XI THE SPINAL CORD AS A CONDUCTOR 489 



We must now explain tlie method by which most of 

 our present information has been obtained, and point 

 out the chief and most definite facts which have been 

 arrived at. 



We have seen previously (p. 475) that when a nerve is 

 cut, a structural change of its fibres starts at the cut. 

 This is spoken of as a "degeneration," and since it 

 indicates a breakdown in the proper nutrition of those 

 fibres which are seen to degenerate may, as in the case of 

 the roots of the .spinal nerves, be used to determine the 

 relationship of nerve fibres to the centres upon which 

 their nutrition depends. The white matter of the spinal 

 cord is composed of fibres which are in all essentials the 

 .same as those of an ordinary medullated nerve, and these 

 fibres of the white matter may similarly degenerate when 

 cut off from the centres on which their nutrition depends. 

 Hence, if the whole spinal cord be cut across transversely, 

 or if transverse cuts be made into any part, or the whole 

 of any one or more of the columns of white matter of 

 which the cord is so largely made up, degenerative changes 

 may start from the point of section, and by the course they 

 pursue up and down the cord, enable us to follow the 

 course of certain fibres or bundles of fibres in that 

 white matter. Now this is exactly what does happen when 

 the cord is cut, and this "degeneration method" has 

 provided the best means for answering the question as to 

 how and along what paths aiferent and efferent impulses 

 travel up and down the spinal cord. 



When the cord is cut across degenerative changes take 

 place in pai'ts of the white matter, both above and below 

 the point of section. These changes only affect limited 

 parts of the white matter, and the parts which are affected 

 abore the cut, that is up towards the brain, are different 

 in position from the parts which are affected below the 

 point of section. The changes which start from the 

 cut and take place uptvards along the spinal cord, towards 

 the brain, are spoken of as ascending degenerations ; 

 those which are observed to occur downivards from below 



