STRUCTURE OF THE GREY SUBSTANCE OF THE SPINAL CORD. 343 



scribed when the character of the several subdivisions of the 

 grey matter are under consideration. 



The nerve fibres of the grey substance are extraordinarily 

 numerous, and form its chief constituent. Many of them possess 

 a medullary sheath, but many must also be regarded as simply 

 axis-cylinders, since no medullary sheath can be demonstrated 

 around them. They differ from the fibres of the white sub- 

 stance by the frequent subdivision that each fibre undergoes, 



Fig. 222. 



Fig. 222. Partially medullated nerve fibres, with frequent division, 

 isolated from the grey substance of the posterior cornu of a spinal 

 cord, treated with ehromate of potash. Human. Magnified 300 

 diameters. 



and by their much smaller breadth, whilst in proportion to 

 the number of divisions the diameter of the fibres diminishes. 

 On this account it is not easy to furnish accurate measure- 

 ments of the breadth of these fibres, since they vary from 

 a diameter of 0*004 or 005 of a millimeter down to an 

 almost immeasurable fineness, the latter being by far the most 

 numerous. The fibres that have thus by frequent subdivision 

 become extremely attenuated unite to form a narrow-meshed 



