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in the whole animal frame. And, what appears still more difficult of attainment, the continuity of that pipe must not be broken, nor any undue pressure exerted on its precious contents, when the body is moved, or bent, or twisted. A series of loopholes is also required through which the spinal cord may send out nerves to all parts of the body. Still further, the same spine must afford a fulcrum, stay, or basis, for the action of the muscles which are spread over the trunk, and also a support for the ends of the ribs to rest upon. All these purposes are admirably provided for by a column of vertebræ. Each vertebra consists (fig. 19) of a
disc or flat piece of bone (a), with surfaces nearly parallel, several processes (b, c, d), an aperture, or short canal (e), for the spinal cord, and notches or grooves on each side for the nerves to pass
through. The processes of the different vertebræ fit into each other, and they are fastened together by cartilages of great strength, flexibility, and elasticity. The structure is then one of the most perfect that can well be conceived. The breadth of the bases on which the parts severally rest, and the closeness of the junction, give to the column its stability; the number of the parts, and the consequent frequency of joints, its flexibility. Again, the hole in one bone falls into a line with those in the bones adjacent to it, so that a continuous canal is formed; and the change and pressure, produced by bending the body, is divided among so many joints, and thrown to so great an extent on the elastic cartilages, that the line of this canal is not readily broken or obstructed. The notches left for the passage of the nerves also fit together two by two, and thus form a row of small holes on each side of the spine. The numerous processes of the vertebræ serve as handles for the muscles to pull by, and some of them are articulated with the ribs, the whole connection of these various parts being so artfully contrived, as to contribute materially to the most important result, the strength and stability of the compages.
The ribs are twenty-four in number, twelve on each side; they are supported at one end by the spine, and the upper seven on each side are united at the other end, by means of cartilages, to the breast-bone. Within the ribs, and protected by them, are the heart and lungs, which may be reckoned among the most important organs of the body. Below, inclosed by soft integuments, are the stomach, liver, and intestines. The trunk is completed and supported by a broad bony basin called the pelvis, chiefly formed by the two haunch-bones, with which the lower limbs are articulated.
THE FRAMEWORK OF THE HUMAN BODY—THE LIMBS. Tue limbs are four in number, two legs and two arms. In structure, the legs and arms closely resemble each other, and the same analogy extends, in a greater or less degree, to the limbs of all vertebrate animals.
The arm is attached to a bone named the shoulder-blade. They are connected by what is called a ball-and-socketjoint, formed by a hollow cup in the shoulder-blade, into which is fitted a corresponding knob or ball on the head of the first or uppermost bone of the arm. Such a joint manifestly admits of a great variety of motions. The bone thus fitted into the shoulder-blade is a long tube, made hollow for the sake of lightness, and articulated at the elbow, by a hinge-joint, with the bones of the forearm. But here there occurs a difficulty, so to speak, in providing for the necessary motions of the forearm and hand. A moment's consideration will show that the hinge-joint admits of motion only in one plane. By using it, we can bend the arm at the elbow and extend it again, but we can do nothing more. How is it, then, that we are enabled to turn the palm of the hand either up or down? Very singular indeed is the inechanism which gives us this power. Above the elbow, the arm has but one bone; between the elbow and the wrist, it has two. These two bones are so connected as to roll upon each other, and, in doing so, they carry the hand round with them. Other motions depend upon the connection of the numerous bones in the wrist and hand. The wrist itself contains eight, arranged in two rows or arches of four bones each. All these move more or less freely on each other, and thus lessen the shock and danger arising from the transmission of force from the hand to the arm, or from the arm to the hand. Next comes a row of five larger bones, supporting each a finger. These form the body of the hand, between the fingers and the wrist. Last of all, connected by hinge-joints, are the finger-bones, of which the thumb has two, and each of the four fingers three.
It is not difficult to trace the similarity between the structure just described and that of the lower extremities. Into the haunch-bone, forming part of the pelvis, the thighbone is inserted by a ball-and-socket-joint, just as the first bone of the arm is inserted into the shoulder-blade. The thigh-bone is round and hollow, and extends to the knee, being by far the longest, as well as the strongest bone in the body. The knee is a hinge-joint like the elbow. In the leg are two bones, rolling upon each other in the same manner and for the same purposes as in the forearm. Then follow two rows or arches of three and four bones respectively. These form the ankle, and correspond · to the two arches already described in the wrist. In like manner, the five bones of the hand have an exact counterpart in five similar bones which form the skeleton of the foot. To complete the resemblance, the toes, like the fingers, have each a series of three bones connected by hinge-joints, except the great toe, the thumb of the foot, which has two only.
It is impossible to notice here the numerous contrivances by which every variety of limb, whether in man or in the lower animals, is adapted to its special purpose. One is too prominent to escape notice. When, as in the human body, the upper extremities assume the form of arms, a collar-bone (as it is called) extends from each shoulder to the top of the breast-bone. These collar-bones are intended to keep the shoulders apart, and hence the frequency of their fracture when any sudden shock forces the arms inwards on the chest. Birds of powerful flight have the collar-bones strongly developed, whereas in horses and most other quadrupeds they are entirely awanting.
While such adaptations to particular ends are well worthy of our notice and admiration, it is probably in the joints that we see the wisdom and goodness of the Creator most strikingly displayed. Every one of them is of the kind which best suits its particular place. The greatest care has also been taken to make them work smoothly and easily, as well as in convenient directions. The hard bones are not allowed to grate harshly upon each other, but have their surfaces coated with a soft and elastic cartilage, by which the friction is greatly lessened. Still further to provide against the irritation likely to result from frequent and long continued motion, each joint is regularly supplied with a mucilage, more emollient and slippery than oil itself, which lubricates the surfaces in contact, and renders their motion easy and pleasant. This mucilage is secreted by a membrane forming part of the joint itself, and supplied as the necessities of the joint demand. Man, it is true, has contrived to make some of his machines regulate their own supply of oil, but his utmost skill has failed to attain the perfection here shown by the Creator's handiwork, in which the supply is not only regulated, but produced.
THE MUSCLES. It is not enough that the skeleton be skilfully constructed, and jointed together with consummate art. Motion would not follow of its own accord. It will not suffice that the bones be made capable of moving, they must actually be moved. There must be some power to pull the machinery, which has been so carefully fitted for being pulled. This is exactly what the muscles supply. They are the active, as the bones and joints are the passive, apparatus of motion.
The muscles constitute what is usually known as the red flesh of the body. They are composed of bundles of fibres, which possess the singular property of contractility, that is, of contracting or shortening themselves under excitement. Each muscle is attached to two or more bones, some of which must be moved by its contraction. We may easily observe this process in several parts of the body; as, for example, in the forearm, when the fingers are alternately bent and extended.
It often happens that a muscle cannot be conveniently accommodated in the exact position where it is required. It is accordingly placed in some convenient situation, and attached by a string, called a tendon, to the bone which it is intended to move. This tendon is simply a rope for the muscle to pull by, and may be longer or shorter according to the necessities of the case. The hand furnishes a good example. If the muscles which move the fingers had been placed in the palm, or back of the hand, they would have swelled that part to an awkward and clumsy shape. They áre, therefore, placed in the forearın, and act by long tendons, strapped down at the wrist, and passing through the hand to the fingers, and to those joints of the fingers which they are severally intended to move. In like manner, the muscles which move the toes and other joints of the foot, are conveniently, not to say gracefully, disposed in the calf of the leg. Sometimes, again, it is necessary to change the direction of the motion which the contraction of a muscle produces. This is managed by some of the very same means which man employs for the same purpose. The tendons are passed over protuberances on the bones, from wbich they proceed in any direction that may be desired. Where no bone is available, the same end is attained by a strap or ligament. Thus the tendons which pass from the leg to the upper part of the foot are bound down at the ankle by a strong ligament, which both prevents them from starting, and changes the direction of the force which they convey.
A muscle acts only by contraction. Its power is exerted in no other way. Hence, it follows that the same muscle