The specific gravity of the arteries exceeds that of distilled water in the proportion of 106 to 100. They are proportionally lighter and less dense than the veins ; while the veins, possess more power of resistance, and are not so easily ruptured as the arteries.
Physical properties.—Of the physical pro-. perties of the arteries the most remarkable are the firmness of their parietes, their power of resistance, and their elasticity. It is owing to the firmness, which principally resides in their middle tunic; that they preserve their circular form in the empty state.
Their power of resistance has been made the subject of experiment by Wintringham,-I- and, more recently, by Beclard,X from which the following results have been obtained.
Their power of resisting rupture is very great, and is generally in proportion to their thickness, being greater in the aorta than in the pulmonary artery. As the arteries diminish in size, their absolute resistance diminishes; however, as their relative thickness and softness increase, their extensibility and relative resistance undergo a proportionate augmentation. The resistance of all arteries of equal volume is not the same : for instance, that of the iliac artery is greater than that of the carotid. It is in the external tunic that the power of resistance in the longi tudinal direction resides ; the resistance in the circular direction is much greater, arid is owing to the middle and external tunics conjointly ; the internal tunic has very little power of re sistance in either direction. The middle and internal tunics are as remarkable for their fra ' gility as the external is for its toughness and great power of resistance ; hence it is, that when a ligature is tightened on an artery, the two former are divided, while the lager remains ' unbroken, as proved by the experiments of Dr. Jones."' The successful employment of torsion of the arteries as a means of suppressing haemorrhage is in like manner owing to the greater power of resistance possessed by the external tunic as compared with the other two. The process by which arteries are obliterated by torsion is thus explained by M. Amussa t,-t to whom belongs the merit of having been the first to propose and practise it. The divided extremity of an artery is seized between the blades of a forceps, and drawn out beyond the surface of the wound : the vessel is then taken hold of with a second pair of forceps a few lines higher, and held firmly while the operator commences to twist the forceps with which he holds the extremity of the vessel in the direction of its axis, making from five to nine or ten turns, according to the size of the vessel operated upon. On examin ing an artery which has undergone this process, it will be found that the middle and internal... tunics of the twisted portion have been broken in several places by the external tunic, which, remaining unbroken, is formed by the twisting process into,a sort of spiral ligature, so tightly applied round the inner tunics as to set at defiance every attempt to unravel it by twisting the vessel in the opposite direction.
The arteries are highly elastic ; they admit of considerable distension in the longitudinal di rection, and quickly contract to their original length on the cessation of the distending force. . In the transverse direction they yield less, and after distension resume their previous state with greater force. When a fluid is injected with
some force into the arteries in the dead body, they become distended and elongated ; and if, when they are in this state, the force with which the injection was propelled he removed, they will contract to their previous state, or nearly so, expelling a portion of the fluid which had been thrown into them. During life the arteries are in a state of elastic tension, so that, when divided, their cut extremities retract with in their sheath.
The arteries are endowed with the power of contracting in a gradual manner, which they exhibit under the following circumstances when the passage of the blood is stopped in the principal artery of a limb, the vessel gradually contracts, its cavity is reduced in size, and ultimately becomes obliterated by degenerating into a filamentous band of cellular tissue ; while the collateral branches, taking up its function of conveying blood to the distant parts, are proportionally enlarged, rendered more tortuous, and increased in length. In process of time the number of enlarged collateral branches diminishes, and one or more vessels of in creased size become as it were promoted to the station which the principal trunk had held in the circulation while in its normal condition. Several distinguished anatomists and physiolo gists have considered the property of elasticity of the arteries sufficient to account for all the phenomena of the circulation of the blood through these vessels. This opinion has been principally insisted on by Haller, Bichat,Nysten, and, at the present day, by Magendie; elasticity, however, can only account for contractions taking place in consequence of previous dis tension, and is equally evident after death as during life : but observation and experiments have shewn that, in the living body, the arteries possess an additional power of contraction, by which their calibre may be diminished in various degrees ; in some instances even almost to obliteration. And this power of contraction has been considered by many anatomists to indicate the existence of a property of irritability in the arteries, similar to, if not identical with muscularity. The existence of irritability in the arteries was denied by hailer in conse quence of his not having succeeded in render ing it evident by the application of chemical or mechanical stimuli. Bichat, Nysten, and Ma gendie, embraced a similar opinion, on the strength of the following facts :—mechanical or chemical stimuli, even the galvanic fluid, ap plied to the surfaces of the arteries, produce no motions ; when the fibres of the middle tunic are dissected off in successive layers in living animals, they are not observed to display that quivering motion visible among the fibres of muscles similarly treated. When cut longi tudinally, the inner surface of the arteries does not become everted like that of canals, such as the intestines, which have a decidedly muscular tunic : they do not contract when separated from the heart. The finger introduced into a living artery is not constricted ; stimuli applied to the nerves of particular arteries, or to the nervous system generally, do not produce con tractions; strong acids applied to arteries pro duce a corrugation or crisping of their struc ture, not a contraction, like that of muscular structure.