2 04 On the Construction of tlie 



the nervous tissues of a considerable number of animals, such as the 

 ox, calf, guinea-pig, rabbit, mouse, frog, toad, alligator, turtle, lizard, 

 snakes, and insects. 



In examining in serum some darh or double-hordered nerve fibres 

 of a freshly killed, or, as I have often done, of a still living animal,* 

 under the microscope, we find them, as is well known, bordered by- 

 dark, sharply-defined double contours. The space between these 

 contours, representing the greater portion of the entire nerve fibre, 

 appears to a certain degree opaque, except where it borders on 

 them ; there it is seen as a clear stripe, becoming gradually famter 

 in the direction of the axis of the fibre (Fig. 1, a and h). In adding 

 now, at the margin of the covering glass, a drop of water to the 

 serum, a fine dark line is seen to appear in the interspace of the 

 double contour itself (Fig. 2), dividing this, so to say, in two halves. 

 The outer half, thus formed, is distinguished by a reddish, and the 

 inner by a greenish hue, the difference in colour pointing to a dif- 

 ferent chemical composition. In pursuing the progressive changes 

 which the nerve fibre undergoes by the action of the water, it will 

 be found that the inner dark line, forming a part of the original 

 double contour, is gradually dissolved and lost sight of; with it, of 

 course, the inner, greenish shining half of the original double con- 

 tour also disappears, while the outer half, with the loss of its reddish 

 hue, remains. In consequence of these changes the fine dark 

 median line, which at first appeared after the addition of water, 

 dividing the original double contour, now forms the inner contour 

 of the nerve fibre. At the same time, however, an essential change 

 takes place in the main part of the nerve fibre, situated between the 

 two, now very fine double contours. This consists in the appearance 

 of certain irregular figures often described, which, examined with a 

 sufficient amplification and central illumination, resemble somewhat 

 an irregular network of fine tubular elements, as Stilling once de- 

 scribed them ; but, by a closer examination with an oblique illumi- 

 nation, they will be found to represent in reality a great number of 

 fine fibrils, which in their usual wavy or tortuous course frequently 

 cross each other, either singly or in the form of bundles, and thus 

 give rise to the resemblance to a network (Figs. 3 to 6, and 9 to 11). 

 We will now examine whether these fibrils owe their origin to a 



* In the absence of an apparatus especially constructed for this purpose, a flat 

 piece of cork, provided with a small round orifice covered by a suitable plate of 

 thin glass, may be taken, and a frog fastened upon it in such a manner, that the 

 bent knee will come to lie around the orifice. By a rapid dissection, a portion of 

 the ischiatic and also the popliteal nerve are tlien exposed. Taking now some of 

 the lymph from under the skin of the same or from another frog, it is put upon 

 the previously slightly warmed glass plate and the coagulated fibrin removed from 

 it. After this, the nerve is gently pulled over the glass plate into the remaining 

 serum, its fibres quickly separated with finely pointed needles, and then covered 

 by a small, also slightly warmed covering glass. 



