THE NERVOUS SYSTEM 



471 



The differentiation of the neuron is accompanied by the appearance 

 of two sheaths, neurolemma and medullary or myelin, which cover the 

 nervous processes and serve to insulate and nourish them. Each neurite 

 may be covered by a chain of neurolemma cells, or it may not; each may 



■NERVE FIBERS 



■^ ^ WNCLION CELLS 



Fig. 388. — Plexus of ganglion cells and fibers in the tentacle of a coelenterate. 

 From such a nerve-net as this it is believed that the nervous systems of higher animals 

 have evolved. (Redrawn from Ariens-Kappers, after Wolff.) 



or may not have a fatty medullary sheath. The familiar distinction 

 between white and gray matter in the nervous system rests on the presence 

 or absence of medullary sheaths. Ganglia and non-medullated fibers 

 form gray matter; meduUated fibers appear white. That the medullary 



Piii! ['||'iWll''!|l|IIH EPINEURIUM 



. NODE OF RANVIER 



NEURILEMMA 

 MYELIN SHEATH 



NEURITE (AXONE) 

 ENDONEURIUM 

 NUCLEUS OF NEURILEMMA'^' 



A LONGIT SECTION B. CROSS SECTION 



Fig. 389. — A portion of A, longitudinal and B, cross section of a nerve prepared by 

 the vom Rath method. A nerve is a bundle of axons (neurites) covered by an epi- 

 neurium. Each axon is surrounded by an inner myelin (fatty) sheath and an outer 

 cellular neurolemma sheath. 



sheath serves at least for insulation is indicated by the fact that nervous 

 impulses are conveyed more rapidly in medullated than in non-medullated 

 nerves. 



Within the central nervous system of vertebrates, neurites lack the 

 neurolemma sheath, but are usually medullated. The presence of a 

 neurolemma is, therefore, not essential to the secretion of this fatty myelin 

 sheath. Most peripheral nerves are medullated, but the medullary sheath 



