Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/894

Rh ANATOMY [JOINTS AND MUSCLES enclosed in a canal by a strong band, the anterior annular ligament, and their surfaces are invested by a synovial membrane, which facilitates their movements to and fro beneath that ligament ; as they pass downwards in front of the fingers they are enclosed in a strong fibrous sheath lined by a synovial membrane, and the tendon of the super ficial flexor is pierced by the deep flexor, so that the latter may reach the third phalanx into which it is inserted. Four rounded muscles, the lumbricales, arise in the palm from the deep flexor tendons, turn round the radial borders of the first phalanges, and are inserted one into the extensor tendon on the dorsum of each finger ; these muscles bend the first phalanges on the metacarpal bones, but from their insertion into the extensor tendons they also extend the second and third phalanges on the first ; as they are much used in playing stringed instruments, they have been called 1 &quot;fiddlers muscles.&quot; Thefingers are extended or straightened by muscles inserted into the back of the second and third phalanges ; the extensor muscles descend from the back of the fore-arm, one, the common extensor, subdivides into four tendons, one for each finger, but in addition the index and little have each a separate extensor muscle, the tendon of which joins that of the common extensor. The index finger possesses more independent movement than the other digits hence its more frequent use as a &quot; pointer;&quot; the extensor tendons of the little and ring fingers are usually united together, so that these digits are associated in their movements. Abduction and adduction of the fingers are caused by seven small muscles situated in the intervals between the metacarpal bones, hence called interossei; four of these lie on the back of the hand, three on its palmar surface ; they are inserted into the sides of the first phalanges, and eUier pull the fingers away from a line drawn through the middle finger or approximate them to that line. Too great abduction is checked by the trans verse metacarpal ligament. The human hand is a perfect in strument of prehension; not only can the individual fingers- be bent into hooks, but the thumb can be thrown across the front of the palm, so that it can be opposed to the several fingers, and objects can therefore be grasped between it and them ; but further, this power of opposing the thumb permits objects to be held in the palm of the hand, which may be hollowed into a cup or made to grasp a sphere. The movements of the joints are indicated on the surface of the palm by tegumentary folds, an oblique fold for the thumb, and two oblique folds for the metacarpo-phalangeal joints of the fingers ; the joints of the second and third phalanges are also marked on the surface by folds. JOINTS AND MUSCLES OF THE LOWER LIMB. aero-iliac The innominate bones are connected to the spinal column &amp;gt;int. by the sacro-iliac joints and the sacro-sciatic ligaments. The Sacro-iliac Joint is between the side of the sacrum and the internal surface of the ilium, the articular surfaces of which bones are covered by cartilage, and connected together by short, strong ligaments. The sacro-sciatic ligaments stretch from the side of the sacrum and coccyx to the spine and tuberosity of the ischium. The two innominate bones are also connected together at the pubic sympliysis, which is an amphiarthrodial joint. The sacro- iliac joints and pubic symphysis permit only slight move ment; that at the former is around an imaginary axis, drawn transversely through the second sacral vertebra, which allows the base of the sacrum to be thrown forward and its .apex backward in the stooping position of the body; but ( too great movement backward of the apex is checked by .the sacro-sciatic ligaments. As the weight of the trunk, or of what may be carried in the arms or on the back, is transmitted through the haunch-bones to the lower limbs. the sacro-iliac ligaments require to be of great strength, because the sacrum, and with it the entire trunk, are sus pended by them on the two innominate bones. The Hip Joint is a ball-and-socket joint; the ball is the Hip joi head of the femur, and the socket the cup-shaped acetabu- lum in the haunch bono, the depth of the cup being in- creased by a ligament which is attached around the brim. A large capsular ligament, which is especially strong in front, encloses the articular surfaces. The ligament is lined by a synovial membrane, which also invests the neck of the thigh bone. Within the joint is the round or sus pensory ligament attached to the head of the thigh bone and to the sides of the depression at the bottom of the acetabulum. Whilst the hip joint possesses considerable mobility, it has much more stability than the shoulder, owing to the acetabulum being deeper than the glenoid fossa, and the greater strength and tension of the fibres of its capsular ligament. The muscles which move the thigh at the hip joint are situated either behind the joint, where they form the fleshy mass of the buttock, or at the front and the inner side of the thigh. They are inserted either into the femur or fascia lata, and the great and small trochanters serve as their principal surfaces of attachment. The thigh can be bent on the abdomen by the action of the psoas, iliacus, and pectineus, which lie in front of the joint ; it can be extended or drawn into line with the trunk by the glutcBus-maximus and medius ; it can be abducted or drawn away from the opposite thigh by the glutajus maximus, medius, and minimus, which muscles are of large size, and form the fleshy mass of the buttocks. It can be adducted or drawn to touch its fellow, or, if slightly bent, drawn in front of its fellow, by the adductor longus, brevis, and magnus, which muscles are inserted into the linea aspera, and form the fleshy mass on the inner side of the thigh ; and by the pectineus and quad- ratus f emoris. It can be rotated outwards by the obturator and gemelli muscles, the glutseus maximus, pyriformis, and quadratus f emoris; and rotated inwards by the glutseus medius, minimus, and tensor fascia? femoris. In standing erect the hip joints are fully extended, and the mechanical arrangements in and around these articulations are such as to enable them to be retained in the extended position with but a small expenditure of muscular power. As the weight of the body in the erect attitude falls behind the joints, the strong anterior fibres of their capsular liga ments are made tense, and the extended position of the joints is preserved. So long as the centre of gravity falls within the basis of support of the body, i.e., the space between the two feet when standing on both legs, the body will not fall. If the body is made to lean forward, then the capsular ligament is no longer tense, and the glutaDal muscles are put in action to re-extend the trunk on the thigh, and prevent it from falling forward ; if the body is made to lean to one side or the other, the round ligament is made tense, or the strong ilio-tibial band of the fascia lata of the thigh, which stretches from the ilium to tho tibia, is put on the stretch, and falling sideways is pre vented. When, in standing erect either on one or both feet, the balance of the body is disturbed, then various muscles both of the trunk and lower limb are brought into action to assist in preserving the erect position. In the erect position the weight of the trunk is transmitted through the acetabula to the heads of the thigh-bones, but the position and connections of the round ligament enable it to suspend that portion of the trunk the weight of which is thrown upon it, and to distribute the weight over the head of the femur. The Knee is the largest and most complicated joint in Knee. the body. It consists of the femur, tibia, and patella; The patella moves up and down the trochlear surface of