The jugular foramen and postglenoid foramen are the main venous foramina of the skull of placental mammals. Their mutual relations are closely related to the development of the internal and external jugular vein, depending on the given kind. On the basis of measuring studies, it was decided to investigate the relations of the size of these foramina and to quantitatively determine which of them prevails. The studies were performed with macerated human and animal skulls. Altogether 100 skulls of human adults of both sexes, 100 of macaccas, 67 of bisons, 25 of mongrel dogs, 37 of foxes and 25 of rats were examined. The jugular foramen was the outlet of the sigmoid sinus or its equivalents and the postglenoid foramen — the petrosquamous or temporal canal. The jugular foramen was always singular and characterised by a small variability in the morphology, consisting only in the occurrence of the internal division or its lack. The postglenoid foramen, on the other hand, in the majority of the studied kinds was variable and numerous. The number of foramina ranged from 1 in man to 7–10 in bisons. In the course of measurements, it was found that the area of the postglenoid foramen in relation to the area of the jugular foramen is 0.87% in man, 24% in macaccas, 30.7% in dogs, 34.4% in foxes, 53.9% in rats and 248.5% in bisons. The results obtained indicate that in bisons the system of venous sinuses, connected with the postglenoid foramina, has a decisive significance for the drainage of the cranial cavity. In contrast, the postglenoid foramen in man has a marginal significance. The remaining kinds, whose skulls have been investigated, occupy on that score an intermediate position.