THE CIRCULATION OF THE BLOOD. 301 



tional area of the aorta, the stream-bed of the former never becomes 

 as narrow as that of the latter. These facts will become apparent 

 if the vascular apparatus is conceived of as a system of symmetri- 

 cally branching vessels and all vessels of the same diameter sym- 

 metrically disposed one to the other (Fig. 138). The gradual increase 

 in the width of the stream-bed which results from this repeated 

 branching, as well as its relative width in the arteries, capillaries, 

 and veins, is graphically shown in Fig. 139. 



The Movement of the Blood. The immediate cause of the 

 movement of the blood from the beginning of the aorta through the 

 arteries, the capillaries, and the veins to the right side of the heart, is 



FIG. 139. DIAGRAM INTENDED TO GIVE AN IDEA OF THE AGGREGATE SECTIONAL 

 AREA or THE DIFFERENT PARTS OF THE VASCULAR SYSTEM. A. Aorta. C. 

 Capillaries. V. Veins. The transverse measurement of the shaded part may be 

 taken as the width of the various kinds of vessels, supposing them fused together. 

 (Yeo.) 



a difference of pressure between these two points. The fact that the 

 blood flows from the aorta to the venae cavae indicates that there is a 

 higher pressure in the former than in the latter. The same holds true 

 for the pulmonary artery and veins. So long as this is the case, the 

 blood must flow from the point of high to the point of low pressure. 

 The Blood-pressure. To this pressure the term blood-pressure 

 is given, and may be defined as the pressure exerted radially or 

 laterally by the moving blood-stream against the sides of the vessels. 

 That there is such a pressure within the arteries, capillaries, and veins, 

 different in amount in each of these three divisions of the vascular 

 apparatus, is evident from the results which follow division of an artery 



