THE CENTRAL NERVOUS SYSTEM. 343 
*tween-brain, in the median line, is a deep fissure, showing that 
here in reality is the termination of the original tube, the 
further extension craniad being due to the pushing forward of 
the lateral outgrowths, or cerebral hemispheres, which extend 
in ventral view a considerable distance further craniad. The 
hemispheres lie also ‘caudad, laterad, and dorsad of the mid- 
brain and ’tween-brain, so that these parts are almost enveloped 
by the hemispheres. The hemispheres and 'tween-brain are 
sometimes taken together as forming a single portion, the fore- 
brain or prosencephalon. 
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 enclos- 
ing the fourth venieicle)s the mesencephalon (enclosing the 
cerebral aqueduct), the diencephalon 
(enclosing the third ventricle), and 
the two cerebral hemispheres, con- 
stituting together the telencéphalon, 
and enclosing the two lateral ven- 
tricles. A plan of the brain, con- 
sidered as a tube enclosing cavities, 
is given in Figs. 139 and 140. Fig. 
139 shows the plan considered as 
ZA | ZX t6 3. 
Fic. 139.—DIAGRAM OF BRAIN Fic. 140. — DIAGRAM OF BRAIN FROM 
FROM ABOVE. THE SIDE. 
I, myelencephalon; 2, metencephalon; 3, mesencephalon; 4. diencephalon; 5, 
telencephalon. @, canal of the spinal cord; 4, fourth ventricle; ¢, cerebral aqueduct 
(aqueduct of Sylvius); ¢, third ventricle; e, lateral ventricle; /, interventricular fora- 
men (or foramen of Monroe); g, lamina terminalis; 4, olfactory bulb. 
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 flax of structure, and omitting all details. In 
the actual conditions many important modifications even of the 
main features of the plan are met with. 
