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STUDIES IN PHYSIOLOGY 



tubular form. It can be stretched to a considerable extent ; 

 but resumes its original form and size when the force is 

 removed. In the cross section of an artery one can distin- 

 guish three layers. The outer layer is formed of interlacing 

 connective tissue fibers. Beneath this covering is a thick 

 layer composed of muscular and elastic tissue. It is the 

 elastic tissue that allows the arteries to expand when more 

 blood is forced into them by the contraction of the ventricles. 

 After each pulse these elastic walls squeeze the blood forward 

 into the capillaries ; arteries, therefore, are specially adapted 



to keep the capillaries full 

 of blood. The third layer 

 forms a smooth lining for 

 the tube ; it is composed of 

 serous membrane (Fig. 51). 

 Position of the Veins. 

 On the back of the hand 

 one sees through the skin 

 a branching system of blu- 

 ish blood vessels. These 

 are veins. Unlike the 

 arteries, veins have no 



B 

 FIG. 52. Valves in a Vein. 



A = vein laid open to show pouch-shaped 



valves. 

 B section of vein, showing valve open 



by flow of blood toward heart. 

 C= section of vein, showing valve closed p u l se , as One can easily 



by flow of blood back toward ' 



capillaries. P rove b J pressing one's 



finger upon one of them. 



Since blood flows slowly and steadily back to the heart 

 through the veins, there is little danger of any consider- 

 able loss of blood, even if some of them should be injured. 

 Many veins lie near the surface, while most of the arteries, 

 as we have just stated, are buried deeply among the other 

 tissues. 



The Structure of Veins. When the veins are emptied of 

 blood, they immediately collapse. In a cross section of one 

 of these blood vessels (as in that of an artery) can be seen 

 three layers, connective tissue, muscular and elastic tissue, 

 and the serous lining, but, as would be expected, each layer 



