THE CONNEXIONS OF THE OLFACTOKY NERVES. 



627 



fissura hippocampi. There is no fissura hippocampi in the human brain. What is 

 usually described under, this name is an artificial cleft made by pushing the 

 handle of a scalpel into the hippocampal formation at the edge of the exposed 

 part of the fascia dentata (Fig. 556, B and C, at x) and separating the morpho- 

 logical surface of the hippocampus from that of the buried part of the fascia 

 dentata. Cleavage readily occurs along this line because there are numerous 

 nerve-fibres, hippocampal and dentate respectively, upon each side of it. 



As development proceeds a break occurs in the cell-column at the junction 

 of its hippocampal and dentate parts, and the two columns (Fig. 556, C) become 

 partially interlocked. 



The axons of the hippo- 

 campal cells collect upon 

 its ventricular surface to 

 form the alveus, the fibres 

 of which converge towards 

 the margin of the fascia 

 dentata, where they bend 

 into the longitudinal direc- 

 tion (i.e. parallel to the 

 edge of the pallium and 

 the lamina chorioidea) to 

 form a prominent white 

 marginal fringe, the fimbria. 



Corpus callosum-^. 



commissure- 



Anterior commissure - 



Paraterminal body "' 

 Lamina terminalis 



Fascia dentata 



Olfactory bulb / 



Optic chiasma 



Column of fornix 



The fibres of the fimbria 



FIG. 555. MEDIAL ASPECT OF THE RIGHT CEREBRAL HEMISPHERE 

 OF 'A HUMAN FCETUS OF THE FOURTH MONTH. 



The broken red lines indicate the paths taken by callosal fibres in the 

 S Upwards and forwards neopallium to reach the upper end of the lamina terminalis. 



(Fig. 555), and ultimately 



reach the upper end of the lamina terminalis, which provides a bridge to 



conduct a certain number of them across the median plane into the fornix or 



I ' P 



Hippocampus. i ; i V 



Fascia 



denfafa 



Fimbria: 



Alveus. 

 Hippocampus. 



Plexus chorioideus. 



FIG. 556. DIAGRAMS REPRESENTING THREE STAGES IN THE DEVELOPMENT OF THE HIPPOCAMPAL 



FORMATION. 



fimbria of the other hemisphere, so as to link together in functional associa- 

 tion the two hippocampi. These crossing fibres are known as the commissura 

 hippocampi. 



Most of the fibres that go up in the fimbria from the hippocampus do 

 not pass into the hippocampal commissure, but bend downwards in the anterior 

 lip of the foramen interventriculare to. enter the thalamic region. They are 

 collected into a vertical rounded column, which is called the cohimna fornicis ; 

 when it reaches the hypothalamus it bends backward to end in the corpus 

 mamillare. 



The olfactory bulb and tract, the area piriformis, tuberculum olfactorium, 

 corpus paraterminale, and the formatio hippocarnpalis together form a part of 

 the hemisphere, which is concerned mainly with the function of smell. Hence 

 they may be grouped together as the rhinencephalon ; but this term has been used 

 in so many different ways that it is of doubtful utility. 



