PART V 

 THE PROPRIOCEPTIVE SYSTEM 



By the proprioceptive system is meant the mechanism 

 which is concerned in the transmission and reception of 

 impressions which arise in the organism as the result of 

 changes in its relation to the environment and of changes 

 in the relative positions of parts of the body. The de- 

 cerebrate animal, when suspended, adopts a certain posture, 

 the hmbs being partly flexed, difierent muscles being in 

 a different state of tension or tonus. When the posterior 

 roots are cut this state of tonus is at once abohshed, the 

 position of the limbs being now determined by gravity 

 alone. As this effect is not produced when the cutaneous 

 sensory nerves are cut, the afferent impressions which give 

 rise reflexly to tonus must arise in the deeper structures — 

 in the joints and in the muscles themselves. The intact 

 animal is aware of the position of his hmbs and of any 

 changes in position which his hmbs undergo. 



The afferent nerves arise as extensive arborisations 

 surrounding the tendons and bundles, and as branches 

 which are entwined around certain of the muscle-fibres. 

 The latter structures are known as muscle-spindles. Con- 

 scious sensations of position and of passive movement 

 pass, as already described, to the motor area of the 

 cerebrum. Unconscious impressions, as we shall see, pass 

 up to the cerebellum. 



THE LABYRINTH 



While the afferent impulses from muscles and joints give 

 us information regarding the position of our hmbs, by other 



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