180 Prof. A. B. Macallum and Miss M. L. Menten. [July 24, 



That the neurilemma in vertebrate nerve fibres is not the only structure 

 preventing the entrance of the reagent is shown by the fact that even in 

 the fibres of the lateral columns of the cord (guinea-pig, rat, and rabbit) the 

 reagent takes time to penetrate. This must be due to the medulla, for in 

 the pressed-out preparations the short portions of fibres with free ends in 

 many cases show Frommann's lines, an indication that the reagent penetrated 

 at the open free ends, and that where a more or less long fibre, not cut by 

 the pressure into portions, exhibited the striation of its axon, it was caused 

 by an injury at some point on its course, permitting the reagent to diffuse 

 into the fibre there more readily than through the intact medulla. It is to 

 be noted that owing to the absence of the neurilemmal sheath, the medullated 

 fibres of the spinal cord are much more fragile than are those provided with 

 Schwann's sheath, and it is owing to this that pressure will give isolated 

 short segments in the former readily and in the latter only with difficulty 

 and then rarely. It is these segments, as a rule, that in the spinal cord, as 

 already stated, show most often the striation. Such segments may be 

 compared to the segments or short portions of capillary glass tubes filled 

 with egg albumen which, as will be described below, give, when placed in a 

 bath of the silver reagent, a striation like in every respect that found in 

 silver preparations of medullated fibre. 



One may in a similar manner explain the results obtained by applying the 

 mercurous nitrate reagent to nerve fibres to demonstrate the distribution of 

 the chlorides. As already pointed out, this reagent is but little less sensitive 

 a test for the chlorides than is the nitrate of silver, and, when it is carefully 

 applied, it gives quite all the results that the other does. It, for example, 

 brings out the crosses of Ranvier (figs. 10, a — d). In the parts of the axon 

 remote from the nodes it demonstrates the chlorides, but the mercurous 

 chloride precipitate is often not uniformly distributed in the axon (figs. 

 12 — 15), and the latter is frequently curiously altered in form and 

 character (fig. 14). Sometimes the precipitate is on the surface of the axon 

 and between it and the medullary sheath. As a rule, it is rarely found to 

 occur for more than a short distance on each side of the node, and here it 

 may give the Frommann striation. In fibres which, as is rarely the case, the 

 striation is in parts of the axon remote from the nodes (fig. 11, a), it would 

 appear as if the reagent penetrated the fibre by the clefts of Lanterman 

 (fig. 13). Such, and a number of other like results, seem to indicate that 

 there are weak points in the medullated fibre provided with a neurilemma, 

 but that, on the whole, the latter is a bar to the direct penetration to the 

 axon by the reagent. 



Quite otherwise was the result of treatment of teased-out preparations 



