THE NERVOUS SYSTEM 1323 



diverge in each grey crescent. They serve as association-fibres 

 which bring the cells of opposite sides into relation with one another. 

 This commissure contains the central canal of the cord, which for 

 the most part is nearer the ventral portion of the commissure than 

 the dorsal. The part of the commissure surrounding the central 

 canal is called the substantia gelatinosa ceniralis. It consists of 

 neuroglia, a few nerve-cells, and nerve-fibres ; and it is invaded by 

 processes derived from the deep ends of the ciliated columnar 

 epithelial cells which line the central canal. 



Summary ol the Gelatinoos Substances ol the Grey Matter. — These are — 

 (i) The substantia gelatinosa of Rolando, which forms a cap for the caput 

 comu posterioris; (2) the substantia gelatinosa centralis, which surrounds the 

 central canal of the cord; and (3) the substantia gelatinosa externa, which 

 forms the glial sheath of the cord beneath the pia mater. 



White Matter. — ^The white matter of the cord consists of longi- 

 tudinal medullated nerve-fibres, traversed by septa of the pia 

 mater, and embedded in neuroglia. The fibres have no primitive 

 sheath or neurilemma. 



Ventral or White Commissure. — ^This commissure lies at the 

 bottom of the ventral median fissure, and it is separated from the 

 central canal of the cord by a part of the dorsal or grey commissure. 

 It consists of medullated nerve-fibres, destitute of a" neurilemma, 

 some of which pass transversely, but most of them decussate, 

 entering the commissure ventrally on one side, and leaving it 

 dor sally on the opposite side. The fibres, after crossing, enter the 

 grey crescent and the ventral column. They are derived from 

 (i) the direct pyramidal tract, or tract of Turck, (2) the processes 

 of root-cells and of association-cells, and {3) the fibres of the spino- 

 thalamic tract, to be presently described. 



Fibres of Roots of Spinal Nerves — Ventral or Anterior Nerve- 

 Roots. — ^The fibres of the ventral nerve-roots arise within the cord 

 from several sources, (i) Many of them are axons of the mesial 

 cells of the ventral horn of grey matter of the same side. (2) Some 

 are axons of the lateral cells of the ventral cornu. (3) Others are 

 axons of the cells of the intermedio-lateral tract of Clarke of the same 

 side. (4) A few are axons of cells in the dorsal cornu of grey matter 

 of the same side. (5) A few are regarded as being axons of the 

 mesial cells of the ventral cornu of the opposite side, which cross in 

 the ventral or white commissure. All the axons receive their 

 medullary sheaths near the parent-cells, and they form funiculi, 

 which leave the white matter of the ventral column over an area 

 corresponding to the caput of the ventral comu of grey matter, 

 after which each fibre acquires its primitive sheath or neurilemma. 



The most of the axons of the fibres of the ventral nerv^e-roots 

 belong to the ventral (motor) cells of the ventral comu of grey 

 matter of the same side. 



. Dorsal or Posterior Nerve-Roots. — ^The fibres of the dorsal nerve- 

 roots arise from the unipolar (originally bipolar) cells of the spinal 

 ganglia. The single pole or process of each of these cells is T-shaped. 



