348 



AN INTRODUCTION TO ZOOLOGY 



few of its fibres also appear to run to the frontal and temporal lobes. 

 These three commissures are generally regarded as arising from the 

 lamina terminalis or median anterior wall of the embryonic fore- 

 brain. If this be so, then the septum lucidum must be regarded 

 as the attenuated portion of the lamina lying between the corpus 

 callosum and the fornix. All these structures are somewhat dis- 

 placed from their original position owing to the enormous develop- 

 ment of the outgrowths from the antero-lateral parts of the fore- 

 brain to form the cerebral hemispheres. 



AC 



CM 



FIG. 121. Diagram to show relation of fornix and hippocampus, 

 the structures being represented as pulled out laterally. 



A., anterior pillar of fornix ; A.C., anterior commissure ; C.M., corpus mammillare ; F., body of 

 fornix ; H., hippocampus ; H.C., hippocampal commissure, psalterium ; P., posterior pillar of 

 fornix, fimbria. 



It will be seen then that we can regard the hemisphere as 

 composed of an anterior basal mass, the corpus striatum, joined on 

 to the main stem of the whole brain and a wall that passes from this 

 outwards, backwards, upwards and inwards to enclose the ventricle, 

 and so forming a sort of fold termed the mantle or pallium. In this 

 pallium two functionally distinct regions can be distinguished. 

 The first is the ventro-lateral portion, limited externally by the 

 rhinal fissure and hippocampal fissure, and comprising the olfactory 

 bulbs, the pyriform lobes, olfactory tubercles, hippocampi and the 

 fornix. All of this is concerned in the main with olfactory sensations, 



