CHAPTER YIII. 



CIECULATION OF THE BLOOD IN THE VEINS. 



Physiological anatomy of the veins Strength of the coats o,f the veins Valves 

 of the veins Course of the blood in the veins Pressure of blood in the 

 veins Rapidity of the venous circulation Causes of the venous circulation 

 Influence of muscular contraction Air in the veins Function of the valves 

 Venous anastomoses Conditions which impede the venous circulation Ke- 

 gnrgitant venous pulse. 



Physiological Anatomy of the Yeins. The blood, dis- 

 tributed to the capillaries of all the tissues and organs by the 

 arteries, is collected from these parts in the veins and carried 

 back to the heart. In studying the anatomy of the capillary 

 system, or in observing the passage of the blood from the 

 capillaries to larger vessels, in parts of the living organism 

 which can be submitted to microscopic examination, it is 

 seen that the capillaries, vessels of nearly uniform diameter 

 and anastomosing in every direction, give origin, so to speak, 

 to a system of vessels, which, by union with others as we fol- 

 low their course, become larger and larger, and carry the 

 blood away in a uniform current. These are called the 

 venules, or venous radicles. They are the peripheral radicles 

 of the numerous vessels which transport the blood, after it 

 has served the purposes of nutrition or secretion, to the cen- 

 tral organ. 



The venous system may be considered, in general terms, 

 as divided into two sets of vessels : one, which is deep, and 



