THE NERVE FIBRE . 117 



the solution penetrates still farther and the blackened axis cylin- 

 der is found to possess spiral transverse markings which are quite 

 characteristic. The true meaning of these appearances has not 

 been satisfactorily explained.* Because of the apparent greater 

 permeability of the fibre at these points, these peculiarities have 

 been taken to indicate a certain relation of the annular constric- 

 tions to the nutrition of the fibre. 



The neurilemma (nucleated sheath of Schwanri) is the outer- 

 most of the nerve fibre sheaths. It is -of distinctly mesoblastic 

 origin and makes its appearance prior to the medullary sheath. 

 It forms a very delicate membrane, which incloses the myelin sub- 

 stance, and at each node of Ranvier comes into contact with the 

 axis cylinder. 



Attached to the inner surface of the neurilemma in each inter- 

 node, and usually but one for each internodal segment, is an oval 

 nucleus. The nucleus is surrounded by a minute amount of finely 

 granular cytoplasm. This structure is taken to indicate that 

 the embryonal neurilemma is formed by connective tissue cells 

 which become spread out over the surface of the primitive fibre, 

 one cell, as a rule, supplying each internodal segment, and its nu- 

 cleus with a minute amount of undifferentiated protoplasm is, 

 according to this hypothesis, considered to remain as a perma- 

 nent part of the neurilemma. 



2. Medullated Nerve Fibres without a Neurilemma, This type 

 of nerve fibre composes the white matter of the central nervous 

 system. The axis cylinder does not, of course, differ in the least 

 from those of the previous variety and will need no further de- 

 scription. 



The medullary sheath is, also, similar in its finer structure to 

 that of the previous type, but since no neurilemma is present, 

 these fibres possess no nodes of Ranvier. The medullary sheath 

 of the fibres found in the white matter of the brain and spinal 

 cord is, therefore, uninterrupted. Its surface is in direct contact 

 with the neuroglia network, which forms the supporting tissue of 

 these organs. 



3. Non-medullated Nerve Fibres with a Neurilemma (sympa- 

 thetic nerve fibres, Remotes fibres). The most of the fibres of the 

 sympathetic system are of this type. The axis cylinder does not 

 differ from that of the previous types. The medullary sheath is 



* They have been usually considered to be artifacts. 



