VESSELS OF THE SPINAL CORD. 379 



Size of the roots to each other. The posterior root is larger than the an- 

 terior, except in the suboccipital nerve ; and the number of the fibrils is 

 also greater. Further, the posterior is proportionally larger in the cervical 

 than in any other group ; in the dorsal nerves there is but a very slight 

 difference in favor of the hinder root. 



Size of both roots along the cord. Both roots are larger where the nerves 

 for the limbs arise, than at any other part of the cord ; and they are 

 greater in the nerves to the lower than in those to the upper limbs. They 

 are smallest at the lower extremity of the cord. 



Direction and length of the roots. As the apertures of transmission from 

 the spinal canal are not opposite the place of origin of the nerves, the roots 

 must be directed more or less obliquely. This obliquity increases from 

 above down ; for in the upper cervical nerves the roots are horizontal, but 

 in the lumbar and sacral nerves they have a vertical direction around the 

 end of the medulla spinalis. The appearance of the long fibrils around the 

 end of the cord (fig. 124) resembles the extremity of a horse's tail, and 

 bears appropriately the term cauda equina. 



The length of the roots increases in proportion to their obliquity. Be- 

 tween the origin and the place of exit of the roots of the lower cervical 

 nerve the increase amounts to the depth of one vertebra : in the lower 

 dorsal nerve it equals the depth of two vertebra? ; and in the lumbar and 

 sacral nerves each succeeding root becomes a vertebra longer, for the cord 

 does not reach beyond the first lumbar vertebra. 



Place of union of the roots. Commonly the roots unite as before state 

 in the intervertebral foramina ; and the trunk of the nerve bifurcates at 

 the same spot into anterior and posterior primary branches (fig. 123, d 

 and e). But deviations from this arrangement are found at the upper and 

 lower ends of the spinal column in the following nerves. 



The roots of the first two cervical nerves join on the neural arches of 

 the corresponding vertebra? ; and the anterior and posterior primary 

 branches diverge from the trunks in that situation. 



In the sacral nerves the union of the roots takes place within the spinal 

 canal ; and the primary branches of the nerves issue by the apertures in 

 the front and back of the sacrum. 



The roots of the coccygeal nerve are also united in the spinal canal ; and 

 the anterior and posterior branches of its trunk escape by the lower aper- 

 ture of that canal. 



Situation of the ganglia. The ganglia are placed commonly in the in- 

 tervertebral foramina, but where the position of those apertures is irregu- 

 lar, as at the upper and lower extremities of the spinal column, they have 

 the following situation : In the first two nerves they lie on the neural 

 arches of the atlas and axis. In the sacral nerves they are contained in 

 the spinal canal. In the coccygeal nerve the ganglion is also within the 

 canal, and about the middle of the long posterior root (Schlemm). 



VESSELS OF THE SPINAL CORD. The arteries on the surface of the 

 cord are anterior and posterior spinal. 



The anterior spinal artery occupies the middle line of the cord beneath 

 the fibrous band before alluded to in that position. It begins by the union 

 of two small branches of the vertebral artery (p. 174) ; and it is continued 

 to the lower part of the cord by a series of anastomotic branches, which 

 are derived from the vertebral and ascending cervical arteries in the neck, 

 and from the intercostal arteries in the Back. Inferiorly it supplies the 

 roots of the nerves forming the cauda equina, and ends on the central 



