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 ventricle), the mesencephalon (enclosing the 



cerebral aqueduct), the diencephalon 

 (enclosing the third ventricle), and 

 the two cerebral hemispheres, con- 

 stituting together the telencephalon, 

 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 



FIG. 139. DIAGRAM OF BRAIN 

 FROM ABOVE. 



140. DIAGRAM OF 

 THE SIDE. 



BRAIN FROM 



I, 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 fora- 

 men (or foramen of Monroe); g, lamina terminalis; /i, 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 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. 



