244 HUMA PHYSIOLOGY 



III. Circulation in the Blood Vessels. 



The general distribution of the various vessels arteries, 

 capillaries, veins, and lymphatics has been already con- 

 sidered (Fig. 105, p. 209). 



(The structure of the walls of each must be studied 

 practically.) 



The capillaries are minute tubes of about 12 micro- 

 millimetres in diameter, forming an anastomising network 

 throughout the tissues. Their wall consists 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 endo- 

 thelium a fine elastic membrane next appears, while out- 

 side the muscles a sheath of fibrous tissue develops. Thus 

 the three essential layers of the coats of these vessels are 

 produced : 



Tunica intima, consisting of endothelium set on the in- 

 ternal elastic membrane. 



Tunica media, consisting chiefly of the visceral muscular 

 fibres. 



Tunica adventitia, consisting of loose fibrous tissue. 



The coats of the arteries are thick ; those of the veins are 

 thin. 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. In the veins double 

 flaps of the tunica intima form valves which prevent any 

 regurgitation of blood. 



The great characteristic of the walls of the large arteries 

 is the toughness and elasticity given by the abundance of 

 elastic fibrous tissue, of the small arteries, the contractility 

 due to the preponderance of muscular fibres. 



The circulation of blood in the vessels is that of a fluid in 

 a closed system of elastic-walled tubes, at one end of which 

 (the great arteries) a high pressure, and at the other (the 

 great veins) a low pressure is kept up. As a result of this 



