General Considerations and Divisions.The brain, is
contained within the cranium, and constitutes the upper, greatly expanded
part of the central nervous system. In its early embryonic condition
it consists of three hollow vesicles, termed the hind-brain
or rhombencephalon, the mid-brain or mesencephalon,
and the fore-brain or prosencephalon; and the parts
derived from each of these can be recognized in the adult (Fig. 90677). Thus in the process of development the wall of the hind-brain
undergoes modification to form the medulla oblongata, the pons, and
cerebellum, while its cavity is expanded to form the fourth ventricle.
The mid-brain forms only a small part of the adult brain; its cavity
becomes the cerebral aqueduct (aqueduct of
Sylvius), which serves as a tubular communication between the
third and fourth ventricles; while its walls are thickened to form
the corpora quadrigemina and cerebral peduncles. The fore-brain undergoes
great modification: its anterior part or telencephalon expands
laterally in the form of two hollow vesicles, the cavities of which
become the lateral ventricles, while the surrounding walls form the
cerebral hemispheres and their commissures; the cavity of the posterior
part or diencephalon forms the greater part of the third ventricle,
and from its walls are developed most of the structures which bound
that cavity. |
