78 THE CIRCULATORY SYSTEM. 



arteries the media is mainly composed of muscle-cells, with 

 elastic tissue only scantily represented. 



This arrangement confers great elasticity on the largest 

 arteries, so that they convert the spurts of blood intermittently 

 pumped from the heart into a more even and continuous cur- 

 rent ; while the smaller arteries, by the action of the vaso-con- 

 strictor and vaso-dilator nerves upon their muscular walls and 

 their calibre, are kept well under the control of the nervous 

 system, which in this way exercises a full and highly coordi- 

 nated control over the circulation and amount of blood-supply 

 in all parts of the body. 



In some arteries an external elastic lamina, similar to but 

 less well marked than the internal elastic membrane, can be 

 distinguished, marking the outer limit of the tunica media and 

 separating it from the tunica adventitia. 



The tunica adventitia, the outer coat of the arterial wall, is 

 a layer of fibrous and elastic tissue, containing perivascular 

 lymphatics and the vasa and nervi vasorum, or the small 

 bloodvessels and nerve-filaments that supply the walls of the 

 artery itself. The adventitia is of indefinite and variable 

 thickness in different situations ; the portion next to the 

 media is a firm, strong tissue, while the more remote portions 

 become looser and more areolar and merge gradually into the 

 neighboring connective tissues. 



In the largest arteries, as the aorta and pulmonary artery, 

 the subendothelial tissue is thick and firm, and the internal 

 elastic lamina is ill defined. The tunica media is made up 

 mainly of elastic plates and reticula, the muscle-cells being 

 present in relatively small proportion. 



As the arteries decrease in size, the subendothelial tissue and 

 the elastic elements of the media diminish and ultimately 

 disappear, the muscle-cells become fewer and scattered, the 

 adventitia decreases in amount, and the internal elastic lamina 

 becomes thinner and finally absent. The endothelium persists 

 throughout. 



The smallest terminal arteries, then, are lined with endo- 

 thelium, resting on the thin remnants of the subendothelial 

 connective-tissue and elastic layers; outside of this are trans- 

 verse involuntary muscle-cells in a single layer or scat- 

 tered and separated by intervals ; and surrounding all is 



