THE HEART AND THE BLOOD VESSELS 



151 



Epithelial layer 

 ''Muscle Layer 





of the tube is a layer of thin, fiat cells, making up a so-called 

 lining epithelium; outside this is a middle layer of involuntary, 

 smooth muscle, the fibres of which pass around the tube; 

 the outer coating is made of connective tissue disposed in a 

 dense, spongy mass of elastic fibres which thus gives the tube 

 its rigidity. If an artery is stretched it will return to its 

 normal length like a strip of rubber; if it is closed at one end 

 and air forced into the other, the artery will swell to two or 

 three times its first diameter, 

 but will return to its normal 

 size when the pressure is 

 relieved. 



Agents commonly causing 

 the contraction and relaxa- 

 tion of these muscles are dis- 

 cussed on pages 162-163. 



Veins are made of the same 

 tissues as arteries ; they differ 

 in that the walls are very 

 much thinner so that the 

 tube collapses whenever it is 

 empty. The muscle and con- 

 nective tissue coats cannot be 

 easily distinguished as their 

 fibres are mixed together, and 

 both are thin as compared 

 with the same layers in arteries; Fig. 82 C. Veins are much 

 ~oss elastic as well as less rigid than arteries. Some veins are also 

 provided with valves which prevent the blood from flowing 

 in any direction except toward the heart. These valves are 

 made of a thin, flexible layer of connective tissue and epithe- 

 lium in the shape of half cups, fastened to the walls of the 

 veins by their edges; Fig. 83. Thus when the blood flows 

 in one direction the valves flatten against the side of the tube 

 offer very little resistance; bu,t if the blood st^ts ba,cl 



FIG. 82. SHOWING THE STRUCTURE 



OF A BLOOD VESSEL. 

 At A is shown an artery with the different 

 layers removed at different levels. At 

 B is a cross section of an artery, and at 

 C of a vein. (Modified from Landois.) 



