INNER COAT 



Chap. X] BLOOD VASCULAR SYSTEM 169 



arteries it is thinner and contains a larger proportion of musculai 

 tissue. The proper functioning of the arteries depends upon 

 their elasticity and contractility and 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 IK "^^ — outer coat 



1-1 1 \\m all ELASTIC LAYER )h,id0LE 



the arteries adapts them ||j^ Jmt^!l°!'7o''*^y!.''^ ''*^" ^ '^°*^ 



for receiving the addi- 

 tional amount of blood 

 thrown into them at 



each contraction of the Fig. 105. — Diagram of a Cross-section of 



AN Artery, showing the Composition of its 

 heart. Tunics. (Gerrish.) 



3. The strength of an 

 artery depends largely upon the outer fibrous coat ; it is far less 

 easily cut or torn than the other coats, and it serves to resist un- 

 due expansion of the vessel. 



The arteries do not collapse when empty, and when an arten,^ is 

 severed the orifice remains open. The muscular coat, however, 

 contracts somewhat in the neighborhood of the opening, and the 

 elastic fibres cause the artery to retract a little within its sheath, 

 so as to diminish its calibre and permit a blood-clot to plug the 

 orifice. This property of the severed artery is an important factor 

 in the arrest of hemorrhage. 



Blood and nerve supply of the arteries. — The blood which 

 flows through the arteries nourishes only the inner coat. The 

 middle and outer coats are supplied with arteries, capillaries, and 

 veins, called vasa vasorum, or blood-vessels of the blood-vessels. 



The muscular tissue found in the walls of the arteries is supplied 

 with nerves chiefly from the sympathetic system. These nerves 

 are called vasomotor, and are divided into two sets, (1) vaso- 

 constrictor, and (2) vaso-dilator. 



Stimulation of one set of these nerves (vaso-constrictor) causes 

 contraction of the muscle-fibres and constriction of the arteries; 

 stimulation of a second set (vaso-dilator) causes a relaxation of 

 the muscle-fibres, and dilatation of the arteries. The widening 



