CIRCULATION OF THE BLOOD IN THE VESSELS. 281 



tissues in a state of integrity, and prevents changes 

 which would very soon take place if the fluid were 

 perfectly stagnant. From the materials held in solu- 

 tion the living matter embedded in the tissue selects 

 certain constituents, and the fluid deprived of these 

 flows on, at last passing back again into the blood. 

 Its place in the tissue is soon taken by a fresh portion 

 of fluid which flows from the blood. The rate at 

 which this nutrient fluid moves, as well as the amount 

 distributed to the tissue in a given time, is determined 

 in part by the influence of the living matter drawing it 

 onwards {vis a fronte), and in part by the rate at which 

 the blood flows through the vessels and the degree 

 of tension of the thin vascular walls. The amount 

 of fluid distributed to the tissue varies from time to 

 time, according to the force of the heart's action, 

 according to the quantity of blood in the body which 

 is not always the same, the state of the arterial coats, 

 the calibre of the smaller arteries, and a number of 

 other circumstances. 



In page 24, the course which the blood takes as it 

 is driven through the heart has been indicated. It 

 is there stated that the blood is driven into the large 

 arteries (aorta to the system, pulmonary artery to the 

 lungs) by the contraction of the muscular walls of 

 the ventricles of the heart. The force of contraction 

 is more than is sufficient simply to drive the blood 

 onwards at the rate at which it is flowing through 

 the capillary vessels, and the great arteries are con- 

 sequently temporarily distended, their walls, which 

 are elastic, being stretched. As soon as the contrac- 

 tion of the muscular walls of the heart ceases, the 

 elastic arterial parietes recoil upon the column of blood, 

 forcing it in opposite directions, back again towards the 

 ventricle which it has just left, and onwards towards 

 the capillaries it is about to enter. As the blood 

 which tends to be driven back into the ventricle of 

 the heart impinges upon the lips and fills the valves 



