The individual bones are very firmly connected by this mode of union. The edges of the different bones overlap each other at different parts, so that they are mechanically locked together, and can not be driven in by any force ab ertemo.
The bones of the cranium are compo sed of two plates of compact bony sub stance, called the external, and internal or vitreous tables ; and an intervening more or less obvious reticular texture, termed diploe. The proportion of these constituent parts varies very considerably; the diploe is in no case of a very loose or open texture. The thickness of indivi dual skulls is subject to great variety ; and there is much difference in the va rious parts of the same skull. For the internal surface is every where exactly moulded to the form of its contents, in stead of influencing them, as we might have expected a priori. Hence the con volutions of the brain, the vessels which ramify on its surface, &c. all leave prints on the inner table. The ordinary thick ness varies from about the fifth of an inch to almost a mere line.
The common number of the bones of the cranium is, as we hare already stated, 7 : but this is often increased by small portions formed between the others, and surrounded by distinct sutures. These are called ossa, triquetra, or wormiana.
The form of the cranium is elliptical, and pretty regularly so, particularly on the front, upper and back part, and sides. The smaller circle of the ellipse is in front, and the larger behind. It is tolera bly smooth, externally, except its basis, and it is almost entire or unperforated, except at the same part. In this situation, however, it possesses numerous holes, or, as they are technically named, foramina, which transmit blood-vessels to the brain, and the nine pairs of nerves which arise from that organ.
The upper and lateral parts of the cra nium constitute a bony vault or arch, for protecting the brain : this part is distin guished by the name of the van cap.
Individual bones of the head.—The os frontis forms the upper and anterior part of the skull, the eyebrow, and the roof oCthe orbit.
The ossa parietalia are called also ossa bregmatis, since the fontanelles or breg mata are formed between their edges.
They compose the whole upper and most of the lateral parts of the skull, and pos sess an irregularly quadrangular figure.
The ossa temporum compose the lower part of the sides, and the middle of the basis of the cranium. They are divided into a squamous portion, a mamillary, and a petrous portion. The former of these has a process contributing to the zygoma, or bony arch, at the side of the cranium, under which the temporal muscle passes. The second is also remarkable, by forming a large nipple-like protuberance towards the basis cranii. The third, which pro jects into the cavity of the skull, con tains the organ of hearing.
The os spheno-occipitale has generally been described as two bones. The occi pital portion forms the posterior portion of the basis cranii, and a part also of the back of the bony case.
The sphenoid portion is situated in the middle of the base of the skull, and ex tends across itfrom one temple to another. It is extremely irregular in its figure, and divided into a body placed in the middle, two alx on the sides, and two pterygoid processes projecting downwards.
The os ethmoides occupies the middle of the forepart of the basis cranii. It lies in the interval betweenthe two orbits, and contributes to the cavity of the nose. It consists of an irregular assemblage of bony cells and processes, of a very thin and delicate formation. It has a cribri form or horizontal plate towards the brain : a nasal or perpendicular plate ; 2 turbi nated bones ; cells ; and two orbital plates.
The sutures joining these are the co rona', between the os frontis and the two ossa parietalia ; the sagittal, be tween the two ossa parietalia ; the lamb doidal, joining the ossa parietalia to the os occipitis ; the squamous, between the temporal and parietal bones.
The foramina occurring in the cranium, for the transmission of nerves, are ; I, those of the cribriform plate of the eth moid bone 2, f. optics : 3, f. lacera or bitalia : 4, f. rotunda : 5, f. ovalia : 6, meatus auditorii interni : 7, f. lacera in basi cranii : 8, f. condyloidea anteriora : 9, foramen magnum.