FROM THE

SKULL  BELOW (norma basalis) (fig . 4) . Starting from in front, the superior alveolar arcade with the teeth sockets is seen . This in a European skull approaches a semicircle, but in lower races the sides become more parallel; this is known as a hypsiloid arcade . Within the arcade is the hard palate formed by the maxillae in front (fig . 4, m), and the palate bones (p) behind . At the front of the median suture between the maxillae is the anterior palatine canal which, if it is looked into closely, will be seen to lead into four small foramina, two antero-posterior known as Scar pa's foramina, for the naso-palatine nerves, and two lateral called Stensen's foramina for small arteries and the re-mains of the mouth opening of Jacobson's organ (see OLFACTORY SYSTEM) . In young skulls a suture runs outward from the anterior palatine canal to between the lateral incisor and canine sockets, and sometimes another runs from the same place to between the central and lateral incisor teeth . At each postero-lateral angle of the palate are the posterior palatine canals for the descending palatine nerves . The posterior mar-gin of the hard palate is a free edge which forms the lower boundary of the posterior nasal apertures or choanae and attaches the soft palate (see PHARYNX) . Be-hind the alveolar arcade on each side are the external and internal pterygoid plates of the sphenoid; the external is a muscular process for the attachment of the pterygoid muscles, while the internal ends below in the hook-like hamular process which is directed backward and outward . Dividing the posterior nasal aperture into two is the vertical hind edge of the vomer (v), which articulates above with the body of the sphenoid (basi-sphenoid), and just behind this the sphenoid is united by bone with the basioccipital (b), though up to twenty years of age there is a synchrondrosis (see JOINTS) called the basilar suture) between them .

It is therefore very easy to tell an adult's skull from that of a young

person . Passing back in the mid line the foramen magnum (f) is seen, through which pass the spinal cord and its membranes, the vertebral arteries and the spinal accessory nerves . A little in front of this is a small tubercle, the pharyngeal spine, to which the constrictors of the pharynx are attached . On each side of the fora-men magnum and in front of its mid transverse diameter are the condyles (c), which articulate with the atlas, while just above these are the anterior condylar foramina, one on each side, for the exit of the hypoglossal nerves . External to the pterygoid plates the base of the skull is formed by the ali-sphenoid, which projects backward into a point, the spine of the sphenoid, and just in front of this is the small foramen spinosum for the passage of the middle meningeal artery . In front and a little internal to the foramen spinosum is a larger opening, the foramen ovale, through which the third division of the fifth nerve leaves the skull . Into the re-entering angle between the ali-sphenoid and basi-occipital is fitted the petrous part of the temporal, which, however, does not quite fill the gap but leaves a space on each side of the site of the basilar suture to be closed in by fibro-cartilage, and this is known as the middle lacerated foramen . On the lower surface of the petrous bone is the round opening of the carotid canal through which the internal carotid artery and its accompanying sympathetic nerves pass into the skull, while more externally the styloid process projects downward and forward and is more or less ensheathed at its root by the rampart-like ridge of the vaginal process . Between the styloid process and the occipital condyle lies the jugular or posterior lacerated foramen through which pass the lateral and inferior petrosal sinuses, and the glosso-pharyngeal, vagus and spinal accessory nerves . The bone which bounds this foramen behind, and which bears the posterior two-thirds of the occipital condyle, is the ex-occipital part of the occipital . A little behind and external to the styloid process is the tip of the mastoid process, just internal to which is the deep antero-posterior groove for the digastric muscle, and internal to that another slighter groove for the occipital artery . Behind the styloid process and between it and the mastoid is. the stylo-mastoid foramen through which the facial nerve passes, while in front of the process the glenoid cavity can be seen in its entirety, hounded in front by the eminentia articularis and divided into an anterior articular part and a posterior tympanic plate by the Glaserian fissure .

Just internal to the glenoid cavity is the opening of the bony Eustachian

tube . The posterior part of the norma basalis behind the foramen magnum is formed by the supra-occipital part of the occipital bone, so that all the four parts of the bone, which are separate up to the third year, help in the formation of that large opening . Between the foramen magnum and the external occipital protuberance and superior curved line already noticed, the bone attaches the deep muscles of the neck .