SECTION VIII. 

 THE CENTRAL NERVOUS SYSTEM. 



CHAPTER XXXV. 



REFLEX ACTION IN THE MAMMALIA. 



Although certain basic facts concerning the physiology of reflex 

 action can be learned by observations on the spinal frog (p. 90) the 

 variety and complexity of the reflex movements exhibited by the 

 preparation are too limited to enable us to understand much about 

 reflex action in the higher animals. Practically the only reflex 

 movement elicitable in the spinal frog is the flexion reflex, which is 

 of a type quite different from that of the reflexes that are concerned 

 in maintaining such animals as the dog or man in the erect posture 

 (postural reflexes), or in enabling movement to occur from place to 

 place. The functional unit of the nervous system is the reflex arc 

 and the ability to perform complex co-ordinated movements de- 

 pends on the integration of the various reflex arcs among one another 

 the integrative action of the nervous system (Sherrington). In 

 order that we may study the principles that govern this integration 

 it is necessary to simplify the conditions from those obtaining in 

 the intact animal, which is done by breaking the connection between 

 the higher brain centres and those of the lower portion of the spinal 

 cord (the spinal animal). 



Demonstration 14. The most satisfactory preparation to 

 use for a study of the spinal reflexes is one in which the spinal cord 

 has been cut some weeks previous to the observation. Spinal shock 

 will have been recovered from and the following reflex movements 

 can readily be demonstrated :* 



*These can readily be demonstrated with the animal lying on his side, but if 

 graphic records are desired, as for measuring latent periods, etc., it is best to sus- 

 pend the animal in a suitable stand with the posterior extremities hanging free. 

 Threads attached near the paws are carried over directing pulleys (or glass rods) 



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