SPINAL NERVES. 391 



According to the experiments of Sir Charles Bell, Ma- 

 gendie, and others, it has also been stated that the anterior 

 columns of the spinal marrow are the sources of motion, 

 and the posterior those of sensation. 



It is also found that the medullary matter of the cord 

 has longitudinal fibres extending to the brain, and that 

 these medullary fibres are the media of communication 

 between the brain and spinal marrow, being the conductors 

 of all impressions to the brain, constituting conscious per- 

 ception, and transmitters of all volitions from the brain, 

 giving rise to voluntary muscular motions. But the spi- 

 nal marrow has nervous fibres which stop within the 

 cord itself, and do not go to the brain, and even without 

 the brain are conductors, both of sensation and motion 

 but of a sensation which is not conscious, and a motion 

 which is involuntary powers which Mr. Marshall Hall 

 has shown to be derived from the spinal marrow itself, 

 acting as an independent ganglionic centre. The nerves 

 which arise from, and terminate in it, he calls, as else- 

 where mentioned, the excito motory. 



ORIGIN OF THE SPINAL NERVES. 



All the spinal nerves are compound, and arise by double 

 roots one anterior, the other posterior; the former arising 

 from the anterior columns and being nerves of motion ; the 

 latter from the posterior columns, and designed for sensa- 

 tion. Each root consists of several filaments. The anterior 

 roots are smaller than the posterior, except the first, or 

 sub-occipital, and are separated from each other by the 

 ligamentum denticulatum. These roots approach each 

 other, and perforate the dura mater separately, each receiv- 

 ing a sheath from it. On the posterior root a small oval 

 ganglion is seen; and immediately beyond the ganglion, 

 the two roots come together and constitute a proper spinal 

 nerve. The number of spinal nerves is thirty-one, eight 

 cervical, twelve dorsal, five lumbar, and six sacral; though 

 some anatomists make but thirty, and others thirty-two. 



The ganglia are reddish, firm bodies, situated in the 



