The fasciculus longitudinalis medialis in the lizard Varanus exanthematicus. 1. Interstitiospinal, reticulospinal and vestibulospinal components.

With the horseradish peroxidase (HRP) technique the various descending components of the medial longitudinal fasciculus (flm) have been studied in the lizard Varanus exanthematicus. After wheat germ agglutinin conjugated HRP injections at the spinomedullary border, retrogradely labeled fibers passing via the flm could be traced to various parts of the magnocellular rhombencephalic reticular formation, the descending and ventromedial vestibular nuclei and the interstitial nucleus of the flm. By implanting HRP slow-release gels into the flm the trajectory and site of termination of various components of the flm have been analysed. The interstitiospinal tract passes via the dorsal part of the flm. Reticulospinal fibers arising in the nucleus reticularis superior and nucleus reticularis medius take a position ventral to the interstitiospinal fibers. Vestibulospinal projections via the flm are found in its ventral part and arise mainly in the contralateral ventromedial and descending vestibular nuclei. A strong vestibulocollic projection to cervical motoneurons should be noted. The positional relations of the various fiber components within the flm found in a lower vertebrate such as the lizard Varanus exanthematicus are comparable to those in mammals.