648 PHYSIOLOGY OF THE CEREBRUM 



shoulder, elbow, wrist and fingers, thumb, trunk, hip, foot, knee, great toe and 

 smaller toes (cf. Fig. 287). The points corresponding to the movements within 

 any given cross section, however, are not sharply delimited, but overlap each 

 other. 



But the pyramidal tracts are not the only motor pathways from the cere- 

 bral cortex. According to Rothman, ablation of the pyramidal paths alone 

 in the dog causes no essential change in the electrical stimulation of the 

 motor region: the motor impulses are then conveyed by Monakow's bundle 

 (see page 596). In monkeys however the latter plays but an unimportant 

 role in this respect, for after section of the pyramidal paths, only movements 

 of the hand, fingers and toes could be obtained by electrical stimulation of 

 the cortex. 



Even after complete suppression of all those pathways in the monkey, 

 and notwithstanding the failure of subsequent stimulation of the motor region, 

 the motor functions of the limbs were not permanently abolished. Impulses 

 which reach the cord in other ways can always produce slight isolated move- 

 ments of the fingers ; indeed, after complete severance of all the paths of the 

 lateral and anterior columns of the cord on one side, a restitution takes place 

 which, while very incomplete,, makes possible not only associated but isolated 

 movements" as well. There are therefore several pathways by which the cere- 

 bral cortex may influence the movements of the body. 



H. DEVELOPMENT OF THE MOTOR AREAS OF THE CORTEX 



The investigations of Flechsig on the formation of the medullary substance 

 in the nerve paths of the central nervous system, have brought to light the 

 fact that the pyramidal paths in man receive their medullary substance only 

 at the very end of intrauterine life. In the dog these same paths are not 

 provided with their medullary substance until after birth. 



In accordance with this fact the excitability of the motor region in newborn 

 dogs is but slight, so much so that it has been stated by some authors to be alto- 

 gether wanting until the tenth day. This appears certainly to be incorrect, 

 for Paneth has found, for example, that responsive movements can be obtained 

 by stimulation of the cerebral cortex in dogs only one to two days after birth. 



According to Bary, the first movements obtainable exhibit various noteworthy 

 differences from those of somewhat older animals. They are not confined to 

 separate groups of muscles as are the latter, but involve the whole anterior or 

 posterior extremity of the opposite side; the duration of the contraction and the 

 latent period are also much longer. Moreover, in very young animals the excita- 

 bility of the cortex is easily destroyed by all sorts of injuries, narcosis, cooling, 

 exposure, etc. 



From about the tenth day onward special areas for the separate groups of 

 muscles develop on the cortex, and pari passu with this development, the dura- 

 tion of the contraction and the length of the latent period become shorter, and 

 the resistance of the cortex to fatigue also greater. 



On the other hand it should be observed that in the guinea pig, in which the 

 pyramidal paths receive their medullary substance in utero, the cortex is ex- 

 citable before birth. 



Of great interest also is the observation made by Herzen and others that 



