

214 NERVOUS SYSTEM. [SECT. 109. 



with contents sometimes completely resembling the axial fibre 

 of other tubes, sometimes softer and more granular. These non- 

 medullated nerve-tubes are found, in the first place, as continuations 

 of the other sort, where they are connected with nerve-cells ; in 

 the second place, as longer independent tubes, in the form of the 

 so-called processes of the nerve-cells of authors; and, lastly, at 

 the terminations of the dark-bordered nerves. They may again 

 be divided into several sub-divisions, according as they possess 

 nuclei or not, and according as their contents are more or less 

 transparent, or more or less consistent. When it is considered 

 that the dark-bordered fibres also vary very much in their delicacy 

 or fineness of structure, and in their diameter, which varies between 

 o*ooo5'" to o'oi'" and upwards, — so that they may be classified 

 into fine and coarse, delicate and firm, — it is readily seen that 

 the nerve-tubes, notwithstanding their general tubular character, 

 differ a good deal from each other in several respects. 



The envelope or sheath of the nerve-tubes discovered by Schwann can, in 

 man, as on the roots of certain cerebral and of the spinal nerves, be only 

 rarely seen without the aid of re-agents ; but it is readily demonstrated by 

 boiling the nerves in absolute alcohol, or acetic acid, or caustic soda. The 

 sheath is best seen, however, by the aid of fuming nitric acid, and the sub- 

 sequent addition of caustic potass. In this case, the fat of the medullary 

 sheath passes out of the tube in form of pale drops, the axis cylinder is 

 dissolved, and the yellow-coloured sheath remains behind empty, wider, and 

 with swollen walls of o'ooo^' to o"ooo8" in thickness. Whether the finest 

 nerve-tubes, also, of the central organs and peripheral nerves (below 0-002'") 

 possess a structureless sheath, is still undetermined. Analogy with the 

 coarser fibres speaks for the. existence of such sheaths ; but there are some 

 facts known, which seem to prove that there exist sheathless medullated and 

 non-medullated primitive nerve-fibres. I have already, in my Microscopical 

 Anatomy (II. i. 396), remarked, that in the tadpole, according to my obser- 

 vations, several dark-bordered fibres are developed in one and the same struc- 

 tureless sheath, formed by the coalescence of the cell-membranes ; and that 

 something similar, at least, according to B. Wagner's drawings, takes place in 

 the electric organ of the torpedo, in which cases, special coverings around 

 each separate tube can scarcely be assumed to exist. Quite recently, also, 

 Stannius (dotting . Nachr., 1850) has found in the lamprey that the nerve*- 

 fibres of the central organs possess neither sheaths nor medulla, and are, so 

 to speak, nothing but free axis-fibres. Now, although it must be admitted, 

 that the impossibility of demonstrating the sheaths does not by any means 

 prove their non-existence, still the above-mentioned facts are deserving of 

 attention ; and we must, on this question, for the present, refrain from 

 drawing conclusions from analogy. Upon the inner side of the sheath, 

 between it and the medulla, there are, in many nerve-fibres, perhaps in all 

 the peripheral nerves, nuclei ( Schiff), which correspond to those seen in the 

 embryo, and also in the higher animals (see below), and especially in fish 

 (in the electric organ, for example). 



In order to see the medullary sheath, or nerve-medulla, in its normal con- 



