TO THE ELEMENTAET EIBEES OE STEIPED MUSCLE. 
901 
nerves (Plate XLIV. figs. 27 & 28), and are to be seen in relation with almost all the 
dark-bordered fibres, distributed not to muscle only, but to the tissues of the frog generally 
(palate, peritoneum, intestines, skin, heart, tongue, liver, kidney, &c.). I have also seen 
fine fibres corresponding to these in the mouse, and believe them to exist in vertebrate 
animals generally ; but being so very delicate, they are more difficult to demonstrate in 
mammals than in the frog. These fine fibres distributed on the muscles of the frog 
cannot, by any peculiarity in their appearance, be distinguished from the fine fibres 
which are in direct continuity "svith the dark-bordered fibres. It has been shown that in 
many cases, especially in the muscles of the eye, and in those of young frogs generally, 
and in the palate of the frog at all ages, dark-bordered fibres can be traced, gradually 
becoming finer and finer until the fibres are too thin to be followed. Some of these 
fine fibres running in the sheath of the nerve, result from the division of the dark-bor- 
dered fibres (Plate XLII. fig. 11 d), but it is doubtful if all have this origin. At the 
same time there are many appearances I have seen in various peripheral nerves which 
are much in favour of this view. I shall be able to express myself more confidently 
on this question when the researches upon which I am engaged are completed. A great 
number of these fine fibres with nuclei, running in the same sheath with dark-bordered 
fibres, are represented in Plate XLIV. fig. 28, from the bladder of the frog ; but they are 
much more numerous in this organ than in connexion with the dark-bordered fibres dis- 
tributed to striped muscle. I have seen these fine fibres divide in the same direction, 
that hfrom centre towards periphery, as the coarse fibres ; so that in the trunk of the 
nerve there are dark-bordered fibres which divide as they pass towards the periphery, and 
finer fibres which divide as they pass in the same direction. In the bladder of the frog 
there are large bundles of fine fibres passing in the same sheath with the ordinary dark- 
bordered fibres, as well as numerous trunks composed of fine fibres alone ; but in this 
organ there are many ganglia connected with these fine fibres ; so that it is doubtful if the 
fine fibres I have described in connexion with the muscular nerves are of the same 
character as the fine fibres in the bladder of the frog. The bundles of fine fibres ramifying 
in the same sheath with the dark-bordered fibres distributed to the bladder have certainly 
no connexion with the dark-bordered fibre ; but, on the other hand, it is certain that 
some of the fibres we have been considering are continuous with dark-bordered fibres. 
It has been assumed by many observers that a fibre which exhibits the refractive 
power peculiar to the white substance can alone be regarded as a true nerve ; and the 
point beyond which the white substance cannot be traced has been looked upon as the 
termination of the nerve. Although pale fibres have been recognized in certain 
situations, and their continuity with dark-bordered fibres traced, still the general idea 
seems to be that all true nerves exhibit the dark contours. The pale fibres described 
by KtiHNE as lying beneath the sarcolemma, and by Kollikek upon this membrane, are 
regarded as the terminations of dark-bordered^ fibres with which they are continuous. 
These pale fibres, which are composed of many fine fibres and have been traced so short 
a distance from the dark-bordered fibres in certain instances, are, in my opinion, only 
