TISSUES, 



103 



Fibrils of axial 

 cord. 



Neurilemma. 



Segment of 

 Lantermann. 



Lantermann segments. It is claimed by some that 

 these are artifacts. Nodes of Ranvier are con- 

 strictions of this sheath at regular intervals of 80 

 to 900 ft. The smaller the fiber, the greater the 

 distance between these nodes. Long fibers are 

 slender, with long distance between the nodes; 

 short fibers are coarse, with 

 short distance between the 

 nodes. Furthermore, in young 

 fibers and at the distal por- 

 tion of nerve fibers the nodes 

 are relatively closer together. 



The neurilemma is a thin 

 structureless membrane that 

 surrounds the medullary 

 sheath. An oval nucleus is 

 present in this sheath, midway 

 between the nodes of Ranvier. 

 At each node the neurilemma 

 is constricted and touches the 

 axis cylinder, which in turn 

 may be slightly thickened at 

 this point and may give off a 

 collateral. Medullated nerve 



fibers with a neurilemma are found in the cranial 

 and spinal nerves. Medullated fibers without a 

 neurilemma are found in the brain and spinal cord. 

 The neurilemma gives great strength to the fibers. 

 Its absence in the brain and cord accounts for the 

 pulpy, soft nature of this tissue. 



2. Non-medullated nerve fibers with a neurilemma, 

 but without a medullary sheath, mingle with the 

 medullated fibers. The sympathetic system con- 



Fig. 71. Longitudinal 

 section through a nerve 

 fiber from the sciatic nerve 

 of a frog (Bohm and 

 Davidoff). 



