272 Professor E. A. Schdfer [March 20, 



a nerve cell or group of nerve cells, ail the fibres which emanate from 

 them will undergo the degeneration in question. And this is, in 

 fact, what is found to occur. For a long time it was a matter of 

 difficulty to trace with exactitude the course of single nerve fibres, 

 which, in consequence of removal of their nerve cells, or of their 

 being cut off from these nerve cells, had undergone Wallerian 

 degeneration. But a method of staining the granules of the de- 

 generated fibres was devised some years ago by an Italian histologist, 

 Dr. Marchi, by means of which the granules become stained intensely 

 black, while normal nerve fibres remain unstained. By this method, 

 it becomes possible to trace degenerated fibres in all parts of the 

 nervous centres, and not only in battalions but even as single spies. 

 It is hardly likely that Waller himself appreciated the full signifi- 

 cance of his discovery. He could not have foreseen what a master- 

 key he had produced, nor how many doors of the secret passages of 

 the nervous system were to be opened by it. Although it is the 

 Wallerian method which has mainly contributed towards the recent 

 progress of our knowledge regarding the paths of conduction in the 

 central nervous system, it has been assisted by another method, that 

 of Flechsig, which depends upon the fact that different tracts of 

 fibres in tlie central nervous system undergo development at different 

 periods of growth. 



It will of course be understood, from what I have said, that in 

 order to employ the Wallerian method it is necessary to effect in 

 animals section of the nerve fibres, the course of which it is proposed 

 to follow, or to remove altogether the cells with which those nerve 

 fibres are connected ; that is to say, the Wallerian method is an ex- 

 perimental method applied to the elucidation both of the structure 

 and functions of the nervous system. Frequently Nature herself 

 makes such experiments for us, when for example, as the result of 

 disease, lesions become established in the brain or spinal cord, which 

 sever nerve tracts or destroy groups of nerve cells. But such experi- 

 ments are rarely as definite in their character as those which we 

 ourselves plan out and make to order, although they are of value in 

 establishing for the human subject the truth of observations which 

 have been already made in animals Rarely have such natural 

 experiments by themselves led to the actual discovery of an un- 

 questioned fact. An exception must be made in connection with the 

 subject with which I am dealing to-night, namely, " The Path of 

 Volitional Impulses from the Brain to the Spinal Cord ; " for it was 

 first noticed (by Tiirck) in the human subject that lesions of the 

 cerebral hemisphere occasioned by disease are followed by the occur- 

 rence of Wallerian degeneration along a particular tract within the 

 brain and sj)inal cord, which it has become customary to spi ak of as 

 the pyramidal tract. The fibres of this tract originate in the motor 

 cortex of the brain. Hence we can trace them through the sub- 

 jacent white matter ; through the broad band of nerve fibres which 

 is known as the internal capsule ; through the isthmus of brain 



