MUSCULAR POWER. 125 



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 engine, 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 re- 

 garded as impelling powers, are, in reality, but 

 intermediate agents in the distribution 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 fire, which has imparted elasticity to 

 the vapour. 



The living body differs from inorganic ma- 

 chinery in containing within itself a principle 

 of motion not referable, as far as we can per- 

 ceive, 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 sub- 

 stance of the body seems to be equally endowed 

 with this contractile property, although ex- 

 hibiting no distinct appearance of a fibrous 

 structure. This is the case with all the lower 

 zoophytes, such as the Infusoria, Polypi, Me- 

 dusce, and the simpler kinds of Entozoa. 



Among the Polypi and Infusoria we meet 

 with a singular mode of acting upon the sur- 



