CHAPTER LXXI 

 THE PROPRIO-CEPTIVE MECHANISM 



INASMUCH as the force of gravity is continuously acting upon 

 the organism, it is necessary for it to develop some mechanism by which 

 its position in regard to gravity may be appreciated, and also, when 

 moving, its successive positions in space. In the lower organisms, 

 such a mechanism is developed from the epithelium, and is of such a 

 nature that the perceiving tissues are stimulated by the positions 

 of small heavy bodies which press against them by the force of gravity. 

 When the organism moves or changes its position, such bodies will affect 

 the sensitive surface by their inertia, or the latter may be stimulated 

 by the flow of fluid over it. In higher animals, the sensitive receptor 



FIG. 381. TENTACULOCYST (STATOCYST) OF A MEDUSA. (Redrawn after Hevtwig 

 from Dahlgren and Kepner.) 



Stl. is the statolith enclosed in a pedicle which sways with the animal's motion and 

 affects the hairs which project from the surface. 



mechanism has become removed from the surface of the body, and 

 has come to lie within the head in the vestibular apparatus, and 

 within the body in connection with the muscles, tendons, and joints. 

 The proprio-ceptive mechanism is the mechanism of sense of 

 position and movement the mechanism by which we are able to 

 poise our bodies in space, by which, also, we are able to adjust our 

 muscular movements to a great degree of accuracy, especially the 

 movements of the limbs. It is this mechanism which enables a 

 man to shave in the dark or with his eyes shut. By the proprio- 

 ceptive mechanism of his head he is aware of its position; b.y the 



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