THE STRUCTURE OF THE ELEMENTARY TISSUES 



93 



The granular material shortly collects into little masses, which distend 

 portions of the tubular membrane; while the intermediate spaces col- 

 lapse, giving the fibres a varicose, or beaded appearance (fig. 95, c and 

 D), instead of the previous cylindrical form. The whole contents of 

 the nerve-tubules are extremely soft, for when subjected to pressure 

 they readily pass from one part of the tubular sheath to another, and 

 often cause a bulging at the side of the membrane. They also readily 

 escape, on pressure, from the extremities of the tubule, in the form of a 

 grumous or granular material. 



The external nucleated sheath of Schwann, also called the neu- 

 rilemma, is a pellucid membrane forming the outer investment of the 



Fig. 95. 



Fig. 96. 



Fig. 95. -Primitive nerve-fibres. A. A perfectly fresh tubule with a single dark outline. B. A 

 tubule or fibre with a double contour from commencing post-mortem change, c. The changes 

 further advanced, producing a varicose or beaded appearance. D. A tubule or fibre, the central 

 part of which, in consequence of still further changes, has accumulated in separate portions within 

 the sheath (Wagner). 



Fig. 96. Two nerve-fibres of sciatic nerve. A. Node of Ranvier. B. Axis-cylinder, c. Sheath 

 of Schwann, with nuclei, x 300. (Klein and Noble Smith.) 



nerve-fibre. Within this delicate structureless membrane nuclei are 

 seen at intervals, surrounded by a variable amount of protoplasm. The 

 sheath is structureless, like the sarcolemma, and the nuclei appear to be 

 within it : together with the protoplasm which surrounds them they are 

 the relics of embryonic cells, and from their resemblance to the muscle 

 corpuscles of striated muscle may be termed nerve-corpuscles. They are 

 easily stained with logwood and other dyes. 



The medullary or myelin sheath or white substance of Schwann 

 *is the part to which the peculiar opaque white aspect of medullated 

 nerves is due. The thickness of this layer in nerve-fibres varies consid- 



