NEUROGLIA. 



73 



neurokeratin. Its origin from fhe epiblast also differentiates it 

 from connective tissue, which arises from the mesoblast. 



Neuroglia is the supporting tissue of the nerve-cells and nerve- 

 fibers of the brain and spinal cord. It consists of cells and fibers. 

 In describing ciliated epithelium it will be remembered that among 

 the locations in which it was found the ventricles of the brain 

 and the central canal of the spinal cord were mentioned. From 

 the attached ends of these cells branching neuroglia-fibers pass to 

 the surface of the brain and the cord, and terminate at the pia mater 

 in enlargements. Other fibers of the neuroglia arise from cells, 

 neuroglia, glia- or spider-cells, which are stellate in shape. These 

 fibers aid in supporting the nerve-cells and nerve-fibers. 



Development of Nerve-cells and Nerve-fibers. The 

 following description is from Schafer: "All nerve-cells in the 

 body are developed from the cells of the neural groove and neural 

 crest of the early embryo; the neural groove closing to form the 

 neural canal, the cells of which form the spinal cord and brain, 

 and the neural crest giving off, at intervals, sprouts which become 

 the rudiments of the ganglia. The cells which line the neural 

 canal are at first all long, columnar cells, but among these, and 

 probably produced by a metamorphosis of some of these, rounded 

 cells (neuroblasts) make their appearance, and presently from each 

 one a process begins to grow 

 out. This is the axis-cylinder 

 process (neuron) and is char- 

 acterized by its enlarged ex- 

 tremity. As it grows, it may 

 emerge from the anterolateral 

 regions of the canal and be- 

 come a motor neuron or ante- 

 rior root-fiber. The dendrons 

 appear somewhat later than the 

 neuron. The axis-cylinder 

 processes of some of the neu- 

 roblasts remain Avithin the 

 nerve-centers, and are devel- 

 oped into association or intra- 

 central fibers. 



"The sprouts from the 

 neural crest contain the neu- 

 roblasts from which the pos- 

 terior root-fibers are devel- 

 oped. Neurons grow out from 

 these neuroblasts in two directions, so that the cells become bipo- 

 lar, one set, forming the posterior root-fibers, grow into the pos- 

 terolateral portion of the spinal cord, and ramify in the develop- 

 ing gray matter ; the other set, containing the afferent fibers of the 



FIG. 79. Neurogliar cells : a, from spinal 

 cord of embryo of cat ; 6, from brain of adult 

 cat ; stained in chrome silver (Huber). 



