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  Indian J Med Microbiol
 

Figure 1: Osseous anatomy of craniovertebral junction. (a) Inferior view of the occipital condyles, foramen magnum, and clivus. The occipital condyles are ovoid structures located along the lateral margin of the anterior half of the foramen magnum. Their articular surfaces are convex, face downward and laterally, and articulate with the superior facet of atlas. The condylar fossa behind occipital condyle is frequently the site of a canal called condylar canal. The jugular process of the occipital bone extends laterally from the posterior half of the occipital condyle to form the posterior margin of the jugular foramen. The stylomastoid foramen is situated lateral to the jugular foramen. The styloid process is located anterior and slightly medial to the stylomastoid foramen. (b) Posterior, external view of foramen magnum and occipital bone. The convex external surface of the squamosal part of occipital bone has several prominences. The largest prominence, the external occipital protuberance, 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. Mastoids are visible laterally with grooves from muscle attachments. Styloid processes can be seen directed medially and anteriorly. Occipital condyles can be seen directed downward and laterally. (c) Anterior view of ventral occipital bone. 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 sella. On the inferior surface of the basilar part, a small elevation, the pharyngeal tubercle, gives attachment to the fibrous raphe of the pharynx. The clivus is the place of attachment of longus capitis and the rectus capitis anterior. These muscles create well-defined lines of attachment on the ventral surface of the clival bone, superior clival line (for the longus capitis), and inferior clival line (for the rectus capitis anterior). Note that the inferior clival line is located at the same level of the external orifice of the hypoglossal canal. Occipital condyles can be seen here as oval, convex downward, face downward and laterally, and have their long axes directed forward and medially. (d) Superior oblique view of atlas and axis vertebra. Atlas is the first cervical vertebra with two lateral masses formed by superior and inferior articular facets, which are connected in front by a short anterior arch and behind by a longer curved posterior arch. The upper surface of each lateral mass has an oval concave facet that faces upward and medially and articulates with the occipital condyle. The medial aspect of each lateral mass has a small tubercle for the attachment of the transverse ligament of the atlas. The transverse process projects from the lateral masses. (e) Anterior view of atlas and axis vertebra. The anterior arch of atlas has a well-defined tubercle in midline for attachment of longus coli and anterior longitudinal ligament. Axis has a body, which is distinguished by odontoid process. The odontoid and body are flanked by a pair of large oval facets that extend laterally from the body onto the adjoining parts of the pedicles and articulate with the inferior facets of the atlas. The superior facets of axis are anterior to the inferior facets. (f) Lateral view of atlas and axis vertebra. Transverse foramen of axis faces superolaterally, thus permitting the lateral deviation of the vertebral artery as it passes up to the more widely separated transverse foramina in the atlas. The inferior articular facets are situated at the junction of the pedicles and lamina and face downward and forward. (g) Superolateral oblique view showing position of dens in relation to superior articular facets of atlas. (h) Anterior view of ventral aspect of clivus, foramen magnum, atlas, and axis. The clivus is separated on each side from the petrous part of the temporal bone by the petroclival fissure that extends from foramen lacerum to jugular foramen. The atlanto-occipital joints are seen here in the anterior aspect of foramen magnum. Tip of odontoid process lies roughly at the level of atlanto-occipital joints

Figure 1: Osseous anatomy of craniovertebral junction. (a) Inferior view of the occipital condyles, foramen magnum, and clivus. The occipital condyles are ovoid structures located along the lateral margin of the anterior half of the foramen magnum. Their articular surfaces are convex, face downward and laterally, and articulate with the superior facet of atlas. The condylar fossa behind occipital condyle is frequently the site of a canal called condylar canal. The jugular process of the occipital bone extends laterally from the posterior half of the occipital condyle to form the posterior margin of the jugular foramen. The stylomastoid foramen is situated lateral to the jugular foramen. The styloid process is located anterior and slightly medial to the stylomastoid foramen. (b) Posterior, external view of foramen magnum and occipital bone. The convex external surface of the squamosal part of occipital bone has several prominences. The largest prominence, the external occipital protuberance, 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. Mastoids are visible laterally with grooves from muscle attachments. Styloid processes can be seen directed medially and anteriorly. Occipital condyles can be seen directed downward and laterally. (c) Anterior view of ventral occipital bone. 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 sella. On the inferior surface of the basilar part, a small elevation, the pharyngeal tubercle, gives attachment to the fibrous raphe of the pharynx. The clivus is the place of attachment of longus capitis and the rectus capitis anterior. These muscles create well-defined lines of attachment on the ventral surface of the clival bone, superior clival line (for the longus capitis), and inferior clival line (for the rectus capitis anterior). Note that the inferior clival line is located at the same level of the external orifice of the hypoglossal canal. Occipital condyles can be seen here as oval, convex downward, face downward and laterally, and have their long axes directed forward and medially. (d) Superior oblique view of atlas and axis vertebra. Atlas is the first cervical vertebra with two lateral masses formed by superior and inferior articular facets, which are connected in front by a short anterior arch and behind by a longer curved posterior arch. The upper surface of each lateral mass has an oval concave facet that faces upward and medially and articulates with the occipital condyle. The medial aspect of each lateral mass has a small tubercle for the attachment of the transverse ligament of the atlas. The transverse process projects from the lateral masses. (e) Anterior view of atlas and axis vertebra. The anterior arch of atlas has a well-defined tubercle in midline for attachment of longus coli and anterior longitudinal ligament. Axis has a body, which is distinguished by odontoid process. The odontoid and body are flanked by a pair of large oval facets that extend laterally from the body onto the adjoining parts of the pedicles and articulate with the inferior facets of the atlas. The superior facets of axis are anterior to the inferior facets. (f) Lateral view of atlas and axis vertebra. Transverse foramen of axis faces superolaterally, thus permitting the lateral deviation of the vertebral artery as it passes up to the more widely separated transverse foramina in the atlas. The inferior articular facets are situated at the junction of the pedicles and lamina and face downward and forward. (g) Superolateral oblique view showing position of dens in relation to superior articular facets of atlas. (h) Anterior view of ventral aspect of clivus, foramen magnum, atlas, and axis. The clivus is separated on each side from the petrous part of the temporal bone by the petroclival fissure that extends from foramen lacerum to jugular foramen. The atlanto-occipital joints are seen here in the anterior aspect of foramen magnum. Tip of odontoid process lies roughly at the level of atlanto-occipital joints