CIRCULATION OF THE BLOOD. 141 



3. Internal, an endothelial membrane similar to that of the arteries. 

 Veins are distinguished by the possession of valves throughout 

 their course, which are arranged in pairs, and formed by a reflec- 

 tion of the internal coat, strengthened by fibrous tissues ; they 

 always look toward the heart, and when closed prevent a reflux of 

 blood in the veins. Valves are most numerous in the veins of the 

 extremities, but are entirely absent in many others. 



The onward flow of blood in the veins is mainly due to the action 

 of the heart, but is assisted by the contraction of the voluntary 

 muscles and the force of respiration. 



Muscular contraction, which is intermittent, aids the flow of blood 

 in the veins by compressing them. As regurgitation is prevented by 

 the closure of the valves, the blood is forced onward toward the 

 heart. 



Rhythmic movements of veins have been observed in some of the 

 lower animals, aiding the onward current of blood. 



During the movement of inspiration the thorax is enlarged in all 

 its diameters, and the pressure on its contents at once diminishes. 

 Under these circumstances a suction force is exerted upon the great 

 venous trunks, which causes the blood to flow with increased rapidity 

 and volume toward the heart. 



Venous Pressure. As the force of the heart-beat is nearly ex- 

 pended in driving the blood through the capillaries, the pressure in 

 the venous system is not very marked, not amounting in the jugular 

 vein of a dog to more than y 1 ^ that of the carotid artery. 



The time required for a complete circulation of the blood through- 

 out the vascular system has been estimated to be from twenty to 

 thirty seconds, while for the entire mass of blood to pass through 

 the heart fifty-eight pulsations would be required, occupying forty- 

 eight seconds. 



The forces keeping the blood in circulation are : 



1. Action of the heart. 



2. Elasticity of the arteries. 



3. Capillary force. 



4. Contraction of the voluntary muscles upon the veins. 



5. Respiratory movements. 



