All vessels carrying blood to the heart are called veins. With the exception of the pulmonary vein, veins carry de-oxygenated blood. As a rule their wall is made up predominantly of connective tissue with some elastic fibres but very little muscular tissue. Most veins have some valves along their length to prevent a backflow of blood. The large vein which returns blood from the systemic circulation to the heart is known as the vena cava. A prominent and highly visible vein in the horse is the jugular vein, located in the neck and running along the eponymous jugular groove. The jugular vein is the preferred site in the horse for intravenous injections. Veins often lie close to the surface under the skin and may become more prominent upon exertion.