SIXTH PAIR OF NERVES. Le Sixieme Nell, Fr. ; Sechster Nerv, Germ. According to the enumeration of Willis, this name is bestowed upon a single soft round cord, which is, with the exception of the fourth, the smallest of the cranial nerves, and which, passing forwards from the medulla oblongata to the external rectus of the eye, finds its distribution in this muscle.
The anatomy of this nerve is readily sub divided into three portions. The first of these extends frorn its apparent origin to the point where it enters the cavernous sinus ; the second includes its course in that cavity ; and thc third, commencing at the sphenoidal fissure or foramen lacerum anticus, contains the course of the nerve in the orbit, and is terminated by its distribution.
The visible origin of the nerve is by one or two bundles from the medulla oblongata, from the anterior pyramid of which it ap pears at its upper part, or in the transverse depression immediately behind the posterior border of the pons varolii. By careful dis section, the nerve can be traced into the sub stance of this anterior column, and, appa rently, it passes through it towards the grey matter which more deeply surrounds this tract of the medulla. Further than this it is impossible to follow it satisfactorily, although some anatomists have, with Mayo, assigned to it a yet deeper origin. In thefirst part of its course the nerve passes forwards, upwards, and outwards for a very short distance, from near the median line to the posterior extre mity of the cavernous sinus which forms the commencement of the inferior petrosal sinus. In this course it lies upon the concave basilar surface of the sphenoid bone, and is covered above by the projecting pons varolii ; and at the front, where it leave.s the interior of' the skull, the arachnoid membrane is reflected around it. It next passes through an opening in the dura mater, and enters the cavity of the sinus. This aperture is situated just in ternal to the tip of the petrous bone, and is about one-third of an inch anterior to the orifice of the fifth nerve, bnt on a rather lower level.
On entering the sinus, it is somewhat curved or bent into a more horizontal direction, and crosses over the posterior or vertical part of the carotid artery, which here experiences its sigmoid bend by the side of the body of the sphenoid bone. It next lies parallel to, but beneath, the horizontal part of this vessel, and passes almost directly forwards, through and amongst the numerous reticulations which occupy the cavity of the sinus, but it is covered by its lining membrane. At the anterior extremity of the cavernous sinus it enters the orbit by passing betvveen the two heads or processes of origin of the external rectus muscle.
Since the nerve in this course lies within the sinus, it is internal to the three nerves, viz. the third, fourth, and the ophthalmic division of the fifth, which are situated in the dura mater forming its outer wall. Poste riorly, the lowest of these, or the ophthalmic nerve, lies on much the same level, but nearer to the sphenoidal fissure. The latter nerve having passed upwards, the sixth is left again occupying the most inferior and internal position of all the nerves which pass through this orifice, the lower division of the third being to its inner side, and somewhat superior to it, whilst above this is the nasal branch of the fifth. Below the sixth nerve, the oph thalmic vein perforates the dura mater of the sinus by a separate aperture, In the cavernous sinus, the following branches are connected with, or come from, this nerve :— 1. It is connected with the sympathetic nerve by several filaments. Two of these are of considerable size, and may be traced back wards at rather an acute angle from the trunk of the nerve, to join those numerous rami fications of the sympathetic which constitute the carotid plexus surrounding the artery in this venous cavity.