NERVES AND THE REACTIONS OF ORGANISMS 221 



the brain, or as in some neurons in the gray 

 part of the spinal cord ; or the axon may 

 be very long, like those in the neurons 

 extending from the lower part of the 

 spinal cord to the ends of the toes or 

 through the length of the arm. 



257. Kinds of neurons. The follow- 

 ing different types of neurons may be 

 recognized. 



1 . Those that bring impulses toward 

 the cord or brain, the afferent, or 

 sensory, neurons. 



2. Those that carry impulses from 

 the cord or brain, the efferent neu- 

 rons that may stimulate a muscle or 

 a gland. 



3. Those that connect afferent and 

 efferent neurons, which we may call 

 associative neurons. 



4. Neurons in the brain, many of 

 which are not directly related to 

 reflexes but are related to knowing 

 and feeling and the voluntary control 

 of muscles. 



258. Nerve connections in a simple 

 reflex. Suppose that your finger 

 touches something hot. The nerve 

 endings in the skin are disturbed, 

 and the disturbance of the proto- 

 plasm is transmitted to the rest of 

 the neuron in a fraction of a second. 

 The fiber of the affected cell sends 

 off a number of branches in the 



spinal cord (Fig. 83), and the dendrites at the ends of these 

 collaterals form fine networks with dendrites of other neurons. 



FIG. 86. Behavior limited by 

 nerve connections 



The diagram shows the nerve 

 connections of a simple mus- 

 cular reflex, with collateral con- 

 nections to the brain. Such 

 connections make possible auto- 

 matic reflexes as well as volun- 

 tary movements. If the afferent 

 nerve is cut, as at a, only volun- 

 tary movement is possible, and 

 there is no sensation. If the 

 efferent nerve is cut, as at e^ 

 neither reflex nor voluntary 

 movement is possible, but sen- 

 sation remains. If the spinal 

 cord is cut high up, as at a z , e^, 

 neither sensation nor voluntary 

 movement is possible, but the 

 reflexes are not affected 



