CHAPTER XXVIII. 



THE NERVOUS SYSTEM (Cont'd). 



The Brain. 



The first question which naturally arises is, what influence 

 does the brain have on the reflex movements produced through 

 the spinal cord? These influences may be summarized as fol- 

 lows : 



1. The brain enables the animal to will that a particular 

 movement shall or shall not take place, irrespective of the stimu- 

 lation of spinal reflexes. Much of this influence of the brain is 

 of course voluntary in nature, but some of it is subconscious or 

 involuntary. In general it may be said that the cerebrum, 

 through the pyramidal tracts, usually exercises a damping or 

 inhibitory influence on the spinal reflexes. It is for this reason 

 that the reflex response to a certain stimulus is usually much 

 more pronounced in a spinal, as compared with a normal animal. 

 For example, it is impossible to bring about the scratch reflex 

 in many normal dogs, whereas it is always present in spinal 

 animals. 



In man this restraining influence of the pyramidal tracts on 

 spinal reflexes is .very evident in the case of the knee jerk, which, 

 it will be remembered, is the extension of the leg which occurs 

 when the stretched patellar tendon is tapped. Ordinarily the 

 kick is moderate in degree, but in patients whose pyramidal 

 tracts are diseased, as in spastic paraplegia, it becomes very 

 pronounced. 



2. The brain, being the receiving station for the projicient 

 sensations (p. 279), sight, hearing and smell, adds greatly to 

 the number of afferent pathways by which reflex actions can 

 be excited. 



3. Since in higher animals all the afferent impulses usually 



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