Efferent connections of the periaqueductal gray matter in the cat

Electrolytic lesions were stereotaxically placed in the dorsal and ventral areas of the mesencephalic periaqueductal gray in the cat. A dorsal angular approach was made so that mechanical electrode damage was contralateral to the lesion, or a posterior approach through the fourth ventricle was employed to avoid electrode damage. The brains were sectioned coronally and sagittally and stained with a modifield Nauta‐Gygax or Fink‐Heimer stain. Degeneration from the dorsal lesions was chiefly in a radial pattern to the superior colliculus, inferior colliculus and mesencephalic reticular area. Also, fibers angled out of the dorsal gray, crossed the midline and joined the commissure of the superior colliculus, some taking a ventrolateral course through the colliculi and others running ventrally along the borders of the gray. Degeneration was traced caudally to the cuneiform nucleus and adjacent reticular area. Rostrally, fibers traveled in the dorsal longitudinal fasciculus to the pretectal area, lateral habenular nucleus and finally, the posterior hypothalamic area. Ventral lesions showed the same radial pattern of degeneration and fibers in the superior colliculus commissure. Caudally, fibers could be traced to the cuneiform nucleus, reticular area and inferior olive. The rostral course of fibers in the dorsal longitudinal fasciculus was similar, though there were additional connections with the ventral tegmental area of Tsai, the fields of Forel, and the parafascicular nucleus of the thalamus.