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The structure of the spine is not in general different in different animals. In the serpent tribe, however, it is con
ed, the membranes recoil by their elasticity, the gristle is pressed into its place, and the bones resume their position.
We can readily understand how great the influence of these twentyfour joinings must be in giving elasticity to the whole column ; and how much this must tend to the protection of the brain. Were it not for this interposition of elastic material, every motion of the body would produce a jar to the delicate texture of the brain, and we should suffer almost as much in alighting on our feet, as in falling on our head. It is, as we have already remarked, necessary to interpose thin plates of lead or slate between the different pieces of a column to prevent the edges (technically called arrises) of the cylinders from coming in contact, as they would, in
chip or split off. But there is another very curious provision for the protection of the brain; we mean the curved form of the spine. If a steel spring, perfectly straight, be pressed between the hands from its extremities, it will resisi, notwithstanding its elasticity, and when it does give way, it will be with a jerk.
Such would be the effect on the spine if it stood upright, one bone perpendicular to another; for then the weight would bear equally; the spine would yield neither to one side nor to the other; and, consequently, there would be a resistance from the pressure on all sides being balanced. We, therefore, see the great advantage resulting from the human spine being in the form of an italic f. It is prepared to yield in the direction of its curves; the pressure is of necessity more upon one side of the column than on the other; and its elasticity is immediately in operation without a jerk. It yields, recoils, and so forms the most perfect spring; admirably calculated to carry the head without jar, or injury of any kind.
The most unhappy illustration of all this is the condition of old age. The tables of the skull are then consolidated, and the spine is rigid: if an old man should fall with his head upon the carpet, the blow, which would be of no consequence to the elastic frame of a child, may to him prove fatal; and the rigidity of the spine makes every step which he takes, vibrate to the interior of the head, and jar on the brain.
We have hinted at a comparison between the attachment of the spine to the pelvis and the insertion of the mast of a ship into the hull. The mast goes directly through the decks without touching them, and the heel of the mast goes into the step, which is formed of large solid pieces of oak timber laid across the keelson. The keelson is an inner keel resting upon the floor-timbers of the ship and directly over the proper keel. These are contrivances for enlarging the base on which the mast rests as a column; for as, in proportion to the height and weight of a column, its base must be enlarged, or it would sink into the earth; so, if the mast were to bear upon a point, it would break through the bottom of the ship.
The mast is supported upright by the shrouds and stays. The shrouds secure it against the lateral or rolling motion, and the stays and backstays against the pitching of the ship. These form what is termed the standing rigging.
'I he mast does not bear upon the deck or on the beams of the ship; inceed there is a space coverial with canvass between the deck and the
siderably varied; but with a strict reference to the conveniency of the animal. [Pl. IX. fig. 3, 4, 5] For, whereas in
We often hear of a new ship going to sea to stretch her rigging; that is, to permit the shrouds and stays to be stretched by the motion of the ship, after which they are again braced tight; for if she were overtaken
a storm before this operation, and when the stays and shrouds were relaxed, the mast would lean against the upper deck, by which it would be sprung or carried away. Indeed, the greater proportion of masts that are lost are lost in this manner. There are no boats which keep the sea in such storms as those which navigate the gulf of Finland. Their masts are not attached at all to the hull of the ship, but simply rest upon the step.
Although the spine has not a strict resemblance to the mast, the con trivances of the ship-builder, however different from the provisions of nature, show what object is to be attained; and when we are thus made aware of what is necessary to the security of a column on a moveable base, we are prepared to appreciate the superior provisions of nature for giving security to the human spine.
The human spine rests on what is called the pelvis, or basin;ma circle of bones, of which the haunches are the extreme lateral parts; and the sacrum (which is the keystone of the arch) may be felt at the lower part of the back. To this central bone of the arch of the pelvis the spine is connected; and, taking the similitude of the mast, the sacrum is as the step on which the base of the pillar, like the heel of the mast, is socketed or morticed. The spine is tied to the lateral parts of the pelvis by powerful ligaments, which may be compared to the shrouds. They secure the lower part of the spine against the shock of lateral motion or rolling; but, instead of the stays to limit the play of the spine forwards and backwards in pitching, or to adjust the rake of the mast, there is a very beautiful contrivance in the lower part of the column.
The spine forms here a semicircle which has this effect; that whether by the exertion of the lower extremities, the spine is to be carried forward upon the pelvis, or whether the body stops suddenly in running, the jar which would necessarily take place at the lower part of the spine, if it stood upright like a mast, is distributed over several of the bones of the spine; and, therefore, the chance of injury at any particular part is diminished.
For example, the sacrum, or centre bone of the pelvis, being carried forward, as when one is about to run, the force is communicated to the lowest bone of the spine. But, then, the surfaces of these bones stand with a very slight degree of obliquity to the line of motion; the shock communicated from the lower to the second bone of the vertebræ is still in a direction very nearly perpendicular to its surface of contact.
The same takes place in the communication of force from the second to the third, and from the third to the fourth; so that before the shock of the horizontal motion acts upon the perpendicular spine, it is distributed over four bones of that column, instead of the whole force being concentrated upon the joining of any two.
If the colunin stood upright, it would be jarred at the lowest point of contact with its base. Put by forming a semicircle, the motion would produce a jar on the very lowest part of the column, and which is distributed over a considerable portion of the column; and in point of fact, this part of the spine never gives way. Indeed, we should be inclined to of
quadrupeds the number of vertebræ is from thirty o forty in the serpent it is nearly one hundred and fifty whereas in men and quadrupeds the surfaces of the bones are flat, and these flat surfaces laid one against the other, and bound tight by sinews; in the serpent the bones play one within another like a ball and socket, * so that they have a free motion upon one another in every direction; that is to say, in men and quadrupeds, firmness is more consulted; in sei pents, pliancy. Yet even pliancy is not obtained at the expense of safety. The backbone of a serpent, for coherence and flexibility, is one of the most curious pieces of animal mechanism with which we are acquainted. The chain of a watch, (I mean the chain which passes between the spring-barrel and the fusee,) which aims at the same properties, is but a bungling piece of workmanship in comparison with that of which we speak.
IV. The reciprocal enlargement and contraction of the chest to allow for the play of the lungs, depends upon a simple yet beautiful mechanical contrivance, referrible to the structure of the bones which enclose it. [Pl. X. fig. 1.] The ribs are articulated to the backbone, or rather to its side
fer this model to the consideration of nautical men, as fruitful in hints for improving naval architecture.
Every one who has seen a ship pitching in a heavy sea, must have asked himself why the masts are not upright, or rather, why the fore mast stands upright, whilst the main and mizzen masts stand oblique to the deck, or, as the phrase is, rake aft or towards the stern of the ship.
The main and mizzen masts incline backwards, because the strain is greatest in the forward pitch of the vessel; for the mast having received an impulse forwards, it is suddenly checked as the head of the ship rises; but the mast being set with an inclination backwards, the motion falls more in a perpendicular line from the head to the heel. This advantage is lost in the upright position of the foremast, but it is sacrificed to a superior advantage gained in working the ship; the sails upon this mast act more powerfully in swaying the vessel round, and the perpendicular position causes the ship to tack or stay better; but the perpendicular position, as we have seen, causes the strain in pitching to come at right angles to the mast, and is, therefore, irure apt to spring.
These considerations give an interest to the fact, that the human spine, from its utmost convexity near its base, inclines backwards."-Bell's Treatise on Animal Mechanics.
* Der. Phys. Theol. p. 396.
+ In fish, which have more elastic, but less flexible bodies, the structure of the spine differs. The end of each vertebra is a cup containing a viscid fluid, which keeps the bones from approaching nearer to each other than the mean state of the elasticity of the lateral ligaments ; the fluid is incompressible, therefore forms a ball round which the bony cups move ; the ball having no cohesion, the centre of motion is always adapted to the change which the joint undergoes without produring friction.- Paxton.
projections obliquely:* that is, in their natural position, they bend or slope from the place of articulation downwards. But the basis upon which they rest at this end being fixed, the consequence of the obliquity, or the inclination downwards, is, that when they come to move, whatever pulls the ribs upwards, necessarily, at the same time, draws them out; and that, whilst the ribs are brought to a right angle with the spine behind, the sternum, or part of the chest to which they are attached in front, is thrust forward. The simple action, the afore, of the elevating muscles does the business; whereas, if the ribs had been articulated with the bodies of the vertebræ at right angles, the cavity of the thorax could never have been farther enlarged by a change of their position. If each rib had been a rigid bone, articulated at both ends to fixed bases, the whole chest had been immoveable. Keill has observed, that the breastbone in an easy inspiration, is thrust out one-tenth of an inch: and he calculates that this, added to what is gained to the space within the chest by the flattening or descent of the diaphragm, leaves room for forty-two cubic inches of air to enter at every drawing-in of the breath. When there is a necessity for a deeper and more laborious inspiration, the enlargement of the capacity of the chest may be so increased by effort, as that the lungs may be distended with seventy or a hundred such cubic inches. † The thorax, says Schelhammer, forms a kind of bellows, such as never have been, nor probably will be, made by any artificer. I
* For the mode of articulation of the ribs with the vertebræ, see Plate IX. Fig. 1 and 2.
† Anat. p. 229.
# The thorax, or chest, is composed of bones and cartilages, so disposed as to sustain and protect the most vital parts, the heart and lungs, and to turn and twist with perfect facility in every motion of the body; and to be in incessant motion in the act of respiration, without a moment's interval during a whole life. In anatomical description, the thorax is formed of the vertebral column, or spine, on the back part, the ribs on either side, and the breastbone, or sternum, on the fore part. But the thing most to be admired is the manner in which these bones are united, and especially the manner in which the ribs are joined to the breastbone, by the interposition of cartilages, or gristle, of a substance softer than bone, and more elastic and yielding. By this quality they are fitted for protecting the chest against the effects of violence, and even for sustaining life after the muscular power of respiration has become too feeble to continue without this support.
If the ribs were complete circles, formed of bone, and extending from t're spine to the breastbone, life would be endangered by any accidental f-acture; and even the rubs and jolts to which the human frame is con D-sually exposed, would be too much for their delicate and brittle texture
V. The patella, or kneepan is a curious little bone; in its form and office, unlike any other bone of the body. [Pl. X. fig. 2, 3.] It is circular: the size of a crown piece; pretty thick; a little convex on both sides, and covered with. a smooth cartilage. It lies upon the front of the knee: and the powerful tendons, by which the leg is brought forward, pass into it, (or rather it makes a part of their continuation,) from their origin in the thigh to their insertion in
Bat these evils are avoided by the interposition of the elastic cartilage. On their fore part the ribs are eked out, and joined to the breastbone by neans of cartilages, of a form corresponding to that of the ribs, being, as it were, a completion of the arch of ihe rib, by a substance more adapted to yield in every shock or motion of the body. The elasticity of this portion subdues those shocks which would occasion the breaking of the ribs. We lean forward, or to one side, and the ribs accommodate themselves, not by a change of form in the bones, but by the bending or elasticity of the cartilages. A severe blow upon the ribs does not break thein, because their extremities recoil and yield to the violence. It is only in youth, however, when the human frame is in perfection, that this pliancy and elasticity have full effect. When o!d age approaches, the cartilages of the ribs become bony. They attach themselves firmly to the breastbone, and the extremities of the ribs are fixed, as if the whole arch were formed of bone unyielding and inelastic. Then every violent blow upon the side is attended with fracture of the rib, an accident seldom occurring in childhood, or in youth.
But there is a purpose still more important to be accomplished by means of the elastic structure of the ribs, as partly formed of cartilage. This is in the action of breathing, or respiration; especially in the more highly-raised respiration which is necessary in great exertions of bodily strength, and in violent exercise. There are two acts of breathing-expiration, or the sending forth of the breath ; and inspiration, or the drawing in of the breath. When the chest is at rest, it is neither in a state of expiration nor in that of inspiration ; it is in an intermediate condition between these two acts. And the muscular effort by which either inspiration or expiration is produced, is an act in opposition to the elastic property of the ribs. The property of the ribs is to preserve the breast in The intermediate state between expiration and inspiration. The muscles of respiration are excited alternately, to dilate or to contract the cavity of the chest, and, in doing so, to raise or to depress the ribs. Hence it is, that both in inspiration and in expiration the elasticity of the ribs is called into play; and, were it within our province, it would be easy to show, that the dead power of the cartilages of the ribs preserve life by respiration, after the vital muscular power would, without such assistance, be too weak to continue life.
It will at once be understood, from what has now been explained, how, in age, violent exercise or exertion, is under restraint, in so far as it depends on respiration. The elasticity of the cartilages is gone, the circle of the ribs is now unyielding, and will not allow that high breathing, that sudden and great dilating and contracting of the cavity of the chest, which is required for circulating the blood through the lungs, and relieving the heart amidst the more tumultuous flowing of the blood which exercise aud exertion produce. — Bell's Treatise on Animal Mechanics.