I054 



HUMAN ANATOMY. 



nerves between the latter spine and the fourth thoracic, the lower six thoracic nerves 

 between the fourth and ninth dorsal spines, the five lumbar nerves opposite the 



ninth, tenth and eleventh spines, 



Fig. 908. 



First cervical, 

 vertebra 



c'/// 7"^/ 



"/3 



-Skull 



%lcr 



fe 



First thoracic_ 

 vertebra' 



\3 



• First thoracic spine 



VI 





'X 



and the five sacral nerves opposite 

 the twelfth thoracic and the first 

 lumbar spine. 



A convenient rule to locate the 

 levels of origin of the nerve-roots, 

 applicable to the prelumbar nerves, 

 is given by Ziehen as follows : — 

 For the cervical nerves, subtract one 

 from the number of the nerve, the 

 remainder indicating the correspond- 

 ing spinous process ; for the upper 

 (I-Vj thoracic nerves subtract one ; 

 for the lower (VI-XII) thoracic 

 nerves subtract two. All the cer- 

 vical nerves pass out through the 

 intervertebral foramina above the 

 vertebrae after which they are named, 

 except the eighth cervical, which 

 emerges between the seventh cer- 

 vical and the first dorsal vertebrae. 

 All the other spinal nerves escape 

 below the vertebrae from which they 

 are named. Since the nerve-roots 

 pass a considerable distance down- 

 ward within the spinal canal before 

 leaving it, it follows that a lesion of 

 the cord at a given level, as from 

 a fracture-dislocation of the spine, 

 may be associated with a paralysis 

 of the nerve-roots passing out at 

 or below that level, and arising 

 from the cord at a higher point. 

 This must be taken into account in 

 determining the scat of the lesion, 

 since when the ner\'e-roots are not 

 involved the lesion will be as much 

 higher than its corresponding inter- 

 vertebral foramina (as indicated by 

 the upper limits of the paralyzed 

 area) as the length of the intraspinal 

 course of the corresponding nerve- 

 roots. 



Each root-cell in the anterior 

 horn of gray matter is connected 

 with a motor fibre, which passes 

 out in the anterior root of a spinal 

 nerve to its muscle. Motor impulses 

 originating in the cortex of the brain, 

 pass downward along the antero- 

 lateral columns of the cord, chiefly 

 in the lateral pyramidal tract. They 

 first traverse the ganglion cells of 

 the anterior horns before passing 

 out in the anterior or motor roots 

 to their destination. These ganglion cells constitute, at least functionally, the trophic 

 centres for the muscles. Lesions of the anterior horns, therefore, besides causing 



First lumbar 

 vertebra 



/ First lumbar spine 



11/ 



IV 



■Sacrum 



First sacral vertebra 



Coccyx < 



Diagram, based on frozen section, shovi'ing relations of 

 oodies and spines of vcrtebrat to levels at which spinal nerves 

 escape irom vertebral canal. 



