SECTION XVIII 



THE NUTRITIVE AND VASCULAR ARRANGEMENT 

 OF THE CENTRAL NERVOUS SYSTEM 



THE brain and spinal cord are enclosed within three membranes or meninges, 

 named from without inwards, the dura mater, the arachnoid membrane, 

 and the pia mater. The dura mater consists of a strong fibrous membrane, 

 smooth and lined with endothelial cells on its inner surface. In the head 

 its outer surface is closely attached to the bones forming the cranium, of 

 which it represents the periosteum. Strong fibrous partitions are sent from 

 the dura mater into the cavity of the cranium to support the chief parts 

 of the brain. One of these, the falx cerebri, supports the two cerebral hemi- 

 spheres, a second, the tentorium cerebelli, forms a horizontal division between 

 the cerebral hemispheres and the cerebrum, and a smaller one, the falx 

 cerebelli, passes a short distance inwards between the cerebellar hemispheres. 

 In the spinal canal the bones have their own periosteum and the dura mater, 

 which is closely attached round the margins of the magnus, forms a loose 

 sheath round the spinal cord, being slung up in the vertebral canal by the 

 tubular prolongations which it sends along each nerve root to form the outer 

 sheath of the nerve. The dura mater in the cranium may be separated 

 with greater or less difficulty into two layers, and between these two layers 

 are found the venous sinuses, which receive the whole of the blood returned 

 from the brain. These venous sinuses are angular clefts, the chief of which 

 lie along the attached margin of the falx cerebri and the tentorium cerebelli. 

 Most of the blood leaves the skull by the internal jugular veins. In the 

 spinal cord the place of these venous sinuses is taken by a plexus of thin- 

 walled veins, imbedded in fat, lying on the outside of the dura. Under the 

 dura mater we find the subdural space, which is rather potential than actual. 

 It can be regarded as a large lymph space and any contents are drained 

 off into the lymph spaces of the nerve roots and adjoining tissues. 



The arachnoid is a delicate transparent membrane which covers the whole 

 of the surface of the brain and spinal cord. Superficially it is covered with 

 endothelial cells which bound the subdural space. On its deep surface it is 

 connected with the pia mater by fine fibres. It bridges over the inequalities 

 in the surface of the brain so that in various localities a space is left which is 

 filled with cerebro-spinal fluid and is known as the subarachnoid space. 

 In certain situations it sends prolongations into the fissures of the brain. 

 Thus a marked expansion passes by the transverse fissure between the 



461 



