io6 



IRRITABILITY AND CONTRACTILITY. [CH. vin. 



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 

 responding 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 

 contractility ; thus granules in protoplasm or in a vacuole may 



Fig. 130. Frog's pigment cells. 



Fig. 131. Pigment-cells from the retina. A, cells 

 still cohering, seen on their surface ; a, nu- 

 cleus indistinctly seen. In the other cells the 

 nucleus is concealed by the pigment granules. 

 B, two cells seen in profile ; a, the outer or 

 posterior part containing scarcely any pig- 

 ment, x 370. (Henle.) 



often be seen to exhibit irregular, shaking movements due 

 simply to vibrations transmitted to them from the outside. Such 

 movement is known as Brownian movement. 



Instances of contractility are seen in the following cases : 



1. The movements of protoplasm seen in simple animal and 

 vegetable cells ; in the former we have already considered 

 streaming, gliding, and amoeboid movement (see p. 13); in 

 the latter case we have noted the rotatory movements of the 

 protoplasm within the cell wall in certain plants (see p. 14). 



2. The movements of pigment cells. These are well seen 

 under the skin of such an animal as the frog ; under the 

 influence of electricity and of other stimuli, especially of light, 

 the pigment granules are massed together in the body of the cell, 

 leaving the processes quite transparent (fig. 130). If the stimulus 

 is removed the granules gradually extend into the processes 

 again. Thus the skin of the frog is sometimes uniformly dusky, 

 and sometimes quite light coloured. The chamseleon is an animal 



