276 



but regeneration of human nerves does 

 not occur commonly. 



The nerve impulse. In most of our 

 nerve fibers an impulse travels at the rate 

 of about 75 yards per second, about as 

 fast as a bullet shot from a revolver. After 

 the passage of the impulse the nerve fiber 

 must rest for a tiny fraction of a second 

 before it can carry a second impulse. It 

 is possible to measure the amount of heat 

 released, the amount of oxygen used, and 

 the amount of carbon dioxide produced 

 by nerves. All these increase while the 

 nerve carries an impulse. While the de- 

 tails are not fully understood, it is 

 known that electrical energy is some- 

 how involved in the passage of an im- 

 pulse. The electrical currents have been 

 measured. 



When a nerve impulse starts in a neu- 

 ron it travels away from the cell body 

 along the axon to the end brush. There 

 it starts up similar impulses in the den- 

 drites of other neurons close to the end 

 brush. The dendrites carry the impulse 

 into the cell body. In this way impulses 

 may be relayed from neuron to neuron 

 until they reach effectors. The region 

 including end brush and dendrites, 

 where the transfer of a nerve impulse 

 from one neuron to the next takes place, 

 is called a sy?iapse (sin'aps). The method 

 of transfer is not completely understood. 

 But it is known that the synapse always 

 slows up the passage of an impulse. 

 Sometimes an impulse does not get across 

 the synapse at all. 



Neurons may connect with many 

 others. Since the end brush may be very 

 much branched, an impulse from one 

 cell may be sent to the dendrites of sev- 

 eral different neurons. This is true of 



Why Livmg Things Behave As They Do unit v 



Blood Vessel cut at angle 



Fig. 261 A greatly magnified section through 

 the cerebral cortex of jna^fs braifi. Only a jew 

 of the neurons of this complex tissue are shown. 

 Many synapses are shown. (See Figure 262.) 



cell A in the diagram. Figure 263. The 

 dendrites of one cell may also come in 

 contact with the end brushes belonging 

 to many different neurons. This is true 

 of the dendrites in cell C. The diagram 

 should help you to realize how in a re- 

 gion like the cortex of the cerebrum 

 where the neurons lie close together very 

 many connections are possible. At one 

 time, an impulse mav take one path; at 

 another time it may travel to other neu- 



