120 THE MECHANICAL FUNCTIONS. 



rounding fluid by means of very minute and 

 generally microscopic filaments, which the ani- 

 mal, by some unknown power, causes to vibrate 

 with great rapidity. Occasionally these organs 

 are found even in animals belonging to the 

 higher classes. Wherever they are met with 

 they perform, as will hereafter be shown, very 

 important functions ; sometimes assisting in res- 

 piration, at other times contributing to the 

 supply of food, and very generally serving as 

 instruments of progressive motion. 



In animals placed a little higher in the 

 scale, we begin to trace the formation of fibres, 

 which at first are irregularly scattered through 

 the soft substance : but as the organization be- 

 comes more refined, these fibres are collected 

 into bundles, and compose what are properly 

 called muscles. Muscular fibres are attached at 

 their extremities to the parts intended to be 

 moved. In the lower animals these attach- 

 ments are principally to the skin, or other 

 external parts, which are subservient to the 

 purposes of progressive motion. In the higher 

 classes, the solid parts, or skeleton, being dis- 

 posed more in the centre of the system, the 

 muscles are applied to them in the interior of 

 the body, and are more distinctly separated 

 into masses, each having its proper function in 

 the movements of the frame. 



The peculiar property which characterises 



