272 THE GROWTH OF THE BRAIN. 



the internal capsule. Their position in the capsule, both 

 with relation to the rest of the brain and to one another, 

 is highly constant, and there is here found a degree of 

 localisation which is just as exact as that in the cortex 

 itself. This arrangement is shown in Fig. 61. 



It has been seen that the larger number of incoming 

 fibres cross the middle line at the lower end of the bulb 

 and enter the opposite hemisphere. In a like manner, 

 the fibres arising from this hemisphere and carrying the 

 outgoing impulses cross with one another at nearly the 

 same level, and so innervate the nuclei controlling the 

 muscles of the opposite half of the body. 



Just as experiments have shown in the case of the 

 so-called motor areas, that removal of the cortex is 

 accompanied by disturbances of the dermal sensations, 

 so in the sensory areas removal of the cortex affects one 

 or the other of the sense organs named. By this method, 

 combined with, pathological observations, we learn that 

 the sense of smell is probably associated with the mesal 

 surface of the temporal lobe, the sense of hearing with 

 the first and second temporal gyri in the middle part of 

 their course, and the sense of sight with the occipital 

 lobe, and especially with that portion of it called the 

 cuneus. The paths by which the stimuli arriving along 

 these different lines may reach and affect any of the 

 efferent cells elsewhere located in the cortex, are pro- 

 bably found in f the great systems of association and 

 commissural fibres, already described. 



Any sensation can serve to arouse any group of motor 

 centres which are under voluntary control. At the 

 same time certain groups of incoming impulses are 

 usually accompanied by corresponding reactions, or in 

 other words, that there are preferred pathways between 

 the incoming fibres and discharging cells representing 

 those muscular adjustments which are easiest. 



