The anterior loop of the carotid siphon.

The extracavernous-presubarachnoid segment of the carotid artery (EPCA), medial to the clinoid process, was studied in 48 sphenoid bones. Ten specimens were submitted to microdissection; the remainders were cut in 1 mm thick slices after electrolytic decalcification. The studies showed that the EPCA has a mean length of 3.09 mm (0.5 to 6.5 mm). It has a shorter length than the whole medioclinoid (paraclinoid) segment of the artery because the subarachnoid space may enter between the artery and the clinoid process, as well as the venous space of the cavernous sinus. Venous channels of extremely small diameter were found between the EPCA and the clinoid process in 30.3% of the cases and between the EPCA and the sphenoid body in 48.6%. The distal limit of the intracavernous cartoid artery is the projection of the caudal surface of the clinoid process, which is coincident with the vertex of the anterior loop of the carotid siphon in 91.7% of the cases. The origin of the ophthalmic artery was verified to be from the EPCA in 9.7%, at the transition to the subarachnoid space in 6.5%, and intracavernous in 3.2%. Immediately proximal to the EPCA, the anterior horizontal segment of the carotid artery may be reached without entering the venous space of the cavernous sinus through the superior wall in 27.8% and through the lateral wall in 66.6%.