160 REFLEX ACTION AND 



backwards, and are only directed to one side when the 

 forward movement is checked by an interposed obstacle.'* 



If we look now to such reflex movements as are com- 

 monly manifested by one of the higher animals — a Frog, 

 for instance — we shall meet with the same machine-like 

 regularity in the execution of motor responses to ordinary 

 stimuli, the same semblance of an intentional effort to 

 accomplish a certain end — even when the animal has been 

 deprived of its Brain, and when the movements are there- 

 fore as involuntary and unconscious as those of the thoracic 

 segment of the Mantis above referred to. 



After the head and neck of a narcotized Frog had been 

 removed, Vulpian* slightly pinched a toe of one of the 

 stretched-out hind limbs, and observed, as others have 

 done, that this stimulus was quickly followed by a flexion 

 of all the se<. m 'uts of the limb upon one another. The 

 same result constantly followed the application of such a 

 stimulus, and as Vulpian points out: — "It is not an 

 indefinite reaction. All the muscles do not contract ; for 

 if it were so, there would be forcible extension of the limb, 

 as in strychnia poisoning, since the extensor muscles in 

 the frog are together much stronger than the flexors. 



Here, on the contrary, a certain number of 



muscles only contract, while the others remain more or 

 less inert. There is a contraction of muscles combined in 

 such a manner as to produce a particular result, and the 

 result of these harmonized contractions is to withdraw the 

 limb from the exciting cause." 



A much stronger excitation applied to one of the hinder 

 paws of this headless Frog will lead to a different reaction, 

 but still to one which is always the same under similar 

 conditions. We no longer witness a movement of flexion 

 in the limb that has been touched, but both it and the 

 * " La Physiolgie du Systerae Nerveux," p. 415. 



