CHAP, i.] THE SPINAL CORD. 861 



neuroglia affords in the spinal cord (and as we shall see in the 

 central nervous system generally) the support which in a nerve is 

 afforded by the neurilemma. Nodes are, according to most 

 authors, absent, but some say they are present. 



The white matter of the cord consists then of a more or less 

 solid mass of neuroglia, having the structure just described, which 

 is permeated by minute canals, some exceedingly fine and carrying 

 very fine 2/u, fibres, others larger and carrying fibres up to the size 

 of 15/& This mass is further broken up into areas by the smaller 

 and larger vascular connective tissue septa with the edges and 

 endings of which the neuroglia is continuous. Most of the nerve- 

 fibres, as we have said, run longitudinally and in a transverse 

 section of the cord are cut transversely ; but as we shall see 

 fibres are continually passing into and out of the white matter, 

 and in so doing take a more or less transverse course ; these 

 however are few compared with those which run in a longitudinal 

 direction. On the outside of the cord below the pia mater the 

 neuroglia is developed into a layer of some thickness from which 

 nerve fibres are absent ; this is often spoken of as an inner layer of 

 the pia mater ; but being neuroglia and not connective tissue is of 

 a different nature from the pia mater proper. A layer of this 

 superficial neuroglia also accompanies the larger septa, and a 

 considerable quantity is present in the large septum called the 

 posterior fissure. 



The pia mater carries not only blood vessels but also lymphatics ; 

 of these however we shall speak when we come to deal with the 

 vascular arrangements of the whole of the central nervous system. 

 564. In the grey matter we may distinguish the larger, 

 more conspicuous nerve-cells and the rest of the grey matter in 

 which these cells lie. We have already ( 99) described the 

 general features of these larger nerve-cells, and shall have pre- 

 sently to speak of their special characters and grouping. Meanwhile 

 the most important point to remember about them besides the 

 fact that they vary largely in form and size is that while one 

 process may or does become an axis cylinder of a nerve-fibre, the 

 others rapidly branch, and breaking up into fine nerve filaments 

 are lost to view in the rest of the grey matter. 



These larger nerve-cells* form, however, a part only, and in most 

 regions of the cord the smaller part, of the whole grey matter. 

 In a transverse section from the thoracic region (Fig. 96) a few 

 only of these larger nerve-cells are seen in the whole section, and 

 though they appear more numerous in sections from the cervical 

 and especially from the lumbar regions (Figs. 98, 99), yet in all 

 cases they occupy the smaller part of the area of the grey matter. 

 The larger part of the grey matter consists, besides a neuroglia 

 supporting the nervous elements, of nerve filaments running in 

 various directions and forming, not a plexus properly so called, but 

 an interlacement of extreme complexity. These filaments are, on 



