ARTERIES. 53 



The passage of the blood through the capillaries is mainly due to the 

 force of the ventricular systole and the elasticity of the arteries ; but it is 

 probably also aided by a power resident in the capillaries themselves, the 

 result of a vital relation between the blood and the tissues. 



The Veins are the vessels which return the blood to the heart ; they 

 have their origin in the venous radicles, and as they approach the heart, 

 converge to form larger trunks, and terminate finally in the venae cavae. 



They possess three coats 



1 . External, made up of areolar tissue. 



2. Middle, composed of non-striated muscular fibres, yellow, elastic and 

 fibrous tissue. 



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 reflection of the 

 internal coat, strengthened by fibrous tissues ; they always look toward the 

 heart, and when closed prevent a return 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 aspiration. 



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. 



Rhythmical 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 is nearly expended 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. 



7^he time required for a complete circulation of the blood throughout 

 the vascular system has been estimated to be from 20 to 30 seconds, while 

 for the entire mass of blood to pass through the heart 58 pulsations would 

 be required, occupying 48 seconds. 



