Dark or Double-bordered Nerve Fibre. 205 



coagulation of the medullary substance of the nerve fibre, or whether 

 they be real pre-existing elements. 



In separating fresh nerve fibres by means of finely pointed 

 needles from each other, a portion of the soft, semi-fluid medullary 

 substance or nerve-medulla is almost always seen to escape from 

 their torn and open ends. By a superficial examination with low 

 magnifying powers, this appears as a homogeneous, semi-fluid sub- 

 stance, and as such it has been hitherto considered by almost all 

 investigators of the nervous tissues, with the exception of Stilling. 

 By a more careful examination, however, it will be discovered that 

 the mass escaping is mostly composed of exceedingly fine and 

 smooth fibrils, which, surrounded by a semi-fluid, finely granular 

 substance, issue from the open end of the nerve fibre in the form of 

 larger or smaller loops (Fig. 4). If it happens that the escaping 

 mass be torn by the manipulation into larger or smaller portions, the 

 fibrils will appear in the form of coils, frequently spiral in shape. 



But it is not always that these masses of nerve-medulla, issuing 

 from the nerve fibres, show at once their fibrillous composition ; on 

 the contrary, they often appear apparently bordered by a double 

 contour of a fat-like lustre, and then resemble those so-called myelin 

 figures, the origin of which was ascribed (Liebreich) to a decom- 

 position of the protagon contained within the nerve fibre;* while 

 again, their formation from the constituents of the brain in the 

 fresh and undecomposed condition was decidedly denied t (Koehler) 

 (Fig. 7, a and b). I cannot believe in the identity of these arti- 

 ficially produced myehn figures with those masses of medullary 

 substance, escaping from the open ends of fresh nerve fibres, al- 

 though I never had an opportunity of examining the former. In 

 treating the escaping nerve - medulla with ether, acetic acid or 

 other reagents, the characters peculiar to myelin figures will be 

 lost, and they will appear in the above-described form of fibrillous 

 coils. In many instances these fat-like, apparently double -con- 

 toured masses of difierent sizes are already seen in the interior of 

 the nerve fibre, either stationary or floating, together with some 

 fibrillous loops or coils, toward the open end of the fibre, from which 

 they escape (Fig. 6). 



In many cases, the fine wavy fibrils, appearing after the addition 

 of water upon the surface of the nerve-medulla, and irregularly 

 crossing each other, are not seen to extend throughout the whole 

 nerve fibre ; but, in addition, a number of narrow wavy bands or 

 stripes of a greenish lustre are seen. These are particularly noticed 

 at the sides of the fibre, where it is in focus, and are easily resolved 

 into fibrillous bundles by the addition of more water, or, if this be 

 not sufiicient, certainly by acetic acid, alcohol, or ether. 



* Kiihne, ' Lehrbuch der Physiol ogisch en Chemie,' 1866, p. 345. 



t Virchow and Birsch, ' Jaliresbericht fiir das Jahr,' 1867, vol. i., p. 147, 



