DESCRIPTIVE AND SURGICAL ANATOMY

THE SKULL

The Skull is divided into two parts, the Cranium and the Face. The Cranium is composed of eight bones; viz., the occipital, two parietal, frontal, two temporal, sphenoid, and ethmoid. The Face is composed of fourteen bones; viz., the two nasal, two superior maxillary, two lachrymal, two malar, two palate, two inferior turbinated, vomer, inferior maxillary. The ossicula auditus, the teeth, and Wor-mian bones, are not included in this enumeration

THE OCCIPITAL BONE

The Occipital Bone is situated at the posterior and inferior part of the cranium, is trapezoid in form, curved upon itself, and presents for examination two surfaces, four borders, and four angles.

Occipital Bone

External Surface. Midway between the summit of the bone and the posterior margin of the foramen magnum is a prominent tubercle, the external occipital pro­tuberance, for the attachment of the ligamentum nuchae; and descending from it, as far as the foramen, a vertical ridge, the external occipital crest. Passing out­wards from the occipital protuberance on each side are two semicircular ridges, the superior curved lines; and running parallel with these from the middle of the crest, are the two inferior curved lines. The surface of the bone above the supe­rior curved lines presents on each side a smooth surface, which, in the recent state, is covered by the occipito-frontalis muscle, whilst the ridges, as well as the surfaces of the bone between them, serve for the attachment of numerous muscles. The superior curved line gives attachment internally to the Trapezius, externally to the Occipito-frontalis, and Sterno-cleido mastoideus; to the extent shewn in the figure. The depressions between the curved lines to the Complexus internally, the Splenius capitis and Obliquus superior externally. The inferior curved line, and the depressions below it, afford insertion to the Rectus capitis posticus, major and minor.

The foramen magnum is a large aperture, with rounded shelving margins, oblong in form, and wider behind than in front; it transmits the spinal cord and its mem­branes, the spinal accessory nerves, and the vertebral arteries. On each side of the foramen magnum are the occipital condyles, for articulation with the Atlas; they are convex articular surfaces, oval in form, and directed downwards and out­wards; they approach each other anteriorly, and encroach more upon the anterior than the posterior segment of the foramen. On their inner surface is a rough tubercle, for the attachment of the check ligaments; whilst external to them is a rough tubercular prominence, the transverse or jugular process, channelled in front by a deep notch, which forms part of the jugular foramen. The under surface of this process affords attachment to the Rectus capitis lateralis muscle; its upper or cerebral surface presents a deeply curved groove, which lodges part of the lateral sinus, whilst its prominent extremity is marked by a quadrilateral rough surface, covered with cartilage in the fresh state, and articulating with a similar surface on the petrous portion of the temporal bone. On the outer side of each condyle is a depression, the anterior condyloid fossa, perforated at the bottom by the anterior condyloid foramen. This foramen (sometimes double) is directed downwards outwards, and forwards, and transmits the lingual nerve. Behind each condyle is seen an irregular fossa, also perforated at the bottom by a foramen, the posterior condyloid, for the transmission of a vein to the lateral sinus. This fossa and foramen are less regular in form and size than the anterior, and do not always exist. Sometimes they are found on one side only, and sometimes are altogether absent. In front of the foramen magnum is the basilar process, somewhat quadri­lateral in form, wider behind than in front; its under surface, which is rough, presenting in the median line a tubercular ridge, the pharyngeal spine, for the attachment of the tendinous raphe and Superior constrictor of the pharynx; and on each side of it, rough depressions for the attachment of the Recti capitis antici, major and minor.

The Internal or Cerebral Surface is deeply concave. The occipital part is divided by a crucial ridge into four fossae. The two superior, the smaller, receive the posterior lobes of the cerebrum, and present eminences and depressions corresponding to their convolutions. The two inferior, which receive the lateral lobes of the cerebellum, are larger than the former, and comparatively smooth; both are marked by slight grooves for the lodgment of arteries. At the point of meeting of the four divisions of the crucial ridge is an eminence, the internal occipital protuberance, which rarely corresponds to that on the outer surface. From this eminence, the superior division of the crucial ridge, called sulcus longi-tndinalis, runs upwards to the superior angle of the bone; it presents a deep groove for the superior longitudinal sinus, whilst its margins give attachment to the falx cerebri. The inferior division, the internal occipital crest, runs to the margin of the foramen magnum, on the edge of which it becomes gradually lost: this ridge, which is bifurcated below, serves for the attachment of the falx cerebelli, and is slightly grooved for the lodgment of the occipital sinuses. The transverse grooves (sulci transversales) pass outwards to the lateral angles; they are deeply grooved, for the lodgment of the lateral sinuses, their prominent margins affording attachment to the tentorium cerebelli. At the point of meeting of these four grooves is a depression for the torcular Herophili, placed a little to the right of the internal occipital protuberance. In the centre of the basilar portion of the bone is the foramen magnum, and above its margin, but nearer its anterior than its posterior part, the internal openings of the anterior condyloid foramina; the internal openings of the posterior condyloid foramina being a little external and posterior to them, and protected above by a small arch of bone. In front of the foramen magnum is the basilar process, presenting a shallow longitudinal depres­sion, the basilar groove, for supporting the medulla oblongata; whilst on its lateral

Occipital Bone, Internal Surface

margins is observed a narrow channel on each side, which, when united with a similar channel on the petrous portion of the temporal bone, forms a groove, the inferior petrosal, which lodges the inferior petrosal sinus.

Angles. The superior angle is acute, and is received into the interval between the posterior superior angles of the two parietal bones: it corresponds with that part of the head in the foetus which is called the posterior fontanelle. The infe­rior angle is represented by the square-shaped surface of the basilar process. At an early period of life, a layer of cartilage separates this part of the bone from the sphenoid; but in the adult, the union between them is osseous. The lateral margins is observed a narrow channel on each side, which, when united with a similar channel on the petrous portion of the temporal bone, forms a groove, the inferior petrosal, which lodges the inferior petrosal sinus.

Angles correspond to the outer ends of the transverse grooves, and are received into the interval between the posterior inferior angles of the parietal and the mastoid portion of the temporal.

Borders. The superior extends on each side from the superior to the lateral angle, is deeply serrated for articulation with the parietal bone, and forms by this union the lambdoid suture. The inferior border extends from the lateral to the inferior angle; its upper half is rough, and articulates with the mastoid por­tion of the temporal, forming the masto-occipital suture: the inferior half articu­lates with the petrous portion of the temporal, forming the petro-occipital suture: these two portions are separated from one another by the jugular process. In front of this process is a notch, which, with a similar one on the petrous portion of the temporal, forms the foramen lacerum posterius. This notch is often subdi­vided into two parts by a small process of bone.

Structure. The occipital bone consists of two compact laminae, called the outer and inner tables, having between them the diploic tissue; this bone is especially thick, at the ridges, protuberances, condyles, and basilar process; whilst at the bottom of the fossae it is thin, semi-transparent, and destitute of diploe.

Development. The occipital bone has seven centres of development; four for the posterior or occi­pital part, one for the basilar portion; and one for each con­dyloid portion.

Occipital Bone, Development

The four centres for the occipital portion are arranged hi pairs above and below the occipital protuberance, and appear about the tenth week of foetal life; the inferior pair make their appearance first, and join; the superior pair be­come also united: these two segments now join together, and form a single piece. The condyloid portions then os­sify; and, lastly, the basilar portion. At birth, the bone consists of these four parts, separate from one another, the posterior being fissured in the direction of the original segments. At about the fourth year, the occipital and the two condyloid pieces join; and at about the sixth year the bone consists of a single piece. At a later period, between the eighteenth and twenty-fifth years, the occipital and sphenoid become united, forming a single bone.

Articulations. With six bones; two parietal, two temporal, sphenoid, and Atlas.

Attachment of Muscles. To the superior curved line are attached the Occipito-frontalis, Trapezius, and Sterno-cleido-mastoid. To the space between the curved lines, the Complexus, Splenius capitis, and Obliquus superior; to the inferior curved line, and the space between it and the foramen magnum, the Rectus posticus ma­jor and minor; to the transverse process, the Rectus lateral is; and to the basilar process, the Recti antici majores and minores, and Superior Constrictor of the pharynx.

THE PARIETAL BONES

The Parietal Bones form the sides and roof of the skull; they are of an irre­gular quadrilateral form, and present for examination two surfaces, four borders, and four angles.

Parietal Bone, External Surface

Surfaces. The External Surface is convex, smooth, and presents about its centre an eminence, called the parietal eminence, which indicates the point where ossification commenced. Crossing the centre of the bone in an arched direction is a curved ridge, the temporal ridge, for the attachment of the temporal fascia. Above this ridge, the surface of the bone is rough and porous, and covered by the aponeurosis of the Occipito-frontalis; below it the bone is smooth, and affords attachment to the Temporal muscle. At the back part of the superior border is a small foramen, the parietal foramen, which transmits a vein to the superior longi­tudinal sinus. Its existence is not constant, and its position varies considerably.

Parietal Bone, Internal Surface

The Internal Surface, concave, presents numerous eminences and depressions for lodging the convolutions of the brain, and minute furrows for the ramifications of the meningeal arteries: these run upwards and backwards from deep grooves, which commence in the anterior inferior angle, and at the central and posterior part of the lower border of the bone. Along the upper margin is part of a shallow groove, which, when joined to the opposite parietal, forms a channel for the superior longitudinal sinus, the elevated edges of which afford attachment to the falx cerebri. Near the groove are seen several depressions; they lodge the Pacchionian bodies. The internal opening of the parietal foramen is also seen when that aperture exists.

Borders. The superior, the longest, is dentated to articulate with its fellow of the opposite side, forming the sagittal suture. The inferior is divided into three parts; of these, the anterior is thin and pointed, bevelled at the expense of the outer surface, and overlapped by the tip of the great wing of the sphenoid; the middle portion is arched, bevelled at the expense of the outer surface, and overlapped by the squamous portion of the temporal; the posterior portion being thick and serrated for articulation with the mastoid portion of the temporal. The anterior border, deeply serrated, is bevelled at the expense of the outer sur­face above, and of the inner below; it articulates with the frontal bone, forming the coronal suture. The posterior border, deeply denticulated, articulates with the occipital, forming the lambdoid suture.

Angles. The anterior superior, thin and pointed, corresponds with that portion of the skull which in the foetus is membranous, and is called the anterior fon-tanelle. The anterior inferior angle is thin and lengthened, being received in the interval between the great wing of the sphenoid and the frontal. Its inner surface is marked by a deep groove, sometimes a canal, for the middle meningeal artery. The posterior superior angle corresponds with the junction of the sagittal and lambdoid sutures. In the foetus this part of the skull is membranous, and is called the posterior fontanelle. The posterior inferior articulates with the mas-toid portion of the temporal bone, and presents on its inner surface a broad shallow groove for the lateral sinus.

Development. The parietal bone is developed by one centre, which corresponds with the parietal eminence, and makes its first appearance about the fifth or sixth week of fetal life.

Articulations. With five bones; the opposite parietal, the occipital, frontal, temporal, and sphenoid.

Attachment of Muscles. To one only, the Temporal.

THE FRONTAL BONE

This bone, which resembles a cockle-shell in form, consists of two portions— a vertical or frontal portion, situated at the anterior part of the cranium, forming the forehead; and a horizontal or orbito-nasal portion, which enters into the for­mation of the roof of the orbits and nose.

Frontal Bone, External Surface

Vertical Portion. External Surface . In the median line, traversing the bone from the upper to its lower part, is a slightly elevated ridge, and in young subjects a suture, which represents the point of union of its two lateral halves: in the adult, this suture usually disappears. On either side of this ridge, a little below the centre of the bone, is a rounded eminence, the frontal eminence, which indicates the point where ossification commenced. The whole surface of the bone above this part is smooth, and covered by the aponeurosis of the Occipito-frontalis muscle. Below the frontal protuberance, and separated from it by a slight groove, is the superciliary ridge, a curved eminence, broad internally where it is continuous with the nasal eminence, less distinct externally as it arches outwards. Beneath the superciliary ridge is the supra-orbital arch, a curved and prominent margin, which forms the upper boundary of the orbit, and separates the vertical from the horizontal portion of the bone. At the inner third of this arch is a notch, sometimes converted into a foramen by a bony process or ligament, and called the supra-orbital notch or foramen. It transmits the supra-orbital artery, veins, and nerve. The supra-orbital arch terminates externally in the external angular process, and internally in the internal angular process. The external angular is a strong prominent process, which articulates with the malar: running upwards and backwards from it is a sharp curved line, the temporal ridge, for the attachment of the temporal fascia; and beneath it a slight concavity, that forms part of the temporal fossa, and gives origin to the Temporal muscle. The internal angular processes are less marked than the external, and articulate with the lachrymal bones. Between the two is a rough, uneven interval, called the nasal notch, which articulates in the middle line with the nasal, and on either side with the nasal process of the superior maxillary bones.

Frontal Bone, Internal Surface

Vertical Portion. Internal Surface. Along the middle line of this surface is a vertical groove, sulcus longitudinalis, the edges of which unite below to form a ridge, the frontal crest; the groove lodges the superior longitudinal sinus, whilst its edges afford attachment to the falx cerebri. The crest terminates below, at a small opening, the foramen ccecum. which is generally completed be hind by the ethmoid; it lodges a process of the falx cerebri, and occasionally transmits a small vein from the nose to the superior longitudinal sinus. On either side of the groove, the bone is deeply concave, presenting eminences and depressions for the convolutions of the brain, and numerous small furrows for lodging the ramifications of the anterior meningeal arteries. Several small, irregular fossae are also seen on either side of the groove, for the reception of the Pacchionian bodies.

Horizontal Portion. External Surface. This portion of the bone consists ot two thin plates, which form the vault of the orbits, separated from one another by the ethmoidal notch. Each orbital vault consists of a smooth, concave, trian­gular plate of bone, marked at its anterior and external part (immediately beneath the external angular process) by a shallow depression, the lachrymal fossa, for lodging the lachrymal gland; and at its anterior and internal part, by a de­pression, sometimes a tubercle, for the attachment of the fibrous pulley of the superior oblique muscle. The ethmoidal notch separates the two orbital plates: it is quadrilateral; and filled up, when the bones are united, by the cribriform plate of the ethmoid. The edges of this notch present several half-cells, which, when united with corresponding half-cells on the upper surface of the ethmoid, com­plete the ethmoidal cells : two grooves are also seen crossing these edges trans­versely; they are converted into canals by articulation with the ethmoid, and are called the anterior and posterior ethmoidal canals; they open on the inner wall of the orbit. In front of the ethmoidal notch is the nasal spine, a sharp eminence, which projects downwards and forwards, and the grooved base of which forms part of the roof of the nose. It articulates in front with the crest of the nasal bones, behind with the perpendicular plate of the ethmoid. On either side of this spine are the openings of the frontal sinuses. These are two irregular cavities, which extend upwards and outwards, a variable distance, between the two tables of the skull, and are separated from one another by a thin bony septum. They give rise to the prominences above the root of the nose, called the nasal eminences. In the child they are absent, and they become gradually developed as age advances. They are lined by mucous membrane, and communicate with the nose by the infundibulum.

The Internal Surface of the Horizontal Portion presents the convex upper surfaces of the orbital plates, separated from each other in the middle line by the ethmoidal notch, and marked by eminences and depressions for the convolutions of the anterior lobes of the brain.

Borders. The border of the vertical portion is thick, strongly serrated, bevelled at the expense of the internal table above, where it rests upon the parietal, at the expense of the external table at each side, where it receives thp lateral pressure of those bones: this border is continued below, into a triangular rough surface, which articulates with the great wing of the sphenoid. The border of the horizontal portion is thin, bevelled at the expense of the internal table, and articulates with the lesser wing of the sphenoid.

Structure. The vertical portion consists of diploic tissue, contained between two compact laminae, the bone being especially thick in the situation of the nasal eminences and external angular processes. The horizontal portion is thinner, more translucent, and composed entirely of compact tissue.

Development. The frontal bone is developed by two centres, one for each lateral half, which make their appearance, at an early period of foetal life, in the situation of the orbital arches. At birth it consists of two pieces, which afterwards become united along the middle line, by a suture which runs from the vertex to the root of the nose. This suture becomes obliterated within a few years after birth; but it occasionally remains throughout life.

Articulations. With twelve bones; two parietal, sphenoid, ethmoid; two nasal, two superior maxillary, two lachrymal, and two malar.

Attachment of Muscles. To three pairs; the Corrugator supercilii, Orbicularis palpebrarum, and Temporal.

THE TEMPORAL BONES

The Temporal bones, situated at the side and base of the skull, present for examination a squamous, mastoid, and petrous portion.

The Squamous Portion, the most anterior and superior part of the bone, is flattened and scale-like in form, thin and translucent in texture. Its outer surface is smooth, convex, and grooved for the deep temporal arteries; it affords attachment to the fibres of the Temporal muscle, and forms part of the tem­poral fossa. At its back part may be seen a curved ridge—part of the temporal ridge; it serves for the attachment of the temporal fascia, limits the origin of the Temporal muscle, and marks the boundary between the squamous and mastoid portions of the bone. Projecting from the lower part of the squamous portion, is a long and arched process of bone, the zygomatic process. It is at first directed outwards, its two surfaces looking upwards and downwards; it then appears as if twisted upon itself, and takes a direction forwards, its surfaces now looking inwards and outwards. The superior border of this process is long, thin, and sharp, and serves for the attachment of the temporal fascia. The inferior, short, thick, and arched, has attached to it some of the fibres of the Masseter muscle. Its outer surface is convex and subcutaneous. Its inner, concave, also affords attachment to the Masseter. The extremity, broad and deeply serrated, articulates with the malar bone. This process is connected to the temporal bone by three divisions, called the roots of the zygomatic process, an anterior, middle, and posterior. The anterior, which is short, but broad and strong, runs trans­versely inwards into a rounded eminence, the eminentia articularis. This eminence forms the front boundary of the glenoid fossa, and in the recent state is covered with cartilage. The middle root runs obliquely inwards, and terminates at the edge of a well-marked fissure, the Glaserian fissure; whilst the posterior, which it strongly marked, runs from the upper border of the zygoma, in an arched direction, upwards and backwards, forming the posterior part of the temporal ridge. At the junction of the anterior and middle roots is a projection, called the tubercle, for the attachment of the external lateral ligament of the lower jaw; and between these roots is a large oval depression, forming part of the glenoid fossa, for the reception of the condyle of the lower jaw. This fossa is bounded in front by the eminentia articularis; behind, by the vaginal process; and exter­nally by the auditory process; and is divided into two parts by a narrow slit, the Glaserian fissure: the anterior part, formed by the squamous portion of the bone is smooth, covered in the recent state with cartilage, and articulates with the condyle of the lower jaw; the posterior part, rough and uneven, and formed chiefly by the vaginal process of the petrous portion, lodges part of the parotid gland. The Glaserian fissure, dividing the two, leads into the tympanum; it lodges the processus gracilis of the malleus, and transmits the laxator tympani muscle and the anterior tympanic artery. The chorda tympani nerve passes through a separate canal parallel to the Glaserian fissure, on the outer side of the Eustachian tube, in the retiring angle between the squamous and petrous portions of the temporal bone.

The internal surface of the squamous portion is concave, presents nu­merous eminences and depressions for the convolutions of the cerebrum, and two well marked grooves for the branches of the middle meningeal artery.

Temporal Bone, Outer Surface

Borders. The superior border is thin, bevelled at the expense of the internal surface, so as to overlap the lower border of the parietal bone, forming the squamous suture. The anterior inferior border is thick, serrated, and bevelled alternately at the expense of the inner and outer surfaces, for articulation with the great wing of the sphenoid.

The Mastoid Portion is situated at the posterior part of the bone, its outer surface is rough, and perforated by numerous foramina, one of these, of large size, situated at the posterior border of the bone, is termed the mastoid foramen, it transmits a vein to the lateral sinus and a small artery. The position and size of this foramen are very variable, being sometimes situated in the occipital bone, or in the suture between it and the occipital. The mastoid portion is continued below into a conical projection, the mastoid process, the size and form of which varies considerably in different individuals. This process serves for the attach­ment of the Sterno-mastoid, Splenius capitis and Trachelo-mastoid; on the inner side of the mastoid process is a deep groove, the digastric fossa, for the attachment of the Digastric muscle, and running parallel with it, but more internal, the occipital groove, which lodges the occipital artery. The internal surface presents a deeply curved groove which lodges the lateral sinus, and into it may be seen opening the mastoid foramen.

Temporal Bone, Inner Surface

Borders. The superior border of the mastoid portion is rough and serrated for articulation with the posterior inferior angle of the parietal bone. The posterior border, also uneven and serrated, articulates with the inferior border of the occi­pital bone between its lateral angle and jugular process.

The Petrous Portion, so named from its extreme density and hardness, is a pyramidal process of bone, wedged in at the base of the skull between the sphenoid and occipital bones. Its direction from without is forwards, inwards, and a little downwards. It presents for examination a base, an apex, three surfaces, and three borders. The base is applied against the internal surface of the squamous and mastoid portions, its upper half being concealed, but its lower half is exposed by their divergence, which brings into view the oval expanded orifice of a canal leading into the tympanum, the meatus auditorius externus. This canal is situated between the mastoid process and the posterior and middle roots of the zygoma; its upper margin is smooth and rounded, its lower surrounded by a carved plate of

bone, the auditory process, the free margin of which is rough for the attachment of the cartilage of the ear. The apex of the petrous portion, rough and uneven, is received into the angular interval between the spinous process of the sphenoid, and the basilar process of the occipital; it presents the anterior orifice of the carotid canal, and forms the posterior and external boundary of the foramen lacerum medium. The anterior surface of the petrous portion, forms the posterior boundary of the middle fossa of the skull. This surface is continuous with the squamous portion, to which it is united by a suture, the temporal suture, the remains of which are distinct at a late period of life. Proceeding from the base to the apex, this surface presents five points for examination. I. An eminence which indicates the situation of the superior semicircular canal. 2. A shallow groove, sometimes double, leading backwards to an oblique opening, the hiatus Fallopii, for the passage of the petrosal branch of the Vidian nerve. 3. A smaller opening immediately beneath and external to the latter for the passage of the smaller petrosal nerve. 4. Near the apex of the bone is seen the termination of the carotid canal, the wall of which in this situation is deficient in front. 5. Above the foramen is a shallow depression for the reception of the Gasserian ganglion.

The posterior surface forms the front boundary of the posterior fossa of the skull, and is continuous with the inner surface of the mastoid portion of the bone. It presents three points for examination. I. About its centre is a large orifice, the meatus auditorius internus. This aperture varies considerably in size, its margins are smooth and rounded, and it leads into a short and oblique canal which is directed outwards and forwards. It transmits the auditory and facial nerves and auditory artery. 2. Behind the meatus auditorius is a small slit almost hidden by a thin plate of bone, and leading to a canal, the aquaeductus vestibuli; it transmits a small artery and vein, and lodges a process of the dura mater. 3. In the interval between these two openings, but above them, is an angular depression which lodges a process of the dura mater, and transmits a small vein into the cancellous tissue of the bone.

The inferior or basilar Surface is rough and irregular, and forms part of the base of the skull. Passing from the apex to the base, this surface presents eleven points for examination;

1. A rough surface, quadrilateral in form, which serves partly for the attachment of the Levator palati, and Tensor tympani muscles.
2. The opening of the carotid canal, a large circular aperture, which ascends at first vertically upwards, and then making a bend, runs horizontally forwards and inwards. It transmits the internal carotid artery, and the carotid plexus.
3. The aquaeductus cochleae, a small triangular opening, lying on the inner side of the latter, close to the posterior border of the bone; it transmits a vein from the cochlea, which joins the internal jugular.
4. Behind these openings is a depression, the jugular fossa, which varies in depth and size in different skulls; it lodges the internal jugular vein, and with a similar depression on the margin of the occipital bone, forms the foramen lacerum posterius.
5. A small foramen for the passage of Jacobson's nerve (the tympanic branch of the glosso-pharyngeal). This is seen on the ridge of bone dividing the carotid canal from the jugular fossa.
6. The canal for Arnold 's nerve, seen on the inner wall of the jugular fossa
7. Behind the jugular fossa is a smooth square-shaped facet, the jugular surface, which articulates with the jugular process of the occipital bone.

The vaginal process, a very broad sheath-like plate of bone, which extends from the carotid canal to the mastoid process; it divides behind into two laminae, receiving between them the 8th point for examination, the styloid process; a long sharp spine, about an inch in length, continuous with the vaginal process, between the laminae of which it is received, and directed downwards, forwards, and inwards. It affords attachment to three muscles, the Stylo-pharyngeus, Stylo-glossus, and Stylo-hyoideus, and two ligaments, the stylo-hyoid and stylo-maxillary.

I0. The stylo-mastoid
foramen, a rather large orifice, placed between the styloid and mastoid processes;
it is the termination of the aquaeductus Fallopii, and transmits the facial nerve
and stylo-mastoid artery.

11. The auricular fissure, situated between the vaginaland mastoid processes, and transmitting the auricular branch of the pneumo-gastric nerve.

Borders. The superior, the longest, is grooved for the superior petrosal sinus, and has attached to it the tentorium cerebelli: at its inner extremity is a semilunar notch, upon which reclines the fifth nerve. The posterior border is intermediate in length between the superior and the anterior. Its inner half is marked by a groove, which, when completed by its articulation with the occipital, forms the channel for the inferior petrosal sinus. Its outer half presents a deep excavation for the jugular fossa, which, with a similar notch on the occipital, forms the fora­men lacerum posterius. A projecting eminence of bone occasionally stands out from the centre of the notch, and divides the foramen into two parts. The ante­rior border is divided into two parts, an outer, joined to the squamous portion by a suture the remains of which are distinct; an inner, free, articulating with the spinous process of the sphenoid. At the angle of junction of these two parts, are seen two canals, separated from one another by a thin plate of bone, the processus cochleariformis; they both lead into the tympanum, the upper one transmitting the Tensor tympani muscle, the lower one the Eustachian tube.

Structure. The squamous portion is like that of the other cranial bones, the mastoid portion cellular, and the petrous portion dense and hard.

Development of Temporal Bone

Development . The temporal bone is developed by/owr centres, exclusive of those for the internal ear and the ossicula, viz.;—one for the squamous portion including the zygoma, one for the petrous and mastoid parts, one for the styloid, and one for the auditory process (tympanic bone). The first traces of the develop­ment of this bone are found in the squamous portion, they appear about the time when osseous matter is deposited in the vertebrae; the auditory process succeeds next, it consists of an elliptical portion of bone, forming about three-fourths of a circle, the deficiency being above; it is grooved along its concave surface for the attachment of the membrana tympani, and becomes united by its extremities to the squamous portion during the last months of intra-uterine life. The pe­trous and mastoid portions then become ossified, and lastly the styloid process, which remains separate a considerable period, and is occasionally never united to the rest of the bone. At birth the temporal bone, excluding the styloid process, is formed of three pieces, the squamous and zygomatic, the petrous and mastoid, and the auditory. The auditory process joins with the squa­mous at about the ninth month. The petrous and mastoid join with the squa­mous during the first year, and the sty­loid process becomes united between the second and third years. The sub­sequent changes in this bone are the extension outwards of the auditory process so as to form the meatus audi-torius, the glenoid fossa becomes deeper,

THE SPHENOID BONE

The Sphenoid is situated at the anterior part of the base of the skull, articulating with all the other cranial bones, which it binds firmly and solidly together. In its form it somewhat resembles a bat, with its wings extended; and is divided into a central portion or body, two greater and two lesser wings extending outwards on each side of the body; and two processes, the pterygoid processes, which project from it below.

The Body presents for examination four surfaces—a superior, an inferior, an anterior, and a posterior.

Sphenoid Bone, Superior Surface

The superior surface. From before, backwards, is seen a prominent spine, the ethmoidal spine, for articulation with the ethmoid; behind this a smooth surface, presenting in the median line a slight longitudinal eminence, with a de­pression on each side, for lodging the olfactory nerves. A narrow transverse groove, the optic groove, bounds the above-mentioned surface behind; it lodges the optic commissure, and terminates on either side in the optic foramen, for the pas­sage of the optic nerve and ophthalmic artery. Behind the optic groove is a small eminence, olive-like in shape, the olivary process; and still more posteriorly, a deep depression, the pituitary fossa, or sella Turcica, which lodges the pituitary body. This fossa is perforated by numerous foramina, for the transmission of nu­trient vessels to the substance of the bone. It is bounded in front by two small eminences, one on either side, called the middle clinoid processes, and behind by a square-shaped plate of bone, terminating at each superior angle in a tubercle, the posterior clinoid processes, the size and form of which vary considerably in different individuals. The sides of this plate of bone are notched below, for the passage of the sixth pair of nerves; and behind, it presents a shallow depression, which slopes obliquely backwards, and is continuous with the basilar groove of the occipital bone; it supports the medulla oblongata. On either side of the body may be seen a broad groove, curved somewhat like f\ it lodges the internal carotid artery and the cavernous sinus, and is called the cavernous groove. The posterior surface, quadrilateral in form, articulates with the basilar process of the occipital bone. During childhood, a separation between these bones exists .by means of a layer of cartilage; but in after-life this becomes ossified, and the two bones are immoveably connected together.

Sphenoid Bone, Anterior Surface

The anterior surface presents, in the middle line, a vertical lamella of bone, which articulates in front with the perpendicular plate of the ethmoid. On either side of it are the irregular openings leading into the sphenoidal sinuses. These are two large, irregular cavities, hol­lowed out of the interior of the body of the sphenoid bone, and separated from one another by a more or less complete perpendicular septum; their form varies considerably, being often subdivided by irregular osseous laminae. These sinuses do not exist in children; but they increase in size as age advances. They are partially closed, in front and below, by two thin triangular plates of bone, the sphenoidal turbinated bones, leaving a round opening at their upper parts, by which they communicate with the upper and back part of the nose, and occa­sionally with the posterior ethmoidal cells. The lateral margins of this surface present a serrated edge, which articulates with the os planum of the ethmoid, completing the posterior ethmoidal cells; the lower margin, also rough and serrated, articulates with the orbital process of the palate bone; and the upper margin with the orbital plate of the frontal bone. The inferior surface presents, in the middle line, a triangular spine, the rostrum, which is continuous with the vertical plate on the anterior surface, and is received into a deep fissure between the ate of the vomer. On each side may be seen a projecting lamina of bone, which runs horizontally inwards from near the base of the pterygoid process: these plates, termed the vaginal processes, articulate with the edges of the vomer. Close to the root of the pterygoid process is a groove, formed into a complete canal when articu­lated with the sphenoidal process of the palate bone; it is called the pterygo-pala-tine canal, and transmits the pterygo-palatine vessels and pharyngeal nerve.

The Greater Wings are two strong processes of bone, which arise at the sides of the body, and are curved in a direction upwards, outwards, and backwards; being prolonged behind into a sharp-pointed extremity, called the spinous process of the sphenoid. Each wing presents three surfaces and a circumference. The superior or cerebral surface forms part of the middle fossa of the skull; it is deeply concave, and presents eminences and depressions for the convolutions of the brain. At its anterior and internal part is seen a circular aperture, the foramen rotundum, for the transmission of the second division of the fifth nerve. Behind and external to this, a large oval foramen, the foramen ovale, for the trans­mission of the third division of the fifth, the small meningeal artery, and the small petrosal nerve. At the inner side of the foramen ovale, a small aperture may occasionally be seen opposite the root of the pterygoid process; it is the foramen Vesalii, transmitting a small vein. Lastly, in the apex of the spine of the sphe­noid is a short canal, sometimes double, the foramen spinosum; it transmits the middle meningeal artery. The external surface is convex, and divided by a trans­verse ridge, the pterygoid ridge, into two portions. The superior or larger, convex from above downwards, concave from before backwards, enters into the formation of the temporal fossa, and attaches part of the Temporal muscle. The inferior portion, smaller in size and concave, enters into the formation of the zygomatic fossa, and affords attachment to the External pterygoid muscle. It presents, at its posterior part, a sharp-pointed eminence of bone, the spinous process, to which is connected the internal lateral ligament of the lower jaw, and the Laxator tympani muscle. The pterygoid ridge, dividing the temporal and zygomatic portions, gives attachment to the upper origin of the External ptery­goid muscle. At its inner extremity is a long triangular spine of bone, which serves to increase the extent of origin of this muscle. The anterior or orbital surface, smooth and quadrilateral in form, assists in forming the outer wall of the orbit. It is bounded above by a serrated edge, for articulation with the frontal bone; below, by a rounded border, which enters into the formation of the spheno-maxillary fissure; internally, it enters into the formation of the sphenoidal fissure; whilst externally it presents a serrated margin, for articulation with the malar bone. At the upper part of the inner border is a notch, for the transmission of a branch of the ophthalmic artery; and at its lower part a small pointed spine of bone, which serves for the attachment of part of the lower head of the external rectus. One or two small foramina may occasionally be seen, for the passage of arteries; they are called the external orbitar foramina. Circumference: from the body of the sphenoid to the spine (commencing from behind), the outer half of this margin is serrated, for articulation with the petrous portion of the temporal bone; whilst the inner half forms the anterior boundary of the foramen lacerum medium, and presents the posterior aperture of the Vidian canal. In front of the spine, the circumference of the great wing presents a serrated edge, bevelled at the expense of the inner table below, and of the external above, which articulates with the squamous portion of the temporal bone. At the tip of the great wing a triangular portion is seen, bevelled at the expense of the internal surface, for articulation with the anterior inferior angle of the parietal bone. Internal to thia is a broad serrated edge, for articulation with the frontal bone: this surface is continuous internally with the sharp inner edge of the orbital plate, which assists in the formation of the sphenoidal fissure.

The Lesser Wings (processes of Ingrassias) are two thin triangular plates of bone, which arise from the upper and anterior part of the body of the sphenoid; and, projecting transversely outwards, terminate in a more or less acute point. The superior surface of each is smooth, flat, broader internally than externally, and supports the anterior lobe of the brain. The inferior surface forms the back part of the roof of the orbit, and the upper boundary of the sphenoidal fissure, or foramen lacerum anterius. This fissure is of a triangular form, and leads from the cavity of the cranium into the orbit; it is bounded internally by the body of the sphenoid; above, by the lesser wing; and below, by the orbital surface of the great wing; and is converted into a foramen by the articulation of this bone with the frontal. It transmits the third, fourth, ophthalmic division of the fifth and sixth nerves, and the ophthalmic vein. The anterior border of the lesser wing is serrated, for articulation with the frontal bone; the posterior, smooth and rounded, is received into the fissure of Sylvius of the brain. The inner extremity of this border forms the anterior clinoid process. The lesser wing is connected to the side of the body by two roots, the upper thin and flat, the lower thicker, obliquely directed, and presenting on its outer side a small tubercle, for the attach­ment of the common tendon of the muscles of the eye. Between the two roots is the optic foramen, for the transmission of the optic nerve and ophthalmic artery.

Sphenoid Bone

The Pterygoid processes, one on each side, descend perpendicularly from the point where the body and great wing unite. Each pro­cess consists of an external and an internal plate, sepa­rated behind by an intervening notch; but joined partially in front. The externalpterygold plate is broad and thin, turned a little outwards, and forms part of the inner wall of the zygo-matic fossa. It gives attach­ment, by its outer surface, to the External pterygoid; its inner surface forms cart of the pterygoid fossa, and gives attachment to the Internal pterygoid. The internal pterygoid plate is much narrower and longer, curving outwards at its extremity, into a hook-like process of bone, the hamular process, around which turns the tendon of the Tensor-palati muscle. At the base of this plate is a small, oval, shallow depression, the scaphoid fossa, from which arises the Tensor-palati, and above which is seen the posterior orifice of the Vidian canal. The outer surface of this plate forms part of the pterygoid fossa, the inner surface forming the outer boundary of the posterior aperture of the nares. The two pterygoid plates are separated below by an angular notch, in which the pterygoid process, or tuberosity, of the palate bone is received. The anterior surface of the pterygoid process is very broad at its base, and supports Meckel's ganglion. It presents, above, the anterior orifice of the Vidian canal; and below, a rough margin, which articulates with the perpendicular plate of the palate bone.

Development of Sphenoid Bone

Development. The sphenoid bone is developed by ten centres, six for the pos terior sphenoidal division, and four for the anterior sphenoid. The six centres for the post-sphenoid are, one for each greater wing and external pterygoid plate; one for each internal pterygoid plate; two for the posterior part of the body. The four for the anterior sphenoid are, one for each lesser wing and anterior part of the body; and one for each sphenoidal turbinated bone. Ossification takes place in these pieces in the following order: the greater wing and external pterygoid plate are first formed, ossific granules being

deposited close to the foramen rotundum on each side, at about the second month of fcetal life; ossification spreading outwards into the great wing, and downwards into the external pterygoid process. Each internal pterygoid plate is then formed, and becomes united to the external about the middle of foetal life. The two centres for the posterior part of the body appear as separate nuclei, side by side, beneath the sella Turcica; they join about the middle of foetal life into a single piece, which remains ununited to the rest of the bone until after birth. Each lesser wing is formed by a separate centre, which appears on the outer side of the optic foramen, at about the third month; they become united and join with the body at about the eighth month of fcetal life. At about the end of the third year, ossification has made its appearance in the sphenoidal spongy bones.

At birth, the sphenoid consists of three pieces; viz. the greater wing and ptery­goid processes on each side; the lesser wings and body united. At the first year after birth, the greater wings and body are united. From the tenth to the twelfth year, the spongy bones commence their junction to the sphenoid, and be­come completely united by the twentieth year. Lastly, the sphenoid joins the occipital.

Articulations. The sphenoid articulates with all the bones of the cranium, and five of the face; the two malar, two palate, and vomer: the exact extent of articu­lation with each bone is shewn in the accompanying figures.

Attachment of Muscles. The Temporal, External pterygoid, Internal pterygoid, Superior constrictor, Tensor-palati, Laxator-tympani, Levator-palpebrae, Obliquus superior, Superior rectus, Internal rectus, Inferior rectus, External rectus.

THE SPHENOIDAL SPONOY BONES

The Sphenoidal Spongy Bones are two thin, curved plates of bone, which exist as separate pieces up to the fifteenth year. They are situated at the anterior and inferior part of the body of the sphenoid, serving to close in the sphenoidal sinuses in this situation. They are irregular in form, thick, and tapering to a point behind, broader and thinner in front. Their inner surface, which looks towards the cavity of the sinus, is concave; their outer surface convex. Each bone articulates in front with the ethmoid, an aperture of variable size being left in their anterior wall, which communicates with the posterior ethmoidal cells: behind, its point is placed under the vomer, and is received between the root of the pterygoid process on the outer side, and the rostrum of the sphenoid on the inner: externally, it articulates with the palate.

THE ETHMOID

The Ethmoid is an exceedingly light spongy bone, of a cubical form, situated at the anterior part of the base of the cranium, between the two orbits, at the root of the nose. It consists of three parts: a horizontal plate, which forms part of the base of the cra­nium; a perpendicular plate, which forms part of the sep­tum nasi; and two lateral masses of cells.

Ethmoid Bone, Outer Surface

The Horizontal or Cribri­form Plate forms part of the anterior ibssa of the base of the skull, and is re­ceived into the ethmoid notch of the frontal bone between the two orbital plates. Pro­jecting upwards from the middle line of this plate, at its fore part, is a thick smooth triangular process of bone, the crista galli,so called from its resemblance to a cock s-comb. Its base joins the cribriform plate. Its posterior border, long, thin, and slightly curved, serves for the attachment of the falx cerebri. Its anterior, short and thick, articulates with the frontal bone, and presents at its lower part two small projecting alas, which are received into corresponding depressions in the frontal, completing the foramen coecum behind. Its sides are smooth, and sometimes bulging, when it is found to enclose a small sinus. On each side of the crista galli, the cribriform plate is concave, to support the bulb of the olfactory nerves, and perforated by numerous foramina for the passage of its filaments. These foramina consist of three sets, corresponding to the three sets of olfactory nerves; an inner, which are lost in grooves on the upper part of the septum; an outer set, continued on to the surface of the upper spongy bones; whilst the middle set run simply through the bone, and transmit nerves distributed to the roof of the nose. At the front part of the cribriform plate, by the side of the crista galli, is a small fissure, which trans­mits the nasal branch of the ophthalmic nerve; and at its posterior part a trian­gular notch, which receives the ethmoidal spine of the sphenoid.

The Perpendicular Plate

The Perpendicular Plate is a thin central lamella of bone, which descends from the under surface of the cribriform plate, and assists in forming the septum of the nose. Its anterior border articu­lates with the frontal spine and crest of the nasal bones. Its poste­rior, divided into two parts, is connected by its upper half with the rostrum of the sphe­noid; its lower half with the vomer. The inferior border serves for the attachment of the triangular cartilage of the nose. On each side of the perpendicular plate numerous grooves and canals are seen, leading from the foramina on the cribriform plate; they lodge the filaments of the olfactory nerves.

The Lateral Masses of the ethmoid are made up of a number of thin walled cellular cavities, called the ethmoidal cells. In the disarticulated bone, many of these appear to be broken; but when the bones are articulated, they are closed in in every part. The superior surface of each lateral mass presents a number of these apparently half-broken cellular spaces; these, however, are completely closed in when articulated with the edges of the ethmoidal fissure of the frontal bone. Crossing this surface are seen two grooves on each side, converted into canals by articulation with the frontal; they are the anterior and posterior ethmoidal fora­mina. They open on the inner wall of the orbit, and transmit; the anterior, the anterior ethmoidal vessels and nasal nerve; the posterior, the posterior ethmoidal artery and vein. The posterior surface also presents large irregular cellular cavities, which are closed in by articulation with the sphenoidal turbinated bones, and orbital process of the palate. The cells at the anterior surface are completed by the lachrymal bone and nasal process of the superior maxillary, and those below also by the superior maxillary. On the outer surface of each lateral mass is a thin smooth square plate of bone, called the os planum; it forms part of the inner wall of the orbit, and articulates above with the frontal; below, with the superior maxillary and orbital process of the palate; in front, with the lachrymal; and behind, with the sphenoid.

The cellular cavities of each lateral mass, thus walled in by the os planum on the outer side, and by its articulation with the other bones already mentioned, are divided by a thin transverse bony partition into two sets, which do not commu­nicate with each other; they are termed the anterior and posterior ethmoidal cells; the former, the most numerous, communicate with the frontal sinuses above, and the middle meatus below, by means of a long flexuous cellular canal, the infundibulum; the posterior, the smallest and least numerous, open into the supe­rior meatus, and communicate (occasionally) with the sphenoidal sinuses behind. If the inner wall of each lateral mass is now examined, it will be seen how these cellular cavities communicate with the nose. The internal surface of each lateral mass presents, at its upper and back part, a narrow horizontal fissure, the supe­rior meatus of the nose, bounded above by a thin curved plate of bone, the superior turbinated bone of the ethmoid. By means of an orifice at the top part of this fissure, the posterior ethmoidal cells open into the nose. Below the superior meatus is seen the convex surface of another thin convoluted plate of bone, the middle turbinated bone. It extends along the whole length of the inner wall of each lateral mass; its lower margin is free and thick, and its concavity, directed outwards, assists in forming the middle meatus. It is by means of a large orifice at the upper and front part of this fissure, that the anterior ethmoid cells, and through them the frontal sinuses, by means of a funnel-shaped canal, the infundi­bulum, communicate with the nose. It will be remarked, that the whole of this surface is rough, and marked with numerous grooves and orifices, which run nearly vertically downwards from the cribriform plate; they lodge the branches of the olfactory nerve, which are distributed on the mucous membrane covering this surface. From the inferior part of each lateral mass, immediately beneath the os planum, there projects downwards and backwards an irregular lamina of bone, called the unciform process, from its hook-like form: it serves to close in the upper part of the orifice of the antrum, and articulates with the inferior turbi­nated bone.

Development. By three centres; one for the perpendicular lamella, and one for

masses, which are small and ill-developed; but when the perpendicular and hori­zontal plates begin to ossify, as they do about the first year after birth, the lateral masses become joined to the cribriform plate. The formation and increase in the ethmoidal cells, which complete the formation of the bone, take place about the fifth or sixth year.

Articulations. With fifteen bones; the sphenoid, two sphenoidal turbinated, the frontal, and eleven of the face—two nasal, two superior maxillary, two lachry­mal, two palate, two inferior turbinated, and vomer.

THE WOKMIAN BONES

The Wormian bones, called also, from their generally triangular form, osta triquetra, are irregular plates of bone, presenting much variation in situation, number, and size. They are most commonly found in the course of the sutures, especially the lambdoid and sagittal, where they occasionally exist of large size; the superior angle of the occipital, and the anterior superior angle of the parietal, being occasionally replaced by large Wormian bones. They are not limited to the vertex, for they are occasionally found at the side of the skull, in the situation of the anterior inferior angle of the parietal bone, and in the squamous suture; and more rarely they have been found at the base, in the suture between the sphenoid and ethmoid bones. Their size varies, in some cases not being larger than a pin's head, and confined entirely to the outer table; in other cases so large, that a pair of these bones formed the whole of that portion of the occipital bone above the superior curved lines, as described by Beclard and others. Their number is most generally limited to two or three; but more than a hundred have been found in the skull of an adult hydrocephalic skeleton. It appears most probable that they are separate accidental points of ossification, which, during their develop­ment and growth, remain separate from the adjoining bones. In their development, structure, and mode of articulation, they resemble the other cranial bones. NEXT PAGE