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 strise 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 fibers which pass from the nucleus into the nerve run inferiorly and ventralward through the reticular 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 ligament, at the side of the dorsum sellte, 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 fibers.

While it is in the cavernous sinus it receives communications from the carotid plexus of the sympathetic and from the ophthalmic nerve.

All the fibers 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 terminate 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 abducens 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 ej'es in the same direction.

Central connections

The nucleus of the abducens receives impulses from the anterior central gyrus of the opposite side by the pj'ramidal fibers, and it is associated with the sensory nuclei of other nerves by way of the medial longitudinal fasciculus, and that of the trigemiuus especially through the reticular formation.

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