This was the closed venous system.

The arterial system, shown, as you see, quite separate in Figure 31, was full of a thinner, brighter, warmer blood, characterized by the presence of an abundance of the vital spirits.

Warmed in the ventricle, it distributed vital heat to all parts of the body.

The two systems were closed and communicated with each other only through certain pores or perforations in the septum separating the ventricles.

At the periphery, however, Galen recognized (as had been done already by the Alexandrians) that the arteries anastomose with the veins, ".