ST&UCTUBAL AKKAKGEMEKTB OF CEKEBKUM 423 



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 is an index to the gradual 

 shifting of these reactions 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 reac- 

 tion 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-PONTItfE FIBRES. 



These arise from cells in the cortex of 

 the frontal lobe, and pass down in the 

 anterior limb of the internal capsule 

 to gain the mesial part of the crusta of 

 the crus cerebri. The fibres end in 

 the grey matter of the formatio reti- 

 cularis of the pons, the nucleus pontis. FlG 215 Diagrammatic representation 



(3) THE TEMPORO-PONTINE FIBRES. . of the internal capsule, as seen in hori- 



These arise from the two upper zontal section ' ( C NI M -) 

 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 

 and in the crusta is shown in the accompanying diagrams (Figs. 215 and 

 216). 



The fronto-pontine and temporo-pontine fibres, which end in the nucleus 



