A description of the amygdaloid complex in the rat and cat with observations on intra‐amygdaloid axonal connections

The cytoarchitectonic structure and divisions of the amygdaloid complex are described in the rat and cat, with special reference to the axonal connections of each of the amygdaloid nuclei, and to interspecies variations and similarities. Several intra‐amygdaloid connections are also described, based on autoradiographic experiments with small injections of ³H‐amino acids into the individual nuclei. Although it has probably not been possible to determine all of the intra‐amygdaloid projections from these experiments, the connections which have been shown are very specific. The lateral nucleus projects to the basomedial nucleus, the lateral part of the central nucleus, and the periamygdaloid cortex, while the basolateral nucleus projects only to itself, the medial part of the central nucleus, and the nucleus of the lateral olfactory tract. The basomedial nucleus projects to the cellular layer of the medial nucleus and the amygdalo‐hippocampal area, while the molecular layer of these structures and of the posterior cortical nucleus, receives projections from the periamygdaloid cortex or from the endopiriform cortex. There are also interconnections between the medial and posterior cortical nuclei, and commissural connections between the posterior cortical nuclei of the two sides.