The nomenclature and borders of the segments of the internal carotid artery (ICA) remain confusing. A classification of segments of the ICA is proposed based on constant anatomical structures, such as the carotid foramen and canal, the petrous bone, the petrolingual ligament (PLL), and the proximal and distal dural rings. The bilateral ICAs were dissected in 15 cadaveric head specimens using different neurosurgical approaches. The bilateral lacerum foramina were studied in five dry skulls. The bilateral segments of the ICA were also examined on carotid angiograms of 10 normal patients and another with the ophthalmic artery originating from the intracavernous portion of the ICA. The present classification divides the ICA into five segments in the direction of the blood flow. The cervical segment is extradural and extracranial, the petrous segment is extradural and intraosseous, the cavernous segment is interdural and intracavernous, the clinoidal segment is interdural and paracavernous, and the cisternal segment is intradural and intracisternal. The ICA did not pass through the lacerum foramen in any specimen. In all specimens, 1/8 to 5/8 of the lacerum foramen was under the deep dural layer of the cavernous sinus. The term 'lacerum segment' as used previously and called the 'trigeminal segment' by us cannot be justified. The PLL is the posterolateral border of the cavernous sinus and the lacerum and trigeminal segments should be included in the cavernous and petrous segments. The ophthalmic artery may originate from the clinoidal ICA, from the cavernous ICA, or from the middle meningeal artery. Instead of using the term 'ophthalmic segment,' the term 'cisternal segment' should be used for the anatomically distinct ICA in the subarachnoid space. This classification should be minimally affected by anatomical variations.