Structure and nerve cell organisation in the cerebral cortex of the dolphin Stenella coeruleoalba a Golgi study. With special attention to the primary auditory area.

Cytoarchitectonic studies of the primary acoustic area, primary visual area and associative cortex of the convexity of the dolphin Stenella coeruleoalba using the Golgi method revealed a thick molecular layer, an accentuated Layer II, poor stratification of the underlying laminae and the absence of an identifiable Layer IV, as well as little areal variability. The morphology and distribution of nerve cells in the three regions, resembled those already known in other mammals. Distinctive cellular types were, however, present, such as extraverted pyramidal neurons in Layer II and giant multipolar and bi-tufted cells with smooth, beaded dendrites and extended, generalized axonal arborizations in Layers III and V. Spiny stellate cells were located in the inner region of Layer III and in Layer V; these cells exhibited a long descending axon and many recurrent and oblique collaterals. Although the basic structure of the cerebral cortex is thus similar to that observed in insectivores and chiropterids, dolphins have dramatically increased numbers of cerebral convolutions exceeding those found in most advanced terrestrial mammals.