IRRITABILITY. 



443 



pass no farther. It has also been demonstrated that muscular 

 tissue in which there are no nerves will respond to stimuli ; so 

 that of the existence of independent muscular irritability there is 

 no doubt. 



Stimuli. Stimuli may be general or special. 



General Stimuli. These are electrical, chemical, mechanical, 

 and thermic. A current of electricity will stimulate a muscle or a 

 nerve ; certain chemicals will also stimulate them ; but there are 

 some of these agents which will stimulate a nerve and not a 

 muscle ; still others will stimulate a muscle and produce no effect 

 upon a nerve. A blow will stimulate either a muscle or a nerve, 

 and is an instance of a mechanic stimulus, and heat or cold 

 suddenly applied will cause a response in either. 



Special stimuli are those whose influence is restricted to a single 

 nervous apparatus ; thus light affects only the retina ; sound- 

 waves, only the organ of Corti ; and the senses of smell and taste 

 require special stimuli to excite them. 



The manner in which stimuli act is not thoroughly understood. 

 It is compared by Sir William Gowers to the blow that explodes 

 dynamite or the match which ignites a mass of gunpowder. 



Although any of the stimuli above mentioned may be used to 

 demonstrate irritability and to study it, still it has been found that 

 the most reliable and satisfactory results are obtained when an 



FIG. 245. Experiment for determining 

 the irritability of nerves. 



FIG. 246. Daniell cell. 



electric stimulus is used, as this is more readily controlled and 

 measured than any of the other varieties of stimuli, and for this 

 purpose a muscle-nerve preparation is made (Fig. 245). It consists 

 of the gastrocnemius muscle of a frog with the sciatic nerve 

 attached, a portion of the bone being also removed by which it 

 may be clamped in an appropriate holder. The electric current 

 may be applied to the muscle directly, constituting direct stimula- 

 tion, or to the nerve through which it passes to the muscle, indirect 



