ROOTS OF THE SPINAL NERVES Thirty-one pairs of spinal nerves connect the cord with the periphery. Each nerve is joined to the cord by two roots: an anterior, efferent or motor root and aposterior, afferent or sensory root (Figs. 138 and 146). These roots descend more or less from their cord attachment to the inter-vertebral foramen in which they unite to form the spinal nerve. The roots of the first cervical nerve are horizontal; those of the first thoracic nerve descend the width of two vertebra, and those of the twelfth thoracic, the width of four vertebra; while the roots of the coc cygeal nerve extend from the first lumbar vertebra to the second piece of the coccyx, through ten vertebra.
Anterior Root.—In all spinal nerves, except the first, the anterior root (radix anterior) is smaller than the posterior. It is composed of from four to six fasciculi, which soon combine into two bundles. After piercing the dura mater, the anterior root unites with the posterior, beyond the latter's ganglion, and forms a spinal nerve. The anterior root is efferent, or motor, in function.
Point of Exit (Fig. I46).—The anterior root is composed of medullated axones which issue from the narrow longitudinal area at the junction of the anterior one-fourth with the posterior three-fourths of the cord's surface. This area is bounded later ally by the anterior root-line, commonly called the anterior lateral sulcus.
Origin (Fig. 146).—These medullated axones rise from the medial, lateral, and intermedio-lateral columns of cell-bodies on the same side of the cord and from the medial column of the opposite side. These cell-bodies of the anterior column and the intermedio-lateral column constitute the genetic nuclei (nuclei origins) of the spinal nerves. The fibers of large caliber in the anterior roots rise from the cell-bodies in the anterior columnee, the somatic nucleus. They are voluntary motor fibers. In the intermedio-lateral column, which is the visceral nucleus, the small fibers of the anterior roots take their origin. They are probably sympathetic in function, that is, involuntary motor, vasomotor, viscero-motor, inhibitory, secretory, trophic, in hibito-secretory, and inhibito-trophic.
Lesions.—The lower motor neurones (spinal and cerebral) are probably in a state of toxic irritation in laryngismus stridu lus, tetanus, acute ascending paralysis (Landry), strychnine poisoning, etc., hence the twitchings, spasms and convulsions. Their sudden destruction causes flaccid paralysis (lower seg ment paralysis). Gradual degeneration of the lower motor neurones causes muscular atrophy and slowly increasing paraly sis. In spinal meningitis both the anterior and posterior roots are affected.
The posterior root (radix posterior) is the sensory, or afferent root (Figs. 138 and 146). It is larger than the anterior root, except in the case of the first cervical nerve; and is composed of from six to eight fasciculi, which combine at once into two bundles. The posterior root pierces the dura mater separately from the anterior root. It unites with the anterior root in the intervertebral foramen. Near the outer end, it presents a swelling which contains large vesicular bipolar cell-bodies and is called a spinal ganglion (ganglion spinale). The ganglion (Figs. 138 and 146) and posterior root are occasionally absent on the first nerve. The posterior root, lateral to the ganglion, is made up of the dendritic processes (Cajal) of the ganglion cells. These dendrites, which in appearance are axones, extend to the most distant parts of the body; they are the sensory fibers of the spinal nerves. On the proximal side of the ganglion, the posterior root is composed of axones, which rise from the ganglion cells. Both the axonic and dendritic processes may be medullated.
Entrance into Cord (Fig. 146).—The posterior roots of the spinal nerves enter the posterior-lateral sulcus; and, at once, divide into an outer set of small fibers and an inner set of large fibers with some small ones interspersed. The fibers of each set bifurcate into a large ascending and a small descending branch. Collaterals rise from the parent axone and from both branches.