826 THE NERVOUS SYSTEM. 



THE SPINAL NERVES. 



The spinal nerves are so called because they take their origin from the spinal 

 cord, and are transmitted through the intervertebral foramina on either side of the 

 spinal column. There are thirty-one pairs of spinal nerves, which are arranged 

 into the following groups, corresponding to the region of the spine through which 

 they pass : 



Cervical 8 pairs. 



Dorsal 12 " 



Lumbar . 5 " 



Sacral 5 " 



Coccygeal 1 pair. 



It will be observed that each group of nerves corresponds in number with the 

 vertebne in that region, except the cervical and coccygeal. 



Each spinal nerve arises by two roots, an anterior or motor root, and a pos- 

 terior or sensory root. 



Roots of the Spinal Nerves. 



The Anterior Roots. The superficial origin is from a somewhat irregular series 

 of depressions which map out a longitudinal area opposite the anterior cornu of 

 gray matter on the antero-lateral column of the spinal cord, gradually approach- 

 ing toward the anterior median fissure as they descend. To the deep origin the 

 fibres can be traced through the antero-lateral column ; the roots, after penetrat- 

 ing horizontally through the longitudinal fibres of this tract, enter the gray sub- 

 stance of the anterior cornu, where their fibrils diverge in several directions : 

 some, passing inward, are continued across the anterior commissure in front of 

 the central canal, to become continuous with the axis-cylinder processes of the 

 large cells of the anterior cornu of the opposite side ; others terminate in the 

 mesial group of cells of the anterior column of the same side ; other fibrils pass 

 outward, some to become continuous with the axis-cylinder processes of the group 

 of cells in the lateral part of the anterior column ; and others enter the lateral 

 column of the same side, where, turning upward, they pursue their course as 

 longitudinal fibres. The remaining fibrils pass backward to the posterior horn, 

 where they are continuous with the axis-cylinders of the cells at the base of the 

 posterior cornu. 



The Posterior Boots. The superficial origin is from the postero-lateral fissure 

 of the cord. The deep origin is from the gray substance of the posterior cornu, 

 either directly through the substantia gelatinosa, or indirectly, by first passing 

 through the white matter of the posterior column and winding round in front of 

 the caput cornu. Those which enter the gray matter at once for the most part 

 turn upward and downward, and become continuous with the fine nerve-plexus in 

 the central portion of the gray matter ; some few fibres pass transversely through 

 the posterior commissure to the opposite side, and others into the anterior cornu 

 of the same side. Those fibres which enter the gray matter in front of the caput 

 cornu reach the posterior vesicular column (Clark's column) and blend with it, a 

 few fibres passing through it, to become longitudinal in the posterior column of 

 the cord. 



The anterior roots are smaller than the posterior, devoid of ganglionic enlarge- 

 ment, and their component fibrils are collected into two bundles near the inter- 

 vertebral foramina. 



The posterior roots of the nerves are larger, but the individual filaments are 

 finer and more delicate than those of the anterior. As their component fibrils 

 pass outward, toward the aperture in the dura mater, they coalesce into two bun- 

 dles, receive a tubular sheath from that membrane, and enter the ganglion which 

 is developed upon each root. 



The posterior root of the first cervical nerve forms an exception to these cha- 

 racters. It is smaller than the anterior, has frequently no ganglion developed 

 upon it, and when the ganglion exists it is often situated within the dura mater. 



