266 THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



At each beat the left ventricle forces about three ounces of blood 

 into the already full arterial system. The arteries are elastic tubes, 

 and the amount of elastic tissue is greatest in the large arteries. 

 The first effect of the extra three ounces is to distend the aorta still 

 further ; the elastic recoil of the walls drives on another portion of 

 blood, which distends the next section of the arterial wall, and this 

 distension is transmitted as a wave along the arteries, but with 

 gradually diminishing force as the total arterial stream becomes 

 larger. This wave constitutes the pulse-wave. Between the strokes 

 of the pump, or, in other words, during the periods of diastole, the 

 energy imparted to the elastic arterial walls by the heart, and which 

 has produced distension of the arteries, comes into play ; their recoil 

 drives the blood onwards and the arteries return to their original size. 

 The flow, therefore, does not cease during the heart's inactivity, so 

 that although the force of the heart is an intermittent one, the flow 

 through the capillaries and the veins beyond is a constant one, all 

 trace of pulsation having disappeared. The peripheral resistance 

 which keeps up the blood-pressure in the arteries, and like the con- 

 striction at the end of our india-rubber tube, assists in the conversion 

 of the intermittent into a continuous and constant stream, is to. be 

 found in the arterioles or small arteries, just before the blood passes 

 into what we have termed the vast capillary lake. These small 

 arteries with their relative excess of muscular tissue, are in health 

 always in a state of moderate tonic contraction. 



The large arteries contain a considerable amount of muscular as 

 well as elastic tissue. This co-operates with the elastic tissue in 

 adapting the calibre of the vessels to the quantity of blood they 

 contain. For the amount of blood in the vessels is never quite 

 constant, and were elastic tissue only present, the pressure exercised 

 by the walls of the containing vessels on the contained blood would 

 be sometimes very small, sometimes too great. The presence of a 

 contractile element, however, provides for a certain uniformity in the 

 amount of pressure exercised. There is no reason to suppose that 

 the muscular coat assists in propelling the onward current of blood, 

 except in virtue of the fact that muscular tissue is elastic, and there- 

 fore co-operates in the large arteries with the elastic tissue in keeping 

 up the constant flow in the way already described. 



The contractility of the arterial walls fulfils a useful purpose in 

 checking haemorrhage should a small vessel be cut, as it assists in the 

 closure of the cut end, and this in conjunction with the coagulation 

 of the blood arrests the escape of blood. 



Blood-pr e ssur e . 



The circulation of the blood depends on the existence of different 

 degrees of pressure in different parts of the circulatory system ; 



