THE DIENCEPHALON AND THE OPTIC NERVE 



223 



it on their way to the thalamus. The red nucleus and the substantia nigra 

 project upward into it from the mesencephalon. An additional mass of gray 

 matter is found in this region lateral to the red nucleus and ventral to the thala- 

 mus. It is known as the hypothalamic nucleus and has the shape of a biconvex 

 lens. Its function and fiber connections are not well understood; but it is prob- 

 ably a motor coordination center receiving fibers from the thalamus, corpus 

 striatum, and pyramidal tract, and sending fibers downward in the cerebral 

 peduncle. 



THE THIRD VENTRICLE 



Since the third ventricle is chiefly surrounded by structures belonging to the 

 diencephalon, it will be convenient to consider it at this point and to give at 



Interventricular foramen Body of corpus callosum 

 Anterior commissure | ' \ Body of fornix 

 Septum pellucidum^ \ \ \ 



Rostral lamina \ \ \ 

 Rostrum of corpus callosum. > \ \ '< 

 Genu of corpus callosum , ', \ ', \ < 



Hippocampal com. Roofs of third ventricle or tela chorioidea 



Stria med. 



Habenular 



Trigone 



/' Haben. com. 

 ,' Splenium 

 ',' Pineal 

 ! i body / 



Suprapineal recess 

 .Superior colliculus 

 /Primary fissure 



/White center of vermis 



Olfactory bulb , . 



Medial olfactory gyms' /, 



Anterior perf. substance' / 



Lamina terminalis / 



Diagonal band 



Central canal 

 \ Medulla 



Medial aperture of 

 \ fourth ventricle ' 

 \ \Tela chorioidea 

 ' Fourth ventricle 

 'Anterior medullary 

 velum 



Fig. 159. Medial sagittal section of the sheep's brain. 



the same time an account of the parts of the telencephalon which help to form 

 its walls. These include the lamina terminalis, anterior commissure, and the 

 optic chiasma (Figs. 158, 159). The latter, formed by the decusssation of the 

 fibers of the optic nerve, projects as a transverse ridge in the floor of the ven- 

 tricle. The lamina terminalis is a thin plate joining the two hemispheres, which 

 stretches from the optic chiasma in a dorsal direction to the anterior commis- 

 sure. Here it becomes continuous with the thin edge of the rostrum of the 

 corpus callosum, known as the rostral lamina. As indicated on page 26, the 



