THE TROCHLEAR NERVES 



933 



Some of the fibres which spring from the medial portion of the oculo-motor nucleus do not 

 pass into the nerve of the same side, but into that of the opposite side, and it is beheved that 

 they are distributed to the opposite medial rectus muscle. Other fibres which arise from the 

 nucleus descend in the medial longitudinal fasciculus and either terminate about the cells of 

 the nucleus of the facial or join the facial nerve, in which they pass to the upper part of the 

 orbicularis palpebrarum. The eye is opened by the oculo-motor and closed by the facial iierve. 

 Central connections. — The nucleus of the oculo-motor is associated with the middle portion of 

 the anterior central gyrus, the posterior end of the middle frontal gj'rus and with the cortex about 

 the visual area of the occipital lobe of the opposite side of the brain by the pyramidal fibres. It is 

 probably associated with the cerebellum by the fibres in the superior cerebellar peduncles, with 

 the superior calliculus, and with the sensory nuclei of the other cranial nerves by the medial 

 longitudinal fasciculus. To produce the coordinated activities of the eye-moving muscles, it 

 must be associated with the nuclei of the trochlear and abducens. 



THE TROCHLEAR NERVES 



The fibres of each trochlear or fourth nerve (or patheticus) spring from the 

 cells of a nucleus which lies in the gre}^ substance of the floor of the cerebral aquae- 

 duct in line with the oculo-motor nucleus, but in the region of the inferior quadri- 

 geminate bodies. As the fibres pass from their origins they run ventrally and lat- 

 erally in the substance of the tegmentum for a short distance, then they curve 

 medianward and dorsalward, and, in passing through the anterior end of the supe- 

 rior medullary velum they decussate totally with the fibres of the trochlear nerve 



Fig. 733. — Diagrams of Sections through the Origin of the Trochlear Nerve. (Still- 

 ing.) (The upper figure is an obHque section, the lower is a coronal section.) 



Cerebral aqueduct — 



Nucleus of trochlear nerve 



Medial longitudinal 

 fasa;ulus 



Raphe 



Trochlear nertre 



Trochleai nerve 



Cerebral aqueduct 

 Nucleus of masticator 



Brachium conjunctivum 



Lateral lemniscus 



of the opposite side. After the decussation the fibres emerge from the surface of 

 the superior medullary velum, at the side of the frenulum veli, usually in two small 

 bundles, which pierce the pia mater and join together to form the slender trunk of 

 the nerve. This trunk curves forward and ventralward to the base of the brain 

 around the sides of the superior peduncle of the cerebellum and cerebral peduncle 

 of the side opposite to that in which the nerve originates, running parallel with and 

 between the superior cerebellar and posterior cerebral arteries. As it reaches the 

 base of the brain behind the optic tract the nerve enters the cisterna basalis, in 

 which it runs forward, immediately beneath or piercing the free border of the ten- 

 torium cerebelh, to the superior border of the petrous portion of the temporal 

 bone, where it pierces the arachnoid and the dura mater and enters the posterior 

 end of the lateral wall of the cavernous sinus. In the wall of the cavernous sinus 

 it receives communications from the cavernous plexus of the sympathetic and by 

 a small filament from the ophthalmic division of the trigeminus. It gradually 

 ascends, as it passes forward in the lateral wall of the sinus, and, beyond the 

 middle of the sinas, it crosses the lateral side of the trunk of the oculo-motor 

 nerve and gains a higher position. At the anterior end of the sinus the nerve 



