326 THE BODY AT WORK 



of administrative capacity. Although of a different shape, a 

 ganglion-cell of the sympathetic system is as large and as 

 complex in form and structure as a pyramidal cell of the cortex 

 of the brain ; yet the work which it does is of a purely me- 

 chanical order. It receives, reinforces, transmits impulses 

 which reach it from the central nervous system. 



The often-repeated statement that a nerve-fibre is a drawn- 

 out process of a nerve cell-body has prepared the reader to 

 anticipate that it dies when cut off from its central connection. 

 When the axon is dead, the sheath which invests it rapidly 

 loses its tubular character. If the situation of the cell-bodies 

 of a nerve be known, it can be at once foretold on which side of 

 the cut degeneration will occur. Suppose that the median nerve 

 has been severed at the wrist. All nerve-fibres on the distal 

 side of the wound must atrophy, whereas none of the fibres 

 on the proximal side will be affected. The motor fibres have 

 their cell-bodies in the spinal cord, the sensory in the spinal 

 ganglia. Degenerations following lesions in the central ner- 

 vous system have taught pathologists more about the course 

 of the fibres in the white matter than any other class of observa- 

 tions. Degeneration above the lesion is spoken of as ascend- 

 ing, below as descending not that it progresses upwards or 

 downwards. It occurs throughout all the stretch of the fibre 

 which has been isolated from its cell-body at the same time, 

 or nearly so. The thought that impulses can no longer ascend 

 or can no longer descend, as the case may be, has given sanction 

 to the expressions " ascending " and " descending " degenera- 

 tion. 



Restoration to functional activity of tracts of fibres which 

 have degenerated in the brain or spinal cord never occurs, but 

 severed peripheral nerves regenerate. Not that fibres join 

 cut end to cut end, however clean the wound. A wound in 

 the wrist which has divided the median nerve may heal in a 

 few days " by first intention," so far as other tissues are con- 

 cerned ; but the patient does not for two or three months 

 recover the power of using the muscles of the hand which the 

 nerve supplied or the sense of touch in the area of skin to 

 which it was distributed. The ends of the axons on the 

 proximal side of the wound have to grow downwards to 

 establish new connections in the muscles and in the skin. 



