THE NERVOUS SYSTEM 1359 



The dorsal longitudinal bundle is displaced dorsalwards during the 

 formation of the fillet, and it comes into contact with the grey 

 matter on the floor of the fourth ventricle ; whilst the fillet lies on 

 the dorsal aspect of the p^Tamid. 



The dorsal longitudinal bundle is prolonged into the ventral 

 column of the spinal cord on the same side, where it is represented, 

 as stated, by the ventral ground-bundle and anterior marginal 

 bundle of Lowenthal. 



A ventral or anterior longitudinal bundle (tecto-spinaJ tract) is described 

 as lying on the ventral aspect of the dorsal or posterior longitudinal bundle. 

 This bundle, however, is not well defined. It descends into the anterior 

 column of the spinal cord, and is accompanied by the ponto-spinal tract, the 

 fibres of which spring from the cells of the formatio reticularis of the pons. 



Arcuate Tracts. — These tracts form two groups — superficial and 

 deep. 



The superficial arcuate fibres are arranged in two sets — anterior 

 and posterior. 



The anterior superficial arcuate fibres arise from the nucleus 

 gracilis and nucleus cuneatus of the opposite side, and a few arise 

 from the arcuate nucleus of the same side. At the median line 

 they decussate with those of the opposite side, and emerge at the 

 ventral median fissure, where many of them arch over the mesial 

 and ventral aspects of the pyramid. Others pierce the pjrramid, 

 whilst some emerge at the ventro-lateral sulcus between the 

 p\Tamid and olive. The fibres now pass outwards and dorsal- 

 wards, some arching over the lower part of the olive, and they 

 finally enter the restiform body. 



The posterior superficial arcuate fibres arise from the nucleus 

 gracilis and nucleus cuneatus of the same side, and they enter the 

 restiform body also of the same side. 



The deep arcuate fibres are disposed in two sets — ^lemniscal and 

 olivo-cerebeUar. The lemniscal deep arcuate fibres arise from the 

 nucleus gracilis and nucleus cimeatus of the same side. They 

 sweep fon^'ards and inwards towards the raphe, passing obliquely 

 through the dorsal longitudinal bundle. At the median line they 

 decussate with those of the opposite side above the level of the 

 decussation of the pyTamids. Having reached the opposite side, 

 the deep arcuate fibres change their course, and now pass upwards. 

 The ascending tract thus formed constitutes the mesial fillet or 

 lemniscus. 



The decussation which takes place between the deep arcuate 

 fibres in the median line, immediately above the decussation of the 

 pwamids, is called the decussation of tlie fillets {decussatio lemnis- 

 coruni B.). or the superior sensory decussation, as distinguished from 

 the inferior sensory- or spino-thalamie decussation, which takes 

 place in the spinal cord. 



The olivo-cerebellar deep arcuate fibres arise from the inferior 

 oUvary nucleus of the same side. Emerging through the hilum, 

 they pass across the median line to the opposite side. They then 



