1, myelencephalon; 2, metencephalon; 3, mesencephalon; 4,
diencephalon; 5, telencephalon. _a_, canal of the spinal cord; _b_,
fourth ventricle; _c_, cerebral aqueduct (aqueduct of Sylvius); _d_,
third ventricle; _e_, lateral ventricle; _f_, interventricular foramen
(or foramen of Monroe); _g_, lamina terminalis; _h_, olfactory bulb.]
There are thus altogether five principal subdivisions of the brain, each
enclosing a portion of the central cavity. These are the
=myelencephalon=, the =metencephalon= (these two enclosing the =fourth=
ventricle), the =mesencephalon= (enclosing the =cerebral aqueduct=), the
=diencephalon= (enclosing the =third= ventricle), and the two cerebral
hemispheres, constituting together the =telencephalon=, and enclosing
the two =lateral ventricles=. A plan of the brain, considered as a tube
enclosing cavities, is given in Figs. 139 and 140. Fig. 139 shows the
plan considered as viewed from the dorsal side, Fig. 140 in a lateral
view. These figures are of course pure diagrams, showing only what may
be called the _plan_ of structure, and omitting all details. In the
actual conditions many important modifications even of the main
features of the plan are met with.
In the following description the parts of the brain will be taken up in
this order:
A. RHOMBENCEPHALON (primitive hindbrain).
1. _Myelencephalon._
2. _Metencephalon._
B. MESENCEPHALON (primitive midbrain).
3. _Mesencephalon._
C. PROSENCEPHALON (primitive forebrain).
4. _Diencephalon._
5. _Telencephalon._