390 



NERVOUS ACTION. 



tubular fibres have been called medullated by Kolliker, to 

 distinguish them from the non-medullated. The latter 

 occur in organs of special sense, when delicate plexuses 

 are formed as in the retina, or the olfactory and auditory 

 apparatuses, and have a structureless envelope containing a 

 clear granular axis without the white substance." 



Fiff. 209. Tubular or white nerve-fibres. A indicates the dark outline produced in the 

 medullary sheath by exposure ; B shows the double contours in a nerve-fibre ; and E 

 the varicosities resulting from traction or pressure. 



" Gelatinous fibres have been described by Eemak, Henle, 

 and others, and are found associated with the elements 

 already noticed. They are flat, slightly granular, nucleated, 

 and about ^-oV^th of an inch in diameter." Anatomy of 

 the Domestic Animals. Gamgee and Law. 



The nerve-cells which exist in the grey or cineritious 

 matter of the brain and other nervous centres, are the gene- 

 rators of nervous force and recipients of sensory impres- 

 sions, and their presence in any part indicates it to be a 

 nervous centre or ganglion. They generate nervous force, 

 however, only when a proper stimulus is conveyed to them, 

 as referred to under the heads of Physical and Mental 



