THE NERVOUS SYSTEM I4S5 



arising from them. It consists of several groups of cells. As 

 many as seven groups are ascribed to each oculo-motor nucleus 

 by Perlia, which correspond to the seven muscles supplied by the 

 oculo-motor nerve, and are disposed symmetrically. In addition to 

 these, there is a mesially-placed group, the cells of which furnish 

 fibres to both ociilo-motor nerves. The fibres which arise from 

 the individual groups of each nucleus are regarded as supplying 

 particular orbital muscles. Certain of the oculo-m.otor fibres of 

 one side arise from the nucleus of the opposite side, the fibres from 

 either side decussating at the median line. Moreover, each oculo- 

 motor nerve is said to receive fibres from the abducent nucleu; of 

 the opposite side, which ascend in the posterior longitudinal bundle 

 and cross to the other side. 



Two views are entertained in regard to the nerve supply of the 

 internal rectus muscle. According to one view, the muscle of one 

 side is supplied by those fibres which have crossed from the oculo- 

 motor nucleus of the opposite side. The other view is that the 

 muscle of one side is supplied by those fibres which have crossed 

 from the abducent nucleus of the opposite side. According to 

 this latter \-iew the nerve-fibres which supply the external rectus 

 muscle of one side and those which supply the internal rectus 

 muscle of the opposite side arise from the same nucleus, namely, 

 the abducent nucleus, and vice versa. 



Fibres are conveyed by the posterior longitudinal bundle from 

 the oculo-motor nucleus to the facial ner\-e, and thence to the 

 orbicularis palpebrarum and corrugator supercilii muscles. 



Course of the Fibres of the Third Nerve. — The fibres pass forwards 

 from their origin through the tegmentum, the red nucleus, and the 

 inner portion of the substantia nigra, and thereafter make their 

 superficial appearance at the oculo-motor sulcus on the inner 

 aspect of the crus cerebri. 



The third or oculo-motor nerve supplies the following seven 

 muscles : the levator palpebrae superioris ; the superior, inferior 

 and internal recti ; the inferior oblique ; the sphincter pupillae ; and 

 the ciliary muscle. 



Fourth, Trochlear, or Pathetic Nerve. — The fibres of this ner\'e 

 arise from the trochlear nucleus, which is situated in the grey matter 

 of the ventral aspect (floor) of the aqueduct of Syhnus. on a level 

 with the upper part of the lower quadrigeminal body. The nucleus 

 is intimately related to the posterior longitudinal bundle, through 

 means of which it is connected with the oculo-motor nucleus. 



The fibres are at first directed outwards and backwards, and then 

 downwards and mwards to the upper part of the superior medullary 

 velvm, or valve of Vieussens, which they enter. Here the ner\'e 

 crosses to the opposite side, decussating with its feUow. after which 

 it emerges from the upper end of the superior medullary velum, 

 close below the lower quadrigeminal body, and by the side of the 

 frenulum veli. Thereafter the ner^-e turns outwards over the 

 superior pedimcle of the cerebellum, and is then directed forwards 



