Originally: a network of small arterioles derived from the external carotid artery and located within the cavernous sinus at the base of the brain, found in ungulates and certain other mammals (and formerly supposed to exist in humans), now believed to assist in regulating the temperature of the blood entering the brain. In later use also: any of various similar networks of blood vessels, most of which are thought to operate as countercurrent systems that maintain a gradient of temperature or of gases or solutes in the blood.