Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/192

152 sides; the os oscipitis makes the whole hinder part, and some of the base; the os ethmoides is placed in the fore-part of the base; and the os sphenoides is in the middle of it.

These bones are joined to each other by five sutures; the names of which are, the coronal, lambdoid, sagittal, and two squamous.

The coronal suture is extended over the head, from within an inch or so of the external canthus of one eye, to the like distance from the other; which being near the place where the ancients wore their vittæ, coronæ, or garlands, this suture has hence got its name.—Though the indentations of this suture are conspicuous in its upper part, yet an inch or more of its end on each side has none of them; for it is squamous and smooth there.

The lambdoidal suture begins some way below, and farther back than the vertex or crown of the head, whence its two legs are stretched obliquely downwards, and to each side, in the form of the Greek letter Δ, and are generally said to extend themselves to the base of the skull.

This suture is sometimes very irregular, being made up of a great many small sutures, which surround so many little bones that are generally larger and more conspicuous on the external surface of the skull than internally. These bones are generally called triquetra, or Wormiana.

The sagittal suture is placed longitudinally, in the middle of the upper part of the skull, and commonly terminated at the middle of the coronal, and of the lambdoid sutures; between which it is said to be placed, as an arrow is between the string and bow.—However, this suture is frequently continued through the middle of the os frontis, down to the root of the nose; which oftener happens in women than men.

The squamous agglutinations, or false sutures, are one on each side, a little above the ear, of a semicircular figure, formed by the overlopping (like one scale upon another) of the upper part of the temporal bones, on the lower part of the parietal, where, in both bones, there are a great many small risings and furrows, which are indented into each other; though these inequalities do not appear till the bones are separated.

The bones of the skull are joined to those of the face by schyndelesis and sutures.—The schyndelesis is in the partition of the nose.The sutures said to be common to the cranium and face are five, viz. the ethmoidal, sphenoidal, transverse, and two zygomatic.—Parts however of these sutures are at the junction of only the bones of the skull.

The ethmoidal and sphenoidal sutures surround the bones of these names; and in some places help to make up other sutures, particularly the squamous and transverse; and, in other parts, there is but one suture common to these two bones.

The transverse suture is extended quite cross the face, from the external canthus of one orbit, to the same place of the other.

The zygomatic sutures are one on each side, being short, and slanting from above obliquely, downwards and backwards, to join a process of the cheek-bone to one of the temporal bones, which advances towards the face; so that the two processes thus united, form a sort of bridge or jugum, under which the temporal muscle passes; on which account the processes, and sutures joining them, have been called zygomatic.

The advantages of the sutures of the cranium are these: 1. That this capsula is more easily formed and extended into a spherical figure, than if it had been one continued bone. 2. That the bones which are at some distance from each other at birth might then yield, and allow to the head a change of shape, accommodated to the passage it is engaged in. Whence, in hard labour of child-bed, the bones of the cranium, instead of being only brought into contact, are sometimes made to mount one upon the other. 3. That the dura mater may be more firmly suspended by its processes, which insinuate themselves into this conjunction of the bones; for doing this equally, and where the greatest necessity of adhesion is, the sutures are disposed at nearly equal distances, and the large reservoirs of blood, the sinuses, are under or near them. 4. That fractures might be prevented from reaching so far as they would in a continued bony substance. 5. That the connection at the sutures being capable of yielding, the bones might be allowed to separate; which has given great relief to patients from the violent symptoms which they had before this separation happened.

Having gone through the general structure of the cranium, we now proceed to examine each bone of which that brain-case consists.

The has its name from its being the only bone of that part of the face we call the forehead, though it reaches a good deal further. It has some resemblance in shape to the shell of the concha bivalvis, commonly called the cockle; for the greatest part of it is convex externally, and concave internally, with a serrated circular edge; while the smaller part has processes and depressions, which make it of an irregular figure.

The external surface of the os frontis is smooth at its upper convex part; but several processes and cavities are observable below: for, at each angle of each orbit, the bone jutts out to form four processes, two internal, and as many external; which, from this situation, may well enough be named angular. Between the internal and external angular processes of each side, an arched ridge is extended, on which the eye-brows are placed.—Very little above the internal end of each of these superciliary ridges, a protuberance may be remarked, in most skulls, where there are large cavities, called sinuses, within the bone.—Betwixt the internal angular processes, a small process rises, which forms some share of the nose, and thence is named nasal. Some observe a protuberant part on the edge of the bone, behind each external angular process, which they call temporal processes; but these are inconsiderable.—From the under part of the superciliary ridges, the frontal bone runs a great way backwards; which parts may justly enough be called orbitar processes. These, contrary to the rest of this bone, are concave externally, for receiving the globes of the eyes, with their muscles, fat, &c.

In each of the orbitar processes, behind the middle of the superciliary ridges, a considerable sinuosity is observed, where