The Spinal Nerves. 



THE Spinal Nerves are so called, from taking their origin from the spinal cord, 

 and from being 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 

 vertebra? in each region, excepting in the cervical and coccygeal. 



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

 posterior or sensitive root. 



ROOTS OF THE SPINAL NERVES. 



The anterior roots arise somewhat irregularly from a linear series of foramina, 

 on the antero-lateral column of the spinal cord, gradually approaching towards 

 the anterior median fissure as they descend. 



The fibres of the anterior roots are, according to the researches of Mr. Lockhart 

 Clarke, attached to the anterior part of the antero-lateral column; and. after 

 penetrating horizontally through the longitudinal fibres of this tract, enter the 

 gray substance, where their fibrils cross each other and diverge in all directions, 

 like the expanded hairs of a brush, some of them running more or less longi- 

 tudinally upwards and downwards, and others decussating with those of the 

 opposite side through the anterior commissure in front of the central canal 

 (figs. 258 and 259). Kolliker states that many fibres of the anterior root enter the 

 lateral column of the same side, where, turning upwards, they pursue their course 

 as longitudinal fibres. In other respects, the description of the origin of the 

 anterior roots by these observers is very similar. 



The posterior roots are all attached immediately to the posterior white columns 

 only ; but some of them pass through the gray substance into both the lateral and 

 anterior white columns. Within the gray substance, they run, longitudinally, 

 upwards and downwards; transversely,* through the posterior commissure to the 

 opposite side and into the anterior column of their own side (figs. 258 and 259). 



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 outwards, towards the aperture in the dura mater, they coalesce into two 

 bundles, receive a tubular sheath from this membrane, and enter the ganglion 

 which is developed upon each root. 



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

 characters. 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. 



The anterior roots are the smaller of the two devoid of any ganglionic 

 enlargement, and their component fibrils are collected into two bundles, near tbe 

 intervertebral foramina. 



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