Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/884

Rh 834 ANATOMY [.JOINTS AND MUSCLE; &amp;gt; [II tremities, one of which is the head or the origin, tiie other the in-sertion. The belly is the fleshy part of the muscle, and possesses a deep-red characteristic colour; it is the active contractile structure, the source of motor power. The two extremities are called the tendons of the muscle, or sinews; the tendons are bluish- white in colour, possess no power of contractility, and are merely, as it were, the ropes by which the belly of the muscle is attached to the bone or other structure which is moved by its contraction. The term tendon of origin, applied to one extremity of the muscle, signifies the fixed end of the muscle, that to which it draws during its contraction; as a rule this is the end nearest the trunk, the proximal end. The term tendon of insertion in applied to the end which is moved by the contraction; as a rule this is tho end most removed from the trunk, the distal end. Entering the sub stance of each muscle is at least one artery, which conveys blood for its nutrition; this artery ends in a net work of capillary blood-vessels, from which a vein arises and conveys tho blood out of the muscle again; an other small vessel, called a lymphatic, also arises within the muscle, and conveys the fluid lymph out of the muscle. Each muscle also is pene trated by a nerve, through which it is brought into connection with the brain, so as to be subject to the in- Fio.m-ThUectns muscle fluence of the will. The will is the of the thi e h ; to show natural stimulus for exciting muscular action, which action is in many cases so rapid that scarcely an appreci able interval of time intervenes be tween willing and doing the action. The bones form a series of rod-like levers, and, in study ing the mode of action of the muscles, the place of inser tion of the muscle into the bone that is to say, the point of application of the power which causes the movement and its relations to the joint, or fulcrum, or centre of motion, and to the weight or resistance which is to be overcome, have to be kept in view. The relative positions of ful crum, point of application of power, and resistance, are not the same in all the bony levers. As a rule, the muscles are inserted into bones between the fulcrum and the move- able point of resistance, and nearer the fulcrum than the movable point, as may be seen in the muscles which bend the fore-arm at the elbow-joint. Although from the weight- arm of the lever being in these cases much longer than the power-arm, the muscles, as regards the application of the power, act at a disadvantage, yet the movement gains in velocity. Sometimes the muscle is inserted, as is the case in the great muscle which straightens or extends the fore-arm, at one end of the lever, and the fulcrum or joint is placed between it and the movable point. At other times, as in the case of the chief depressor muscle of the lower jaw, whilst the muscle is attached to one end of the lever, the fulcrum is at the opposite end. When a muscle is so placed that its tendon of insertion is perpen dicular to the bone to which it is attached, it acts to great advantage; when placed obliquely or nearly parallel, a loss of power occurs. Many muscles at the commencement of contraction lie obliquely to the bones which they move, the constituent parts of a muscle. 11, the fleshy belly; to, tendon of ori gin; ti, tendon of inser tion ; n, nerve of supply. ; a, artery of supply ; v, vein ; I, lymphatic vessel ; P, the patella. but as contraction goes on they become more nearly per pendicular, so that they act with more advantage near the close than at the commencement of contraction. If a muscle passes over only one joint, it acts on that joint only; but if it passes over two or more joints, it acts on them in succession, beginning with the joint next the point of insertion. A given movement may be performed by the contraction of a single muscle, but as a rule two or more muscles are associated together, and they are not unfre- quently so arranged that one muscle initiates the move ment, which is then kept up and completed by the rest. Muscles producing movement in one direction have opposed to them muscles which by their contraction effect tho opposite movement; when both groups act simultaneously and with equal force, they antagonise each other, and no motion is produced; when a muscle is paralysed or divided, its antagonistic muscle draws and permanently retains tho part to its own side. The rapidity of action of a muscle is proportioned to the length of its fasciculi, its power of contraction to their number. Each muscle is invested by a sheath formed of connective tissue. In the limbs and in the neck not only has each muscle a sheath, but a strong fibrous membrane envelopes the whole of the muscles, and assists materially in giving form and compactness to the region. This membrane is called generally a fascia or aponeurosis, but special descriptive names are given to it in the different regions e.g., cervical fascia, brachial aponeurosis, fascia lata, or fascia of the thigh. In some localities muscles arise from the fascia, and in others they are inserted into it. The fascia is separated from the skin by a layer of subcutaneous fatty tissue, and in this layer muscles are in some localities de veloped. In the fat of the inner border of the palm of the hand a small muscle, the palmaris brevis, is found, which is inserted into the skin covering the ball of the little finger ; at each side of the neck, also, lies a thin muscle called platysma myoides, and the muscles on the face and scalp which move the skin of the face and head belong to the same category. These muscles form the group of sub cutaneous or dermal muscles which, except in the localities above referred to, are not represented in the human body, but are well known in the bodies of the mammalia gene rally as the panniculus carnosus. In arranging the muscles for descriptive purposes, either a morphological, a topographical, or a physiological method may be pursued. The morphological arrangement is to be preferred when the object is to compare the muscular system in man with that in different animals, and the basis of the arrangement should be into muscles of the axial, the appendicular, and the axi-appendicular skeletons, and sub-cutaneous muscles; a topographical arrangement is most suitable for the purposes of the practical surgeon ; a physio logical arrangement, when the object is to study the action of the muscles in connection with the movements of the joints. In Plates XV. and XVI., a front and back view of the voluntary muscles of the body is given. JOINTS AND MUSCLES OF THE AXIAL SKELETON. The Tntervertibml Joints are complex in construction. Intcrv The bodies of the true vertebra are connected together by bral J an amphiarthrodial joint : the fibro-cartilaginous plate or intervertebral disc is tough and fibrous in its peripheral part, but soft and pulpy within. (Fig. 15.) Eemains of the chorda dorsalis are said to occur in the soft pulp, and some times a distinct cavity, lined by a synovial membrane, is found in the centre of the disc, which in the finner whales is expanded into a large central cavity containing many ounces of synovia. A diarthrodial joint connects the supe rior and inferior articular processes of adjacent vertebrae on each side. Elastic yellow ligaments, the ligamenta sub/Java,