The paragraphs which follow are intended primarily for the student who has no previous knowledge of the skull, and may well be omitted by those who are already familiar with the broad outlines of its structure.
The skull forms the skeleton of the head. It is made up of a large number of bones which, with the exception of the mandible or lower jaw, are so intimately connected to one another that no movement is possible between them. The lines along which the individual bones meet one another are, for the most part, very irregular and are frequently serrated like the edge of a saw (fig. 313). These immovable joints between the bones of the skull are termed sutures. They are easily seen in the skulls of young adults, but, as old age approaches, contiguous bones tend to fuse with each other and the suture lines become more or less obliterated.
When the mandible is left out of account the remainder of the skull, strictly speaking, constitutes the cranium, but in this textbook, as well as in many others, the terra skull is widely used with the same significance. The upper part of the cranium forms a box to enclose and protect the brain, and is often termed the calvaria. The remainder of the skull forms the facial skeleton, of which the upper part is immovably fixed to the calvaria and the lower part consists of the freely movable mandible.
The skull, considered as a whole, is of much greater importance to the student of medicine than the individual bones of which it is made up. Nevertheless, the position of the individual constituents must be located before the student can be in a position to follow the more detailed description.
The skull as a whole may be viewed from above (norma verticalis), from below (norma basalis), from behind (norma occipitalis), from in front (norma frontalis) and from the side (norma lateralis). The roof of the calvaria, or skull-cap, may be removed and the interior of the skull may be examined. In the erect attitude the lower margins of the orbital openings and the upper margins of the external auditory meatuses lie on the same horizontal plane, and it is important that the student should bear this in mind when he is examining the various aspects of the skull.
The region of the forehead is formed by the Frontal bone (fig. 309), which passes backwards in the vault of the skull as far as the coronal suture, where it meets the anterior borders of the right and left Parietal bones These two bones together form the greater part of the top of the head, and they articulate with each other at the serrated sagittal suture. Posteriorly they extend backwards to meet the occipital bone, which forms the back of the head. Owing to the lambda figure formed by the meeting of the sagittal with the parieto-occipital sutures, the latter are named the lambdoid suture. Each parietal bone extends downwards on the side of the vault until it meets the upper limit of the greater wing of the sphenoid bone in front, and the squamous part of the temporal bone behind. When the skull cap is removed, the section passes through the frontal bone and usually cuts across the lower part of the parietal bone, but it may involve the squamous part of the temporal bone. Posteriorly the section cuts the occipital bone. Consequently, the skull cap consists of (1) a large part of the frontal bone, (2) most of the two parietal bones, (3) possibly, small parts of the squamae of the temporal bones, and (4) a small part of the occipital bone.
The posterior cranial fossa (fig. 310) is almost circular in outline and occupies roughly two-fifths of the base of the skull. It is formed to a very large extent by the Occipital bone, The large opening in its floor, termed the foramen magnum, is placed entirely within that hone and allows the brain stein to become continuous with the spinal cord. The anterior part of the fossa is formed by the basilar part of the occipital bone, which is fused in front with the posterior part of the sphenoid bone. On each side the lateral wall of the fossa is formed by the posterior surface of the petrous part of the temporal bone above, and by the condylar (lateral) part of the occipital bone, below. The mastoid part of the temporal bone, which lies immediately behind the petrous part, helps the squamous part of the occipital bone to complete the fossa.
When the skull is viewed from in front (norma frontalis, fig. 311) the orbits, which lodge the eyeballs, and the anterior aperture of the nose, can easily be identified. The part below the mouth is formed entirely by the body of the Mandible is the part above the mouth is formed almost entirely by the Maxilla, or upper jaw. This bone forms the upper boundary of the mouth, and the lower and lateral boundaries of the anterior nasal aperture. In addition it forms the medial part of the lower margin of the orbit, which it helps the zygomatic bone to complete, while its frontal process ascends in the medial margin of the orbit to reach the Frontal bone. The frontal processes of the two maxillae are separated from each other by the two Nasal bones, which form the upper boundary of the anterior nasal aperture.
The inferior aspect of the cranium (norma basalis, fig. 312) is usually termed the external aspect of the base of the skull. It should be examined next. Posteriorly the Occipital bone, with the foramen magnum, can be located without difficulty. Lateral to the foramen magnum the occipital bone articulates with the mastoid part of the Temporal bone. Anterolaterally it articulates with the petrous part, which extends forwards almost to the root of the pterygoid process. In the anterior part of the inferior aspect of the cranium, the bony palate, which lies in the roof of the mouth, can be seen within the arch of the teeth of the maxilla. Four bones contribute to its formation, viz. the two Maxillae and the two Palatine bones. The anterior three-fourths of the bony palate are formed by the palatine processes of the maxillae, which meet each other in the median plane ; the posterior fourth is formed by the horizontal plates of the palatine bones. The latter are now seen, in part, for the first time; their perpendicular plates are still hidden as they ascend, on each side, from the lateral border of the horizontal plate to forth the posterior part of the lateral wall of the nose.
The Lacrimal bone, which lies in the anterior part of the medial wall of the orbit, the Vomer, which forms a large part of the nasal septum (fig. 331), and the Inferior Concha, which lies in the lateral wall of the nose, can be seen only when the orbits and the nose are examined. With these exceptions all the bones of the skull have now been identified, and the student is in a position to undertake profitably a more detailed study of the skull as a whole.
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