136 



Prof. J. A. MacWilliam. On the 



[Oct. 24, 



Fig. 17. — Carotid (ox), relaxed. Longitudinal strip. Loaded a second time after 

 an interval of 10 minutes. 



which there comes a progressive diminution in the amount of elonga- 

 tion produced by each successive addition of weight. Hence a line 

 joining the lower ends of the vertical lines representing extension is a 

 curved line at first convex towards the axis and then concave. (Fig. 18.) 



In the early part of the process of stretching a contracted artery the 

 resistance is solely muscular ; later, when the muscular resistance has 

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

 normal length in a passive or relaxed artery, further stretching 

 brings into play the resistance of the elastic and other elements in the 

 arterial walls, and, as we have seen, a strip from such an artery resists 

 elongation more and more, with successive increments of stretching 

 force. 



The behaviour of a transverse strip from a contracted artery is so 

 constant and characteristic that it is always easy to tell from the 

 tracing obtained whether the artery was contracted or relaxed. I am 



