ORIGIN OF SPINAL NERVES. 495 



It may be added, however, that there is no sufficient 

 evidence for the supposition that an uninterrupted con- 

 tinuity of nerve-fibres is essential to the conduction of 

 impressions on the spinal nerves to and from the brain : 

 such impressions may be as well transmitted through the 

 nerve-vesicles of the cord as by the nerve -fibres ; and the 

 experiments of Brown-Sequard, again to be alluded to, 

 make it probable that the grey substance of the cord is the 

 only channel through which sensitive impressions are con- 

 veyed to the brain. 



The Nerves of the Spinal Curd consist of thirty- one pairs, 

 issuing from the sides of the whole length of the cord, their 

 number corresponding with the intervertebral foramina 

 through which they pass. Each nerve arises by two roots, 

 an anterior and posterior, the latter being the larger. The 

 roots emerge through separate apertures of the sheath 

 of dura mater surrounding the cord ; and directly after 

 their emergence, while the roots lie in the intervertebral 

 foramen, a ganglion is formed on the posterior root. The 

 anterior root lies in contact with the anterior surface of 

 the ganglion, but none of its fibres intermingle with those 

 in the ganglion. But immediately beyond the ganglion 

 the two roots coalesce, and by the mingling of their fibres 

 form a compound or mixed spinal nerve, which, after 

 issuing from the intervertebral canal, divides into an 

 anterior and posterior branch, each containing fibres from 

 both the roots (fig. 134). 



According to Kolliker the posterior root-fibres of the 

 cord enter into no connection with the nerve-corpuscles in 

 the ganglion, but pass directly through, in one or more 

 bundles, which are collected into a trunk beyond the gang- 

 lion, and then join the motor root. From most, if not all, 

 of the ganglionic corpuscles, one or two, rarely more, 

 nerve-fibres arise and pass out of the ganglion, in a peri- 

 pheral direction, in company with the posterior root-fibres 

 of the cord. Each spinal ganglion, therefore, is to be 



