THE CEHEBKUM OE PROSENCEPHALON. 173 



as we shall see later, of the high development of mantle not yet described. 

 The corpus callosum always lies dorsal to the olfactory commissure and is 

 naturally correspondingly longer and thicker the more the mantle is ex- 

 tended. It is longest in apes and in man and shortest in rodents and in 

 inseetiYora. 



The physiological significance of the brain-cortex has come to be known 

 through a great number of admirable studies on the mammalian brain in the 

 course of the last twenty-five years. The experiments upon animals and 

 the observations upon man, observations which daily lead to new and inter- 

 esting results, have yielded the following: The cortex may he accepted as that 

 part of the hrain which serves as the basis of the highest psychic functions. 

 Upon the normal existence and condition of the brain-cortex depend all of those 

 abilities which may be acquired by study, nearly all of those acliviiies tchich are 

 executed by the use of memory-pictures, and especially all of those psychic 

 processes which we term associations. 



One may conceive the whole cortical apparatus as a gigantic association- 

 center, to which from without through relatively narrow tracts such impres- 

 sions may be conducted as have alreadj' found their first termini in deeper 

 centers: primary brain-centers. From this cortical association-center tracts 

 pass down to more posterior brain-regions -si-hich are adapted to call forth 

 movements, etc., through their agency. The sum of all these tracts is desig- 

 nated Corona radiata. 



That which determines the size and extent of the cortex-bearing mantle 

 is not the usually narrow tracts, but the development of the association- 

 fibers which aiford the possibility to receive the afferent sensory impressions 

 in the very greatest variety of 'n'ays, to inhibit or suppress, to evaluate, to 

 associate with previously received ones, and finally to harmonize the activi- 

 ties with acquired memory-pictures. We know also that special activities are 

 performed by special cortical areas; that the cortex subdivides into a num- 

 ber of separate regions which are functionally different. Numerous investi- 

 gations of the last years have made us more definitely acquainted with the 

 surface of the mammalian brain-mantle. The results of these investigations 

 taught us that certain areas, varying according to the species, are more 

 developed, and certain ones less so. Our knowledge of the physiological 

 significance of these cortical areas is, in many cases, slight, but it is the task 

 of the immediate future to study the development of these cortical areas: 

 a task which, happily, is already u.ndertaken for certain species of mammals. 



Thus, since, according to the present state of our knowledge, the cortex 

 may be accepted as the location of those psychic functions which are con- 

 sciously executed after consideration, through use of memory-pictures, so 

 is the demonstration of a cortical bundle to the nucleus of a special sensory 

 apparatus of great interest to comparative psychology. 



