One result of the varied arrangement of the tendinous fibres with regard to the muscular, is the production of symmetry and beauty of form ; a second is convenient package; a third is the adaptation of the particular muscle to the kind and amount of exercise which is required of it. Where a great mass of fibrous tissue runs into a muscle, the number of fibres and their ob liquity is very much increased, while the length of each is diminished ; and, as a general result, the power of such a muscle is augmented, while the extent of its contractions is limited. The same mass of contractile material may be arranged as a few long fibres (as in the sar torius), or as many short ones (as in the masseter). In the former case its con tractions would be characterized by their ex tent, in the latter by their power; for, cirteris purifiers, the extent is as the length, the power as the thickness.
The terms origin and insertion are employed with great convenience in ordinary anatomical language to denote the more fixed and the more moveable attachments of muscles. in human anatomy general consent has sanctioned their use, and even, with few exceptions, their par ticular application to each muscle in the body, although this assignment is in many cases arbi trary, in consequence of its being impossible to determine which attachment is the more fre quently the fixed one.
The arrangement of the fibres in the heart has been already fully treated of in this work. (See II EA nr, Fibres of.) In the muscular coat of the alimentary canal, of the bladder, and uterus, the unstriped fibres are disposed, as in the heart, so as to enclose a cavity, but without having, as in that organ, any point at which they can be said to com mence or terminate. In the alimentary tube they are arranged in two lamina., the respective fibres of which take a course at right angles to each other. In the bladder the arrangement is reticulate. The elementary fibres form sets of variable thickness, which at numerous points send off ,detachments to join neigh bouring bundles, whence has sprung the notion that the fibres are branched. It is mani festly, however, the sets of them only that are branched, the • unstriped like the striped fibres being invariably simple from end to end. In the uterus the disposition of the fibres is essen tially similar, calculated to allow of great variety in the capacity of the cavity they encircle.
Of the arcolar tissue of muscles.—This tissue is much more abundant in the voluntary than in the involuntary muscles. In the former it forms an external investment, which sends septa into the intervals between the larger and smaller packets of fibres, and thus enables them to move in some degree independently of one another. The density of these general and partial sheaths is proportioned to the amount of pressure to which the organ may be subject, as is exemplified in the superficial muscles of their back, and in those superficial muscles generally where a fibrous aponeurosis does not perform the same office. The areolar tissue does not usually clothe every individual fibre from end to end, giving it a cellular sheath, except in cases where the elementary fibres are of large dimensions. Besides the protection the areolar tissue affords to the mus cular fibres, it admits of motions between them. But it must also serve the important office of limiting undue motions of one part of a muscle on another part, by its form ing a connecting bond between neighbouring bundles. But a principal use of it appears to be that of furnishing a resisting nidus in which the delicate vessels and nerves can traverse the interstices of the fibres, and by which they can be protected from hurtful dragging during the unequal and oscillating movements of the fibres of a voluntary muscle during its state of activity. This idea is supported by the fact that scarcely any areolar tissue exists in the heart or in the unstriped muscles generally. In the heart, though the contraction is power ful, it is instantaneous or nearly so, and there fore probably more uniformly diffused, so that neighbouring fibres must be less moved on one another than in the more sustained contraction of a voluntary muscle. Moreover the mutual intertwining, of even the elementary fibres in this organ is, in many parts of it, so intricate, as to contribute much to their mutual support. And in the other involuntary muscles, the con tractions are slowly and evenly progressive along the fibres of the same set.
Of the bloodvessels muscles.—The arteries and veins of muscles commonly run together, and most of the arterial branches, to within two removes from the capillaries, ate accom panied by two vene (mites. They invariably pass more or less across the direction of the fibres, divide and subdivide, first in the in tervals between the larger sets, then between the smaller sets, till the ultimate twigs insinuate themselves between the fibres composing the smallest bundles, and break up into their capil lary terminations. In this course the vessels
supply the areolar tissue, their own coats, and the attendant nerves. The capillary plexus of the areolar membrane consists of irregular but pretty equal-sized meshes,and contrasts strongly with that of the muscular tissue itself. The proper capillaries of muscle are quite charac teristic in their arrangement, so that a person who has once seen them can never afterwards mistake them. They consist of longitudinal and transverse vessels, the longitudinal always following the course of the elementary fibres, and lying in the intervals between them, the transverse being short communications placed at nearly equal distances between the longitu dinal ones, and crossing nearly or quite trans. versely over or under the fibres. The manner in which the longitudinal vessels are placed in relation to the fibres, is seen in fig. 286, where I have represented them as they are seen on a transverse section. They usually occupy the interstice between three or mo fibres, but sometimes also the space betwee the contiguous surfaces of two fibres. Th length of the longitudinal vessels does no usually exceed the twentieth of an inch ; in oth words the terminal twigs of the artery and vci pertaining to the same capillary are seldon further than that apart. The length of the transverse anastomosing capillaries necessarily varies with the thickness of the fibres over which they pass (fig. 299). The diameter of the capillaries of muscle varies like that of others with the size of the blood-particles of the animal. It is, however, only just sufficient to allow of the particles to pass. If a frag ment of a frog's muscle, perfectly fresh, be examined, series of blood-particles will be seen in the longitudinal capillaries. These yet they cannot be said to have hitherto contri-' buted, in any great degree, to the elucidation of these mysterious questions. The best mode of inspecting the arrangement of the ultimate vous twigs, is to select a very thin muscle, (as one of the abdominal muscles of any small anima!, or one of the muscles of the eye of a small bird,) to steep it in weak acetic acid, and then thin it out under the compressorium. The primitive tubules of the nerve may then be readily distinguished with a power of 100 to 200 linear. They separate from one another, at first in sets, afterwards in twos, threes, or fours, and if these be followed they will be found ultimately separating from one another, form ing arches, and returning either to the same bundle from which they set out or to some neighbouring one (fig. 300). In this loop like course they accompany to some extent the minute bloodvessels, but do not accurately follow them in their last windings, since their distribution is in a different figure. They pass among the fibres of the muscle, and touch the sarcolem ma as they pass; but as far as present researches have informed us, they are entirely precluded by this structure from all contact with the contractile material, and from all im mediate intercourse with it. flow then shall we explain the transmission of the nervous in fluence to a material thus enclosed ? If it were wise or safe to go a single step in advance of pure observation on so abstruse a question, we might suggest, resting on the seemingly sure ground of exact anatomy, that this in fluence must be of a nature capable of ema nating beyond the limits of the organ which furnishes it. But further than this, as to how, or to what extent this influence may so emanate, particles are compressed and elongated, some times to a great extent, evidently by the nar rowness of the canal which contains them. It may seem at first sight not doubtful that in the living creature these elastic blood-discs are similarly elongated in their passage through the vessels of muscle, but the admirable re searches of Poiseuille will perhaps serve to explain this appearance without our being driven to suppose the presence of so formid able an obstacle to the capillary circulation through these organs. It is more probable that the contraction of the vessels and the com pression of the blood-discs occur, on some of the contents of the vessels being permitted to escape by the severing of the fragment for microscopic examination. The coats of the capillaries of muscle consist of a simple diapha nous membrane, in which a few irregular-shaped cytoblasts occur at infrequent intervals.