The medial compartment is much larger, and protrudes more caudally and ventrally. Each have NS and WDR neurons but terminate either caudally or rostrally respectively. V3 neurons are excitatory commissural interneurons that extend caudally projecting primary axons. In type I the coccyx is curved anteriorly with its apex facing downward and caudally. Its function is to move the tongue caudally. This process points dorsally and caudally from the junction of the laminae. They extend quite far caudally into the abdomen. Its function is also to move the tongue caudally. Approximate boundaries are the cingulate sulcus dorsally and the parieto-occipital sulcus caudally. Placoid scales have a pointed tip that is oriented caudally and are homologous to teeth. The diencephalon expands more evenly, but caudally of the diencephalon the canal remains narrow. Motor axons leaving the abducens nucleus run ventrally and caudally through the pons. The spinal cord is continuous with the brain and lies caudally to the brain. For a short time, the neural tube is open both cranially and caudally. The two hemicords usually reunite caudally to the cleft.