how useless; nay, how pernicious it must be, to apply a trepan on this part of the skull; for this instrument, instead of piercing into the cavity of the cranium, would reach no further than the sinuses.
The upper circular part of the os frontis is joined to the ossa parietalia, from one temple to the other, by the coronal suture. From the termination of the coronal suture to the external angular processes, this bone is connected to the sphenoid by the sphenoidal suture. At the external canthi of the eyes, its angular processes are joined by the transverse suture to the ossa malarum, to which it adheres one third down the outside of the orbits; whence to the bottom of these cavities, and a little up on their internal sides, these orbitar processes are connected to the sphenoidal bone by that same suture.— On the inside of each orbit, the orbitar process is indented between the cribriform part of the ethmoid bone, and the os planum and unguis.The transverse suture afterwards joins the frontal bone to the superior nasal processes of the ossa maxillaria superiora, and to the nasal bones. And, lastly, its nasal process is connected to the nasal lamella of the ethmoid bone.
The frontal bone serves to defend and support the anterior lobes of the brain. It forms a considerable part of the cavities that contain the globes of the eyes, helps to make up the septun narium, organ of smelling, &c. From the description of the several parts, the other uses of this bone are evident.
In a ripe child, the frontal bone is divided through the middle; the superciliary holes are not formed; often a small round piece of each orbitar process, behind the superciliary ridge, is not ossified, and there is no sinus to be seen within its substance.
Each of the two Ossa Parietalia, or bones serving as walls to the encephalon, is an irregular square; its upper and fore sides being longer than the one behind or below. The inferior side is a concave arch; the middle part receiving the upper round part of the temporal bone.—The angle formed by this upper side and the fore one, is so extended, as to have the appearance of a process.
The external surface of each os parietale is convex. Upon it, somewhat below the middle heighth of the bone, there is a transverse arched ridge, of a whiter colour generally than any other part of the bone; from which, in bones that have strong prints of muscles, we see a great many converging furrows, like so many radii drawn from a circumference towards a centre. From this ridge of each bone the temporal muscle rises; and, by the pressure of its fibres, occasions the furrows just now mentioned.—Below these, we observe, near the semicircular edges, a great many risings and depressions, which are joined to like inequalities on the inside of the temporal bone, to form the squamous suture. The temporal bone may therefore serve here as a buttress, to prevent the lower side of the parietal from starting outwards when its upper part is pressed or struck.
Near the upper sides of these bones, towards the hind part, is a small hole in each, through which a vein passes from the teguments of the head to the longitudinal sinus.—In several skulls, one of the ossa parietalia has not this hole; in others, there are two in one bone; and in some not one in either. Most frequently this hole is through both tables; at other times the external table is only perforated.—The knowledge of the course of these vessels may be of use to surgeons, when they make any incision near this part of the head, lest, if the vessels are rashly cut near the hole, they shrink within the substance of the bone, and so cause an obstinate hæmorrhagy, which neither ligatures nor medicines can stop.
On the inner concave surface of the parietal bones, we see a great many deep furrows, disposed somewhat like the branches of trees: The furrows are largest and deepest at the lower edge of each os parietale, especially near its anterior angle, where sometimes a full canal is formed. They afterwards divide into small furrows, in their progress upwards.-In some skulls a large furrow begins at the hole near the upper edge, and divides into branches, which join with those which come upwards, shewing the communications of the upper and lower vessels of the dura mater.—In these furrows we frequently see passages into the diploe. On the inside of the upper edge of the ossa parietalia, there is a large sinuosity, frequently larger in the bone of one side than of the other, where the upper part of the falx is fastened, and the superior longitudinal sinus is lodged.—Generally part of the lateral sinuses makes a depression near the angle, formed by the lower and posterior sides of these bones; and the pits made by the prominent parts of the brain are to be seen in no part of the skull more frequent, or more considerable, than in the internal surface of the parietal bones.
The ossa parietalia are amongst the thinnest bones of the cranium: but enjoy the general structure of two tables and diploe the completest, and are the most equal and smooth.
These bones are joined at their fore-side to the os frontis by the coronal suture; at their long inferior angles, to the sphenoid bone, by part of the suture of this name; at their lower edge, to the ossa temporum, by the squamous suture, and its posterior additamentum; behind, to the os occipitis, or ossa triquetra, by the lambdoid suture; and above, to one another, by the sagittal suture.
In a child born at the full time, none of the sides of this bone are completed; and there never is a hole in the ossified part of it near to the sagittal suture.
The large unossified ligamentous part of the cranium observable between the parietal bones, and the middle of the divided os frontis of new-born children, called by the vulgar the open of the head, was imagined by the ancients to serve for the evacuation of the superfluous moisture of the brain; and therefore they named it bregma, or the fountain; sometimes adding the epithet pulsatilis, or beating, on account of the pulsation of the brain felt through this flexible ligamento-cartilaginous substance. Hence very frequently the parietal bones are called ossa bregmatis.
All the bregma is generally ossified before seven years of age. Several authors say, they have observed it unossified in adults; and physicians, who order the application of medicines at the meeting of the coronal and sagit-
