Irving Hardesty 349 



the framework suggests tliat the heavy cones pictnred by Wynn may after 

 all refer to the arrangement pictured here. His pictures probably rep- 

 resent the conical arrangement of the reticulated framework, the fine 

 meshes of which had been clogged by the precipitate of the Weigert 

 method. 



The Schmidt-Lanterinann clefts seen in the usual osmic prepara- 

 tions of the peripheral nerves are interpreted as representing the cones 

 in longitudinal section. The cones, being condensed portions of the 

 framework, W'hich for that reason contain less myelin than other portions 

 of the sheath, are therefore less blackened by the osmic acid. Light pass- 

 ing through them more readily results in the familiar appearances. 



As is well known, in the ordinarily fixed material, every peripheral 

 fiber does not show the clefts, and when they are shown they seldom 

 appear in straight, even course slanting from the axone to the periphery. 

 Further, the clefts never appear parallel along a considerable extent of a 

 fiber, indicating that the cones do not all lie in the same direction. Again, 

 it is invariably claimed that the clefts are not present in the fresh con- 

 dition of the medullary sheath; that after death the sheath soon under- 

 goes changes resulting in their appearance. 



i\.ll the statements concerning cones and clefts in the medullary 

 sheaths have been made with reference to the peripheral nerve fibers 

 alone. I have found neither in the fibers of the spinal cord. In the 

 peripheral fibers it seems to me that a partial explanation of the cones is 

 suggested in the many varieties of arrangement of the framework of 

 sheath to be found in the fixed and stained material. Considering types 

 A and B of Fig. 8 as the two extremes, a study and arrangement in series 

 of the intermediate forms may be made which will suggest that the inter- 

 mediate forms are gradations from type B into type A; in other words, 

 that the form of framework shown in type B, though less frequent in the 

 preparations, may represent more nearly the normal arrangement and 

 that type A is probably derived from type B through a procession of 

 post-mortem changes. In the intermediate forms may be noted: (1) 

 those in which the lamellated reticulum of type B, arranged more or less 

 parallel with the axone, contains occasional small, oval or globular spaces 

 interrupting the parallel arrangement; (2) tliose in which the spaces 

 are more numerous, some of them much larger than others and so shaped 

 and situated with reference to each other as to suggest that the larger 

 spaces arise from a coalescence of the smaller; (3) those sheaths in 

 wliich the larger spaces predominate, giving the framework a marked 

 blistered or honeycombed appearance with the smaller spaces in the 

 partitions between the larger; and finally, those which conform in 



