The Spinal Cord

The Spinal CordIn man the spinal cord is a long cylindrical structure lying within the vertebral or spinal canal. At the level of the joint between the vertebral column and the skull the spinal cord is continuous with the brain. Below, the functionally active part of the cord terminates somewhat abruptly in a conical extremity at the upper part of the lumbar region, i.e. it occupies about the upper two-thirds of the spinal canal (Fig. 33); a fine non-nervous strand however - the filum terminale - continues the cord to near the end of the vertebral column. Two enlargements disturb the regular cylindrical form of the cord (Fig. 34): one in the upper part where the large nerves to the arms are given off; the other at the lower end where the nerves to the legs emerge. Along the front of the cord is a well-marked longitudinal groove termed the anterior median fissure; an extremely narrow and shallow longitudinal groove the posterior median fissure similarly extends down the back of the cord.

The relative depths of these two fissures are most clearly evident in a transverse section of the cord; in such a section it will be seen that a septum of neuroglia (the posterior median septum) extends nearly to the middle of the cord from the posterior median fissure (Fig. 35).

Although on superficial inspection the cord appears to be a solid structure, yet on careful examination, a fine canal will be found running down its centre; this, the central canal of the cord, indicates the original development of the cord from a tube.

At regular intervals the spinal nerves emerge from the sides of the cord, each nerve, as already described, coming off by two roots, a posterior sensory gangliated, and an anterior motor non-gangliated.

The cord is enveloped by three membranous sheaths. The innermost, known as the pia mater, is closely applied to the surface of the cord and carries its bloodvessels. It is separated by a space the subarachnoid space-from the next membrane, the arachnoid, which is thin and delicate; the subarachnoid space communicates with the ventricles of the brain and through these with the central canal of the cord, and contains a fluid known as the cerebro-spinal fluid. The outermost membrane forms a tough fibrous covering and is called the dura mater; it is separated from the arachnoid by a fine capillary space - the subdural space - which is moistened with lymph. To avoid repetition it may be pointed out here that the same three membranes are continued over the surface of the brain.

The substance of the cord is made up of nerve cells and nerve fibres supported. by neuroglia; the nerve cells are massed together around the centre and constitute the grey matter; most of the nerve fibres are medullated, and are grouped into large tracts on the periphery of the grey matter to form the white matter of the cord.