THE CENTRAL NERVOUS SYSTEM. 



1025 



FIG. SSi. 



those of the thoracic re- 

 gion from 12-26 mm., the 

 longest belonging to the 

 V-VII thoracic nerves; 

 those of the lumbar region 

 rapidly decrease from 15.5 

 -5.5 mm., followed by a 

 more gradual diminution 

 to less than 4 mm. in the 

 sacral region. 



In consequence of the 

 disproportion between the 

 length of the spinal cord 

 and that of the vertebral 

 canal, the discrepancy be- 

 tween the level at which 

 the nerves are attached 

 to the cord and that of 

 the intervertebral foramina 

 through which they leave 

 the canal becomes more 

 marked towards the lower 

 end of the series. The 

 growth of the cord, how- 

 ever, is not uniform since, as shown by Pfitzner, during the later years of childhood elongation 



Veins 



Transverse section of vertebral canal, at level of middle of first lumbar 

 vertebra; spinal cord (conus medullaris), surrounded by nerve-bundles, is seen 

 within dural sheath. 



FlG. 882. 



Conus 



i In 



medullaris 



Descending 

 spinal nerves 



Front wall of 



dural sheath 

 Filum internum 



End of spinal cord with roots of lower nerves descend- 

 ing in cauda equina to gain their respective foramina; 

 1-5 In, 7-5 sn, en, lumbar, sacral and coccygeal nerves. 



65 



of the thoracic region occurs to such an extent 

 that this part of the cord once more equals, 

 if indeed not exceeds, the corresponding 

 portion of the spine. While the cervical cord 

 keeps fairly abreast the cervical portion of 

 the vertebral column, the lumbar and sacral 

 segments are left far behind. The results of 

 these changes are seen in the course of the 

 root-fibres, which in the neck, below the third 

 nerve, run somewhat downward to their points 

 of emergence, and in the thoracic region pass 

 more horizontally, while those of the lumbar 

 and sacral nerves descend almost vertically 

 for a considerable distance in the case of the 

 last sacral nerve 28 cm. (Testut) before 

 reaching their appropriate levels. 



The large and conspicuous leash of 

 descending root-fibres, seen upon open- 

 ing the dural sheath, constitutes the 

 cauda equina, in the midst of which 

 the glistening silvery filum terminale 

 is distinguishable. It is evident, there- 

 fore, that in most cases the level of the 

 cord-segment and that of the vertebra 

 bearing the same designation do not 

 correspond. Likewise, it must be re- 

 membered that, although in general the 

 spinal nerves are named in accordance 

 with the vertebrae immediately below 

 which they escape, in the neck there 

 are eight cervical spinal nerves and 

 only seven vertebrae, the first or sub- 

 occipital nerve emerging between the 

 atlas and the skull, and the eighth 

 between the last cervical and first thoracic 

 vertebra; hence, except the last one, they 

 correspond with the vertebra below. 



