The walls T.S. artery and T.S. vein consist of three coats :

(a) Tunica externa : It is outermost layer and made up of connective tissue. The matrix has both white and yellow fibres arranged longitudinally.

Fig. Structure of vein and artery and a capillary network

(b) Tunica media : It has a number of layers of smooth muscle fibres arranged in circles or in concentric rings. Tunica media is more developed in artery than in vein.

(c) Tunica interna : It has two parts, the outer layer which is more developed in artery and has yellow fibres. The other layer is endothelium. It has closely packed cells.