594 THE CRANIAL NERVES. 



After receiving one or two delicate filaments from the cavernous plexus of 

 the sympathetic, the third nerve divides near the orbit into two parts, which 

 are continued into that cavity between the heads of the external rectus 

 muscle, and separated one from the other by the nasal branch of the 

 ophthalmic nerve. 



The upper, the smaller part, is directed inwards over the optic nerve to 

 the superior rectus muscle of the eye and the elevator of the eyelid, to both 

 which muscles it furnishes branches. 



The lower and larger portion of the nerve separates into three branches ; 

 of these one reaches the inner rectus ; another the lower rectus ; and the 

 third, the longest of the three, runs onwards between the lower and the 

 outer rectus, and terminates below the ball of the eye in the inferior 

 oblique muscle. The last-mentioned branch is connected with the lower 

 part of the lenticular ganglion by a short thick cord, and gives two filaments 

 to the lower rectus muscle. 



The several branches of the third nerve enter the muscles to which they 

 are distributed on the surface which in each looks towards the eyeball 



POSITION OP CERTAIN NERVES at the cavernous sinus, and as they enter the 

 orbit. There are several nerves, besides the third, placed close together at 

 the cavernous sinus, and entering the orbit through the sphenoidal fissure. 

 To avoid repetition hereafter, the relative positions of these nerves may 

 now be described. The nerves thus associated are the third, the fourth, 

 the ophthalmic division of the fifth, and the sixth. 



At the cavernous sinus. In the dura mater which bounds the cavernous 

 sinus on the outer side, the third and fourth nerves and the ophthalmic 

 division of the fifth are placed, as regards one another, in their numerical 

 order both from above downwards and from within outwards. The sixth 

 nerve is placed separately from the others close to the carotid artery, 

 on the floor of the sinus and internally to the fifth nerve. Near the sphe- 

 noidal fissure, through which they enter the orbit, the relative position of 

 the nerves is changed, the sixth nerve being here close to the rest, and 

 their number is augmented by the division of the third and the ophthalmic 

 nerves the former into two, the latter into three parts. 



In the sphenoidal fissure. The fourth and the frontal and lachrymal 

 branches of the fifth, which are here higher than the rest, lie on the same 

 level, the fourth being the nearest to the inner side, and enter the orbit 

 above the muscles. The remaining nerves pass between the heads of the 

 outer rectus muscle, in the following relative position to each other ; the 

 upper division of the third highest, the nasal branch of the fifth next, the 

 lower division of the third beneath these, and the sixth lowest of all. 



FOURTH PAIR OF NERVES. 



The fourth (nervus trochlearis, n. patheticus) is the smallest of the cranial 

 nerves, and is distributed entirely to the upper oblique muscle of the 

 orbit. 



From the remoteness of its place of origin, the part of this nerve within 

 the skull is longer than that of any other cranial nerve. It enters an aperture 

 in the free border of the tentorium, outside that for the third nerve, and 

 near the posterior clinoid process. Continuing onwards through the outer 

 wall of the cavernous sinus, the fourth nerve enters the orbit by the 

 sphenoidal fissure, and above the muscles. Its position with reference to 

 other nerves in this part of its course has been already described. 



