« VorigeDoorgaan »
. .' , ', \~-''
OF MECHANICAL ARRANGEMENT IN THtf "HUMAN FRAME.
We proceed therefore to propose certain examples taken out of this class; making choice of such, as, amongst those which have come to our knowledge, appear to be the most striking, and the best understood; but obliged, perhaps, to postpone both these recommendations to a third, that of the example being capable of explanation without plates or figures, or technical language.
OF THE BONES.,
I. I challenge any man to produce, in the joints and pivots of the most complicated, or the most flexible, machine, that was ever contrived, a construction more artificial, or more evidently artificial than that which is seen in the vertebrae of the human neck. Two things were to be done. The head was to have the power of bending forward and H 2 backward, backward, as in the act of nodding, stooping looking upward or downward; and, at the fame time, of turning itself round upon the body to a certain extent, the quadrant we will fay, or rather, perhaps, a hundred and twenty degrees of a circle. For these two purposes, two distinct contrivances are employed. First, The head rests immediately upon the uppermost of the vertebræ, and is united' to it by a binge joint; upon which joint the head plays freely forward and backward, as far either way as is necessary, or as the ligaments allow: which was the first thing required. But then the rotatory motion is unprovided for. Therefore, secondly, to make the head capable of this, a further mechanism is introduced; not between the head and the uppermost bone of the neck, where the hinge is, but between that bone, and the bone next underneath it. It is a mechanism resembling a tenon and mortice. This second, or uppermost bone but one, has what anatomists call a process, viz. a projection, somewhat similar, in size and shape, to a tooth; which tooth, entering a corresponding hole or socket in the bone above it, forms a pivot or axle, upon which that upper bone, together with the head which it supports, turns freely in a circle 5 and as far in the circle, as the attached muscles permit the head to turn. Thus are both motions perfect, without interfering with each other. When we nod the head, we use the hinge joint, which lies between the head and the first bone of the neck. When we turn the head round, we use the tenon and mortice, which runs between the first bone of the neck and* the second. We see the same contrivance, and the fame principle, employed in the frame or mounting of a telescope. It is occasionally requisite, that the object end of the instrument be moved up and down, as well as horizontally, or equatorially. For the vertical motion there is a hinge upon which the telescope plays: for the horizontal or equatorial motion, an axis upon which the telescope and the hinge turn round together. And this is exactly the mechanism which is applied to the motion of the head: nor will any one here doubt of the existence of counsel and design, except it be by that debility of mind, which can trust to its own reasonings in nothing. ..'.>
We may add, that it was, oil another account also, expedient, that the motion of the head backward and forward should be performed upon the upper surface of the first vertebra: for, if the first vertebra itself had bent forward, it would have brought th^e spinal marrow, at the very beginning of its course, upon the point of the tooth.
H.. Another mechanical contrivance, not unlike the last in its object, but different and original in its means, is seen in what anatomists call the sort-arm; that is, in the arm between the elbow and the wrist. Here, for the perfect use of the limb, two motions are wanted; a motion at the elbow backward and forward, 'which is called a reciprocal motion; and a rotatory motion, by which the palm of the handy as: occasion requires, may be turned upward. , How is this managed? The fore.acm, it is well known, consists of two bones, lying along-side^ each other, but touching only towards the ends. One, and only one, of these bones, is joined to the cubit, or upper part of the arm, at the elbow; the0 other alone, to the hand at the wrist. The first, by means at the elbow, of a hinge joint (which • . ;, allows
allows only of motion in the fame plane), swings backward and forward, carrying along with it the other bone, and the whole forearm. In the mean time, as often as there is occasion to turn the palm upward, that other bone to which the hand is attached, rolls upon the first, by the help of a groove or hollow near each end of one bone, to which is fitted a corresponding prominence in the other. If both bones had been joined to the cubit or upper arm at the elbow, or both to the hand at the wrist, the thing could not have been done. The first was to be at liberty at one end, and the second at the ether: by which means the two actions may be performed together. The great bone which carries the fore-arm, may be swinging upon its hinge at the elbow, at the very time, that the lesser bone, which carries the hand, may be turning round it in the grooves. The management also of these grooves, or rather of the tubercles and grooves, is very observable. The two bones are called the radius and the ulna. Above, i. e. towards the elbow, a tubercle of the radius plays into a socket of the ulna; whilst below, i. c. towards the wrist, the radius finds the socket, and the ulna the tubercle. A single bone in the fore-arm, with a ball and
H 4 socket