250 PHYSIOLOGY OF MUSCLES AND NERVES. 



on the nerve and which are active in the nerve itself 

 vJiile tin- latter transmits the excitement. To use a 

 common but appropriate simile, the nt-rve is but the 

 -park which causes the explosion in the powder-mine; 

 or. to carrv the simile further, the sul])hur train which, 

 being tireil av one end, carries the fire- to the mine, and 

 there causes the explosion. The forces which are set 

 free within the muscle are chemical, due to the oxida- 

 tion of its substances ; the irritant originating from tin- 

 nerve is only the incitement in consequence of which 

 the chemical forces inherent in the miiM-le come into 

 play. Physicists call such processes the freeing of 

 forces. The nerve-irritant, therefore, frees the rauscle- 

 forces, and these translate themselves into warmth and 

 mechanical work. In every such freeing, the freeing 

 force is generally very small when compared with tin- 

 forces set free, and which may be dormant for incalcu- 

 lable periods ; though when they are once set free, they 

 are, capable of enormous effects. A huge block of stone 

 may for years hang in unstable equipoise on the edge 

 of a precipice till some insignificant disturbance makes 

 it fall, carrying destruction to all in the way of its de- 

 scent. It is even supposed that the slight disturbance 

 caused in the air by the sound <>f a mule-bell is suf- 

 ficient to start the ball of snow which at last thunders 

 down into the valley in the form of a mighty, all- 

 destroying avalanche, This freeing by small forces is 



only possible in the case f unstable equipoise. But 

 there is also a chemical unstable equipoise. Carbon 

 and oxygen may lie for thousands of years side by side 

 without combining. C|o>ely mingled, as in gunpowder, 

 <>r still more closely, as in nitro-glycerine, they are in 



unstable equipoise; the sli^hte-t blow suffices to cause 



