This work is based on the microscopic study of 30 trochlear nerve trunks (15 heads). In 17 cases, the trunk arose from two nerve bundles, in 8 cases from one bundle, and for the other 5 nerves, three or four bundles. The mean total length of the trochlear nerve was 86 mm. The nerve may be separated into the 3 following parts: infratentorial, intracavernous, intraorbital. In all 30 cases studied, the first part of the nerve was infratentorial, thus leading us to suggest the term "infratentorial part" for this segment of the nerve. In 27 cases, contact was found with the superior cerebellar artery, in the infratentorial part. In the intracavernous part of ten nerves we found two rami tentorii and in eight cases fibers were exchanged with the ophthalmic nerve. In the orbit, 18 trochlear nerves crossed the posterior ethmoidal artery. 23 trochlear nerves ended on the medial face of the superior oblique muscle. The remaining 7 ended at the superior border of the muscle.