Page 348 - Clinical Anatomy

ECA6 7/18/06 6:54 PM Page 333

The spinal cord

The spinal cord is 18in (45cm) long. It is continuous above with the medulla
oblongata at the level of the foramen magnum and ends below at the lower
level of the 1st, or the upper level of the 2nd lumbar vertebra. Inferiorly, it
tapers into the conus medullaris from which a prolongation of pia mater, the
filum terminale, descends to be attached to the back of the coccyx.
The cord bears a deep longitudinal anterior fissure, a narrower posterior
septum and on either side, a posterolateral sulcus along which the posterior
(sensory) nerve roots are serially arranged (Fig. 234).
These posterior roots each bear a ganglion which constitutes the first
cell-station of the sensory nerves.
The anterior (motor) nerve roots emerge serially along the anterolateral
aspect of the cord on either side. Both the anterior and posterior nerves
arise by a series of rootlets from the cord.
At each intervertebral foramen the anterior and posterior nerve roots
unite to form a spinal nerve which immediately divides into its anterior and
posterior primary rami, each transmitting both motor and sensory fibres.
The length of the roots increases progressively from above downwards
due to the disparity between the length of the cord and the vertebral
column; the lumbar and sacral roots below the termination of the cord at
vertebral level L2 continue as a leash of nerve roots termed the cauda equina.

Age differences
Up to the 3rd month of fetal life the spinal cord occupies the full extent of
the vertebral canal. The vertebrae then outpace the cord in the rapidity of
their growth so that, at birth, the cord reaches only the level of the 3rd
lumbar vertebra (Fig. 235).
Further differential growth up to the time of adolescence brings the
cord to its definitive position at the approximate level of the disc between
the 1st and 2nd lumbar vertebrae (Fig. 236).

Structure (Fig. 234)

In transverse section of the cord is seen the central canal around which is the
H-shaped grey matter, surrounded in turn by the white matter which con-
tains the long ascending and descending tracts.
Within the posterior horns of the grey matter, capped by the substantia
gelatinosa, terminate many of the sensory fibres entering from the posterior
nerve roots. In the large anterior horns lie the motor cells which give rise to
the fibres of the anterior roots.
In the thoracic and upper lumbar cord are found the lateral horns on each
side, containing the cells of origin of the sympathetic system.
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