700 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the right. So far as can be judged from experiments on man, the higher 

 sense-organs, the eye and the ear, are more perfect, physiologically, on the right 

 side. Since the connection of the sense-organs is largely with the cortex of the 

 contralateral hemisphere, this means that the impulses going mainly to the left 

 hemisphere are better differentiated than those going to the right. For these 

 impulses to reach a motor area in the same hemisphere would appear to be 

 easier than to reach the corresponding area on the opposite side, and it is thus 

 possible to see how, on the basis of the slightly better sense-organs of the right 

 side, the left-brained man might have been developed. The observations of 

 Flechsig J on the pyramidal tracts also show that this tract, before medullation 

 at least, may be unevenly developed on 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 exercised. 



Doubtless there are other factors concerned, and, moreover, it has yet to be 

 demonstrated that the sense-organs of the left side are superior in persons left- 

 handed. Nor has the inequality of the crossed pyramidal tracts in the adult 

 been established with reference to these questions. 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 permanent 

 loss of function, but this loss may be transient when the lesion occurs in the 

 very young subject, so that during the growing period the sound hemisphere 

 can in a measure take up the function of the one that has been injured. 



Assuming this general plan for the arrangement of the cortex to be correct, 

 it is evident that a given cell, the neuron of which forms part of the pyrami- 

 dal 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 Figure 198. 



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

 trolled by it become more or less paralyzed. The discharging cell may, how- 

 ever, remain intact, but the pathways by which impulses arrive at it be dam- 

 aged. This is the type of lesion which produces symptoms of aphasia. When 

 an interruption of associative pathways occurs some one or more of these tracts 

 is broken, and hence this discharging cell does not receive a stimulus adequate 

 to cause a response. 



The physiological simplicity of the elements in any part of the central sys- 

 tem, either when different portions of the system from the same animal or 

 when the corresponding portions of different animals are compared, depends 

 on the number of paths by which the impulses are brought to the discharging 

 cells. 



Composite Character of Incoming Impulses. To these conclusions 

 based on the anatomy are to be added others suggested by clinical observa- 

 1 Leitungsbahnen im Gehirn und JRuckenmark, 1876. 



