THE NERVOUS SYSTEM. 349 



as in the roots of certain cerebral nerves (those of the muscles of the 

 eye for instance), and of the spinal nerves that it appears distinct from 

 the contents ; its presence however is, with certainty, and readily de- 

 monstrated by the aid of chemical reagents. When the nerves are 

 boiled in absolute alcohol, soon after the removal of a considerable 

 part of the fatty matter of the pulp, the sheaths become tolerably dis- 

 tinct, as dark boundary lines ; and they are rendered remarkably and 

 beautifully so by a short boiling in acetic acid, during which, the 

 remaining contents of the nerve-sheaths, with the exception of the 

 central fibre, escape from them, whilst at the same time numerous (fat) 

 crystals (Fig. 139, 1) are formed. When boiled in alcohol and treated 

 in the cold with caustic soda, the nerve-fibres also exhibit the sheaths 

 very beautifully, as pale, frequently undulating contours of the color- 

 less, remaining contents ; and when such fibres are boiled for a moment 

 in caustic soda, numerous, elongated fragments of perfectly empty, 

 somewhat swollen nerve-sheaths, are detached, which, from their deli- 

 cacy, present a striking resemblance to the empty tubules of the mem- 

 Irana propria, of the tubuliuriniferi (Fig. 139, 2). The sheaths, how- 

 ever, are rendered most distinct by means of fuming nitric acid and the 

 subsequent addition of caustic potassa. In this case the fatty matter 

 of the medullary sheath escapes from the tubes in the form of color- 

 less drops, the axis cylinder is dissolved, and the yellow sheaths are 

 left empty, dilated and with swollen walls of 0-0004-0-0008 of a line 

 in thickness. In nerves treated with corrosive sublimate, according to 

 Czermaak(" Zeitsch. f. wissensch. Zoologie,"1850), the sheaths are, also, 

 often very prettily shown. It has not yet been determined whether the 

 finest nerve-fibres in the central organs, and in the peripheral nerves 

 (under O'OOl of a line) possess a structureless sheath. Analogy with 

 the coarser fibres is in favor of the existence of such sheaths, but on 

 the other hand there are some facts which would seem to indicate that 

 there are also, sheathless primitive nerve-fibres, both of the medullated 

 and of the non-medullated kinds. I have already (in my " Microscopical 

 Anatomy," II. 1, 396) remarked, that according to my observations in the 

 Tadpole, several dark-bordered fibres are developed in one and the 

 same structureless sheath formed by the coalescence of cell-membranes; 

 and that a similar thing (at least from R. Wagner's figures) occurs in 

 the electric organ of the Torpedo : in which cases special tunics can 

 scarcely be supposed to exist around each separate fibre. And, quite 

 recently, Stannius (" Gott. Nachr.," 1850) has found, in Petromyzon, 

 that the nerve-fibres of the central organs possess neither membra- 

 nous sheath nor pulp, and are, as it may be expressed, nothing more 

 than free axis-fibres. When to this, it is also added, that the impossi- 

 bility of demonstrating membranes, by no means certainly proves their 

 non-existence, still, the facts stated are worthy of all consideration, 



