Intrinsic Factors in the Act of Progression in the Mammal. 309 



integrated in the act of progression, may be obtained by the artificial 

 stimulation of exteroceptive or of proprioceptive end-organs, the suggestion 

 has naturally arisen that the act of progression may be entirely determined 

 by cyclic variations of the stimuli which arise in peripheral sense organs, 

 and are themselves conditioned by the movements which they engender 

 — in other words, that the act of progression is automatic and conditioned by 

 the integration of reflex movements which follow each other successively, 

 and each of which determines the stimulus which calls the following 

 movement into being. 



In discussing the nature of progression, Philippson* has laid stress upon 

 the stimulation of the skin as a factor in the determination of the act. 

 Before this, Sherringtonf had observed that in the spinal dog and cat, 

 the ipsilateral flexion reflex may be evoked, soon after the trans-section, 

 by pressure so directed upwards upon the pads of the foot that the toe-joints 

 are extended — the stimulus being comparable to the pressure of the ground 

 in progression. But later, when the shock of the operation has passed, the 

 same stimulus will evoke the ipsilateral extension reflex — the " extensor 

 thrust" — if the stimulated limb be in a state of flexion. If it be extended, 

 the reaction is one of flexion as before. Philippson believes that this reflex 

 plays an important part in the mechanism and determination of progression. 

 He supposes that the first contact of the limb with the ground evokes the 

 extensor thrust. This reaction is reinforced by the crossed extension which 

 accompanies the flexion of the opposite limb. These two determine an 

 increase of pressure of the foot upon the ground, and this peripheral 

 exteroceptive stimulus then causes a reflex flexion of the same limb and an 

 extension of the opposite limb, which then is about to come, in its turn, into 

 contact with the ground. The former limb is now flexed and carried 

 forward while the latter is in contact with the ground, and the stretching of 

 the skin thus caused in the inguinal region determines the appearance of 

 extension in the flexed limb. This extension brings the limb on to the 

 ground, where the contact determines the extensor thrust. And so the cycle 

 begins again. On this hypothesis, it is, however, difficult to explain the 

 mechanism of Freusberg'sj " mark-time " reflex. There the " late spinal " 

 dog, when suspended free from the ground, performs movements of pro- 

 gression with its hind limbs. Philippson thinks that here the crossed 

 extension due to the ipsilateral flexion, combined with the inguinal stretching 



* ' L'autonomie et la centralisation dans le systeme nerveux des animaux,' Bruxelles, 

 1905. 



t ' Koy. Soc. Proc.,' 1900, vol. 66, p. 66. 



1 1 Arch. f. d. ges. Physiol.,' 1874, vol. 9, p. 358. 



