eye movements to the opposite direction Medial surface (Figure 5-2) CT The cingulate sul- prcs CENTRAL SULCUS cus terminates posteri- PL orly in the pars SFG pM marginalis (pM) (plu- cins sps ral: partes marginales). cins CinG PCu On axial imaging, the MRI pMs: are visible on 95% s callosu rpu m pos of CTs and 91% of co MRIs4, are usually the most prominent of the paired grooves strad- Cu dling the midline, and LG they extend a greater distance into the pons hemispheres4. On axial CT, the pM is located slightly posterior to the widest biparietal diameter4; on the typi- cally more horizontally Figure 5-2 Medial aspect of the right hemisphere “CT” & “MRI” bars depict typical axial slice orientation for CT & MRI scans. oriented MRI slices the See Table 5-1 and Table 5-2 for abbreviations pM assumes a more posterior position. The pMs curve posteriorly in lower slices and anteriorly in higher slices (here, the paired pMs form the “pars bracket” - a characteristic “handlebar” configuration straddling the mid- line). Central sulcus on axial imaging See Figure 5-3. Identification is important to localize the motor strip (contained in the PreCG). The central sulcus (CS) is visible on 93% of CTs and 100% of MRIs4. It curves posterior- ly as it approaches the interhemispher- ic fissure (IHF), and often terminates in the paracentral lobule, just anterior SFG cs to the pars marginalis (pM) within the cs pars bracket (see above)4 (i.e. the CS of- PreCG ten does not reach the midline). PL PostCG Pointers: pocs • parieto-occipital sulcus (pos) (or fissure): more prominent over the medial surface, and on axial imag- ing is longer, more complex, and more posterior than the pars pM marginalis5 • post-central sulcus (pocs): usually bifurcates and forms an arc or pa- renthesis (“lazy-Y”) cupping the pM. The anterior limb does not en- ter the pM-bracket and the posteri- Figure 5-3 CT scan (upper cut) showing gyri/sulci. or limb curves behind the pM to See Table 5-1 and Table 5-2 for abbreviations enter the IHF

NEUROSURGERY 5.1. Surface anatomy 85 Table 5-1 Cerebral sulci Table 5-2 Cerebral gyri and lobules (abbreviations) (abbreviations) cins cingulate sulcus AG angular gyrus cs central sulcus CinG cingulate gyrus ips-ios intraparietal-intraoccip- Cu cuneus ital sulcus LG lingual gyrus los lateral occipital sulcus MFG, SFG middle & superior frontal gyrus pM pars marginalis OG orbital gyrus pocn pre-occipital notch PCu precuneous pocs post-central sulcus PreCG, PostCG pre- and post-central gyrus pof parieto-occipital fissure PL paracentral lobule (upper SFG and PreCG pos parieto-occipital sulcus and PostCG) prcs pre-central sulcus IFG inferior frontal gyrus POp pars opercularis sfs, ifs superior, inferior frontal PT pars triangularis sulcus POr pars orbitalis sps superior parietal sulcus sts, its superior, inferior tem- STG, MTG, ITG superior, middle & inferior temporal gyrus poral sulcus SPL, IPL superior & inferior parietal lobule tos trans occipital sulcus SMG supramarginal gyrus

5.1.2. Surface anatomy of the cranium

CRANIOMETRIC POINTS See Figure 5-4. Pterion: region vertex where the following bones are approximat- bregma P ed: frontal, parietal, ARIET L AL temporal and sphenoid stephanion TA ON (greater wing). Esti- pterion FR mated as 2 finger- cs stl breadths above the zy- ophyron sqs lambda gomatic arch, and a glabella thumb’s breadth be- nasion hind the frontal pro- S TEMPORAL rhinion ls W L cess of the zygomatic A G pms IT bone (blue circle in Fig- sms IP C ure 5-4). C OID O Asterion: junc- NASAL G T ZY AS oms tion of lambdoid, occip- M prosthion inion itomastoid and MAXILLA parietomastoid su- asterion tures. Usually lies inferior alveolar point opisthion within a few millime- ters of the posterior-in- gnathion BLE gonion ferior edge of the or menton MANDI junction of the trans- verse and sigmoid si- Figure 5-4 Craniometric points & cranial sutures. nuses (not always Named bones appear in all upper case letters. reliable6 - may overlie Abbreviations: GWS = greater wing of sphenoid bone, NAS = nasal bone, stl = either sinus). superior temporal line, ZYG = zygomatic. Vertex: the top- Sutures: cs = coronal, ls = lambdoid, oms = occipitomastoid, pms = parietomas- most point of the skull. toid, sms = squamomastoid, sqs = squamosal Lambda: junc- tion of the lambdoid and sagittal sutures. Stephanion: junction of coronal suture and superior temporal line. 86 5. Neuroanatomy and physiology NEUROSURGERY Glabella: the most forward projecting point of the forehead at the level of the su- praorbital ridge in the midline. Opisthion: the posterior margin of the in the midline. Bregma: the junction of the coronal and sagittal sutures. Sagittal suture: midline suture from coronal suture to lambdoid suture. Although often assumed to overlie the superior sagittal sinus (SSS), the SSS lies to the right of the sagittal suture in the majority of specimens7 (but never by > 11 mm). The most anterior mastoid point lies just in front of the sigmoid sinus8.

RELATION OF SKULL MARKINGS TO CEREBRAL ANATOMY Taylor-Haughton lines Taylor-Haughton (T-H) 2 cm lines can be constructed on an 1/2 angiogram, CT scout film, or skull x-ray, and can then be re- s u c constructed on the patient in the l u O.R. based on visible external s 9 l 3/4 landmarks . T-H lines are a r t shown as dashed lines in Figure n e 5-5. c 1. Frankfurt plane, AKA re ©2001 Mark S Greenberg, M.D. baseline: line from inferi- su All rights reserved. fis Unauthorized use is prohibited. or margin of orbit ian ylv through the upper mar- s gin of the external audi- tory meatus (EAM) (as distinguished from Re- EAM id’s base line: from infe- Frankfurt rior orbital margin plane through the center of the EAM)10 (p 313) posterior ear line 2. the distance from the na- condylar line sion to the inion is mea- sured across the top of the calvaria and is divid- ed into quarters (can be Figure 5-5 Taylor-Haughton lines done simply with a piece and other localizing methods of tape which is then folded in half twice) 3. posterior ear line: perpendicular to the baseline through the mastoid process 4. condylar line: perpendicular to the baseline through the mandibular condyle 5. T-H lines can then be used to approximate the sylvian fissure (see below) and the motor cortex (also see below) Sylvian fissure AKA lateral fissure Approximated by a line connecting the lateral canthus to the point 3/4 of the way posterior along the arc running over convexity from nasion to inion (T-H lines). Angular gyrus Located just above the pinna, important on the dominant hemisphere as part of Wernicke’s area. Note: there is significant individual variability in the location2. Angular Located 6 cm above the EAM. Motor cortex Numerous methods utilize external landmarks to locate the motor strip (pre-central gyrus) or the central sulcus (Rolandic fissure) which separates motor strip anteriorly from primary sensory cortex posteriorly. These are just approximations since individual variability causes the motor strip to lie anywhere from 4 to 5.4 cm behind the coronal suture11. The central sulcus cannot even be reliably identified visually at surgery12. • method 1: the superior aspect of the motor cortex is almost straight up from the EAM near the midline • method 213: the central sulcus is approximated by connecting:

NEUROSURGERY 5.1. Surface anatomy 87 A. the point 2 cm posterior to the midposition of the arc extending from nasion to inion (illustrated in Figure 5-5), to B. the point 5 cm straight up from the EAM • method 3: using T-H lines, the central sulcus is approximated by connecting: A. the point where the “posterior ear line” intersects the circumference of the skull (see Figure 5-5) (usually about 1 cm behind the vertex, and 3-4 cm be- hind the coronal suture), to B. the point where the “condylar line” in- tersects the line representing the sylvian fissure cs • method 4: a line drawn 45° to Reid’s base line B starting at the pterion D1 FM F A points in the direction of V3 O the motor strip14 (p 584-5) Aq T Twining D2 V4 D3 RELATIONSHIP OF VENTRICLES D4 TO SKULL Figure 5-6 shows the rela- tionship of non-hydrocephalic ventricles to the skull in the lat- opisthion eral view. Some dimensions of in- baseline terest are shown in Table 5-315. sigmoid sinus In the non-hydrocephalic sella turcica adult, the lateral ventricles lie 4- 5 cm below the outer skull sur- face. The center of the body of the Figure 5-6 Relationship of ventricles to skull landmarks* lateral ventricle sits in the midp- upillary line, and the frontal * Abbreviations: (F = frontal horn, B = body, A = atrium, O = oc- horn is intersected by a line pass- cipital horn, T = temporal horn) of lateral ventricle. FM = fora- men of Monro. Aq = sylvian aqueduct. V3 = third ventricle. V4 ing perpendicular to the calvaria → along this line16. The anterior = fourth ventricle. cs = coronal suture. Dimensions D1-4 horns extend 1-2 cm anterior to see Table 5-3 the coronal suture. Average length of third ventricle ≈ 2.8 cm. Table 5-3 Dimensions from Figure 5-6 Dimension Description Lower limit Average Upper limit (see Figure 5-6) (mm) (mm) (mm) D1 length of frontal horn anterior to FM 25 D2 distance from clivus to floor of 4th ventricle at 33.3 36.1 40.0 level of fastigium* D3 length of 4th ventricle at level of fastigium* 10.0 14.6 19.0 D4 distance from fastigium* to opisthion 30.0 32.6 40.0 * the fastigium is the apex of the 4th ventricle within the cerebellum

88 5. Neuroanatomy and physiology NEUROSURGERY 5.1.3. Surface landmarks of spine levels

Estimates of cervical levels for anterior cervical Table 5-4 Cervical levels17 spine surgery may be made using the landmarks shown in Table 5-4. Intra-operative C-spine x-rays are essential Level Landmark to verify these estimates. C1-2 angle of mandible The scapular spine is located at about T2-3. ≈ C3-4 1 cm above thyroid carti- The inferior scapular pole is T6 posteriorly. lage (≈ hyoid bone) Intercristal line: a line drawn between the high- est point of the iliac crests across the back will cross the C4-5 level of thyroid cartilage midline either at the interspace between the L4 and L5 C5-6 crico-thyroid membrane spinous processes, or at the L4 spinous process itself. C6 carotid tubercle C6-7 cricoid cartilage

5.2. Cranial foramina & their contents

Table 5-5 Cranial foramina and their contents* Foramen Contents nasal slits anterior ethmoidal nn., a. & v superior orbital fissure Cr. Nn. III, IV, VI, all 3 branches of V1 (ophthalmic division divides into nasociliary, fron- tal, and lacrimal nerves); superior ophthalmic vv.; recurrent meningeal br. from lacrimal a.; orbital branch of middle meningeal a.; sympathetic filaments from ICA plexus inferior orbital fissure Cr. N. V-2 (maxillary div.), zygomatic n.; filaments from pterygopalatine branch of max- illary n.; infraorbital a. & v.; v. between inferior ophthalmic v. & pterygoid venous plexus foramen lacerum usually nothing (ICA traverses the upper portion but doesn’t enter, 30% have vidian a.) carotid canal internal carotid a., ascending sympathetic nerves incisive foramen descending septal a.; nasopalatine nn. greater palatine foramen greater palatine n., a., & v. lesser palatine foramen lesser palatine nn. internal acoustic meatus Cr. N. VII (facial); Cr. N. VIII (stato-acoustic) - (see text & Figure 5-7 below) hypoglossal canal Cr. N. XII (hypoglossal); a meningeal branch of the ascending pharyngeal a. foramen magnum (); Cr. N. XI (spinal accessory nn.) entering the skull; ver- tebral aa.; anterior & posterior spinal foramen cecum occasional small cribriform plate olfactory nn. optic canal Cr. N. II (optic); ophthalmic a. foramen rotundum Cr. N. V2 (maxillary div.), a. of foramen rotundum foramen ovale Cr. N. V3 (mandibular div.) + portio minor (motor for CrN V) foramen spinosum middle meningeal a. & v. jugular foramen internal jugular v. (beginning); Cr. Nn. IX, X, XI stylomastoid foramen Cr. N. VII (facial); stylomastoid a. condyloid foramen v. from transverse sinus mastoid foramen v. to mastoid sinus; branch of occipital a. to dura mater * Abbreviations: a. = artery, aa. = arteries, v. = vein, vv. = , n. = nerve, nn. = nerves, br. = branch, Cr. N. = cranial nerve, fmn. = foramen, div. = division

Porus acusticus AKA internal auditory canal (see Figure 5-7) The filaments of the acoustic portion of VIII penetrate tiny openings of the lamina cribrosa of the cochlear area18. Transverse crest: separates superior vestibular area and facial canal (above) from the inferior vestibular area and cochlear area (below)18. Vertical crest (AKA Bill’s bar): separates the meatus to facial canal anteriorly (con- NEUROSURGERY 5.2. Cranial foramina & their contents 89 taining VII and nervus intermedius) from the vestibular area posteriorly (containing the superior division of vestibular nerve). The “5 nerves” of the IAC: 1. facial nerve (VII) (mnemonic: “7-up” as VII is in superi- facial canal (Cr. N. VII with NI*) or portion) vertical crest (”Bill’s bar”) 2. nervus intermedi- us: the somatic superior vestibular area (superior sensory branch of vestibular nerve) (to utricle & the facial nerve superior & lateral semicircular canals) primarily inner- transverse crest (crista falciformis) vating mechanore- ceptors of the hair inferior vestibular area (to saccule) (inferior follicles on the in- foramen singulare (to vestibular ner surface of the nerve) pinna and deep posterior semicircular canal) mechanoreceptors tractus spiralis foraminosus (cochlear of nasal and buccal area) (acoustic portion of Cr. N. VIII) cavities and chemoreceptors in the taste buds on Figure 5-7 Right internal auditory canal (porus acusticus) & nerves the anterior 2/3 of * NI = nervus intermedius the tongue 3. acoustic portion of the VIII nerve (mnemonic: “Coke down” for cochlear portion) 4. superior branch of vestibular nerve: passes through the superior vestibular area to terminate in the utricle and in the ampullæ of the superior and lateral semi- circular canals 5. inferior branch of vestibular nerve: passes through inferior vestibular area to ter- minate in the saccule

5.3. Cerebellopontine angle anatomy

retractor on cerebellar hemisphere V Meckel's cave foramen of pons Luschka flocculus choroid plexus VII IAC VIII IX foramen of jugular foramen Magendie X cerebellar XI tonsil XII PICA olive medulla

Figure 5-8 Normal anatomy of right cerebellopontine angle viewed from behind (as in a suboccipital approach)18

90 5. Neuroanatomy and physiology NEUROSURGERY 5.4. Occiptoatlantoaxial-complex anatomy

≈ 50% of head rotation occurs at the C1-2 (atlantoaxial) joint. Ligaments of the occipito-atlanto-axial complex

apical odontoid ligament cruciate ligament, ascending band anterior atlantooccipital posterior membrane C1 atlantooccipital anterior transverse membrane ligament longitudinal ligamentum ligament flavum cruciate ligament, descending band spinal C2 tectorial cord membrane posterior C3 longitudinal ligament Figure 5-9 Sagittal view of the ligaments of the craniovertebral junction Modified with permission from “In Vitro Cervical Spine Biomechanical Testing” BNI Quarterly, Vol.9, No. 4, 1993 Stability of this joint complex is primarily due to ligaments, with little contribution from bony articulations and joint capsules (see Figure 5-9 through Figure 5-11): 1. ligaments that connect the atlas to the occiput: A. anterior at- lanto-occipi- tal mem- ascending clivus band brane: cephal- ad extension right alar ligament of the anteri- or longitudi- nal ligament. Extends from accessory anterior mar- (deep) portion gin of fora- C1 of tectorial membrane men magnum (FM) to ante- transverse CRUCIATE band rior arch of C1 LIGAMENT B. posterior at- lanto-occipi- descending C2 tal mem- band brane: con- nects the pos- terior margin Figure 5-10 Dorsal view of the cruciate and alar ligaments Viewed with tectorial membrane removed. of the FM to Modified with permission from “In Vitro Cervical Spine Biomechanical posterior arch Testing” BNI Quarterly, Vol.9, No. 4, 1993 of C1 C. the ascending band of the cruciate ligament 2. ligaments that connect the axis (viz. the odontoid) to the occiput: A. tectorial membrane: some authors distinguish 2 components 1. superficial component: cephalad continuation of the posterior longitu-

NEUROSURGERY 5.4. Occiptoatlantoaxial-complex anatomy 91 dinal ligament. A strong band connecting the dorsal surface of the dens to the ventral surface of the FM above, and dorsal surface of C2 & C3 bodies below 2. accessory (deep) portion: located laterally, connects C2 to occipital condyles B. alar (“check”) ligaments19 1. occipito-alar portion: connects side of the dens to occipital condyle 2. atlanto-alar portion: connects side of the dens to the lateral mass of C1 C. apical odontoid liga- ment: connects tip of dens to the FM. Little odontoid right alar process ligament mechanical strength 3. ligaments that connect the transverse axis to the atlas: ligament A. transverse (atlanto- axial) ligament: the horizontal component of the cruciate liga- tubercle ment. Traps the dens tectorial against the anterior at- membrane las via a strap-like mechanism (see Figure 5-11). Provides the ma- posterior arch C1 jority of the strength (“the strongest liga- Figure 5-11 C1 viewed from above, showing the trans- 20 verse and alar ligaments ment of the spine” ) Modified with permission from “In Vitro Cervical Spine Bio- B. atlanto-alar portion of mechanical Testing” BNI Quarterly, Vol.9, No. 4, 1993 the alar ligaments (see above) C. descending band of the cruciate ligament The most important structures in maintaining atlanto-occipital stability are the tec- torial membrane and the alar ligaments. Without these, the remaining cruciate liga- ment and apical dentate ligament are insufficient.

5.5. Spinal cord anatomy

5.5.1. Spinal cord tracts

Figure 5-12 depicts a cross-section of a typical spinal cord segment, combining some elements from different levels (e.g. the intermediolateral grey nucleus is only present from T1 to ≈ L1 or L2 where there are sympathetic (thoracolumbar outflow) nuclei). It is schematically divided into ascending and descending halves, however, in actuality, as- cending and descending paths coexist on both sides.

Table 5-6 Descending (motor) tracts (↓) in Figure 5-12 Number (see Path Function Side of body Figure 5-12) 1 anterior corticospinal tract skilled movement opposite 2 medial longitudinal fasciculus ? same 3 vestibulospinal tract facilitates extensor muscle tone same 4 medullary (ventrolateral) reticulospinal tract automatic respirations? same 5 rubrospinal tract flexor muscle tone same 6 lateral corticospinal (pyramidal) tract skilled movement same

92 5. Neuroanatomy and physiology NEUROSURGERY Table 5-7 Bi-directional tracts in Figure 5-12 Number (see Path Function Figure 5-12) 7 dorsolateral fasciculus (of Lissauer) 8 fasciculus proprius short spinospinal connections

Table 5-8 Ascending (sensory) tracts (↑) in Figure 5-12 Number (see Path Function Side of Figure 5-12) body 9 fasciculus gracilis joint position, fine touch, same 10 fasciculus cuneatus vibration 11 posterior spinocerebellar tract stretch receptors same 12 lateral pain & temperature opposite 13 anterior spinocerebellar tract whole limb position opposite 14 spinotectal tract unknown, ? nociceptive opposite 15 anterior spinothalamic tract light touch opposite

Figure 5-12 also depicts some of the laminae according to the scheme of Rexed. Lam- ina II is equivalent to the substantia gelatinosa. Laminae III and IV are the nucleus pro- prius. Lamina VI is located in the base of the posterior horn.

S = sacral

MOTOR bi-directional T = thoracic SENSORY (descending paths C = cervical (ascending

paths) { paths) { {

7 8 9 10 intermediolateral 6 S TC grey nucleus I II III (sympathetic) IV 5 V 11 STC VI 12

VII X VIII IX CTS IX dentate ligament 4 13 3 14 2 cm 15 1 2.5-4 anterior spinal anterior motor artery nerve root Figure 5-12 Schematic cross-section of cervical spinal cord

SENSATION

PAIN & TEMPERATURE: BODY Receptors: free nerve endings (probable). 1st order neuron: small, finely myelinated afferents; soma in dorsal root ganglion (no synapse). Enter cord at dorsolateral tract (zone of Lissauer). Synapse: substantia ge-

NEUROSURGERY 5.5. Spinal cord anatomy 93 latinosa (Rexed II). 2nd order neuron axon cross obliquely in the anterior white commissure ascending ≈ 1-3 segments while crossing to enter the lateral spinothalamic tract. Synapse: VPL thalamus. 3rd order neurons pass through IC to postcentral gyrus (Brodmann’s areas 3, 1, 2).

FINE TOUCH, DEEP PRESSURE & PROPRIOCEPTION: BODY Fine touch AKA discriminative touch. Receptors: Meissner’s & pacinian corpuscles, Merkel’s disks, free nerve endings. 1st order neuron: heavily myelinated afferents; soma in dorsal root ganglion (no synapse). Short branches synapse in nucleus proprius (Rexed III & IV) of posterior gray; long fibers enter the ipsilateral posterior columns without synapsing (below T6: fascicu- lus gracilis; above T6: fasciculus cuneatus). Synapse: nucleus gracilis/cuneatus (respectively), just above pyramidal decussa- tion. 2nd order neuron axons form internal arcuate fibers, decussate in lower medulla as medial lemniscus. Synapse: VPL thalamus. 3rd order neurons pass through IC primarily to postcen- tral gyrus.

ANT E R IOR P OST E R IOR

V1 trigeminal C2 nerve V2 { V3 superior clavicular occipitals C2 C3 C3 INTERCOSTALS posterior C4 C4 lateral T2 T3 T4 C5 T4 medial C5 axillary T6 T6 RADIAL T2 T8 T2 T8 post. cutaneous dorsal cutan. T10 T 10 T12 T1

6 1 T T C 12 musculocutan. C6

medial cutan. S5 ©2001 Mark S Greenberg, M.D.

L1 4 All rights reserved. clunials S Unauthorizradialed use is prohibited. S3 C8 C8 L2 median ilio- L3 L4 inguinal ulnar S1 L3 C7 lateral cutan. posterior C7 nerve of thigh FEMORAL anterior cutaneous L4 cutaneous saphenous L5 L5 SCIATIC L4 COMMON PERONEAL lat. cutan. sup. peroneal deep peroneal TIBIAL sural S1 S1 med. plantars {lat. DERMATOMES DERMATOMES CUT ANEOUS (anterior) NER VES (posterior)

Figure 5-13 Dermatomal and sensory nerve distribution (Redrawn from “Introduction to Basic Neurology”, by Harry D. Patton, John W. Sundsten, Wayne E. Crill and Phillip D. Swanson, © 1976, pp 173, W. B. Saunders Co., Philadelphia, PA, with permission)

LIGHT (CRUDE) TOUCH: BODY Receptors: as fine touch (see above), also peritrichial arborizations.

94 5. Neuroanatomy and physiology NEUROSURGERY 1st order neuron: large, heavily myelinated afferents (Type II); soma in dorsal root ganglion (no synapse). Some ascend uncrossed in post. columns (with fine touch); most synapse in Rexed VI & VII. 2nd order neuron axons cross in anterior white commissure (a few don’t cross); enter anterior spinothalamic tract. Synapse: VPL thalamus. 3rd order neurons pass through IC primarily to postcen- tral gyrus.

5.5.2. Dermatomes and sensory nerves

Figure 5-13 shows anterior and posterior view, each schematically separated into sensory dermatomes (segmental) and peripheral sensory nerve distribution.

5.5.3. Spinal cord vasculature

basilar artery spinal cord

radicular artery at C3 radicular artery at C6

right vertebral left vertebral artery artery right common left common carotid carotid radicular artery at C8 right subclavian left subclavian brachiocephalic trunk left posterior spinal artery radicular artery at T5 aorta

posterior inter- costal artery (dorsal branch) posterior artery of spinal arteries Adamkiewicz radicular (arteria radicularis artery }anterior magna) posterior arteria radicularis intercostal magna artery (posterior branch) anterior spinal aorta artery Axial view

Figure 5-14 Schematic diagram of spinal cord arterial supply Modified from Diagnostic Neuroradiology, 2nd ed., Volume II, pp. 1181, Taveras J M, Woods EH, editors, © 1976, the Williams and Wilkins Co., Baltimore, with permission)

Although a radicular artery from the aorta accompanies the nerve root at many lev- els, most of these contribute little flow to the spinal cord itself. The anterior spinal artery is formed from the junction of two branches, each from one of the vertebral arteries. Ma- jor contributors of blood supply to the anterior spinal cord is from 6-8 radicular arteries

NEUROSURGERY 5.5. Spinal cord anatomy 95 at the following levels (“radiculomedullary arteries”, the levels listed are fairly consis- tent, but the side varies21 (p 1180-1)): • C3 - arises from • C6 - usually arises from deep cervical artery ≈ 10% of population lack an an- • C8 - usually from costocervical trunk } terior radicular artery in lower cervical spine22 • T4 or T5 • artery of Adamkiewicz AKA arteria radicularis anterior magna A. the main arterial supply for the spinal cord from ≈ T8 to the conus B. located on the left in 80%23 C. situated between T9 & L2 in 85% (between T9 & T12 in 75%); in remaining 15% between T5 & T8 (in these latter cases, there may be a supplemental radicular artery further down) D. usually fairly large, gives off cephalic and caudal branch (latter is usually larger) giving a characteristic hair-pin appearance on angiography The paired posterior spinal arteries are less well defined than the anterior spinal artery, and are fed by 10-23 radicular branches. The midthoracic region has a tenuous vascular supply (“watershed zone”), possess- ing only the above noted artery at T4 or T5. It is thus more susceptible to vascular in- sults.

ANATOMIC VARIANTS Arcade of Lazorthes: normal variant where the anterior spinal artery joins with the paired posterior spinal arteries at the .

5.6. Cerebrovascular anatomy

5.6.1. Cerebral vascular territories

CORONAL VIEW AXIAL VIEW anterior cerebral artery

RAH

MCA

AChA internal carotid PCommA anterior choroidal basilar artery artery posterior cerebral artery RAH = recurrent artery of Heubner Figure 5-15 Vascular territories of the cerebral hemispheres

Figure 5-15 depicts approximate vascular distributions of the major cerebral arter- ies. There is considerable variability of the major arteries24 as well as the central distri- 96 5. Neuroanatomy and physiology NEUROSURGERY