934 THE NERVOUS SYSTEM 



enters the orbit above the lateral rectus and immediately turns medialward 

 between the periosteum of the roof of the orbit and the levator palpebrse superioris. 

 At the medial border of the roof it turns forward to its termination, and enters 

 the orbital or superior surface of the superior oblique muscle to which its fibres 

 are distributed. 



The central connections of the nucleus of the trochlear nerve are similar to those of the 

 oculo-motor save that its cells probably do not send fibres which connect with the facial nerve. 



The trochlear is peculiar in that — (1) it is the smallest of the cranial nerves; (2) it is the 

 only nerve having its superficial attachment upon the dorsal aspect of the encephalon; (3) it 

 is the only cranial nerve whose fibres undergo a total decussation, and (4) in that it terminates 

 in a muscle of the side of the body opposite that in which it has its origin. Gaskell has suggested 

 that this latter condition has probably been brought about, phylogenetically, by the trans- 

 ference of the muscles which have carried their nerves with them. It should be remembered 

 that most of the fibres arising from the medial group of the cells of the nucleus of the oculo-motor, 

 cross the opposite side. This is thought to be especially true for those supplying the medial 

 rectus muscle. 



THE ABDUCENS 



The abducens (or sixth nerve) on each side arises from the cells of a nucleus 

 which lies in the grey substance of the floor of the fourth ventricle in the region of 

 the inferior part of the pons. The nucleus is situated close to the middle line, 

 ventral to the acoustic medullary striae and beneath the colliculus facialis and 

 it is in direct linear series with the nuclei of the oculo-motor, trochlear and hypo- 

 glossal nerves. It is the third of the eye-moving nerves. The fibres which pass 

 from the nucleus into the nerve run inferiorly and ventralward through the ret- 

 icular formation, the trapezium, and the pyramidal fasciculi, and they emerge 

 from the ventral surface of the medulla in the sulcus at the inferior border of the 

 pons and the upper end of the pyramid of the medulla. From this superficial 

 attachment the nerve runs upward and forward in the subarachnoid space between 

 the pons and the basisphenoid and at the side of the basilar artery. A little below 

 the level of the upper border of the petrous portion of the temporal bone it pierces 

 the dura mater, passes beneath the petro-sphenoidal hgament, at the side of the 

 dorsum sellse, and enters the cavernous sinus, in which it runs forward along the 

 lateral side of the internal carotid artery. At the anterior end of the sinus it 

 passes through the superior orbital (sphenoidal) fissure between the heads of 

 the rectus lateralis, below the inferior branch of the oculo-motor nerve, and above 

 the ophthalmic vein. In the orbit it runs forward on the inner or ocular surface 

 of the rectus lateralis, and finally it pierces this muscle and terminates upon its 

 fibres. 



While it is in the cavernous sinus it receives communications from the carotid 

 plexus of the sympathetic and from the ophthalmic nerve. 



All the fibres arising in the nucleus of the sixth nerve do not pass into the sixth nerve. Some 

 of them ascend in the medial longitudinal fasciculus of the same and opposite sides, and ter- 

 minate about cells of the medial group of the nucleus of the oculo-motor nerve, by which the 

 impulses are conveyed to the opposite medial rectus muscle. Thus impulses reaching the abdu- 

 cens nucleus can throw into simultaneous action the lateral rectus of the same side and the 

 medial rectus of the opposite side, and thus turn both eyes in the same direction. 



Central connections. — The nucleus of the abducens receives impulses from the anterior 

 central gyrus of the opposite side by the pyramidal fibres, and it is associated with the sensory 

 nuclei of other nerves by way of the medial longitudinal fasciculus, and that of the trigeminus 

 especially through the reticular formation. 



THE TRIGEMINUS 



The trigeminus is the largest of the cranial nerves with the exception of the 

 optic. It is usually described as the fifth cranial nerve and as possessing both a 

 sensory and a motor root. For reasons already given, the "motor root" is here 

 described separately and given the separate name, masticator nerve. The 

 fibres of the trigeminus, which are all sensory, spring from the cells of the semi- 

 lunar (Gassorian) ganglion, which corresponds with the ganglion of the dorsal 

 root of a spinal nerve, and they enter the brain stem through the side of the 

 anterior third of the pons. 



