140 



THE TISSUES 



h 



\ 



ii 



/ 



i^ 



/ 



outside the neurilemma, a nucleated sheath of connective-tissue 

 origin, known as the sheath of Henle (Fig. 85). 



n]P||fl| Two views as to the relation of the 



./''M axolemma to the neurilemma are illus- 



' trated in Fig. 84. According to one the 



neurilemma is continuous, merely dipping 

 into the nodes of Ranvicr, where it touches 

 the axolemma or the axone. According to 

 the second both neurilemma and axolem- 

 ma are interrupted at the node, but unite 

 with each other there to enclose completely 

 the medullary substance of the internode. 

 According to the views illustrated in 

 Fig. 86, that part of the axone which lies 

 between two nodes is enveloped by a cell 

 or by several cells forming a syncytium. 

 The outer homogeneous membrane there 

 pictured would thus be of the nature of a 

 cell membrane or cuticle, and would cor- 

 respond to the neurilemma. The trabeculae 

 (protoplasmic strands and neurokeratin 

 network, of which some of the larger 

 strands would represent the incisures of 

 Schmidt) would correspond to the spongio- 

 plasm, and just along the outer side of the 

 axone would constitute the axolemma. 

 The myelin would thus be enclosed within 

 the cylindrical neurilemma cell which sur- 

 rounds the axone. 



Recent experiments of Bethe and others 

 tend to prove an interruption of the peri- 

 fibrillar substance at the node of Ranvier. 

 They consider the axone at the node as 

 probably crossed by a sieve-like plate, 

 through the holes of which the fibrils pass, 

 but which completely interrupts the peri- 

 fibrillar substance. The accuracy of these 

 observations has been disputed. 



Medullated nerve fibres vary- 

 greatly in size. The finer fibres have 

 a diameter of from 2 to 4/i, those 

 of medium size from 4 to lo/x, the 

 largest from 10 to 20^1. They have 

 few branches, and these are always given off at the nodes of Ranvier- 

 {b) Medullated axones without a neurilemma are the medullated 



Fig. 84. Fig. 85. 



Fig. 84. — -Diagram of Structure of 

 a Medullated Nerve Fibre of a Peri- 

 pheral Nerve, showing two different 

 views (one on each side) as to rela- 

 tions of neurilemma and axolemma 

 and their behavior at the nodes of 

 Ranvier. (Szymonowicz.) a, Neuro- 

 fibrils; b, cement substance; c, axone; 



d, incisure of Schmidt; c, nucleus of 

 neurilemma; /, medullary sheath; g, 

 sheath of Schwann; h, axone; i, axo- 

 lemma; j, sheath of Schwann; k, node 

 of Ranv'ier. 



Fig. 85. — Piece of Medullated 

 Nerve Fibre from Human Radial 

 Nerve. X400. Osmic-acid fixation 

 and stain. (Szymonowicz.) a, Me- 

 dullary sheath; b, axone; c, sheath 

 of Henle; d, nuclei of Henle's sheath; 



e, nucleus of neurilemma. 



