In general, the human head can be said to consist anatomically of two distinct parts, the neurocranium and the orbita-jaw cranium (Gesichts-Kiefer-Schadel; HOFER, 1956). The latter has shown a tendency to become reduced and degenerate during the course of human evolution, whereas the former has developed and enlarged. These features are presumably related to bipedalism. However, the relationship between the two parts has not been analyzed sufficiently up to now. The author attempted to compare the distribution patterns of the main arteries of the head in humans and some quadrupedal animals. In addition, some experiments were conducted on blood flow in the human head, using the model (tube system) shown in Fig. 5.
The distribution pattern of the main arteries in the human head is closely related to the development of the brain and neurocranium and the degeneration of the orbit-jaw cranium, and arranged so that less blood is sent to the latter part, and more to the former.
At the same time, on the basis of the comparative distribution patterns of the main arteries between orthograde humans and pronograde mammals, it is suggested that human bipedalism may have promoted the development of the human brain and neurocranium.
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