688 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



eration is mainly in the internal capsule and crusta of the same side, though 

 by way of fibres crossing in the callosum it may be traced on the other side also. 

 At the decussation of the pyramids the fibres occupying the internal capsule 

 of the same side as the lesion, for the most part cross the middle line (see Fig. 

 187). The portion which remains uncrossed passes as the direct pyramidal 

 tract of the ventral columns in man, while the crossed bundle, which is much 

 the larger, lies in the dorso-lateral field of the lateral column, forming the 

 crossed pyramidal tract. This, however, is only the principal, but not the com- 

 plete, distribution of the degenerated fibres. 



The direct pyramidal tracts disappear in the cervical region, having entered 

 the substance of the cord by way of the ventral commissure, and probably 

 having there undergone decussation. The crossed pyramidal tract shows the 

 greatest diminution in area after passing caudad of the cervical and lumbar 

 enlargements, and hence it is inferred that the pyramidal fibres largely 

 terminate in these regions of the cord. Most important, however, is the 

 observation of Sherrington, 1 that even with a unilateral cortical lesion degen- 

 eration occurs in both crossed pyramidal tracts, and that at the level of the 

 two enlargements the degenerations in the crossed pyramidal tract on the same 

 side as the lesion is larger than above or below these enlargements, thus 

 showing a local increase in the degenerated fibres running on this side. 

 Sherrington's first explanation of this bilateral degeneration in the pyramidal 

 tracts was based on the assumption that fibres which had once crossed at the 

 decussation of the pyramids recrossed at lower levels. If, however, such 

 were the case, the recrossing would carry a number of the degenerated fibres 

 across the middle line, and decrease by so many the fibres in the opposite 

 half. The diminution of the fibres in number on the first side of the cord 

 does not warrant this inference : Sherrington therefore put forward the view 

 that the pyramidal fibres recrossing in the cord are derived in large part from 

 a division of the pyramidal fibres into two branches, one of which may cross 

 to the opposite side of the cord, while the other continues its first course ; 

 such dividing fibres he designates as " geminal fibres," the number of which 

 is by no means small. 



The observations of Sherrington were made on monkeys (Macacus) and 

 dogs, and probably the arrangements of these fibres in man is similar. The 

 observations are particularly significant as giving an anatomical basis for the 

 control of the movements in both halves of the body from each cerebral 

 hemisphere. 



The continuous degeneration, coupled with the histological evidence for 

 the absence of intervening nerve-cells, indicates that the cell-bodies in the cortex 

 have neurons that extend all the way to the cell-groups of the spinal cord, 

 even as far as the sacral region. The neurons of one group of these cortical 

 cells pass, however, to the cell-groups in the cervical enlargement, while those 

 from others pass to the groups in the lumbar enlargement. It thus happens 

 that if the spinal cord be cut across in the middle of the thoracic region, and 

 1 Journal of Physiology, 1889, vol. x. 



