The hippocampus, including the dentate gyrus, has the shape of a curved tube, which has been compared to a seahorse, and a ram's horn (Cornu Ammonis). Its abbreviation CA is used in naming the hippocampal subfields CA1, CA2, CA3, and CA4. It can be distinguished as an area where the cortex narrows into a single layer of densely packed pyramidal neurons, which curl into a tight U shape. One edge of the "U," – CA4, is embedded into the backward-facing, flexed dentate gyrus. The hippocampus is described as having an anterior and posterior part (in primates) or a ventral and dorsal part in other animals. Both parts are of similar composition but belong to different neural circuits. In the rat, the two hippocampi resemble a pair of bananas, joined at the stems by the commissure of fornix (also called the hippocampal commissure). In primates, the part of the hippocampus at the bottom, near the base of the temporal lobe, is much broader than the part at the top. This means that in cross-section the hippocampus can show a number of different shapes, depending on the angle and location of the cut.