פירוש על קידושין 49:14

The dispute between Rabbi and the sages is only about a person who is sprinkled on his tongue. According to Rabbi sprinkling on the tongue purifies, just as being touched on the tongue by a sheretz defiles. Rabbi reads the verse as connecting sprinkling with the source of impurity—if the water lands on a part of the body that can be defiled, the person is pure.
The other sages say that sprinkling on the tongue is compared with immersing. Just as a tongue need not be immersed for it is not considered an external organ, so too if the person is sprinkled on the tongue, he is not pure.