112 



ANATOMY FOR NURSES. 



[CHAP. IX. 



into the other end, the artery swells out to a very great extent, 

 but will return at once to its former size when the fluid is let 

 out. This great elasticity of the arteries adapts them for 

 receiving the additional amount of blood thrown into them 

 at each contraction of the heart. Again, if we stimulate the 

 muscular coat of any of the smaller arteries, the artery will 

 shrink in size, the circularly disposed fibres contracting and 

 narrowing the calibre of the vessel. This contractility is under 

 the control of the nervous system, and as the organs of the 

 body that are at rest do not require so much blood as those that 

 are working actively, the nervous system, the master-regulator 

 of the body's work, is able to diminish or increase the supply of 

 blood to the capillaries in different parts by acting upon this 

 contractile muscular tissue in the arterial walls. The arteries 

 do not collapse when empty; and when an artery is severed, the 

 orifice remains open. The muscular coat, however, contracts 

 somewhat in the neighbourhood of the 

 opening, and the elastic fibres cause the 

 artery to retract a little within its sheath. 1 

 The walls of the arteries are supplied with 

 both blood-vessels and nerves. The blood- 

 vessels are known as the vaso-vasorum ves- 

 sels and the nerves as the vaso-motor nerves. 

 The veins. The veins have three coats, 

 and on the whole resemble the arteries in 

 FIG. 86. A, part of a structure. They differ from them, how- 

 vein, laid open, with two ever j n h av mo. much thinner walls, and 



pairs of valves; B, longi- 

 tudinal section of vein, 

 showing valves closed. 



yellow elastic tissue. 



in their walls containing relatively much 

 more white fibrous tissue and much less 

 They are, therefore, not so elastic or con- 

 tractile as the arteries, and their walls collapse when empty. 

 Many of the veins, especially those of the limbs, are provided 

 with valves, which are mechanical contrivances adapted to pre- 

 vent the reflux of the blood. The valves are semilunar folds of 

 the internal coat of the veins; the convex border is attached to 

 the side of the vein, and the free edge points towards the heart. 

 Should the blood in its onward course towards the heart be, for 

 any reason, driven backwards, the refluent blood, getting be- 

 tween the wall of the vein and the flaps of the valve, will press 



1 This property of the severed artery is an important factor in the arrest of 

 hemorrhage. 



