ORIGIN OF THE SPINAL NERVES. 



493 



dorsal portion and the upper part of its lumbar, at the 

 origins of the large nerves which, after forming the lum- 

 bar and sacral plexuses, are distributed to the lower 

 extremities. The chief cause of the greater size at these 

 parts >of the spinal cord is increase in the quantity of grey 

 matter ; for there seems reason to believe that the white 

 or fibrous part of the cord becomes gradually and pro- 

 gressively larger from below upwards, doubtless from the 

 addition of a certain number of upward passing fibres from 

 each pair of nerves. 



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 Cord 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, where the roots lie in the intervertebral 

 foramen, a ganglion is found 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 



