462 MOVEMENTS. 



erally received, it is proper to apply it to any tissue or organ 

 that performs its vital function, so-called, under a natural 

 or artificial stimulus. The nerves receive impressions and 

 carry a stimulus to the muscles, causing them to contract. 

 This property, which is always present during life under 

 normal conditions, and persists for a certain period after 

 death, is called nervous irritability. It has lately been 

 shown that the application of a proper stimulus will induce 

 secretion by the glands ; and Bernard has called this glandular 

 irritability. 1 The application of a stimulus to the muscular 

 tissue causes the fibres to contract ; and this is muscular irri- 

 tability. As it always involves contraction, and is extinct 

 only when the muscles can no longer act, it is equally proper 

 to call this property contractility. 'No property, such as we 

 understand by this definition of irritability, is manifested 

 by tissues or organs that have purely passive or mechanical 

 functions, such as bones, cartilages, and fibrous or elastic 

 membranes. Irritability can only be applied properly to 

 nerves or nerve centres, contractile structures, and glands. 



During life and under normal conditions, the muscles 

 will always contract in obedience to a proper stimulus ap- 

 plied either directly or through the nerves. In the natural 

 action of the organism, this contraction is always induced by 

 nervous influence through reflex action or volition. Still, a 

 muscle may be living and yet have lost its contractility. 

 For example, after a muscle has been for a long time par- 

 alyzed and disused, the application of the most powerful 

 galvanic excitation will fail to induce contraction. But 

 when we examine such a muscle with the microscope, it is 

 found that the nutrition has become profoundly affected, 

 and that the contractile substance has disappeared, giving 

 place to inert fatty matter. Muscular contractility persists 

 for a certain time after death and in muscles separated from 

 the body ; and this fact has been taken advantage of by phys- 

 iologists in the study of the so-called vital properties of the 



1 See page 24. 



