100 



IRRITABILITY AND CONTRACTILITY 



[CH. VIII. 



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. 12) ; in the latter case we 

 have noted the rotatory movements of the protoplasm within the cell 

 wall in certain plants (see p. 13). 



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

 the skin of such an animal as the frog ; under the influence of elec- 

 tricity 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 



FIG. 130. Frog's pigment cells. 



Fio. 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.) 



coloured. The chamseleon is an animal which has become almost 

 proverbial, since it possesses the same power to a marked degree. 

 This function is a protective one ; the animal approximates in colour 

 that of its surroundings, and so escapes detection. 



In the retina we shall find a layer of pigment cells (fig. 131), the 

 granules in which are capable of moving in the protoplasm in a some- 

 what similar way ; the normal stimulus here also is light. 



3. Ciliary movement ; here we have a much more orderly move- 

 ment which has already been described (see p. 29). 



4. In Vorticellse, a spiral thread of protoplasm in their stalk 

 enables them by contracting it to lower the bell at the end of the 

 stalk. 



5. In certain of the higher plants, such as the sensitive and carni- 

 vorous plants, movements of the stalks and sensitive hairs of the 

 leaves occur under the influence of stimuli. 



6. Muscular movement. This for the student of human physio- 

 ; logy fa the; mofct important of the series; it is by their muscles that 



