The venous valves are believed to play an important role in venous function, but their number, position, and spacing in limb veins are reported to be irregular. In this study, the relationship between the number of valves and the length of veins in which they occur was investigated for humans and nonhuman primates. In addition, valve distributions within the superficial veins of the human upper limb were compared with those of the lower limb. Upper and lower limb veins were dissected from adult humans, and forelimb veins were obtained from seven genera of primates. The mean valve index (number of valves per unit length of vein) of the forelimb lateral superficial vein exhibited relatively little variation between humans and primates of a wide range of body sizes and locomotor repertoires. The mean valve index for the lateral superficial vein of the human upper limb is similar to that of the short saphenous vein of the lower limb, and in both veins, the maximum valve index occurs in the most proximal portion of the vein. The relative constancy of the valve index across a range of primate species, and between the upper and lower limbs of humans, may reflect the known constancy of circulatory pressures in mammals. The minimum numbers of valves in superficial veins are only slightly greater than required to limit capillary pressure to below 10 mm Hg, above which pressure the tissue fluid flow would be compromised.

The ontogeny and comparative anatomy of the forelimb superficial veins were investigated in humans, non-human primates and other mammals. Adult humans and the orangutan (Pongo) possess two autonomous forelimb veins, one on the lateral (preaxial) margin of the limb, the other on the medial (postaxial) margin. All other adult primates and mammals examined possess a lateral vein alone. In African apes (Pan and Gorilla) and in 24% of human forelimbs the lateral vein is short, being essentially confined to the antebrachial region, whereas in other mammals and in 76 % of human limbs the lateral vein runs from the carpus to the clavicular region. In humans the medial vein develops before the lateral vein, whereas in the rabbit and the pig the medial vein is present in early embryos but is subsequently lost. We propose that in humans, and probably also in the orangutan, the possession of a medial vein is a neotenic retention of a primitive tetrapod condition. These animals, which retain their medial vein, are united by losing a late stage in their ontogeny. Other animals subsequently pass through a stage in which the medial vein is lost, but Pongo and Homo retain this vein to adulthood. The loss of an ontogenetic stage can arise independently, and the presence of a medial vein therefore affords only weak evidence for a close phylogenetic relationship between humans and the orangutan. The polymorphic lateral vein of humans may be a character state that is intermediate between the derived (short) lateral vein of the African apes and the primitive long lateral vein of other non-human primates and mammals.