MUSCULAR POWER. 99 



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 ani- 

 mals belonging to the higher classes. Wherever they arc 

 met with, they perform, as will hereafter be shown, very 

 important functions; sometimes assisting in respiration, at 

 other times contributing to the supply of food, and very ge- 

 nerally 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 

 becomes more refined, these fibres are collected into l)undles, 

 and compose what are properly called m/iiscles. Muscular 

 fibres are attached at their extremities to the parts intended 

 to be moved. In the lower animals, these attachments are 

 principally to the skin, or other external pnrts, v/hich arc 

 subservient to the purposes of progressive motion. In the 

 higher classes, the solid parts, or skeleton, being disposed 

 more in the centre of the system, the niuscles 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 characterizes the muscular 

 fibre is that of suddenly shortening itself, so as to bring its 

 two ends, and the parts to which those ends are connected, 

 nearer to one another. This contraction is performed with 

 astonishing quickness and force, and the accumulated effect 

 of a large collection of these fibres, such as constitutes a mus- 

 cle, is therefore capable of overcoming great resistances, or 

 of raising enormous weights. Those muscles, which, by 

 means of their nerves, as will hereafter be noticed, are sub- 

 servient to voluntary motion, are excited into action by an 

 exertion of the will of the animal. There are, however, a 

 great number of other muscles, the contractions of which 

 are involuntary, that is, are produced by other causes than 

 the will.* 



* These two classes of muscles do not differ in their outward appearance: 

 but Dr. Hodgkin has lately pointed out a curious difference in tJie micro- 



