"For we all often stumble and fall and offend in many things.
And if anyone does not offend in speech [never says the wrong things], he is a fully developed character and a perfect man, able to control his whole body and to curb his entire nature."
James 3:2 AMP
James says that if we can control our mouths, we can control our bodies.
In verse 3 James gives us more information about the tongue, giving us an example of controlling a horse.
And he also shows us how a ship is controlled.
Putting a bit in a horse's mouth and directing a ship by its rudder are examples of controlling both inside and outside desires.
As we control our mouths, we can control ourselves, inside and out!