|Henry Gray (18251861). Anatomy of the Human Body. 1918.|
|floccular fissure; when the two lateral walls fuse, the right and left floccular fissures join in the middle line and their central part becomes the post-nodular fissure.|
| On the ventricular surface of the cerebellar lamina a transverse furrow, the incisura fastigii, appears, and deepens to form the tent-like recess of the roof of the fourth ventricle. The rudiment of the cerebellum at first projects in a dorsal direction; but, by the backward growth of the cerebrum, it is folded downward and somewhat flattened, and the thin roof-plate of the fourth ventricle, originally continuous with the posterior border of the cerebellum, is projected inward toward the cavity of the ventricle.|
The Mid-brain or Mesencephalon.The mid-brain (Figs. 650 to 654) exists for a time as a thin-walled cavity of some size, and is separated from the isthmus rhombencephali behind, and from the fore-brain in front, by slight constrictions. Its cavity becomes relatively reduced in diameter, and forms the cerebral aqueduct of the adult brain. Its basal laminæ increase in thickness to form the cerebral peduncles, which are at first of small size, but rapidly enlarge after the fourth month. The neuroblasts of these laminæ are grouped in relation to the sides and floor of the cerebral aqueduct, and constitute the nuclei of the oculomotor and trochlear nerves, and of the mesencephalic root of the trigeminal nerve. By a similar thickening process its alar laminæ are developed into the quadrigeminal lamina. The dorsal part of the wall for a time undergoes expansion, and presents an internal median furrow and a corresponding external ridge; these, however, disappear, and the latter is replaced by a groove. Subsequently two oblique furrows extend medialward and backward, and the thickened lamina is thus subdivided into the superior and inferior colliculi.
FIG. 652 Exterior of brain of human embryo of five weeks. (From model by His.) (See enlarged image)
The Fore-brain or Prosencephalon.A transverse section of the early fore-brain shows the same parts as are displayed in similar sections of the medulla spinalis and medulla oblongata, viz., a pair of thick lateral walls connected by thin floor-and roof-plates. Moreover, each lateral wall exhibits a division into a dorsal or alar and a ventral or basal lamina separated internally by a furrow termed the sulcus