THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 321 



This corresponds to the tractus olfacto-corticalis of selachians, 

 amphibia and reptiles (tractus olfactorius septi). A diffuse bundle 

 of fine fibers arising in the precommissural body runs backward 

 below the anterior commissure and is widely distributed in the 

 hypothalamus (cf. p. 275). This tract is evidently identical with 

 the tractus olfacto-hypothalamicus medialis of lower vertebrates. 

 Fibers arising from the whole arch of the hippocampus (Fig. 160) 

 collect toward the commissural region in the lamina terminalis. 



rec.s. 



c.d. 



c.v. 



Fig. 162. Transverse section through the cerebral hemispheres of Ornithorhyn- 

 chus. From G. Elliot Smith, c.d., hippocampal commissure; c.v., anterior commis- 

 sure plus neopallial fibers; f.d., fascia dentata; hip., hippocampus; I. p., lobus 

 pyriformis; para., paraterminal body; rec.s., recessus superior. 



Those from the caudal portion of the hippocampal arch run forward 

 on its ventricular surface forming the fimbria. When the fimbria 

 arrives at the lamina terminalis a part of its fibers go to form the 

 hippocampal commissure (Fig. 164, w). The remainder of the 

 fimbria fibers together with numerous fiber bundles from the 

 whole anterior portion of the hippocampal formation collect into 

 a large bundle close upon the upper surface of the anterior com- 



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