240 Dr. Beale Croonian Lecture. [May 11, 



Division of the dark'bordered nerve-fibres as they approach their distri- 

 bution. 



It is to be specially noted that the dark-bordered fibres very frequently 

 divide, and, in consequence of the fibres being exceedingly thin at the points 

 of division, which occur, for the most part, just where a bundle of fibres 

 divides into two branches, they are seen only in very carefully prepared speci- 

 mens. Although Wagner long ago showed that dark-bordered fibres un- 

 derwent subdivision, the numerous subdivisions which I have demonstrated 

 in all dark -bordered fibres near their peripheral distribution and also as they 

 pass towards the nerve-centre, have not been generally observed. The 

 number of fibres into which a single dark-bordered fibre divides is very great 

 in a comparatively short course. The resulting subdivisions pursue very 

 different directions, and can often be followed for a considerable distance as 

 they run with other dark-bordered fibres. From this it follows that many 

 different parts of a muscle at a distance from one another may be supplied 

 with nerves which result from the subdivision of a single dark-bordered 

 fibre.. 



The fibres resulting from the subdivision of the dark-bordered fibres are 

 of less diameter than the parent trunks ; but the area of the section of two 

 fibres would invariably be much greater than that of the parent trunk. For 

 the most part the fibres divide dichotomously ; but sometimes a fibre is 

 seen to divide into as many as three or four divisions, and in muscle five, six, 

 or even more dark-bordered fibres have been seen to result from the division 

 of one. The finer dark-bordered fibres often run in the same bundle with 

 coarse dark-bordered fibres, the former being in fact much nearer to their 

 ultimate destination than the latter. The dark-bordered fibres distributed 

 to the muscles of the frog often divide into two very fine fibres, as repre- 

 sented in several of these figures. These fibres may be traced onwards for 

 some distance. They do not exhibit the dark-bordered character. They 

 appear pale and granular, and connected with them at varying intervals are 

 nuclei. These pale nucleated fibres in the frog are often less than the 



* of an inch in diameter. They are nevertheless compound, and 

 consist of bundles of still finer fibres. These in fact, although much 

 narrower, correspond to the pale, granular, but nucleated intermuscular 

 nerves first described by me in the muscles of the mouse (Phil. Trans. 1860). 

 The very fine compound fibres still continue to divide and subdivide, and 

 assist to form plexuses and networks in precisely the same manner as the 

 dark-bordered fibres, of which they are the continuation. It is quite cer- 

 tain that these pale fibres are true nerve-fibres, for they are directly conti- 

 nuous with the dark-bordered fibres. Instead of breaking up into one or 

 more bundles of fine fibres, a dark-bordered fibre not unfrequently divides 

 into a finer dark -bordered, and a bundle of fine fibres, as represented in this 

 drawing from the frog's mesentery. 



