1911.] 



Act of Progression in the Mammal. 



317 



The evidence given by the records throws some light upon the problem 

 of the nature of the central activities. In a typical example there is first a 

 period in which a state of maintained flexion, although a broken one, is the 

 predominant feature. The terminating period of the reaction is one 

 characterised by maintained extension — at the end unbroken. Between 

 these there lies a period in which the phasing of the act is most perfect. 

 This period — lying as it does intermediate between those of maintained 

 flexion and of maintained extension — may be regarded as a period of 

 balance. 



Any hypothesis regarding the nature of the central activities must at 

 present be tentative, but this appearance of " balance " supports a view put 

 forward by the author in previous papers.* As regards the act of pro- 

 gression the central mechanism may be regarded as consisting of antagonistic 

 centres. In one hind limb it may be supposed that the one of these 

 determines a state of maintained flexion and a concomitant state of 

 maintained inhibition of extension, while the other determines a state of 

 maintained extension and a concomitant state of inhibition of flexion. It 

 is inessential at present whether the lumbar centres are two in number and 

 situate in opposite sides of the spinal cord; or whether they are four in 

 number and situated in antagonistic pairs on each side of the cord ; or 

 whether they are more than four in number.f All that it is desired to 

 insist upon here and now is that the centres are paired, and that each pair 

 consists of antagonistic opposites. 



If this be granted, and if there be presumed some such state as " fatigue " 

 — in its broadest sense — accompanying the expression of activity of either 

 of the antagonistic centres, it is possible to frame a tentative hypothesis 

 of the nature of the central changes which condition the phasic act of 

 progression. 



It may then be supposed that some neural disturbance rearranges the 

 conditions of activity of the antagonistic centres in such a manner as to 

 destroy their balance. The one centre then expresses its activity in a 

 movement — for example, flexion — of the limb, and at the same time inhibits 

 the activity of its antagonistic centre. But this expression of activity is 

 accompanied by a state of " fatigue," which progresses in extent and tends 

 to restore the balance of the centres. At the same time the inhibition of 



* 'Q. Journ. Exp. Physiol.,' 1910, vol. 3, p. 21 ; 1 Q. Journ. Exp. Physiol.,' 1911, vol. 4, 

 p. 19. 



t For instance, there may be antagonistic factors (contraction and relaxation producing) 

 in one and the same centre. Thus the activity of such a centre as that for tibialis 

 anticus might be exhibited either as contraction or as relaxation of the muscle. 



