860 STRUCTURE OF SPINAL CORD. [BOOK in. 



pletely filled up by a large septum of this kind, indeed as we 

 have said is in reality not a fissure but a large septum ; but the 

 anterior fissure is too wide for such an arrangement ; the whole 

 membrane dips down into this fissure, following the surface of the 

 cord and being reflected at the bottom. From these primary 

 septa, secondar}^ finer septa still composed of ordinary fibrillated 

 connective tissue, carrying blood vessels, branch off ; but these are 

 soon merged into the peculiar supporting tissue called, as we have 

 said, neuroglia. This consists in the first place of small branching 

 cells, lying in various planes. The branching is excessive, so that 

 the body of the cell is reduced to very small dimensions, indeed 

 at times almost obliterated, the nucleus disappearing while the 

 numerous branches are continued as long fine filaments or fibres 

 pursuing a devious but for the most part a longitudinal course. 

 In the second place these cells and fibres or filaments are im- 

 bedded in a homogeneous ground substance. Relatively to the 

 fibres and ground substance the bodies of the cells (which are 

 called Deiter's cells), especially bodies such as bear obvious nuclei, 

 are very scanty; hence in sections, especially in transverse 

 sections, of the cord the neuroglia has often a dotted or punctated 

 appearance, the dots being the transverse sections of the fine lon- 

 gitudinally disposed fibres imbedded in the ground substance. 

 Examined chemically the neuroglia is found to be composed not 

 like connective tissue of gelatine, but of a substance which appears 

 to be closely allied to keratin, the chief constituent of horny 

 epidermis, hairs and the like, 435, and which has therefore been 

 called neurokeratin, (see also 68). And indeed this neuroglia, 

 though like connective tissue a supporting structure, is not, like 

 connective tissue, of mesoblastic, but of epiblastic origin. The 

 walls of the neural canal of the embryo which are transformed 

 into the spinal cord of the adult consist at first of epithelial, 

 epiblastic cells; and while some of these cells become nervous 

 elements, others become neuroglia. The epithelial cells which are 

 destined to form neuroglia become exceedingly branched, while 

 their originally protoplasmic cell-substance becomes transformed to 

 a large extent into neurokeratin. 



The neuroglia fills up the spaces between the radiating larger 

 septal prolongations of the pia mater and the finer branched septa 

 which starting from the larger ones carry minute blood vessels into 

 the interior of the white matter. In these spaces it is so arranged 

 as to form delicate tubular canals, of very variable size, running 

 for the most part in a longitudinal direction. Each of these tubular 

 canals is occupied by and wholly filled up with a medullated 

 nerve fibre of corresponding size. A medullated nerve fibre of the 

 white matter of the spinal cord resembles a medullated nerve 

 fibre of a nerve ( 68) in being composed of an axis-cylinder and a 

 medulla; but it possesses no primitive sheath or neurilemma. 

 This is absent and indeed is not wanted; the tubular sheath of 



