CHAP. XI.] ROOTS OF THE SPINAL NERVES. 301 



lumbar, and sacral), from which nerves are furnished to the extre- 

 mities and the anterior part of the trunk. 



The first spinal nerve, called by Winslow the suboccipital, offers 

 an exception to this arrangement. Generally it arises by two roots, 

 of which, however, the anterior is the larger. Sometimes it has 

 only one root, corresponding to the anterior. 



The spinal nerves are arranged naturally in classes, according to 

 the regions of the spine in which they take their rise. We number 

 eight in the cervical region, the suboccipital included; twelve in 

 the dorsal region; five in the lumbar, and six in the sacral regions. 

 All the nerves after the second pass obliquely outwards and down- 

 wards, from their emergence from the spinal cord to their exit 

 from the vertebral canal; and this obliquity gradually increases 

 from the higher to the lower nerves, so that the inferior ones 

 are nearly perpendicular; and, as their intra-spinal course is of 

 some length, they are collected into a leash, which constitutes the 

 cauda equina. 



All the spinal nerves arise from the cord by separate fasciculi of 

 filaments, which, as they approach the dura mater, converge to each 

 other, and are united together to constitute the anterior or the 

 posterior roots. The posterior roots arise at a pretty uniform dis- 

 tance from the posterior median fissure in all regions of the cord, 

 indicating but a very trifling change in the thickness of the pos- 

 terior columns throughout their entire course. Not so the anterior 

 ones : they are farthest from the anterior median fissure in the 

 neck, but very near it in the dorsal region ; this difference being due 

 to the variation in the thickness of the antero-lateral columns in the 

 different regions. The ganglia on the posterior roots are always 

 proportionate in size to the roots themselves. 



In tracing the mode of connexion of the roots of the spinal 

 nerves with the cord, great care is required, from the sudden change 

 of consistence which their fascicles experience on penetrating the 

 substance of the cord. They lose the sheath of pia mater which 

 gave firmness to that part which is external to the cord, and soon 

 break up into their component fibrillaB. For this reason, the speci- 

 men employed for the dissection should be quite recent, and slightly 

 hardened by previous immersion in spirit. 



The anterior roots penetrate the lateral part of the antero-lateral 

 columns. Their fibres soon radiate, some passing upwards and in- 

 wards, others horizontally inwards towards the centre of the cord, 

 mingling, no doubt, with the elements of the vesicular matter com- 

 posing the anterior horn. It is a matter of uncertainty, whether the 



