They suspect brain overheating may be a cause of sudden infant death syndrome.

Researchers from the Institute of Neurology in London believe the blood vessels and airways in the head may be arranged to act as a heat exchanger that cools blood before it enters the brain.

The brain is particularly sensitive to the dangerous effects of overheating.

Physiologists know that some mammals, such as cats and sheep, have a complex network of arteries - the rete caroticum - extending from the left and right internal carotid arteries that feed the brain.

As the network weaves through the large cavernous sinus in their heads heat is transferred from the blood in the arteries to the cooler blood running in the veins that return blood to the heart after it has circulated through the brain. This ensures the brain does not get too hot.

Humans and other primates do not have this complex structure.

But Professor George du Boulay and his colleagues at the Institute of Neurology in London suspected that the internal carotid arteries that run from the neck to the brain in primates might still play a similar role.

The blood passing through them might lose enough heat to the nearby airways and jugular veins to help cool the brain.

Monkey tests

The theory was tested on monkeys

New Scientist magazine reports that, as a test, the team threaded a tiny thermometer called a thermocouple up through the internal carotid artery of a macaque monkey.

They compared the temperature of blood in this artery with other tissues.

They found that blood in the artery was consistently about 1.5 °C below rectal or brain tissue temperatures before it even reached the cavernous sinus.

Professor du Boulay said: "We were very surprised to find such a large temperature difference.

"It is very, very likely that the arterial blood is being cooled by the airway and that further cooling comes from the cavernous sinus."

Professor du Boulay said that if the cooling system fails to lower brain temperature in an infant, it might even explain some cot deaths.

He speculates that cool air may not reach the nose or face of a baby lying face down, which might be enough to make the brain's heat exchanger fail and trigger cot death.

Louis Sokoloff, head of research into brain metabolism at the National Institutes of Health near Washington DC, said the study backed up previous research suggesting heat exchange takes place between vessels and airways in the head.