PROBLEM 2. Why Cojnplex Behavior Is Possible 



275 



treme end it is subdivided into the end 

 brush or terminal branches. Axons, and 

 sometimes dendrites, too, may be very 

 long compared to their width. Though 

 they are very thin projections from a 

 microscopic cell, and therefore invisible, 

 some axons or dendrites are two feet or 

 more in length. Impulses travel into the 

 cell body by way of a dendrite and out 

 of a cell body by way of an axon. 



Cell bodies of neurons are quite vari- 

 able in shape and size. They have a con- 

 spicuous nucleus, but the neurons of man 

 lose their power of multiplication soon 

 after birth. The protoplasm in the cell 

 body, axon, and dendrites has the same 

 properties as all other protoplasm but it 

 has one property, irritability, developed 

 to a very high degree. The neuron is able 

 to transmit impulses through the body 

 with great speed. 



Cell bodies outside the brain and cord. 

 You have just read that the cell bodies of 

 neurons lie in the gray matter of the 

 brain and cord. Some cell bodies of neu- 

 rons, however, lie outside of these cen- 

 ters. They form small groups called 

 ga?iglia (gan'glee-a). The singular is ga7i- 

 glion. These ganglia are found in defi- 

 nite regions, either near the spinal cord 

 or close to the internal organs. Those 

 close to the organs are part of the auto- 

 nomic system. You will read about them 

 later. The nervous system of insects con- 

 sists of chains of ganglia from which the 

 nerves extend. See Figure 260. 



Relation of nerves to nerve cells. If 

 you again refer to Figure 254 you will 

 be reminded that all the nerves originate 

 (are attached to) in the brain or cord. 

 There are roughly forty pairs of nerves. 

 By separating into finer and finer 



Head ganglion ( brain ) 



Nerve chain of ganglia 



Larva Adult 



Fig. 260 Chains of ganglia in the honeybee 

 larva and adult. They are found along the ven- 

 tral side. Insects have no true brain or cord. 



branches these nerves reach all the cells 

 of the body. A nerve is nothing more 

 than a bundle of axons with a covering. 

 An axon is therefore often spoken of as 

 a nerve fiber. Even a very fine branch of 

 a nerve is a bundle of axons, not a single 

 axon. A large nerve consists of many 

 thousands of axons, or nerve fibers. Most 

 axons have a covering or sheath. This 

 sheath is glistening white. 



A nerve fiber gets food through its 

 cell body. If a fiber is cut, the portion 

 which is detached from the cell body 

 soon dies. The portion attached to the 

 cell body may grow out. In other words, 

 the cell body with its nucleus is the only 

 portion of a neuron in which there can 

 be renewed growth. In this way a whole 

 nerve could grow again or regenerate^ 



