[This figure represents the lower surface or base of the skull. The hole is the foramen magnum through which the spinal marrow descends into the spine; and on each side of the hole are the articulating processes, called the condyles.]

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 vertebræ of the human neck. Two things were to be done : the head was to have the power of bending forward and backward, as in the act of nodding, stooping, looking upward or downward; and, at the same time, of turning itself round upon the body to a certain extent-the quadrant, we will say, 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 part of the vertebræ, and is united to it by a hingejoint; 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

[This figure represents the uppermost vertebra, or atlas; and the condyles, mentioned in the former figure, sink into the articulating surfaces of this vertebra, permitting the nodding motions. a and b are the articulating surfaces; c is a surface which receives the tooth of the vertebra below; d the circle through which the spinal marrow passes.]

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 mortise. 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; 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 mortise, which runs between the first bone of the neck and the second.2+

[Here the tooth-like process of the second vertebra, which is called dentata, is passed through the ring of the first, and is held there by a transverse ligament, like a spindle in the bush. No doubt the object of this complexity is to permit the free motion of the head, without too great a laxity at any one joining, and thereby to protect the most vital organ of the body, the medulla oblongata, or spinal marrow, which passes from the head into the tube of the spine.]

We see the same contrivance and the same principle employed in the frame or mounting of a tele

24 The meaning of our author is obvious here; but the tenon and mortise are terms used for the firm joining of beams, as in the carpentry of a roof; not for rotatory motion.

scope. It is occasionally requisite that the objectend 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, on 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 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 seen in what anatomists call the fore-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 hand, as occasion requires,