THE NEKVOUS SYSTEM. 



451 



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. 



(1.) The external nucleated sheath of Schwann is a pellucid mem- 

 brane, 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 surrounds them, they are the relics of embryonic 



C TJ 



P 



FIG. 309. 



FIG. 310. 



FIG. 811. 



FIG. 309. 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. 310. Two nerve-fibres of sciatic nerve. A. Node of Banvier. B. Axis-cylinder, c. Sheath 

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



FIG. 311. A node of Ranvier in a medullated nerve fibre, viewed from above. The medullary 

 sheath is interrupted, and the primitive sheath thickened. Copied from Axel Key and Eetzius. 

 X 750. (Klein and Noble Smitli.) 



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. 



(2.) The medullary sheath or white substance of Schwann is the part 

 to which the peculiar white aspect of some nerves is principally due. It 

 is a thick, fatty, semi-fluid substance, as we have seen, possessing a 

 double contour. It is said to be made up of a fine reticulum (Stilling, 



