The spinal cord lies within the spinal column, extending from the medulla oblongata to the level of the second lumbar vertebra. The adult spinal cord is 2 cm wide at the midthoracic region and 42–45 cm long (Figure 13.17). Like the brain, the spinal cord is enclosed and protected by the meninges – that is, the dura mater, arachnoid mater and pia mater. The dura and arachnoid mater are separated by a potential space known as the subdural space. The arachnoid mater and pia mater are separated by the subarachnoid space, where the CSF continually circulates. Below the termination of the spinal cord, the subarachnoid space contains CSF, the filum terminale and the cauda equina (the anterior and posterior roots of the lumbar and sacral nerves) (Doherty and Forbes [50], Pagana and Pagana [177], Tortora and Derrickson [243]). So as to avoid damage to the spinal cord, it is imperative that lumbar puncture is performed at the lowest palpable intervertebral space (Figure 13.18). The higher the space, the greater the risk of spinal cord injury.

The spinal needle has to pass through the following structures: skin, subcutaneous tissue, supraspinal ligament, interspinous ligament, ligamentum flavum, epidural space, dura mater and arachnoid mater.

CSF is formed primarily by filtration and secretion from networks of capillaries, called choroid plexuses, located in the ventricles of the brain. Eventually, absorption takes place through the arachnoid villi, which are finger‐like projections of the arachnoid mater that push into the dural venous sinuses. CSF is a clear, colourless liquid with various functions and properties. In an adult, the total volume of CSF is 80–150 mL; it is reabsorbed at a rate of 20 mL/hour or 480 mL/day (Tortora and Derrickson [243]). CSF's constituents include:

There are three basic functions of CSF: