Page:EB1911 - Volume 19.djvu/66

 As the common white mica obtainable in thin, transparent cleavage sheets of large size it was formerly used in Russia for window panes and known as “Muscovy glass”; hence the name Muscovite, proposed by J. D. Dana in 1850. It crystallizes in the monoclinic system; distinctly developed crystals, however, are rare and have the form of rough six-sided prisms or plates: thin scales without definite crystal outlines are more common. The most prominent feature is the perfect cleavage parallel to the basal plane (𝑐 in the figure), on which the lustre is pearly in character. The hardness is 2–2, and the spec. grav. 2·8–2·9. The plane of the optic axes is perpendicular to the plane of symmetry and the acute bisectrix nearly normal to the cleavage; the optic axial angle is 60–70°, and double refraction is strong and negative in sign.

Muscovite frequently occurs as fine scaly to almost compact aggregates, especially when, as is often the case, it has resulted by the alteration of some other mineral, such as felspar, topaz, cyanite, &c.; several varieties depending on differences in structure have been distinguished. Fine scaly varieties are damourite, margarodite (from Gr. , a pearl), gilbertite, sericite (from  , silky), &c. In sericite the fine scales are united in fibrous aggregates giving rise to a silky lustre: this variety is a common constituent of phyllites and sericite-schists. Oncosine (from , intumescence) is a compact variety forming rounded aggregates, which swell up when heated before the blowpipe. Closely related to oncosine are several compact minerals, included together under the name pinite, which have resulted by the alteration of iolite, spodumene and other minerals. Other varieties depend on differences in chemical composition. Fuchsite or “chrome-mica” is a bright green Muscovite containing chromium; it has been used as a decorative stone. Oellacherite is a variety containing some barium. In phengite there is more silica than usual, the composition approximating to H2KAI3(Si3O8)3.

Muscovite is of wide distribution and is the commonest of the micas. In igneous rocks it is found only in granite, never in volcanic rocks; but it is abundant in gneiss and mica-schist, and in phyllites and clay-slates, where it has been formed at the expense of alkali-felspar by dynamo-metamorphic processes. In pegmatite-veins traversing granite, gneiss or mica-schist it occurs as large sheets of commercial value, and is mined in India, the United States and Brazil (see ), and to a limited extent, together with felspar, in southern Norway and in the Urals. Large sheets of Muscovite were formerly obtained from Solovetsk Island, Archangel.

MUSCULAR SYSTEM (Anatomy ). The muscular tissue (Lat. musculus, from a fancied resemblance of certain muscles to a little mouse) is of three kinds: (1) voluntary or striped muscle; (2) involuntary or unstriped muscle, found in the skin, walls of hollow viscera, coats of blood and lymphatic vessels, &c.; (3) heart muscle. The microscopical differences of these different kinds are discussed in the article on. Here only the voluntary muscles, which are under the control of the will, are to be considered. The voluntary muscles form the red flesh of an animal, and are the structures by which one part of the body is moved at will upon another. Each muscle is said to have an origin and an insertion, the former being that attachment which is usually more fixed, the latter that which is more movable. This distinction, however, although convenient, is an arbitrary one, and an example may make this clear. If we take the pectoralis major, which is attached to the front of the chest on the one hand and to the upper part of the arm bone on the other, the effect of its contraction will obviously be to draw the arm towards the chest, so that its origin under ordinary circumstances is said to be from the chest while its insertion is into the arm; but if, in climbing a tree, the hand grasps a branch above, the muscular contraction will draw the chest towards the arm, and the latter will then become the origin. Generally, but not always, a muscle is partly fleshy and partly tendinous; the fleshy contractile part is attached at one or both ends to cords or sheets of white fibrous tissue, which in some cases pass round pullies and so change the direction of the muscle’s action. The other end of these cords or tendons is usually attached to the periosteum of bones, with which it blends. In some cases, when a tendon passes round a bony pulley, a sesamoid bone is developed in it which diminishes the effects of friction. A good example of this is the patella in the tendon of the rectus femoris (fig. 1, P.).

Every muscle is supplied with blood vessels and lymphatics (fig. 1, 𝑣, 𝑎, 𝑙), and also with one or more nerves. The nerve supply is very important both from a medical and a morphological point of view. The approximate attachments are also important, because unless they are realized the action of the muscle cannot be understood, but the exact attachments are perhaps laid too great stress on in the anatomical teaching of medical students. The study of the actions of muscles is, of course, a physiological one, but teaching the subject has been handed over to the anatomists, and the results have been in some respects unfortunate. Until very recently the anatomist studied only the dead body, and his one idea of demonstrating the action of a muscle was to expose and then to pull it, and whatever happened he said was the action of that muscle. It is now generally recognized that no movement is so simple that only one muscle is concerned in it, and that what a muscle may do and what it really does do are not necessarily the same thing. As far as the deeper muscles are concerned, we still have only the anatomical method to depend upon, but with the superficial muscles it should be checked by causing a living person to perform certain movements and then studying which muscles take part in them.

Muscles have various shapes: they may be fusiform, as in fig. 1, conical, riband-like, or flattened into triangular or quadrilateral sheets. They may also be attached to skin, cartilage or fascia instead of to bone, while certain muscles surround openings which they constrict and are called sphincters. The names of the muscles have gradually grown up, and no settled plan has been used in giving them. Sometimes, as in the coraco-brachialis and thyro-hyoid, the name describes the origin and insertion of the muscle, and, no doubt, for the student of human anatomy this is the most satisfactory plan, since by learning the name the approximate attachments are also learnt. Sometimes the name only indicates some peculiarity in the shape of the muscle and gives no clue to its position in the body or its attachments; examples of this are biceps, semitendinosus and pyriformis. Sometimes, as in the flexor carpi ulnaris and corrugator supercilii, the use of the muscle is shown. At other times the position in the body is indicated, but not the attachments, as in the tibialis anticus and peroneus longus, while, at other times, as in the case of the pectineus, the name is only misleading. Fortunately the names of the describers themselves are very seldom applied to muscles; among the few examples are Horner’s muscle and the