THE NERVOUS SYSTEM 463 



crease and decrease in the number of nerves entering and leaving 

 the cord in these regions. 



The Grey Matter of the Cord may be regarded as a pair of 

 long columns extending throughout its length. These columns, 

 when examined microscopically, are found to consist mainly of 

 nerve-cells, some of which are connected with the dorsal, others 

 with the ventral roots of the spinal nerves. A collection of 

 nerve-cells possessing afferent and efferent nerves is essentially 

 a ganglion, and, regarded in this light, the grey matter of the 

 spinal cord may be considered to be built up of a series of gan- 

 glia placed end to end and communicating. In the ventral 

 cornu of the grey matter the cells are largest and arranged in 

 groups. The cells, as we have already seen (p. 435), are multi- 

 polar ; their axon is the origin of the fibre of the ventral or motor 

 spinal root. All the motor nerves obtain their nerve-cell origin 

 in the inferior cornu, and wherever the outflow of nerves is the 

 greatest, as in those parts of the cord opposite to the limbs, 

 there these cells are largest and most definitely grouped. There 

 is another group of cells in the ventral cornu which is not 

 connected with the building up of motor nerves ; their axons pass 

 from the grey into the white matter, and travel up, down, and 

 across the cord, where they constitute short tracts of white 

 matter, which knit together the various segments. These are 

 known as ' association fibres.' Between the ventral and dorsal 

 cornua is a portion of grey matter known as the ' intermedio- 

 lateral column/ In this part are the cells connected with the 

 nerves which link up with the sympathetic system, and supply 

 the viscera by means of the white rami communicantes. The cells 

 in the dorsal horn of the cord are smaller, and the cell-groups 

 not as well marked. There is, however, a group of large cells 

 extending throughout the dorsal grey matter, known as ' Glarke's 

 column.' It furnishes fibres which pass into the white substance 

 of the cord, and form there what is known as the direct cerebellar 

 tract. 



The White Matter of the Cord consists of medullated nerve 

 fibres, generally running longitudinally ; structurally, these 

 fibres differ from those found in the body-nerves by the fact 

 that they possess no neurilemma, the medullary sheath being 

 contained within a supporting material known as neuroglia. 

 The white matter of the cord is divided into certain columns — 

 dorsal, lateral, and ventral. These anatomical columns give no 

 notion of the physiological paths which exist between the brain, 

 cord, and body. These paths in the white matter have been 

 mainly worked out by the degeneration method — viz., by study- 

 ing the degeneration which follows division of the spinal nerves. 

 If, for example, cutting the dorsal roots above the spinal gan- 



