MOVEMENT OF THE BLOOD IN THE ARTERIES. 485 



some days (in which nothing but the elasticity remains), be distended with an 

 equal force, the former becomes much more contracted than the latter, after the 

 distending force is removed. 



514. Several experiments also indicate the existence of that power of slow 

 contraction in the arteries, which has been distinguished by the appellation 

 Tonicity ( 331). Thus, when a ligature is placed upon an artery in 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. 

 Again, when part of an artery in a living animal is isolated by means of two 

 ligatures, and is punctured, the blood issues from the orifice, and the inclosed 

 portion of the artery is almost completely emptied of its contents. Further, 

 every Surgeon knows, that the contraction of divided arteries is an efficient means 

 of the arrest of hemorrhage from them, especially when they are of small caliber ; 

 so that, in the case of the temporal artery for example, the complete division of 

 the tube is often the readiest means of checking the flow of blood from it, when 

 it has been once wounded. This contraction is much greater than could be 

 accounted for by the simple elasticity of the tissue ; and is more decided in small, 

 than in large vessels. The empty condition of the arteries, generally found 

 within a short time after death, seems to be in part due to the same cause ; 

 since their caliber is usually much diminished, and is sometimes completely 

 obliterated. A remarkable example of the same slow contraction is that which 

 takes place in the end of the upper portion of an arterial trunk, when the 

 passage of blood through it is interrupted by a ligature; for the current of blood 

 then passes off by the nearest large lateral branch; and the tube of the artery 

 shrivels, and soon becomes impervious, from the point at which the ligature is 

 applied, back to the origin of that branch. This last fact is important, as proving 

 how little influence the vis a tergo possesses over the caliber of arterial tubes ; 

 since, without any interruption to the pressure of blood occasioned by it, the 

 tube becomes impervious. It is to the moderate action of the tonicity of arteries, 

 that their contraction upon the stream of blood passing through them (which 

 serves to keep the tubes always full) is due. If the tonicity be excessive, the 

 pulse is hard and wiry ; but if it be deficient, the pulse is very compressible, 

 though bounding, and the flow of blood through the arteries is retarded. Dr. 

 C. J. B. Williams has performed some ingenious experiments ( 532), which 

 prove that the force required to propel fluid through a tube whose sides are 

 yielding, is very much greater than that which will carry it through a tube of 

 even smaller size, with rigid parietes; consequently, a loss of tonicity in the 

 bloodvessels retards the flow of blood through them ; whilst an increase hastens 

 it. We have seen that between the Irritability of arteries and their Tonicity, 

 there is much less difference than exists in most other muscular structures; 

 since the former is so long in manifesting itself, that it almost approaches to the 

 character of the latter. But in the Arteries, as in other muscles, the tonic 

 contraction may be most efficiently induced by cold. Thus Hunter observed 

 that the exposure of an artery of a warm-blooded animal to the air for some time, 

 would occasion its gradual contraction to such an extent as to effect the ob- 

 literation of its canal. This statement has been verified by many subsequent 

 experimenters; and it has also been confirmed by the observations of Schwann 

 upon the small arteries of the mesentery of frogs, which he caused to contract 

 slowly by the application of cold water, and then saw dilate again ; as much as 

 half an hour being required, however, before they recovered their original size. 

 On the other hand, the application of moderate warmth causes a relaxation of 

 this tonic contraction. And thus Cold and Heat are two of our most valuable 

 remedial agents, when the Tonicity of the Vascular system is deficient or in 

 excess. 



515. We have now to inquire more closely into the influence exerted by the 



