THE NERVOUS SYSTEM 



449 



a division of the cord into segments, each segment extending above 

 and below its nerve roots one-half the distance to the next adjacent 

 roots. There are 31 segments corresponding to the 31 spinal nerves; 

 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and i coccygeal. 



Origin of the Fibres which Make up the White Matter of the Cord 



It has already been observed that the white matter of the cord is 

 composed mainly of medullated nerve fibres, most of which run in a 

 longitudinal direction. From the study of the neurone it follows that 

 each of these fibres must be the axone of some nerve cell. The 

 bodies of these cells, the medullated axones of which form the white 

 matter of the cord are situated as follows: 



(i) 



Cells outside the spinal 

 cord. (Extrinsic cells.) 



B. 



Cells situated in the gray 

 matter of the cord. (In- 

 trinsic cells.) 



(3) 



(5) 



Cells outside the central nervous system 

 (spinal ganglion cells). 

 ] (2) Cells in other parts of the central nervous 

 system (the brain). 



Root cells, such as those of the anterior 

 horn, whose axones form the ventral root 

 (efferent peripheral neurones) . These pass 

 out directly, and thus do not become longi- 

 tudinal column fibres. 

 (4) Intermediate neurones, whose axones enter 

 into formation of the fibre columns of the 

 cord (column cells.) 



Cells of Golgi, type II, the axones of which 

 ramify in the gray matter. (These cells do 

 not give rise to fibres of the white matter, 

 but are conveniently mentioned here among 

 the other cord cells.) 



(i) The Spinal Ganglion Cell and the Origin of the 



Posterior Columns 



It has already been seen that the central processes of these cells 

 enter the cord as dorsal root fibres and spHt into ascending and de- 

 scending longitudinal arms composing the greater part of the dorsal 

 funiculus and the zone of Lissauer. They are described more in 

 detail later (pp. 467 and 468). 



(2) Cells Situated in Other Parts of the Central Nervous 

 System which Contrlbute Axones to the White Columns 

 OF the Cord. 

 These cells are situated in the motor areas of the cortex of the 



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