We examined the shape of the jugular fossa and its protrusion into the tympanic cavity in 51 human skulls and in 355 various monkey skulls. All human specimens had a dome-shaped fossa, and the right fossa was larger than the left in 60 per cent of the specimens. Fossae protruded into the tympanic cavity in 20 per cent of these specimens.
In contrast, none of the monkey specimens had a dome-shaped fossa. Some monkeys had saucershaped jugular fossa; the frequency of such fossa became higher as phylogeny progressed. Furthermore, the jugular fossae in monkeys did not protrude into the tympanic cavity. The shapes of both the jugular fossa and sulcus of the transverse sinus were generally symmetrical.
The shape of the jugular fossa and its positional relationship to the tympanic cavity were considered from the viewpoint of the influence of phylogeny and the possible relationship to various otological problems.