322 THE CIRCULATION OF THE BLOOD. [CHAP, xxvill. 



arteries are in their greatest state of contraction, the quantity of 

 blood circulating in them being reduced to a minimum. This state 

 of contraction Hunter assumed to be the result of muscular force, 

 and with good reason, as, after stretching, the artery did not con- 

 tract to its previous dimensions. The stretching destroyed the 

 muscular force, leaving whatever contraction would take place, on 

 the removal of the stretching, to be effected by the elastic force. 

 Thus a piece of the aorta of a horse, when slit up and opened on a 

 plane surface, measured five inches and a half; on being stretched, 

 it lengthened to ten inches and a half; the stretching power being 

 removed, it contracted again to six inches, " which," says Hunter, 

 " we must suppose to be the middle state of the vessel."* These 

 powers inherent in the arterial wall, of yielding under a distending 

 force, and reacting upon its contents with a force equal to that 

 of the primitive disturbing one, and also that of muscular con- 

 traction, exercise an important influence in promoting or directing 

 the circulation of the blood through the arterial system. 



The elastic element of the arterial tunic is always developed in 

 the direct ratio of the size of the artery ; and the muscular element, 

 although perhaps not bearing an inverse proportion to the size of 

 the artery, yet becomes more prominent and distinct as the elastic 

 tissue diminishes in coarseness and in strength. Thus it is in the 

 smaller arteries that we notice the most perfect arrangement of 

 muscular fibres, and in these the fibrous tissue is reduced to its in- 

 ternal longitudinal fibrous layer, the external circular fibres having 

 disappeared. 



Blood-vessels and nerves are freely distributed to the arterial 

 tunics. To the former, allusion has already been made in describ- 

 ing their external tunic. We have no evidence that these blood- 

 vessels penetrate further than to a slight depth into the fibrous 

 tunic. It is probable, therefore, that they are destined to nourish 

 the external tunic, and a portion (chiefly the muscular element) 

 of the fibrous tunic, leaving the remainder of the arterial wall to 

 imbibe its nutrition directly from the blood itself. The general 

 arrangement of the nerves on the outer coat of arteries has been 

 already described (vol. i. p. 223, and vol. ii. Chap, xx.) The 

 plexuses formed are chiefly conducted by the arteries to parts 

 beyond: but they also furnish filaments penetrating to the 

 muscular fibres, and bringing these into relation with the nervous 

 system. 



The arterial system may be described as taking its starting point 

 * Hunter on the Blood, inflammation, &c., 4to. cd. p. 124, et scq. 



