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Intracranial vasculature

Basic Principles of Skull Organization


     The human skull encloses and protects the brain and the organs of special sense. It frames the nasal and oral cavities, and includes the jaws, which lodge the dentition. It sits poised atop the vertebral column and serves as a platform from which the cervical viscera and their skeletal elements are suspended.

     Blood vessels and the 12 pairs of cranial nerves traverse the skull on their way to and from the brain. Several of these nerves follow complicated paths and form an intricate plexus within the bone itself. This situation arises during development, when the neurovascular elements are established first and become entrapped as the skull condenses and ossifies around them.

     A normal adult human skull consists of 22 individual bones, excluding the ossicles of the middle ear and the hyoid bone. Six of these – occipital, sphenoid, frontal, ethmoid, vomer, and the mandible - are unpaired, bilaterally symmetrical, and lie athwart the mid-sagittal plane. The rest – temporal, parietal, maxilla, zygomatic, palatine, nasal, lacrimal, and inferior nasal concha - occur in mirror-image pairs.

     Individual bones are bound to each other by cartilaginous joints and by fibrous joints termed sutures. These joints form as ossifying bones expand toward one another during development and also occur between major portions of a given bone where these are ossified independently. They not only hold adjoining bones securely together but also allow continued growth of the skull.

     In a young adult, the apposed bony surfaces interdigitate to varying degrees. Between flat bones of the cranial vault (occipital and parietal, for example) the margins fit together in a series of interlocking process, much like pieces of a jigsaw puzzle, thus forming many small mortise-and-tenon joints (white arrow). Ultimately, as growth is completed, the joints themselves become ossified and eventually obliterated – or “closed.” However, some are obliterated before birth, others in infancy, and still others during adulthood, and this sequence is used by anthropologists and forensic pathologists to help determine the age of an individual at the time of death from examination of skeletal remains.  As bone grows it can be affected by tissues that are in direct contact with it.  A good example of this are the branches of the middle meningeal artery whose trajectory is reflected by narrow grooves on the inner surface of the skull (black arrow above).

     As a general rule, the skull is composed of two layers of compact (cortical) bone – the inner (large black arrow, right) and outer (large white arrow, right) “tables” - separated by cancellous bone – the “diploë” containing red marrow (small black arrow, right). The thickness of these strata varies considerably from region to region, however. Where the bone is flat and thin, as in the dome of the skull, the inner and outer tables may be fused together, without any intervening diploë.

     Certain bones are pneumatized, that is hollowed out by air spaces continuous with either the nasal cavities or the middle ear cavities. These air sinuses, or air cells, usually are named for the bones involved (frontal air sinus [small white arrow, above], mastoid air cells, etc.). They form and expand at different times during pre- and postnatal development, and they vary considerably in size from one individual to another.

     The bones of the skull, like other bones, are closely covered by periosteum. That on the outer surface is called the epicranium that on the inner surface the endocranium. Epi- and endocrania are continuous with one another along the walls of passageways through the skull – canals (tunnels), foramina (holes) and fissures (cracks) – as well as at sutures.

      The endocranium is provided by dura mater, a dense layer of tissue derived from the somatic intraembryonic mesoderm. The dura consists of two elements. An outer zone of fibrous connective tissue - the cranial layer - follows intimately the detailed contours of the inside of the skull, serving as its periosteum. The inner zone - the meningeal layer - is surfaced by a mesothelium that faces the arachnoid mater across the subdural space. The meningeal layer follows the gross contours of the brain, forming sickle-shaped sagittal partitions between the two cerebral hemispheres (the falx cerebri) and the two cerebellar hemispheres (the falx cerebelli), a roughly transverse partition between cerebrum and cerebellum (the tentorium cerebelli), and a membrane that stretches from side-to-side to cover the sella turcica (the diaphragma sellae).

     The  dynamic nature and adaptability of bone belies its metabolic requirements that are served by the intracranial vasculature.  Any appreciation of those vessels begins with a consideration of the connective tissue coverings of the brain; the pachymeninges (dura mater) and the leptomeninges (arachnoid and pia mater).  Collectively these three layers are referred to as the meninges.  The next page provides an overview of these structures.

Copyright 2008 DF Siwek & RJ Hoyt