THE CIRCULATION OF THE BLOOD 339 



capillary sectional area of i to 600. In passing from the capillary into the 

 venous system the sectional area of individual veins increases, though 

 the total sectional area decreases and in direct proportion to their distance 

 from the capillaries. 



The stream-bed in the aorta is relatively narrow, but widens gradually as 

 it approaches the capillaries, where it attains its maximal width; it again 

 narrows gradually as it passes into the veins, until in the venae cavse it be- 

 comes almost as narrow as in the aorta. As the combined sectional areas of 

 the venae cavae are greater than the sectional area of the aorta, the stream- 

 bed of the former never becomes as narrow as that of the latter. 



The gradual increase in the width of the stream-bed from the beginning 

 of the aorta to the middle of the capillary system, and the gradual decrease 

 in the width of the stream-bed from the middle of the capillary systenf to the 



FIG. 154. DIAGRAM DESIGNED TO GIVE AN IDEA OF THE AGGREGATE SECTIONAL AREA of 

 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.) 



terminations of the venae cavae, which result from the repeated branching 

 and subsequent uniting, as well as its relative width in the arteries, capil- 

 laries, and veins, are shown graphically in Fig. 154. 



The Intra-ventricular Pressure. When the heart contracts and the 

 intra-ventricular pressure rises above the pressure in the aorta, the aortic 

 valves are suddenly forced open and the blood is driven into and through the 

 arteries, capillaries, and veins to the right side of the heart and in accordance 

 with foregoing considerations with a definite velocity and against a certain 

 resistance, which in turn gives rise to a side pressure. 



The Velocity. The velocity of the blood in the systemic vascular ap- 

 paratus will gradually decrease from the aorta to the middle of the capillary 

 system in a ratio inversely proportional to the total area of any given cross- 

 section of the stream-bed, until in the capillaries it will attain its minimal 

 value, which is especially small because the resistance to the flow of blood in 

 the capillaries increases inversely as the square of their diameters, while in 



