596 THE HUMAN BODY. 



latiou of the sciatic nerve, impulses must travel from the 

 lumbar enlargement of the cord to the vaso-constrictor centre 

 in the medulla and reflex afferent impulses from it down the 

 cord to the region of the gray matter from which the anterior 

 roots conveying motor fibres for the blood-vessels pass out 

 Although part of the whole course of such impulses lies 

 in the gray core, yet most of it, in the normal physiological 

 working of the Body lies, so far as the cord is concerned, 

 in its white columns, and we have now to try and track 

 these paths: as also paths of special conduction between 

 different regions of the spinal gray matter themselves. The 

 gray matter of the cord being directly continuous with the 

 gray matter of the medulla oblongata and through it with that 

 of some other parts of the brain can transmit impulses after 

 all the white columns of the cord have been divided, but with 

 such conduction we are not for the present concerned. 



To determine the special paths in the white substance of 

 the cord from and to the brain several methods have been 

 employed. Experiment on animals as* to loss of sensation or 

 the power of voluntary movement in parts supplied by nerves 

 arising from the cord posterior to a partial transverse section 

 give on the whole unsatisfactory results : partly because of the 

 difficulty in exactly limiting the section and partly because 

 of the general shock to the nervous system resulting from the 

 operation. Still something has been learned in that way, 

 and something also from observations on persons suffering 

 from more or less localized diseases of the spinal cord. Direct 

 stimulation of parts of the cord exposed by transverse section 

 have also given some results; but more satisfactory evidence 

 as to tracts of conduction between the brain and cord have 

 been obtained by the Wallerian method and by the study 

 of development. Removal or disease of certain parts of 

 the brain and partial cross-sections of portions of it or of 

 the cord itself, give rise to degeneration of localized groups of 

 fibres in parts of the cord posterior to the disease or injury: 

 these are tracts of descending degeneration. Partial cross- 

 section of other parts of the cord or of the posterior spinal 

 roots lead to degenerations above the injury or ascending de- 

 generations: and in general all the fibres which degenerate as 

 a result of a given injury acquire in embryonic development 

 their medullary sheaths at the same time, which is different 

 from the period at which other groups acquire theirs. Finally, 



