The spinal cord
The spinal cord [38, 160] is well protected within the spinal canal [43] , which runs through the spine. In adults, it is about 42 cm long in women and 45 cm in men [13, 14, 15, 16, 17, 19]. The spinal cord is the origin of all spinal nerves [38, 125] , which number 31 pairs [14].
The spinal cord, like the brain, is surrounded by meninges [43] : the pia mater [38, 85] , the arachnoid [64, 94] , and the dura mater [43, 69, 94]. It is bathed in CSF [95] , and it is pierced in the center by a rudimentary canal (the ependymal canal) [14, 20].
External Configuration :
Due to the faster intrauterine development of the spine, the nerve roots of the spinal nerves are displaced compared to the intervertebral foramina from which they emerge. This is why the spinal cord ends at the level of the second lumbar vertebra (L2) [21, 22, 23, 24, 25] , although it gives off roots extending as far as the fifth sacral or even the first coccygeal roots [26, 27, 28, 29, 30].
When a lumbar puncture is to be performed, it is generally done below the second lumbar vertebra so as not to injure the spinal cord [2, 31, 32, 33].
The spinal cord follows the path of the spine, thus describing two curvatures: a cervical one with a posterior concavity (lordosis) and a thoracolumbar one with an anterior concavity (kyphosis) [34].
It also presents two enlargements (bulges) [32, 35, 36, 37, 38] : a cervical and a lumbar one, related to the innervation of the upper and lower limbs. It ends at the bottom with the conus medullaris [50] , which gives rise to the cauda equina [91] (a cluster of lumbosacral descending roots).
Internal Configuration :
On a cross-section [43] , the spinal cord has a central region: the gray matter, which includes the bodies of nerve cells, and a peripheral part: the white matter, which is made up of axonal extensions and their myelin sheaths [104, 113, 124, 125].
The gray matter takes the shape of a butterfly, with two anterior horns housing the bodies of the motor neurons and two posterior horns which receive the sensory fibers. In the thoracolumbar spinal cord, there are also lateral horns for the cell bodies of the sympathetic fibers [40, 41, 42].
The white matter is organized into three pairs of columns (funiculi) [36, 43, 44, 45] (anterior, posterior, and lateral).
The spinal cord also has a ventral fissure, which is the most pronounced, a posterior sulcus, and two lateral sulci from which two pairs of nerve roots arise: one anterior (motor) and the other posterior (sensory). These two roots join together to form a spinal nerve.