THE RHINENCEPHALON 



867 



1. Gyrus fornicatus 



Hippocampus 



structures which are either wholly or in part devoted to the functions of the 

 olfactory apparatus: — 



Part of gyrus cinguli and cingulum. 

 Isthmus of the gyrus fornicatus. 

 i f hippocampal gyrus, 



uncus. 



dentate gyrus (fascia) . 

 fimbria. 



2. The medial and lateral longitudinal striae upon the corpus callosum. 



3. The fornix. 



4. The mammillary body, the mammillo-thalamic fasciculus to the anterior 



nucleus of the thalamus and the mammillo-peduncular fasciculus. 



5. Part of anterior cerebral commissure. 



6. Part of septum pellucidum. 



7. Most of medullary stria of thalamus. 



8. Most of habenular nucleus. 



The gyrus fornicatus comprises the greater mass of the limbic lobe. As seen 

 above, it is a term used to collectively represent a number of conjoined structures. 



Fig. 681. — Diagram showing Position of Structures Comprising the Limbic Lobe as 

 Seen from the Mesial Aspect of the Cerebral Hemisphere. 



Fornix 



Fasciola cinerea 



Gyrus cinguli 

 Medial and lateral 

 longitudinal strise of 

 corpus callosum 

 Septum pellucidum 



Subcallosal gyrus 

 Olfactory bulb 



Mammillo-thalamic fasciculus 

 CVicq d'azyri) 



M ammill ary body 



Medial olfactory stria 



Lateral olfactory stria 



Dentate fascia or gyrus 



Being an incomplete ellipse in form, its two ends are united to form a closed ring 

 by means of the connection of the parolfactory area with the gyrus cinguli and 

 the connection of the anterior perforated substance with the uncus of the hippo- 

 campal gyrus. It is best described in terms of its three component parts indi- 

 cated above: 



The g3mis cinguli begins in junction with the area parolfactoria below the 

 anterior end of the corpus callosum, and curves above so as to entirely embrace 

 the upper surface of the latter. It is separated from the frontal lobe by the sulcus 

 cinguli (calloso-marginal fissure), from the parietal lobe by the subparietal sulcus, 

 and from the corpus callosum below by the sulcus of the corpus callosum. By the 

 latter it is separated from the longitudinal striae of the upper surface of the corpus 

 callosum. 



The gyrus cinguli covers over, and its cells are closely associated with, the cingulum, a well- 

 marked arcuate band of white substance, which follows the gyrus in its bend around the rostrum 

 and backward to turn around the splenium of the corpus callosum in the isthmus of the gyrus 

 fornicatus, and then to course forward into the hippocampal gyrus and the uncus. The cingulum 

 is largely an association fasciculus between the gyri of the temporal lobe and those gjnri on the 

 mesial surface of the cerebral hemisphere, its fibres for the most part running short courses, being 

 continually added to it and continually leaving it. However, it contains olfactory axones 

 running in two directions: (1) fibres from the medial olfactory stria and fibres arising in the 

 parolfactory area, the gyrus subcaUosus and the anterior perforated substance which course 

 posteriorly for distribution in the cortex of the gjTus cinguli and hippocampal gyrus; (2)fibre3 

 arising in the hippocampal gyrus, especially the uncus, to course dorsalward through the isthmus 

 and then forward as association fibres. Some fibres arising from the cortical cells of the gyrus 

 cinguli pass inferiorly through the cingulum, through the corpus callosum and, anteriorh'', 

 through the septum pellucidum to join the fornix below {-perforating fibres of the fornix). 



The isthmus of the gyrus fornicatus is the constricted portion connecting the 

 posterior end of the gyrus cinguli with that of the hippocampal gj'rus (fig. 619 



