76 Journal of Comparative Neurology. 



lin, a peculiarity which they share with all non-nervous nuclei ; 

 while the nuclei of the larger nerve cells do not take up haema- 

 toxylin. Here allusion may be made to preparations of Weigert, 

 who succeeded in staining the neuroglia separately by a method 

 the details of which have not yet been published (Anat. Anz., 

 1890). 



The instances adduced teach us that much of the most re- 

 cant investigation makes it appear at least possible to place many 

 of the tissue elements hitherto looked upon as distinctively non- 

 nervous nearer to the nervous elements histologically and espe- 

 cially genetically. 



I will, however, in what follows limit myself to those con- 

 tributions which during the last few years have promoted our 

 knowledge of the significance of the pure nervous elements, and 

 I will adduce the most important facts which they teach us con- 

 cerning the structure of the nervous system as a whole. 



It is customary to distinguish nerve cells and nerve fibres. 

 The histological idea of a nerve cell is especially hard to define. 

 If we remember that every functional nerve cell is directly con- 

 nected by one of its processes with at least one nerve fibre and 

 it is impossible to decide where the process becomes transformed 

 into the nerve fibre, it seems very appropriate to consider the 

 nerve cell with the fibres springing from it as a unit. Every 

 nerve fibre, however, is said to break up up at the end opposite 

 the cell into a fine terminal tuft, or brush {Endbaeumchen, Koili- 

 ker), so that we arrive finally at the conclusion that the whole 

 nervous system consists of numerous nerve units, or neurones 

 (Waldeyer, deutsch. med. Wochenschr. , 1891). Every nerve 

 unit consists of three parts: the nerve cell, the nerve fibre, and 

 the terminal brush. These three elements will now be treated 

 separately. 



I. The nerve cell. I have already remarked that it is 

 scarcely possible accurately to define this histologically. The 

 views, moreover, with reference to the anatomical and physio- 

 logical significance of the nerve cell are no longer separated, as 

 has been the case heretofore. Max Schultze had pointed out 

 (1869) the fibrillary structure of nerve fibres; i. e. of the most 

 important and uniformly present element of such fibres, the axis- 

 cylinder. These fibrillae may be followed into the cells, where 



