Cavernous portion of the trochlear nerve with special reference to its site of entrance.

In a study of 80 cavernous sinuses in 40 cadavers, the trochlear nerve entered the cavernous sinus in 87.5% of cases before the crossing, in 7.5% at the crossing and in 5.0% after the crossing of the free and the attached margins of the tentorium cerebelli. In 77.5% of specimens the trochlear nerve showed a marked bend with flattening at the site of its entrance into the cavernous sinus. The nerve ran between the superficial and deep layers of dura, partly between them, in the thickness of the deeper layer, or deep to the deep layer but adherent to it. These findings do not conform with the description of its course by other workers in the past. In 72.5% of specimens the size of the trochlear nerve was larger during its course in the cavernous sinuses but in 20.0% of specimens such an increase in thickness was noted even in the posterior cranial fossa. Only in one specimen was the trochlear nerve adherent to the ophthalmic division of the trigeminal nerve. Cases showing the trochlear nerve entering the cavernous sinus after the crossing of the two margins of tentorium cerebelli and the splitting and branching of the trochlear nerve in the cavernous sinus have not hitherto been reported.