THE NERVOUS SYSTEM. 263 



anything is to be seen ; chemical experiment and the 

 coarse perception of the naked eye in this instance alone 

 discover the occurrence of peculiar changes. It may 

 therefore be said that in this quarter really the most has 

 yet to be done. We have rather presumptive evidence 

 than facts. 



If now we briefly sum up what I have laid before you 

 concerning the blood, we see, either that certain sub- 

 stances find their way into it, which exercise an injurious 

 influence upon its cellular elements and render them in- 

 capable of performing their functions ; or that from a 

 definite point, either from sources external to the body, 

 or from some organ, matters are conveyed into it, which 

 thence exercise an injurious influence upon other organs ; 

 or finally that its constituents are not replaced and rege- 

 nerated in a regular manner. Nowhere in this whole 

 series do we find any one condition, indicating that defi- 

 nite changes once set on foot in the blood itself can be 

 permanently maintained, in other words that a perma- 

 nent dyscrasia is possible, unless new agencies derived 

 from a definite source are continually brought to bear 

 upon the blood. This is the reason why I began by call- 

 ing your attention to this point of view, which I conceive 

 to be of extreme importance in practice also, namely, 

 that in all forms of dyscrasiae the chief point is to search 

 for their local origin. 



Let us now proceed to the consideration of another 

 subject which comes next in historical importance, namely 

 the structure and arrangement of the nervous system. 



The great mass of the nervous system consists of 

 fibrous constituents. It is to them that nearly all the 

 finer physiological discoveries, which the last fifteen 

 years have brought with them, have reference, whilst 

 the remaining portion of the nervous system, in quantity 



