Ansa and fasciculus lenticularis of carnivora

The fiber systems of the globus pallidus and the nucleus entopenduncularis were studied in normal brains of the cat and of the brown bear, stained by the Häggqvist method. The globus pallidus in both animals is traversed by bundles of the internal capsule. No division into external and internal segments can be made. Pallidal fibers do not form fascicles resembling the ansa or fasciculus lenticularis of primates. They are thicker in the bear (3.5–5 μ) than in the cat (2–3.5 μ). The entopeduncular nucleus in the bear is prominent and lies close to the globus pallidus. Fibers arising in its grey network are rather thick (5–6.5 μ) and at its rostral level assemble to form the ansa lenticularis. Subsequently they traverse the internal capsule in slender ribbons to assemble again in the fasciculus lenticularis. Corresponding fibers in the cat are thinner (3–4.5 μ) and do not form distinct fascicles. Their course resembles that in the bear but they reach Forel's field rostral to the subthalamic nucleus. The fasciculus thalamicus and fasciculus pallidohypothalamicus are composed of these fibers. Edinger's comb system is well developed in both animals. Its dorsal part consists of pallidal fibers, representing the fasciculus pallido‐subthalamicus. The findings furnish further evidence that the nucleus entopeduncularis actually represents the internal pallidal segment of the primates.