962 THE NERVOUS SYSTEM 



As they leave the nucleus the fibres of the oculomotor nerve form a series of 

 fasciculi which curve ventrally through the red nucleus and the medial part of the 

 substantia nigra, to the oculo-motor sulcus on the inner surface of the cerebral 

 peduncle, where they emerge in from six to fifteen small bundles which pierce the pia 

 mater and collect into the trunk of the nerve. Immediately after its formation along 

 the oculo-motor sulcus, the trunk of the nerve passes between the posterior cerebral 

 and the superior cerebellar arteries, and, running downwards, forwards, and laterally 

 in the posterior part of the cisterna basalis, it crosses the anterior part of the attached 

 border of the tentorium cerebelli at the side of the dorsum sella:, and, piercing the 

 arachnoid and the inner layer of the dura mater, it enters the wall of the cavernous 

 sinus about midway between the anterior and posterior clinoid processes. Immedi- 

 ately after its entry into the wall of the sinus it lies at a higher level than the fourth 

 nerve, but the latter soon crosses on its outer side and gets above it, and directly 

 afterwards the third nerve divides into a smaller superior and a larger inferior branch 

 (fig. 701). Before its division it receives communications from the cavernous plexus 

 ofthe sympathetic about the internal carotid artery, and from the ophthalmic 

 division of the fifth nerve. Both branches proceed forwards, and the nasal 

 branch of the fifth nerve, which has passed upwards, on the outer side of the 

 inferior branch of the third nerve, lies between them. At the anterior end of the 

 cavernous sinus the two branches pass through the superior orbital (sphenoidal) 

 fissure, between the heads of the external rectus muscle, and enter the orbital 

 cavity. In the orbit, the superior branch lies between the superior rectus and the 

 optic nerve; it supplies the superior rectus and then turns round the inner border of 

 that muscle and terminates in the levator palpebra- superioris. The inferior branch 

 runs forwards, beneath the optic nerve, and divides into three branches which supply 

 the inferior and internal recti and the inferior oblique. The branch to the inferior 

 oblique muscle is connected with the ciliary ganglion by a short thick offset, the short 

 root of the ciliary ganglion, by mediation of which the oculo-motor nerve sends im- 

 pulses to the ciliary muscle and the sphincter muscle of the iris. The inferior branch 

 also gives some small twigs to the inferior rectus. The branches of the third nerve 

 which supply the recti muscles enter the muscles on their ocular surfaces, but the 

 branch to the inferior oblique muscle enters the posterior border of that muscle. 



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 believed that 

 they are distributed to the opposite internal 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 

 oculi. 



Central Connections. The nucleus of the third nerve is associated with the anterior portion 

 of the somsesthetic area 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 of the superior cerebellar peduncles, and with the sensory nuclei of the other 

 cranial nerves by the medial longitudinal fasciculus. 



THE FOURTH PAIR THE TROCHLEAR NERVES 



The fibres of each fourth or trochlear nerve spring from the cells of a nucleus which 

 lies in the grey substance of the floor of the aquseductus cerebri in line with the 

 oculo-motor nucleus, but in the region of the inferior quadrigeminate bodies. As the 

 fibres pass from their origins they run ventrally and laterally in the substance of the 

 tegmentum for a short distance, then they curve medianwards and dorsal wards, and, 

 in passing through the anterior end of the superior medullary velum they decussate 

 totally with the fibres of the fourth nerve 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 forwards and 

 ventralwards 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 forwards, immediately beneath or 

 piercing the free border of the tentorium cerebelli, to the superior border of the 

 petrous portion of the temporal bone, where it pierces the arachnoid and the dura 



