THE DIENCEPHALON AND THE OPTIC NERVE 



215 



the stria medullaris thalami. This fascicle, together with the closely related 

 habenular trigone and the pineal body, belong to the epithalamus and will be 

 described later. 



The dorsal surface of the thalamus is slightly convex and is divided by a faint 

 groove into two parts: a lateral area, covered by the lamina affixa and forming 

 a part of the floor of the lateral ventricle; and a larger medial area, which forms 

 the floor of the transverse fissure of the cerebrum. The oblique groove separat- 

 ing these two areas corresponds to the lateral border of the fornix (Figs. 154, 155). 

 The lamina affixa is part of the ependymal lining of the lateral ventricle superim- 



Fornix 

 Stratum zonale 



Chorioid plexus of lateral ventricle 

 Lamina affixa 



Internal medullary 

 lamina 



Chorioid plexus of 

 third ventricle 



Third ventricle 



Lenticular nucleus- 



Internal capsule' ? 



Hypothalamic 

 nucleus 



^Transverse fissure of the cerebrum 

 Stria medullaris 



-, Corpus callosum 

 Lateral ventricle 



Caudate nucleus 



Stria terminalis 

 and vena ter- 

 minalis 



._ External medull- 

 ary lamina 



Anterior nucleus 

 of thalamus 



$--, ^Lateral njifleus 

 of thalamus 



- Medial nucleus 

 of thalamus 



Red nucleus"- 



Optic tract 



% Basis pedunculi 



Fig. 155. Diagrammatic frontal section through the human thalamus and the structures which 



immediately surround it. 



Substantia nigra ' 



posed upon this part of the thalamus. It is not present in the sheep, where the 

 fornix is larger and the entire dorsal surface of the thalamus belongs to the floor 

 of the transverse fissure. These features are well illustrated in Figs. 179 and 

 180, as is also the position of the transverse fissure. This fissure intervenes be- 

 tween the thalamus and the cerebral hemisphere, and contains a fold of pia 

 mater, known as the tela chorioidea, of the third ventricle. 



The medial surface of the thalamus forms the lateral wall of the third ven- 

 tricle (Figs. 158, 159). It is covered by the ependymal lining of that cavity. 

 The medial surfaces of the two thalami are closely approximated, being separated 



