CONTRACTILE TISSUES 61 



and is consequently named cardiac. The third order, 

 which is responsible for the movements of the limbs, 

 for breathing, balancing, facial expression, speech, etc., 

 is best called skeletal. 



Smooth Muscle. The properties of cardiac muscle 

 may well be taken up in connection with the physiology 

 of the heart. Those of skeletal muscle will be discussed 

 in the next chapter. It will be desirable at this place to 

 say something of smooth muscle. We have just said 

 that this kind of contractile tissue is most conspicuous 

 in the internal organs. For example, it is responsible 

 for the movements of the alimentary canal, the con- 

 tractions of the gall-bladder, the urinary bladder, and 

 the uterus. But is not restricted to these localities. 



FIG. 11. Cells of smooth muscle. 



It occurs in the blood-vessels, the bronchial tubes, the 

 eye, and, sparsely distributed, in the skin. 



The cells of smooth muscle are nowhere massed to form 

 layers of any great thickness. They are generally 

 arranged to form sheets adapted to enter into the struc- 

 ture of hollow organs or vessels. The individual cells 

 are slender and elongated, tapering to pointed ends. 

 In each is a single nucleus. The terms smooth and plain 

 applied to these cells have reference to the contrast 

 which exists between them and the fibers of skeletal 

 muscle; the difference will be apparent later. When 

 smooth muscle cells contract each one shortens and 

 thickens and, since the majority have a parallel direction, 

 a corresponding change in dimensions may take place 

 over an area of considerable extent. 



