THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 333 



hemispheres may be summarized by giving a brief review of the 

 chief parts of the hemisphere of a higher mammal or man, with an 

 indication of their homologues in lower vertebrates. The base of 

 the hemisphere is formed chiefly by the corpus striatum which 

 includes the caudate and lentiform nuclei and is traversed by 

 ascending and descending fiber tracts to the general cortex and 

 hippocampus. In addition to these, many fibers come up to the 

 corpus striatum from the lower parts of the brain and end in it, 

 and a smaller number from the pyramidal cells of the general 

 cortex also end here. Fibers arising from the cells of the striatum 

 end in the centers of the thalamus, forming a tractus strio-thalami- 

 cus. The nucleus caudatus, from its position next to the ventricle 

 and from the fact that a large part of its fibers end in the nucleus 

 lentiformis, strongly reminds one of the unspecialized epistriatum 

 of fishes. 



The ventral surface of the striatum is covered by the nucleus 

 amygdalae which is continuous with the pyriform lobe or sphe- 

 noidal cortex. These areas together correspond to the lateral 

 olfactory area of fishes, amphibia and reptiles. From this region 

 arise three tracts; first, the stria medullaris or taenia thalami, to 

 the nucleus habenulae (tractus olfacto-habenularis) ; second, the 

 thaenia semicircularis, to the hypothalamus (tractus olfacto-hypo- 

 thalamicus lateralis) ; third, a tract to the hippocampus by way of 

 of the precommissural body (gyrus subcallosus) and the fimbria 

 (tractus olfacto-corticalis). 



The inner surface of the corpus striatum forms the floor of the 

 lateral ventricle, the anterior horn of which extends in front of 

 the striatum into the frontal lobe of the hemisphere and reaches 

 into the olfactory bulb, except where it becomes obliterated in the 

 adult as in man. This is homologous with the primitive lateral 

 or olfactory ventricle in lower vertebrates. The ventral and 

 mesial wall of the anterior horn in front of the lamina terminalis 

 is formed by the anterior and mesial part of the secondary olfactory 

 center, homologous with the mesial olfactory nucleus in fishes. 

 In man this region includes the tuberculum olfactorium and the 

 small region in front of the lamina terminalis known as the gyrus 

 subcallosus, better called the precommissural body. In the lamina 



