CENTRAL NERVOUS SYSTEM. 269 



the muscles represented. This, however, is an initial symptom only, and 

 gradually disappears, though not always with the same completeness. In 

 man, of course, the tendency to recover is least. 



The anatomical relations behind this difference are the following: The 

 efferent cells in the ventral horns are dominated principally by two set- of 

 impulses, those arriving directly over the dorsal roots of that segment in 

 which they are located, and those coining over the long paths by way of the 

 cerebral cortex and pyramidal tracts. In the lower mammals this second 

 pathway is insignificant, and when interrupted, therefore, the disturbance in 

 the control of the ventral-horn cells is but slight. Passing up the series, 

 however, this pathway tends to become more and more massive and important, 

 as the figures previously given show (see p. 252), until in man and the 

 monkey a damage of it such as is effected by injury to the cortex causes a 

 high degree of paresis if not permanent paralysis, because by this injury a 

 large proportion of the impulses is thus cut off from the efferent cells. 



It has been previously shown that the cortical areas do not vary according 

 to the mass of the muscles which they control. Experiments also show that 

 it is the fore-limbs which are most disturbed in their reactions when the 

 lesion involves the cortical centres for both fore- and hind-limbs, and this falls 

 under the law that the more highly adaptable movements (/. c, those of the 

 fore-limb as contrasted with those of the hind-limbs) are most under the con- 

 trol of the cortex. If the examination be restricted to the fore-limb alone, it 

 is found that the finger and hand movements or those of the more distal seg- 

 ments are in turn the ones most disturbed. Thus, in the limbs the more 

 distal groups of muscles are those best controlled from the cortex. It fol- 

 lows, then, that for the arm, paralysis of shoulder movements as the result 

 of cortical lesion is least complete, while as we travel toward the extremity 

 of the arm the liability to disturbance of its function as the result of cortical 

 injury increases steadily. 



Turning now to the "sensory" areas of the cortex, the principles under- 

 lying their physiological significance and connections appear to be similar. 

 The lower the animal in the vertebrate series the more probable that its reac- 

 tions can be controlled by the afferent impulses which have not passed through 

 the cerebral cortex. 



None of the senses except vision can be analyzed sufficiently to bring out 

 the significance of the subdivisions of the cortical area; hence the illustra- 

 tions are taken from that sense alone. 



It lias already been shown that without cerebral hemispheres a bony 

 fish can distinguish the colors of wafers thrown on water and discriminate 

 between a bit of string and a worm. In the same case, a frog is able to direct 

 its movements and to catch flies — i, e. } to detect objects in motion and read 

 to them normally. A pigeon can direct its movements in some measure, and 

 even select a special objeel as a perch ; but it is not able to respond to the sight 



of food Or its fellows, or those objects which might be Supposed to excite the 

 bird to flight. In the dog the vision which remains permits only the response 



