Comparative anatomy of the cerebral convolutions: The great limbic lobe and the limbic fissure in the mammalian series

The cerebral hemisphere is made up of a very heterogeneous central mass in which there are groupings of extremely diverse components and of an external layer that I along with Burdach call the mantle of the hemisphere. The folds in the mantle are what form the convolutions. In lissencephalic animals (without convolutions), the mantle is smooth over the greatest extent of its area; in gyrencephalic animals, the mantle is composed of a set of convolutions. The mantle completely envelops the central mass or body of the hemisphere, with the exception of a very well-defined area visible on the medial surface after the hemisphere has been isolated by a medial cut and the cerebral peduncle has then been severed. This region is roughly oval in shape and is bordered above and in front by the groove of the corpus callosum and below and in back by the great cleft of Bichat. Foville and Gratiolet have referred to this region as the great window of the hemisphere because a large opening can be seen above, behind, and below the optic layer and peduncle; this opening leads to the cavity of the lateral ventricle. A different opening, the foramen of Monro, is also visible in this area; however, these two openings form only a minimal part of the region in question. This region essentially includes the cross-section of: the corpus callosum, the transparent septum, the anterior portion of the fornix, the commissures, the cerebral peduncle, the floor of the third ventricle, the lateral wall of the third ventricle, the edge of the dentate body along the inferior margin of the cleft of Bichat, and the cleft of Bichat itself, which leads into the lateral ventricle. All of the convergent and divergent fibers that enter or exit the hemisphere pass through this area. The portion of the medial surface that is not covered by the mantle is not an opening, but in a sense it is the doorway for entering and exiting the hemisphere. I will henceforth call it the threshold of the hemisphere; this can be described in Latin nomenclature, very frequently used in other countries, by the word limen. The threshold of the hemisphere is surrounded on all sides by the mantle, which forms an edge around it that can be compared with the circular edge at the opening of a purse. I therefore call this edge the limbus of the hemisphere (edge), and I call the convolution that forms it the limbic convolution. This large convolution is composed of two arcs. The complete length of the superior arc runs along the convex surface of the corpus callosum; the inferior arc follows along the cleft of Bichat up to the tip of the temporal lobe. The superior arc is called the convolution of the corpus callosum, whereas the inferior arc is called the hippocampal convolution or the uncinate convolution and constitutes the final convolution of the temporal lobe. The usual distinction that is made between these two arcs is motivated to some extent by their locations in two different regions and also by the dividing line formed between them by the extension of the calcarine fissure (fissure of the hippocampus