

THE FLOW OF BLOOD IN THE ARTERIES 201 



weight becomes steadily less the greater the total load. The coefficient of 

 elasticity of the arterial wall therefore increases as the stretching force in- 

 creases,, so that the curve of elongation resembles an hyperbola (Wertheim). 



As to the rate of cubic distention of the arteries caused by increasing 

 internal pressures, which is a matter of much greater importance for under- 

 standing the circulation, the results of investigators differ widely. While 

 Marey and others have found that the cubic enlargement runs the same 

 course as the elongation of the strip cut from the wall, Roy asserts that this 

 is the case only in arteries taken from animals and men who have suffered 

 from some wasting disease, and finds that with perfectly sound arteries the 

 increase in volume with equal increments of internal pressure rises at first 

 up to a certain limit (variously given for the dog from 32 to 120 mm. Hg.), 

 but with still higher internal pressure the distensibility falls (Fig. 75). 



At any rate, it is certain that the cubic enlargement of the arteries beyond 

 that given by a certain internal pressure, which as a rule is not higher than 

 a medium, normal blood pressure, becomes less and less with equal incre- 

 ments of pressure. From which it follows that when the arterial blood pressure 





FIG. 75. The cubic enlargement of the aorta of a rabbit, under a uniformly increasing internal 



pressure, after Roy. 



is already high, any steady increase in the quantity of blood discharge from 

 the heart will cause a more than proportionate rise in the blood pressure and 

 will correspondingly increase the work of the heart. 



From results thus far at hand it appears that with any given increase in 

 pressure the arteries are distended relatively more the farther they are situ- 

 ated from the heart. 



In the body arteries as well as veins are always on the stretch longitudinally, 

 whatever the internal pressure ; for the moment they are cut out of the body they 

 at once retract, becoming shorter and thicker. It is always possible to find a 

 stretching force which will give an exsected vessel the dimensions it would have 

 if it were completely fixed in situ, when empty. This stretching force expressed 

 as pressure in millimeters of Hg is looked upon by R. Fuchs as the measure of 

 the longitudinal tension when there is no internal pressure; it amounts to from 

 50 to 90 mm. Hg. in the thoracic aorta of the dog, and is therefore below the 

 mean blood pressure. 



The elasticity of the arteries. is perfect i. e., if they be subjected to a high 

 internal pressure, and the excessive pressure be then removed, they immediately 

 return to the original volume. 



Furthermore, the resistance of the arteries to high pressure is exceedingly 

 great. The internal pressure necessary to rupture the carotid of the dog is four 



