CHAPTER VIII 



IRRITABILITY AND CONTRACTILITY 



Irritability or Excitability is the power that certain tissues possess 

 of responding by some change to the action of an external agent. This 

 external agent is called a stimulus. 



Undifferentiated cells like white blood-corpuscles are irritable; 

 when stimuli are applied to them they execute the movements we 

 have learnt to call amoeboid. 



Ciliated epithelium cells and muscular fibres are irritable ; they 

 also execute movements under the influence of stimuli. 



Nerves are irritable ; when they are stimulated, a change is pro- 

 duced in them; this change is propagated along the nerve, and is 

 called a nervous impulse ; there is no change of form in the nerve 

 visible to the highest powers of the microscope ; much more delicate 

 and sensitive instruments than a microscope must be employed to 

 obtain evidence of a change in the nerve ; it is of a molecular nature. 

 But the irritability of nerve is readily manifested by the results the 

 nervous impulse produces in the organ to which it goes; thus the 

 stimulation of a motor nerve produces a nervous impulse in that nerve 

 which, when it reaches a muscle, causes the muscle to contract: 

 stimulation of a sensory nerve produces a nervous impulse in that 

 nerve which, when it reaches the brain, causes a sensation. 



Secreting glands are irritable ; when irritated or stimulated they 

 secrete. 



The electrical organs found in many fishes like the electric eel, 

 and torpedo ray, are irritable ; when they are stimulated they give 

 rise to an electrical discharge. 



Contractility is the power that certain tissues possess of respond- 

 ing to a stimulus by change of form. Contractility and irritability 

 do not necessarily go together; thus both muscle and nerve are 

 irritable, but of the two, only muscle is contractile. 



Some movements visible to the microscope are not due to con- 

 tractility ; thus granules in protoplasm ,qr. in a .vaauole may often be 

 seen to exhibit irregular, shaking moYenoefr-ts due ^irciply t<") vibration^ 



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