282 PHYSIOLOGY. 



knots become more marked; on the contrary, if the blood be pressed 

 toward the heart, it passes freely. From this Harvey deduced that 

 the direction of the venous blood is from the periphery to the heart. 



When an artery is obstructed, the blood accumulates between the 

 heart and the obstacle ; on the contrary, the accumulation in the case 

 of a vein is between the obstructed point and the general capillaries. 

 In the arteries, therefore, the blood flows from the heart to the 

 extremities; in the veins, from the extremities toward the heart. 



If an artery be completely severed and the animal's blood be 

 permitted to flow, all of its blood will eventually pass through the 

 opening. Would this occur if there were not a continual passage of 

 the blood from the heart to the arteries, then to the veins, and finally 

 to the heart again ; that is to say, a true circulation ? 



This great physiologist also observed that if poison be injected 

 at but a single point there will follow a general constitutional dis- 

 turbance, explained only by the movement of this vital fluid through- 

 out the entire body. 



To be able to ascertain by vision the direct passage of the blood 

 from the arteries into the veins was not allowed Harvey. It was 

 left to Malpighi, who, in 16G1, while examining the lung and mesen- 

 tery of a frog with the aid of a microscope, was able to note the 

 circulation of the blood in the capillary blood-vessels. The spec- 

 tacle of capillary circulation within the web of a frog's foot or tail 

 of a tadpole is within the reach of every student. Harvey was 

 denied this from lack of lenses powerful enough to demonstrate it. 



Now that the general plan of the circulation has been noted, 

 attention is naturally turned toward the principles governing the 

 flow of the blood. The mechanical act of impulsion can be readily 

 imitated by physical apparatus, but physics do not account for a cer- 

 tain part of the body receiving blood, now more, now less, abundantly ; 

 becoming congested or pale, warm or cold ; and at the same time the 

 impetus remaining perceptibly the same. 



By employing a simple piece of apparatus, designed by E. H. 

 Weber, the main, simple, physical phenomena of the circulation may be 

 simulated. To imitate the Harveian circuit, take a piece of small 

 intestine, sufficiently long, and join the two ends so that there is formed 

 a closed and circular conduit. A part of this elastic conduit is limited 

 by two valves which open according to the direction it is desired 

 ihsrt the current of liquid should go. The arrangement of the valves is 



