THE TROCHLEAR NERVE. 



1229 



Leaving the upper and lateral part of the nucleus as axones of the trochlear neurones, 

 the strands of fibres pass outward and backward within the gray matter of the floor 

 of the aqueduct until they near the inner concave surface of the mesencephalic root 

 of the fifth nerve, which, after being condensed into one or two bundles, they follow 

 downward as far as the superior extremity of the fourth ventricle. Then bending 

 sharply medially, the fourth nerve, so far as the great majority of its fibres are 

 concerned, enters the superior medullary velum, in which it decussates with its fellow 

 of the opposite side and crosses the mid-line to emerge at its superficial origin on 

 the dorsal surface of the brain-stem (Fig. 957) just below the inferior corpora quad- 

 rigemina, between the frenum of the velum and the mesial border of the superior 

 cerebellar peduncle. 



Cortical and Central Connections. — The trochlear nucleus is directly connected wifti the 

 cerebral cortex by fibres which descend from the inferior frontal convolution through the corona 

 radiata, the internal capsule and the cerebral peduncle and cross to the nuc'eus of the opposite 



Fig. 1053. 



Olfactory tracts 

 Optic chiasm 

 Gasserian ganglion 

 Int. carotid artery 

 in. nerve 



IV. nerve 



Crura cerebri 



Floor of 

 IV. ventricle" 



XI. nerve, 

 spinal portion' 



Middle peduncle 

 of cerebellum 



Medulla 

 oblongata 



Frontal nerve 



Supratrochleai 

 nerve 



Supraorbital 

 nerve 



Levator pal- 

 pebrae superioris 



Rectussuperior 



Lachrymal nerve 



Rectusexternus 

 Br. of communi- 

 cation bet.lachry. 

 nial and teinporo. 

 n.alar br. niaxil* 

 lary nerve 

 Malar br. tempo 

 ro-malar nerve 

 Temporal br. 

 temporo-malar 

 nerve 



XI. nerve 



X. nerve 



IX. nerve \ \ VII. nerve 



VIII. nerve Temporal bone, cut 



Ophthalmic div. V. nerve 

 Maxillary div. V. nerve 

 Mandibular div. V. nerve 



Geniculate ganglion of VII. nerve (a part of great 

 superficial petrosal nerve is seen passing beneath 

 Gasserian ganglion) 



Dissection showing right trochlear nerve throughout its length, also oculomotor and frontal and lachrymal branches 

 of trigeminal nerve ; roof and outer wall of orbit have been removed. 



side. By means of the posterior longitudinal bundle it is brought into relation with the nucleus 

 of the third and of the sixth nerve, thus insuring harmonious action of the eye muscles; further, 

 by means of the same path, it is probably connected with the auditory nuclei by way of the 

 superior olive and its peduncle. 



Course and Distribution. — Emerging at its superficial origin, the nerve is 

 directed outward over the superior cerebellar peduncle, then winds forward around 

 the outer surface of the cerebral peduncle, parallel to and between the posterior 

 cerebral and superior cerebellar arteries, and appears at the base of the brain (Fig. 

 1053). Proceeding forward to the floor of the cranium, the nerve enters the dura 

 immediately beneath the free border of the tentorium, slightly behmd and external 

 to the posterior cUnoid process and the third nerve, and continues in the outer wall 

 of the cavernous sinus, at first having the third nerve above it and the ophthalmic 

 division of the fifth below, and then crossing above the third from below inward, to 

 gain the medial end of the sphenoidal fissure. It enters the orbit above the heads of 



