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Occasionally there is a small foramen (canaliculus innominatus) to the medial side of the foramen spinosum for the transmission of the small superficial petrosal nerve.
The external surface of the great wing enters into the formation of the walls of the orbital, temporal, infra-temporal, and pterygo-palatine fosse by three welldefined areas; of these the upper two, i.e. the orbital and the temporal, are separated by an oblique jagged ridge, the margo zygomaticus (zygomatic border), for articulation with the fronto-sphenoidal process of the zygomatic bone. Occasionally the lower part of this ridge articulates with the zygomatic process of the maxilla. The facies orbitalis (orbital surface) lies to the medial side of this crest and is directed forwards and a little medially; of quadrilateral shape, it forms the posterior and lateral wall of the orbit; plane and smooth, it is bounded posteriorly by the sharp inferior free margin of the superior orbital fissure, towards the medial extremity of which a pointed spine (spina recti lateralis), for the attachment of the inferior common ligament of origin of the ocular muscles, can usually be seen. It is limited superiorly by the edge of a rough triangular area which articulates with the frontal bone; anteriorly by the zygomatic border; whilst inferiorly a free, well-defined oblique margin constitutes the posterior and lateral boundary of the fissura orbitalis inferior (inferior orbital fissure), which separates this part of the bone from the orbital surface of the maxilla. Below this border there is a grooved surface which leads medially toward the orifice of the foramen rotundum. In the articulated skull this forms part of the posterior wall of the pterygo-palatine fossa.
To the lateral side of the zygomatic border, which bounds it in front, is the facies temporalis (temporal area), concavo-convex from before backwards. It slopes medially below, where it is separated from the spheno-maxillary area by a wellmarked muscular ridge, the crista infratemporalis (infra-temporal crest). Behind, the temporal surface is bounded by the margin of the great wing which articulates with the squamous part of the temporal (margo squamosus), and above by the edge which unites it with the sphenoidal angle of the parietal and with the frontal bone. The temporal surface enters into the formation of the floor of the fossa of the same name, and affords an extensive attachment to the fibres of origin of the temporal muscle. The facies sphenomaxillaris (spheno-maxillary surface), the third of the areas above referred to, is situated below the infra-temporal crest, and corresponds to the under surface of the posterior half of the great wing; it extends as far back as the angular spine and posterior border. Opening on it are seen the orifices of the foramen spinosum and ovale. It is slightly concave from side to side, and is confluent medially with the lateral surface of the lateral pterygoid plate. In front, it is bounded by a ridge which curves upwards and laterally from the anterior part of the lateral pterygoid plate to join the infra-temporal crest. In the articulated skull this ridge forms the posterior boundary of the pterygomaxillary fissure. The spheno-maxillary surface overhangs the infra-temporal fossa, and affords an origin for the superior head of the external pterygoid muscle. The processus pterygoidei (pterygoid processes) spring from the inferior surface of each lateral aspect of the body as well as from the under side of the roots of the great wings, and pass vertically downwards. Each consists of two lamina, the lateral and medial laminæ of the pterygoid process, fused together anteriorly, and enclosing between them posteriorly the pterygoid fossa.
The lateral pterygoid plate, thin and expanded, is directed obliquely backwards and laterally, its lower part being often somewhat everted. Its posterior edge is sharp, and often has projecting from it one or two spines, to one of which processus pterygospinosus) the pterygo-spinous ligament, which stretches towards the angular spine, is attached. Laterally it furnishes an origin for the inferior head of the external pterygoid muscle, and on its medial side, where it forms the lateral wall of the pterygoid fossa, it supplies an attachment for the internal pterygoid muscle.
The medial pterygoid plate is narrower and somewhat stouter. By its medial aspect it forms the posterior part of the lateral wall of the nasal cavity; laterally it is directed towards the pterygoid fossa. Its posterior edge ends
below in the hamulus pterygoideus (pterygoid hooklet), which, reaching a lower level than the lateral plate, curves backwards and laterally, furnishing a groove on its lower surface in which the tendon of the tensor veli palatini muscle glides; superiorly, the sharp posterior margin of the medial plate bifurcates, so as to enclose the shallow scaphoid fossa from which the tensor veli palatini muscle arises, and wherein may occasionally be seen the inferior aperture of the foramen Vesalii. To the medial edge of this fossa, as well as to the posterior border of the medial pterygoid plate, the pharyngo-basilar fascia is attached. Here, too, the cartilage of the auditory tube is supported on a slight projection, and the pharyngo-palatinus muscle receives an origin, whilst the superior constrictor of the pharynx arises from the inferior third of the same border and from the pterygoid hamulus. Superiorly and medially the medial plate forms an incurved lamina of bone, the processus vaginalis (vaginal process), which is applied to the inferior surface of the lateral aspect of the body, reaching medially towards the root of the rostrum, from which, however, it is separated by a groove, in which, in the articulated skull, the ala of the vomer is lodged. The angle formed by the vaginal process and the medial edge of the scaphoid fossa forms a projection called the pterygoid tubercle, immediately above which is the posterior aperture of the pterygoid canal, through which the nerve and artery of the canal (O.T. Vidian) are transmitted. On its inferior surface the vaginal process displays a groove (sulcus pterygopalatinus), which in the articulated skull is converted into the pharyngeal canal by its union with the palate bone. In front, at its root, the pterygoid process displays a broad smooth surface (facies sphenomaxillaris), which is confluent above with the root of the great wing around the foramen rotundum, and forms the posterior wall of the pterygo-palatine fossa. Here, to the medial side of the foramen rotundum, is seen the anterior opening of the pterygoid canal. Below, the pterygoid lamina are separated by an angular cleft, the pterygoid fissure; in this is lodged the pyramidal process of the palate bone, the margins of which articulate with the serrated edges of the fissure.
Connexions. The sphenoid articulates with the occipital, temporal, parietal, frontal, ethmoid, sphenoidal concha, vomer, palate and zygomatic bones, and occasionally with the maxillæ.
Ossification. The sphenoid of man is formed by the fusion of two parts, the presphenoid and the post-sphenoid, each associated with certain processes. In most manimals the orbito-sphenoids or small wings fuse with the pre-sphenoid, whilst the alisphenoids or great wings, together with the medial pterygoid lamina, ankylose with the post-sphenoid. The ossification of these several parts takes place in cartilage, with the exception of the medial pterygoid lamina, which is developed from an independent centre in the connective tissue of the side wall of the oral cavity (Hertwig).
At the end of the second month a centre appears in the root of the great wing between the foramen ovale and foramen rotundum; from this the ossification spreads laterally and
b a b
backwards and also downwards into the lateral pterygoid lamina. According to Fawcett the pterygoid laminæ or the common root of the two laminæ in the adult is practically the only part of the ala temporalis preformed in cartilage; the whole of the lateral pterygoid lamina and that part of the alisphenoid projected into the orbital and temporal fossæ are ossified in membrane; so too are the foramen ovale and foramen spinosum. Meanwhile two centres appear about the same time in the basi-sphenoid in relation to the floor of the d, Medial pterygoid lamina; e, Basi-sphenoid. fossa hypophyseos and on either side of the cranio-pharyngeal canal, around which they ossify, ultimately leading to the obliteration of that channel. Somewhat later a sphenotic centre appears on each side, from which the lateral aspect of the body and the lingula are developed. Fusion between these four centres is usually complete by the sixth month.
In the pre-sphenoid a pair of lateral nuclei make their appearance about the middle of the third month, just lateral to the optic foramina; from each of these the orbito
sphenoids (small wings) and their roots are developed. About the same time another pair of centres, placed medial to the optic foramina, constitute the body of the presphenoid. At first the superior surface of the body of the pre-sphenoid is exposed in the interval between the orbito-sphenoids, but by the ultimate coalescence of the medial borders of the orbito-sphenoids to form the jugum sphenoidale the body of the pre-sphenoid is almost completely covered over superiorly. By the coalescence of these in front, and their ultimate union with the basi-sphenoid behind, a cartilaginous interval is enclosed, of triangular shape, which, however, becomes gradually reduced in size by the ingrowth of its margins so as to form two medially placed foramina, as may be frequently observed in young bones-one opening on the surface of the tuberculum sellæ, the other being placed anteriorly. (Lawrence," Proc. Soc. Anat.," Journ. Anat. and Physiol. vol. Ixviii. p. 19.)
As has been seen, the medial pterygoid laminæ are developed in membrane and are the first parts of the sphenoid to ossify. (Fawcett, Anat. Anz., vol. xxvi. 1905, p. 280.) Each is derived from a single nucleus which appears about the ninth or tenth week, and fuses with the inferior surface of the great wing, there forming a groove which is converted into the pharyngeal canal when the alisphenoid and medial pterygoid lamina fuse later with the body of the post-sphenoid. The hamulus, however, chondrifies before it ossifies during the third month. Fawcett also regards the lateral pterygoid plate as of membranous origin.
At birth the sphenoid consists of three parts: one comprising the orbito-sphenoids together with the body of the pre-sphenoid and the basi-sphenoid, the others consisting of the alisphenoids, one on each side. Fusion of the latter with the former occurs near the end of the first year. The dorsum sellæ at birth consists of a cartilaginous plate which separates the body of the post-sphenoid from the basi-occipital. This slowly ossifies, but the cartilage does not entirely disappear till the age of twenty-five, by which time bony ankylosis of the basi-cranial axis is complete. For a considerable time the under surface of the body of the pre-sphenoid displays a bullate appearance, with the sides of which the sphenoidal concha articulate. It is only after the seventh or eighth year is reached that the spongy tissue within this part of the bone becomes absorbed to form the sphenoidal sinuses.
The sphenoidal conchæ, or bones of Bertin, best studied in childhood, are formed by the fusion of four distinct ossicles (Cleland), the centres for which appear in the later months of utero gestation. Each bone consists of a hollow, three-sided pyramid, the apex of which is in contact with the anterior part of the vaginal process of the medial pterygoid lamina, whilst the base fits on to the posterior surface of the labyrinth of the ethmoid. The inferior surface of each forms the roof of the corresponding nasal cavity, and completes the formation of the spheno-palatine foramen, whilst the lateral aspect is united with the palate bone and forms the medial wall of the pterygo-palatine fossa, and Occasionally constitutes a part of the orbital wall posterior to the lamina papyracea of the ethmoid. The superior surface of each sphenoidal concha is applied to the anterior and inferior surface of the body of the pre-sphenoid on the corresponding side of the rostrum. It is by the absorption of this surface that the contained sphenoidal sinus is ultimately extended. In the bases of the pyramids are formed the apertures through which the sinuses open in to the nasal cavity in the adult. Up to the age of five these ossicles remain independent, but subsequently, owing to their firm ankylosis with the surrounding bones, they are merely represented in the adult disarticulated skull by the irregular fragments adherent to the separated borders of the ethmoid, palate, and sphenoid bones.
The ethmoid bone lies in front of the sphenoid, and occupies the interval between the orbital parts of the frontal, thus entering into the formation of the anterior cranial fossa as well as the medial walls of the orbits and the roof and medial and lateral walls of the nasal cavities. The bone, which is extremely light, consists of two cellular parts-the labyrinthi ethmoidales (ethmoidal labyrinths), which are united superiorly to a median lamina perpendicularis (perpendicular plate) by a thin horizontal lamina which, from its perforated condition, is called the lamina cribrosa (cribriform plate). The general arrangement of the parts of the bone resembles the capital letter T; the median plate corresponds to the vertical limb, the cribriform plate to the horizontal limb of the T, whilst the ethmoidal labyrinths may be
regarded as comparable to the enlarged down-turned extremities of the horizontal limb of the letter.
The study of this bone will be much facilitated by cutting through the cribriform plate on one side of the perpendicular plate, thus removing the ethmoidal labyrinth of one side and exposing more fully the central perpendicular lamina.
The perpendicular plate, of irregular pentagonal shape, forms the superior part of the nasal septum. Its superior border projects above the level of the cribriform plate so as to form a crest, which is much elevated anteriorly, where it terminates in a thick, vertical, triangular process, called the crista galli, the interior of which is filled with fine spongy bone, but is occasionally pneumatic. The superior edge of this process is sharp and pointed, and affords attachment to the falx cerebri. In front of this process there is a groove which separates the processus alares (alar processes) which project from the crista galli on either side. By articulation with the frontal bone this groove is converted into a canal, the foramen cæcum; this, however, is not always blind, but frequently transmits a vein to the roof of the nose. The posterior border of the perpendicular plate is thin, and articulates with the crest of the sphenoid. The posterior inferior border in the adult is ankylosed with the vomer; and the anterior inferior border, which is usually thicker than the others, unites with the carti
laginous nasal septum. The anterior superior border articulates with the spine of the frontal bone and with the median crest formed by the union of the two
nasal bones. The per- (orbital surface)
Middle concha of the nose
FIG. 148. THE ETHMOID SEEN FROM THE RIGHT SIDE.
foramina which pierce the cribriform plate; these are for the lodgment of the olfactory nerves.
The ethmoidal labyrinth is composed of exceedingly thin bone, enclosing a large number of air-cells; these are arranged in three groups-an anterior, a middle, and a posterior, the walls of which have been broken in front, above, behind, and below, in the process of disarticulation. Laterally they are closed in by a thin oblong lamina, the lamina papyracea or orbital plate, which forms a part of the medial wall of the orbit, and articulates above with the orbital part of the frontal, which here roofs in the ethmoidal cells. (The line of this suture is pierced by two canals, the anterior and posterior ethmoidal foramina, both of which transmit small ethmoidal vessels and nerves. In front, the lamina papyracea articulates with the lacrimal bone; whilst below, by its union with
Anterior ethmoidal groove
the orbital surface of the maxilla, the air-sinuses in both situations are completed. Posteriorly, the lamina papyracea articulates with the sphenoid, and, at its posterior inferior angle for a variable distance, with the orbital process of the palate bone, both of which serve to close in the air-cells. The medial aspect of the ethmoidal labyrinth displays the convoluted concha of the nose, usually two in number, though occasionally there may be three-rarely more. cases where there are two conchæ or ethmoturbinals they are separated posteriorly by a deep groove. A channel is thus formed in the posterior part of the lateral and upper aspect of the nasal cavity, called
OF THE LEFT LABYRINTH OF THE ETHMOID.
the superior meatus, which is roofed in by FIG. 149.-SECTION SHOWING THE NASAL ASPECT the concha nasalis superior (superior concha), whilst its floor is formed by the superior surface of the concha nasalis media (middle concha). The posterior ethmoidal cells open into this meatus. In front of the superior meatus, which only grooves the posterior half of this aspect of the bone, the surface is rounded from above downwards and before backwards, and forms the medial wall of the anterior and middle ethmoidal cells. Running obliquely from above downwards and backwards over the medial surface of the superior concha, are a number of fine grooves continuous above with the foramina in the cribriform plate; these are fewer and more scattered in front, do not pass on to the middle concha, and are for the olfactory nerves.
The middle concha is nearly twice the length of the superior. Its anterior extremity is united for a short distance to the ethmoidal crest on the
Middle/ Concha of the nose
medial side of the frontal
process of the maxilla. By its thickened, free convoluted border it overhangs a a deep groove which runs along the
inferior surface of the ethmoidal labyrinth. This is the middle Perpendicu- meatus of the nose. It receives lar plate the openings of the middle ethmoidal cells, which project into the meatus, forming a rounded elevation called the ethmoidal bulla. Uncinate process In front of and below the bulla is a groove, the hiatus semilunaris, which by articulation above with adjacent bones is converted into a canal, the infundibulum, which Inferior concha runs upwards and forwards and forms a channel of communication with the frontal sinus and the anterior ethmoidal cells. Curving downwards, backwards, and a little laterally from the roof of the anterior part of this meatus, in front of the infundibulum, is the processus uncinatus. This bridges across the irregular opening on the medial wall of the maxillary sinus, and articulates inferiorly with the ethmoidal process of the inferior concha. The posterior extremity of the middle concha articulates with the ethmoidal crest on the perpendicular part of the palate bone.
FIG. 150.-SHOWING THE ARTICULATION OF THE INFERIOR
The lamina cribrosa (cribriform plate) is the horizontal lamina which con
So called to distinguish them from the maxillo-turbinals and naso-turbinals of comparative anatomy.