Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

Special Part
Axial skeleton, sceleton axiale - Bones of the neurocranium
Occipital bone

The occipital bone, os occipitale, is an unpaired bone (Fig. 38) that contributes to The formation of the base and vault of the cranium. The upper portion of the occipital squama ossifies directly from Connective Tissue, whereas the remaining parts (basilar and lateral) ossify through an intermediate cartilaginous stage. The external surface of the occipital bone is convex, while the internal surface is concave. The anteroinferior region features the large foramen magnum. The occipital bone is anatomically divided into four parts: the basilar part, pars basilaris; two lateral parts, partes laterales; and the occipital squama, squama occipitalis. Up to 3-6 years of age, these components remain as separate bones before fusing to form a single bone.

The basilar part, pars basilaris, is short, thick, and quadrilateral. It forms the anterior boundary of the foramen magnum, which is oval or circular in shape (Yu. V. Zadvornov, 1972). The upper surface of the basilar part is concave, forming a shallow groove facing the cranial cavity—the clivus, which Supports the Medulla Oblongata. A small pharyngeal tubercle, tuberculum pharyngeum, is located near the center of the inferior external surface. The slightly uneven lateral margins of the basilar part articulate with the petrous PARTS OF THE temporal bones to form the petro-occipital fissures, which are filled with Cartilage in childhood and undergo ossification with age.

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Fig. 37. Types of frontal sinuses (after Testut):

1 - sinus frontalis dexter, 2 - septum intersinuale frontale; 3 - sinus frontalis sinister; A - laterally directed sinuses; B - superiorly directed sinuses; C - moderate development of sinuses; D - underdeveloped sinuses

The lateral parts, partes laterales, form the lateral BOUNDARIES OF THE foramen magnum and connect the basilar part with the squama. On the internal (cerebral) surface, near the lateral margin, runs a narrow groove for the petrosal sinus; together with a corresponding groove on the Temporal bone, it forms a canal lodging the inferior petrosal sinus, sul. sinus petrosi inferioris.

The inferior external surface of each lateral part features the occipital condyle, condylus occipitalis, for articulation with the superior articular surface of the atlas. Posterior to the occipital condyle lies the condylar fossa, fossa condylaris, containing an opening at its base that leads into the inconstant condylar canal, canalis condylaris. The lateral margin of the lateral part presents the jugular notch, incisura jugularis, projecting from which is the small intrajugular process, processus jugularis. The jugular notch, together with the corresponding notch on the temporal bone, forms the jugular foramen, foramen jugulare, which is divided by the intrajugular process into anterior and posterior compartments. The anterior compartment gives origin to the Internal jugular vein, while the posterior compartment transmits the Cranial Nerves (CN IX-XI). Posterior to the jugular process on the internal surface of the lateral part lies the deep groove for the transverse sinus, sul. sinus transversus. In the anterior region of the lateral part is the jugular tubercle, tuberculum jugulare; posterinferior to it, between the jugular and occipital processes, lies the hypoglossal canal, canalis nervi hypoglossi.

Fig. 38. Occipital bone, os occipitale:

A - external surface; B - internal surface

The occipital squama, squama occipitalis, is triangular in shape, curved, and forms the posterior boundary of the foramen magnum. The lateral margin of the squama is divided into two sections: the upper (lambdoid margin, margo lambdoideus) and the lower (mastoid margin, margo mastoideus). At the center of the external surface of the squama lies the external occipital protuberance, protuberantia occipitalis externa. Extending laterally from it are the superior nuchal lines, linea nuchalis superior. Superior to these are the highest nuchal lines, linea nuchalis suprema. Passing inferiorly from the external occipital protuberance toward the foramen magnum is the external occipital crest, crista occipitalis externa. Midway between the foramen magnum and the external occipital protuberance, the inferior nuchal lines diverge laterally. These lines serve as attachment sites for Muscles. The internal surface of the squama features the cruciate eminence, eminentia cruciformis, at the center of which is the internal occipital protuberance, protuberantia occipitalis interna. The cruciate eminence divides the internal surface of the squama into four fossae: the two inferior fossae lodge the cerebellar hemispheres, while the superior fossae lodge the occipital lobes of the cerebrum. Radiating from the cruciate eminence are the grooves for the transverse sinus, sul. sinus transversi, laterally; the groove for the superior sagittal sinus, sul. sinus sagittalis superior, superiorly; and the internal occipital crest, crista occipitalis interna, inferiorly.

Ossification. The initial ossification centers in the occipital bone appear at the beginning of the 3rd month of intrauterine development within the membranous and cartilaginous parts. In the cartilaginous part, There are five ossification centers: one in the basilar part, two in the lateral parts, and two in the cartilaginous portion of the squama. In the membranous part of the squama, There are two ossification centers. By the end of the 3rd month, the upper and lower sections of the squama fuse, whereas the basilar part, squama, and lateral parts unite between 3 and 6 years of age. The basilar part typically fuses with the body of the Sphenoid bone around the twentieth year of life.