Microsurgical anatomy of the upper basilar artery and the posterior circle of Willis

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✓ The microvascular anatomy of the posterior part of the circle of Willis, important in surgery of pituitary tumors and basilar aneurysms, was defined in 50 cadaver brains. Significant findings were as follows: 1) Anomalies of the posterior half of the circle of Willis were found in 46% of cases. 2) Hypoplastic P-1 (posterior cerebral segment) and posterior communicating segments gave origin to the same number and size of perforating arteries, having the same termination as normal-sized segments. Thus hypoplastic segments should be handled with care and divided to aid in exposure of the basilar bifurcation only after careful consideration. 3) An average of four perforating branches arose from P-1; most from the superior and posterior surfaces. No branches arose from the anterior surface of the basilar bifurcation. The most proximal P-1 branch originated 2 to 3 mm distal to the basilar bifurcation. It was most commonly a thalamoperforating artery. The largest P-1 branch was usually a thalamoperforating or a posterior choroidal artery. 4) An average of seven branches emerged from the superior and lateral surfaces of the posterior communicating artery. The anterior half was a richer source of perforators than the posterior half. The largest communicating branch in 80% of specimens supplied the premamillary area. 5) The anterior choroidal artery originated from the carotid artery on both sides in all cases. A double anterior choroidal artery was present in 4% of cases.

The posterior portion of the circle of Willis sends a series of perforating arteries into the diencephalon and midbrain which may become stretched around suprasellar tumors or posterior circle aneurysms. The risks of occlusion of these vital perforating vessels during pituitary or aneurysm surgery include visual loss through involvement in the visual pathways in the optic nerves and tracts and the lateral geniculate bodies; somatesthetic disturbances due to involvement of the afferent pathways in the medial lemniscus or thalamus; motor weakness due to involvement of the corticospinal tracts in the internal capsule or peduncle; memory deficits due to involvement of hypothalamic pathways entering and exiting from the mamillary bodies; autonomic imbalance caused by disturbance of sympathetic and parasympathetic centers in the anterior and posterior diencephalon; diplopia due to involvement of the extraocular nerves or nuclei in the midbrain; alterations of consciousness due to ischemia of the midbrain reticular formation; abnormal movements due to involvement of cerebellothalamic circuits in the midbrain and thalamus; and endocrine disturbances due to involvement of the hypothalamic-pituitary axis. Magnification now makes it possible to visualize and preserve these small vessels. The purpose of this study was to define the microsurgical anatomy of the branches of the posterior half of the circle of Willis, important in aneurysm and pituitary surgery.

Materials and Methods

Fifty randomly selected adult brains removed at autopsy provided the material for this study. The posterior half of the circle of Willis was examined using ×3 to ×40 magnification. The arteries examined included the internal carotid above the posterior communicating, the anterior choroidal, posterior communicating, proximal posterior cerebral, upper centimeter of the basilar, and the proximal superior cerebellar (Table 1). The anterior choroidal artery was included because its branches are directed posteriorly into the area supplied by the posterior part of the circle of Willis. In the results described below, percentages are drawn from 50 circles of Willis and basilar arteries and 100 of each of the other arteries mentioned above.

TABLE 1

Diameter, length and number of perforating branches of parts of the circle of Willis (50 brains)

ArteryDiameter (mm)Length (mm)No. Perforating Branches
AverageRangeAverageRangeAverageRange 
internal carotid artery (above posterior communicating)4.32.5–7.02.30–6 
anterior cerebral artery*  
 A-1 segment2.60.9–4.012.7 7.2–18.08.02–15 
 A-2 segment (proximal 5 mm)1.20–4 
anterior communicating artery*1.50.2–3.42.6 0.3−7.01.60–4 
recurrent artery*1.00.2–2.94.20–12 
anterior choroidal artery1.00.5–2.3
posterior communicating artery1.30.3–3.112.6 7.0–23.07.04–12 
posterior cerebral artery  
 P-1 segment2.60.9–4.07.0 3.0–20.04.11–13 
 P-2 segment2.71.6–4.0
basilar artery (upper 1 cm)4.13.0–5.532.0 15.0–40.08.03–18 
superior cerebellar artery (proximal 1 cm)1.90.9–3.04.01–12 

Data obtained from Perlmutter and Rhoton.33

The posterior cerebral segment between the basilar bifurcation and the posterior communicating is referred to as “P-1,” and the segment just distal to the communicating as “P-2.” The posterior communicating artery is referred to here as the communicating artery. A hypoplastic arterial segment is defined as one having a diameter of 1 mm or less.

Results
Anomalies

A normal posterior circle, defined as one in which both P-1 segments have a diameter larger than their communicating arteries that are not hypoplastic, was found in 54% (Fig. 1A). Anomalies consisting of either a hypoplastic communicating artery or a fetal arrangement in which the communicating artery provided the major supply to the posterior cerebral artery and was larger than P-1 were found in 46% in our study as compared to 35% to 84% in previous studies (Table 2).3,24,35 A hypoplastic communicating artery was found in 32% (unilateral 26% (Fig. 1B), bilateral 6% (Fig. 1C)) and a fetal configuration in which the posterior cerebral artery arose predominantly from the carotid artery was found in 22% (unilateral 20% (Figs. 1D, 2), bilateral 2% (Fig. 1E)) as compared to previously reported incidences of 22% to 53% and 15% to 40%,3,24,35 respectively. In this study, 8% had a hypoplastic communicating artery on one side and a fetal complex on the other side (Fig. 1F) as compared to 4% to 12% reported by others.3,24,35 Communicating arteries and P-1 segments were present on both sides in all 50 circles of Willis examined. Absence of either segment is rare.3

TABLE 2

Frequency of posterior circle anomalies

AnomalyAlpers, et al. (1959)Riggs & Rupp (1963)Kaplan & Ford (1966)Saeki & Rhoton (1977)
no. of brains examined350 9945050
normal posterior circle of Willis (%)65.4 311654
abnormal posterior circle of Willis34.6 698446
hypoplastic posterior communicating22 534432
 unilateral13 22*26
 bilateral9 31*6
fetal type15 224022
 unilateral11 162820
 bilateral4 6122
hypoplastic posterior communicating one side, fetal type on other side3 608
posterior communicating absent0.6 000

Data not given.

Fig. 1.
Fig. 1.

Superior view of the basilar (B.A.), superior cerebellar (S.C.A.), P-1, and distal segments of posterior cerebral (P.C.A.), posterior communicating (P.Co.A.), internal carotid, and proximal anterior choroidal (A.Ch.A.) arteries. The arterial branches below the posterior perforating substance (P. Perf. S.), mamillary bodies (Mam. B.), optic tracts (O.Tr.), chiasm, and nerves (O.N.) are shown in half tone. The third and fourth nerves (III and IV) course between the superior cerebellar and posterior cerebral arteries. Arterial branches to the upper pons, posterior mesencephalon, interpeduncular fossa, posterior perforating substance, mamillary bodies, tuber cinereum, optic tracts and chiasm arise from the basilar, P-1, posterior communicating and internal carotid arteries. A.C.A. = anterior cerebral artery; C.A. = carotid artery; P.Ch.A. = posterior choroidal artery; Premam. A. = premamillary artery; Th.Pe.A. = Thalamoperforating artery. A: Normal configuration of the posterior half of the circle of Willis; both P-1's are larger than communicating arteries and the latter are not hypoplastic (diameter greater than 1 mm). The right superior cerebellar artery is duplicated. The largest right P-1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. Only two perforating arteries arise on right P-1. The left posterior choroidal arises on P-2. Both premamillary arteries (largest communicating trunk to premamillary area) arise from the middle one-third of the posterior communicating arteries. Anterior choroidal arteries arise as a single trunk. B: Hypoplastic left communicating artery. Thalamoperforating artery arises on P-1 medial to the posterior choroidal on both sides. The left premamillary artery arises from the posterior and the right from the anterior portion of the posterior communicating artery. The superior cerebellar arteries are duplicated on both sides. C: Posterior communicating arteries are hypoplastic bilaterally. The largest right P-1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. The thalamoperforating artery arises medial to the posterior choroidal artery on the left P-1. The premamillary artery arises from the anterior one-third of the right posterior communicating artery and from the middle third on the left. The left anterior choroidal arises from the carotid as two trunks.

Fig. 1.
Fig. 1.

(continued)D: Fetal origin of the right posterior cerebral artery. The thalamoperforating artery on the right arises near the basilar bifurcation. The right posterior choroidal artery arises on P-2. The left posterior choroidal artery arises medial to the thalamoperforating artery. The right premamillary artery arises from the anterior portion of the communicating artery. The left premamillary area is supplied by a group of nearly equal-sized arteries. The anterior choroidal artery bifurcates immediately after origin on the left. E: Bilateral fetal origin of the posterior cerebral artery. The right posterior choroidal artery arises lateral to the thalamoperforating artery. The largest P-1 branch gives rise to the left thalamoperforating and choroidal arteries. The right premamillary artery arises from the middle portion of the communicating artery. A premamillary arterial complex is present on the left. F: Fetal type of right posterior cerebral origin and hypoplastic left communicating artery. The right posterior choroidal arises lateral to the well developed thalamoperforating artery. No thalamoperforating branches are present on the left. The right premamillary artery arises from the anterior and the left from the posterior portion of the communicating artery. G: Superior view of the suprasellar area. Arterial branches stretched around the superior extension of a pituitary tumor. Anterior cerebral arteries send branches to the superior surface of the optic nerves and chiasm. The posterior communicating, internal carotid, and posterior cerebral arteries send branches into the area below and behind the chiasm. Recurrent arteries arise just distal to the anterior communicating artery.

The wide range of reported incidence of anomalies has resulted from a lack of uniform definition of hypoplasia and the different methods of selecting brains in previous studies. Alpers, et al., who defined hypoplasia the same as in this study, reported an incidence of anomalies approximating ours.3 Series based on brains from patients with cerebrovascular disease have a higher incidence of anomalies than series with randomly selected brains.2

Basilar Artery

The basilar artery, from its origin near the pontomedullary junction to termination near the pontomesencephalic junction, averaged 32.0 mm in length (range 15 to 40 mm). Diameter was remarkably constant except for widening at the bifurcation giving it a cobralike appearance in 16% of cases. Average diameter just below the superior cerebellar artery origin was 4.1 mm and between the superior cerebellar artery and P-1, 4.5 mm.

The posterior and lateral surfaces of the upper centimeter of the basilar artery were a rich source of perforating arteries (Fig. 1). An average of eight (range 3 to 18) branches of 0.1 to 0.5 mm diameter arose from the upper centimeter; approximately one-half arose from the posterior surface and one-fourth from each side. The more medial branches, called “median”7,38 or “paramedian”15 branches, enter the midbrain and pons near the midline, and the lateral ones, called “transverse”38 or “circumferential”15 branches, terminate in the lateral pons, peduncle, and posterior perforating substance. No perforating branches arose from the anterior surface of the basilar artery (Figs. 2 and 3). The upper paramedian arteries emerged 2 or 3 mm below the basilar bifurcation and intermixed with the medial P-1 branches forming a complex arterial plexus in the interpeduncular fossa (Fig. 4).

Fig. 2.
Fig. 2.

Inferior view of the circle of Willis. Fetal type of left posterior cerebral artery (P.C.A.). The left third nerve (III) courses under P-1, right under P-2. The left thalamoperforating (Th.Pe.A.) and the posterior choroidal (P.Ch.A.) arteries originate from P-1. The right premamillary artery (Premam. A.) emerges from the anterior and the left from the middle third of the posterior communicating artery (P.Co.A.). Small communicating branches course superiorly and medially and terminate in the premamillary area. The left thalamoperforating and right premamillary arteries are well developed in spite of the small trunk of origin. No branches arise on the anterior surface of the basilar artery (B.A.). Perforating branches arise from the anterior cerebral (A.C.A.) and anterior communicating arteries. C.A. = carotid artery; M.C.A. = middle cerebral artery; O.N. = optic nerve.

Fig. 3.
Fig. 3.

Inferior view of the circle of Willis with bilateral hypoplastic posterior communicating arteries (P.Co.A.). O.N. = optic nerve; O.Tr. = optic tract; A-1, A-2 = anterior cerebral artery. Upper: Well developed premamillary arteries (Premam. A.) arise from the communicating arteries, regardless of the hypoplastic trunk vessels. Right anterior choroidal artery (A.Ch.A.) arises from the carotid (C.A.) above and swings lateral to the communicating artery. Lower: The basilar artery (B.A.) bifurcates at the pontomesencephalic junction. The superior cerebellar arteries (S.C.A.) bifurcate on the level of the anterior pontine segment. Thalamoperforating branches (Th.Pe.A.) originate from P-1 near bifurcation. Well developed premamillary arteries originate from the posterior communicating arteries. No branches arise from the anterior surface of the basilar artery.

Fig. 4.
Fig. 4.

Superior view of the basilar bifurcation and midbrain. There are no perforators on the superior surface of the basilar bifurcation. The upper portion of the basilar artery (B.A.) is a rich source of paramedian branches to the interpeduncular fossa and pontine area. S.C.A. = superior cerebellar artery; P-1 = posterior cerebral artery.

The basilar bifurcation may be as far caudal as 1.3 mm below the pontomesencephalic junction (Fig. 3 lower) and as far rostral as the mamillary bodies. The artery bifurcated opposite the interpeduncular fossa in 88% and the upper pons in 10%. One bifurcation indented the mamillary bodies. The distance between the basilar bifurcation and mamillary bodies varied from 0 to 14 mm, the average being 8.1 mm. Stopford38 found that of 160 specimens studied, 156 bifurcated at the upper border of the pons, two just below this level, and two over 1 cm below.

Posterior Cerebral Artery P-1 Segment

The posterior cerebral segment between the basilar bifurcation and the posterior communicating artery, which we refer to as “P-1,” has previously been called the “precommunical,” “circular,” “mesencephalic,” or “P-1 segment,” and the “pars basalaris.”25

The P-1 diameter was relatively constant throughout its length. An average of four perforating branches (range 1 to 13), the largest with a diameter of 1.6 mm, arose mainly from the superior and posterior surfaces of the P-1, coursed superiorly and posteriorly, and divided into numerous branches that terminated, in descending order of frequency, in the posterior mesencephalon, interpeduncular fossa, cerebral peduncle, posterior perforating substance, and mamillary bodies (Fig. 5). No perforators arose from the anterior side of the basilar apex (Fig. 2), but one or two perforators arose from the anterior surface of 35 of 100 P-1 segments, and terminated in the posterior perforating substance and mamillary bodies. Stephens and Stilwell37 noted that at least one perforating branch commonly originated from the anterior surface of P-1.

Fig. 5.
Fig. 5.

Superior view of the area of basilar bifurcation and P-1. The posterior part of the optic chiasm is split at junction with optic tracts (O.Tr.) to give this view. A: Anterior cerebral arteries (A-1) pass above the chiasm; the anterior third ventricle lies above the mamillary bodies. The thalamoperforating arteries (Th.Pe.A.) terminate in the retromamillary area and interpeduncular fossa. The right posterior choroidal artery (P.Ch.A.) originates from the distal portion of P-1. Small branches arise from the posterior cerebral artery (P.C.A.) and terminate in the peduncle. The left third nerve (III) courses medial to the posterior communicating artery (P.Co.A.). The right posterior communicating joins P-1 medial to the third nerve. The left anterior choroidal artery (A.Ch.A.) passes to the peduncle and optic tract. B: The left superior cerebellar artery (S.C.A.) bifurcates just lateral to its origin. Both third nerves course below the posterior communicating artery. The right thalamoperforating, with its many branches, and posterior choroidal arteries arise from P-1. Perforating arteries originate from the superior surface of the left posterior communicating artery, course superiorly and terminate in the premamillary area, optic tract, and tuber cinereum. Both anterior choroidal arteries branch to the optic tract and peduncle. Both third nerves pass between the posterior cerebral and superior cerebellar arteries and course below the posterior communicating artery. C: Fetal type of right posterior cerebral origin. One trunk, arising from P-1, gives rise to both the thalamoperforating and posterior choroidal arteries on both sides. Both third nerves pass between the superior cerebellar and posterior cerebral arteries. D: Several thalamoperforating branches arise from the superior and posterior surfaces of P-1, creating a complicated plexus of vessels that terminate in the interpeduncular fossa. The largest branch is referred to as the thalamoperforating artery. The left posterior choroidal artery originates from the posterior cerebral artery just distal to the communicating artery. The fifth nerve lies below the posterior cerebral arteries bilaterally. The posterior communicating arteries pass medial to the third nerve on both sides. Ped = peduncle; S. Nigra = substantia nigra.

Several relatively constant branches arising from P-1 were: 1) the thalamoperforating artery which by definition enters the brain through the posterior perforated substance; 2) the medial posterior choroidal artery supplying the thalamus, tela choroidae of the third ventricle, and choroid plexus of the lateral ventricle, referred to here as the “posterior choroidal artery;” 3) the branch to the quadrigeminal plate called the “quadrigeminal artery” by Peele31 and Stephens and Stilwell;37 and 4) rami to the cerebral peduncle and mesencephalic tegmentum.

The thalamoperforating arteries consist of one or more arteries originating from P-1 and the posterior part of the posterior communicating artery that enter the brain by passing through the posterior perforating substance which is located behind the mamillary bodies and in the upper part of the interpeduncular fossa (Fig. 5). Most list the thalamoperforating artery as arising from P-1, but Peele31 agrees that it may arise from the posterior part of the communicating artery. These arteries have been called thalamoperforating arteries,37,40 posterior thalamoperforating arteries,17,41 and interpeduncular thalamoperforating arteries.28 When one P-1 branch entering this area predominates, it is referred to as the thalamoperforating artery (Fig. 5D). The thalamoperforating artery originated on the medial 1 mm of P-1 in 8% of the specimens (Fig. 1D), and on the lateral 1 mm in 5%, but the majority arose on the central part of P-1. The thalamoperforating arteries have been reported to terminate predominantly in the interpeduncular fossa,18,37 posterior perforating substance,17,28 or the retromamillary area.41 Their branches also terminated in the posterior hypothalamus and the medial portion of the upper midbrain.37

The P-1 branch arising nearest to the basilar bifurcation arose on an average of 2.2 mm (range 0 to 6.0 mm) from it, and an average of 4.9 mm (range 0.5 to 12.5 mm) from the junction of P-1 and the communicating artery. The branch nearest the bifurcation was the largest P-1 perforating artery in 56% of the 100 P-1 segments studied and in almost all cases it was a thalamoperforating artery. If the first branch was not a thalamoperforating artery, it was one which terminated in the peduncle or posterior mesencephalic area.

Four P-1 segments had no thalamoperforating branches (Fig. 1F), but those on the opposite side were well developed. Westberg41 and Percheron32 also noted that the thalamoperforating arteries may arise entirely from one side. Sixteen of 100 P-1 segments, five right and 11 left, had no branches to the posterior mesencephalic area which we defined as posterior choroidal arteries. In these cases the arteries to the posterior mesencephalic area arose from P-2 near its origin (Figs. 1D and 5D).

Occlusion of the thalamoperforating arteries is reported to cause choreoathetoid movements of the opposite extremities without significant loss of cutaneous sensibility.39 The classical thalamic syndrome is produced by occlusion of the thalamogeniculate arteries.17,38,39 The thalamogeniculates originate from the posterior cerebral artery distal to P-1 and terminate in the posterior portion of the lateral thalamic nuclear mass.17,37,39 The thalamoperforating arteries may provide useful clues in the angiographic diagnosis of thalamic and midbrain tumors.16,18,41

The largest P-1 branch was a thalamoperforating artery (42%), a posterior choroidal artery (40%), or a large trunk from which both arteries arose (18%) (Figs. 1A, C, E, and 5C). The largest P-1 perforator arose from the superior or posterior surface in 88% of the specimens (Figs. 3B and 5D) and from the anterior surface in 12%. It did not originate from the inferior surface in a single case. Common terminations of the largest P-1 perforating branch were the posterior mesencephalic area, the interpeduncular fossa and the posterior perforating substance.

Average diameter of the largest P-1 branch was 0.8 mm (range 0.3 to 1.6 mm). It arose an average of 3.3 mm from the basilar bifurcation (range 0.3 to 8.5 mm), and 3.6 mm (range 0 to 12.5 mm) from the communicating junction.

P-1 segments with the larger branches tended to have few perforating branches. One-third of all hemispheres had only one or two perforators arising from P-1, and they tended to be those with larger branches (Figs. 1A and 5C). If the largest P-1 branch was relatively small there tended to be more P-1 branches.

The average P-1 segment had one branch (range 0 to 7) located medial to its largest branch, and two (range 0 to 8) located lateral to it. More perforating vessels arose on P-1 lateral to the largest perforator in 58% of the specimens, more were medial in 26%, and an equal number of perforators were medial and lateral to the largest perforator in 16%.

Posterior Communicating Artery

The posterior communicating artery, which forms the lateral boundary of the circle of Willis, arises from the posteromedial surface of the internal carotid artery and sweeps backward and slightly medially above the third nerve to join the posterior cerebral artery. The diameter of the carotid origin was slightly larger than the P-1 junction, but the difference was as great as 1 mm in only one case.

Four to 12 branches (average seven) of diameter 0.1 to 0.6 mm arose along its course, mostly from the superior and lateral surfaces (Figs. 1, 2, 3, 5B and 6D). Previous studies report five to 12 branches.24,37 In 50% of cases, one or two perforators emerged from its medial side. Branch origins were distributed relatively evenly along the course of the artery. There was an average of four branches on the anterior and three on the posterior half. More branches arose on the anterior half of the communicating artery in 54%, on the posterior half in 25%, and the split between the two halves was equal in 21%.

Fig. 6.
Fig. 6.

Right anterior and lateral views of the suprasellar area. A: Right anterolateral view. Perforating arteries pass from the carotid to terminate in the optic chiasm and tract and the anterior hypothalamus. Right posterior communicating artery (P.Co.A.) is medial to the carotid artery (C.A.). Anterior choroidal artery (A.Ch.A.) arises from the carotid and passes above the posterior cerebral artery (P.C.A.). A-1 branches enter chiasm. Anterior communicating branches (A.Co.A.) pass to hypothalamus. The third nerve (III) lies below the right posterior cerebral artery. The right recurrent artery of Heubner (Rec.A.) arises from A-1. O.N. = optic nerve; M.C.A. = middle cerebral artery. B: Right lateral view with the temporal lobe removed. The tentorium is intact (Tent. Edge = medial tentorial edge). The third nerve courses lateral to the posterior communicating artery, passing between the posterior cerebral and superior cerebellar (S.C.A.) arteries. The middle cerebral artery ends as a blind stump above the carotid artery and optic tract (O.Tr.). Perforating branches pass from the anterior communicating artery to the hypothalamus. The third nerve enters dura lateral to the clivus. Anterior choroidal artery passes lateral to the communicating artery and above the posterior cerebral artery. C: Right lateral view with the tentorium removed. Note relationship of the anterior and posterior communicating arteries to the hypothalamus (Hypo.) and foramen of Monro (For.M.). Fetal type of posterior cerebral artery. Both communicating arteries send branches to the hypothalamus. The posterior communicating artery passes under the optic tract medial to the third nerve. Note the double anterior choroidal artery and the entrance of the third and fourth (IV) nerves into the cavernous sinus. D: Right lateral view. Premamillary (Premam.A.) arises from the posterior communicating artery. Thalamoperforating arteries (Th.Pe.A.) arise from P-1. The optic tract passes around the peduncle to the lateral geniculate. Arterioles pass from the carotid to the optic nerve. Basilar artery (B.A.) medial to third nerve. E: Lateral view. The lateral wall of right cavernous sinus (Cav.S.) is removed, exposing the third, fourth, fifth (V1, V2, V3), and sixth (VI) nerves and intracavernous carotid in the sinus. The anterior choroidal artery arises distal to the posterior communicating and gives branches to the optic tract and peduncle. The anterior communicating branches pass to the hypothalamus. Clin = anterior clinoid.

The branches coursed superiorly and medially to penetrate the brain, in descending order of frequency, in the tuber cinereum, posterior perforating substance, optic tract, peduncle, mamillary bodies, optic chiasm, and interpeduncular fossa (Fig. 5B). These penetrating arteries supply the posterior hypothalamus, anterior thalamus,12,37 posterior limb of the internal capsule,1,4,39 and subthalamus.24,39 The anterior group supplies the hypothalamus, ventral thalamus, anterior one-third of the optic tract, and posterior limb of the internal capsule; the posterior group supplies the posterior perforating substance and subthalamic nucleus. Obstruction of the branches to the subthalamic nucleus leads to contralateral hemiballism.39

In 80% of the specimens the largest communicating branch terminated in the area between the mamillary bodies and the optic tracts, called the “premamillary area.” The largest branch terminating in this area has been referred to as the “premamillary artery,”37 “anterior thalamoperforating artery,”41 or the “thalamotuberal artery.”15,19 We call it the “premamillary artery.” The vessel varied in diameter from 0.3 mm to 1.0 mm, the average being 0.6 mm. It originated on the middle one-third of the communicating artery in 50%, the anterior one-third in 17%, and the posterior one-third in 13% (Figs. 1 and 2). The premamillary artery has also been reported to arise on the posterior half of the communicating artery.24,37 The premamillary arteries supply the lateral and anterior portion of the thalamus,24,37,41, and the lateral19 and anterior17 hypothalamus. The premamillary artery may be helpful in the angiographic diagnosis of tumors in or near the third ventricle, thalamus, and hypothalamus.16,18,41

In the majority of the specimens, two or three communicating branches terminated in the premamillary area (Fig. 2), but in 20% no single artery dominated and several smaller arteries of nearly equal size supplied the area, in which case the arterial group was referred to as the “premamillary arterial complex” (Fig. 1D, E). This finding is in agreement with other studies noting that there may be more than one premamillary branch.15,17

Internal Carotid Artery

The supraclinoid portion of the carotid artery usually gives off the superior hypophyseal, ophthalmic, posterior communicating, and anterior choroidal arteries. This study yielded data on the portion of the internal carotid artery above the posterior communicating artery. The carotid artery was transected just below the communicating artery at autopsy and the branches below the communicating, including the superior hypophyseal and ophthalmic were often disrupted in brain removal. Two or three perforating arteries (range 0 to 6) excluding the anterior choroidal, usually arose from the carotid artery distal to the communicating artery (Figs. 1 A–F and 6A, D). These branches, 0.1 to 0.4 mm in diameter, terminated in the optic tract, chiasm and nerve, anterior hypothalamus, anterior perforating substance, and medial temporal lobe. The perforators terminating in the tuber cinereum form a fine net-like anastomosis with the perforators from the posterior communicating artery.24 This site around the pituitary stalk has been called the “circuminfundibular anastomosis.”12

Anterior Choroidal Artery

One hundred anterior choroidal arteries were studied from their carotid origin to the choroidal fissure. All arose from the carotid artery. The artery has been reported to arise from other arteries in up to 24% of cases, including the middle cerebral and posterior communicating arteries (Table 3).4,8,20,30 The racial difference in the origin of the artery has been reviewed because the work of Otomo30 on Japanese cadavers differed from that of Carpenter, et al.,8 and Herman, et al.,20 on non-oriental cadavers. Our result was similar to the findings of Beevor4 and Otomo.30 The diameter of the artery varied from 0.5 to 2.3 mm (average 1.0 mm) in our study as compared to 0.5 mm to 1.5 mm in previous studies.8,17,30

TABLE 3

Arteries of origin of the anterior choroidal artery (%)

Author, YearNo. Arteries ExaminedInternal Carotid ArteryBifurcation Internal CarotidMiddle Cerebral ArteryPosterior Communicating Artery 
Beevor (1907)174100 0 0 0  
Carpenter, et al. (1954)6076.6 3.3 11.7 6.7  
Otomo (1965)77899.2 0.4 0 0.4  
Herman, et al. (1966)7485 7 8 0  
Saeki & Rhoton (1977)100100 0 0 0  

The artery originated an average of 2.7 mm distal to the origin of the posterior communicating artery (range 1.0 to 5.0 mm) (Fig. 3A) and a few millimeters proximal to the internal carotid bifurcation.

The anterior choroidal artery was the first branch on the internal carotid distal to the posterior communicating in 67% of the specimens, the second in 15%, the third in 11%, and the fourth in 7%. Carpenter, et al.,8 found it to be the first branch after the communicating in 90% of their cases, the second branch in 4%, the third in 4%, and the fourth in 2%. Our examination with magnification revealed more carotid branches than Carpenter and his associates found with the naked eye.

Double anterior choroidal arteries were found in 4% of instances (Figs. 1C, D, and 6C). Their origins were of two types: One consisted of two separate arteries arising from the carotid (Figs. 1C and 6C) and the other arose from the carotid as a single artery but divided immediately into two trunks (Fig. 1D).

This artery supplies the following areas: 1) the temporal lobe: the uncus, pyriform cortex, and part of the amygdaloid nucleus; 2) the visual system: the optic tract, a portion of the lateral geniculate body, and the optic radiations; 3) the internal capsule and the basal ganglia: medial globus pallidus and caudate tail, and genu and posterior part of the capsule;17 4) the diencephalon: part of the lateral thalamic nuclear mass and subthalamus; and 5) the midbrain: the middle one-third of the cerebral peduncle and substantia nigra1,8,36 (Figs. 5A, B, and 6E).

Usually the anterior choroidal artery gives off one or two branches that terminate in the medial wall of the temporal lobe. If there is a double artery the more distal branch usually terminates in the temporal lobe, and the more proximal branch nourishes the remaining anterior choroidal field.

The classically reported clinical features of involvement of the anterior choroidal artery were contralateral hemiplegia, hemianesthesia, and hemianopsia.1 However, the surgical occlusion of this artery for the treatment of parkinsonism may not cause a deficit.11,34 The inconsistent results of the occlusion of this artery are explained by the anastomoses with the posterior communicating artery on the optic tract and peduncle and the posterior choroidal artery on the lateral geniculate body and the choroid plexus.8,36

Superior Cerebellar Artery

The superior cerebellar artery arose from the basilar artery at a level between the P-1 origin and 7 mm below. Average separation between the P-1 and superior cerebellar artery origin was 2.5 mm. Two superior cerebellar arteries arose at the level of the P-1 origin. Mani, et al.,27 reported that the superior cerebellar artery arose from P-1 on one or both sides in 4% of cases.

Its course around the brain stem is divided into three segments: 1) the anterior pontine segment between the pons and clivus; 2) the ambient segment lying lateral to the brain stem; and 3) the quadrigeminal segment within the quadrigeminal cistern. The anterior pontine and ambient segments, which were the subject of this study, gave origin to an average of four perforating vessels (range 1 to 12) which terminated in the medial and lateral brain stem, respectively. Most arose from the anterior pontine segment.

The superior cerebellar artery divides into two main branches: a lateral or marginal and a medial (Figs. 3B and 5B), an average of 18.8 mm (range 0.5 to 35 mm) from its origin. The medial branch courses superior to the marginal. The diameter of the two branches was nearly equal (average 1.2 mm).

The two branches arose independently from the basilar artery, a phenomenon referred to as duplication,21,27 unilaterally in five brains (Fig. 1A), and bilaterally in one (Fig. 1B). Others found this variation in from 3% to 28% unilaterally and 1% to 8% bilaterally.5,27,36 Mani, et al.,27 stated that in eight of 100 brains with duplication, the upper trunk arose from the posterior cerebral artery.

Third and Fourth Cranial Nerves

The relationship between the third and fourth cranial nerves and the posterior cerebral and superior cerebellar arteries was constant in all cases; the third nerve consistently passed between the posterior cerebral and superior cerebellar arteries near their origin, and the fourth nerve passed between the two on the lateral margin of the brain stem (Figs. 5B, C, 6 and 7). The relationship was unaltered even when the superior cerebellar origin was duplicated. When the superior cerebellar artery arose as a double trunk the nerves passed between the superior trunk of the superior cerebellar artery and the posterior cerebral artery (Fig. 1A, B).

Fig. 7.
Fig. 7.

Right lateral view of the anterior and middle fossa with the tentorium and lateral wall of cavernous sinus (Cav.S.) removed. The third (III) and fourth (IV) nerves course between the posterior cerebral (P.C.A.) and superior cerebellar (S.C.A.) arteries, lateral to the posterior communicating and anterior choroidal (A.Ch.A.) arteries. The olfactory nerve is in the anterior fossa above the optic nerve (O.N.). The optic nerve continues posteriorly as the optic tract (O.Tr.), which runs above the posterior part of the circle of Willis. The third nerve penetrates the dura lateral to the dorsum. The third, fourth, and fifth (V1, V2, V3) nerves pass forward in the lateral wall of the cavernous sinus. The anterior choroidal artery passes below the optic tract, and the basilar artery (B.A.) passes between the peduncle (Ped.) and dorsum. Clin = anterior clinoid; Gr.P.N. = greater petrosal nerve; Pe.A. = perforating artery; V.Ga. = Gasserian ganglion.

Discussion

Approximately 15% of saccular aneurysms occur in the vertebrobasilar system; the majority (63%) occur at the basilar bifurcation.6 The incidence of anomalies consisting of either a hypoplastic communicating or a fetal posterior cerebral origin is more common with aneurysms than in normal groups.3,9,36

Transection of a hypoplastic posterior communicating artery or P-1 segment has been recommended to gain access to basilar bifurcation aneurysms10,13,14,23,42 on the assumption that they have fewer branches and the brain is less dependent upon them. However, in our study the number and diameter of perforating branches were relatively constant, regardless of the trunk size (Figs. 2, 3, and Table 4); therefore, a hypoplastic posterior communicating artery or P-1 segment supplied the same perforating area as a larger vessel despite its smaller size. The importance of preservation of these vessels deserves emphasis because of the important role of a hypoplastic vessel in supply of the local area. If hypoplastic segments are divided, care should be taken not to sacrifice any small perforators. Jamieson23 wisely suggested that an effort be made to spare these anastomotic channels. Yaşargil, et al.,42 also emphasized the importance of not occluding branches as the clips are placed for division of the communicating artery.

TABLE 4

Comparison of number and diameter of P-1 and posterior communicating branches on circles with a hypoplastic posterior communicating artery or a fetal posterior cerebral origin in 50 brains

FactorHypoplastic PCoA*Fetal ArrangementTotal Series
posterior communicating artery
 diameter
  average0.62.61.3
  range0.3–0.91.9–3.10.3–3.1
 no. of perforating branches
  average7.77.47.0
  range5–126–94–12
 diameter of largest branch
  average0.50.50.6
  range0.3–0.60.3–0.90.3–1.1
P-1 segment
 diameter
  average2.91.42.6
  range2.2–4.00.9–2.50.9–4.0
 no. of perforators
  average4.15.24.1
  range1–132–81–13
 diameter of largest perforator
  average0.70.80.8
  range0.4–1.60.3–1.30.3–1.6
 no. of arteries1912100

PCoA = posterior communicating artery.

Preoperative grading of the patient's clinical state has proved helpful in determining the prognosis with aneurysms.10,14,42 The patient with basilar bifurcation aneurysms has been graded more gravely than the patient with aneurysms in other areas because of the greater tendency of vital perforators to be involved in aneurysm dissection and clipping. In basilar bifurcation aneurysms, Drake14 and Laine26 have found the more posterior the aneurysm the poorer the prognosis, because the tendency for vital perforators to be involved becomes greater as the aneurysm projects more posteriorly. This fits with our finding that no perforators were located on the anterior surface of the basilar bifurcation and thus surgical results are better with anterior aneurysms. The rich plexus on the posterior basilar surface 2 to 3 mm below the bifurcation entering the interpeduncular fossa and terminating in the medial midbrain makes this the most dangerous site. The basilar apex is intermediate in risk. The thalamoperforating artery and other branches arising in this area are easier to identify at surgery and are fewer in number than those on the posterior basilar artery.

Drake13 stated that (in the subtemporal approach for the basilar aneurysm) the neck of the aneurysm of the bifurcation was found by following the inferior side of the posterior cerebral artery medially as it curved around the peduncle. In our study the inferior surface of P-1 was the most infrequent site of origin for perforators, making it the safest approach to the basilar bifurcation.

Yaşargil, et al.,42 noted that some apical aneurysms may be approached through the interval between the optic nerve and carotid, if the space is sufficiently wide and the aneurysm projects superiorly or anteriorly. If this approach is used, care should be taken to preserve the vital perforating branches arising on the carotid artery and crossing this interval to supply the optic nerve and tract and diencephalon.

Previous considerations of complications in pituitary surgery have dealt with vascular complications mainly in terms of carotid artery injury implying that psychoses, visual impairment, disturbances of consciousness, and other complications were caused by direct neural injury.22 Deaths due to carotid hemorrhage during chromophobe adenomal removal, and to circulatory embarrassment following clipping of the carotid artery have been reported.22 Occlusion of the perforating branches reviewed here has been neglected in discussion about complications in pituitary surgery. The arterial branches reviewed in this study and which would be stretched around the margin of suprasellar tumors have the potential, when occluded, to cause personality disorders, memory disturbances, extraocular palsies, visual loss, and altered states of consciousness (Fig. 1G).

Millikan29 recently pointed out the poor correlation between the angiographically demonstrable spasm following subarachnoid hemorrhage and the occurrence of a “spasm syndrome” usually manifested by depressed level of consciousness, autonomic disturbances, and varying types and degrees of focal deficits. These inconsistencies are most likely explained by the fact that angiography provides information about spasm in the larger arteries but not about the perforating arteries discussed here, which, if constricted, could cause all of the clinical manifestations of the spasm syndrome. These small branches are only inconstantly visualized at angiography and it seems likely that better correlation will await the wider use of more exacting angiography that will allow accurate deductions about their physiological state.

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    Carpenter MBNoback CRMoss ML: The anterior choroidal artery. Its origin, course, distribution and variations. Arch Neurol Psychiatry 71:7147221954Arch Neurol Psychiatry 71:

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    Chason JLHindman WM: Berry aneurysms of the circle of Willis. Results of a planned autopsy study. Neurology (Minneap) 8:41441958Neurology (Minneap) 8:

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    Chou SNOrtiz-Suarez HJ: Surgical treatment of arterial aneurysms of the vertebrobasilar circulation. J Neurosurg 41:6716801974J Neurosurg 41:

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    Dawson BH: The blood vessels of the human optic chiasma and their relation to those of the hypophysis and hypothalamus. Brain 81:2072171958Dawson BH: The blood vessels of the human optic chiasma and their relation to those of the hypophysis and hypothalamus. Brain 81:

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    Drake CG: Bleeding aneurysms of the basilar artery. Direct surgical management in four cases. J Neurosurg 18:2302381961Drake CG: Bleeding aneurysms of the basilar artery. Direct surgical management in four cases. J Neurosurg 18:

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This paper was supported in part by the Heart Association of Broward County and National Institutes of Health Grant No. NS 10978-02.

Article Information

Address reprint requests to: Albert L. Rhoton, Jr., M.D., Division of Neurological Surgery, Box J-265 Health Center, University of Florida, Gainesville, Florida 32610.

© AANS, except where prohibited by US copyright law."

Headings

Figures

  • View in gallery

    Superior view of the basilar (B.A.), superior cerebellar (S.C.A.), P-1, and distal segments of posterior cerebral (P.C.A.), posterior communicating (P.Co.A.), internal carotid, and proximal anterior choroidal (A.Ch.A.) arteries. The arterial branches below the posterior perforating substance (P. Perf. S.), mamillary bodies (Mam. B.), optic tracts (O.Tr.), chiasm, and nerves (O.N.) are shown in half tone. The third and fourth nerves (III and IV) course between the superior cerebellar and posterior cerebral arteries. Arterial branches to the upper pons, posterior mesencephalon, interpeduncular fossa, posterior perforating substance, mamillary bodies, tuber cinereum, optic tracts and chiasm arise from the basilar, P-1, posterior communicating and internal carotid arteries. A.C.A. = anterior cerebral artery; C.A. = carotid artery; P.Ch.A. = posterior choroidal artery; Premam. A. = premamillary artery; Th.Pe.A. = Thalamoperforating artery. A: Normal configuration of the posterior half of the circle of Willis; both P-1's are larger than communicating arteries and the latter are not hypoplastic (diameter greater than 1 mm). The right superior cerebellar artery is duplicated. The largest right P-1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. Only two perforating arteries arise on right P-1. The left posterior choroidal arises on P-2. Both premamillary arteries (largest communicating trunk to premamillary area) arise from the middle one-third of the posterior communicating arteries. Anterior choroidal arteries arise as a single trunk. B: Hypoplastic left communicating artery. Thalamoperforating artery arises on P-1 medial to the posterior choroidal on both sides. The left premamillary artery arises from the posterior and the right from the anterior portion of the posterior communicating artery. The superior cerebellar arteries are duplicated on both sides. C: Posterior communicating arteries are hypoplastic bilaterally. The largest right P-1 branch gives rise to both the thalamoperforating and the posterior choroidal arteries. The thalamoperforating artery arises medial to the posterior choroidal artery on the left P-1. The premamillary artery arises from the anterior one-third of the right posterior communicating artery and from the middle third on the left. The left anterior choroidal arises from the carotid as two trunks.

  • View in gallery

    (continued)D: Fetal origin of the right posterior cerebral artery. The thalamoperforating artery on the right arises near the basilar bifurcation. The right posterior choroidal artery arises on P-2. The left posterior choroidal artery arises medial to the thalamoperforating artery. The right premamillary artery arises from the anterior portion of the communicating artery. The left premamillary area is supplied by a group of nearly equal-sized arteries. The anterior choroidal artery bifurcates immediately after origin on the left. E: Bilateral fetal origin of the posterior cerebral artery. The right posterior choroidal artery arises lateral to the thalamoperforating artery. The largest P-1 branch gives rise to the left thalamoperforating and choroidal arteries. The right premamillary artery arises from the middle portion of the communicating artery. A premamillary arterial complex is present on the left. F: Fetal type of right posterior cerebral origin and hypoplastic left communicating artery. The right posterior choroidal arises lateral to the well developed thalamoperforating artery. No thalamoperforating branches are present on the left. The right premamillary artery arises from the anterior and the left from the posterior portion of the communicating artery. G: Superior view of the suprasellar area. Arterial branches stretched around the superior extension of a pituitary tumor. Anterior cerebral arteries send branches to the superior surface of the optic nerves and chiasm. The posterior communicating, internal carotid, and posterior cerebral arteries send branches into the area below and behind the chiasm. Recurrent arteries arise just distal to the anterior communicating artery.

  • View in gallery

    Inferior view of the circle of Willis. Fetal type of left posterior cerebral artery (P.C.A.). The left third nerve (III) courses under P-1, right under P-2. The left thalamoperforating (Th.Pe.A.) and the posterior choroidal (P.Ch.A.) arteries originate from P-1. The right premamillary artery (Premam. A.) emerges from the anterior and the left from the middle third of the posterior communicating artery (P.Co.A.). Small communicating branches course superiorly and medially and terminate in the premamillary area. The left thalamoperforating and right premamillary arteries are well developed in spite of the small trunk of origin. No branches arise on the anterior surface of the basilar artery (B.A.). Perforating branches arise from the anterior cerebral (A.C.A.) and anterior communicating arteries. C.A. = carotid artery; M.C.A. = middle cerebral artery; O.N. = optic nerve.

  • View in gallery

    Inferior view of the circle of Willis with bilateral hypoplastic posterior communicating arteries (P.Co.A.). O.N. = optic nerve; O.Tr. = optic tract; A-1, A-2 = anterior cerebral artery. Upper: Well developed premamillary arteries (Premam. A.) arise from the communicating arteries, regardless of the hypoplastic trunk vessels. Right anterior choroidal artery (A.Ch.A.) arises from the carotid (C.A.) above and swings lateral to the communicating artery. Lower: The basilar artery (B.A.) bifurcates at the pontomesencephalic junction. The superior cerebellar arteries (S.C.A.) bifurcate on the level of the anterior pontine segment. Thalamoperforating branches (Th.Pe.A.) originate from P-1 near bifurcation. Well developed premamillary arteries originate from the posterior communicating arteries. No branches arise from the anterior surface of the basilar artery.

  • View in gallery

    Superior view of the basilar bifurcation and midbrain. There are no perforators on the superior surface of the basilar bifurcation. The upper portion of the basilar artery (B.A.) is a rich source of paramedian branches to the interpeduncular fossa and pontine area. S.C.A. = superior cerebellar artery; P-1 = posterior cerebral artery.

  • View in gallery

    Superior view of the area of basilar bifurcation and P-1. The posterior part of the optic chiasm is split at junction with optic tracts (O.Tr.) to give this view. A: Anterior cerebral arteries (A-1) pass above the chiasm; the anterior third ventricle lies above the mamillary bodies. The thalamoperforating arteries (Th.Pe.A.) terminate in the retromamillary area and interpeduncular fossa. The right posterior choroidal artery (P.Ch.A.) originates from the distal portion of P-1. Small branches arise from the posterior cerebral artery (P.C.A.) and terminate in the peduncle. The left third nerve (III) courses medial to the posterior communicating artery (P.Co.A.). The right posterior communicating joins P-1 medial to the third nerve. The left anterior choroidal artery (A.Ch.A.) passes to the peduncle and optic tract. B: The left superior cerebellar artery (S.C.A.) bifurcates just lateral to its origin. Both third nerves course below the posterior communicating artery. The right thalamoperforating, with its many branches, and posterior choroidal arteries arise from P-1. Perforating arteries originate from the superior surface of the left posterior communicating artery, course superiorly and terminate in the premamillary area, optic tract, and tuber cinereum. Both anterior choroidal arteries branch to the optic tract and peduncle. Both third nerves pass between the posterior cerebral and superior cerebellar arteries and course below the posterior communicating artery. C: Fetal type of right posterior cerebral origin. One trunk, arising from P-1, gives rise to both the thalamoperforating and posterior choroidal arteries on both sides. Both third nerves pass between the superior cerebellar and posterior cerebral arteries. D: Several thalamoperforating branches arise from the superior and posterior surfaces of P-1, creating a complicated plexus of vessels that terminate in the interpeduncular fossa. The largest branch is referred to as the thalamoperforating artery. The left posterior choroidal artery originates from the posterior cerebral artery just distal to the communicating artery. The fifth nerve lies below the posterior cerebral arteries bilaterally. The posterior communicating arteries pass medial to the third nerve on both sides. Ped = peduncle; S. Nigra = substantia nigra.

  • View in gallery

    Right anterior and lateral views of the suprasellar area. A: Right anterolateral view. Perforating arteries pass from the carotid to terminate in the optic chiasm and tract and the anterior hypothalamus. Right posterior communicating artery (P.Co.A.) is medial to the carotid artery (C.A.). Anterior choroidal artery (A.Ch.A.) arises from the carotid and passes above the posterior cerebral artery (P.C.A.). A-1 branches enter chiasm. Anterior communicating branches (A.Co.A.) pass to hypothalamus. The third nerve (III) lies below the right posterior cerebral artery. The right recurrent artery of Heubner (Rec.A.) arises from A-1. O.N. = optic nerve; M.C.A. = middle cerebral artery. B: Right lateral view with the temporal lobe removed. The tentorium is intact (Tent. Edge = medial tentorial edge). The third nerve courses lateral to the posterior communicating artery, passing between the posterior cerebral and superior cerebellar (S.C.A.) arteries. The middle cerebral artery ends as a blind stump above the carotid artery and optic tract (O.Tr.). Perforating branches pass from the anterior communicating artery to the hypothalamus. The third nerve enters dura lateral to the clivus. Anterior choroidal artery passes lateral to the communicating artery and above the posterior cerebral artery. C: Right lateral view with the tentorium removed. Note relationship of the anterior and posterior communicating arteries to the hypothalamus (Hypo.) and foramen of Monro (For.M.). Fetal type of posterior cerebral artery. Both communicating arteries send branches to the hypothalamus. The posterior communicating artery passes under the optic tract medial to the third nerve. Note the double anterior choroidal artery and the entrance of the third and fourth (IV) nerves into the cavernous sinus. D: Right lateral view. Premamillary (Premam.A.) arises from the posterior communicating artery. Thalamoperforating arteries (Th.Pe.A.) arise from P-1. The optic tract passes around the peduncle to the lateral geniculate. Arterioles pass from the carotid to the optic nerve. Basilar artery (B.A.) medial to third nerve. E: Lateral view. The lateral wall of right cavernous sinus (Cav.S.) is removed, exposing the third, fourth, fifth (V1, V2, V3), and sixth (VI) nerves and intracavernous carotid in the sinus. The anterior choroidal artery arises distal to the posterior communicating and gives branches to the optic tract and peduncle. The anterior communicating branches pass to the hypothalamus. Clin = anterior clinoid.

  • View in gallery

    Right lateral view of the anterior and middle fossa with the tentorium and lateral wall of cavernous sinus (Cav.S.) removed. The third (III) and fourth (IV) nerves course between the posterior cerebral (P.C.A.) and superior cerebellar (S.C.A.) arteries, lateral to the posterior communicating and anterior choroidal (A.Ch.A.) arteries. The olfactory nerve is in the anterior fossa above the optic nerve (O.N.). The optic nerve continues posteriorly as the optic tract (O.Tr.), which runs above the posterior part of the circle of Willis. The third nerve penetrates the dura lateral to the dorsum. The third, fourth, and fifth (V1, V2, V3) nerves pass forward in the lateral wall of the cavernous sinus. The anterior choroidal artery passes below the optic tract, and the basilar artery (B.A.) passes between the peduncle (Ped.) and dorsum. Clin = anterior clinoid; Gr.P.N. = greater petrosal nerve; Pe.A. = perforating artery; V.Ga. = Gasserian ganglion.

References

1.

Abbie AA: The clinical significance of the anterior choroidal artery. Brain 56:2332461933Abbie AA: The clinical significance of the anterior choroidal artery. Brain 56:

2.

Alpers BJBerry RG: Circle of Willis in cerebral vascular disorders. The anatomical structure. Arch Neurol 8:3984021963Arch Neurol 8:

3.

Alpers BJBerry RGPaddison RM: Anatomical studies of the circle of Willis in normal brain. Arch Neurol Psychiatry 81:4094181959Arch Neurol Psychiatry 81:

4.

Beevor CE: The cerebral arterial supply. Brain 30:4034251907Beevor CE: The cerebral arterial supply. Brain 30:

5.

Blackburn IW: Anomalies of the encephalic arteries among the insane. A study of the arteries at the base of the encephalon in two hundred and twenty consecutive cases of mental disease, with special reference to anomalies of the circle of Willis. J Comp Neurol 17:4935171907J Comp Neurol 17:

6.

Bull JWD: Contribution of radiology to the study of intracranial aneurysms. Br Med J 2:170117081962Bull JWD: Contribution of radiology to the study of intracranial aneurysms. Br Med J 2:

7.

Busch W: Beitrag zur Morphologie und Pathologie der Arteria basialis (Untersuchungsergebnisse bei 1000 Gehirnen). Arch Psychiatr Nervenkr 208:3263441966Busch W: Beitrag zur Morphologie und Pathologie der Arteria basialis (Untersuchungsergebnisse bei 1000 Gehirnen). Arch Psychiatr Nervenkr 208:

8.

Carpenter MBNoback CRMoss ML: The anterior choroidal artery. Its origin, course, distribution and variations. Arch Neurol Psychiatry 71:7147221954Arch Neurol Psychiatry 71:

9.

Chason JLHindman WM: Berry aneurysms of the circle of Willis. Results of a planned autopsy study. Neurology (Minneap) 8:41441958Neurology (Minneap) 8:

10.

Chou SNOrtiz-Suarez HJ: Surgical treatment of arterial aneurysms of the vertebrobasilar circulation. J Neurosurg 41:6716801974J Neurosurg 41:

11.

Cooper IS: Surgical occlusion of the anterior choroidal artery in parkinsonism. Surg Gynec Obstet 99:2072191954Cooper IS: Surgical occlusion of the anterior choroidal artery in parkinsonism. Surg Gynec Obstet 99:

12.

Dawson BH: The blood vessels of the human optic chiasma and their relation to those of the hypophysis and hypothalamus. Brain 81:2072171958Dawson BH: The blood vessels of the human optic chiasma and their relation to those of the hypophysis and hypothalamus. Brain 81:

13.

Drake CG: Bleeding aneurysms of the basilar artery. Direct surgical management in four cases. J Neurosurg 18:2302381961Drake CG: Bleeding aneurysms of the basilar artery. Direct surgical management in four cases. J Neurosurg 18:

14.

Drake CG: Further experience with surgical treatment of aneurysms of the basilar artery. J Neurosurg 29:3723921968Drake CG: Further experience with surgical treatment of aneurysms of the basilar artery. J Neurosurg 29:

15.

Foix CHillemand P: Les arteres de l'axe encéphalique jusqu' au diencéphale inclusivement. Rev Neurol (Part 2) 32:7057391925Rev Neurol (Part 2) 32:

16.

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