900 
PEOFESSOE BEALE ON THE DISTEIBUTION OF NEEVES 
meet with finer and still finer plexuses — the finest visible fibres being probably com- 
posed of more than a single fibre. 
The chief differences observed between the arrangement of the nerves in the muscular 
fibres of the mouse, described by me in the Philosophical Transactions for 1860, and those 
of the frog, are, that the fibres are much finer in the latter than in the former, and the fibres 
themselves, and the nuclei in connexion with them, are much more abundant and closer 
together in the mouse than in the frog. The position of the fibres with regard to the sar- 
colemma and the muscular tissue, and their relation to the capillaries, is the same in both. 
I now pass on to the consideration of the fine fibres connected with the nerves, and 
often seen running in the sheath with the dark-bordered fibres, which have already been 
alluded to in this paper. These fibres have been regarded as fibres of connective tissue, 
but they have not been carefully studied. The evidence in favour of these being real 
nerve-fibres is as strong as that upon which the nervous character of the fine fibres, 
which are the direct prolongations of the dark-bordered fibres, rests. 
Fine Nerve-Jibres imbedded in the Sheath. 
The following observations are of the greatest importance with regard to the general 
question of the mode of distribution of nerve-fibres. 
As a dark-bordered nerve crosses the muscular fibres, it often gives off several very fine 
branches. In Plate XLIII. fig. 19 a nerve is represented from which two branches are 
seen to proceed and divide upon the surface of the same muscular fibre, and other 
branches are given off a short distance from the point at which these branches leave the 
trunks. These fibres are not connected with those portions of the darJc-borde)'ed fibres 
seen in the figure, although their arrangement exactly corresponds with the fine fibres 
prolonged from the dark-bordered fibres. Compare the fibres marked 5, 5 in fig. 19 -with 
b in figs. 13&14. These fine nerve-fibres are very numerous. They often run pai'allel 
with the dark-bordered fibres for some distance, perhaps cross them several times, divide 
and give off fibres which pass off in different directions. The line which appears to be 
the outline of the tubular membrane of nerve-fibres near their periphery, is frequently 
found to consist of two or more very fine fibres, which are in many instances comiected 
with nuclei, which, it is generally believed, are the nuclei of the tubular membrane. I 
have specimens in which five or more very fine fibres can be demonstrated between the 
dark-bordered fibre and the outline of the tubular membrane (Plate XLI. et seg., figs. 7, 
8, 9, a, 11, 15, 18, 21, 24, 25, 26, 27, 28, 29, & 31). These branches leave the trunk at 
intervals and pass to their distribution, or pursue their course with other fibres. Usually 
two or more pass off at the same point; and in the rare instances in which one fibre 
passes by itself to its distribution, one can never feel confident that it is really single. 
In those fortunate specimens in which it can be followed for a short distance, it can be 
seen to divide into two branches which pursue opposite courses ; and very often at the 
point of division a third fibre can be seen. 
The fine fibres I have described are very numerous near the point of distribution of the 
