8. Of the extremities if the elementary fibres, and their attachment to other struc tures.—Every fibre is fixed to fibrous tissue, or to something analogous to it; but an accurate examination of this difficult subject gives no countenance to the ordinarily received opinion that this tissue is prolonged over the whole fibre from end to end, as its cellular sheath; ripr is this view reconcileable with the physical require ments of the case. After many trials I have never succeeded in isolating a muscular fibre ith the tendinous fibrillae pertaining tp it, in iffier Mammalia or I3irds; but this ceases abruptly at the circeniference of the terminal disc, and here some small part pf the tendinous material appears to 'be joined to it. The same disposition may be well seen in the legs of certain insects (fig. 293). In other cases, where the muscle is fixed obliquely to a membranous surface, each fibre is obliquely truncated at its extremity, at an angle deter mined by the inclination of its axis, instances of which may be seen in the limbs of Crusr tacea, and elsewhere.
9. Development.—Th p researches of Valentin and Schwann have shewn that a muscle con sists in the earliest stage of a mass of nu, cleated cells, which first arrange themselves in a linear series, with more or less regularity, and then unite to constitute the elementary fibres. As this process of agglutination of the cells is going forward, a deposit of contractile material gradually takes place tvithin them, commencing on the inner surface and ad vancing towards the centre, till the whole is solidified. The deposition occurs in granules, which, as they conic intp view, are seen to be disposed in the utmost order, according to the two directions already so often mentioned. These granules are the sarcous elements, and being of the same size as in the perfect the transverse stripes resulting from their apopo, sitipn are of the same width as in the adult; but as they are very few in number, the fibres, which they compose are of corresponding tenuity. From the very first period of their for.: !nation these granules are parts of a mass and pot independent of one another, for as soon as solid matter is deposited in the cells, (Ai n t indications of a regular arrangement in gra nules are usually to be met with. It is com mon for the longitudinal lines to become well defined before the transverse ones. When both are strongly marked, as is always the case at birth, the nuclei of the cells, which were before visible, disappear, being shrouded from view by the dark shadows caused by the mul titudinous refractions of the light transmitted through the mass of granules; but, as before gated, they can still he shown to exist by im mersion in a weak acid, which, while it swells the fibrinous material of the granules and obliterates their intervening lines, has no action upon the nuclei.
b. Of the unstriped elementary variety possesses far less interest than the other in consequence of its apparent simplicity of structure. The fibres consist of flattened bands, generally of a pale colour, bulged at frequent intervals by elongated corpuscles similar to those of striped muscle, and capable of being displayed by the same process (fig. 298). The
texture of these fibres seems to be homoge neous. By transmitted light they have usually a soft, very finely mottled aspect, and without a darkly-shaded border. Sometimes the mot tling is so decided as to appear granular, and occasionally these granules are arranged in a linear series for some distance. This condition is probably an approximation towards the struc ture of the striped fibre, for I have observed the granules to be about the size of the sarcous elements already described It is generally to be seen more or less distinctly in the gizzard of Birds, and I have now and then met with it in the fresh muscle of the stomach, intestinal canal, urinary bladder, and uterus of Nam malia. The ordinary diameter of the unstriped fibre is from to of an inch. From this account of the appearance of these fibres, it might he expected that their discrimination from other tissues would be often difficult. And, in fact, it is so to an inexperienced eye. The peculiar texture, however, the size, the soft margin, and, above all, the presence of nume rous elongated oval corpuscles, with two or three granules in their centre, are characters which, taken together, I believe to be decisive. As a number of fibres usually take a parallel course together, the bulgings occasioned by the corpuscles give rise to partial longitudinal shadows, extending for some way beyond the corpuscles, in the intervals of the fibres. As these irregular longitudinal shadows occur pretty uniformly throughout an aggregation of fibres, and as some are necessarily out of focus, while others are in focus, the whole mass com monly presents a very confused reticulate ap pearance, which has given rise to an almost universal notion that these fibres do, in reality, interlace one with another, and do not run parallel. This notion, however, is, in most cases, erroneous. It is doubtful whether these fibres are invested in a sarcolemma: none has hitherto been detected in an unequivocal man ner. It is also still a matter of speculation how they terminate, or whether they in all instances have a termination. In the case of the more or less circular set of fibres, inclosing the small intestine, for example, it is uncertain whether each fibre surrounds the canal once, returning into itself as a ring, or, more than once, as a spiral, or whether it passes only par tially round it, the rest of the circle being completed by others. Whether the areolar tissue (the representative of the fibrous) that is always found in connection with these fibres, serves to give them an attachment by union with their extremities, or by involving them in its meshes, is also altogether unknown. In the gizzard of Birds the ends of the fibres ate united to white fibrous tissue, thus making an approximation to the striped fibre, as they do in colour. But I have not been able, after diligent search, to detect the true transverse stripes, which Ficinus describes to exist in this organ.