26 



P recent ral or Motor Area 



[CHAP. 



fibres radiate, thick and stout, but the zone of substance, the so-called radiaiy zone, capping 

 that process, is of outstanding depth and richly coloured on account of its immense fibre 

 wealth. Analysing its constituents more closely, we find that it owes its density to a remark- 

 able development of all the systems of fibres which it contains : thus, the large association 

 fibres which intersect the radiations at all levels are more numerous and of greater size 

 than they are in any other part of the cortex; the radiary projections themselves form 

 stout bundles, and are richly stocked with fibres of every calibre, but especially with ones 

 of large size ; and the collaterals of axis cylinders of large nerve cells, situated in the 

 deeper parts of the cortex, are very numerous and aid in increasing the density of the 

 interradiary plexus. 



\ 



FIG. 2. Kadiary zone in the prel'rontal cortex at a magnification of x |^. 

 Notice that there are absolutely no fibres of large calibre and almost none which we can call of medium-size. The 



slender radiations are composed of fine wavy fibres, and the interradiary plexus only contains a few of the same 



elements. 

 To see two extremes of fibre representation contrast this with figure 1. 



Difficulties at once arise when we attempt to attach physiological significance to the 

 individual constituents of the radiary zone ; for as the plexus of fibres is such an exceed- 

 ingly dense one, it is impossible to trace even the larger fibres for any distance : we can 

 only say, in the first place, that physiological experiment warrants the assumption that the 

 majority of the fibres in the radiating fasciculi are endogenous and convey centrifugal 



