GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 209 



upon the hip-disease of children is usually felt, not at the hip- 

 joint, but at the knee. 



The Degeneration of Nerve-Fibres separated from their 

 Centre. A nerve-fibre may in its course be connected with 

 more than one nerve-cell, but one cell always has a special 

 influence in maintaining its normal structure and functional 

 activity. If cut off from this cell the separated portion 

 undergoes degenerative changes, easily recognized in medul- 

 lated fibres by a breaking up and, later, a disappearance of 

 the medullary sheath. If, for example, the sciatic nerve of 

 a warm-blooded animal be completely cut across, all of the 

 nerve and its branches beyond the point of section will show 

 marked changes in three days or less: the medullary sheath 

 separates into small cuboidal pieces, these in a day or two 

 more round off at their corners and then are gradually ab- 

 sorbed, so/that at the end of ten days or a fortnight they have 

 entirely disappeared. Meantime the nuclei of the internodes 

 multiply and the usually sparse protoplasm around them in- 

 creases, and encroaches upon and causes the absorption of the 

 axis cylinder, so that after some weeks little or no trace of 

 true nervous elements can be found. Some three or four days 

 after making the section the peripheral portion of the nerve 

 ceases to be excitable. If the part of the nerve above the sec- 

 tion be examined, its fibres will be found to have undergone 

 no degeneration except close to the place of section, and it re- 

 mains excitable; pinching it causes pain, and if any muscle 

 branch arising from it be irritated, the muscles contract. If 

 instead of cutting a whole mixed nerve-trunk, such as the sci- 

 atic, we divide only a ventral spinal root (as 5, c, Fig. 71), it is 

 found that all the fibres in that part of the root which is cut 

 off from the spinal cord degenerate and become unirritable, 

 and degenerated fibres can be found in the mixed trunk into 

 which the cut root is continued; while the fibres of the part 

 of the root still attached to the cord do not degenerate. 

 The nutritional integrity of the anterior root-fibres depends 

 therefore on anatomical continuity with the spinal cord, and 

 probably with cells there, of the type shown in Fig. 81. On 

 the other hand, if the dorsal root only be cut across, the por- 

 tion of it attached to the cord degenerates, while that still 

 connected to the spinal ganglion and the fibres beyond the 

 ganglion remain unaltered : the nutritive centres for the dor- 

 sal root-fibres are the cells of the corresponding root-ganglion. 



