IN THE HUMAN FRAME. 53 



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 mor- 

 tice, which runs between the first bone of the neck and the 

 second. We see the same contrivance, and the same prin- 

 ciple, employed in the frame or mounting of a telescope. 

 It is occasionally requisite, that the object-end of the in- 

 strument 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 nothincr. 



We may add, that it was, on another account also, ex- 

 pedient, that the motion of the head backward and for- 

 ward should be performed upon the upper surface of the 

 first vertebrae; for if the first vertebrae itself had bent for- 

 ward, 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 from ' the elbow to the wrist. [PI. VIII. fig. 

 1, 2.] 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, may be 

 turned upward. How is this managed 1 The fore-arm, 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 ; the other alone, to the hand at 

 the wrist. The first, by means, at the elbow, of a hinge 

 joint (which allows only of motion in the same plane,) 

 swings backward and forward, carrying along with it the 

 other bone, and the whole fore^arm. In the meantime, 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 

 F 



