250 PHYSIOLOGY OF MUSCLES AND NERVES. 



on tlie nerve and which are active in the nerve itself 

 while the latter transmits the excitement. To use a 

 common but aj)propriate simile, the nerve is but the 

 spark which causes the explosion in the powder-mine ; 

 or, to carry the simile further, the sulphur train which, 

 being fired at 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 the 

 nerve is only the incitement in consequence of which 

 the chemical forces inherent in the muscle come into 

 play. Physicists call such processes the freeing of 

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

 forces, and these translate themselves into warmth and 

 mechanical work. In every such freeing, the fireeing 

 force is generally very small when compared with the 

 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 of 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 of unstable equipoise. But 

 there is also a chemical unstable equipoise. Carbon 

 and oxygen may lie for thousands of years side by side 

 without combining. Closely mingled, as in gunpowder, 

 or still more closely, as in nitro-glycerine, they are in 

 unstable equipoise ; the slightest blow suffices to cause 



