MUSCULAR POWER. 127 



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 muscle, is therefore 

 capable of overcoming great resistances, or 

 of raising enormous weights. Those muscles, 

 which, by means of their nerves, as will here- 

 after 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 contractions 

 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,! has been 

 established by nature as the primary moving 

 power of the animal machine. This agent is 



* 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 in Life, p. 443. 



t The principal instances, which have been adduced iu 

 support of the opinion that muscularity occasionally exists in 

 vegetable structures, are the alternate movements of the leaflets 

 of the Hedysarum gyrans, which have been fancifully com- 

 pared to the movements of the ribs in respiration ; the quick 

 motions of the stamina of the Berberis, Opiintia, and many 

 plants of the genera Carduus, and Centanrea ; the closing of the 



