THE CIRCULATION 



233 



until the original conduit was represented by five to six thou- 

 sand million little pipes, the friction which the pumping-station 

 would have to overcome would be very great. But little force 

 would remain in the water when it reached the smallest pipe. 

 Still greater is the resistance to the flow of blood, which is 

 slightly viscous, and contains solid corpuscles, which increase 

 friction. Two thousand miles of capillary tubing in the body 

 of a man, without reckoning the vessels of his liver and lungs ! 

 Water is supplied to houses in rigid tubes. Arteries are 

 elastic, and their elasticity is self -regulating. The cause of 

 this will be apparent if a section of an artery is examined. It 



_ Lining 

 Epithelium 



Perforated 

 Elastic Membrane 



Muscle 

 Fibres 



Elastic Fibres 

 cut across 



FIG. 12. A PORTION OF THE WALL OP A SMALL ARTERY CUT TRANSVERSELY AND HIGHLY 



MAGNIFIED. 



Its inner coat consists of a lining sheet of epithelial scales supported by connective tissue and 

 a strong elastic membrane. This membrane is perforated with holes which place the 

 lymph-spaces on its two sides in continuity. The middle coat is composed of plain muscle 

 fibres and patches of elastic membrane ; the outer coat of elastic fibres, mostly longitudinal, 

 and connective tissue. 



contains much elastic tissue. It also contains plain muscle- 

 fibres. The smaller the artery, the greater is the amount of 

 muscle relatively to the other constituents of its wall. The 

 wall of a vein contains very little muscle, and not much elastic 

 tissue. The muscle of all arterial walls is in a chronic state of 

 tone. To some extent the degree of tone is varied automatically. 

 Pressure within an artery acts as a stimulus to the muscle- 

 fibres of its wall. Any increase leads the fibres to contract 

 more strongly. Any diminution induces them to relax. The 

 arteries resist distension ; they do not narrow to any great 

 extent when pressure falls. But more important than this 

 automatic mechanism for maintaining a uniform pressure in 



