GENERAL INSPECTION OF THE BRAIN.
Let us first examine the dorsal surface of the brain. This is strongly arched in the sagittal as well as in the frontal direction—fades convexa cerebri (Fig. 14). A deep median vertical cleft (fissura longitudinalis cerebri) divides the whole into two symmetrical halves, the hemispheres of the end-brain. On probing to the bottom of the fissure, one learns that the separation is not complete, since in the middle of the cleft the two halves are united by a broad zontal commissure, the corpus callosum. In front of the latter, the fissure passes to the ventral surface of the brain ; behind the commissure, the fissure likewise penetrates deeply and ends in a large transverse cleft (fissura transversa cerebri), which separates the hemispheres from the subjacent bellum. The surface of the hemispheres hibits clefts and furrows of varying depths and the intervening convolutions.
The ventral surface of the brain, known as the basis cerebri, is much more com plexly modelled. In the first place, we per ceive to what extent the hemispheres occupy also the base of the brain. In the anterior part, the fissura longitudinalis cerebri runs in the mid-line, as far backward as an X Fm. te.—Brain viewed from above; frontal pole below. shaped structure, the chiasma opticum. On folding the chiasma slightly backward, one sees a thin gray and easily torn lamella stretching from the front border of the chiasma into the depth of the fissura longitudi nalis cerebri ; this is the lamina ferminalis. Forwards from the chiasma lead the nerves of sight (nervi Odd), while posteriorly and laterally, on each side, extends the visual path, the tractus optici. Lateral from the chiasma and the optic tract lies a gray field, penetrated by larger and smaller openings, the substantia perforata anterior. The anterior boundary of this field presents a triangular area, the trigonum olfactorium, from whose front point a narrow white-stripe, the tractus olfactorius, leads forward to end in the broadened terminal bulbus olfactorius. The olfactory nerve-fibres (fila olfactoria) extend from the ventral surface of the bulb as delicate white thread-like strands, that have been torn in removing the brain. Bulbus olfactorius, tractus olfactorius, trigonum olfactorium, substantia perforata anterior are all parts of the rhinencephalon. These will be more closely considered in connection with the rhinencephalon.
Behind the chiasma opticum rises a gray hump, the tuber cinereum, that tapers to the infundibulum bearing a bean-shaped gray body, the hypophysis or pituitary body.
The hypophysis lies in the sella turcica of the body of the sphenoid and may readily become separated in consequence of the tearing of the thin infundibulum, when the brain is taken out, so that only the conical pointed part of the infundibulum presents, while the hypophysis remains within the sella turcica. Laterally, the tuber cine reum is bounded by the tractus optici, whose further course is over the forward and outwardly coursing cerebral stalks, the pedunculi cerebri, and then to pass deeply. Behind the tuber cinereum, rise two white pyriform structures, the corpora mamil laria or candicantia. Behind these and between the pedunculi cerebri lies the fossa interfieduncularis, which is prolonged backward into the recessus posterior and forward into the recessus anterior. The floor of this depression is formed by the substantia perforata posterior, a gray surface modelled by numerous apertures and divided into halves by a median furrow. Towards the cerebral peduncle it is bounded by a groove, the sulcus nervi oculomotorii, from which emerge the fibres of the oculo motor nerve.
Behind these deeply sunken structures, appears a white, broad, transverse bridge, the pons Varolii, which in front and behind is sharply bounded, in the middle is impressed by a broad median furrow, the sulcus basilaris, and at the sides narrows and then extends laterally and backward to sink into the cerebellum. Behind the pons lies the tapering bulb, the medulla oblongata, which is prolonged into the spinal cord. It presents the median longitudinal furrow, the fissura mediana anterior, that is bounded on each side by a white strand, the pyramid or pyramis. Beyond the pyramidal tract, the sulcus lateralis anterior extends as a shallow groove, beyond which, in turn, lies an elongated egg-shaped elevation, the Oliva or olivary eminence. The medulla covers the median part of the cerebellum, occupying a broad furrow, known as the vallecula cerebelli, behind which appears the strongly arched ventral surface of the cerebellum. A deep median cleft, the incisura cerebelli posterior, separates the two halves of the little brain, the hemisphaeria cerebelli, which exhibit numerous, more or less parallel narrow tracts, the folia. On slightly raising the cerebellum, the fissura transversa cerebri appears as a deep cross cleft, separating the cerebellum from the cerebrum and opening into the fissura longitudinalis cerebri.