the trigeminal nerve (V/2) through the foramen rotundum and anastomoses with the artery of the foramen rotundum of the internal maxillary artery (IMA).

The posterior ramus divides into a lateral and medial branch. The posteromedial ramus courses together with the third division of CN V towards the foramen ovale and anastomoses with the accessory meningeal artery (AMA) a branch of the IMA. It also supplies the CN VI, the medial third of the Gasserian ganglion and the motor roots of the CN V3. The posterolateral ramus reaches the foramen spinosum and supplies the middle and lateral third of the Gasserian ganglion and anastomoses with a branch of the middle meningeal artery (MMA).

Parkinson (1965) was able to identify the ILT in 80% of the cases, others in 65%–84% (Harris and

Rhoton 1976; Tran-Dinh 1987; Lasjaunias and

Berenstein 1987; Rhoton et al. 1979). Unlike the MHT, the ILT was not observed to show variants and always arose as a single trunk (Tran-Dinh 1987; Willinsky et al. 1987). According to Lang and Schafer (1976), the MHT is called Truncus carotico-cavernosus posterior and the ILT can be called Truncus carotico-cavernosus lateralis. As mentioned above, Lasjaunias et al. (1977, 1978a,b) and Willinsky et al. (1987) as well as Brassier (1987) have suggested a different concept that considers the intracavernous ICA branches embryological remnants, assuming that observed normal dispositions and variants rather represent various phases in the embryological development of the primitive maxillary and trigeminal arteries, possibly influenced by individual hemodynamic balance between ICA and ECA, right and left or anterior and posterior.

Tran-Dinh (1987) who studied the various classifications and terminology extensively proposed a simplification into primary and secondary branches. The fourth Nomenclatura Anatomica (1977) contains a list of intracavernous ICA branches but no systematic classification. Table 3.1 provides an overview of various terms used for the anastomotic branches of the C4 and C5 segment.

Capsular arteries according to Mc Connell

(1953) and Parkinson (1965), and a medial group according to Tran-Dinh (1987) can angiographically be identified in less than 30% of cases (Osborn 1991) and were seen in cadaver dissections in 28% of the cases (Harris and Rhoton 1976). The inferior

capsular artery courses inferomedially to supply the floor of the sella turcica and anastomoses with the contralateral side and with branches of the inferior hypophyseal artery. The superior capsular artery passes along the roof of the sella. The capsular arteries may also arise from the inferior hypophyseal artery. They do not contribute to the supply of the CNs and usually play no role in the supply of DCSFs.

Recurrent artery of the foramen lacerum (RAFL), a small branch of the lateral surface of the vertical portion of the C5 segment, descends to the foramen lacerum and usually anastomoses with the carotid branch of the APA. It supplies the pericarotid autonomic nerve plexus and the carotid wall (Martins et al. 2005) and may connect with the posterior ramus of the ILT (cavernous branch of MMA) (Lasjaunias et al. 2001).

Ophthalmic Artery

The ophthalmic artery (OA) is the most proximal major intracranial branch of the ICA and arises just as this vessel is emerging from the cavernous sinus on the medial side of the anterior clinoid process, and enters the orbital cavity through the optic foramen, below and lateral to the optic nerve. The origin is intradural in about 90% and extradural in about 10% of the time, from either the cavernous or the clinoid segment of the ICA (Punt 1979). The most commonly reported variants are the origins from the MMA (Heyre 1974) and from the ACA (Lasjaunias et al. 2001). Other possible origins are the accessory meningeal artery and the basilar artery (Lasjaunias et al. 2001), the MMA, the anterior deep temporal artery or directly from the ECA (Newton and Potts 1974).

The OA usually courses anterolaterally below the optic nerve and enters the optic canal where it pierces the dural sheath of the optic nerve, usually inferolateral but sometimes directly below the nerve (Hayreh 1962a). The intraorbital portion of the OA may be divided into three segments, the first extending from its entry to where the artery crosses under the nerve. The second segment crosses the nerve (in 80% from lateral to medial), and the third extends from here to its termination. The OA gives off a number of branches including the short and long posterior ciliary arteries and the central retinal artery (CRA). The central retinal artery has a diameter of about 200 microns and may have a tortuous course along the inferior surface of the optic nerve before it pierces the optic nerve sheaths 10–15 mm posterior to the globe and runs for 1–3 mm with

the subarachnoid space of the optic nerve (Miller 1998). The CRA gives numerous small branches to the optic nerve and finally passes through the retrolaminar portion of the nerve where it gives off its terminal branches to supply the inner layers of the retina (Hayreh 1963).

The intraorbital course of the OA and its branching pattern was studied in detail by Hayreh (1962b). He found major variations, depending on whether the OA crosses over or under the nerve. In the former case, the first major branch is the central retinal artery, followed by the lateral posterior ciliary artery, lacrimal artery, muscular arteries, medial posterior ciliary arteries, supraorbital artery, anterior and posterior ethmoidal arteries and medial palpebral artery. In the latter case, the OA sends small perforating branches to the optic nerve, followed by the lateral posterior ciliary, central retinal artery, muscular arteries, medial posterior ciliary arteries, lacrimal artery, posterior ethmoidal artery, supraorbital artery and anterior ethmoidal and medial palpebral artery (Hayreh 1962b). The OA also gives rise to small meningeal arteries of which

the anterior falx artery originates from the anterior ethmoidal artery. The terminal branches are the supratrochlear and the dorsal nasal arteries (Miller N 1998). The posterior ciliary arteries vary in number and form the anastomosing ring, the Circle of Zinn and Haller (Hayreh 1962b). They are called short posterior ciliary arteries and give rise to the cilioretinal arteries which supply the retina in the region of the optic disc (Randall 1887). The long posterior ciliary arteries supply the internal structure of the anterior portion of the eye (Ducasse et al. 1986). Important anastomoses are formed by the anterior and posterior ethmoidal arteries, the lacrimal artery and the deep and superficial recurrent ophthalmic arteries. The deep recurrent ophthalmic artery usually arises from the first part of the intraorbital OA, courses backwards through the SOF and consistently anastomoses with the anteromedial ramus of the ILT. The angiographic appearance of this vessel is characteristic when it projects below the C3 and C4 portion of the ICA (Lasjaunias et al. 1978b). The superficial recurrent ophthalmic artery may arise from the intraorbital OA or from

the lacrimal artery and usually courses through the most lateral part of the SOF to supply the intradural parts of CN III and IV and continues as an artery of the free margin of the tentorium (Lasjaunias et al. 1978b). Two others can establish anastomoses between OA and MMA, the recurrent meningeal artery and the meningolacrimal artery (Lasjaunias et al. 1975b). The recurrent meningeal artery is an adult remnant of the common meningoorbital vascular system, courses through the SOF and connects the lacrimal artery with the anterior branch of the MMA. The meningolacrimal artery also arises from the anterior branch MMA to enter the orbit through the meningolacrimal foramen (foramen of Hyrtl, also called cranioorbital, meningoorbital, stapedial- ophthalmo-lacrimal foramen), which usually lies just lateral to the SOF (Moret et al. 1977).

Ethmoidal Arteries

The ethmoidal arteries arise from the ophthalmic artery and can be divided into an anterior and a posterior group. Their origin, course and supplied regions were studied on 30 injected adult heads by Lang and Schaefer (1979). After branching off, the anterior ethmoidal artery normally turns in a single loop by first coursing forwards and then, reversing towards the anterior ethmoidal foramen, it passes into the canal portion. Occasionally, a common ethmoidal artery or a common source for the ethmoidal arteries is present. As a rule, the smaller posterior ethmoidal artery arises from the ophthalmic artery; occasionally it is absent or can even very rarely arise from the MMA (Lang and Schafer 1979). The artery usually courses over the superior oblique muscle, passes through the posterior ethmoid canal, supplies the posterior ethmoidal cells and enters the cranium, the dura mater and gives off branches to descend to the nasal cavity through the cribriform plate where they anastomose with branches from the sphenopalatine artery (Gray 1918). Before or while entering the olfactory fossa the artery usually gives off branches to supply the dura and bone of the planum sphenoidale, lesser wing of the sphenoid and adjacent dura of the anterior cranial fossa.

The anterior ethmoidal artery accompanies the nasociliary nerve through the anterior ethmoidal canal, supplying the anterior and middle ethmoidal cells and frontal sinus. It then enters the cranium to supply the dura mater with the anterior meningeal artery, which gives off the anterior falx artery and nasal branches to supply the lateral wall and the septum of the nose (Gray 1918). The ethmoidal arteries

are usually involved in the arterial supply of DAVFs located on the floor of the anterior cranial fossa, but may also contribute to the arterial supply of DCSFs.

Table 3.2. ICA and ECA branches relevant for supply of DCSF and their main anastomoses to adjacent territories (modified after Martins et al. 2005)

MHT

Tentorial trunk

–Medial tentorial artery (to contralateral, lateral tentorial, dorsal meningeal, MMA, ILT)

–Lateral tentorial artery (to medial tentorial, dorsal meningeal, posterior meningeal, mastoid)

Dorsal meningeal artery (to contralateral, medial clival, tentorial, MMA, APA)

Inferior hypophyseal artery (to contralateral, capsular)

Medial clival artery (to contralateral, dorsal meningeal, medial tentorial)

ILT

Superior ramus (to medial tentorial)

Anterior Ramus

–Anteromedial ramus (to deep recurrent ophthalmic)

–Anterolateral ramus (to AFR)

Posterior Ramus

–Posteromedial ramus (to AMA)

Posterolateral ramus (to MMA)

Ophthalmic artery

Anterior ethmoidal arteries

Posterior ethmoidal arteries

Deep recurrent ophthalmic artery (to ILT)

Superficial recurrent ophthalmic artery (to medial tentorial)

Recurrent meningeal artery (to MMA)

Recurrent artery of the foramen lacerum (to APA, ILT, vidian)

ECA

Ascending pharyngeal artery

–Carotid ramus (to RAFL)

–Jugular ramus (to lateral clival)

–Hypoglossal ramus (to medial clival)

Maxillary artery

–Artery of pterygoid canal (to AMA, APA, C5 segment, RAFL)

–Pterygovaginal artery

Middle meningeal artery (to ILT, AMA, OA)

Accessory meningeal artery (to MMA, ILT, medial tentorial)