THE CRANIAL OR ENCEPHALIC SERVES. 709 



with the sixth pair and the ophthalmic branch of the trigeminal nerve, into the 

 smallest of the great suprasphenoidal foramina. The common oculo-motor 

 nerve afterwards arrives, by the orbital hiatus, at the bottom of the ocular 

 sheath, where it separates into several branches destined to the follo.wing 

 muscles of the eye : the elevator of the upper eyelid, superior rectus, 

 internal rectus, inferior rectus, posterior rectus except its internal fasciculus 

 and the small oblique. The branch to the latter is remarkable for its great 

 length ; it reaches its destination in passing to the outside of, and then below 

 the inferior rectus. The motor roots of the ophthalmic ganglion furnished 

 by this nerve, are given off from the same point as the branch for the small 

 oblique muscle. 



The nerves of, the third pair are purely motor, as is shown by their 

 connections with the inferior plane of the cerebral peduncles, and their 

 exclusive distribution to contractile organs. They incite all the muscles 

 lodged in the ocular sheath, except the external rectus, the great oblique, 

 and posterior rectus. 



(It also sends a motor filament to the lenticular ganglion, supplies the 

 circular muscular fibres of the iris and ciliary muscle, and presides over 

 contraction of the pupil.) 



4. Fourth Pair, or Pathetici. (Fig. 335, 6.) 



The pathetic or internal oculo-motor (or trochlearis) nerve is the smallest 

 of all the cranial nerves. Its description is extremely simple. It arises 

 from the band of Eeil, immediately behind the corpora quadrigemina, by 

 two short roots, which it has been impossible for me to follow very deeply 

 in the substance of the isthmus. 



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 trigemini, accompanying it to the suprasphenoidal foramina, 

 the smallest of which it enters. 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, supplying that organ 

 with the stimulant principle of muscular contractility. 



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 

 I 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 relationship 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 

 )blique ; when the head is inclined on the left shoulder, an inverse move- 

 lent takes place in the two eyes. This simultaneous rotation of the ocular 

 lobes around their antero-posterior diameter, when the head is inclined to 

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