CHAP. IL] THE BRAIN. 989 



S. indicates the course of the most anterior and dorsal part of the temporo- 



occipital tract. 

 The fine dotted lines converging to the corpus callosum c.c. indicate the course of 



the callosal fibres. 



capsule, are continually changing their direction as they pass 

 through the capsule. Moreover while the fibres from different 

 parts of the ' motor area ' assume definite positions in relation to 

 each other as they pass into the capsule, their relative positions 

 are not constant, but vary somewhat. To this point however we 

 shall return when we come to speak of the function of this tract. 



In the crus these fibres run exclusively in the pes and form a 

 compact strand (Fig. 114, Py) occupying the central and larger 

 portion of the pes between a small median portion on the inside 

 and a lateral portion on the outside. Maintaining this position 

 along the crus they enter the pons, but here the previously com- 

 pact strand is split up, by the interlacing transverse fibres of the 

 pons, into a number of scattered bundles, which however as a 

 whole still keep their central position. They form the greater 

 part of but not all the bundles seen cut transversely in transverse 

 sections of the pons (Figs. 112, 113). Farther backwards they 

 become the pyramid of the bulb, and so give rise in the spinal cord 

 to the direct and crossed pyramidal tracts. These fibres from the 

 motor area of the cortex of the cerebrum are thus the source of the 

 pyramidal tracts of the spinal cord, and hence the whole strand of 

 fibres from the cortex downwards has been called the pyramidal 

 tract. We have said ( 575) that we have reasons for thinking 

 that the pyramidal tract in the spinal cord makes connections 

 through the grey matter of the anterior horn with the anterior roots 

 of all the spinal nerves in succession ; and similarly we have reason 

 to think that along its course in the crus, in the pons, and in the 

 bulb, before it reaches the cord, the tract also makes connections 

 with the nuclei of those cranial nerves which are motor in function. 

 During the passage of the tract through the internal capsule the 

 fibres destined for cranial nuclei occupy the knee, while those 

 belonging to the spinal cord run in the hind limb. Some authors 

 limit the term pyramidal tract to the spinal moiety, since this 

 alone forms the pyramid ; but this is undesirable. 



This tract is well marked out by the degeneration method, 

 and the degeneration in it is a descending one, the trophic centres 

 of the fibres being cells in the grey matter of the cortex. Removal 

 of or injury to the cortex of the whole motor area gives rise to a 

 degeneration along the whole tract, and removal of or injury to 

 part of the area gives rise to degeneration of some of the strands. 

 The tract is also well marked out by the embryological method ; 

 the fibres belonging to it acquire their medulla at times different 

 from those of other fibres. 



Anterior or frontal cortical. Fibres from the grey matter 

 of the cortex in front of the motor area also pass to the internal 



