ANATOMY other, and thereby the conveyance of whatever influence travels from neurone to neurone may be accentuated or hindered. The details of these groupings and of nerveendings will be considered in connexion with the splanchnology of the nervous system. (For further particulars of the neurone theory, see Hoche, Die Neuronenlehre, Berlin, 1899.) Each bundle of processes which leaves the central masses of nerve-tissue in its course towards its peripheral termination is called The courses these in nerves he Nerves. followeda nerve.by dissection, but theofmodes whichcanthey end can only be ascertained by methods of staining, sectioncutting, and microscopical examination. Within the central masses of nerve-tissue the courses of the fibres are intricate and obscure, and methods of staining and section give only imperfect differential criteria whereby individual bundles may be traced. The two methods which are most instructive are those which may be respectively called the experimental and the embryological:—(1) When an axon is detached from its cell it degenerates through all its length to its terminal tuft; so by dividing the axons coming from any group of cells their course, as distinguished from otherwise arising axons, becomes apparent in a series of successive sections stained to show such as are degenerated. (2) It is found by observation on developing nerves that some fibres acquire their myelin sheath sooner than others ; so by making successive sections of an immature part, and staining the myelin sheath, any group of fibres already myelinated can be differentiated from those not as yet invested with this layer. Muscle is contractile tissue derived from the cells of the middle embryonic layer and consisting of elongated fibre-like cells, whose reticulum is regularly distissue^ Pose(l) an(l whose enchylema or trophoplasm is charged with myosin and other proteids. These protoplasm-elements are so organized that when the cell contracts its two extremities are approximated. The three varieties of muscle—unstriped, cardiac, and striped—as well as some of the views which are currently held respecting their minute structure have been referred to in vols. i. p. 856 and xii. p. 8; but as yet the real nature of its organization has not been made out, and it is not improbable that many of the appearances described by observers are factitious, produced by the actions of reagents. The observations of Kambn y Cajol and van Gehuchten have led them to regard the fibre as consisting of a reticulum comparable with the cytomiton of a cell in whose interstices is an inter-reticular enchylema. Rollett, on the other hand, regards the reticulum as being composed of partitions of non-contractile sarcoplasm around a series of contractile sarcostyles. Merkel and Engelmann believe that the fibre consists of alternating masses of two materials which differ in their optical properties. One of these is anisotropous, and this may possibly during contraction absorb, or during relaxation discharge, a fluid yielded to it by the isotropous band, altering thereby its width and surface tension. In view of the conflict of opinion new methods of research are required before any positive pronouncement can be made as to the ultimate histological analysis of the muscle fibre. (For discussions of these views, see van Gehuchten, Anat. Auz. 1888-89; v. Kolliker, Gewebelehre, 1890; Rollett, Wiener Sitzungsber. 1889; Engelmann in Hermann's Physiology, i. 2; and M‘Dougall, Journ. of Anat. Jan. 1898.) Whatever may be their intrinsic structure, the mode of grouping of striped or voluntary fibres is easily ascertained. I hey are by a fine areolar tissue, called endomysium, Structure jsurrounded n w]1jcj1 p]ie nutrient capillary vessels ramify and by the means of which they are united into bundles. Each muscle. such fascicle of fibres is surrounded by an areolar sheath or perimysium. Groups of these bundles are united into larger masses, which are so arranged as to be capable of acting as motor units in the economy of the body. Each of these is invested with an areolar sheath, and is called a muscle. The ends of each muscular fibre are attached to the connective tissues with which they are in contact. In general, there is a filament of white fibrillar tissue firmly united to the end of each fibre, and the
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collective masses of these filaments at the extremities of a muscle are called its tendons. By these the muscle is tied to the parts of the skeleton, from and on which it acts. The lengths of the fleshy fascicles of muscular tissue are proportional to the mobility of the point of insertion, being just long enough to draw that part to the extreme of its possible excursion ; so, if the distance between origin and insertion exceed this, the tendons at either or both ends are of sufficient length to occupy the remaining space. It must be remembered, however, that, embryonically, tendon and muscle arise independently, and that their connexion is secondary. Muscles are apt to degenerate from want of use. In extreme cases the contractile tissue vanishes, leaving its connective framework as a fibrous mass. This takes place by natural selection, as muscle is an expensive structure to maintain owing to the amount of nutrition it requires. On this account, also, the fleshy bellies of muscles tend to become accumulated as near the base of supplies as possible; that is, as near the main vessel of the limb or the axis of the body. The arrangement of the fascicles is not the same in all muscles. In some they run in direct lines, from origin to insertion, as in the rectus abdominis ; in others, such as the pectorals, they converge from a wide origin to a narrow insertion so as to concentrate their useful work in moving one point. Muscles of the former group are called prismatic or bandlike ; those of the latter, triangular or pyramidal. In a third group the fibres pass obliquely from origin to insertion, so that their action is decomposable into two components, one of approximation and one of translation, the relative sizes of which depend on the angle of the slant of the fibre with regard to the axis of the bone into which they are inserted. Owing to the exigencies of the formation of the skeleton most muscles belong to some species or other of this third class of rhomboidal muscles. A very common special case of this group is that of penniform muscles, in which a large number of short fibres are attached obliquely to an elongated tendon, as the barbs of a feather are to its axis. By this formation the number of short fibres in a slender muscle can be greatly increased without its becoming unduly thickened at any point. There is a certain small loss of useful work owing to the obliquity of the fibres, but this approximation-element is generally a negligible quantity, being only equal to the product of the force into the sine of the angle, which is usually very small. One or more nerves or blood-vessels enter into each muscle. The point of entrance is at that spot at which the muscle moves least on the contiguous structures, and is usually on Nervous the aspect which is farthest from the surface, and, in and the case of the vessels, nearest to the main artery of vascular the limb. The nerve on entering breaks up into supply. branches in various ways, the exact pattern of division depending on the arrangement of the fibres in the muscle. (See Disse in Journ. of Anat. Oct. 1897.) Ultimately, a nerve-twig passes into each muscular fibre and terminates in an end plate. The proportion between the number of nerve fibres entering a muscle and that of muscle fibres is in the external rectus oculi 1 :8, in the soleus 1 : 350. In general, the nerve to each muscle comes from a single source, but in a few cases muscles are diploneural. Thus the brachialis anticus is supplied both by median and musculo-spiral nerves. The double supply generally indicates the fusion of two muscle germs, which have originated independently. When the area of origin of a muscle includes parts which are discontinuous or independently movable, the portion arising from each such part remains partially separate from the rest, and the muscle becomes two-headed. This condition may also occur from the interjection of a vessel or nerve, or from the synthesis of separate segmental elements in the one muscle. In like manner, a double insertion may arise from the separate mobility of the structures into -which the muscle is inserted. Some muscles present two or more successive fleshy bellies, and are called in consequence digastric or polygastric. In such cases the several bellies have been separate morphological units, and have independent nerve-supplies. Connective tissues are derived from the middle embryonic lamella and make up more than two-thirds of the body, forming its framework and the supporting skeleton of the more metabolic and physiologically active elements of each organ. Every epithelial layer is based on a lamella of this tissue; every muscular fibre is sheathed by a connective layer, and is attached to, or acts on, the connective tissues around its extremity. In this tissue there are three form-elements— (1) cells, which are usually flattish, often processed, rarely or never contractile, and consist of a distinct cell-wall and a comparatively small amount of protoplasm ; (2) intercellular substance, copious in amount and traversed by
