NOTES AND MEMORANDA. 97 



starts from the periphery, and advances towards the axis- 

 cylinder. The segments at the same time swell up by 

 imbibition, and their discontinuity ceases by their edges 

 becoming confluent. The medullary sheath is thus con- 

 verted into a continuous layer formed of concentric coats, 

 and increased in diameter. This increase of volume in the 

 medullary sheath, being accompanied by loss of refraction, 

 brings more distinctly into view the sheath of Schwann and 

 the axis-cylinder. The former appears as a delicate outline 

 bounding the altered medullary sheath, usually in contact 

 Avith it, but sometimes separated by a fine interval filled by 

 fluid. A similar separation often takes place between the 

 internal surface of the medullary sheath and the axis-cylinder, 

 which appears as a pallid and homogeneous streak bounded 

 by two contours, and is of unequal breadth. It appears to 

 contract somewhat, and to form a rather compact coagulum. 

 The appearances above described soon become notably 

 changed. Owing to the continually increasing absorption 

 of water, the altered medullary sheath, which is at first 

 solid, becomes viscous, then semi-fluid, and finally com- 

 pletely liquid. At tlie same time, its laminated structure is 

 dissolved, and gives place to a material of a frothy appear- 

 ance. This material escapes from the cut ends of the fibres 

 in considerable masses of irregular or rounded shape, remain- 

 ing at first connected with the nerve fibre, but they soon 

 become detached and float away. A current of this frotliy 

 matter becomes set up with increasing rapidity, running 

 between the axis-cylinder and the sheath of Schwann. Con- 

 sequently the latter must form a completely closed tube, not 

 interrupted in any part of its course, as the liquefied mate- 

 rial always remains in the above narrow channel, not escaping 

 even at Ranvier's nodes ; so that at this point also the sheath 

 of Schwann must be regarded as continuous, which had not 

 been hitherto proved wdth certainty. Another interesting 

 point concerning the structure of the sheath of Schwann and 

 Ranvier's nodes may be observed; the current of the liquefied 

 medullary sheath may be observed to stagnate for a moment 

 at these points, and afterwards to pass the narrow channel 

 with increased velocity. There must have been, therefore, 

 a considerable resistance to the stream at this point, the 

 sheath of Schwann being not only narrowed, but also spe- 

 cially resistant. The axis-cylinder continues to appear as a 

 longitudinal band of variable breadth after the medullary 

 sheath has begun to liquefy; but soon the axis-cylinder is 

 itself altered in a similar manner, gradually liquefying and 

 escaping from the cut ends of the nerve fibre in the form of 



VOL. XVIII. NEW SER. G 



