250 Dr. Beale Croonian Lecture. [May 11, 



cleated nerve-fibres, forming lax networks, branches of which may often be 

 followed for a very long distance, and then traced into neighbouring nerve- 

 trunks. 



Of the finest nerve-fibres which influence the muscle. 



It is probable that the active part of the nerve-fibre, as regards the ele- 

 mentary muscular fibre, commences only at the point where the dark- 

 bordered character of the nerve-fibre ceases, and therefore that the most 

 important and most active portion of the peripheral nerve-fibres distributed 

 to muscle, has escaped the observation of many observers. The fibres are 

 extremely delicate, and, like other very fine nerve-fibres, can only be rendered 

 visible by special methods of preparation. Probably every fibre, however 

 fine, is compound, being composed of several finer fibres. Nuclei are in- 

 variably found in relation with these fibres, and they vary in number in 

 different cases. The structure and general appearance of the finest nerve- 

 fibres will be understood by reference to the figures. 



From the foregoing observations I conclude that the nerve-fibres which 

 are to be regarded as the fibres of distribution are far more delicate and 

 much finer than has been hitherto supposed. The remarks which I make 

 on this head with reference to the ultimate nerve-fibres distributed to volun- 

 tary muscle, will apply to the ultimate nerve-fibres distributed to other 

 organs. 



In mammalia the ultimate fibres appear as narrow, long, slightly granular, 

 and scarcely visible bands with oval masses of germinal matter, situated at 

 short but varying intervals, as described in my paper published in the Phil. 

 Trans, for 1860. In many reptiles (frog, newt, lizard, snake, chameleon), 

 however, these ultimate nerve-fibres are narrower but much firmer than in 

 mammalia; and they are more readily demonstrated, as they do not give 

 way under the influence of considerable pressure and stretching. Although 

 fine nerve-fibres have been described in certain situations before I drew 

 attention to these fine pale nucleated fibres in muscle, it was not generally 

 supposed that the active peripheral portion of nerves exhibited these cha- 

 racters; nor indeed has this fact yet received the assent of many distin- 

 guished anatomists. The arrangement of the fine nerve-fibres in the 

 summit of the papillao of the frog's tongue, described in my last paper 

 presented to the Royal Society (Phil. Trans. June 1864), and in the mucous 

 membrane of the human epiglottis, will, I venture to think, tend to con- 

 vince many that the really active peripheral portion of the nervous system 

 consists of excessively fine nucleated nerve-fibres arranged as a plexiform 

 network. 



"With reference to the diameter of these finest branches of the nerve- 

 fibres, many can be demonstrated and followed for long distances which are 

 less than the j^jVuTfth f an inch in diameter ; and there is reason to think 

 that fibres much finer than this actually exist, and serve as efficient con- 

 ductors of impressions to and from nerve-centres and peripheral parts. 



