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VETERINARY PHYSIOLOGY 



drawn up, then extended, again drawn up, and so on. The 

 cause of this is that the first movement of muscles and 

 joints leads to the stimulation of nerves from the muscles 

 and joints to the spinal cord (fig. 37), and it is these which 

 brinor about the reversal of action. This leads to a fresh set 

 of impulses from the muscles and joints which re-establishes 

 the first action. So the rhythm is maintained for a consider- 

 able time after the stimulus which started it has stopped. 



These impulses from the muscles and joints do not always, 

 as in the scratch reflex, bring about a reversal effect. In 

 many cases they tend to keep up the position produced, and 



Fig. 38. — Reflex Figures struck in Decerebrated Cat on stimulating a fore 

 and a hind paw. (Sherrington.) 



hence to fix a particular posture. This is best seen in 

 animals which have had their cerebrum removed above the 

 tectum (fig. 38). 



Postural Reflexes. — By putting such an animal in 

 certain postures, sustained reflex movements may be caused, 

 or they may be inhibited. 



Magnus has shown that, in decerebrated cats, the position 

 of the head and the cervical vertebras modifies the tonus of 

 the muscles of the limbs, and so determines the position 

 taken by the animal. If the head is bent forward, as when 

 a cat looks under an object in pursuit of its prey, the tonus 

 of the fore limbs decreases, that of the hind limbs increases, 



