324 NERVOUS SYSTEM OF VERTEBRATES. 



a mesial portion whose connections in the tuberculum olfactorium, 

 precommissural body and hippocampus have just been described, 

 and a lateral portion which enters the pyriform lobe. This 

 lateral portion, the external root or external olfactory radiation 

 of authors, is distributed through the whole length of the pyriform 

 lobe and to the caudal portion of the ventral wall of the hemisphere 

 from which arises the stria medullaris thalami mentioned above. 

 In higher mammals the anterior part of this region is known 



Fig. 165. Ventral surface of the brain of Ornithorhynchus to show the position 

 of the pyriform lobe. Outline after a figure by G. Elliot Smith, b., olfactory bulb; 

 p., pyriform lobe; t. tuberculum olfactorium; g.c., general cortex or neopallium. 



as the nucleus amygdalae, the posterior part as the pyriform lobe 

 or sphenoidal cortex and from it arises the taenia semicircularis 

 to the hypothalamus (Fig. 166). This is homologous with the 

 tractus olfacto-hypothalamicus lateralis in fishes. As indicated 

 above a tract from this same region goes to the hippocampus 

 by way of the precommissural body and the fimbria. 



This general survey shows that there is no essential difference 

 in the arrangement of the central olfactory apparatus in mono- 

 tremes and in amphibia and reptiles. The large size of the anterior 



