118 THE MECHANICAL FUNCTIONS. 



purposes of progressive motion. In the higher 

 classes, the solid parts, or skeleton, being disposed 

 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 the 

 muscular fibre is that of suddenly shortening itself, 

 so as to bring its two ends, and the parts to which 

 those ends are attached, 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 that which 

 constitutes a muscle, is therefore capable of over- 

 coming great resistances, or of raising enormous 

 M^eights. Those muscles, which, by means of their 

 nerves, as will hereafter be noticed, are subservient 

 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 con- 

 tractions of which are involuntary ; that is, are 

 produced by other causes than the will.* 



Muscular contractility, of which there exists no 

 trace in the vegetable kingdom, t has been esta- 

 blished by nature as the primary moving power of 

 the animal machine. This agent is resorted to on 



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

 appearance : but Dr. Hodgkin has lately pointed out a curious 

 difference in the microscopic structure of the fibres of some of the 

 involuntary muscles. See Appendix to his Translation of Edwards 

 on the influence of Physical Agents on Life, p. 443. 



t The principal instances, which have been adduced in support 

 of the opinion that muscularity occasionally exists in vegetable 

 structures, are the alternate movements of the leaflets of the Hedy- 

 s(truin yyrans, which have been fancifully compared to the move- 



