Occipitomarginal Venous Sinus
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The occipitomarginal sinus is part of the venous sinus system that drains blood from the brain to return it to the heart. The sinus is comprised of two distinct parts, and is considered to be two distinct sinuses by most anatomists (and is thus often referred to as the occipital marginal sinus): a vertical occipital sinus which represents an inferior continuation of the superior sagittal sinus, and which branches into right and left marginal sinuses that pass around the foramen magnum. The marginal sinuses variably join the sigmoid sinuses near or at the jugular bulbs and internal jugular veins. Importantly, the vertebral venous plexus (which provides an alternate route for blood return to the thorax) also form anastamoses with the internal jugular veins in this area. The occipitomarginal sinus is effectively absent in orangutans, chimpanzees and gorillas. However, it only occurs in low frequencies (4-6%) in modern human populations. The relatively common occurrence of an enlarged occipitomarginal sinus in fossil hominins has led to the suggestion that expansion of these sinuses was selectively advantageous in the context of bipedal posture and locomotion, on the argument that increased intrathoracic pressure attendant with habitual bipedalism inhibited blood return to the heart via the internal jugular veins (thus more blood was shunted to the vertebral venous plexus). More recently, humans secondarily lost the expanded occipitomarginal venous system due to other changes in cranial venous drainage (having to do with a proliferation of emissary veins) that relaxed the need for blood return via the vertebral route, such that the expanded occipitomarginal sinus is only found in low frequencies today. Careful studies of venous blood return in modern humans, however, have failed to support a strong association between occipitomarginal sinuses and blood flow in the vertebral venous plexus.
Falk, 1986. Evolution of cranial blood drainage in hominids: Enlarged occipital/marginal sinuses and emissary foramina. Am J Phys Anthropol 70: 311-324.
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