186 



THE CIRCULATORY APPARATUS 



The typical artery consists of three coats: internal 

 or tunica intima; middle or tunica media; an external 

 or tunica adventitia. Aside from the latter, most 

 arteries are covered by a sheath of connective tissue 

 derived from the fascia of the part of the body in 

 which they are found, and held to the artery by 

 fibrous tissue. 



FIG. 73 



Transverse section of part of the wall of the posterior tibial artery. X 75. 

 A', endothelial and subendothelial layers of inner coat. B, elastic layer (fenes- 

 trated membrane) of inner coat, appearing as a bright line in section. C, 

 muscle layer (middle coat). D. outer coat, consisting of connective-tissue 

 bundles. In the interstices of the bundles are some connective- tissue nuclei, 

 and, especially near the muscular coat, a number of elastic fibers cut across. 

 (Schafer.) 



The muscular tissue (media) is not so well-marked in 

 the larger arteries, but the elastic tissue predominates 

 and is more closely arranged; in the smaller arteries 

 the elastic tissue is in excess, while only a single 

 layer of muscle tissue is present; the larger arteries 

 possess a thicker outer or fibrous coat than the smaller 

 vessels. 



All arteries possess elasticity and contractility due 

 to the presence in their walls of the elastic (intima) 

 and muscle (media) coats. 



Elasticity. This elastic property possessed by 

 arteries permits their wall to expand and recoil to 

 adapt itself to the pressure and reaction of the column 

 of blood thrown into the arterial system from the 

 ventricle of the heart at each contraction or systole 

 and subsequent relaxation or diastole. 



