COMMON THINGS MAN. 



variety of motion and inflection, something would still be wanting. 

 The chief seat of the sensibility of touch is the palm of the hand 

 and the palmar sides of the fingers ; and the mechanism of the 

 hand is so contrived as to accommodate itself to the play of this 

 sensibility. The thumb is mounted so as to face the fingers, and 

 the articulations of both are such as to enable them to be inflected 

 towards each other, and towards the palm, so that when an object 

 is embraced or grasped by the fingers, all the part of the hand 

 possessing most sensibility of touch is brought into contact with it. 

 If we grasp the hand of a friend or a beloved relative, the palms 

 come into contact, and we are conscious of a mutual sensation con- 

 veyed through the nervous system. If, while the mechanism of 

 the hand remains the same, the nerves which now overspread the 

 palm and the palmar sides of the fingers were spread over the back 

 of the hand, all this sensibility would cease. 



It is obviously essential that the palm of the hand, which is- 

 thus its prehensive side, should be capable of being turned in 

 all directions, so as to present itself to the objects to be grasped 

 or touched. But the hinge joints of the wrist and elbow would 

 only enable the palm to be inflected inwards towards the hollow of 

 the arm. 



It is true that the rotatory motion which can be given to the 

 arm upon the shoulder would vary the play of the palm, but the 

 motion would still be imperfect for the purposes of prehension 

 and touch. An expedient is, therefore, provided, which may be 

 fairly said to confer upon the hand the utmost perfection as an 

 organ of prehension. This expedient consists of a simple and 

 beautiful mechanical arrangement in the structure of the fore arm, 

 which is composed between the elbow and wrist, not of one, but 

 of two bones, of nearly equal length, placed side by side. One of 

 these, called the ulna, is articulated with the upper bone of the 

 arm at the elbow by a hinge joint ; the other, called the radius, 

 is articulated to the hand at the wrist, with a like hinge motion. 

 But the radius having the hand thus appended to it is so arranged 

 that it can revolve round the ulna, carrying the hand with it, thus 

 having the faculty of presenting the palm in any desired direction 

 without changing the general position of the arm. 



11. In fig. 11, the bones of the arm and hand are represented ; 

 the ulna, hinged upon the elbow, being on the left, and the radius, 

 with the hand hinged upon it at the wrist, being on the right. 

 The two bones are tied together by intermediate ligaments (6, 7), 

 the ligament by which the hand is tied to the radius appearing 

 at 10. The palm of the hand and the hollow of the elbow are 

 supposed to be presented to the observer. 



When to all these conditions it is added that the successive 

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