Birds and reptiles always carry a long and thick artery accompanying the sciatic nerve (i.e. the sciatic artery) whereas mammals do not. We attempted to demonstrate a difference in courses of the nerve and the arteries of foetuses in relation to hip joint posture. Eight mid-term human foetuses (15–18 weeks), five mouse foetuses (E18), and five chick embryos (11 days after incubation) were examined histologically. Thin feeding arteries in the sciatic nerve were consistently observed in human foetuses in spite of the long, inferiorly curved course of the nerve around the ischium. The tissue around the human sciatic nerve was not so tight because of the medial and inferior shift of the nerve away from the hip joint. The foetal hip joint position differed among the species, being highly flexed in humans and almost at right angle flexion in mice and chicks. Because of deep adduction of the hip joint in the mouse, the knee was located near the midline of the body. The mouse sciatic nerve ran through the tight tissue along the head of the femur, whereas the chick nerve ran through the loose space even in the gluteal region. In birds, evolution of the pelvis including the hip joint without adduction seemed to make the arterial development possible. In mammals, highly flexed or adducted hip joint seemed to be one of the disturbing factors against development of the long and thick artery. A slight change in posture may cause significant arterial variation. (Folia Morphol 2013; 72, 1: 41–50)
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