He struck him on the forehead with his cudgel. (Not on his forehead)
With reference to parts of the body and following a preposition (On, By, In etc) “The” is used instead of a possessive word (My, His, Her, Your, Their.)
They pulled her by the hair.
I shook him by the hand.
She looked me in the eye.
He hit her on the head.
When the reference is a general one and no specific person is indicated the article “The” is used.
Drugs can dull the brain and affect the heart.
The use of “The” is preferred when the reference is to unpleasant conditions of the body (Aches, Pains, Wounds)
I have a cold in the head.
She’d a pain in the shoulder.
Smoking causes cancer on the lung.