The fourth cranial or trochlear nerve is the smallest of the cranial nerves. It arises at the outer side of the crus cerebri. Its superficial origin is from the superior medullary velum, right behind the testes or posterior pair of corpora quadrigemina. Its deep origin is from a nucleus in the floor of the cerebral aqueduct by the oculomotor nerve. In the substance of the superior medullary velum, it decussates with the opposite fourth cranial nerve. It then winds around the outer side of the crus cerebri and appears at the base of the brain at the anterior border of the pons. It is a somatic motor nerve and supplies the superior oblique muscle of the eye.

Motor

Superior oblique-depresses and abducts eyeballs

Superior orbital fissure

Orbit

Nervus trochlearis

Nerve IV

Nervus trochlearis [IV]