36 ELECTRO-PHYSIOLOGY CHAP. 



" sheath of Schwann," but is rather comparable with the con- 

 nective tissue (" endoperineurium ") which, in vertebrates, unites 

 the axis-cylinders, along with their special sheaths, into primitive 

 bundles of fibres, or nerve-trunks. Only that fine, homogeneous, 

 nucleated sheath which partially invests the peripheral axis- 

 cylinders (medullated and non-rnedullated) of vertebrates and 

 some invertebrates is to be termed the sheath of Schwann. 



In the muscular nerve of crayfish, the neural sheath, both in 

 the larger trunks and in parts where the axis-cylinders run singly, 

 exhibits, even in the fresh state, and still more after treatment 

 with the gold method, an obvious stratification, resembling in 

 places the highly developed and richly nucleated connective- 

 tissue sheath on the capsule of the Pacinian corpuscle (Fig. 150). 

 A similar concentric layering appears in the neural sheath of several 

 Orthoptera (locusts). In other cases, on the contrary (e.g. many 



FIG. 150 



.Isolated muscular nerve from the abductor muscle of crayfish. (Gold and formic acid.) 



insects), the substance in which the axis-cylinder is embedded is 

 finely granulated, like protoplasm (3). 



These relations between the nerve-fibres (axis-cylinders) of 

 invertebrates and their sheaths only appear fully on staining the 

 former by proper methods. The gold method, which was so much 

 employed, after Cohnheim, has been superseded by the methylene - 

 blue method of Ehrlich, more particularly for invertebrate animals. 

 Biedermann has failed to determine a special and individual sheath 

 within the common integument of the finer axis-cylinders of in- 

 vertebrate nerves, unless the compacter layers of connective tissue 

 immediately surrounding each axis-cylinder in the muscular nerve- 

 trunk of the crayfish be recognised as such. As a rule, invertebrate 

 nerves present a naked axis-cylinder within a common sheath, or 

 substratum of connective tissue, which is histologically distinct 

 from the specific sheaths of the nerve-fibres of higher animals, 

 even when it invests a single axis-cylinder. In vertebrates, similar 



