CIRCULATION. 



667 



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. 



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 

 Hunter}- and others. 



4. It has been shewn by PoiseuilleJ that 

 when a portion of an artery from an animal 

 recently dead, and one from an animal 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 larger 

 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 



' Parry, loc. citat. 



t On the Blood and on Inflammation. 

 $ Magendie's Journ. vol. via. 



beyond the place at which they have been tied 

 in the living body, and the contraction of ar- 

 teries which 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. Influence of the vital powers of the arte- 

 ries on the circulation. 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. 



In the first place, Haller, Spallanzani, 

 Thomson, and many others have observed in 

 the transparent parts of animals that the im- 

 pulse of the heart is transmitted to the very 

 ends of the small arteries, which may be less 

 than gigth part of an inch in diameter, and that 

 in some states of the circulation the impulse of 

 the heart is continued on through the capillary 

 vessels and into the commencements of the 

 veins. The fact that this generally occurs when 

 the action of the heart is weakened, and when 

 the vessels are consequently not sufficiently 

 distended by its impulse to react by then- 

 elasticity and convert the remitting into a 

 uniform force, is a distinct proof that in the 

 natural state of the circulation a greater pro- 



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