NEUROGLIA. ?I 



lateral cells from whose angles extend the numerous delicate fibrilla: that 

 later become the chief constituents of the neuroglia. So long as neuroglia 

 is being produced, the gliogenetic cells are present and concerned in the 

 production of additional fibrillae, their cytoplasm becoming progressively 

 reduced until in their final condition of the small glia cells, little more than 

 the nucleus remains. During these changes very many fibrillje lose their 

 connection with the cells and, in conjunction with the glia threads still 

 attached, form an intricate interlacement in 

 which the neuroglia cells, now greatly re- 

 duced and for the most part devoid of proc- 

 esses, lie scattered at uncertain intervals. 



The mature neuroglia everywhere 

 consists of essentially the same tissues, 

 the differences noted in certain localities 

 depending largely upon variations in its 



FIG. 95. Young neuroglia cells; astrocytes 

 from brain of child. X 300. 



FIG. 96. Ependyrnal cells and adjacent 

 neuroglia surrounding central canal of 

 spinal cord of cat. X 75. (Rubaschkin.) 



compactness. Its chief constituent is the intricate feltwork of glia-fibres 

 which are usually free but to some extent connected with the glia-cells. 

 Where, however, the neuroglia borders the brain-ventricles and the central 

 canal of the spinal cord it presents special features. In these situations it 

 forms the ependyma, which appears as a single-layered epithelial lining. 

 Within the cord, the cells are pyramidal, their bases looking towards the lumen 

 of the tube and their apices towards the nervous tissue. At least during the 

 earlier years in man, and throughout life in many lower animals, the free 

 surfaces of the cells are beset with hair-like processes resembling the cilia of 

 epithelial cells. The pointed distal ends of the ependymal cells are prolonged 

 into processes continuous with neuroglia fibres that are soon lost in the 

 surrounding glia-complex. Where the ependy ma lines the ventricular spaces, 

 the cells are low cuboidal elements that constitute a continuous and single- 

 layered investment, whose primary relation to the surrounding neuroglia is 

 often lost or, at best, obscured. 



The Nerve Trunks. The component fibres of the peripheral nervous 

 system are assembled into larger or smaller cords, the "nerves "of gross 

 anatomy, which extend to various parts of the body. Those that supply both 

 muscles and sensory surfaces (integument or mucous membranes), as, for 



