CHAPTER XXV. 



CONTKACTILE TISSUES. 



IN the lower forms of organisms the motions executed by proto- 

 plasm suffice for all their requirements. Thus the amoeba man- 

 ages to pass through its entire lifetime with no other kind of 

 motion at its disposal but the flowing circulation and the bud- 

 ding out of its soft protoplasm. A vast number of minute organ- 

 isms depend wholly upon the protoplasmic stream and the twitch- 

 ing of cilia for their digestive and progressive movements. Before 

 we leave the class of animals which never pass beyond the uni- 

 cellular stage, we find, however, examples in which a portion of 

 their protoplasm is specially adapted to the performance of sud- 

 den and rapid motions. The protoplasm so modified in function 

 deserves the name of contractile material. Thus, though the 

 protoplasm which lies within the stalk of the bell animalcule is 

 morphologically undifferentiated, it can contract with such rapid- 

 ity that the eye cannot follow the motion. 



As we ascend in the scale of animal life, the necessity of having 

 motions of varied rapidity and duration at the command of the 

 animal becomes more and more urgent, and so we find not only 

 one, but several kinds of tissue specially adapted for carrying out 

 motions of different rate and duration. 



As a general rule the more rapid the contraction it performs 

 the more the tissue differs from the original type of protoplasm ; 

 and the slower and more persistent the contraction, the more the 

 tissue elements resemble protoplasmic cells. Thus, in the minute 

 bloodvessels, as we have seen, a very prolonged form of contrac- 

 tion, only varied by partial relaxations, is the rule, and gives rise 

 to the tone of the arterioles, and the contractile elements differ 

 but little from ordinary protoplasmic cells. The intestinal move- 

 ments are rapid compared with those of the arterial muscles, and 



