CHAP. VIII.] THE VASCULAR SYSTEM. 89 



ments differs in different arteries ; but, as a general rule, the 

 larger arteries are the more elastic, and the smaller the more 

 muscular. The elasticity and contractility of the arteries may 

 be demonstrated by the following example : - 



If we tie a piece of a large artery at one end and inject fluid 

 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, con- 

 tracts 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 

 vessels and the nerves as the vaso-motor nerves. 



The veins. The veins have three coats, and on the whole 

 resemble the arteries in structure. They differ from them, 

 however, in having much thinner walls, and in their walls con- 

 taining relatively much more white fibrous tissue and much less 

 yellow elastic tissue., 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. 



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

 hemorrhage. 



