558 OF THE CIRCULATION OF BLOOD. 



vigorously on the direct application of stimuli to itself; and Valentin and 

 others have recently succeeded in producing evident contractions in the Aorta, 

 by irritation of the Sympathetic nerve, and of certain roots of the Spinal 

 nerves. Further, although many experiments have failed in producing con- 

 tractions of this tissue, by stimuli directly applied to itself, yet others have 

 distinctly witnessed them; and, in any question of this kind, the positive 

 evidence must be held to outweigh the negative. Thus Verschuir states, that 

 he has seen arteries contract, when stimulated by the mineral acids, by elec- 

 tricity, and by the application of the point of a scalpel. Dr. Thomson also 

 saw them contract, on the application of ammonia, and when punctured with 

 the point of a fine needle, in the living body. It has been ascertained by the 

 direct and careful experiments of Poisseuille, that, when the artery is dilated 

 by the blood injected into it from the heart, it reacts with a force superior to 

 the impressing impulse ; and he has also shown that, if a portion of an artery 

 from an animal recently dead (in which the vital contractility seems to be 

 preserved), and one from an animal that has been dead some days (in which 

 nothing but the elasticity remains), be distended with an equal force, the for- 

 mer becomes much more contracted than the latter, after the distending force 

 is removed. 



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

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

 Tonicity ; but which does not seem anything else than a particular manifesta- 

 tion 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 inclosed 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 ; 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 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 liga- 

 ture ; for the current of blood then passes oft* 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 d 

 tergo possesses over the calibre of arterial tubes ; since, without any interrup- 

 tion 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 he deficient, the pulse is very compressible, though bounding, 

 and the flow of blood through the arteries is retarded. Dr. Williams has 



