284: 



THE MICROSCOPE AND ITS KEVELATIONS. 



closely in their general appearance to epithelium-cells, yet they seem to 

 have quite a different nature, being distinguished by their elongated 

 nuclei, as well as by their contractile endowments. 



681. Nerve- substance. Wherever a distinct Nervous System can be 

 made out, it is found to consist of two very different forms of tissue 

 namely the cellular, which are the essential components of the ganglionic 

 centres' and the fibrous, of which the connecting trunks consist. The 

 typical form of the nerve-cells or ' ganglion -globules' may be regarded as 

 globular; but they often present an extension into one or more long pro- 

 cesses, w'hich give them a ' caudate ' or 6 stellate ' aspect. These pro- 

 cesses'have been traced into continuity, in some instances, with the axis- 

 cylinders of nerve-tubes (Fig. 475); whilst in other cases they seem to 

 inosculate with those of other vesicles. The cells, which do not seem to 

 possess a definite cell-wall, are for the most part composed of a finely- 

 granular substance, which extends into their prolongations; and in the 

 midst of this is usually to be seen a large well-defined nucleus. They also 



FIG. 475. 



FIG. 476. 



FIG. 477. 



Vertical Section of Skin of Finger, showing 

 Ganglion-cells and Nerve- Gelatinous Nerve- the branches of the cutaneous nerves, a, 6, 

 fibres from a ganglion of fibres, from Olf ac- inosculating to form a plexus, of which the ulti- 

 Lamprey. tory Nerve. mate fibres pass into the cutaneous papillae, c, c. 



generally contain pigmeitt-granules, which give them a reddish or yellow- 

 ish-brown color, and thus impart to collections of ganglionic cells in the 

 warm-blooded Vertebrata that peculiar hue, which causes it be known as 

 the cineritious or gray matter, but which is commonly absent among the 

 lower animals. Each of the tubular nerve-fibres, on the other hand, of 

 which the trunks are made up, consists, in its fully developed form, of a 

 delicate membranous sheath, within which is a hollow cylinder of a 

 material known as the 'white substance of Schwann/ whose outer and 

 inner boundaries are marked-out by two distinct lines, giving to each 

 margin of the nerve-tube what is described as a i double contour.' The 

 contents of the membranous envelope are very soft, yielding to slight 

 pressure: and they are so quickly altered by the contact of water or of 

 any liquids which are foreign to their nature, that their characters can 

 only be properly judged-of when they are quite fresh. The centre or axis 

 of the tube is then found to be occupied by a transparent substance 

 which is known as the 'axis-cylinder:' and there is reason to believe that 



