CIRCULATION OF THE BLOOD. 51 



ton. These fibers are spindle-shaped cells, as shown in 

 Fig. 19, with a nucleus near the center, and do not have 

 the cross-markings of the fibers of the skeletal muscles ; 

 they are in consequence called nonstriated, smooth, or 

 plain muscle fibers. They are arranged circularly in the 

 walls of the arteries. These fibers have, in common with 

 all muscle fibers, the power of shortening. When they 

 shorten they reduce the size of the artery, and, there- 

 fore, for the time, less blood can flow through the 

 artery. When the muscle fibers cease to shorten, the 

 artery widens, and allows more blood to pass through it. 



Illustration of the Action of Muscles in Arterial Walls. To 



illustrate the action of the muscles in the walls of an artery, let the 

 water run through a hose or large 

 rubber tube. Now, if a row of per- 

 sons take hold of this tube, the grip Endotheiium 



of their hands is like that of the 



muscles. When the hands tighten internal Elastic 



their grip, the caliber of the hose La y er 



or tube is diminished, and less water 



is allowed to flow through it. When Circular Mus- 

 cle Fibers 

 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 small, thin-walled rubber tube and 

 wind a red thread around it. Now, 



if the thread could be made to Fl * 21 - Coats of a Small Artery. 

 shorten, it would diminish the cali- 

 ber of the tube. The representation would be more exact if the thread 

 were cut into many short pieces, and if each piece were thicker in 

 the middle, and were then glued to the tube. If the whole were 

 covered by a layer of tissue paper, the structure of the artery would be 

 roughly represented. 



