THE PROPRIO-CEPTIVE MECHANISM 659 



It is possible that sense of movement is referable to the semi- 

 circular canals, and sense of position to the end-organ of the utricle. 

 Here the solid particles the otoliths play a part by acting upon the 

 hair cells with varying pressures, according to the position of the head. 



The Proprio-ceptive Mechanism of the Body. The sensations 

 concerned in " kinaesthetic sense," or the " muscular sense," as it is 

 sometimes called, arise from the muscles themselves, the joints, and 

 the tendons. As the receptor mechanism special sensory nerve ter- 

 minations have been described in muscle, the neuro-muscular spindles. 

 These are long, fusiform structures in connection with the muscle 

 fibres. Into them passes a medullated nerve, which finally breaks 

 up into a non-medullated plexus surrounding the modified muscle 

 fibre (Fig. 386). JJ 



In connection with the tendons are found the organs of Golgi 

 small fibrous capsules containing a plexus of non-medullated nerve- 

 fibres derived from a branch of medullated nerve. Somewhat similar 

 varicose nerve terminations are also found in the synovial membranes 

 and ligaments of joints. ;. 



l-'iu. 386. KUBO-TK3?0I3TOU8 NF.KVE END-OKUAX IN RABBIT. (Redrawn after 

 Hviber and De Witt from Dahlgren and Kepner.) 



The nerve-supply to these structures is large. It is computed that 

 two-thirds of the fibres of the mixed sciatic nerve are connected with the 

 proprio-ceptive system, and sensory in function. By means of these 

 terminations, information is conveyed to the central nervous system 

 as to the degree of contraction or relaxation of a muscle or sets of 

 muscle, and also as to the degree of extension or flexion of a joint. 

 The position of a limb may therefore be fairly well localized, even with 

 the eyes shut. Such information is of great importance in guiding 

 and co-ordinating the movements of a limb. Such impulses do 

 not necessarily affect the consciousness, but they are of value in aiding 

 the extero-ceptive mechanism in determining the size and shape of 

 bodies as, for example, the amount and nature of muscular movement 

 necessary to feel completely over the surface. This sense is of 

 particular value in determining the size of a body in the dark. The 

 weight of a body is also gauged by determining the amount of muscular 

 contraction necessary to prevent it falling or to raise it. The condi- 

 tion of this mechanism may be investigated by testing the judgment 

 of the individual in regard to moderate weights, and also in regard 

 to ;i deep pressure.'' Deep pressure sensation e.g., the pressure of a 



