THE FRONTAL LOBES 227 



the actions of the limbs are chiefly automatized loco- 

 motor movements, the premotor cortex is very small; 

 but in all primates, from the lowest to the highest, 

 where there is greater mobility of the whole body and 

 especially of the limbs, this cortex is vastly greater 

 proportionally and the central (Rolandic) fissure is 

 pushed far backward in the hemisphere (Elliot Smith, 

 1924; F. Wood Jones, 191 6, p. 162). 



The exact nature of this premotor organization is 

 by no means clear. It may be similar in principle to 

 that of some lower centers of motor co-ordination. In 

 a quadruped the reflex co-ordination of locomotor 

 movements of the limbs is partly effected in seg- 

 mental spinal centers, for bilateral adjustment of a 

 single pair of limbs, and partly it is in the brain stem, 

 for adjustment of the proper rhythm of fore and hind 

 legs (Laughton, 1924). The apparatus of running, 

 walking, and galloping is structurally present as 

 permanent preformed neuromuscular mechanisms in 

 the subcortical apparatus. Which (if any) of these 

 will be activated is determined elsewhere in the 

 nervous system. When the proper button is pressed 

 one or another of these mechanisms is activated and 

 the proper co-ordinations of movement follow auto- 

 matically. 



The various parts of the excitable cortex of a 

 primate ("arm area," "leg area," etc.) are connected 

 in one-to-one relation with particular subcortical 

 centers of motor co-ordination. Each is a "button" 



