THE SPINAL CORD 345 



slender filament, the filum terminale, which may be traced 

 backward to the middle of the tail, and which is produced 

 in development by the more rapid growth of the bony 

 canal than of the spinal cord within it. 



A faint median groove, most distinct towards the cephalic 

 end of the cord, divides it into right and left halves. This 

 is the dorsal or posterior median sulcus. A short distance 

 ■ to each side of this, the dorsal roots of the spinal nerves 

 enter the cord along a still fainter groove, the dorsolateral 

 or posterior lateral sulcus. 

 (c) The nerve roots have a regional distribution — eight cervical , 

 twelve thoracic, seven lumbar, four sacral, and six caudal. 



Since the first spinal nerve emerges between the skull 

 and the atlas, the cervical nerves are numbered to correspond 

 with the vertebrae lying behind the intervertebral foramina 

 from which they proceed, though the remaining spinal 

 nerves are designated according to the vertebrae lying in 

 front of the corresponding intervertebral foramina. The 

 nerve transmitted by the intervertebral foramen between the 

 seventh cervical and first thoracic vertebrae is described as 

 the eighth cervical. 



The disproportionate growth which produces the filum 

 terminale also carries the connections of the posterior spinal 

 nerves with the spinal cord to levels further forward than 

 their emergence from the vertebral column. Hence a group 

 of these nerves runs backward at each side of the filum 

 terminale, constituting the formation known as the cauda 

 equina. These features are less pronounced in the rabbit 

 than in the human species, in which the spinal cord termin- 

 ates near the boundary between the first and second lumbar 

 vertebrae in the adult. 

 (d) The origin and primary divisions of the nerve roots may be 

 worked out by removing carefully the lateral portions of 

 the arches of one or two vertebrae. Each spinal nerve has 

 a posterior or dorsal root (radix posterior), composed of 

 afferent nerve fibres (p. 73), which enters the dorsolateral 

 surface of the spinal cord as a linear series of rootlets. These 



