138 HAND-BOOK OF PHYSIOLOGY. 



there are few or none in those which are not subject to muscular pressure. 

 Among those veins which have no valves may be mentioned the superior 

 and inferior vena cava, the trunk and branches of the portal vein, 'the 



FIG. 120. A, vein with valves open. B, vein with valves closed: stream of blood passing off by 

 lateral channel. (Dalton.) 



hepatic and renal veins, and the pulmonary veins; those in the interior 

 of the cranium and vertebral column, those of the bones, and the trunk 

 and branches of the umbilical vein are also destitute of valves. 



CIRCULATION IN THE ARTERIES. 



Functions of the External Coat of Arteries. The external coat 

 forms a strong and tough investment, which, though capable of exten- 

 sion, appears principally designed to strengthen the arteries and to guard 

 against their excessive distension by the force of the heart's action. It is 

 this coat which alone prevents the complete severance of an artery when 

 a ligature is tightly applied; the internal and middle coats being divided. 

 In it, too, the little vasa vasorum (p. 131) find a suitable tissue in which 

 to subdivide for the supply of the arterial coats. 



Functions of the Elastic Tissue in Arteries. The purpose of 

 the elastic tissue, which enters so largely into the formation of all the 

 coats of the arteries, is, (a) to guard the arteries from the suddenly 

 exerted pressure to which they are subjected at each contraction of the 

 ventricles. In every such contraction, the contents of the ventricles are 

 forced into the arteries more quickly than they can be discharged into 

 and through the capillaries. The blood therefore, being, for an instant, 

 resisted in its onward course, a part of the force with which it was im- 



