134 INTRODUCTION TO GENERAL PHYSIOLOGY 



capillaries. The blood is collected again by small vessels, veins, 

 which join to form larger ones, and then open into the main venous 

 channel, the vena cava. The veins have thinner walls than the 

 arteries, but are to some extent muscular. The vena cava carries 

 the blood, which has by now given up a large part of its oxygen to 

 the tissues and received carbon dioxide from them, to the right 

 heart. The arrangement here is similar to that described for the 

 left heart, except that the walls are not so thick and powerfully 

 contractile. The blood driven out from it by a large artery passes 

 through the lungs. A capillary system is formed here around the 

 air sacs, and the gaseous exchange described in Chapter II. is 

 effected. The restored blood then arrives by the pulmonary veins 

 at the place from which we started. 



The history of the discovery of the circulation of the blood is a 

 very interesting one (see P., pp. 668-669). The real proof was given 

 by Harvey in 1616, although, in the absence of the microscope, it 

 was impossible for him to see the actual passage of the blood from 

 the arteries to the veins through the capillaries. This was done by 

 Leeuwenhoek in 1686, by means of the microscope which he had 

 invented. 



The general mechanics of the circulation can best be understood 

 by making experiments on a model (E., p. 218). The actual cir- 

 culation itself must be examined in the web of the frog's foot 

 under the microscope (E., p. 221), and the structure of the heart 

 with its valves by dissection of a sheep's heart (E., p. 222). 



\\'e sec then that the blood is sent through a number of channels 

 in multiple arc, as the electrician would say, by means of a pump 

 with the appropriate valves. This pump, the heart, consists of a 

 hollow space surrounded by muscular walls, which diminish the 

 size of the cavity when they contract. In order that the blood 

 may be sent round the circulation, it is clear that a pressure must 

 be produced, since a liquid only flows from a place where the 

 pressure is higher to one where it is lower. This pressure is pro- 

 duced by the heart, which drives blood into the elastic arteries, 

 producing tension in their walls. This tension continues to drive 

 the blood onwards during the interval between the beats, so that the 

 general arterial blood pressure does not fall greatly in this interval. 

 The way in which the supply of blood to different parts is 

 regulated in accordance with their needs will be described later, 

 but it can be seen from Fig. 7 how the widening of one alternative 

 channel will result in a greater supply of blood to that part with a 

 diminished supply to other parts, while a narrowing of it will result 

 in a diminished local supply, with a greater one to other parts. 



The uppermost of these parallel channels in the figure repre- 

 sents that of the heart muscle itself. As would be expected, the 



