THE STORY OP THE HEART 61 



the river begins to flow out over level ground into 

 a larger number of small channels. The circulation 

 in the veins, that is, the return of the blood 

 to the right side of the heart, is due to various 

 causes. There is first of all the force of the blood 

 which is perpetually being sent out from the left 

 side of the heart, each oncoming rush of blood, so to 

 speak, forcing onwards, and, in the case of the lower 

 part of the body, upwards, in the veins, the blood 

 which has preceded it. Again, the muscular move- 

 ments of the body compressing the veins, assist the 

 return of the blood, any tendency to backward flow 

 being prevented by the presence in these vessels of 

 pocket-like valves. The mouths of the pockets open 

 towards the heart, so that whilst blood easily passes 

 in that direction, any back-flow fills the pockets, 

 which, meeting in the middle, offer an obstacle to 

 regurgitation of the blood. 



THE BLOOD. With regard to the chemical com- 

 position of the blood we find that it contains a large 

 proportion of water, so much nitrogenous or proteid 

 matter held in solution, this matter being derived 

 from the food, and a certain amount of mineral 

 matter in addition. Blood when drawn from the 

 body gives us an idea of its physical characteristics 

 and composition, for it then separates ultimately 

 into two parts, the clot which falls to the bottom of 

 a vessel, and a layer of straw-coloured liquid which 

 rises above. This liquid is termed serum, the clot 

 consisting of the corpuscles of the blood which are en- 

 tangled in a substance apparently developed when 

 the blood is removed from the body, and known as 

 "fibrin." An old experiment taught that if fresh 

 blood from an animal was switched with willow 

 twigs it did not clot. The reason for this change in 



