SENSE AREAS AND ASSOCIATION AREAS. 227 



nution in the width of the granular layer of cells. In the occipital 

 lobes the region round the calcarine fissure has a structure differ- 

 ent from that of the contiguous cortex, and a similar difference is 

 claimed for the auditory region. Campbell believes that the ex- 

 treme end of the frontal lobe (prefrontal region) has a compara- 

 tively undeveloped structure, but Bolton,* on the contrary, states 

 that it has a typical structure and believes that it plays a part of 

 the greatest importance in the higher or general processes of as- 

 sociation. It is the last region of the cortex to be evolved. In 

 mental decadence or dementia it is, according to this author, the 

 first region to undergo dissolution, and in conditions of amentia it 

 is undeveloped. 



Fig. 102. — ^Diagram to show the composition of the corpus callosuin as a system of oom- 

 missural fibers, without projection fibers. — {Cajal.) 



The Corpus Callosum. — The corpus callosum is the most 

 conspicuous of the bands of commissural fibers that connect one 

 cerebral hemisphere with the other. Similar tracts of the same 

 general nature are the anterior commissure and the fornix. 

 The position and great development of the corpus callosum 

 has made it the object of experimental as well as anatomical 

 investigation. When the corpus is divided by a section along the 

 longitudinal fissure (v. Koranyi) no perceptible effect of either a 

 motor or sensory nature is observed in the animal. When it is 

 stimulated electrically (Mott and Schafer) from above, symmetri- 

 cal movements on the two sides of the body may be obtained. If 

 the motor cortex on one side is removed, stimulation in the lon- 

 gitudinal fissure causes movements only on the side controlled 

 by the uninjured cortex. These facts are in harmony with the 



* Bolton, "Brain," 1910, p. 26. 



