SPINAL COED. 285 



The fibres making up the white matter are medullated. 

 They all lack the sheath of Schwann, and in consequence of 

 this show no nodes of Ranvier or segmentation. Not until we 

 reach the roots do the fibres show a neurilemma and nodes of 

 Ranvier. In observing a cross-section of the cord (Fig. 213), 

 we notice a difference in thickness in the fibres. In the fascic- 

 ulus cuneatus and the funiculus ventralis are to be found the 

 largest fibres ; while those of the smallest diameter are seen in 

 the fasciculus gracilis and the funiculus lateralis. In such a 

 section it is to be noted that the great majority of the fibres are 

 cut transversely i. e., they run parallel to the long axis of the 

 cord. Diagonal and transverse fibres are relatively rare. 



The supporting framework of the cord, as well as of the 

 whole central nervous system, consists of neuroglia. This is of 

 ectodermal origin, and arises in a way quite similar to the rest 

 of the nervous system. In the study of neuroglia there are to 

 be considered the neuroglia cells and neuroglia fibres (glia cells 

 and glia fibres). In the medulla of adults we find glia cells of 

 two kinds, the so-called ependyma cells and the astrocytes 

 (Deiters' cells). 



Ependyma cells are cylindrical cells bordering on the central 

 canal. They form either a single layer or are arranged in two 

 or three rows. In embryonic life these cells are ciliated on the 

 surface toward the central canal, but these cilia disappear later on. 

 Toward the surface of the cord each cell sends out a long, fili- 

 form process (ependyma fibre),which enters the gray substance, 

 and in the embryo reaches the surface. In post-embryonal 

 life the ependyma fibres reach the periphery of the cord only 

 in the region of the septum dorsale. The ependyma cells are 

 phylogenetically and ontogenetically the oldest neuroglia cells, 

 from which the astrocytes take their origin. A part of the 

 cells, which arise by division of the ependyma cells, leave the 

 region of the central canal, and, moving peripheralward in the 

 gray and white matter, become astrocytes. 



The astrocytes are small nucleated cells containing little 

 protoplasm. They are more or less stellate in outline, and 

 owe their name to this peculiarity. According to the length 



