140 140 1 Fig. 44. Пе O bones are very closely packed and tightly bound together with ligaments, so that the wrist is as strong as if it consisted of but a single bone; while, at the same time, the elasticity obtained by having so many bones movable on each other, neutralises, to a great extent, a shock occasioned by falling on the hands. Next to the carpal bones come the 5 metacarpal1 (11— 11), which form the comparatively long bones of the back of the hand. Last of all, come the 14 phalanges 2 or bones of the fingers (12-14), a name also applied to the corresponding bones of the foot. (2) THE LOWER EXTREMITIES—THE LEGS.-The plan of the bony framework of the legs is very similar to that of the arms. The principal point of difference is, that the structure of the former is stronger, in order to support the weight of the body; strength and solidity being in the lower limbs of even more importance than freedom of action, which we saw to be so well provided for in the arms. The whole framework to which the lower extremities are attached, and which also forms a basin-like cavity for supporting the contents of the abdomen, is called the pelvis3 (6 in fig. 43). In a full-grown person, the pelvis consists of a single bone, called the os innominatum or 'nameless bone.' On the under side of the spreading part of the pelvis is a cup-like socket, to receive the rounded head of the thigh-bone. The deepness of the socket here, while it does not allow so much freedom of action to the leg, makes the joint a very secure one (compare what is said on the shoulder-joint, page 48). In the thigh there is only one bone, the femur (7 in fig. 43), which is jointed at the knee to the two bones of the leg proper. These are the tibia,5 or shin-bone (8 in fig. 43), the larger of the two, and the fibula (9 in fig. 43). It is the tibia alone which supports the thigh-bone, so that the fibula, as its name indicates, forms merely a sort of connection between the knee-joint and the ankle, and serves for the attachment of muscles. On the knee-joint, which is a pure hinge-joint, is a small separate bone, the patella, or knee-pan (10 in fig. 43), the object of which is to change the direction of the tendons of the muscles that come down from the front of the thigh to be inserted in the tibia, so as to enable them to act more advantageously, according to the principle afterwards explained (see p. 52). THE FOOT consists of 26 bones, which are arranged on a plan very similar to, if not identical with, that of the hand. In the hand, we had 1 From Greek meta, after, or coming after. 2 From Greek phalangx, the lines of an army drawn up in battle array. 6 5 Latin, the shin-bone.' D carpal and metacarpal bones; in the foot, we have tarsal and metatarsal; and in both there are phalanges. The tarsal1 bones, seven in number, compose the heel and the hinder part of the instep. Fig. 45 exhibits the beautiful arch which is formed by the bones of the foot, called the ment B, which binds E to F. D, E, and F correspond to bones 3, 4, and 5 in the hand; while in front, and at the outer side of E, are four bones corresponding to 7, 8, 9, and 10 in the hand. In the front part of the instep are the five metatarsal bones corresponding to 11-11 in the hand. To the metatarsal bones are jointed the phalanges, or bones of the toes, each toe having three, except the great toe; just as in the hand each of the fingers had three phalanges, but the thumb only two. Such is an outline of the bones composing the framework of the body. The coatings of flesh or muscle which cover this skeleton, and the skin, the outer covering of the whole, are now to be described. The Muscles. MUSCULAR TISSUE is the soft, fleshy covering of the bones, by means of which all the movements of the body are made. This faculty which it possesses of causing motion is due to its contractility. A mass of this tissue attached to certain bones, is called a muscle. When a muscle is closely examined, it is seen to consist of a great number of fibres regularly arranged lengthwise. It is further seen that these Fig. 46. fibres are connected together in bundles, of different sizes, enclosed 1 From low Latin tarsus, Greek tarsos, the instep. in sheaths of another kind of tissue. Fig. 46 represents such a bundle of fibres. The individual fibres, again, are streaked in two ways-lengthwise and crosswise. When separated from each other, they often split up into fibrilla, or little fibres, as seen in fig. 47. Other fibres, when extended, separate, according to the cross streaks, into discs, as seen in fig. 47, a,b; and when this separation takes place both ways at once, the fibre is separated into a Fig. 47. mass of particles, b' in fig. 47. When a muscle is being contracted, these cross discs of the individual fibres become more closely squeezed together, and the fibre becomes thicker as it is shortened, in a manner similar to what is seen to take place in the body of a worm, when it is drawing itself up after having put forward its head. The muscles are attached to the bones by means of tendons,2 which are white, smooth, non-elastic bands at the ends of the muscles, of a different structure from the muscle itself. Of the two component parts of fibrous tissue, the elastic and the non-elastic, only the non-elastic, which is white, is found in tendons; a tendon being formed of fibres of this white tissue, one fibre coming from each fibre of the muscle. The tendon itself is a tough, white mass, attached to the bones by the individual fibres becoming fixed in depressions in the bones, and also by becoming amalgamated with the hard outer part of the bone called the periosteum. The muscle being thus attached at its ends to two bones, a distinction is made between the two ends; the point of attachment to the less movable bone is called the origin, while that to the bone specially to be moved is called the insertion ; thus, the muscle which bends the elbow is attached to the shoulder and to the fore-arm; the point of attachment of this muscle at the shoulder is its origin, and the point of attachment in the fore-arm its insertion. Muscles such as the one here alluded to—that is, those that bend joints, the best example being the muscles that bend the fingers on the palm-have a particular name, flexors, or benders, from Latin flecto, to bend. Where such muscles exist, there is always, of course, another set that reverse this bending action, or extend the bones connected with the joint, and are hence called extensors, from Latin extendo, to extend; such are the muscles that open the hand, by extending the fingers. Muscles which act in opposition to each other in this way, as flexors and extensors, are said to be antagonistic, or the one is said to be the antagonist of the other. The various bones acted on by the muscles are like so many levers 1 Latin, diminutive of fibræ, threads. 2 From Latin tendo, to stretch. (PHYSICS, p. 11); the fulcrum is generally a joint; the power is a muscle, the force being exerted by contraction, and the weight the farther end of the bone or limb. They are all levers of the third kind (PHYSICS, P. 9), the power acting between the weight and the fulcrum, and therefore m m B Fig. 48. A acting at a mechanical disadvantage—that is, a power greater than the resistance has to be exerted in order to accomplish the movement. There is another disadvantage which the muscles labour under in working, arising from their being attached to the bones in an oblique direction; but this is partly done away with by means of the peculiar formation of the bones —that is, by a thickening at the joints. Thus, in fig. 48, A, when a bone (0) is to be moved, the tendon (i) of the muscle (m) comes over the thickened part of the bone at the joint, and is attached immediately below it, so that the attachment of the muscle is more nearly perpendicular than it would otherwise have been. By this means the bones turn more freely on each other; otherwise the ends of two bones would merely be pressed together, as in fig. 48, B. As regards the mechanical disadvantage from the nature of the lever, it is compensated for, we know, by the gain in velocity (PHYSICS, p. 10), as will be seen from the consideration of one case, the raising of the fore-arm. The lever to be raised is the fore-arm, bc, the weight being whatever is in the hand; the fulcrum is at the elbow-joint; the power is in the muscle d, inserted с Fig. 49. at e. Now, it is clear that when the muscle is contracted a little, the hand will be raised a distance proportioned to the length of the arm, and, of course, must move proportionally quicker; while, at the same time, the power exerted by the hand requires an exertion on the part of the muscle greater exactly in the same proportion (PHYSICS, page 11). This is the general principle on which the muscles of the body act. Principal Muscles of the Body.-Space will not permit a detailed description of the various muscles of the body: a few of the principal muscles only can be mentioned. The various emotions of pleasure, anger, wonder, scorn, &c., are expressed in the face by a smile, a frown, a raising of the eyebrows, a curl of the lip, &c., which are caused by the action of the muscles round the eyes and the mouth, of those in the cheeks, and of those that raise and depress the eyebrows and the nose, &c. The most important muscles of the skull are those attached to the lower jaw, by which the food is chewed. The largest of these, called the temporal muscle, is attached to the flat surface of the temporal and parietal bones, and is assisted by another, called the masseter, a Greek word meaning 'the chewer.' The head is kept balanced on the neck by means of strong muscles, the constant exertion of which is necessary to keep it erect. The body is kept erect on the legs by a set of muscles attached to the thighs and to the bones of the pelvis, which do the double duty of moving the legs and of supporting the trunk, as well as of raising it to an erect posture on being bent down. arm. .4 The arm is raised from the side by a large muscle on the shoulder, called the deltoid from its resemblance to the Greek letter delta, s. From under it arise two muscles which respectively bend and extend the foreThat which bends the fore-arm is called the biceps1 or two-headed muscle, and that which extends it the triceps2 or three-headed muscle. The biceps forms a great part of the fleshy mass in front of the arm, and the triceps forms the fleshy mass at the back. The turning movements of the hand are effected principally by the radius. The turning of the palm of the hand downward is called pronation,3 while turning it upward is called supination; and the muscles by which these movements are performed are called respectively pronators and supinators. The wrist and hand are bent upon the fore-arm by three muscles, which have their origin from the inner end of the humerus, and one of which spreads out into a fan-like membrane on the palm of the hand. Under this palmar membrane lie the flexor muscles of the fingers. The antagonists of these flexors are the common extensors of the fingers, a special extensor of the fore-finger, the muscles of the thumb, accumulated in the ball of the thumb, which move it in every direction, and several other muscles of less importance. The thigh is raised and advanced by means of two muscles, which descend in front of the pelvis--one from the lumbar vertebræ, and one from the upper expanded surface of the pelvis. The antagonists of these, the muscles which draw back the thigh, have their origin on the back and under surface of the pelvis. The muscles which bend the knee proceed from the lower border of the pelvis and the back of the thigh-bone, and are inserted in the sides of the tibia and fibula, a little below the knee. The tendons of these are felt behind the knee, and are known as the hamstrings. The extensors of the 1 From Latin bis, twice, and caput, a head. 2 Latin tres, three, and caput, a head. 3 From Latin pronus, with the face downward. |