ITS HISTOLOGICAL RELATIONS. 97 



entirely homogeneous and transparent, exhibiting a sharply de- 

 fined contour. But after lying for some time or having come in 

 contact with water, they exhibit a double contour, resembling 

 broad dark bands, with a clear stripe running down the middle. 

 This peculiarity, together with the size of their diameter, especially 

 distinguishes them from the so-called sympathetic fibres. Distinct 

 morphological elements may be recognised in each of these fibres ; 

 and, in the first place, we must distinguish between the coat or 

 sheath and the contents. 



The coat or limitary membrane of the nerve-tubes can scarcely 

 be recognised in very recent nerves, for this membrane is an 

 entirely structureless, transparent, somewhat elastic membrane, 

 which can only be rendered visible by the application of certain 

 chemical reagents which we shall presently mention. 



The substance contained in the nerve-tubes (the nerve-medulla 

 or pulp) appear at first, as we have already observed, to be 

 perfectly homogeneous, whence it has been assumed by many 

 observers to be an entirely homogeneous, closely mixed matter. 

 It is quite immaterial to our purpose, whether we admit the cor- 

 rectness of this view, or adopt the opinion held by others, that the 

 subsequently visible separation of the medulla into a cortical 

 substance and a cylindrical axis-fibre, exists pre-formed in fresh 

 nerves ; but, judging from a chemical point of view, we are com- 

 pelled to assume the most decided morphological separation. 

 Thus, the application of cold, or the addition of water, or of certain 

 chemical agents, gives rise to a kind of coagulation in the medulla, 

 causing the outermost layer of the contents of the nerve-tubes to 

 become darker, somewhat grumous, or even granular ; whilst in 

 the interior, that is to say, in the long axis of the nerve-fibre, there 

 remains a clear, somewhat sharply defined filiform stripe the 

 central or axis-fibre, or axis-cylinder. The actual medullary 

 substance, or nerve-pulp, assumes very different forms when 

 subjected to rough pressure, appearing in different cases in 

 nodular, cylindrical, or clavate shapes as it protrudes from the 

 sheath. Within the tubes the substance is frequently accumulated 

 in an irregular manner, distending the sheath unequally at different 

 points, which gives rise to those varicose nerve-fibres which we so 

 frequently meet with in examining the brain. 



The finer or sympathetic nerve-fibres have a diameter varying 

 between G'00212 and O'OOSOO'". These fibres bear less resem- 

 blance to tubes than to solid cylinders, and have generally no 

 trace of any contents. They occur principally in the sympathetic, 

 VOL. ill. H 



