722 



NEUROLOGY 



that they are conducted. Hence the gray substance forms the essential constituent 

 of all the ganglionic centers, both those in the isolated ganglia and those aggregated 

 in the brain and medulla spinalis; while the white substance forms the bulk of the 

 commissural portions of the nerve centers and the peripheral nerves. 



Neuroglia. Neuroglia, the peculiar ground.substance in which are imbedded the 

 true nervous constituents of the brain and medulla spinalis, consists of cells and 

 fibers. Some of the cells are stellate in shape, with ill-defined cell body, and their 

 fine processes become neuroglia fibers, which extend radially and unbranched 

 (Fig. 623, .B) among the nerve cells and fibers which they aid in supporting. Other 

 cells give off fibers which branch repeatedly (Fig. 623, A). Some of the fibers start 

 from the epithelial cells lining the ventricles of the brain and central canal of 

 the medulla spinalis, and pass through the nervous tissue, branching repeatedly 

 to end in slight enlargements on the pia mater. Thus, neuroglia is evidently a 

 connective tissue in function but is not so in development; it is ectodermal in 

 origin, whereas all connective tissues are mesodermal. 





FIG. 623. Neuroglia cells of brain shown by Golgi's method. A. Cell with branched processes. B. Spider cell 

 with unbranched processes. (After Andriezen.) 



Nerve Cells (Fig. 624). Nerve cells are largely aggregated in the gray substance 

 of the brain and medulla spinalis, but smaller collections of these cells also form 

 the swellings, called ganglia, seen on many nerves. These latter are found chiefly 

 upon the spinal and cranial nerve roots and in connection with the sympathetic 

 nerves. 



The nerve cells vary in shape and size, and have one or more processes. They 

 may be divided for purposes of description into three groups, according to the 

 number of processes which they possess: (1) Unipolar cells, which are found in 

 the spinal ganglia; the single process, after a short course, divides in a T-shaped 

 manner (Fig. 624, E). (2) Bipolar cells, also found in. the spinal ganglia (Fig. 625), 

 when the cells are in an embryonic condition. They are best demonstrated in the 

 spinal ganglia of fish. Sometimes the processes come off from opposite poles of 

 the cell, and the cell then assumes a spindle shape; in other cells both processes 

 emerge at the same point. In some cases where two fibers are apparently con- 

 nected with a cell, one of the fibers is really derived from an adjoining nerve cell 

 and is passing to end in a ramification around the ganglion cell, or, again, it may 

 be coiled spirally around the nerve process which is issuing from the cell. (3) 

 Multipolar cells, which are pyramidal or stellate in shape, and characterized by 

 their large size and by the numerous processes which issue from them. The 



