146 



Prof. J. A. Mac William. On the [Oct. 24, 



a passive condition, the resistance must evidently be entirely muscular, 

 since the purely elastic property of the arterial wall tends to make it 

 take up the position seen in a passive artery. Increased opposition to 

 further distension will be offered by the elasticity of the arterial wall 

 resisting distension beyond its position in the passive (relaxed) artery. 



In arteries with relatively slight contraction present, the maximum 

 distensibility is found to occur at a relatively low level — which may be 

 about the normal blood pressure or much below the normal blood 

 pressure of the animal. (Fig. 28.) When portions of the same 

 artery are tested on successive days when post-mortem contraction is 

 diminishing and passing off, it is found that the maximum distensi- 

 bility of the arterial tube is manifested at lower and lower levels until 

 at length, when contraction is quite abolished, the greatest expansion 

 occurs with the first rise of pressure above zero. Thus the artery 

 which showed increasing expansion up to 420 mm. in fig. 27, showed 

 some days later (when its contraction was weak) maximum expansion 

 t 40 — 60 mm., and later still immediately above 0. 



Here it is necessary to remark upon the extraordinary persistence of 

 a residuum of contraction in the arterial wall — in some cases even after 

 signs of putrefaction are evident. Putrefaction begins in adherent 

 blood, periarterial tissue, &c, much earlier than in the tunica media, 

 and the survival of the latter is favoured by cleansing the artery after 

 excision from blood and serum, and by removing connective tissue, &c, 

 from around the vessel ; still more by keeping the artery so prepared 

 immersed in olive oil. Roy remarked that the elasticity curve of any 

 given artery remains the same until putrefaction is far advanced — 

 which is intelligible in view of what has just been stated. 



In warm weather pieces of artery kept in a corked bottle, moistened 

 with normal saline, may have a putrefactive smell in 2 days, while still 

 contracted and showing a striking increase of contraction on stimulation 

 by cutting, &c. In such circumstances, when arteries are kept for 

 several days in defibrinated blood, it is important that the blood 

 should be replaced by fresh blood from time to time. 



Effects of Repeated Distension. — When a strongly-contracted artery 

 has once been distended by high pressures, it is more easily stretched 

 by a repetition of the rise of pressure, while the general character of the 

 expansion remains the same — unless the first distension was very great. 

 This is seen on comparing figs. 29 and 30. 



20 40 60 80 100 IB 140 160 180 200 220 240 260 280 300 

 FlG. 29. — Carotid (ox), strongly contracted; 48 hours p.m. Length 14 mm. 



