THE SPINAL COLUMN AND ITS CONTENTS 95
Cross-section of the spinal cord (Fie. 3. 15; see also Fie. 3. 14) The spinal cord is somewhat elliptical in cross-section, with its greater diameter from side to side. It has a deep anterior median sulcus, which contains the anterior spinal artery. A smaller posterior sulcus is continued within the substance of the cord as a posterior median septum. A small central canal transmits cerebrospinal fluid (CSF) and is continuous above with the fourth ventricle. It extends inferiorly for 5-6 mm into the filum terminale. While this canal is truly central in the upper lumbar cord it lies anterior to the midline in the cervical and thoracic cord and is posteriorly placed in the conus. A fusiform dilation of the central canal in the conus about 10 mm long and triangular in cross-section is termed the terminal ventricle. Around the central canal is an H-shaped area of grey matter, the anterior horns of which have cell bodies of the motor neurons and the posterior horns have cells in the sensory pathways. In the thoracic and upper lumbar regions, lateral horns are found that have cell bodies for sympathetic neurons.
Outside the grey matter is the white matter of the cord, which has long ascending and descending tracts (see Fig. 3. 15). The posterior (dorsal) columns, between the posterior horns, have tracts that convey sensations of pressure, vibration, position, movement and touch. Other sensations are carried in the lateral columns to the thalamus - the spinothalamic tracts - and to the cerebellum - the spinocerebellar tracts. Descending tracts from the motor cortex to the motor neurons of the anterior horns of the spinal cord - pyramidal tracts - also pass in the lateral white columns. Other pyramidal tracts are found in the anterior columns. THE SPINAL MENINGES (Fig. 3. 16) Three layers of meninges - the pia, arachnoid and dura mater - cover the spinal cord as well as the brain. All three layers extend along the nerve roots to the intervertebral foramina. The pia mater is applied closely to the surface of the spinal cord, entering its anterior and posterior sulci just as it does on the brain surface. The pia is thickened laterally between the nerve roots as the denticulate ligament up to the level of the twelfth thoracic nerve. It is so called because of teeth-like projections that attach to the dura at intervals. Inferiorly the pia is thickened with glial tissue as the filum terminale. The arachnoid mater lines the dura and forms an incomplete posterior median septum. The subarachnoid space, which contains CSF, lies between the arachnoid and the pia mater. The subarachnoid space extends along the nerve roots to the intervertebral foramina, allowing visualization, on myelography, of the nerve roots in their sheaths to this point. The subdural space is a potential space between the arachnoid and the dura and contains only lubricating serous fluid.
