SECTION XIII 

 THE FUNCTIONS OF THE CEREBELLUM 



THE carrying out of co-ordinated movements is associated with and regu- 

 lated by afferent impressions which can be divided into two main groups. 



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

 ment of the animal, working on sensory structures or ' receptors,' of varying 

 qualitative sensibility, in the surface of the body. These receptors may be 

 excited by the mechanical stimuli of pressure, by changes of temperature, 

 or 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, if of sufficient intensity, 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 impressions 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 ' proprioceptive ' system, since it is excited, not directly 

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

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

 immediate result of stimulation of the other system, the ' exteroceptive,' 

 which is distributed over the surface of the body. Since it is excited by 

 the movement of the muscles themselves, i.e. by the first result of the reaction 

 to external stimulus, it serves as a governing mechanism to regulate the 

 extent of each motor discharge. Its excitation' not only prevents over-action 

 of the muscles, but may evoke a compensatory reflex in an opposite direction 

 to the reflex immediately excited from the skin. A marked feature of this 



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