DISSECTION OF THE BACK AND THORAX. 143 



sensory, or ganglionic root consists of filaments which arise from along 

 the supero-lateval fissure of the cord. These filaments perforate the 

 dura mater, and converge towards the intervertebral foramen, where 

 they form a cord on which there is superposed a reddish oval ganglion. 

 Immediately beyond the ganglion the cord mixes its fibres with the 

 inferior root. 



The inferior, motor, or aganglionic root consists of fibres detached 

 from the cord along its infero-lateral fissure. These, wiiich are fewer 

 and smaller than those of the superior root, perforate the dura mater 

 by openings distinct from those for the superior root ; and, converging 

 towards the intervertebral foramen, they join the superior root immedi- 

 ately external to the point at which the ganglion is placed on it. The 

 fibres of the inferior root, thus, have no connection with the ganglion. 

 Where the superior and inferior roots meet in the intervertebral foramen 

 they mix their fibres and form a short common cord, which almost 

 immediately divides into two — the superior and the inferior primary 



Fig. 18. 



PoRTioif or Spinal Coed with the Boots of the Nerves (quahi). 



1. Inferior median Assure ; 2. Superior median Assure ; 3. Infero-lateral Assure (exaggerated) ; 



4. Supero-lateral fissure ; 5. Inferior roots, passing under tlie ganglion (on the left side these are cut) ; 



b. Superior roots, the fibres of which pass into the ganglion— 6' ; 7. The united or compound nerve ; 



8. Superior primaiy branch ; 9. Inferior primary branch. 



branch; and each of these contains fibres from both roots. Both 

 branches emerge by an intervertebral foramen, and, roughly speaking, 

 the series of superior primary branches supply the skin and muscles 

 above their points of emergence, while the inferior primary branches are 

 distributed to the skin and muscles below their points of emergence, 

 including the limbs. From the common trunk formed by the union of 

 the two roots, a filament re-enters the spinal canal to be distributed to 

 the bones and vessels. 



In the region of the neck the spinal accessory nerve (page 262) passes 

 along each side of the cord, between the superior and inferior roots of 

 the spinal nerves. It is formed by rootlets that spring out of the side 

 of the cord. 



The Vessels of the Spinal Cord : — ■ 



The Middle Spinal Aetbry begins beneath the cord, in the ring of 

 the atlas. It is here formed on the mesial plane by the fusion of right 

 and left branches, each of which is the posterior branch formed by 



