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much in character; sometimes being very soft and tender, at others as in the tendons and ligaments, which are almost wholly composed of it-attaining great strength and density.
13. Among the most important of the tissues imbedded in and ensheathed by the connective tissue, are some the presence and action of which can be readily determined during life.
If the upper arm of a man whose arm is stretched out be tightly grasped by another person, the latter, as the former bends up his fore-arm, will feel a great soft mass which lies at the fore part of the upper arm, swell, harden, and become prominent. As the arm is extended again, the swelling and hardness vanish.
On removing the skin, the body which thus changes its configuration is found to be a mass of red flesh, sheathed in connective tissue. The sheath is continued at each end into a tendon, by which the muscle is attached, on the one hand, to the shoulder-bone, and, on the other, to one of the bones of the fore-arm. This mass of flesh is the muscle called biceps, and it has the peculiar property of changing its dimensions-shortening and becoming thick in proportion to its decrease in length-when influenced by the will as well as by some other causes,1 and of returning to its original form when let alone. This temporary change in the dimensions of a muscle, this shortening and becoming thick, is spoken of as its contraction. It is by reason of this property that muscular tissue becomes the great motor agent of the body; the muscles being so disposed between the systems of levers which support the body, that their contraction necessitates the motion of one lever upon another.
14. These levers form part of the system of hard tissues which constitute the skeleton. The less hard of these are the cartilages, composed of a dense, firm substance, ordinarily known as gristle." The harder are the bones, which are masses either of cartilage, or of connective tissue, hardened by being impregnated with phosphate and carbonate of lime. They are animal tissues which have become, in a manner, naturally petrified; and when the salts of lime are extracted, as they may be, by the
1 Such causes are called stimuli.
action of acids, a model of the bone in soft and flexible animal matter remains.
More than 200 separate bones are ordinarily reckoned in the human body, though the actual number of distinct bones varies at different periods of life, many bones which are separate in youth becoming united together in old age. Thus there are originally, as we have seen, thirty-three separate bodies of vertebræ in the spinal column, and the upper twenty-four of these commonly remain distinct throughout life. But the twenty-fifth, twenty-sixth, twentyseventh, twenty-eighth, and twenty-ninth early unite into one great bone, called the sacrum; and the four remaining vertebræ often run into one bony mass called the coccyx. In early adult life, the skull contains twenty-two naturally separate bones, but in youth the number is much greater, and in old age far less. Twenty-four ribs bound the chest laterally, twelve on each side, and most of them are connected by cartilages with the breast-bone. In the girdle which supports the shoulder, two bones are always distinguishable as the scapula and the clavicie. The pelvis, to which the legs are attached, consists of two separate bones called the ossa innominata in the adult; but each os innominatum is separable into three (called pubis, ischium, and ilium) in the young. There are thirty bones in each of the arms, and the same number in each of the legs, counting the patella, or knee pan.
All these bones are fastened together by ligaments, or by cartilages; and where they play freely over another, a coat of cartilage furnishes the surfaces which come into contact. The cartilages which thus form part of a joint are called articular cartilages, and their free surfaces, by which they rub against each other, are lined by a delicate synovial membrane, which secretes a lubricating fluid-the synovia.
15. Though the bones of the skeleton are all strongly enough connected together by ligaments and cartilages, the joints play so freely, and the centre of gravity of the body, when erect, is so high up, that it is impossible to make a skeleton or a dead body support itself in the upright position. That position, easy as it seems, is the result of the contraction of a multitude of muscles which oppose and balance one another. Thus, the foot affording
the surface of support, the muscles of the calf (Fig. 2, I) must contract, or the legs and body would fall forward.
FIG. 2.-A DIAGRAM ILLUSTRATING THE ATTACHMENTS OF SOME OF THE MOST IMPORTANT MUSCLES WHICH KEEP THE BODY IN THE ERECT POSTURE.
I. The muscles of the calf. II. Those of the back of the thigh. III. Those of the spine. These tend to keep the body from falling forward.
1. The muscles of the front of the leg. 2. Those of the front of the thigh. 3. Those of the front of the abdomen. 4, 5. Those of the front of the neck. These tend to keep the body from falling backwards. The arrows indicate the direction of action of the muscles, the foot being fixed.
But this action tends to bend the leg; and to neutralize this and keep the leg straight, the great muscles in front of the thigh (Fig. 2, 2) must come into play. But these, by the same action, tend to bend the body forward on the legs; and if the body is to be kept straight, they must be neutralized by the action of the muscles of the buttocks and of the back (Fig. 2, III).
The erect position, then, which we assume so easily and without thinking about it, is the result of the combined and accuratel oportioned action of a vast number of muscles. What is it that makes them work together in this way?
16. Let any person in the erect position receive a violent blow on the head, and you know what occurs. On the instant he drops prostrate, in a heap, with his limbs relaxed and powerless. What has happened to him? The blow may have been so inflicted as not to touch a single muscle of the body; it may not cause the loss of a drop of blood: and, indeed, if the "concussion," as it is called, has not been too severe, the sufferer, after a few moments of unconsciousness, will come to himself, and be as well as ever again. Clearly, therefore, no permanent injury has been done to any part of the body, least of all to the muscles, but an influence has been exerted upon a something which governs the muscles. And this influence may be the effect of very subtle causes. A strong mental emotion, and even a very bad smell, will, in some people, produce the same effect as a
These observations might lead to the conclusion that it is the mind which directly governs the muscles, but a little further inquiry will show that such is not the case. For people have been so stabbed, or shot in the back, as to cut the spinal cord, without any considerable injury to other parts and then they have lost the power of standing upright as much as before, though their minds may have remained perfectly clear. And not only have they lost the power of standing upright under these circumstances, but they no longer retain any power of either feeling what is going on in their legs, or, by an act of their volition, causing motion in them.
17. And yet, though the mind is thus cut off from the
lower limbs, a controlling and governing power over them still remains in the body. For, if the soles of the disabled feet be tickled, though no sensation will reach the body, the legs will be jerked up, just as would be the case in an uninjured person. Again, if a series of galvanic shocks be sent along the spinal cord, the legs will perform movements even more powerful than those which the will could produce in an uninjured person. And, finally, if the injury is of such a nature that the cord is crushed or profoundly disorganized, all these phenomena cease; tickling the soles, or sending galvanic shocks along the spine, will produce no effect upon the legs.
By examinations of this kind carried still further, we arrive at the remarkable result that the brain is the seat of all sensation and mental action, and the primary source of all voluntary muscular contractions; while the spinal cord is capable of receiving an impression from the exterior, and converting it not only into a simple muscular contraction, but into a combination of such actions.
Thus, in general terms, we may say of the cerebrospinal nervous centres, that they have the power, when they receive certain impressions from without, of giving rise to simple or combined muscular contractions.
18. But you will further note that these impressions from without are of very different characters. Any part of the surface of the body may be so affected as to give rise to the sensations of contract, or of heat or cold; and any or every substance is able, under certain circumstances, to produce these sensations. But only very few and comparatively small portions of the bodily framework are competent to be affected, in such a manner as to cause the sensations of taste or of smell, of sight or of hearing and only a few substances, or particular kinds of vibrations, are able so to affect those regions. These very limited parts of the body, which put us in relation with particular kinds of substances, or forms of force, are what are termed sensory organs. There are two such organs for sight, two for hearing, two for smell, and one, or more strictly speaking two, for taste.
19. And now that we have taken this brief view of the structure of the body, of the organs which support it,