Pathogenesis of Paralysis of the Third Cranial Nerve

Paralysis of the third cranial nerve presents a discouraging problem from a clinical standpoint, for too often, even after intensive investigation, the cause remains obscure.1Even when a cause is found, the exact manner in which conduction through the nerve is interrupted remains uncertain. In order to clarify the mechanism by which the third cranial nerve is interrupted and to establish the site of involvement in various diseases, we examined the brains of a number of patients seen at the Mayo Clinic who had exhibited impaired function of the nerve during life. We shall present a few representative examples of the more frequent causes of third-nerve paralysis. Anatomic Aspects To comprehend the pathogenesis of paralysis of the third cranial nerve, complete cognizance of its course and relationships is necessary. The oculomotor nerves extend from the midbrain into the orbits (Fig. 1a). They leave the midbrain between the cerebral