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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 cartilaginous 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) pendicular plate, which is usually deflected to
one or other side, has Middle meatusgenerally smooth surfaces, except above, where they are channelled by short and shallow grooves leading to the foramina which pierce the cribriform plate; these are for the lodgment of the olfactory nerves.
FIG. 147.-THE ETHMOID SEEN FROM BEHIND.
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 disarticulati
in by a thin oblong lamina, the lamin
are closed ich forms e orbital
a part of the medial wall of the
part of the frontal, which b suture is pierced by tw both of which transmit papyracea articulates
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 concha 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 the superior meatus, which is roofed in by FIG. 149.-SECTION SHOWING THE NASAL ASPECT the concha nasalis superior (superior concha),
OF THE LEFT LABYRINTH OF THE ETHMOID.
whilst its floor is formed by the superior
Middle cha of
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 medial side of the frontal process of the maxilla. By its thickened, free convoluted border it overhangs 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, Lacrimal process 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
of the nose
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 he inferior concha. The posterior extremity of the middle l crest on the perpendicular part of the
e) is the horizontal lamina which con
naso-turbinals of comparative anatomy.
Anterior ethmoidal groove
nects the ethmoidal labyrinths with the perpendicular plate. It occupies the interval between the orbital parts of the frontal bone, roofing in the nasal cavities inferiorly, and superiorly forming, on each side of the crista galli, two shallow
olfactory grooves, in which, in the recent condition, the olfactory bulbs of the cerebrum are lodged. Numerous foramina for the transmission of the olfactory nerves pierce this Slit for naso-ciliary part of the bone; those to the medial and lateral sides of the groove are the largest and most regular in their arrangement. Along the lateral edges of the cribriform plate two notches can usually be distinguished; when articulated with the frontal bone these form the medial openings of the ethmoidal foramina. Leading forwards from the anterior of these there is often a groove which crosses to the side of the crista galli, where it ends in a slit which allows of the transmission of the anterior ethmoidal nerve to the nose. Posteriorly, the cribriform plate articulates with the spine of the sphenoid.
Connexions. The ethmoid articulates with the sphenoid and sphenoidal conchæ, the frontal, the
two nasals, two maxillæ, two lacrimals, two inferior concha, two palates, and the vomer.
Ossification takes place in the cartilage of the nasal capsule. Each labyrinth has one centre, which appears about the fourth or fifth month in the neighbourhood of the lamina papyracea. According to Fawcett ossification first commences in a process which passes outside the naso-lacrimal duct to reach the frontal process of the maxilla. From this the lamina around the ethmoidal air-cells are formed, which are complete at birth, the air-sinuses in this instance not being formed by the absorption of spongy bone. From these centres the conchæ are also developed, and these too are ossified at the ninth month.
At birth the ossified labyrinths are united to the cartilaginous septum by a fibrous layer. Two centres make their appearance in the septal cartilage on either side of the root of the crista galli about the end of the first year; from these, the crista galli and the perpendicular plate are ossified, as well as the medial part of the cribriform plate, the lateral portions of which are derived from a medial extension of the labyrinths.
Ossification is usually complete about the fifth or sixth year. About the twenty-fifth year bony union has taken place between the cribriform plate and the sphenoid, but ankylosis between the perpendicular plate and the vomer is not usual till the fortieth or forty-fifth year.
Conchæ Nasales Inferiores.
The inferior concha (O.T. inferior turbinated bones) are two shell-like laminæ of bone lying along the lower part of the lateral wall of the nasal cavity on either
FIG. 152. THE RIGHT INFERIOR CONCHA. A, Medial Surface; B, Lateral Surface.
side. Of elongated form, the bone displays two curved borders enclosing a medial and lateral surface.
The superior or attached border is thin and sharp in front and behind, where
it articulates with the inferior conchal crests on the medial surface of the body of the maxilla and the perpendicular part of the palate bone, respectively. Between these two articulations the central part of the superior border rises in the form of a sharp crest, the anterior part of which forms the upstanding lacrimal process which articulates above with the descending process of the lacrimal bone, as well as with the edges of the naso-lacrimal groove of the maxilla, thus completing the osseous canal of the naso-lacrimal duct. The posterior end of this crest is elevated in the form of an irregular projection called the ethmoidal process. This unites with the uncinate process of the ethmoid bone (see Fig. 150). Spreading downwards from the middle of the superior border, on its lateral side, is a thin irregular plate of bone, the maxillary process, which partially conceals the lateral concave surface of the bone, and, by its union with the medial wall of the maxillary sinus, assists in the completion of the partition which separates that cavity from the inferior nasal meatus.
The inferior or free border, gently curved from before backwards and turned slightly laterally, is rounded and full, and formed of bone which is deeply pitted and of a somewhat cellular character. The anterior and posterior extremities of the bone, formed by the convergence of the superior and inferior borders, are thin and sharp; as a rule the posterior end is the more pointed of the two. The medial surface projects into the nasal cavity; convex from above downwards, and slightly curved from before backwards, it forms the floor of the middle meatus. It is rough and pitted, and displays some scattered and longitudinally directed vascular grooves. The lateral surface overhangs the inferior meatus of the nose. Concave from above downwards, and to some extent from before backwards, it is directed towards the lateral wall of the nasal cavity. It is smooth in front, where it corresponds to the opening of the canal for the naso-lacrimal duct; behind and towards its inferior border it is irregular and pitted. In the disarticulated bone this surface is in part concealed by the downward projecting maxillary process.
Connexions.-The inferior concha articulates with the maxilla, lacrimal, ethmoid, and palate bones.
Ossification.-The inferior concha (the maxillo-turbinal of comparative anatomy) is derived from the cartilage forming the lateral wall of the nasal capsule, the upper portion of which forms the ethmo- turbinals. It ossifies, however, from a separate centre, which appears about the fifth month of fœtal life, and later contracts a union by a horizontal lamella on its lateral side with the maxilla.
The lacrimal bone, a thin scale of bone about the size of a finger-nail, forms part of the medial orbital wall behind the frontal process of the maxilla. Irregularly quadrangular, it has two surfaces-a medial and lateral-and four borders.
Its lateral or orbital surface has a vertical ridge, the crista lacrimalis posterior (posterior lacrimal crest), running downwards upon it. In front of this is the sulcus lacrimalis (lacrimal groove) for the lodgment of the lacrimal sac. medial wall of this groove descends below the level of the bulk of the bone, and forms the descending process, which helps to complete the osseous canal for the naso-lacrimal duct, and articulates inferiorly with the inferior concha. The inferior end of the lacrimal crest terminates in a hooklike projection, the hamulus lacrimalis (lacrimal hooklet), which curves round the posterior and lateral edge of the naso-lacrimal notch of the maxilla, and thus defines the upper aperture of the canal for the naso-lacrimal duct. To the free edge of the crest, behind the lacrimal groove, are attached the reflected portion of the medial palpebral liga- FIG. 153. RIGHT LACRIMAL ment and the lacrimal part of the orbicularis oculi, the crest being sometimes thickened at the site of this attachment. The part of the bone behind the lacrimal crest is smooth and continuous with the surface of the lamina papyracea of the ethmoid. The medial surface is irregular and
BONE (Orbital Surface).
Lacrimal crest Lacrimal groove
cellular above; it closes in some of the anterior ethmoidal cells and helps to complete the infundibulum. Where it is smoother it forms a part of the lateral wall of the middle meatus of the nose immediately behind the frontal process of the maxilla, and above the inferior concha. The superior border articulates with the orbital part of the frontal; the anterior edge with the posterior border of the frontal process of the maxilla, with which it completes the lacrimal groove for the lodgment of the lacrimal sac. The inferior margin articulates with the orbital surface of the maxilla, and in front by its descending process with the inferior concha. Posteriorly the bone articulates with the anterior border of the lamina papyracea of the ethmoid.
Connexions. The lacrimal bone articulates with four bones-the frontal, ethmoid, inferior concha, and the maxilla.
Ossification. The lacrimal is developed from a single centre, which makes its appearance about the end of the second or the beginning of the third month of intrauterine life in the membrane around the cartilaginous nasal capsule.
The vomer or ploughshare bone, a bone of irregular quadrilateral shape, is placed in the posterior part of the nasal septum. It has four borders and two
surfaces. The superior border, which can readily be distinguished by the presence on either side of an everted lip or ala, for septal slopes from behind upwards and forwards, and articulates with the inferior surface of the body of the sphenoid, the pointed rostrum of which is received into the groove bounded by the projecting alæ. Laterally these alæ are wedged in between the sphenoidal processes of the palate bones in front, and the vaginal processes at the root of the medial laminæ of the
Palate Maxilla FIG. 154. THE VOMER AS SEEN FROM THE RIGHT SIDE.
pterygoid processes behind. The posterior border, which slopes from behind downwards and forwards, is free, and forms a sharp, slightly curved edge; this constitutes the posterior margin of the nasal septum, and serves to separate the openings of the choanæ (O.T. posterior nares). The inferior border, more or less horizontal in direction, articulates with the nasal crest formed by the maxillæ and palate bones. The anterior edge is the longest; it slopes obliquely from above downwards and forwards. In its upper half it is ankylosed to the perpendicular plate of the ethmoid; in its lower half this margin is grooved for the reception of the septal cartilage of the nose. The anterior extremity of the bone forms a truncated angle, which articulates with the posterior border of the incisor crest of the maxillæ, and sends downwards a pointed process which passes between the incisor foramina. The right and left surfaces of the bone are smooth and covered by mucous membrane. It is not uncommon to find them deflected to one or other side. A few vascular grooves may be noticed scattered over these surfaces, and one, usually more distinct than the others, running obliquely downwards and forwards, indicates the course of the naso-palatine nerve.
Vomer at Birth, displaying its formation by two Osseous Laminæ united inferiorly. The figure to the right exhibits the appearance of the bone, in vertical section, at the point marked in the left figure.
Connexions. The vomer articulates with the sphenoid, the ethmoid, the palates, and the maxillæ. In front it supports the septal cartilage.
Ossification.-The vomer commences to ossify in membrane at the end of the second month. A nucleus appears on each side of the middle line, below the nasal septum, medial to the plane of the anterior paraseptal cartilages and posterior to them. During the third month the nuclei, which have increased in height and length,