82 THE CEREBRUM. 



in its concavity the section of the mid-brain. It is separated from 

 the fusiform gyrus by the collateral fissure and the ectorhinal 

 sulcus; and bounded medially by the hippocampal fissure. The 

 anterior end of the gyrus is flexed inward and backward over the 

 end of the hippocampal fissure and the whole anterior part consti- 

 tutes the uncus hippocampi. - The region of the uncus is some- 

 what irregular and appears to comprise the gyrus circumambiens 

 and the gyrus semilunaris described by Retzius. It represents 

 the greater part of the lobus pyraformis of osmatic mammals 

 and is probably the chief receptive center of smell; it receives the 

 lateral stria of the olfactory tract and fuses with a low oblique 

 ridge, the gyrus subcallosus (or peduncle of the corpus callosum). 



If the hippocampal gyrus be drawn downward somewhat, a 

 rudimentary gyrus may be seen between the hippocampal and 

 chorioidal fissures. That is the dentate fascia. It is continuous 

 with the reflected part of the uncus in front, and behind is in con- 

 tinuity with the fasciola cinerea and gyrus supracallosus. Like 

 the hippocampal gyrus it forms a part of the limbic lobe. We 

 shall recur to this lobe on the medial surface of the hemisphere. 



Having studied the basal structures of the cerebral hemispheres, 

 it is now in order to examine the median structures in the inferior 

 surface of the fore-brain. They occupy the interpeduncular or 

 hypophyseal region. They constitute the hypothalamus and 

 form part of the floor of the third ventricle. 



The hypothalamus is the name applied to the fore-brain 

 structures under the thalamus. Posteriorly it blends with the 

 mid-brain. Its free portion is divided into two parts, viz., the 

 pars optica hypothalami and the pars mammillaris hypo- 

 thalami. The former belongs to the telencephalon, the latter to 

 the diencephalon. They include the following: 



Pars Optica Hypothalami 



Lamina cinerea terminalis 



Optic chiasma (chiasma opticum) 



Tuber cinereum 



Infundibulum and 



Hypophysis. 



Pars Mammillaris Hypothalami < Corpora mammillaria. 



