DEGENERATIONS IN THE SPINAL COED. 31 



posterior roots have only a limited course up the postero-lateral column, passing-, as they 

 ascend, into the grey matter, in which they and their collaterals form the fine nervous 

 plexus around many of the nerve-cells which has been already mentioned. From some of 

 the large fibres the plexus around the cells of Clarke's column (and perhaps also around 

 the cells of the nucleus cuneatus of the medulla oblongata) is in this way formed, whilst 

 others furnish an enveloping 1 plexus to the cells of the anterior horn and to those of other 

 parts of the grey matter. (5.) The descending- main branches of the posterior root-fibres 

 have, like many of the ascending branches, only a limited course, during which they give off 

 collaterals, by means of which, and also ultimately by directly passing into the grey matter, 

 they assist in forming the enveloping plexus around the nerve-cells of the segments of the 

 cord immediately below that at which the parent root has entered (compare figs. l(j and 20). 



2. Degenerations in the spinal cord resulting from lesions of parts of 

 the brain. Removal of the cortex of the so-called " motor " region of the cerebral 

 hemisphere is followed by degeneration extending along the pyramidal tract in the 

 brain and cord (fig. 24). In the spinal cord this tract has for the most part crossed 

 to the opposite side, but in man a portion runs for a time as the direct tract along 



Fig. 25. DIAGRAMS SHOWING THE DEGENEKATIOKS IN THE SPINAL CORD * 



WHICH RESULT FROM REMOVAL OP PORTIONS OF THE CEREBRAL CORTEX 



OF THE MONKEY (France). 



A, upper thoracic cord from a monkey in which the left marginal 

 gyrus had been removed some months before death. Sclerosis in both 

 crossed pyramidal tracts, but most on the opposite side. 



B, mid-thoracic cord from a monkey in which the right gyrus fornicatus 

 had been destroyed some months before death. Sclerosis in the opposite 

 pyramidal tract. 



the anterior median fissure on the same side as the lesion (fig. 

 14). Although this direct pyramidal tract is usually considered 

 to be confined to the anterior column, it is found in the 

 monkey as the result of lesions of the motor regions of the V,( ,1 - ? / 



cortex that there are a few scattered fibres in the circum- 

 ferential part of the antero-lateral column which also under- 

 go degeneration (descending cerebral fibres of the antero- 

 lateral column, see p. 25). The crossed tract lies in the posterior part of the 

 lateral column between the direct (dorso-lateral) cerebellar tract and the posterior 

 horn, and in this situation the degeneration can be traced downwards, decreasing 

 in amount, and, in the lower parts of the cord, with the disappearance of the 

 dorso-lateral cerebellar tract, approaching the surface of the cord near the tip of the 

 posterior horn. In the cervical region the crossed pyramidal degeneration may reach 

 the surface of the cord in front of, i.e., ventral to the dorso-lateral cerebellar tract, 

 which is partly enclosed by the degenerated fibres ; this, at least, is the case in the 

 monkey (fig. 25, A). After a unilateral lesion of the cortex there is also a certain 

 amount of degeneration in the lateral tract of the cordon the same side as the lesion. 

 This is especially the case when the lesion has involved the marginal convolution 

 (fig. 25, A). These are not, as might be supposed, fibres which have passed from 

 the pyramid of the medulla oblongata directly to the lateral column of the same 

 side of the cord, but they may be traced from the lesion of the cerebral cortex 

 across the corpus callosum into the internal capsule of the opposite side, whence 

 they run down the crusta and pyramid bundles of the pons and bulb to re-cross at 

 the decussation of the pyramids, and thus to reach the pyramidal tract of the 

 cord on the same side as the lesion. Although most numerous in the higher 

 parts of the nerve-centres, a few of these re-crossed fibres are found to extend right 

 along the spinal cord. 



On the other hand, section of the pyramidal tract in the cord of the new-born animal is 

 followed by atrophy of the large cells of the " motor " area of the cortex (v. Monakow). 



