NERVES. 361 



clastic, and its contraction gives rise to the corrugated ap- 

 pearance presented by the tubes of motor nerves which have 

 been disturbed and broken. (Plate XLIV. Jig. 1.) 



The existence of the third constituent of the nerve tube, 

 the " axis cylinder " of Rosenthal and Purkinje, is best deter- 

 mined by the immersion of the fibres in either ether or acetic 

 acid, which breaks it up into granules and vesicles. (Plate 

 XLIV. fig. 3.) 



In albumen the nerve tubes and nervous tissue in general 

 undergo but little alteration, and it is therefore in this fluid 

 that its examination is best conducted. 



But the tubes of the white fibrous material of the cerebrum, 

 cerebellum, and spinal marrow just described, form one ele- 

 ment only of its structure ; another is invariably present, 

 forming indeed the greater portion of its substance ; and it is 

 somewhat strange that it should have been overlooked by 

 observers : this element consists of globules of every possible 

 size, which, when free from pressure and undisturbed, are 

 perfectly spherical but which are put out of shape by the 

 slightest compression or disturbance. It is not easy to de- 

 termine whether these globules are true cells or not : they pre- 

 sent the greatest possible variety of size : they have the colour 

 and consistence of oil ; but nevertheless appear to be hollow, 

 and frequently present a spot which bears much resemblance 

 to a nucleus. (See Plate XLIV. Jig. 6.) 



It now only remains to be observed, that the nerves of 

 special sense, as the optic, olfactory and auditory, present a 

 structure precisely analogous to that of the white substance 

 of the cerebrum. 



SYMPATHETIC SYSTEM. The nerves entering into the 

 formation of the sympathetic or organic system differ both in 

 appearance and structure from those derived from the cere- 

 bro-spinal system : thus the great sympathetic cord itself, 

 as well as the organic nerves connected with it, present a 

 reddish grey colour, are soft and gelatinous, and do not 

 readily admit of division in the longitudinal direction, 

 although they are easily torn across by any extending 

 force : these differences of colour and of consistence are de- 



