Fig. 3.1
Suboccipital muscles. Stepwise dissection. (a) The right trapezius and sternocleidomastoid have been preserved. The left trapezius and sternocleidomastoid have been reflected along with the galea aponeurotica to expose the underlying semispinalis capitis, splenius capitis, and levator scapulae. (b) The right sternocleidomastoid and trapezius have been reflected to expose the splenius capitis. The left splenius capitis has been removed to expose the underlying semispinalis and longissimus capitis. (c) The right splenius capitis has been removed to expose the semispinalis and longissimus capitis. The left semispinalis and longissimus capitis have been removed to expose the suboccipital triangle formed by the superior oblique, which passes from the C1 transverse process to the occipital bone; the inferior oblique, which extends from the transverse process of C1 to the spinous process of C2; and the rectus capitis posterior major, which extends from the occipital bone below the inferior nuchal line to the spinous process of C2. The vertebral artery courses in the depths of the suboccipital triangle as it passes behind the superior facet of C1 and across the upper edge of the posterior atlantal arch. (d) Both semispinalis capitis muscles have been reflected laterally to expose the suboccipital triangles bilaterally. (e) The muscles forming the left suboccipital triangle have been removed. The vertebral artery ascends slightly lateral from the transverse process of C2 to reach the transverse process of C1 and turns medially behind the superior facet of C1 to reach the upper surface of the posterior arch of C1. The C2 ganglion is located between the posterior arch of C1 and the lamina of C2. The dorsal ramus of C2 produces a medial branch that forms the majority of the greater occipital nerve. (f) The muscles forming both suboccipital triangles have been removed. The rectus capitis posterior minor, which extends from the posterior arch of C1 to the occipital bone below the inferior nuchal line, has been preserved. The vertebral arteries cross the posterior arch of the atlas and penetrate the posterior atlanto-occipital membrane to reach the dura. A artery, Atl atlanto-, Cap capitis, Car carotid, CN cranial nerve, Inf inferior, Int internal, Jug jugular, Lev levator, Longiss longissimus, M muscle, Maj major, Memb membrane, Min minor, Obl oblique, Occip occipital, Post posterior, Proc process, Rec rectus, Scap scapulae, Semispin semispinalis, Spin spinalis, Splen splenius, Sternocleidomast sternocleidomastoid, Sup superior, Trans transverse, V vein, Vert vertebral
The posterior fossa posteriorly is surrounded by the muscles attached to the occipital bone and upper cervical vertebrae (Fig. 3.1). The trapezius covers the back of the head and neck. It extends from the medial half of the superior nuchal line, the external occipital protuberance, and the spinous processes of the cervical and thoracic vertebrae and converges on the shoulder to attach to the scapula and the lateral third of the clavicle. The sternocleidomastoid passes obliquely downward across the side of the neck from the lateral half of the superior nuchal line and mastoid process to the upper part of the sternum and the adjacent part of the clavicle. This muscle divides the side of the neck into an anterior triangle and a posterior triangle. The anterior triangle is bounded posteriorly by the anterior border of the sternocleidomastoid, above by the mandible, and anteriorly by the median line of the neck; the posterior triangle is bounded in front by the posterior border of the sternocleidomastoid, below by the middle third of the clavicle, and behind by the anterior margin of the trapezius. The splenius capitis, situated deep to and partially covered by the trapezius and sternocleidomastoid, extends from the bone below the lateral third of the superior nuchal line to the spinous processes of the lower cervical and upper thoracic vertebrae. Two muscles, both of which are situated deep to the splenius capitis and sternocleidomastoid and attach below to the upper thoracic and lower cervical vertebrae, are the semispinalis capitis, which attaches above in the area between the superior and inferior nuchal lines beginning medially at the external occipital crest and extending laterally to the occipitomastoid junction, and the longissimus capitis muscle, which attaches above to the posterior margin of the mastoid process.
The suboccipital muscles, located in the next layer, are a group of muscles situated deep to the splenius, semispinalis, and longissimus capitis in the suboccipital area. This group includes the superior oblique, which extends from the area lateral to the semispinalis capitis between the superior and inferior nuchal lines to the transverse process of the atlas; the inferior oblique, which extends from the spinous process and lamina of the axis to the transverse process of the atlas; the rectus capitis posterior major, which extends from and below the lateral part of the inferior nuchal line to the spine of the axis; and the rectus capitis posterior minor, which is situated medial to and is partially covered by the rectus capitis posterior major, extends from the medial part and below the inferior nuchal line to the tubercle on the posterior arch of the atlas. The suboccipital triangle is a region bounded above and medially by the rectus capitis posterior major, above and laterally by the superior oblique, and below and laterally by the inferior oblique (Fig. 3.1). It is covered by the semispinalis capitis medially and by the splenius capitis laterally. The floor of the triangle is formed by the posterior atlanto-occipital membrane and the posterior arch of the atlas. The structures in the triangle are the terminal extradural segment of the vertebral artery and the first cervical nerve.
The platysma is a broad sheet extending downward from the lower part of the face and across the clavicle to the fascia covering the pectoralis major and deltoid. The anterior vertebral muscles insert on the clival part of the occipital bone anterior to the foramen magnum. This group includes the longus colli, which attach to the anterior surface of the vertebral column between the atlas and the third thoracic vertebra; the longus capitis, which extends from the clivus in front of the foramen magnum to the transverse processes of the third through the sixth cervical vertebrae; the rectus capitis anterior, which is situated behind the upper part of the longus capitis and extends from the occipital bone in front of the occipital condyle to the anterior surface of the lateral mass and transverse process of the atlas; and the rectus capitis lateralis, which extends from the jugular process of the occipital bone to the transverse process of the atlas (Fig. 3.1).
The posterior fossa extends from the tentorial incisura, through which it communicates with the supratentorial space, to the foramen magnum, through which it communicates with the spinal canal. It is surrounded by the occipital, temporal, parietal, and sphenoid bones (Figs. 3.2 and 3.3). It is bounded in front by the dorsum sellae, the posterior part of the sphenoid body, and the clival part of the occipital bone; behind by the lower portion of the squamosal part of the occipital bone; and on each side by the petrous and mastoid parts of the temporal bone, the lateral part of the occipital bone, and above and behind by a small part of the mastoid angle of the parietal bone. Its intracranial surface is penetrated by the jugular foramen, internal acoustic meatus, hypoglossal canal, the vestibular and cochlear aqueducts, and several venous emissary foramina (Fig. 3.4).
Fig. 3.2
Occipital bone and foramen magnum. (a) Inferior view. (b) Posteroinferior view. (c) Anterior-inferior view. (d) Superior view. (e) Posterosuperior view. (f) Oblique posterosuperior view. The occipital bone surrounds the oval-shaped foramen magnum, which is wider posteriorly than anteriorly. The occipital bone is divided into a squamosal part located above and behind the foramen magnum, a basal (clival) part situated in front of the foramen magnum, and paired condylar parts located lateral to the foramen magnum. The convex external surface of the squamosal part has several prominences. The largest prominence, the external occipital protuberance (inion), is situated at the central part of the external surface. The superior nuchal line radiates laterally from the protuberance. A vertical ridge, the external occipital crest, descends from the external occipital protuberance to the midpoint of the posterior margin of the foramen magnum. The inferior nuchal lines run laterally on both sides from the midpoint of the crest. The internal surface is divided into four unequal fossae by the sulcus of the superior sagittal sinus, the internal occipital crest, and the sulci for the transverse sinuses. The basilar part of the occipital bone, which is also referred to as the clivus, is a thick quadrangular plate of bone that extends forward and upward to join the sphenoid bone just below the dorsum sellae. The clivus is separated on each side from the petrous part of the temporal bone by the petroclival fissure that ends posteriorly at the jugular foramen. The condylar parts of the occipital bone, on which the occipital condyles are located, are situated lateral to the foramen magnum on the external surface. The hypoglossal canal is situated above the condyle. The jugular foramen is bordered posteriorly by the jugular process of the occipital bone and anteriorly by the jugular fossa of the petrous temporal bone. The jugular tubercle lies on the internal surface above the hypoglossal canal. A artery, Ac acoustic, Car carotid, Cond condyle, Digast digastric, Ext external, Fiss fissure, For foramen, Hypogl hypoglossal, Inf inferior, Jug jugular, Occipitomast occipitomastoid, Occip occipital, Petrocliv petroclival, Pharyng pharyngeal, Proc process, Protrub protuberance, Sag sagittal, Sig sigmoid, Sup superior, Trans transverse
Fig. 3.3
Osseous relationships. (a) The jugular foramen is located between the temporal and occipital bones. (b) The view directed from posterior and superior shows the shape of the foramen, which is not seen on the direct superior view. The glossopharyngeal, vagus, and accessory nerves pass through the intrajugular portion of the foramen located between the sigmoid and petrosal parts. (c) Jugular foramen viewed from directly below. (d) The view directed from anterior and backward reveals the shape of the jugular foramen. The posterior margin of the foramen is formed by the jugular process of the occipital bone, which connects the basal (clival) part of the occipital bone to the squamosal part. Ac acoustic, Car carotid, Coch cochlear, Cond condyle, Fiss fissure, For foramen, Hypogl hypoglossal, Int internal, Intrajug intrajugular, Jug jugular, Mast mastoid, Occip occipital, Pet petrous, Petrocliv petroclival, Post posterior, Proc process, Sig sigmoid, Squam squamosal, Stylomast stylomastoid, Temp temporal, Vest vestibular
Fig. 3.4
(a) Superior view of the posterior cranial fossa. The osseus walls of the posterior fossa are formed by the occipital, temporal, and sphenoid bones. (b) Nerves and arteries of the posterior fossa. Only 2 of the 12 pairs of cranial nerves course entirely outside the posterior fossa. The tentorium, which is attached along the petrous ridges, roofs the posterior fossa. A artery, Ac acoustic, A.I.C.A anteroinferior cerebellar artery, Bas basilar, CN cranial nerve, For foramen, Int internal, Jug jugular, Occip occipital, P.C.A posterior cerebral artery, P.I.C.A posteroinferior cerebellar artery, S.C.A superior cerebellar artery, Temp temporal, Tent tentorial, Vert vertebral
The cortical surfaces are divided on the basis of the structures they face, or along which they may be exposed, to make this description more readily applicable to the operative setting The first surface, the tentorial surface, faces the tentorium and is retracted in a supracerebellar approach; the second surface, the suboccipital surface, is located below and between the lateral and sigmoid sinuses and is exposed in a suboccipital craniectomy; and the third surface, the petrosal surface, faces forward toward the posterior surface of the petrous bone and is retracted to expose the cerebellopontine angle. Each of the surfaces has the vermis in the midline and the hemispheres laterally and is divided by a major fissure named on the basis of the surface that it divides. The hemispheric lobules forming each of the three surfaces commonly overlap onto and form a part of the adjacent surfaces [10]. The fissures dividing the three cortical surfaces are to be distinguished from the fissures between the cerebellum and the brainstem (Fig. 3.5).
Fig. 3.5
Tentorial, suboccipital, and petrosal cerebellar surfaces. (a) The tentorial surface faces the lower surface of the tentorium. The anterior vermis is the most superior part of the tentorial surface. This surface slopes downward to its posterior and lateral margins. The fissure separates the hemispheric surface between the quadrangular and simple lobules and the vermis between the declive and culmen. (b) Suboccipital surface. The parts of the hemispheric surface from above to below are the superior and inferior semilunar and biventral lobules and the tonsils. These lobules extend beyond the suboccipital surface to the other surfaces of the cerebellum. The prebiventral fissures between the inferior semilunar and the biventral lobules separate the hemispheres into superior and inferior parts, and the prepyramidal fissure between the pyramid and tuber separates the vermis into superior and inferior parts. The petrosal (horizontal) fissure, the most prominent fissure on the petrosal surface, extends onto the suboccipital surface and divides the superior half of the suboccipital surface between the superior and inferior semilunar lobules. The cerebellomedullary fissure extends superiorly between the cerebellum and medulla. (c) Petrosal surface. The petrosal surface faces forward toward the petrous temporal bone and is the surface that is retracted to surgically expose the cerebellopontine angle. The petrosal fissure divides the petrosal surface into superior and inferior parts. The cerebellopontine fissures are V-shaped fissures formed where the cerebellum wraps around the pons and the middle cerebellar peduncles. These fissures have a superior and an inferior limb, which meet at a lateral apex. The petrosal fissure extends laterally from the apex of the cerebellopontine fissures. Ant anterior, Cer. Med cerebellomedullary, Cer. Pon cerebellopontine, CN cranial nerve, Fiss fissure, Horiz horizontal, Inf inferior, Pet petrosal, Post posterior, Quad quadrangular, Suboccip suboccipital, Sup superior, Tent tentorial
3.1 Tentorial Surface
The tentorial surface faces and conforms to the lower surface of the tentorium (Fig. 3.5). The anteromedial part of this surface, the apex, formed by the anterior vermis, is the highest point on the cerebellum. This surface slopes downward from its anteromedial to its posterolateral edge. On the tentorial surface, the transition from the vermis to the hemispheres is smooth and not marked by the deep fissures on the suboccipital surface between the vermis and hemispheres. Deep notches, the anterior and posterior cerebellar incisurae, groove the anterior and posterior edges of the tentorial surface in the midline. The brainstem fits into the anterior cerebellar incisura and the falx cerebelli fits into the posterior incisura (Fig. 3.6). The tentorial surface faces and conforms to the lower surface of the tentorium (Fig. 3.7). The anteromedial part of this surface, the apex, formed by the anterior vermis, is the highest point on the cerebellum. This surface slopes downward from its anteromedial to its posterolateral edge. On the tentorial surface, the transition from the vermis to the hemispheres is smooth and not marked by the deep fissures on the suboccipital surface between the vermis and hemispheres. Deep notches, the anterior and posterior cerebellar incisurae, groove the anterior and posterior edges of the tentorial surface in the midline. The brainstem fits into the anterior cerebellar incisura and the falx cerebelli fits into the posterior incisura (Figs. 3.6 and 3.7).
Fig. 3.6
Tentorial surface and cerebellomesencephalic fissure. (a) The surface slopes downward from the apex to the posterior and lateral margins. The anterior cerebellar incisura, the notch where the brainstem fits into the anterior part of the tentorial surface, is located anteriorly, and the posterior cerebellar incisura, the notch where the falx cerebelli fits into the cerebellum, is located posteriorly. (b) Enlarged view of the cerebellomesencephalic fissure, which extends downward between the midbrain and the cerebellum. The superficial part of the posterior lip is formed by the culmen in the midline and the quadrangular lobule laterally. The quadrigeminal cistern extends caudally from the pineal into the cerebellomesencephalic fissure. (c) The culmen has been removed to expose the central lobule and its wings, which form part of the posterior lip of the cerebellomesencephalic fissure. (d) The central lobule and its wings, the lingula, the superior medullary velum, and medial part of the superior cerebellar peduncles have been removed to expose the fourth ventricle. The lower half of the roof is formed in the midline by the nodule and laterally by the inferior medullary velum, which passes laterally above, but is separated from the rostral pole of the tonsils by the cerebellomedullary fissure. A.I.C.A anteroinferior cerebellar artery, Cent central, Cer cerebellar, Cer. Mes cerebellomesencephalic, Chor choroid, CN cranial nerve, Coll colliculus, Dent dentate, Fiss fissure, Flocc flocculus, Inf inferior, Lat lateral, Mid middle, Med median, medullary, Nucl nucleus, Ped peduncle, Plex plexus, Post posterior, Quad quadrangular, Sulc sulcus, Sup superior, Tent tentorial, Vel velum, Vent ventricle
Fig. 3.7
Tentorial surface and cerebellomesencephalic fissure. (a) The tentorial cerebellar surface faces the tentorium and slopes downward from its apex located below the tentorial apex. The cerebellomesencephalic fissure extends forward between the cerebellum and midbrain. The SCA exits the cerebellomesencephalic fissure and supplies the tentorial surface. (b) The right half of the posterior lip of the cerebellomesencephalic fissure has been removed. The anterior wall of the fissure is formed in the midline by the collicular plate and lingula and laterally by the superior cerebellar peduncles. (c) The right half of the lingula and superior medullary velum has been removed to expose the fourth ventricle. Additional white matter has been removed below the right superior peduncle to expose the dentate nucleus in which the superior peduncular fibers arise. (d) Enlarged view. The dentate nucleus appears to wrap around the rostral pole of the tonsil. A.I.C.A anteroinferior cerebellar artery, Cer. Mes cerebellomesencephalic, Chor choroidal, CN cranial nerve, Coll colliculus, Dent dentate, Fiss fissure, Inf inferior, Lat lateral, Med medullary, Mid middle, Nucl nucleus, Ped peduncle, S.C.A. superior cerebellar artery, Str straight, Sup superior, Tent tentorial, Vel velum, Vent ventricle
The anterior border, separating the tentorial and petrosal surfaces, has a lateral part (the anterolateral margin) that is parallel to the superior petrosal sinus and separates the hemispheric part of the tentorial and petrosal surfaces, and a medial part (the anteromedial margin) that faces the midbrain and forms the posterior border of the fissure between the midbrain and cerebellum. The anterior angle formed by the junction of the anterolateral and anteromedial margins is directed anteriorly above the origin of the posterior root of the trigeminal nerve. The posterior border between the tentorial and the suboccipital surfaces also has a lateral and a medial part. The lateral part (the posterolateral margin) is parallel and adjacent to the lateral sinus and separates the hemispheric part of the suboccipital and tentorial surfaces, and the short medial part (the posteromedial margin) faces the posterior cerebellar incisura and separates the vermic part of the two surfaces. The lateral angle, formed by the junction of the anterolateral and posterolateral margins, is located at the junction of sigmoid, lateral, and superior petrosal sinuses. Veins often converge on the anterior and lateral angles. The hemispheric part of the tentorial surface includes the quadrangular, simple, and superior semilunar lobules, and the vermian part includes the culmen, declive, and folium. The vermian and the related hemispheric parts from above to below in sequence are the culmen and the quadrangular lobule, the declive and the simple lobule, and the folium and the superior semilunar lobule. The tentorial surface is divided at the site of its major fissure, the tentorial fissure, into anterior and posterior parts. This fissure, located between the quadrangular and the simple lobules on the hemisphere and the culmen and the declive on the vermis, has also been called the primary fissure. The postclival fissure separates the simple and the superior semilunar lobules. The interfolial fissures on this surface pass anterolaterally from the midline and are continuous with the fissures on the superior half of the petrosal surface.
3.2 Suboccipital Surface
The suboccipital surface, located below and between the lateral and sigmoid sinuses, is the most complex of the three surfaces (Fig. 3.8). Operative approaches to the fourth ventricle and most cerebellar tumors are commonly directed around or through this surface. It has a deep vertical depression, the posterior cerebellar incisura, which contains a fold of dura, the falx cerebelli. The vermis is folded into and forms the cortical surface within this incisura. The lateral walls of the incisura are formed by the medial aspects of the cerebellar hemispheres. Deep clefts, the vermohemispheric fissures, separate the vermis from the hemispheres. The vermian surface within the incisura has a diamond shape. The upper half of the diamond-shaped formation has a pyramidal shape and is called the pyramid. The folium and the tuber, superior to the pyramid, form the apex of the suboccipital part of the vermis. The lower half of the diamond-shaped formation, the uvula, projects downward between the tonsils, thus mimicking the situation in the oropharynx. The rostromedial margin of the tonsils borders the tapering edges of the uvula. The nodule, the lowermost subdivision of the vermis, is hidden deep to the uvula. The strip of vermis within the incisura is broadest at the junction of the pyramid and uvula. Inferiorly, the posterior cerebellar incisura is continuous with the vallecula cerebelli, a cleft between the tonsils that leads through the foramen of Magendie into the fourth ventricle. The hemispheric portion of the suboccipital surface is formed by the superior and inferior semilunar and biventral lobules and the tonsils, and the vermic portion is formed by the folium, tuber, pyramid, and uvula. The vermian and the related hemispheric parts from above to below are the folium and the superior semilunar lobules, the tuber and the inferior semilunar lobules, the pyramid and the biventral lobules, and the uvula and the tonsils. The suboccipital surface is divided at its major fissure, the suboccipital fissure, into superior and inferior parts. The suboccipital fissure has a vermian and a hemispheric part. The vermian part of this fissure, the prepyramidal fissure, separates the tuber and the pyramid, and the hemispheric part, the prebiventral fissure, separates the biventral and the inferior semilunar lobules. The prebiventral and prepyramidal fissures are continuous at the vermohemispheric junction, and together they form the suboccipital fissure. The petrosal fissure, the major fissure on the petrosal surface, extends from the petrosal surface onto the suboccipital surface and separates the superior and inferior semilunar lobules laterally and the folium and the tuber medially. The tonsillobiventral fissure separates the tonsil and the biventral lobule.
Fig. 3.8
Suboccipital surface of the cerebellum and the cerebellomedullary fissure. (a) The suboccipital surface is located below and between the sigmoid and lateral sinuses and is the surface that is exposed in a wide suboccipital craniectomy. The vermis sits in a depression, the posterior cerebellar incisura, between the hemispheres. The vallecula extends upward between the tonsils and communicates through the foramen of Magendie with the fourth ventricle. (b) Enlarged view. The lower parts of the vermis behind the ventricle are the pyramid and uvula. (c) The right tonsil has been removed to expose the lower part of the roof formed by the inferior medullary velum and tela choroidea. The uvula hangs downward between the tonsils, thus mimicking the situation in the oropharynx. The choroid plexus arises on the inner surface of the tela and extends through the foramen of Luschka behind the glossopharyngeal and vagus nerve. The inferior medullary velum arises on the surface of the nodule, drapes across the superior pole of the tonsil, and blends into the flocculus laterally. (d) Both tonsils have been removed to expose the inferior medullary velum and tela choroidea bilaterally. The telovelar junction is the junction between the velum and tela. Cer. Med cerebellomedullary, Chor choroid, CN cranial nerve, Fiss fissure, Flocc flocculus, For foramen, Inf inferior, Lat lateral, Med medullary, Ped peduncle, P.I.C.A. posteroinferior cerebellar artery, Plex plexus, Suboccip suboccipital, Sup superior, Telovel telovelar, Vel velum
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