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 Ornithorhynchus. From G. Elliot Smith, am., tractus cor- 

 tico-habenularis; b., nucleus habenulae; d, fascia dentata; g, neopallium; o f , 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 medullaris 

 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 



