REFLEX ACTION 
469 
beginning and grows in an ordered manner. Not only is 
this true in man, but also in the frog and fish. The tissue 
of the embryo, which is to grow into brain and spinal cord, 
gradually changes until the adult parts are formed. Dur¬ 
ing this early period of growth, the nerve cells send out 
processes which become nerve fibers, so 
that at birth the nervous system is ready 
to go to work. Indeed, nearly all the nerve 
cells which the human being is ever to use 
are made before birth. These cells gradu¬ 
ally become more active and the different 
parts of the brain work more perfectly as 
we go through the periods of childhood, 
youth, and maturity. The brain becomes 
a more perfect working organ by making 
the brain cells do their specific work over 
and over and over, until each group of cells 
can be relied upon to do a definite thing. 
363. Reflex Action. — Reflex action is 
the simplest form of nervous activity in 
man. For example, when the finger is 
placed on a hot stove and suddenly with¬ 
drawn the following actions take place. 
The heat stimulus affects the nerve end¬ 
ings in the finger and that stimulus is Figure 414— Nerve 
carried to the spinal cord. If this were Cells. 
all that occurred, the finger would burn, Stained to bring 
because this stimulus and the nerve fibers out the mmute P arts 
. . J . in addition to the 
over which it travels have no control over nuc i eU s. 
the muscles. The removal of the finger 
is brought about by another set of nerve cells — the cells 
which have their fibers ending in the muscle of the hand 
and arm. All these changes take place involuntarily, and 
the reaction to the stimulus is known as reflex action. 
Specific names are used in describing these several changes; 
