THE MECHANISM OF THE CIRCULATION 443 



less degree (for part of the blood in the arteries is escaping into the 

 capillaries), now retracts on the blood within it, and as the arteries at 

 each division present a larger area, and therefore a broader stream, 

 whilst more and more blood is entering the great capillary sea, the inter- 

 rupted current observed in the larger vessels, due to the intermittent 

 action of the heart, is gradually converted in the smaller vessels into a 

 uniform, steady, and continuous stream. Thus the pulse, which is very 

 perceptible in the larger vessels, becomes imperceptible in the smaller 

 ones. Subsidiary purposes for which the elasticity is useful are that it 

 enables the limbs to be freely bent and stretched and otherwise moved 

 without risk of rupture. It also enables the arteries to accommodate 

 themselves to the considerable variations that occur in the absolute 

 quantity of blood in the system. Lastly, the elasticity of the vessels 

 reduces the chances of death by hemorrhage, partly by retracting the 

 cut artery in its sheath, and partly by diminishing its calibre. The elas- 

 ticity of the vessels is not a new or active force in effecting the circulation; 

 it is passive, and represents the stored -up energy of the heart, which, 

 during contraction, is expended in dilating the large vessels, and is given 

 out again by them during the period of quiescence or relaxation of the 

 heart. 



The Pulse. — The pulse is the wave -like movement which traverses 

 the arteries with each beat of the heart, and which is perceived when a 

 vessel is slightly compressed with the fingers against an unyielding surface. 

 The arteries, in health, are always distended with blood, as shown by the 

 spurt which takes place when one is divided, and when a fresh quantity 

 is injected into it by the heart. 



The Contractility of the Arteries, unlike the property of elasticity, 

 is feebly marked in the larger trunks and their primary branches, whilst it 

 becomes more conspicuous in the smaller vessels. It is the means by which 

 the supply of blood to the several organs is regulated in accordance with 

 their condition of activity or repose. To take an example amongst many 

 that might be given, the stomach during fasting receives a supply of blood 

 sufficient to minister to the nutrition of the tissues of which its walls are 

 composed. The vessels in this condition are contracted, and the colour of 

 the organ is a pale pink; but after a hearty meal, when the process of diges- 

 tion is in course of active performance, when gastric juice is being poured 

 forth and absorption is proceeding, a much larger supply of blood is needed. 

 Under the influence of certain (dilatator) nerves, by means of which the 

 muscular tissue of the wall of the vessels is caused to relax, their calibre 

 enlarges, a freer current of blood passes through them, and the whole organ 

 becomes congested and of a deep rose-red hue. When the digestive process 



