In the base of the cranium the bones are placed in simple contact, and are so disposed that forces, descending from above, will neces sarily drive them closer to each other. To understand this rightly, we must suppose the sphenoid and occipital to form (which, in fact, they do) but one bone at an early period of life. The temporal bone is placed alongside the occipital, in such a way that the petrous process is wedged into the angle between the basilar process of the occipital, and the great wing of' the sphenoid; while the latter, again, is wedged into the angle between the petrous and squarnous processes of the temporal bone. It has been said that on the upper surface of the outer margin of the great wing, rests the lower part of the squamous process ; in case of force descending through the parietal bone this will be the fulcrum, and the lever (the squamous process) being directed outwards, the mastoid and petrous processes will neces sarily be squeezed more forcibly against the occipital bone and its basilar process.
The peculiar appearance presented by the articulations on the outer surface of the cal varia, has procured for them the name of sutures, a term which is applied frequently to the joinings in the base, although they are essentially different in appearance and in fact. Those which are situated in the calvaria, and to which the name is more suitable, are the corona!, lambcloidal, and sagittal sutures.
Tile coronal suture extends between the two great wings of the sphenoid bone across the upper part of the skull, and connects the fron tal to the two parietal bones (fig. 373, a). The hanbdoida/ (o) consists of two diverging lines formed by the articulation of the posterior border of the two parietals with the superior half of the occipital ; and extends from the superior to the lateral angles of that bone. The sagitta/ is the line of union between the parietals themselves, and runs longitudinally from the superior part of the lambdoidal to the centre of the coronal suture. On each side of the skull is the squamous suture (fig. 373, c); it has none of the serrated characters of the other sutures, but is an arched line ex tending from the great wing of the sphenoid to the mastoid process of the temporal bone, and traversing so much of the border of its squarnous process as embraces the parietal bone.
The squamous suture and the lanabdoidal suture are connected by a short transverse line formed by the articulation of the mastoid angle of the parietal bone with the mastoid process of the temporal, and which is called addita mention suturec squamosee (fig. 373, g). From the lateml angle of the occipital bone to its jugular process, that is, from the termination of the lainbdoidal suture (where it is joined by the before-mentioned supplement of the squa mous suture) to the jugular foramen, there is a line formed by the posterior border of the mastoid process and the occipital bone termed additamentum suturte lambdoidalis.
The transverse frontal suture (fig. 373 a) is situated transversely, but forms several angles in its course. It extends from one
external angular process of the frontal bone to the other; commencing at either angle, after uniting that angle to the malar bone, it enters the orbit, and unites the frontal bone to the great wing and to the small wing of the sphe noid ; it then passes out of the other side of the orbit, joining the same bone to the eth timid, lachrymal, nasal process of the superior maxillary and nasal bones themselves ; enters the orbit of the opposite side and retires from it, articulating the frontal to bones analogous to those in the other orbit.
Other sutures are occasionally enumerated, such as the sphenoidal, vvhich entirely sur rounds the sphenoid bone; and the ethmoidal, which bounds the cribriform plate of the eth moid bone. Both of these, so far as they deserve the name of sutures, are comprehended in the transverse frontal suture.
The articulations of the temporal with the occipital, sphenoid, and parietal bones have been designated as the petro-occipital, petro sphenoidal, spheno-temporal, and spheno-pari etal sutures ; but, with the exception of the last, (which is squamous, and truly a part of that suture,) they are not sutures.
It ought further to be remarked that, while the bones of the calvaria are much thinner than those of the base, they are compamtively thicker in their borders to allow of that serra tion from which the term suture is derived.
To study, in combination with each other, the facts enumerated in the foregoing descrip tion, it is necessary to take a survey of the external and internal suffices of the skull itself.
For this purpose the external surface may be divided into four regions : the superior, the inferior, and the two lateral.
The superior region extends from the nasal process of the frontal bone to the occipital protuberance, and is bounded on each side by the Utica temporalis; a curved line, which, commencing at the external angular process of the frontal bone, passes backwards, traverses the parietal below its protuberance, and is re ceived on the extreme point of the root of the zygoniatic process of the temporal bone. To proceed from before to behind, there are, on the median line, the nasal process and the rough notch for the articulation of the nasal bones ; the nasal protuberance ; the glabella bounded laterally by the frontal processes; the line indicating the junction of the two fcetal portions of the frontal bone ; the centre of the coronal suture; the whole length of the sa gittal suture, with the fommen parietale on each side of it; the superior angle of the occipital bone ; a part of the occipital bone itself; and, lastly, the occipital protuberance. Laterally, and on each side, there are the frontal process, the superciliary ridge, the depression between them, and the supra-or bitary foramen ; the frontal protuberance ; the coronal suture; the parietal protuberance; the lambdoidal suture; and so much of the side of the occipital bone as is above the tmnsverse ridge.