_I_-_XII_, the twelve cranial nerves in order; _I_, olfactory bulb; _II_, N. opticus; _III_, N. oculomotorius; _IV_, N. trochlearis; _V_, N. trigeminus (1, mandibular nerve; 2, maxillary nerve; 3, ophthalmic nerve; 4, sensory root); _VI_, N. abducens; _VII_, N. facialis; _VIII_, N. acusticus; _IX_, N. glossopharyngeus; _X_, N. vagus; _XI_, N. accessorius; _XII_, N. hypoglossus. _a_, tractus olfactorius; _b_, substantia perforata anterior; _c_, optic chiasma; _d_, infundibulum; _e_, hypophysis; _f_, lobus pyriformis, or tractus postrhinalis; _g_, pedunculi cerebri; _h_, substantia perforata posterior; _i_, pons; _j_, sulcus basilaris of pons; _k_, semilunar (or Gasserian) ganglion; _l_, trapezium; _m_, area ovalis; _n_, area elliptica; _o_, pyramidal tracts; _p_, anterior or ventral fissure; _q_, foramen cæcum; _r_, anterior lateral sulcus; _s_, first cervical nerve.] Just craniad of the midbrain is a small rather irregular area, bounded craniad by the band-like =optic chiasma= (_c_), from which arise the optic nerves (_II_), and showing caudad of this band a rounded irregular projection (_e_). This is a fourth division of the brain which is likewise covered dorsad by the hemispheres; it is the ’tween-brain or =diencephalon=. Its cavity is known as the third ventricle. Just craniad of the ’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 midbrain 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 forebrain or =prosencephalon=. [Illustration: FIG. 139.--DIAGRAM OF BRAIN FROM ABOVE. FIG. 140.--DIAGRAM OF BRAIN FROM THE SIDE. 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.]