THE STRUCTURE OF THE SPINAL CORD. 533 



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 trans- 

 verse 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 bloodvessels but also lymphatics ; of these, 

 however, we shall speak when we come to deal with the vascular arrange- 

 ments of the whole of the central nervous system. 



477. In the gray matter we may distinguish the larger, more conspicu- 

 ous nerve-cells and the rest of the gray matter in which these cells lie. We 

 have already ( 95) described the general features of these larger nerve-cells, 

 and shall have presently 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 gray 

 matter. 



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

 of the cord the smaller part, of the whole gray matter. In a transverse sec- 

 tion from the thoracic region (Fig. 119) a few only of these larger nerve-cells 

 are seen in the whole section, and though they appear more numerous in sec- 

 tions from the cervical and especially from the lumbar regions (Figs. 121, 

 122), yet in all cases they occupy the smaller part of the area of the gray 

 matter. The larger part of the gray matter consists, besides a neuroglia 

 supporting the nervous elements, of nerve-filaments running in various direc- 

 tions and forming, not a plexus properly so called, but an interlacement of 

 extreme complexity. These filaments are, on the one hand, the fine medul- 

 lated fibres spoken of above as being recognized with difficulty, and, on the 

 other hand, non-medullated filaments ranging from fairly wide and con- 

 spicuous naked axis-cylinders down to fibrils of extreme tenuity, the latter 

 arising apparently either from the division of axis-cylinders and nerve-fibres 

 passing into or out of the gray matter or from the continued branching of 

 processes of nerve-cells. By the modes of preparation now available it has 

 been shown that the fine medullated fibres, so far from being rare, are in 

 certain parts of the gray matter so abundant as even to preponderate over 

 the non-medullated fibres or fibrils. Lastly, besides the conspicuous nerve- 

 cells spoken of above, which, though of various sizes, may all perhaps be 

 spoken of as large, a very large number of other cells of small size, some of 

 which at all events must be regarded as true nerve-cells, are present in the 

 gray matter. 



The neuroglia in which all these structures, nerve-cells, fine medullated 

 nerve-fibres, naked axis-cylinders, and fine filaments are imbedded, is identical 

 in its general characters with that of the white matter, but, as naturally fol- 

 lows from the nature of the nervous elements which it supports, is differently 

 arranged. Instead of forming a system of tubular channels it takes on the 

 form of a sponge-work with large spaces for the larger nerve-cells and fine 

 passages for the nervous filaments. At the junction of the gray matter with 

 the white matter, the neuroglia of the one is continuous with that of the 



