630 THE BRAIN. 



outside. Maintaining this position along the cms they enter the pons, but 

 here the previously compact 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. 135, 136). Further backward 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 downward has been called the pyramidal 

 tract. We have said ( 488) that we have reasons for thinking that the 

 pyramidal tract in the spinal cord makes connections through the gray 

 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 pyra- 

 mid ; but this is undesirable. 



This tract is well marked out by the degeneration method, and the de- 

 generation in it is a descending one, the trophic centres of the fibres being 

 cells in the gray 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 embryologi- 

 cal method ; the fibres belonging to it acquire their medulla at times different 

 from those of other fibres. 



Anterior or frontal cortical. Fibres from the gray matter of the cortex 

 in front of the motor area also pass to the internal capsule, but occupy the 

 fore limb (Fig. 145, /row). Thence they pass to the crus, of which they 

 form the small inner, median portion of the pes (Fig. 137, Fr.\ and from 

 the crus pass into the pons ; in transverse sections of the pons they are seen 

 as scattered bundles (Fig. 136, F. C.) to the median side of the pyramidal 

 fibres. But here they seem to end ; the degeneration of the tract is a de- 

 scending one, and ceases here. Most probably the fibres end in the nerve- 

 cells of the gray matter, which, as we have seen, is abundant in the pons. 

 It is also probable that through these nerve-cells the fibres of this tract are 

 connected with transverse fibres passing along the middle cerebellar pedun- 

 cle into the cerebellum of the opposite side ; but this has not been definitely 

 proved. 



Posterior or temporo-occipital cortical. Fibres from the gray matter of 

 parts of the cortex behind the motor area also converge to the internal cap- 

 sule, forming the hinder end of the hind limb behind the pyramidal tract 

 (Fig. 144, ). These fibres also contribute to form the crus cerebri, passing 

 into the pes, of which they occupy the outer lateral portion (Fig. 137, Pr. 0.}. 

 From the crus they pass into the pons, where, like the fibres of the preceding 

 tract, they appear to end, and probably in a like manner. This tract has 

 been described as one of ascending degeneration, but in all probability like 

 the preceding is one of descending degeneration. 



The above three tracts of fibres may, therefore, all be regarded as start- 

 ing from or having their trophic centres in the cortical gray matter of the 

 hemispheres, as all helping to form, first, the internal capsule and then the 

 pes of the crus cerebri. But while the pyramidal tract passes, in part, to the 



