The dorsal cavity is the space containing the brain, spinal cord and cerebrospinal fluid. It gets its name from being situated along the posterior (dorsal) aspect of the body.
The boundaries of the dorsal cavity: The skull bones and cranial fascia form the boundaries of this semi-closed hydraulic system. The cranial fascia is normally given another name , the dura. It is a tough, relatively inelastic layer of connective tissue that attaches to the bones of the skull and those of the spinal canal in different ways. Have a look at figures 1 and 2.
The dura attaches loosely to the central panels of the cranial bones but adheres tightly along the suture lines of the skull and at the foramen magnum. The foramen magnum is the hole at the base of the skull through which the spinal cord passes to enter the vertebral column. Inside the skull, the dura forms vertical folds (falx cerebri) that move into the brain box to separate the cerebral and cerebellar hemispheres. It creates, as well, horizontal folds (tentorium cerebelli) that separate the cerebrum from the cerebellum. See figure 3. All these elegant dural folds form part of the larger connective tissue web.
Within the bony spinal canal, the dura creates a tube that houses the spinal cord. This tube runs from the base of the skull down to and then into the canal of the sacrum where it fuses with the filum terminale. The filum terminale, a fibroelastic string, emerges from the sacral canal to blend with the connective tissue coating the outside surface of the coccyx. The skull bone, then, attaches to the tailbone.
As figure 2 revealed, the dural tube anchors itself at points along both the anterior and posterior aspects of the spinal canal. In addition, it is drawn away from the cord and tethered by the paired nerve roots that exit from each level of the vertebral column to innervate the torso and limbs. See figure 5.
The other two wrappings of the dorsal cavity are the arachnoid that lines the inner surface of the dura and the pia that invests the tissue of the brain and spinal cord. Thin and delicate, both layers are well supplied with blood. Fine ligaments connect them to one another. The space between them is referred to as the subarachnoid space and is filled by the cerebrospinal fluid. All these interconnections between the 3 wrappings of the dorsal cavity illustrate the complex web of interaction that typifies connective tissue everywhere. Collectively, the dura, arachnoid and pia make up the meninges.
The contents of the dorsal cavity: The brain, spinal cord and cerebrospinal fluid (CSF) fill the dorsal cavity. Within the substance of the brain are four chambers – the right and left lateral ventricles and the 3rd and 4th ventricles. Each produces CSF. Protruding into the ventricles are dense loops of capillaries (the choroid plexuses) that produce the CSF, a crystal clear fluid. On average, there are 100 cc of CSF lying within the brain box and another 10 to 20 cc surrounding the spinal cord. About 20 cc of CSF is produced each hour so that the entire volume of CSF renews itself 4 or 5 times a day.
The CSF has its own pattern of circulation, moving down through the ventricular system and into the subarachnoid space that surrounds the spine and brain. It drains away through minute folds of the arachnoid villae that protrude through the dura into the veins of the brain. See figure 8.
There is much yet to learn about the functions of this fluid. We know it provides oxygen and nutrients to the cord and nerve roots and removes the waste products of cerebral metabolism (carbon dioxide, lactate and hydrogen ions). It cushions the central nervous system against trauma and sudden movements. And it provides such buoyancy that the 1500 g brain weighs only 50 g as it floats in its water jacket of CSF.
“Craniosacral Therapy”, John E. Upledger & Jon D. Vredevoogd, 1983, Eastland Press, ISBN 0-939616-01-7
“The Sensitive Nervous System”, David Butler, 1st Edition, 2000, ISBN 0 646 40251 X
Atlas of Human Anatomy, 4th edition, Frank H. Netter, Sunders Elsevier, 2006, ISBN-13: 978-1-4160-3385-1
© 2010, Taoist Tai Chi Society of Canada