260 THE GROWTH OF THE BRAIN. 



devoted to special senses, like sight, hearing, and 

 smell, the immediate motor reactions which arise from 

 the stimulation of these areas occur in the muscles con- 

 trolling the corresponding external sense organs. On 

 the other hand, the mass of muscles which has for its 

 principal sense organ the skin, is by comparison with 

 the muscles of the external sense organs very large, and 

 the locality from which the reaction of the former can 

 be obtained is so elaborated that this region has been 

 called motor, to distinguish it by contrast from the other 

 regions designated as sensory. Such designations, 

 therefore, as motor or sensory are misleading, and 

 although the sensory side in one locality may be most 

 highly developed, and the motor in another, yet in all of 

 them are to be found pathways for both incoming and 

 outgoing impulses. 



Before discussing the cortical areas in detail, these 

 great regions, as they appear in the brains of the 

 monkey (and probably of man), should be described. 

 The monkey is chosen for comparison because the 

 most important experiments which have been made in 

 this direction have been upon a species of macaque 

 (Macacus sinicu$\ and though in a general way the 

 relations in this animal are similar to those in other 

 monkeys and in man, yet, as there are variations in 

 detail, it is well to remember the species of animal which 

 has been employed. Fig. 55 shows the relation of the 

 motor and sensory centres in this monkey as determined 

 by Horsley. 



As a matter of convenience regions may be termed 

 motor or sensory, if only it is remembered that by these 

 terms we indicate the more evident rather than the ex- 

 clusive character there found. There are two peculiarities 

 in the distribution of the motor areas in the monkey 

 which are worthy of remark. In the first place, a large 



