Fifty cavernous sinuses from cadavers were studied in detail using magnification, with special attention to the relationships important in surgical approaches on the intracavernous structures, and to understanding arterial contributions to arteriovenous fistulas involving the cavernous sinus. Significant findings were: 1) The three main branches of the intracavernous portion of the carotid artery were the meningohypopyseal artery, present in 100% of the specimens, the artery of the inferior cavernous sinus (84%), and McConnell's capsular arteries (28%). In addition, the ophthalmic and dorsal meningeal arteries arose from the carotid artery within the cavernous sinus in 8% and 6%, respectively. The three main branches of the meningohypopyseal trunk were the tentorial artery, present in 100%, the dorsal meningeal (90%), and the inferior hypophyseal (80%). 2) The carotid artery was separated from the trigeminal nerve just proximal to the sinus by only dura in 84% of the specimens, and the artery was exposed in the floor of the middle fossa lateral to the trigeminal nerve in 38%. 3) The intracavernous portion of the carotid artery indented the lateral side of the pituitary gland in 28% of dissections but could be as far as 7 mm from it. 4) A triangular area, described by Parkinson, through which the intracavernous portion of the carotid artery could be exposed surgically was found in all specimens. 5) The sixth cranial nerve may split into as many as five rootlets as it passes lateral to the intracavernous portion of the carotid artery. 6) The three major venous spaces within the sinus were posterosuperior, anteroinferior, and medial to the intracavernous portion of the carotid artery.