1901.] Properties of t lie Arterial and Venous Walls. 149 



It is to be noted that arteries show a striking general resemblance in 

 (a) the expansion of the arterial tube when distended by internal 

 pressure in each case, and (b) the elasticity of strips from the arterial 

 wall when stretched by weights. This applies to both contracted and 

 relaxed arteries. 



Changes in the Length of Arteries during Variations in Internal Pressure. 



We cannot at once predict the exact behaviour of an artery as 

 regards changes in its length when subjected to variations in internal 

 pressure from a study of the behaviour of longitudinally-cut strips of 

 the arterial wall stretched by weights. For the relations of the tissues 

 of the arterial wall are very different when they are in the form of a 

 strip to what they are while forming a tube which is distended by 

 internal pressure. 



We have already seen that in an artery containing air at atmospheric 

 pressure, the occurrence of contraction is accompanied by a lengthening 

 of the. tube, while relaxation is attended by a considerable amount of 

 .shortening. 



An artery subjected to a considerable rise of pressure in its interior 

 undergoes expansion in both its transverse and longitudinal diameters — 

 in contrast to an excised artery at atmospheric pressure, which shortens 

 when it widens from relaxation. 



In the artery distended by internal pressure it is evident that the 

 tendency to shortening which is associated with a marked increase in 

 the transverse diameter of the tube is overborne by the stretching 

 •effect of the internal pressure. 



The method I have employed for studying the changes in the length 

 of an artery is as follows : — 



A segment of artery was connected with a system of pressure 

 bottles as already described. The portion of artery so prepared was 

 placed in the vertical position, and the brass cannula tied into its upper 

 end was rigidly fixed. The part of the wooden plug projecting from 

 the lower end of the artery was then grasped by a small clamp 

 attached to the lever of a Helmholtz myograph ; the lever was directed 

 at right angles to the smoked surface so that the writing point traced 

 a vertical line. The pressure was usually raised 50 mm. Hg. at a time, 

 beginning at zero, and it was usually kept at each level for 1 minute. 

 The elongation is magnified 6 times by the lever. The pressure was 

 raised to 300—400 mm. Hg. 



Contracted Artery. — The total elongation in a strongly contracted 

 artery is relatively small in amount ; there is progressively increasing 

 augmentation as the pressure is raised. 



When the pressure is lowered the artery speedily returns to its 

 original length — commonly indeed it becomes shorter than before. 



