HISTOLOGICAL ELEMENTS 



823 



The Neurons. A typical nerve-cell (Figs. 318, 320, 322) is a knot of 

 granular protoplasm, containing a large nucleus, inside of which lies 

 a highly refractive nucleolus. A centrosome and attraction sphere 

 (p. 5) have also been found in some nerve-cells, though not as yet 

 demonstrated in all. Pigment may also be present, especially in old 

 age. By certain methods of staining it may be shown that fibrils 

 (neuro-fibrils) run through the protoplasm of the cell, forming a felt- 

 work in it, and entering the dendrites on the one hand and the axis- 

 cylinder process on the other (Figs. 318, 321, 325). In the axis- 

 cylinders of nerve-fibres the fibrils (Fig. 319) appear to preserve their 



identity down to the distribution of 

 the fibre. In the ground substance 

 between the fibrils lie round, an- 

 gular, or spindle-shaped bodies 

 (Nissl's bodies) which stain with 

 basic dyes (Fig. 3 30).* These bodies 

 vary in appearance in different 

 kinds of nerve-cells, and in the 

 same nerve-cell under different con- 

 ditions. According to Macallum, 

 they contain organically combined 

 iron. In a multipolar cell, like those 



Fig. 318. Anterior Horn Cell from Man 

 showing Fibrils (Bethe). 



Fig. 319. Medullated Nerve- 

 Fibre showing Fibrils of Axis- 

 Cylinder (Bethe). The fibrils are 

 seen passing, without interrup- 

 tion, across a node of Ranvier. 



in the anterior horn of the spinal cord, several processes it may be five 

 or six, or even more pass off from the cell-body (Frontispiece). The 

 most complete pictures of them are given by preparations impregnated 

 according to the method of Golgif (Figs. 320, 323). One of the pro- 



* In Nissl's method the sections are stained in a solution of methylene blue, 

 and decolourized in anilin-alcohol. 



f The method depends upon the deposition of mercury, or silver, in or 

 around the cell-bodies and their processes in tissues which have been hardened 

 in bichromate of potassium and then soaked in a solution of mercuric chloride 

 or silver nitrate. In Pal's improvement of Golgi's method a solution of sodic 

 sulphide follows the mercuric chloride. 



