158 THE TISSUES. 



ing horn is given off. It is thought that the meshes of this neuro- 

 keratin network contain the highly refractive substance similar to 

 fat, composing the greater portion of the medullary sheath. The 

 medullary sheath is interrupted at intervals of from 80 to 900 /*, the 

 constrictions thus formed being known as the nodes of Ranvier. The 

 smaller the fiber, the less the distance between the nodes. In a fiber 

 with a diameter of 2 p. the internodal segments are usually about 

 90 [J. in length. 



In peripheral nerves the medullary sheath is in its turn sur- 

 rounded by a clear, structureless membrane, the neurilemma or 

 sheath of Schwann. Nerve-fibers contain here and there relatively 

 long, oval nuclei (neurilemma-nuclei) which are surrounded by a 

 small quantity of protoplasm, and are situated in small excavations 

 between the neurilemma and the medullary sheath. In the higher 

 vertebrates a single nucleus is found midway between each two 



Connective . 



tissue. 





Medullary 

 sheath. 



Fig. 119. Transverse section through the sciatic nerve of a frog; X ^20. At a 

 and b is a diagonal fissure between two Lantermann's segments ; as a result, the medul- 

 lary sheath here appears double. (Compare Fig. 118. ) 



nodes ; in the lower vertebrates (fishes) several scattered nuclei 

 (5-16) may be found in each internodal segment. At the nodes, 

 where the medullary sheath is interrupted, the neurilemma is 

 thickened and contracted down to the axial cord (contraction-ring). 



Just beneath the contraction-ring, Ranvier found that the axis- 

 cylinder presents a slight, biconic swelling (renflement biconiquc). 

 Thus the sheath of Schwann represents a continuous tube through- 

 out the length of the fiber in contrast to the medullary sheath. In 

 the nerve-fibers of the spinal cord and brain there is no neurilemma, 

 although the medullary sheath is present. 



In the fresh nerve-fiber the axial cord fills the space (axial 

 space) within the medullary sheath, and appears transparent. 

 After treatment with many fixing fluids the neuroplasm coagulates 

 and shrinks, no longer filling the entire axial space, but appears in 

 the* latter as a wavy cord composed of an apparently homogeneous 



