16 THE NERVOUS SYSTEM 



absence of the olfactory nerves is one of the stated causes 

 of anosmia. 



The frontal lobes are separated from one another by the 

 great longitudinal fissure. On the basal surface of the brain, 

 the posterior part of this fissure is hidden by the optic chiasma 

 (Fig. 8), from which the optic nerves arise anteriorly and the 

 optic tracts posteriorly. The tracts pass backwards and laterally 

 round the mid-brain to reach the lower visual centres (p. 51). 



The interpeduncular fossa is bounded by the optic chiasma 

 in front, by the mid-brain behind, and by the optic tract on 

 each side. Its most anterior part is termed the tuber cinereum 

 and it gives attachment to the stalk of the hypophysis (pituitary 

 body}. Behind the tuber cinereum lie the two corpora mamill- 

 laria^ one on each side of the median plane. The posterior 

 perforated substance occupies the posterior angle of the inter- 

 peduncular fossa. 



The structures included in this area on the basal surface 

 of the brain form the anterior part of the floor of the third 

 ventricle, and this statement may be confirmed by reference 

 to Fig. 5. 



The anterior perforated substance (ant. perf. spot) lies lateral 

 to the optic chiasma and forms the floor of the angle 

 between the optic nerve and the optic tract. Its relationship 

 to the internal carotid and the middle cerebral arteries is 

 referred to on page 119. It may be noted that, whereas the 

 posterior perforated substance lies in the median plane, the 

 anterior perforated substance is bilateral. Both areas are 

 pierced by small blood-vessels. 



The uncus is a well-marked elevation which lies postero- 

 medial to the temporal pole and lateral to the anterior per- 

 forated substance. It forms the anterior extremity of the 

 hippocampal gyrus and so is part of the gyrits fornicatus 

 (limbic lobe}. It is said to contain the higher centres for the 

 sense of smell. 



On its lateral side, the hippocampal gyrus is bounded by 

 the collateral fissure, which is separated from the calcarine 



