1901.] Properties of the Arterial and Venous Walls. 145 



pressures (e.g., 420 mm.), the muscular resistance being more and more 

 overborne by the successive increments of internal pressure. The 

 expansion caused by each rise of pressure is very gradual — in marked 

 contrast to what is seen in a fully-relaxed artery ; the oil in the 

 horizontal graduated tube keeps progressing along the tube during the 

 period of raised pressure. The increase in capacity with unit increase 

 of internal pressure may go on increasing up to pressures vastly higher 

 than the normal blood-pressure of the animal. (Fig. 27.) Beyond a 



f-r-r 



20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 



Fig-. 27. — Carotid (ox), strongly contracted (48 hours p.m.). 



certain pressure, varying in different cases, diminished expansion 

 occurs, though in a strongly-contracted artery an enormous pressure 

 is required to reach this point — at least when the pressure is raised at 

 the rate adopted in the experiments now being described. 



When an artery is taken which presents a slighter muscular resistance 

 to distension — on account of its muscular coat being relatively thin, 

 or its contraction being less strongly developed, the increase of 

 capacity with unit increase of internal pressure becomes progessively 

 augmented up to a certain point — which is more easily reached — and 

 then declines. (Fig. 28.) The turning-point — indicating the maximum 



20 40 60 80 100 120 140 160 180 200 220 240 

 Fig. 28. — Carotid (ox), weak contraction (5 days Length 12 mm. 



distensibility of the tube — may come at various levels of pressure 

 according to circumstances ; it may or may not correspond to the 

 normal height of the blood pressure in the animal from which the 

 artery was taken. The change seems to take place some time after the 

 muscular resistance is so far overcome, that the artery has become 

 distended to its normal size in the passive condition; the elastic 

 resistance of the arterial wall (now becoming stretched beyond its size 

 in the passive state) comes into play to resist further distension. In 

 the process of distension, when the arterial wall is being moved from 

 the position which it occupies in a contracted artery to its position in 



