The arachnoid trabeculae are delicate strands of connective tissue that loosely connect the two innermost layers of the meninges

Meninges

The meninges is the system of membranes which envelopes the central nervous system. The meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.-Dura...

  -- the arachnoid mater

Arachnoid mater

The arachnoid mater, literally from Latin "spider -like mother", is one of the three meninges, the membranes that cover the brain and spinal cord...

 and the pia mater

Pia mater

Pia mater often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. The word finds its roots in Latin, meaning literally "tender mother." The other two meningeal membranes are the dura mater and the arachnoid mater....

. They are found within the subarachnoid space

Subarachnoid space

In the central nervous system, the subarachnoid cavity is the interval between the arachnoid membrane and pia mater....

 where cerebrospinal fluid

Cerebrospinal fluid

Cerebrospinal fluid , Liquor cerebrospinalis, is a clear, colorless, bodily fluid, that occupies the subarachnoid space and the ventricular system around and inside the brain and spinal cord...

is also found. Embryologically, the trabeculae are the remnants of the common precursor that forms both the arachnoid and pial layers of the meninges.