Which Bones Form the Sigmoid Sulcus?
The sigmoid sulcus is located on the internal surface of three cranial bones:
- Parietal bone (Os parietale) — a paired flat bone. The sulcus is situated on its concave internal surface (Facies interna).
- Temporal bone — the sulcus courses along the mastoid process (Processus mastoideus), which continues into the petrous part (Pars petrosa).
- Occipital bone (Os occipitale) — the sulcus lies on the intracranial surface of the lateral parts (Partes laterales).
Topography on the Parietal Bone
The internal surface (Facies interna) of the parietal bone presents:
- Sigmoid sulcus (Sulcus sinus sigmoidei).
- Sulcus for the superior sagittal sinus (Sulcus sinus sagittalis superioris).
- Sulcus for the middle meningeal artery (Sulcus arteriae meningeae mediae).
The parietal bone itself features four borders (sagittal, frontal, occipital, squamous) and four angles (frontal, occipital, sphenoidal, mastoid).
Topography on the Temporal Bone and External Communication
On the temporal bone, the sigmoid sulcus is located on the mastoid process (Processus mastoideus). This process is prominent, projects posteroinferiorly and laterally, and is bounded by the occipital border (Margo occipitalis).
Key structures in this area include:
- Mastoid foramen (Foramen mastoideum) — transmits an emissary vein connecting the sigmoid sulcus to the external surface of the skull.
The mastoid process also features the mastoid notch (Incisura mastoidea) and the sulcus for the occipital artery (Sulcus arteriae occipitalis).
Topography on the Occipital Bone
The sulcus is located on the intracranial surface of the lateral parts (Partes laterales) of the occipital bone.
This surface features:
- Jugular tubercle (Tuberculum jugulare).
- Hypoglossal canal (Canalis nervi hypoglossi) — located within the substance of the bone.
The jugular notch (Incisura jugularis) is located along the lateral margin of the lateral part. Together with the petrous part of the temporal bone, it forms the jugular foramen (Foramen jugulare). Posteriorly, the notch continues into the jugular process (Processus jugularis), and anterior to it lies the intrajugular process (Processus intrajugularis), which separates the venous and neural compartments within the foramen.
The lateral parts of the occipital bone bear the occipital condyle (Condylus occipitalis). Posterior to the condyle, a condylar fossa (Fossa condylaris) may be present, at the depth of which the condylar canal (Canalis condylaris), transmitting an emissary vein, occasionally opens.