532 THE SPINAL CORD. 



nective tissue fairly rich in elastic elements and abundantly supplied with 

 bloodvessels ; it is indeed essentially a vascular membrane and furnishes the 

 nervous elements of the cord with their chief supply of blood. It sends in 

 at intervals partitions or septa of the same nature as itself radiating toward 

 the central gray matter. The narrow posterior fissure is completely 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, secondary finer septa still composed of ordinary fibrillated connective 

 tissue, carrying bloodvessels, 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 imbedded in a homo- 

 geneous 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 longitudinally- 

 disposed fibres imbedded in the ground substance. Examined chemically, 

 the neuroglia is found to be composed not like connective tissue of gelatin, 

 but of a substance which appears to be closely allied to keratin, the chief 

 constituent of horny epidermis, hairs and the like, 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 orig- 

 inally protoplasmic cell substance becomes transformed to a large extent 

 into neurokeratin. 



The neuroglia fills up the spaces between the radiating larger septal pro- 

 longations of the pia mater and the finer branched septa which starting from 

 the larger ones carry minute bloodvessels 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 

 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 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 /j. 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 



