AND BLOOD PRESSURE 



119 



has been so far overcome that the strip is stretched to what would 



be the normal length of a passive or relaxed artery, further stretch- 



ing brings into play the resistance of the elastic elements and the 



extension curve then becomes like that of a 



relaxed artery. On repeating the stretching 



the curve is like that of a relaxed artery 



throughout. As the difference between con- 



tracted and relaxed arteries depends on the 



muscular element, it is much more evident 



when transverse rather than longitudinal 



strips are used. The increments of cubic 



capacity of relaxed arteries, just as of veins, 



when subjected to equal increments of in- 



ternal pressure, is greatest at first, and suc- 



cessively diminishes as the pressure is raised. 



On repeating the distension a second time 



the relaxed artery is found to yield much 



more. 



Contracted arteries, on the other hand, 

 yield relatively little at first, and go on 

 yielding with each increase up to very high 

 pressure, but only very gradually. The dis- 

 tension of an artery whose muscular coat is 



thin augments up to a Certain point, and 



then becomes less as the muscular resistance 

 is wholly overcome, and the elastic elements come into play. 

 Second distensions of contracted arteries cause much greater ex- 

 pansions. Mac William says that Roy's conclusion which has been 

 generally given in the text-books that maximum distensibility 



internal pressure. Length 



Flo. 11. Carotid (ox), strongly contracted (48 hours p.m.). Increase in 

 capacity on rise of pressure. 



of arteries corresponds to normal blood pressure was based on 

 experiments with arteries more or less in a post-mortem state 

 of contraction. Fully contracted muscular arteries and relaxed 



