THE NERVOUS TISSUE. 



1115 



brilliant. 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 ; their single process, after a short course, divides 

 in a T-shaped manner. (2) Bipolar cells, also found in the spinal ganglia (Fig. 648). 

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



sympathetic ganglion-cells of a frog. 

 Sometimes the processes come off from 

 opposite poles of the cell, and the cell 

 then assumes a spindle-shape ; at others 

 they both emerge at the same point. In 

 some cases where two fibres are appar- 

 ently connected with a cell, one of the 



FIG. 650. Pyramidal cell from the cerebral 

 corter of a mouse. (After Ram6n y Cajal.) 



FIG. 651. Cell of Purkinje from the cerebellum of 

 a cat. (After Ram6n y Cajal.) 



fibres is really derived from an adjoin- 

 in^ nerve-cell and is passing to end in a 

 ramification around the ganglion-cell, 

 or, again, it may be coiled spirally 

 round the nerve process which is issuing 

 from the cell. (3) Multipolar cells, 

 which are caudate or stellate in shape, 

 and characterized by their large size and by the tail-like processes which issue 

 from them. The processes are of two kinds : one of them is termed the axis- 

 cylinder process or axon, because it becomes the axis-cylinder of a nerve-fibre 

 (Figs. 649, 650, 651). The others are termed the protoplasmic processes or 

 dendrons ; they begin to divide and subdivide as soon as they emerge from the 

 cell, and finally end in minute twigs and become lost among the other elements of 



the nervous tissue. 



The white or fibrous nerve-substance or nerve-fibre is found universally in the 

 nervous cords, and also constitutes a great part of the brain and spinal cord, 

 fibres of which it consists are of two kinds, the medullated or white fibres, and 

 non-meduttated or gray fibres. 



