TISSUES 103 



the walls of the aHmentary canal, the blood vessels, and the gland 

 ducts. 



124. Nervous Tissues. — Nervous tissues contain cell bodies from 

 which extend processes or nerve fibers which vary in length and in the 

 degree to which they are branched (Fig. 42). The function of these 

 tissues is to register the effect of stimuli and to conduct this effect from 

 cell to cell until it finally reaches a cell which gives the appropriate 

 response. The nerve cells of a brain may themselves initiate impulses 

 which stimulate another part of the body to action. Finally, nervous 

 tissues have the power to conserve the effect of stimulation and to use 

 it in modifying future action. The effect of a stimulus conducted along a 

 fiber is known as an impulse. Irritability and conductivity are properties 

 of all protoplasm but are developed to the highest degree in nerve tissues. 

 A fiber which transmits an impulse to the cell body of which it is a part 

 is known as a dendron or dendrite, while a fiber which transmits an impulse 

 in the opposite direction is an axon, or axis cylinder process. Some nerve 

 fibers have a fatty sheath and are said to be medullated; others, which lack 

 this, are nonmedullated. 



