342 



CHORDATE ANATOMY 



mit nervous impulses in only one direction. In contrast with neurosensory 

 cells, however, a neuron always has at least two nervous processes, of 

 which the dendrite carries impulses toward the cell body, while the 

 neurite (neuraxon) carries impulses away from the cell body. Each 

 neuron has but one neurite, and may have one or many dendrites. In 



;> NERVE FIBERS 



*^OANCUON CELLS 



Fig. 303. — 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.) 



ontogenesis the neurite grows away from the source of stimulation and 

 the dendrites towards the stimulus. 



The differentiation of the neuron is accompanied by the appearance 

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



EPINEURIUM 



NODE OF RANVIER 



NEURILEMMA 

 MYELIN SHEATH 



NEURITE (AXONE) 

 ENDONEURIUM 

 NUCLEUS OF NEURILEMMA'^ 



A LONGIT SECTION B. CROSS SECTION 



Fig. 304. — 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 neurilemma sheath. 



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

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

 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-meduHated fibers 



