132 THE MECHANICAL FUNCTIONS. 



frequently employed in pairs, and are made to 

 act on opposite sides of the line of the intended 

 motion, which is, in this case, the diagonal be- 

 tween the direction of the two equal forces. 

 Thus, in order to bring a bone at p, Fig. 39, 

 down to the point Q, the two muscles A and 

 B, extending from the fixed points M and N, 

 may be employed ; for as they exert forces in 

 the directions P M and p N, there will result 

 a force in the intermediate direction p o : and 

 the effect desired will be accomplished more 

 quickly, and with a smaller extent of contrac- 

 tion in the muscles producing it, than if the 

 same power had been applied by means of a 

 straight muscle in the direction p o.* It is by 

 means of two sets of muscles, acting thus ob- 

 liquely, that the ribs are brought in closer ap- 

 proximation every time that the chest is ele- 

 vated in breathing. Thus carefully does nature 

 dispose the muscular fibres so as to obviate 

 the necessity of their being contracted beyond 

 a certain extent : and thus does she economize, 

 as much as possible, the expenditure of muscu- 

 lar power, wherever there is a constant call for 

 its exertion. 



The principle which I have just explained, 

 whereby certain advantages result from the ob- 



* See a paper by Dr. Monro, in the Transactions of the Royal 

 Society of Edinburgh. Vol. iii. p. 250. 



