368 



which, meeting two obstacles, the ground, and the body to which the im- 

 pulse is to be communicated, exert their action on one of the two which 

 is the more easily moved. The force is equal to the contraction of the 

 extensors, which elongate the body previously in a state of decurtation, 

 and advance the moveable obstacle by the whole difference, in regard to 

 length, of a man, whose limbs are in a state of flexion, and of the same 

 man while these parts are in a state of extension. It is, in the same man- 

 ner, and by a similar mechanism, that by pushing against the shore, with 

 an oar, we force a boat from it. The vertebral column represents an 

 elastic curve which straightens itself, between the feet which rest 

 against the bottom of the boat and the end of the pole or oar, pushed 

 against the shore, or the bottom of the water. 



If, on the contrary, we wish to draw towards us a body, we seize it 

 with extended arms 5 we then bend them forcibly : the spring, which is 

 in a state of tension, shortens itself, the effort is wholly performed by 

 the flexors: it is less fixed, and of less duration than that of the exten-. 

 sors, because the axis of the bones do not correspond to one another, in 

 a straight line, and because the action is generally partial. 



We can throw, to a distance, a projectile, the arm remaining pendu- 

 lous, and performing a mere oscillatory motion, or by a whirling motion 

 of the arm. This last action is much more powerful, because the mus- 

 cles which go from the trunk to the upper extremity, concur in it. In 

 the former, the previous oscillations give to the arm a motion which is 

 peculiar to it, which is added to the force of muscular contraction, and 

 which augments its effects. 



Professor Barthez was av/are, that the motions, by which the upper 

 extremity stiffens itself, and assumes a state of extension to project a 

 moveable body, or to repel a resistance that is opposed to it, perfectly 

 resemble leaping, and are attended, like that action, with a sudden ex- 

 tension of the joints which were previously bent. In motions applied 

 to a resistance that cannot be overcome, the body is not repelled with 

 the force communicated to it in leaping, by the abrupt extension of the 

 lower extremities. The scapula is too moveable on the trunk, its arti- 

 culation with the humerus is too unsteady, and the action of this bone 

 is not directed, with regard to the shoulder, in a sufficiently favourable 

 manner, to render the impulse equally great, even though the powers 

 should be equal, and they are far from being so. In every repulsion, and 

 every attraction, whether we bring towards us an object or remove it 

 from us, by acting upon it, with our superior extremities, these limbs 

 represent an elastic arch, which is curved or straightened by the action 

 of its flexors or extensors, and these motions, like the greater number 

 of those which we have hitherto considered, present a precise applica- 

 tion of the levers of the third kind. 



The action of seizing a body with the hand, is facilitated, 1st. by the 

 action of the radius on the ulna, which performs pronation and supina- 

 tion, motions which belong exclusively to the hands, and of which the 

 feet are incapable; 2dly. by the mobility of the wrist, which, properly 

 speaking, is capable of flexion and extension in two directions ; for, the 

 extension of the head does not consist in merely bringing it into a pa- 

 rallel line with the axis of the limb, but it is, besides, capable of turning 

 it round towards the back part of the fore-arm, a phenomenon not observ- 

 able in any other articulations ; Sdly. by the obscure motions, on one 

 another, of the bones of the carpus, by which the palm of the hand be- 



