616 
PEOFESSOE BEALE OX THE DISTEIBUTIOX OF ^'EPA'ES 
branches, apparently proceeding from the axis cylinder, pass through the white sub- 
stance and run parallel with the fibre for a long distance, then leave this fibre and 
pursue their course in close relation with another. This transparent tissue becomes 
much spread out as the nerve-fibres approach their distribution, and numerous delicate 
fibres, connected both with nerves and vessels, may be seen in it. This delicate texture 
might be described as a tissue in which the ners'es and capillaries are imbedded. In 
conjunction with the nerves and capillaries, it forms an exceedingly thin expansion or 
membrane on the external sm’face of the elementaiy fibre. In some specimens it would 
appear that this is incorporated with the sarcolemma itself, but this point is doubtful. 
I have some preparations in which it certainly appears as if the injection had passed 
between this membrane and the sarcolemma, and, as before stated, I have certainly 
removed capillaries and nerves from the surface of the elementary fibre, leading the 
sarcous material apparently invested with its sarcolemma ; but this point is of coui'se 
very difficult to demonstrate. 
Of the Axis Oylinder and the White Substance. 
I have been led to conclude that the axis cylinder dmdes and subdi^Ides into many 
branches, so that one fibre in the trunk of a nerve may cany impressions to or from a 
comparatively large extent of surface. I have not yet been able to determine by actual 
observation if a branch of one axis cylinder may run in apposition with one or more 
branches from the axis cylinder of another trunk at a distance from the first, or derived 
from another source ; but from appearances I have observed I think this possible. The 
axis cylinder often exhibits a tendency to split, but I have not yet been able to ascer- 
tain if it consists originally of a great many fibres w'hich ultimately become fused into 
one, or of one fibre which gives off numerous branches ; but from w hat I have seen, I 
feel sure that towards the distribution of the nerve the axis cylinder divides and sub- 
divides very freely, the amount of investing substance which surrounds it (w hitc sub- 
stance of Schwann) becoming less and less, until in the finer ramifications it is not 
possible to say that a fibre consists of an axis cylinder and w hite substance ; for its 
general appearance and refractive power are the same in every part, except w here the 
nuclei are situated. The arrangement of the so-called tubular membrane has been 
already explained. It is not unreasonable to suppose that the definite character of the 
wliite substance of Schwann and the axis cylinder, in the larger nerve-fibres, results 
from changes w^hich occur during the gradual growth and altered relations of the fibres 
w hich take place during the development and growlli of the entire organism. The full 
discussion of these points demands a knowledge of the history of the development of the 
structures referred to. This w^e do not yet possess ; but nevertheless I may perhaps be 
allowed to state briefly the conclusion to w'hich 1 have arrived from investigations w Inch, 
how'ever, cannot be regarded as completely satisfactory. The axis cylinder, towards the 
ultimate ramifications of the nerve-fibres, loses its fibrous or band-like character, and in 
the ultimate subdivisions, the entire extent of the nerve-fibre possesses the same compo- 
