A TEXTBOOK OF PHYSIOLOGY 



manometers are affected to almost a like extent. On screwing up 

 the clip (increasing the resistance in the arterioles), the flow becomes 

 less and less intermittent as the resistance increases, and eventually 

 becomes continuous. The pressure rises in the arteries, the systolic 

 and diastolic variations in pressure becoming greatly reduced. Thfr 

 variations in pressure disappear from the veins. This represents what 

 takes place in the vascular system. 



The E:asticity of Arteries. An artery 

 possesses great lability, that is, it is easily 

 distended; it has great elasticity, recovering 

 its shape after the distending force is removed. 

 The curves of extensibility and elasticity may 

 be worked out on excised arteries either by 

 weighing and unweighing a strip of vessel wall, or 

 by recording the expansion of a short length of 

 artery when submitted to a pressure of water. 

 Such curves vary according as the artery is 

 relaxed or contracted. The muscle in its well 

 alters the lability by its contraction or relaxa- 

 tion (Figs. 89, 80). 

 The breaking 

 strain of a healthy 

 artery is very great. 

 In some experi- 

 ments the caret "d 

 of a goat success- 

 fully withstood a 

 pressure of 2,250 

 mm. Hg that b, 



about fourteen FlG 8 9.-ELONGATioN OF CON- 

 TRACTED ARTERY, WITH RISE 

 OF INTERNAL PRESSURE (0 TO 

 300 MM. Ho). LENGTH 

 = 16 MM. (MacWilliam.) 



FIG. 90. ELONGATION 

 OF RELAXED ARTERY, 

 WITH RISE OF INTER- 

 N AL . PRESSURE. 

 LENGTH 21 MM. 

 (MacWilliam.) 



times the normal 



blood -pressure. It 



takes an internal 



pressure of 3,000 



to 8,500 mm. Hg 



to rupture the carotid artery of a dog. In the case of the human carotid 



the smallest rupturing pressure was found to be 1,290 mm. Hg. The- 



larger arteries rupture more easily than the smaller, and thus the aorta 



breaks asunder at a lower tension than the radial. 



In the vascular system, an area of vessels of capillary size is placed 

 between the large arteries and veins. This area opposes a great 

 resistance to flow. The effect of the peripheral resistance, as it is 

 called, is to raise the pressure on the arterial side and lower it on the 

 venous. The resistance to flow is situated chiefly, not in the capil- 

 laries, but in the small arteries, where the velocity is high. " Skin 

 friction " that is, the friction of the moving concentric layers of 

 blood against one another and against the layer which wets the wall 

 of these bloodvessels is proportional to the surface area and to the 

 'viscosity of the blood; is nearly proportional to the square of th& 



