The arachnoid mater of the brain is the second of the three membranes of the brain. It is intermediate in position between the dura mater and the pia mater. It envelops the brain. Like the dura mater, it sends processes into the longitudinal and transverse fissure, between the hemispheres of the cerebellum and slightly into the lateral sulcus. It also surrounds the nerves, forming tubular sheaths for them as far as their points of exit from the skull. Unlike the pia mater, it does not dip into the sulci or fissures between the convolutions, but passes directly from one convolution to the other, bridging over the sulci. It forms a loose investment for the brain. It is continued downward over the spinal cord. Because it is a serous membrane, it is a smooth, polished membrane to the naked eye. It is connected by delicate connective tissue with both the dura and pia mater, but much more intimately with the latter.

The arachnoid mater of the spinal cord is a thin, veil-like membrane between the dura mater and the pia mater. The arachnoid mater in the spinal cord is more delicate than the arachnoid of the brain, but it resembles it in sending tubular prolongations along the nerves. It is attached posteriorly to the dura mater by prolongations of connective tissue. Below, it is prolonged upon the cauda equina. The arachnoid mater forms a long sac, the cavity of which lies between the arachnoid mater and the pia mater, and is known as the subarachnoid space.

Arachnoidea mater

Arachnoïde