The brain and spinal cord are completely surrounded by three layers of tissue,the meninges,lying between the skull and the brain,and between the vertebral foramina and spinal cord. Named from outside inwards they are the:
- Dura mater.
- Arachnoid mater.
- Pia mater.
The dura and arachnoid maters are separated by a potential space,the subdural space. The arachnoid and pia maters are separated are separated by the subarachnoid space,containing cerebrospinal fluid.
DURA MATER.
The cerebral dura mater consists of two layers of dense fibrous tissue. The outer layer takes the place of the periosteum on the inner surface of the skull bones and the inner layer provides a protective covering for the brain. There is only a potential space between the two layers except where the inner layer sweeps inwards between the cerebral hemispheres to form the falx cerebri; and between the cerebrum and cerebellum to form the tentorium cerebelli.
ARACHNOID MATER.
This is a layer of fibrous tissue that lies between the dura and pia maters. The arachnoid mater passes over the convolutions of the brain and accompanies the inner layer of dura mater in the formation of the falx cerebri,tentorium cerebelli and falx cerebelli.
PIA MATER.
This is a delicate layer of connective tissue containing many minute blood vessels. It adheres to the brain,completely covering the convolutions and dipping into each fissure.
