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HAND-BOOK OF PHYSIOLOGY. 



occupying the axis of the tube (axis-cylinder), becomes greyish, while 

 the outer, or cortical portion (white substance of Schwann), becomes opaque 

 and dimly granular or grumous, as if from a kind of coagulation. At the 

 same time, the fine outline of the previously transparent cylindrical tube 

 is exchanged for a dark double Contour (Fig. 303, B), the outer line being 

 formed by the sheath of the fibre, the inner by the margin of curdled or 

 coagulated medullary substance. The granular material shortly collects 

 into little masses, which distend portions of the tubular membrane; while 

 the intermediate spaces collapse, giving the fibres a varicose, or beaded 

 appearance (Fig. 303, c and D), instead of the previous cylindrical form. 



FIG. 303. 



FIG. 304. 



FIG. 303. 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. 304. Two nerve-fibres of sciatic nerve. A. Node of Ranvier. B. Axis-cylinders, c. Sheath 

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



The whole contents of the nerve-tubules are extremely soft, for when sub- 

 jected 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 nucleated sheath of Schwann is a pellucid membrane, forming the 

 outer investment of the 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 sur- 

 rounds them, they are the relics of embryonic cells, and from their resem- 

 blance to the muscle corpuscles of striated muscle, may be termed nerve- 

 corpuscles. 



