The archistriatum of the pigeon: Organization of afferent and efferent connections

The avian archistriatum, usually considered homologous with the mammalian ‘amygdaloid complex’, was studied in the pigeon by the aid of silver impregnation methods for degenerating nerve fibers and nerve endings. The results obtained suggest a subdivision of the archistriatum into 4 major regions: the archistriatum anterior, intermedium, posterior (with a postero-ventral portion) and mediale. The archistriatum posterior and mediale give rise to descending pathways terminating in the medial and lateral hypothalamus, and may therefore be comparable to the mammalian amygdala. The archistriatum anterior is the only part of the archistriatum receiving afferents from the anterior commissure and from the tractus fronto-archistriatalis, whereas the archistriatum intermedium encompasses most of the terminal fields of the tractus archistriatalis dorsalis. The anterior and intermediate regions appear to be associated mainly with the ‘somatic sensorimotor’ system, since they issue the long ipsi- and contralateral projections of the tractus occipitomesencephalicus to the brain stem and rostral spinal cord. These long descending connections strongly recall the picture of Bagley's bundle of ungulates, a fiber bundle often considered a variant form of part of the pyramidal tract of primates.