In most parts of the body the arteries lie in a sheath of connective tissue, from which fine fibrous processes pass to the tunica externa.  The connection, however, is not a close one, and the artery when divided transversely is capable of retracting for a considerable distance within its sheath.  In some of the larger arteries the sheath assumes the form of a definite membrane.

The arteries are nourished by small vessels—­the vasa vasorum—­which ramify chiefly in the outer coat.  They are also well supplied with nerves, which regulate the size of the lumen by inducing contraction or relaxation of the muscular coat.

The veins are constructed on the same general plan as the arteries, the individual coats, however, being thinner.  The inner coat is less easily ruptured, and the middle coat contains a smaller proportion of muscular tissue.  In one important point veins differ structurally from arteries—­namely, in being provided with valves which prevent reflux of the blood.  These valves are composed of semilunar folds of the tunica intima strengthened by an addition of connective tissue.  Each valve usually consists of two semilunar flaps attached to opposite sides of the vessel wall, each flap having a small sinus on its cardiac side.  The distension of these sinuses with blood closes the valve and prevents regurgitation.  Valves are absent from the superior and inferior venae cavae, the portal vein and its tributaries, the hepatic, renal, uterine, and spermatic veins, and from the veins in the lower part of the rectum.  They are ill-developed or absent also in the iliac and common femoral veins—­a fact which has an important bearing on the production of varix in the veins of the lower extremity.

The wall of capillaries consists of a single layer of endothelial cells.

HAEMORRHAGE

Various terms are employed in relation to haemorrhage, according to its seat, its origin, the time at which it occurs, and other circumstances.

The term external haemorrhage is employed when the blood escapes on the surface; when the bleeding takes place into the tissues or into a cavity it is spoken of as internal.  The blood may infiltrate the connective tissue, constituting an extravasation of blood; or it may collect in a space or cavity and form a haematoma.

The coughing up of blood from the lungs is known as haemoptysis; vomiting of blood from the stomach, as haematemesis; the passage of black-coloured stools due to the presence of blood altered by digestion, as melaena; and the passage of bloody urine, as haematuria.

Haemorrhage is known as arterial, venous, or capillary, according to the nature of the vessel from which it takes place.