Cerebrospinal Fluid Flows down a Pressure GradientThrough the Ventricular System into the Subarachnoid Space

CSF flows, by bulk, down a pressure gradient from its site of formation at the choroid plexuses through the ventricular system and subarachnoid space into the venous system. Fluid formed in the lateral ventricles passes into the third ventricle through the interventricular foramina (foramina of Monro) (see Figures 15-1, 15-2, and 15-3).

Recall that the brain and spinal cord are encased in bone (the skull and spinal canal, respectively) and covered by a series of three membranes called the meninges (sec Chapter 3). From outer to inner, these membranes are the dura, arach­noid, and pia (see Figure 15-1). The subarachnoid space lies between the arachnoid and pia, and when the CSF exits the brain through the apertures (foramina) of the fourth ven­tricle, the CSI fills this space, spreading out over the entire outer surface of the brain and spinal cord. Thus the entire CNS is essentially floating in a fluid-filled, membranous bag. As the CSF circulates up over the dorsal convexity of the brain, it is absorbed into the venous system near the midline.

The pressure, cell count, and chemical constituents of CSF can be sampled by placing a Styleltcd spinal needle into the subarachnoid space. Anatomically, the most convenient place to perform this varies with species. In humans it is usually per­formed in the lumbar spinal column because the human spinal cord tapers to a cone (conus medullaris) near the first lumbar vertebra (humans have five lumbar vertebrae) as the dura and arachnoid continue down to around the second sacral vertebra. This provides a relatively large subarachnoid space in the human midlumbar spinal column from which to sample. In most veter­inary species, however, the conus medullaris extends to about the sixth or seventh lumbar vertebra, leaving only a small sub­arachnoid space. Therefore, most veterinary spinal taps are performed by sampling from the subarachnoid space between the skull and the first cervical vertebra in anesthetized animals. In this location the subarachnoid space, formed as the arach­noid stretches from the caudal cerebellar surface to the dorsal surface of the medulla, is called the cisterna magna (“big cis­tern”) and is much deeper than other portions of the sub­arachnoid space (see Figure 15-3). Spinal taps provide valuable information about such neuropathological lesions as intra­cranial space-occupying masses and inflammation.