LECT. XVI. MUSCULAR FORCE. 307 



one only of the cases examined by Borelli; that being suffi- 

 cient to explain his mode of calculating the forces of 

 muscles. The weight of the fore-arm of man is about 2 

 kilog., which we may consider as applied at the middle of 

 the fore-arm ; or, what is the same thing, we may assume 

 that this weight is 1 kilog., applied at twice the distance 

 from the point of support, namely, the hand. It is known 

 that a man can support with his extended arm a weight of 

 about 13 kilog., and consequently the total resistance to 

 which the equilibrium should be made is 14 kilog. But 

 the power of the muscles for the arm of the lever has a 

 length which we know to be about the twentieth part of that 

 of the fore-arm ; therefore, the value of x will be 280, or 

 the product of 14 multiplied by 20, so that to carry a weight 

 of 14 kilog. with the hand, the flexor muscles ought to 

 make an effort equal to 280 kilogrammes. 



The generalities which we have now explained suffice 

 to enable you to comprehend the mechanism of the different 

 movements of animals. If I had wished to speak with all 

 possible minuteness of the theory of walking, of running, 

 of swimming, of flying, &c., the subject would have passed 

 beyond the limits, not only of a single lecture, but even of 

 an entire course. 



In all cases, whatever be the animal and the manner of 

 its progression, the essential part of its mechanism of loco, 

 motion invariably consists in an elongation or a contraction, 

 movements which the two branches of the arc or arcs, 

 formed by certain parts of the animal, execute. Under 

 some circumstances, these arcs are formed by the body of 

 the animal, which then becomes vermiform, or arched, as 

 in animals which swim or crawl. In other cases the move- 

 ments of extension and of flexion result from the successive 

 approach and retirement which the two sides of an angle 

 formed by the limbs of the animal undergo. One of these 



