An experimental silver study of the ascending projections of the central gray substance and adjacent tegmentum in the rat with observations in the cat

Abstract The ascending projections of the central gray substance and adjacent tegmentum in the rat and cat were studied with the Fink‐Heimer (1967) modification of the Nauta method. Following lesions of the circumaqueductal gray, degeneration was traced along the dorsal longitudinal fasciculus of Schütz, a reciprocal association pathway between the central gray and the hypothalamus, into the dorsal and posterior hypothalamus, midline and intralaminar regions of the thalamus, and the zona incerta of the subthalamus. Another group of fibers travelled ventrally from the central grey to reach the ventral tegmental area of Tsai; some of these fibers continued rostrally to terminate in the lateral hypothalamus. Other fibers distributed to the midbrain tegmentum as components of Weisschedel's radiatio grisea tegmenti. Lesions which extended into the adjacent tegmentum produced a larger quantity of degeneration than did lesions confined to the central gray, but the patterns of degeneration were similar in both cases. The ascending projections of the central gray in the rat and cat were found to have similar distributions but were slightly more massive in the cat. No evidence of fiber degeneration was detected in the ventromedial hypothalamus. Negative results, however, must be interpreted with caution, for they may be due to an excessively small caliber of axons and/or peculiar modes of axon termination in hypothalamic tissue.