5. Internal structure.—Though the elemen tary fibres of all animals are visible to the naked eye, and in some animals, as the Skate (ltaia Batus), are often as thick as a small pin, nothing of their internal organization can be distinguished without the aid of a powerful lens. There is indeed, in certain lights, a splendid pearly iridescence, resulting from the arrangement of their structure, and quite characteristic among the soft tissues; but this is not explained till a high power of the micro scope is brought to bear upon the fibres. They are then seen, when viewed on the side, to be marked by innumerable alternate light and dark lines,whose delicacy and regularity nothing can surpass, and which take a parallel direction across them; and if the focus be altered so as to penetrate the fibre, they are found to be pre sent within it just as on its surface, thus differ ing from those on the trachea of insects, which exist only at the surface. At the extreme border of the fibre the light lines are sometimes seen to project a trifling degree more than the dark ones, thus giving a slight scallop, or regular indentation, to the edge. If often happens, in tearing the fibres roughly with needles before examination, that they crack across, or give way entirely, along one or several of these dark lines, the line of fracture or cleavage running more or less completely through the fibre in a plane at right angles with its axis; and occasionally two or more of such complete cleavages will occur close together, the result of which is the separa tion of so many plates or discs (fig. 287, B), of which the light lines at the surface are the edges, and the corresponding light lines seen within are what may be termed the focal sections. Thus it is evident that there is a tendency in the mass of the fibre to separate, when torn or pulled rf ter death, along the transverse planes, of which the dark transverse stripes are the edges. When such a separation takes place, a series of discs result, but to say that the fibre is a mere pile of discs is incorrect, for the discs are only formed by its disintegration. Neverthe less they are marked out, and their number and form are imprinted, in the very structure of the fibre, in its perfect state. ( Figs. 287 and 288.) But again, it always happens that longitu dinal lines, more or less continuous and pa. rallel, according to the integrity of the fibre and the strength and distinctness of the trans verse lines, are also to be discerned ; and like the transverse ones, not on the surface only, but throughout the whole of its interior. And it is found that there is a remarkable proneness in the fibre to split in the direction indicated by these lines also; by which splitting it is resolved into a great number offbrilla. These fibrilla, like the discs, do not exist as such in the fibre, and to obtain them its structure must be neces sarily broken up to a certain extent, for the union which naturally subsists between these parts must be destroyed. It is therefore most correct to say that there is an indication in the entire state of the fibre of a longitudinal ar rangement of its parts, occasioning a cleavage in that direction on the application of violence. ( Fig. 287.) Sometimes the fibre will split into discs only, more often into fibrilla only, but there are always present in it the transverse and the longitudinal lines which mark the two cleav ages. It is the most common to find a crack or fracture taking both directions irregularly, running partly in the transverse dark lines, partly in die longitudinal dark lines, some times being crosswise on the exterior, more or less lengthwise within. These cracks are often short, even, well defined ; at other times the parts near them are much stretched, or quite disorganized,— differences depending on the brittleness or toughness of the particular fibre, which qualities vary very much in different specimens, according to the state of nutrition, period of examination, and other circumstances.
Hence it is clear that the discs and fibrilla consist of the same parts, and merely result from the different direction in which the mass breaks up. To detach a fibrilla entire is to remove a particle from every disc, and to take away a disc is to abstract a particle of every fibrilla. Thus, every disc consists of a particle of every fibrilla, and every fibrilla of a particle of every disc. Therefore every
fibrilla of the same fibre has the same number of particles, and every disc in like manner is composed of the same number of particles.
If, now, isolated discs and fibrilla be examined under a high magnifying power, they will be found to bear out, in the fullest manner, the description that has been given. The discs are marked on the edge by the fragments of the longitudinal lines, and if regarded on their flat surface, present a finely granular appearance, the granules being equal in diameter with the (fig. 288). In fact, the dark lines be tween the granules are the fragments of the longitudinal lines of the interior of the fibre. Again, the fibrilla, whether taken from the surface or from the interior, are always found to be marked at intervals by transverse dark lines, which are nothing more than the frag ments of the transverse lines seen on and in the fibre. They uniformly correspond with them in distance and force (fig. 287, c). Thus, whether the fibre cleave crosswise or lengthwise, the resulting fragments bear in their structure their respective portions of the lines, taking an op posite course, and evincing a co-existent ar rangement in the opposite direction; and when a detached disc or fibrilla is itself broken, the fracture follows the lines thus imprinted in its structure.
It remains to inquire, what is the nature and meaning of the dark lines so often men tioned ? They can be best examined in the separated discs or fibrilla ; and they appear to be un doubtedly the results of an unequal refraction of the light transmitted through the object. The light spaces intercepted between them, and which by their union constitute the discs and fibrilla, have the aspect of small lenses or particles of higher refractive power than the connecting material, which consequently is in darkness when the inclosed spaces are in focus. By placing the object out of focus, however, the light and dark parts are reversed, which is precisely what occurs with true lenses. I have had a series of beaded rods of glass con structed, which have exactly the same ap pearance as the fibrilla; and when two of these are regarded between the observer and the window, one being in front of the other, and their beads corresponding, the dark circum ferences, visible round the beads of each rod when seen separately, are found to he converted into transverse bars, crossing the rods at right angles in the interval of the beads; or, in other words, forming the elements of the transverse stripes.
My friend, Dr. Gruby, of Vienna, informed me that he had bad spiral rods of glass con structed, which, when placed in front of one another, have the same appearance as that often met with in the fibres, and he conceives the fibrilla to be, consequently, spiral threads: an opinion advanced by Muys, to explain the phenomenon of contraction, but unnecessary for that purpose, and which is quite at variance with all I have observed on the subject. Such spiral rods, however apposed, can never pre sent lines absolutely transverse, such as always exist on the unniniilated fibre, and generally on the detached fibrilla; and the minute zig gags the stripes so often form, and which might, if constant, be possibly explained by the notion of spiral rods, are the mere result of a stretching and disturbance of the direction of the axes of the particles composing the discs and fibrilla. But the cleavage of the fibre into discs is especially opposed to the idea of a spiral form of each fibrilla.
I think it is clear that the dark lines in both directions are not occasioned by a difference of colour, but solely by a variety in refraction ; but on what this difference in refraction de pends it is more difficult to explain. Is the connecting material of a different refractive power, or of the same nature as the particles it unites? If of the same nature, it must be of smaller dimensions, and minute interspaces must be left ; but of the existence of such in terspaces there is no conclusive evidence. It seems more probable that the connecting ma terial is less dense, and fills up every interval; but I do not pretend to determine what may be its nature, or whether it differs chemically from the parts it serves to join.