The Brain said: I control all parts of the body so I should be boss
The Legs said: I carry you where you want to go so I should be boss
The Stomach said: I digest all the food to provide energy so I should be boss
The Eyes said: Without me you would be helpless so I should be boss
The Heart said: I pump the blood around the body so I should be boss
The Lungs said: I provide the oxygen the body needs so I should be boss
Then the rectum applied for the job as boss, to which all the other parts of the body laughed so much that the rectum got mad, closed up and refused to work.
After a few days...
The brain went foggy, the legs got wobbly, the stomach was full, the eyes got crossed and unable to see the heart and lungs struggled to keep up. so they had to concede and agree that the rectum was the boss.
All goes to show that you don't need a brain to be a boss....
Just an Asshole.
Of course that's just a joke and the real answer is that the body would not work at all if all the parts do not work together to serve a common goal.