388 OF NERVOUS IRRITABILITY 



CHAPTER II. 



OF NERVOUS IRRITABILITY AND ITS MODE OF 



ACTION. 



WE have already mentioned, in a previous chapter, that every 

 organ in the body is endowed with the property of irritability ; that 

 is, the property of reacting in some peculiar manner when subjected 

 to the action of a direct stimulus. Thus the irritability of a gland 

 shows itself by increased secretion, that of the capillary vessels by 

 congestion, that of the muscles by contraction. Now the irritability 

 of the muscles, indicated as above by their contraction, is extremely 

 serviceable as a means of studying and exhibiting nervous pheno- 

 mena. We shall therefore commence this chapter by a study of 

 some of the more important facts relating to muscular irritability. 



The irritability of the muscles is a property inherent in the muscular 

 fibre itself. The existence of muscular irritability cannot be ex- 

 plained by any known physical or chemical laws, so far as they 

 relate to inorganic substances. It must be regarded simply as a 

 peculiar property, directly dependent on the structure and consti- 

 tution of the muscular fibre; just as the property of emitting light 

 belongs to phosphorus, or that of combining with metals to oxygen. 

 This property may be called into action by various kinds of stimu- 

 lus ; as by pinching the muscular fibre, or pricking it with the point 

 of a needle, the application of an acid or alkaline solution, or the 

 discharge of a galvanic battery. All these irritating applications 

 are immediately followed by contraction of the muscular fibre. 

 This contraction will even take place under the microscope, when 

 the fibre is entirely isolated, and removed from contact with any 

 other tissue ; showing that the properties of contraction and irrita- 

 bility reside in the fibre itself, and are not communicated to it by 

 other parts. 



Muscular irritability continues for a certain time after death. The 

 stoppage of respiration and circulation does not at once destroy 

 the vital properties of the tissues, but nearly all of them retain 



