166 HANDBOOK OF PHYSIOLOGY. 



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

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



Wounds of arteries and veins. In the former case haemorrhage may 

 be almost stopped by pressure between the heart and the wound, in the 

 latter by pressure beyond the seat of injury. 



9. The direct observation of 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. 



10. The results of injecting certain substances into the blood. 

 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 is 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 it in the tail of the tadpole and 

 lung of the frog. 



