354 NERVOUS SYSTEM OF VERTEBRATES. 



bination of tactile and visual impressions, for example, require 

 the cooperation of association centers which receive fibers from 

 both somaesthetic and visual areas. The order of myelinization 

 of association centers corresponds to the order of development 

 of more and more complex actions and mental states in the child. 

 The central zones of the association fields, then, serve for the 

 higher psychic states and more complex processes of thought. 

 The specific functions of the three association fields are deter- 

 mined in large part at least by their position with reference to the 

 senso-motor areas. The middle association field (island of Reil) 

 situated as it is between the auditory area and that part of the 

 somaesthetic area which receives sensation from the lips, tongue, 

 throat, etc., is chiefly the association center for speech. The 

 anterior and posterior fields have much more complex relations 

 and functions. The posterior field, situated between the somaes- 

 thetic, visual and auditory areas, receives from those areas 

 impressions concerning the external world. The functions of this 

 field are to construct external objects from the several kinds of 

 sense impressions and to form ideas concerning the relations of 

 objects and physical processes to one another and to the self. 

 In a word, the objective relations of the individual and all those 

 processes which we commonly call "intellectual" are the peculiar 

 province of the great posterior association field. That this is at 

 least in general the true interpretation of this field is evidenced by 

 the mental symptoms in cases of disease affecting the posterior 

 association field. The most common phenomena met with are 

 various forms of loss of memory and of the power of association. 

 If the sensory areas are not affected the perception of sense 

 impressions by the touch, sight and hearing is not impaired, but 

 these impressions can not be combined into objects which are 

 recognized as previously experienced. The individual presents 

 to the observer the phenomena of mind-blindness, mind-deafness 

 and the like, while he himself loses the power of connecting his 

 several impressions into orderly experience. Whatever set of 

 associations are thus affected the corresponding set of sense 

 impressions cease to interest and finally cease to attract the atten- 

 tion of the individual and he appears listless with regard to things 



