CH. xix.] HARVEY'S DISCOVERIES. 22J 



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. 



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

 instance : 



8. If an artery is wounded, haemorrhage 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. 



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. Our increased knowledge of the structure of the heart and 

 its valves has shown that its structure is such as to permit the 

 blood to pass in one direction only. 



i r . 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 trans- 

 it parts of animals, such as the tail of a tadpole or the web 

 { 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, 



Q 2 



