ACTION OF THE AKTERIES. 141. 



The muscularity of the arteries is shown by their contraction on ex- 

 posure, their subsequent dilatation being due to their elasticity, this con- 

 tractile property being continued for some time after death. It is also 

 proved by the great diminution of diameter which arteries exhibit when 

 under the influence of an electric current. The quantity of muscular 

 and elastic tissue in different arterial tubes is usually in an inverse pro- 

 portion. In the great arteries the elastic tissue abounds, in the smaller 

 the muscular increases. By their muscular coat the quantity of blood 

 in these tubes can, within certain limits, be regulated. 



At each injection of blood into it an artery distends. It then con- 

 tracts, and thus gives origin to a pulsation. Its increase is Action of the 

 both in diameter and length, the tendency being to lift it at arteries. 

 each pulsation. The distention does not occur at the same instant in 

 all these tubes, but those nearest to the heart yield first, and the more 

 distant a little later. There is therefore what may be termed a wave of 

 distention passing throughout the length of each arterial tube, and an- 

 other actual wave in the blood itself. These pass onward at different 

 rates of speed. The interval of wave-motion from the heart to the 

 wrist is about one seventh of a second. Of course this wave-motion is 

 to be distinguished from the absolute movement of the blood, which is 

 much slower. In the carotid artery the flow of the blood is about one 

 foot in one second. 



A pressure or impact, communicated to a liquid in a long tube, is 

 transmitted to the more distant end with vastly more rapidity than the 

 liquid itself could flow through the same distance. Thus, if we were to 

 suppose a very long metal tube to be filled completely with water, its 

 two ends having been tightly closed by tying pieces of bladder over 

 them, the tap of a finger on one of the pieces of bladder would be almost 

 instantly felt by a finger laid on the other. Indeed, it has been pro- 

 posed to establish telegraphic communication on this principle, though 

 such attempts would prove abortive from the interference of collateral 

 circumstances. This example may serve, however, to illustrate the es- 

 sential difference between the flow of a liquid in a tube and the passage 

 of a pulsation through such a liquid contained in such a tube. 



The capillaries may be regarded as tubular continuations of the arte- 

 ries and the commencement of the veins. They ramify 



, . J .. J The capillaries. 



through the organic structures. They are of pretty uniform 

 diameter, and may therefore be looked upon as cylinders. Their usual 

 size is about -g-^-Q of an inch ; their mode of distribution varies with the 

 structure and functions of the part they occur in : thus, in muscles they 

 run parallel ; in the papillas they are looped. 



They consist essentially of a delicate structureless membrane, analo- 

 gous to cell membrane, and the sarcolemma of voluntary muscles. It 



