

THE CIRCULATION OP THE BLOOD. 



20. The blood can also be seen to move in the small ca- 

 pillaries by means of a microscope. The fine web of a 

 frog's foot is generally used for this purpose. Microscopi- 

 cal observations, however, should be received with great al- 

 lowance, for no two observers have ever found the same ap- 

 pearances. One makes out the globules of the blood to be 

 round, another square, and another tubular. Some physi- 

 ologists, it is to be feared, find just what they wish to find. 

 Owing to the refraction of light, the microscope is little to 

 be depended on in examining the minute structure of bodies. 



21. ^IThe motion of the blood is chiefly owing to the ac- 

 tion of the heart. This contracts with great force, and in 

 the following manner.) The auricles both contract at the 

 same instant, forcing the blood which is received from the 

 lungs, and the general circulation, into the ventricles ; these 

 then contract at the same moment ; the right one sending 

 the blood through the lungs, the left one through the aorta. 

 This alternate action is constant, as long as life continues. 

 The reason why the auricles act together, and the ventricles 

 the same, is probably owing to the fact, that both have a 

 common septum or dividing wall, so that one cannot contract 

 without the other. 



22. The ventricles contract more suddenly and powerfully 

 than the auricles, and they are three times as long in dila- 

 ting or expanding, as contracting. When the ventricles 

 contract, the apex or point of the heart rises up and strikes 

 against the left wall of the chest, between the sixth and 

 seventh ribs, and this can be felt by placing the hand on the 

 left side. The left ventricle has much thicker and stronger 

 walls than the right, because it has a greater distance to 

 throw the blood ; but the right ventricle will hold more than 

 the left, because the venous system is more capacious than 

 the arterial. 



23. (The velocity of the blood in the arterial system grows 

 slower in proportion to its distance from the heart, while 

 that in the veins is accelerated the nearer it approaches the 



23 



