THE PROOFS OF THE CIRCULATION 79 



through the right side of the heart, the lungs, the left 

 side of the heart, the arteries, and capillaries, back to the 

 vena cava ; but not the other way. In the second place, 

 we know that in the living body the blood is continually 

 flowing in the arteries towards the capillaries, because 

 when an artery is tied, in a living body, it swells up and 

 pulsates on the side of the ligature nearest the heart, 

 whereas on the other side it becomes empty, and the 

 tissues supplied by the artery become pale from the want 

 of a supply of blood to their capillaries. And when we cut 

 an artery the blood is pumped out in jerks from the cut 

 end nearest the heart, whereas little or no blood comes 

 from the other end. When, however, we tie a vein the 

 state of things is reversed, the swelling taking place on 

 the side farthest from the heart, etc., &c., showing that 

 in the veins the blood flows from the capillaries to the 

 heart. 



But certain of the lower animals, the whole, or parts, 

 of the body of which are transparent, readily afford direct 

 proof of the circulation ; in these the blood may be seen 

 rushing from the arteries into the capillaries, and from the 

 capillaries into the veins, so long as the animal is alive 

 and its heart is at work. The animal in which the circu- 

 lation can be most conveniently observed is the frog. 

 The web between its toes is very transparent, and the 

 corpuscles suspended in its blood are so large that 

 they can be readily seen as they slip swiftly along with 

 the stream of blood, when the toes are fastened out, 

 and the intervening web is examined under a microscope 

 (Fig. 24). 



19. The Capillary Circulation.— The essential charac- 

 teristics of blood flow through the capillaries may be 

 easily studied by observation under the microscope of the 

 web of a frog's foot. In the smallest capillaries the cor- 

 puscles pass along singly, sometimes following each other 

 in close file, at other times leaving quite considerable 



