THE CENTRAL NERVOUS SYSTEM 



not be looked upon as the token of any specific lesion; they are the 

 common structural response of the cell to injurious influences of the 

 most varied nature. 



Grey and White Matter. Nerve-cells are the most distinctive his- 

 tological feature of the grey nervous substance. Sown thickly in the 

 cerebral cortex, the basal ganglia, the floor of the fourth ventricle, and 

 the cervical and lumbar enlargements of the cord, they are scattered 

 more sparingly wherever the grey matter extends. They also occur 

 in the spinal ganglia, and their cerebral homologues (such as the Gas- 

 serian ganglion), in the ganglia of the sympathetic system, and the 

 sporadic ganglia in general. But wide as is their distribution, and 

 great as is the size of the individual cells, some of which have a diameter 

 of 140 //, or even more, they yet make up but a small portion of the whole 

 of the central nervous substance, the total weight of the 9,000 millions of 

 nerve-cell bodies in the human brain being less than 27 grammes 

 (Donaldson). And although it is not to be wondered at that objects 

 so notable when viewed under the microscope should have struck the 



imagination of physiologists, it is probable 

 that the very high powers which it is so 

 common to attribute exclusively to them 

 are, in part at least, shared with the network 

 or feltwork formed by their processes. 



The grey matter, in addition to this ex- 

 ceedingly delicate feltwork of non-medullated 

 fibres and filaments representing the den- 

 drites and such axons and collaterals as 

 terminate within itself, contains also, as may 

 be seen in preparations stained by Weigert's 

 method,* great numbers of exceedingly fine 

 medullated fibres, many of which are the 

 collaterals of fibres that are passing out to the 

 white matter. 



Only medullated nerve-fibres are met with 

 in the white matter of the cerebro-spinal axis. 

 They are commonly stated to be devoid of a 

 neurilemma (or neurolemma), and in the sense 

 that there is no continuous separate mem- 

 branous sheath corresponding to the sheath 

 of Schwann of the peripheral medullated 

 fibres this is correct. Sheath cells, however, 

 are present, and form a reticulum around each fibre in the meshes 

 of which myelin is contained. In diameter the medullated fibres of 

 the white matter vary from 2 /* to 20 /*. In Malapterurus electricus 

 the fibre in the cord which supplies the electrical organ is of immense 

 size; and in the anterior column of many fishes may also be seen a 

 single gigantic fibre on each side with a diameter of nearly 100 ft. It 

 cannot be said that any relation between the functions of neurons 

 and the calibre of their axons has been definitely established. Many 

 afferent fibres, it is true, are small this is notably the case with the 

 fibres of the posterior column, and many motor fibres are large. But 

 the distinction can by no means be generalized, for the fibres of the 

 direct cerebellar tract (p. 866), which certainly are afferent, are amongst 

 the largest in the spinal cord ; and the vaso-motor fibres, which pass from 

 the cord by the anterior (ventral) roots (Fig. 341) into the sympathetic, 

 are smaller than the fibres of the posterior column. Even the motor 



* Weigert's is a special method of staining the medullary sheath with 

 haematoxylin. 



Fig. 341. Transverse Section 

 of a Bundle of Nerve- 

 Fibres from the Anterior 

 (Ventral) Root of the First 

 Coccygeal Nerve of the Cat 

 (Dale). The great differ- 

 ence in the diameter of the 

 fibres is well shown. The 

 small fibres are vaso-motor. 



