Il8 NORMAL HISTOLOGY. 



point of view. A fresh nerve is slightly teased apart on 

 a slide, a large drop of a one-half-per-cent. solution of 

 nitrate of silver added and allowed to remain for four 

 minutes ; this is washed off with one-half-per-cent. salt 

 solution, the specimen transferred to a drop of glycerin 

 on another slide, the fibres carefully teased apart, and 

 covered. The preparation is now exposed to sunlight or 

 diffuse daylight until it becomes brown. If now ex- 

 amined, at tolerably regular intervals along the fibres, 

 tiny brown or black crosses, called Ranvier s crosses, will 

 be seen ; the transverse arm of the cross being the stained 

 cement substance between the neurilemma segments at 

 the constrictions ; the longitudinal arm, which coincides 

 with the axis of the fibre, and which is longer or shorter, 

 depending upon the length of time to which the fibre 

 was exposed to the action ot the silver, is the axis- 

 cylinder. If the specimen be allowed to remain longer 

 than the above time in contact with the silver, the longi- 

 tudinal arm of the cross will be longer. 



It is to be observed that the axis cylinder is first stained 

 at that part which passes through the constrictions, and 

 not along the segments. Certain other soluble substances 

 which stain the axis cylinder comport themselves in the 

 same way. We infer from this that at the constrictions 

 certain substances in solution can pass into the fibre and 

 come in contact with the axis cylinder, or the nerve- 

 element proper of the fibre. This inference is significant 

 in connection with the nutrition of the nerves, since we 

 are justified in assuming that nutritive substances in solu- 

 tion may pass also to the axis cylinder in the same way. 



It is not improbable that the constrictions serve yet 



