CHAPTER IV 

 CONTRACTILE TISSUES 



It is a peculiar fact about physiology that very different 

 orders of presentation have commended themselves to 

 different writers and have been used with success. 

 Whatever one chooses to place first, one is likely soon to 

 wish that the student were in possession of some other 

 part of the subject to serve him as a background. But 

 there is much to be said in favor of the introduction 

 early in a book of the physiology of movement. We 

 have to reckon with it in all the remaining sections of our 

 survey. So in the present instance we shall take up the 

 question of motion at this time. 



It has been said previously that most of the movements 

 executed by animal cells are the expression of contraction 

 as that term is understood by the biologist. Such move- 

 ments may be carried out by single cells or by tissues 

 composed of cells whose action is concerted. We could 

 know nothing about the behavior of single cells if we 

 had not the assistance of the microscope. Thanks to 

 that instrument we have found out that the two ex- 

 hibitions of contractility which are most common 

 among one-celled animal forms are to be seen also in the 

 higher organisms. These are, respectively, ameboid and 

 ciliary movement. 



Ameboid Movement. This manifestion of the con- 

 tractile property takes its name from a single-celled 

 aquatic animal, the ameba. It is one of the simplest 

 types conceivable, a minute mass of jelly-like substance 

 with a nucleus which confirms its right to rank as a cell. 

 It is usually colorless and transparent except for numer- 

 ous granules within. Its movement consists in the most 



53 



