1903.] on the Paths of Volition. 271 



cord and other lower nerve centres, from which the motor nerve 

 fibres which carry those impulses to the muscles directly emerge. 



But before actually proceeding to consider this question, I must 

 say a word as to the methods which are employed to solve it. On 

 the evening of Friday, the 31st of May, 1861, a discourse was 

 delivered in this theatre, which contained an account of a discovery 

 of first-rate importance in the physiology of the nervous system. 

 This discourse was prepared by Augustus Waller,* the distinguished 

 father of a distinguished son, himself well known to members of the 

 Royal Institution as having occupied for a time the position of 

 Fullerian Lecturer in Physiology. Waller had noticed (some ten 

 years previously) that after any nerve had been cut, one part of the 

 cut nerve, that namely which was furthest from the nerve centre, 

 showed after a few days a remarkable degeneration of its fibres, 

 which has ever since been known as the " Wallerian degeneration '* 

 (Diagram 7). The drawing which I here exhibit to you (7) is one 

 made by Mrs. Waller from one of her husband's preparations. I 

 have had it, as well as the portrait which you have just seen, copied 

 from ' Some Apostles of Physiology,' which I have placed upon 

 the table, and which we owe to the erudition and literary research 

 of Professor Stirling and the liberality of Dr. Whitehead of Man- 

 chester ; a book which is not only notable on account of its artistic and 

 literary merit, but also by reason of the fact that only 100 copies of 

 it have been printed (and none of these placed on the market), so 

 that it will no doubt be, in time to come, an object of particular 

 interest to the bibliomaniac. 



Waller, as I have said, observed this granular degeneration in the 

 peripheral part of the cut nerve, whereas in the central part there 

 was no such appearance to be seen. Proceeding further, he noticed 

 that when a nerve upon which there is a ganglion (a collection of 

 nerve cells, such, for example, as that upon a posterior root of one of 

 the spinal nerves) is cut so as to sever the part of the nerve which is 

 connected with the spinal cord from the ganglion, it is now the more 

 central part of the nerve which undergoes degeneration, whereas the 

 part which is still in connection with the ganglion remains unaltered. 

 The Wallerian doctrine, stated in present-day language, postulates 

 that a nerve fibre undergoes degeneration when it is cut off from the 

 cell with which it is connected, and from which it has originally 

 grown. From it and in connection with it has developed a corollary 

 which postulates that, at any rate within the nerve centres, the nerve 

 fibre only conducts nervous impulses away from the cell with which 

 it is naturally connected. This is, I may point out in passing, the 

 revival of the valvular idea relating to nerve paths which had been 

 suggested by Descartes, and to which I have already alluded. If the 

 doctrine of nerve degeneration is true, it follows that if we cut away 



* In point of fact the discourse, although prepared by Waller, was read for 

 him, ag he was prevented by illness from being himself present. 



