Page:Proceedings of the Royal Society of London Vol 69.djvu/206

192 Contracted arteries show increasing increments of volume per unit rise of pressure up to a certain maximum, then diminishing incre- ments. In a strongly contracted artery an enormous pressure (several hundred mm. Hg), vastly higher than the normal blood-pressure of the animal, is required in order to obtain the maximal increase of volume ; when contraction is weaker, the level of pressure at which extensibility is greatest is much lower (?.{/., 100 120 mm.), and when contraction is feeble the maximal increase in volume may occur at 40 60 mm.; it comes immediately above zero when contraction has been com- pletely abolished, as a result of keeping the artery for a number of days in blood, heating to 40 Q for a number of hours, or to 50 55 for a few minutes, free/ing for some hours, treatment with ammonia vapour, sulphocyanide, &c.

Both contracted and relaxed arteries show increased extensibility when they have already been distended by a previous rise of pressure.

The main features in the behaviour of the artery are shown by diagrams, and numerical details are given.

Elongation of an Artery -ir/icu Distended ly a llixe of Internal Pressure. Relaxed arteries elongate vastly more than contracted ones when the internal pressure is raised, e.g., from to 300 mm. Hg.

The character of the tracing is very different in the two cases. In the relaxed artery the maximum elongation per unit rise of pressure comes early usually with the first rise, e.g., from to 50 mm. Hg.

Li the contracted artery the elongation with the first rise is small ; it gradually increases with subsequent rises up to a maximum, which in the case of a strongly contracted artery is only reached with pres- sures much higher than those stated above.

Tracings are given to illustrate the changes in relaxed and contracted arteries.

Pulsatile Expansion of Artmi'*. With a succession of sudden, brief elevations of pressure within an artery the pulsatile expansion of the tube is vastly greater in a relaxed artery than in a contracted one. In the former the maximum expansion is got when the brief rise of pres- sure starts from zero ; the expansion becomes smaller at higher pres- sures. The observations were made with the aid of a plethysmograph.

Among the most important points described in the paper are

(1.) The remarkable persistence of vitality in the arteries of a healthy animal for days after the death of the animal.

(2.) The great importance of the presence or absence of post-morteni, contraction in influencing not only the measurements of the artery but its behaviour in various respects

(a.) Eesponse to changes in temperature.

(b.) Extensibility of strips of the arterial wall when weighted.

(c.) Relation of cubic capacity to changes in internal pressure.