xiiv Journal of Comparative Neurology. 



the sections being naturally more readily permeable to the oxidizing 

 agent than the bundles of fibrils, and consequently more readily decol- 

 orized." That this principle operates to a certain extent must be ad- 

 mitted, -yet it must play a very subordinate role. It would, I think, be 

 difficult to convince anyone who has watched the differentiation by one 

 of the slower methods of Weigert sections cut through the entire body 

 and containing various kinds of tissue that the rate of decolorizing is 

 proportional, however roughly, to the permeability of the tissue. It 

 ill accords with such a view to find that the deeper layers of the myelin 

 sheaths clear before their periphery, that the axis cylinder sometimes 

 decolorizes to a clear yellow still earlier in the process, that the nucle- 

 ated blood corpuscles of the fishes (a tissue which is certainly suffic- 

 iently permeable) may retain their brilliant blue color after all the nerves 

 are fully decolorized and the large muscle fibers, transversely cut and 

 hence with their protoplasm directly exposed to the action of the re- 

 agents, decolorize nearly as late as the medullated nerves and sometimes 

 even later. On the contrary these variations rest upon chemical differ- 

 ences in the tissues which cause them to react differently to the dye. 

 That it is not merely a question of permeability, is clearly shown by 

 the fact that a change from one decolorizer to another is often sufficient 

 to cause a reversal of those conditions, e. g., to cause muscles to decol- 

 orize before instead of after the nerves. Such chemical differences, not 

 only between different tissues, but between the same tissue in different 

 animals, are real factors, as is shown by the fact that histological meth- 

 ods which yield a satisfactory stain, say in the Amphibia, may fail com- 

 pletely when applied to the fish, and that even different species of fishes 

 have not the same susceptibility to stains. This receives the most fre- 

 quent illustration perhaps among workers with metylene blue. These 

 staining reactions are far too compHcated to be reduced to chemical 

 terms until we know much more of the chemistry of the tissues which 

 take up the stains than we do at present. 



I desire in conclusion to express my deep obligation to Dr. Oliver 

 S. Strong for advice and valuable suggestions freely given in the course 

 of these researches. c. j. h. 



PATHOLOGY. 

 Mills' Practical Neurology.* 



American science is to be congratulated in having been the first to 

 supply a comprehensive treatise on neural pathology in terms of the 



' Charles K. Mills. The Nervous System and its Diseases. J. B. Lip- 

 pincott and Co., 1898. 



