NERVE-FIBRES. 



69 



FIG. 92. Medullated nerve-fibres in 

 transverse section. X 385. 



supporting reticulum. The latter, arranged for the most part as connected 

 membranous lamellae, resists pancreatic digestion and fat-dissolving reagents 

 and is composed of a substance named neurokeratin. The reactions ex- 

 hibited by myelin indicate its fatty nature, 

 this substance existing during life perhaps 

 in the form of an extremely fine emulsion 

 supported by the framework. When fresh, 

 myelin appears clear and highly refracting 

 and confers upon the nerve-fibres which it 

 covers, "medullated fibres" as they are 

 called, their characteristic whitish color. It is 

 prone to post-mortem changes, so that after 

 death it loses its former uniformity and pre- 

 sents irregular contractions and collections, or extrudes in irregular globules 

 at the ends of the broken fibres. The medullary sheath is not uniformly 

 continuous, but is almost completely interrupted at regular intervals marked 

 by annular constrictions. These constrictions, the nodes of Ranvier, corre- 

 spond to very narrow zones at which the medullary sheath is practically 

 wanting and the neurilemma dips in and comes into close relation with the 



axis-cylinder. The medullary sheath, how- 



B ever, does not suffer complete suppression 



at the nodes, but is represented by a part 

 of its framework which traverses the con- 

 strictions. The latter occur at regular 

 intervals along the fibre which they thus 

 divide into a series of internodal segments. 

 In a general way, the segments are longer 

 (about i mm. ) in large fibres, and shorter 

 in those of small diameter, in which they 

 are reduced to . i mm. or less in length. The 

 axis-cylinder passes uninterruptedly across 

 the nodes and is continuous from its origin 

 in the nerve-cell to its ending in the terminal 

 arborization (telodendrion). The neuri- 

 lemma also suffers no break at the nodes, 

 but continues from one segment to the 

 other. After treatment with osmic acid, 

 the medullary sheath frequently is broken 

 by clear narrow clefts that extend obliquely 

 from the neurilemma towards the axis- 

 cylinder, and thus subdivide each internodal 

 segment into a number of smaller tracts, 

 known as the Schmidt- Lantcrmann seg- 

 ments. The significance of this subdivision 

 is uncertain, the details being regarded 

 by some as artefacts. Within each inter- 

 nodal segment, beneath the neurilemma, lies a small cell, the neurilemma 

 cell, which comprises an elongated oval nucleus surrounded by a meagre 

 amount of cytoplasm. These cells represent the remains of the ectodennic 

 elements (sheath cells'} that were active during the growth of the nerve-fibre 

 in providing its envelope. 



According to the presence or absence of the medullary coat throughout 

 the greater part of their course, nerve-fibres are designated as medullated or 



Node of Ranvier 



FIG. 93. Medullated nerve-fibres after 

 treatment with osmic acid ; A, fibre show- 

 ing reticulum within medullary coat ; /?, 

 one showing same coat divided into seg- 

 ments. X 500. 



