SIXTH PAIR OF NERVES. 



621 



rotation, as was mentioned to be the case in 

 the former example. 



We must agree with the author that the 

 phenomena noticed in this rare and remark- 

 able case originated neither in disease, nor 

 were they the result of accident. The com- 

 plete absence of a glenoid cavity in the nor- 

 mal situation for it, the existence of the 

 malformation on both sides, the perfect 

 resemblance to each other of the abnormal 

 sockets, in form, size, and position, the in- 

 tegrity of the tendon of the biceps and of the 

 capsular and glenoid ligaments, and the pe- 

 culiar form of the head of the humerus, as 

 well as of the acromion process of the sca- 

 pula on each side, all support the opinion that 

 the malformation was intra-uterine and con- 

 genital. 



(Robert Adams.} 



SIXTH PAIR OF NERVES. Le Sixieme 

 Nerf, 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 from its apparent origin to the 

 point where it enters the cavernous sinus ; 

 the second includes its course in that cavity ; 

 and the 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 the first 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 leaves 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 ori 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 between 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. 



2. An anastomosis, or junction with the 

 ophthalmic branch of the fifth, is described by 

 most anatomists, and may be readily verified 

 in the recent subject. One or more branches, 

 having very much the same direction and ap- 

 pearance with the preceding to the sympa- 

 thetic, pass backwards from the sixth nerve, 

 in the anterior part of the sinus ; leaving it at 

 a very acute angle, inclining outwards as they 

 go, and finally, entering the wall of the sinus 

 to join .the ophthalmic branch, not far from 

 the Gasserian ganglion. These branches also 

 exist in the sheep, and some other of the lower 

 animals.* 



* A very similar description might be extended 

 to the analogous junction of the fourth nerve with 

 this division of the fifth. Thus, in the sheep, three 

 or even four considerable branches leave the ophthal- 

 mic nerve at an acute angle to join the fourth 

 nerve. They effect this junction very obliquely, 

 and may be traced forwards (distad ) for at least 

 some distance. May not some of these filaments, 

 traced backwards from the fourth nerve to the wall 

 of the cavernous sinus, which they enter to join the 

 ophthalmic division, have been the tentorial branches 

 of Bidder, which he describes as coming from the 

 former nerve to be distributed to the dura mater of 

 the tentorium ? 



