142 THE BLOOD AND ITS CIRCULATION. 



(Fig. 41). As regards their relation to the heart, we have 

 seen already that at the summit of the arterial cone is found 



a muscular reservoir, the 

 left ventricle; and at the 

 summit of the venous cone 

 a similar reservoir, the 

 right auricle. This con- 

 stitutes the system of gen- 

 * eral circulation, the greater 



Fig. 41. Diagram of the whole extent of rfrm/Jfifinn Tn iflditinn 

 the arterial Ind venous cone, with the in- CITC \ \ OH ' J 



terposition of the capillaries.* to this double cone, as rep- 



resenting the general cir- 

 culation, a similar ^one may be placed representing the pul- 

 monary circulation : as in the case of the first-mentioned 

 system, the two extremities of this double cone will each 

 communicate with a muscular reservoir ; the right ventricle 

 on the one hand, and the left auricle on the other. By giv- 

 ing these two systems of cones a curved form, so as to bring 

 their different summits to the same central point, as is the 

 case with the heart in the living body, a graphic description 

 of the whole circulatory system may be given, under the 

 figure of two incomplete circles joined at their free extremi- 

 ties, thus forming a sort of figure of 8 (Fig. 42). 



This figure shows plainly that the four muscular reservoirs 

 which form the heart are so arranged that the pulmonary 

 double cone is in communication with the double cone of 

 the general circulation. For this purpose the left auricle, 

 communicating with the system of the pulmonary veins, 

 opens into the left ventricle at the beginning of the system 

 of the general circulation ; this is the left heart. On the 

 other hand, the right auricle, communicating with the gen- 

 eral venous system, opens into the right ventricle at the 

 point of departure of the pulmonary arterial cone ; this is 

 the right heart. 



Knowing the mechanism of the heart, we can, by means 

 of this simple sketch or diagram of the peripheral organs, 

 account for the circulation, and determine the two essential 

 conditions of the blood when in motion; these are its pres- 

 sure and its velocity in the different parts of the circulatory 

 apparatus. 



Pressure. At each contraction the ventricle pours from 



* V, Ventricle. O, Auricle, a, Arterial cone, v, Venous cone, c, c, Capil- 

 laries. 



