224 



ARTERY. 



they are more numerous and larger in young 

 than in adult and old subjects. 



Absorbents are not visible on the coats of 

 any arteries except the larger trunks ; however, 

 the removal of coagula formed in the interior 

 of all arteries after the application of ligatures 

 may be regarded as proving the existence of 

 absorbents in every part of the arterial system. 

 The arteries are plentifully supplied with 

 nerves, of which the aortic system receives more 

 in proportion than the pulmonary artery, and 

 the smaller arteries more than the larger trunks. 

 The trunk of the aorta, the pulmonary artery, 

 and the arteries of the head, neck, thorax, ab- 

 domen, and those of the genital organs, receive 

 their supply from the nerves of organic life. 

 These form a very intricate plexus on their 

 surface. The arteries of the extremities receive 

 their supply of nerves from those of animal 

 life in their neighbourhood. Two sets of nerves 

 have been described as being furnished to the 

 arteries ; one set, consisting of softer nerves, of 

 a flattened form, are said to be lost in the cel- 

 lular or external tunic, nervi molles ; the other 

 set, more firm and round, penetrate the middle 

 tunic, in which they form a thin membraniform 

 expansion, containing distinct fibres. Meckel* 

 justly considers the internal nerves as subdivi- 

 sions of the larger flattened external branches. 

 No nerves have yet been discovered on the 

 umbilical arteries, and the arteries of the brain 

 are supposed to be without any. The nerves 

 of the arteries become less apparent in old age. 

 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,f and, 

 more recently, by Beclard,| 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, and is owing 

 to the middle and external tunics conjointly; 

 the internal tunic has very little power of re- 



* Op. cit. 



t Experimental 



Inquiry on some 

 Animal Structure. Lond. 1740. 

 Anatomic Generale, p. 373. 



parts of the 



sislance in either direction. The middle and 

 internal tunics are as remarkable for their fra- 

 gility as the external is fur 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 latter 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. Ainussat,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 ho'ds 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 be 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 



* Treatise on Haemorrhage. Lond. 1805. 

 f Archives Generates de Medecinc, t. xx. Aout, 

 1829, p. 606. 



