THE NERVOUS SYSTEM. 345 



OF THE NERVOUS SYSTEM. 



109. The nervous system, regarded in the more general anatomical 

 sense, constitutes a connected whole, consisting of two principal masses 

 the spinal cord and brain, and of numerous cords nerves extending 

 from them to almost all the organs of the body. The two former or 

 the central nervous system, the central organs, are to be regarded not 

 merely from an anatomical point of view, as affording origin to the 

 nerves, but, also, in a physiological sense, as exciters of the movements, 

 and seat of the sensations, as well as of the mental or psychical actions, 

 and consequently as belonging to a higher or governing order of parts, 

 whilst to the latter must be ascribed more of a ministerial office the 

 communication of the contractions and sensations. This mode of regard- 

 ing the two divisions of the nervous system, however, is only partially 

 correct, because, in the first place, in the central organs, as in the 

 nerves, very many subordinate parts exist ; and, secondly, because in 

 the peripheral nervous system, the so-termed ganglia, physiologically 

 and anatomically, represent central organs. The older division also of 

 the nervous system into animal and vegetative, after the observations of 

 recent times, can no longer be maintained ; and the latter, the sym- 

 pathetic or ganglionic nervous system, can only be regarded as a por- 

 tion of the peripheral system, though undoubtedly peculiarly consti- 

 tuted. 



ELEMENTS OF THE NERVOUS SYSTEM. 



110. The nerve-tubes or fibres (Figs. 137-139), also termed primi- 

 tive tubes, or primitive fibres of the nerves, are soft, fine, cylindrical 

 filaments having a diameter of 0-0005-0-01 of a line ; they constitute 

 the principal part of the nerves and of the white substance of the central 

 organs, although they are not wanting in the greater part of the gray 

 substance of the latter and in the ganglia. When examined in the re- 

 cent state and by transmitted light (Fig. 137) they appear as clear as 

 water, transparent, and with simple dark contours ; by reflected light 

 glistening, opaline, like fat, in larger quantities together, white, and for 

 the most part their appearance does not indicate that they are composed 

 of different constituent parts. But it is readily seen upon the applica- 

 tion of various methods, that they consist of three, entirely distinct, 

 component structures, viz. : of a delicate coat, and a viscid fluid, in the 

 centre of which is a soft but elastic fibre. The coat, or sheath of the 

 nerve-fibres (limitary membrane, Valentin) (Fig. 139, 1, 2, 3, 4, a) is 

 an excessively delicate, flexible, but elastic, perfectly structureless, and 

 transparent membrane, which, in quite unaltered nerve-fibres, except in 

 certain situations, is altogether invisible. But on the application of 



