ON THE FUNCTIONAL ACTIVITY OF NERVE CELLS. 523 



In such a case, the changes in the nerve, as a whole, are readily ob- 

 servable, and moreover it is often possible to find fibres optically isolated 

 at the extreme edge of the bundle, so that individual appearances may be 

 studied. 



A.t other times, however, one of the popliteal nerves was taken, and 

 after placing it in position on the slip, one end was quickly and lightly 

 separated with mounted needles. In nearly every nerve thus treated 

 were visible one or more fibres separated from their fellows for a short 

 distance, rendering individual observation easier, the only drawback being 

 that in this case the nerve must of necessity be more or less injured. 



When treating nerves for examination, after fixation, by the ' thimble ' 

 method described above, obviously the size of the nerve was not a matter 

 of importance, provided that the vapour was given sufl&cient time to 

 penetrate the bundle. 



After medullated nerve fibres have been exposed in either of the above 

 ways to the vapour of ether, or chloroform, or to carbon dioxide gas, for 

 periods varying from 2-5 minutes (according to the size of the nerve 

 chosen) certsdxv slight microscopical changes maybe observed to have taken 

 place in them. In most cases these changes are but slight, and the fibres 

 are not all equally affected. Some may show little or no change from the 

 normal, while the rest show a more or less distinctly visible change. 



In observing a preparation, therefore, it is advisable to search it 

 throughout for any of the differences from the normal state, to be subse- 

 quently described, for it is not uncommon to only find a few fibres 

 typically affected throughout a whole nerve trunk. 



The ' effects ' produced by ether vapour and by carbon dioxide are very 

 similar, and usually slight, but the chloroform effect is generally marked 

 in the majority of cases, and is a more thorough change, affecting most of 

 the fibres of a nerve trunk. 



These effects are not easily visible in the fresh specimen, but are 

 usually recognisable in tissue subsequently fixed and darkened by osmic 

 vapour, which was found to be the most satisfactory of several fixing 

 agents tried. 



The histological changes fall under two headings : — 



{a) Changes in the appearance and consistency of the medullary 



sheath ; 

 (h) Changes at the nodes. 



Ether Vapour. — The earliest effect is noticeable as soon as twenty 

 seconds after the commencement of administration, a faint granularity 

 appearing in the medullary sheath and slowly increasing for about thirty 

 seconds. It is not an invariable result of the action of ether vapour, and 

 is not always visible in the fresh specimen, but is usually present in the 

 osmic preparation. It must be borne in mind that the myelin tends to 

 become granular after death, and that granularity is often caused by 

 manipulation, thus : — Granularity is invariably present at the cut ends of 

 a nerve. 



Another change common to etherised nerve, which is made evident by 

 the blackening action of the osmic acid, is the increased distinctness of 

 the medullary segments or incisures, which appear rather more distinctly 

 than in normal nerve, as light V-shaped markings on a darker ground. 



The most distinctive change, however, is at the nodes of Ranvier. 

 (As has been before remarked, the change may be but slight, and does not 



