54 
OF MECHANICAL ARRANGEMENT 
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 up¬ 
ward. How is this managed? The fore-arm, it is well 
known, consists of two bones lying alongside each other, 
but t-ouching 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, 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 other: 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 les¬ 
ser 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. e. towards the wrist, 
the radius finds the socket, and the ulna the tubercle. A 
single bone in the fore-arm, with a ball and socket joint at 
the elbow, which admits of motion in all directions, might, 
in some degree, have answered the purpose of both moving 
the arm and turning the hand. But how much better it is 
accomplished by the present mechanism, any person may 
convince himself, who puts the ease and quickness, with 
which he can shake his hand at the wrist circularly, (mov- 
