590 



THE NEEVOUS SYSTEM. 



with that part of the oculomotor nucleus from which the nerve for the medial 

 rectus of the opposite side derives its fibres. If this view is correct, it affords 

 a ready and simple anatomical explanation of the harmonious action of the 

 lateral and medial recti muscles in producing movements of the two eyeballs 

 simultaneously to the right and to the left. From the investigations of E. H. 

 Fraser it would appear that no fibres from the abducens nucleus go directly into 

 the oculomotor nerve. The same observer has shown that many fibres from 

 Deiters' nucleus, a part of the vestibular nucleus of the acoustic nerve to be 

 described later in this account, enter the oculomotor and the trochlear nuclei 

 through the path afforded by the medial longitudinal bundle. 



Optic tract 



Posterior commissure 



Nucleus 

 hypoglossi 



Nucleus 

 gracilis 



Anterior 

 column of 

 spinal 

 medulla 



FIG. 524. DIAGRAM REPRESENTING SOME OF 

 THE CONSTITUENT ELEMENTS OF THE 

 FASCICULUS LONGITUDINALIS MEDIALIS. 



FIG. 525. DIAGRAM OF THE CONNEXIONS ( 

 THE MEDIAL LEMNISCDS AND ALSO OF CER 



TAIN OF THE THALAMO-CORTICAL FIBRES. 



Lemniscus Lateralis. The lateral lemniscus is a definite tract of longitudinal 

 fibres, which extends upwards through the lateral part of the tegmental substance 

 of the superior portion of the pons and the mesencephalon. It is formed by the 

 fibres of the corpus trapezoideum and striae medullares in the inferior part of the 

 pons, turning abruptly upwards and taking a course towards the quadrigemina] 

 region. But the details of the arrangement and connexions of this important 

 fasciculus must be left for fuller consideration when we are discussing the central 

 connexions of the acoustic nerve. 



Lemniscus Medialis. The medial lemniscus has already been followed through 

 the medulla oblongata and pons, and its position in each of these portions of the 

 brain-stem has been defined (pp. 561 and 562). In the tegmentum of the inferioi 

 part of the mesencephalon it is carried up in the form of a more or less flattened 



