Projections from the amygdaloid complex to the cerebral cortex and thalamus in the rat and cat

Projections asre described from the basolateral, lateral and anterior cortical nuclei of the amygdaloid complex, and from the prepiriform cortex, to several discrete areas of the cerebral cortex in the rat and cat and to th mediodorsasl thalamic nucleus in the rat. These projections are very well‐defined in their origin, and in their area and laminar pattern of termination. The basolateral amygdaloid nucleus can be divided into anterior and posterior divisions, based on cytoarchitectonic and connectional distinctions. In both the rat and cat the posterior division projects to the prelimbic area (area32) and the infralimbic area (area 25) on the medial surface of the hemisphere. The anterior division projects more lightly to these areas, but also sends fibers to the dorsal and posterior agrangular insular areas and the perirhinal area on the lateral surface. Furtheremore, in the cat the perirhinal area is divided into two areas(area 35 and 36) and the anterior division projects to both of these and also to a ventral part of the grangular insular area; this last area is adjacent to, but separate from the auditory insular area and the second cortical taste area. In most of these areas, the fibers from the basolateral nucleus terminate predominantly in two bands: one in the deep part of layer I and layer II, and a heavier band in layer V (in the rat) or layers V and VI (in the cat). The lateral amygdaloid nucleus projects heavily to the perirhinal area, and also to the posterior agranular insular area. These fibers terminate predominantly in the midle layers of the cortex, although the cellular lamination in these two areas is relatively indistinct. The anterior cortical amygdaloid nucleus and the prepiriform cortex both project to th infralimbic area and the ventral agranular insular area, and the anterior cortical nucleus also projects to the posterior agranular area and the perirhinal area. In all of these areas, the fibers from these olfactory‐relasted structures terminate in the middle of layer I. In the rat, the two divisions of the basolateral nucleus also project to the medial segment of the mediodorsasl thalamic nucleus, with the anterior division projecting mainly to the posterior part of this segment and the posterior division to the anterior part. The endopiriform nucleus, deep to the prepiriform cortex, projects to the central segment of the mediodorsasl nucleus, since little or no projection could be demonstrated from the prepiriform cortex itself. Projections to the mediodorsal nucleus have not been found in the cat.