Anomaly of the Abducens Nerve

An anomaly of interest to both the anatomist and historian of medicine was noted in the course of dissection of the cadaver of a 77-year-old white male. No abducens nerve existed on the right side. Careful examination of both the brain stem and the ocular muscles revealed that in its stead, two distinct branches of the oculomotor nerve supplied the right lateral rectus muscle. When discussing the morphology of the cranial nerves that supply the extra-ocular muscles Neal wrote, "In the great majority of vertebrates the number and nerve relations of the eye muscles remain unchanged. Nature seems to have pursued with regard to them the policy of 'letting well enough alone.'"" Several anomalies of the sixth nerve have been described, however. Whitnall cites the following variations in the origin or course of the nerve to the lateral rectus muscle: origin from the olive; origin by means of two roots which may not unite until they reach the cavernous sinus; origin 8 mm. above the inferior border of the pons; branch received from the otic or pterygopalatine ganglion; entrance into the orbit outside of the common tendinous ring (of Zinn); branch furnished to the superior rectus in addition to the usual supply from the oculomotor; root to the ciliary nerve or ciliary ganglion.! Bremer has described aberrant roots and recurrent branches in the human embryo as well as total absence of the abducens in one 18 mm. embyro whose lateral rectus was supplied by the oculomotor. Bremer states, however, that this embyro was in a poor state of preservation.8 The current textbooks of orbital and ocular anatomy as well as those of ophthalmology mention only one documented case of total absence of the abducens nerve. Many of these authorities cite the name, Generali, yet no reference to a source or date is mentioned. A careful search of the pertinent material, available in the Historical Library of the Yale School of Medicine, revealed that the first mention of Generali's work in English is in the 1876 edition of Quain's Anatomy, but no source is included.'