The internal carotid artery has a sleeve of increased innervation density within the cavernous sinus in monkeys.

The numerical density of nerve terminals of the internal carotid artery was measured using interrupted serial sections and compared with densities sampled from the major cerebral arteries of cynomolgus and rhesus monkeys. In its course through the carotid canal and the foramen lacerum the artery received few terminals. Nerve terminal density increased substantially within the cavernous sinus in 13 of 19 animals, reaching a peak shortly before emerging to join the circle of Willis. The density dropped rapidly on leaving the sinus. The increase was present in both species and rose to a mean nerve terminal density at least ten times that of any other artery measured. In 6 monkeys terminal incidence was unchanged through the sinus. The possible relevance of the nerve terminal sleeve to cerebral vasodynamics and to vascular head pain is discussed.