470 TEXT-BOOK OF EMBRYOLOGY. 



also for very complex coordinations between these stimuli. In this way an 

 extensive symbolization and formulation of individual experience (memory, 

 language, etc.) can take place. The formulated experience of one generation 

 can be immediately transmitted (by education in the broad sense of the term) 

 to the plastic late-developing neopallia of the next generation. In this 

 way a racial experience may be rapidly built up without the direct inter- 

 vention of the slow processes of heredity and natural selection and each gen- 

 eration profit by the accumulated experience of past generations to a much 

 greater extent. The nervous mechanism, the pallium, is provided by in- 

 heritance; experience is not inherited but " learned." The pallial associative 

 mechanisms are continuously modified by their activities, thus affecting the 

 character of subsequent pallial reactions (associative memory). Such reac- 

 tions are usually termed psychical or conscious, as distinguished from the 

 reflex reactions of other parts of the nervous system. 



In the course of these developments the pallium or cerebral hemispheres 

 have enormously increased in size until in man they overlap all the other parts 

 of the brain. Naturally the extensive connections of the neopallium with the 

 rest of the brain have profoundly modified the latter. Among the new struc- 

 tures which have on this account been added to the older structures of the rest of 

 the brain, the following may be mentioned: (i) The centripetal connections of 

 the neopallium, consisting mainly of what are usually termed the thalamic radi- 

 ations. These consist essentially of a system of neurones passing from the 

 above mentioned termini in the thalamus of general somatic, acoustic and optic 

 ascending systems to certain areas in the cerebral hemispheres. In this system 

 we can distinguish (a) the continuation of the fillet (general somatic) to the cen- 

 tral region (somaesthetic area) of each hemisphere; (b) the optic radiation from 

 the lower thalamic optic center (lateral geniculate body) to the calcarine 

 (visual) area of the hemisphere; (c) the acoustic radiation from the medial 

 geniculate body of the thalamus to the upper temporal region (auditory area) 

 of the hemisphere. Associated with these last two connections are the increase 



lobes. In Amphibia and Reptiles the hippocampal formation includes all or nearly all of the 

 mesial surface. As the early neopallium appears in the lateral hemisphere walls, the neo- 

 pallial commissural fibers first pass across the median line in the ventral or anterior com- 

 missure. With the increase of the neopallium and its extension on the mesial hemisphere 

 walls, its commissural fibers pass across more dorsally via the archipallial or fornix com- 

 missure (psalterium) forming the neopallial commissure or corpus callosum, the great de- 

 velopment of which nearly obliterates the anterior hippocampal formation. 



Com. ant., Anterior commissure; corp. callosum, corpus callosum; Fimbr., fimbria; Fiss. hippo- 

 campi, hippocampal fissure; Lam. t., lamina terminalis; Lob. olf. ant., anterior olfactory lobe; 

 Lob. pyriformis, pyriform lobe; Psalt., psalterium (fornix commissure); Sept. pell., septum 

 pellucidum; Tub. olf., tuberculum olfactorium. Only a part of the gray (cortex) of the hip- 

 pocampal formation appears, as the gyrus dentatus, on the mesial surface; the remainder forms 

 an eminence, the cornu Ammonis, on the ventricular surface. This invagination is indicated 

 externally by the hippocampal fissure. The exposed fiber bundle forming the edge of this 

 formation (fimbria) passes forward (fornix and its commissure) and thence descends, as the 

 anterior pillar of the fornix, behind the anterior commissure. The anterior pillar is partly 

 indicated by a few lines in this region in the figure. 



