316 Mr. Graham Brown. Intrinsic Factors in the [July 21, 



of the extensor is strongly reminiscent of rebound contraction. It does not 

 persist for the whole of the duration of the synchronous flexor relaxation, 

 but relaxes soon. There is then sometimes an additional relaxation 

 synchronous with the contraction of the flexor. The termination of this 

 flexor contraction is then succeeded by the extensor contraction — and so on. 



In the third period of the reaction the rebound-like contraction of the 

 extensor disappears and is replaced by a condition of maintained contraction 

 which is broken by relaxations synchronous with the flexor contractions. 

 These are much smaller than before and gradually disappear. The transition 

 of the type of flexor contractions is sometimes sudden. The period finally 

 ends with a maintained and unbroken contraction of the extensor. 



It may be said that there is no great difference between the movements of 

 progression as evidenced in the de-afferented muscles and those not 

 de-afferented. The greater regularity of the movements as observed in the 

 former preparation may possibly be due to the absence of the peripheral 

 part of the mechanism. But it is also possible that the difference is an 

 accidental one ; for records as regular have been obtained from the 

 preparation in which the muscles are not de-afferented, although the average 

 regularity is less. 



V. Conclusions. 



These experiments show that the phasing of the acts of progression is 

 determined neither by the peripheral shin stimuli nor by the self-generated 

 proprioceptive stimuli of the muscles which take part in them. 



The section of the spinal cord generates an arrhythmic stimulus. This 

 causes the contraction of certain limb muscles. In the preparation used, of 

 these the recording muscles are the only mobile parts of the two limbs. The 

 characteristic alternating contraction of the two antagonists cannot be 

 determined by their own contraction and the consequent setting up of 

 a series of refractory phases in the activity of the centres by means of the 

 stimulation of a sensory apparatus contained in the muscles, because the 

 afferent nerves which arise in these muscles were put out of action in the 

 preparation used. Not only must the locus of the changes which condition 

 the refractory phase of progression be in the spinal cord, but the mechanism 

 which determines them must also be centred. 



There are, therefore, two points of interest in connection with the 

 mechanism of progression — the question of the nature of the central changes 

 in activity, and the question of the part played in the act by the pro- 

 prioceptive mechanism. The stimuli which arise in the skin probably play 

 but a small part in the act, and then are of importance only in certain of its 

 types and not in all. 



