OCR Text |
Show 1876.] PROF. T. H. HUXLEY ON CERATODUS FORSTERI. 29 The brain of Ceratodus nearly fills the cranial cavity, the interspace left between it and the walls of the latter being, to a great extent, occupied by a peculiar reticulated tissue. The medulla oblongata is long and slender, but widens, anteriorly, in the region of the fourth ventricle. This cavity is arched over by a tela vasculosa (Tv, figs. 1 and 2), separated into two lateral convexities by a slight median depression. In front, each convexity is continued into a blind rounded cornu, which lies over the origin of the fifth and seventh auditory nerves. The two cornua diverge, and the cerebellum is continued backwards as a triangular lamella between them. The cerebellum is relatively very small, being represented merely by the thin arched roof of the anterior part of the fourth ventricle. In front of it is a rounded elevation, obscurely divided by a longitudinal depression into two. These are the only indications of the optic lobes of the mid brain, or mesencephalon. In front of this is the fore brain. The hinder division (or thalamencephalon) is narrower than the mesencephalon, and passes below into the infundibulum, which terminates in the large, oval, flattened, pituitary body (hypophysis). This is lodged in an excavation of the cartilaginous floor of the skull representing the sella turcica. Dorsally, the thalamencephalon is continued upwards and forwards into the subcylindrical peduncle of the pineal gland (epiphysis or conarium). This is a large heart-shaped body, the base of which is turned downwards and backwards. The apex is connected by fibrous and vascular tissue with a depression in the cartilaginous roof of the skull. Next follows the largest division of the brain, answering to the cerebral hemispheres and the olfactory lobes. The former are represented by a single oval lobus communis, the middle area of the roof of which is occupied by a broad thick tela vasculosa. From the anterior dorsal aspect of the prosencephalon proceeds, on each side, the large olfactory lobe, which, flattening in front, and becoming obliquely truncated, terminates against the posterior walls of the olfactory sacs. A backward prolongation of the mesethmoid cartilage separates the two lobes. In the lateral view of the brain (fig. 2) the manner in which the olfactory lobes take their origin from the dorsal aspect of the prosencephalon is well seen. The short and thick infundibulum, terminating in the flattened oval pituitary body, and the origins of the second, third, fifth, eighth, ninth, and tenth nerves are shown. The ventral view (fig. 3 ) displays the origins of the small optic nerves (II) which arise close together from the floor of the thalamencephalon. Whether any chiasma exists could not be ascertained. In the middle line of its ventral aspect the prosencephalon presents a deep longitudinal fissure, lodging anterior cerebral arteries. The walls of the fissure have been separated by turning the left division of the prosencephalon to one side; and the floor of the ventricle (v.c), which is contained in the prosencephalon, has been removed. W h e n the dorsal wall of the brain was cautiously laid open by a median section, it was found to contain one large ventricular cavity |