Gross Anatomy

Abstract

The brain stem as defined here comprises the mesencephalon and the rhombencephalon. As in most reptiles, the brain stem of Varanus exanthematicus is rather strongly curved (Figs. 2, 17). The fossa rhomboidea is covered by a richly vascularized tela choroidea which is attached to the dorsal aspect of the alar plate and to the cerebellum. The cerebellum in Varanus exanthematicus is tilted forward, i.e., everted, resulting in a dorsal position of part of the granular layer (see Bangma 1983; Bangma et al. 1984; ten Donkelaar and Bangma 1987). Its base forms the roof of the rostralmost part of the fourth ventricle, and continues rostrally into the velum medullare anterius. Laterally, the cerebellar peduncles interconnect the cerebellum and the brain stem. Just caudal to the bilateral domes of the large tectum mesencephali the two tori semicirculares are found, hidden under the corpus cerebelli (see Fig. 17). The most rostral parts of the cerebellum and of the rhombencephalon constitute a narrowed part of the brain stem, which is known as the isthmus rhombencephali (see Figs. 7, 8). Here, both trochlear nerves leave the brain stem at its dorsal side. Further rostrally, the fourth ventricle narrows considerably, continues diamond-shaped over a very short distance as the aqueductus cerebri (see Figs. 6, 17, 20), and widens again beneath the tectum mesencephali, as the caudal part of the third ventricle, where both lateral recesses extend into the domes of the tectum (see Figs. 4, 5, 20). Rostral to the level where the oculomotor nerves leave the tegmentum mesencephali, the mesodiencephalic junction is marked at the ventral side by the two small, dome-shaped swellings of the mamillary bodies, rostral to which the hypothalamus protrudes ventrally. This part of the diencephalon, which is continuous with the mesencephalic tegmentum, is covered dorsally by the rostral part of the tectum mesencephali (Figs. 2, 17).