140 THE ANATOMY OF THE HORSE. 



stated, does not extend be3-ond the middle of the sacrum. These last 

 nerves have therefore a length of several inches within the spinal canal ; 

 and as they pass back together, each to reach its aperture of exit, they 

 have an arrangement which resembles the hairs of a horse's tail, and is 

 therefore termed the cauda equina. 



KooTS of the Nerves. Each spinal nerve has two roots connecting 

 it with the spinal cord — a superior and an inferior. The superior, 

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

 the supero-lateral 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, motw\ or aganglionic root consists of fibres detached 

 from the cord along its infero-lateral fissure. These, which 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 

 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 accessm'y nerve (page 255) 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 Artery 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 the bifurcation of the cerebro-spinal artery. The middle spinal 

 artery passes backwards beneath the inferior median fissure of the cord. 

 Its branches are distributed to the cord and its membranes. As it passes 

 backwards giving off its branches, it is reinforced by other arteries 

 entering at the intervertebral foramina. Thus, at each intervertebral 



