medulla oblongata. The fibres of the trigeminus, or the cells 



' Sappey. ' Anatomie Descriptive.' 



