THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 293 



thalamus. Lying in front of and below the corpus striatum 

 and forming part of the lower and mesial wall of the hemispheres 

 at the anterior end are the olfactory bulb and olfactory lobe. 

 On the ventral surface of the striatum is the nucleus amygdalae 

 which is continuous caudally with the pyriform lobe and the hip- 

 pocampus in the temporal region of the hemisphere. These sev- 

 eral structures, together with the fornix and hippocampal com- 

 missure constitute the second main portion of the hemisphere, 

 and may be spoken of collectively as the central olfactor}^ 

 apparatus. All the rest — ^much the greater part— of the cer- 

 ebrum is concerned with sensory impulses from the external 

 world which come from various parts of the body including 

 the special sense organs of sight and hearing; with the cor- 

 relation of these impulses with one another and with habitual 

 tendencies produced by previous actions; with voluntary impulses 

 sent out to arouse, direct or inhibit actions in response to stimuU; 

 with sensations; and with thought processes. This portion of 

 the cerebral hemispheres may be spoken of as the somatic pallium. 

 The phylogenetic history of these three portions of the forebrain 

 has formed one of the most obscure chapters of comparative 

 morphology. If it is possible to frame a connected account of 

 the evolution of these structures, it will render the study of the 

 human cerebrum simpler and its relations more intelHgible. 



In order to gain a clear view of the vertebrate forebrain it will 

 be necessary to begin with the lowest classes and give as concise 

 an account as possible of the centers and fiber tracts and their 

 functional relations in one class of vertebrates after* another, 

 endeavoring to fix with as great certainty as possible the homology 

 of the most important structures and to harmonize those incon- 

 sistencies which arise from differences of nomenclature or inter- 

 pretation of known facts. In such a survey the guiding principle 

 must be functional relationship. When a given center or fiber 

 tract is clearly recognized anatomically, the questions must be 

 asked, what is its function; with what other centers or fiber 

 tracts is it related ; with what kind of impulses is it concerned ? 



The forebrain of cyclostomes consists of paired lateral 

 lobes and of a mesial portion connected with the diencephalon. 



