CHAP, ii.] THE CONTRACTILE TISSUES. 117 



strangled by a ligature tied tightly round it ; its transverse 

 diameter is suddenly narrowed, and the double contour lost, the 

 fibre above and below being united by a narrow short isthmus 

 only. This is called a node, a node of Ranvier, and upon exami- 

 nation it will be found that each fibre is marked regularly along its 

 length by nodes at intervals of about a millimeter. If the fibre be 

 examined with further care there will be seen or may be seen, 

 about midway between every two nodes, an oval nucleus lying 

 embedded as it were in the outline of the fibre, with its long 

 axis parallel or nearly so to the axis of the fibre. 



If some of the fibres be torn across it may sometimes be seen 

 that at the torn end of a fibre, though the double contour ceases, 

 the outline of the fibre is continued as a delicate transparent 

 membranous tubular sheath ; this is the primitive sheath or 

 neurilemma 1 . Lying in the axis of this sheath and sometimes 

 projecting for some distance from the torn end of a fibre, whether 

 the sheath be displayed or no, may, in some cases, be seen a dim 

 or very faintly granular band or thread, about one-third or half 

 the diameter of the fibre ; this is the axis-cylinder; it becomes lost 

 to view as we trace it back to where the fibre assumes a double 

 contour. This axis-cylinder stains readily with ordinary staining 

 reagents, and being in this and in other respects allied in nature to 

 the cell-substance of a leucocyte or to the muscle-substance of a 

 muscular fibre, has often been spoken of as protoplasmic. 



Lying about the torn ends of the fibres may be seen drops or 

 minute irregular masses, remarkable for exhibiting a double 

 contour like that of the nerve fibre itself; and indeed drops of 

 this double contoured substance may be seen issuing from the torn 

 ends of the fibres. Treated with osmic acid these drops and 

 masses are stained black ; they act as powerful reducing reagents, 

 and the reduced osmium gives the black colour. Treated with 

 ether or other solvents of fat they moreover more or less readily 

 dissolve. Obviously they are largely composed of fat and we shall 

 see that the fat composing them is of a very complex nature. Now 

 a nerve fibre shewing a double contour stains black with osmic 

 acid ; but the staining is absent or very slight where the double 

 contour ceases as at a torn end or at the nodes of Ranvier; the axis- 

 cylinder stains very slightly indeed with osmic acid and the sheath 

 hardly at all. So also when a transverse section is made through 

 a nerve or a nerve cord, each fibre appears in section as a dark 

 black ring surrounding a much more faintly stained central area. 

 Further, when a double contoured nerve fibre is treated with ether, 

 or other solvents of fat, the double contour vanishes, and the 



1 This word was formerly used to denote the connective tissue sheath wrapping 

 round the whole nerve. It seemed undesirable however to use two such analogous 

 terms as sarcolemma and neurilemma for two things obviously without analogy, 

 and hence neurilemma is now used for that part of the nerve which is obviously 

 analogous to the sarcolemma in muscle, viz. the sheath of the fibre. 



