2. When a part of an artery in a living ani mal is isolated from other organs by means of two ligatures and punctured, the blood issues from the orifice, and the enclosed portion of artery is nearly completely emptied of its con tents.
3. The empty condition of the arteries gene rally found after death is believed to be, in part at least, produced by a slow contraction of the whole of the large arterial tubes; for it has been observed, that some hours after death the arteries are much diminished in size, and this occasionally to such an extent as to be rendered impervious, as was observed in the umbilical arteries of the navel string by John Huntert and others.
4. It has been shewn by Poiseuillet that when a portion of an artery from an animal recently dead, and one from an aniinal that has been dead for some days, are distended with an equal force, the portion of the artery from the recently dead animal becomes more contracted after the distending force is removed than the other one.
5. In the last place, when a large artery is divided, the cut extremities frequently become so completely constricted as wholly to prevent the issue of blood, and this kind of contrac tion is well known to occur in a greater degree after laceration of an artery than after division by the knife : hence the less danger to be ap prehended from hemorrhage in lacerated than in incised wounds ; and thence the possibility of producing the closure of one of the lamer arteries by the mere compression or torsion of its cut end.
In the three last-mentioned proofs of to nicity the contraction of the artery followed the application of some kind of irritation ; for the exposed artery was dissected out by the scalpel, and ligatures were tightened round it, the coats of the artery were stimulated by dis tension in Poiseuille's experiment, and in the twisting or torsion as well as in the division of an artery by laceration or cutting there is always an irritation applied to the contracting part. The tonicity or tonic contractility therefore was in some of these instances first called into ope ration and in others increased by irritation, and ought not therefore to be distinguished from irritability as regards its cause, but only as relates to its phenomena.
The evacuation of the blood from arteries beyond the place at which they have been tied in the livinu body, and the contraction of ar teries whic takes place in the dead body, as well as the rigidity of muscles soon after death, or their retraction when divided in the living body, all seem to indicate a tendency in ir ritable parts to undergo a slow and continued contraction during the persistance of their vital powers. This tendency to contraction seems
to differ from the shortening and subsequent relaxation which are the more or less imme diate effects of stimulation in truly irritable parts, and it seems to be more dependent upon the removal of the forces by which the parts in which it occurs are kept in a state of distension than upon any other cause.
It is obviously in consequence of this ten dency to. contract when not distended by a force from within that the arteries are always nearly accommodated to the quantity of blood contained in them. But while we are con strained to admit the existence of the peculiar slow contractile power in arteries appropri ately denominated tonicity, we would caution the accurate physiologist against considering as the effect of this property rather than of irri tability any of those contractions of the arterial tubes which are induced or increased by me chanical, galvanic, or other stimuli.
e. Iyluence of the vital powers of the arte ries on the circu.lation.—Let us now inquire in what manner the flow of the blood is influ enced by the irritability and tonicity of the arteries.
Some of those who have regarded the arteries as contributing by their active powers to propel the blood have conceived it sufficient for them to prove that there is a necessity for some additional force in the circulation besides that of the heart, in consequence of the total ex penditure of the heart's force from the windings of the small vessels, the friction of the blood against the side, and other resistances to be overcome in the capillary system. This expen diture of the heart's power admitted by many on insufficient grounds has been very generally overrated. Although the causes just men tioned may diminish to a certain extent the propelling power of the heart, there are various very simple experiments which shew that the heart's action is propagated with a propelling effect through the whole vascular system, so as to act in the extreme vessels and veins.