1911.] 



Act of Progression in the Mammal. 



313 



III. Movements of Progression in the Low Spinal Preparation in which the 

 Muscles are not De-afferentecl. 



Before passing to the consideration of the movements of progression in 

 de-afferented muscles it is well to examine briefly the movements which 

 occur in the same preparation when the muscles have not been de-afferented. 

 The procedure of operation was that described above, with the exception that 

 the lumbar posterior spinal roots were not divided. 



When such a decerebrate preparation is rendered a low spinal preparation 

 by the rapid severance of the spinal cord at or about the level of the 12th 

 thoracic segment the recording muscles shew movements undoubtedly those 

 of progression. 



Three periods may be distinguished in typical instances of the reaction thus 

 evoked as movement at the ankle. 



Immediately after the section of the cord there is a period during 

 which the flexor muscle, tibialis anticus, remains more or less in a state 

 of maintained contraction. The maintenance of this contraction is not perfect. 

 There are usually phases of incomplete relaxation. These become more 

 frequent and* more complete towards the end of this period, and at its 

 termination the relaxation phases are complete but rather irregular in rhythm. 

 During the first period the extensor, gastrocnemius, plays little part, and may 

 exhibit no movement at all. Towards the end gastrocnemius begins to exhibit 

 movements synchronous with the relaxations of tibialis anticus. 



In the second period the movements of the antagonistic muscles are very 

 regular and alternate. The flexor record demonstrates regular phases of 

 contraction separated by regular phases in which the muscle remains relaxed. 

 The extensor record exhibits contractions of the muscle synchronous with the 

 relaxation phases of tibialis anticus. These contractions are of rapid initiation 

 and short duration. They seem to commence at the moment in which the 

 movement of the flexor changes from contraction to relaxation, and they 

 resemble the " extensor rebound " observed after the cessation of a state of 

 extensor inhibition. 



In the third period of the reaction the movements of the flexor become 

 smaller in extent and appear at greater intervals of time. There is a change 

 in the type of movement of the extensor. The extensor record changes 

 completely in appearance. The muscle tends to remain in a state of con- 

 traction, and this is broken by phases of relaxation which are synchronous 

 with, but commence before, the contractions of the flexor. 



This third period of the reaction is the terminal one. The movements of 

 the flexor cease. The extensor remains in a state of contraction, still, however, 



2 A 2 



