The jugular and the limb veins of the Giraffe, Okapi, Camel and Ostrich have been examined. Certain arrangements of valve cusps at the orifices of the larger tributaries of these veins have been found in the Giraffe, Okapi and Camel, the cusps being arranged in various ways one above the other along the axis of the main vessel.
The term Bicuspid or Tricuspid SERIED Valves is suggested to describe these.
Experiments have shown that these valves are efficient in preventing backflow of blood into the tributaries when a sudden rise of pressure occurs in the main vein, unless the pressure in both is very low to start with. Once closed, the valves will withstand high pressures, even with negative pressure in the tributary.
Seried Valves are not found at the mouths of small tributaries. Thus they are absent from the jugular vein of the Giraffe which receives only small vessels.