78 



STRUCTURE OF ARTERIES. 



Endothelium 



tighten their grip, the caliber of the hose or tube is dimin- 

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

 hands relax, the tube, being elastic, allows more liquid to flow 

 through it. 



To represent a small artery, take a small thin-walled rub- 

 ber tube, and wind a red thread around it. Now, if the thread 

 could be made to shorten, it 

 would diminish the caliber of 

 the tube. The representation 

 would t 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. Each 

 of the plain muscle fibers has 

 a Nucleus near its center. 



These plain muscle fibers are 

 further like the skeletal mus- 

 cles 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 

 brain when we are studying, to the digestive organs after 

 eating, etc. 



The Blood-Flow in the Capillaries. The arteries divide 

 and subdivide, and become capillaries, which have connecting 

 branches, forming a close network of tiny thin-walled tubes. 

 These penetrate and pervade nearly every tissue of the body. 



The Oute 

 Coat 



Fig. 26. Coats of a Small Artery. 

 (After Landois & Stirling.) 



