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 " 



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

 the vertebra 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. 



EOOTS 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, according to the researches of Mr. Lockhart 

 Clarke, are 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. 

 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, longi- 

 tudinally, upwards and downwards, transversely, through the posterior com- 

 missure to the opposite side, and into the anterior column of their own side. 



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 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 

 characters. It is smaller than the anterior, has frequently no gnnglion de- 

 veloped 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 en- 

 largement, and their component fibrils are collected into two bundles, near the 

 intervertebral foramina. 

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