

CHAPTER II. 

 THE CONTRACTILE TISSUES. 



THE greater number of the movements of the complex animal 

 body are carried on by means of the skeletal striated muscles. A 

 skeletal muscle when subjected to certain influences contracts, i. e. 

 shortens, bringing its two ends nearer together ; and the shortening, 

 acting through various bony levers or by help of other mechanical 

 arrangements, produces a movement of some part of the body. The 

 striated tissue of which the skeletal muscles are composed is the 

 chief contractile tissue. The peculiar muscular tissue of the heart 

 is another contractile tissue; under certain influences the fibres 

 into which it is arranged, shorten and thus give rise to the beat of 

 the heart. A similar shortening or contraction of the fusiform 

 fibre cells of plain muscular tissue, gives rise to movements such as 

 changes of calibre &c. of the alimentary canal, the urinary bladder, 

 the uterus, the arteries and the like. 



At first sight 'contraction' of any one of these forms of differen- 

 tiated muscular tissue seems wholly unlike an amoeboid movement 

 of an amoeba or of a white blood corpuscle. And yet the transition 

 from the one to the other is very slight. A typical amoeba may be 

 regarded as spherical in form, and when it is executing its move- 

 ments the pseudopodic bulging of its protoplasm may be seen 

 to occur now on this now on that part of its circumference and to 

 take now this and now that direction. The fibre cell of plain 

 muscular tissue is a nucleated protoplasmic mass of a distinctly 

 fusiform shape, and when it executes its movements, i. e. contracts, 

 the bulging of its protoplasm is always a lateral bulging in a 

 direction at right angles to the long axis of the fibre cell. Since as 



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