530 THE CEAXIAL NERVES. 



and are separated one from the other bj^ the uasal brancli of the oph- 

 thalmic nerve. 



The iqipej-, 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 of 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 W"hich 

 they are distributed on the surface "which in each is turned towards 

 the eyeball. 



Varieties. — A branch of tlae third nerve has been found supplying the external 

 rectus in a case in which the sixth nerve was wanting (Generali, quoted by 

 Henle). The branch to tlie inferior oblique muscle was seen by Arnold to pass 

 through the lowe- part of the lenticular ganglion ; and by Henle to piereo the 

 inferior rectus. 



In the outer wall of the sinus the third nerve is said to be connected with the 

 fii-st division of the fifth nerve, and with the cavernous plexus of thespnpathetic. 



Position of certain nerves at the cavernous sinus, and as 

 THEY ENTER THE ORBIT. — There are several nerves, besides the third, 

 pilaced close together at the cavernous sinus, and entering the orbit 

 through the sphenoidal fissure. To avoid repetition hereafter, the rela- 

 tive positions of these nerves may now be described. The nerves thus 

 associated are the third, the fourth, the ophthalmic divisions 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 out- 

 wards. 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 sphenoidal 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 sj)hpnoidal 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 lemaining nerves pass between the 

 heads of the outer rectus muscle, in the following order from above 

 downwards ; the upper division of the third, the nasal branch of the 

 fifth, the lower dinsion of the third, and, lowest of all, the sixth. 



FOURTH PAIR OF NERVES. 



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

 cranial nerves, and is distributed entirely to the upper obhque muscle 

 of the orbit. 



Surface attachment. — Each nerve appears at the outer side of the 



