Part 1 (1/2)
A Book of Exposition.
by Homer Heath Nugent.
PREFACE
It is a pleasure to acknowledge indebtedness to my wife for a.s.sistance in editing and to Dr. Ray Palmer Baker, Head of the Department of English at the Inst.i.tute, for suggestions and advice without which this collection would hardly have been made.
INTRODUCTION
The articles here presented are modern and unhackneyed. Selected primarily as models for teaching the methods of exposition employed in the explanation of mechanisms, processes, and ideas, they are nevertheless sufficiently representative of certain tendencies in science to be of intrinsic value. Indeed, each author is a recognized authority.
Another feature is worthy of mention. Although the material covers so wide a field--anatomy, zoology, physics, psychology, and applied science--that the collection will appeal to instructors in every type of college and technical school, the selections are related in such a way as to produce an impression of unity. This relation is apparent between the first selection, which deals with the student's body, and the third, which deals with another organism in nature. The second and fourth selections deal with kindred aspects of modern industry--the manufacture of paper and the Linotype machine, by which it is used. The fifth selection is a protest against certain developments of the industrial regime; the last, an attempt to reconcile the spirit of science with that of religion. While monotony has been avoided, the essays form a distinct unit.
In most cases, selections are longer than usual, long enough in fact to introduce a student to each field. As a result, he can be made to feel that every subject is of importance and to realize that every chapter contains a fund of valuable information. Instead of confusing him by having him read twenty selections in, let us say, six weeks, it is possible by a.s.signing but six in the same period, to impress him definitely with each.
The text-book machinery has been sequestered in the Biographical and Critical Notes at the end of the book. Their character and position are intended to permit instructors freedom of treatment. Some may wish to test a student's ability in the use of reference books by having him report on allusions. Some may wish to explain these themselves. A few may find my experience helpful. For them suggestions are included in the Critical Notes. In general, I have a.s.sumed that instructors will prefer their own methods and have tried to leave them unhampered.
THE EXPOSITION OF A MECHANISM
THE LEVERS OF THE HUMAN BODY[1]
_Sir Arthur Keith_
In all the foregoing chapters we have been considering only the muscular engines of the human machine, counting them over and comparing their construction and their mechanism with those of the internal-combustion engine of a motor cycle. But of the levers or crank-pins through which muscular engines exert their power we have said nothing hitherto. Nor shall we get any help by now spending time on the levers of a motor cycle. We have already confessed that they are arranged in a way which is quite different from that which we find in the human machine. In the motor cycle all the levers are of that complex kind which are called wheels, and the joints at which these levers work are also circular, for the joints of a motor cycle are the surfaces between the axle and the bushes, which have to be kept constantly oiled. No, we freely admit that the systems of levers in the human machine are quite unlike those of a motor cycle. They are more simple, and it is easy to find in our bodies examples of all the three orders of levers. The joints at which bony levers meet and move on each other are very different from those we find in motor cycles. Indeed, I must confess they are not nearly so simple.
And, lastly, I must not forget to mention another difference. These levers we are going to study are living--at least, are so densely inhabited by myriads of minute bone builders that we must speak of them as living. I want to lay emphasis on that fact because I did not insist enough on the living nature of muscular engines.
[Ill.u.s.tration: Fig. 1.--Showing a chisel 10 inches long used as a lever of the first order.]
We are all well acquainted with levers. We apply them every day. A box arrives with its lid nailed down; we take a chisel, use it as a lever, pry the lid open, and see no marvel in what we have done (Fig. 1). And yet we thereby did with ease what would have been impossible for us even if we had put out the whole of our unaided strength. The use of levers is an old discovery; more than 1500 years before Christ, Englishmen, living on Salisbury Plain, applied the invention when they raised the great stones at Stonehenge and at Avebury; more than 2000 years earlier still, Egyptians employed it in raising the pyramids. Even at that time men had made great progress; they were already reaping the rewards of discoveries and inventions. But none, I am sure, surprised them more than the discovery of the lever; by its use one man could exert the strength of a hundred men. They soon observed that levers could be used in three different ways. The instance already given, the prying open of a lid by using a chisel as a lever, is an example of one way (Fig. 1); it is then used as a lever of the first order. Now in the first order, one end of the lever is applied to the point of resistance, which in the case just mentioned was the lid of the box. At the other end we apply our strength, force, or power. The edge of the box against which the chisel is worked serves as a fulcrum and lies between the handle where the power is applied and the bevelled edge which moves the resistance or weight. A pair of ordinary weighing scales also exemplifies the first order of levers. The knife edge on which the beam is balanced serves as a fulcrum; it is placed exactly in the middle of the beam, which we shall suppose to be 10 inches long. If we place a 1-lb. weight in one scale to represent the resistance to be overcome, the weight will be lifted the moment that a pound of sugar has been placed in the opposite scale--the sugar thus representing the power. If, however, we move the knife-edge or fulcrum so that it is only 1 inch from the sugar end of the beam and 9 inches from the weight end, then we find that we have to pour in 9 lb. of sugar to equalise the 1-lb. weight. The chisel used in prying open the box lid was 10 inches long; it was pushed under the lid for a distance of 1 inch, leaving 9 inches for use as a power lever. By using a lever in this way, we increased our strength ninefold. The longer we make the power arm, the nearer we push the fulcrum towards the weight or resistance end, the greater becomes our power. This we shall find is a discovery which Nature made use of many millions of years ago in fas.h.i.+oning the body of man and of beast. When we apply our force to the long end of a lever, we increase our power. We may also apply it, as Nature has done in our bodies, for another purpose. We have just noted that if the weight end of the beam of a pair of scales is nine times the length of the sugar end, that a 1-lb. weight will counterpoise 9 lb. of sugar. We also see that the weight scale moves at nine times the speed of the sugar scale. Now it often happens that Nature wants to increase, not the power, but the speed with which a load is lifted. In that case the ”sugar scale” is placed at the long end of the beam and the ”weight scale” at the short end; it then takes a 9-lb. weight to raise a single pound of sugar, but the sugar scale moves with nine times the speed of the weight scale. Nature often sacrifices power to obtain speed. The arm is used as a lever of this kind when a cricket ball is thrown.
Nothing could look less like a pair of scales than a man's head or skull, and yet when we watch how it is poised and the manner in which it is moved, we find that it, too, acts as a lever of the first order. The fulcrum on which it moves is the atlas--the first vertebra of the spine (Fig. 2). When a man stands quite erect, with the head well thrown back, the ear pa.s.sages are almost directly over the fulcrum. It will be convenient to call that part of the head which is behind the ear pa.s.sages the _post-fulcral,_ and the part which is in front the _pre-fulcral._ Now the face is attached to the pre-fulcral part of the lever and represents the weight or load to be moved, while the muscles of the neck, which represent the power, are yoked to the post-fulcral end of the lever. The hinder part of the head serves as a crank-pin for seven pairs of neck muscles, but in Fig. 2 only the chief pair is drawn, known as the _complex_ muscles. When that pair is set in action, the post-fulcral end of the head lever is tilted downwards, while the pre-fulcral end, on which the face is set, is turned upwards.
[Ill.u.s.tration: Fig. 2.--The skull as a lever of the first order.]
The complex muscles thus tilt the head backwards and the face upwards, but where are the muscles which serve as their opponents or antagonists and reverse the movement? In a previous chapter it has been shown that every muscle has to work against an opponent or antagonist muscle. Here we seem to come across a defect in the human machine, for the _greater straight_ muscles in the front of the neck, which serve as opposing muscles, are not only much smaller but at a further disadvantage by being yoked to the pre-fulcral end of the lever, very close to the cup on which the head rocks. However, if the _greater straight_ muscles lose power by working on a very short lever, they gain, in speed; we set them quickly and easily into action when we give a nod of recognition. All the strength or power is yoked to the post-fulcral end of the head; the pre-fulcral end of its lever is poorly guarded. j.a.panese wrestlers know this fact very well, and seek to gain victory by pressing up the poorly guarded pre-fulcral lever of the head, thus producing a deadly lock at the fulcral joint. Indeed, it will be found that those who use the jiu-jitsu method of fighting have discovered a great deal about the construction and weaknesses of the levers of the human body.
Merely to poise the head on the atlas may seem to you as easy a matter as balancing the beam of a pair of scales on an upright support. I am now going to show that a great number of difficulties had to be overcome before our heads could be safely poised on our necks. The head had to be balanced in such a way that through the pivot or joint on which it rests a safe pa.s.sageway could be secured for one of the most delicate and most important of all the parts or structures of the human machine. We have never found a good English name for this structure, so we use its clumsy Latin one--_Medulla oblongata_--or medulla for short. In the medulla are placed offices or centres which regulate the vital operations carried on by the heart and by the lungs. It has also to serve as a pa.s.sageway for thousands of delicate gossamer-like nerve fibres pa.s.sing from the brain, which fills the whole chamber of the skull, to the spinal cord, situated in the ca.n.a.l of the backbone. By means of these delicate fibres the brain dispatches messages which control the muscular engines of the limbs and trunk. Through it, too, ascend countless fibres along which messages pa.s.s from the limbs and trunk to the brain. In creating a movable joint for the head, then, a safe pa.s.sage had to be obtained for the medulla--that part of the great nerve stem which joins the brain to the spinal cord. The medulla is part of the brain stem.
This was only one of the difficulties which had to be overcome. The eyes are set on the pre-fulcral lever of the head. For our safety we must be able to look in all directions--over this shoulder or that. We must also be able to turn our heads so that our ears may discover in which direction a sound is reaching us. In fas.h.i.+oning a fulcral joint for the head, then, two different objects had to be secured: free mobility for the head, and a safe transit for the medullary part of the brain stem.
How well these objects have been attained is known to all of us, for we can move our heads in the freest manner and suffer no damage whatsoever.
Indeed, so strong and perfect is the joint that damage to it is one of the most uncommon accidents of life.
Let us see, then, how this triumph in engineering has been secured. In her inventive moods Nature always. .h.i.ts on the simplest plan possible. In this case she adopted a ball-and-socket joint--the kind by which older astronomers mounted their telescopes. By such a joint the telescope becomes, just as the head is, a lever of the first order. The eyegla.s.s is placed at one end of the lever, while the object-gla.s.s, which can be swept across the face of the heavens, is placed at the other or more distant end. In the human body the first vertebra of the backbone--the atlas--is trimmed to form a socket, while an adjacent part of the base of the skull is shaped to play the part of ball. The kind of joint to be used having been hit upon, the next point was to secure a safe pa.s.sage for the brain stem. That, too, was worked out in the simplest fas.h.i.+on.
The central parts of both ball and socket were cut away, or, to state the matter more exactly, were never formed. Thus a pa.s.sage was obtained right through the centre of the fulcral joint of the head. The centre of the joint was selected because when a lever is set in motion the part at the fulcrum moves least, and the medulla, being placed at that point, is least exposed to disturbance when we bend our heads backwards, forwards, or from side to side. When we examine the base of the skull, all that we see of the ball of the joint are two knuckles of bone (Fig. 3, A), covered by smooth slippery cartilage or gristle, to which anatomists give the name of occipital condyles. If we were to try to complete the ball, of which they form a part, we should close up the great opening--the _foramen magnum_--which provides a pa.s.sageway for the brain stem on its way to the spinal ca.n.a.l. All that is to be seen of the socket or cup is two hollows on the upper surface of the atlas into which the occipital condyles fit (Fig. 3, B). Merely two parts of the brim of the cup have been preserved to provide a socket for the condyles or ball.
[Ill.u.s.tration: Fig. 3.--A, The opening in the base of the skull, by which the brain stem pa.s.ses to the spinal ca.n.a.l. The two occipital condyles represent part of the ball which fits into the cup formed by the atlas. B, The parts of the socket on the ring of the atlas.]