344 NERVOUS SYSTEM OF VERTEBRATES. 



of many cases has shown clearly that the functions affected depend 

 upon the specific regions of the cortex injured. A sufficient 

 number of facts of this sort have been collected from clinical 

 observations, post-mortem examinations and surgical operations 

 to render fairly certain and accurate the determination of the area 

 of the cortex involved in case of a brain tumor, degeneration of 

 cortical substance or other cause of disturbance. If the patient 

 shows symptoms of disturbance in the functions of sight, hearing, 

 bodily sensation, or voluntary movement including speech, certain 

 specific areas of the cerebral cortex may be pointed out as the seat 

 of the disease, and in the case of bodily sensation or movement 

 certain subdivisions of the cortical area may be assigned to certain 

 parts of the body. 



These clinical observations on man are supported by experi- 

 mental investigations on animals in which either certain areas of the 

 cortex in the living animal are directly stimulated and the effect 

 noted, or the degenerations produced by the extirpation of certain 

 cortical areas are studied by the method of Marchi. These 

 investigations combined with the study of degenerations in human 

 brains where the symptoms have been recorded during fife, have 

 led to the mapping out of functional areas on the cortex and to 

 the description of the course of the tracts connected with those areas. 

 Now the histological study and comparison of the areas thus 

 marked out shows that in the normal brain certain differences exist 

 between the different regions of the cortex and these differences 

 are in some cases sufficiently marked to be of assistance in deter- 

 mining the limits of the various areas. Thus the cortex in vari- 

 ous regions differs in total thickness; in the relative thickness 

 of its layers, in the presence, number and arrangement of giant 

 pyramids and other cells; and above all, in the character of fibers 

 connected mth it. The skilled observer can distinguish between 

 sections from various cortical areas. In the case of the cortical 

 area for hearing, the thickness alone is sufl&cient guide and in the 

 visual area the tangential fibers connected with it are so well 

 marked that the limits of the area may be seen in dissections 

 with the naked eye. 



The differences in the fiber tracts connected with the several 



