952 



THE NER VE SYSTEM 



the lenticular nucleus, in part in the suhlenticular zone, to form the ansa lenticu- 

 laris. The reenforcement of this sublenticular white-fibre tract by cortico- 

 thalamic fibres from the temporal lobe to thalamus forms the ansa peduncularis. 

 The topographic relations of the various tract masses as seen in a flatwise section 

 is schematically shown in Fig. 706; on the whole, they correspond to the cortical 

 areas with which they are connected. 



Geniculate fibret 



Motor area of 

 cortex 



Internal 

 capsule 



Decussation of pyramids 



Direct pyramidal Iract 

 Crossed pyramidal tract 



Anterior nerve roots 



Fir,. 706. The motor tract. (Modified from Poirier.) 



The external capsule (Fig. 704) is a thin lamina of white substance interposed 

 between the ectal face of the lenticular nucleus and the claustrum. Dorsally, 

 frontad and caudad, at the corresponding borders of the lenticular nucleus, it 

 joins the internal capsule mass, while ventrally it is continuous with the white 

 centrum of the temporal lobe. Its comparatively few projection fibres course 

 to and from the ventral parts of the thalamus; its chief constituents are associa- 

 tion axones for the circuminsular cortical areas. 



Intimate Structure of the Cerebral Cortex and its Special Types in Different Regions 



(Fig. 707). A section of the cerebral cortex reveals a tendency on the part of its constituent 

 cells to arrange themselves in layers which alternate with zones less rich in cellular elements. 

 Among the cells course the axones arising from them or terminating in their neighborhood. 

 The axones arc chiefly amyelinic, though some are myelinic for a part of their intracortical 



