30 IRRITABILITY. 



circulating fluid. It is displayed by the sensitive 

 plant, when its leaves, on being touched, collapse ; 

 by the sunflower, when it bends to the refreshing 

 light ; and by the night-folded blossom, expanding 

 with the return of day. 



In animals, we see this property residing to a 

 wonderful degree in the fibres of the muscles, whether 

 the muscles be of the class termed involuntary, such 

 as the heart ; of a mixed character, such as those of 

 respiration ; or purely voluntary, as those of loco- 

 motion. Many consider the irritability of muscular 

 fibres as depending immediately upon the influence 

 of the nerves with which they are supplied, and not 

 as an independent faculty. Now, that volition acts 

 as a stimulus to the voluntary muscles through the 

 medium of the nerves, is obvious, for we can move a 

 limb at pleasure. That the agitation of the nervous 

 system, from whatever cause, influences the action 

 of the voluntary muscles, and even those of volition, 

 is also manifest. Yet in all this there is nothing to 

 contradict the opinion, that irritability is an inde- 

 pendent property of muscular fibre, since it is not 

 annihilated by the cutting off of all communication 

 between the nerves and the brain, or the spinal 

 marrow, which are their great centres. If, for 

 instance, the heart of a frog be removed from the 

 body, it continues to act for a considerable period, 

 and even after action has apparently ceased, its 

 contraction will be repeated on the application of 

 stimuli. The same fact may be observed, also, in 



