It is remarkable that the direction of the cleavage should vary so much in different spe cimens, without it being possible to say on 'what the variety depends : and the question has still to be determined, whether the trans verse and longitudinal modes of union between the particles are the same. It is most likely that they are, and the differences in the regu larity and breadth of the transverse and longi tudinal lines are easily explained on that sup position.
The transverse dark intervals between the particles, being all ranged on the same plane, the edge of which is directed to the observer, when he looks on the side of a fibre, appear as a sharp line, while the longitudinal dark intervals not being on a plane, are seen irre gularly one in front of the other, as a little consideration will spew. Hence the latter seldom seem so definite or regular as the former. Nevertheless their union, seen on the surface of a detached disc, often presents much regularity, and forms curved or straight lines, such as result when a number of balls of equal size are huddled together on a level.
It may be concluded from what has now been advanced, that the discs and tibrillw, (or, in other words, the general mass of the fibre,) are made up of a number of particles, which I have termed primitive particles, or sarcous elements, and which would be obtained in a detached form by a general separation occur ring along the transverse and longitudinal lines visible in the fibre. The existence of these particles, as well as their form and size, is in dicated in the structure of the fibre, while yet entire; but they are united together, and have no independent existence, each being by its very nature a part of the mass, which is ren dered incomplete by the removal of a single element. It. results also from this descrip tion that these particles have no definite outline on all their aspects, being united together; and that they only obtain such an outline on being severed ; on which account it is perhaps impossible to say whether, in the perfect fibre, they be rounded, square, or polygonal.
An example of the strong lateral union of these particles to one another was presented by the specimen from which the following sketch was taken. It consisted of two or three ele mentary fibres from the leg of a newly-born rabbit, which had been kept for some months in weak spirit. They were lying in a curved form on the field of the microscope, and pre sented on the convex edge transverse series of the particles, which, hav ing lost their longitudinal while they retained their lateral union, stood out in relief, as represented in fig. 289, a, a, a.
It sometimes happens that a linear series of them (a fibrilla) is separated, which has the appearance of a necklace of beads, with constricted intervals, while at other times the intervals, though dark, are of equal width with the light or highly refracting particles. Again, it is pos sible, by steeping in acid a transverse section of a dried muscle, to separate the particles considerably from one another, and to see that they are granules acting as lenses, being much more refractive than the material connecting them. Such transverse sections are an artificial division into discs, and the intervals between the par ticles widen out most in specimens taken from birds (fig. 290).
It is in these sarcous elements that the con tractile power resides, and, as they are apt to retain after death the varying effects of the con traction they have undergone during the rigor mortis, it is not easy to give an exact measure ment of their size or shape. An average drawn from very numerous observations shews, how ever, that they are very nearly alike in these respects in all animals and at all periods of life. Their diameter in the longitudinal direction of the fibre, as indicated by the distance between the transverse lines. is thus shown to be :" Their diameter in the opposite direction or that marked by the distance between the long itudinal lines is less, often by a half, but liable to variety from the cause already spe cified.
In a paper, entitled " On Fibre," read before the Royal Society, on the 16th December and the Cth January last,t Dr. Barry describes the fibrilla to be a flat filament rounded at the edges, and deeply grooved along the middle line on both its surfaces. He states that this fiat filament consists of two spiral threads placed side by side, with their coils interlacing : that it " is so situated in the fascicrilus (ele mentary fibre) of voluntary muscle, as to pre sent its edge to the observer ; " and that the curves of the spiral thread, then seen, seem to have been the appearance that " suggested the idea of longitudinal bead-like enlargements producing the stria'." In Dr. Barry's opinion
the dark longitudinal stria are spaces (pro bably occupied by a lubricating fluid) between the edges of flat filaments, and the dark trans verse stria, rows of spaces between the curves of the spiral threads," of which each flat fila ment consists. " In a postscript, the author observes, that there are states of voluntary muscle in which the" (doubly-spiral, fiat,) "lon gitudinal filaments have no concern in the production of the transverse stria, these stria' being occasioned by the windings of spirals, within which very minute bundles of longi tud i nal" (doubly-spiral, flat,)" filaments are con tained and have their origin." This description, so entirely opposed to the more simple view above given (and which was already in type when the paper " On Fibre" was read) demands a brief notice. The paper of which it forms part, might perhaps have been more explicitly entitled, " On the double spiral structure of the organic world ;" for, in it, the doubly-spiral flat filament, giving the appearance of transverse striae to voluntary muscle, is discovered to exist in the interior of the blood-corpuscles of all animals, and " apparently in every tissue in the body. The author enumerates a great variety of organs in which he has observed the same kind of fila ments." "And if the author's view of iden tity in structure between the larger and the smaller filaments be correct, it follows that spirals are much more general in plants them selves than has been hitherto supposed ; spirals would thus appear, in fact, to be as universal as a fibrous structure." " Valentin had pre viously stated, that in plants all secondary de posits take place in spiral lines. In the in ternal structure of animals, spirals have here tofore seemed to be wanting, or very nearly so. Should the facts recorded in this memoir, how ever, be established by the researches of other investigators, the author thinks the question in future may perhaps be, where is the ' secon dary deposit' in animal structure, which is not connected with the spiral form ? The spiral in animals, as he conceives he has shown, is in strictness not a secondary formation, but the most primary of all ; and the question now is, whether it is not precisely so in plants." As these speculations profess to be grounded solely on observations of particular structures, of which muscle is one, I shall make no apo logy for applying my few remarks solely to the account of this structure, which is all that can properly be considered here. A renewed ex amination of this tissue has confirmed, fully and decisively to my own mind, the account I gave of it in 1840, and which was the result of two years' study. 1. I find that when the natural and ready cleavage happens to he into fibrillae (and I do not pretend to explain why this cleavage should be at one time into fibrillae and at another time into discs, I only know the fact,) these solitary and isolated fibrillae do not present any such central longitudinal groove, as Dr. Barry describes, to indicate their double nature: that the cross lines are usu ally transverse, and not oblique, by which I mean that the spaces they bound have a rectan gular outline, so sharp and definite, that the mind rests entirely satisfied that there cannot be two opinions concerning them, between any who have examined the object in one of Powell's best microscopes, and with the use of that admirable definer and clarifier of the image, the achromatic condenser. That right angles can be produced by a spiral, whether double or single, however distorted by accident or violence,' it is impossible to conceive. That these transverse lines may sometimes become oblique by irregular traction, such as is almost necessarily applied in preparing the object, is most easy to understand, if we bear in mind, that the substance in the different spaces which they circumscribe is one united mass. 2. The transverse cleavage of the elementary fibre, which I first showed to be occasionally so com plete as to separate it into discs, cannot be reconciled with Dr. Barry's statement. For the surfaces of such discs present, as in fig. 288, a fine granular aspect, and no ends of doubly spiral threads. And the definite and beautiful appearance presented by a transverse section of the fibre in all animals, but espe cially in Birds (fig. 290), is totally at variance with his views : for the particles there dis played are highly refracting, round, and not aggregated in pairs. The condition repre sented in fig. 289 is not less opposed to them. Other proofs might be adduced, but they would lead to greater detail than is compatible with the form of the present publication ; and perhaps they will be allowed to be unneces sary.