THE THIRD AND FOURTH NERVES. 



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

 distributed on the surface which in each is turned towards the eyeball, with the 

 exception of that to the inferior oblique, which penetrates the hinder border of its 

 muscle. 



The fibres of the third nerve, about 15,000 in number, are generally large ; but there are 

 some small ones, most of which pass into the ciliary ganglion. In the rootlets of the nerve 

 Thomsen and G-askell have described a peculiar structure which is regarded by them as the 

 remains of a degenerated ganglion (Virchow's Archiv, cix, 1887 ; Journ. Phys., x, 1889, 

 p. 167). 



Varieties. A communication between the third and sixth nerves as they pass the 

 cavernous sinus has been described, but its existence is denied by most recent observers. 

 The upper division of the third nerve may have a communication with the nasal (Svitzer, 

 Sommering, Testut). The third nerve has been seen in a few cases giving a branch to the 

 external rectus (Cruveilhier, Fasebeck, C. Krause). and in one instance a branch of the third 

 supplied the place of the sixth nerve which was wanting (Generali). A filament to the 

 superior oblique muscle is noticed by Volkmann. The branch to the inferior oblique muscle 

 was seen by Arnold to pass through the lower part of the lenticular ganglion, and by Henle 

 to pierce the inferior rectus. 



POSITION OF CERTAIN NERVES AT THE CAVERNOUS SINUS, AND AS THEY ENTER 

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



Fig. 150. THE NERVES IN THE OUTER WALL OF THE 



CAVERNOUS SINUS, SEEN IN TRANSVERSE SECTION. 



(Langer.) 



^ OPHTn. NERVE 



n INT. CAR. Af! 



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 in the cavity of the sinus, and internally to the 

 ophthalmic 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 sphenoidal /insure. 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 external rectus muscle, in the following order 

 from above downwards ; the upper division of the third, the nasal branch of the fifth, 

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



IV. TROCHLEAR NERVE. 



The fourth, trochlear, or pathetic nerve is the smallest of the cranial nerves, and 

 has the longest course within the cranial cavity. It is distributed solely to the 

 superior oblique muscle of the eye. 



The nucleus of the fourth nerve continues downwards the column of cells giving 

 origin to the third nerve, being placed in the ventral grey matter of the aqueduct of 



