378 OF THE CIRCULATION OF BLOOD. 



Tonicity; but which does not seem any thing else than a particular mani- 

 festation of the general property of vital contractility, and is certainly of a 

 nature quite distinct from ordinary elasticity. 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 enclosed portion of the artery is almost completely emptied of 

 its contents. The exposure of arteries to the air was found by Hunter to occa- 

 sion their contraction to such an extent, that obliteration of their tube was the 

 result; and this statement has been subsequently confirmed. 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 

 calibre ; 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 ; anti 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 calibre 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 cur- 

 rent 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 

 calibre 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. Williams has performed some 

 ingenious experiments, 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 an even smaller tube, with rigid parietes ; conse- 

 quently, a loss of tonicity in the blood-vessels retards the flow of blood through 

 them; whilst an increase hastens it. The Tonicity of the arteries differs 

 from their ordinary Contractility, in being augmented by cold, and diminished 

 by warmth. Hence cold and heat are two most valuable remedial agents, 

 when this property is deficient or in excess. 



504. It is still to be inquired, in what manner the Contractility of the Arte- 

 ries is to be regarded as influencing the flow of Blood through them. It is at 

 once evident, that any general contraction of the arterial tubes would have 

 rather the effect of opposing than of assisting the flow ; but if the fibrous coat 

 of the Arteries is in some degree disposed to the alternate contraction and 

 relaxation, which are so remarkable in the Heart, they may exert a force which 

 shall be supplementary to that of the Heart's impulse, relaxing to receive 

 the blood from it, and contracting upon their contents, with a power superior 

 to that by which they were distended. It is difficult to say whether or not 

 this be the case ; though there would certainly appear some evidence in favour 

 of the supposition. The loss of the Heart's power over the currents of blood, 

 in proportion to their degree of subdivision, occasioned by the increased fric- 



