According to Dr. Thomson the contraction of the larger arteries is in general not percepti ble before from three to ten minutes after the application of the stimulus. When galvanism is used, the shocks need not be strong, but must be frequently repeated in order to induce contraction.
Many have remarked the gradual ,or sudden contraction of the trunks of arteries which have been laid bare in Man as well as in the lower animals. When exposed, an artery is some times equally contracted for some length along its tube ; at other times its surface assumes a waved appearance from the occurrence of irre gular contractions or alternate contractions and dilatations, and not unfrequently the coat of the artery is much constricted at one point only, as if a tight cord had been passed round it. Appearances of this kind, which seem to indicate very distinctly the possession of the property of irritability by the arteries, are well known to many surgeons ; they were noted by Drs. Jones and Tbomson, in the experiments upon which Dr. Jones's work on Ilemorrhage was founded; and also by Dr. Parry, who nevertheless refuses to consider them as irri table contractions. At p. 74 of his work on the Powers of the Arteries, Dr. Parry, referring to Experiment 13th, says, " thus a very narrow ring of the carotid became, while it was under examination, contracted as if a small ligature had been half tightened around it." So also in Experiment 24th, he relates that a part of the carotid artery of a ewe was diminished by a third of its original diameter under exposure, after having been half an hour denuded, while the neighbouring parts had be come rather dilated, and that while he Nvas pro ceeding to measure one of these dilated por tions, he " saw it shrink to nearly the same size as the constricted part." It appears to us manifest, that, whether these irregular diminu tions of the diameter of the artery, obviously occasioned by a shortening of its fibres, are at tributed to the exposure of the artery to the air, or the violence done during the dissection of it by the scalpel, they must equally be regarded as the consequence of stimulation of one kind or other, and are therefore of the nature of mus cular contractions.
Hoffmann first noticed the contractions of the arteries from the application of acrid che mical stimuli to their coats; and it appears from numerous subsequent experiments, that contractions are more readily induced in this than in any other way. Were there no other
proofs of the contractility of the arteries than those derived from the effect of chemical agents, we should not feel inclined to place much reliance on them, on account of the pos sibility of there having been induced a perma nent alteration of the texture from chemical action ; but the results of such experiments form an important confirmation of those which are performed with mechanical and galvanic irritation. We cannot, however, acquiesce in the opinion of Wedemeyer* and others who compare the distinct and well-marked contrac tions of particular parts of the arterial tubes, such as those above alluded to, to the general constriction of other textures, and more parti cularly to the shrinking of the skin which occurs from the influence of cold, passions of the mind, &c.
From these considerations we are induced to adopt the opinion that the contractions which under certain circumstances occur in the ar teries resemble muscular contractions more nearly than any other vital phenomenon. The positive evidence of direct experiment obviously proves that the contractions in general follow the application of some stimulus to the artery ; but these contractions difler from that of mus cular parts chiefly in the length of time which elapses after the application of the stimulus before the change of size begins, in the slol,v nessl,vith which the contraction is succeeded by relaxation, and in the want of obvious cor respondence between the force of the stimulus and the extent of contractions which follow it.
Besides the rnore marked contractions of parts of their tubes, the arteries are subject in various circumstances to undergo a slow and gradual diminution of their diameter through out their whole length, which is considered by many physiologists to indicate the possession by them of a property of the nature of contrac tility different frorn irritability in its pheno mena and the causes which call it into action. A power of a similar kind, to which the name of Tonicity is applied, is believed to reside in the voluntary muscles.* The experiments and observations generally stated in proof of the tonic power of arteries are the following :— 1. When a ligature is placed upon an artery of a living animal, the part of the artery beyond the ligature becomes gradually smaller, and is emptied to a certain degree, if not completely, of the blood it contained.