Sphenoid Bone

SPHENOID BONE (os sphenoidale, os cuneiforme, alatum, multiforme, pterygoideum, os carinae, os colatorii), or wedge-shaped bone, in the adult human forms a single whole with the occipital bone and in this form is properly called the basic bone (os basilare, s. tribasilare). Usually, however, the sphenoid bone is understood to mean only its wedge-shaped part, i.e., the bone located transversely at the base of the skull, connecting with all of its bones, as well as with the maxilla, palatine bone, zygomatic bone, and vomer. It consists of a body and three pairs of processes. Corresponding to phylogeny, the human sphenoid bone also distinguishes a praesphenoid, i.e., the anterior part of the body, basisphenoid—the posterior, alisphenoid—the greater wings, orbitosphenoid—the lesser wings, and the pterygoid process (pterygoid)—joining later in the form of a medial plate. The body of the sphenoid bone (corpus) is of irregular shape, can be compared with some approximation to the shape of a cube, and consequently has six surfaces. The posterior surface connects with the body of the occipital bone by means of hyaline cartilage in youth and by means of firm bony fusion in adulthood (ossification occurs at the age of 13–30 years). The superior surface in its middle part bears a depression, variously expressed (fossa hypophyseos). By its appearance, it received the name of the Turkish saddle (sella turcica) and houses the pituitary gland. On the floor of the saddle fossa, closer to its anterior part, there may be (in adults in 0.3%) an opening of the canal (canalis craniopharyngeus) penetrating through the body of the sphenoid bone and opening on its lower surface (near the junction of the wings of the vomer) by the pharyngeal opening. The canal contains a continuation of the dura mater in the form of a fibrous sleeve enclosing connective tissue and blood vessels (veins). In front of the fossa protrudes a poorly noticeable tubercle (tuberculum sellae), on the sides of which are located more or less expressed processes (processus clinoidei medii). In front of these formations, a transversely running groove is noticeable, continuing anteriorly and laterally into the optic canals (sulcus optici, foramina optica) and demarcated by a relatively sharp edge (limbus sphenoidalis) from the platform located even more anteriorly (jugum sphenoidale); this latter extends to the sides and continues into the lesser wings, and anteriorly protrudes in the form of a flat, very differently shaped projection (spina ethmoidalis) connecting with the cribriform plate of the ethmoid bone. The fossa of the sella turcica is bounded posteriorly by a plate protruding sharply in the frontal plane (dorsum sellae), the posterior surface of which is somewhat inclined and continues into the clivus Blumenbachii (see Occipital bone). The upper edge of the plate is uneven and ends on the sides with well-expressed processes (processus clinoidei posteriores). The lateral surfaces of the body of the sphenoid bone are largely occupied by the bases of the greater and lesser wings departing here. Free is only their upper-external part, along which is located an S-shaped curved groove (sulcus caroticus) intended for the internal carotid artery, directed from behind and below, forward and upward. The relief of the groove is particularly emphasized at the very beginning by a protruding from the lateral side (at the base of the greater wings) process in the form of a bony plate with a sharp edge (lingula sphenoidalis), slightly bent with its end in a medial direction. There is no boundary between the anterior and inferior surfaces of the body of the sphenoid bone: they pass into one another and face the nasal cavity. In the middle of both protrudes a crest (crista sphenoidalis), ending anteriorly in the form of a sharp keel (rostrum sphenoidale). On the sides of the crest, rough areas (margo ethmoidalis) are located vertically, adjoining the labyrinth of the ethmoid bone. Between the crest and these roughnesses on the anterior and partly on the lower surface are

Fig. 1. Base of the skull from the inside: connection of the bones of the skull. (According to Spalteholz.)—Narrow and curved triangular-shaped plates (conchae sphenoidales, s. ossicula Bertini), essentially belonging by embryogenesis to the ethmoid bone and only later fusing with the sphenoid. Above and outside them there are notches in the form of round-shaped openings (aperturae sinuum) leading into the sphenoid sinus (sinus sphenoidalis) located in the body of the sphenoid bone. The sinus is quite volumetric, highly varying in shape and size. Its position is determined by the limits of spread: a strongly developed sinus reaches the optic foramen, surrounding it, and also reaches the foramen ovale and foramen rotundum. In childhood, the sinus is expressed insignificantly. By the age of 20, it acquires definite dimensions and shape, being essentially a double chamber, since a vertically standing septum (septum sinuum) divides it in the sagittal direction into non-communicating cavities. The septum is usually curved and deflected in one direction or another (more often to the right). Three types of sinus are distinguished (Shevkunenko). The first is when pneumatization reaches in the anteroposterior level of the sella turcica, so that the hypophysis lies posterior to the sinus; the second—pneumatization reaches the level of the posterior edge of the sella turcica, so that two sections are obtained: portio sellaris and portio antesellaris; the third—pneumatization reaches the level of the spheno-occipital synchondrosis; then, there is still another section of the sinus—portio retrosellaris. The sinus from the inside is lined with a mucous membrane penetrating here from the nasal cavity and firmly adhering to the periosteum with a fibrous surface (membrana Schneideriana). Blood and lymph vessels of the mucosa are in connection with those of the posterior section of the nasal cavity. Innervation—at the expense chiefly of the posterior ethmoidal nerve. On the lower surface of the body of the sphenoid bone, in the posterior section, there is a median groove, on the sides of which in the sagittal direction lateral grooves (sulci vomero-basilares) are located in the form of a groove open to the midline and supplemented from below by the vaginal processes (processus vaginalis) of the pterygoid processes. The lesser wings of the sphenoid bone (alae parvae) at their base are pierced by an opening (foramen opticum) through which the optic nerve passes. The upper surface of the wings faces the cranial cavity and participates in the formation of the anterior cranial fossa and the posterior section of the orbit; their anterior, serrated edge (margo frontalis) connects with the frontal bone, the posterior, smooth one protrudes freely into the cranial cavity and is the boundary between the anterior and middle cranial fossae (Fig. 1). On the medial part of the posterior edge there is a well-expressed protrusion (processus clinoideus), hanging over the anterior end of the carotid sulcus. Between this process and the middle clinoid process, a bony bridge is sometimes thrown, as a result of which an opening is formed (foramen carotico-clinoideum); this bony bridge can extend to the posterior clinoid process, and since an antilingula is located opposite the lingula, the artery in such a case turns out to be enclosed in a bony half-canal. The outer end of the wings is pointed and reaches the place of connection of the greater wings with the frontal bone. The wing as a whole limits from above the superior orbital fissure (fissura orbitalis superior) (Fig. 2).

Figure 2. Sphenoid bone from above: 1—corpus; 2—ala parva; 3—ala magna; 4—dorsum sellae; 5—proc. clinoideus post.; 6—sulcus caroticus; 7—lingula sphenoidalis; 8—foramen ovale; 9—foramen spinosum; 10—facies cerebralis; 11—foramen rotundum; 12—proc. clinoideus ant.; 13—fissura orbitalis sup.; 14—proc. clinoideus med.; 15—foramen opticum; 16—sella turcica; 17—tuberculum sellae; 18—sulcus chiasmatis. (According to Spalteholz.)

The greater wings (alae magnae) depart from the lower-lateral surfaces of the sphenoid bone with a wide base, which is pierced by a number of openings (Fig. 3 and 4): in front is the round opening (foramen rotundum) opening into the pterygopalatine fossa and intended for the second branch of the trigeminal nerve; closer to the posterior edge of the wing is located the oval opening (foramen ovale) for the third branch of the trigeminal nerve and

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