112 



NORMAL HISTOLOGY. 



surfaces and enter the subendothelial layer on the one side and the 

 median coat of the artery on the other. 



The tunica media, or middle coat of the arteries, consists essen- 

 tially of smooth muscular tissue, with the cells arranged trans- 

 versely to the long axis of the vessels, so that by their contraction 

 they serve to diminish the calibre of the arteries. 



The adventitia is an external sheath or layer of fibrous tissue 





Portion of a transverse section of a human lingual artery from an adult. (Grunstein.> 

 a, intima ; b, media ; c, adventitia ; d, endothelium ; e, subendothelial stratum (delicate 

 areolar tissue) ; /, tunica elastica interna (fenestrated membrane belonging to the intima) : 

 g, stratum subelasticum containing elastic fibres (h) that pass from the fenestrated mem- 

 brane into the media; i, concentric elastic fibres within the media; ,;', smooth muscular 

 fibres of media with elongated nuclei ; k, white fibrous tissue in media; I, elastic fibres 

 radiating from the media into the external elastic tunic ; m, stratum submusculare (are- 

 olar fibrous tissue) ; n, tunica elastica externa ; o, stratum elasticnm longitudinale (fibrous 

 tissue containing elastic fibres running parallel with the axis of the vessel) ; p, stratum 

 elasticum concentricum (fibrous tissue containing elastic fibres encircling the vessel). 

 The vasa vasorum supplying the tissues of the vascular wall are not represented. 



which merges with the areolar tissue of the parts surrounding the 

 arteries and serves to support the latter without restricting the 

 mobility necessary for their functional activity. 



