THE CRURA CEREBRI. 301 



Close to the substantia nigra, the bundles of white fibres are smaller and some- 

 what separated by projections of the grey matter extending between them. They 

 have received the name of stratum intermedium. These are usually stated to have a 

 different origin and destination from the other fibres of the crusta, passing, accord- 

 ing to Meynert, between the lenticular nucleus of the corpus striatum (see p. 131), 

 and the substantia nigra and reticular formation of the bulb and pons. It is 

 doubtful, however, if this is true for any of these fibres, and is certainly not the 

 case with the majority, for (in monkeys) after a lesion of the Eolandic region, most 

 if not all the fibres of the stratum intermedium undergo degeneration along with the 

 fibres of the pyramidal tract, to which, therefore, they must be regarded as belonging. 

 The pyramidal tract of the mesencephalon, or continuation of the pyramid- 

 bundles of the pons, occupies about the middle third of the crusta. Superiorly 

 its fibres pass through the middle part of the internal capsule to the fronto-parietal 

 or Rolandic region of the hemisphere. By far the majority arise from the cortical 

 cells, but a few have their cell-origin in the cord or bulb, and degenerate after 

 lesions of these parts. 



It is remarkable that whereas in the lower part of its course (spinal cord and 

 bulb) the fibres of the pyramidal tract acquire a medullary sheath later than the 

 other fibres of the white columns, in the upper part (crus cerebri and cerebrum) it is 

 acquired earlier than in the other fibres. 



The outer or lateral third of the crusta is formed of fibres which are traceable 

 downwards to the lateral longitudinal bundles of the pous, and upwards to the 

 posterior part of the internal capsule, but their origin and destination have not yet 

 been satisfactorily made out. They are probably connected superiorly with the 

 occipito-temporal regions of the cerebral cortex, and according to Flechsig they arise 

 below from the cells of the nuclei pontis. 



The mesially situated bundles of the crusta are also distinct from the pyramidal 

 tract proper (Flechsig), being developed at a later period. They are perhaps 

 connected through the anterior part of the internal capsule with the prefrontal 

 region of the hemisphere. Finally, one well-marked bundle in the crusta is con- 

 nected with the fillet (mesial fillet, pp. 66, 67, and 103). This bundle is at the 

 lateral border of the pyramidal tract in the upper part of the crusta, but lower down 

 crosses obliquely over or between the fibres of that tract to attain the mesial border 

 of the crusta, whence it is traceable to the fillet. It contains, according to Spitzka, 

 the afferent cerebral tracts of the cranial nerves. Traced upwards it is lost in the 

 subthalamic region. 



Lastly the crusta includes some fibres derived originally from the cerebellum and 

 joining the pyramidal tract in its passage through the pons, such fibres being 

 scattered amongst the fibres of the other tracts. 



The substantia nigra is a mass of grey matter which is characterised by the 

 presence of a number of very darkly pigmented irregular nerve-cells, which give the 

 substance in which they are scattered the appearance from which it derives its name. 

 It forms a layer which separates the crusta from the tegmentum. It is thicker 

 near the mesial border of the peduncle than laterally, where the tract of the fillet 

 may be but incompletely separated by it from the longitudinal bundles of the crusta 

 It commences at the upper margin of the pons, and can be traced as far forwards as 

 the posterior border of the corpora albicantia. At the origin of the third nerve it is 

 traversed in its mesial part by some of the issuing fibres of the nerve-root. The 

 grey matter of the substantia nigra projects here and there between the adjacent 

 bundles of the crusta ; one considerable projection in particular in the lower part 

 of the mesencephalon serving to mark off the mesial portion of the crusta from the 

 rest. The cells in this projection are much smaller, and relatively more numerous 

 than in the rest of the substantia nigra. 



