The anatomy of the brain of the bottlenose dolphin (Tursiops truncatus). Rhinic lobe (Rhinencephalon): The archicortex.

The hippocampal formation or archicortical division of the rhinecephalon of the bottlenose dolphin, Tursiops truncatus, is described from the standpoint of its gross topographic relations and cytoarchitecture. A feature of the dolphin brain, which lacks olfactory bulbs and peduncles, is the striking reduction of the archicortical relative to the paleocortical formations. The small, poorly developed archicortex covered by massive epihippocampal portions of the hemispheres (parietal and temporal lobes), appears greatly reduced relative to the large, well developed olfactory lobes which are covered by small epistriatal portions of the hemispheres (orbital lobes). The archicortex exhibits three junctional zones with the paleocortex, two laterally in the unci and one anteriorly in the septal area. Despite the small size of the hippocampal formations, the general topographic disposition of its cytoarchitectonic areas and their cellular organization in Tursiops have many features that are similar to those in other placental mammals. The archicortex is subdivisible into four major sectors: temporal, retrosplenial, supracallosal and subcallosal. With the exception of the temporal sector, cytoarchitectonic areas of the other sectors are variously attenuated and poorly differentiated, particularly the dentate area and the hippocampal areas H5 and H4. Here, the dentate area and hippocampal areas H5 and H4 which are present along the paradentate bank of the hippocampal sulcus, extend to the level of the oblique sulcus of the parahippocampal gyrus and then disappear. Hippocampal areas H3, H2 and H1 are also clear in the floor and along the parahippocampal bank of the hippocampal sulcus in the temporal sector. These areas are less definable as they extend beyond the oblique sulcus into the retrosplenial sector and are difficult to recognize as distinct areas in the supracallosal and subcallosal sectors of the archicortex. The archicortex is demarcated bilaterally from limbic formations in the border of the hemisphere by segments of the rhinic cleft which are very clear. Equally clear is the cytoarchitectonic demarcation of the archicortex from the neocortex in the border (limbus) of each hemisphere, i.e., where the subiculum abuts against the presubiculum. The subicular area, best expressed in the temporal sector, extends anteriorly over the corpus callosum to the subcallosal gyrus and, throughout its extent from the uncal to the septal junction, is clearly demarcated from limbic neocortex by a transition zone characterized by archicortical cells merging with cells in the deep layer of the bordering neocortex. Overall, the archicortical formations of the dolphin and other whale brains we have examined exhibit many regional peculiarities that we have described, both grossly and architectonically, with emphasis on the comparative anatomical approach.