MUSCULAR POWER. 11.'} 



point where its action is impressed, to the parts 

 which are designed ultimately to receive the action 

 of the force employed. It is an established prin- 

 ciple in physics, that mere machinery is incapable 

 of generating mechanical force ; and that such force 

 must always be originally derived from an extra- 

 neous source. Some impulse from without, whether 

 it be the pressure of the wind, the fall of a stream 

 of water, or the action of men or horses, or any 

 other kind of foreign agency, must be resorted to, 

 both to set the engine m motion, and to continue 

 its movements when they are once begun. Nor is 

 the case essentially different when the source of 

 motion apparently resides in some internal part of 

 the machine itself; in a watch, for instance, which 

 is actuated by the main spring ; or in a steam en- 

 gine, which is set in motion by the elastic vapour 

 contained in its cylinder ; the spring in the one 

 case, and the vapour in the other, although they 

 may in one sense be regarded as impelling powers, 

 are, in reality, but intermediate agents in the dis- 

 tribution of a force originating from other sources. 

 In the watch, the force may be traced to the hand 

 which coiled the spring : in the steam-engine, to the 

 lire, which has imparted elasticity to the vapour. 



The living body differs from inorganic machinery 

 in containing within itself a principle of motion not 

 referable, as far as we can perceive, to any of the 

 primary forces which exist in the inanimate world. 

 This principle has been termed contractility. In 

 animals of the simplest construction, every part of 

 the substance of the body seems to be equally 

 endowed with this contractile property, although 

 exhibiting no distinct appearance of a fibrous struc- 



VOL. I. I 



