CENTRAL NERVOUS SYSTEM. 259 



is always active daring our periods of consciousness, and it is to be thought 

 of as a region over which the local point of intensest activity is continually 

 shifting — this focal point, wherever it may be, having about it a halo of less 

 active cells as extensive as the cortex itself. 



When the subject is right-handed, it appears that injury to the left cere- 

 bral hemisphere is productive of more disturbance than injury to the right 

 hemisphere. At the same time, lesion of the left hemisphere is far more fre- 

 quent than that of the right. So far as can be judged from experiments on 

 man, the higher sense-organs, the eye and the ear, are more perfect, physio- 

 logically, on the right side. Since the connection of the sense-organs is largely 

 with the cortex of the contra-lateral hemisphere, this means that the impulses 

 going mainly to the left hemisphere are better differentiated than those going 

 to the right, and it would appear to be easier for these impulses to reach a 

 motor area in the same hemisphere than to reach the corresponding area on 

 the opposite side. It is further true that in right-handed persons the cortical 

 activities of the left hemisphere in the region of the body sense-area, must 

 always be greater than those of the opposite hemisphere, and these two 

 circumstances cannot fail to have a profound influence. The observations of 

 Flechsig 1 on the pyramidal tracts also show that these tracts, before medulla- 

 tion at least, may be unevenly developed on the two sides of the cord, and the 

 ease of control may thus be rendered unequal — a condition which must be 

 dominant in the. determination of the side of the body which shall be most 

 exercised. Be this as it may, the lesions which cause aphasia or apraxia 

 (inability to determine the meaning and use of objects), are predominantly 

 in the left hemisphere in persons who are right-handed, while there is 

 some evidence that the right hemisphere is more important in left-handed 

 persons. 



In the adult, damage to one hemisphere is usually followed by a perma- 

 nent loss of function, to a greater or less degree, but this loss maybe more 

 transient and less serious when the lesion occurs in the very young subject. 

 so that during the growing period the sound hemisphere can in a measure 

 replace the one that has been injured. 



Assuming this general plan for the arrangement of the cortex to be cor- 

 rect, it is evident that a, given cell, the axone of which forms part of the 

 pyramidal tract, must in the human cortex be subject to a large series of 

 impulses coming to it over as many paths. Schematically, it would be as 

 represented in Fig. 1 1 . -> >. 



The discharging cell may be destroyed ; then, of course, the muscles con- 

 trolled by it become paralyzed for voluntary movements. The discharging 

 cell may, however, remain intact, but the pathways by which impulses arrive 

 at it be damaged. This is the type of lesion which produces symptoms oi 

 aphasia. When an interruption of associative pathways occurs some one or 

 more of these tracts is broken, and hence the discharging cell doe- not receive 

 a stimulus adequate to cause a response. 



1 Leitung8bahnen im Oehri/n und Riickenmark, 1876. 



