SECTION XIII 

 THE FUNCTIONS OF THE CEREBELLUM 



IN discussing the spinal mechanisms for the carrying out of co- 

 ordinated movements we have seen that in every case the reflex 

 discharge is associated and regulated by afferent impressions. These 

 afferent impressions can be divided into two main groups. 



In the first group may be placed those due to the changes in 

 the environment of the animal, working on sensory structures or 

 ' receptors,' of varying qualitative sensibility, in the surface of the 

 body. Among these receptors are those which are excited by the 

 mechanical stimuli of pressure, those excited by changes of tempera- 

 ture, and those excited by nocuous or harmful impressions, such as 

 would, in the presence of consciousness, give rise to pain. At the fore 

 end of the body we have in addition the special receptor organs 

 excited by waves of light or of sound. The action of any of these 

 impressions falling with sufficient intensity on any part of the body is 

 to evoke an appropriate reflex movement, such as the flexor reflex in 

 response to nocuous stimulus applied to the foot, or the stepping, or 

 extensor, reflex excited by steady pressure on the sole of the foot. 



The integrity of the nerve paths carrying these afferent impres- 

 sions and of the motor paths to the muscles is not, however, sufficient. 

 A secondary set of afferent impulses is essential in order to guide 

 and regulate the extent of the resultant discharge. These secondary 

 afferent impulses start in the deep tissues, viz. the muscles, joints, 

 and ligaments, which are provided with special sense-organs capable 

 of being stimulated by the mechanical changes of tension or pressure 

 set up by the movements themselves. The importance of these 

 impressions for the carrying out of muscular movements is shown 

 by the ataxia which is the result of injury to the corresponding 

 afferent nerves. Degeneration of the nerves to muscles, or section 

 of the afferent roots, causes marked ataxia of the movements of the 

 limb, whereas no such result follows section of all the cutaneous 

 nerves supplying the surface of the limb with sensibility. To this 

 system of afferent nerves Sherrington has given the name of the ' pro- 

 prioceptive ' system, since it is excited, not directly by changes in 

 the environment, but by alteration in the animal itself. It is respon- 

 sible for reactions differing in many respects from those which are the 



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