The insular formations of the dolphin brain: Quantitative cytoarchitectonic studies of the insular component of the limbic lobe

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Abstract
The large insula of the bottlenose dolphin consists of radial gyri arising, in fanlike fashion, from the transverse insular gyrus, and is covered completely by the frontal, parietal, and temporal opercula. On cytoarchitectonic grounds, the dolphin insula is divided into anterior, middle, and posterior sectors that may be the equivalent of the three similar sectors present in the primate insula. Rostrocaudally, these sectors become increasingly more homogeneous and less laminated. Within each sector progressive differentiation occurs in the direction of the circular sulcus. A transitional cortex, the peripaleocortex in the transverse insular gyrus, is interposed between the prepiriform and the periamygdalar cortex and the proisocortex of the insula proper. This peripaleocortex consists of outer and inner cellular strata separated by a hypocellular lamina dissecans. The outer cell stratum is continuous with layers II and III of the insular proisocortex; the more prominent inner stratum is continuous with proisocortical layers V and VI; the intervening lamina dissecans becomes partially filled, mostly with modified pyramidal cells of medium size that may constitute an incipient layer IV. A band of myelinated fibers corresponding to the external band of Baillarger is found within the lamina dissecans. The anterior insular sector is characterized by distinct lamination and a well‐defined, ribbonlike layer Va. In the middle sector, the cortex is internodense and lamination is less clear. The posterior sector is even less laminated and tends to be externodense. Within each sector, lamination becomes clearer in the direction of the circular sulcus. Furthermore, the rostrocaudal architectonic changes suggest a possible transition from a motor‐type to a sensory‐type cortex. Beyond the insula, the architecture of the opercular cortices reflect, in turn, the influences of the insular sectors.