320 PHYSIOLOGY 



fibres which connect different segments of the cord, the internuncial or intra-spinal 

 fibres. Next come those fibres which connect the spinal cord with the cerebellum. 

 Last of all to receive a medullary sheath are the fibres which take a direct course from 

 the cerebral cortex to the spinal cord. These are called the pyramidal tracts, and in 

 man are not medullated until the first month after birth (Fig. 156). 



(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 opera- 

 tion, 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 contain ; ng bichromates and are then immersed in a mixture of osmic 



FIG. 157. Cells from the oculo-motor nuclei thirteen days after section of the 



nerve on one side. 



a, cell from healthy side ; 6, cell from side on which nerve was 

 divided. (FLATATJ.) 



acid and bichromate. By this method ordinary fat is stained, but myelin is left un- 

 stained (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 (cp. Fig. 164). 



(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, affec'ed, and the extent to which this occurs is 

 more pronounced according as the lesion is nearer to the cell (Fig. 157). F, for instance. 



