6. Of the corpuscles of the elementary fibre. The elementary fibres always contain, among their primitive particles, a number of corpuscles, which either are, or are analogous to, the nuclei of the cells of development, of which this and other structures have originally consisted. These corpuscles are visible in the early stages of growth (fig. 291), but disappear towards the close of fcetal life, as the lines resulting from the deposit of the contractile particles grow dark. The addition of a little acid, how ever, swells the fibre, obliterates the cross lines. and brings the corpuscles into view, not only at this early period, but at every subsequent one, even to old age. In insects, the nuclei in the earliest stage are a single or double series in the axis of the fibre, and in the per fect fibre they hold the same position (figs. 292 and 293, c). In the Vertebrate classes they have a like correspondence, being scat tered equally throughout the mass, in both foetal and adult states. Where the fibre is small, however, they usually abound more towards the surface. They are oval and flat, and of so little substance, that though many times larger than the primitive particles, and lying amongst them, they do not interfere with their mutual apposition and union. These corpuscles are frequently the cause of irregular dark longitudinal streaks, seen in the fibre by transmitted light. They usually contain some central granules or nucleoli (fig. 295). It is doubtful whether the corpuscles or nuclei originally present remain through life, or whe ther successive crops advance and decay during the progress of growth and nutrition. But it is certain, that, as development proceeds, fresh corpuscles are deposited, since their absolute number is far greater in the adult than in the foetus, while their number, relatively to the bulk of the fibre, at these two epochs, remains nearly the same.
7. Of the sarcolemma, or tunic of the ele mentary fibre.—This is a simple transparent homogeneous membrane of extreme tenuity. but very tough and elastic, which, in the form of a perfect tube, invests every elementary fibre, adheres to its surface, and isolates it from sur rounding parts. It is universally present in vo luntary muscles, and may be demonstrated in a variety of ways. When the fibres have been im mersed in alcohol, which causes them to shrink, it is often seen wrinkled on their surface ; or when they are cracked or broken across, it fre quently remains entire and connects the severed fragments (fig. 294). This method of showing it is best followed in the case of the large and brittle fibres of the Skate ; or, it may be seen cut across in a general transverse section of a dried muscle (fig. 290). When the texture of the fibre is destroyed by maceration, the broken mass is sometimes retained by the sheath, which thus becomes visible. When the fibre swells by acid, this tunic resists, and the swollen mass emerges at its broken and open end : but, if this is not effected with suf ficient celerity, the sarcolemma may give way at different points, being burst by the mass, which thus forms hernia:. Such protruding masses being unequally stretched have their transverse and longitudinal lines distorted from their true direction and thrown into very ele gant curves (fig. 295). Again, if a fibre still retaining its irritability, be immersed in water, this fluid, on being absorbed, excites tion, by which it is immediately expelled from among the primitive particles. When thus
forced out it usually collects between the fibre and its sheath, raising the latter in the form of bulls: (figs. 301 and 302, and Art. AIusctiLa R Morro's). The progress of this teresting phenomenon evinces the adhesion that exists between the fibre and its sheath. The bullm immediately subside, by the transudation of their fluid, when the part is placed in thick syrup. I once met with a singular demonstration of the existence and properties of the colemma, in finding it filled with merous trichina (fig. 296), which had taken the place of the contractile terial, the sheath preserving all its cha . . • rriCleFIKIl; Malay and trdnsparelicy• I discovered this remarkable brane in Insects, Crustacea, and all the tribes of Vertebrata, in 1939, not knowing that Professor Schwann had viously described it in connection with the velopment of muscle in Insects and Fish.fi He believes it to be a persistent portion of the membrane of the original cells of de occasionally accomplished in Fishes, and in certain muscles of insects. In these examples the minute detachment of the fibrous tissue may be seen to pass and become attached to the truncated extremity of the fibre. The fibre ends by a perfect disc, and with the whole surface of this disc the tendon is connected and continuous (fig. 297). The sarcnlernma velopment, united to form a single tube, the septa at first resulting from their apposition having been absorbed. This opinion is un doubtedly ingenious ; but, as I have yet no data from which to judge of its correctness, I neither admit nor deny it. I have seen the sarcolemma in human muscle as early as the period of birth, and have traced it at all epochs, to old age, when the atrophy of its contents has often seemed to render it more easy of detection. It also remains in muscles wasted by disease or accident at other periods of life, and no difference appears to occur in it whether the specimens examined are pale or dark-coloured, firm or flaccid. It is thickest in those classes that possess the thickest elementary fibres, viz. in Crustacea and Fish, and so thin in Birds, whose fibres are the smallest, that it is often difficult to detect it at all.
With regard to the use which this singular structure may serve in the economy of the organ, our present ignorance of the manner in which motion is excited renders any explana tion that might be offered of doubtful value. But it has appeared probable to me, first, that it may act as a mechanical protector and iso lator of the contractile tissue enclosed within it; secondly, that its exquisitely smooth external surface may facilitate those rapid minute motions of neighbouring fibres, one against another, which may be shown to occur in contracting muscle (see MuscuLa Alerrots); and, thirdly, that from its apparent similarity in structure to the membrane of the nervous tubules, which run among the fibres, and be tween which and the proper contractile tissue it seems certainly to intervene, as well as from its extensive contact and union with the surface of the contractile tissue, it may be the conducting medium of that infipence, whose mode of propagation the late disco very of the loop-like termination pf the nerves in muscle has hitherto only seemed to render more inexplicable than ever.