Nervous and mental diseases . er layer, or visceral pia, is a varying space, the pial space,filled with a delicate, open, reticular network of fibrous tissue containingcerebrospinal fluid or lymph. It is an enormous lymph-space. At thegyral grooves the visceral pia dips to the bottom of the sulci. Iteverywhere closely adheres to the brain-cortex, which it follows 72 PACHYMENINGITIS AND PIAL HEMORRHAGE. through the transverse fissure into the ventricular chambers, furnishing thevelum interpositam and bearing the choroid plexuses. The arachnopiabridges over the sulci. This arrangement at the gre
Nervous and mental diseases . er layer, or visceral pia, is a varying space, the pial space,filled with a delicate, open, reticular network of fibrous tissue containingcerebrospinal fluid or lymph. It is an enormous lymph-space. At thegyral grooves the visceral pia dips to the bottom of the sulci. Iteverywhere closely adheres to the brain-cortex, which it follows 72 PACHYMENINGITIS AND PIAL HEMORRHAGE. through the transverse fissure into the ventricular chambers, furnishing thevelum interpositam and bearing the choroid plexuses. The arachnopiabridges over the sulci. This arrangement at the great fissures and atthe base of the brain in the intervals between the cerebrum, cerebellum,and medulla forms lymph-reservoirs, which are continuous with the pialspaces of the spinal cord through the foramen magnum. Delicate proc-esses of the pia also accompany the cranial nerves and vessels from theskull, and are continuous with the extracranial lymph-channels. Between the pial layers the cerebral vessels ramify. As they pene-. Fig. 25.—Diagram of cerebral meninges and cortex. Z>, dura mater; S. D., subdural space; P. S., pialspace; A. P., arachnopia; V. P., visceral pia; A, arterial perivascular space; Z, venous perivascularspace (Tuke). trate the cortex both arteries and veins are accompanied by deli-cate sheaths of the visceral pia, which form the perivascular lymph-spaces of the brain. Some of the great pyramidal cells of the cortexare encapsuled by diverticula from these perivascular lymph-channels,and are thus nourished (Tuke). In a manner, therefore, these mostimportant cortical elements may be considered as appendages of the pia,with which they have such intimate anatomical relations. The pia alsopushes up the Pacchionian bodies into the vascular area of the duranear the venous sinuses at the vertex. These bodies are supposed tofurnish au outlet for the meningeal lymph. 74 DISEASES OF THE CEREBRAL MENINGES. The brain is constantly changing in bulk through variations
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