228 THE CIRCULATION OF THE BLOOD [CH. XX. 



becomes pale, flaccid, and bloodless, and on removal of the ligature 

 the blood again flows into the heart. 



5. If the aorta is tied, the heart becomes distended with blood, 

 and cannot empty itself until the ligature is removed. 



6. The preceding experiments were performed on animals, but by 

 the following experiment he showed that the circulation is a fact in 

 man also ; if a ligature is drawn tightly round a limb, no blood can 

 enter it, and it becomes pale and cold. If the ligature is somewhat 

 relaxed so that blood can enter but cannot leave the limb, it becomes 

 swollen. If the ligature is removed, the limb soon regains its normal 

 appearance. 



7. Harvey also drew attention to the fact that there is general 

 constitutional disturbance resulting from the introduction of a poison 

 at a single point, and that this can only be explained by a movement 

 of the circulating fluid all over the body. 



8. If an artery is wounded, hsemorrhage may be stopped by 

 pressure applied between the heart and the wound ; but in the case 

 of a wound in a vein, the pressure must be applied beyond the seat 

 of injury. 



Since Harvey's time many other proofs have accumulated; for 

 instance : 



9. If a substance which, like ferrocyanide of potassium, can be 

 readily detected, is injected at a certain point into a blood-vessel, it 

 will after the lapse of a short interval have entirely traversed the 

 circulation and be found in the blood collected from the same point. 



10. Perhaps the most satisfactory proof of the circulation is one 

 now within the reach of every student, though beyond that of Harvey. 

 It consists in actually seeing the passage of the blood from small 

 arteries through capillaries into veins in the transparent parts of 

 animals, such as the tail of a tadpole or the web of a frog's foot. 

 Harvey could not follow this part of the circulation, for he had no 

 lenses sufficiently powerful to enable him to see it. Harvey's idea 

 of the circulation here was that the arteries carried the blood to the 

 tissues, which he considered to be of the nature of a sponge, and the 

 veins collected the blood again, much in the same way as drainage 

 pipes would collect the water of a swamp. The discovery that the 

 ends of the arteries are connected to the commencements of veins by 

 a definite system of small tubes we now call capillaries, was made 

 by Malpighi, in the year 1661. He first observed them in the tail of 

 the tadpole, and Leeuwenhoek, seven years later, saw the circulation 

 in the lung of the frog. 



We can now proceed to study some of the principles on which 

 the circulation depends : 



The simplest possible way in which we could represent the 

 circulatory system is shown in fig. 211 A. Here there is a closed 



