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The trochlear nerve has quite a complex origin.

It starts from its nucleus in the mesencephalon, from there it turns dorsal and crosses the midline in the velum medullare superius. Just below the inferior colliculus the nerve exits the brain stam, it goes ventralwards around the pedunculus cerebellaris superior and the mesencephalon in the cisterna ambiens. Under the incisura tentorii the nerve goes forwards and penetrates the dura mater in the sinus cavernosus.

As you can see, quite complex, I can't see this in the picture on this page. So if anyone could add a decent picture, or a few pictures, which shows this all that would be great.