NERVE IMPULSES AND REFLEX ACTION 313 



from the skin: a part of the impulse might go into the side 

 branch, e, and give rise to a reflex movement, while the rest of 

 the impulse could take the path, a, leading to the brain and 

 there produce sensation. Influenced by this sensation, the 

 brain might send down a message through the fibre, g, which 

 would result in conscious movement, in addition to the re- 

 flex response. Suppose, for example, a barefooted boy steps 

 on a thistle. He jumps off quickly (a reflex action). In 

 a fraction of a second he feels the prick and acts accordingly, 

 but he jumped before he was conscious of the pain. 



The neurons described are those in the spinal cord and 

 spinal ganglia, but there are a large number in the brain it- 

 self. The details of their structure are slightly different, but 

 their general relation and method of working is the same. 



Outgoing Paths. Although the gray and white materials 

 of the brain cortex have been previously referred to merely 

 as cells and fibres respectively, it is necessary to realize that 

 the brain material is a mass of neurons and that the whole 

 nervous system is made up of these same units. The white 

 material is not independent of the gray ; it merely consists of 

 the axis cylinder fibres of the neuron cell bodies of the gray 

 matter. 



The cell bodies of these brain neurons receive all messages 

 brought to them, and are the centers of the thinking pro- 

 cesses. Decisions are made in them and they start impulses 

 outward to any part of the body which they have decided to 

 move or influence. These impulses pass directly downward 

 over an axis cylinder (Fig. 164 g) or nerve fibre, which finally 

 ends somewhere in the cord in a bunch of arborizations, like 

 those already described; Fig. 164 h. Very near these fine 

 endings are the dendrites of another neuron body, i, and into 

 these the descending impulses pass. The long axis cylinder 

 of this last neuron (Fig. 164 /) passes out through the anterior 

 or ventral root of a spinal nerve, and extends directly, with 

 no further interruptions or "relay neurons", to the special 



