Wilhelmus J. A. J. Smeets

The Brain Stem

The brain stem (truncus cerebri) comprises the rhombencephalon and the mesencephalon; caudally it grades into the spinal cord while rostrally it borders the diencephalon. In all vertebrates the brain stem harbours the centres of origin and termination of all cranial nerves, except for cranial nerve I. In cartilaginous fish the brain stem contains a fairly well developed reticular formation and a number of relay centres and their associated ascending and descending connections. The cerebellum, although ontogenetically and phylogenetically a derivative of the rostral part of the rhombencephalon, is not included in the brain stem and will be considered separately in Chap. 7. However, the cerebellum and the brain stem are strongly interconnected and many of the cerebellar afferent systems originate, and almost all cerebellar efferent fibres terminate, within the brain stem.

The Overall Histological Pattern

The grey matter of the spinal cord consists of two lateral, wing-like expansions in cyclostomes, whereas in chondrichthyans it is differentiated into distinct dorsal and ventral horns, an arrangement similar to that found in all other vertebrates. The spinal grey matter in cartilaginous fishes contains cells of a wide size range, although in adults the larger elements are mainly confined to the ventral horns. Myelinated fibres are absent from the cyclostomes but the white matter of the chondrichthyan cord comprises numerous well-myelinated fibres.

The Tectum Mesencephali

In the majority of cartilaginous fishes the tectum mesencephali is strongly developed and differentiated into two bilateral lobes, which surround expansions of the ventricular cavity. In the species studied this cavity is most spacious in Squalus and Hydrolagus, but much reduced in Raja.