Two Intracranial Ligaments Supporting the Brain of the Brachiopterygian Fish Polypterus senegalus

Two hitherto unknown ligaments that assist in holding the brain in position occur in the brachiopterygian fish Polypterus senegalus. These rounded fibrous cords, placed one behind the other, are called ligamentum intracraniale transversum and ligamentum intracraniale obliquum. The former spans the orbitotemporal division of the cranial cavity and passes through the velum transversum. The latter, which is paired, extends from the roof to the sidewall of the endocranium and runs in the rhombomesencephalic sulcus in company with the trochlear nerve. It is argued that these intracranial ligments, like the spinal denticulate ligaments, are derivatives of a pair of fibrous bands which in early craniate phylogeny extended throughout the length of the neuraxial meninx and enhanced its tensile strength. It is also argued that at least the transverse intracranial ligament was already in existence some 350 million years ago.