

THE TROCHLEAR NERVE 



885 



of and lateral to the posterior clinoid process, passing between the free and attached 

 borders of the tentorium cerebelli. It runs along the lateral 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, and a communicating branch from 

 the ophthalmic division of the trigeminal. It then divides into two branches, 

 which enter the orbit through the superior orbital fissure, between the two heads 

 of the Rectus lateralis. Here the nerve is placed below the trochlear nerve and 

 the frontal and lacrimal branches of the ophthalmic nerve, \vhile the nasociliary 

 nerve is placed between its two rami. 



The superior ramus, the smaller, passes medialward over the optic nerve, and 

 supplies the Rectus superior and Levator palpebra? superioris. The inferior ramus, 

 the larger, divides into three branches. One passes beneath the optic nerve to the 

 Rectus medialis; another, to the Rectus inferior; the third and longest runs for- 

 ward between the.Recti inferior and lateralis to the Obliquus inferior. From 

 the last a short thick branch is given off to the lower part of the ciliary ganglion, 

 and forms its short root. All these branches enter the muscles on their ocular 

 surfaces, with the exception of the nerve to the Obliquus inferior, which enters 

 the muscle at its posterior border. 



THE TROCHLEAR NERVE (N. TROCHLEARIS; FOURTH NERVE) (Fig. 776). 



The trochlear nerve, the smallest of the cranial nerves, supplies the Obliquus 

 superior oculi. 



It arises from a nucleus situated in the 

 floor of the cerebral aqueduct, opposite 

 the upper part of the inferior colliculus. 

 From its origin it runs downward through 

 the tegmentum, and then turns backward 

 into the upper part of the anterior medul- 

 lary velum. Here it decussates w r ith its 

 fellow of the opposite side and emerges 

 from the surface of the velum at the side 

 of the frenulum veli, immediately behind 

 the inferior colliculus. 



The nerve is directed across the super- 

 ior cerebellar peduncle, and then winds 

 forward around the cerebral peduncle, 

 immediately above the pons, pierces the 

 dura mater in the free border of the ten- 

 torium cerebelli, just behind, and lateral 

 to, the posterior clinoid process, and 

 passes forward in the lateral wall of the 

 cavernous sinus, between the oculomotor 

 nerve and the ophthalmic division of the 

 trigeminal. It crosses the oculomotor 

 nerve, and enters the orbit through the 

 superior orbital fissure. It now becomes 

 the highest of all the nerves, and lies 

 medial to the frontal nerve. In the orbit 

 it passes medialward, above the origin of 

 the Levator palpebra? .superioris, and 

 finally enters the orbital surface of the 

 Obliquus superior. 



In the lateral wall of the cavernous sinus the trochlear nerve forms communica~ 

 tions with the ophthalmic division of the trigeminal and with the cavernous plexus 



Motor root 

 Sensory root' 

 Fio. 776. Nerves of the orbit. Seen from above. 



