394 SPECIAL HISTOLOGY. 



tion 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 colourless, remaining con- 

 tents ; and when such fibres are boiled for a moment in caustic 

 soda, numerous, elongated fragments of perfectly empty, some- 

 what swollen nerve-sheaths, are detached, which, from their 

 delicacy, present a striking resemblance to the empty tubules 

 of the membrana propria of the tubuli uriniferi (fig. 139, 2). 

 The sheaths, however, are rendered most distinct by means of 

 fuming nitric acid and the subsequent addition of caustic potass. 

 In this case the fatty matter of the medullary sheath escapes 

 from the tubes in the form of colourless drops, the axis cylinder 

 is dissolved, and the yellow sheaths are left empty, dilated and 

 with swollen walls of 0-0004— 0-0008'" in thickness. In 

 nerves treated with corrosive sublimate, according to Czermak 

 (' Zeitsh f. wissensch. Zoologies 1 850) 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 - 00l'") possess a structureless sheath. 

 Analogy with the coarser fibres is in favour 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 H. Wag- 

 ner'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 membranous 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 

 impossibility of demonstrating membranes, by no means 

 certainly proves their non-existence, still, the facts stated 

 are worthy of all consideration, and we must, in this ques- 



