Pagina-afbeeldingen
PDF
ePub

CHAPTER VIII.

OF MECHANICAL ARRANGEMENT IN THE HUMAN FRAME.

WE proceed therefore to propose certain examples taken out of this clafs; making choice of fuch, as, amongst those which have come to our knowledge, appear to be the most striking, and the best understood; but obliged, perhaps, to poftpone 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 conftruction more artificial, or more evidently artificial than that which is feen in the vertebræ of the human neck. Two things were to be done. The head was to have the power of bending forward and backward,

H 2

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 thefe two

purposes, two diftinct contrivances are employed. First, The head refts immediately upon the uppermost of the vertebræ, and is united to it by a hinge joint; upon which joint the head plays freely forward and backward, as far either way as is neceffary, or as the ligaments allow: which was the first thing required. But then the rotatory motion is unprovided for. Therefore, fecondly, to make the head capable of this, a further mechanifm 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 mechanifm refembling á tenon and mortice. This fecond, or uppermoft bone but one, has what anatomifts call a procefs, viz. a projection, fomewhat fimilar, in fize and shape, to a tooth; which tooth, entering a correfponding hole or focket in the bone above it, forms a pivot or axle, upon

*

which that upper bone, together with the head which it fupports, turns freely in a circles, and as far in the circle, as the attached mufcles 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 fecond. We fee the fame contrivance, and the fame principle, employed in the frame or mounting of a telescope. It is occafionally requifite, 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 counfel and defign, except it be by that debility of mind, which can truft to its own reasonings in nothing.

We may add, that it was, or another account alfo, expedient, that the motion of the head backward and forward fhould be performed upon the upper furface of the first vertebra: for, if the first vertebra itself had bent forward, it would have brought the spinal marrow, at the very beginning of its course, upon the point of the tooth.

II. Another mechanical contrivance, not unlike the last in its object, but different and original in its means, is feen in what anatomifts call the fore-arm; that is, in the arm between the elbow and the wrift, 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 hand, as occafion requires, may be turned upward. How is this managed? The forearm, it is well known, confifts of two bones, lying along-fide 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; the other alone, to the hand at the wrift. The first, by

means at the elbow, of a hinge

joint (which

allows

allows only of motion in the fame plane), fwings backward and forward, carrying along with it the other bone, and the whole forearm. In the mean time, as often as there is occafion to turn the palm upward, that other bone to which the hand is attached, rolls upon the firft, by the help of a groove or hollow near each end of one bone, to which is fitted a correfponding 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 wrift, the thing could not have been done. The firft was to be at liberty at one end, and the fecond at the cther: by which means the two actions may be performed together. The great bone which carries the fore-arm, may be fwinging upon its hinge at the elbow, at the very time, that the leffer bone, which carries the hand, may be turning round it in the grooves. management alfo of thefe grooves, or rather of the tubercles and grooves, is very obfervable. The two bones are called the radius and the ulna. Above, i. e. towards the elbow, a tubercle of the radius plays into a focket of the ulna; whilft below, i. c. towards the wrift, the radius finds the focket, and the ulna the tubercle. A fingle bone in the fore-arm, with a ball and focket

H 4

The

« VorigeDoorgaan »