DISTRIBUTION OF NON-MEDULLATED NERVE-FIBRES. 23 



I have obtained but a limited number of preparations adapted 

 to solve these problems in a thoroughly unexceptionable 

 manner. 



The method which I adopt in producing any specimens 

 is the usual one of colouring with gold ; the treatment of the 

 gold-coloured membrane with tartaric acid, which was used 

 in the case of the cornea of the rabbit, has yielded in my 

 hands only in a very few cases moderately useful results, 

 and the same is true of the more rapid reduction of the gold 

 salt by means of gentle heat. In both cases the very serious 

 inconvenience arises, that the membrane shrinks in a remark- 

 able degree, curls itself up, and so forth, in such a way that 

 no further preparation is possible without considerable 

 mechanical injury. 



The method which I pursue is as follow^s : — I separate 

 the nictitating membrane on the living frog [Rana esculenta), 

 in the neighbourhood of its thick muscular attachments, with 

 a few cuts of the scissors, transfer it with the greatest caution 

 into a few drops of a half per cent, solution of chloride of gold, 

 where I allow it to remain froin three quarters of an hour to 

 an hour. I then remove it to distilled water, iu which it is kept 

 exposed to the light a sufficient number of days to acquire a 

 perfectly dark red or black colour. The objects may without 

 damage remain for a week or more in Avater, and, indeed, 

 this long soaking brings with it the advantage that the epi- 

 thelium of the surfaces becomes macerated and loosened, and 

 may then be removed for large extents of surface by simple 

 pencilling with a thick camel's-hair brush under water. 

 Objects so prepared are examined in glycerine. 



We now turn to the consideration of the nerves of the 

 nictitating membrane. 



The trunks of the meduUated nerve- fibres are united to 

 form a plexus ; that is to say they divide into branches, 

 which, constantly diminishing in size, contain only single or 

 a few meduUated fibres (distinguished, as is well known, by 

 their tendency to divide), and by these branches they are con- 

 nected. To these trunks and branches we shall give the name 

 of nerves of the first order. In these the neurilemma appears 

 generally in gold preparations strongly developed, and pos- 

 sesse<l of oblong nuclei. Helfreich (J29) has described these 

 meduUated nerves in detail. 



In a fresh nictitating membrane, which has been prepared 

 Avith the necessary caution, and is examined in serum, the 

 space within the neurilemma filled Avith lymph, Avhich 

 Strieker (29) has described, may often be seen with the 

 greatest distinctness, both on the larger and on the 



