424 



PHYSIOLOGY 



structure, and are continued as the pyramids of the pons and medulla to 

 she upper part of the spinal cord, where most of them decussate to the other 

 tide to form the crossed pyramidal tracts. Some of the fibres do not cross 

 at the pyramidal decussation, but are continued down in the same position 

 in the anterior columns of the spinal cord of the same side, forming the direct 

 or anterior pyramidal tracts. These fibres cross for the most part lower down 

 in the cord, so that the direct pyramidal tract is not seen below the cervical 



region. The pyramidal tracts are not 

 found in lower vertebrates, and make 

 their first appearance in the mammalia. 

 Their development corresponds with the 

 gradual increase in the direct interference 

 of the cerebral cortex in the reactions 

 of the organism as a whole and are an in- 

 dex to the gradual shifting of these reac- 

 tions from the inevitable to the educated 

 reflex. The fibres of the pyramidal 

 tract end at various levels of the spinal 

 cord and can be traced to the lower end 

 of the sacral region. According to 

 Schafer they end in the posterior cornua, 

 so that their action is to set going a 

 reaction which could otherwise be 

 elicited by stimulation of the afferent 

 fibres entering by the posterior root at 

 the level of the cord where they end. 



(2) THE FRONTO-PONTINE FIBRES. 



These arise from cells in the cortex 

 FIG. 215. Diagrammatic representation of the frontal lobe, and pass down in the 



anterior limb of the inte al ca ? sule 



to gain the mesial part of the crusta of 

 the crus cerebri. The fibres end in the grey matter of the formatio 

 reticularis of the pons, the nucleus pontis. 



(3) THE TEMPOROPONTINE FIBRES. These arise from the two upper 

 temporal convolutions, especially from that area which is associated with 

 hearing. They pass inwards under the lenticular nucleus through the hinder 

 limb of the internal capsule to gain the outer part of the crusta. In this 

 situation this tract passes down into the pons, where it ends in the nucleus 

 pontis. 



As part of these projection fibres we ought probably to reckon the fibres 

 which take origin or end in the corpus striatum. The afferent fibres of this 

 body are derived chiefly from the thalamus, forming the thalamo-striate 

 fibres. Other fibres arise in the nuclei of the corpus striatum and pass down 

 in the dorsal portion of the crusta to end for the most part in the pons, the 

 strio-pontine fibres. 



The relative position of these various fibres in the internal capsule 



