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ethmoid. On each side of the root of this process the nasal part of the bone is grooved obliquely from above downwards and forwards, and enters into the formation of the narrow roof of the nasal cavity. Anteriorly the nasal notch is limited by a rough, U-shaped serrated surface, the medial part of which articulates with the nasal bones, whilst on each side the frontal processes of the maxillæ are united with it. Behind this, amid the broken cells, the passages leading into the frontal sinuses are readily distinguished, and here the medial edges of the orbital plates articulate with the lacrimal bones. The orbital part is thin and brittle. Anteriorly, it is bounded by the supraorbital margin, just within which, midway between the medial angular process and the supraorbital notch, there is a small shallow depression (fovea trochlearis), often displaying a spicule of bone arising from its edge (trochlear spine), which affords attachment to the pulley of the superior oblique muscle of the eyeball. Laterally, the orbital part is overhung by the supraorbital margin and the zygomatic process, and in the hollow so produced (fossa glandula lacrimalis) the lacrimal gland is lodged. The extremity of the zygomatic process articulates with the frontal process of the zygomatic bone
Behind this the irregular edge of the orbital surface is united with the great wing of the sphenoid by a triangular area, which also extends on to the inferior aspect of the temporal surface of the frontal bone. The apex of the orbital surface, for the space of about half an inch, articulates with the small wing of the sphenoid. The cerebral surface of the bone forms a fossa in which lie the anterior and inferior parts of the frontal lobes of the cerebrum, the gyri of which impress their form on the inner table of the bone. Here, too, on each side of the median plane, may be seen depressions, called foveola granulares, for the lodgment of arachnoideal granulations (O.T. Pacchionian bodies). Descending from the centre of the upper margin of the bone is a median groove, the sagittal sulcus; narrowing below, this ends in a ridge the frontal crest-which nearly reaches the anterior part of the ethmoidal notch, where it terminates in a small orifice, the foramen cæcum, placed usually in the suture between the anterior part of the ethmoid and the frontal. This foramen may, or may not, transmit a small vein from the nose to the commencement of the superior sagittal sinus. This sinus, which is interposed between the layers of the falx cerebri, is at first attached to the frontal crest, but subsequently occupies the sagittal sulcus. Deeply concave from side to side and from above downwards, the lateral parts of the fossa are seen to be traversed by small grooves for the anterior branches of the middle meningeal arteries. Below, the
orbital parts bulge into the floor of the fossa, so that the ethmoidal notch appears in a depression between them. On each side of the notch faint grooves for the meningeal branches of the ethmoidal vessels may be seen. The circumference of the fossa is formed by the serrated edges of the bone which articulate with the parietals above, and on each side below with the great and small wings of the sphenoid.
Connexions. The frontal articulates with twelve bones, viz., posteriorly, with the parietals and sphenoid; laterally, with the zygomatic bones; inferiorly and medially, with the nasals, maxillæ, lacrimals, and ethmoid.
Ossification.-Ossification begins in membrane from one centre for each half. This makes its appearance about the sixth or seventh week in the region above the processus
zygomaticus. From these the two halves of the frontal part of the bone are developed, and by extension medially and posteriorly from their lower part the orbital parts are also formed. Serres, Rambaud and Renault, and v. Ihering describe the occurrence of three pairs of secondary centres somewhat later: one pair for the frontal spine, on either side of the foramen cæcum; a centre on either side in correspondence with the position of each trochlear pit; and a centre for each zygomatic process. Fusion between these secondary and the primary centres is usually complete about the sixth or seventh month of foetal life. At birth the two symmetrical halves of the bone are separated by the metopic suture, obliteration of which, commencing as a rule on a level with the frontal tubera, gradually takes place, So that about the fifth or sixth year it is more or less completely closed, traces only of the suture being left above and below. In about 8 per cent. of Europeans, however, the suture persists in the adult (see ante). At birth the supraorbital notches lie near the middle of the supraorbital margins. Traces of the frontal sinuses may be met with about the second year, but it is only about the age of seven that they can be definitely recognised. From that time they increase in size till the age of puberty, subsequent to which time they attain their maximum development.
a, Metopic suture still open. b, Position of primary centre. c, Centre for zygomatic
process. d, Centre for region of trochlea. e, Centres for nasal spine.
The parietal bones, two in number, are placed one on each side of the vault of the cranium. Each articulates with its fellow of the opposite side, the frontal anteriorly, the occipital posteriorly, and the temporals and sphenoid inferiorly. Each bone possesses a parietal and cerebral surface, four borders, and four angles.
The parietal surface, convex from above downwards and from before backwards, displays towards its centre a more or less pronounced elevation, the tuber parietale (parietal tuberosity). This marks the position of the primitive ossific centre, and not infrequently corresponds to the point of maximum width of the head. At a variable distance from the inferior border of the bone, and more or less parallel to it, two curved lines can usually be distinguished. The linea temporalis superior (superior temporal line) serves for the attachment of the temporal fascia; the linea temporalis inferior (inferior temporal line) defines the attachment of the temporal muscle, the extent and development of which necessarily determine the position of the line. The surface below the lines enters into the formation of the floor of the temporal fossa, and is called the planum temporale; it also affords origin to the temporal muscle, and is often faintly marked by grooves which indicate the course of the middle temporal artery.
Above the superior temporal line the bone is covered only by the tissues of the scalp. Near its superior border, and about an inch from its occipital angle, is the small parietal foramen, through which pass a small arteriole and an emissary vein.
The cerebral surface is concave from before backwards and from above downwards. It is moulded over the surface of portions of the frontal, parietal, occipital, and temporal lobes of the cerebrum, and displays impressions corresponding to the arrangement of the gyri of those portions of the brain. It also presents a series of well-marked grooves for the lodgment of the veins which accompany the branches of the middle meningeal artery (F. Wood Jones); these radiate from the sphenoidal angle of the bone, the best marked running upwards at some little distance behind and parallel to its anterior border. Close to the superior margin there is a series of depressions for arachnoideal granulations, and there also the bone is channelled so as to form a groove (sulcus sagittalis), which is completed by articulation with its fellow of the opposite side. Within the groove lies the superior sagittal venous sinus, and to its edges the falx cerebri is attached. Close to the mastoid angle there is also a curved groove, the transverse sulcus, in which the highest portion or bend of the transverse venous sinus is lodged.
The anterior, superior, and posterior borders are deeply serrated. The anterior border articulates with the frontal bone, forming with it the coronal suture. In the superior part of this suture the frontal bone overlaps the parietal, while the parietal overlies the frontal below. The posterior border is united with the occipital bone to form the lambdoid suture. The superior border articulates with its fellow of the opposite side by means of the sagittal suture; in the interval between the two parietal foramina this suture is usually simple in its outline. The frontal angle is almost rectangular, and corresponds to the site of the anterior fontanelle. The occipital angle, usually more or less rounded, corresponds in position to the posterior fontanelle. The inferior border (margo squamosus) is carved, and shorter than the others; it lies between the sphenoidal and mastoid angles. Sharp and bevelled at the expense of its outer table, it displays a fluted arrangement, and articulates with the squama temporalis of the temporal bone. The sphenoidal angle, pointed and prominent, articulates with the great wing of the sphenoid. It is wedged into the angle formed by the union of that bone with the frontal, and is bevelled at the expense of its inner table anteriorly,
whilst inferiorly it is thinned at the expense of its outer table. The mastoid angle is a truncated angle lying between the inferior and posterior borders. It is deeply serrated, and articulates with the mastoid part of the temporal bone. Not infrequently there is a channel in this suture which transmits an emissary vein.
Connexions. The parietal bone articulates with its fellow, with the frontal, occipital, mastoid and squama temporalis of the temporal, and with the sphenoid. Occasionally the Depressions for arachnoideal granulations (O.T. Pacchionian bodies)
Groove for superior sagittal sinus
Groove for transverse sinus (O.T. lateral sinus)
sphenoidal angle may not reach the great wing of the sphenoid, being separated from it by the articulation of the squama temporalis of the temporal with the frontal (Appendix B).
Ossification.-Ossification takes place in membrane by two centres which appear, one superior to the other, about the end of the second month (Toldt); these gradually unite during the fourth month and correspond in position to the future tuber parietale; from this, ossification spreads in a radial manner towards the edges of the bone, where, however, the membranous condition still for some time persists, constituting the fontanelles. These correspond in position to the angles of the bone. Ossification is also somewhat delayed in the region of the parietal foramina, constituting what is known as the sagittal fontanelle, a membranous interval which is not infrequently apparent even at birth.
The occipital bone, placed at the posterior and lower part of the cranium, consists of four parts, arranged around a large oval hole, called the foramen occipitale magnum or foramen magnum. At birth these parts are all separate. The expanded curved plate posterior to the foramen is the squama occipitalis or tabular part. The thick rod-like portion anterior to the foramen is the basilar part. On either side the foramen is bounded by the lateral or exoccipital parts.
The squamous or tabular part in shape somewhat resembles a Gothic arch, and is curved from side to side and from above downwards. It forms inferiorly a small portion of the middle of the posterior boundary of the foramen magnum, and unites, on each side of that, with the lateral parts of the bone. About the centre of the parietal surface of the squama there is a prominence -the external occipital protuberance, which varies considerably in its distinctness and projection, and serves for the attachment of the ligamentum nucha. From the protuberance, on each side, two lines curve towards the lateral angles of the bone. These are known respectively as the linea nuchæ suprema and linea nuchæ superior (highest and superior curved lines). To the upper of the two the galea aponeurotica (O.T. epicranial aponeurosis) is attached, whilst the lower serves for the origin of the trapezius and occipitalis muscles and the insertion of the sternomastoid and splenius capitis muscles. The two lines together serve to divide the external surface of the squama occipitalis into an upper or occipital plane (planum occipitale), covered by the hairy scalp, and a lower or nuchal plane (planum nuchale), serving for the attachment of the fleshy muscles of the back of the neck. As a rule the occipital part bulges backwards beyond the external occipital protuberance; exceptionally, however, the latter process is the most outstanding part of the bone. The nuchal plane, irregular and rough, is divided into two halves by a median ridge-the crista occipitalis externa (external occipital crest), which extends from the external occipital protuberance above to the posterior border of the foramen magnum below. Crossing the nuchal plane transversely, about its middle, is the inferior nuchal line, which passes laterally and forwards on each side towards the corresponding lateral margin of the bone. The areas thus marked out serve for the attachment of the semispinalis capitis (O.T. complexus), obliquus capitis superior, and rectus capitis posterior major and minor muscles.
The cerebral surface of the squamous part, concave from side to side and from above downwards, is subdivided into four fosse by a crucial arrangement of grooved ridges called the eminentia cruciata. In the upper pair of fossæ are lodged the occipital lobes of the cerebrum, whilst the hemispheres of the cerebellum occupy the lower pair. Near the centre of the eminence is the internal occipital protuberance, an irregular elevation, the sides of which are variously channelled according to the disposition of the grooves. Leading from this to the posterior margin of the foramen magnum is a sharp and well-defined ridge, the internal occipital crest, which serves for the attachment of the falx cerebelli, a process of dura mater which separates the two cerebellar hemispheres. Passing upwards from the internal occipital protuberance there is usually a well-marked ridge, to one or other side of which, more frequently the right (with the bone in the normal position and viewed from behind), there is a well-defined groove, the sulcus sagittalis, the lateral lip of which is generally less prominent. Placed in this groove is the superior sagittal venous sinus, and attached to the lips is the falx cerebri. At right angles to the foregoing, and at the level of the internal occipital protuberance, with which they become confluent, are two transverse grooves, the sulci transversi. These grooves, which have more or less prominent edges, lie between the upper and lower pairs of fossa, and serve for the attachment of the tentorium cerebelli as well as the lodgment of the transverse sinuses. Commonly the right transverse groove is confluent with groove to the right side of the median ridge, but exceptions to this rule are not infrequent. The angle formed by the union of the venous sinuses lodged in these grooves constitutes the confluens sinuum (O.T. torcular Herophili), which may accordingly be placed to one or other side of the internal occipital protuberance, more frequently the right; in some cases, however, it may occupy a central position. The superior angle, more or less sharp and pointed, is wedged in between the two parietal bones, its position corresponding to the site of the posterior fontanelle. Each lateral angle articulates with the posterior extremity of the mastoid portion. of the corresponding temporal bone. The superior borders, much serrated, articulate with the parietal bones, forming the lambdoid suture; and the lateral borders, extending from the lateral angles to the jugular process inferiorly, are connected with the medial sides of the mastoid portions of the temporals.
The lateral (or exoccipital) parts of the occipital bone are placed on