608 



CRANIAL NERVES. 



THIRD NERVE. (Figs. 339, 340.) 



The Third Nerve (motor oculi) supplies all the muscles of the eyeball except 

 the Superior Oblique and External Rectus; it also sends motor filaments to the 

 iris. It is a rather large nerve, of rounded form and firm texture, having its 

 apparent origin from the inner surface of the crus cerebri, immediately in front 

 of the pous Varolii. 



The deep origin may be traced into the substance of the crus, where some of 

 its fibres are connected with the locus niger; others run downwards, among the 



longitudinal fibres of the pons; 



Fig. 339. Nerves of the Orbit. Seen from above. whilst others ascend, to be con- 



nected with the tubercula quad- 

 rigemina, and valveof Vieussens. 

 According the Stilling, the fibres 

 of the nerve pierce the peduncle 

 and locus niger, and arise from 

 a gray nucleus in the floor of the 

 aquaeduct of Sylvius. One merg- 

 ing from the brain, the nerve is 

 invested with a sheath of pia 

 mater, and inclosed in a prolon- 

 gation from the arachnoid. It 

 then pierces the dura mater on 

 the outer side of the anterior 

 clinoid process, where its serous 

 covering is reflected from it, and 

 it passes along the outer wall of 

 the cavernous sinus, above the 

 other orbital nerves, receiving in 

 its course one or two filaments 

 from the cavernous plexus of the 

 sympathetic. It then divides 

 into two branches, which enter 

 the orbit through the sphenoidal 

 fissure, between the two heads 

 of the External Rectus muscle. 

 On passing through the fissure, 

 the nerve is placed below the 

 fourth, and the frontal and lach- 

 rymal branches of the ophthal- 

 mic nerve. 



The superior division, the 



smaller, passes inwards across the optic nerve, and supplies the Superior Kectus 

 and Levator Palpebrae. 



The inferior division, the larger, divides into three branches. One passes 

 beneath the optic nerve to the Internal Rectus; another to the Inferior Rectus; 

 ami the third, the largest of the three, passes forwards between the Inferior 

 and External Recti, to the Inferior Oblique. From the latter, a short thick 

 branch is given off to the lower part of the lenticular ganglion, forming its 

 inferior root, as well as two filaments to the Inferior Rectus. All these branches 

 enter the muscles on their ocular surface. 



rriirrrnt Ft'Litarnt 

 lu Dura-Matt r 



FOURTH NERVE. (Fig. 339.) *. 



The Fourth, or Trochlear Nerve, the smallest of the cranial nerves, supplies 

 the Superior Oblique muscle. It arises from the upper part of the valve of 

 Vieussens, immediately behind the testis, and divides, beneath the corpora 

 quadrigernina, into two fasciculi ; the anterior one arising from a nucleus of 



