BLOOD VESSELS 431 



III. CIRCULATION IN THE BLOOD AND LYMPH 



VESSELS. 



The general distribution of the various vessels — arteries, 

 capillaries, veins, and lymphatics — has been already con- 

 sidered (fig. 162, p. 383). 



1. STRUCTURE. 



{The structure of the walls of each kind of Vessel 

 must he studied practically.) 



The capillaries are minute tubes of about 12 micromilli- 

 metres in diameter, forming an anastomosing network 

 throughout the tissues. Their walls appear to be composed 

 of a single layer of endothelium. 



On passing from the capillaries to arteries on the one 

 side, and to veins and lymphatics on the other, non-striped 

 muscle fibres make their appearance encircling the tube. 

 Between these fibres and the endothelium, a fine elastic 

 membrane next appears, while, outside the muscles, a sheath 

 of fibrous tissue develops. Thus the three essential coats of 

 these vessels are produced : — 



Tunica intima, consisting of endothelium set on the 

 internal elastic membrane. 



Tunica media, consisting chiefly of circularly arranged 

 visceral muscular fibres. 



Tunica adventitia, consisting of loose fibrous tissue. 



A. Arteries. — The coats of the arteries are thick. In 

 the large arteries, the muscular fibres of the media are 

 largely replaced by elastic fibres, so that the vessels may 

 better stand the strain of the charge of blood which is shot 

 from the heart at each contraction. 



The great characteristic of the walls of the large arteries 

 is the toughness and elasticity given by the abundance of 

 elastic fibrous tissue, and of the sr)iall arteries, the con- 

 tractihty due to the preponderance of muscular fibres. 



