274 ARCHIVES OF NEUROLOGY AXD PSYCHIATRY 



1. Excitation of the motor cells supplying a muscle by impulses 

 derived from the receptors in the muscle itself (Sherrington 56 ). 



2. Long spinal reflexes from synergic muscles (Sherrington, 56 

 Magnus 61 ). 



3. Other proprioceptive and general exteroceptive facilitation whose 

 central mechanism is as yet rather obscure (Yerkes, 62 Richter 63 ). 



4. Vestibular and proprioceptive influences exerted through the 

 mechanisms of the cerebellum. 



5. Probably facilitation derived from thalamic mechanisms in emo- 

 tional excitement (Head' 14 ). 



6. Kinetic influences of obscure origin integrated in the corpus 

 striatum (Wilson, 42 Hunt 60 ). 



Interference with any of these mechanisms is able to produce a 

 change in the excitability of the final common path, and in the intact 

 organism it seems certain that every act involves the participation of 

 all of them, both by excitation and inhibition. 



These considerations make it possible to form a tentative hypothesis 

 concerning the function of the precentral gyrus. Its demonstrated 

 facilitating effects, and its lack of direct participation in the conditioned 

 reflex arc seem to throw it into a class with these other postural and 

 tonic systems. Cerebral paralysis, is, I believe, to be interpreted as 

 showing that a normal function of the stimulable cortex is to supply a 

 substratum of facilitating impulses which act in some way to render 

 the final common paths excitable by the more finely graduated impulses, 

 descending from the cortex by extrapyramidal paths and producing the 

 finer shades of adaptive movement. In other words, impulses descend- 

 ing from the precentral gyrus do not initiate the finer adaptive move- 

 ments through the lower motor neurons, but only "prime" these cells 

 so that they may be excited bv impulses from other sources. The 

 source of this activity and the probable interrelations of the stimulable 

 areas with other parts of the motor system and with sensory projection 

 areas present problems too complex for discussion here. Unquestion- 



61. Magnus. R., and de Kleijn, A.: Die Abhangigkeit des Tonus der Extre- 

 mitatenmuskeln von der Kopfstellung, Arch. f. d. ges. Physiol. 145:455-548, 1912. 

 Magnus, R. : Welche Teile des Centralnervensystems mussen f iir das Zustande- 

 kommen der tonischen Hals- und Labyrinthreflexe auf die Korpermusku- 

 latur vorhanden sein ? Arch. f. d. ges. Physiol. 153:224-250, 1914. 



62. Yerkes, R. M. : Inhibition and Reinforcement of Reactions in the Frog, 

 J. Comp. Xeurol. & Psychol. 14:124, 1904. 



63. Richter, C. P.: A Behavioristic Study of the Activity of the Rat, Comp. 

 Psychol. Monogr. 1:1-55. 1922. 



64. Head, H.: Studies in Neurology, London, 1920; Release of Functions in 

 the Nervous System, Proc. Roy. Soc. 92 B: 184-209, 1921. 



