\\u THB NERVOUS SYSTEM 183 



numl)cr of grey non-mcdullatcd nerve fibres, ami in addition 

 numerous nerve cells. The nerve cells vary in size and shape : 

 those in the anterior horn are the largest, being about - J th of 

 an inch in diameter. Each of these contains a large round 

 nucleus and gives off a number of fine processes branching 

 out in various directions, and in addition a single large un- 

 branched process which proceeds outwards from the grey 

 matter of the anterior horn through the white matter into the 

 anterior root of one of the spinal nerves. The single process 

 is continuous with, and in fact forms the axis cylinder of, a 

 nerve fibre of the anterior root of the nerve. In this way 

 most of the nerve fibres of the anterior root derive their axis 



FIG. 84. A nerve cell from a ganglion of the posterior root of a spinal nerve. 

 /, The cell substance ; , nucleus ; ', iis nuclcolus ; c, the connective tissue capsule 

 nf the cells; ", nuclei of tin: capsule; tt.f, nerve fibre dividing into two at d\ 

 til, iiciirilciiiina ; ji, sheath. 



cylinders from the single unbranchcd processes of nerve cells 

 in the anterior horn. So that the anterior root springs largely 

 from the grey matter of the anterior horn. Similarly some 

 of the fibres of the posterior root pass into the grey matter of 

 the posterior horn and become connected with the nerve cells 

 of the grey matter, though in a different way. The nerve cells 

 of the |M>sterior horn, smaller than those of the anterior horn, 

 arc like them in giving off a number of fine processes, but differ 

 from them in having no axis cylinder process. While the fibres 

 of the anterior root may be considered as axis cylinder processes 

 of the cells of the anterior horn, the fibres of the posterior root 

 may l>e considered as axis cylinder processes of the cells of 

 tli- ganglion of the poMi-rior root, and therefore end in the 

 grey matter of the cord in a different way. The great majority 



