Circulation of the Blood. 



61 



run through a hose or large rubber tube. Now, if a row of persons take 

 hold of this tube, the grip of their hands is like that of the muscles. 



Connective Tissue 



Endothelium 



Muscle Fiber 



Fig. 35. Plain Muscle Fiber. Separate and in Wall of Artery. 



Endothelium 



When the hands tighten their grip, the size of the hole in the tube is 

 made smaller, and less water is allowed to flow through it. When the 

 hands relax, the tube, being elastic, 

 allows more liquid to flow through it. 



Illustration of a Small Artery. 

 To represent a small artery, take a intern^ Elastic 

 small, thin-walled rubber tube and 

 wind a red thread around it. Now, if Circular Mus _ 

 the thread could shorten, it would cle Fibers 

 make the tube smaller. 



The Action of Plain and Striated 

 Muscles Fibers compared. These 

 plain muscle fibers are further like 

 those of the skeletal muscles in that 

 they are under the control of the 

 nerves, but they are involuntary in 

 their action. We cannot interfere 

 with the action of these muscles, no 

 matter how strongly we may will to do so. Without our thinking about 

 it, more blood goes to the muscles of the legs when we walk, more to 



The Outer 

 Coat 



Fig. 36. Coats of a Small Artery. 



