1901.] Properties of the Arterial and Venous Walls. 121 



to note in regard to the relaxation beginning at 50°, that the arterial 

 strip is subjected to only a very slight amount of tension during the 

 experiment — the spiral springs employed being long and weak ; the 

 tension may be regarded as being nearly constant throughout.. 

 Instead of springs, weights of small amount (2 grammes) were some- 

 times used. In the case of the unopened artery where no tension is 

 employed, the only force (outside the muscle fibres) to cause an 

 expansion movement is the elasticity of the arterial wall, which tends 

 to make it assume the position which it occupies in the passive artery. 



The extent of the relaxation occurring at 50 — 55° varies with the 

 amount of contraction present at the time the temperature reaches 

 that level. 



When relaxation of contracted artery has begun at about 50° to he? 

 completed, perhaps by the time the gradually rising temperature has 

 reached 58 — 60°, then a further rise to 60° or 60 — 65° leads to the 

 appearance of the contraction already described in the case of a 

 relaxed artery. (Figs. 6 and 7.) When the process of relaxation has 

 not been completed at the time the changes underlying the 60 — 65° 

 contraction begin, the latter is often much more slightly marked in 

 the tracing than usual, the tendency to shortening being probably 

 opposed and partially masked by the relaxation which is still going on. 

 (Fig. 11.) 



While relaxation at about 50° C. is the most constant and out- 

 standing feature in the behaviour of a contracted artery when heated, 

 there are other important features which vary according as we are 

 dealing with (a) the earlier stage of post-mortem contraction (a few 

 hours or a day or two after death), or (b) its later stage. 



(a) During the earlier stage — while the artery is still excitable — 

 heating commonly induces important changes long before the level 

 of 50° is reached — often indeed pretty soon after the rise of tempera- 

 ture has begun. The tracing given by a transverse strip may show 

 large curves indicating marked changes of length ; there is commonly 

 a curve before 40° is reached, and one after 40° before the extensive 

 relaxation beginning about 50°. Contraction often begins at about 

 25° to reach its maximum about 35° and then relax ; at 40° relaxation 

 is usually well marked. As the temperature rises, a further contrac- 

 tion (sometimes very extensive) takes place, especially between 45° 

 and 50°, and often particularly marked about 47°, to give place about 

 50° to relaxation. At 60 — 65° contraction again occurs — to relax 

 slowly and partially when the temperature falls. (Figs. 4 and 8.) 

 Thus when a transversely cut strip of artery from a recently killed 

 animal is heated up to or beyond 65° and then allowed to cool, there 

 are usually three phases of shortening and three phases of lengthening ; 

 the first shortening at 25 — 35°, the second at 45 — 50°, the third at 

 60 — 65°; the first lengthening at 35 — 45°, the second between 50° 



