CHAPTER LXXXIX 



THE REFLEXES OF THE SPINAL ANIMAL AND 

 SPINAL SHOCK 



Having become familiar with the peculiar properties of each of the 

 structures which go to make up the reflex arc, we may now proceed to 

 consider the function of the arc as a whole. It may be well first of all 

 to consider briefly the experimental method by which such studies may 

 be made. The object aimed at is to simplify the conditions as 

 much as possible, for it will be evident that, in the intact nervous sys- 

 tem, with the brain exercising a dominating influence over the great 

 majority of all -the reflexes, it would be impossible by applying a given 

 stimulus, to predict exactly what kind of reflex response it might call 

 forth. The reflex will be conditioned upon the accompanying influence 

 which the brain exercises on the reflex involved. 



In order to render the reflex unconditioned, we must remove the in- 

 fluence of higher centers. This can be done experimentally for the re- 

 flexes of a great part of the body by cutting the spinal cord above the 

 level of the segment in which the reflex .under investigation resides. 

 Some of the reflexes elicitable from the cord isolated in this way in- 

 volve only one or two neighboring segments, whereas others spread 

 over several. The reflexes which have been most extensively employed 

 are those which involve the musculature of the hind limbs. Since some 

 of the receptors concerned come from the skin of the flank and shoul- 

 der areas, the section is usually made at the upper end of the thoracic 

 region of the spinal cord. 



Spinal Shock in Laboratory Animals 



Immediately after the operation a profound condition of depression sets 

 in, involving all the reflex arcs in the separated portion of cord. This 

 condition is known as spinal shock. It supervenes in all classes of ani- 

 mals having a spinal cord, but is much more profound in the higher 

 than in the lower animals. As a result of this depression, the part of 

 the body below the section exists in a limp and flaccid condition, and the 

 application of even very strong stimuli to the skin will evoke no form 

 of reflex movement. In the case of the lower animals, such as the frog, 



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