THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 323 



respond to the crossed tractus olfacto-corticalis of fishes and to 

 the crossed sphenoido-hippocampal tract in higher mammals. 



It was stated above that the region corresponding to the lateral 

 olfactory nucleus of lower vertebrates is crowded down upon the 



Fig. 164. — Sagittal section of the commissural and precommissural regions of 

 the right hemisphere of Ornithorhynchiis. From G. Elliot Smith, am., tractus cor- 

 tico-habenularis;6., nucleus habenulae; J, fascia dentata; ^, neopallium ; o', olfactory 

 peduncle; p., precommissural body; t., tuberculum olfactorium; v., anterior com- 

 missure plus neopallial fibers; w., hippocampal commissure; 2, column of fornix; 

 2', fibers of the same from the anterior end of the hippocampus passing beneath 

 the anterior commissure; 3, fasciculus praecommissuralis, probably coming from 

 the pyriform lobe; 4, tractus olfacto-hypothalamicus; 5, fasciculus marginalis, 

 a part of the olfactory tract which goes to the hippocampus; 6, stria meduUaris 

 thalami. 



ventral surface of the hemisphere to form the pyriform lobe. This 

 lobe appears on the ventral aspect of the brain as a ridge which 

 extends the whole length of the lower wall of the hemisphere (Fig. 

 165). As the olfactory tract comes from the bulb it divides into 



