THE CRANIAL OB ENCEPHALIC NERVES. 813 



Bide, it is directed outwards, downwards, and forwards, to disengage itself from 

 the deep position it at first occupies, and lies beside the superior branch of the 

 trigeminus, accompanying it to the supra-sphenoidal foramina, the smallest of which 

 it enters (pathetic canal). This opening is exclusively intended for it, and carries 

 it to the bottom of the ocular sheath, when it gains the deep face of the great 

 oblique muscle, in which it ramifies. 



The physiological study of this nerve gives rise to some very interesting 

 remarks, which we will sum up here in a few words. The two oblique muscles of 

 the eye pivot the ocular globe in the orbit, without causing the slightest deviation 

 either upwards, downwards, or otherwise, of the pupillary opening. But this 

 rotatory movement is altogether involuntary, and is only accomplished in certain 

 determinate conditions. " Guerin, Szokalski, Hueck, and Helie have remarked, 

 that when the head is alternately inclined to the right or left, while the vision is 

 fixed on any object, the ocular globes describe around their antero-posterior axis 

 an inverse rotatory movement that has the effect of preserving a constant relation- 

 ship between the object from which the luminous rays proceed and the two 

 retinae. In this rotatory motion, the great oblique muscle of one side has for its 

 congener the small oblique of the other side : thus, when the head is inclined on 

 the right shoulder, the right eye revolves inwards and downwards on its axis, 

 under the influence of the superior oblique muscle, while the left eye turns on 

 itself outwards and downwards, through the action of the inferior oblique ; when 

 the head is inclined on the left shoulder, an inverse movement takes place in the 

 two eyes. This simultaneous rotation of the eyes around their antero-posterior 

 diameter, when the head is inclined to 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 should perceive two images — a superior corre- 

 sponding 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 merged in 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 attention 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 which they supply, 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 excliisively 

 destined to the superior oblique muscle, and the long branch sent by the common 

 oculo-motor nerve to the inferior oblique does not give any filament to the neigh- 

 bouring parts. This branch is, therefore, also the exclusive nerve of the inferior 

 oblique, and may be 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 Paie, or Trigeminal Nerves (Figs. 451, 452, 453, 454). 



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 com- 



' Sappey, Anatomie Descriptive. 



54 



