142 



Prof. J. A. MacWilliam. On the 



[Oct. 24, 



desired pressure of air by means of a movable mercury reservoir 

 communicating with a pressure bottle, from which in turn a tube leads 

 to the arterial cannula ; this tube has a lateral connection with a 

 mercury manometer to show the amount of pressure acting upon the 

 internal surface of the artery. The artery thus prepared is inclosed in 

 a sort of plethysmograph consisting of a glass vessel filled with olive 

 oil, and provided with a long graduated tube of small bore projecting 

 horizontally at one side ; the size of the bore employed varied in 

 different experiments — commonly 1 mm. for contracted arteries, and 

 3 mm. in experiments on relaxed arteries (ox). Variations in the 

 volume of the artery were accurately indicated by the movement of oil 

 in the horizontal tube. In the figures the rises of pressure are taken 

 as abscissae and the expansions in volume (indicated by the number of 

 millimetres the oil moves in the graduated tube) as ordinates. The 

 length of the portion of artery used was measured between the ligatures 

 fastening the cannula and the wooden plug in situ at either end of the 

 artery. 



The pressure was usually raised by increments of 20 mm. Hg, and 

 kept steady at each level for 1 minute. 



As might be anticipated, great differences were observable in the 

 behaviour of relaxed and of contracted arteries. 



Relaxed Artery. — Completely relaxed arteries when subjected to equal 

 increments of internal pressure do not respond by equal increments of 

 cubic capacity ; the increase in volume is greatest at first and succes- 

 sively diminishes as the pressure is raised — the distensibility of the 

 vessel is greatest at the first rise above zero. This result is similar to 

 what has already been described as holding good in the case of veins ; 

 it is in entire opposition to what Roy found in healthy arteries examined 

 soon after death. The expansion of volume caused in the relaxed 

 artery by each successive elevation of pressure goes on pretty quickly 

 at first each time the pressure is raised, then proceeds much more 

 slowly during the latter portion of the period during which the 

 pressure is kept steady at any particular level. (Figs. 24 and 26.) 



When the pressure is raised a second time after an interval, the dis- 

 tensibility of the artery is found to be greatly increased. (Fig. 25.) 



I have tested the elasticity of arteries (chiefly of ox and sheep) in 

 which relaxation has been induced in the different ways already de- 

 scribed. No doubt these various modes of treatment induce changes in 

 the arterial wall very different in their nature in some respects. But they 

 have one feature in common, inasmuch as they all abolish contraction, 

 and concurrently they lead to a characteristic change in the behaviour 

 of the tube towards internal pressure, in virtue of which the artery 

 becomes most distensible at low pressures — immediately above zero 

 pressure, &c. 



While an idea of the behaviour of arteries relaxed by keeping and by 



