STRUCTURE AND ORGANISATION OF NERVE 



37 



mF 



ran 



but coarser sheaths of connective tissue surrounding a single axis- 

 cylinder appear exceptionally. 



Thus the nerve - fibres of the electrical organ of Torpedo 

 exhibit a tolerably thick sheath, and an extreme development of 

 the same, consisting of many concentric layers closely packed 

 together, is also characteristic of the two giant nerve-fibres which 

 supply the electrical organ of Malapterurus. These nerves are as 

 thick as a sewing-needle, and yet contain only a single medullated 

 primitive fibre. 



The sheath of Schwann and the medullary sheath are the 

 only " specific " sheaths of nerve-fibres. As we have stated, a 

 true sheath of Schwann rarely 

 appears in the nerve - fibres of 

 invertebrates, and then only in 

 cases where there is a compara- 

 tively broad axis - cylinder. In 

 nearly all crayfish nerves, if not 

 excessively fine, there are, along 

 with a number of very delicate 

 axis-cylinders which never exhibit 

 a special sheath, others of much 

 greater diameter; these, on treat- 

 ment with methylene blue, for the 

 most part become paler in colour, 

 and exhibit, in Eemak's words, a 

 visible " tubular " structure, i.e. a 

 delicate, apparently structureless, 

 nucleated sheath with its content, 

 the axis -cylinder proper, to the 

 finer structure of which we shall 

 return later. 



These structural relations of 

 invertebrate nerves have much in 



FIG. 151. A peripheral bundle of the human 

 sympathetic nerve, fixed with osmic acid. 

 Two medullated fibres (mF)lie in a bundle 

 of Eemak's fibres. Epineural sheath be- 

 yond. (Schiefferdecker.) 



common with the fine nerve-trunks found in the sympathetic 

 system of vertebrates, which contain a bundle of non-medullated 

 fibres (axis - cylinders) the gray fibres of Eemak within a 

 strong sheath of connective tissue (epineural sheath), Fig. 151. 

 Each of these fibres appears when isolated as a transparent flattish 

 band homogeneous, or with delicate longitudinal striations, in the 

 fresh state, with here and there a long oval nucleus. M. Schultze 



