NATURE’S SUPREME TRIUMPH: THE HUMAN MACHINE

THE study of Anatomy is the study of the architecture of the human body. It is the study of how the house is built and what its walls are made of. It does not tell us how it works, or any of the intimate details of the family circle that lives inside it. This is left to Physiology. However, when an architect builds a house, he must be certain that it will not collapse on its inmates, and for this he must know many things about the materials he is using, and how best they can be put together in order that each may be used to the best advantage. Anatomy by itself is only the study of the geography of the body, but we shall find it much more interesting if we do not confine ourselves strictly to this, but always keep in mind the use and ‘team work ‘of each organ that we study. The intimate family affairs, however, we will leave to the next section on Physiology.

LET us start with the bones or skeleton, without which we ./should collapse into an indeterminate mass of jelly. The bones serve several purposes, for they are not only used to keep us rigid and to support our weight, but also as levers upon which the muscles can work so that we can move about and do all that we want to do. The two essentials that an engineer would require in a support and a lever would be lightness and strength, and it will be found that this holds good in the body, for most of our bones are hollow tubes, the outside of which is hard like ivory while the inside contains a substance which is used for making blood. Nature is never wasteful of space. Lightness and strength are combined, therefore, for everyone knows that it is almost as difficult to bend a hollow steel tube as to bend a solid rod of steel which would weigh perhaps six times as much, while it would certainly not be six times as strong.

Each bone must be movable and must be moved with the least possible amount of difficulty. To accomplish this, the ends of the bones that are jointed are covered with a very smooth substance called cartilage, while between the two

cartilage-covered ends of the bones lies a small space which is filled by fluid that serves the purpose of ‘oiling the bearings.’ Sometimes when the joint is damaged, this fluid collects and distends the joint and the familiar condition of ‘water on the knee ‘is found.

A joint must not only be freely movable but it must also be strong and must not be easily dislocated. Therefore between the two bone ends are strong bands of ligaments which prevent the bone ends being separated, because they are made of tissue which will not stretch unless subjected to enormous force. The ligaments are collected together so that they form a sort of sleeve for the joint, which is thus converted into a closed cavity from which the joint fluid cannot escape.

Each joint is made specially for some particular purpose. Some, like the shoulder where free movement is necessary, have had to increase their power of movement at the expense of their strength, whereas others, like the joints in the spine where only the smallest movement is allowable, have become immensely strong.

Movement between the bones is effected by the muscles. Each muscle is firmly attached to the bone on each side of a joint. If the muscle contracts or becomes shorter, its two points of attachment will be brought closer together and therefore the joint will be moved. The muscles are used for two purposes, one of which is to move the joints and the other to keep them still and rigid and to prevent movement when the limb is being used as a support.

Let us take for example the knee joint, which is used chiefly for supporting the body when we are standing. If the muscles at the back of the knee contract, the knee will be bent, while if those in front contract, the knee will be straightened. If, however, both contract at the same time, not only will no movement take place but the knee will be made into a perfectly rigid support, just as if the knee joint did not exist and the two bones were continuous.

THE LEG: A TRIUMPH OF ENGINEERING

THE legs must be capable of sufficient strength to support the body, they must be movable to allow walking and running, and they must have some ‘spring ‘in them to prevent jarring of the delicate mechanisms of the body when, for example, a jump is made. Let us start at the foot and work upwards.

If the skeleton of the human foot is examined, it will be found to consist of a large number of small bones closely held together by very strong ligaments. These bones are so arranged that they form a series of arches, all of which are capable of being slightly flattened out if weight is put upon them, by virtue of the slight amount of movement allowed at each joint. It is this power of being slightly flattened out and then returning to their original shape that gives our feet the ‘spring ‘that protects our nervous systems from excessive jolting.

There is an arch which runs along the inner border of the foot from the heel to the big toe, another on the other side of the foot from the heel to the little toe, and a third which runs across the middle of the foot transversely. This last is only half an arch and is made into a complete arch by the other foot when both are placed together side by side. ( In order to prevent the arch from falling in and its good effects being abolished, Nature has been most ingenious in her provisions and has worked according to the best engineering principles. First, she has so arranged the shape of the bones that they fit closely one into the other like the bricks on the lower surface of a bridge. Secondly, she has fixed very strong ligaments, incapable of stretching, between the bones forming the arch. The important thing is that these are situated on the under surface of the arch where they are most useful in preventing its collapse, and not on the upper surface where they could do no useful work.

Thirdly, a ‘tie,’ composed of muscles and ligaments, has been fixed to the pillars of the arch, preventing their separation and the collapse of the superstructure. Fourthly, and last, two straps composed of the tendons of two muscles have been passed under the keystone of the arch in order to hold it up. These two muscles come down to the foot from the back of the leg so that they work to the best possible mechanical advantage.

It will be noted that much of the power of the arches to withstand strain depends on the muscles which are always exerting a slight pull upon them. This continual action of the muscles is known as ‘tone.’ If by any chance the muscles become weak or exhausted by a bad attitude in standing (as, for example, when the toes are turned out), they may be incapable of holding the arch up, when the ligaments, after a time, will stretch and the condition known as flat foot will develop. Of course, the proper way to cure such a condition is

not by wearing specially built shoes, but by making the muscles more efficient by exercise.

THE HINGE THAT ALLOWS US TO BEND OUR LEGS THE only movement that is necessary, or indeed desirable, at the knee joint is one in which the lower part of the leg moves forward and backward on the thigh like a hinge. Sideways movement must be avoided at all costs and the joint must be very strong. Sideways movement is prevented by the shape of the bones and by very strong ligaments which lie on each side of the joint. The ligament in front has a large, fiat circular bone developed in it, upon which we kneel. The ligament behind is not so strong, for it has little strain to bear. Inside the joints there are two very strong ligamentous bands which cross one another at right angles, and they are specially adapted so that, although they prevent dislocation under almost all circumstances, they still allow the hinge movement to take place. The upper surface of the tibia, which is the lower bone, is almost flat, and in order to make this more cup-shaped and the joint more stable, two flat, circular rings of cartilage with a hole in the centre and with their outer edges thicker than their inner edges have been placed between the bone ends. Sometimes, as in a ‘missed kick ‘at football, a little piece of one of these cartilages becomes broken off and sticks in an awkward place inside the joint, preventing all movement. This is the torn cartilage that is such a common accident of the football field.

WHY MAN CAN WALK ON TWO LEGS

THE upper end of the thigh bone is formed into an almost completely circular globe, covered with smooth, shining cartilage. This globe fits exactly into a perfect cup, which is situated on the outer side of the hip bone or pelvis. This joint is one of the most important in the body, for it is by means of this very strong but yet movable joint that the erect attitude has been made possible in man, as opposed to walking on all fours as most of the animals do. This has been achieved by fitting the ball into a very deep socket and by placing a very strong ligament which is nearly a quarter of an inch thick on the front of the joint between the hip and thigh bones. This prevents the thigh being bent backwards. If, therefore, the muscles at the back of the hip contract, they will have to work in opposition to this ligament and, when

balance between them has been made, the hip joint will be immovable and the body weight will be carried with the least amount of difficulty.

In the work she requires of us in standing, Nature has again been most economical. The ability to stand has indeed been man’s great salvation, for it enabled him to use and develop his hands for other things besides being walked upon. The original use which was made of his hands was for climbing trees, but once this was accomplished, many other uses were readily found, and it has been said that it was this that led to the enormous development of the human brain as compared with that of the lower animals.

The hip bone or pelvis is in two halves, which when fitted together form a basin. In front the two halves meet one another in a joint, but behind the lower part the spine comes between, and between each hip bone and the spine there is a joint. Here the requirements of the joints are quite different from those in the limbs, for great strength is required while movement, except to a minimum degree, is most undesirable. What is wanted is stability. To suit these principles, the joints have been modified and the ligaments are enormously strong, while the cartilaginous surfaces of the bones which were smooth in the limb to diminish friction, are here roughened to increase it. The joints between the lower part of the spine and the pelvic bones are very neatly contrived, for the spine, which transmits the body weight, is wedge-shaped in its lower part and fits into the wedge-shaped socket formed by the two pelvic bones. The weight of the body will, therefore, tend to drive the wedge further into the socket.

Within reasonable limits, therefore, the greater the weight to be borne the more stable does the joint become. The pelvis is one of man’s great difficulties, for through it his head must pass during birth. Owing to the large size of the human head, which results from the growth of the brain, birth is made very difficult, and in some rare cases impossible, because the pelvis is too small to allow the head to pass. Nature allows for this, however, for she softens the ligaments at the joints we have mentioned and allows the surfaces to separate before the birth of the child takes place.

The muscles lying in the pelvis are of extreme interest, for when man was a four-footed animal, his body lay horizontally and certain muscles in his pelvis were used for moving the

lower part of his spine, which was, of course, his tail. When he learnt to stand, however, he was in extreme danger, for he possessed nothing in his pelvis which was capable of preventing his intestines falling through it. He soon learnt, however, to control them with the muscles that originally wagged his tail, and these have now been formed into a complete diaphragm which closes the outlet of the pelvis and holds up the viscera.

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