■ THE NERVOUS SYSTEM. 389 



central organs, are to be regarded not merely from an anato- 

 mical point of view, as affording origin to the nerves, but, also, 

 in a physiological sense, as excitors of the movements, and scat 

 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 regarding 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, physiologi- 

 cally 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 main- 

 tained; and the latter, — the sympathetic or ganglionic nervous 

 system, can only be regarded as a portion of the peripheral 

 system, though undoubtedly peculiarly constituted. 



ELEMENTS OF THE NERVOUS SYSTEM. 



§ no. 



The nerve-tubes or fibres (figs. 137 — 139), also termed 

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

 cylindrical filaments, having a diameter of O0005 — 0*01'" ; 

 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 grey substance of the latter and in 

 the ganglia. When examined in the recent state and by trans- 

 mitted 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 application 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, 



