94 THE VASCULAK SYSTEM 



tissue, elastic fibres being scarcely demonstrable except by means 

 of the specific stains for this tissue. 



The wall of the larger veins closely resembles that of the cor- 

 responding artery, except that the venous wall is much thinner 

 and contains far less elastic tissue. The tunica intima of the 

 medium and large veins presents a lining endothelium, a thin 

 layer of delicate connective tissue fibres, and an incomplete in- 

 ternal elastic membrane. The last named is never so prominent 

 as in the artery. 



The tunica media contains smooth, muscle fibres, the most of 

 which are circularly arranged. A somewhat smaller proportion of 

 delicate connective tissue completes this coat. 



The adventitia of the larger veins consists of interlacing 

 bundles of dense white fibres, among which is a network of fine 

 elastic fibres. Occasional small bundles of longitudinal smooth 

 muscle fibres occur in the adventitia of the largest veins. In 

 these vessels also, a very incomplete external elastic membrane 

 may be demonstrated by the specific stains for elastic tissue. 



'- '-. ,' - -^i ; ^ ---- ^3 ;.- 



FIG. 95. TRANSECTION OF THE WALL OF THE HUMAN VENA CAVA. 



a, tunica intima ; 6, tunica media ; c, tunica adventitia. The inner portion of which 

 contains numerous bundles of longitudinal smooth muscle fibres which have been cut 

 across. Hematein and eosin. x 90. 



Nerve fibres and minute blood vessels, vasa vasorum, occur in this 

 coat and distribute their terminal branches to the two outer coats of 

 the vessel. The intima of the vein, as in the artery, is non-vascular. 



\b 



