THE NERVOUS SYSTEM. 395 



tion, for the present, abstain from all conclusions drawn from 

 analogy. 



In order to see the medullary sheath or nerve-pulp in its 

 normal condition, a nerve of an animal just killed, without any 

 addition, must be quickly brought under the microscope ; in 

 which case some isolated fibres will always be seen quite 

 unchanged, although, as the nerve dries, they are very rapidly 

 altered. Besides this method, I would also recommend the 

 examination of the nerves in the transparent parts of animals, 

 either alive or just killed (nictitating membrane, mucous mem- 

 brane of the Frog, tail of Tadpole, &c.), the observing of them 

 on warmed pieces of glass (Stark.), and after treatment with 

 chromic acid, which frequently preserves, particularly the cere- 

 bral fibres, quite uninjured. The nerve-pulp or medulla is 

 obviously a viscid, fluid, extensible, glutinous substance, to 

 be compared in point of consistence with thick oil of turpentine, 

 and Avhich under pressure assumes all possible figures, appear- 

 ing in the form of globules, filaments, and membranous masses, 

 of very different aspects, Avith pale or dark borders, and opaque 

 or clear. In chemical composition it consists principally of 

 fatty matter. 



The central filament of the nerve-fibres, which was perhaps 

 seen as early as by Fontana, and with which we have become 

 better acquainted under the name of " primitive band " given 

 to it by Remak, or " cylinder-axis " as it has been termed by 

 Rosenthal and Purkinje, is indisputably the most difficult of 

 investigation, and the least known portion of the nerves. 

 There is no microscopist avIio has not frequently seen this axis- 

 fibre, but it may, without fear of contradiction, also be asserted, 

 that there is none, not even excepting Remak himself, its dis- 

 coverer, who can boast that he has studied and learned its 

 relations in every particular. For this reason, but few, as 

 Hannover and J. Miiller, are unconditionally agreed with 

 Remak and Purkinje in regarding the axis-cylinder as a con- 

 stant element in recent nerve, whilst most observers have 

 adopted the views of Valentin (< Repert.' 1838, p. 76, 1839, 

 p. 79), and Henle ( f Allg. Anat/), who regard it as not always 

 present, but rather as a secondary formation, which does not 

 exist during life, and as the uncoagulated central portion of 

 the contents of the nerve-fibre, which, during life, are homo- 



