It is necessary, for example, that the heart, the lungs, and many other organs of the body should keep at work all the time.

If the large brain had to attend to all of these different kinds of work besides thinking about what we see, hear, and read, and other things which we do, it would have too much work to do, and would not be able to do it all well.

Besides, the large brain sometimes falls asleep.

So the large brain lets the little brain do the kinds of work which have to be attended to all the time, and the little brain keeps steadily at work when we are asleep as well as when we are awake.
~21.

What the Spinal Cord Does.~--If you tickle a person's foot when he is asleep, he will pull it up just as he would if he were awake, only not quite so quickly.