« PrécédentContinuer »
The ligamenta tarsometatarsea plantaria are the weakest bands of the series, and consist of scattered fibres passing from the cuboid to the bases of the two metatarsals. Some fibres, which are almost transverse, extend from the third cuneiform to the fifth metatarsal, and additional fibres reach the fifth metatarsals from the long plantar ligament.
Occasionally the tarsal end of the ligamentum cuneometatarseum interosseum laterale is attached to the medial margin of the cuboid.
The synovial stratum is restricted to this articulation, and merely sends a prolongation forwards between the opposing articular surfaces of the fourth and fifth metatarsal bases.
The intermetatarsal articulations are found between adjacent lateral aspects of the bases of the four lateral metatarsal bones. The articular facets are small, oval, or rounded surfaces which occupy only a limited portion of the flattened contiguous surfaces of the bones. Each joint is provided with an articular capsule, which, however, is not a complete investment, because the three joint cavities are in free communication on their proximal aspects with the tarso-metatarsal joint cavities -one with the lateral and two with the intermediate. The definite fibres of each fibrous stratum are situated chiefly in the transverse direction
The ligamenta basium dorsalia are short bands which extend from one base to the other.
The ligamenta basium plantaria and the ligamenta basium interossea are similarly arranged, but the interosseous ligaments are the strongest and most important members of this series.
The synovial stratum of each capsule is an extension from the lateral and intermediate tarso-metatarsal joints.
Frequently a bursa is found between the bases of the first and second metatarsal bones. It produces an appearance of indistinct facetting upon these bones, and it may communicate with the first cuneo-metatarsal joint.
The ligamentum metatarsale transversum (transverse metatarsal ligament) lies upon, and is attached to, the non-articular plantar aspects of the heads of all the metatarsal bones. It differs from the corresponding ligament in the palm in the fact that it binds all the metatarsal bones together, whereas in the palm the thumb is left free. It is closely associated with the plantar fibrous plates of the metatarsophalangeal joints, to the plantar surfaces of which it contributes prolongations.
Metatarso-phalangeal Joints.-Each of these joints is a modified ball-andsocket in which a shallow cup upon the bases of the first phalanges receives the somewhat globular head of a metatarsal bone.
Each joint retains a modified articular capsule which invests the joint. Its only distinct bands of the fibrous stratum are the ligamenta collateralia. These are strong cord-like bands which are situated on the medial and lateral sides of each joint, where they extend between adjacent rough surfaces.
On the dorsal aspect, ligaments distinct from the dorsal expansion of the extensor tendons can hardly be said to exist. The plantar aspect of the capsule consists of a thick fibrous plate strengthened by transverse fibres to form the plantar accessory ligament, which in the case of the great toe presents developed within it two large sesamoid bones. In the other toes this plate remains fibrous throughout, and is grooved on its plantar aspect for the accommodation of the long flexor tendons. It will thus be seen that the metatarso-phalangeal joints are constructed upon a plan very similar to that of the corresponding joints in the hand.
A synovial stratum lines the capsule of each articulation; and the epiphyseal lines of the metatarsals and phalanges are extra-capsular.
ARTICULATIONES DIGITORUM PEDIS.
Interphalangeal Joints.-Each toe possesses two interphalangeal joints except the great toe, which has only one. Not infrequently in the little toe the distal joint is obliterated through ankylosis. All the joints of this series are uniaxial or hinge joints. The nature of the articular surfaces closely resembles the corresponding joints in the fingers.
Each joint possesses an articular capsule which is either very thin or limited to the synovial stratum on the dorsal aspect. The plantar surface of the capsule is strengthened by a fibrous plate. The ligamenta collateralia are well-defined bands similar to those already described in connexion with the metatarso-phalangeal joints.
A synovial stratum lines each capsule in the series. The epiphyseal lines are extra-capsular.
Mechanism of the Foot.-The bones of the foot are arranged in the form of a longitudinal and a transverse arch. The longitudinal arch is built on a very remarkable plan. Posteriorly the mass of the calcaneus constitutes a rigid and stable pier of support, while anteriorly, by increasing the number of component parts, the anterior pier acquires great flexibility and elasticity without sacrificing strength or stability. The summit of the arch is formed by the talus, which receives the weight of the body from the tibia, and the resilience of the arch is assured by the calcaneo - navicular, calcaneo-cuboid, and long plantar ligaments, together with the plantar aponeurosis, which act as powerful braces or tie-bands, preventing undue separation of the piers of the arch, and consequent flattening of the foot. The weight of the body is distributed over all the five digits, owing to the arrangement of the bones of the foot in two parallel columns, a medial and a lateral. The former, consisting of the talus, navicular, and the three cuneiforms, with the three medial metatarsal bones, distributes weight through the talo-navicular joint, while the latter (ie. the lateral column), comprising the calcaneus, cuboid, and the two lateral metatarsal bones, acts in a similar manner through the talo-calcanean joint. The main line of immobility of this arch passes from the heel forwards through the middle toe, but its anterior section, which is slender, is supported on either side by two metatarsal bones, with their proximal tarsal associations, in all of which greater freedom of movement is found. The transverse arch is most marked at the level of tarso-metatarsal articulations. The intersection of these two arches at right angles to each other introduces an architectural feature of great importance in connexion with the support of heavy weights. These longitudinal and transverse arches of the foot are in effect "vaults" intersecting each other at right angles, and in relation to the area which is common to both "vaults" the body weight is superposed exactly as the dome of a cathedral is carried upon two intersecting_vaults.
Movements at the Joints of the Tarsus, Metatarsus, and Phalanges.—Considered in detail, the amount of movement which takes place between any two of these bones is extremely small, and, so far as the tarsus and metatarsus are concerned, it is mostly of the nature of a gliding motion.
At the metatarso-phalangeal and interphalangeal joints movement is much more free, and is of the nature of flexion (bending of the toes towards the sole of the foot, i.e. plantar flexion) and extension. The latter movement when continued so as to raise the toes from the ground, and bend or approximate them towards the front of the leg, is termed dorsiflexion. Coincident with dorsiflexion there is always associated a certain amount of spreading of the toes, which is called abduction, and similarly with prolonged flexion there follows a diminution or narrowing of the transverse diameter of the anterior part of the foot by drawing the toes together-a movement termed adduction. In the foot the movements of abduction and adduction take place in regard to a plane which bisects the foot antero-posteriorly through the second toe, for this toe carries the first and second dorsal interosseous muscles.
Notwithstanding the small amount of possible movement in connexion with individual tarsal and metatarsal joints, yet the sum total of these movements is considerable as regards the entire foot. In this way the movements of inversion and eversion of the foot result. By inversion we mean the raising of the medial border of the foot so that the sole looks medially, while the toes are depressed towards the ground, and the lateral border of the foot remains downwards. This takes place chiefly at the talo-calcanean joint, but the transverse tarsal joints also participate.
Eversion is chiefly the opposite of inversion, and the return of the foot to the normal position of the erect attitude; but under certain conditions it may be carried further, so that the lateral border of the foot is raised from the ground, while the medial border is depressed. In both of these movements there is rotation between the talus and calcaneus about an oblique axis which passes from the medial side of the neck of the talus to the lateral and inferior part of the calcaneus.
Of course, all the movements of the foot are subordinated to its primary functions as an organ of support and progression. For these purposes its longitudinal and transverse arches are of extreme importance. The longitudinal arch resting on the calcaneus behind and the heads of
the metatarsal bones in front receives the weight of the body, as already explained, on the summit of the talus in the line of the third toe. Hence it is that the medial malleolus appears to be unduly prominent on the medial side of the ankle. The transverse arch buttresses the longitudinal one, and therefore, whether the body weight fall to the lateral or the medial side of the longitudinal arch, it is supported by a mechanism at once stable, flexible, and elastic, or resilient, and capable of reducing to a minimum all jars that may be received by the fore part of the foot. As the heel is raised in the act of walking, the weight is gradually transferred from the lateral to the medial side of the foot, until the foot finally leaves the ground with a propulsive movement, which results from flexion of the phalanges of the great toe. In this connexion it is worthy of note that the longitudinal line of greatest strength is on the medial side of the longitudinal arch, ¿e. in relation to the great toe.
THE MUSCULAR SYSTEM.
By A. M. PATERSON.
THE movements of the various parts and organs of the body are brought about by the agency of muscle-cells, which are characterised by a special histological structure and by the special function of contracting in length under the influence of a proper stimulus.
There are three classes of muscle-cells: (1) the striated, and usually voluntary muscle-cells, out of which the skeletal muscular system is constructed; (2) the nonstriated, involuntary muscle-cells, occurring in the walls of vessels and hollow viscera, etc.; and (3) the cardiac muscle-cells, striated but involuntary, of which the substance of the heart is composed.
The following section deals solely with the skeletal muscles, the structure, arrangement, and mechanical action of which are based upon a common plan.
The cells of which the skeletal muscles are composed are long, narrow, and characterised by a peculiar striation, which is different from the striation of the muscle-cells of the heart; they also differ both in structure and function from the non-striated muscle-cells which occur in viscera and vessels.
A typical skeletal muscle consists of a fleshy mass enveloped in a membranous aponeurosis or fascia, and provided at its extremities or borders with membranous or tendinous attachments to bone, cartilage, or fascia.
Each muscle is made up of a number of fasciculi or bundles, arranged together in different muscles in different ways, so as to give rise to the particular form of the muscle in question. The fasciculi are clothed and connected together by a delicate connective tissue, the perimysium externum, continuous externally with the fascia enclosing the muscle.
Each muscular bundle or fasciculus is composed of a number of narrow, elongated muscle-cells or fibres, held together by a still more delicate connective tissue, the perimysium internum. This tissue is connected on the one hand with the sarcolemma or cell-wall of the muscle-cell, and on the other hand with the coarser tissue of the perimysium externum enclosing the muscular bundles.
By means of these connective tissue envelopes the muscle-cells, the essential agents of motor activity, are brought into firm and intimate relation with the Osseous or other attachments of the muscle. Through the agency of sarcolemma, perimysium internum, perimysium externum, fascia, and tendon, the musclecell when it contracts can produce a precise and definite effect upon the structure to be moved.
Each muscle is supplied by one or more nerves, which, in their course through the muscle, separate into smaller and smaller branches, ultimately, by their terminal filaments (axons), forming special end-organs in relation to each muscle-cell.
While muscle may thus be looked upon as an organ endowed with particular properties, and executing a definite movement in response to a stimulus by the simultaneous contraction of its constituent cells, the various muscles may further be considered in groups, associated together by mode of development, nerve-supply, and co-ordination of action. For example, we speak of the hamstring muscles of
the thigh, the muscles of the back, and the prevertebral muscles,-groups in which separate muscles are associated together by development, nerve-supply, and action. In their development the separate muscles arise from the subdivision of a larger stratum, as in the limbs, or from the fusion of segmental elements (myotomes), as in the case of the axial muscles. The peripheral nerves supplying skeletal muscles are distributed, through the plexuses or directly, so as to associate particular muscles morphologically and physiologically, and to secure a co-ordinated movement by the simultaneous contraction of several muscles.
Beneath the skin there are two (or in some regions three) layers of tissue which require consideration in relation to the muscular system: the superficial fascia (panniculus adiposus), the deep fascia, and, in animals, the panniculus carnosus (rudimentary in man, and represented chiefly by the platysma in the neck).
Fascia Superficialis (Superficial Fascia).-The superficial fascia is a continuous sheet of areolar tissue which underlies the skin of the whole body. It is closely adherent to the cutis vera, and is sometimes termed panniculus adiposus, from the fact that, except beneath the skin of the eyelids, penis, and scrotum it is usually more or less impregnated with fat. The cutaneous vessels and nerves ramify in this fascia; and its deep surface, membranous in character, is in loose connexion with the subjacent deep fascia. It is in this layer that dropsical effusions chiefly
Fascia Profunda (Deep Fascia).-Underneath the skin and superficial fascia is a fibrous membrane, bluish white in colour, devoid of fat, and in closest relation to skeleton, ligaments, and muscles. This is the deep fascia. It covers, invests, and in some cases forms the means of attachment of the various muscles. It has a special tendency to become attached to all subcutaneous bony prominences, and to be continuous with the connecting ligaments. It forms septal laminæ, which separate groups of muscles and individual muscles; enclose glands and viscera ; and form sheaths for vessels and nerves. Around joints it gives rise to bands which strengthen the capsule or limit the mobility of the joint, or, as in the region of the wrist and ankle, bind down the tendons passing over the joint. It not only ensheathes vessels and nerves, but is perforated by those which pass between superficial and deeper parts.
The term aponeurosis is used in relation to muscles. It is synonymous with deep fascia, either as an investing fascia, or as a membranous layer which (e.g. vertebral aponeurosis) performs at one and the same time the purpose of a deep fascia and the expanded membranous attachment of a muscle.
The panniculus carnosus is a thin muscular layer enveloping the trunk of animals with a hairy or furry coat. It is strongly developed in the hedgehog. In man it is represented mainly by the (rudimentary) platysma. It is placed between the superficial and the deep fascia.
Burse. Where a tendon passes over a bony surface, or where the superficial fascia and skin cover a bony prominence, there is generally formed a mucous (synovial) sac, or bursa, containing fluid, for the purpose of lubricating the surface over which the tendon or fascia glides. Allied to these are the mucous or synovial sheaths which envelop the tendons passing over the wrist and the ankle joints.
Description of Muscles. In studying the muscular system it is necessary to note the following characters in reference to each individual muscle: (1) The shape of the muscle-flat, cylindrical, triangular, rhomboidal, etc.; and the character of its extremities-membranous, tendinous, or fleshy. (2) The attachments of the muscle. The origin is the more fixed or central attachment: the insertion is the more movable or peripheral attachment. (3) The relations of the surfaces and borders of the muscle to bones, joints, muscles, and other important structures. (4) Its vascular and nervous supply; and (5) its action. It must be borne in mind that hardly any single muscle acts alone. Each muscle, as a rule, forms one