Chapter 144. Spinal Cord Length and Spinal Meninges - Review of Medical Embryology Book

WHEN THE CROWN-RUMP LENGTH of the embryo is about 30 mm (in month 3 of development), the spinal cord extends the entire length of the embryo, and the spinal nerves pass through the intervertebral foramina at their level of origin
WITH INCREASING AGE, the vertebral column and dura mater lengthen more rapidly than the neural tube, and the terminal end of the spinal cord gradually shifts to a higher level
AT BIRTH, THE END OF THE CORD is located at the level of the third lumbar vertebra (L3)

Due to disproportionate growth, the spinal nerves run obliquely from their segment of cord origin to the corresponding level of the vertebral column
The dura remains attached to the bony column at the coccygeal level

Below this point, the CNS is represented by only the filum terminale internum marking the tract of spinal cord regression
Nerve fibers (roots of lumbar and sacral nerves) below the terminal end of the cord are collectively called the cauda equina (horse's tail)

WHEN CEREBROSPINAL FLUID is taken in a lumbar puncture, the needle is inserted at the lower lumbar level to avoid injuring the lower end of the spinal cord

At this level, there is no spinal cord, only meninges, cerebrospinal fluid, and the mobile nerves of the cauda equina, through which the needle will slip

THE SPINAL CORD WIDENS in 2 regions to form the cervical and lumbar enlargements which are located at the level of origin of nerves that give rise to the brachial plexus for the upper extremity and the lumbosacral plexus for the lower extremity

THE PRIMITIVE MENINX is a covering or membrane of condensed mesenchyme that surrounds the neural tube

The inner cell layer of the meninx is probably derived from neural crest and remains there to form the pia-arachnoid

The pia mater and the arachnoid membrane together make up the so-called leptomeninges

FLUID SPACES IN THE LEPTOMENINGES form and then coalesce to create the subarachnoid space which, in the adult, is traversed by fine, delicate strands of connective tissue (the arachnoid trabeculae). Thus, the trabeculae pass between the pia mater and the arachnoid membrane