562 THE NEKVOUS SYSTEM. 



group, until a very large part of each half of the medulla is seen to be traversed by the; 

 arcuate fasciculi (Fig. 495). The internal arcuate fibres decussate in the median plai 

 with the internal arcuate fibres of the opposite side. They then change their directk 

 and turn upwards, and the lemniscus, as already stated, takes form and gradual 

 increases in volume as it ascends. This great and important tract is thus laid do\* 

 between the pyramid and the fasciculus longitudinalis medialis ; and the consequence 

 this is that the latter tract is pushed still farther backwards, and, when the lemnisci 

 is fully established, it comes to lie immediately beneath the gray matter of the floor 

 the fourth ventricle (Fig. 495). But the lemniscus is not in direct contact with ill 

 fasciculus longitudinalis, for a bundle of fibres, the continuation of which has ber 

 seen in the anterior funiculus of the medulla spinalis, the fasciculus tectospinali 

 separates them, as well as fibres coming from sensory nuclei of the cerebral nerves whi< 

 are crossing the raphe' to join the medial lemniscus (Fig. 495). 



It is important that we should realise at this stage the full significance of tl 

 decussation of the lemniscus and have a clear conception of the connexions of tK 

 fibres which take part in it. The funiculus posterior, which ends in the cuneate ai 



fracile nuclei, is derived from the posterior roots of the spinal nerves. The lemnisc 

 bres therefore carry on the continuity of part of the posterior funiculus, fr 

 gracile and cuneate nuclei, which are thrown across its path in the lower part 

 the medulla oblongata, constituting merely a nodal interruption. At this point t 

 lemniscus is transferred to the opposite side of the medulla oblongata. But it w . 

 be remembered that a large proportion of the fibres of the entering posterior ner\ 

 roots of the spinal nerves end in connexion with the cells of the posterior column 

 gray matter of the spinal medulla. It must not be supposed that the path repi 

 sented by these latter fibres comes to a termination thereby ; from these poster: 

 column cells other fibres arise which cross, in the anterior white commissure, to 1 1 

 opposite side of the spinal medulla and proceed up the spinal medulla to t 

 lateral part of the medulla oblongata. These fibres constitute the spino-thalar 

 tract already referred to. The practical bearing of this is that, owing to the crossi 

 of the lemniscus medialis and lower down of the spino-thalamic tract, unilate^ 

 lesions of the medulla oblongata are apt to produce complete hemi-ancesthesia ; whv-, 

 unilateral lesions of 'the spinal medulla produce only partial hemi-ancesthesia. 



The pyramid forms a massive tract in front of and quite distinct from i> 

 lemniscus medialis. The lemniscus medialis, the tecto-spinal bundle, and the rned t 

 longitudinal bundle are, in the first instance, not marked off from each oth 

 They appear as a broad flattened band applied to the raphe. One edge of t5 

 band is directed backwards and reaches the gray matter on the floor of the fou: i 

 ventricle, while the other edge looks forwards, and is in contact with the pyraii . 

 In the upper part of the medulla oblongata the lemniscus and the mecl 

 longitudinal fasciculus begin to draw asunder from each other. The intermedi s 

 longitudinal fibres become reduced in number and the two strands grow densei - 

 the one on the dorsal aspect of the pyramid, and the other immediately beneath 3 

 gray matter of the floor of the fourth ventricle (Fig. 495). 



The fasciculus longitudinalis medialis is largely formed of fibres homolog s 

 with those which in the spinal medulla constitute the fasciculus anterior propr 5. 

 As they are followed upwards these fibres are thrust back by the two decussatio : 

 the lower decussation pushing them behind the pyramids, and the upper decussat a 

 displacing them still farther backwards to a position behind the lemniscus m.edif 3. 



Corpus Restiforme. The gracile and cuneate nuclei gradually give plaw o 

 the restiform body in the superior part of the posterior district of the media 

 oblongata. Fibres from various quarters converge to form this great strand, 

 first takes shape as a thin superficial layer of longitudinal fibres, which are gathe d 

 together on the lateral aspect of the cuneate nucleus ; but after that nucleus has c< .e 

 to an end, and as the superior part of the medulla oblongata is reached, the restif- a 

 body is seen to have grown into a massive strand, which presents a kidney-sha n 

 or oval outline on transverse section (Fig. 495) ; and it ultimately enters the w e 

 central core of the cerebellum as its inferior peduncle. The fibres which build p 

 the restiform body are the following : (1) the fasciculus spinocerebellaris [posteri 1 ; 

 (2) arcuate fibres coming from the nucleus gracilis and nucleus cuneatus of 1 h 



