250 HANDBOOK OF PHYSIOLOGY. 



heart becomes distended, the distention lasting until the ligature is 

 removed ; 



5. That if a ligature round a limb be dr,awn very tight, no blood can 

 enter the limb, and it becomes pale and cold. If the ligature be some- 

 what relaxed, blood can enter but cannot leave the limb; hence it be- 

 comes swollen and congested. If the ligature be removed, the limb 

 soon regains its natural appearance; 



6. That the valves in the veins only permit the blood to flow toward 

 the heart; 



7. That there is general constitutional disturbance resulting from 

 the introduction of a poison at a single point, e.g., snake poison; 



To these may now be added many further proofs which have accu- 

 mulated since the time of Harvey, e.g. : 



8. That in wounds of arteries and veins;, in the former case hemor- 

 rhage may be almost stopped by pressure between the heart and the 

 wound, in the latter by pressure beyond the seat of injury; 



9. That the passage of blood-corpuscles from small arteries through 

 capillaries into veins in all transparent vascular parts, as the mesentery, 

 tongue, or web of the frog, the tail or gills of a tadpole, etc., may actu- 

 ally be observed under the microscope. 



Further, it is obvious that the mere fact of the existence of a hollow 

 muscular organ (the heart) with valves so arranged as to permit the 

 blood to pass only in one direction, of itself suggests the course of the 

 circulation. The only part of the circulation which Harvey could not 

 follow was that through the capillaries, for the simple reason that he 

 had no lenses sufficiently powerful to enable him to see it. Malpighi 

 (1661) and Leeuwenhoek (1668) demonstrated this in the tail of the tad- 

 pole and lung of the frog. 



