STRUCTURE OF THE SPINAL CORD 361 



(c) THE WALLERIAN METHOD. A nerve fibre, when cut off 

 from the nerve-cell of which it is a process, degenerates. This 

 degeneration is marked by a breaking up of the medullary sheath 

 and a conversion of the phosphorised fat, myelin, of which it is 

 composed, into ordinary fat. Later on the fat is absorbed and the 

 nerve becomes replaced by a strand of fibrous tissue in the case of 

 peripheral nerves, of neuroglia in the central nervous system. If 

 the white matter of one half of the spinal cord be divided in the dorsal 

 region, and the animal be killed about three weeks after the operation, 

 sections of the cord both above and below the lesion will show the 

 presence of degenerated fibres. In order to display these fibres 

 pieces of the cord are hardened in a solution containing bichromates 

 and are then immersed in a mixture of osmic acid and bichromate. 

 By this method ordinary fat is stained, but myelin is left unstained 

 (Marchi's method). Degenerated fibres are therefore stained black 

 in virtue of their content in fat. The black staining has different 

 distribution according as we take a section of the cord above or 

 below the lesion. The existence of the degeneration shows that those 

 fibres which are degenerated in the cervical region are axons of 

 nerve- cells situated below the lesion, while the fibres in the lumbar 

 cord which are degenerated must have their nerve- cells in some part 

 of the nervous system which is above the lesion. If the animal be 

 kept alive for a considerable time, six months or more, before being 

 killed, the occurrence of degeneration in any given area of the cord 

 will be shown by the absence of normal nerve fibres in this area. 

 In such a case some method of staining the medullary sheath, such 

 as that of Weigert or Heller, is employed, when the degenerated 

 area will be evident owing to its inability to take the stain. This 

 method, however, is not so satisfactory as the Marchi method, since 

 it is impossible in this way to detect in a section the presence of one 

 or two degenerated nerve fibres, whereas by the use of the Marchi 

 method they would appear as black dots in the unstained section 

 (op. Fig. 165). 



(d) METHOD OF RETROGRADE DEGENERATION. When a 

 nerve fibre is divided there is no degeneration as a rule in the part 

 of the nerve fibre central to the lesion. The nerve- cell is, however, 

 affected, and the extent to which this occurs is more pronounced 

 according as the lesion is nearer to the cell (Fig. 158). If, for instance, 

 an anterior root be divided and three weeks later the animal be killed 

 and sections made of the corresponding segment of the cord and 

 stained with toluidine blue or methylene blue, a striking difference 

 will be observed between the cells of the anterior horn of the two 

 sides of the cord. On the side of the lesion the nucleus of the 

 cells will be somewhat swollen, and may be displaced towards the 



